High-Speed Circuits for Lightwave Communications

The following sections are included:

• INTRODUCTION

• FOREWORD

• CONTENTS

Recent increase in the demand for bandwidth in the telecommunications network has stimulated both research and commercial activity in high-speed electronics. This paper summarizes the technical issues that bear on high-speed circuits in this context, as well as the device technologies available for these circuits in both laboratory and commercial processes. Finally a tutorial treatment is given of the main design requirements pertinent to circuits required for lightwave systems.

This paper gives an overview on very-high-speed ICs for optical-fiber systems with restriction to Si-based technologies. As a main aim, the circuit and system designer shall get an impression what operating speeds have already been achieved and, moreover, get a feeling for potential limitations. It is shown that all ICs in 10 Gb/s TDM systems can be fabricated in Si-bipolar production technologies, while for the speed-critical ICs in 20 Gb/s systems, present SiGe laboratory technologies are required if the circuit specifications, apart from the data rate, must remain unchanged. With uncritical circuits like time-division multiplexer (MUX) and demultiplexer (DEMUX), record data rates of 60 Gb/s were achieved with a SiGe laboratory technology, using an adequate mounting and measuring technique. Recent measuring results even showed that all ICs in a 40 Gb/s TDM system (i.e., also the speed-critical ones) can be realized in advanced SiGe technologies. However, compared to ICs in 10 and 20 Gb/s systems, some circuit specifications must be relaxed. This is possible by the use of optical amplifiers and improved opto-electronic components as well as by system modifications, which further make possible the elimination of some of the speed-critical circuits. It should be noted that all the experimental results presented are measured on mounted chips, using conventional wire bonding, and that most of the circuits have been used in experimental TDM links.

Design considerations concerning Si preamplifiers for 10-Gb/s optical transmission systems are discussed. A preamplifier with 300-Ω transimpedance was designed by focusing on attaining low transimpedance fluctuation in the frequency response despite bias variation caused by the photo current. To ensure good design accuracy, we optimized the current density of the transistor and the open-loop voltage gain using measured results to obtain the desired bandwidth. We also developed a low-loss pad structure that has U-grooves to improve the bandwidth. The U-grooves increase the pad parasitic resistance and reduce signal-loss from pad to Si substrate. A preamplifier IC fabricated using a Si bipolar technology with f_T of 35-GHz provided a bandwidth of 10.2 GHz, transimpedance fluctuation within 0.5 dB, an input dynamic range of up to 1.6 mA_p−p, and a low averaged input noise current density of .

This paper describes recent advances in high-speed digital IC design technologies based on GaAs MESFETs for future high-speed optical communications systems. We devised new types of a data selector and flip-flops, which are key elements in performing high-speed digital functions (signal multiplexing, decision, demultiplexing, and frequency conversion) in front-end transmitter/receiver systems. Incorporating these circuit design technologies with state-of-the-art 0.12-/μm gate-length GaAs MESFET process, we developed a DC-to-44-Gbit/s 2:1 data multiplexer IC, a DC-to 22-Gbit/s static decision IC, and a 20-to-40-Gbit/s dynamic decision IC. The fabricated ICs demonstrated record speed performances for GaAs MESFETs. Although further operating speed margin is still required, the GaAs MESFET is a potential candidate for 20- to 40-Gbit/s class applications.

Using our 0.2 and 0.3 μm AlGaAs/GaAs/AlGaAs quantum well HEMT technology, we have developed a chip set for 20–40 Gbit/s fiber-optical digital transmission systems. In this paper we describe nine analog and digital receiver ICs: a 22 GHz high-gain transimpedance amplifier, a 20 Gbit/s OEIC front-end optical receiver, a 25 Gbit/s automatic-gain-control amplifier, a limiting amplifier with a differential gain of 26 dB and a bandwidth of 27.7 GHz, a 20–40 Gbit/s clock recovery, a 20 Gbit/s low-power Master-Slave-D-Flipflop with 24 mW power dissipation, a parallel data decision and a 1:4 demultiplexer, both for bit rates of 40 Gbit/s, and a 30 GHz static frequency divider, respectively. All chips were characterized on wafers with 50Ω coplanar test probes.

We describe experimental ultra-high-speed HBT circuits for lightwave communications applications. High speed circuits such as multiplexer/demultiplexers, variable gain amplifiers, (VGAs), and transimpedance amplifiers operating at high bit rates (> 30 Gb/s) are required for the realization of high-performance lightwave systems using TDM or WDM. We have demonstrated 40 Gb/s 4:1 multiplexers, > 30 Gb/s 1:4 demultiplexers, DC-26 GHz VGAs, DC-25 GHz transimpedance amplifiers, 30 Gb/s data and clock regenerators, 40 Gb/s differentiate-and-rectify timing recovery circuits, and 40 Gb/s delay-and-multiply timing recovery circuits, for use in such systems using a manufacturable hybrid digital/microwave HBT process.

Technology and performance of electronic crosspoint switches with data rates above 1 Gb/s/channel are reviewed. Switch applications and architectures are described, as well as the principal problems in achieving low output jitter. Recent results for different IC technologies are summarized. As a particular example, a 10 Gb/s crosspoint switch implemented in GaAlAs/GaAs-HBT technology is described, including details of design, packaging, testing methodology, and performance results.

Heterojunction bipolar transistor (HBT) integrated circuits are just beginning to appear on the commercial market, mostly as small-scale microwave and broadband parts. The high-speed portion of Nortel's OC-192 fiber communications product makes extensive use of this new technology, and this article describes the rationale for using HBT ICs in such a system, and the philosophy behind HBT IC introduction.

High-speed integrated circuits (ICs) are essential for expanding the capacity of lightwave communications. InP-based heterostructure field effect transistors (HFETs) and heterojunction bipolar transistors (HBTs) are very promising for producing high-speed digital and analog ICs. This paper reviews the current status of InP-based lightwave communication ICs in terms of device, circuit, and packaging technologies. A successful 40-Gbit/s, 300-km optical fiber transmission using InP HFET ICs demonstrates the feasibility of the ICs. Furthermore, we estimate future IC performance based on the relationship between electron device figures-of-merit and IC speed. To keep up with the performance trend, technological problems, like inter- and intra-chip interconnections, have to be solved.

InP-based HBT technology has proven to be a strong candidate for ultra high speed electronic as well as optoelectronic integrated circuits. The cut-off frequencies of the available devices exceed 100 GHz. To challenge the technology, a variety of circuits, suitable for a demonstrator for the 40 Gb/s fiber optical transmission system have been designed, fabricated, and tested. All the circuits show potential for 40 Gb/s applications. The electrical parts have been implemented in MSI/LSI AlInAs/InGaAs-HBT technology, while the monolithic optoelectronic receivers were realized in SSI- InP/InGaAs HBT technology. The verification of performance of the circuits have been mainly limited by available measurement equipment. All the electronic parts were operational with 3 volt supply voltage. All the circuits were fully functional after the first processing round. No redesign was necessary.

InP-based OEIC receivers look promising for high-speed (≥ 10 Gb/s) optical communications systems and for WDM networks because of the inherent advantages of integration, and the intrinsic speed of the devices available. This paper reviews recent developments.

Transferred-substrate heterojunction bipolar transistors (HBTs) have demonstrated very high bandwidths and are potential candidates for very high speed integrated circuit (IC) applications. The transferred-substrate process permits fabrication of narrow and aligned emitter-base and collector-base junctions, reducing the collector-base capacitance and increasing the device f_max. Unlike conventional double-mesa HBTs, transferred-substrate HBTs can be scaled to submicron dimensions with a consequent increase in bandwidth. This paper introduces the concept of transferred-substrate HBTs. Fabrication process in the AlInAs/GaInAs material system is presented, followed by DC and RF performance. A demonstration IC is shown along with some integrated circuits in development.