Driven primarily by cloud service and data-center applications, short-reach optical communication has become a key market segment and growing research area in recent years. Short-reach systems are characterized by direct detection-based receiver configurations and other low-cost and small form factor components that induce transmission impairments unforeseen in their coherent counterparts. Innovative signaling and digital signal processing (DSP) play a pivotal role in enabling these components to realize their ultimate potentials and meet data rate requirements in cost-effective manners. This paper presents an overview of recent DSP developments for short-reach communications systems and discusses future trends.

Digital Signal Processing for Short-Reach Optical Communications: A Review of Current Technologies and Future Trends

An overview of the most recent developments and improvements to the low-loss TriPleX Si3N4 waveguide technology is presented in this paper The TriPleX platform provides a suite of waveguide geometries (box, double stripe, symmetric single stripe, and asymmetric double stripe) that can be combined to design complex functional circuits, but more important are manufactured in a single monolithic process flow to create a compact photonic integrated circuit All functionalities of the integrated circuit are constructed using standard basic building blocks, namely straight and bent waveguides, splitters/combiners and couplers, spot size converters, and phase tuning elements The basic functionalities that have been realized are: ring resonators and Mach–Zehnder interferometer filters, tunable delay elements, and waveguide switches Combination of these basic functionalities evolves into more complex functions such as higher order filters, beamforming networks, and fully programmable architectures Introduction of the active InP chip platform in a combination with the TriPleX will introduce light generation, modulation, and detection to the low-loss platform This hybrid integration strategy enables fabrication of tunable lasers, fully integrated filters, and optical beamforming networks

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Low-Loss Si3N4 TriPleX Optical Waveguides: Technology and Applications Overview

In this paper, we review the historical evolution of predictions of the performance of optical communication systems. We will describe how such predictions were made from the outset of research in laser based optical communications and how they have evolved to their present form, accurately predicting the performance of coherently detected communication systems.

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Performance limits in optical communications due to fiber nonlinearity

In this paper, an overview is given on the recent developments in next-generation high-speed PON. A summary is presented on the progress being made by the IEEE P802.3ca task force on the standardization of 25, 50, and 100G EPON. An outline of the physical layer specification for next-generation high-speed PON as well as MAC layer functions such as channel bonding to achieve 100 Gbps line rates is discussed. Also, the wavelength plan for 100G EPON coexistence with the various PON generations are discussed.

Recent Progress on Standardization of Next-Generation 25, 50, and 100G EPON

Unlike ultralong coherent optical systems that seriously suffer from fiber nonlinearities, short-reach noncoherent systems such as data center interconnections, which utilize small, cheap, and low-bandwidth components, are sensitive to nonlinearities that are mainly produced by devices responsible for electrical signal amplification, modulation, and demodulation. One of the most promising schemes for these applications is the four-level pulse amplitude modulation format combined with intensity modulation and direct detection; however, it can be significantly degraded by linear and nonlinear intersymbol interference. Linear and nonlinear signal degradation can efficiently be handled by different types of equalizers. In many cases, the straightforward linear equalizer cannot lower the error rate at the acceptable level. Therefore, much stronger equalizers based on nonlinear models such as the Volterra series are proposed. Volterra filter that can also be orthogonalized by the Wiener model is well described in the existing literature, and, in this paper, we investigate the most critical points related to high-speed Volterra filter design and implementation. Several experiments are carried out in order to indicate filter requirements/complexity, acquisition, and stability. We also provide a simple guidance for filter complexity reduction and useful hints for channel acquisition.

Volterra and Wiener Equalizers for Short-Reach 100G PAM-4 Applications

Advanced modulation formats combined with digital signal processing and direct detection is a promising way to realize high capacity, low cost and power efficient short reach optical transmission system. In this paper, we present a detailed investigation on the performance of three advanced modulation formats for 100 Gb/s short reach transmission system. They are PAM-4, CAP-16 and DMT. The detailed digital signal processing required for each modulation format is presented. Comprehensive simulations are carried out to evaluate the performance of each modulation format in terms of received optical power, transmitter bandwidth, relative intensity noise and thermal noise. The performance of each modulation format is also experimentally studied. To the best of our knowledge, we report the first demonstration of a 112 Gb/s transmission over 10km of SSMF employing single band CAP-16 with EML. Finally, a comparison of computational complexity of DSP for the three formats is presented.

Experimental study of PAM-4, CAP-16, and DMT for 100 Gb/s Short Reach Optical Transmission Systems

We experimentally demonstrated 140-Gb/s transmission over 20-km standard single-mode fiber employing pulse-amplitude modulation (PAM)-4 and direct detection (DD) at 1.3 $\mu \text{m}$ . DD faster than Nyquist is employed to compensate for channel impairments. The optimal length of taps of decision directed least mean square and the optimal tap coefficient for digital postfilter are investigated. A receiver sensitivity of −5.5 dBm at bit error rate of $3.8\times 10^{-3}$ is realized for 140-Gb/s PAM-4 signal after 20-km transmission. To the best of our knowledge, this is the highest reported baud rate (70 GBd) of direct detected PAM-4 signal and the highest per channel bit rate with single polarization and DD for short reach communications.

140-Gb/s 20-km Transmission of PAM-4 Signal at 1.3 $\mu \text{m}$ for Short Reach Communications

We proposed a novel 100Gb/s PON scheme using 4-PAM modulation with commercial low-cost 10Gbps TOSA/ROSA compatible with 40Gb/s TWDM-PON Experimental results show that with EQ compensating bandwidth limitation and CD, 30km transmission is available

30km Downstream Transmission Using 4×25Gb/s 4-PAM Modulation with Commercial 10Gbps TOSA and ROSA for 100Gb/s-PON

In this paper, we experimentally demonstrate single channel 128Gbit/s and four lanes transmission with a total capacity of 500Gbit/s for short reach applications employing PAM4 format and direct detection with commercial 25Gbps devices.

Experimental demonstration of 500Gbit/s short reach transmission employing PAM4 signal and direct detection with 25Gbps device

With the introduction of PAM4 modulation, a novel 100GE optical transceiver module has been proposed using the low-cost 4x10Gbps DML TOSA and PIN ROSA. 4x25Gbps PAM4 signal transmission has been demonstrated over 2km SMF on-line with -11dBm sensitivity and 4dB link margin.

Li Zeng

Papers

Experimental study of PAM-4, CAP-16, and DMT for 100 Gb/s Short Reach Optical Transmission Systems

140-Gb/s 20-km Transmission of PAM-4 Signal at 1.3 $\mu \text{m}$ for Short Reach Communications

30km Downstream Transmission Using 4×25Gb/s 4-PAM Modulation with Commercial 10Gbps TOSA and ROSA for 100Gb/s-PON

Experimental demonstration of 500Gbit/s short reach transmission employing PAM4 signal and direct detection with 25Gbps device

A Low-Cost 100GE Optical Transceiver Module for 2km SMF Interconnect with PAM4 Modulation