Research On Multi-pass Absorption Enhanced Fiber-optic Photoacoustic Gas Sensing

With the development of society,trace gas detection has become an indispensable tool in human life.It is active in atmospheric environment detection,analysis of dissolved gas in oil in the power system,fire monitoring in coal mine goafs,non-invasive medical diagnosis,etc.Photoacoustic（PA）spectroscopy（PAS）owns the advantages of high sensitivity,good selectivity and fast response.In order to meet the application requirements of low gas volume and high sensitivity from many industrial fields and human lives,Some research results have been achieved in this paper based on the design of miniaturized multi-pass cells and the optimization of microphone.First,a fiber-optic Fabry-Perot（F-P）diaphragm acoustic sensor based on a titanium film is proposed.The diaphragm had a radius of 4.5 mm and a thickness of 3μm.The experimental results showed that the resonance frequency of the fiber-optic microphone（FOM）was 410 Hz,and the sensitivity at 110 Hz reached 126.6 nm/Pa.Second,an ultra-highly sensitive FOM is proposed.The FOM used a cylindrical hollow polyvinyl chloride（PVC）tube as an acoustic transducer.The hollow-core tube had an outer diameter of 17.85 mm,a thickness of 0.45 mm,and a length of 100.22 mm.The FOM can overcome the shortcomings of the traditional diaphragm-based or intensity-based demodulation-based FOM,such as Q point drift and being sensitive to environmental interference.Experimental results showed that the sensitivity of FOM exceeded 6800nm/Pa at the resonance frequency.The sound pressure sensitivity and the minimum detectable sound pressure level（MDSPL）at 1000 Hz reached 315.3 nm/Pa and-20.4 d B/Hz^1/2(1.9μPa/Hz^1/2),respectively.Third,a PAS trace gas detection system based on multiple reflections on the cell wall was proposed.In the designed PA cell,the laser was incident obliquely from the side wall of the cylindrical PA cell.Multiple reflections occurred on the inner surface of the cell wall to increase the absorption path.The PA signal can thus be enhanced.The inner diameter and the length of the PA cavity were only 6 mm and 30 mm,respectively.The corresponding volume of the cavity was only 848μL while the absorption length reached over 258 mm,realizing the amplification of the gas absorption path in a miniaturized PA cell.The absorption path-volume ratio reached 300(10³ m^-2).When the deflection angle of the collimator was 5.1°,the magnification of the PA signal is 6.4.The normalized noise equivalent absorption（NNEA）coefficient reached 1.1×10^-8 cm^-1·W·Hz^-1/2.When the lock-in integration time was 400 s,the minimum detection limit（MDL）of the system to acetylene detection was 31 ppb.Fourth,a fiber-optic PA gas sensing system based on multi-pass absorption enhancement is proposed.The system used a miniaturized Herriott type multi-pass cell and a titanium diaphragm based FOM.The gas absorption length reached 374 mm while the volume of the PA cavity is only 622μL,and the corresponding absorption path-volume ratio reached 600(10³m^-2).When the average time is 100 s,the MDL of the system to acetylene detection reached 8.4ppb.