A practical high performance 140 Mbit/s FSK coherent system

The use of frequency shift keying (FSK) modulation format reduces considerably the requirements on the performance of optical sources and leads to simple system implementation. Several laboratories have reported systems operating close to the quantum limit. The authors describe the design, implementation and evaluation of a practical 140 Mbit/s FSK heterodyne coherent transmission system with a balanced dual channel receiver, automatic carrier acquisition, tracking and endless polarisation control. The carrier acquisition, tracking and polarisation control are controlled by a built-in microprocessor giving the system flexibility and the ability to monitor the operation of these subsystems. A sensitivity of -57.4 dBm has been obtained, only 3.4 dB away from the quantum limit. System stability has been demonstrated over a period of 250 hours at an optical power level below sensitivity and an error floor shown to be lower than BER=3.5*10/sup -14/. This system therefore fulfils the requirements for transmission systems with span lengths on the order of 250 km. This enables previously unexploited markets for unrepeated transmission links to be addressed, opening a new era in the history of long haul communications.