This paper presents the use of space-time coding techniques to fully exploit polarization diversity in coherent optical systems without the need for polarization tracking. Our approach is analogous to a situation of two transmit antennas and two receive antennas in a wireless communication system. Utilizing the two polarizations intelligently, the highly effective technique of orthogonal space-time block coding is applied to combat polarization uncertainty and signal losses due to free-space non-idealities. The use of space-time block coding and differential space-time block coding are modeled and tested successfully. Significant performance gain is demonstrated by our new coherent optical multiple-input multiple-output (COMIMO) system over many traditional coherent optical communication techniques.
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