Diffraction efficiency as a function of the grating vector orientation in electro-optic and optically active photorefractive crystals

We study the diffraction efficiency of volume phase gratings in Bi12GeO20, when the grating vector, lying on a (110) plane, makes an angle with the (110) crystallographic direction. An external bias electric field, parallel to the (110) axis, is applied during recording, thus the recording conditions do not remain constant as we vary the grating vector orientation. The basic parameters determining the diffraction efficiency are the grating vector orientation, the optical activity, and the field induced linear birefringence (primary and secondary). Analytic expressions for the diffraction efficiency have been derived by taking into account all the above mentioned parameters, provided that linearly polarized light is incident on the crystal. Experimental results are given.