  We show that the existence of optical activity in an optical material can
lead to essential enhancement of acousto-optic (AO) figure of merit for this
material. The reason is that the ellipticity of interacting optical eigenwaves
approaches unity near the optic axis and so additional elasto-optic (EO) tensor
components with relatively large values become involved into the effective EO
coefficient. We demonstrate on the example of lead germanate crystals,
Pb$_5$Ge$_3$O$_{11}$, that the increase in the efficiency of AO diffraction
manifests itself for all the types of isotropic and anisotropic interactions,
whenever the incident optical wave propagates close to the optic axis. We find
that, in the particular case of diffraction in the interaction plane XZ of
Pb$_5$Ge$_3$O$_{11}$ crystals, the maximal enhancement of the AO figure of
merit takes place under conditions of the types V and VI of isotropic
diffraction, with the AO figure of merit increasing from zero up to
13.3x10$^{-15}$ s$^3$/kg, and the type IX of anisotropic diffraction when the
AO figure of merit increases more than twice (from 12.5x10$^{-15}$ up to
26.5x10$^{-15}$ s$^3$/kg). The maximal AO efficiency in the XZ interaction
plane is reached at the types I and II of isotropic AO interactions. In these
cases the AO figure of merit increases from 6.8x10$^{-15}$ up to
37.9x10$^{-15}$ s$^3$/kg and from 31.1x10$^{-15}$ to 37.9x10$^{-15}$ s$^3$/kg,
respectively.
