Cross-phase modulation in fiber links with multiple optical amplifiers and dispersion compensators

We have theoretically and experimentally investigated the cross-phase modulation (XPM) effect in optical fiber links with multiple optical amplifiers and dispersion compensators. Our theory suggests that the XPM effect can be modeled as a phase modulator with inputs from the intensity of copropagating waves. The frequency response of the phase modulator corresponding to each copropagating wave depends on fiber dispersion, wavelength separation, and fiber length. The total XPM-induced phase shift is the integral of the phase shift contributions from all frequency components of copropagating waves. In nondispersive fibers, XPM is frequency-independent; in dispersive fibers, XPM's frequency response is approximately inversely proportional to the product of frequency, fiber dispersion, and wavelength separation. In an N-segment amplified link, the frequency response of XPM is increased N-fold, but only in very narrow frequency bands. In most other frequency bands, the amount of increase is limited and almost independent of N. However, in an N-segment amplified link with dispersion compensators, the frequency response of XPM is increased N-fold at all frequencies if the dispersion is compensated for within each fiber segment. Thus, the XPM-induced phase shift is smaller in systems employing lumped dispersion compensation than in systems employing distributed dispersion compensation.