The Investigation To The Formation Of Holographic Grating In Photopolymers

Photopolymer is a new material for holographic storage; it has a number of compelling properties such as high diffraction efficiency, relatively simple fabrication process, and long persistence. So far, the researches on photopolymer in our country involve mainly in experimental investigations, and only a few studies on recording mechanism have been done. The key work in this thesis is theoretically investigating the recording mechanism of photopolymer, in order to guide the optimization of recording conditions and the material components.Previous diffusion models of grating formation in photopolymer were investigated to find the basic model that can reflect the recording mechanism of photopolymer, and the signification of parameters in the model. The applicable range of different models was investigated too. Based on the first harmonic diffusion model, a simplified diffusion model specifying the formation of holographic grating has been proposed in this thesis. Numerical simulations with the analytical expression of the refractive-index modulation according to the diffusion model have been done, showing the effects of each parameter in the model on the saturated refractive-index modulation of photopolymer gratings. The methods to enhance the diffraction efficiency of holographic gratings by optimize the recording conditions have been discussed based on the relation among the parameters in the model and recording intensity and fringe spacing.Two special phenomena, dark enhancement and uniform post-exposure, were considered as a part of the diffusion model. The analytical expressions of the refractive-index modulation to these phenomena were induced by the diffusion model. The numerical simulations to the three recording modes (holographic exposure, dark enhancement and uniform post-exposure) have been conducted, and the results indicate that the effects of these three recording modes on saturated refractive-index modulation are different, which are also effected by the exposure intensity. If the exposure intensity is moderate, the highest saturated refractive-index modulation can be got by holographic exposure. However, if the exposure intensity is too high, the saturated refractive-index modulation can be got by dark enhancement before the refractive-index modulation reaches saturation,and the uniform post-exposure makes the saturation course fast. It is possible to choose a suitable recording mode according to one's demand.The experiments were designed and conducted to validate the theoretical results. The material in experiments was a novel photopolymer invented by our group. The experimental results agreed well with theoretical predictions. This indicates that the simplified diffusion model of the formation of holographic grating suits to our photopolymer material. The parameters in the model can be got by fitting experimental data with the model, and the effects of material components on holographic performances have been investigated which provides helpful suggestions for the development of the material. The effects of dark enhancement on the holographic recording course were also investigated by experiments. Experiments of non-continuous holographic exposure were designed and conducted based on the dark diffusion of monomers in dark enhancement. The experimental result indicated that the diffraction efficiency of a non-continuously exposed grating is higher than that of continuously exposed one in the same recording condition. Finally, we recorded holographic gratings with three recording modes (holographic exposure, dark enhancement and uniform post-exposure) with the same exposure intensity. The experimental results indicated that the exposure intensity 2 mW/cm2 is suitable to this material, and the highest saturated diffraction efficiency was got by holographic exposure mode.