Objective  For a laser communication system ground principle prototype, the system uses coherent high-speed communication system, communication laser is polarized light, the optical system needs to be coated with dielectric film to ensure the stability of the polarization state. Affected by the thickness of the dielectric film and the coating process, the dielectric film has a large impact on the precision of the surface shape of the pendulum mirror, and it is very easy to cause the deterioration of the shape behind the coating. Therefore, the servo pendulum mirror is designed as a flat back structure to ensure the symmetry of the front and back sides of the pendulum mirror, and the front and back sides are coated simultaneously to reduce the influence of the dielectric film on the surface shape accuracy of the pendulum mirror. For the above reasons, the servo pendulum mirror is thicker, heavier and not easy to use the central support solution, so the peripheral flexible support structure is used.

  Methods  In order to ensure the mirror precision and support stiffness of the pendulum mirror, a peripheral support scheme is designed, and according to the flexible support design theory, a peripheral flexible support structure is proposed to reduce the structural stiffness and reduce the stress generated by the structural deformation by forming a hinge structure through notching the mechanical structure at the bonding of the pendulum mirror and the mirror base. Since the shape of the pendulum mirror, the location of the bonding point and the flexible support structure have many parameters and are coupled with each other, the main parameters of the pendulum mirror are first analyzed and optimized by the orthogonal experiment method to determine the shape and size of the pendulum mirror and the location of the bonding point, and then the flexible support structure of the pendulum mirror is optimized.

  Results and Discussions   It can be seen that the maximum surface shape error PV value of the pendulum mirror assembly is λ/16.34 and RMS value is λ/83.28 under the combined effect of standard earth gravity load and 5 ℃ uniform temperature rise and temperature drop (Tab.9). value is λ/5, and the RMS value is λ/42.87 to meet the surface accuracy requirement. The maximum RMS value of the surface shape error of the pendulum mirror assembly in the temperature range of (23±5) ℃ is λ/43.28 (Tab.10), which meets the requirement of the index, proving that the flexible support structure around the pendulum mirror assembly can ensure good thermal stability.

  Conclusions  In order to ensure the dynamic stiffness and face shape accuracy of the large-thickness flat-backed servo pendulum mirror system under the harsh environment, a peripheral flexible support structure scheme is proposed. The shape of the pendulum mirror, the position of the bonding point and the peripheral flexible support structure are designed parametrically, and the parameters are optimized according to the orthogonal experiment method to obtain a peripheral flexible support structure that meets the design requirements. After the finite element analysis, the fundamental frequency of the pendulum mirror assembly is 446.66 Hz (Fig.6), which meets the design index requirement of component mode frequency greater than 300 Hz; the PV value of the pendulum mirror surface shape is λ/5 and the RMS value is λ/42.87 in the temperature range of (23±5) ℃ (Tab.9), which is better than the index requirement of λ/40. The surface shape of the pendulum mirror was examined at different temperatures using ZYGO laser interferometer (Fig.10), and the test results showed that the RMS value of the surface shape of the pendulum mirror was better than the design value of λ/40. Therefore, the parametric design of the pendulum mirror shape, bonding point location and the surrounding flexible support structure make the structural stiffness and thermal stability of the pendulum mirror assembly meet the design requirements of the system.