Raman Spectroscopy For Gas Detection Using A Folded Near-Concentric Cavity

Raman spectroscopy is used in many fields with advantages of no sample pretreatment and simultaneous multiple species detection, while the disadvantage of low sensitivity limits its further a pplication. In order to improve the sensitivity of Raman gas detection, a method for enhancing the gas Raman signal based on a folded near-concentric cavity is reported in this paper. By inserting a high reflectivity plane mirror into the center of multiple reflectors, the cavity body is folded from the center, so that the beams in the center of the cavity overlap with each other to increase the light density and luminous flux, thereby improving the detection sensitivity of the system. Ray tracing and flux analysis are performed on the different cavity modes using TracePro software(laser: 300 mW (R) 532 nm, reflectance: 99% (R) 532 nm). The results show that the optical cavity folding method can significantly increase the optical flux at the center of the reflecting cavity. When the number of reflections in the folded near concentric cavity is 68 times, the luminous flux at the center of the waist is 22. 35 W, which is about 1. 5 times enhancement compared with the unfolded near-concentric reflecting cavity. In order to verify the simulation results, a gas detection Raman spectroscopy system with folded near-concentric cavity is set up. The experimental results show that the folded near-concentric cavity has the best enhancement, reaching 49 times; next is near concentric cavity about 36 times; the third is folding the concentric cavity, a pproximately 24 times; the last is the concentric cavity, just 17 times. The signal-to-noise ratio of the gas Raman signal collected by the folded near-concentric cavity is 1. 4 times higher than that of the unfolded near-concentric cavity. According to the relative intensity of the carbon dioxide Raman peak in the air, we can calculate the limit of detection for CO2 using 3-sigma criterion standard with the value of 0. 13 mg.L-1 (66 ppm).