In this paper,we describe successful joint experiments with Deutsche Telecom on long-haul transmission of 100G and beyond over standard single mode fiber (SSMF) and with in-line EDFA-only amplification.The transmission link consists of 8 nodes and 950 km installed SSMF in DT's optical infrastructure.Laboratory SSMF was added for extended optical reach.The first field experiment involved transmission of 8 × 216.8 Gbit/s Nyquist-WDM signals over 1750 km with 21.6 dB average loss per span.Each channel,modulated by a 54.2 Gbaud PDM-CSRZ-QPSK signal,is on a 50 GHz grid,which produces a net spectral efficiency (SE) of 4 bit/s/Hz.We also describe mixed-data-rate transmission coexisting with 1T,400G,and 100G channels.The 400G channel uses four independent subcarriers modulated by 28 Gbaud PDM-QPSK signals.This yields a net SE of 4 bit/s/Hz,and 13 optically generated subcarriers from a single optical source are used in the 1T channel with 25 Gbaud PDM-QPSK modulation.The 100G signal uses real-time coherent PDM-QPSK transponder with 15% overhead of soft-decision forward-error correction (SD-FEC).The digital post filter and 1-bit maximum-likelihood sequence estimation (MLSE) are introduced at the receiver DSP to suppress noise,linear crosstalk,and filtering effects.Our results show that future 400G and 1T channels that use Nyquist WDM can transmit over long-haul distances with higher SE and using the same QPSK format.
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