In this paper, we investigate advanced digital signal processing (DSP) at the transmitter and receiver side for signal preequalization and post-equalization in order to improve spectrum efficiency (SE) and transmission distance in an optical access network. A novel DSP scheme for this optical superNyquist filtering 9 Quadrature Amplitude Modulation (9 QAM) like signals based on multi-modulus equalization without post filtering is proposed. This scheme recovers the Nyquist filtered Quadrature Phase-Shift Keying (QPSK) signal to a 9-QAM-like one. With this technique, SE can be increased to 4 b/s/Hz for QPSK signals. A novel digital super-Nyquist signal generation scheme is also proposed to further suppress the Nyquist signal bandwidth and reduce channel crosstalk without the need for optical pre-filtering. Only optical couplers are needed for super-Nyquist wavelength-division-multiplexing (WDM) channel multiplexing. We extend the DSP for short-haul optical transmission networks by using high-order QAMs. We propose a high-speed Carrierless Amplitude/Phase64 QAM (CAP-64 QAM) system using directly modulated laser (DML) based on direct detection and digital equalization. Decision-directed least mean square is used to equalize the CAP-64QAM. Using this scheme, we generate and transmit up to 60 Gbit/s CAP-64QAM over 20 km standard singlemode fiber based on the DML and direct detection. Finally, several key problems are solved for real time orthogonal-frequency-division-multiplexing (OFDM) signal transmission and processing. With coherent detection, up to 100 Gbit/s 16 QAM-OFDM real-time transmission is possible.

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.

This is the second part of a special issue on “100G and Beyond: Trends in Ultrahigh-speed Communications.”The first part of this special issue contained nine fpapers written by service providers, telecommunications equipment manufacturers, and top universities and research institutes. This special issue includes comprehensive reviews as well as original technical contributions covering the rapid advances and broad scope of ultrahigh-speed technologies in optical fiber communications. All papers in this issue have been invited. After peer review, five papers were selected to be published. We hope this issue serves as a timely and high-quality networking forum for scientists and engineers.

The first paper,“FSK Modulation Scheme for High-Speed Optical Transmission,”by Nan Chi et al. from Fudan University, describes the generation, detection, and performance of frequency-shift keying (FSK) for high-speed optical transmission and label switching.

The second paper,“Computationally Efficient Nonlinearity Compensation for Coherent Fiber-Optic System,”by Li et al. from the University of Central Florida, describes how split-step digital backward propagation (DBP) can be combined with coherent detection to compensation for fiber nonlinear impairments.

The third paper,“Flipped-Exponential Nyquist Pulse Technique to Optimize the PAPR in Optical Direct Detection OFDM System,”by Xiao et al. from Hunan University, describes the use of advanced coding to reduce peak-to-average power ratio of the OFDM signal and extend the transmission distance.

The fourth paper,“100Gb/s Nyquist-WDM PDM-16QAM Transmission over 1200-km SMF-28 with Ultrahigh Spectrum Efficiency,”by Dong et al. from ZTE USA, describes the use of pre- and post-equalization to improve transmission system performance and realize ultrahigh spectrum efficiency.

The fifth paper,“Field Transmission of 100G and Beyond: Multiple Baud Rates and Mixed Line Rates Using Nyquist-WDM Technology,”by Jia et al. from ZTE USA, describes a field trial experiment of mixed 100G, 400G, and 1 Tbit/s signal transmission. Joint experiments between ZTE and Deutsche Telecom (DT) have been conducted on long-haul transmission of 100G and beyond over standard single-mode fiber (SSMF) and inline EDFA-only amplification.

We would like to thank all authors for their valuable contributions and all the reviewers for their timely and constructive feedback on all submitted papers. We hope that the contents of this issue are informative and useful for all readers.

In this paper, we evaluate transmission in a 1 Tb/s (10 × 112 Gb/s) Nyquist-WDM PM-RZ-QPSK superchannel over a widely-deployed SMF-28 fiber with and without maximum a-posteriori (MAP) equalization. Over 1000 km can be reached with BER below the HD FEC limit and with a spectral efficiency of 4 b/s/Hz.

Fiber optics underpins the communication infrastructure of today’s information society. Rapid progress in advanced modulation formats, high-gain coding, optical amplification, coherent detection with digital signal processing, and new types of transmission fibers have significantly affected optical communications. Increasing transmission capacity and bit rate per channel is the trend for optical transmission systems and networks. Commercial transmission capacity has increased more than one hundred thousand times since the first optical transmission system was deployed in the 1980s. Spectral efficiency of a single channel has increased from 0.025 b/s/Hz to 2 b/s/Hz. Bit rate per channel for commercial products has increased from 155 Mb/s to more than 100 Gb/s. Larger capacity is driven by the proliferation of broadband FTTH access networks, broadband wireless communications, and high-speed data communication systems in data centers and high performance computing. High bit rate per channel simplifies the management of complex optical networks. Although 100G is just the beginning of the commercial manufacturing and deployment stage, major optical networking research groups have been focusing on standards and technologies beyond 100G. The challenge of generating 400 Gb/s and 1 Tb/s per channel and transmitting at these speeds is one of the hottest topics in recent conferences on optical communications. Many forward-looking solutions have been proposed, and experiments have been carried out to achieve these high bit rates.

Globally, many research groups have been developing novel enabling technologies for meeting the requirements of high capacity and high bit rate operation using spectrally efficient multiplexing and modulation formats. These advanced techniques include single-carrier polarization multiplexing QPSK (which is currently used in 100G commercial products), multicarrier optical orthogonal frequency division multiplexing, multicore or multimode spatial multiplexing, and coherent detection based on digital signal processing. For high-speed optical signal transmission, a traditional transponder with direct modulation and detection has a simple, low-cost architecture. However, the transmission distance at high bit rate is limited by the rigid requirements of high optical signal-to-noise ratio, polarization mode dispersion, and optical/electrical filtering effects. Coherent detection based on digital signal processing is becoming the trend for optical signal receivers because it can lift these limitations. The change from direct detection to coherent detection is revolutionary. Receiver architecture, transmission fiber and distance, and network management will be completely reshaped from previous direct-detection systems.

This special issue includes comprehensive reviews and original technical contributions that cover the rapid advances and broad scope of technologies in optical fiber communications. The invited papers of Part I of this issue come from service providers, telecommunication equipment manufacturers, and top universities and research institutes. After peer review, eleven papers were selected for this special issue. We hope it serves as a timely and high-quality networking forum for scientists and engineers.

The first two papers come from service providers. In the first paper,“High Spectral Efficiency 400G Transmission,”Dr. Xiang Zhou from AT&T labs gives an overview of the generation and transmission of 450 Gb/s wavelength-division multiplexed channels over the standard 50 GHz ITU-T grid at a net spectral efficiency of 8.4 b/s/Hz. In the second paper, “Direct-Detection Optical OFDM Superchannel for Transmitting at Greater Than 200 Gb/s,”Dr. Peng Wei Ren et al. from KDD&I propose and experimentally demonstrate a direct-detection optical orthogonal-frequency-division-multiplexing (OFDM) superchannel and optical multiband receiving method to support a data rate higher than 200 Gb/s and to support longer distance for direct-detection systems.

Papers 3-9 come from universities that are renowned for research on optical transmission. In the third paper,“Spatial Mode Division Multiplexing for High-Speed Optical Coherent Detection Systems,”Professor William Shieh from the University of Melbourne proposes using spatial mode division multiplexing to increase transmission capacity. In the fourth paper,“Exploiting the Faster-Than-Nyquist Concept in Wavelength-Division Multiplexing Systems by Duobinary Shaping,”Dr. Jianqiang Li from Chalmers University of Technology presents a novel algorithm at the coherent receiver that is based on digital signal processing and is designed to tolerate strong filtering effects. In the fifth paper, “Super Receiver Design for Superchannel-Coherent Optical Systems,”Dr. Cheng Liu from Georgia Institute of Technology presents a novel super-receiver architecture for Nyquist-WDM superchannel coherent systems. This receiver detects and demodulates multiple WDM channels simultaneously and performs better than conventional coherent receivers in Nyquist-WDM systems. In the sixth paper,“Design of Silicon-Based High-Speed Plasmonic Modulator,”Professor Yikai Su from Shanghai Jiao Tong University proposes a silicon-based high-speed plasmonic modulator. This modulator is based on a double-layer structure with a 16 um long metal-dielectric-metal plasmonic waveguide at the upper layer and two silicon single-mode waveguides at the bottom layer. In the seventh paper,“Key Technology in Optical OFDM-PON,”Professor Xiangjun Xin from Beijing University of Posts and Telecommunications proposes a novel optical access network based on OFDM. In the eighth paper,“Compensation of Nonlinear Effects in Coherent Detection Optical Transmission Systems,”Professor Fan Zhang from Beijing University reviews two kinds of nonlinear compensation methods: digital backward propagation, and nonlinear electrical equalizer based on the time-domain Volterra series. The last paper comes from a telecommunication equipment manufacturer. In“Performance Assessment of 1 Tb/s Nyquist-WDMPM-RZ-QPSK Superchannel Transmission over 1000 km SMF-28 with MAP Equalization,”Dr. Ze Dong from ZTE (USA) evaluates the transmission performance of a 1 Tb/s (10 × 112 Gb/s) Nyquist-WDM PM-RZ-QPSK superchannel over a widely deployed SMF-28 fiber with and without MAP equalization.

We thank all authors for their valuable contributions and all reviewers for their timely and constructive feedback on submitted papers. We hope the contents of this issue are informative and useful for all readers.