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Sensitive multiplex spectroscopy in the molecular fingerprint 2.4 µm region with a Cr2+:ZnSe femtosecond laser

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Abstract

An ultrashort-pulse Cr2+: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 Fourier-transform spectrometer. Recorded in 13 s, from 70-cm length absorption around 4150 cm-1, acetylene and ammonia spectra exhibit a 3800 signal-to-noise 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.

©2007 Optical Society of America

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Figures (4)

Fig. 1.
Fig. 1. Experimental set-up. CM: chirped mirror. OC: output coupler. Atten: attenuator.
Fig. 2.
Fig. 2. (a). Spectrum (FWHM: 76 cm-1) and (b) interferometric autocorrelation (FWHM: 250 fs) of a 130-fs pulse of the Cr2+:ZnSe mode-locked laser. Pump power: 1.9 W, output power: 50 mW, repetition rate 200 MHz.
Fig. 3.
Fig. 3. Part of a C2H2 spectrum at 0.12 cm-1 resolution in the 2.4 µm region. The baseline modulation is not noise-related but due to a channel spectrum from an unwedged window. The strongest spectral features are the P, Q and R branches of the ν11 5 band [12] of 12C2H2, centered at 4090 cm-1. The intensity alternation is obvious in the R-branch. The upper left inset is a zoom showing acetylene and residual water vapor lines coming from a 5 meters air path between the cell and the interferometer. The H2O lineshapes reveal an atmospheric-pressure broadening. The rotational assignment for the H2O lines [13] gives (J,K a,K c).
Fig. 4.
Fig. 4. Spectrum of NH3 in the 2.4 µm region illustrating the spectral bandwidth and SNR capabilities of the spectrometric technique. The ν12 and ν23 combination bands are observed. The upper graph shows the whole spectral domain covered in a singe recording while the lower graph shows a portion expanded in wavenumber and intensity scales.
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