The performance of a beam expander, as a key component for improving laser divergence angle in high-energy laser emission systems, directly affects the collimation and beam quality of laser emitted by the system. However, when all-weather operation is required, changes in the ambient temperature affect the wavefront stability of the internal beam expander in high-energy laser emission systems. The conventional design concept of passive thermal compensation is to compensate for the thermal displacement of optical surfaces via the reverse expansion deformation of two different material support structures in a thermal environment. However, the structural form of supporting structures connected by different materials can generate thermal stress in the thermal environment, thus resulting in the uncontrollable deformation of the structure and increasing the design risk of the system. In this study, a new design method for injection thermal compensation is proposed, which compensates for thermal deformation via the flexible force of the silicone-rubber layer. This method offers good thermal compensation and avoids the irregular deformation and stress generation of the support structure during thermal compensation. We hope that our basic research can provide new ideas and data support for the thermal compensation design of coaxial reflective systems.

In this study, a laser-beam expander was regarded as the research object. First, the effect of temperature change on the wavefront root mean square (RMS) and Zernike fringe coefficients of the beam expander in the temperature range of 0 ℃ to 40 ℃ was investigated via integrated optomechanical analysis. Subsequently, a temperature test platform was established, and the accuracy of the integrated simulation results was discussed using wavefront test data from 0 ℃ to 40 ℃. Next, to accommodate the significant changes in the power of the system during temperature rise and fall, a method for designing the thermal compensation of the injection rubber was proposed. The relationship between the thickness and diameter of the silicone rubber layer and the thermal-compensation effect was investigated via integrated optomechanical analysis, and the suitable thickness of the silicone-rubber layer for the thermal compensation of laser-beam expanders was determined. Finally, experimental testing and simulation analysis were performed, which verified that the laser-beam expander designed with thermal compensation presents favorable thermal-environment adaptability and satisfies the usage requirements.

The simulation analysis results are consistent with the experimental test results (Fig. 11), thus indicating that the first-order astigmatism and coma of the system vary marginally within 0 ℃ to 40 ℃, and that the power change caused by the change in the distance between the primary and secondary mirrors contributes primarily to the wavefront increase of the system (Fig. 7 and Fig. 8). The analysis on the thermal compensation of the beam expander shows that both the thickness and diameter of the rubber layer affect thermal compensation, and that the effect of the rubber-layer thickness on thermal compensation is more significant. The thickness of the silicone-rubber layer ranges from 0.15 mm to 0.25 mm. As the thickness increases, the system wavefront RMS and power decrease. When the thickness of the adhesive layer exceeds 0.25 mm, overcompensation occurs, and the power changes from positive to negative (or from negative to positive), whereas the RMS of the system increases with the rubber-layer thickness (Fig. 12). By considering thermal compensation in the design of the laser-beam expander, the wavefront RMS and beam quality equivalent β factors at 0 ℃, 20 ℃, and 40 ℃ are 0.373λ@633 nm, 0.0319λ@633 nm, 0.397λ@633 nm and 1.385, 1.331, and 1.402, respectively, thus demonstrating the good thermal stability of the beam expander (Fig. 14).

In the present study, the wavefront variation of laser-beam expanders in the temperature range of 0 ℃ to 40 ℃ is revealed. Because the power of beam expanders is sensitive to temperature change, a new passive thermal-compensation design method suitable for coaxial reflective optical systems is proposed. The compensation design involves injecting silicone rubber on the back of optical components to compensate for the change in optical spacing with the expansion or contraction flexible force of the silicone-rubber layer in the thermal environment. It can effectively reduce the occurrence of uncontrollable thermal stress and deformation in the thermal environment caused by the connection of different material support structures in conventional, passive, mechanical, non-thermal designs. After considering thermal compensation, the wavefront variation of the laser-beam expander within the temperature range of 0 ℃ to 40 ℃ remain less than 0.0078λ@633 nm. Additionally, β does not exceed 0.071, which signifies that the usage requirements are satisfied.