Design of automatic coupling system based on all-fiber water vapor Raman lidar

The coupling efficiency between the backscattering light field received by the telescope and the single-mode fiber is one of the important parameters affecting the performance of the all-fiber water vapor Raman lidar system. In the process of using Raman lidar to detect water vapor, the telescope receives the backscattered light signal of the system, which is focused and coupled into the single-mode fiber through the microscopic objective lens. The mode field diameter of the selected single-mode fiber is only 4μm. The weak offset will lead to a decrease in coupling efficiency. When the no-load or space-borne lidar detects the atmosphere, the fluctuation of the airflow may cause the platform vibration to produce a position offset, which will reduce the efficiency of coupling the water vapor Raman scattering echo signal into the single-mode fiber. Based on the above problems, this paper designs a single-mode fiber automatic coupling system. In the closed-loop mode, the controller uses the piezoelectric effect to control the three-axis motion platform to automatically track the maximum brightness in the shooting spot, and realizes the coupling alignment between the single-mode fiber and the nitrogen Raman scattering echo (386.7nm) and the water vapor Raman scattering echo (407.8nm). The coupling efficiency is 49.7%, and the automatic adjustment accuracy is sub-micron. The influence of axial offset and lateral offset on the coupling efficiency is analyzed at the incident light wavelength of 407 nm. This provides a new solution for the continuous, stable and efficient acquisition of water vapor signals by all-fiber detection water vapor Raman lidar system.