Polarisation independent detection by synchronous intra-bit polarization switching in optical coherent systems

A novel approach to polarization diversity transmission is discussed in connection with achieving polarization-independent coherent detection with minimum IF bandwidth, as required in high-bit-rate optical systems. This is obtained by stepping the polarization of the field between two orthogonal states at the same frequency as the bit rate. This approach is promising, avoiding complicated receivers, and particularly suitable for a distribution network where switching at the transmitter can be shared among several receivers, reducing system complexity. Computer simulations have been performed for both amplitude- and frequency-shift keying. Critical system parameters are examined and excess penalty is evaluated. In addition to square-wave modulation, sinusoidal modulation has been considered. An optimum value of the modulation depth exists for which the penalty is reduced with respect to the square-wave