Objective Lidar is the main way to obtain three-dimensional geographic information within the military, and the data results obtained through this way are also widely used in resource exploration, land use, environmental monitoring and national key construction projects, providing extremely important original information for the national economy, social development and scientific research, and has achieved significant economic benefits, showing good application prospects. The lower temperature of the near space can reach –60 ℃, the optical antenna as the core component of LiDAR, its optical components have strict requirements for temperature changes. Temperature variations can lead to thermal deformation of the element, resulting in problems of defocusing and focal plane translation, which reduces the coupling efficiency. Improving the coupling efficiency can increase the detection rate, and the off-axis reflective optical system can be realized without obstruction, which can improve the energy utilization. However, unlike conventional refractive or coaxial reflection systems, each optical element in the off-axis reflection system does not have rotational symmetry, and its temperature deformation after temperature change is not uniform, so solving the effect of temperature change on the focus of the off-axis parabolic mirror is the key to improve the coupling efficiency of LiDAR.
Methods In this paper, the LiDAR is modeled, and the temperature field is simulated by the finite element analysis method for the model. The Zernike polynomials are used to fit the surface shape data obtained after the analysis to obtain the shape and position change of the reflector after temperature deformation, and the optical design software is used to obtain the optimal focal point position. Finally, the optical fiber position is adjusted by the focusing device to achieve the effect of focusing.
Results and Discussions The PV value of the mirror of the designed LiDAR is less than 10/λ. Its optimal focus position curve is obtained by optical design software, and a temperature-adaptive focusing structure is designed according to this curve, through which the RMS radius of the mirror compensated by the focusing structure decreases from 26.495 μm to 15.93 μm, the spot radius is reduced by 39.9%, and the coupling efficiency is improved from 15.8% to 91%. This focusing method does not need to keep track of the changes in the temperature of the reflector and reduces a certain amount of weight and cost compared to focusing with a motor. However, the method requires a certain temperature response time, can not adjust the focus to the best position at the first time after the temperature change. If the temperature changes frequently, the motor should be used to quickly adjust. Due to the working height of the blimp is more fixed, and its ambient temperature does not change much, the method is feasible.
Conclusions Aiming at the problem that the off-axis parabolic mirrors used in the near-space lidar will deform at low temperatures which leads to a decrease in the efficiency of the reflected light when it is coupled into the optical fiber, the self-focusing technique is investigated, and the auto-compensating lidar assembly is designed to offset the effect of temperature on the system. The deformation of the receiving system under thermal load is analyzed using the finite element method to obtain the discrete deformation data of the mirror surface, and the Zernike polynomials are used to fit the surface shape after the deformation of the mirror surface, and the simulation of the optical design software concludes that the change of the focal length of this LiDAR receiving system after optimization has a linear relationship with the temperature, and the compensation position is determined by the optical design software. A temperature adaptive adjustment mechanism is used to reduce the effect of out-of-focus amount caused by thermal deformation, which improves the coupling efficiency by more than 80%.
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