Low-stress Near-infrared dichroic Beam-splitting Coatings on Erbium Glass Substrates for Laser Ranging Systems

Erbium glass lasers offer advantages such as compact size， high efficiency， and excellent beam quality. Additionally， the 1 535 nm wavelength falls within the atmospheric transmission window， exhibits strong penetration capability， and lies in the eye-safe range， significantly reducing operational risks. These lasers are widely used in laser ranging systems and also hold important applications in fields such as lidar and military medicine. However， the weak stability， hydrophilicity， and high thermal expansion coefficient of erbium glass often lead to excessive stress in thin-film devices， resulting in poor adhesion—a critical technical bottleneck limiting the development of erbium glass lasers. Research on reducing residual stress and improving thin-film adhesion on erbium glass substrates remains scarce. To address this， this study employs the control variable method combined with magnetron sputtering technology to optimize three key process parameters—argon （Ar） flow rate， target （TG） power， and inductively coupled plasma （ICP） power—thereby reducing thin-film stress and enhancing the adhesion of erbium glass thin-film devices.First， the optical constants of the erbium glass substrate were determined using a Cary7000 spectrophotometer. Based on optical thin-film design theory， Ta2O5 and SiO2 were selected as coating materials for the near-infrared band. The initial film structure was designed as Sub |（0.5LH0.5L）S| Air， with the reflection bandwidth formula confirming a bandwidth of 355.2 nm at the central wavelength of 1 535 nm， meeting the specifications. The reflectivity formula indicates that， for a given ratio of high-to-low refractive indices， reflectivity increases with the number of film periods （S）. To achieve a reflectivity of 99.9%， at least 12 periods were required， resulting in the finalized structure： Sub|（0.5LH0.5L）12|Air. The film design was globally optimized using Macleod software， yielding the following configuration： Sub| 1.06H 0.76L 0.97H 0.86L 1.16H 1.11L 0.93H 0.92L H 1.08L H 0.93L 0.96H 1.06L 1.03H 0.94L 0.93H 1.02L 1.08H 1.02L 0.87H 0.96L 0.75H 0.49L |Air. Theoretical spectra showed an average reflectivity of 0.076% in the 800~1 000 nm band and 99.92% in the 1 500~1 600 nm band. Based on the robustness principle and considering deposition process constraints， manufacturability was improved by introducing film thickness error analysis. Subsequently， a beam-splitting film sample was prepared following standard deposition processes.After deposition， adhesion testing was performed using 3M tape pressed onto the film surface and rapidly peeled off vertically five times， revealing delamination. Comprehensive analysis indicated that film delamination was primarily caused by thin-film stress， which consists of intrinsic stress and thermal stress. To address this issue， the study focused on optimizing deposition parameters and temperature variations during coating. By analyzing the influence of deposition parameters on single-layer film stress using the Stoney formula and thermal stress equations， the following trends were observed：The stress of Ta2O5 and SiO2 single-layer films decreased with increasing Ar flow rate. The stress of Ta2O5 and SiO2 single-layer films initially decreased and then gradually increased with rising TG power. The stress of Ta2O5 and SiO2 single-layer films increased with higher ICP power. A low-stress film preparation process was identified， and the curvature radii of the samples were measured before and after deposition using an ET-4000A step profiler. The calculated multilayer film stress was -105 MPa， and subsequent adhesion tests showed no delamination.The final beam-splitting film prepared on the erbium glass substrate exhibited an average reflectivity of 0.16% in the 800~1 000 nm band and 99.87% in the 15 00~1 600 nm band， meeting the specifications. The slightly higher reflectivity in the 800~1 000 nm band compared to the design spectrum was attributed to surface roughness-induced interfacial scattering in the multilayer film. This was further analyzed using a Zygo laser interferometer to measure surface roughness. After undergoing environmental tests in accordance with specifications， the film showed no defects such as cracking， delamination， or bubbles. As laser energy increases， the thermo-mechanical coupling interaction between the laser and the film raises the risk of film failure due to residual stress. Further reducing film stress remains a key focus for future research.