Dual-parameter measurements of curvature and strain using cladding-etched two-mode fiber Bragg grating

Abstract

Even though single-mode fibers are commonly used for optical fiber sensing, few-mode fibers, as potential alternatives, have also been investigated for multiple-parameter measurements, mainly because such fibers support only a few transverse modes with their own propagation constants. These modes respond in varied situations to different parameters and, therefore, can be retrieved and recognized. In this study, a two-mode fiber Bragg grating (TM-FBG) structure was fabricated, with its cladding layer surrounding the grating area partially etched away for enhanced sensitivity for curvature and strain variations. The experimental results show that within a curvature range of 0–19 m−1, the difference in reflection peaks between the linear polarized LP01 and LP11 modes decreases at a rate of 0.46 dB/m−1. The TM-FBG sensor is insensitive to fluctuations induced by the source and also independent of the grating’s central wavelength. Within the specific range of 0–100 mN for strain measurement, the reflections of the two modes have similar responses of 27 nm/N.