Experimental Demonstration of Differential Coding Gain for Dual Polarization Two-Eigenvalue Signals

We experimentally investigated differential precoding for dual polarization signals with two eigenvalues for a fiber link with Raman amplification, using QPSK and 8-PSK modulation schemes with symbol rates of 1 and 2 GBd. By using differential precoding in nonlinear domain for higher-order modulation schemes, we reduced the phase variance of the eigenvalues’ spectral amplitude by a factor of up to 5.4 and 7.4 for 1 and 2 GBd respectively, while LMMSE equalization provides a significant benefit for lower-order modulation schemes. With differential precoding we achieved bit error rates below SD-FEC limit at 8 Gbps for up to 5030 km link length. For 20 Gbps the range could be doubled to 1609 km. Adding LMMSE equalization to the proposed scheme also offers a tolerance to launch powers below the theoretical optimum for correct detection of the differential phase.