Three-photon absorption in optical parametric oscillators based on OP-GaAs

Modulated ringdown comb interferometry for sensing of highly complex gases

Gas samples relevant to health1-3 and the environment4-6 typically contain many molecular species that span a huge concentration dynamic range. Mid-infrared frequency comb spectroscopy with high-finesse cavity enhancement has allowed the most sensitive multispecies trace-gas detections so far2,7-13. However, the robust performance of this technique depends critically on ensuring absorption-path-length enhancement over a broad spectral coverage, which is severely limited by comb-cavity frequency mismatch if strongly absorbing compounds are present. Here we introduce modulated ringdown comb interferometry, a technique that resolves the vulnerability of comb-cavity enhancement to strong intracavity absorption or dispersion. This technique works by measuring ringdown dynamics carried by massively parallel comb lines transmitted through a length-modulated cavity, making use of both the periodicity of the field dynamics and the Doppler frequency shifts introduced from a Michelson interferometer. As a demonstration, we measure highly dispersive exhaled human breath samples and ambient air in the mid-infrared with finesse improved to 23,000 and coverage to 1,010 cm-1. Such a product of finesse and spectral coverage is orders of magnitude better than all previous demonstrations2,7-20, enabling us to simultaneously quantify 20 distinct molecular species at above 1-part-per-trillion sensitivity varying in concentrations by seven orders of magnitude. This technique unlocks next-generation sensing performance for complex and dynamic molecular compositions, with scalable improvement to both finesse and spectral coverage.

Nanosecond optical parametric oscillator with midinfrared intracavity difference-frequency mixing in orientation-patterned GaAs

We report on efficient midinfrared difference-frequency generation (DFG) in orientation-patterned GaAs by intracavity mixing the signal and idler pulses of a narrowband nanosecond optical parametric oscillator based on periodically poled LiNbO₃. The maximum average DFG output power reached 215 mW at 8.15 µm for a repetition rate of 35 kHz. The temperature tuning range spanned over 8026-8710 nm. The maximum overall conversion efficiency from 1 to 8 µm amounted to ∼1.3%.

Infrared Comb Spectroscopy of Buffer-Gas-Cooled Molecules: Toward Absolute Frequency Metrology of Cold Acetylene

We review the recent developments in precision ro-vibrational spectroscopy of buffer-gas-cooled neutral molecules, obtained using infrared frequency combs either as direct probe sources or as ultra-accurate optical rulers. In particular, we show how coherent broadband spectroscopy of complex molecules especially benefits from drastic simplification of the spectra brought about by cooling of internal temperatures. Moreover, cooling the translational motion allows longer light-molecule interaction times and hence reduced transit-time broadening effects, crucial for high-precision spectroscopy on simple molecules. In this respect, we report on the progress of absolute frequency metrology experiments with buffer-gas-cooled molecules, focusing on the advanced technologies that led to record measurements with acetylene. Finally, we briefly discuss the prospects for further improving the ultimate accuracy of the spectroscopic frequency measurement.

High-average-power picosecond mid-infrared OP-GaAs OPO

We report a high-average-power mid-infrared picosecond (ps) optical parametric oscillator (OPO) based on orientation-patterned gallium arsenide (OP-GaAs), with wide wavelength tunability. The OP-GaAs OPO is synchronously pumped by a thulium-doped-fiber (TDF) master oscillator power amplifier (MOPA), seeded by a gain-switched laser diode. At a pump power of 35.3 W and a repetition rate of 100 MHz, a maximum OPO total average output power of 9.7 W (signal 5.7 W (0.60 kW peak power), idler 4.0 W (0.42 kW peak power)) is obtained at signal and idler wavelengths of 3093 nm and 5598 nm, and a thermally induced power roll-off is observed. To mitigate the thermal effects, an optical chopper is placed before the OPO to provide burst mode operation and a reduced thermal load. We achieved a linear growth in OPO output power over the full range of available pump powers in this instance confirming thermal effects as the origin of the roll-off observed under continuous pumping. We estimate the maximum peak powers of the signal and idler are estimated to be over 0.79 kW and 0.58 kW, respectively in this instance. A wide mid-infrared wavelength tuning range of 2895-3342 nm (signal) and 4935-6389 nm (idler) is demonstrated.

Stabilized all-fiber source for generation of tunable broadband fCEO-free mid-IR frequency comb in the 7 - 9 µm range

A compact and robust all-fiber difference frequency generation-based source of broadband mid-infrared radiation is presented. The source emits tunable radiation in the range between 6.5 µm and 9 µm with an average output power up to 5 mW at 125 MHz repetition frequency. The all-in-fiber construction of the source along with active stabilization techniques results in long-term repetition rate stability of 3 Hz per 10 h and a standard deviation of the output power better than 0.8% per 1 h. The applicability of the presented source to laser spectroscopy is demonstrated by measuring the absorption spectrum of nitrous oxide (N₂O) around 7.8 µm. The robustness and good long- and short-term stability of the source opens up for applications outside the laboratory.

Phase-stabilized 100 mW frequency comb near 10 μm

Long-wavelength mid-infrared (MIR) frequency combs with high power and flexible tunability are highly desired for molecular spectroscopy, including investigation of large molecules such as C60. We present a high power, phase-stabilized frequency comb near 10 μm, generated by a synchronously pumped, singly resonant optical parametric oscillator (OPO) based on AgGaSe2. The OPO can be continuously tuned from 8.4 to 9.5 μm, with a maximum average idler power of 100 mW at the center wavelength of 8.5 μm. Both the repetition rate (f rep) and the carrier-envelope offset frequency (f ceo) of the idler wave are phase-locked to microwave signals referenced to a Cs clock. We describe the detailed design and construction of the frequency comb, and discuss potential applications for precise and sensitive direct frequency comb spectroscopy.

All-fiber mid-infrared source tunable from 6 to 9 μm based on difference frequency generation in OP-GaP crystal

We report the first fully fiberized difference frequency generation (DFG) source, delivering a broadly tunable idler in the 6 to 9 μm spectral range, using an orientation-patterned gallium phosphide (OP-GaP) crystals with different quasi-phase matching periods (QPM). The mid-infrared radiation (MIR) is obtained via mixing of the output of a graphene-based Er-doped fiber laser at 1.55 μm with coherent frequency-shifted solitons at 1.9 μm generated in a highly nonlinear fiber using the same seed. The presented setup is the first truly all-fiber, all-polarization maintaining, alignment-free DFG source reported so far. Its application to laser spectroscopy was demonstrated by the absorption spectrum measurement of ν₄ band of methane in 7.5 - 8.3 µm spectral range. The system simplicity and compactness paves the way for applications in field-deployable optical frequency comb spectroscopy systems for gas sensing.

Efficient half-harmonic generation of three-optical-cycle mid-IR frequency comb around 4 µm using OP-GaP

We report a broadband mid-infrared frequency comb with three-optical-cycle pulse duration centered around 4.2 µm, via half-harmonic generation using orientation-patterned GaP (OP-GaP) with ~43% conversion efficiency. We experimentally compare performance of GaP with GaAs and lithium niobate as the nonlinear element, and show how properties of GaP at this wavelength lead to generation of the shortest pulses and the highest conversion efficiency. These results shed new light on half-harmonic generation of frequency combs, and pave the way for generation of short-pulse intrinsically-locked frequency combs at longer wavelengths in the mid-infrared with high conversion efficiencies.

Thulium-fiber-laser-pumped, high-peak-power, picosecond, mid-infrared orientation-patterned GaAs optical parametric generator and amplifier

We report a high-peak-power, picosecond, mid-infrared optical parametric generator (OPG) and optical parametric amplifier (OPA) based on orientation-patterned gallium arsenide pumped by a thulium:fiber master-oscillator-power-amplifier employing direct diode-seeded amplification. An OPG tuning range of 2550-2940 nm (signal) and 5800-8300 nm (idler) is demonstrated with peak powers as high as 3 (signal) and 2 kW (idler), and with pulse energies of 0.26 and 0.16 μJ, respectively. When seeded with a 0.6  cm^-1 linewidth tunable Cr:ZnSe laser, the OPA idler linewidth is narrowed to 1.4  cm^-1 and a small-signal parametric gain of 60 dB is achieved. A maximum peak power of 13.3 (signal) and 3.2 kW (idler) is obtained at an overall conversion efficiency of 36%. The corresponding maximum pulse energies for the signal and idler are 1.07 and 0.26 μJ, respectively.

Midinfrared frequency comb by difference frequency of erbium and thulium fiber lasers in orientation-patterned gallium phosphide

We generate over 60 mW of pulses with wavelengths from 6 to 11 micrometers by difference frequency mixing between erbium and thulium fiber amplifiers in orientation-patterned GaP with a photon conversion efficiency of 0.2. By stabilizing the repetition rate of the shared oscillator and adding a frequency shifter to one arm, the output becomes a frequency comb with tunable carrier envelope offset.

Gas-phase broadband spectroscopy using active sources: progress, status, and applications

Broadband spectroscopy is an invaluable tool for measuring multiple gas-phase species simultaneously. In this work we review basic techniques, implementations, and current applications for broadband spectroscopy. We discuss components of broad-band spectroscopy including light sources, absorption cells, and detection methods and then discuss specific combinations of these components in commonly-used techniques. We finish this review by discussing potential future advances in techniques and applications of broad-band spectroscopy.