Design Of Mach-zehnder Interferometer For Multi-Longitudinal-mode High-spectral-resolution Lidar

The multi-longitudinal-mode(MLM)high-spectral-resolution-lidar(HSRL)is a new type of HSRL system,which abandons the requirement that the HSRL must use the single-longitudinal-mode(SLM)pulse laser.The MLM-HSRL uses the MLM pulsed laser emitted by the high power laser as the excitation light source,and uses the spectral discriminator with periodic transmittance function to separate the molecular Rayleigh scattering signals and aerosol Mie scattering signals excited by the MLM pulsed laser to realize the fine detection of aerosol optical parameters.The spectral discriminator is the core of the HSRL system.The spectral change of the excitation light source of the HSRL will inevitably change the spectra of the atmospheric elastic scattering signals,resulting in the different requirement of spectral discriminators between the MLM-HSRL and the SLM-HSRL.Therefore,it is necessary to build a set of methods of analysis,design and realization for the construction of the spectral discriminator of the MLM-HSRL.In this paper,the tunable field-compensation Mach Zehnder interferometer(MZI)is selected as the spectral discriminator of the MLM-HSRL.Based on the analysis of the influence factors of the performance of the MZI,the design and optimization methods are proposed,and the theoretical simulation and experimental verification are carried out.Different from the spectral discriminator of the SLM-HSRL with high spectral separation ratio,the effective transmittance is the best performance evaluation parameter of the spectral discriminator of the MLM-HSRL.The effective transmittance of the MZI is detemmined by its transmittance curve and atmospheric elastic scattering signals,the influence factors of the performance of the MZI are analyzed in this paper from three aspects of the atmospheric elastic scattering signals,configuration parameters of the MZI and experimental environment,which are as follows:(1)the optical path difference of the MZI should be matching the double length of the laser optical cavity and the integral times of laser wavelength;(2)additional optical path difference must be suppressed;(3)the effective transmittance need to be corrected according to the specific parameters of the optical elements used in the MZI;(4)the dynamic matching between the transmittance curve of the MZI and the spectra of the atmospheric elastic scattering signals excited by the MLM laser should be realized.According to the influence factors,the following solutions are executed:(1)design a combined displacement system to realize the accuracy adjustment of the optical path difference of the MZI;(2)the aspherical lens is selected to collimate the incident beam;(3)the additional optical path difference of the MZI caused by the beam divergence angle is restrained by adding a field compensation glass into the long arm of the MZI;(4)the dynamic matching between the transmittance curve of the MZI and the spectra of atmospheric elastic scattering signals excited by the MLM laser is realized by optical path difference scanning.The spectral separation performance of the designed MZI is verified by the simulation and the experiments.The results show that:(1)the beam divergence angle is close to 0° after collimated by aspheric lens for the point light source with a divergence angle of 12.7°;(2)the additional optical path difference produced by the beam divergence angle less than 30mrad can be effectively suppressed when the length of compensation glass is ideal.(3)Zemax simulation for the MLM laser is carried out,and the results show that the interference contrast decreases to 0 slowly with the increase of optical path difference after the field compensation and then obtain the second maximum value of 0.561 when the optical path difference is twice of laser optical cavity length;(4)the parameters of the optical elements are measured and the effective transmittance is corrected;(5),the interference fringe contrast of the MZI is measured.The results show that the interference fringe contrast under the irradiation of the SLM laser changes continuously with the optical path difference,while the interference fringe contrast under the irradiation of the MLM laser obtains the maximum value 0.453 at the optical path difference of 1010mm,which determines the length of laser optical cavity should be 505mm.Finally,the simulation inversion of aerosol backscattering coefficient is carried out.