Direct detection of closely spaced optical FM-FDM Gb/s microwave PSK signals

A modulation scheme combining microwave phase-shift-keyed (PSK) and optical frequency modulation (FM) techniques is used in a two-channel optical frequency-division-multiplexed (FDM) system. Optical filtering and FM demodulation are carried out by a tunable fiber Fabry-Perot etalon. A transmission rate of 2 Gb/s and a channel spacing of less than 30 GHz are achieved with a crosstalk level of about -25 dB. This work shows that the modulation technique of combining optical FM/FDM and RF/microwave subcarrier multiplexing is transparent to either digital or analog signals. The modulation technique not only avoids the low-frequency nonuniform FM response problem, but also facilitates close optical channel spacing.<
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