Coding techniques to mitigate dispersion-induced ISI in optical data transmission

The authors present line-coding methods that reduce the effects of dispersion-induced intersymbol interference (ISI) in high-speed (4 to 8 Gb/s) data transmission over direct-detection single-mode optical fibers. In addition to their use for synchronization purposes, line codes can be designed to eliminate certain worst-case patterns that would otherwise preclude reliable data transmission at very high rates. The authors study the use of these codes with directly modulated 1.55 μm wavelength DFB (distributed feedback) lasers using conventional fiber (dispersion minimum at 1.3 μm). Performance is quantified in terms of eye opening at the same information rate (which does not usually correspond to the same clock rate due to code overhead), normalized by any increased noise bandwidth. Examples of simple line codes (with symbol-by-symbol detection and feedback-free decoders) that significantly improve eye opening and thereby increase noise margins are given. The two examples presented at 4 Gb/s and 8 Gb/s realize respective coding gains of 1.65 dB and 5 dB over uncoded systems operating at the same data (information) rate