We present a new method to solve the coupled equations for two-wave mixing
(TWM) in sillenites. The first-order solutions, based on the approximation
of a weak grating and strong optical activity, can describe BSO crystals we
ll. The result is generally valid and the maximum gain for arbitrary cut is
shown, The fundamental properties of TWM are systematized for the practica
lly relevant cut (1 1 1). For the optimization of BTO crystals, characteriz
ed by not very large optical activity, dynamic gratings are taken into acco
unt. We find an approximation scheme, related to the parallel subsystem, wh
ich works for quite large coupling coefficients and for a wide range of cry
stal thicknesses. (C) 2001 Elsevier Science B.V. All rights reserved.