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Abstract:The mid-infrared part of the electromagnetic spectrum is the so-called molecular fingerprint region because gases have tell-tale absorption features associated with molecular rovibrations. This region can be for instance exploited to detect small traces of environmental and toxic vapours in atmospheric and industrial applications. Novel Fourier transform spectroscopy without moving parts, based on time-domain interferences between two comb sources, can in particular benefit optical diagnostics and precision spectroscopy.

Received Month X, XXXX; revised Month X, XXXX; accepted Month X, XXXX; posted Month X, XXXX (Doc. ID XXXXX); published Month X, XXXX Broadband mid-IR output suitable for producing 1000-nm-wide frequency combs centered at 4.9 m was achieved in a degenerate subharmonic optical parametric oscillator (OPO) based on 500-m-long Brewster-angled orientation-patterned GaAs crystal. The OPO was synchronously pumped at 182 MHz repetition rate by 100-fs pulses from a Cr 2+ :ZnSe laser with the central wavelength of 2.45m and the average power of 100 mW. © 2010 Optical Society of America OCIS Codes: (190.4975) Parametric processes; (190.4410) Nonlinear optics, parametric processes Extending the spectral range of optical frequency combs to the mid-infrared (mid-IR) 'fingerprint' region where molecules have their tell-tale absorption features associated with ro-vibrational transitions is critical for numerous applications including frequency standards, precision spectroscopic measurements and trace molecular detection. With the aid of coherent Fourier transform spectroscopy using broadband frequency combs, one can access simultaneously a great variety of molecular signatures, accurately and with high sensitivity, spectral resolution and speed [1][2][3][4][5].Several methods were developed lately for extending broadband frequency combs (and more generally, supercontinuum sources) to the mid-IR. These include direct laser sources [6], sources based on supercontinuum generation in optical fibers driven by self-phase modulation [7][8][9] and engineered  (2) nonlinear-optical devices [10], optical rectification [11], difference-frequency generation [12][13][14], optical parametric oscillators [15,16] and amplifiers [17].This work extends the results of two earlier works on producing broadband frequency combs in synchronouslypumped subharmonic OPOs, based on periodically poled lithium niobate -with a 780-nm femtosecond (fs) Ti:Sapphire laser [18], and fs 1560-nm erbium-fiber laser [19] as a pump. In the latter case, a spectral span of 2.5 -3.8 m was achieved. Here we report on our first results on exploring even longer wavelength range -with a degenerate OPO based on orientation-patterned GaAs (OP-GaAs) crystal.OP-GaAs has excellent characteristics for mid-IR parametric frequency conversion. It has large secondorder nonlinear optical coefficient d14=94 pm/V [20] and good mid-IR transparency with a long-wavelength cutoff at 17 m. In addition it has smaller, as compared to lithium niobate, absolute value of group velocity dispersion at  > 3 m.The OPO was synchronously pumped (Fig.1) by a femtosecond Cr 2+ :ZnSe laser with the following parameters: central wavelength 2.45 m, repetition rate 182 MHz, pulse duration ~100 fs. The laser was mode locked using a semiconductor saturable absorber mirror (SESAM) [5,6] and used a sapphire plate for the 2-nd order dispersion compensation. Compared to [5,6], we used only a 5-mm sapphire plate for dispersion compensation and higher (6%) outcoupling, allowing to increase the output a...

An ultrashort-pulse Cr 2+ :ZnSe laser is a novel broadband source for sensitive high resolution molecular spectroscopy. A 130-fs pulse allows covering of up to 380 cm -1 spectral domain around 2.4 µm which is analyzed simultaneously with a 0.12 cm -1 (3.6 GHz) resolution by a Fouriertransform spectrometer. Recorded in 13 s, from 70-cm length absorption around 4150 cm -1 , acetylene and ammonia spectra exhibit a 3800 signal-tonoise ratio and a 2.4·10 -7 cm -1 ·Hz -1/2 noise equivalent absorption coefficient at one second averaging per spectral element, suggesting a 0.2 ppbv detection level for HF molecule. With the widely practiced classical tungsten lamp source instead of the laser, identical spectra would have taken more than one hour.