Polarization-maintaining fiber-optic-grating torsion sensor

A compact fiber torsion sensor, in which a short section of polarization-maintaining (PM) fiber stub containing a normal straight fiber Bragg grating (FBG) is spliced to another single-mode fiber without any lateral offset, is proposed and experimentally demonstrated. Two well-defined resonances (orthogonal-polarimetric FBG core-modes) with ~0.5 nm wavelength separation have been achieved in reflection due to the birefringence of the PM fiber, and they exhibit a strong polarization and rotation dependence. Both the orientation and the angle of rotation can be determined unambiguously via dual-path power detection of the orthogonal-polarimetric core reflections. The unwanted power fluctuations and temperature perturbations can be effective referenced out by dual-path power normalization.