We study the process of two-wave mixing (TWM) in optically active, electra-
optic, and elasto-optic BSO and BTO crystals. We calculate the TWM gain for
arbitrary crystal cut and optimize the energy exchange. For the (111) cut,
by choosing an appropriate coordinate system, we obtain a simple analytica
l solution for the components of the coupling tensor which allows us to opt
imize analytically the TWM gain with respect to the grating orientation and
the initial light polarization.