Temperature-compensated fiber directional-bend sensor based on a sandwiched MMF–PMPCF structure

An optical fiber directional-bend sensor based on an inline Mach–Zehnder interferometer is proposed and demonstrated. The device consists of a piece of a multimode fiber (MMF) splicing with a polarization-maintaining photonic crystal fiber (PMPCF) and sandwiched by lead in/out single-mode fibers (SMFs). Owing to the larger diameter of the MMF, some high-order modes in fiber are efficiently coupled and transmitted through the PMPCF, and finally interfere with each other in the output SMFs. The experimental results show that a well-defined interference fringe envelope can be obtained in the transmitted spectrum and, when the fiber is bent, both the intensity and the fringe visibility of the interference pattern are changed with the bending curvature. Meanwhile, the bend sensitivities are varied with different bending directions, and the maximum sensitivity is achieved up to −8.33 dB/m⁻1 within the bend range from 0 to 1.7 m⁻¹. The proposed device also demonstrates a very low-intensity cross-talk of environment temperature.