高效能VCSEL光互连技术：世界纪录的低能耗传输

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"这篇博士学位论文详细探讨了垂直腔面发射激光器(VCSEL)在光互连中的动态能效问题。研究者系统地研究了基于GaAs的VCSEL在850和980nm波长下的静态和动态特性，以此来制定实现能源高效数据传输的一般规则。这些规则适用于任何波长的氧化物限制的VCSEL，并在25 Gbit/s的数据传输实验中得到验证，实验实现了无错误传输，每比特能量仅为56 fJ，这是目前世界纪录的水平，比2015年国际半导体技术路线图（ITRS）的要求低了一半。此外，论文还展示了在长达1000米的传输距离、高达85°C的温度下以及40 Gbit/s的速率下，VCSEL的能源高效运行。这种稳定高温运行消除了对主动冷却的需求，进一步降低了功率消耗，提高了能源效率。" 本文档是所有关注信息技术并寻求经济且环保的卓越技术解决方案的科学家和工程师的必读之作。由德国柏林的Dieter Bimberg教授推荐，强调了该研究的重要性和影响力。 "SpringerTheses"系列是全球杰出博士研究的精选集，旨在展示物理科学领域的优秀博士论文。入选的论文由两位专家提名和背书，因其科研卓越和对相关研究领域的重要影响而被选中。为了使非专业人士更容易理解，出版版本包括一个扩展的介绍和学生导师的前言，解释了这项工作的特殊相关性。该系列为新进入相关研究领域的人提供宝贵资源，同时也为其他寻求详细背景信息的科学家提供帮助。它还正式记录了当今年轻一代科学家的宝贵贡献，只有经过邀请提名并满足特定标准的论文才能入选该系列。

Chapter 1

Introduction

This chapter provides a short introduction into the problem of the exponentially

increasing energy consumption in data centers and supercomputers mainly caused

by the growth of the global Internet trafﬁc and computational bandwidth demand.

The resulting need for energy-efﬁcient optical interconnects, in particular optical

interconnects based on vertical-cavity surface-emitting lasers (VCSELs), is brieﬂy

explained, followed by a detailed overview of the state-of-the-art of VCSELs for

data communication and VCSEL-based optical interconnects. Then the future per-

formance requirements of VCSELs and the contribution of this dissertation to the

ﬁeld of energy-efﬁcient VCSELs for optical interconnects are explained, before the

chapter ends with a s ection about the organization of this dissertation.

1.1 Internet and Energy Consumption

The global Internet trafﬁc has been increasing exponentially in the past decades and

its growth is predicted to continue to increase exponentially in the next decades as

well [1–3]. The number of Internet users is growing due to social and economic

development in developed and in developing countries. But not only the number of

people with access to the Internet is increasing, also new Internet-based services and

devices that require access t o the Internet are evolving and leading to an additional

increase of Internet trafﬁc in addition to the conventional Internet use via desktop

computers. The global Internet trafﬁc per year is shown for 2010–2018 in Fig. 1.1

[3].

One characteristic of the global bandwidth demand is, that each increase of the

upload or download capacity for a single Internet user, group of users, companies,

or research institutes stimulates innovative new applications which in turn further

increases the bandwidth demand.

About 80 % of the data transferred via the I nternet in 2018 will be videos ﬁles

[3]. While Internet video platforms like YouTube and others are well established and

have been used for several years, videos on demand or Internet television (TV) via for

P. Moser, Energy-Efﬁcient VCSELs for Optical Interconnects,

Springer Theses, DOI 10.1007/978-3-319-24067-1_1

2 1 Introduction

2010 2011 2012 2013 2014 2015 2016 2017 2018

0,0

0,5

1,0

1,5

Year

Internet Traffic (Zettabyte/Year)

global Internet Traffic

1 Zettabyte = 10 bytes

Fig. 1.1 Global Internet trafﬁc per year from 2010 to 2018. The predicted values for the years

2015–2018 can be found in Ref. [3]

instance Netﬂix and Amazon Video are comparably new in Europe. The provided

quality of such videos on demand has become already very high enabled by fast

streaming or download speeds, resulting in large volumes of transferred data. With

high resolution computer screens and smart TVs even three dimensional (3D) high

deﬁnition videos with high quality sound are expected from many Internet users to

be accessible online.

Mobile Internet trafﬁc growths faster than the average Internet trafﬁc [3]. For

many people in the industrial nations and the newly industrialized countries, perma-

nent Internet access via mobile devices such as smart phones or tablets has become a

basic necessity. Seamless access to the Internet via wiﬁ networks at airports, in train

stations, throughout airports and during ﬂights, at universities, in libraries, adminis-

tration buildings, in shopping malls and department stores, in restaurants, in hotels,

and many other public places enables immediate and nearly instantaneous infor-

mation transfer for many simultaneous users. For example ubiquitous connectivity

enables one to send and receive email and tweets, document or track every moment

of one’s life, gain access to cloud services and other stored content, and use a smart

phone for navigation. Smart phones, tablet computers, and other new product families

that enable and require access to the Internet lead therefore to a seemingly limitless

growth of the Internet trafﬁc.

It is predicted that more than 50 % of the global Internet trafﬁc in 2018 will be due

to devices other than personal computers such as smart TVs, smart phones, tablet

computers, and other portable devices with Internet access [3]. It has already become

common that Internet users are simultaneously connected to the Internet with several

devices, such as a desktop computer, a smart phone, and a smart TV or Internet radio.

1.1 Internet and Energy Consumption 3

Further development and innovation of devices with Internet access, will increase this

trend of multiple Internet access. Smart homes are envisioned where air conditioning,

heating, surveillance technology, windows, lighting, and other features of a home

are automatically or remotely controlled. Also many items in the home may be

smart and connected including toasters, refrigerators (that automatically order food

when supplies run low) and stores will deliver goods via mobile vehicles or drones.

Although an automatic control of many devices without the need for Internet access

is possible, in reality many features such as video surveillance will be controlled via

web browsers or smart phones when the home owner is not at home. Then people

even may generate Internet trafﬁc at their homes while being somewhere else.

The growing Internet trafﬁc has led to a corresponding dramatic growth of the

energy consumption, especially in data centers and supercomputers. While in 2010

most of the energy consumption of the Internet can be attributed to the access net-

works, it is predicted that data centers will require the largest fraction of the Internet

energy consumption in 2020 [2]. The enormous energy consumption of data centers

is already today a pressing ecological and economical problem. New massive data

centers are built in sub-polar regions or similar higher northern and southern latitudes

where the electricity cost is low and free cooling is available via access to naturally

cold water in close proximity to the data center facilities from rivers, lakes, or the sea.

This has led to the construction of new data centers in northern Europe, for example

a Facebook datacenter in Luleå in Sweden. Other examples include data centers in

Norway [4] where hydroelectricity is abundant, and in Iceland where hydrothermal

energy may serve a large fraction of the data center energy needs.

Because the energy consumption caused by the Internet is growing faster than

the global electricity production [2], new technologies are required that enable data

transmission at larger data rates and simultaneously signiﬁcantly lower energy con-

sumption per bit. Most of the energy in a data center is consumed by inter or intra-rack

data transfer and for cooling the hardware. About 80 % of the trafﬁc in a data center

stays in the data center, 5 % goes to other data centers, and only 15 % of the trafﬁc

actually goes to the users.

Depending on the assumed annual improvement of the energy efﬁciency of the

hardware the energy consumption for transferring one single bit of information via

the Internet is predicted be 1–4 µJ per bit in 2020 [2]. The largest fraction of this

energy will be consumed in the interconnects inside the data centers. Thus fast (high

bit rate) and energy-efﬁcient optical interconnects are a key-enabling technology for

an ecological, economical, and sustainable Internet.

1.2 Optical Interconnects

Optical interconnects have already partly replaced electrical interconnects in data

centers and supercomputers due to their larger maximum bandwidth-distance product.

4 1 Introduction

driver

circuit

VCSEL

electrical

interface

optical

interface

photo-

diode

amplifier

optical

interface

electrical

interface

light guiding medium: optical fiber, polymer, ...

TOSA: transmitter optical sub-assembly

ROSA: receiver optical sub-assembly

Fig. 1.2 Schematic of an optical interconnect consisting of a transmitter optical sub-assembly

(TOSA) and a receiver optical sub-assembly (ROSA) connected by a light guiding medium. A

vertical-cavity surface-emitting laser (VCSEL) is used as light source that converts the electrical

signal into an optical signal

Optical interconnects consist of the transmitter optical sub-assembly (TOSA), a

light guiding medium such as single or multimode optical ﬁber that transports the

information, and the receiver optical sub-assembly (ROSA). In general the TOSA

consists of an electrical interface, an integrated driver circuit that provides the elec-

trical power to the light source, an optical interface that couples the light into the

light guiding medium, then again an optical interface that couples the light to the

detector or photo receiver of the ROSA. The electrical signal may then be modiﬁed

by electrical ampliﬁers followed again by an electrical interface. A schematic of an

optical interconnect that employs a vertical-cavity surface-emitting laser (VCSEL)

as light source is shown in Fig. 1.2.

Electrical interconnects are running into intrinsic limitations once the bandwidth-

distance product is increased beyond a certain value. Therefore 850-nm VCSEL-

based optical interconnects have already replaced electrical interconnects for short-

reach (SR) data transmission at 10–14 Gb/s from several meters of multimode optical

ﬁber (MMF) up to 300m of MMF. At longer transmission distances typically dis-

tributed feedback (DFB) edge-emitting laser diodes and single-mode ﬁber (SMF)

are used. Because the energy consumption of an edge-emitting laser is at least one

order of magnitude larger than that of a VCSEL it is desired to extend the MMF

transmission distance of the VCSEL-based optical interconnects in order to replace

the SMF-based DFB-laser optical interconnects.

The performance of the complete optical interconnect such as the maximum error-

free bit rate and the total energy consumption depend on the combined performance

1.2 Optical Interconnects 5

of the individual optical, electronic, and optoelectronic components of the optical

interconnect. Thus different optical interconnect designs that are optimized to fulﬁll

different performance requirements will consist of different combinations of the elec-

tronic and optoelectronic components. In order to optimize an optical interconnect

for a speciﬁc performance goal the impact of each component in a given different

optical interconnect conﬁguration, e.g. different combinations of the electronic and

optoelectronic components, has to be known or predicted by systems-level modeling

or testing. Since in the past the main focus of the development of optical intercon-

nects was primarily on increasing the bit rate, good experience exists here. For future

optical interconnects, performance goals such as high bit rates, long MMF transmis-

sion distance, and temperature-stable operation, all at low current densities, have

to be achieved at simultaneously high energy efﬁciencies, e.g. low dissipated and

consumed energy per transmitted bit of information.

Prior to the work in this dissertation very little systematic analysis of the dynamic

energy efﬁciency of VCSELs for optical interconnects had been accomplished.

Therefore it has not been possible to predict the optimal optical interconnect design

for future energy-efﬁcient optical interconnects. The main purpose of this disser-

tation is to provide a systematic investigation of the dynamic energy efﬁciency of

VCSELs in terms of consumed and dissipated energy per bit. Via the proposed and

demonstrated evaluation methods and the revealed intrinsic trade-offs between dif-

ferent performance goals, the dynamic energy efﬁciency performance of VCSELs

shall be made accessible for systems designers in order to optimize the overall energy

efﬁciency of complete optical interconnects.

The state-of-the-art of VCSELs for optical interconnects and VCSEL-based opti-

cal interconnects as well as future optical interconnect performance goals are sum-

marized in the next two sections.

1.3 State-of-the-Art of Vertical-Cavity Surface-Emitting

Lasers for Optical Interconnects

Optical interconnects based on vertical-cavity surface-emitting lasers (VCSELs) are

especially well-suited for data transmission at large bit rates and low consumed

energy per bit, because of the VCSELs’ small mode-volume compared to edge-

emitting semiconductor lasers.

In order to enable extremely energy-efﬁcient optical data transmission with only

a few femto-joules per bit via even smaller mode-volumes new device concepts such

as photonic-crystal [5–7] and metal-cavity lasers [8] are subject of intense research.

While photonic crystal structures inevitably have comparably large lateral dimen-

sions, metal-cavity lasers may allow extremely small geometric device dimensions

for optical interconnects with small footprint and high bandwidth density. Those

nano-lasers are at a much earlier stage of development as compared to the mature

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