Finite element formulation for self-writing of polymer optical fiber sensors

Abstract

This thesis presents a multi-physics finite element model of the self-writing process of photopolymerization in photosensitive gels in order to better predict the self-generation of polymer optical fiber sensors. The finite element model incorporates the electromagnetic, chemical and mechanical physics of this photonic reaction. The output of the model is thus the dynamics of photopolymerization and self-focusing including densification and the induced index of refraction change. An experimentally verifiable benchmark trial of a UV light source focused into UV curable resin was modeled using COMSOL Multiphysics, to develop a basis for calibration of different photosensitive gels. This model was then extrapolated to demonstrate the effects of photopolymerization when a single fiber is focused into an epoxy resin as well as the effects of curing an epoxy-filled gap between two aligned fibers. Results were obtained for linear and saturation models of the change in the index of refraction. These models are applied to single and two fiber benchmark examples. The saturating effects are consistent with previous experimental research showing a 2% change in refractive index with similar lightwave confinement within the cured portion of the resin.The confinement achieved with this model has also verified the ability to predict selffocusing and tapering effects that are also seen in previous experimental studies. The effect of densification within the sample and how this is affected by the geometric constraints on the system are demonstrated as well. This thesis provides a versatile finite element model for predicting the dynamics of the photopolymerization process for a variety of resins and experimental geometries and has presented an effective tool for use in determining the response of a polymer sensing element cured using the photopolymerization process.