km Transmission of 100 GHz Spaced , 8 × 495 - Gb / s PDM Time - Domain Hybrid QPSK - 8 QAM Signals

Employing time-domain hybrid QPSK-8QAM and training-assisted phase recovery, we successfully transmitted eight 495-Gb/s DWDM signals at net 4.125 bit/s/Hz spectral efficiency (SE) over 120×100km of ultra-large-area fiber, achieving a record SE⋅distance product of 49500 (bit/s/Hz)⋅km. 1. Introduction In response to the continuing bandwidth demand growth at 30 to 40% year-over-year, carriers are seeking spectrally-efficient systems to lower their cost-per-transmitted bit. Recently the IEEE announced formation of an 802.3 group to reach consensus on whether the next Ethernet standard should be at 400 Gbit/sec or 1 Terabit/sec. With 400Gb/s appearing increasingly likely as the next-generation transport standard, several experimental demonstrations of long-haul transmission of 400Gb/s per channel systems [1-9] have been reported over the past three years, and several of these demonstrations have utilized unconventional channel spacings to maximize SE, in anticipation of flex-grid ROADMs [1-3]. However, the ability to place 400Gb/s signals on the conventional 50GHz or 100GHz channel grid has advantages in terms of network management, and may be most cost-effective, as it could allow increasing the network capacity through upgrade of current ITU-T grid-based optical networks to 400Gb/s per channel. Recently, we demonstrated WDM transmission over >3000km at 400Gb/s with 50GHz spacing [9]. That result utilized time-domain hybrid QAM [9,10], which for a given channel bit rate allows tuning of the modulation SE (bit/symbol) to match the allowed channel bandwidth and satisfy the required transmission reach. In this paper we demonstrate the application of time-domain hybrid QAM to 400-Gb/s channels on the 100GHz grid for ultra-long-haul reach. We report the successful transmission of 8×495Gbits/s PDM QPSK-8QAM DWDM signals over 120×100km of ultra-large-area (ULA) fiber at a net SE of 4.125bit/s/Hz (after excluding the 20% soft-decision FEC overhead). Novel, improved carrier-phase recovery and equalization algorithms and time-domain hybrid QAM were key enablers. Compared to our 3200km transmission of 494Gb/s hybrid 32-64QAM signals [9], we gain 7.9dB in OSNR sensitivity due to QPSK-8QAM and ∼1dB due to the larger effective area (150 um 2) and lower loss of the ULA fiber. Thus 12,000km transmission with 1.5dB Q margin is consistent with prior results, and the SE⋅distance product of 49500 (bit/s/Hz)⋅km represents, to our knowledge, a new record for terrestrial and submarine system demonstrations.