FSK/IM scheme for unrepeatered optical systems

A frequency shift keyed/intensity modulated (FSK/IM) optical transmitter is described which uses direct frequency shift keying of a laser and a Mach-Zehnder optical discriminator to produce a very low chirp intensity modulated source for high bit rate, high dispersion systems. The FSK/IM transmitter was demonstrated over a 200 km span of conventional nondispersion shifted fibre at 2.488 Gbit/s, with very low dispersion penalty, an error floor of better than 10^-15 BER, and a long term stability of better than 0.3 dB over 100 hours of operation. The performance and stability obtained make possible the introduction of repeaterless transmission over installed nondispersion shifted fibre links: system loss and dispersion requirements for spans of 200 km can be readily satisfied with sufficient margin for long term system viability