Temperature-insensitive polarimetric torsion sensor based on a pair of angularly cascaded LPFGs

A novel torsion sensor is proposed based on the polarization characteristics of a pair of angularly cascaded long period fiber gratings (ALPFG). A set of special designed fiber rings serve as the detached sensor head and the ALPFG provides the polarization filter. The input polarization is continually adjustable through rotating the dial of the fiber rings, which alters the output intensity from ALPFG, and the torsion sensitivity can reach −15.355 dB/rad. We study the periodicity of output intensity theoretically and experimentally and find the angular lag between the fluctuations of the two resonant dips. Through post-proceeding, we break the sensitivity-range limit and double the measuring range by taking advantage of the angular lag. This system design not only theoretically provides a more sensitive sensor head design, but also possesses more flexible, healthy and temperature-insensitive characteristics. Therefore, such sensor can be a good candidate for torsion sensing.