Limiting repeater separation of digital and optical transmission systems at high bit rates

The optical channel consists of lasers, glass fibres, avalanche photo-diodes, pre-amplifiers, etc; it is assumed that the light signal on entry to the fibre approximates to independent square impulses, that the transmission characteristics of the fibres are approximately those of a Gaussian low-pass filter, and that the transmission property of the receiver is Gaussian. At 1 Gbit/s, it is calculated that for very wide band filter fibres, without quantised feed-back, the binary transmission signal is the optimum. For narrow band fibres, this and the multi-step system are equivalent in performance.