High-performance vector torsion sensor based on high polarization-dependent in-fiber Mach-Zehnder interferometer.

We propose a high-performance vector torsion sensor based on an in-fiber Mach-Zehnder interferometer (MZI), which consists of a straight waveguide inscribed in the core-cladding boundary of the SMF by a femtosecond laser in only one step. The length of the in-fiber MZI is 5 mm, and the whole fabrication time does not exceed 1 min. The asymmetric structure makes the device have high polarization dependence, and the transmission spectrum shows a strong polarization-dependent dip. Since the polarization state of the input light entering the in-fiber MZI varies with the twist of the fiber, torsion sensing can be achieved by monitoring the polarization-dependent dip. Torsion can be demodulated by both the wavelength and intensity of the dip, and vector torsion sensing can be achieved by setting the appropriate polarization state of the incident light. The torsion sensitivity based on intensity modulation can reach 5763.96 dB/(rad/mm). The response of dip intensity to strain and temperature is weak. Furthermore, the in-fiber MZI retains the fiber coating, so it maintains the robustness of the complete fiber structure.