Noise Suppression Research In TDLAS System

Gas detection plays a very important role in the petrochemical industry,medicine,and atmospheric environmental monitoring.Among many gas detection technologies,tunable diode laser absorption spectroscopy(TDLAS)technology is widely used for its high sensitivity,good selectivity,fast response,and real-time detection.The performance improvement of gas detection is extremely important.One of the most important indicators affecting the performance improvement of TDLAS systems is the detection limit.The improvement in detection limits is mainly limited by system noise.Interference fringe noise is the most important noise of TDLAS system.The most important methods to suppress interference fringes are two frequency modulation and optical path modulation.At present,there is still no systematic theoretical research.In this paper,the interference fringes in TDLAS system using Wavelength Modulation Spectroscopy(WMS)technology are deeply studied.The current methods of several interference fringe suppression methods are theoretically analyzed and an improved method is proposed.In this dissertation,methane detection is taken as an example to carry out research through theoretical derivation,model simulation and experimental verification.The main work and innovations of the dissertation are as follows:1.Infrared gas absorption detection techniques including gas molecule spectral absorption theory and WMS technology were introduced.The detection system and temperature control are introduced.The detection system includes a photoelectric detection system and a mechanical optical system.The significance of temperature control on detection sensitivity improvement is given,and research on temperature control stability,response speed and spectral test are made.2.Based on the mathematical model of interference fringes in TDLAS system,the main factors affecting interference noise are analyzed and experimentally verified.It is pointed out that laser temperature noise and cavity length change have important influence on interference noise.3.An improved two frequency modulation method was proposed,which optimizes the modulation current(selecting any zero point of the Bessel function)while optimizing the jitter current(choosing the first zero of the Bessel function).The model simulation proves that when using the improved two frequency modulation method,the requirements of the model parameter control level can be relaxed,and the signal-to-noise ratio can be maximized.The experimental results of 50 min in the air background confirmed that the noise was reduced from 2.1 ppm to 180 ppb using the two frequency modulation technique,and the noise was further reduced to 110 ppb using the improved two frequency modulation method.Allan variance prediction,when the integration time is 11 sec,can obtain a minimum detection concentration of 17 ppb.4.The derivation of the optical path modulation theory is proposed for the first time.It is concluded that the optimal modulation amplitude of the optical path modulation is 0.383,0.879 and 1.377 times the wavelength,instead of 0.25 times the wavelength.It is proved by theoretical derivation that the optical path modulation method does not depend on the length of the etalon,and the optical path modulation accuracy mainly depends on the accuracy of the optical path modulation amplitude.An improved optical path modulation method was first proposed,which optimizes the modulation current(selecting any zero point of the Bessel function)while optimizing the optical path modulation amplitude(selecting the first zero point of the Bessel function).Experimental results in the air background for 50 min show that the improved optical path modulation method can reduce the system noise to 113 ppb.Allan variance predicts a minimum detection concentration of 18 ppb at an integration time of 9 sec.5.This dissertation points out that the fitting algorithm can suppress interference fringes.Combined with the two frequency modulation and fitting algorithm,or a combination of optical path modulation and fitting algorithms,the suppression effect of the fitting algorithm on noise can be further improved.The experimental results of 50 min in air background show that the proposed fitting algorithm can suppress interference fringes,and the system noise is reduced from 1.85 ppm to 103 ppb;both two frequency modulation and optical path modulation can improve the fitting method.The effect is reduced from 103 ppb to 44 ppb and 42 ppb.Allan variance predicts a minimum detection concentration of 11 ppb at an integration time of 50 sec.