  Nonlinear wave interaction of low amplitude gravitational waves in flat
space-time is considered. Analogy with optics is established. It is shown that
the flat metric space-time is equivalent to a centro-symmetric optical medium,
with no second order susceptibility. The lowest order nonlinear effects are
those due to the third order nonlinearity and include self-phase modulation and
high harmonic generation. These processes lead to an efficient energy dilution
of the gravitational wave energy over an increasingly large spectral range.
