Research On The Mechanism,Characteristics And Suppression Of The Optical Fringes In TLAS System

Optical fringes constitute one of the major obstacles in the gas detection based on tunable laser absorption spectroscopy(TLAS).In order to suppress optical fringes and improve the TLAS system,the theoretical analysis of optical fringes is performed,and the mathematical model of optical fringes in a wavelength modulation spectroscopy(WMS)system are developed.Based on the theoretical analysis,three approaches for optical fringes suppression are proposed,and relevant experiment results are showed.First,the mathematical model and mechanism of optical fringes are performed,and the common sources of optical fringes in the TLAS systems are summarized.Convolving with the basic theory of TLAS,the performance of optical fringes in the TLAS is analyzed.Moreover,the characteristics of optical fringes are analyzed,and three approaches for optical fringes suppression are proposed.The mathematical model of optical fringes in the WMS system is performed,and the relation between optical fringes and modulation index is analyzed.A convenient method for modulation index optimization was presented,using the signal-fringe ratio(SFR)as a criterion.The principle about optical fringes reflection and superposition.Based on theoretical analysis,a method for optical fringes compensation are proposed.The characteristics and mechanism of Empirical Mode Decomposition are summarized.The usage of Empirical Mode Decomposition(EMD)in the optical fringes separation is analyzed and preformed.Finally,applications of proposed approaches are show in the real-world TLAS systems.An Oxygen sensor is developed and the method for modulation index optimization is applied.By comparing the sensor performances with and without using optimized modulation index,we showed that the optical fringes are reduced by using the optimized modulation index.More specifically,the long-term fluctuation of the sensor measurement is dramatically reduced by a factor of 8,a detection limit of as low as 120 ppm(with effective optical path length 32 cm),characterized by the Allan variance,was derived.The method for optical fringes compensation is applied in the suppressing optical fringes produced by white cell.The optical fringes are reduced in the compensated signal,and the relevance of liner fitting between the measurement signal and standard signal is improved from 0.823 to 0.993.EMD is used to extract the narrow optical fringes from the signal of capnograhpy sensor.The measurement results show that optical fringes,which is derived by EMD,are similar with the real optical fringes.The relevance of liner fitting between two lines is 0.89.The main innovation of this work is that mechanism and characteristic of optical fringes in TLAS systems are analyzed,and three approaches for optical fringes suppression are proposed.Furthermore,experiment results validate the feasibility of optical fringes reeducation by proposed approaches.