Design Of Demodulation System For Weak Photoacoustic Signal Based On Photoacoustic Spectroscopy

Photoacoustic spectroscopy has the characteristics of large dynamic range,high sensitivity,good selectivity,fast response,strong anti-interference ability and easy maintenance.It is widely used in transformer oil for dissolved gas detection,atmospheric pollutant detection and petroleum smelting.An important part of photoacoustic spectroscopy is the demodulation and extraction of weak photoacoustic signals.Since there are all kinds of unavoidable noises inside the general equipment,the signal to be measured is extremely weak,and its surroundings are full of strong interference,and the weak signal to be measured carries important information,which makes the weak signal demodulation is particularly critical.In this thesis,a research and preparation of high-precision photoacoustic spectroscopy instrument is studied.A weak photoacoustic signal demodulation system based on photoacoustic spectroscopy is designed.Firstly,based on the principle of photoacoustic effect,the process of generating and detecting photoacoustic signals is introduced.Through the solution of the thermal power density analytical formula and the wave equation,the heat energy generated by the gas molecules after the photons are absorbed is described in detail,and the complete physical process of the sound waves is excited.The relationship between the voltage amplitude of the photoacoustic signal detected by the microphone in the non-resonant photoacoustic cell and the gas absorption coefficient,the incident light power,the photoacoustic cell constant,and the sensitivity of the microphone is derived.Photoacoustic signals are generated according to the principle of photoacoustic effect in photoacoustic spectroscopy,and photoacoustic signals are detected by high-sensitivity fiber-optic acoustic wave sensors.The focus is on how to phase-demodulate the principle of phase-demodulation based on FFT(Fourier Transform).Signal conversion electrical signal,in this process,the demodulation principle will be combined with the virtual instrument technology idea,the demodulation algorithm will be converted into Lab VIEW graphical programming language,and the noise source and characteristic law of the demodulation system will be further analyzed,using software algorithm.The hardware-related detection performs noise reduction processing to demodulate the gas concentration information.The advantage of this demodulation system is that it is resistant to external interference,high sensitivity,high precision,and insensitive to jitter of the source power.Finally,the photoacoustic spectroscopy detection system was tested by CO₂ gas.The sensitivity of the system at 100ppm CO₂ gas concentration was 0.68ppm,which indicated that the demodulation system can detect 50ppm to 100ppm CO₂ gas concentration more accurately.This demodulation system is feasible,and there is still a large space for further improvement of the experimental system to achieve better system sensitivity.