Low Refractive-Index and Temperature Sensitive Torsion Sensor Based on Cascaded Long-Period Fiber Gratings Inscribed in a Four-Mode Fiber

Torsion sensing is a promising technique in a variety of different scientific and industrial applications. In this paper, we present a novel torsion sensor based on the cascaded long-period fiber grating inscribed in a four-mode fiber (CLPFG-FMF). The change in rotatory vector makes resonant wavelength shift, for the twisted CLPFG-FMF. The key factor, which severely impacts on the properties and performance of torsion sensor, is also investigated and discussed. Experimental results show that the resonant wavelength linearly shifts with the torsion rate and the cascaded angle plays an important role in enhancing torsion sensitivity. The proposed torsion sensor achieves optimal sensitivities of -0.519 nm/(rad/m) in counterclockwise and 0.501 nm/(rad/m) in clockwise. The repeatability and stability of the CLPFG-FMF with cascaded angle of 0° are studied as well. The refractive index sensitivity of the CLPFG-FMF with cascaded angle of 0° is −5.20 nm/RIU from 1.3431 to 1.4374 refractive index ranges and the temperature sensitivity is −0.0173 nm/°C from 25 °C to 90 °C. The high sensitivity of torsion sensing is expected to be useful for broad applications across engineering fields.