  We consider the optical fiber channel modelled by the nonlinear
Shr\"{o}dinger equation with zero dispersion and additive Gaussian noise. Using
Feynman path-integral approach for the model we find corrections to conditional
probability density function, output signal distribution, conditional and
output signal entropies, and the channel capacity at large signal-to-noise
ratio. We demonstrate that the correction to the channel capacity is positive
for large signal power. Therefore, this correction increases the earlier
calculated capacity for a nondispersive nonlinear optical fiber channel in the
intermediate power region.
