Research On Ultra-high Sensitivity Gas Detection Technology Based On Resonant Multi-pass Photoacoustic Cell

Photoacoustic spectroscopy（PAS）technology is widely used in the detection of trace gases due to its high sensitivity,low cross-interference,fast response and maintenance-free.Traditional PAS is based on a photoacoustic cell with the single absorption optical path and a condenser microphone,which limits the improvement of detection sensitivity.In this paper,the amplitude and detection accuracy of the photoacoustic signal were improved from the three dimensions of light excitation enhancement,acoustic resonance enhancement and photoacoustic signal detection enhancement.The following are the research results obtained.First,the principle of excitation light power enhancement of the multi-pass cell（MPC）was theoretically analyzed,and the position coordinates of the light spot and the optical path length of a single reflection were calculated in detail.The beam analysis,simulation and parameter optimization of the photoacoustic cell based on Herriott type were carried out.The correctness of theoretical analysis and simulation was verified by experiments.Through the detection experiment of trace CH₄ gas,it was verified that the photoacoustic signal enhancement technology based on MPC can improve the gas detection sensitivity.Second,a fiber-optic acoustic sensor was design and optimize to improve the gas detection sensitivity from the perspective of photoacoustic signal detection enhancement.A fiber-optic cantilever acoustic sensor based on the Fabry-Perot（FP）structure was designed and fabricated.A high-sensitivity cantilever acoustic sensor was fabricated on a stainless-steel diaphragm with a thickness of 10μm by laser micromachining technology.The frequency response of the experimental test showed that at the first-order resonance frequency of 2602 Hz,the sensitivity is 8193 nm/Pa,which made up for the shortcomings of traditional capacitive microphones,such as low sensitivity,poor anti-electromagnetic interference and difficulty in realizing long-distance measurement.Third,the multi-pass structure was embedded in the photoacoustic cell to improve the Minimum detection limit（MDL）from the perspective of excitation enhancement of the light.A fiber-optic photoacoustic trace gas analysis system based on a resonant multi-pass photoacoustic cell was built by combining the fiber-optic acoustic sensor.The chamber volume of only 157 m L,the beam was reflected 20 times and the absorption optical path was 4 m,which was about 16 times higher than the performance of the single-pass photoacoustic cell.Experiments showed that the normalized noise equivalent absorption（NNEA）coefficient of the system was 8.3×10^-10 cm^-1W/Hz^1/2.The MDL reached 3.7 ppb with an averaging time of500 s.Fourth,based on the theory of acoustic resonant cavity,a high-Q differential photoacoustic cell model was designed and optimized to improve the reliability of the detection system from the perspective of acoustic resonance enhancement.A set of photoacoustic spectroscopy trace gas analysis system based on a differential multi-pass photoacoustic cell was built.The laser was incident on the photoacoustic cell at an inclination angle of 0.8°and the optical path length was as high as 4.92 m,which improved the amplitude of the photoacoustic signal and suppressed the flow noise.Experiments showed that the NNEA was equal to 1.79×10^-10cm^-1W/Hz^1/2.In addition,the high-sensitivity detection of CH₄ was achieved,the MDL reached 0.6ppb with an averaging time of 500 s.This system realized the continuous flow monitoring of CH₄ in the environment,it has a good development prospect in the application of real-time online monitoring of trace gases.