512-Gb/s quad-carrier PM-QPSK transmission over 2400-km SMF-28 subject to narrowing 100-GHz optical bandwidth

For the first time, a long-reach 400G transmission proposal based on quad-carrier PM-QPSK with a net spectral efficiency of 4 b/s/Hz was experimentally demonstrated. Under the assumption of narrowing100-GHz bandwidth after passing through multiple optical nodes, 512-Gb/s quad-carrier PM-QPSK signals were successfully delivered over 2400-km SMF-28 with EDFA-only amplification, and the worst-case BER received at edge channel was small than the soft-decision pre-FEC limit of 2×10-2.