Nonlinear optical crosstalk in WDM CATV systems

The trend to deliver higher capacity targeted services in CATV systems has increased interest in WDM transmission. During the past year, numerous forward and return path WDM CATV systems have been deployed. In these multi-wavelength systems, fiber exhibits nonlinear effects such as cross-phase modulation (XPM), four-wave mixing (FWM), and stimulated Raman scattering (SRS). These nonlinear processes contribute to optical interference (i.e., crosstalk) among the multiple optical channels propagating along the same fiber. With the growing number of WDM channels it is desirable to transport these optical channels at an optimum fiber launch power while maintaining an acceptable level of carrier-to-noise ratio and nonlinear optical crosstalk. In this paper, we investigate the dependence of optical crosstalk on fiber group-velocity dispersion, fiber launch power, subcarrier modulation frequency, optical channel spacing, and optical channel number. It is determined that FWM in standard fiber can generate significant levels of optical crosstalk, and depending on the channel spacing, it can contribute larger crosstalk levels than SRS and XPM.