FBG and FOPS for local and global structural health monitoring on a single fiber

Abstract

Fiber Bragg grating (FBG) sensors and fiber optic polarimetric sensors (FOPS) have been widely researched and implemented for structural health monitoring (SHM). FBG essentially provides localized strain information, while FOPS gives a global indication of the structural health of materials. An FBG written on the polarization maintaining (PM) fiber can thus be used for both global structural monitoring and local strain sensing. However each sensor has to be used with its own hardware and processing. For gratings written on PM fibers two Bragg reflections, corresponding to two modes of polarization, are observed. While both Bragg wavelengths shift under longitudinal strain in unison, their relative peak amplitude does not change. In this paper, a novel concept is proposed which makes the peak amplitudes responsive to the longitudinal strain. This relative amplitude of both the peaks is used for the first time to determine the state of polarization (SOP) with no additional optical systems. With this additional information on SOP, PM–FBGs can be used for both, local and global SHM simultaneously. Further, a new design has been proposed which gives improved information on the damaged location in beam structures. This can be further extended to other complex geometries.