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.

Comment on “Reducing the refractive index by replacing a [AlPO4]° unit with [BPO4]° in fused silica”

“…Clearly shown there, and included in Fig. here, is that the refractive index along the BPO4-SiO2 join increases markedly with the addition of BPO4 into SiO2, in opposition to the assertion in Xiang, et al that "the refractive index of the SiO2-BPO4 is close to the pure silica, thus not altering the refractive index profile of the silica-based fiber core" [11]. Additionally, as noted, the refractive index of AlPO4 doped SiO2 decreases with increasing AlPO4 concentration.…”

“…Xiang, et al, compute the refractive index of AlPO4 doped SiO2 to be higher than SiO2 across the entirety of the visible spectral range (see Fig. 6 in Ref [11]). [15], calculated values based on additivity of the end-member indices, and theorized results [11].…”

“…The procedure is as follows: The base SiO2 refractive index was set to that of Ref. [15] (1.456, note that a wavelength was not provided) and its density was assumed to be 2200 kg/m 3 as in [11]. Then, both the refractive index and density for glassy BPO4 can become fit parameters in the model.…”

“…However, both are significantly lower in SiO2. Using the silica values from [11] and 2640 kg/m 3 , nBPO4 = 1.504 is deduced from [11]. Similarly, using a density of 2233 kg/m 3 for AlPO4 [10], nAlPO4 = 1.640 is deduced from [11].…”

“…Recently reported in Xiang, et al, is molecular modeling of silica glasses doped with AlPO4 and BPO4 [11]. Their theoretical results suggest that doping SiO2 with BPO4 results in a lower refractive index than when silica is doped with AlPO4.…”