Automation of an "Aculight" continuous-wave optical parametric oscillator

Mid-infrared Doppler-free saturation absorption spectroscopy of the Q branch of CH4ν3 = 1 band using a rapid-scanning continuous-wave optical parametric oscillator

We have developed a mid-infrared Doppler-free saturation absorption spectroscopy apparatus that employs a commercial continuous-wave optical parametric oscillator (CW OPO), complemented by a home-built automation and wavelength scanning system. Here, we report a comprehensive spectral scan of the Q branch transitions of the ν3 = 1 band of methane (CH4) with an average linewidth (FWHM) of 4.5 MHz. The absolute frequency calibration was achieved using previously reported transition frequencies determined using optical frequency combs, while a Fabry-Perot etalon was used for the relative frequency calibration. We report 15 transitions with improved accuracies of 1.13 MHz (3.76 × 10-5 cm-1).

Modeling and experimental characterization of two-wave mixing in Yb-doped fiber amplifiers

Two-wave mixing between forward- and backward-propagating signal light has recently been observed in frequency-modulated single-frequency fiber laser systems. The phenomenon is a potential limiting factor for power scaling of such frequency-tunable lasers. In this contribution, we derive a perturbative coupled-mode theory for two signals that counter-propagate in an Yb-doped fiber with a constant frequency detuning. We apply the theory to analyze experimental results dedicated to extracting the central material parameter that relates the Yb inversion to a (real) refractive-index change. The perturbative theory is derived to all orders, and argued to be convergent. The experimental results and our analysis support previous estimates of the ratio between changes in the gain coefficient and the refractive index.

Infrared Laser Stark Spectroscopy of Methyl Fluoride in 4He Nanodroplets

We measured the rotationally resolved infrared spectra of helium solvated methyl fluoride at 3 μm and 10 μm, wherein lies C-H and C-F stretching bands, respectively. The linewidths (FWHM) were found to increase with increasing vibrational energy and range from 0.002 cm-1 in the v3 band (C-F stretch) at ~1047 cm-1, to 0.65 cm-1 in the v4 band (asymmetric C-H stretch) at ~2997 cm-1. In between these two bands we observed the lower and upper components of the Fermi triad bands (ν1/2ν2/2ν5) at ~2859 and ~2961 cm-1. We carried out Stark spectroscopy on the lower band on account of its narrower linewidths (0.04 vs. 0.14 cm-1, respectively). The objective of performing Stark spectroscopy was to see if there is any evidence for a rotational linewidth dependence on the external field strength, due to a reduced difference in between methyl fluorides rotational energy gap and the roton-gap of superfluid helium. We did not find any evidence for such an effect, which we largely attribute to the rotational energy gap not increasing significantly enough by the external field. We point to another molecule (formaldehyde) whose energy levels are predicted to show a more promising response to application of an external field.

Theory and experiment of transient two-wave mixing in Yb-doped single-frequency fiber amplifiers induced by frequency modulation

This paper presents a theoretical and experimental characterization of an instability phenomenon observed in single-frequency fiber amplifiers when the frequency of the seed laser is modulated. The instability manifests itself as fluctuating elastic back-reflections that occur only when the frequency is decreasing with time. The theory is a generalization of a coupled-mode model developed for a single-frequency fiber amplifier back-seeded with a constant frequency shift relative to the main signal. It can explain most observed features of the experiments in a qualitative and semi-quantitative way. Open questions and directions for further developments are also discussed.

On-chip mid-infrared optical sensing with GeSbSe waveguides and resonators

We fabricated single mode Ge₂₈Sb₁₂Se₆₀ waveguides and resonators using e-beam lithography and achieved a propagation loss of 3.88 dB/cm at 3.66 µm. We compared BCl₃ and CHF₃ etch chemistries and determined CHF₃ produced 1.5 dB/cm higher propagation losses at 3.6 µm due to C-H bond absorption. We use fabricated waveguides to detect an aromatic aldehyde dissolved in a non-polar solvent with a limit of detection of 1.09 µmol/mL. We then reduce this detection limit to 0.25 µmol/mL using the enhancement produced by a chalcogenide ring resonator.

Helium Droplet Infrared Spectroscopy of the Butyl Radicals

Butyl radicals ( n-, s-, i-,} and tert-butyl) are formed from the pyrolysis of stable precursors (1-pentyl nitrite, 2-methyl-1-butyl nitrite, isopentyl nitrite, and azo- tert-butane, respectively). The radicals are doped into a beam of liquid helium droplets and probed with infrared action spectroscopy from 2700-3125 cm-1, allowing for a low temperature measurement of the CH stretching region. The presence of anharmonic resonance polyads in the 2800-3000 cm-1 region complicates its interpretation. To facilitate spectral assignment, the anharmonic resonances are modeled with two model Hamiltonian approaches that explicitly couple CH stretch fundamentals to HCH bend overtones and combinations: a VPT2+K normal mode model based on CCSD(T) quartic force fields and a semi-empirical local mode model. Both of these computational methods provide generally good agreement with the experimental spectra.

High-resolution, broadly-tunable mid-IR spectroscopy using a continuous wave optical parametric oscillator

We report on the design and automation of a mid-infrared, continuous wave, singly-resonant optical parametric oscillator. Hands-free controls and the implementation of a tuning algorithm allowed for hundreds of nanometers of continuous, effective-mode-hop-free tuning over the range of 2190-4000 nm. To demonstrate the applicability of this light source and algorithm to mid-IR spectroscopy, we performed a sample spectroscopy measurement in a C₂H₂ gas cell and compared the experimentally-measured absorption spectrum to HITRAN 2016 simulations. We found excellent agreement with simulation in both peak heights and peak centers; we also report a reduced uncertainty in peak centers compared to simulation.

Fast, precise, and widely tunable frequency control of an optical parametric oscillator referenced to a frequency comb

Optical frequency combs (OFCs) provide a convenient reference for the frequency stabilization of continuous-wave lasers. We demonstrate a frequency control method relying on tracking over a wide range and stabilizing the beat note between the laser and the OFC. The approach combines fast frequency ramps on a millisecond timescale in the entire mode-hop free tuning range of the laser and precise stabilization to single frequencies. We apply it to a commercially available optical parametric oscillator (OPO) and demonstrate tuning over more than 60 GHz with a ramping speed up to 3 GHz/ms. Frequency ramps spanning 15 GHz are performed in less than 10 ms, with the OPO instantly relocked to the OFC after the ramp at any desired frequency. The developed control hardware and software are able to stabilize the OPO to sub-MHz precision and to perform sequences of fast frequency ramps automatically.

Mid-infrared optical parametric oscillators and frequency combs for molecular spectroscopy

Review of mid-infrared optical parametric oscillators and frequency combs for high-resolution spectroscopy, including applications in trace gas detection and fundamental research.

Broadband standoff detection of large molecules by mid-infrared active coherent laser spectrometry

A widely tunable active coherent laser spectrometer (ACLaS) has been demonstrated for standoff detection of broadband absorbers in the 1280 to 1318 cm^-1 spectral region using an external cavity quantum cascade laser as a mid-infrared source. The broad tuning range allows detection and quantification of vapor phase molecules, such as dichloroethane, ethylene glycol dinitrate, and tetrafluoroethane. The level of confidence in molecular mixing ratios retrieved from interfering spectral measurements is assessed in a quantitative manner. A first qualitative demonstration of condensed phase chemical detection on nitroacetanilide has also been conducted. Detection performances of the broadband ACLaS have been placed in the context of explosive detection and compared to that obtained using distributed feedback quantum cascade lasers.