  Self-phase modulation of spherical gravitational wavepackets propagating in a
flat space-time in the presence of a tenuous distribution of matter is
considered. Analogies with respect to similar effects in nonlinear optics are
explored. Self phase modulation of waves emitted from a single source can
eventually lead to an efficient energy dilution of the gravitational wave
energy over an increasingly large spectral range. An explicit criterium for the
occurrence of a significant spectral energy dilution is established.
