Photonic bandgap fibers for transverse strain sensing

This research examines the change in bandgap characteristics of Photonic Bandgap (PBG) fibers under transverse loading for applications such as fabrication and service life monitoring of composite structures. Photonic Bandgap (PBG) fibers rely on Bragg reflection conditions in the plane of optical fiber cross-section and therefore offer great potential as transverse strain sensors which are insensitive to axial loading and temperature variations.;A numerical study of the effect on the bandgap in PBG fibers under transverse loads is thus performed in this dissertation. First the fundamental equations for lightwave propagation in classical step-index fibers, microstructured holey-fibers and PBG fibers are reviewed. The behavior of each for sensing purposes is also discussed. The structural deformation and electromagnetics modeling of a PBG fiber is then performed using the Finite Element Method (FEM) because this method offers the ability to examine arbitrary fiber configurations, specifically through deformation where the fiber is no longer circularly symmetric.;The FEM models were run for both uniaxial crush loads and uniform pressure loads for both silica and a doped PMMA material targeting strains up to approximately 6% at the boundary of the fiber. The results showed that degradation of the bandgap occurs with loading and that axis specific loading information may be obtained in fibers whose material normal and shear Pockel's constants differ by approximately 50% or more, although the exact difference required is not known. In the case of the PMMA uniform pressure load it was determined that the combination of loading and fiber characteristics may cause the bandgap to switch modes which may interfere with actual sensor implementation and should be avoided. The cross-section of the fiber studied was not rotationally symmetric which resulted in nonsymmetric optical output from the uniform pressure case. While fibers of this construction are likely to not be rotationally symmetric by design, the actual manufacture of the fibers results in a cross section that more closely approximates this condition.