Optical frequency combs (OFCs) 1,2 offer significant advantages for Fourier transform spectrometers (FTS) 3,4 compared to thermal sources 5,6 . The high spectral brightness and spatial and temporal coherence of combs allow acquisition of broadband molecular spectra with high signal-tonoise ratios in recording times orders of magnitude shorter than traditional Fourier transform infrared (FTIR) spectrometers and removes the need to collimate light from isotropic sources. Furthermore,

Detection and analysis of volatile compounds in exhaled breath represents an attractive tool for monitoring the metabolic status of a patient and disease diagnosis, since it is non-invasive and fast. Numerous studies have already demonstrated the benefit of breath analysis in clinical settings/applications and encouraged multidisciplinary research to reveal new insights regarding the origins, pathways, and pathophysiological roles of breath components. Many breath analysis methods are currently available to help explore these directions, ranging from mass spectrometry to laser-based spectroscopy and sensor arrays. This review presents an update of the current status of optical methods, using near and mid-infrared sources, for clinical breath gas analysis over the last decade and describes recent technological developments and their applications. The review includes: tunable diode laser absorption spectroscopy, cavity ring-down spectroscopy, integrated cavity output spectroscopy, cavity-enhanced absorption spectroscopy, photoacoustic spectroscopy, quartz-enhanced photoacoustic spectroscopy, and optical frequency comb spectroscopy. A SWOT analysis (strengths, weaknesses, opportunities, and threats) is presented that describes the laser-based techniques within the clinical framework of breath research and their appealing features for clinical use.

Broadband precision spectroscopy is indispensable for providing high fidelity molecular parameters for spectroscopic databases. We have recently shown that mechanical Fourier transform spectrometers based on optical frequency combs can measure broadband high-resolution molecular spectra undistorted by the instrumental line shape (ILS) and with a highly precise frequency scale provided by the comb. The accurate measurement of the power of the comb modes interacting with the molecular sample was achieved by acquiring single-burst interferograms with nominal resolution precisely matched to the comb mode spacing. Here we give a full theoretical description of this sub-nominal resolution method and describe in detail the experimental and numerical steps needed to retrieve ILS-free molecular spectra, i.e. with ILS-induced distortion below the noise level. We investigate the accuracy of the transition line centers retrieved by fitting to the absorption lines measured using this method. We verify the performance by measuring an ILS-free cavity-enhanced low-pressure spectrum of the 3ν1+ν3 band of CO2 around 1575 nm with line widths narrower than the nominal resolution. We observe and quantify collisional narrowing of absorption line shape, for the first time with a comb-based spectroscopic technique. Thus retrieval of line shape parameters with accuracy not limited by the Voigt profile is now possible for entire absorption bands acquired simultaneously.

Dual-comb spectroscopy can provide broad spectral bandwidth and high spectral resolution in a short acquisition time, enabling time-resolved measurements. Specifically, spectroscopy in the mid-infrared wavelength range is of particular interest, since most of the molecules have their strongest rotational-vibrational transitions in this “fingerprint” region. Here we report time-resolved mid-infrared dual-comb spectroscopy, covering ~300 nm bandwidth around 3.3 μm with 6 GHz spectral resolution and 20 μs temporal resolution. As a demonstration, we study a CH4/He gas mixture in an electric discharge, while the discharge is modulated between dark and glow regimes. We simultaneously monitor the production of C2H6 and the vibrational excitation of CH4 molecules, observing the dynamics of both processes. This approach to broadband, high-resolution, and time-resolved mid-infrared spectroscopy provides a new tool for monitoring the kinetics of fast chemical reactions, with potential applications in various fields such as physical chemistry and plasma/combustion analysis.

We present a versatile mid-infrared frequency comb spectroscopy system based on a doubly resonant optical parametric oscillator tunable in the 3-5.4 μm range and two detection methods, a Fourier transform spectrometer (FTS) and a Vernier spectrometer. Using the FTS with a multipass cell we measure high-precision broadband absorption spectra of CH4 and NO at ~3.3 μm and ~5.2 μm, respectively, and of atmospheric species (CH4, CO, CO2 and H2O) in air in the signal and idler wavelength range. The figure of merit of the system is on the order of 10 −8 cm −1 Hz −1∕2 per spectral element, and multiline fitting yields minimum detectable concentrations of 10-20 ppb Hz −1∕2 for CH4, NO and CO. For the first time in the mid-infrared, we perform continuous-filtering Vernier spectroscopy using a low finesse enhancement cavity, a grating and a single detector, and measure the absorption spectrum of CH4 and H2O in ambient air at ~3.3 μm. © 2016 Optical Society of America OCIS codes : (190.4410) Nonlinear optics, (190.7110) Ultrafast nonlinear optics; (190.4970 Optical frequency comb sources in the mid-infrared (MIR) wavelength range (3-12 µm) have large potential for molecular spectroscopy, since the fundamental absorption bands of many species lie in this fingerprint region [1,2]. The maximum achievable wavelength of low repetition rate (<1 GHz) direct comb sources is still limited to <3 µm, and longer wavelengths (>3 µm) can only be reached through nonlinear frequency conversion. Sources based on difference frequency generation (DFG) offer wide wavelength coverage in the MIR, but suffer from poor conversion efficiency [3,4]. Higher average output power is provided by synchronously-pumped optical parametric oscillators (OPOs) [5,6]. In particular, OPOs based on orientation-patterned gallium arsenide (OP-GaAs) crystals pumped by Cr:ZnSe or Tm:fiber femtosecond lasers have made it possible to reach wavelengths beyond ~4.8 µm, a barrier for the well-established oxidebased materials [7,8]. Both DFG and OPO sources have been used for MIR optical frequency comb spectroscopy with different detection methods, namely a Fourier transform spectrometer [9-13], a virtually imaged phased array (VIPA) [14,15], mode-resolved Vernier spectroscopy [16], and dual comb spectroscopy [17,18]. However, the spectral range of all previous demonstrations was limited to <4.8 µm.Here we report a versatile optical frequency comb spectroscopy system based on a doubly resonant optical parametric oscillator (DROPO) with an OP-GaAs crystal operating in the 3-5.4 µm wavelength range. The system incorporates two detection methods, a fast-scanning Fourier transform spectrometer (FTS) in combination with a multipass cell, and a continuous-filtering Vernier spectrometer. The FTS provides ultra-broadband spectral coverage, absolute frequency calibration and high precision in the absorption measurement. We demonstrate this by acquiring absorption spectra of CH4, NO, and ambient air in the signal and idler ranges and comparing the results to the theoretical mod...

We present a continuous-filtering Vernier spectrometer operating in the 3.15-3.4 µm range, based on a femtosecond doubly resonant optical parametric oscillator, a cavity with a finesse of 340, a grating mounted on a galvo scanner and two photodiodes. The spectrometer allows acquisition of one spectrum spanning 250 nm of bandwidth in 25 ms with 8 GHz resolution, sufficient for resolving molecular lines at atmospheric pressure. An active lock ensures good frequency and intensity stability of the consecutive spectra and enables continuous signal acquisition and efficient averaging. The relative frequency scale is calibrated using a Fabry-Perot etalon or, alternatively, the galvo scanner position signal. We measure spectra of pure CH 4 as well as dry and laboratory air and extract CH 4 and H 2 O concentrations by multiline fitting of model spectra. The figure of merit of the spectrometer is 1.7×10 −9 cm −1 Hz −1/2 per spectral element and the minimum detectable concentration of CH 4 is 360 ppt Hz -1/2 , averaging down to 90 ppt after 16 s.

We present a fast-scanning Fourier transform spectrometer (FTS) in combination with high-repetition-rate mid-infrared supercontinuum sources, covering a wavelength range of 2–10.5 µm. We demonstrate the performance of the spectrometer for trace gas detection and compare various detection methods: baseband detection with a single photodetector, baseband balanced detection, and synchronous demodulation at the repetition rate of the supercontinuum source. The FTS uses off-the-shelf optical components and provides a minimum spectral resolution of 750 MHz. It achieves a noise equivalent absorption sensitivity of ∼10−6 cm−1 Hz−1/2 per spectral element, by using a 31.2 m multipass absorption cell.

Optical cavities provide high sensitivity to dispersion since their resonance frequencies depend on the index of refraction. We present a direct, broadband, and accurate measurement of the modes of a high finesse cavity using an optical frequency comb and a mechanical Fourier transform spectrometer with a kHz-level resolution. We characterize 16000 cavity modes spanning 16 THz of bandwidth in terms of center frequency, linewidth, and amplitude. We retrieve the group delay dispersion of the cavity mirror coatings and pure N2 with 0.1 fs 2 precision and 1 fs 2 accuracy, as well as the refractivity of the 3ν1+ν3 absorption band of CO2 with 5 × 10 -12 precision. This opens up for broadband refractive index metrology and calibration-free spectroscopy of entire molecular bands.OCIS codes : (140.4780) Optical resonators; (300.6300) Spectroscopy, Fourier transform; (260.2030) Physical optics, dispersion.Fabry-Perot cavities in combination with narrow linewidth continuous wave (cw) lasers are versatile tools for ultra-sensitive measurements of displacement, absorption, and dispersion. For example, high precision measurements of minute length variation of Fabry-Perot cavities enable detection of gravitational waves [1]. A pressure sensor based on the measurement of gas refractivity inside a cavity can outperform a manometer [2]. Cavity-enhanced molecular absorption [3,4] and dispersion [5,6] spectroscopies, which rely on the measurement of intracavity absorption losses and dispersion induced shifts of the cavity modes, respectively, provide complementary information about the molecular transitions and high sensitivity to absorption/dispersion. However, cw lasers allow such measurements only over narrow bandwidths, typically in the sub-THz range. Optical frequency combs, whose spectra consist of thousands of equidistant narrow lines, can probe cavity modes over a much broader bandwidth. In cavity-enhanced optical frequency comb absorption spectroscopy, spectra of entire molecular bands can be acquired with high resolution in short acquisition times [7][8][9][10]. Combs are also an ideal tool for measurements of broadband cavity dispersion induced either by the cavity mirror coatings or intracavity samples. However, previous demonstrations [11-13] did not fully benefit from the high frequency accuracy provided by the comb and suffered from poor spectral resolution (at the THz level).Recent advances in comb-based Fourier transform spectroscopy have provided means to measure spectra over the entire comb bandwidth with resolution directly given by the comb linewidth, using either dual-comb spectrometers [14][15][16][17] or mechanical Fourier transform spectrometers (FTS) [18]. Here we use a frequency comb and a mechanical FTS with sub-nominal resolution [18] to directly measure broadband transmission spectra of a high finesse cavity with high signal-to-noise ratio and frequency precision and accuracy. We fully characterize the cavity modes in terms of amplitude, width, and center frequency. From the shift of the cavity mod...