Abstract

Rapid development of cost-effective and reliable sources generating coherent radiation in the mid-infrared wavelength region was the main driving factor for development of miniaturized gas sensors capable of selective concentration measurements in out-of-lab environment. Widely used laser spectroscopy techniques rely on detecting slight variations in the amplitude of a laser source interacting with the measured gas sample. Such configurations require periodical calibration to maintain repeatability. This problem can be addressed by implementing detection techniques allowing for encoding the gas concentration in frequency (e.g. deviation), like dispersion spectroscopy or photothermal spectroscopy (PT) [1]. PT spectroscopy is in principle similar to photoacoustic spectroscopy and requires the gas sample to be excited with a radiation source. Part of the absorbed energy will increase the temperature of the sample. This will lead to localized change of density and hence modulate the refractive index (RI) of the gas. Because the PT-induced change in the RI is small, most published PT sensors relied on using sensitive interferometric signal retrieval. Unfortunately, interferometric sensor configurations usually require using expensive frequency shifters or PTS’s to sustain quadrature operation in phase-sensitive detection.

© 2019 IEEE