The Diffraction Efficiency Of Holographic Multi-gratings Based On Coupled Wave Theory

Bayfol material is a kind of ideal holographic recording material, which shows high diffraction efficiency, easy exposure and no necessity of wet or heat treatments. In this work, firstly, we analyze the diffraction efficiency on the basis of Kogelnik coupled wave theory, and study the influence of refractive index modulation, wavelength and angle detuning on the efficiency of the single grating. In the Bragg condition, the efficiency is valued by the refractive index modulation. Off the Bragg condition, the efficiency is affected by the Bragg mismatch, and the increase of the Bragg mismatch will lead to a rapid decrease of the efficiency. In addition, we also discuss the influence of wavelength and angle detuning on single grating efficiency as for different diffraction efficiency.Secondly, due to the limited spectrum width of grating, we propose to write multiple gratings into the material. For Bayfol material with 16 micro and refractive index modulation 1.5, the efficiency of two gratings is the linear superposition of each efficiency because the coupling between gratings is relatively small when the slanted angle is more than five degree, and the diffraction spectrum width of double gratings is more than half width of grating diffraction. We study the diffraction efficiency of double gratings, the quadruple gratings and the multiple gratings through the linear superposition. For the certain refractive index modulation, we write multiple gratings into the material to expand the spectrum width and to make efficiency fairly uniform.The nonlinear coupling can not neglect if the angle between gratings is less than five degree. Therefore, considering that the grating coupling is proportional to the product of the intensity of the adjacent grating, we derive the diffraction efficiency of double gratings, and get the coefficient through comparing with the experiment results. The results show that the nonlinear coupling can explain the experimental phenomena better than the linear coupling when the angle between gratings is fairly small.