Cross polarization interference cancellation for coherent fiber optics

Coherent optical communication systems offer advantages over IM-DD systems such as higher receiver sensitivity and more efficient use of the optical channel bandwidth. One disadvantage of coherent systems is the need to address the polarization sensitivity of the coherent optical receiver. This receiver can only detect the component of the signal with the same polarization as the optical local oscillator, and the orthogonal component is lost. On the other hand, this property can be used to increase the system capacity by allowing the transmission of independent signals with orthogonal polarization. A receiver can reconstruct both signals as long as they remain orthogonally polarized by tracking the polarization of each. Since orthogonality is lost to some extent during propagation through optical fiber, a polarization tracking system would incur a signal crosstalk penalty. A new and practical method using cross polarization interference cancellation is proposed to correct for the loss of orthogonality incurred during propagation between two optical signals transmitted with orthogonal polarization states. The system optimally reconstructs both signals at the receiver and eliminates the need for polarization tracking.; A new result for the bit error rate of DPSK using delay demodulation is also derived. Previous work on the BER performance of DPSK has shown that delay demodulators with narrowband IF bandpass filters and sampling detectors have superior performance than those with wideband IF bandpass filters and integrating detectors in the presence of AWGN alone. This has not been shown to be true when phase noise effects are included. Phase noise is an important consideration in coherent optical communication systems and the most widely accepted model is a Wiener phase noise process. A closed form BER expression along with detailed monte-carlo simulation results are presented for the DPSK delay demodulator with wideband IF bandpass filtering and integration detection filtering including phase noise effects using the Wiener process model. It is found that this receiver has noise performance comparable to receivers with narrowband IF bandpass filtering and sampling detectors for SNR and phase noise in the range of practical interest but with potentially less degradation due to ISI.