High-sensitivity vector twist sensor based on quasi-third-order PMF-Solc-Sagnac interferometer

In recent years, optical fiber sensors are widely used in various fields. Optical fiber twist sensor have also been extensively studied. However, in some special situation with requirements of high sensitivity and directional measurement, traditional optical fiber sensing framework is difficult to satisfy the detection requirements. Therefore, this work presents and validates a twist sensor based on quasi-third-order Solc-Sagnac interferometer. The sensing structure is constructed by two segments of PMF with similar lengths at a 90° fast axis fusion angle. Not only does this configuration achieves the amplification of sensitivity, but also enables vector measurement of twist. The twist is applied on the shorter segment of the two PMFs. When the sensing segment undergoes twist deformation, the birefringence of this segment will change, in turn causes the dip wavelength to shift. In this work, through three sets of experiments, the influences of both the length difference between two PMF segments and the sensing segment length on the sensitivity are verified. The maximum sensitivity of 2.156 nm/° (3.706 nm/(rad/m)) is obtained, which is 15.33 times higher than the highest sensitivity of 0.1406 nm/° (0.2417 nm/(rad/m)) by the traditional single-loop Sagnac interferometer. The directional measurement of twist is realized due to the asymmetric optical path structure, meanwhile the dual-parameter measurement of twist and temperature is realized by using the coefficient matrix. In conclusion, the quasi-third-order Solc-Sagnac interferometer constructed has the significant advantages during measurement, including high sensitivity and directional measurement.