A Novel Twist Sensor Based on Long-Period Fiber Grating Written in Side-Helical Polished Structure

We propose a novel method to write a long-period fiber grating in helical polished structure (HPLPFG) as a twist sensor. The helical structure is polished on one side of twisting fiber by CO2 laser following either right-hand or left-hand helix direction. The sensor can discriminate the twist direction and improve the twist sensitivity, due to its asymmetric and helical refractive index distribution. The twist and temperature characteristics of the sensor have been experimentally investigated. The twist sensitivities of the sensor (right-hand direction helix) are 0.199 nm/(rad/m) and 0.141 nm/(rad/m), and the twist sensitivities of the sensor (left-hand direction helix) are -0.119 nm/(rad/m) and -0.196 nm/(rad/m) in clockwise and anti-clockwise directions, respectively. The temperature sensitivities of the two sensors are 0.060 nm/degrees C and 0.063 nm/degrees C, respectively. The novel sensor has practical application potential in twist sensing.