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Nonlinear effects has been appointed as the main limitation in coherent optical fiber transmission. Digital Back-Propagation algorithms and Maximum Likelihood Sequence Estimation are two of the current studied methods to cope with such impairment and extend the systems maximum reach. In this article, we analyzed both methods in a 112 Gb/s Dual Polarization Quadrature Phase Shifting Keying (DP-QPSK) optical coherent system through simulations and experimental results. In order to reduce the huge required computational complexity, a modified back-propagation algorithm is also analyzed.

We propose and experimentally demonstrate a hardware-efficient, feed-forward, wide-range frequency offset estimator for DSP-based optical coherent receivers. Using a simple relationship of signal spectrum, this estimator is capable to estimate offsets in a range compliant with OIF requirements. Obtained results show that this estimator has a high tolerance to spectrum asymmetry caused by electrical and optical signal filtering, even when using return-to-zero pulse shaping.

Soft decision driven joint carrier synchronization and signal detection, employing expectation maximization, is experimentally demonstrated. Employing soft decision offers an improvement of 0.5 dB compared to hard decision based digital PLL carrier synchronization and demodulation