Shear Strain Influence on Fiber Bragg Grating Measurement Systems

Fiber Bragg grating (FBG) sensors have become commercially available sensors for the measurement of temperature, strain, and many other quantities. One interesting application is the embedding of these sensors, during which shear strains can arise inside the sensor. As we have recently demonstrated 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 paper, we use the theory behind this analysis to compute the direct influence of shear strains on the output of a FBG measurement system, and show cases where shear strain effects are relevant. Furthermore, we compare the sensitivity of different interrogation algorithms toward shear strain influences on the measurement system output. To model the experimentally relevant unpolarized light sources, we derive a model using the monochromatic waves of coupled-mode theory. We apply the unpolarized light to the FBG shear strain problem and show that for unpolarized light, shear strain has to be taken into account as well. We find absolute measurement errors in the range of 100 pm. For typical normal strain measurements, this would be of an order of 10% of relative error.