In-line, fiber-optic polarimetric twist/torsion sensor.

This letter presents an optical fiber twist/torsion sensor that utilizes dissimilar polarization-preserving characteristics of standard single-mode and high-birefringence fibers. When only one polarization mode of the high-birefringence fiber is excited, spatial orientation of the E-field vector follows the fiber's principal axis orientation, even when the fiber is twisted around its longitudinal axis. This is contrary to a standard single-mode fiber (SMF), where the E-field vector maintains its spatial orientation regardless of the fiber torsional twist. The proposed sensor consists of a short section of standard SMF inserted in between two lead-in polarization-mainlining fibers. Only one mode of the lead-in fiber is excited at the input, while the ratio of both polarization modes is measured at the output side of the fiber assembly in order to determine the torsional twist of a standard SMF. This sensor can be used for measuring unambiguously twist/torsion angles of between ±95°. The sensor demonstrated very low temperature dependence.