Research On Gas Detection Technology Based On Absorption Spectroscopy

Detecting the type and concentration of gas has significant practical importance.Gas detection technology based on absorption spectroscopy has gained wide attention in the field of gas detection due to its high accuracy,wide detection range,and remote detection capabilities.Tunable Diode Laser Absorption Spectroscopy(TDLAS)technology has the advantages of high sensitivity,non-contact measurement,and strong selectivity,yet has become the mainstream gas measurement method.In this technology,the lock-in amplifier is used as a device for demodulating gas concentration information,which directly affects the accuracy of the measurement output.Traditional analog lock-in amplifiers have strict requirements for equipment and low flexibility in output results due to serious effects from phase differences.While commercial lock-in amplifiers also have the characteristics of large size and high price,which was not conducive to the integration of TDLAS technology.To address these issues,this paper designs a quadrature lock-in amplifier based on FPGA,simulations and system tests were also performed in TDLAS measurement system.The main works are as follows:1.This paper overview the spectroscopy theory,introduce the theory and characteristics of molecular absorption spectroscopy,and explains the principles of absorption spectroscopy formation;Analysis of the influence of gas concentration on light intensity through Lambert-Beer law;Based on spectroscopy theory,introduces the wavelength modulation of TDLAS measurement technology to obtain the relationship between gas concentration and harmonic signal amplitude,providing a basis for lock-in amplifier demodulation of gas concentration signals.2.The system designs a TDLAS gas measurement scheme based on FPGA with quadrature lock-in amplifier as the core;The DDS technique is used to generate two quadrature reference signals,and the final result is obtained by offsetting the effect of phase difference through arithmetic;The feasibility of the scheme is verified using simulation software;The flexibility of the measurement system is improved.3.The performance of different types of light sources was analyzed,and ultimately a Distributed Feedback(DFB)laser was adopted as the system light source,so that the emitted laser could be absorbed by the test gas and the absorbed light signal could be detected;The DFB laser driver circuit was designed according to its tunable characteristics,a 10 k Hz high-frequency sine signal was modulated with a 10 Hz low-frequency sawtooth signal as the laser’s driving signal,shifting the measurement range to the high-frequency region and suppressed the influence of low-frequency noise on the measurement results.4.Finally,targeting the detection of harmful gas methane,the paper systematically tests the gas concentration measurement by absorption spectroscopy using TDLAS technology;The stability of the input laser current was studied under the temperature control module.By injecting methane gas of different concentrations,the response of the orthogonal lock-in amplifier demodulation module to the absorption spectroscopy is measured,verifying the feasibility of the system.