Demonstration of polarisation independent coherent transmission by synchronous intra-bit polarisation spreading

Polarization-independent heterodyne detection of an FSK (frequency shift keying) signal has been demonstrated by the synchronous intrabit polarization spreading technique, which consists in the switching or proper modulation of the state of polarization of the laser output field inside the bit period. This approach is particularly suitable for distribution networks, as the polarization diversity function is shifted to the transmitter, thus resulting in a simpler and cheaper heterodyne receiver. A transmission experiment, making use of a 1.55 μm DFB (distributed feedback) laser diode, directly FSK modulated at 155 Mb/s, a properly developed high-speed polarization modulator, and single filter heterodyne detection, has been successfully carried out. The penalty resulting from preliminary bit-error-rate measurements ranges from 4 to 6.5 dB, including the 3 dB intrinsic loss common to all polarization spreading techniques