We present a new perturbative approach to the study of signal-noise
interactions in nonlinear optical fibers. The approach is based on the
hydrodynamic formulation of the nonlinear Schr\"odinger equation that governs
the propagation of light in the fiber. Our method is discussed in general and
is developed in more details for some special cases, namely the
small-dispersion regime, the continuous-wave (CW) signal and the solitonic
pulse. The accuracy of the approach is numerically tested in the CW case.Comment: Revised version, submitted to Optics express, 15 pages, 6 figure
