Electrical signal processing techniques in long-haul, fiber-optic systems

Intersymbol interference in long-haul fiber-optic systems can severely degrade performance and consequently limit both the maximum distance and data rate. The sources of intersymbol interference include nonlinearity in the transmit laser, chromatic dispersion in systems operated at wavelengths other than the dispersion minimum of the fiber, polarization dispersion, and bandwidth limitations in the receiver. The authors discuss several techniques for reducing intersymbol interference in single-mode fiber systems with single-frequency lasers and show which techniques are appropriate at high data rates in direct and coherent detection systems. In particular, the performance of linear equalization, nonlinear cancellation, and maximum likelihood detection is analyzed. Computer simulation results for an 8 Gb/s system using measured laser and receiver characteristics showed that in direct detection systems, a 6-tap linear equalizer can reduce the penalty due to chromatic and polarization dispersion by more than 1 dB (or increase the dispersion-limited distance by more than 20%). Nonlinear cancellation and maximum likelihood detection can reduce the penalty even further, more than doubling the dispersion-limited distance in some cases