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

Abstract

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.

Fiber-Optic Sensors for Measurements of Torsion, Twist and Rotation: A Review

“…This additional rotation is, however, relatively small in comparison to the twist imposed to the fiber and which causes rotation through opto-elastic effect. For example, twisting of SMF by 1 degree causes rotation of the polarization plane within the fiber by about 0.069° [84]. This polarization plane rotation is the opposite of the mechanical rotation of the fiber, thus, the rotation angle measured by an analyzer at the fiber output must be multiplied by a factor of 1.069 [84] to obtain the correct twist angle readout.…”

“…For example, twisting of SMF by 1 degree causes rotation of the polarization plane within the fiber by about 0.069° [84]. This polarization plane rotation is the opposite of the mechanical rotation of the fiber, thus, the rotation angle measured by an analyzer at the fiber output must be multiplied by a factor of 1.069 [84] to obtain the correct twist angle readout. The appearance of twist induced circular birefringence (optical activity) is, thus, not limiting in measurements of rotation angles by using the E-field vector displacement measurement principle.…”

“…The only adverse effect that comes from the appearance of circular birefringence is, perhaps, in a slight increase of the system’s temperature sensitivity. While the twist induced circular birefringence is temperature sensitive, its contribution to the angle readout is small, and will, thus, cause only very limited temperature dependence (for example when the fiber is twisted through 90°, a 50 K temperature change of the same fiber will induce error in angle readout of less than 0.4° [84]).…”

Efficient design of Fiber Optic Polarimetric Sensors for crack location and sizing

“…A configuration of Hi-Bi fibers was presented as Sagnac interferometer which worked as temperature and strain independent torsion sensor [8]. A simple and effective design of fiber-optic polarimetric twist/torsion sensor was also presented [9]. Though FOPS has already been studied applying dynamic and static tests for global SHM of different structures [10][11][12], it could not be used for local and long distance SHM applications.…”

Phase-shifted helical long-period grating-based temperature-insensitive optical fiber twist sensors

“…The most common approach to the fabrication of fiber twist sensors is the natural characteristics of the polarization in the fiber. The orthogonal polarization modes or single polarization mode can be excited through various types of fiber components, such as UV-inscription tilted fiber gratings (TFGs) [4], polarization-maintaining fiber Bragg gratings (PMFBGs) [5], or high-birefringence fibers [6]. The direction of the polarization mode is changed by rotating the fiber, and the twist angle is measured by interrogating the visibility or intensity of the optical light.…”