Fourier transform and Vernier spectroscopy using an optical frequency comb at 3–54  μm

Abstract

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...

Mid-infrared continuous-filtering Vernier spectroscopy using a doubly resonant optical parametric oscillator

“…2. The MIR comb source is a doubly resonant optical parametric oscillator (DROPO) based on an orientation-patterned GaAs (OP-GaAs) crystal (BAE Systems) pumped by a Tm:fiber femtosecond laser (IMRA America) with a repetition rate of 125 MHz, which delivers up to 2 W of power around 1.95 µm [10].…”

“…In the non-degenerate operation mode, the DROPO output (p-polarized) consists of a signal comb with ~250 nm bandwidth tunable between 3.1 and 3.6 μm and an idler comb with ~350 nm bandwidth tunable in the 4.6-5.4 μm range, providing up to 28 mW of signal and 20 mW of idler power [10]. The f rep of the pump laser is locked to the FSR of the DROPO cavity using a dither-lock method.…”

“…Fourier transform spectroscopy (FTS) can be implemented either with a mechanical interferometer [3,[7][8][9][10][11] or using the dual-comb technique [4,12,13]. Both approaches enable measurements with frequency resolution and precision provided by the comb with no influence of the instrumental line shape and are suitable for broadband precision spectroscopy [4,14].…”

“…We have recently implemented the CF-VS technique in the MIR range (3.15-3.4 μm) using a spectrometer based on a doubly resonant optical parametric oscillator (DROPO) with an orientation-patterned GaAs crystal pumped by a Tm:fiber femtosecond laser, an enhancement cavity, a diffraction grating, and a photodetector [10]. In this proof-of-principle experiment, we measured a spectrum of laboratory air at atmospheric pressure with absorption sensitivity of 6.2 × 10 −7 cm −1 in 2 ms, demonstrating the potential of the technique for fast and sensitive measurement in the molecular fingerprint region.…”

High-power frequency comb source tunable from 27 to 42 μm based on difference frequency generation pumped by an Yb-doped fiber laser

“…In the first demonstration, we measured a broadband spectrum of acetylene at 3.0 -3.1 μm with an FTS. In our future work we will combine this DFG source with a continuous-filtering Vernier spectrometer [29], which will allow fast and robust detection of multiple atmospheric species with concentration detection limits below ppb. …”