Cavity-enhanced optical frequency comb spectroscopy in the mid-infrared - application to trace detection of H2O2

We demonstrate the first cavity-enhanced optical frequency comb spectroscopy in the mid-infrared wavelength region and report the sensitive real-time trace detection of hydrogen peroxide in the presence of a large amount of water. The experimental apparatus is based on a mid-infrared optical parametric oscillator synchronously pumped by a high power Yb:fiber laser, a high finesse broadband cavity, and a fast-scanning Fourier transform spectrometer with autobalancing detection. The comb spectrum with a bandwidth of 200 nm centered around 3.75 {\mu}m is simultaneously coupled to the cavity and both degrees of freedom of the comb, i.e., the repetition rate and carrier envelope offset frequency, are locked to the cavity to ensure stable transmission. The autobalancing detection scheme reduces the intensity noise by a factor of 300, and a sensitivity of 5.4 {\times} 10^-9 cm^-1 Hz^-1/2 with a resolution of 800 MHz is achieved (corresponding to 6.9 {\times} 10^-11 cm^-1 Hz^-1/2 per spectral element for 6000 resolved elements). This yields a noise equivalent detection limit for hydrogen peroxide of 8 parts-per-billion (ppb); in the presence of 2.8% of water the detection limit is 130 ppb. Spectra of acetylene, methane and nitrous oxide at atmospheric pressure are also presented, and a line shape model is developed to simulate the experimental data.Comment: submitted to special FLAIR 2011 issue of Appl. Phys.

MID-INFRARED FREQUENCY COMB SPECTROSCOPY USING A VIRTUALLY IMAGED PHASED ARRAY

Here we present a new mid-infrared frequency comb system for rapid spectral acquisition using a virtually imaged phased array (VIPA) spectrometer.\footnote{L. Nugent-Glandorf et al., \textit{Opt. Lett.} \textbf{37,} 3285 (2012)} A difference-frequency generation comb, tuneable from 4.4 $\mu$m to 4.7 $\mu$m, was used to interrogate a single-pass absorption cell containing either N$_2$O or CO dilute in either N$_2$ or air. Precision molecular spectroscopy capabilities at timescales of less than 1 ms will be presented, and progress toward cavity-enhanced and time-resolved comb spectroscopies\footnote{A.J. Fleisher et. al., \textit{J. Phys. Chem. Lett.} \textbf{5,} 2241 (2014)} will be discussed

QUANTUM-NOISE-LIMITED CAVITY RING-DOWN SPECTROSCOPY IN THE MID-INFRARED

We report a highly sensitive mid-infrared spectrometer capable of recording cavity ring-down events in the quantum (shot) noise limit. A linear optical cavity of finesse 31,000 was pumped by a distributed feedback quantum cascade laser (DFB-QCL) operating at 4.5 $mu$m until a cavity transmission threshold was reached. A fast optical switch then extinguished optical pumping and initiated a cavity decay which exhibited root-mean-square noise proportional to the square root of optical power (quantum noise) for several cavity time constants until a detector noise floor was reached. This spectrometer has achieved a noise-equivalent absorption of NEA = $2.6times10^{-11}$ wn Hz$^{-1/2}$ and a minimum absorption coefficient of $alpha = 2.3times10^{-11}$ wn in 3 seconds. Applications for such a highly sensitive spectrometer operating in the mid-infrared region, including ultra-trace molecular spectroscopy of chem{CO_2} isotopologues and the direct interrogation of weak mirror birefringence and polarization-dependent losses, will be discussed

Collision-dependent line areas in the a1∆g← X3Σ−g band of molecular oxygen

We report precise line areas for individual rotationally resolved transitions within the $a^1Delta_gleftarrow$ $X^3Sigma^-_g$ electronic band of molecular oxygen recorded as a function of pressure for both neat samples of chem{O_2} as well as samples of chem{O_2} dilute with a variety of collisional partners. Using optical frequency comb referenced frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) near 1.27 $mu$m we measure line areas with a quality-of-fit QF $leq$ 50,000 using a partially correlated quadratic-speed-dependent Nelkin-Ghatak profile. This spectrometer has achieved this high QF by both suppressing coupled cavity effects and by preserving a high-fidelity frequency axis with absolute frequency accuracy approaching 1 part in 10$^9$. With this instrument we are also currently exploring collision-induced absorption (CIA) and perturbative line mixing effects in chem{O_2} over the entire 7800-7940 wn spectral range

DIRECT ABSORPTION SPECTROSCOPY WITH ELECTRO-OPTIC FREQUENCY COMBS

The application of electro-optic frequency combs to direct absorption spectroscopyfootnote{D.A. Long et al., textit{Opt. Lett.} textbf{39,} 2688 (2014)} has increased research interest in high-agility, modulator-based comb generation. This talk will review common architectures for electro-optic frequency comb generators as well as describe common self-heterodyne and multi-heterodyne (i.e., dual-comb) detection approaches. In order to achieve a sufficient signal-to-noise ratio on the recorded interferogram while allowing for manageable data volumes, broadband electro-optic frequency combs require deep coherent averaging,footnote{A.J. Fleisher et al., textit{Opt. Express} textbf{24,} 10424 (2016)} preferably in real-time. Applications such as cavity-enhanced spectroscopy, precision atomic and molecular spectroscopy, as well as time-resolved spectroscopy will be introduced

PRECISION CAVITY-ENHANCED DUAL-COMB SPECTROSCOPY: APPLICATION TO THE GAS METROLOGY OF CO2, H2O, and N2O.

With inherent simplicity, mutual phase coherence, and a high degree of user control, electro-optic frequency combs are amenable to both dual-comb spectroscopyfootnote{I. Coddington et al., textit{Optica} textbf{3,} 414 (2016)} and cavity-enhanced comb spectroscopy.footnote{B. Bernhardt et al., textit{Nat. Photonics} textbf{4,} 55 (2010)} This combination of fast, multiplexed spectroscopy, with an effective absorption pathlength $>$1 km, is used here to perform line-by-line metrology of the gas-phase absorption spectra of CO$_2$, H$_2$O, and N$_2$O in the near-infrared. We report absolute transition frequency with precision better than 1 MHz in 1 s of spectral acquisition per transition using a comb with an instantaneous optical bandwidth of 6 GHz, tunable over the entire 6240-6370 wn range. A full model for the electric field transmitted through the enhancement cavity (even in the presence of strong molecular absorption and dispersion) will be discussed