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Analysis of telescope coupling efficiency for all-fiber spectroscopic Raman lidar

Telescope coupling efficiency is one of the important parameters for a lidar system because of the use of single-mode fiber Bragg grating, which seriously affects the signal-to-noise ratio of lidar. A newly developed telescope coupler of lidar based on single-mode fiber array is designed to enhance the coupling efficiency and signal power. By analyzing the coupling mathematic expression between the free space plane-wave and single-mode fiber, the relationship between the numerical aperture and the alignment tolerance, especially the relation between lateral offset and tilt angle error, is established, and then the system parameters are optimized to improve the coupling efficiency. For further improvement of the signal-to-noise ratio of a lidar system, seven single-mode fibers array structured as a telescope coupler is considered to receive the lidar returning signal, and its configuration feature is analyzed using the Gaussian beam propagating matrix. Simulation results show that this coupler may provide optimum coupling efficiency and signal power for all-fiber Raman lidar, and then the coupling efficiency of backscattering signal stimulated by near-field beam is given for the case of considering telescope focal length and central obscuration.