Objective  The high-energy laser system is a directed energy system that realizes damage to a target by firing a high-energy laser beam at the target and generating a very high energy density on the surface of the target. Installed on a carrier aircraft, it can realize rapid detection and precise strike on the target by using the high mobility of the carrier aircraft, and plays an important role in the field of modern optoelectronic confrontation. The optical middle cabin is an important part of the high-energy laser system, and its main role is to realize the capture and tracking of the target. Due to the size limitations of the optical middle cabin for a typical small diameter cylindrical envelope, we can't use the traditional single-layer layout of the optical path, and put forward the "optical path stacking", cage-type layered structure (Fig.3), through the 45° mirrors layered transmission of the optical path, to solve the problem of the system volume is too large and meet the system requirements. However, the optical middle cabin contains the fine tracking and the main laser branch, and it is difficult to ensure the consistency of the optical axis when the temperature changes, it is necessary to simulate and analyze the optical middle cabin to explore the temperature adaptability of the optical middle cabin.

  Methods  The optical-mechanical-thermal integration simulation analysis is carried out for the optical middle cabin, and the finite element simulation analysis model is established (Fig.4) to obtain the mirror distortion (Fig.5). Because the mirror deflection in the mirror distortion will cause the shift of the optical axis, the method of chi-square coordinate transformation is used to calculate the mirror deflection amount, and the mirror deflection amount of a single mirror is obtained (Tab.4). Later, the error transfer matrix of the optical axis by mirror deflection is proposed according to the theory of prism mounting error transfer matrix, and the deflection amount of each mirror is brought in to obtain the azimuthal deviation of the optical axis of the system's fine tracking branch and the main laser branch to be 109.634 μrad, and the pitch deviation to be 132.952 μrad.

  Results and Discussions   The optical axis deviation of each optical axis in the optical middle cabin should be less than 150 μrad, system wave aberration should be less than 1/12λ@632.8 nm. After simulation analysis, the system optical axis deviation is obtained to meet the index requirements. In order to verify the accuracy of the simulation results, the optical middle cabin is tested by using a large diameter collimator to build a light path, and using an air conditioner to warm up the overall environment to 25 ℃, the position of the fine tracking spot and the main laser spot is detected, and the azimuthal deviation of both is calculated to be 104.019 μrad, and the pitch deviation is 125.009 μrad, which is 5.40% and 6.35%, respectively, and verify the reasonableness of the simulation results. After that, the optical middle cabin is placed between the interferometer and the high-precision reflection, and the wave aberration of the optical middle cabin system is detected by changing the environment temperature, and the worst wave aberration is 1/15λ, which meets the requirements of the index.

  Conclusions  In order to meet the lightweight and miniaturization requirements of airborne equipment, we adopt the idea of "optical path stacking" and propose a cage-type layered structure, and carry out structural design of the optical middle cabin to meet the system volume requirements. In order to investigate the temperature adaptability of the optical middle cabin, finite element simulation analysis of the optical middle cabin is carried out to obtain the mirror distortion, and then the displacement of the rigid body in the distortion is calculated to obtain the deflection of the mirror. According to the theory of prism mounting error transfer matrix, the error transfer matrix of the optical axis deflection by mirror deflection is proposed, and the optical axis consistency deviation between each branch is calculated, and the optical axis azimuthal deviation of the fine tracking branch and the main laser branch is obtained to be 109.634 μrad, and the pitch deviation is obtained to be 132.952 μrad. Experiments are carried out in the laboratory, the experimental results show that the azimuthal deviation of the fine tracking branch and the main laser optical axis is 104.019 μrad, and the pitch deviation is 125.009 μrad, which are 5.40% and 6.35% of the simulation results, respectively, and verify the reasonableness of the simulation results.