  The main methods for controlling the biphoton field, as well as the problems
for which the width and the shape of the spectrum of the biphoton field are of
decisive importance, are discussed. The method for controlling the spectrum of
the spontaneous parametric downconversion of light based on the spatial
modulation of the refractive indices of a nonlinear crystal in which the
generation of biphotons has been analyzed. Modulation is due to the
thermo-optic and electro-optic effects.
