Objective With the widespread development of laser technology, lasers are now extensively used in various fields such as industry, medicine, and the military. Optical antennas play a crucial role in high-energy laser transmission, and their thermal deformation has a significantly greater impact on the overall wavefront aberration than other components in the optical path. Compared to Cassegrain antennas, off-axis optical antennas have a higher level of analytical difficulty due to their parabolic mirrors rather than flat mirrors. Current research on the thermal stability of off-axis optical antennas mainly focuses on the steady-state thermal analysis of temperature variations in space. The issues of surface shape changes of the primary and secondary mirrors under transient thermal loads of high-energy lasers and the resultant changes in the distance between these mirrors also need to be addressed. Presently, there are few integrated analytical models that study the increase in wavefront aberration in off-axis optical systems caused by photothermal effects from high-energy lasers. This study focuses on off-axis two-reflector optical antennas, analyzing the changes in wavefront aberration in off-axis optical systems due to photothermal effects induced by high-energy lasers.
Methods An off-axis two-reflector optical system finite element analysis model was established under the environment of high-energy laser operation. Transient thermal analysis and static analysis under laser loading were completed, and the displacement cloud map varying with laser loading time was obtained. The rigid body displacement of the off-axis primary and secondary mirrors was solved using homogeneous coordinate transformation. The wavefront fitting of the mirrors was completed using Zernike polynomial fitting. The change along the Z-axis in the rigid body displacement of the mirrors and the Zernike polynomial coefficients were imported into ZEMAX. The thermal analysis results of the off-axis two-reflector optical system were obtained under laser loading with an average power of 1000 W, a repetition frequency of 0.2 Hz, and an incident light diameter of 30 mm for 300 seconds.
Results and Discussions Based on the node data of the off-axis two-mirror surface deformation analyzed by the software, the rigid body displacement values of the primary and secondary mirrors were obtained using the least squares method (Fig.6). The wavefront aberration of this off-axis two-reflector optical system is mainly caused by the surface shape errors of the components. By fitting the deformation nodes with Zernike polynomials, the first nine Zernike polynomial coefficients were obtained (Fig.7). The values of the Zernike coefficients for the primary mirror were smaller than those for the secondary mirror, indicating that the surface shape of the primary mirror is better than that of the secondary mirror under laser irradiation. The absolute values of the system's coefficients were positively correlated with the main laser loading time. After 300 seconds of 1000 W laser irradiation, the system's wavefront aberration RMS was 0.0646λ (Fig.8). Using the same method, simulation analysis showed that after 30 minutes of 10 W laser irradiation, the system's wavefront aberration RMS was 0.0583λ (Fig.9). Experimental verification of the wavefront aberration changes in the off-axis two-reflector antenna under these conditions showed that the system's wavefront aberration RMS was 0.063λ after 30 minutes of 10 W laser irradiation (Fig.10). The error between the simulation results and the experimental results was 7.46% (Tab.4), indicating that the simulation results obtained by this analysis method have reference value.
Conclusions Focusing on the off-axis two-reflector optical antenna, an analysis model was established for the off-axis two-reflector optical antenna under the transient thermal load of a 1 000 W high-power laser. The changes in the rigid body displacement and surface shape of the primary and secondary mirrors with increasing laser loading time were analyzed, quantifying the impact of high-power laser thermal effects on the image quality degradation of the off-axis two-reflector optical antenna. The analysis results showed that after 300 seconds of1000 W laser transmission, the RMS wavefront aberration of the off-axis optical antenna deteriorated to 0.0646λ, and the PV value increased by 0.2664λ. Using microcrystalline glass as the material, the off-axis two-reflector antenna maintained good image quality under 1 kW laser irradiation. Using the same analysis method, the simulation results of the off-axis two-reflector optical antenna under 10 W loading showed an RMS wavefront aberration deterioration of 0.058λ. Experimental verification with 10 W laser irradiation showed an RMS wavefront aberration of 0.063λ, with a simulation deviation of less than 7.93%. This analysis model and its results provide valuable references for assessing the direct relationship between high-energy laser transmission time and image quality degradation in off-axis two-reflector optical antennas.
DownLoad: