Hybrid integration of FEC and channel equalization for ultra-high bandwidth efficient coherent optical interconnection

To fully address the inter-symbol interference (ISI) for ultra-high bandwidth efficiency (BE) application, a hybrid integrated forward error correction (FEC) equalization (IFE) scheme is proposed and experimentally demonstrated in a coherent optical interconnection system. The estimation for the channel response affected with severe ISI is realized by an adaptive linear minimum mean square error (LMMSE) equalizer, assisted with soft-input soft-output (SISO) a posteriori information by low density parity check (LDPC) decoder, allowing simultaneously accurate ISI deduction and reliable FEC decoding. The proof-of-concept experiments are investigated in a 60 GBaud polarization division multiplexed 16 quadrature amplitude modulation system within a 5-GHz ultra-narrow bandwidth receiver. The results show that the proposed IFE scheme successfully obtains more than 3-dB optical signal-to-noise ratio (OSNR) gain compared with the serial, independent structure of the equalizer and the FEC decoder at the BER threshold of 1 × 10⁻². The validity comparison of ISI reduction ability between traditional soft-decision maximum likelihood sequence estimation (MLSE) and the proposed IFE scheme has been experimentally investigated, with the proposed scheme demonstrating greater robustness.