Optical Fiber Repeatered Transmission Systems Utilizing SAW Filters

Optical fiber systems under development worldwide promise economical transmission of long-haul digital traffic at bit rates of 100 to 1000 Mbit/s and more. Such systems employ a succession of long fiber spans followed by regenerators, which restore the magnitude, shape, and timing of message pulses after degradation in the transmission spans. The regeneration process is controlled by a timing wave that is extracted from the pulse train itself. For the task of timing extraction at the high bit rates of interest, SAW filters comprise the most mature technology currently available. We will show how the major system constraints, in combination with the limitations of SAW technology, tend to define the required filter characteristics within fairly narrow limits. The comparative merits of different types of SAW filters for retiming will be discussed, with attention to both the literature and new results. A comparison will also be made between SAW retiming and the alternative phaselocked-loop technology, which is in common use at somewhat lower bit rates.