Modeling the nonlinear photoabsorptive behavior during self-written waveguide formation in a photopolymer

Photopolymer materials can be used as recording media for self-written waveguides (SWWs) as they can exhibit a large refractive index change and high photo-sensitivity. In free radical photo-polymerization systems, the dyes, functioning as the photosensitizer, strongly influence the material properties. During photo-illumination the spatial and temporal evolution of the dye concentration is an important factor leading to nonlinear absorption. In this paper, based on an investigation of the photochemical mechanisms, we analyze the nonlinear photoabsorptive effect during the photo-initiation processes. The time varying exposing light distribution is calculated and used to iteratively estimate the evolving cross-sectional refractive index and loss coefficient values. The model enables a more accurate and physical description of the optically induced growth of SWWs in such systems. Then SWWs formed in dry acrylamide/polyvinyl alcohol (AA/PVA) based photopolymer samples, containing different initial dye concentrations, are experimentally examined. The nonlinear absorptive behavior is quantified by comparing the model predictions and the experimental results.