  We consider the optical fiber channel modelled by the nonlinear
Schr\"{o}dinger equation with additive white Gaussian noise and with large
signal-to-noise ratio. For the small dispersion case we present the approach to
analyze the stochastic nonlinear Schr\"{o}dinger equation. Taking into account
the averaging procedure (frequency filtering) of the output signal detector we
find the first corrections in small dispersion parameter to the correlators of
the input signal recovered by the backward propagation. These correlators are
the important ingredients for the calculation of the channel capacity and the
optimal input signal distribution. We assert that the information channel
characteristics essentially depend on the procedures of the output signal
filtering and the recovery of the transmitted signal.
