ELECTRICAL SIGNAL-PROCESSING TECHNIQUES IN LONG-HAUL FIBEROPTIC SYSTEMS

The purpose of this paper is to demonstrate the potential for electrical signal processing to mitigate the effect of intersymbol interference 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 laser transmitter, chromatic dispersion in systems operated at wavelengths other than the dispersion minimum of the fiber, polarization dispersion, bandwidth limitations in the receiver, and mode partition fluctuations. It is expected that longer, repeaterless spans made feasible through the use of optical amplifiers will increase the need for the processing techniques described in this paper. In this paper we discuss several techniques for reducing intersymbol interference in single-mode fiber systems with single-frequency lasers, and we show which techniques are appropriate at high data rates in direct and coherent detection systems. In particular, we analyze the performance of linear equalization (tapped delay lines), nonlinear cancellation (variable threshold detection), maximum likelihood detection, coding, and multilevel signaling. Our results, for a simulated binary 8 Gbps system, show that simple techniques can be used to substantially reduce intersymbol interference, increasing system margin by several dB. In particular, a 6-tap linear equalizer increases the dispersion-limited distance (due to chromatic or polarization dispersion) by 20% (or reduces the optical power penalty by as much as a factor of two) in direct detection systems, even when the distortion is nonlinear. A novel, but simple, nonlinear cancellation technique (adjusting the decision threshold in the detector based on previously detected bits) can more than double the dispersion-limited distance and/or data rate. © 1990 IEEE