Effects of H<sub>2</sub>O and SF<sub>6</sub> on CO Molecular Relaxation in a Cantilever-Enhanced Fiber-Optic Photoacoustic Sensor
Xinyu Zhao, Hongchao Qi, Yajie Zhang, Yufu Xu, Chenxi Li, Min Guo, Fengxiang Ma, Wei Peng, Ke Chen
Abstract
The effects of H 2 O and SF 6 on the relaxation rate of CO molecules are studied using a fiber-optic photoacoustic sensor (FOPAS) that integrates a resonant multipass photoacoustic cell with a fiber cantilever. The CO photoacoustic signal is enhanced by approximately 1 order of magnitude under the induction of H 2 O. SF 6 has a high density compared to N 2, which causes the photoacoustic resonance frequency to shift to low frequency. The photoacoustic responses of CO/SF 6 and CO/N 2 at low frequency are tested with and without H 2 O to verify that SF 6 gas has a promoting effect on the relaxation process. The impact of the increasing H 2 O concentration on the CO/N 2 photoacoustic signal manifests in three distinct stages, which are reflected in gradual increase, sharp increase, and eventual saturation. The corresponding H 2 O concentration ranges of the three different stages are 2000–12,000, 12,000–18,000, and above 18,000 ppm, respectively. The SF 6 ratio in CO/SF 6 and CO/N 2 gas is precisely controlled, and the influence mechanism of the background gas mixture changes on the resonance frequency, photoacoustic response, and Q -factor is studied. Furthermore, the responsivity and noise of the fiber-optic photoacoustic CO sensor are evaluated under the induction of H 2 O and SF 6 . The detection limits of CO/N 2 and CO/SF 6 are 10 ppb with averaging times of 100 and 400 s, which makes this study applicable to the detection environment with N 2, SF 6, and N 2 –SF 6 mixtures as background gas.