Temperature-independent multi-parameter sensor based on polarization maintaining fiber Bragg grating

Dynamic multi-parameter detection is of great significance in predicting fatigue damage to structures such as tunnels, bridges, and pipelines. Developing a high-sensitivity, environmentally friendly, low-cost, and easy-to-operate multi-parameter dynamic detection technology has always been the goal of the industry. The polarization-maintaining fiber Bragg grating (PM-FBG) has a special grating structure composed of fiber Bragg grating (FBG) directly written into high birefringence and polarization-maintaining fiber, and it supports two distinct polarization eigenmodes with two effective refractive indices. The PM-FBG couples the light beams polarized along the two principal axes corresponding to slow axis and fast axis at two different Bragg wavelengths. The two peaks of PM-FBG have different responses to external changes, which may be used to solve the cross-sensitivity problem of FBG sensor and realize the simultaneous multi-parameter measurement of the temperature, longitudinal strain, transverse strain, or twist. In order to solve the problems of complex structure and principle and high production cost of FBG-based multi-parameter sensors, a novel multi-parameter fiber-optic sensor with high sensitivity and temperature independence is designed based on PM-FBG in this work. The PM-FBG sensor proposed can simultaneously measure the changes of displacement and twist in two vertical directions at a certain point and has the function of temperature self-compensation. The external structure of the sensor is fabricated by using three-dimensional printing technology through the fused deposition method and the raw material for creating different components through using polylactic acid. Experimental results show that the fast axis and slow axis of the sensor have different temperature responses, with linear sensitivities of 11.4 pm/℃ and 10.6 pm/℃, respectively, and the temperature compensation coefficient and average torsional sensitivity of the PM-FBG sensor are 0.8 pm/℃ and 0.20 dB/(°), respectively. The fast axis and slow axis of the PM-FBG sensor have the same response to displacement, with a sensitivity of 31.5 pm/mm and an adjustable range of 0–20 mm. The sensitivity to displacement, torsion, and temperature sensitivities of the sensor are all superior over those of commercial FBG sensors. By changing the temperature field around the sensor, its displacement- and torsion-sensing performances are not affected, thereby realizing the temperature self-compensation. Consequently, the proposed sensor has potential applications in the multi-parameter dynamic detection due to its simple structure, high sensitivity, good mechanical strength, and low cost.