Design and fabrication of the high-precision beam splitter with stress compensation analysis for infrared band-pass filters

Abstract

This paper uses thin film interference principles to introduce a stress-compensated beam splitter design for infrared band-pass filters. The beam splitter provides high transmittance ($>$96%) at 1540.53 nm while maintaining high reflectance ($>$99.5%) at 1563 nm. The fabrication process utilizes an electron beam ion-assisted deposition technology. Tantalum pentoxide (Ta₂O₅) and silicon dioxide (SiO₂) are chosen for their respective high and low refractive indices, forming the basis of the design. However, a significant challenge in fabrication lies in managing the root mean square (RMS) value of wavefront error induced by high packing density during electron beam ion-assisted deposition process. To address this, in this work, we developed a stress model to analyze the overall stress within the multilayer Ta₂O₅/SiO₂ structure because the fluctuation in stress exerted on the substrate affects the RMS value of wavefront error. From the stress model analysis, we employ backside coating which emerges as a viable solution to mitigate stress, ensuring structural integrity. The backside coating, chosen through a stress model, decreases the compressive stress from −46.94 MPa to −1.88 MPa and also reduces the RMS wavefront error from 63.60 nm to 8.26 nm. After stress compensation, the beam splitter’s transmission properties are evaluated using a spectrophotometer. The experimental results validate the performance of the fabricated beam splitter, meeting the specifications outlined in the study.