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

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.