100Gand Beyond:Trends in Ultrahigh-Speed Communications(Part I)

Gee-Kung Chang

Jianjun Yu

F iber 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 channelfor 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 spatialmultiplexing,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 digitalsignal 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 willbe completely reshaped from previous direct-detection systems.

This special issue includes comprehensive reviews and original technicalcontributions that cover the rapid advances and broad scope of technologies in opticalfiber communications.The invited papers of Part Iof 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&Tlabs gives an overview of the generation and transmission of 450 Gb/s wavelength-division multiplexed channels over the standard 50 GHz ITU-Tgrid 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&Ipropose 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 Lifrom 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 noveloptical 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 MAPequalization.

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.