Research On Theory And Methodology Of Multi-Longitudinal-Mode High-Spectral-Resolution Lidar

Multi-longitudinal-mode（MLM）high-spectral-resolution lidar（HSRL）is a new type of HSRL,which employs the free-running MLM pulsed laser as the excitation source rather than the seeded single-frequency pulsed laser in the single-frequency HSRL.In order to fulfill the requirements for accurate measurement of atmospheric aerosols and overcome the application bottleneck of single-frequency HSRL,the theory and method of MLM-HSRL are discussed in this thesis,and the critical techniques in the construction of MLM-HSRL and the feasibility of the fine detection of atmospheric aerosol are also analyzed.The main contents of the thesis are summarized as follows:（1）The key techniques of HSRL in the application of measuring atmospheric aerosol optical properties are developed.Based on the key factors in the actual construction of single-frequency HSRL,the frequency stabilization of the single-frequency pulsed laser and the design of ultranarrowband filter as a spectral discriminator are analyzed.The techniques of high-precision frequency stabilization and high-efficiency spectral filtering restrict the application of single frequency HSRL.In order to improve the application range of H SRL in the field of atmospheric and environmental monitoring,the principle of MLM-HSRL is introduced in this thesis,and the key techniques in the construction of MLM-HSRL are emphatically analyzed.The algorithm for retrieving aerosol optical parameters of MLM-HSRL is also deduced.（2）By calculating the spectral distribution of MLM laser and its corresponding elastic echo signals,the spectra and its characteristics are analyzed,and then the effective transmittance of the spectral discriminator of Mach-Zehnder interferometer（MZI）is studied and calculated,which plays an important role in retrieving aerosol properties.Because of the periodic characteristics of MLM laser and its Mie-Rayleigh scattering spectra,the MZI with periodic transmittance function is selected as the spectral discriminator to separate the Mie and Rayleigh scattering signals.The concept of the complex degree of coherence is introduced into the theoretical calculation of the effective transmittance of MZI.The calculation results show that the effective transmittance of MZI in both Mie channel and Rayleigh channel is determined by the modulus and phase of the complex degree of coherence,where the phase affects the phase factor of the effective transmittance,and the modulus affects the amplitude of the effective transmittance.In this thesis,the specified UV Nd:YAG pulsed laser with the linewidth of 1 cm^-1（30 GHz）,optical cavity length of 40 cm,pulse width of 10 ns,is selected as the transmitter.The calculation results show that mode interval of the pulsed laser is 375 MHz,linewidth of each longitudinal mode is 100 MHz,and mode number is 81.The theoretical analysis shows that when the optical path difference（OPD）of MZI is equal to an even multiple of the cavity length of the MLM laser,the spectral discriminator can effectively separate the aerosol Mie scattering and atmospheric molecular Rayleigh scattering signals excited by MLM laser,while the optimal design principle is that the OPD of MZI is twice of the cavity length of the MLM laser.The calculation results of the complex degree of coherence show that,when the free spectral range（FSR）of MZI equals to the mode interval of the MLM laser,the optimal modulus of the complex degree of coherence for the specified MLM laser has a fixed value of 0.776,while the optimal value of the modulus of the complex degree of coherence for their corresponding atmospheric echo spectra is a function of the backscatter ratio R_a,i.e.,the modulus of the complex degree of coherence is 0.699 when R_a is 10,and it is 0.761 when R_a is 50.Based on the calculation of complex degree of coherence,a new method for calculating the effective transmittance of MZI and a new algorithm for retrieving aerosol optical parameters of MLM-HSRL are proposed.（3）System simulation and performance analysis are carried out.Based on the relationship between the visibility（complex degree of coherence）of the interference fringes of MZI and its OPD,a method for measuring the longitudinal mode of MLM pulsed laser is proposed.The simulation results using Zemax show that the first maximum of the visibility of interference fringes appears when the OPD equals two times of the cavity length of the laser,which is consistent with the theoretical calculation,and makes a theoretical foundation for the determination of the basic OPD of MZI in the later research.Meanwhile,the standard atmospheric model and the real atmospheric model by an actual lidar measurement are utilized as the simulation model respectively,to verify the performance of aerosol measurement by the proposed MLM-HSRL.The simulation results show that the signal to noise ratio（SNR）for both Mie channel and Rayleigh channel are larger than 100 within the height of 10 km,which proves that the designed system can realize the accurate measurement of tropospheric aerosols.