One solution to the polarization matching problem in coherent FSK receivers is data-induced polarization switching.1 A highly biréfringent device causes the transmitted polarization to change between orthogonal states when the frequency is shifted according to the data pattern. A very highmodulation index of 20 and a dual-filter receiver have been used previously to transmit a 50-Mbit/s signal to a dual-filter FSK receiver.1 We avoid the difficulty of building low-distortion filters and use, for the first time, we believe, a delay-line demodulator in conjunction with this polarization-switching technique. Our modulation index of only 2.6 allows much more efficient use of the receiver bandwidth. Finally, we achieve the necessary birefringence with fiber only. A 500-Mbit/s signal is thus transmitted over 153 km, with only 2-dB sensitivity variation due to polarization changes.

© 1989 Optical Society of America

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