  We have developed the approach for analyzing of anisotropy of acoustooptic
figure of merit (AOFM) for lithium niobate crystals in the case of isotropic
acoustooptic (AO) diffraction. The working relations for the effective
elastooptic coefficients and the AOFM have been derived. We have found that,
under the conditions of isotropic AO diffraction, the maximum AOFM value for
LiNbO3 is equal tp 11.62x10-15s3/kg. It is peculiar for the geometry of AO
interaction of the shear acoustic wave propagating in the YZ plane (the
velocity 3994 m/s) with the optical wave polarized in the same plane. We have
demonstrated that the maximum AOFM values are achieved mainly due to essential
anisotropy and high values of the elastooptic coefficients of LiNbO3.
