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Results obtained using a fiber optical star network using densely-spaced frequency-division multiplexing (FDM) and heterodyne detection techniques are presented. The system consists of three optical sources frequency-shift keyed at 45 Mb/s and spaced by 300 MHz. Frequency selection of the desired channel is done by a heterodyne FM receiver at a bit-error rate of 10/sup -9/ is -61 dBm or 113 photons/b, which is 4.5 dB from the shot-noise limit. Cochannel interference is found negligible for this channel spacing and modulation rate. Frequency stabilization of the FDM signals to a comb of equally-spaced frequencies, imperative in such a densely-spaced FDM system, is also demonstrated. The results indicate that this system can provide a throughput of 4500 Gb/s. >

The authors discuss results of research aimed at understanding the use of monolithic tunable lasers and fiber Fabry-Perot filters in implementing an optical cross-connect switch. The switch architecture uses optical frequency division multiplexing where laser signals are transmitted at different frequencies and then summed in a star coupler. At each star coupler output, a tunable receiver is used to retrieve any input signal of choice. The experimental receiver consists of an optical tunable filter followed by a direct-detection receiver. The laboratory demonstration consists of four frequency-locked transmitters with monolithic tunable lasers, each dithered with a slightly different frequency for the dual purpose of frequency locking and positive channel identification upon signal reception. The tunable filter is a tunable two-stage fiber Fabry-Perot filter design consisting of a narrow filter followed by a wide filter. The tuning of the filters is computer controlled, and the combined filter has a tuning range of 15000 GHz with a finesse approximately=5170. Therefore, it is capable of covering over 1000 channels of 2.9-GHz each. Receiver sensitivity was measured to be -30 dBm at 1.7 Gb/s and -29 dBm at 2.5 Gb/s (bit error rate=10/sup -10/). >