224-Gb/s Carrier-Recovery-Free Doubly Differential 2ASK-8PSK for Short-Reach Optical Networks

We propose and experimentally demonstrate a carrier-recovery-free 224-Gb/s dual-polarization doubly differential (DD) two-amplitude/eight-phase shift keyed (2ASK-8PSK) signal for 100-km fiber transmission with coherent detection. An 11-tap multi-symbol DD (MSDD) decoding scheme helps to reduce the penalty caused by two differential operations in conventional DD decoding, allowing an optical signal-to-noise-ratio improvement of 3.7 dB for DD quadrature phase shift keying and 4.3 dB for DD 16 quadrature amplitude modulation (QAM). By employing such decoding, a frequency offset (FO) tolerance of 16 GHz has been achieved in a DD 2ASK-8PSK system for a bit-error-rate of $\sim 1\times 10^{-3}$ . Compared with a 224-Gb/s 16QAM system employing conventional carrier recovery algorithms, the proposed system is more robust to FOs, and the FO tolerance range is only limited by the effective receiver bandwidth.