A pre-twisted taper in dual-side hole fiber for torsion measurement with high sensitivity

We propose and demonstrate a novel torsion sensor based on a fiber Mach-Zehnder interferometer (MZI) in this paper. The MZI consists of a short section of dual-side hole fiber (DSHF) sandwiched between two single-mode fibers, in which the DSHF is twisted and tapered by a hydrogen oxygen flame to form a spiral structure in the MZI. The spiral structure makes MZI not only sensitive to torsion, but also has the ability to distinguish the direction of torsion. Experimental results show that the torsion sensitivity of the interferometer is as high as -1.67 nm/(rad $\cdot \text{m}^{{-1}}$ ) and 1.65 nm/(rad $\cdot \text{m}^{{-1}}$ ) in the clockwise (CW) and counterclockwise (CCW) rotations, respectively. Furthermore, the temperature sensitivity of the structure is 7.66 pm/°C, corresponding to a low temperature cross-sensitivity of 0.008 rad $\cdot \text{m}^{{-1}}/^{\circ }\text{C}$ . The proposed structure is particularly suitable for the situations where high measurement sensitivity is required and the torsion direction can be identified.