Measurement errors from internal shear strain within fiber-Bragg-grating sensors

Abstract

Fiber-Bragg-grating (FBG) sensors have become commercially available sensors for the measurement of temperature, strain and many other quantities. The sensor information is encoded in the spectral reflection characteristic of these devices. Their usage as strain sensors is one of the most prominent fields of application. Strains from a structure which is to be monitored are transferred into the fiber-Bragg-grating, by surface bonding or embedding. In general an arbitrary state of strain may thus occur within the FBG, represented by a full strain tensor with normal strain components, as well as with shear strain components. The influence of normal strains is well understood and has been treated theoretically by many authors. The influence of shear strains is however not well understood. As we were recently able to theoretically demonstrate by a full tensor coupled mode analysis, shear strains do influence the spectral response of fiber-Bragg-sensors and thus have to be considered. In this work, an introduction to the modeling of shear strains within fiber-Bragg-gratings is given. We discuss reasonable approximations for the simplification of the theoretical model. We compute, to our knowledge for the first time, the direct influence of shear strains on the output of a FBG measurement system and show the cases when shear strain effects are relevant. Furthermore, we compare the sensitivity of different interrogation algorithms towards shear strain influences on the measurement system output.