Space-time codes for fiber communications: Coding gain and experimental validation

Dual polarization coherent systems are considered to be an attractive solution for optical fiber transmissions beyond 100Gb/s. Polarization multiplexing (PolMux) doubles the channel capacity and the spectral efficiency by transmitting two signals on the orthogonal polarization states. However optical polarization-dependent impairments, such as polarization mode dispersion (PMD) or polarization dependent loss (PDL) in optical fibers, may induce severe performances degradation. As PolMux systems can be seen as 2×2 MIMO channels, Space-Time codes can be used to enhance the transmission performance. The performances of the Golden code and the Silver code, which are the two best Space-Time codes in wireless communications, are evaluated in the case of optical PolMux OFDM systems by numerical simulations and validated by experiments. We show that Space-Time coding are inefficient against dispersion effects such as PMD but can dramatically mitigate PDL effects. Moreover, the obtained results show that, unlike in wireless communications, the Silver code outperforms the Golden code.