Client-side optics are facing an ever-increasing upgrading pace, driven by upcoming 5G related services and datacenter applications. The demand for a single lane data rate is soon approaching 200 Gbps. To meet such high-speed requirement, all segments of traditional intensity modulation direct detection (IM/DD) technologies are being challenged. The characteristics of electrical and optoelectronic components and the performance of modulation, coding, and digital signal processing (DSP) techniques are being stretched to their limits. In this context, we witnessed technological breakthroughs in several aspects, including development of broadband devices, novel modulation formats and coding, and high-performance DSP algorithms for the past few years. A great momentum has been accumulated to overcome the aforementioned challenges. In this article, we focus on IM/DD transmissions, and provide an overview of recent research and development efforts on key enabling technologies for 200 Gbps per lane and beyond. Our recent demonstrations of 200 Gbps short-reach transmissions with 4-level pulse amplitude modulation (PAM) and discrete multitone signals are also presented as examples to show the system requirements in terms of device characteristics and DSP performance. Apart from digital coherent technologies and advanced direct detection systems, such as Stokes–vector and Kramers–Kronig schemes, we expect high-speed IM/DD systems will remain advantageous in terms of system cost, power consumption, and footprint for short reach applications in the short- to mid- term perspective.

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200 Gbps/Lane IM/DD Technologies for Short Reach Optical Interconnects

Recently, remarkable efforts have been made in developing wireless communication systems at ultrahigh data rates, with radio frequency (RF) carriers in the millimeter wave (30–300 GHz) and/or in the terahertz (THz, >300 GHz) bands. Converged technologies combining both the electronics and the photonics show great potential to provide feasible solutions with superior performance compared to conventional RF technologies. However, technical challenges remain to be overcome in order to support high data rates with considerably feasible wireless distances for practical applications, particularly in the THz region. In this work, we present an experimental demonstration of a single-channel THz radio-over-fiber (RoF) system operating at 350 GHz, achieving beyond 100 Gbit/s data rate over a 10-km fiber plus a >20-m wireless link, without using any THz amplifiers. This achievement is enabled by using an orthogonal frequency division multiplexing signal with a probabilistic-shaped 16-ary quadrature amplitude modulation format, a pair of highly directive Cassegrain antennas, and advanced digital signal processing techniques. This work pushes the THz RoF technology one step closer to ultrahigh-speed indoor wireless applications and serves as an essential segment of the converged fiber-wireless access networks in the beyond 5G era.

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26.8-m THz wireless transmission of probabilistic shaping 16-QAM-OFDM signals

With the continuously growing popularity of cloud services, the traffic volume inside the data centers is dramatically increasing. As a result, a scalable and efficient infrastructure for data center networks (DCNs) is required. The current optical DCNs using either individual fibers or fiber ribbons are costly, bulky, hard to manage, and not scalable. Spatial division multiplexing (SDM) based on multicore or multimode (few-mode) fibers is recognized as a promising technology to increase the spatial efficiency for optical DCNs, which opens a new way towards high capacity and scalability. This tutorial provides an overview of the components, transmission options, and interconnect architectures for SDM-based DCNs, as well as potential technical challenges and future directions. It also covers the co-existence of SDM and other multiplexing techniques, such as wavelength-division multiplexing and flexible spectrum multiplexing, in optical DCNs.

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Enabling Technologies for Optical Data Center Networks: Spatial Division Multiplexing

We report on an on – off keying intensity-modulation and direct-detection C-band optical transceiver capable of addressing all datacenter interconnect environments at well beyond 100 Gbaud. For this, the transmitter makes the use of two key InP technologies: a 2:1 double heterojunction bipolar transistor selector multiplexer and a monolithically integrated distributed-feedback laser traveling-wave electro-absorption modulator, both exceeding 100-GHz of 3-dB analog bandwidth. A preamplified 110-GHz PIN photodiode prior to a 100-GHz analog-to-digital converter complete the ultrahigh bandwidth transceiver module; the device under study. In the experimental work, which discriminates between intra- and inter-data center scenarios (dispersion unmanaged 120, 560, and 960 m; and dispersion-managed 10 and 80 km of standard single-mode fiber), we evaluate the bit-error rate evolution against the received optical power at 140, 180, and 204 Gbaud on – off keying for different equalization configurations (adaptive linear filter with and without the help of short-memory sequence estimation) and forward error correction schemes (hard-decision codes with 7% and 20% overhead); drawing conclusions from the observed system-level limitations of the respective environments at this ultrahigh baudrate, as well as from the operation margins and sensitivity metrics. From the demonstration, we highlight three results: successful operation with >6-dB sensitivity margin below the 7% error-correction at 140 Gbaud over the entire 100 m–80 km range with only linear feed-forward equalization. Then, the transmission of a 180-Gbaud on – off keying carrier over 80 km considering 20% error-correction overhead. Finally, a 10-km communication at 204 Gbaud on – off keying with up to 6 dB sensitivity margin, and regular 7% overhead error-correction.

140/180/204-Gbaud OOK Transceiver for Inter- and Intra-Data Center Connectivity

85-Gbaud PAM-8 transmission with BER below HD-FEC limit under 20-GHz bandwidth limitation is achieved applying NL-MLSE based on Volterra filter. NL-MLSE also achieves 3.84-Tbps O-band transmission using 4-λ LAN-WDM and SDM over 2-km 4-core fiber.

255-Gbps PAM-8 Transmission under 20-GHz Bandwidth Limitation using NL-MLSE Based on Volterra Filter

Kernel adaptive filtering (KAF) is proposed for nonlinearity-tolerant optical direct detection. for $7\times 128$ Gbit/s PAM4 transmission over 33.6km 7-core-fiber, KAF only needs 10 equalizer taps to reach KP4-FEC limit (BER@2.2e-4), whereas decision-feedback-equalizer needs 43 equalizer taps to reach HD-FEC limit (BER@3.8e-3).

Kernel Adaptive Filtering for Nonlinearity-Tolerant Optical Direct Detection Systems

We report on the first experimental demonstration of 200-Gbps (net rate 166.7-Gbps) 1.55-μm DMT IMDD transmission over 1.6 km fiber using a single monolithically-integrated-EML, DAC and photodiode, achieving an effective electrical spectrum efficiency of 4.93 bit/s/Hz.

200-Gbps DMT Transmission over 1.6-km SSMF with A Single EML/DAC/PD for Optical Interconnects at C-Band

By employing enhanced DPCM, 15-Gbaud PAM4 digital mobile fronthaul is experimentally demonstrated to support 122 LTE-A channels with up to 4096QAM. Compared to PCM based CPRI, the supported number of channels increases 5 times and EVM is obviously improved.

15-Gbaud PAM4 Digital Mobile Fronthaul with Enhanced Differential Pulse Coding Modulation Supporting 122 LTE-A Channels with up to 4096QAM

Kernel adaptive filtering (KAF) is proposed for nonlinearity-tolerant optical direct detection. For 7x128Gbit/s PAM4 transmission over 33.6km 7-core-fiber, KAF only needs 10 equalizer taps to reach KP4-FEC limit (BER@2.2e-4), whereas decision-feedback-equalizer needs 43 equalizer taps to reach HD-FEC limit (BER@3.8e-3).

Shilin Xiao

Papers

Kernel Adaptive Filtering for Nonlinearity-Tolerant Optical Direct Detection Systems

200-Gbps DMT Transmission over 1.6-km SSMF with A Single EML/DAC/PD for Optical Interconnects at C-Band

15-Gbaud PAM4 Digital Mobile Fronthaul with Enhanced Differential Pulse Coding Modulation Supporting 122 LTE-A Channels with up to 4096QAM

Kernel Adaptive Filtering for Nonlinearity-Tolerant Optical Direct Detection Systems