Characterization and birefringence effect on embedded optical fiber Bragg gratings

This study characterizes the performance of embedded optical fiber Bragg gratings (FBGs) used as strain sensors. Focus is provided to FBGs embedded in a quasi-isotropic lay-up of carbon fiber epoxy lamina both parallel and perpendicular to adjacent structural fibers. It studies the birefringence induced during curing and quantifies the residual transverse strain differences on the fibers by measuring the split from a single reflected Bragg wavelength into two. The association between light polarization and loading directions relative to the optical fiber (in-plane parallel, in-plane transverse, and out-of-plane transverse) are analyzed. Birefringence was seen to increase when a compressive out-of-plane load was applied to the embedded optical fiber. In contrast, in-plane loads did not lead to an increase in birefringence as indicated by reflected wavelengths that split during curing shifting equally and linearly during tensile load tests. An effective strain-optic coefficient was determined that resulted in strong correlations between FBG and surface mounted electrical strain gauge measurements.