Wavelength encoding to reduce four-wave mixing crosstalk in multi-wavelength channels

The trend toward higher bit rates in lightwave communications has increased interest in dispersion-shifted fiber (DSF) to minimize dispersion penalties. In addition, wavelength-division multiplexing (WDM) is being used to gain even greater capacity, and broad bandwidth erbium-doped fiber amplifiers are extending link distances between signal regeneration. Taken together, these techniques can result in severe performance degradation due to four-wave mixing (FWM). Several different ways of reducing the FWM degradation in WDM systems have been proposed, but all of the approaches increase system complexity and make it difficult to add channels to the system. We show that wavelength-coded multiplexing can provide multiple channels on a simple fiber link while significantly reducing four-wave mixing interference compared to a WDM system. The FWM spectrum is symmetric and use of an antisymmetric code cancels the interference. Wavelength-coded multiplexing is also known as spectral code-division multiplexing (SCDM). We have calculated the error probability in the SCDM system considering only shot noise, thermal noise and FWM crosstalk, all of which are assumed to have a Gaussian distribution. For comparison, we also derived an expression for the error probability in a WDM system.