Design And Experiment Of A Novel Lock-in Amplifier For TDLAS

With the rapid development of modern industrial technology,toxic and harmful gas leakage accidents occur from time to time,which have bad influence on the environment and human health.Therefore,it is necessary to carry out the concentration detection of related gases.At present,among many gas detection methods,spectral method has the advantages of non-contact,fast response and high accuracy,which is the development direction of current gas detection.Among them,tunable diode laser absorption spectroscopy(TDLAS)is based on the"fingerprint absorption" characteristics of gases in the infrared region.It uses wavelength tuning and harmonic detection methods,and has high detection sensitivity and limit.It has been applied to the detection of trace gases such as CO、SO2、NxO,etc.In TDLAS technology,lock-in amplifier can effectively reduce the adverse effect of signal noise,especially 1/f noise,which directly affects the performance of the whole detection system.Based on the analysis and comparison of existing research,a new type of lock-in amplifier is designed,and the signal generation,detection and harmonic extraction of TDLAS system based on this lock-in amplifier are studied.The main work is as follows:(1)The study analyzes the characteristic of gas absorption "fingerprint".Based on the basic theory of TDLAS,combined the technology of wavelength modulation and harmonic detection method,established the TDLAS gas concentration detection model,which can simulate and analysis the relation between the gas concentration and the second harmonic.It provides theoretical foundation for the design of phase-locked amplifier circuit to extract the second harmonic signals based on TDLAS detection method.(2)It is the key of TDLAS to extract the second harmonic signal by using the lock-in amplifier.In order to obtain the required in-phase and double frequency reference signal,the existing lock-in amplifier needs to use complex frequency doubling and phase-shifting circuits.A new type of lock-in amplifier is proposed,which takes advantage of the synchronization and programmability of FPGA,and outputs the scanning signal through high-speed D/A.When the sinusoidal signal is modulated,a double frequency signal which is strictly in phase with the modulated signal is generated.Combined with the subsequent detection circuit,the second harmonic absorption signal related to gas concentration can be effectively extracted.This method not only simplifies the circuit structure of lock-in amplifier,but also reduces the noise caused by analog circuit,which is expected to improve the performance of concentration detection.(3)A TDLAS gas detection system based on a new lock-in amplifier is designed and built.FPGA controls high-speed D/A chip AD9767 to generate scanning signal and modulation signal,and drives semiconductor laser after conditioning circuit and MLD203CHB pressure current conversion,the optical signal absorbed by the gas is converted into a phase-locked amplifier to extract the second harmonic.The temperature control circuit,lock-in amplifier circuit and photoelectric detection circuit are designed,and the module test is carried out,which lays the foundation for the subsequent gas detection.(4)Taking the natural gas whose main component is methane as the target gas,a preliminary concentration detection experiment was carried out in a gas chamber with an optical path of 0.6m.The concentration of natural gas was diluted by dichotomy,and the detected second harmonic peak also corresponded to it.The detection limit of CH4 was 10%,which preliminarily verified the performance of the designed lock-in amplifier.After optimizing the laser linewidth,detection optical path and detector performance,it is expected to greatly improve the detection performance.