Sensitive light-induced thermoelastic spectroscopy based on transmitted light amplification

This paper investigates the light-induced thermoelastic spectroscopy (LITES) based on transmitted light amplification to realize high-precision gas detection. The modulated laser beam passes through a multi-pass cell and is then coupled to an optical amplifier. The multi-pass cell reflects the laser beam 100 times, has an optical length of 16 m, and its transmitted light intensity is 1.67 mW. A narrowband fiber optical filter with a bandwidth of 0.8 nm is utilized to suppress optical noise. Based on the transmitted light amplification, the signal-to-noise ratio (SNR) is improved by a factor of 3.6. To investigate the enhancement of second harmonic (2 <i>f</i>) signals under weak light intensities, a fiber optical attenuator is adopted to attenuate the transmitted light intensity. While the transmitted light intensity is attenuated to 0.048 mW, a high SNR of 1823 and a minimum detection limit (MDL) of 0.110 ppm can be obtained. Hence, LITES based on transmitted light amplification enables high-precision measurements while the light intensity is only at the scale of μW. This approach facilitates a significant increase in the number of beam reflections as well as the optical length of the multi-pass cell and resonant cavity for LITES sensors.