Optical fiber twist sensor based on a double-helix structure and quasi-distributed displacement system

The inherent axial torsion of cable-type sensor arrays can cause errors in the projection direction of vector signals. Therefore, accurate characterization of torsional deformation in such arrays is of critical importance. A dual-helix optical fiber twisting measurement method was proposed for the first time, so as to address the issue of measurement errors accumulating with sensor length in traditional optical fiber twist sensors. Saint Venant's theory was applied to a fiber twist sensing model to analyze the relationship between fiber displacement and twist angle. A quasi-distributed fiber displacement measurement system based on dual modulation using a Mach-Zehnder modulator (MZM) was constructed. Subsequently, a proof-of-concept verification was carried out, in which two optical fibers were wound in opposite helical directions on a silicone substrate with a diameter of 4 cm and a pre-twisted pitch of 25 cm. Experimental results show that the sensor can effectively measure twisting angles, with a sensitivity coefficient of 40.76 kHz/° over a twisting range of ±90°, and a measurement accuracy of 0.39°. The main advantage of this twisting measurement method is that the twisting error does not accumulate with sensor length, and the common-mode noise (CMN) caused by external environmental factors can be effectively suppressed. This approach, therefore, offers significant potential for the torsional measurement of long-distance underwater cable-style sensor arrays.