Design of an off-axis reflective beam expander for multi-wavelength Raman lidar

Lidar has been proven to be a powerful tool for the atmospheric detection due to the advantage of high spatial and temporal resolution. And multi-wavelength lidar can obtain more atmospheric optical properties, which can be inverted to the particle microphysical properties. However, in the multi-wavelength lidar, classical transmission beam expanders of simple structure are difficult to achieve simultaneous beam expansion at multiple wavelengths because of the impact brought by chromatic aberration. To solve this problem, we design an off-axis reflective beam expander with only two spherical mirrors and applied it in the multi-wavelength Raman lidar system. The main parameters of the beam expander have been analyzed in detail and optimized by the Zemax software. And we also design a simple mechanical structure for adjustment of the beam expander which was demonstrated experimentally. According to the receiver field-of-view (FOV), the divergence angle of the emitted laser is less than 0.4 mrad. The experimental results show that the beam expander can be applied well in the multi-wavelength Raman lidar system. Keywords: Off-axis beam expander, Multi-wavelength Raman