Realization Of Self-tuning Technique Of Mach-Zehnder Interferometer For Multi-longitudinal-Mode High-spectral-resolution Lidar And Its Experiment

Multi-longitudinal-mode(MLM)high spectral resolution lidar(HSRL)is a new type of HSRL for fine edtection of aerosol optical properties.It directly selects MLM pulsed laser output from high-power lasers as excitation light source,and an interferometric optical filter with periodic transmittance is selected as the spectral discriminator.The center for Lidar Remote Sensing Research Center of Xi’an University of Technology has carried out research on the aerosol accurate detection using MLM-HSRL system and designed a Mach-Zchnder interferometer(MZI)with large optical path difference as the spectral discriminator of the MLMHSRL system.The optical path difference of MZI must meet two conditions:one is that it must be equal to twice the length of the laser resonator,which requires an accuracy of the magnitude order of mm;the other is that it must be equal to integral multiple of laser wavelength,which requires an order or nm accuracy.Only in this way,MZI can achieve the best match between its free spectral range and the longitudinal mode interval of the MLM laser,and then finely separate the aerosol Mie scattering signal and the atmospheric molecular Rayleigh scattering signal excited by the MLM laser.In the thesis,due to the influence of external environmental factors such as laboratory temperature,pressure,and vibration on the MLM-HSRL system,the optical path difference of MZI cannot be stabilized at an integer multiple of the laser wavelength,resulting in the mismatch between the free spectral range of MZI and the longitudinal mode interval of the MLM lasers,and makes output signals from the two channels of the MLM-HSRL system unstable.To overcome such a problem during the experiments,a self-tuning implementation method is proposed to adjust the optical path difference of MZI in real time to meet the requirement of the integral multiples of laser wavelength,realizing the dynamic matching between the free spectral range of MZI and the longitudinal mode interval of the MLM lasers,and providing a solution for fine separation of aerosol Mie scattering signal and molecular Rayleigh scattering signal excited by the MLM laser.In order to realize the self-tuning functionality of the optical path difference of MZI,the thesis discusses the change of effective transmittance of MZI caused by the mismatch due to the free spectral range shift of MZI,and simulates and analyzes how the zero offset,λ/32 offset,λ/16 offset and λ/4 offset of the free spectral range of MZI affect the effective transmittance of MZI in the case of 9 and 101 laser longitudinal modes.The simulation results show that the bigger offset of the free spectral range and the more longitudinal modes,the more decreases of the effective transmittance of MZI.Moreover,the ratio of the two-channel output signal of the MLMHSRL system decreases with the decreasing effective transmittance of MZI.In the thesis,the research results are used as the criterion for the optimal optical path difference of the spectral discriminator of the MLM-HSRL system.In the thesis,a self-tuning system of the optical path difference of MZI for the MLM-HSRL system is built,and a piezoelectric ceramic nano-displacement platform is added to the existing roof mirror load platform in the design of MZI,so that the optical path difference of MZI can be adjusted in nm magnitude order.At the same time,a software system for self-tuning of the optical path difference of MZI is designed by using the NI data acquisition system as well,which can simultaneously realize the data acquisition of the two channels outputs of the MLM-HSRL and the comprehensive monitoring function of the nm displacement using the piezoelectric ceramic nano-displacement platform.After completing the software and hardware design of the self-tuning system for the optical path difference of MZI,the thesis carried out the experimental observations of the MLM-HSRL system.Preliminary experiments show that the optimal adjustment value of the optical path difference of MZI within the range of 640 nm is 360 nm,which can satisfy the condition that the optical path difference is an integer multiple of the laser wavelength;the optimal change step of the piezoelectric ceramic nano-displacement platform is 20 nm.The experimental observation results show that before the self-tuning of the optical path difference of MZI,the effective transmittance of MZI varies from 0.608 to 0.746 and the ratio of the two-channel output signal of the MLM-HSRL system mainly varies irregularly between 1.0 and 3.0,after the self-tuning of the optical path difference of MZI,the effective transmittance of MZI varies from 0.714 to 0.746 and the ratio of the two-channel output signal of the MLM-HSRL system is mainly concentrated between 2.5 and 3.0.The research results show that the self-tuning system designed for the optical path difference of MZI in the MLM-HSRL system can greatly compensate the influence of the external environment on the optical path difference of MZI,and can realize the long-term observation of aerosol optical properties using the MLM HSRL system.