Effects of selectively thickening core wall on birefringence and loss in polarization-maintaining hollow core photonic bandgap fiber

Selectively thickening core wall is an effective approach to improve the birefringence of hollow core photonic bandgap fiber (HC-PBGF). However, the variation of core wall thickness has a significant influence on the transmission loss and transmission bandwidth. In this paper, the effects of selectively thickening core wall on birefringence and loss performance of the HC-PBGF are systematically investigated. Numerical analysis results indicate that when the core wall thickness in the y-direction is 2.4 to 3.3 times as that in the x-direction, the birefringence of a 19-cell HC-PBGF can be improved to 10^− 4, meanwhile the transmission loss can be controlled below 10 dB/km. The high birefringence (≥ 10^− 4), low transmission loss (< 10 dB/km) performance can be maintained in a wide waveband from 1460 to 1610 nm, even under a tight bending radius of 6 mm. The applicability of selectively thickening core wall on improving birefringence is also confirmed in the 7-cell HC-PBGF and 37-cell HC-PBGF, where the birefringence of 7-cell HC-PBGF and 37-cell HC-PBGF can be enhanced to 5.5 × 10^− 4 and 5.0 × 10^− 5, respectively. Moreover, to further reduce the transmission loss, two modified structures (adding nodes or anti-resonant layer) are discussed. The numerical analysis results demonstrate that the structure by adding nodes on the thickened core wall is more advantageous in terms of reducing transmission loss and expanding transmission bandwidth.