Beyond 200-Gb/s/λ DMT Signal Transmission With NGMI Optimization and Volterra Equalization

The band-limited and power fading effects are two main limitations to intensity-modulation direct-detection (IM-DD) transmission systems due to the low-cost transceivers and interactions from fiber dispersion. Entropy loading (EL) has the ability to squeeze out the last bit by continuous adjustment of entropy. However, it is a passively process to adapt to the channel response. Moreover, the normalized generalized mutual information (NGMI) target based EL may be overestimated because the practical binary forward error correction (FEC) codes have a gap to the theoretic limit, which may result in unsatisfied transmission capacity or post-FEC performance. To optimize the NGMI, we analyze the NGMI upper and lower boundaries defined by the achievable information rate (AIR) and FEC overhead (OH). We propose to use the AIR as target and optimize the NGMI with the FEC OH as constraint. We conduct an experiment to target the beyond 200-Gb/s discrete multi-tone signal transmission over 2-km or 10-km single mode fiber using a 32-GHz electro-absorption modulated laser (EML). However, we find that the channel response and generalized mutual information (GMI) cannot support the target AIR even with NGMI optimization. We propose a joint EL and second-order Volterra filter (VF) to alleviate the power fading effects and maximize the transmission capacity. To achieve the AIR target and use below 20% SD-FEC, we find the optimized NGMI is 0.92 to simultaneously meet the 2-km and 10-km transmission requirement. We successfully transmit beyond 200-Gb/s DMT signal over 2-km or 10-km single mode fiber with BER below 2.6 × 10⁻² in the EML-based direct-detection system.