Design And Implementation Of Multimode Fiber Mach-Zehnder Interferometer System For Doppler Lidar

The Doppler wind lidar has been widely used in the survey of wind speed and direction around the world.Optical fiber Mach-Zehnder interferometer,as a frequency measuring system for wind measurement lidar,has the advantages of lightness and high stability.However,when the single fiber of the ordinary fiber Mach-Zehnder interferometer is coupled with the multimode fiber of the telescope receiving system,the energy coupling loss is very serious,which causes a large attenuation of the echo signal and affects the detection height.In view of this,it is proposed to use a multimode fiber Mach-Zehnder interferometer as a frequency discrimination system to improve the coupling efficiency and achieve the improvement of energy and detection height.The thesis first analyzes a series of transmission characteristics such as the propagation mode,mode distribution,and energy ratio of Gaussian beams in multimode fibers,and discusses the characteristics of mode excitation after light rays with different divergence and inclination coupling into the fiber.In order to reduce the interference quality of the multimode fiber Mach-Zehnder interferometer,this thesis proposes to adjust the incident angle and the mode scrambling method to achieve the quasi-single-mode output of the multimode fiber,and determine the angle of incidence of the quasi-single-mode output coupled to the multimode fiber And the scrambling radius for quasi-single-mode transmission.After that,the experiment on the determined scrambled structure was conducted,and the measured output energy was about 30%of the total energy,which was higher than the efficiency of the all-fiber Mach-Zehnder interferometer system.In the study of the interferometer frequency discrimination system,firstly,through the discussion of the wind field detection theory of the all-fiber Mach-Zehnder interferometer and the effective refractive index of the multimode fiber,the arm length difference of the interferometer is determined.Then,based on the theory of inter-mode interference,the difference in transmittance function after different modes of interference was analyzed,the influence of the difference in inter-mode interference on the transmittance function of the entire interferometer was discussed,and the multimode fiber Mach-Zehnder interferometer was calculated the transmittance function of the dual output terminals found that the contrast of the interferometer decreased significantly,and the phenomenon of spreading of the transmittance appeared at the same time.According to the obtained transmittance function,the interferometer is used as an important parameter of the frequency discriminator,and the influence of temperature on the interferometer is analyzed.It is shown that when the system controls the temperature within a certain change range,the temperature can be quantitatively analyzed the wind speed error caused by the influence of the interferometer and the calibration of the interferometer arm length difference.Finally,the construction of the interferometer system is completed,and the actual interferometer transmittance function is obtained through experimental tests,and the frequency detector system parameters such as the wind field detection range,sensitivity,and signal-to-noise ratio of the system are determined,and the experiments prove that the scrambling structure significantly improves the interference effect.