Comparison of Equalizing and Nonequalizing Repeaters for Optical Fiber Systems

This paper is concerned with the effects of intersymbol interference on the performance of equalizing and nonequalizing optical fiber system repeaters. Recent advances in the fabrication of graded-index optical fibers and improvements in laser lifetime should result in the ability to design fiber systems that are essentially loss-limited for data rates below 100 M b / s . Nevertheless, it is interesting to study the effects of small amounts of intersymbol interference due to fiber delay distortion on the performance of optical fiber system repeaters.* Repeaters which can accommodate small amounts of intersymbol interference increase the yield of usable fibers in real cables where the index gradings are not ideal. In a previous paper, 1 the effects of intersymbol interference on equalizing repeaters were studied. Such repeaters compensate for the fiber delay distortion (pulse spreading) by incorporating a highfrequency rollup filter in their linear channels. This rollup results in an enhancement of the receiver noise. Using the results of that paper, curves can be plotted of receiver sensitivity loss vs the spreading of the received optical pulses. * Throughout this paper we shall assume that optical power pulses that overlap at the receiver add linearly, as justified in Ref. 3. 957