Research On Trace Gas Detection Technology Based On T-type Resonant Photoacoustic Cell

With the development of technology and the improvement of people’s living standards,trace gas detection has become a particularly important part of industry and life.In the field of atmospheric monitoring,disease prevention and security,trace gas detection plays an irreplaceable role.With the development of laser technology,photoacoustic(PA)spectroscopy(PAS)has received wide attention as a gas detection method with fast response and high sensitivity.In order to solve the problems of large volume,long response time and high processing difficulty of traditional H-type resonant PA cell,a systematic study of the PAS trace gas detection technology based on T-type resonant PA cell is presented in this paper.Compared with the traditional H-type resonant PA cell,the T-type resonant PA cell is smaller in size,using less gas,and has a shorter gas equilibration time because of the reduction of a buffer volume.At the same time,the T-type resonant PA cell does not require a hole in the middle of the resonant cavity to be connected with a microphone,so the processing is simpler.The cell constant of the T-type resonant PA cell is larger under the premise of the same resonant cavity size.In this thesis,the buffer volume structure of the T-type resonant PA cell was analyzed using the theoretical acoustic formula,and its size was optimized.And the conclusion is that when the length of the buffer volume and the resonant cavity were equal,the influence of external environmental noise was minimized.The acoustic module of COMSOL Multiphysics finite element analysis software was used to simulate and analyze the T-type resonant PA cell,and the results of the equation analysis were verified.In addition,the simulation analysis of the PA signal of the T-type resonant PA cell using the thermo viscous module of COMSOL Multiphysics software showed that the PA signal reaches the maximum value when the resonant cavity size was 120 mm long and 4 mm in diameter,and the buffer cavity was 122 mm long and 7 mm in radius.An experimental system for the trace gases detection by PAS based on an optimized Ttype resonant PA cell was established,which mainly included a T-type resonant PA cell,a highsensitivity cantilever-based fiber-optic microphone,a distributed feedback(DFB)semiconductor laser,and a signal amplification and processing circuit.The first-order resonance frequency of the optimized T-type resonant PA cell was 686 Hz,which was consistent with the simulation analysis.The detection of different concentrations of methane gases showed that the PA signal has good linearity,and the sensitivity of the system for methane detection was 1.8 pm/ppm by linear fitting.Using this system for air measurement in the laboratory,the experimental results can clearly observe the second harmonic signal generated by methane in air,and the methane concentration in air can be obtained by calculation as 2.2ppb.The Allan-Werle variance was used to calculate the minimum detective limit(MDL)for methane,and the MDL was 9 ppb when the average time was 500 s.An compact gas sensor for remote detection based on the T-type resonant PA cell was proposed.By special design,the excitation light source and the fiber microphone were located on the same side of the PA cell,so the system can realize remote telemetry in a narrow space.The experimental results showed that the resonant frequency of the cantilever-based microphone was1680 Hz and that of the PA cell is 1342 Hz,which were consistent with the simulation analysis results.The experimental results showed that the second harmonic signals of methane with different concentrations were linear.By linear fitting,the detection sensitivity of the system for methane was 0.6 pm / ppm.The MDL of methane was calculated by AllanWerle variance.When the average time was 1000 s,the MDL was 15.9 ppb.