A sensor based on high-sensitivity multi-pass resonant photoacoustic spectroscopy for detection of hydrogen sulfide

A high-sensitive tunable diode laser sensor based on photoacoustic spectroscopy and multi-pass absorption spectroscopy is developed for hydrogen sulfide (H2S) detection using a target absorption line at 6287.005 cm-1. Two spherical concave mirrors are utilized as the reflectors to make the laser beam pass through the photo -acoustic cell for 24 times. The wavelength modulation spectroscopy technique with second harmonic detection (WMS-2f) method is employed to reduce external interference and increase the detection sensitivity. With the enhancement of the effective laser power in the multi-pass PA cell, the photoacoustic 2f signal of the target H2S absorption line has a -8 times improvement compared to the sensor system without multi-pass cell. Based on the results of Allan variance analysis, the minimum detection concentration of the system could reach 0.68 ppm with a 10-s average time and it can be further lowered down to -0.1 ppm when a 1-minute averaging time is used. The linear responsivity of the multi-pass PA spectroscopy sensor is estimated to be -0.00112 mV/ppm by linear fitting for a continuous flow gas measurement results. The excellent performance validates the potential of the sensor for field application.