For some parametric interactions with identically zero exponential gain for
the signal wave the intensity of the idler wave can grow as a second power
of the propagation coordinate. Such an amplification is revealed for the p
arametric mixing of four copropagating waves in BaTiO3; two of them are ord
inarily polarized and the two others are extraordinarily polarized. This mi
xing is used to build up a coherent oscillator. A reasonable qualitative ag
reement of the experimental results with the calculated data is demonstrate
d.