High performance 140 Mbit/s FSK coherent system

A 140 Mbit/s FSK heterodyne coherent transmission system is described having automatic frequency and polarisation acquisition and tracking. A receiver sensitivity of −57.4 dBm (88 photons/bit), using 7B8B block code, and −58.2 dBm (84 photons/bit), using biphase coding, has been obtained, only 3.4 and 3.2 dB away from the shot noise limit, respectively.