The design of optical path of lidar will directly affect the attenuation of echo signal in the transmission or reflection process, thus affecting the performance of lidar. In order to improve the compactness of the whole structure, reduce the weight and ensure the rigidity and strength, the topology parameter optimization design of the micro-pulse lidar structure is carried out during the design process.

The variable density topology optimization can be used to optimize and reassemble the structure of optical devices efficiently and conveniently, especially for the complex and discrete connections of optical devices, by defining and calculating the density function. This method has been widely used because it has the advantages of unconstrained design domain and simple design variables, and can better adapt to the situation of large computation and complex function design. In this paper, the pseudo-density value is used to represent the density of each element in the design domain which is the mass of material. Specifically, the density of material in the design domain is represented by the element in the density field β by the finite element meshing method, with a pseudo-density value representing the partition unit, a Y value for β element representing a corresponding material, and an N value for β element representing no material, as illustrated by the solid isotropic material with penalization (SIMP) interpolation schematic in Fig. 3. Structural topology optimization can deal with different types of design objectives and constraints. In continuum topology optimization, the shapes of outer boundary and inner boundary and the number of inner holes can be optimized at the same time according to predefined design goals. The SIMP model is used to optimize the optical support device by restricting the mass, volume and stress of the structure. By defining the design domain, design load, constraints and boundary conditions of the laser transmitting module and the optical receiving module, the structure function and sensitivity of the optical path are analyzed by using the SIMP model. The sequential quadratic programming (SQP) optimization algorithm is used to update the design variables of the optical path iteratively, so as to improve the stability of the whole structure of the optical path and achieve the goal of mass and compactness optimization. According to the optimization process, the SQP gradient optimization algorithm is adopted after the initial topology optimization is completed, and the optimized skeleton is obtained by continuously replacing and iteratively adjusting.

The lightweight structure model can be obtained by topology optimization of the optical device connection support structure. The initial mass of the structure is 68 kg, and the whole structure of the optical path including the primary mirror is taken as the topology parameter optimization region. By this topology design, the triangular lightweight rib units near the support hole and the through hole are retained, and some units farther away from the support hole are eliminated, thus the topology parameter optimization of the support structure of ultra-lightweight optical devices is realized. In the preliminarily optimized optical structure, when the gravity direction is parallel to the orientation of the optical axis, the deformation of the primary mirror will be larger, so the constraint is further optimized. After further optimization, the maximum offset generated on the image space orientation of the primary mirror surface node is not greater than 66 nm, and the overall mass of the primary mirror is constrained to 20%?35%, thus minimizing the flexibility of the primary mirror-centered structure. The reinforcement under the primary mirror of the telescope is optimized by the topology method, which can reduce the mass and deformation of the telescope under the premise of ensuring the stability of the structure of the telescope. The optimized double-layer light path structure support is shown in Fig. 7.

The pseudo-density topology parameter optimization method is used to optimize the support structure of lidar optical device. It can realize the effects of mass reduction, volume reduction, proper arrangement of stress conduction path and proper material distribution. The optimized optical device support structure skeleton is suitable for the frequent movement by vehicles or aircraft and other field monitoring activities. The design parameters are analyzed according to the iterative times of topology optimization by changing the constraint conditions such as stress and volume ratio. By adjusting the position and material of the optical path, the results of the optical path topology optimization are verified in the aspects of reducing the mass and flexibility. In the design and reconstruction, the structural characteristics should be adjusted, and the structural mass should be gradually reduced when the strength design requirements are met. The effectiveness of the topology design is verified by the scale optimization design and the comparison of the structural details before and after optimization, which further proves that the new optical path structure has the function of reducing weight and improving rigidity. Furthermore, the additional vibration caused by overload can be reduced, and the stability can be improved by reducing the weight of the optical device supporting structure.