Multiplex Molecular Fingerprinting with a Mid-Infrared Cr2+:ZnSe Femtosecond Laser

A 130-fs Cr:ZnSe laser is used to record, in 13 s, molecular spectra covering simultaneously 135 nm at 3.6 GHz resolution. The high signal-to-noise ratio of 3800 suggests 0.2 ppbv detection level for HF molecule. ©2008 Optical Society of America OCIS codes: 140.4050 Mode-locked lasers, 140.3070 Infrared and far-infrared lasers, 300.6300 Spectroscopy, Fourier transforms, 300.6390 Spectroscopy, molecular The solid-state modelocked lasers have now become reliable sources of broadband radiation, possessing unique features such as high spectral power, brightness and perfect beam quality. This makes them attractive for high resolution spectroscopy, as frequency combs [1-3] or as broadband sources with excellent beam quality [4,5]. Trace gas detection or molecular spectroscopy of weak gas lines puts a number of specific requirements to the apparatus: (i) the measurement setup should provide the highest possible sensitivity and signal-to-noise ratio (SNR) to detect faint lines; (ii) the bandwidth coverage should be sufficient to observe sets of lines for accurate identification of different constituents; (iii) the resolution and frequency accuracy must be sufficient to resolve individual lines, and (iv) for quantitative measurements, the complete line profile must be resolved (this also improves the SNR). The traditional measurements techniques meet difficulties in achieving these requirements simultaneously. Especially in the most interesting molecular fingerprint region, the spectral irradiance of the incoherent thermal sources and their poor focusability do not allow high sensitivity measurements. In this paper, we report the implementation of a new spectrometric method based on a femtosecond laser taking the full benefit of the direct access to the mid-infrared wavelength range. We apply the femtosecond CrZnSe modelocked laser in the molecular fingerprint region to high resolution spectroscopy. The radiation of a CrZnSe modelocked laser is absorbed by the gas sample and analysed by a commercial Fourier transform (FT) spectrometer. Examples of ammonia spectra covering up to 135 nm in the 2400 nm region are shown.