The aim of this work is to study and to optimize the diffraction efficiency in sillenite BTO crystals.A theoretical analysis, where the effects of coupling beams are significant, is carried on.In addition, non-mobile transmission gratings are considered.The diffraction efficiency is obtained by using the coupled wave equations in on-Bragg regime using Method Runge-Kutta of order 4 in Matlab.Also, the non-uniformity of the gratings inside the crystal sample, the optical activity of the material and the polarization of the recording beams are taken into account.