High-spatial resolution measurements of transverse stress in a fiber Bragg grating using four-state analysis, low-coherence interferometry, and layer-peeling

Abstract

Fiber Bragg grating (FBG) sensors have been shown to be a good means of nondestructive monitoring of the stress and/or strain of the materials in which they are embedded. Many FBG transverse stress/strain measurement systems can resolve only a single stress and/or strain value for the entire length of the FBG and often require the use of polarization-maintaining fiber. We demonstrate a new method for measuring the two components of transverse stress with high spatial resolution in a distributed FBG sensor. A directional compressive load is applied by placing weights on top of the FBG, creating a transverse stress in the core of the FBG. Small metallic strips are placed under the FBG to create a localized stress in the FBG. The relative index of refraction as a function of position in the FBG is determined with a low-coherence Michelson interferometer and a layer-peeling algorithm. With this method we are able to measure changes in the refractive index with resolution better than 5x10^-6, limited by the signal-to-noise ratio of the measurement system, with a spatial resolution of 16 μm. To determine transverse stress, we repeat the measurement for four different polarization states. A four-state analysis is then used to determine the birefringence as a function of position in the grating. This measurement assumes that the applied transverse load is much larger than any other birefringence in the grating, so that the principal axes do not change with position in the grating. This measurement offers the advantage that it can be implemented with a simple layer-peeling algorithm, and it does not require the use of expensive polarization maintaining fiber. Measurements of the externally induced birefringence agree well with values predicted by the stress-optic properties and the geometry of the fiber.