Simultaneous Measurement of Knuckle Flexion and Temperature With a Few-Mode FBG

This article presents a knuckle flexion and temperature sensing structure based on a few-mode fiber Bragg grating (FM-FBG) and polydimethylsiloxane film. Fiber Bragg grating inscribed on a dual-mode fiber fusion bonded to a single-mode fiber excites the fundamental and first-order modes. The coupling mode is LP01-11 between the fundamental mode and the first-order mode. According to the phase matching condition, the Bragg grating selectively reflects a specific wavelength of light that matches its periodic structure. The reflected wavelengths of the three modes hardly drifted during the bending measurements, and the intensities of the LP01 and LP11 modes remained almost constant, while the intensities of the LP01-11 modes changed significantly. The LP01-11 mode shows a high intensity response to fiber bending with almost no change in wavelength. The experimental results show that the bending sensitivity is -0.038 dB/°, and the intensity of the LP01-11 mode reflectance spectrum varies linearly; the temperature sensitivity is 10.97 pm/°C, and the central wavelength of the reflectance spectrum of the LP01-11 mode is red-shifted.