Orthogonal polarization mode coupling for pure twisted polarization maintaining fiber Bragg gratings

Abstract

Spectral characteristics of orthogonal polarization mode coupling for pure twisted polarization maintaining fiber Bragg gratings (PM-FBG) are proposed and analyzed experimentally and theoretically. Different from the polarization mode coupling in PM-FBG due to side pressure, a resonant peak at the middle of two orthogonal polarization modes is found when the PM-FBG is twisted purely which is attributed to the cross coupling of polarization modes. Its intensity increases with the twisting rate. A new coupled mode equation is built to describe the pure twist polarization mode coupling, in which both the normal strain induced by strain-applied parts and the tangential strain induced by twisting are taken into consideration and expressed in a unified coordinate. The novel phenomenon and its explanation are believed to be helpful for PM-FBG applications in fiber sensor and laser technologies.

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

“…When the fiber twist-to-linear-birefringence ratio is sufficiently high, twist/torsion can, by itself, act as a perturbation that can provoke mode coupling among polarization modes of the PM fiber, which can be monitored further, for example, by observation of the spectral response of the FBG written in the same fiber [101]. Whilst this effect is not pronounced soundly in PM fibers, it seems it can still provide a sufficiently distinctive spectral response that can be used for twist/rotation sensing.…”

Long period fiber grating based on periodically screw-type distortions for torsion sensing

“…They are lightweight, small, flexible, compact, and most importantly, feature a high torsion sensitivity [1]. As a result, several optical in-fiber torsion sensors have been proposed using various types of fiber components, such as corrugated long-period fiber gratings (LPFGs) [2], high-birefringence fibers [3], UV inscription-tilted fiber gratings [4], polarization-maintaining fiber Bragg gratings [5], Sagnac loop structures [6,7], and distributed Bragg reflector fiber grating lasers [8]. However, many of these proposed structures are torsion-independent and can distinguish the angle but not the direction (clockwise or counter-clockwise) of the applied torsion.…”

“…However, many of these proposed structures are torsion-independent and can distinguish the angle but not the direction (clockwise or counter-clockwise) of the applied torsion. As such, despite their high torsion sensitivity, they are impractical for certain applications [2,4,5]. CO 2 laser-induced LPFGs (C-LPFG) exhibit clear torsion dependence because of unilateral exposure to the laser beam.…”

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.…”