Abstract
A spectroscopic method of molecular detection based on dispersion measurements using a frequency-chirped laser source is presented. An infrared quantum cascade laser emitting around 1912 cm(-1) is used as a tunable spectroscopic source to measure dispersion that occurs in the vicinity of molecular ro-vibrational transitions. The sample under study is a mixture of nitric oxide in dry nitrogen. Two experimental configurations based on a coherent detection scheme are investigated and discussed. The theoretical models, which describe the observed spectral signals, are developed and verified experimentally. The method is particularly relevant to optical sensing based on mid-infrared quantum cascade lasers as the high chirp rates available with those sources can significantly enhance the magnitude of the measured dispersion signals. The method relies on heterodyne beatnote frequency measurements and shows high immunity to variations in the optical power received by the photodetector.
Highlights
Laser absorption spectroscopy (LAS) based on tunable semiconductor lasers and/or non-linear frequency conversion sources has become a widely used technique for the analysis of gasphase chemicals
Due to the extremely small refractive index change associated with the absorption of trace amounts of molecular species, little progress has been made to adopt this approach to routine molecular detection
We report on a new measurement method for molecular dispersion based on a frequency-chirped laser source and heterodyne detection
Summary
Laser absorption spectroscopy (LAS) based on tunable semiconductor lasers and/or non-linear frequency conversion sources has become a widely used technique for the analysis of gasphase chemicals. Due to the extremely small refractive index change associated with the absorption of trace amounts of molecular species, little progress has been made to adopt this approach to routine molecular detection. Interferometric methods such as the “hook” method [4], developed at the early stage of gaseous sample dispersion studies, have been used until now [5]. The atmosphere exhibits two wide spectral windows in the mid-IR region (3-5 and 8-12 μm), enabling molecular sensing with minimal spectral interferences (primarily from water vapor) For these reasons, mid-IR spectroscopy is optimum for gas-phase chemical detection at trace levels. The following sections present the theoretical aspects of the method, the corresponding experimental studies and results, and a discussion on the merits of chirped-laser dispersion spectroscopy (CLaDS)
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