High-performance dense FDM coherent optical network

The results obtained with a densely spaced frequency division multiplexing (FDM) optical star network providing random channel selection by a digitally tuned high-sensitivity heterodyne receiver are presented. The experimental system consists of six 200-Mb/s frequency-shift keyed (FSK) modulated channels, spaced by 2.2 GHz and multiplexed by a star coupler. Channel selection is performed by a computer-controlled random-access heterodyne receiver having a sensitivity of 74 photons/b at a BER of 10^-9, which is 1.7 dB from the quantum limit. The digital-tuning random-access capability (which depends on the frequency-tuning current relationship of the local oscillator (LO) laser) is protected against frequency drifts of the LO laser. The receiver can also detect the absence of a selected channel. In this case, the receiver locks to the next available channel and displays which channel is received instead. The results obtained indicate that this system has potential throughput of 2000 Gb/s