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2024 | Automated control and stabilization of ultrabroadband laser pulse angular dispersion We present an innovative automatic control of angular dispersion for high-power laser systems. A novel, to the best of our knowledge, diagnostic has been developed to visualize angular dispersion in ultrashort near-infrared laser pulses for on-shot analysis. The output of a commercial ultrabroadband oscillator was prepared with an arbitrary chromatic dispersion and sent through a compensation system composed of 4° glass wedges in motorized mounts. These wedges were rotationally controlled in discrete steps about the beam axis in accordance with the diagnostic, via an automated feedback loop, to successfully eliminate angular dispersion to a precision of 5 nrad/nm. The system can be implemented to maintain a zero or nonzero target dispersion for experiments. |
| Continuous ultraviolet to blue-green astrocomb Cosmological and exoplanetary science using transformative telescopes like the ELT will demand precise calibration of astrophysical spectrographs in the blue-green, where stellar absorption lines are most abundant. Astrocombs—lasers providing a broadband sequence of regularly-spaced optical frequencies on a multi-GHz grid—promise an atomically-traceable calibration scale, but their realization in the blue-green is challenging for current infrared-laser-based technology. Here, we introduce a concept achieving a broad, continuous spectrum by combining second-harmonic generation and sum-frequency-mixing in an MgO:PPLN waveguide to generate 390–520 nm light from a 1 GHz Ti:sapphire frequency comb. Using a Fabry-Pérot filter, we extract a 30 GHz sub-comb spanning 392–472 nm, visualizing its thousands of modes on a high-resolution spectrograph. Experimental data and simulations demonstrate how the approach can bridge the spectral gap present in second-harmonic-only conversion. Requiring only 100 pJ pulses, our concept establishes a new route to broadband UV-visible generation at GHz repetition rates. |
| Mid-infrared optical coherence tomography with a stabilized OP-GaP optical parametric oscillator We demonstrate mid-infrared time-domain optical coherence tomography (OCT) with an orientation-patterned GaP optical parametric oscillator. Instantaneous broadband mid-infrared spectra provide reduced scattering for OCT applications including cultural heritage, quality assurance, and security. B-scan calibrations performed across the wavelength tuning range show depth resolutions of 67 µm at 5.1 µm and 88 µm at 10.5 µm. Volumetric imaging inside a plastic bank card is demonstrated at 5.1 µm, with a 1 Hz A-scan rate that indicates the potential of stable broadband OPO sources to contribute to mid-infrared OCT. |
| Cross-dispersion spectrograph calibration using only a laser frequency comb High-resolution cross-dispersion spectrographs are widely used in spectroscopy, but the two-dimensional format of the spectrum requires sophisticated calibration, conventionally performed by illuminating the instrument with a broadband hollow-cathode lamp and cross-referencing the result to an emission-line atlas. Here, we introduce a new technique to completely calibrate a high-resolution echelle spectrograph using only a laser frequency comb. Selected individual comb lines are removed from a broadband 20 GHz laser frequency comb—revealing their exact location in the spectrograph echellogram—and wavelength-tagged with sub-fm accuracy. In a complementary procedure, the comb is reduced to contain one line per echellogram order, enabling the spectrograph’s free spectral range to be visualized and the exact concatenation between orders to be determined. In this way, the complete calibration of the high-resolution spectrograph is achieved using only a laser frequency comb, directly providing GPS-referenced accuracy without the need to bootstrap the calibration by indexing the comb modes using a broadband atomic-line source. |
2023 | Fiber-delivered heterodyne spectroscopy with a mid-infrared frequency comb By exploiting the excellent short-term phase stability between consecutive pulses from a free-running optical parametric oscillator frequency comb, we report the first example of hollow-core fiber-delivered heterodyne spectroscopy in the 3.1–3.8 µm wavelength range. The technique provides a means of spectroscopically interrogating a sample situated at the distal end of a fiber, with all electronics and light sources situated at the proximal end and with an inherent capability to suppress spectroscopically interfering features present in the free-space and in-fiber delivery path. Using a silica anti-resonant, hollow-core delivery fiber, we demonstrate high quality transmission and attenuated total reflectance spectroscopy of a plastic sample for fiber lengths of up to 40 m, significantly exceeding the few-meter lengths typically possible using solid-core fibers. The technique opens a route to implementing multi-species spectroscopic monitoring in remote and / or hostile industrial environments and medical applications. |
| Hong-Ou-Mandel interference with a diode-pumped 1-GHz Ti:sapphire laser Correlated photon pairs generated through spontaneous parametric down-conversion (SPDC) are a key resource in quantum optics. In many quantum optics applications, such as satellite quantum key distribution (QKD), a compact, high repetition rate pump laser is required. Here we demonstrate the use of a compact, GHz-rate diode-pumped three-element Kerr-lens-modelocked Ti:sapphire laser for the generation of correlated photon pairs at 790 nm. We verify the presence of indistinguishable photons produced via SPDC using Hong-Ou-Mandel (HOM) interferometry and observe a dip in coincidence counts with a visibility of 81.8%. |
| GPU-accelerated full-field modelling of highly dispersive ultrafast optical parametric oscillators We demonstrate GPU-accelerated modelling of ultrafast optical parametric oscillators (OPOs) via the χ(2) nonlinear envelope equation with 1265× improvement in execution time compared with a CPU-based approach. Incorporating an adaptive step-size algorithm and absorbing boundary conditions, our model is capable of simulating OPOs containing long (>10 mm) nonlinear crystals or significant intracavity dispersion with outputs generated in less than 1 minute, allowing the investigation of systems that were previously computationally prohibitive to explore. We implement real-world parameters such as optical coatings, material absorption, and non-ideal poling domains within quasi-phase matched nonlinear crystals, producing excellent agreement with the spectral tuning behaviour and average power from a previously reported prism-based OPO. Our digital twinning approach provides a low-cost iterative development platform for ultrafast OPOs. |
| Multi-target two-photon dual-comb LiDAR By substituting two-photon cross-correlation in a wide-bandgap photodiode for the coherent gating conventionally used in dual-comb ranging, two-photon dual-comb LiDAR exchanges data-intensive interferometric acquisition for a single time-stamp from which an absolute distance can be inferred. Here, we report the application of two-photon dual-comb LiDAR to obtain real-time ranging to three independent targets with only a single silicon-photodiode detector. We show precisions of 197–255 nm (2 seconds averaging time) for static targets, and real-time simultaneous ranging to three dynamic targets driven by independent sinusoidal, saw-tooth and square waveforms. Finally, we demonstrate multi-target ranging to three points on a rigid body to provide simultaneous pitch and yaw angular measurements with precisions of 27.1 arcsec (130 µrad) on a 25 mm baseline. |
| Misalignment-free, Kerr-lens-modelocked Yb:Y2O3 2.2-GHz oscillator, amplified by a semiconductor optical amplifier We present a fully bonded, misalignment-free, diode-pumped Yb:ceramic (Yb:Y2O3) oscillator producing 190-fs pulses at a repetition frequency of 2.185 GHz. Self-starting Kerrlens-modelocked operation was obtained from both outputs of the ring cavity with an average combined output power of 14-30mW for pump powers from 380-670mW. The fully bonded design provided self-starting, turnkey operation, with a relative intensity noise of 0.025% from 1 Hz-1 MHz. Tuning of the pulse repetition rate over a 120 kHz range was demonstrated for a 2 degrees C change in temperature. Chirped-pulse amplification in a semiconductor optical amplifier was shown to increase the pulse average power to 69mW and the pulse energy (peak power) from 2.5 pJ (12 W) to 32 pJ (71 W). |
| Design, construction and characterisation of a diode-pumped, three-element, 1-GHz Kerr-lens-modelocked Ti:sapphire oscillator We present a design and construction prescription for a 1-GHz repetition rate Ti:sapphire laser pumped with a single green pump diode and with a resonator comprising as few as three optical elements. In a three-element configuration, the laser produces 111-fs pulses and exhibits self-starting Kerr-lens modelocking at pump powers above 850 mW. At 1.1 W of pump power, the average output of the laser is over 116 mW, and the slope efficiency is measured to be 13%. With the addition of a fourth dispersion-compensating element to optimise the second-order group-delay intracavity dispersion, we have demonstrated a reduction of the pulse durations to 87 fs with an average power of 108 mW. |
| Reflecting the past, imag(in)ing the past: macro-reflection imaging of painting materials by fast MIR hyperspectral analysis Imaging spectroscopy has been developed in the last two decades in the visible and infrared spectral range for detecting pigments and binders on paintings. The near-infrared (NIR) region has been proved effective for the discrimination of lipids and proteinaceous binders. More recently, the mid-infrared (MIR) range has also been tested on paintings. Reflection imaging prototypes already developed could be further optimized for cultural heritage analysis, for example by: enhancing the instrument configuration and performance; adopting compressive strategies to increase data processing speeds; using data validation to confirm that the processed image reflects the composition of a painted surface; and lowering price to enable more cost-effective analysis of large surface areas. Here, we demonstrate a novel hyperspectral Fourier transform spectrometer (HS FTS), which enables an imaging strategy that provides a significant improvement in acquisition rate compared to other state-of-the-art techniques. We demonstrate hyperspectral imaging across the 1400-700 cm(-1) region in reflection mode with test samples and the painting 'Uplands in Lorne' (Acc. No.: GLAHA43427) by D.Y. Cameron (1865-1945). A post-processing analysis of the resulting hyperspectral images, after validation of reference samples by conventional Fourier transform infrared spectroscopy, shows the potential of the method for efficient non-destructive classification of different materials found on painted cultural heritage. This research demonstrates that the HS FTS is a convenient and compact tool for non-invasive analysis of painted cultural heritage objects at spatio-spectral acquisition rates potentially higher than current FTS imaging techniques. Ultimately, when combined with fast graphics processing unit-based reconstruction, the HS FTS may enable fast, large area imaging. |
2022 | Feed-forward stabilization of a single-frequency, diode-pumped Pr:YLF-Cr:LiCAF laser operating at 813.42 nm We introduce a simple and compact diode-pumped Pr:YLF-Cr:LiCAF laser, operating at 813.42 nm and providing a 130-mW, single-frequency output tunable over a 3-GHz range. The laser has a short-term intrinsic linewidth estimated to be 700 Hz (β-separation method), while exhibiting a free-running wavelength stability of below 1 pm in one hour. Using a feed-forward technique we demonstrate the integration of the laser output into a fully stabilized, 1-GHz Ti:sapphire laser frequency comb, resulting in a heterodyne beat note between the laser and the comb with a bandwidth of 65 kHz. Combining feed-forward control with a low-bandwidth servo feedback loop permits stable long-term locking with an rms beat note variation of 15 kHz over 2 minutes. This performance makes the laser a potential candidate for the lattice laser in a 87Sr optical lattice clock. |
| Design of quasi-phase-matching nonlinear crystals based on quantum computing Quasi-phase-matching (QPM) makes it possible to design domain engineered nonlinear crystals for highly efficient and multitasking nonlinear frequency conversion. However, finding the optimal crystal domain arrangement in a meaningful time is very challenging sometimes impossible by classical computing. In this paper, we proposed a quantum annealing computing method and used D-Wave superconducting quantum computer to design aperiodically poled lithium niobate (APPLN) for coupled third harmonic generation (CTHG). We converted the optical transformation efficiency function to an Ising model which can be solved by D-Wave quantum computer. The crystal design results were simulated by using nonlinear envelope equation (NEE), which showed very similar conversion efficiencies to the crystals designed by using simulated annealing (SA) method, demonstrating that quantum annealing computing is a powerful method for QPM crystal design. |
| Three-element, self-starting Kerr-lens-modelocked 1-GHz Ti:sapphire oscillator pumped by a single laser diode We present a Kerr-lens-modelocked, three-element, diode-pumped Ti:sapphire laser producing 111-fs pulses at a repetition frequency of 1.02 GHz. Self-starting soliton-modelocked operation with an output power of 106 mW was obtained when the laser was pumped at 1.0 W with a single 527-nm laser diode. The output exhibits a relative intensity noise of 0.06% (1 Hz - 1 MHz) and locking of the repetition rate to an external reference is demonstrated with a phase error of 1.7 mrad (1 Hz-1 MHz). The simplicity of the laser makes it an attractive candidate as a module for integration into larger systems. |
| Compressive hyperspectral imaging in the molecular fingerprint band Spectrally-resolved imaging provides a spectrum for each pixel of an image that, in the mid-infrared, can enable its chemical composition to be mapped by exploiting the correlation between spectroscopic features and specific molecular groups. The compatibility of Fouriertransform interferometry with full-field imaging makes it the spectroscopic method of choice, but Nyquist-limited fringe sampling restricts the increments of the interferometer arm length to no more than a few microns, making the acquisition time-consuming. Here, we demonstrate a compressive hyperspectral imaging strategy that combines non-uniform sampling and a smoothness-promoting prior to acquire data at 15% of the Nyquist rate, providing a significant acquisition-rate improvement over state-of-the-art techniques. By illuminating test objects with a sequence of suitably designed light spectra, we demonstrate compressive hyperspectral imaging across the 700-1400 cm(-1) region in transmission mode. A post-processing analysis of the resulting hyperspectral images shows the potential of the method for efficient non-destructive classification of different materials on painted cultural heritage. Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License. |
| Hollow-core fiber delivery of broadband mid-infrared light for remote spectroscopy High-resolution multi-species spectroscopy is achieved by delivering broadband 3-4-μm mid-infrared light through a 4.5-meter-long silica-based hollow-core optical fiber. Absorptions from (HCl)-Cl-37, (HCl)-Cl-35, H2O and CH4 present in the gas within the fiber core are observed, and the corresponding gas concentrations are obtained to 5-ppb precision using a high-resolution Fourier-transform spectrometer and a full-spectrum multi-species fitting algorithm. We show that by fully fitting the narrow absorption features of these light molecules their contributions can be nulled, enabling further spectroscopy of C3H6O and C3H8O contained in a Herriott cell after the fiber. As a demonstration of the potential to extend fiber-delivered broadband mid-infrared spectroscopy to significant distances, we present a high-resolution characterization of the transmission of a 63-meter length of hollow-core fiber, fully fitting the input and output spectra to obtain the infra-fiber gas concentrations. We show that, despite the fiber not having been purged, useful spectroscopic windows are still preserved which have the potential to enable hydrocarbon spectroscopy at the distal end of fibers with lengths of tens or even hundreds of meters. Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License. |
| Development of a laser frequency comb and precision radial velocity pipeline for SALT's HRS The Southern African Large Telescope (SALT) is developing precision radial velocity capability for its high-resolution spectrograph (HRS). The instrument's high-stability (HS) mode includes a fibre double scrambler and makes provision for simultaneous thorium-argon (ThAr) injection into the calibration fibre. Given the limitations associated with ThAr lamps, as well as the cost and complexity of turn-key commercial laser frequency combs (LFCs), we are in the process of designing and building a bespoke LFC for the Red channel of the HRS (555-890 nm). At a later stage we plan to extend the wavelength range of the LFC to include parts of the blue channel (370-555 nm) as well. A data reduction pipeline capable of delivering precision radial velocity results for the HS mode is also currently under development. We aim to have the LFC and PRV pipeline available for science operations in early 2024. |
| ANDES, the high resolution spectrograph for the ELT: science case, baseline design and path to construction The first generation of ELT instruments includes an optical-infrared high resolution spectrograph, indicated as ELT-HIRES and recently christened ANDES (ArmazoNes high Dispersion Echelle Spectrograph). ANDES consists of three fibre-fed spectrographs (UBV, RIZ, YJH) providing a spectral resolution of similar to 100,000 with a minimum simultaneous wavelength coverage of 0.4-1.8 μm with the goal of extending it to 0.35-2.4 μm with the addition of a K band spectrograph. It operates both in seeing- and diffraction-limited conditions and the fibre-feeding allows several, interchangeable observing modes including a single conjugated adaptive optics module and a small diffraction-limited integral field unit in the NIR. Its modularity will ensure that ANDES can be placed entirely on the ELT Nasmyth platform, if enough mass and volume is available, or partly in the Coude room. ANDES has a wide range of groundbreaking science cases spanning nearly all areas of research in astrophysics and even fundamental physics. Among the top science cases there are the detection of biosignatures from exoplanet atmospheres, finding the fingerprints of the first generation of stars, tests on the stability of Nature's fundamental couplings, and the direct detection of the cosmic acceleration. The ANDES project is carried forward by a large international consortium, composed of 35 Institutes from 13 countries, forming a team of more than 200 scientists and engineers which represent the majority of the scientific and technical expertise in the field among ESO member states. |
2021 | Dynamic measurements at up to 130-kHz sampling rates using Ti:sapphire dual-comb distance metrology By using fully-locked Ti:sapphire combs operating with repetition-frequencies of 513 MHz, we demonstrate high-speed dual-comb distance metrology with update rates up to 130 kHz, equivalent to a sampling interval of 7.7 μs. This measurement bandwidth is achieved by limiting detection to a wavelength range much less than the pulse bandwidth, enabling interferometric precision to be reached in a time of 2.6 ms and yielding a precision of 2 nm in 100 ms. The repetition frequency achieves an instantaneous non-ambiguity range of 29.2 cm, while the high sampling rate provides the ability to make dynamic measurements, which is demonstrated by using the system to directly sample audio waveforms by recording the displacement of a mirror mounted on a loudspeaker. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. |
| Two-photon dual-comb LiDAR The interferometric signals produced in conventional dual-comb laser ranging require femtosecond lasers with long-term carrier-envelope offset frequency stability, and are limited to an upper sampling rate by radio-frequency aliasing considerations. By using cross-polarized dual combs and two-photon detection, we demonstrate carrier-phase-insensitive cross-correlations at sampling rates of up to 12x the conventional dual-comb aliasing limit, recording these in a digitizer-based acquisition system to implement ranging with sub-100 nm precision. We then extend this concept to show how the high data burden of conventional dual-comb acquisition can be eliminated by using a simple microcontroller as a ns-precision stopwatch to record the time intervals separating the two-photon cross-correlation pulses, providing real-time and continuous LiDAR-like distance metrology capable of sub-100 nm precision and dynamic acquisition for unlimited periods. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. |
| Towards a space-qualified Kerr-lens mode-locked laser We report a 1.5-GHz Kerr-lens mode-locked (KLM) Yb:Y2O3 ring laser constructed by directly bonding the cavity components onto an aluminum baseplate. Stable unidirectional operation with an output power >= 10 mW was obtained for pump-diode currents of 300-500 mA, corresponding to a total electrical power consumption of 1.5 W. After repetition rate stabilization, a comparison with a conventionally constructed identical laser showed a 50% reduction in phase noise. In free-running operation the bonded laser showed superior passive repetition rate stability. The bonding process follows an already proven integration approach in space-borne instrumentation, mapping a development pathway for KLM lasers in aerospace applications. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. |
| Investigation of a diode-pumped Ti:sapphire laser modelocked using carbon nanotubes We report a diode-pumped femtosecond Ti:sapphire laser in which a suspension of single-walled carbon nanotubes spin-coated onto a cavity mirror is the saturable absorber element. Laser performance is presented for single-diode pumping at 462 nm with 51-fs pulses being achieved with an output power of 27 mW. The laser exhibits stable operation with a relative intensity noise of 0.26% and a pulse repetition frequency of 79.24 MHz. Measurements reporting saturable and non-saturable losses each of similar to 0.1% illustrate that this saturable absorber element is particularly appropriate for use in a low gain system such as a diode-pumped Ti:sapphire laser. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. |
| Laser-frequency-comb calibration for the Extremely Large Telescope: an OPO-based infrared astrocomb covering the H and J bands The Extremely Large Telescope (ELT) will address an unprecedented optical wavelength range from 370 to 2400 nm, and its high-resolution spectrograph (HIRES) will require a laser frequency comb calibrator of comparable coverage. An architecture based around a Ti:sapphire master comb in principle enables wavelengths across this range to be obtained by a combination of second- and third-order nonlinear effects. In this scheme, near-infrared wavelength coverage can be addressed by downconversion of the comb to 1600 nm using an optical parametric oscillator (OPO), followed by broadband supercontinuum generation in highly nonlinear fiber. Here we present an example of this approach in the form of a 10 GHz astrocomb comprising a Fabry-Perot-filtered supercontinuum derived from a degenerate OPO and spanning 1.15-1.80 μm. We characterize the astrocomb using Fourier-transform spectroscopy, enabling the mode orders within the filtered comb to be identified. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. |
| Development of a deep-ultraviolet pulse laser source operating at 234 nm for direct cooling of Al+ ion clocks We report on the development of a 250-MHz 234 nm deep-ultraviolet pulse source based on a flexible wavelength-conversion scheme. The scheme is based on a frequency-doubled optical parametric oscillator (FD-OPO) together with a cascaded frequency conversion process. We use a chi((2)) nonlinear envelope equation to guide the design of an intra-cavity OPO crystal, demonstrating a flexible broadband tunable feature and providing as high as watt-level of a frequency-doubled signal output centered at 850 nm, which is served as an input wave for the cascaded frequency conversion process. As much as 3.0 mW of an average power at 234 nm is obtained, with an rms power stability of better than 1% over 20 minutes. This deep-ultraviolet pulse laser source can be used for many applications in quantum optics and for direct laser cooling of Al+ ion clocks. © 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement |
| Continuous wavelength tuning from 3.9-12 μm from an optical parametric oscillator based on orientation-patterned GaP grown on GaAs We report for the first time nonlinear frequency conversion-specifically optical parametric oscillation-in OP-GaP layers grown by hydride vapor-phase epitaxy on 3-inch OP-GaAs templates. Continuous tuning with wavelength coverage from 3.9-12 μm was achieved by using stepped and fan-out gratings having periods from 18.0-35.2 μm, which propagated 150 μm of a 1.2-mm-thick layer before overgrowth. Anti-reflection-coated OP-GaP crystals were pumped at 1040 nm with an ultrafast Yb-fiber laser, yielding idler output powers up to 150 mW (60 mW) in spectra centered at a wavelength of 5.6 μm (10.7 μm). Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. |
| Supercontinuum generation in orientation-patterned gallium phosphide Supercontinuum generation in bulk media is not normally observed at the nJ-level pulse energies available from high-repetition-rate femtosecond oscillators. Here, we present results demonstrating how a visible supercontinuum can be produced in bulk orientation-patterned gallium phosphide from 100-MHz 1040-nm femtosecond pulses with energies of up to 32 nJ. High-order parametric gain near 550 nm, seeded by self-phase-modulated spectral sidebands, underpins this new and simple supercontinuum process which yields an output spectrum spanning from the blue/green to the red. |
2020 | High resolution ZrF4-fiber-delivered multi-species infrared spectroscopy Using coherent broadband mid-infrared light from a picosecond optical parametric oscillator we introduce a flexible, easy to use, high-resolution technique which can be utilized to conduct remote stand-off, or fiber delivered, multi-species spectroscopy in a spectroscopically cluttered environment. In particular, both narrow line-like and broad continuum-like species can be handled simultaneously. If only species with narrow line-like absorptions are present, this can be done without the need for an explicit reference spectrum. We demonstrate the approach by performing absorption spectroscopy of H2O, CH4, CH3OH and C2H7NO (MEA) at high optical resolution (approximate to 0.033 cm(-1)) and via fiber delivery, opening the possibility of conducting multi-species spectroscopy in remote and hazardous environments. Spectral co-fitting of all absorption features and of the spectrum of the light-source provides a robust means of determining species concentrations, with detection limits of 290 ppb and 890 ppb obtained for CH4 and MEA respectively using a 10.5-m Herriott cell and 32 seconds measurement time. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. |
| Supercontinuum generation in orientation-patterned gallium phosphide Supercontinuum generation from nanojoule femtosecond lasers is well known in photonic-crystal fibers, channel waveguides, and micro-resonators, in which strong confinement shapes their dispersion and provides sufficient intensity for self-phase modulation, four-wave mixing, and Raman scattering to cause substantial spectral broadening. Until now, supercontinuum generation in bulk media has not been observed at equivalent energies, but here we introduce a new mechanism combining second- and third-order nonlinearities to produce broadband visible light in orientation-patterned gallium phosphide. A supercontinuum from the blue/green to the red is produced from 32 nJ 1040 nm femtosecond pulses, and a nonlinear-envelope-equation model including chi((2) )and chi((3) ) nonlinearities implies that high-order parametric gain pumped by the second-harmonic light of the laser and seeded by self-phase-modulated sidebands is responsible. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. |
| Dither-free stabilization of a femtosecond doubly resonant OPO using parasitic sum-frequency mixing Stable operation of a doubly resonant femtosecond optical parametric oscillator (OPO) requires submicron matching of the ON) and pump laser cavity lengths, which is normally implemented using a dither-locking feedback scheme. Here we show that parasitic sum-frequency mixing between the pump and resonant pulses of a degenerate femtosecond OPO provides an error signal suitable for actuating the cavity length with the precision needed to maintain oscillation on a single fringe and at maximum output power. Unlike commonly used dither-locking approaches, the method introduces no modulation noise and requires no additional optical components, except for one narrowband filter. The scheme is demonstrated on a Ti:sapphire-pumped sub-40-fs PPKTP OPO, from which data are presented showing a tenfold reduction in relative intensity noise compared with dither locking. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. |
| Open-path remote sensing for multi-species gas detection using a broadband optical parametric oscillator We demonstrate path-integrated simultaneous concentration measurements of water, methane and ethane, measuring spectra across the 3.1-3.5-μm range using 0.05 cm(-1) resolution Fourier-transform spectroscopy in-line with an ultrafast optical parametric oscillator and a simple, non-compliant target. Illumination spectra were extracted from a fitting procedure which simultaneously minimized the rms error between the experimental spectrum and a synthetic spectrum calculated from the envelope and a fitted mixture of PNNL or HITRAN absorbance data for water, methane and ethane. Simultaneous methane, ethane and water measurement at 30-m range were initially performed. Indoor measurements launched light from the OPO through a 20-cm-long gas cell containing a 1.5 +/- 0.15% ethane-in-air mixture. Light was reflected from a rough Al-foil target. Best-fit concentrations were determined to be 1.15% (water), 1860 ppb (methane) and 1.37 % (ethane). The methane background value is consistent with reported ambient levels. Respective water and ethane values were consistent with the ambient relative humidity. The second experiment demonstrated real-time methane emission measurement at 70-m range. A 2% methane:air mix was released for 100 seconds at a rate of 103 μgs(-1) at a distance of 65 m from the OPO. The signal was recorded from a simple target of rough aluminum foil situated 70 m from the OPO, with the beam passing near the emission point. This work demonstrates our ability to extract concentration data from a single spectrum with no need for averaging, which provides a real-time and quantitative monitoring capability. |
2019 | Laser spectroscopy-Using active FTIR to distinguish between multiple gases An active long-range FTIR spectroscopy system can acquire high-resolution gas absorption spectra such as hydrocarbon emissions. |
| Characterization of a carrier-envelope-offset-stabilized blue- and green-diode-pumped Ti:sapphire frequency comb Diode-pumping of Ti:sapphire provides a low-cost route to high-quality frequency-comb sources, exploiting the potential of direct diode modulation for wideband control of the carrier-envelope-offset frequency. We present here an f(REP)- and f(CEO)-locked, directly diode-pumped Ti:sapphire frequency comb, producing 66-fs pulses at 800 nm and employing f-to-2f interferometry and current modulation of a 462-nm blue laser diode to achieve a stabilization band-width extending to similar to 70 kHz. Characterizations of the f(REP) and f(CEO) phase noise are compared to relative intensity noise spectra of the pump diodes to provide insights into how the diode design and performance transfer into the comb stability, suggesting a lower contribution to f(REP) and f(CEO) noise from the blue laser diode than from the green diode. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. |
| Open-path multi-species remote sensing with a broadband optical parametric oscillator Open-path remote sensing is critical for monitoring fugitive emissions from industrial sites, where a variety of volatile organic compounds may be released. At ranges of only a few tens of metres, spatially coherent broadband mid-infrared sources can access sufficiently large absorption cross-sections to quantify hydrocarbon gas fluctuations above ambient background levels at high signal:noise ratios. Here we report path-integrated simultaneous concentration measurements of water, methane and ethane implemented in the 3.1-3.5-μm range using 0.05cm(-1)-resolution Fourier-transform spectroscopy with an ultrafast optical parametric oscillator and a simple, non-compliant target. Real-time concentration changes were observed at a range of 70 m by simulating a fugitive emission with a weak localized release of 2% methane in air. Spectral averaging yielded a methane detection sensitivity of 595 ppb.m, implying a system capability to resolve few-ppb concentrations of many volatile organic compounds at observation ranges of 50-100 m. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. |
| Autonomous multi-species environmental gas sensing using drone-based Fourier-transform infrared spectroscopy Unmanned aerial vehicles (UAVs)-or drones-present compelling new opportunities for airborne gas sensing in applications such as environmental monitoring, hazardous scene assessment, and facilities' inspection. Instrumenting a UAV for this purpose encounters trade-offs between sensor size, weight, power, and performance, which drives the adoption of lightweight electrochemical and photo-ionisation detectors. However, this occurs at the expense of speed, selectivity, sensitivity, accuracy, resolution, and traceability. Here, we report on the design and integration of a broadband Fourier-transform infrared spectrometer with an autonomous UAV, providing ro-vibrational spectroscopy throughout the molecular fingerprint region from 3 - 11 μm (3333 - 909 cm(-1)) and enabling rapid, quantitative aerial surveys of multiple species simultaneously with an estimated noise-limited performance of 18 ppm (propane). Bayesian interpolation of the acquired gas concentrations is shown to provide both localization of a point source with approximately one meter accuracy, and distribution mapping of a gas cloud, with accompanying uncertainty quantification. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. |
| Open-Path Mid-Infrared Remote Sensing of Atmospheric Gases Using a Broadband Optical Parametric Oscillator Using active Fourier-transform spectroscopy with a mid-infrared ultrafast optical parametric oscillator we demonstrate quantitative, open-path, simultaneous concentration measurements of water, methane and ethane at over 30-m range with a simple target. © 2019 The Author(s) |
| Multi-Species Environmental Gas Sensing Using Drone-Based Fourier-Transform Infrared Spectroscopy We report a broadband FTIR spectrometer integrated with an autonomous UAV enabling quantitative aerial surveys of multiple gas species simultaneously with a demonstrated sensitivity of 37 ppm and an estimated noise-limited performance of 18 ppm. © 2019 The Author(s) |
2018 | White powder identification using broadband coherent light in the molecular fingerprint region We show that a variety of white powder samples, including painkillers, amino acids, stimulants and sugars are readily discriminated by diffuse reflectance infrared spectroscopy involving no preparation of the sample and no physical contact with it. Eleven powders were investigated by illuminating each sample with broadband coherent light in the 8-9-μm band from an OPGaP femtosecond optical parametric oscillator. The spectra of the scattered light were obtained using Fourier-transform spectroscopy. Similarities between different spectra were quantified using Pearson's correlation coefficient, confirming that spectral features in the 8-9-μm wavelength region were sufficient to discriminate between all eleven powders evaluated in the study, offering a route to simple and automated non-contact chemical detection. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. |
| Systematic spectral shifts in the mid-infrared spectroscopy of aerosols Infrared spectroscopy in the spectral fingerprint region from 6.5 to 20 μm has been applied for decades to identify vapor- and condensed-phase chemicals with high confidence. By employing a unique broadband laser operating from 7.2- to 11.5-μm we show that, in this region, wavelength-dependent Mie-scattering effects substantially modulate the underlying chemical absorption signature, undermining the ability of conventional infrared absorption spectroscopy to identify aerosolized liquids and solids. In the aerosol studied, Mie theory predicts that the positions of spectroscopic features will blue-shift by up to 200 nm, and this behavior is confirmed by experiment, illustrating the critical importance of considering Mie contributions to aerosol spectroscopy in the mid infrared. By examining the spectroscopy of light scattered from an aerosol of the chemical diethyl phthalate, we demonstrate excellent agreement with a Mie-scattering model informed by direct measurements of the particle-size-distribution and complex refractive index. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. |
| Long-wave infrared generation from femtosecond and picosecond optical parametric oscillators based on orientation-patterned gallium phosphide Optical parametric oscillators synchronously pumped with 1-A μm femtosecond and picosecond lasers are used to generate long-wave mid-infrared radiation using the nonlinear material orientation-patterned gallium phosphide. The output spectra from the femtosecond OPO are measured, demonstrating tuning based on grating period and temperature from 5.5 to 13.0 A μm. The picosecond OPO produces 137 mW at 7.87 A μm, representing the first picosecond-pumped OPO using orientation gallium phosphide. |
| Towards Deep UV Femtosecond Optical Frequency Combs Based on High Repetition Rate High Average Power Femtosecond Yb:Fiber Lasers Femtosecond optical frequency combs have been undertaking a fast development since the end of last century, which are typically based on femtosecond lasers with wavelength varies from 800 nm to 2 μm. Optical frequency conversions based on.(2) and.(3) nonlinear effects, such as optical parametric interaction, second harmonic generation (SHG), sum-frequency mixing (SFM), four-wave mixing (FWM), etc. extend the spectral range of frequency combs spanning from deep UV (DUV) to mid-infrared (MIR) wavelengths [1]. These combs have been introducing revolutionary promotions to the fields of precision optical spectroscopy, optical metrology and laser-mater interactions. High average power high repetition rate Yb: fiber laser synchronously pumped femtosecond optical parametric oscillators (OPOs) can generate wide range tunable optical frequency combs with high comb energy at some specific spectra, enabling both deep UV spectroscopy for atoms and midinfrared spectroscopy for molecules when combined with SHG and SFM. Particularly we developed such an OPO system to generate 234 nm comb for aluminium ion cooling of an optical clock [2]. |
| Singly-and doubly-resonant femtosecond optical parametric oscillators for precision spectroscopy from the near-to-mid-infrared Optical parametric oscillators (OPOs), pumped by Ti:sapphire and Yb-doped femtosecond lasers, provide unique capabilities to address a broad range of parameters of interest to precision spectroscopy. We review here a variety of OPOs under development that offer tuning from 1.5 to 13 μm, repetition rates from 100 MHz to 10 GHz and pulse durations from < 25 fs to a few picoseconds. Spectroscopic techniques revealing the individual frequency comb modes are discussed, along with dual-comb spectroscopy at 3 μm and from 6 - 8 μm. |
| ELT-HIRES the High Resolution Spectrograph for the ELT: Fabry-Perots for use as calibration sources High resolution spectroscopy enables the detection of atmospheres of exoplanets. To reach the required radial velocity precision of about 1 m/s, calibration with even more precise sources is mandatory. HIRES will employ several calibration sources, the most important ones are an Laser Frequency Comb (LFC) and Fabry-Perots (FP). The LFC needs to be filtered with a set of FP. One possible solution is to illuminate this set of FP with a broadband light source and use them as calibrators, when they are not used for filtering the LFC. It has been demonstrated that passively-stabilized FP can perform better than 10 cm/s per night. We give an overview of the currently used FP in different surveys and compare their individual features. For the FP which may be used in HIRES we discuss different configuration. We show that the Finesse and FSR of the FP needs to be optimized with regard to the resolution of the spectrograph and we outline how we aim to fulfill the requirements of HIRES. |
| Chemical detection using broadband femtosecond optical parametric oscillators in the 6-12-micron spectral fingerprint region The broad and slowly varying spectral features of liquids and solids require broadband sources in the mid- to long-wave infrared for their detection and identification. We present here a range of measurements made using uniquely tunable femtosecond optical parametric oscillators, which have enabled stand-off Fourier-transform spectroscopy to be implemented across a large part of the spectral fingerprint region. In this way we have achieved active stand-off detection of liquids on surfaces, of powders and of airborne liquid particles in aerosol form. We discuss the optical parametric oscillator technology, the spectroscopy implementations and the detection capabilities and limitations of the techniques. |
| Dual-comb spectroscopy in the spectral fingerprint region using OPGaP optical parametric oscillators Dual-comb spectroscopy using OPGaP optical parametric oscillators is demonstrated with cross-correlation-based spectral averaging to obtain high-fidelity spectra of H2O and CH4 at approximately 0.3 cm(-1) resolutions from 1285-1370 cm(-1) and 1500-1585 cm(-1). |
| Identification of white powder samples using broadband coherent light in the molecular fingerprint region Stand-off Fourier-transform spectroscopy of eleven white powder samples illuminated by a broadband femtosecond source was used to construct a library of spectra, which are demonstrated to enable automated chemical detection using a simple correlation algorithm. |
| 10-GHz Femtosecond Degenerate Optical Parametric Oscillator We report a synchronously pumped degenerate optical parametric oscillator (DRO) with a 90-nm bandwidth at 1.634 μm, producing 400-mW average power femtosecond pulses at 10 GHz, the highest repetition frequency femtosecond DRO demonstrated to date. |
2017 | Dual-comb spectroscopy in the spectral fingerprint region using OPGaP optical parametric oscillators The new material orientation patterned gallium phosphide (OPGaP) enables coherent and broadband infrared spectroscopy in the spectral fingerprint region from 6 to 12 μm, accessing the largest molecular absorption cross-sections and permitting sensitive, quantitative and species-specific measurements. Here, we show how dual-comb spectroscopy-a form of high-speed Fourier-transform spectroscopy involving no moving parts and capable of very high resolutions-can be extended to the 6-8 μm wavelength band using femtosecond optical parametric oscillators (OPOs). By acquiring dual-comb interferograms in a time comparable with the mutual decoherence time of the OPO combs we implement cross-correlation-based spectral averaging to obtain high-fidelity spectra of H2O and CH4 at approximately 0.3 cm(-1) resolutions from 1285 to 1370 cm(-1) and 1500-1585 cm(-1). Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. |
| Pure down-conversion photons through sub-coherence-length domain engineering Photonic quantum technology relies on efficient sources of coherent single photons, the ideal carriers of quantum information. Heralded single photons from parametric down-conversion can approximate on-demand single photons to a desired degree, with high spectral purities achieved through group-velocity matching and tailored crystal nonlinearities. Here we propose crystal-nonlinearity-engineering techniques with sub-coherence-length domains. We first introduce a combination of two existing methods: a deterministic approach with coherence-length domains and probabilistic domain-width annealing. We then show how the same deterministic domain-flip approach can be implemented with sub-coherence-length domains. Both of these complementary techniques create highly pure photons, outperforming previous methods, in particular for short nonlinear crystals matched to femtosecond lasers. |
| Broadband Fourier-transform spectrometer enabling modal subset identification in Fabry-Perot-based astrocombs A multi-GHz frequency comb (astrocomb) is typically realized by filtering modes of a sub-GHz frequency comb (source comb) in a Fabry-Perot etalon, which can lead to ambiguities in determining which subset of source comb modes has been filtered. Here we demonstrate a broadband Fourier-transform spectrometer (FTS) with a resolving power of R = 430,000 at 550 nm, and apply it to the identification of comb subsets from a filtered 1-GHz supercontinuum. After apodization the FTS demonstrated an instrument line shape width of 1.26 GHz which enabled individual comb-line positions to be identified with an uncertainty of 17.6 MHz, a relative precision of 5 x 10(-8). Correcting for air dispersion allowed the instrument to determine the comb-mode spacing to an accuracy of 300 Hz and filtered subsets of source comb modes to be uniquely distingished across the entire comb bandwidth from 550 to 900 nm. The inherently broadband design of the FTS makes it suitable in future applications for calibrating ultra-broadband astrocombs employed by instruments such as ELT HIRES. |
| Dual-comb mid-infrared spectroscopy with free-running oscillators and absolute optical calibration from a radio-frequency reference By using free-running independent femtosecond OPOs with a repetition-rate difference of 500 Hz we demonstrate methane absorption spectroscopy with spectral coverage simultaneously spanning the methane P, Q and R branches and with a resolution of 0.5 cm(-1). Absolute optical frequency calibration with an accuracy of 0.25 cm(-1) (0.27 nm) is achieved from simultaneous repetition-rate and carrier-envelope-offset frequency measurements, without the need for any optical reference. The calibration technique allows registration and averaging of consecutively acquired dual-comb spectra, leading to a high quality and low-noise absorbance measurement in good agreement with the HITRAN database. © 2017 Optical Society of America |
| Autocorrelation z-scan technique for measuring the spatial and temporal distribution of femtosecond pulses in the focal region of lenses In this work we present an Autocorrelation z-scan technique to measure, simultaneously, the spatial and temporal distribution of femtosecond pulses near the focal region of lenses. A second-order collinear autocorrelator is implemented before the lens under test to estimate the pulse width. Signals are obtained by translating a Two Photon Absorption (TPA) sensor along the optical axis and by measuring the second-order autocorrelation trace at each position z. The DC signal, which is typically not considered important, is taken into account since we have found that this signal provides relevant information. Experimental results are presented for different lenses and input wavefronts. © 2017 Optical Society of America |
| A decade of astrocombs: recent advances in frequency combs for astronomy A new regime of precision radial-velocity measurements in the search for Earth-like exoplanets is being facilitated by high-resolution spectrographs calibrated by laser frequency combs. Here we review recent advances in the development of astrocomb technology, and discuss the state of the field going forward. |
| Time-domain measurements reveal spatial aberrations in a sub-surface two-photon microscope We show that in a nonlinear microscopy system the effects of chromatic and spherical aberrations are revealed by a difference in the focal positions corresponding to the shortest pulse duration and the minimum lateral resolution. By interpreting experimental results from a high-numerical-aperture two-photon microscope using a previously reported spatio-temporal model, we conclude that the two-photon autocorrelation of the pulses at the focal plane can be used to minimize both the chromatic and spherical aberrations of the system. Based on these results, a possible optimization strategy is proposed whereby the objective lens is first adjusted for minimum autocorrelation duration, and then the wavefront before the objective is modified to maximize the autocorrelation intensity. © 2017 Optical Society of America |
| Wavelength calibration of a high resolution spectrograph with a partially stabilized 15-GHz astrocomb from 550 to 890 nm A visible astrocomb spanning 555-890 nm has been implemented on the 10-m Southern African Large Telescope, delivering complete calibration of one channel of its highresolution spectrograph and an accurate determination of its resolving power. A novel cocoupling method allowed simultaneous observation of on-sky, Th-Ar lamp and astrocomb channels, reducing the wavelength calibration uncertainty by a factor of two compared to that obtained using only Th-Ar lines. The excellent passive stability of the master frequency comb laser enabled broadband astrocomb generation without the need for carrier-envelope offset frequency locking, and an atomically referenced narrow linewidth diode laser provided an absolute fiducial marker for wavelength calibration. The simple astrocomb architecture enabled routine operation by non-specialists in an actual telescope environment. On-sky spectroscopy results are presented with direct calibration achieved entirely using the astrocomb. |
| Comparison of astrophysical laser frequency combs with respect to the requirements of HIRES Precise astronomical spectroscopy with the forthcoming E-ELT and its high resolution spectrograph HIRES will address a number of important science cases,1 e.g. detection of atmospheres of exoplanets. Challenging technical requirements have been identified to achieve these cases, principal among which is the goal to achieve a radial velocity precision on the order of 10 cms-1. HIRES will experience systematic errors like intrapixel variations and random variations like fiber noise, caused by the non-uniform illumination of the coupling fibers, with these and other systematic errors affecting the performance of the spectrograph. Here, we describe the requirements for the calibration sources which may be used for mitigating such systematic errors in HIRES. Precise wavelength calibration with wide-mode-spacing laser frequency combs (LFCs), so called astrocombs, has been demonstrated with different astronomical spectrographs. Here we present a comparison of currently used astrocombs and outline a possible solution to meet the requirements of HIRES with a single broadband astrocomb. |
| Pure single photon generation from nonlinear processes |
| Long-Term Stability of Low Phase Noise Active Harmonically Mode-locked Fibre Laser for Timing Distribution Applications |
| Mid-Infrared Dual-Comb Spectroscopy with Microwave-Referenced Wavelength Calibration |
| Infrared Fingerprint-Region Aerosol Spectroscopy |
| Compressive Sampling for Spectral Imaging |
| High Resolution On-Sky Spectroscopy Calibrated with a Partially Stabilized 15-GHz Astrocomb from 550-890-nm |
| Time-Domain Measurements Reveal Spatial Aberrations in a Sub-Surface Two-Photon Microscope |
| Dual-comb mid-infrared spectroscopy with free-running oscillators and complete optical calibration from a radio-frequency reference By using free-running femtosecond OPOs with a repetition-rate difference of similar to 500 Hz we demonstrate methane absorption spectroscopy at a resolution < 0.08 cm(-1), fully calibrated in the optical domain from repetition-rate and carrier-envelope-offset frequency measurements. |
| Infrared fingerprint-region aerosol spectroscopy Mie theory predicts aerosols should exhibit different mid-infrared spectral signatures to bulk materials. We verify this experimentally using broadband pulses from a 7.1-8.85-μm femtosecond optical parametric oscillator, obtaining close agreement with theoretical calculations. |
| Compressive sampling for spectral imaging An iterative algorithm is used to reconstruct the spectra of light passing through a scanning Michelson interferometer, without using a Fourier transform, potentially allowing significantly fewer images to be recorded in spectral imaging. |
| Passively Stable Astrocomb from 550-890-nm for High Resolution On-Sky Spectroscopy A broadband visible astrocomb was implemented on the 10-m Southern African Large Telescope, delivering complete calibration of one channel of its high-resolution spectrograph. The passively stable master comb removed the need for overnight CEO-frequency locking. |
| Time-Domain Measurements Reveal Spatial Aberrations in a Sub-Surface Two-Photon Microscope We experimentally demonstrate that in a sub-surface microscope the effects of chromatic and spherical aberrations are revealed by a difference in the focal positions corresponding to the shortest pulse duration and the maximum autocorrelation amplitude. |
| Molecular spectroscopy from 5-12 μm using an OP-GaP OPO We report a femtosecond optical parametric oscillator (OPO) based on the new semiconductor gain material orientation patterned gallium phosphide (OP-GaP) and being the first example of a broadband OPO operating across the molecular fingerprint region. OP-GaP crystals with lengths of 1 mm and several patterning periods were diced, polished, and antireflection (AR) coated for near-to mid-infrared wavelengths. We configured a synchronously pumped OP-GaP OPO in a 101.2-MHz resonator with high reflectivity from 1.15-1.35 μm, pumped with 150-fs pulses from a 1040-nm femtosecond laser (Chromacity Spark). The coating of one spherical mirror was optimized for transmission at the pump wavelength of 1040 nm and for high reflectivity at the resonant signal wavelength in a range from 1.15-1.35 μm, while the other spherical mirror collimated the idler beam emerging from the OP-GaP crystal and was silver coated to provide high reflectivity for all idler wavelengths. This collimated idler beam was output-coupled from the cavity by transmission through a plane mirror coated with high transmission for the idler wavelengths (5-12 μm) and high reflectivity for the signal wavelengths (1.15-1.35 μm) on an infrared-transparent ZnSe substrate. Idler spectra centered from 5.4-11.8 μm and extending to 12.5 μm were collected. The maximum average power was 55 mW at 5.4 μm with 7.5 mW being recorded at 11.8 μm. Details of Fourier transform spectroscopy using water vapor and a polystyrene reference standard are presented. |
2016 | Molecular fingerprint-region spectroscopy from 5 to 12 μm using an orientation-patterned gallium phosphide optical parametric oscillator We report a femtosecond optical parametric oscillator (OPO) based on the new semiconductor gain material orientation-patterned gallium phosphide (OP-GaP), which enables the production of high-repetition-rate femtosecond pulses spanning 5-12 μm with average powers in the few to tens of milliwatts range. This is the first example of a broadband OPO operating across the molecular fingerprint region, and we demonstrate its potential by conducting broadband Fourier-transform spectroscopy using water vapor and a polystyrene reference standard. © 2016 Optical Society of America |
| Roadmap on ultrafast optics The year 2015 marked the 25th anniversary of modern ultrafast optics, since the demonstration of the first Kerr lens modelocked Ti:sapphire laser in 1990 (Spence et al 1990 Conf. on Lasers and Electro-Optics, CLEO, pp 619-20) heralded an explosion of scientific and engineering innovation. The impact of this disruptive technology extended well beyond the previous discipline boundaries of lasers, reaching into biology labs, manufacturing facilities, and even consumer healthcare and electronics. In recognition of such a milestone, this roadmap on Ultrafast Optics draws together articles from some of the key opinion leaders in the field to provide a freeze-frame of the state-of-the-art, while also attempting to forecast the technical and scientific paradigms which will define the field over the next 25 years. While no roadmap can be fully comprehensive, the thirteen articles here reflect the most exciting technical opportunities presented at the current time in Ultrafast Optics. Several articles examine the future landscape for ultrafast light sources, from practical solid-state/fiber lasers and Raman microresonators to exotic attosecond extreme ultraviolet and possibly even zeptosecond x-ray pulses. Others address the control and measurement challenges, requiring radical approaches to harness nonlinear effects such as filamentation and parametric generation, coupled with the question of how to most accurately characterise the field of ultrafast pulses simultaneously in space and time. Applications of ultrafast sources in materials processing, spectroscopy and time-resolved chemistry are also discussed, highlighting the improvements in performance possible by using lasers of higher peak power and repetition rate, or by exploiting the phase stability of emerging new frequency comb sources. |
| Stand-off identification of aerosols using mid-infrared backscattering Fourier-transform spectroscopy We show that it is possible to identify the chemical composition of an aerosol by using Fourier-transform spectroscopy of backscattered mid-infrared light at a range of 0.2 m. An ultrafast mid-infrared optical parametric oscillator provides the illumination source for the measurement across a wavelength range from 3.2-3.55 μm containing characteristic optical absorption features for two different chemicals. Normalized detection sensitivity below 10 ppm m(-1) Hz(-1/2) is obtained. © 2016 Optical Society of America |
| Two-photon laser-assisted device alteration in CMOS integrated circuits using linearly, circularly and radially polarized light The rapidly developing semiconductor industry demands constant innovations in optoelectronic imaging of semiconductor integrated circuits to keep up with continuing device scaling. It was recently shown that two photon laser-assisted device alteration (2pLADA) can deliver precision fault isolation. Here we describe an investigation into the influence of the incident light polarization on the 2pLADA spatial resolution. Linear polarization provides a highly confined but elliptical focal spot, while circular polarization diminishes the lateral resolution but benefits from a symmetrical focal spot. Radially polarized light potentially provides the highest lateral imaging resolution in all directions at the expense of the longitudinal resolution. By comparing 2pLADA results obtained using linear, circular and radial polarizations we show that certain polarizations have advantages in particular applications. Therefore a polarization optimized 2pLADA tool can achieve a sufficiently high performance to isolate faults of transistors separated by as little as 100 nm and maybe smaller. © 2016 Elsevier Ltd. All rights reserved. |
| Frequency comb metrology with an optical parametric oscillator We report on the first demonstration of absolute frequency comb metrology with an optical parametric oscillator (OPO) frequency comb. The synchronously-pumped OPO operated in the 1.5-μm spectral region and was referenced to an H-maser atomic clock. Using different techniques, we thoroughly characterized the frequency noise power spectral density (PSD) of the repetition rate f(rep,) of the carrier-envelope offset frequency f(CEO), and of an optical comb line nu(N). The comb mode optical linewidth at 1557 nm was determined to be similar to 70 kHz for an observation time of 1 s from the measured frequency noise PSD, and was limited by the stability of the microwave frequency standard available for the stabilization of the comb repetition rate. We achieved a tight lock of the carrier envelope offset frequency with only similar to 300 mrad residual integrated phase noise, which makes its contribution to the optical linewidth negligible. The OPO comb was used to measure the absolute optical frequency of a near-infrared laser whose second-harmonic component was locked to the F = 2 -> 3 transition of the Rb-87 D2 line at 780 nm, leading to a measured transition frequency of nu(Rb) = 384,228,115,346 +/- 16 kHz. We performed the same measurement with a commercial fiber-laser comb operating in the 1.5-μm region. Both the OPO comb and the commercial fiber comb achieved similar performance. The measurement accuracy was limited by interferometric noise in the fibered setup of the Rb-stabilized laser. © 2016 Optical Society of America |
| Carrier-envelope offset frequency stabilization in a femtosecond optical parametric oscillator without nonlinear interferometry By exploiting the correlation between changes in the wavelength and the carrier-envelope offset frequency (f(CEO)) of the signal pulses in a synchronously pumped optical parametric oscillator, we show that f(CEO) can be stabilized indefinitely to a few megahertz in a 333 MHz repetition-rate system. Based on a position-sensitive photodiode, the technique is easily implemented, requires no nonlinear interferometry, has a wide capture range, and is compatible with feed-forward techniques that can enable f(CEO) stabilization at loop bandwidths far exceeding those currently available to OPO combs. © 2016 Optical Society of America |
| First light of a laser frequency comb at SALT We present preliminary results of the commissioning and testing of SALT-CRISP (SALT-Calibration Ruler for Increased Spectrograph Precision), a Laser Frequency Comb (LFC) built by Heriot-Watt University and temporarily installed at the Southern African Large Telescope (SALT). The comb feeds the High Stability mode of SALT's High Resolution Spectrograph (HRS) and fully covers the wavelength range of the red channel of the HRS: 555-890 nm. The LFC provides significantly improved wavelength calibration compared to a standard Thorium-Argon (ThAr) lamp and hence offers unprecedented opportunities to characterise the resolution, stability and radial velocity precision of the HRS. Results from this field trial will be incorporated into subsequent LFC designs. |
| Environmental Stability of Actively Mode-Locked Fibre Lasers Lasers developed for defence related applications typically encounter issues with reliability and meeting desired specification when taken from the lab to the product line. In particular the harsh environmental conditions a laser has to endure can lead to difficulties. This paper examines a specific class of laser, namely actively mode-locked fibre lasers (AMLFLs), and discusses the impact of environmental perturbations. Theoretical and experimental results have assisted in developing techniques to improve the stability of a mode-locked pulse train for continuous operation. Many of the lessons learned in this research are applicable to a much broader category of lasers. The AMLFL consists of a fibre ring cavity containing a semiconductor optical amplifier (SOA), an isolator, an output coupler, a circulator, a bandpass filter and a modulator. The laser produces a train of 6-ps pulses at 800 nm with a repetition rate in the GHz regime and a low-noise profile. This performance is realisable in a laboratory environment. However, even small changes in temperature on the order of 0.1 degrees C can cause a collapse of mode-locked dynamics such that the required stability cannot be achieved without suitable feedback. Investigations into the root causes of this failure were performed by changing the temperature of components that constitute the laser resonator and observing their properties. Several different feedback mechanisms have been investigated to improve laser stability in an environment with dynamic temperature changes. Active cavity length control will be discussed along with DC bias control of the Mach-Zehnder modulator (MZM). |
| Stand-off detection of aerosols using mid-infrared backscattering Fourier transform spectroscopy The spectrum of mid-infrared light scattered from an actively illuminated aerosol was used to distinguish between different chemicals. Using spectrally broad illumination from an optical parametric oscillator covering 3.2 - 3.55 μm, characteristic absorption features of two different chemicals were detected, and two similar molecules were clearly distinguished using the spectra of backscattered light from each chemical aerosol. |
| Frequency Comb Metrology with a Near-Infrared Optical Parametric Oscillator We show the first absolute optical frequency metrology demonstration performed with an OPO. The OPO frequency comb shows similar to 300-mrad integrated CEO phase noise and similar to 70-kHz optical linewidth at 1557-nm when fully-stabilized to a radio-frequency reference. |
| Carrier-envelope offset frequency stabilization in a femtosecond optical parametric oscillator without nonlinear interferometry The carrier-envelope offset frequency of signal pulses in a femtosecond optical parametric oscillator was stabilized with a position-sensitive photodiode. The technique is easily implemented, requires no nonlinear interferometry and has a wide capture range. |
| Broadband Mid-Infrared Dual Comb Spectroscopy with Independent Asynchronous Optical Parametric Oscillators By using free-running femtosecond optical parametric oscillators with a repetition-rate difference > 200 Hz we demonstrate methane absorption spectroscopy at a resolution < 0.2 cm(-1) and simultaneously covering the P, Q and R branches. |
| Femtosecond pulses tunable between 5.5-10.4 μm from a 1-μm-pumped OP-GaP optical parametric oscillator We exploit the wide infrared transparency, versatile phasematching and low two-photon absorption of orientation patterned gallium phosphide (OPGaP) to demonstrate a 1-μm-pumped optical parametric oscillator producing broadband idler pulses centered from 5.5-10.4 μm. |
| Stand-off identification of aerosols using mid-infrared backscattering Fourier-transform spectroscopy We show that aerosolized liquid chemicals can be identified from their optical absorption spectra by illuminating them with broadband mid-infrared pulses from an optical parametric oscillator and analyzing the backscattered light using Fourier-transform spectroscopy. |
| 4.3-cycle near-infrared pulses from a degenerate 1-GHz optical parametric oscillator We report transform-limited 23-fs pulses at 1.6-μm from a degenerate 1-GHz optical parametric oscillator (OPO), supported by a 169-nm FWHM bandwidth. This OPO forms the basis for a frequency comb for astronomical spectrograph calibration. |
| Two-photon laser-assisted device alteration in CMOS integrated circuits using linearly, circularly and radially polarized light We investigate the influence of the incident light polarization on the 2pLADA spatial resolution. By comparing 2pLADA results obtained using linear, circular and radial polarizations we show that certain polarizations offer advantages in particular applications. |
| Aberration analysis based on pinhole-z-scan method near the focal point of refractive systems In this work we present a method used to study the spherical and chromatic aberrations contribution near the focal point of a refractive optical system. The actual focal position is measured by scanning a pinhole attached on the front of a power detector, which are scanned along the optical axis using a motorized stage with 1 μm resolution. Spherical aberration contribution was analyzed by changing the pupil aperture, by modifying the size of the input iris diaphragm and for each case, measuring the actual laser power vs the detector position. Chromatic aberration was analyzed by performing the same procedure but in this case we used an ultra-broad-band femtosecond laser. The results between ML and CW operation were compare. Experimental results are presented. |
| EELT-HIRES the high-resolution spectrograph for the E-ELT The first generation of E-ELT instruments will include an optical infrared High Resolution Spectrograph, conventionally indicated as EELT-HIRES, which will be capable of providing unique breakthroughs in the fields of exoplanets, star and planet formation, physics and evolution of stars and galaxies, cosmology and fundamental physics. A 2-year long phase A study for EELT-HIRES has just started and will be performed by a consortium composed of institutes and organisations from Brazil, Chile, Denmark, France, Germany, Italy, Poland, Portugal, Spain, Sweden, Switzerland and United Kingdom. In this paper we describe the science goals and the preliminary technical concept for EELT-HIRES which will be developed during the phase A, as well as its planned development and consortium organisation during the study. |
2015 | High-resolution subsurface microscopy of CMOS integrated circuits using radially polarized light Under high numerical aperture (NA) conditions, a linearly polarized plane wave focuses to a spot that is extended along the E-field vector, but radially polarized light is predicted to form a circular spot whose diameter equals the narrower dimension obtained with linear polarization. This effect provides an opportunity for improved resolution in high-NA microscopy, and here we present a performance study of subsurface two-photon optical-beam-induced current solid-immersion-lens microscopy of a complementary metal-oxide semiconductor integrated circuit, showing a resolution improvement by using radially polarized illumination. By comparing images of the same structural features we show that radial polarization achieves a resolution of 126 nm, while linear polarization achieves resolutions of 122 and 165 nm, depending on the E-field orientation. These results are consistent with the theoretically expected behavior and are supported by high-resolution images which show superior feature definition using radial polarization. © 2015 Optical Society of America |
| Femtosecond optical parametric oscillator frequency combs Techniques to measure and manipulate the carrier-envelope phase within femtosecond optical parametric oscillators (OPOs) allow their outputs to be stabilized in a way that produces a frequency comb structure, potentially tunable throughout the transparency band of the gain material. In this review we describe the fundamental principles of phase control, on which the development of singly-and doubly-resonant OPO frequency combs is based. We give examples of practical embodiments of such combs, and discuss in detail several applications, including spectroscopy, metrology, quantum computation and astrophotonics. |
| Few-cycle near-infrared pulses from a degenerate 1 GHz optical parametric oscillator We report the generation of transform-limited 4.3-cycle (23 fs) pulses at 1.6 μm from a degenerate doubly resonant optical parametric oscillator (OPO) pumped by a 1 GHz mode-locked Ti: sapphire laser. A chi((2)) nonlinear envelope equation was used to inform the experimental implementation of intracavity group-delay dispersion compensation, resulting in resonant pulses with a 169 nm full width half-maximum spectral bandwidth, close to the bandwidth predicted by theory. © 2015 Optical Society of America |
| Mid-infrared 333 MHz frequency comb continuously tunable from 1.95 to 4.0 μm We report a 333 MHz femtosecond optical parametric oscillator in which carrier-envelope offset stabilization was implemented by using a versatile locking technique that allowed the idler comb to be tuned continuously over the mid-infrared range from 1.95 to 4.0 μm. A specially designed multi-section, multi-grating, periodically poled KTP crystal provided simultaneously phase-matched parametric down-conversion and pump + idler sum-frequency generation, enabling strong heterodyne signals with the pump supercontinuum (employed for locking) to be obtained across the tuning range of the device. The idler comb offset was stabilized to a 10 MHz reference frequency with a cumulative phase noise from 1 Hz-64 kHz of <1.3 rad maintained across the entire operating range, and average idler output powers up to 50 mW. © 2015 Optical Society of America |
| Atomically referenced 1-GHz optical parametric oscillator frequency comb The visible to mid-infrared coverage of femtosecond optical parametric oscillator (OPO) frequency combs makes them attractive resources for high-resolution spectroscopy and astrophotonic spectrograph calibration. Such applications require absolute traceability and wide combtooth spacing, attributes which until now have remained unavailable from any single OPO frequency comb. Here, we report a 1-GHz Ti:sapphire pumped OPO comb whose repetition and offset frequencies are referenced to Rb-stabilised microwave and laser oscillators respectively. This technique simultaneously achieves fully stabilized combs from both the Ti: sapphire laser and the OPO with sub-MHz comb-tooth linewidths, multihour locking stability and without the need for super-continuum generation. © 2015 Optical Society of America |
| Mode-resolved 10-GHz frequency comb from a femtosecond optical parametric oscillator We report a 10-GHz frequency comb generated by filtering a 333.3-MHz OPO frequency comb with a Fabry-Perot (FP) cavity, which was directly stabilized to the incident fundamental comb. This result is supported by a detailed analysis of the Vernier-effect-induced multiple peaks in the transmitted comb power as the FP cavity spacing is detuned. Modes of the generated 10-GHz comb were clearly resolved by a Fourier transform spectrometer with a spectral resolution of 830 MHz, considerably better than the Nyquist sampling limit. The potentially broad tuning range of this mode-resolved OPO frequency comb opens unique opportunities for precise frequency metrology and high-precision spectroscopy. © 2015 Optical Society of America |
| Multi-color carrier-envelope-phase stabilization for high-repetition-rate multi-pulse coherent synthesis Using a zero-offset carrier-envelope locking technique, we have synthesized an octave-spanning composite frequency comb exhibiting 132-attosecond timing jitter between the constituent pulses over a one-second observation window. In the frequency domain, this composite comb has a modal structure and coherence which are indistinguishable from those of a comb that might be produced by a hypothetical single mode locked oscillator of equivalent bandwidth. The associated phase stability enables the participating multi-color pulse sequences to be coherently combined, representing an example of multi-pulse synthesis using a femtosecond oscillator. © 2015 Optical Society of America |
| High power tunable femtosecond ultraviolet laser source based on an Yb-fiber-laser pumped optical parametric oscillator We report a high average power tunable 51 MHz femtosecond ultraviolet (UV) laser source based on an intra-cavity sum frequency mixing optical parametric oscillator (OPO) pumped by a fiber laser. The UV laser is generated by sum frequency generation (SFG) between the second harmonic of a mode-locked Yb-fiber laser and the signal of the OPO. A non-collinear configuration is used in the SFG to compensate the group velocity mismatch, and to increase the SFG conversion efficiency dramatically. Tunable ultraviolet pulses within the wavelength range from 385 to 400 nm have been produced with a maximum average power of 402 mW and a pulse width of 286 fs at 2 W Yb-fiber laser pump, corresponding to 20.1% near-infrared to UV conversion efficiency at 387 nm. To our knowledge, this is the first demonstration of tunable femtosecond UV pulse generation from a fiber laser pumped OPO, and is also the highest average power tunable UV femtosecond pulses from an OPO. © 2015 Optical Society of America |
| 1-GHz harmonically pumped femtosecond optical parametric oscillator frequency comb We present the first example of a femtosecond optical parametric oscillator frequency comb harmonically-pumped by a 333-MHz Ti:sapphire laser to achieve a stabilized signal comb at 1-GHz mode spacing in the 1.1-1.6-μm wavelength band. Simultaneous locking of the comb carrierenvelope- offset and repetition frequencies is achieved with uncertainties over 1 s of 0.27 Hz and 5 mHz respectively, which are comparable with those of 0.27 Hz and 1.5 mHz achieved for 333-MHz fundamental pumping. The phase-noise power-spectral density of the CEO frequency integrated from 1 Hz-64 kHz was 2.8 rad for the harmonic comb, 1.0 rad greater than for fundamental pumping. The results show that harmonic operation does not substantially compromise the frequency-stability of the comb, which is shown to be limited only by the Rb atomic frequency reference used. © 2015 Optical Society of America |
| 1-GHz harmonically pumped femtosecond optical parametric oscillator frequency comb We present the first example of a femtosecond optical parametric oscillator frequency comb by using a 333-MHz Ti: sapphire laser to achieve a stabilized comb at 1-GHz mode spacing in the 1.1-1.6-μm wavelength band. |
| Mid-infrared 333-MHz frequency comb continuously tunable from 1.95 μm to 4.0 μm Idler pulses from a 333-MHz femtosecond optical parametric oscillator were carrier-envelope-offset stabilized using a versatile locking technique which allowed the resulting comb to be tuned continuously over a range from 1.95 μm to 4.0 μm. |
| Composite 1-GHz Optical Parametric Oscillator Frequency Comb from 400-1900 nm We report a fully-stabilized synchronously-pumped optical parametric oscillator frequency comb at 1-GHz repetition frequency, comprising pump, signal and idler combs as well as combs at their sum-frequency and second-harmonic frequencies. |
| Stand-off Detection of Liquid Thin Films using Active Mid-Infrared Hyperspectral Imaging A hyperspectral imaging system was implemented using active illumination in the 3-4-μm band from an MgO:PPLN ultrafast optical parametric oscillator. Using a staring configuration based on a high-resolution mid-IR camera it was possible to distinguish between liquid chemicals based on their absorption characteristics, demonstrating the potential for standoff detection of a wide range of liquids. |
| Observations of complex frequency comb structure in a harmonically-pumped femtosecond optical parametric oscillator Various schemes allow femtosecond optical parametric oscillators to produce pulses at harmonics of their pump laser repetition frequency, each apparently offering the possibility of generating widely-spaced, tunable frequency combs. Using a 100-MHz Ti:sapphire pump laser, we have compared two alternative optical parametric oscillator architectures, both leading to 300-MHz pulses but one configured in a cavity three times shorter than the pump laser and the other in a cavity one-third longer. Heterodyne measurements between the pump and each of these two systems show that they possess different carrier-envelope offset characteristics, with implications on the coherence and stabilization of the resulting combs. |
| Stand-off Detection of Liquid Thin Films using Active Mid-Infrared Hyperspectral Imaging The idler output of an ultrafast optical parametric oscillator is used with a mid-infrared camera to distinguish between water and deuterium oxide, demonstrating the potential for standoff detection of a wide range of liquids. |
| High-resolution sub-surface microscopy of CMOS integrated circuits using radially polarized light Comparison of high-resolution sub-surface microscopy shows that illumination with linear polarization resolves an edge with resolutions of 95 nm and 120 nm, depending on E-field orientation, while radial polarization achieves a resolution of 98 nm. |
| Compression of 1030-nm femtosecond pulses after nonlinear spectral broadening in Corning Ⓡ HI 1060 fiber: Theory and experiment We present the design and implementation of femtosecond pulse compression at 1030 nm based on spectral broadening in single-mode fiber, followed by dispersion compensation using an optimized double-pass SF11 prism pair. The source laser produced 1030-nm 144-fs pulses which were coupled into CorningⓇ HI 1060 fiber, whose length was chosen to be 40 cm by using a pulse propagation model based on solving the generalized nonlinear Schrödinger equation. A maximum broadening to 60-nm bandwidth was obtained, following which compression to 60 ± 3 fs duration was achieved by using a prism-pair separation of 1025 ± 5 mm. |
2014 | Recent advances in ultrafast optical parametric oscillator frequency combs We discuss recent advances in the stabilization and application of femtosecond frequency combs based on optical parametric oscillators (OPOs) pumped by femtosecond lasers at 800 and 1060 nm. A method for locking to zero the carrier-envelope-offset of a Ti:sapphire-pumped OPO comb is described. The application of Yb:KYW-laser-pumped dual-combs for mid-infrared spectroscopy is detailed, specifically methane spectroscopy at approximately a 0.7% concentration at 1 atm. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. |
| Active FTIR-based stand-off spectroscopy using a femtosecond optical parametric oscillator We presented the first demonstration of stand-off Fourier transform infrared (FTIR) spectroscopy using a broadband mid-infrared optical parametric oscillator, with spectral coverage over 2700-3200 cm(-1). For vapor-phase water and nitromethane (NM), stand-off spectra was recorded using a concrete target at from 1-m to 2-m range and showed good agreement with reference spectra, and in NM a normalized detection sensitivity of 15 ppm . m . Hz(-1/2) was obtained. Spectra from 50-μL droplets of liquid thiodiglycol were detected at a stand-off distance of 2 m from aluminum, concrete and painted metal surfaces. Our results imply that OPO-based active FTIR stand-off spectroscopy is a promising new technique for the detection of industrial pollutants and the identification of chemical agents, explosives or other hazardous materials. © 2014 Optical Society of America |
| Characterization of a liquid-crystal ultrafast pulse shaper for ultra-broadband applications By combining broadband common-path spectral interferometry with an iterative fitting algorithm the phase response of a liquid-crystal spatial light modulator has been characterized from 400 to 770 nm, equivalent to a frequency bandwidth of 0.36 PHz. The resulting calibration not only maps the response of the device as a function of wavelength and voltage, but also provides sufficient information to recover the thickness of the liquid-crystal cell and the wavelength dependent refractive index of the liquid-crystal. The technique has applications in amplitude and phase shaping of pulses from broadband supercontinuum and optical parametric oscillator/amplifier sources. © 2014 Elsevier B.V. All rights reserved. |
| Rb-87-stabilized 375-MHz Yb: fiber femtosecond frequency comb We report a fully stabilized 1030-nm Yb-fiber frequency comb operating at a pulse repetition frequency of 375 MHz. The comb spacing was referenced to a Rb-stabilized microwave synthesizer and the comb offset was stabilized by generating a super-continuum containing a coherent component at 780.2 nm which was heterodyned with a Rb-87-stabilized external cavity diode laser to produce a radio-frequency beat used to actuate the carrier-envelope offset frequency of the Yb-fiber laser. The two-sample frequency deviation of the locked comb was 235 kHz for an averaging time of 50 seconds, and the comb remained locked for over 60 minutes with a root mean squared deviation of 236 kHz. © 2014 Optical Society of America |
| Measuring Propagation Delays of Critical Paths Using Time-Resolved LADA Laser-assisted device alteration is an established technique used to identify critical speed paths in integrated circuits. By using a synchronized pulsed laser, logic transition waveforms have been acquired that can be used to measure propagation delays of the critical path signals. A method for determining the polarity of the transitions is presented for LADA sites on bulk 28nm CMOS technology. The time-resolved LADA logic waveforms are compared to, and are in close agreement with, laser voltage probe waveforms acquired from the same device. |
| Active FTIR-based standoff detection in the 3-4 micron region using broadband femtosecond optical parametric oscillators We present the first demonstration of stand-off Fourier transform infrared spectroscopy using a broadband mid-infrared femtosecond optical parametric oscillator, with spectral coverage over 2700-3200 cm(-1). Remote spectroscopy and chemical detection from 2700-3100 cm(-1) is demonstrated for a thiodiglycol drop on concrete and anodized aluminum surfaces at a stand-off distance of 2 meters, as well as open-path spectroscopy of atmospheric water vapor from a concrete target at the same range. Comparison of the measured stand-off spectra with archived reference spectra for thiodiglycol and water vapor show good agreement. This technique provides greater spatial coherence and spectral brightness than a thermal source, and wider spectral coverage than a typical quantum-cascade laser, thereby presenting opportunities for application in the detection of industrial pollutants and the environmental identification of chemical warfare agents, explosives or other hazardous materials. |
| Femtosecond Optical Parametric Oscillator Frequency Combs at Harmonics of the Pump Laser Repetition Frequency Various schemes allow femtosecond OPOs to produce pulses at harmonics of their pump laser repetition frequency, ostensibly facilitating access to widely-spaced tunable frequency combs. Here, we show not all approaches possess equivalent carrier-envelope offset characteristics. |
| A Phase Coherent Near-Octave-Spanning Zero-Offset Composite Frequency Comb Using a zero-offset carrier-envelope locking technique, we have demonstrated that multiple pulse sequences of different colors from a femtosecond optical parametric oscillator can be coherently combined to synthesize a near-octave-spanning composite frequency comb. |
| Asynchronous Mid-Infrared Optical Parametric Oscillator Frequency Combs and Applications in Spectroscopy Principles of asynchronous optical parametric oscillator frequency combs are introduced and their performance in dual-comb mid-infrared molecular spectroscopy is presented, including a specific demonstration of methane absorption spectroscopy with a resolution of 0.2 cm(-1). |
| Stand-Off Spectroscopy and Chemical Sensing using a Femtosecond Optical Parametric Oscillator Fourier-transform spectroscopy using a femtosecond optical parametric oscillator is demonstrated. Specifically, this system is used to detect a thiodiglycol droplet from concrete and aluminum surfaces and atmospheric water vapor at a 2-m stand-off distance. |
2013 | Two-photon laser-assisted device alteration in silicon integrated-circuits Optoelectronic imaging of integrated-circuits has revolutionized device design debug, failure analysis and electrical fault isolation; however modern probing techniques like laser-assisted device alteration (LADA) have failed to keep pace with the semiconductor industry's aggressive device scaling, meaning that previously satisfactory techniques no longer exhibit a sufficient ability to localize electrical faults, instead casting suspicion upon dozens of potential root-cause transistors. Here, we introduce a new high-resolution probing technique, two-photon laser-assisted device alteration (2pLADA), which exploits two-photon absorption (TPA) to provide precise three-dimensional localization of the photo-carriers injected by the TPA process, enabling us to implicate individual transistors separated by 100 nm. Furthermore, we illustrate the technique's capability to reveal speed-limiting transistor switching evolution with an unprecedented timing resolution approaching < 10 ps. Together, the exceptional spatial and temporal resolutions demonstrated here now make it possible to extend optical fault localization to sub-14 nm technology nodes. ©2013 Optical Society of America |
| Femtosecond-laser pumped CdSiP2 optical parametric oscillator producing 100 MHz pulses centered at 6.2 μm We report the first, to the best of our knowledge, femtosecond-laser-pumped optical parametric oscillator (OPO) based on the newly developed nonlinear crystal, CdSiP2. The OPO was synchronously pumped by a mode-locked Yb:KYW/Yb:fiber master-oscillator power amplifier, providing 1.053 μm pump pulses with durations of 130 fs at a repetition rate of 100 MHz. The 0.5-mm-thick CdSiP2 crystal was cut for a type-I noncritical interaction, providing a broad phase-matching bandwidth and ensuring excellent temporal overlap among the pump, signal, and idler pulses. The OPO generated signal pulses with a spectral coverage over 1260-1310 nm and mid-infrared idler pulses with a broad spectral coverage at 5.8-6.6 μm (6.2 THz). With a 2% output coupler (OC), the threshold pump power was 600 mW. At the maximum pump power of 1.6 W, 0.55 W was absorbed due to two-photon absorption and residual linear absorption in the CdSiP2 crystal, 0.75 W was transmitted, and 53 mW signal power was extracted through the OC. We estimate that the generated idler power was 24 mW. © 2013 Optical Society of America |
| Mid-infrared dual-comb spectroscopy with an optical parametric oscillator We present the first implementation of mid-infrared dual-comb spectroscopy with an optical parametric oscillator. Methane absorption spectroscopy was demonstrated with a resolution of 0.2 cm(-1) (5 GHz) at an acquisition time of similar to 10.4 ms over a spectral coverage at 2900-3050 cm(-1). The average power from each individual mid-infrared comb line was similar to 1 μW, representing a power level much greater than typical difference-frequency-generation sources. Mid-infrared dual-comb spectroscopy opens up unique opportunities to perform broadband spectroscopic measurements with high resolution, high requisition rate, and high detection sensitivity. © 2013 Optical Society of America |
| 1.4 GHz femtosecond comb generation by Fabry-Perot filtering of optical parametric oscillator frequency comb Laser frequency combs based on optical parametric oscillators provide widely tunable sources of regularly spaced frequencies from the visible to the mid-infrared, with potential applications in precision metrology, spectroscopy and astronomy. Reported is the extension of the comb mode spacing into the GHz domain by using a Fabry-Perot cavity, the length of which was electronically stabilised to a harmonic of the original pulse repetition frequency via a dither-locking technique. A 1500 nm femtosecond optical parametric oscillator frequency comb, the repetition frequency and carrier-envelope offset frequency of which were locked to 280 MHz and 10 MHz, respectively, was filtered by a high-finesse cavity to generate a pulse sequence at 1.4 GHz, corresponding to a factor of five increase in the mode spacing of the comb. |
| High-power asynchronous midinfrared optical parametric oscillator frequency combs We introduce a new stabilization scheme providing a pair of high-power, carrier-envelope-offset (CEO) frequency-stabilized, broadband, asynchronous frequency combs operating at 3.3 μm. The two channels, each with 100 mW average power and originating from a single synchronously pumped optical parametric oscillator, share all the components for midinfrared generation and CEO-frequency detection, and can be stabilized independently at repetition frequencies up to 5 kHz apart. This unique source is fully compatible with midinfrared dual-comb spectroscopy, and the approach can be readily extended to other wavelengths. © 2013 Optical Society of America |
| High average power, widely tunable femtosecond laser source from red to mid-infrared based on an Yb-fiber-laser-pumped optical parametric oscillator We report on the highly efficient generation of widely tunable femtosecond pulses based on intracavity second harmonic generation (SHG) and sum frequency generation (SFG) in a MgO-doped periodically poled LiNbO3 optical parametric oscillator (OPO), which is pumped by a Yb-doped large-mode-area photonics crystal fiber femtosecond laser. Red and near infrared from intracavity SHG and SFG and infrared signals were directly obtained from the OPO. A 2 mm beta - BaB2O4 is applied for Type I (oo -> e) intracavity SHG and SFG, and then femtosecond laser pulses over 610 nm similar to 668 nm from SFG and 716 nm similar to 970 nm from SHG are obtained with high efficiency. In addition, the oscillator simultaneously generates signal and idler femtosecond pulses over 1450 nm similar to 2200 nm and 2250 nm similar to 4000 nm, respectively. © 2013 Optical Society of America |
| Towards optical attosecond pulses: broadband phase coherence between an ultrafast laser and OPO using lock-to-zero CEO stabilization The carrier-envelope-offset frequencies of the pump, signal, idler and related sum-frequency mixing pulses have been locked to 0 Hz in a 20-fs-Ti:sapphire-pumped optical parametric oscillator, satisfying a critical prerequisite for optical attosecond pulse synthesis. |
| Femtosecond pulse generation at 50 W average powers from an Yb:KYW-Yb:YAG planar-waveguide MOPA An Yb:YAG planar-waveguide power amplifier seeded by an Yb:KYW master oscillator is reported. The system produced 700-fs pulses at 1032 nm at average output powers of 50 W and a frequency of 53 MHz. |
| Highly-efficient 1-GHz-repetition-frequency femtosecond Yb3+:KY(WO4)(2) laser for super-continuum generation We present a 1.024-GHz-repetition-rate femtosecond Yb3+:KY(WO4)(2) laser with 61% optical-to-optical efficiency and 69% slope efficiency, generating a super-continuum of bandwidth 400 nm in silica photonic-crystal fibre. RIN measurements of the laser yielded values <0.1%. |
| Asynchronous mid-infrared broadband optical parametric oscillator for dual-comb spectroscopy Two asynchronous, broadband 3.3-μm pulse trains with a stabilized repetition-rate difference of up to 5-kHz were generated from a single optical parametric oscillator. With additional carrier-envelope-offset stabilization, it could be applied to coherent dual-frequency-comb spectroscopy. |
| Towards Optical Attosecond Pulses: Broadband Phase Coherence between an Ultrafast Laser and an OPO via Lock-to-zero CEO Stabilization The carrier-envelope-offset frequencies of the pump, signal, idler and related sum-frequency mixing pulses have been locked to 0 Hz in a 20-fs-Ti: sapphire-pumped optical parametric oscillator, satisfying a critical prerequisite for broadband optical pulse synthesis. |
| Two-Photon Laser-Assisted Device Alteration in Silicon Integrated Circuits using a 1.28-μm Femtosecond Raman-Soliton Fiber Laser By inducing two-photon absorption to perturb the switching characteristics of sensitive transistors located within the active layer of a proprietary 28-nm silicon test chip, we demonstrate time-resolved nonlinear laser-assisted device alteration. |
| Asynchronous mid-infrared optical parametric oscillator frequency combs We report high-power, carrier-envelope-offset (CEO) frequency stabilized, asynchronous dual frequency combs at 3.3- μm. The two channels, each with 100 mW average power, share all the components for mid-infrared generation and CEO-frequency detection. |
| Characterization of a liquid-crystal pulse shaper over 0.36-PHz bandwidth |
| Broadband phase coherence between an ultrafast laser and an OPO using lock-to-zero CEO stabilization |
| Asynchronous Mid-IR Optical Parametric Oscillator Frequency Combs |
2012 | Femtosecond pulses at 50-W average power from an Yb:YAG planar waveguide amplifier seeded by an Yb:KYW oscillator We report the demonstration of a high-power single-side-pumped Yb:YAG planar waveguide amplifier seeded by an Yb:KYW femtosecond laser. Five passes through the amplifier yielded 700-fs pulses with average powers of 50 W at 1030 nm. A numerical simulation of the amplifier implied values for the laser transition saturation intensity, the small-signal intensity gain coefficient and the gain bandwidth of 10.0 kW cm(-2), 1.6 cm(-1), and 3.7 nm respectively, and identified gain-narrowing as the dominant pulse-shaping mechanism. © 2012 Optical Society of America |
| Broadband phase coherence between an ultrafast laser and an OPO using lock-to-zero CEO stabilization The carrier-envelope-offset frequencies of the pump, signal, idler and related second-harmonic and sum-frequency mixing pulses have been locked to 0 Hz in a 20-fs-Ti: sapphire-pumped optical parametric oscillator, satisfying a critical prerequisite for broadband optical pulse synthesis. With outputs spanning 400 - 3200 nm, this result represents the broadest zero-offset comb demonstrated to date. © 2012 Optical Society of America |
| Octave-spanning super-continuum from a silica photonic crystal fiber pumped by a 386 MHz Yb:fiber laser We report octave-spanning super-continuum generation in a silica photonic crystal fiber (PCF) pumped by a compact, efficient, mode-locked all-normal dispersion Yb:fiber laser. The laser achieved 45% optical-to-optical efficiency by using an optimized resonator design, producing chirped 750 fs pulses with a repetition rate of 386 MHz and an average power of 605 mW. The chirped pulses were compressed to 110 fs with a loss of only 4% by using multiple reflections on a pair of Gires-Tournois interferometer mirrors, yielding an average power of up to 580 mW. The corresponding peak power was 13.7 kW and produced a super-continuum spectrum spanning from 696-1392 nm. © 2012 Optical Society of America |
| Highly efficient 1 GHz repetition-frequency femtosecond Yb3+:KY(WO4)(2) laser We present a highly efficient, diode-pumped, femtosecond Yb3+:KY(WO4)(2) (Yb:KYW) laser operating at a 1.024 GHz repetition rate. The output was centered at a wavelength of 1042 nm and had a bandwidth of 3.8 nm, leading to transform-limited pulses with durations of 278 fs determined by fringe-resolved autocorrelation measurements. The optical-to-optical conversion efficiency and slope efficiency were 61% and 69%, respectively, and the relative intensity noise was <0.1%. © 2012 Optical Society of America |
| Asynchronous midinfrared ultrafast optical parametric oscillator for dual-comb spectroscopy Two asynchronous, broadband 3.3 μm pulse trains with a stabilized repetition-rate difference of up to 5 kHz were generated using an ultrafast optical parametric oscillator. The two oscillation channels, each producing similar to 100 mW average power, ran essentially independently, and weak non-phase-matched sum-frequency mixing between them provided a timing signal that indicated when the asynchronous pulses coincided. The system has immediate applications in incoherent asynchronous optical sampling and, with additional carrier-envelope-offset stabilization, could be applied to coherent dual-frequency-comb spectroscopy. © 2012 Optical Society of America |
| Generation of stabilized asynchronous pulse trains from a synchronously pumped optical parametric oscillator Two asynchronous, broadband 3.3-μm pulse trains with a stabilized repetition-rate difference of up to 5 kHz were generated with an optical parametric oscillator. This system is potentially suitable for coherent dual-frequency-comb spectroscopy. |
| Controlling the Cross-section of Ultrafast Laser Inscribed Optical Waveguides The refractive index profile, or cross-section, of an optical waveguide is its most defining property. It directly determines the number of transverse modes supported by the waveguide and the properties of these modes. Proper control of the waveguide cross-section is therefore essential if the performance of the waveguide, or waveguide device, is to be optimised. This chapter describes how the waveguide cross-section affects the properties of the guided modes, why it is important to control the waveguide cross-section from a device engineering point of view and the various experimental techniques that have been developed to control the cross-section of ultrafast laser inscribed waveguides. |
2011 | Strain field manipulation in ultrafast laser inscribed BiB3O6 optical waveguides for nonlinear applications A novel technique was used to control the spatial overlap of the orthogonal linearly polarized waveguide modes in ultrafast laser inscribed BiB3O6 waveguides. We report that the strain fields induced by the expansion of material in the laser focus can be considered independently in the design of "type II" waveguides guiding orthogonal linearly polarized light. The waveguide with the optimal mode overlap was used for type I birefringently phase-matched second-harmonic generation of a continuous wave laser source at 1047 nm. © 2011 Optical Society of America |
| Frequency stability of a femtosecond optical parametric oscillator frequency comb Carrier-envelope-offset (CEO) and pulse-repetition frequencies of a Ti:sapphire-pumped femtosecond optical parametric oscillator were locked to uncertainties of 0.09 Hz and 0.16 mHz respectively, with the CEO beat signal linewidth being stabilized to 15 Hz (instrument limited). In-loop phase-noise power spectral density measurements showed a contribution of our servo electronics to the comb-line frequency uncertainty of up to 110 Hz. Complementary time-series data implied an in-loop comb instability of 2 x 10(-11) (1-s gate time), matching the Rb-stabilized reference used and verifying that dual servo-control of the CEO and repetition frequencies was effective in stabilizing the comb to at least this precision. © 2011 Optical Society of America |
| Solid-immersion-lens-enhanced nonlinear frequency-variation mapping of a silicon integrated-circuit By inducing two-photon absorption within the active layer of a proprietary silicon test chip, we demonstrate here solid-immersion-lens-enhanced nonlinear frequency-variation mapping of a 500-MHz ring oscillator circuit at 1560 nm. This work compares the performance of conventional single-photon linear methodologies against advanced two-photon alternatives and reports a maximum laser-induced change in the oscillator stage-delay of approximately 1 ps, and a signal injection resolution of 260 nm. © 2011 American Institute of Physics. [doi: 10.1063/1.3658873] |
| Asynchronous modelocked Yb:KYW lasers for dual-comb spectroscopy A simple scheme for directly stabilising the repetition-rate difference of two laser frequency combs is reported. For the intended application in dual-comb spectroscopy, the RMS phase error achieved for the locking was 0.1 rad, corresponding to a repetition-rate uncertainty of 0.16 Hz and a timing error of 18 as in a measurement time of 100 ms. |
| Ultra-broadband pulse evolution in optical parametric oscillators Ultrashort-pulse evolution inside a optical parametric oscillator is described by using a nonlinear-envelope-equation approach, eliminating the assumptions of fixed frequencies and a single chi((2)) process associated with conventional solutions based on the three coupled-amplitude equations. By treating the interacting waves as a single propagating field, the experimentally-observed behaviors of singly and doubly-resonant OPOs are predicted across near-octave-spanning bandwidths, including situations where the nonlinear crystal provides simultaneous phasematching for multiple nonlinear processes. © 2011 Optical Society of America |
| 650-nJ pulses from a cavity-dumped Yb:fiber-pumped ultrafast optical parametric oscillator Sub-250-fs pulses with energies of up to 650 nJ and peak powers up to 2.07 MW were generated from a cavity-dumped optical parametric oscillator, synchronously-pumped at 15.3 MHz with sub-400-fs pulses from an Yb:fiber laser. The average beam quality factor of the dumped output was M-2 similar to 1.2 and the total relative-intensity noise was 8 mdBc, making the system a promising candidate for ultrafast laser inscription of infrared materials. © 2011 Optical Society of America |
| Broadband conversion in an Yb:KYW-pumped ultrafast optical parametric oscillator with a long nonlinear crystal We report the generation of 200-nm-bandwidth mid-infrared pulses at 3.5-μm from an optical parametric oscillator incorporating a 25-mm MgO:PPLN crystal and synchronously-pumped by chirped pulses from a fiber-amplified Yb:KYW laser. A long nonlinear crystal permits efficient transfer of the pump bandwidth into the idler pulses, achieves exceptional passive stability and enables pumping using chirped pulses directly from a fiber-amplifier, avoiding the need to use lossy pulse-compression optics. © 2011 Optical Society of America |
| Ultrafast laser writing of optical waveguides in ceramic Yb:YAG: a study of thermal and non-thermal regimes We report the improvement of ultrafast laser written optical waveguides in Yb:YAG ceramics by tailoring the presence of heat accumulation effects. From a combination of ytterbium micro-luminescence and micro-Raman structural analysis, maps of lattice defects and stress fields have been obtained. We show how laser annealing can strongly reduce the concentration of defects and also reduce compressive stress, leading to an effective 50% reduction in the propagation losses and to more extended and symmetric propagation modes. |
| Advances in ultrafast optical parametric oscillators Synchronously-pumped ultrafast optical parametric oscillators are a generic technology capable of providing broadly-tunable high-repetition-rate ultrafast pulses across the visible and the infrared. Recent research in the authors' group and elsewhere has extended the performance of ultrafast optical parametric oscillators in terms of their pulse-energy, phase-stability and other parameters such as their tunability. This article reviews the current status of ultrafast optical parametric oscillators, concentrating particularly on recent advances in energy scaling and carrier-envelope-phase stabilization techniques. [GRAPHICS] Spectra of the cavity-dumped pulses obtained by using cavity-length tuning at a cavity dumping frequency of 982 kHz © 2011 by Astro Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA |
2010 | Optical coherence tomography for non-destructive investigation of silicon integrated-circuits The development of an ultra-high-resolution high-dynamic-range infrared optical coherence tomography (OCT) imaging system is reported for the novel purpose of sub-surface inspection of silicon integrated-circuits. This approach utilises an almost octave-spanning supercontinuum source and a balanced-detection scheme in a time-domain OCT configuration to achieve an axial resolution of 2.5 μm in air, corresponding to similar to 700 nm in silicon. Examples of substrate thickness profiling and device feature inspection capabilities for additional circuit navigation and characterisation are presented. © 2009 Elsevier B.V. All rights reserved. |
| Laser Action From an Ultrafast Laser Inscribed Nd-Doped Silicate Glass Waveguide We report single-transverse-mode laser oscillation from waveguides inscribed in a commercially available Nd-doped silicate glass substrate using an ultrafast fiber laser operating at 1064 nm. When pumped at 808 nm, laser action was observed at 1062 nm, with a slope efficiency of 15.0% and a maximum output power of 7.5 mW. Analysis of the laser performance implied a waveguide loss of 0.17 +/- 0.06 dB . cm(-1.) |
| Locking the carrier-envelope-offset frequency of an optical parametric oscillator without f-2f self-referencing The carrier-envelope-offset (CEO) frequency of pulses from a femtosecond optical parametric oscillator (OPO) was stabilized for 30 min without the need for f-2f self-referencing in either the OPO or its pump laser. Interference between the high-frequency modes of the pump supercontinuum and the modes of the non-phase-matched pump + signal sum-frequency-mixing pulses provided the beat signal used for locking. The -3 dB bandwidth of the locked CEO-frequency was measured as 1.1 kHz, and the cumulative phase error recorded over 1 s was 0.38 rad, representing 1-order-of-magnitude improvement in comparison to previous results. © 2010 Optical Society of America |
| Ultrafast laser inscription of bistable and reversible waveguides in strontium barium niobate crystals We report the fabrication of buried optical channel waveguides in strontium barium niobate nonlinear ferroelectric crystals by direct ultrafast laser inscription. These waveguides are strongly polarized and can be reversibly switched on and off by changing the temperature of the crystal, a characteristic we attribute to the bistable enhancement of the electro-optic coefficients at the ferro to paraelectric phase transition. © 2010 American Institute of Physics. [doi:10.1063/1.3429584] |
| Wavelength stabilization of a synchronously pumped optical parametric oscillator: Optimizing proportional-integral control We describe a formal approach to the wavelength stabilization of a synchronously pumped ultrafast optical parametric oscillator using proportional-integral feedback control. Closed-loop wavelength stabilization was implemented by using a position-sensitive detector as a sensor and a piezoelectric transducer to modify the cavity length of the oscillator. By characterizing the frequency response of the loop components, we constructed a predictive model of the controller which showed formally that a proportional-only feedback was insufficient to eliminate the steady state error, consistent with experimental observations. The optimal proportional and integral gain coefficients were obtained from a numerical optimization of the controller model that minimized the settling time while also limiting the overshoot to an acceptable value. Results are presented showing effective wavelength and power stabilization to levels limited only by the relative intensity noise of the pump laser. © 2010 American Institute of Physics. [doi: 10.1063/1.3385684] |
| Common-path self-referencing interferometer for carrier-envelope offset frequency stabilization with enhanced noise immunity A nonlinear interferometer design for stabilizing the carrier-envelope offset frequency of a Ti:sapphire frequency comb with superior immunity to air currents and acoustic noise is presented. The scheme uses a pair of Wollaston prisms for group-delay dispersion compensation, providing an all-common-path optical configuration. Out-of-loop phase noise measurements for an unshielded interferometer setup showed up to 15 dB improvement compared to a Michelson interferometer based system. Further simplification of the self-referencing scheme providing a compact single-Wollaston-prism design has been demonstrated. © 2010 Optical Society of America |
| Strain Field Manipulation in Ultrafast Laser Inscribed Nonlinear BiB3O6 Optical Waveguides Type I phase matching is demonstrated in an ultrafast laser inscribed BiB3O6 optical waveguide. A novel technique was implemented to achieve good spatial overlap of the horizontally and vertically polarized modes in "strain-field" type waveguides. ©2010 Optical Society of America |
| Carrier Envelope Offset Stabilization of an Optical Parametric Oscillator Without f-2f Self-Referencing Signal pulses from a femtosecond optical parametric oscillator were carrier-envelope- offset stabilized for similar to 10 minutes with a bandwidth of <2 kHz and a cumulative phase error over 1 second of 2.05 radians, without f-2fself-referencing. ©2010 Optical Society of America |
| Proportional-Integral Control for Wavelength Stabilization of a Synchronously-Pumped Optical Parametric Oscillator We present a rigorous method for the wavelength stabilization of a synchronously-pumped optical parametric oscillator using proportional-integral control. With wavelength stabilization active, the relative intensity noise is limited only by that of the pump laser. © 2010 Optical Society of America |
| Ultrafast Laser Inscribed Nd-Doped Silicate Glass Waveguide Laser We report laser oscillation from waveguides inscribed in Nd-doped silicate glass. Laser action was observed at 1062nm for a pump of 808nm, with a slope efficiency of 15.0% and maximum output power of 7.5mW. © 2010 Optical Society of America |
| Optical Coherence Tomography for Non-Destructive Investigation of Silicon Integrated-Circuits We present the development of an ultra-high-resolution high-dynamic-range infrared optical coherence tomography imaging system for the novel purpose of sub-surface inspection of silicon integrated-circuits. Examples of substrate thickness profiling and device feature inspection are demonstrated. © 2010 Optical Society of America |
| Noise-insensitive self-referencing interferometer for carrier-envelope offset frequency stabilization of a Ti:sapphire laser A novel design of f:2f self-referencing interferometer employing Wollaston prisms for group-delay dispersion compensation exhibits up to 15 dB reduction in phase noise in specific frequency bands compared to a Michelson interferometer arrangement. ©2010 Optical Society of America |
| OPTICAL FREQUENCY COMBS AND APPLICATIONS AT NPL |
| Two-Photon X-Variation Mapping Based on a Diode-Pumped Femtosecond Laser By inducing two-photon absorption within the device layer of a proprietary silicon test chip, the first nonlinear variant of X-variation mapping is reported by demonstrating frequency mapping of a ring oscillator circuit at 1.55 μm. |
2009 | Yb:fiber-laser-pumped high-energy picosecond optical parametric oscillator We report a high-energy extended-cavity MgO:PPLN optical parametric oscillator, synchronously-pumped by a femtosecond Yb:fiber laser. The oscillator operated at a signal wavelength of 1530 nm with a repetition-frequency of 15.3 MHz (9.8 m length) achieved using intracavity relay-imaging optics. The signal pulses had an average power above 1.0 W, durations of 1.5 ps and energies greater than 70 nJ, making it a potential source for rapid femtosecond waveguide inscription in infrared materials. © 2009 Optical Society of America |
| Coherent ultrafast pulse synthesis between an optical parametric oscillator and a laser We have demonstrated coherent pulse synthesis between the carrier-envelope, phase-locked, second-harmonic pulses from a synchronously pumped femtosecond optical parametric oscillator and those from its self-mode-locked Ti:sapphire pump laser. By using a single nonlinear crystal for parametric and second-harmonic generation, we maximized the common-mode rejection of environmental noise, obtaining a temporal overlap between the pulses with a precision of 30 as (1% of the optical period) in an observation time of 20 ms. Mutual coherence between the two parent pulses was verified optically by spectral interferometry, and synthesis was tested by measuring the autocorrelation of the combined pulses, with and without carrier-envelope phase locking. © 2009 Optical Society of America |
| 70 nm resolution in subsurface optical imaging of silicon integrated-circuits using pupil-function engineering We present experimental evidence for the resolution-enhancing effect of an annular pupil-plane aperture when performing nonlinear imaging in the vectorial-focusing regime through manipulation of the focal spot geometry. By acquiring two-photon optical beam-induced current images of a silicon integrated-circuit using solid-immersion-lens microscopy at 1550 nm we achieved 70 nm resolution. This result demonstrates a reduction in the minimum effective focal spot diameter of 36%. In addition, the annular-aperture-induced extension of the depth-of-focus causes an observable decrease in the depth contrast of the resulting image and we explain the origins of this using a simulation of the imaging process. |
| Ultrafast optical parametric oscillators for spectroscopy The combination of high spatial coherence, wide tunability and broad intrinsic bandwidth makes femtosecond optical parametric oscillators (OPOs) uniquely attractive sources for spectroscopy in the visible and infrared. In the mid-infrared the idler pulse bandwidths from such systems can extend over several hundred nanometres, making Fourier-transform spectroscopy possible, and transferring the wavelength calibration and resolution constraints from the OPO to the detection system. Unlike thermal sources of mid-infrared radiation, the spatial coherence of the output from femtosecond OPOs is extremely high, with the potential for spectroscopic measurements to be made over long free-space path lengths, in fiber or at the focus of a microscope objective. Using OPOs based on MgO:PPLN, and pumped by a self-modelocked Ti:sapphire laser, we have shown free-space and photonic-crystal-fiber-based spectroscopy of methane to concentrations as low as 50 ppm. The spectral bandwidth of the idler pulses used for gas sensing exceeds 200 nm, allowing the principal methane absorption lines around 3.3 μm to be acquired without wavelength tuning the OPO. Practical Ti: sapphire and Yb:fiber pumped based OPOs have been demonstrated that offer combinations of flexible tuning, high stability, low-threshold operation and high-energy output pulses. |
| High-Pulse-Energy Optical Parametric Oscillator in the Near- and Mid-Infrared We report an extended-cavity femtosecond optical parametric oscillator. based oil MgO:PPLN and synchronously pumped by a commercial Yb:fibre laser. Preliminary results show 15 MHz-repetition-frequency, 475 fs-duration pulses at 1.5 μm, with energies of 36 nJ. © 2009 Optical Society of America |
| Coherent Pulse Synthesis Between a Femtosecond Optical Parametric Oscillator and Its Pump Laser: Towards Isolated Attosecond Optical Pulses We demonstrate coherent Synthesis between pulses from an optical parametric oscillator and its Ti:sapphire pump laser with a mutual timing-jitter of 30 attoseconds in 20 milliseconds, sufficient for attosecond waveform synthesis. © 2009 Optical Society of America |
| 70nm Resolution in Sub-Surface Two-Photon Optical Beam Induced Current Microscopy through Pupil-Function Engineering in the Vectorial Focusing Regime We present experimental evidence for the resolution-enhancing effect of an annular pupil-plane aperture in two-photon semiconductor microscopy in the vectorial-focusing regime. At an illumination wavelength of 1550nm we achieved a resolution of 70nm (lambda/22). © 2009 Optical Society of America |
| IMPROVEMENTS TO THE ROBUSTNESS OF A TI:SAPPHIRE-BASED FEMTOSECOND COMB AT NPL This paper reports a number of improvements to a Ti:sapphire-based frequency comb. Changes to the spectral broadening set up, f:2f self-referencing arrangement and servo system are described, including a novel scheme for group-delay dispersion compensation using Wollaston prisms. In combination, these changes improved the signal-to-noise ratio of the carrier-envelope offset beat by 15 dB and increased its frequency stability by more than four orders of magnitude, as well as enabling it to be locked continuously for many hours without optical adjustment. |
| Nanoscale Optical Microscopy in the Vectorial Focusing Regime By using extreme numerical-aperture solid-immersion microscopy at 1553 nm we demonstrate, under certain circumstances, polarization-sensitive imaging with resolution values approaching 100 rim which substantially surpass the classical scalar diffraction-limit embodied by Sparrow's resolution criterion. |
| Towards Versatile Coherent Pulse Synthesis using a Femtosecond Laser and Synchronously Pumped Optical Parametric Oscillator Pulses from a femtosecond optical parametric oscillator and its Ti:sapphire pump laser were phase-locked as a prerequisite to coherent synthesis from different wavelengths. Mutual coherence was demonstrated using spectral interferometry and cross-correlation. |
| Designer Femtosecond Pulse Shaping Using Grating-Engineered Quasi-Phasematching in Lithium Niobate The generation of tailored femtosecond pulses with fully engineered intensity and phase profiles is demonstrated using second-harmonic generation of an Er: fibre laser in an aperiodically-poled lithium niobate crystal in the undepleted pump regime. Second harmonic pulse-shapes including Gaussian, stepped, square and multiple pulses have been characterised using cross-correlation frequency-resolved optical gating and shown to agree well with theory. |
2008 | Optical super-resolution with aperture-function engineering We demonstrate optical super-resolution (resolution better than conventional diffraction-limited resolution) in a simple optical configuration by using annular apertures to manipulate the pupil function of the system. The theoretical basis of the technique is described, and it is shown how good agreement between theory and experiment can be achieved by suitable selection of the principal system parameters. The ready implementation and suitability for theoretical interpretation makes the demonstration a good candidate for an undergraduate laboratory experiment in classical optics. (c) 2008 American Association of Physics Teachers. |
| Shaping ultrafast laser inscribed optical waveguides using a deformable mirror We use a two-dimensional deformable mirror to shape the spatial profile of an ultrafast laser beam that is then used to inscribe structures in a soda-lime silica glass slide. By doing so we demonstrate that it is possible to control the asymmetry of the cross section of ultrafast laser inscribed optical waveguides via the curvature of the deformable mirror. When tested using 1.55 μm light, the optimum waveguide exhibited coupling losses of approximate to 0.2 dB/facet to Corning SMF-28 single mode fiber and propagation losses of approximate to 1.5 dB.cm(-1). This technique promises the possibility of combining rapid processing speeds with the ability to vary the waveguide cross section along its length. © 2008 Optical Society of America. |
| Nanoscale optical microscopy in the vectorial focusing regime It has been known since 1959 that the focal-plane intensity distribution produced by focusing polarized light with a high-numerical-aperture lens should be highly asymmetric(1). Remarkably, the consequences of this fundamental effect in direct image acquisition have remained unexploited, although vectorial effects have been observed in the contexts of free-space focusing(2), molecular fluorescence(3) and photolithography(4). By using extreme-numerical-aperture ( values of 3.5), solidimmersion microscopy(5-7) we have obtained images of a silicon integrated circuit showing, for the first time, the dramatic influence of polarization on their spatial resolution, with values from 100 nm to 250 nm. Our data show that polarization-sensitive imaging can substantially surpass the scalar diffraction limit embodied by classical formulae such as Sparrow's criterion. Such performance will have an impact on activities such as integrated-circuit failure analysis, where optical inspection faces serious challenges from the sub-100-nm feature sizes routinely used in production devices. |
| Mid-infrared gas sensing using a photonic bandgap fiber We demonstrate methane sensing based on Fourier transform infrared spectroscopy using a hollow-core photonic bandgap fiber guiding in the mid-infrared and idler pulses from a femtosecond optical parametric oscillator. Transmission measurements are presented for several fibers, and sensing is demonstrated using a fiber whose bandgap overlaps the methane fundamental absorption lines. The gas filling process of the air core is described, and qualitative methane concentrations measurements to 1000 ppm (parts in 10(6)) are reported. Operation down to 50 ppm based on our current experiment is predicted. (c) 2008 Optical Society of America. |
| Control of the carrier-envelope phases of a synchronously pumped femtosecond optical parametric oscillator and its applications The intrinsic synchronization, multi-color outputs and related carrier-envelope phases (CEP) among pulses bring advantages to synchronously pumped femtosecond optical parametric oscillators and the pumping sources for broadband frequency comb generation and ultrashort waveform coherent synthesis. In this paper, we discuss our latest research results in this field, which cover the following aspects: the phase relationship and energy conservation law in an OPO and related experimental verification; control of the pumping Ti:sapphire femtosecond laser's CEP by self-referencing technology, and its repetition-rate locking by piezoelectric transducer (PZT); CEP locking of the pulses from the OPO by beating the non-phase-matched visible outputs against pump supercontinuum to obtain a driving signal for a fast PZT on the OPO end mirror; the generation of a broadband frequency comb spanning from 400 nm to 2.4 μm with 1.2 kHz bandwidth; and the realization of coherent interference between phase controlled pump pulses and signal second harmonic pulses. |
| Designer femtosecond pulse shaping using grating-engineered quasi-phase-matching in lithium niobate The generation of tailored femtosecond pulses with fully engineered intensity and phase profiles is demonstrated using second-harmonic generation of an Er:fiber laser in an aperiodically poled lithium niobate crystal in the undepleted pump regime. Second-harmonic pulse shapes, including Gaussian, stepped, square, and multiple pulses have been characterized using cross-correlation frequency-resolved optical gating and have been shown to agree well with theory. © 2008 Optical Society of America. |
| Towards versatile coherent pulse synthesis using femtosecond laser and optical parametric oscillators Pulses from a tunable near-infrared femtosecond optical parametric oscillator and its Ti:sapphire pump laser were phase-locked by matching their carrier-envelope phase-slip frequencies to one quarter of their common pulse repetition frequency. Interferometric second-order crosscorrelation and spectral interferometry traces demonstrated their mutual coherence for periods of at least 20 ms, compared with individual coherence times of 0.1 ms estimated from their phase-noise power spectra. This result is a prerequisite for versatile coherent pulse synthesis. Implications for the synthesis of arbitrary waveforms from multi-colour pulses are discussed. |
| Frequency comb generation and carrier-envelope phase control in femtosecond optical parametric oscillators We describe progress in the measurement and control of the carrier-envelope phase-slip frequencies of pulses generated by a femtosecond optical parametric oscillator. Example applications of such control are presented and include the generation of a frequency comb spanning nearly three optical octaves, and the creation of a train of 30-fs pulses via coherent pulse synthesis. Future prospects for frequency combs based on femtosecond optical parametric oscillators are discussed. |
| Towards coherent pulse synthesis using independently tunable femtosecond oscillators Pulses at 780 nm from a femtosecond optical parametric oscillator and its Ti:sapphire pump laser were phase-locked as a prerequisite to coherent synthesis at different wavelengths. Coherence was demonstrated using spectral interferometry and interferometric cross-correlation. (c) 2008 Optical Society of America |
| Designer Femtosecond Pulse Shaping Using Grating-Engineered Quasi-Phasematching in Lithium Niobate Tailored femtosecond pulses with fully engineered intensity and phase profiles are demonstrated using second-harmonic generation of an Er:fiber laser in an aperiodically-poled lithium niobate crystal. The profiles created include square, stepped, double and triple pulses. © 2007 Optical Society of America |
| Ultrafast laser inscription of a three dimensional fan-out device for multicore fiber coupling applications A three dimensional fan-out device has been fabricated using ultrafast laser inscription. The device allows each core of a multicore fibre to be addressed individually by a single mode fiber held in an FVA. © 2008 Optical Society of America |
| Optical Super-Resolution through Aperture-Function Engineering and Vectorial-Focusing Effects We demonstrate optical super-resolution by using custom obscuration apertures and polarization effects to manipulate the pupil-function and focal-plane point-spread-function in a two-photon microscope used for semiconductor flip-chip imaging. Experimental results suggest sub-100nm performance. © 2008 Optical Society of America |
| Nanoscale Optical Microscopy in the Vectorial Focusing Regime By using extreme numerical-aperture solid-immersion microscopy at 1553nm we demonstrate, under certain circumstances, polarization-sensitive imaging with resolution values approaching 100nm which substantially surpass the classical scalar diffraction-limit embodied by Sparrow's resolution criterion. © 2008 Optical Society of America |
| Solid immersion lens applications for nanophotonic devices Solid immersion lens (SIL) microscopy combines the advantages of conventional microscopy with those of near-field techniques, and is being increasingly adopted across a diverse range of technologies and applications. A comprehensive overview of the state-of-the-art in this rapidly expanding subject is therefore increasingly relevant. Important benefits are enabled by SIL-focusing, including an improved lateral and axial spatial profiling resolution when a SIL is used in laser-scanning microscopy or excitation, and an improved collection efficiency when a SIL is used in a light-collection mode, for example in fluorescence micro-spectroscopy. These advantages arise from the increase in numerical aperture (NA) that is provided by a SIL. Other SIL-enhanced improvements, for example spherical-aberration-free sub-surface imaging, are a fundamental consequence of the aplanatic imaging condition that results from the spherical geometry of the SIL. The theory of SIL imaging exposes the unique properties of SILs that provide advantages in applications involving the interrogation of photonic and electronic nanostructures. Such applications range from the sub-surface examination of the complex three-dimensional microstructures fabricated in silicon integrated circuits, to quantum photoluminescence and transmission measurements in semiconductor quantum dot nanostructures. |
| Weak-guidance-theory review of dispersion and birefringence management by laser inscription A brief review of laser inscription of micro- and nanophotonic structures in transparent materials is provided in terms of a compact and convenient formalism based on the theory of weak optical waveguides. We derive physically instructive approximate expressions allowing propagation constants of laser-inscribed micro- and nanowaveguides to be calculated as functions of the transverse waveguide size, refractive index step, and dielectric properties of the host material. Based on this analysis, we demonstrate that dispersion engineering capabilities of laser micromachining techniques are limited by the smallness of the refractive index step typical of laser-inscribed structures. However, a laser inscription of waveguides in pre-formed micro- and nanostructures suggests a variety of interesting options for a fine dispersion and birefringence tuning of small-size waveguides and photonic wires. |
2007 | Frequency-doubling in femtosecond laser inscribed periodically-poled potassium titanyl phosphate waveguides Frequency doubling has been achieved in femtosecond-laser-inscribed single-mode waveguides written in two periodically-poled potassium titanyl phosphate crystals. A conversion efficiency of 0.22 %W-1 was obtained for first-order quasi-phase matching at 980 nm and an efficiency of 0.02 %W-1 for third-order quasi-phase matching at 800 nm. © 2007 Optical Society of America. |
| Direct optimization of femtosecond laser ablation using adaptive wavefront shaping We introduce an approach based on using an iterative simulated annealing algorithm to drive the outcome of femtosecond laser ablation towards a specific target shape by using a two- dimensional deformable mirror. Unlike previous work combining adaptive optics and laser machining we use the machining outcome itself as the fitness parameter for the optimization procedure. Single- pulse ablation features with programmable aspect ratios and dimensions as small as 2.5 μm are faithfully reproduced by the technique in a chromium- on- glass test sample. |
| Mid-infrared methane detection in a photonic bandgap fiber using a broadband optical parametric oscillator We demonstrate methane sensing using a photonic bandgap fiber-based gas cell and broadband idler pulses from a periodically-poled lithium niobate femtosecond optical parametric oscillator. The hollow core of the fiber was filled with a methane: nitrogen mixture, and Fourier transform spectroscopy was used to measure transmission spectra in the 3.15-3.35 μm methane absorption region. The method has applications in gas sensing for remote or hazardous environments and potentially at very low concentrations. (c) 2007 Optical Society of America. |
| Ultrafast-laser inscription of a three dimensional fan-out device for multicore fiber coupling applications A fan-out device has been fabricated using ultrafast-laser waveguide-inscription that enables each core of a multicore optical fiber (MCF) to be addressed by a single mode fiber held in a fiber V-groove array (FVA). By utilizing the unique three-dimensional fabrication capability of this technique we demonstrate coupling between an FVA consisting of a one-dimensional array of fibers and an MCF consisting of a two-dimensional array of cores. When coupled to all cores of the MCF simultaneously, the average insertion loss per core was 5.0 dB in the 1.55 μm spectral region. Furthermore, the fan-out exhibited low cross-talk and low polarization dependent loss. (c) 2007 Optical Society of America. |
| Three-dimensional nanometric sub-surface imaging of a silicon flip-chip using the two-photon optical beam induced current method Two- and three-dimensional sub-surface optical beam induced current imaging of a silicon flip-chip is described and is illustrated by results corresponding to 166 nm lateral resolution and an axial performance capable of localising feature depths to around 100 nm accuracy. The experimental results are compared with theoretically modelled performance based on analytic expressions for the system point spread functions valid for high numerical apertures, and are interpreted using numerical geometric ray tracing calculations. Examples of depth-resolved feature profiling are presented and include depth cross-sections through a matrix of tungsten vias and a depth-resolved image of part of a poly-silicon wire. © 2007 Elsevier Ltd. All rights reserved. |
| Coherent synthesis using carrier-envelope phase-controlled pulses from a dual-color femtosecond optical parametric oscillator By using a dual-color femtosecond optical parametric oscillator (OPO), a coherent waveform was synthesized from two coresonant near-infrared signal pulses whose center wavelengths had a separation of 100 nm. Immediately after the OPO cavity the pulses had independent carrier-envelope phase-slip frequencies, and synthesis was achieved by shifting these frequencies using an acousto-optic modulator driven by an internally generated difference frequency. Soliton self-frequency shifted pulses from a photonic crystal fiber and a cross-correlation frequency-resolved optical gating (XFROG) measurement were used to analyze the result of the synthesis experiment and revealed that the synthesized waveform was a train of high-contrast 30 fs pulses. (c) 2007 Optical Society of America. |
| Composite frequency comb spanning 0.4-2.4 μm from a phase-controlled femtosecond Ti : sapphire laser and synchronously pumped optical parametric oscillator A repetition-rate-stabilized frequency comb ranging from the violet to the mid-infrared (0.4-2.4 μm) is obtained by phase locking a femtosecond Ti:sapphire laser and a synchronously pumped optical parametric oscillator to a common supercontinuum reference. The locking results have bandwidths lower than 3 kHz. By changing the locking frequencies, different relative and absolute offsets of the constituent frequency combs are achievable. (c) 2007 Optical Society of America. |
| Monolithic optical parametric oscillator using chirped quasi-phase matching We describe a highly efficient monolithic, Q-switched, nanosecond optical parametric oscillator based on a magnesium-oxide-doped periodically poled lithium niobate crystal and containing multiple quasi-phase-matched gratings. The crystal consisted of a single unchirped grating and five gratings containing progressively increasing amounts of longitudinal chirp. The monolithic design makes the device highly compact, stable, and robust, and it demonstrated a pump-to-signal conversion efficiency of around 50%, generating 50 μJ pulses at 1.55 μm. with a spectral bandwidth of 20 nm. Sonogram. traces are presented showing the effect of crystal chirp on the temporal and spectral performance. © 2007 Optical Society of America |
| Testing the parametric energy conservation law in a femtosecond optical parametric oscillator An experimental verification of energy conservation in a parametric oscillator is reported with an optical frequency precision of approximately 200 kHz (< 10(-6) nm). This high precision is made possible by simultaneously measuring the frequency offsets of the pump, signal and idler frequency combs in a singly-resonant femtosecond optical parametric oscillator system without any phase control. (c) 2006 Optical Society of America. |
| Three-dimensional nanoscale subsurface optical imaging of silicon circuits Three-dimensional subsurface imaging through the back side of a silicon flip chip is reported with a diffraction-limited lateral resolution of 166 nm and an axial performance capable of resolving features only 100 nm deep. This performance was achieved by implementing sample-scanned two-photon optical beam induced current microscopy using a silicon solid immersion lens and a peak detection algorithm. The excitation source was a 1530 nm erbium:fiber laser, and the lateral optical resolution obtained corresponds to 11% of the free-space wavelength. (c) 2007 American Institute of Physics. |
| Frequency Doubling in Femtosecond-Written Periodically-Poled Potassium Titanyl Phosphate Waveguides Frequency doubling is demonstrated in femtosecond-laser-created single-mode waveguides written in a periodically-poled potassium titanyl phosphate crystal. Conversion efficiencies of 0.22%/W (0.02%/W) were obtained for first (third) order phasematching at 980nm (800nm). |
| Mid-Infrared Methane Sensing Using an Optical Parametric Oscillator and a Photonic Bandgap Fiber as a Gas Cell Mid-infrared methane sensing is demonstrated using a photonic bandgap fiber-based gas cell and broadband idler pulses from a periodically-poled lithium niobate fermosecond optical parametric oscillator as the light source for Fourier transform infrared spectroscopy. © 2006 Optical Society of America |
| Nearly Three-Octave-Spanning Frequency Comb from a Phase-Controlled Femtosecond Ti:sapphire Laser and Synchronously Pumped Optical Parametric Oscillator A repetition-rate-stabilized frequency comb ranging from the violet to the mid-infrared is obtained by phase-locking a femtosecond Ti:sapphire laser and synchronously pumped optical parametric oscillator to a common supercontinuum reference. © 2007 Optical society of America |
| Coherent Synthesis using Carrier-Envelope Phase Controlled Pulses from a Dual-Color Femtosecond Optical Parametric Oscillator A coherent waveform is synthesized from two co-resonant optical parametric signal pulses with different center wavelengths and independent earn er-envelope phase-slip frequencies. XFROG measurements confirm the synthesized waveform is a train of high-contrast 30 femtosecond pulses. © 2007 Optical society of America |
| Nanometric three-dimensional sub-surface imaging of a silicon flip-chip By implementing two-photon optical-beam-induced-current microscopy using a solid-immersion lens, imaging inside a silicon flip chip is reported with 166nm lateral resolution and an axial resolution capable of resolving features only 100nm deep. © 2006 Optical Society of America |
| Three-dimensional nanometric sub-surface imaging of a silicon flip-chip using the two-photon optical beam induced current method By implementing two-photon optical-beam-induced-current microscopy using a solid-immersion lens, imaging inside a silicon flip chip is reported with 166nm lateral resolution and an axial resolution capable of resolving features only 100nm in height. |
2006 | Internal gain from an erbium-doped oxyfluoride-silicate glass waveguide fabricated using femtosecond waveguide inscription A channel waveguide is fabricated inside an erbium-doped oxyfluoride silicate glass sample using femtosecond pulses in the low repetition rate regime. The waveguide cross section is controlled using the multiscan fabrication technique. The 1.85-cm-long waveguide exhibits a total background insertion loss of 4.3 dB when-coupled to Corning SMF-28 fibers. Under the maximum available pump power, the device exhibits an internal gain of 1.7 dB at 1537 mn. |
| Dual-color operation of a femtosecond optical parametric oscillator exhibiting stable relative carrier-envelope phase-slip frequencies A femtosecond optical parametric oscillator is demonstrated that can operate in a regime where two signal pulses with well-separated center wavelengths are simultaneously resonant. Measurements show that the oscillator output contains a stable modulation at a frequency corresponding to the difference in the carrier-envelope phase-slip frequencies of the co-resonant pulses. The physical origin of this internal beat signal is attributed to second-order mixing effects, and its frequency is shown to be consistent with theory. (c) 2006 Optical Society of America. |
| Optical waveguide fabrication in z-cut lithium niobate (LiNbO3) using femtosecond pulses in the low repetition rate regime Waveguide structures are fabricated in z-cut lithium niobate (LiNbO3) using focussed femtosecond pulses. Two different types of waveguide structure are fabricated depending on the pulse energy used. In the first, guiding occurs in regions directly surrounding a visible laser-damage region. In the second, guiding occurs in a material modification region created at the focus. High confinement guiding at 1550 nm is demonstrated in the first type of waveguide but found to be temporary, thus indicating that at least part of the refractive index change is due to phenomena such as stress that are subject to relaxation. Finally, the polarization dependent guiding properties of the structures are investigated. |
| Optical probing of a silicon integrated circuit using electric-field-induced second-harmonic generation By using the electric-field-induced second-harmonic generation effect, we have detected electrical signals present on a complementary metal-oxide-semiconductor (CMOS) integrated circuit in a noncontact geometry. Femtosecond pulses with a wavelength of 2.16 μm were incident on the device and the second harmonic at 1.08 μm exhibited a field-dependent behavior. The conversion efficiency from the fundamental to the second harmonic was estimated to be -103 dB. (c) 2006 American Institute of Physics. |
| Silicon flip-chip imaging with a resolution of 325-nm using solid-immersion lenses and the two-photon optical-beam induced current method. This paper reports the imaging of a silicon flip-chip with high resolution by detection of the photocurrent generated by the two-photon absorption of 1530nm light from a femtosecond Er:fiber laser. High resolution imaging was made possible by the inclusion of a silicon solid immersion lens, which increased the numerical aperture of the microscope. Using this technique, features on a sub-micron scale are clearly resolvable with excellent contrast, and the resolution of the system was found to be 325nm. |
| Probing electrical signals in silicon CMOS devices using electric field induced second harmonic generation We report the use of electric field induced second harmonic generation to probe electrical signals in a CMOS chip. The second harmonic of incident 2.3 μm illumination provided by a femtosecond optical parametric oscillator was measured and shown to depend quadratically on both optical intensity and on the applied DC electric field. By using a near infrared photomultiplier tube it was possible to monitor directly the electrical waveform in the chip on the oscilloscope. |
2005 | Active waveguide fabrication in erbium-doped oxyfluoride silicate glass using femtosecond pulses By directly writing waveguides inside bulk erbium-doped oxyfluoride silicate glass using a focused femtosecond pulse train, we demonstrate the potential for the fabrication of especially broadband, compact telecommunications optical amplifiers operating in the c-band (1529-1561 nm). Characterization results include coupling losses to single-mode fiber, waveguide losses, and gain measurements. Clear relationships between the fabrication pulse energy and the waveguide losses and coupling losses are also established. |
| Adaptive beam profile control using a simulated annealing algorithm We present a programmable beam-shaping method based on the combination of a deformable mirror membrane mirror and a simulated annealing algorithm. The algorithm iteratively adjusts the control voltages of 37 independent electrodes to reduce the variance between the chosen shape and the actual beam shape. The experimental results show that the system is capable of adaptively creating, on demand, Gaussian and super-Gaussian beam profiles that closely match the desired target parameters. ©2005 Optical Society of America. |
| Mid-infrared absorption spectroscopy of methane using a broadband femtosecond optical parametric oscillator based on aperiodically poled lithium niobate We describe the implementation of optical absorption spectroscopy in which a Ti:sapphire pumped femtosecond optical parametric oscillator based on aperiodically poled lithium niobate was used as a broadband source to directly acquire a mid-infrared absorption spectrum of methane gas. Fourier transform spectroscopy was performed using the idler output from the optical parametric oscillator to directly acquire spectra spanning over 600 nm (14.4 THz or 480 cm(-1)) with around 2.2 nm (55 GHz or 1.9 cm(-1)) resolution. Data are presented of absorption measurements in methane at pressures of 2 bar, 250 mbar and 25 mbar. This approach combines the advantages of spectroscopy using a broadband thermal source with the high power and excellent beam quality of a modelocked laser source. |
| Fibre interferometer for multi-wavelength interferometry with a femtosecond laser In this paper we propose a novel fibre interferometer for the simultaneous measurement of optical phase at multiple wavelengths generated within the spectral envelope of a femtosecond laser. The use of narrow bandwidth fibre Bragg gratings allows particular wavelengths to be selected with sufficiently narrow bandwidth to give the necessary coherence length for interferometric phase measurements over practical distances exceeding 30 mm. Multi-wavelength interferometry provides a means of making absolute measurements by extending the unambiguous measurement range beyond the wavelength of light. Preliminary experimental results are presented for a two-wavelength sensor where an absolute range of 0.13 mm with 3 nm resolution has been achieved producing a dynamic range of 43000:1. This approach has the potential to be applied to high speed transient events. |
| General second-harmonic pulse shaping in grating-engineered quasi-phase-matched nonlinear crystals We describe a spectrogram-based simulated annealing algorithm for designing quasi-phase-matched crystals capable of producing second harmonic generation pulses of any chosen amplitude and phase profile. The approach applies a new and rapid analytic method for calculating the amplitude and phase of the second harmonic generation pulses generated by a quasi-phase-matched crystal containing an arbitrary grating design. The performance of the algorithm is illustrated by examples of femtosecond second harmonic pulses designed according to various target shapes including single, double and triple Gaussian pulses, positive and negative linear chirp and square, triangular and stepped profiles. © 2005 Optical Society of America. |
| Single-pulse femtosecond laser machining of glass We describe a study of single-pulse machining of glass with 150 fs pulses at 800 nm (Ti:sapphire) and present results demonstrating in a quantitative way the dependence of the machining quality on the focal position and the pulse energy of the laser beam. We report experimental results indicating the creation of sub-diffraction limited structures. A theoretical calculation of the ablation threshold of glass is inferred from our experimental results to be 5.55 J cm(-2). Results are also presented that illustrate the dependence of the machining quality on the pulse duration used. A technique for optimizing the focal position at energies below the machining threshold is detailed and the results of a related study investigating the self-focusing of femtosecond beams in air are described. |
| Broadband femtosecond-seeded optical parametric amplifier based on chirped frequency conversion in MgO: APPLN A femtosecond-seeded optical parametric amplifier based on MgO:APPLN is described which employs a chirped-pulse amplification approach to produce a low coherence length, nanosecond output at 1.55 μm with > 80nm bandwidth. (c) 2005 Optical Society of America. |
| Solid-immersion imaging of a silicon flip-chip with a resolution of 325nm using the optical-beam induced current method We report high resolution solid-immersion sub-surface imaging of a flip-chip by detecting the two-photon photocurrent generated by a 1530nm femtosecond Er:fiber laser. Features show high contrast and a resolution of 325nm. (c) 2005 Optical Society of America. |
| Femtosecond waveguide fabrication in bulk lithium niobate (LiNbO3) We report fabrication of embedded waveguides supporting a well confined 1550 nm propagation mode in a z-cut lithium niobate crystal using focussed femtosecond pulses. (c) 2005 Optical Society of America. |
| Active waveguide fabrication in novel Erbium doped glasses using focussed femtosecond pulses We present the results of optical characterisation experiments conducted on Erbium Doped Waveguide Amplifiers (EDWAs) fabricated in novel erbium. doped bulk glasses using femtosecond pulses to modify the refractive index of the glass. (c) 2005 Optical Society of America. |
| Active waveguide fabrication in erbium doped oxyfluoride-silicate glass using femtosecond pulses We demonstrate the potential for the fabrication of broadband Erbium Doped Waveguide Amplifiers (EDWAs) using femtosecond waveguide fabrication in erbium doped oxyfluoride-silicate glasses. |
| Two-photon optical-beam-induced current solid-immersion imaging of a silicon flip chip with a resolution of 325 nm We report high-resolution subsurface imaging of a silicon flip chip by detection of the photocurrent generated by the two-photon absorption of 1530-nm light from a femtosecond Er:fiber laser. The technique combines the focal sensitivity of two-photon excitation with the enhanced optical resolution of high-numerical-aperture solid-immersion imaging. Features on a sub-1-mum scale are clearly resolvable with high contrast, showing a resolution of 325 nm. © 2005 Optical Society of America. |
2004 | Fiber interferometer for simultaneous multiwavelength phase measurement with a broadband femtosecond laser We present a fiber interferometer for the simultaneous measurement of phase at multiple wavelengths from a single broadband femtosecond laser. Narrow-bandwidth fiber Bragg gratings isolate a particular frequency from the broad-bandwidth laser pulse produced. The multiwavelength phase data permit the unambiguous measurement range to be significantly increased compared with the wavelengths used in the interferometer. Preliminary experimental results are presented for a two-frequency sensor with an absolute range of 0.13 mm and associated dynamic range of 43,000:1. © 2004 Optical Society of America. |
| Mid-infrared absorption spectroscopy across a 14.4 THz spectral range using a broadband femtosecond optical parametric oscillator We describe the implementation of optical absorption spectroscopy in which a Ti:sapphire pumped femtosecond optical parametric oscillator based on periodically poled lithium niobate was used as a broadband source to directly acquire a midinfrared absorption spectrum of methane gas. Fourier-transform spectroscopy was performed using the idler output from the optical parametric oscillator to directly acquire spectra spanning over 600 nm (14.4 THz or 480 cm(-1)) with around 3 nm (78 GHz or 2.6 cm(-1)) resolution. This approach combines the advantages of spectroscopy using broadband thermal sources with the high power and excellent beam quality of a mode-locked laser source. © 2004 American Institute of Physics. |
| Flow imaging by use of femtosecond-laser-induced two-photon fluorescence A novel technique is demonstrated for the imaging of turbulent flows in which a single window to the flow is the only optical access required. A femtosecond laser is used to excite two-photon fluorescence in a disodium-fluorescein-seeded water jet. The fluorescence signal is generated at only the focal point of the laser because of the highly nonlinear nature of the two-photon absorption, and it is collected in a direction counterpropagating to the excitation beam. Tight focusing of the laser is used to limit the probe volume, and the two-dimensional mean and rms concentration images are collected by raster scanning the laser. © 2004 Optical Society of America. |
| Idler-resonant femtosecond tandem optical parametric oscillator tuning from 2.1 μm to 4.2 μm A Ti:sapphire-pumped idler-resonant femtosecond tandem optical parametric oscillator is reported that is based on periodically poled lithium niobate and operates with wavelength tuning from 2.1 to 4.2 mudm (idlers) and 1.25 to 1.40 mum (signal). The configuration uses two cascaded gratings arranged so that the nonresonant signal from the first grating acts as a pump for the second grating. Novel phase-matching behavior is described and explained, including full tandem operation, degenerate parametric downconversion of the signal pulses, and simultaneous tandem operation with seeded optical parametric amplification. Signal spectra show characteristic depletion profiles, implying substantial conversion from the signal to the tandem idler output. © 2004 Optical Society of America. |
| Programmable spectral phase control of femtosecond pulses by use of adaptive optics and real-time pulse measurement We describe a programmable,spectral-phase, pulse-shaping system for femtosecond pulses based on a deformable membrane mirror. Accurate spectral phase design as well as pulse intensity modulation was achieved with direct control of the mirror surface by use of a negative-feedback, mirror-surface control mode. Convergence to the chosen spectral-phase design was typically achieved within several seconds. The pulses were measured with a real-time, second-harmonic-generation, frequency-resolved optical gating system. © 2004 Optical Society of America. |
| General ultrafast pulse measurement using the cross-correlation single-shot sonogram technique The cross-correlation single-shot sonogram technique offers exact pulse measurement and real-time pulse monitoring via an intuitive time-frequency trace whose shape and orientation directly indicate the spectral chirp of an ultrashort laser pulse. We demonstrate an algorithm that solves a fundamental limitation of the cross-correlation sonogram method, namely, that the time-gating operation is implemented using a replica of the measured pulse rather than the ideal delta-function-like pulse. Using a modified principal-components generalized projections algorithm, we experimentally show accurate pulse retrieval of an asymmetric double pulse, a case that is prone to systematic error when one is using the original sonogram retrieval algorithm. © 2004 Optical Society of America. |
| Femtosecond soliton pulse delivery at 800nm wavelength in hollow-core photonic bandgap fibers We describe delivery of femtosecond solitons at 800nm wavelength over five meters of hollow-core photonic bandgap fiber. The output pulses had a length of less than 300fs and an output pulse energy of around 65nJ, and were almost bandwidth limited. Numerical modeling shows that the nonlinear phase shift is determined by both the nonlinearity of air and by the overlap of the guided mode with the glass. © 2004 Optical Society of America. |
| Low threshold, high repetition frequency, femtosecond optical parametric oscillator based on chirped-pulse frequency conversion We report the first demonstration of a femtosecond optical parametric oscillator based on chirped-pulse frequency conversion in a long crystal of aperiodically poled potassium titanyl phosphate. The minimum pump threshold power was 14.4 mW and a signal slope efficiency of 35% was achieved. Continuous tuning from 1194-1455 nm was obtained for an average pump power of 750 mW. |
| Measuring ultrafast laser pulses |
2003 | Designer femtosecond pulses using adaptive optics We describe a femtosecond pulse shaper using a deformable membrane mirror. The pulses are measured with a real time second-harmonic-generation frequency-resolved optical gating system. Pulse shapes are modified according to a prescribed spectral phase. Accurate spectral phase design as well as pulse intensity modulation was achieved by using negative feedback mirror-surface control. Convergence to the chosen spectral phase design was typically achieved within several seconds. © 2003 Optical Society of America. |
| Real time femtosecond optical pulse measurement using a video-rate frequency-resolved optical gating system We describe an improved instrument for measuring at video rate (30 frames per second) the second-harmonic frequency-resolved optical gating trace of femtosecond pulses from a mode-locked laser oscillator. The system comprises separate scanning acquisition and pulse retrieval elements which together enable the exact pulse profile to be viewed in real time with a typical refresh rate of 1 Hz. Details are given of the optical system used, the electronic synchronization circuits and the acquisition and retrieval software employed. © 2003 American Institute of Physics. |
| Engineered quasi-phase-matching for second-harmonic generation An approach for analytically calculating the second-harmonic output field from an arbitrary quasi-phase-matched non-linear crystal is presented. By applying this method within a simulated annealing minimization routine we demonstrate a way of designing quasi-phase-matched gratings whose conversion response can be tailored to closely match any chosen profile. The influence on the final conversion response of important parameters such as the number of crystal domains and the algorithm temperature is presented in detail. |
| Investigation of the two-photon optical beam induced current effect in silicon integrated circuits We describe a simulation to model the two-photon optical beam induced current effect in silicon integrated circuits. Our model includes explicitly both the single-photon and two-photon absorption coefficients and explains how it is possible to obtain a three-dimensional image using this technique despite the relatively weak focal dependence. A full comparison with experimental results is also included. © 2003 Published by Elsevier Science B.V. |
| Low-threshold, high-repetition-frequency femtosecond optical parametric oscillator based on chirped-pulse frequency conversion We report a quasi-phase-matched optical parametric oscillator that incorporates a chirped nonlinear crystal and uses prechirped pulses matched to the crystal chirp to improve the conversion efficiency and reduce the operational threshold. A 20-mm crystal of aperiodically poled KTiOPO4 is phase matched to stretched Ti:sapphire pump pulses. The Ti:sapphire laser produces 104-MHz output pulses at 850 nm that are stretched from 190 to 900 fs with an average output power of 750 mW The system has demonstrated a pump depletion of more than 80%, a signal slope efficiency of 35%, and a threshold of 14.4 mW The cavity showed tuning from 1194 to 1455 nm. over a length range of 130 mum. The approach described demonstrates the potential of using chirped-pulse-chirped-crystal quasi-phase matching in long nonlinear crystals as a method to reduce ultrafast optical parametric oscillator thresholds. © 2003 Optical Society of America. |
| Low-threshold femtosecond optical parametric oscillator based on chirped-pulse frequency conversion We report what is to our knowledge the first demonstration of a femtosecond optical parametric oscillator based on chirped-pulse frequency conversion in a long crystal of aperiodically poled potassium titanyl phosphate. The minimum pump threshold power was 15 mW, and a signal slope efficiency of 35% was achieved. Continuous tuning from 1190 to 1450 nm was obtained for an average pump power of 800 mW. © 2003 Optical Society of America. |
2002 | Functional imaging |
| High idler conversion in femtosecond optical parametric oscillators In this work we theoretically demonstrate high idler conversion in a femtosecond optical parametric oscillator based on a crystal with two consecutive quasi-phasematching gratings, each one with a different poling period. In the first parametric interaction the signal and the idler are generated, while in the second one, part of the energy in the signal is transferred to the idler. Two singly resonant cavities, one for the signal and the second for the idler, are analysed and the advantages and disadvantages of each configuration are discussed. © 2002 Elsevier Science B.V. All rights reserved. |
| Three-dimensional imaging of a silicon flip chip using the two-photon optical-beam induced current effect We describe two- and three-dimensional imaging of a flip-chip silicon integrated circuit using backside optical probing and femtosecond two-photon excitation at a laser wavelength of 1.275 mum. Using a x50 microscope objective, we typically achieved micron resolutions or better in both lateral and axial directions. Using axial scanning and a peak-detection algorithm we have demonstrated optical depth profiling across components on the chip. © 2002 American Institute of Physics. |
| Efficient femtosecond optical parametric oscillators based on aperiodically poled nonlinear crystals We discuss the use of aperiodically poled nonlinear crystals to improve conversion efficiency in a synchronously pumped ultrafast optical parametric oscillator. We show theoretically that with these crystals the conversion efficiency can be considerably higher than that obtained when a periodically poled crystal with the same length is used. Moreover, we show that pump threshold can be simultaneously improved by use of longer crystals. © 2002 Optical Society of America. |
| Soliton self-frequency shift effects in photonic crystal fibre Using a femtosecond Ti:sapphire laser operating at a wavelength of 810 nm we have demonstrated infrared generation in photonic crystal fibre at distinct wavelengths which can be attributed to the soliton self-frequency shift effect. The maximum observed shift produced spectra centred at 1260 nm and the frequency-shifted light accounted for up to 80% of the fibre output power. We show that the shifts can be explained by the dispersion properties of the fundamental and higher-order waveguide modes of the fibre. |
| Single-shot sonogram: a real-time chirp monitor for ultrafast oscillators We report a static single-shot configuration for measuring the sonogram trace of an ultrashort optical pulse. When it is combined with a CCD camera, the instrument provides an intuitive video-rate monitor of the pulse chirp, and it is equally compatible with a high-repetition-rate mode-locked oscillator and a low-repetition-rate amplified source. The performance of the instrument is described, and iterative retrieval of the measured sonogram trace, which yields complete and unambiguous pulse characterization, is demonstrated. © 2002 Optical Society of America. |
| Pulse compression and gain enhancement in a degenerate optical parametric amplifier based on aperiodically poled crystals We describe theoretically a method of obtaining pulse compression and simultaneously improving amplification in a degenerate optical parametric amplifier by use of quasi-phase-matched engineered crystals. The scheme combines the possibility of increasing the signal spectral bandwidth that is offered by a degenerate parametric amplifier with the ability of engineered aperiodically poled crystals to compensate for group-velocity mismatch. © 2002 Optical Society of America. |
| Observation of soliton self-frequency shift in photonic crystal fibre Results from a study in which Raman-shifted outputs were observed from photonic crystal fibres pumped by 800 nm femtosecond pulses are reported. Under optimum conditions. a spectral shift to wavelengths of 1260 nm was recorded. |
2001 | Ultrashort pulse characterization using a scanning Fabry-Perot etalon enabling rapid acquisition and retrieval of a sonogram at rates up to 1.52 Hz Rapid characterization of ultrashort pulses has been demonstrated by measuring a two-photon absorption sonogram trace. The use of a scanning Fabry-Perot frequency filter has made it possible to acquire and retrieve a sonogram at rates as high as 1.52 Hz. We discuss details of the experimental setup, including the synchronization electronics and the acquisition/retrieval software. Excellent agreement is obtained between retrieved pulse data and independent experimental pulse measurements. © 2001 American Institute of Physics. |
| Rapid measurement of ultrashort-pulse amplitude and phase from a two-photon absorption sonogram trace By measuring the sonogram of an ultrashort pulse with a two-photon detector we have demonstrated a robust and sensitive complete characterization method for 80fs; pulses derived from a self-mode-locked Ti:sapphire laser. Acquisition and retrieval rates as high as 0.5 Hz are demonstrated, and the design of a rapid-acquisition configuration is described, where we discuss the use of white-light interferometry to achieve a nearly dispersionless bandpass filter. Excellent agreement is obtained between retrieved pulse data and independent experimental pulse measurements. © 2001 Optical Society of America. |
| Femtosecond second-harmonic pulse compression in aperiodically poled lithium niobate: a systematic comparison of experiment and theory We present detailed experimental results of simultaneous frequency doubling and pulse compression of chirped pulses from a femtosecond optical parametric oscillator using a second-harmonic crystal of aperiodically poled lithium niobate comprising eight different linearly chirped gratings. Our results are compared with a numerical model that incorporates the complex amplitude of the input pulse determined with frequency-resolved optical gating. We use the results of this model to analyze and discuss several aspects of the pulse-generation process. © 2001 Optical Society of America. |
| Practical measurement of femtosecond optical pulses using time-resolved optical gating We report a practical experimental implementation of time-resolved optical gating (TROG) using a dispersive pulse (DP) propagation geometry and a two-photon absorption nonlinear detector. A measurement of 50 fs pulses from a self-mode-locked Ti:sapphire laser is demonstrated and a general procedure for calibrating the dispersion axis of the TROG trace is presented. A complete iterative pulse retrieval algorithm is described and the limitations of the DP-TROG technique are discussed. © 2001 Elsevier Science B.V. All rights reserved. |
| Laser physics - Toward attosecond pulses |
| Amplitude and phase measurement of Mid-IR femtosecond pulses using XFROG We describe a cross-correlation-based frequency-resolved optical gating (XFROG) technique for simultaneously measuring the amplitude and phase of two ultrashort pulses with different wavelengths but derived from a common modelocked oscillator. |
| Rapid retrieval of ultrashort pulse amplitude and phase from a sonogram trace By measuring the sonogram of an ultrashort pulse we have demonstrated a robust and sensitive characterization method with acquisition rates as high as 0.5 Hz. Retrieved data are compared to independent experimental measurements. |
2000 | Measurement of group velocity dispersion using white light interferometry: A teaching laboratory experiment A teaching laboratory experiment is described which uses a basic Michelson interferometer arrangement to make fast and accurate measurements of the group velocity dispersion of an optical material using a method based on recording white-light fringes. We present a brief analysis of the theory behind the technique and describe two example measurements, one of the material dispersion of a KTP crystal and another of the reflectivity dispersion of a silver-coated mirror. Details are also given of our implementation of the data analysis using the MATLAB programming environment © 2000 American Association of Phsics Teachers. |
| Amplitude and phase measurement of mid-infrared femtosecond pulses by using cross-correlation frequency-resolved optical gating We describe a cross-correlation-based frequency-resolved optical gating (XFROG) technique for simultaneously measuring the amplitude and phase of two ultrashort pulses that have different wavelengths but are derived from a common mode-locked oscillator. A measurement is presented in which 4.0-μm mid-IR pulses from a synchronously pumped femtosecond optical parametric oscillator (OPO) are characterized by mixing with the 770-nm OPO pump pulses. Details of the pulse-retrieval algorithm are included, together with examples of pulse data retrieved from the experimentally measured XFROG trace. © 2000 Optical Society of America. |
| Five-optical-cycle pulse generation in the mid infrared from an optical parametric oscillator based on aperiodically poled lithium niobate We describe an optical parametric oscillator based on aperiodically poled lithium niobate that generates nearly transform-limited 53-fs duration pulses at a center wavelength of 3 μm, corresponding to only 5 optical cycles. Results are presented illustrating the effect of pump- and grating-period chirp on the idler pulses, and a configuration capable of producing idler bandwidths in excess of 700 nm is discussed. © 2000 Optical Society of America OCIS codes: 190.7110, 190.4410, 190.4970, 190.2620. |
| Sonogram characterisation of picosecond pulses at 1.5 μm using waveguide two photon absorption Low energy picosecond pulses at 1.5 μm have been completely characterised by sonogram measurements based on mio photon absorption in a commercial InGaAsP 1.3 μm laser diode. The intensity and phase of pulses with 12pJ energy are in excellent agreement with those obtained from an independent frequency resolved optical gating characterisation. |
| Autocorrelation of femtosecond pulses from 415-630nm using GaN laser diode Using the two-photon response of a commercial 393nm GaN laser diode the authors have measured intensity and interferometric second-order autocorrelations of visible femtosecond pulses in the 415-630nm wavelength region for pulse energies as low as 1 pJ. |
1999 | Algorithm for complete and rapid retrieval of ultrashort pulse amplitude and phase from a sonogram We describe an algorithm based on principal-component generalized projections that is capable of unambiguous retrieval of the amplitude and phase of an ultrashort pulse from a measurement of its sonogram. Running on a personal computer, the algorithm can complete more than 50 iterations per second which enables real-time display of an ultrashort pulse intensity and phase profile. Retrieval is demonstrated for a range of pulses with commonly encountered phase errors including linear chirp, self-phase modulation, and cubic spectral phase, and experimental results illustrate good agreement between the retrieved pulse characteristics and independent spectral measurements. |
| High-repetition-rate ultrashort-pulse optical parametric oscillator continuously tunable from 2.8 to 6.8 μm We describe a synchronously pumped femtosecond optical parametric oscillator based on periodically poled LiNbO3 that is broadly tunable in the mid infrared. A transmission window of periodically poled lithium niobate beyond the conventionally accepted infrared absorption edge of 5.4 μm has been exploited to produce idler pulses that are tunable across a wavelength range of 4 μm, with milliwatt-level output powers at wavelengths as long as 6.8 μm. We also present experimental tuning results that are in good agreement with the theoretical phase matching predicted from published infrared-corrected Sellmeier equations for LiNbO3. © 1999 Optical Society of America OCIS codes: 190.0190, 190.2620, 190.4970, 190.4360, 320.2250, 320.7110. |
| Simultaneous second-harmonic generation and femtosecond-pulse compression in aperiodically poled KTiOPO4 with a RbTiOAsO4-based optical parametric oscillator We have obtained both extracavity and intracavity simultaneous second-harmonic generation and compression of signal pulses at 1.25 μm from a synchronously pumped RbTiOAsO4-based optical parametric oscillator with an aperiodically poled crystal of KTiOPO4. The 290-fs input pulses yield temporally compressed frequency-doubled pulses with durations of 120 fs and average output powers of as much as 120 mW. Experimental results are compared with a numerical model in which the temporal and spectral pulse shape and phase of the second-harmonic pulses are calculated with data obtained by characterization of the input pulses by use of the frequency-resolved optical gating technique. We also used the model to optimize the crystal parameters that would result in higher conversion efficiencies and that would enhance pulse compression. © 1999 Optical Society of America [S0740-3224(99)00109-5]. |
| Simultaneous femtosecond-pulse compression and second-harmonic generation in aperiodically poled KTiOPO4 We report both extracavity and intracavity simultaneous second-harmonic generation and compression of pulses at 1.25 μm from a synchronously pumped RbTiOAsO4-based optical parametric oscillator, using an aperiodically poled crystal of KTiOPO4. The 290-fs input pulses were temporally compressed to 120 fs, with average output powers as great as 120 mW. The experimental results are compared with a numerical model that uses data obtained by characterization of the input pulses by use of the frequency-resolved optical gating technique. © 1999 Optical Society of America. |
| Few cycle EM pulses Modern ultrafast laser lasers can produce pulses so short that they exist for no more than a few oscillations of the carrier wave. Once thought of as exotic phenomena, these pulses introduce new challenges and have potential uses novel applications as diverse as metrology and quantum control. New measurement techniques are now able to take a snapshot of the electric field of these pulses, allowing their intensity and phase characteristics to be manipulated and established theor ies describing how optical pulses propagate are being challenged as pulse durations become so short that concepts of a separate carrier wave and its envelope are no longer appropriate. |
| Periodically poled RbTiOAsO4 femtosecond optical parametric oscillator tunable from 1.38 to 1.58 μm We describe a synchronously pumped optical parametric oscillator based on periodically poled RbTiOAsO4 that is continuously tunable in the signal wavelength from 1.375 to 1.575 μm and with a maximum output power of 110 mW. The signal pulses were produced with nearly transform-limited duration as short as 215 fs. |
| High-average-power, 1-MW peak-power self-mode-locked Ti : sapphire oscillator We describe a novel self-mode-locked Ti:sapphire ring laser that produces 13-fs pulses with peak powers exceeding 1 MW, pulse energies of 13 nJ, and average mode-locked output powers of 1.5 W at a cavity repetition frequency of 110 MHz. A complete resonator analysis describing the optimum mode-locking configuration is presented, together with fringe-resolved autocorrelation and sonogram measurements of the output pulses. © 1999 Optical Society of America. |
1998 | Commercial semiconductor devices for two photon absorption autocorrelation of ultrashort light pulses Optical autocorrelation of ultrashort pulses using two photon absorption (TPA) in commercial semiconductor devices provides a convenient, sensitive, and inexpensive alternative to standard techniques using nonlinear crystals. A summary of readily available commercial devices suitable for TPA autocorrelation of picosecond and femtosecond pulses in the near-IR from 0.7-3 μm is presented. |
| Experimental comparison of conventional pulse characterisation techniques and second-harmonic-generation frequency-resolved optical gating Conventional methods to characterise the output of modelocked lasers rely on phase-sensitive intensity measurements in the time domain (interferometric autocorrelation) and power measurements in the frequency domain. We describe a full comparison between measurements made using second-harmonic-generation frequency-resolved optical gating (FROG) and those made independently with a conventional autocorrelator and spectrometer. Our measurements of pulses from a self-modelocked Ti:sapphire laser operated throughout the region of net negative intracavity dispersion show that pulse durations inferred from autocorrelation measurements by assuming a sech(2)(t) pulse intensity profile are consistently lower than those determined by the general FROG technique. © 1998 Elsevier Science B.V. All rights reserved. |
| Femtosecond optical parametric oscillators based on periodically poled lithium niobate We describe two configurations of collinearly pumped femtosecond optical parametric oscillator based on periodically poled lithium niobate and tunable in the infrared from 975 nm to 4.98 μm. Maximum output powers of 240 mW for the signal and 106 mW for the idler were recorded with 25 mW of average power measured at 4.88 μm. An overall conversion efficiency of 35% and slope efficiencies for the signal of 46% at a wavelength of 1.04 μm and 70% at 1.11 μm were measured. Interferometric autocorrelations of the signal and idler pulses at various wavelengths within the tuning range have been obtained and imply nearly transform-limited pulse durations of about 140 fs for the signal and about 190 fs for the idler. |
| Near- to mid-infrared picosecond optical parametric oscillator based on periodically poled RbTiOAsO4 We describe a Ti:sapphire-pumped picosecond optical parametric oscillator based on periodically poled RbTiOAsO4 that is broadly tunable in the near to mid infrared. A 4.5-mm single-grating crystal at room temperature in combination with pump wavelength tuning provided access to a continuous-tuning range from 3.35 to 5 μm, and a pump power threshold of 90 mW was measured. Average mid-infrared output powers in excess of 100 mW and total output powers of 400 mW in similar to 1-ps pulses were obtained at 33% extraction efficiency. © 1998 Optical Society of America. |
| Widely tunable, near-to mid-infrared femtosecond and picosecond optical parametric oscillators using periodically poled LiNbO3 and RbTiOAsO4 We describe the operation and characterization of Ti:sapphire laser-pumped femtosecond and picosecond optical parametric oscillators based the new quasi-phase-matched nonlinear materials of periodically poled LiNbO3 and RbTiOAsO4 with broad tunability in the near-to mid-infrared. We discuss the merits of the two materials for use in ultrafast optical parametric oscillators (OPO's) and compare and contrast their properties to the birefringent materials. We demonstrate an extended spectral coverage from <1 μm to >5 μm, pump power thresholds as low as 45 mW, average mid-infrared output powers in excess of 100 mW, and pulse durations of 100-200 fs and 1-2 ps at similar to 80 MHz repetition rate. We also report the efficient operation of Ti:sapphire-pumped femtosecond OPO's in all-solid-state configurations by utilizing diode-laser-based input pump sources. |
| Versatile femtosecond laser sources for time-resolved studies: configurations and characterizations The generation of tunable ultrashort laser pulses in different regions of the optical spectrum has particular significance for the study of the rapid events that arise in photochemistry and photobiology. High-resolution techniques which use such pulses to probe transient phenomena require hi,ah-repetition-rate sources which can operate over extended wavelength regions in the visible and infrared. Dramatic progress in the development of ultrafast sources has taken place during the 1990s and ave present here a review of those which now represent practical tools for time-resolved studies and describe powerful characterization techniques for the pulses which they produce. |
| Femtosecond optical parametric oscillator based on periodically poled lithium niobate We describe a femtosecond optical parametric oscillator based on periodically poled lithium niobate and pumped by a self-mode-locked Ti:sapphire laser. Signal and idler outputs almost continuously tunable from 975 nm to 4.55 μm were generated by a combination of grating tuning and cavity-length tuning, and an explanation of the tuning properties is given in terms of the gain bandwidth. A threshold of 45 mW was measured and, in the absence of optimized output coupling, signal powers of 90 mW and idler powers of 70 mW were obtained, with 140 mW of green light at 540 nm generated by phase-matched frequency doubling of the signal. Dispersion compensation produced near-transform-limited signal pulses of duration 140 fs. Observations regarding temperature tuning and pump depletion are also presented. © 1998 Optical Society of America. |
| All-solid-state mid-infrared femtosecond optical parametric oscillator based on periodically-poled lithium niobate We describe an all-solid-state collinearly-pumped femtosecond optical parametric oscillator based on periodically-poled lithium niobate and tunable in the infrared from 996 nm to 1.22 μm (signal) and 2.6 to 4.98 μm (idler). Maximum output powers of 240 mW for the signal and 106 mW for the idler are recorded with 25 mW of average power measured at 4.88 μm. An overall conversion efficiency of 35% and slope efficiencies for the signal of 40% at a wavelength of 1.04 μm and 70% at 1.1 μm are measured. Using dispersion compensation, interferometric autocorrelations at signal and idler wavelengths of 1.0 and 4.88 μm, implying pulse durations of 210 and 190 fs, respectively, have been obtained. © 1998 Elsevier Science B.V. |
1997 | A real-time FROG-trace acquisition system for non-amplified femtosecond oscillators |
| Broadly tunable infrared femtosecond optical parametric oscillator based on periodically poled RbTiOAsO4 We present results from what we believe is the first reported example of an optical parametric oscillator based on periodically poled RbTiOAsO4. The oscillator is pumped by a femtosecond self-mode-locked Ti:sapphire laser and, with a single-grating a-mm-long crystal and one mirror set, a combination of pump and cavity-length tuning provided wavelength coverage from 1060 to 1225 nm (signal) and 2.67 to 4.5 μm (idler). Average output powers were as much as 120 mW in the signal and 105 mW in the idler and interferometric autocorrelations recorded at signal and idler wavelengths of 1.1 and 3.26 μm, respectively, imply pulse durations of 125 and 115 fs, respectively. © 1997 Optical Society of America. |
| Compact, efficient 344-MHz repetition-rate femtosecond optical parametric oscillator We describe configurations of a novel synchronously pumped femtosecond optical parametric oscillator based on the crystal RbTiOAsO4 and operating with a signal-pulse-repetition frequency as high as 344 MHz. Average signal powers as high as 600 mW and pulse durations of 78 fs are demonstrated at a wavelength of 1.25 μm, and a characterization of the signal output using frequency-resolved optical gating implies asymmetric near-sech(2)(t) intensity-profile pulses with significant amounts of spectral cubic phase. © 1997 Optical Society of America. |
| Light-emitting diodes as measurement devices for femtosecond laser pulses We present results showing that, when it is used as a photodetector, a light-emitting diode (LED) has a power-dependent response that can be used for sensitive detection and characterization of picosecond and femtosecond laser pulses. A characterization of a typical LED is presented at 800 nm, and we demonstrate how this effect can be used to construct an extremely compact novel autocorrelator based on a Wollaston prism. © 1997 Optical Society of America. |
| Femtosecond pulses tunable beyond 4 μm from a KTA-based optical parametric oscillator Femtosecond pulses with idler (signal) wavelengths tunable from 2.5-4.1 μm (1027-1195 nm) have been generated from a Ti:sapphire-pumped parametric oscillator based on KTA and configured with non-collinear phasematching for partial compensation of Poynting-vector walkoff. The average idler output power at 4 μm was typically around 10 mW with a maximum of 110 mW measured near 2.5 μm Interferometric autocorrelation of the idler indicates near-transform-limited pulses of duration 94 fs at 3.5 μm. |