Optimization of the diffraction efficiency in non-uniform gratings in sillenite crystals (Bi₁₂SiO₂₀ and Bi₁₂TiO₂) considering the variation of fringe period, optical activity and polarization angles in a strong non-linear regime

We included the non-uniformity of the grating and of the magnitude and phase of light modulation throughout the sample thickness to optimize the diffraction efficiency. The variation of fringe period, optical activity, birefringence, absorption of light, and polarization angle were considered. We studied strong nonlinear conditions and two crystal orientations one is with the grating vector parallel to the face [001] and the other is with the grating vector perpendicular to the same face. We included applied fields There is a complex relationship among all these parameters, and the prediction of the conditions for the optimum value of the diffraction efficiency is complicated. We report the optimal sample thickness for different situations, considering two wavelengths for reading: green (532 nm) and red (632 nm).