5.2        Characterization of a Fiber Bragg Grating under Diametric Loading

5.2.1        Introduction

When uniform transverse stresses are applied to a FBG gauge (Fig. 5-15), the refractive index becomes non-uniform in the transverse plane of the fiber. This leads to birefringence. The Bragg wavelength condition splits in two solutions, one for each refractive index along the fast and slow axis of the fiber

Fig. 5-15 : Transverse Stresses

where

If the fiber has a natural birefringence due to internal stresses included during the fiber preform fabrication, the birefringence due to loading can enhance or remove the natural birefringence of the fiber. If the transverse strains induce an axial deformation of the fiber, the Bragg wavelengths equations above need to be adapted.

Several articles report on experiments of transverse stresses applied to optical fiber. For example Wagreich et al [5-11] have conducted diametric load experiments on low-birefringent fiber. Good agreement between the proposed theory and experimental results were obtained. Lawrence, Nelson, and Udd [5-12, 5-13] performed similar work but in a polarization maintaining (PM) fiber. In this case, the proposed theory was unable to explain the experimental results.

Since we are interested in utilizing FBG's written in PM fibers in composite structures for monitoring transverse stresses, we need to characterize the FBG gauge when placed in a transverse stress field. A diametric loading technique has also been chosen to investigate FBG response in low and high-birefringent fibers. A simple theoretical model is proposed that explains also the results of FBG in high-birefringent fibers.