Reducing the refractive index by replacing an [AlPO4]° unit with [BPO4]° in fused silica

Abstract

The radiation resistance of rare-earth doped optical fibers is critical to applications in space-based environments such as laser radars, optical communications, and laser altimeters. Usually, doping various elements, such as aluminum (Al), phosphorus (P), and boron (B), is necessary to fine-tune the structural, electronic, and optical properties, but often results in degraded radiation resistance. Thus, achieving both excellent optical and radiation properties remain a challenge. Here, we theoretically investigate and compare the electronic, structural, and optical properties of [BPO4]° and [AlPO4]° units in silica glass. We prove that both [BPO4]° and [AlPO4]° units are stable in the SiO2 matrix. As the radiation resistance of [SiO4/2]° is excellent, inferring from the material's structure, the SiO2-BPO4 and SiO2-AlPO4 should have good radiation resistance. From the calculation, the SiO2-BPO4 is structurally and electronically similar to the SiO2-AlPO4. Importantly, the refractive index of SiO2-BPO4 is lower than SiO2-AlPO4, achieving refractive index tuning while maintaining its radiation resistance. Our results provide some guidance for the design of BPO4-based radiation-resistant active fibers.