Optical transmission technologies have gone through several generations of development. Spectral efficiency has significantly improved, and industry has begun to search for an answer to a basic question: What are the fundamental linear and nonlinear signal channel limitations of the Shannon theory when there is no compensation in an optical fiber transmission system? Next-generation technologies should exceed the 100G transmission capability of coherent systems in order to approach the Shannon limit. Spectral efficiency first needs to be improved before overall transmission capability can be improved. The means to improve spectral efficiency include more complex modulation formats and channel encoding/decoding algorithms, prefiltering with multisymbol detection, optical OFDM and Nyquist WDM multicarrier technologies, and nonlinearity compensation. With further optimization, these technologies will most likely be incorporated into beyond-100G optical transport systems to meet bandwidth demand.

In this work, we focus on enhancing the network reach in terabit superchannel transmission by using a noise-suppressed Nyquist wavelength division multiplexing (NS-N-WDM) technique for polarization multiplexing quadrature phase-shift keying (PM-QPSK) subchannels at different symbol-rate-to-subchannel-spacing ratios up to 1.28. For the first time, we experimentally compare the transmission reach of this emerging technique with that of no-guard-interval coherent optical orthogonal frequency-division multiplexing (NGI-CO-OFDM) on the same testbed. At BER of 2 × 10 ^-3 and 100 Gbit/s per channel, an NGI-CO-OFDM terabit superchannel can transmit over a maximum of 3200 km SMF-28 with EDFA-only amplification, and an NS-N-WDM terabit superchannel can transmit over a maximum of 2800 km SMF-28 with EDFA-only amplification. Assuming different coding gain, 11 × 112 Gbit/s per channel with hard-decision (HD) forward-error correction (FEC) and 11 × 128 Gbit/s per channel NS-N-WDM transmission with soft-decision (SD) FEC can be achieved over a maximum of 2100 km and 2170 km, respectively. These are almost equal and were achieved using digital noise filtering and one-bit maximum likelihood sequence estimation (MLSE) at the receiver DSP. Characteristics including the back-to-back (BTB) curves, the ADC bandwidth requirement, and the tolerance to unequal subchannel power of an NS-N-WDM superchannel were also evaluated.

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.