  Pulse broadening for optical solitons due to birefringence is investigated.
We present an analytical solution which describes the propagation of solitons
in birefringent optical fibers. The special solutions consist of a combination
of purely solitonic terms propagating along the principal birefringence axes
and soliton-soliton interaction terms. The solitonic part of the solutions
indicates that the decay of initially localized pulses could be due to
different propagation velocities along the birefringence axes. We show that the
disintegration of solitonic pulses in birefringent optical fibers can be caused
by two effects. The first effect is similar as in linear birefringence and is
related to the unequal propagation velocities of the modes along the
birefringence axes. The second effect is related to the nonlinear
soliton-soliton interaction between the modes, which makes the solitonic
pulse-shape blurred.
