  Propagation of few-cycle laser pulses in optical fiber is investigated beyond
the unidirectional approximation. Considered medium parameters include
dispersion, Kerr and Raman nonlinearities. High intensities of ~10^10 kW/cm^2
lead to generation of the octave-spanning continuum. The error of the
unidirectional approximation is seen in simulations as two pulses propagating
forwards and backwards, both remain weak under the considered conditions.
Direct analytical estimate of the error is derived and verified numerically. It
can be applied to a priory justification of the unidirectional approach.
