Birefringence Performance of Side-hole Optical Fiber by Pressure Based on Polarized Light Interference

Amid the rapid advancements in optical fiber communications and sensing technologies, polarization-maintaining fibers have increasingly been utilized in fiber sensing systems. As a novel type of polarization-maintaining fiber, Side-Hole Fiber (SHF), with its unique microstructure and superior performance, holds broad application prospects in fields such as communication, sensing, and medical technology. The sensors made from SHF are highly sensitive, capable of monitoring multiple parameters simultaneously, and are easily integrated into the materials being measured. They play an extremely important role in the field of structural health monitoring, thereby attracting widespread attention. In recent years, many researchers have studied and analyzed the structure and birefringence properties of SHF. However, systematic analysis and research on the impact of pressure on the birefringence performance of SHF have not been reported. In this paper, the impact of radial pressure on the birefringence characteristics of SHF was analyzed systematically based on coupled mode theory and the photoelastic effect. To facilitate the experimental component of the study, a mechanical loading apparatus was engineered to apply varying levels of radial pressure on the SHF using different weights. Furthermore, we established an experimental system grounded in the principle of polarization interference, designed specifically to measure the birefringence of SHF under different pressure conditions. The experimental setup comprised a broadband light source, a polarizer, the SHF under test, and a spectrometer. Light from the broadband source, after passing through the polarizer, was transmitted through the SHF. The interference spectrum was subsequently captured by the spectrometer. Birefringence was quantified by analyzing the mean wavelength of troughs and the average interval between adjacent peaks within the interference spectrum. Experimental results indicated that while keeping the pressure magnitude constant, the birefringence values varied according to a cosine function with respect to the direction of application, achieving maximum and minimum values at even and odd multiples of pi/2, respectively. When the direction of application was held constant, the birefringence values exhibited a linear relationship with the magnitude of pressure. Specifically, for angles theta within the range (k pi-pi/4,k pi + pi/4)(where k is an integer), birefringence values increased linearly with pressure. Conversely, for theta in the range (k pi + pi/4,k pi + 3 pi/4), birefringence values decreased linearly with pressure. At theta = k pi + pi/4 the birefringence values remained essentially unchanged. The correlation coefficient r between the experimental and simulation results was 0.992 2, indicating a high degree of consistency within the permissible error range.