Examining Directional Strain Sensitivity of Shaped Optical Fibre Embedded in Polyurethane Strip

Abstract: Distributed strain sensing using a straight optical-fibre cable suffers from a decreased strain senstivity away from the fibre's axis. In this study, the directional senstivity is enhanced via sinusoidally shaping the fibre that is embedded in a polyurethane strip. First, the directional senstivity is quantified via a relatively simple analytical model. Second, static-strain measurements using a Brillouin Optical Frequency Domain Analysis (BOFDA) system are carried out on a physical sample of the strip. Two different directions are examined, namely in-line and broadside orientations. The former involves deforming parallel to the plane where the fibre is sinusoidally embedded, whereas the latter means deforming the plane normal to it. It can be observed that the response to the deformation in these orientations is opposite, i.e. the fibre experiences negative strain, so shortening, an in-line deformation, and a positive strain, so stretching, for a broadside deformation. It can be found that the fibre is slightly more senstive in the in-line direction which agrees with the behaviour as predicted by the model. Also, we see that the measured strain along the optical fibre is mainly influenced by the elastic properties of the embedding material due to, most importantly, the Poisson's ratio as well as the geometrical parameters of the sinusoidally shaped fibre.