High sensitivity twist sensor based on Sagnac interference with a helical elliptical core fiber

Twist detection is of great significance in industrial production, aerospace, biomedicine and other fields. Aiming at the problem of the low sensitivity of the twisted sensor, this paper proposed and prepared a kind of fiber twist sensor with high sensitivity based on Sagnac interference. The sensitive area of the sensor is a helical elliptical-core polarization maintaining (HEPM) fiber, the optical fiber with an elliptical core has linear birefringence property, and when the fiber is rotated, it has circular birefringence property. The HEPM fiber is plugged into the light loop to form Sagnac interference. Since circular birefringence is proportional to the degree of fiber distortion, the interference spectral trough will respond to different angles of fiber torsion. In addition, the sensor can not only identify the twist direction but also maintain good stability in different twist directions. The experimental results show that the free spectral range (FSR) of the interference spectrum increases and the sensitivity of twist sensing increases as the length of the used HEPM fiber decreases. The maximum twist sensitivity that can be achieved is 8740.64 pm/(rad/m) when the length of HEPM fiber is 15 cm in the twist range of plus or negative 120°, which is higher than the sensitivity of existing studies. The proposed sensor can be used for high-sensitivity distortion sensing. The temperature stability of the sensor is studied experimentally, and a sensing matrix is established that can simultaneously measure the torsion and temperature to eliminate the error caused by temperature. In summary, the proposed sensor not only has high sensitivity but also has an excellent linear response and can identify the twist direction, which has great potential in practical distortion detection.