To satisfy the development requirements of high speed, miniaturization, and networking in space laser communication, the single-wavelength common aperture scheme of polarization splitting is primarily adopted in coherent laser communication systems. The optical system comprises an optical telescope, a splitting component, a communication-transmitting mirror group, a communication-receiving mirror group, a capture-tracking mirror group, and other components. In the light-splitting component, the polarizing beam splitter should have a high extinction ratio to isolate the transmitted and received signal light. If the polarizing beam splitter exhibits polarization crosstalk and wavefront distortion, then the polarization state and isolation of the signal light will be affected, thereby reducing the sensitivity of system detection. In recent years, the preparation of high-performance polarizing beam splitter films and the correction of the surface shape of optical components have been extensively investigated locally and abroad. However, reports regarding polarizing beam splitters with high extinction ratios and surface accuracies are few. In this study, polarizing beam splitters with high extinction ratios at 1540 nm and 1563 nm were investigated based on the requirements of laser communication systems.

In this study, based on an analysis of material properties and research on film design theory, Ta₂O₅ and SiO₂ were selected as high- and low-refractive-index materials, respectively. Polarizing light-splitting films and antireflective films were designed on both sides of the substrate to achieve extinction ratios greater than 5000∶1 at 1540 nm and 1563 nm, respectively. The Fabry?Perot structure was selected as the basic film system for the polarizing beam splitter film. After software optimization, the thickness of the film system was distributed uniformly, which facilitated the monitoring of the wavelength distribution and reduced the difficulty of subsequent film preparation. During the thin-film preparation, the relationship between the thicknesses of Ta₂O₅ and SiO₂ and the sensitivity of the monitoring wavelength was analyzed, and the light-control monitoring strategy was optimized. The LightRatioPeak light-value ratio method was used to precisely control the film thickness. Using the double-sided stress-balance method, the relationship between root-mean-square (RMS) change and SiO₂ thickness was established, and the reflection surface shape of the polarizing beam splitter was accurately corrected.

Result and Discussions To achieve the desired film thickness, a monitoring sheet was used to deposit two layers of films. A Ta₂O₅ film was deposited first, followed by a SiO₂ film. The Macleod software was used to calculate the sensitivity relationship between the reflection spectrum of Ta₂O₅ with different physical thicknesses from 120 nm to 190 nm and the monitoring wavelength (Fig. 8), which was reasonably selected based on the distribution of the film structure (Fig. 7). The same method was used to calculate the relationship between the reflection spectrum and monitoring wavelength sensitivity of SiO₂ with different physical thicknesses of 200?300 nm deposited on a 170-nm-thick Ta₂O₅ layer (Fig. 9). The monitoring wavelength was selected based on two principles: 1) the deposition of both films stops when the light intensity exceeds the extreme point to improve the accuracy of monitoring the film thickness, and 2) the number of different monitoring wavelengths to monitor the film thickness is reduced, which can weaken the effect of dispersion. The selection of the monitoring-wavelength scheme for the polarizing-splitting film is shown in Table 3, and the monitoring curve of the P-light antireflection film is shown in Fig. 11. The stress generated by the film causes severe deformation to the substrate; consequently, the reflection surface shape of the polarizing beam splitter increases to 0.0728λ ( Fig.12 ), which affects the light-transmission accuracy. To solve this problem, the experimental data for the substrate RMS change and deposited SiO₂ thickness were fitted and analyzed (Fig. 13). Based on the linear relationship between RMS change and SiO₂ thickness, the thickness of the deposited SiO₂ was accurately calculated. Based on the interferometer test, the reflection and transmission surface shapes of the final polarizing beam splitter measured 0.0084λ and 0.0106λ, respectively (Fig. 14). Based on a test performed using a spectrophotometer, the extinction ratios of the polarizing beam splitter at 1540 nm and 1563 nm are 7264∶1 and 5420∶1, respectively (Fig. 15). The results of environmental reliability test show no significant scratches, cracks, or other adverse effects on the surface of the polarized beam splitter sample under the intense illumination of a lamp (Fig. 16).

To satisfy the application requirements of laser communication systems, a polarizing beam splitter with a high extinction ratio was developed. Vacuum electron-beam evaporation ion-assisted deposition technology was used in conjunction with the LightRatioPeak light-value ratio method to monitor the film thickness. An optical-control monitoring strategy was designed and optimized using the Macleod software to accurately control the film thickness. Based on the principle of double-sided stress balance, the stress compensation was calculated and analyzed to prepare a high-profile polarizing beam splitter. The test results show that the extinction ratios at 1540 nm and 1563 nm are 7264∶1 and 5420∶1, respectively, and that the RMS of the transmission and reflection surfaces are 0.0106λ and 0.0084λ, respectively, which satisfy the requirements of laser communication systems.