Molecular dispersion spectroscopy for chemical sensing using chirped mid-infrared quantum cascade laser

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.

A quantum cascade laser-based Mach–Zehnder interferometer for chemical sensing employing molecular absorption and dispersion

“…Nevertheless, considering the narrow ro-vibrational transitions in small gas molecules, a range of new measurement approaches expanding on direct absorption spectroscopy have been demonstrated to feature important advantages in terms of ruggedness as well as sensitivity [13,14]. Some of these novel approaches exploit molecular dispersion, rather than absorption, to decouple the recorded quantity (phase or frequency) from the laser intensity [15,16].…”

Femtosecond optical parametric oscillators toward real-time dual-comb spectroscopy

“…The measurement of the optical phase shift due to the target gas sample can offer a variety of advantages over measurements based on absorption sensing. For instance, the linear relationship between the dispersion spectrum and the sample concentration is beneficial; on the contrary, the absorption saturates with increasing concentration [45,46]. For dual-comb-based spectrometers, as well as for Michelson-based Fourier spectrometers, the dispersion signal is, to some extent, immune to the intensity fluctuations of the light sources and can provide more reliable and accurate background free data than measuring the absorption signal [43,47].…”

Remote Sensing of Atmospheric Methane with Simultaneous Ranging Using Chirped Laser Dispersion Spectroscopy

“…Chirped laser dispersion spectroscopy is a new technique that probes the optical dispersion induced when probe laser light interacts with a targeted species [1]. When the laser frequency coincides with a molecular transition, light is absorbed, however, in addition the light also experiences dispersion which is related to resonant absorption via the Kramers-Kronig relationship.…”