NTT Technical Review

May 2019 Vol. 17 No. 5
View from the Top

Katsuhiko Kawazoe, Senior Vice President, Head of Research and Development Planning,NTT

Feature Articles: Ultrahigh-speed Transmission Technology to Support High-capacity
Communications Infrastructure of the Future

Ultrahigh-speed Transmission Technology for Future High-capacity Transport Networks

Ultrahigh-speed Optical Communications Technology Combining Digital Signal Processing
and Circuit Technology



Low-noise Amplification and Nonlinearity Mitigation Based on Parametric Repeater
Technology

Ultrahigh-speed Optical Front-end Device Technology for Beyond-100-GBaud Optical
Transmission Systems



Toward Terabit-class Wireless Transmission: OAM Multiplexing Technology

Regular Articles


Recent Progress in Applications of Optical Multimode Devices Using Planar Lightwave
Circuits

Global Standardization Activities
Standardization Efforts of International Electrotechnical Commission Related to Surge
Protection Components, and Receipt of METI Minister’s Award

Short Reports

NTT Develops Non-rigid Object Recognition Technology that Achieves Recognition of
Database Images—Not Only Rigid 3D Objects but also Non-rigid Objects—at Low Cost

External Awards/Papers Published in Technical Journals and Conference Proceedings

View from the Top

Creating Happiness with ICT by
Making the World Smart andTechnology Natural
Katsuhiko Kawazoe
Senior Vice President, Head of Research andDevelopment Planning, NTT
Overview

The NTT laboratories heretofore have focused on research supporting
NTT’s own services and systems. However, their strategy has been shiftedto the promotion of the B2B2X (business-to-business-to-X) model and now
emphasizes value creation with partner companies. What is the mindset atthe NTT laboratories to advance research and development (R&D) with theaim of creating innovation for people to live their lives more affluently andin a more natural manner? Katsuhiko Kawazoe, Senior Vice President andthe head of Research and Development Planning, NTT, was asked about his
attitude toward R&D that will transform the world.
Keywords: smart world, natural technology, happiness

What kinds of research and development (R&D)
and technology make people happy in thetrue sense?
—It has been half a year since you took up your current position. What do you think so far?
My prior position as the head of a laboratory groupwas somewhat an extension of my specialties.
However, the current position as the head of Researchand Development Planning involves managing all theNTT laboratories while also supervising R&D acrossthe entire NTT Group. In other words, I now determine what kind of new world our R&D in different
fields should create, and lead the labs to generate newvalue inside and outside the NTT Group and society
through the daily accumulation of research in thelabs. As you probably know, we are promoting thebusiness-to-business-to-X (B2B2X) model and have
NTT Technical Review

Vol. 17 No. 5 May 2019

expanded our business activities accordingly.
Promoting the B2B2X model is one of the pillars of
our medium-term management strategy “Your Value
Partner 2025,” which encourages us to support thesecond B in creating new value and to conduct R&D
that contributes to that value creation. Our lab members have already been pursuing various efforts inline with this strategy. Moreover, I believe it is vital tofurther accelerate our R&D by collaborating withmore partners, and I’d like to promote this belief
under the catchphrase “Making the world smart andtechnology natural.”
As for the meaning of natural in that catchphrase,
for example, a smartphone is really a convenient tool.
However, when we contact someone via smartphone,
we first consider what application the other party isusing on their smartphone. Then, we consider whether the line is faster by using 4G (fourth-generation
wireless) or Wi-Fi and how to send images according1

View from the Top

to the usage environment, and we change the phonesettings accordingly.
This procedure is no problem for smartphone users
who do not think it is a burden, but some users do
consider it a burden. For the latter users, althoughmost of the current smartphones will automaticallyconnect to a faster network, tasks such as selecting
the application and pre-setting are still a considerable
burden. We want to reach out to those users—that is,pause just for a while as various functions evolve andenable anyone to use services smoothly. The word
natural has come up to describe that aspiration.
Making technology natural may mean thinking about
what function really makes people happy.
Show new possibilities and create the next
developments while facing the times—The catchphrase “Making the world smart andtechnology natural” reflects your true feelings. Whatkind of research will make the world smart?
Some studies are aimed at realizing a smart world,
however, that is not what this is all about. I think that
it is also necessary to bring smart into the process ofadvancing research itself. Currently, many departments are in charge of R&D, especially development,
in various parts of the NTT Group. Under those circumstances, the NTT laboratories are very special.
Here, special refers to a researcher’s way of thinking
and the subject that the researcher is tackling—which
is clearly different from development in general.
Therefore, I think it is meaningful to go back to thestarting point of the research and re-evaluate whatrole researchers should play and what research can
only be done by NTT.
We have created many technologies so far. A recent
example is our successful demonstration of wavelength-division-multiplexed optical transmission at
1 Tbit/s per wavelength over a long distance—a world
first—by using digital signal processing technology
and ultra-broadband optical front-end integrated
device technology. This optical transmission technology enables transmission rates that are ten times thecapacity—namely, 100 Gbit/s per wavelength—of
current practical systems. It is therefore expected toachieve the high-capacity communications networks
that will help expand Internet of Things and 5G services. Simply put, it has tremendous potential interms of achieving the ultimate low-latency transmission in an end-to-end manner, which would enable
4K/8K video to be transmitted in real time without
NTT Technical Review

Vol. 17 No. 5 May 2019

having to compress it. I have the feeling that this willlead to paradigm shifts in various industries.
The origin of such technology is basic research.
That basic research leads to applied research, thefruits of which are further developed for practical use.
That’s why I want to enhance our basic research. On
the basis of that, I think it is important to create a flowfrom basic research to practical development andthen to commercialization of services and the creation of new business. This flow will allow ourresearch to flourish in the form of value creation for
the world, and that is exactly what I mean by “making
the world smart.”
Incidentally, software has been highlighted in many
ways such as software-defined networking and open
source software in the past several years; however, I
think it is necessary to reaffirm the importance ofhardware. That’s because software runs and generates
its value on innovative hardware. There are various
limitations due to technical difficulties and costs, butthere are some things only hardware can provide,
such as ultra-wideband optical front-end integrated
devices, and I believe that it is essential to produce
such things and take up the challenge of improving
them. I think that as part of the smart flow I mentioned before, it is necessary to view R&D in total,
including not only software but also hardware.
We have been talking from a viewpoint of problem
solving in society. However, I think that we must
consider another viewpoint of growing a sense ofhappiness, too. Matters that generate a sense of happiness are not universal. For example, Japanese people think that sushi is delicious; but, many people incountries that do not like cold meals generally do not2View from the Top

think it is delicious. However, people from suchcountries are increasingly visiting Japan and enjoying
sushi. That situation shows that people’s values canchange through experience and be shared with others.
I think there are many ways that people can sharevalues, and sharing values has been my major
research theme as a researcher, and it has also becomea research policy. I’d be very happy if we could be
involved in inspiring people and making them happy.
And I think that hope will become one of the themesof our smart research.
From a global perspective, our current mediumterm management strategy lays out “strengthening
competitiveness of global business” and “globalization of R&D.” We aim to enhance basic research asefforts to create innovative things and something
entirely new, and accordingly, we have established a
new research corporation, NTT Research, Inc., in
Silicon Valley in the US. In addition to spreading theachievements of the NTT laboratories globally, we
are also promoting the globalization of our R&D targets. The research themes originate not only in Japanbut also all over the world.
Taking a global view of ideas for creating new technologies, services, and business for solving various
problems scattered around the world, we are grasping
the research themes that are revealed from that viewpoint. I believe the globalization of R&D is not only
about expanding seeds that have grown domestically
but also actively incorporating seeds that have grownaround the world and those that can be expected to

NTT Technical Review

Vol. 17 No. 5 May 2019grow in the future.
Believe in one’s identity whileadvancing diversity—How were these values created?
I lived in New York, where people of many different
cultures are gathered from around the world, until thesecond grade of elementary school. There were few
Japanese there at that time, and it was difficult forchildren to develop their own identity in the community. In those environments, I had the experience of
gradually opening my heart to others and being
accepted by them. As for my research activities when
I was in graduate school, I sometimes wondered what
it is to be human and what research activities would
be best to carry on with. At that time, I studied anthropology and learned that it was important to recognizethat people have different values and senses of happiness. That recognition is at the root of my current
stance.
In regard to R&D, there are various types ofresearch in a wide range of fields that cannot be
described in one word. Moreover, there are researchers involved in such research around the world. Under
those circumstances, researchers should aim to pursue research with the pride and conviction that newservices and value will not be created if they do notresearch this theme or have the attitude “Who will dothis research if I don’t do it myself?” At one time,
there was an argument that we do not necessarily
have to aim to be number one. That is still true and I
think it is possible to create new value by thoroughly
pursuing anything that anyone can think up. However,researchers tend to take pride in achieving things
through difficult means. If the result is the same, it
doesn’t matter if it was difficult or easy to create.
I think whether the results of research really benefit
the world is important. Many researchers don’t actually mind struggling to solve difficult problems andeven find it stimulating to do so. There is no doubtthat they are all very hard workers. However, it issometimes unrealistic to doggedly stick to a pursuit
because doing so doesn’t always lead to the happinessof society. I think that their efforts will be fruitful if
researchers keep a broad view and do not lose sight of
the goal of serving the world. That’s why I want tostick to playing the role of giving advice—in thephase from basic research to applied research—on
whether or not research will lead to the happiness ofpeople in society, which is another need we must3

View from the Top

consider. And I think it would be of great value to
NTT to deeply explore that direction.
On the contrary to what I said above, not all
researchers take this viewpoint and I think it is impossible to force them to do so. Diversity is very important, and some people have exceptional talent. My
duty is therefore to gather those people together andlead them to achieve the goals that are set.
—Since you have been a researcher yourself, youmust have learnt many lessons. Would you like to saya word to your researchers?
My research theme when I joined NTT was satellite
communications. I was really happy to work on a
fantastic theme associated with the space. However,NTT changed its business model from owning communications satellites to leasing them from othercompanies. I was really shocked because I thoughtthat it was a theme I could tackle over a lifetime.
When I was pondering what to do in the future, a colleague mentioned that the research related to content
was pretty close to what I was doing with satellites.
Although I didn’t really know how and where satellites and content could be connected, my colleague
convinced me that it was better to research content
from then on because the age of content was coming.
With those words in mind, I was able to extend myresearch themes to IP (Internet protocol) televisionand our ultrahigh presence communications technology called “Kirari!”.
As researchers, you may be forced to take an aboutturn in the future. I must admit a research theme thatlasts forever is great, very important, and worthwhile,but, one day, the theme may not be the target ofresearch anymore. However, that does not mean thatyou are not needed anymore because there is no longer work to do. I believe that research is in the mindset, and themes will follow accordingly. The mindset
in that sense means the intention of researchers, forexample, the intention to invent and propose newprinciples, the intention to understand a certain phenomenon and clarify its mechanism, and the intention
to create as a form what has never been created
before, and these intentions do not depend on the
research theme. I’d like you to continue to believe

NTT Technical Review

Vol. 17 No. 5 May 2019that as long as you maintain your own enthusiasm,
you can find new themes wherever you look. If you
think about how you apply your way of thinking, yourapproach, and your knowledge accumulated so far,
you will see infinite possibilities and infinite themes.
I want you to take up those challenges. You should
believe in yourselves so that you can find new themes.
I want you to reconsider, as a researcher, your identity, your existence value, and why you are here, andthink seriously about what you need to do. There wasa time when we decided our direction and goals, andeveryone worked together toward those goals; however, now is the time to pursue various possibilities inmany ways. Do not give up believing in yourself,
your research theme, or the potential of your challenges. I want to create an environment for supporting that belief you should have in yourselves.

Interviewee profile
Career highlights
Katsuhiko Kawazoe joined NTT in 1987. He
became Vice President of Research andDevelopment Planning in 2008, head of NTT
Service Evolution Laboratories in 2014, and headof NTT Service Innovation Laboratory Group in
2016. He assumed his present position in June
2018.

4

Feature Articles: Ultrahigh-speed Transmission Technology to
Support High-capacity Communications Infrastructure of the Future

Ultrahigh-speed Transmission
Technology for Future High-capacity
Transport Networks
Yutaka Miyamoto, Shuichi Yoshino, and Akira Okada
Abstract

Ultrahigh-speed transmission technologies in radio and optical fiber transport systems are essential toaccommodate the ever-increasing demand for bandwidth in future network infrastructure. Advanced
digital modulation/demodulation techniques as well as ultrahigh-speed front-end integration technologies are optimized to fully exploit the characteristics of different types of transmission media such as airand optical fiber, considering novel degrees of freedom such as space division multiplexing. This article
introduces state of the art research and development that achieves ultrahigh-speed communications at
speeds of over 1 Tbit/s per carrier (over a hundred times the current speed) in both optical fiber transmission and radio transmission.
Keywords: transmission, digital modulation/demodulation, parametric optical amplification

1. High-capacity transport networks andapplications of ultrahigh-speed communications
Common broadband network services such asvideo streaming and electronic commerce (e-commerce) are now available all over the world via personal computers (PCs) and smartphones in daily life.
The novel transmission technologies have thus
become essential for future network infrastructure inorder to achieve further network service evolutions.
The fifth-generation (5G) mobile communications
service will start in fiscal year 2019, and broadband
communications at a line rate up to 20 Gbit/s with low
latency will be expected to support emerging newservices such as self-driving vehicles and factory
automation. Furthermore, it is expected that the application of new technologies such as machine learningand artificial intelligence will facilitate the emergence of new applications such as detailed weather
forecasting and preventive medicine, thanks to theInternet of Things technology achieved throughrecent advances in cost-effective low-power semiconductor integrated circuits used in sensor technology.
We believe that future transport networks will have to
NTT Technical Review

Vol. 17 No. 5 May 2019support the creation of such new application services
supporting our daily life.
The application area of ultrahigh-speed transmission technology in today’s high-capacity transport
networks is shown in Fig. 1. In radio transmission,
significant advances in digital modulation/demodulation techniques have been made in fixed microwave
transmission systems, and these had been used tosupport long-haul core networks until the commercial
installation of optical fiber transmission systems
began in the 1980s [1]. Further advances have been
made in these fixed microwave transmission systemtechnologies to enable economical link systems inareas where it is difficult to install a wired cable transport system.
In addition, dramatic technological advances havebeen made in mobile communications, achieving
high-speed wireless local area networks and mobile
phones in the last quarter of a century. Today, wirelessaccess services using PCs and smartphones (4G) witha throughput of at least 1 Gbit/s have spread throughout the world. Furthermore, next-generation 5G systems will offer faster wireless access with the highreliability and low latency required for self-driving
5

Feature Articles

Core network

Sub

ma

Metropolitan network

Datacenter
network

Wireless accessnetwork

rine

net

wor

k

Wired accessnetworkFig. 1. Application area of ultrahigh-speed transmission technology.

vehicles and factory automation.
An ultrahigh-speed optical fiber transmission system was first installed in NTT’s core network in the
1980s, and remarkable progress has been made overthe last 40 years. Today, optical fiber is widely used
for transoceanic systems over 10,000 km, long-haulnational backbone networks, metropolitan networks,
and access networks. Optical fiber transmission systems have also become indispensable recently forincreasing the capacity of networks such as datacenter (DC) networks and mobile backhauls. Opticalfiber transmission systems have so far mostly usedsingle-mode fiber (SMF) as a transmission medium.
SMF is designed to have only one optical waveguide(core) per fiber supporting a single waveguide mode.
Recent long-haul core networks support a capacity inexcess of 10 Tbit/s per fiber by using 100-channel
wavelength division multiplexing (WDM) of
100-Gbit/s channels [2]. Furthermore, low-power
optical communications with a compact configuration and a channel capacity over 200 Gbit/s haverecently been introduced in DC interconnection networks [3].
2. Technical challenges in ultrahigh-speed
communications technology
This section describes the common technical chalNTT Technical Review

Vol. 17 No. 5 May 2019lenges in introducing ultrahigh-speed communications technologies in both radio transmission systems
and optical fiber transmission systems. The system
capacity C of a communications system is generally
expressed by the following equation according to
Shannon’s well-known theorem:
C = N • B • log (1 + SNR),
where N represents the number of multiplexed channels, B represents the signal bandwidth, and SNR
represents the signal-to-noise ratio. There are threemajor approaches to improving the system capacity Cof a communications system as expressed by thisequation:
Approach 1: Increase the symbol rate by expanding the signal bandwidth B of each channel.
Approach 2: Improve the SNR by reducing noisein the system and/or increasing the signal power, orby adopting a digital modulation technique such as
quadrature amplitude modulation (QAM)* that ismore efficient than binary modulation.
Approach 3: Improve the channel capacity byusing a frequency or spatial degree of freedom toachieve higher multiplicity (increasing N).
*

QAM: A highly efficient digital modulation scheme whereby the
amplitude and phase of a signal’s electrical field are modulated tomultiple signal levels.

6

Feature Articles

Symbol rate
>128GB

Higher symbol rate
per channel

96GB
10T
(target)

64GB

ith s
1T
n w tnes
Multi-level m
o
i
c
s
odul
a
32GB
400G
s
SNR enhanc ation &mi omp
s
ement
n
100G
c
tra d Subcarrier
QPSK 16Q
el r an
l
A
M
l
6
4
Q
A
M 256QA
multiplexing
ra e
MPa -poww
Spatiallo
Modulation level
multiplexing
Degree ofmultiplexing(frequency domain,
spatial domain)

Target area
Current achievement

Degree of spatial
multiplexing
40SDM

Increase spatial
multiplicity.

(a) Optical fiber transmission

11SDM

AM

Hig

rQ

r

e
igh

H

e
ord

her

256QAM

2 GHz

1024QAM

ban

dw

idth

10 GHzModulation level

Signal bandwidth(b) Millimeter-wave radio transmission
QPSK: quadrature phase-shift keying
SDM: space division multiplexing

Fig. 2. Three approaches in ultrahigh-speed communications.

To increase the channel speed in both wireless andoptical fiber communications, it is very important tochoose the right combination of the feasible abovementioned approaches considering their application
area and the maturity of the technologies.
The future technical trends of the abovementioned
approaches in line-of-sight millimeter-wave radiotransmission and optical fiber transmission are shownin Fig. 2. In the case of using single-channel polarization-division multiplexed (PDM) quadrature phaseshift keying (QPSK) (Approach 1 only) to realize
NTT Technical Review

Vol. 17 No. 5 May 2019

channel capacities over 1 Tbit/s, it is necessary toachieve a symbol rate of at least 300 Gbaud. However, it is very difficult to achieve such a high symbol
rate over 300 Gbaud using the latest front-end devices
and digital signal processing (DSP) circuits withanalog-to-digital converter (ADC) circuits and digital-to-analog converter (DAC) circuits. We therefore
need to reduce the required symbol rate by adopting
higher-order QAM and/or by using subcarrier multiplexing in the frequency or spatial domain.
An example of the latest digital coherent optical
7

Feature Articles

WDM
multiplexer

WDM
demultiplexer

Optical transmission line

OpticalamplifierOptical transmitter(Y-pol.)DAC

LD

Optical I/Q
modulator

DAC

ADC

ADC

Digitaldata signal

ADC

PDM

PDM

LD

Receiver DSP

DSP

Optical I/Q
modulator(X-pol.)DAC
DSP

Optical receiver front-end circuit
90-degree
hybridDAC
Digitaldata signal

Optical receiver

Optical transmitter front-end circuit

90-degree
hybridTransmitter DSP

ADCPol. DEMUX

Carrier recoveryyX-pol.
tx
Y-pol.

PDM-16QAM
modulation

PDM-QPSK
modulation

Waveform distortion andcrosstalk between twopolarization components

DSP
Equalization and demodulation by adaptive DSPDEMUX: demultiplexer
IQ: in-phase and quadrature
LD: laser diodePol.: polarization

Fig. 3. Example configuration of digital coherent optical transceiver circuit in an optical fiber transport system.

transceiver circuit configuration in an optical fibertransmission system is illustrated in Fig. 3 [2, 3]. In
this system, each channel is modulated in PDMQPSK format, and approximately 100 channels of
100-Gbit/s optical signals are wavelength-divisionmultiplexed to form a 10-Tbit/s aggregate capacity
signal and transmitted through an optical inline
amplified link over 1000 km.
Here, the optical transmitter consists of a transmitter DSP circuit and an optical transmitter front-end
circuit. Each polarization component from a continuous-wave signal laser diode (LD) is independently
modulated in QPSK format at a symbol rate of theorder of 32 Gbaud, and 100-Gbit/s PDM-QPSK signals (approximately 64 Gbit/s per polarization in
NTT Technical Review

Vol. 17 No. 5 May 2019gross capacity with error-correcting coding at a coding rate of R = 5/6) can be generated. If the modulation scheme is adaptively changed to 16QAM, it ispossible to double the channel capacity to 200 Gbit/s
at the same symbol rate.
Similarly, the optical receiver consists of a receiver
DSP circuit and an optical receiver front-end circuit.
An intradyne reception of PDM-QPSK signals is
performed using a wavelength-tunable local oscillating LD that has a frequency offset within a few gigahertz. Four lanes of received electrical tributary signals are fed to ADCs, and linear waveform distortionsof transmitted signals after the optical fiber transmission are digitally equalized to demodulate the QPSK(or 16QAM) signals. Finally, the error correcting
8

Feature Articles

Digital signal
processor
1 Ultrahighspeed modem

Transmitter

ReceiverSignal generation(encoding, modulation)

Signal recovery(decoding, demodulation)

Signal shaping((pre-) equalization)

Signal shaping(equalization)

DAC

ADC

2 Optical parametricamplification (opticalfiber transmission)

Front-end

Waveform
distortion,
noise, andinterference

Digital signal
processor

Front-end

Transmission
line

3 Ultrahigh-speed
integrated front-end

Parallel transmission schemes based on frequency, polarization, mode, etc.
4 OAM (millimeter-wave

wireless transmission)

Fig. 4. Key technologies to achieve future ultrahigh speeds for millimeter-wave radio transmission and optical fibertransmission.

code recovers the original data bit stream. In this case,the throughput in the DSP circuit exceeds 2 Tbit/s(≈8×2×(6/5)×100 Gbit/s) between the DSP circuit
part and the DAC/ADC circuit part, when the channel
capacity is 100 Gbit/s assuming 8-bit quantization, asampling rate of two samples per symbol, and anerror correction coding rate of R = 5/6.
In order to implement a compact and cost-effective
DSP circuit, it is preferable to integrate the DAC/ADC circuit part and the DSP circuit part on a singlechip large-scale integrated (LSI) circuit. Furthermore, it is essential to suppress the skew of the fourparallel broadband analog electrical signals (in-phaseand quadrature signal components for each of the Xand Y-polarization planes) between the DSP-ASIC(application-specific integrated circuit) and optical
front-end circuits for high-quality digital modulationand demodulation.
3. Fundamental technologies forultrahigh-speed communications infrastructure
The Feature Articles in this issue focus on the four
fundamental technologies of ultrahigh-speed transmission to achieve a capacity of 1 Tbit/s (Fig. 4), andthey introduce the current status and future prospectsof these technologies.
NTT Technical Review

Vol. 17 No. 5 May 2019

3.1 U
 ltrahigh-speed digital modulation/demodulation circuit technology
The ultrahigh-speed transmission technology usedin digital modulation and demodulation circuits hascommon technical issues for both optical fiber transmission systems and radio transmission systems. The
article “Ultrahigh-speed Optical Communications
Technology Combining Digital Signal Processing
and Circuit Technology” [4] discusses the latest
achievements relating to the required functions ofultrahigh-speed digital modulation/demodulation
circuits with capacities of over 1 Tbit/s per wavelength, especially in optical fiber transmission systems. In general, higher-order QAM digital modulation and demodulation require a higher SNR andmore precise DSP calibration techniques for simple
and stable operation. Here, we discuss the effectiveness of a novel coded modulation that makes full useof the features of higher-order QAM digital modulation.
3.2 L
 ow-noise high-power parametric amplifier
relay technology
Low-noise high-power parametric amplification
technology is promising for reducing system noise, as
needed for higher-order QAM signal transmission.
Optical signal processing based on the parametric
9Feature Articles

amplification greatly reduces the required complexityof DSP for high-speed higher-order QAM signaltransmission.
When the gate length of CMOS (complementary
metal oxide semiconductor) LSI circuits is reduced tojust a few nanometers (about 10 monolayers of silicon atoms), it is predicted that the conventional scaling law in terms of transistor switching speeds andpower consumption (namely Moore’s Law) will
gradually saturate. To achieve higher channel speeds,
this situation will naturally require new system architectures and novel technologies to reduce the amountof DSP and power consumption. In future opticalnodes, parametric optical amplifiers will achieve
simultaneous coherent optical signal processing of
WDM high-speed channels to reduce system noise
and the required DSP complexity of waveform distortion compensation of high-speed channels. They arealso expected to reduce the system’s overall power
consumption. The article “Low-noise Amplification
and Nonlinearity Mitigation Based on Parametric
Repeater Technology” [5] in this issue introduces a
low-noise, highly efficient parametric optical amplification system that uses PPLN (periodically-poled
Lithium Niobate) crystals developed at NTT’s laboratories.
3.3 U ltrahigh-speed optical front-end integration
technology
The article “Ultrahigh-speed Optical Front-end
Device Technology for Beyond-100-GBaud Optical
Transmission Systems” [6] introduces ultrahighspeed device interconnections between the front-end
circuits and DSP circuits. The novel architecture of
integrated front-end circuits equipped with an analog
multiplexing/demultiplexing function in the frontend circuits can relax the required operation speed of
the DAC/ADC in DSP circuits. It also reduces the
input and output speed at the front-end module interface to achieve stable interconnections between frontend circuits and DSP circuits. We present the latest
performance results of the proposed optical front-end
integrated technology for optical fiber communications at symbol rates in excess of 100 Gbaud.
3.4 U ltrahigh-speed technology based on orbital
angular momentum mode multiplexing
The article “Toward Terabit-class Wireless Transmission: OAM Multiplexing Technology” [7] introduces novel spatial multiplexing technology for realizing line-of-sight millimeter-wave wireless communications at speeds on the order of 1 Tbit/s. A new
NTT Technical Review

Vol. 17 No. 5 May 2019spatial degree of freedom based on the orbital angularmomentum (OAM) of an electromagnetic field isintroduced to increase the spatial multiplicity.
In recent research on optical fiber transmission,mode-multiplexed optical transmission technology
using multiple orthogonal waveguide modes was proposed to increase the transmission capacity beyond
the physical limit of existing SMF. It has recently
been shown that multiple-input multiple-output(MIMO)-DSP technology has great potential to realize long-haul inline amplified transmission over 6000
km [8]. In wireless systems, on the other hand, it isgenerally impossible to define orthogonal waveguidemodes as used in optical fiber, although conventional
MIMO spatial multiplexing based on transmission
path differences is widely used in commercial wireless communications.
A novel spatial multiplexing scheme using OAM
was recently proposed as a way to achieve 100-Gbit/schannel transmission in millimeter-wave wirelesscommunications [9]. This demonstration attracted
much attention as a candidate for further increasing
the speed of backhaul networks in 5G and post-5G
applications. To achieve such high-speed mode-multiplexed transmission systems, it is essential to effectively implement DSP circuits considering the tradeoff between the DSP complexity and reliable
dynamic characteristics to accommodate high-speed
channel fluctuations in each transmission line (freespace, optical fiber, etc.). Further progress inadvanced DSP architecture and novel combinationsof the abovementioned fundamental technologies are
expected.
4. Future prospects
This article introduced the latest ultrahigh-speed
communications technologies and future trends torealize a channel capacity over 1 Tbit/s in futuretransport networks. Further advances are expected inhighly efficient DSP technologies for ultrahigh-speed
channel transmission with careful consideration ofpractical DSP complexity economically implemented
in single-chip LSI circuits. In millimeter-wave communications, a new degree of freedom offered by
the OAM of electromagnetic waves in free spaceis expected to increase the channel speeds to over
1 Tbit/s. In optical fiber communications, long-haultransmission of high-speed channels beyond 1 Tbit/s
will be achieved by using a suitable combination ofadvanced DSP functions and integrated optical frontend technology. Optical parametric amplifier
10Feature Articles

technology and spatial multiplexing technology arepromising for achieving further increases in the channel speed while maintaining the current DSP complexity and power consumption in optical fiber communications. Through continuous efforts in researchand development and timely commercialization ofthese new technologies, terabit-per-second-class signals will be able to be easily handled in future transport networks.

[5]

[6]

References
[1]
[2]

[3]

[4]

K. Kohiyama and S. Komaki, “A 64/256QAM Digital Microwave
Transmission System,” J. IEICE, Vol. 68, No. 8, pp. 889–895, 1985 (inJapanese).
S. Suzuki, Y. Miyamoto, T. Tomizawa, T. Sakano, K. Murata, S. Mino,
A. Shibayama, M. Shibutani, K. Fukuchi, H. Onaka, T. Hoshida, K.
Komaki, T. Mizuochi, K. Kubo, Y. Miyata, and Y. Kamio, “R&D on
the Digital Coherent Signal Processing Technology for Large-capacity Optical Communication Networks,” J. IEICE, Vol. 95, No. 12, pp.
1100–1116, 2012 (in Japanese).
Y. Kisaka, M. Tomizawa, and Y. Miyamoto, “Digital Signal Processing (DSP) Circuits for Optical Transport Beyond 100G,” NTT Technical Review, Vol. 14, No. 9, 2016.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201609fa2.html
T. Kobayashi, F. Hamaoka, M. Nakamura, H. Yamazaki, M. Nagatani,
and Y. Miyamoto, “Ultrahigh-speed Optical Communications Technology Combining Digital Signal Processing and Circuit Technolo-

Yutaka Miyamoto

Senior Distinguished Researcher, Director,
Innovative Photonic Network Research Center,
NTT Network Innovation Laboratories.
He received a B.E. and M.E. in electrical engineering from Waseda University, Tokyo, in 1986
and 1988, and a Dr. Eng. from the University ofTokyo. He joined NTT Transmission Systems
Laboratories in 1988, where he engaged inresearch and development of high-speed opticalcommunications systems including the first
10-Gbit/s terrestrial optical transmission system(FA-10G) using EDFA (erbium-doped opticalfiber amplifier) inline repeaters. He was withNTT Electronics Technology Corporation
between 1995 and 1997, where he engaged in theplanning and product development of high-speed
optical modules at data rates of 10 Gbit/s andbeyond. Since 1997, he has been with NTT Network Innovation Labs, where he has been
researching and developing optical transporttechnologies based on 40/100/400-Gbit/s channels and beyond. He has been investigating andpromoting scalable optical transport networks
with Pbit/s-class capacity based on innovative
optical transport technologies such as digital
signal processing, space division multiplexing,
and cutting-edge integrated devices for photonic
pre-processing. He is a member of the Institute ofElectrical and Electronics Engineers (IEEE) anda Fellow of the Institute of Electronics, Information and Communication Engineers (IEICE).

NTT Technical Review

Vol. 17 No. 5 May 2019

[7]

[8]

[9]

gy,” NTT Technical Review, Vol. 17, No. 5, pp. 12–19, 2019.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201905fa2.html
T. Umeki, T. Kazama, T. Kobayashi, K. Enbutsu, R. Kasahara, and Y.
Miyamoto, “Low-noise Amplification and Nonlinearity Mitigation
Based on Parametric Repeater Technology,” NTT Technical Review,Vol. 17, No. 5, pp. 20–26, 2019.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201905fa3.html
M. Nagatani, H. Wakita, Y. Ogiso, H. Yamazaki, M. Ida, and H.
Nosaka, “Ultrahigh-speed Optical Front-end Device Technology for
Beyond-100-GBaud Optical Transmission Systems,” NTT Technical
Review, Vol. 17, No. 5, pp. 27–33, 2019.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201905fa4.html
D. Lee, H. Sasaki, Y. Yagi, T. Yamada, T. Kaho, and H. Hamada,“Toward Terabit-class Wireless Transmission: OAM Multiplexing
Technology,” NTT Technical Review, Vol. 17, No. 5, pp. 34–39, 2019.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201905fa5.html
K. Shibahara, T. Mizuno, D. Lee, Y. Miyamoto, H. Ono, K. Nakajima,
S. Saitoh, K. Takenaga, and K. Satoh, “DMD-unmanaged Long-haul
SDM Transmission over 2500-km 12-core × 3-mode MC-FMF and
6300-km 3-mode FMF Employing Intermodal Interference Cancelling Technique,” Proc. of the 41st Optical Fiber Communication
Conference and Exhibition (OFC 2018), Th4C.6, San Diego, CA,
USA, Mar. 2018.
D. Lee, H. Sasaki, H, Fukumoto, Y. Yagi, T. Kaho, H. Shiba, and T.
Shimizu, “An Experimental Demonstration of 28 GHz Band Wireless
OAM-MIMO (Orbital Angular Momentum Multi-input and Multioutput) Multiplexing,” Proc. of the 2018 IEEE 87th Vehicular Technology Conference (VTC 2018-Spring), Porto, Portugal, June 2018.

Shuichi Yoshino

Director, General Manager, NTT Network
Innovation Laboratories.
He received a B.E. and M.E. in mechanicalengineering from Kanazawa University in 1990
and 1992. He joined NTT in 1992 and worked on
the development of a satellite Internet system andwireless networking technologies. He has contributed to the practical application of active
radio frequency identification devices for logistics and wireless access technologies for gas
meter reading systems.

Akira Okada

Vice President, Head of NTT Device Technology Laboratories.
He received a B.S. and M.S. in physics in 1988
and 1990, and a Ph.D. in materials science in1993 from Keio University. He joined NTT in
1993 and conducted research on polymer-based
waveguide devices, full-mesh wavelength division multiplexing networks, optical packet
switching, and optical modules for access networks. From October 1997 to October 1998, he
was a visiting scholar at Stanford University, CA,
USA. He is a member of IEEE, IEICE, and theJapan Society of Applied Physics.

11

Feature Articles: Ultrahigh-speed Transmission Technology to
Support High-capacity Communications Infrastructure of the Future

Ultrahigh-speed OpticalCommunications Technology
Combining Digital Signal Processing
and Circuit Technology
Takayuki Kobayashi, Fukutaro Hamaoka,
Masanori Nakamura, Hiroshi Yamazaki,
Munehiko Nagatani, and Yutaka Miyamoto
Abstract

This article introduces ultrahigh-speed optical communications technology that achieves the opticaltransport networks needed to support an advanced information society. Most data traffic is carried on the
Ethernet; 400G Ethernet has recently been standardized, and discussions have also started on standardizing signal speeds in excess of 1 Tbit/s. This technology is used to economically transmit high-speed
client signals such as Ethernet signals on optical fiber networks. The combination of advanced digital
signal processing and ultrahigh-speed circuit technology can greatly improve the speed and quality of
optical signals and is expected to result in cost-efficient ultrahigh-speed optical communications at rates
in excess of 1 Tbit/s per channel.
Keywords: optical communications, digital signal processing, high-speed Ethernet

1. Introduction
Ultrahigh-speed optical communications technology is the fundamental technology that determines thetransmission performance of optical networks. In a
backbone network, multiple ultrahigh-speed client
signals such as 400G Ethernet (400GE; standardized
in 2017) are accommodated in high-speed opticalchannels based on digital coherent transmission technology [1], and multiple high-speed optical channels
are combined by wavelength division multiplexing(WDM) to implement a long-distance high-capacity
optical network. Meanwhile, the rate of internal datatransmission between servers in datacenters that support various services such as social networking andvideo distribution services is becoming very large,
and so is the demand for communications between
multiple datacenters. Compared with the backbone
NTT Technical Review

Vol. 17 No. 5 May 2019

network, the transmission distances are shorter, but
greater economy is required, and high-speed datatransmission is implemented by employing a technique called intensity-modulation direct-detection(IM-DD), which has a simple transceiver configuration.
2. Technical issues in increasing the opticalsignal data rate
Optical communications technology capable ofaccommodating 100-Gbit/s and 400 Gbit/s client signals per channel has already been put to practical use[2]. During the Ethernet standardization activities,
discussions were initiated and are continuing on thenext transmission speed standard, with 800 Gbit/s
and 1.6 Tbit/s being cited as possible candidates. In the
future, optical networks are expected to accommodate
12

Multicarrier

Feature Articles

Commercial technologies
100G/400G

3
2
1

Time

32

Spectrum

Higher-order modulation(more bits per symbol)

G

H
ig
he
r

ba
ud

96

ra
te

ud
Ba

64

QPSK 16QAM 64QAM

QAM: quadrature amplitude modulation
QPSK: quadrature phase-shift keying

Fig. 1. Three key elements of high-speed optical signals.

high-speed client signals at speeds of over 1 Tbit/s
per channel.
The three key elements of high-speed optical signals are shown in Fig. 1. Conventional IM-DD systems transmit information using binary light intensity
signals (either on or off), but by using the digital
coherent technique shown in Fig. 2, it is now possible
to use optical amplitude and phase information totransmit signals with four or more values per pulse. In
addition, by performing digital signal processing in
the transceiver, it is possible to equalize and compensate for wavelength deformation caused by polarization separation and wavelength dispersion and polarization mode dispersion that occur inside the opticalfibers, resulting in substantial increases in the transmission range and data transmission rate.
Currently, practical optical signals carrying data at
a rate on the order of 100 Gbit/s per channel areachieved by using polarization multiplexing to combine 4-value quadrature phase-shift keying (QPSK)
signals at a rate of 32 Gbaud. In addition, 400-Gbit/s
optical signals are achieved by using two carriers(wavelengths) to carry two 16-quadrature amplitude
modulation (QAM)*1 polarization multiplexed signals at 32 Gbaud in a single transmission channel.
There are also three key elements for speeding up
optical signals (Fig. 1). The first is to use a higher
baud rate (light pulse speed). If the baud rate isincreased, this provides a corresponding increase in
the transmission rate per wavelength. If signals have
NTT Technical Review

Vol. 17 No. 5 May 2019the same number of bits per symbol, then the baudrate can be increased without any excessive deterioration of reception sensitivity, but in order to transmit
and receive these high baud rate signals, it is necessary to use devices that are capable of operating athigh speed, including electrical devices such as digital-to-analog converters (DACs), analog-to-digitalconverters (ADCs), and optical devices such as optical modulators and balanced photo detectors (BPDs).
Also, in the frequency domain, the bandwidth occupied by optical signals increases in proportion to thebaud rate. This means that fewer signals can be combined by WDM, so unless this approach is combinedwith the use of a higher number of bits per symbol as
described later, the overall capacity of the transmission system will not increase.
The second element is the use of more bits per symbol. By increasing the number of optical amplitudelevels and phases used for signal transmission, it ispossible to increase the number of bits that can betransmitted in a single optical pulse. For a given baudrate, the transmission speed improves in proportion to
the number of bits per symbol*2. With more bits per
symbol, devices such as DACs and ADCs must havegreater resolution and linearity, and the required
signal-to-noise ratio (SNR) also increases. This
*1 QAM: A highly efficient digital modulation scheme where data
are transmitted as a series of multilevel codes by modulating theamplitude and phase of a carrier signal at multiple levels.

13

Feature Articles

Transmitted bit sequence
010011100011· · ·Optical transmitter

Optical receiver

Error correction
encoding
Symbol mappingUp-sampling

Digital signal
processing

Symbol demapping
Error correction decoding

DSP block forconstellation
shaping

Training seq. insertion

Frequency/phase offset
compensation

Digital signal
processing

Waveform shaping
Pre-equalization

LD1

Optical front-end
module

IQmodulator(X-pol.)IQmodulator(Y-pol.)Pol.
MUXPolarization multiplexed
transmitter

ADC: analog-to-digital converter
BPD: balanced photo detector
DAC: digital-to-analog converter

DSP: digital signal processor
IQ: in-phase and quadrature
LD: laser diodePolarization demultiplexing& equalization
Chromatic dispersion
estimation and compensation
Front-end error correction

DSP block forcompensation of
optical/frequency
response ofmodulesDAC DAC DAC DAC(XI) (XQ) (YI) (YQ)

Received bit sequence
010011100011· · ·

PBS

ADC
(XI)

ADC
(XQ)

ADC
(YI)

ADC
(YQ)

BPD

BPD

BPD

BPD

Optical90ºhybridOptical90ºhybridLD forlocal oscillator

MUX: multiplexer
PBS: polarization beam splitter
Pol.: polarization

Fig. 2. Schematic diagram of coherent optical transceiver.

decreases the maximum transmission distance, andreduces the resilience to signal distortion. Although
QAM with a square constellation is currently used, it
is possible to improve the required SNR by using amodified constellation.
The third element is the use of multicarrier transmission. By configuring one channel from opticalsignals of multiple wavelengths (carriers), it is possible to increase the capacity per channel in proportion to the number of carriers. This is an effective
method for configuring a logical high-speed channel
in an optical network. For example, it is possible toachieve a channel speed of 1 Tbit/s by bundling five
200-Gbit/s per wavelength optical wavelengths at different wavelengths into a single channel. However,since this increases the required number of transceivers, the abovementioned increases in baud rate andnumber of bits per symbol result in a higher transmission rate per wavelength but require the application of
NTT Technical Review

Vol. 17 No. 5 May 2019multicarrier technology that takes transmission performance and economy into consideration.
Thus, in order to attain ultrahigh-speed opticaltransmission at rates in excess of 1 Tbit/s per channel,
it is essential to increase the baud rate and the numberof bits per symbol, and while developing high-speed
devices, it is necessary to use highly sensitive digital
signal formatting and signal processing technologythat enable the device requirements to be relaxed.

*2 Improvement proportional to bits per symbol: For example, thenumber of bits per symbol is doubled by switching from QPSKwith a four-point constellation (4 = 22) to 16QAM with a 16point constellation (16 = 24); however, switching from 16QAM
to 64QAM only increases the number of bits per symbol by afactor of 1.5 (from 4 to 6).

14

Feature Articles

20

w/o Pre-EQL
w/ Pre-EQL

18
SNR (dB)

Optical power (dB/div)

Signal after
calibration

16
14

TheorySingle
WDM

12

−100

−50
0
50
Relative frequency (GHz)(a) Constellation and optical spectrum ofcalibrated 64-Gbaud 64QAM signal

100

10

20

24

28
32
36
Optical SNR (dB)40(b) Evaluation results of constellation-shaped
82-Gbaud 256QAM signal

EQL: equalization

Fig. 3. Experimental results employing calibration technology and constellation shaping.

3. Digital calibration technology andhigh-sensitivity technology to maximize
device performance
Signal degradation may occur when transmitting
and receiving high-speed signals, even if the bandwidth of the electrical device or optical device meetsthe respective requirements. For example, as shownin Fig. 2, a digital coherent transmitter requires four
DACs and two IQ (in-phase and quadrature) modulators for each polarization component. The receivingside requires four BPDs and four ADCs. Due to differences such as variations within manufacturing
tolerances, the characteristics of these devices and theconnections between them may not necessarily be
uniform when they are all interconnected. These sortsof errors can have a major impact on the signal quality of high baud rate signals and signals with a large
number of bits per symbol.
At our laboratory, we have managed to achieve
substantial improvements in signal quality by digitally correcting device imperfections on the receivingside, and by using digital signal processing on thetransmitting side to perform ultra-precise pre-equalization and calibration of the frequency characteristics and variation of devices in the transmitter. The
optical spectrum of a 64-Gbaud 64QAM signal
before and after calibration is shown in Fig. 3(a). Preequalization smooths out the optical spectrum andimproves its quality so that the indistinct signal constellation is resolved into a set of 64 clearly distinguishable points [3].
Also, with regard to the loss of sensitivity at a
NTT Technical Review

Vol. 17 No. 5 May 2019higher number of bits per symbol, we are studying
how the signal constellation can be adapted in order
to increase the signal sensitivity. Conventionally,
multilevel QAM signals such as QPSK, 16QAM, and
64QAM have used signal constellations in which
every point appears with equal probability. Althougha multilevel QAM signal can be generated from theoriginal bit sequence by using simple mapping anddemapping processes, this does not yield an optimal
signal constellation from an information theoreticalpoint of view.
Attention has recently been focused on information-theoretical signal point-shaping techniques thatcan be used to achieve near-optimal signal constellations by arranging the constellation points of multilevel QAM signals based on probability distributions.
Although this technique requires dedicated mappingand demapping processes, it enables the same number of data bits to be transmitted with a lower minimum SNR than that of multilevel QAM. Also, since
the number of data bits to be transmitted can bechanged by changing the probability distribution of
constellation points without changing the base multilevel QAM signal, there is no need to modify thesignal processing algorithm used for modulation anddemodulation.
Our laboratory has conducted a successful test demonstration of 800 Gbit/s per wavelength transmission
by using this pre-equalization calibration techniqueand 256QAM-based constellation shaping as shownin Fig. 3(b) [4]. With this technique, an optical
network can use one or two wavelengths to accommodate client signals of 800 Gbit/s and 1.6 Tbit/s,
15

Feature Articles

Digital pre-processing
AMUX

Target signal

+1

x 1/r

fc/2

fc

f

fc/2

Band divide+rff
Clock cfc

Pre-emphasis

f

fc/2

f

fc

Electrical high-speed signal
fc/2

DAC

f

fc

Target signal

Flip & add
fc/2

DAC

fc/2

Clock
+1
fc

f

fc

f

fc

Undesiredimage signal
f
−r

fc/2

f

fc

(a) Scheme using spectrum flip processing

OutputDigital pre-processing
Low-speed
signal
1

0

Real

+

DAC

AMUX

Imaginary

+

DAC

AMUX

Input
1

+

Frequency

Clock

Real

Low-speed
signal20Integrated circuit

Imaginary

+
+

DAC

LD

Clock input
2 × 2 mm2

OpticalIQ-modulator

AMUX
+

DAC

Input
2

AMUX

OutputInput Input
1
2
Clock input

13.6 × 13.6 × 5 mm3

Optical high-speed signal
Low-speed Low-speed
signalsignal21Frequency

Frequency(b) Scheme using addition/subtraction of low-speed signal

Fig. 4. Bandwidth doubling technologies using digital pre-processing and AMUX circuit.

which are candidate data rates for the next-generation Ethernet standard.
4. Bandwidth doubling technology forhigh-speed signal generation
One of the most important devices for generating
high-speed optical signals is the DAC. DACs are integrated with digital signal processing LSIs (largescale integrated circuits), but the analog bandwidth of
DACs fabricated using Si CMOS (silicon complementary metal oxide semiconductor) is currently
around 40 GHz. While the above calibration technique enables some correction, it becomes a bottleneck at high baud rates beyond 100 Gbaud. Our laboratory proposed bandwidth doubling technology thatcan double the analog bandwidth of a DAC. This
technology involves preliminary signal processing of
the desired wideband signal and outputting signals
NTT Technical Review

Vol. 17 No. 5 May 2019from two DACs. Using an analog multiplexer(AMUX) circuit [5] to combine these signals makes
it possible to generate high-speed signals with twice
the bandwidth.
We proposed the two bandwidth doubling methods
shown in Fig. 4, and by applying these two methods
to digital coherent transmission technology, we conducted an experimental demonstration of ultrahighbaud rate signal transmission using DACs with a
32-GHz analog bandwidth. We show in Fig. 5(a) the
results of a transmission experiment using a 120Gbaud signal generated using the spectral method
shown in Fig. 4(a). In this experiment, we successfully transmitted data at a net rate*3 of 600 Gbit/s per
wavelength over a long distance of approximately*3 Net rate/line rate: The net rate represents the number of data bits
transmitted per second as an optical signal. The line rate refers to
the entire data signal, including signals for supervisory control
and redundant bits for error correction.

16

PDM-8QAM
8D-16QAM optimal
8D-16QAM offset

10
9
8
7

PDM-8QAM Q-limit

65

8D-16QAM Q-limit
0

1000

2000
3000
Distance (km)4000(a) Transmission experiment result of 120-Gbaud signal
using spectrum flip processing (net rate: 600 Gbit/s)

0
−10
−20
−30
−40
−50
−100 −75 −50 −25 0 25 50 75 100
Frequency (GHz)

Quadrature (arb.unit)

Pre LDPC Q-factor (dB)11

Power spectral density (dB)

Feature Articles

2
1
0
−1
−2
−2 −1 012In-phase (arb. unit)(b) 120-Gbaud QPSK signal generation using
addition/subtraction of low-speed signal

LDPC: low-density parity check

Fig. 5. High-speed signal experiments using bandwidth doubling technologies.

4000 km [6]. Also, for the addition/subtraction processing method shown in Fig. 4(b), we made a prototype circuit integrated with two AMUX circuits, andwe successfully generated 120-Gbaud QPSK signals
as shown in Fig. 5(b) [7].
In theory, it is difficult to generate high-quality signals at 100 Gbaud or above using 40-GHz DACs,
because the DACs require a bandwidth of at least half
the baud rate. With our technique, it is possible toextend the bandwidth of existing DACs, enabling thesignal baud rate to be increased. In addition, by combining the abovementioned calibration and sensitivity
enhancement techniques, we have found that it is possible to achieve ultrahigh-speed signal transmission
at over 1 Tbit/s per wavelength without resorting tomulticarrier technology [8].
5. Application of ultrahigh-speed opticalcommunications technology toshort-distance communications
The simple and highly economical system configuration of the IM-DD method has attracted attention as
an optical communications technique for short-distance communications on the order of a few tens ofkilometers at most, where most traffic occurs between
servers or between datacenters. The recently standardized 400GE system uses PAM4 (4-level pulse
amplitude modulation) to produce four parallel
100-Gbit/s signals per wavelength and thus requires
four sets of transceiver equipment. At our laboratory,
we have performed successful short-distance transmissions over 20 km at a bit rate of 400 Gbit/s using
NTT Technical Review

Vol. 17 No. 5 May 2019a single transceiver by combining bandwidth doubling technology with an ultra-wideband indium
phosphide (InP) modulator [9, 10]. This is the world’s
highest transmission rate for IM-DD using a single
wavelength and a single polarization.
The results of this transmission test are shown in
Fig. 6. In this experiment, we used an InP opticalmodulator with an exceptionally wide bandwidth
together with a bandwidth doubler to generate awideband electrical signal and were able to generate
optical signals while maintaining the wide bandwidthof the electrical signal. Also, by using DMT (discrete
multitone) as the modulation method, we generated
256 subcarrier signals by digital signal processing,and by allocating appropriate signal bits to each carrier according to the frequency characteristics of theelectrical and optical devices, we achieved nearoptimal bit allocation.
For example, as shown in Fig. 6(b), a high-SNR
6.96-GHz subcarrier is used for 64QAM signals, anda low-SNR 9.10-GHz subcarrier is used for 16QAM
signals. The use of bandwidth doubling technology
and a wideband modulator has made it possible toallocate even multilevel signals such as 16QAM tothe high frequency domain. With this technology, it isexpected that transmission rates of over 400 Gbit/s
per wavelength can be achieved by IM-DD with asimple and economical transceiver configuration.
6. Summary
In this article, we introduced ultrahigh-speed optical transmission technology that combines digital
17

Feature Articles

AMUX-MZM
LD

EDFA
PD

TODC

400.16 Gbit/s

7

20

15

6
5
4

10

3

6.96 GHz

32.59 GHz

54.42 GHz 79.10 GHz

25

Probed SNR (dB)

ADC
(Digitaloscilloscope)

8

EDFA
Bits per subcarrier

Off-line PC

Ch 1
DAC
Clock
(Arbitrary
waveform x2
generator)
Ch 2

1
SSMF DCF
20/40 km

0

0

10

20

30

40

50

60

70

80

0

Subcarrier frequency (GHz)(a) Setup of 400-Gbit/s IM-DD transmission
experiment
DCF: dispersion compensation fiber
EDFA: erbium-doped fiber amplifier
MZM: Mach-Zehnder modulator
PC: personal computer(b) Signal quality of 400-Gbit/s signal after
20-km transmission

PD: photodiodeSSMF: standard single-mode fiber
TODC: tunable optical dispersion compensator

Fig. 6. Short reach application of digital-preprocessed analog-multiplexed DAC technology.

signal processing and high-speed circuit technology
to achieve optical networks that will form theinfrastructure of an advanced information society. We
have shown that this technology is capable of ultrahigh-speed optical transmission at over 1 Tbit/s per
wavelength. In the future, we will keep working on
improving the speed and will continue with our
research and development so that this technology canprovide a highly reliable communications infrastructure.
References
[1]

[2]

[3]

[4]

Y. Miyamoto, A. Sano, E. Yoshida, and T. Sakano, “Ultrahigh-capacity Digital Coherent Optical Transmission Technology,” NTT Technical Review, Vol. 9, No. 8, 2011.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201108fa2.html
Y. Kisaka, M. Tomizawa, and Y. Miyamoto, “Digital Signal Processor(DSP) for Beyond 100G Optical Transport,” NTT Technical Review,Vol. 14, No. 9, 2016.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201609fa2.html
A. Matsushita, M. Nakamura, F. Hamaoka, S. Okamoto, and Y. Kisaka, “High-spectral-efficiency 600-Gbps/Carrier Transmission Using
PDM-256QAM Format,” J. Lightw. Technol., Vol. 37, No. 2, pp.
470–476, 2019.
M. Nakamura, A. Matsushita, S. Okamoto, F. Hamaoka, and Y. Kisaka, “Spectrally Efficient 800 Gbps/Carrier WDM Transmission withNTT Technical Review

Vol. 17 No. 5 May 2019

100-GHz Spacing Using Probabilistically Shaped PDM-256QAM,”
Proc. of the 44th European Conference on Optical Communication(ECOC 2018), Rome, Italy, Sept. 2018.
[5] M. Nagatani, H. Yamazaki, F. Hamaoka, H. Nosaka, and Y. Miyamoto, “Bandwidth Doubler Technology for Increasing the Bandwidth of
an Optical Transmitter,” NTT Technical Journal, Vol. 29, No. 3, pp.
62–66, 2017 (in Japanese).
[6] M. Nakamura, F. Hamaoka, A. Matsushita, H. Yamazaki, M.
Nagatani, A. Hirano, and Y. Miyamoto, “Low-complexity Iterative
Soft-demapper for Multidimensional Modulation Based on Bitwise
Log Likelihood Ratio and Its Demonstration in High Baud-rate Transmission,” J. Lightw. Technol., Vol. 36, No. 2, pp. 476–484, 2018.
[7] F. Hamaoka, M. Nakamura, M. Nagatani, H. Wakita, H. Yamazaki, T.
Kobayashi, H. Nosaka, and Y. Miyamoto, “Electrical Spectrum Synthesis Technique Using Digital Pre-processing and Ultra-broadband
Electrical Bandwidth Doubler for High-speed Optical Transmitter,”Electron. Lett., Vol. 54, No. 24, pp. 1390–1391, 2018.
[8] M. Nakamura, F. Hamaoka, M. Nagatani, H. Yamazaki, T. Kobayashi,
A. Matsushita, S. Okamoto, H. Wakita, H. Nosaka, and Y. Miyamoto,“1.04 Tbps/carrier Probabilistically Shaped PDM-64QAM WDM
Transmission over 240 km Based on Electrical Spectrum Synthesis,”
Proc. of the 42nd Optical Fiber Communication Conference andExhibition (OFC 2019), paper M4I.4., San Diego, CA, USA, Mar.
2019.
[9] M. Nagatani, H. Wakita, Y. Ogiso, H. Yamazaki, M. Ida, and H.
Nosaka, “Ultrahigh-speed Optical Front-end Device Technology for
Beyond-100-GBaud Optical Transmission Systems,” NTT Technical
Review, Vol. 17, No. 5, pp. 27–33, 2019.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201905fa4.html[10] H. Yamazaki, M. Nagatani, H. Wakita, Y. Ogiso, M. Nakamura, M.
Ida, T. Hashimoto, H. Nosaka, and Y. Miyamoto, “Transmission of
400-Gbps Discrete Multi-tone Signal Using > 100-GHz-bandwidth
Analog Multiplexer and InP Mach-Zehnder Modulator,” Proc. of
ECOC 2018, Rome, Italy, Sept. 2018.

18

Feature Articles

Takayuki Kobayashi

Distinguished Researcher, Photonics Transport
Network Laboratory, NTT Network Innovation
Laboratories.
He received a B.E. and M.E. in communications engineering from Waseda University,
Tokyo, in 2004 and 2006. In 2006, he joined NTT
Network Innovation Laboratories, where heresearched high-speed and high-capacity digital
coherent transmission systems. He moved to
NTT Access Network Service Systems Laboratories in 2014 and studied fifth-generation mobile
optical network systems. He moved back to NTT
Network Innovation Laboratories in 2016 andhas been working on high-capacity optical transmission systems. His current research interests
are long-haul optical transmission systems
employing spectrally efficient modulation formats enhanced by digital signal processing. He
served as Technical Program Committee (TPC)
Member in the electrical subsystems category of
the Optical Fiber Communication Conference
from 2016 to 2018. He has been a TPC Memberin the point-to-point optical transmission category of the European Conference on Optical Communication since 2018. He is a member of theInstitute of Electronics, Information and Communication Engineers (IEICE) of Japan and theInstitute of Electrical and Electronics Engineers(IEEE)/Photonics Society.

Fukutaro Hamaoka

Senior Research Engineer, Photonics Transport
Network Laboratory, NTT Network Innovation
Laboratories.
He received a B.E., M.E., and Ph.D. in electrical engineering from Keio University, Kanagawa, in 2005, 2006, and 2009. From 2009 to 2014,
he was with NTT Network Service Systems
Laboratories, where he was engaged in the
research and development (R&D) of high-speed
optical communications systems, including digital coherent optical transmission system. His
research interests include high-speed and highcapacity optical transport systems with digital
signal processing. He was a recipient of the 2007
The Japan Society of Applied Physics Young
Scientist Presentation Award. He is a member of
IEICE.

Hiroshi Yamazaki

Distinguished Researcher, NTT Device Technology Laboratories and NTT Network Innovation Laboratories.
He received a B.S. in integrated human studies
in 2003 and an M.S. in human and environmental
studies in 2005, both from Kyoto University, anda Dr. Eng. in electronics and applied physics
from Tokyo Institute of Technology in 2015. He
joined NTT Photonics Laboratories, where hehas been involved in research on optical waveguide devices for communications systems. He isconcurrently with NTT Network Innovation
Laboratories and NTT Device Technology Laboratories, where he is involved in research on
devices and systems for optical transmission
using advanced multilevel modulation formats.
He has been an Associate Editor of IEEE Journalof Lightwave Technology since 2016. He is amember of IEICE.

Munehiko Nagatani

Distinguished Researcher, NTT Device Technology Laboratories and NTT Network Innovation Laboratories.
He received an M.S. in electrical and electronics engineering from Sophia University, Tokyo,
in 2007. He joined NTT Photonics Laboratories
in 2007, where he engaged in R&D of ultrahighspeed analog and mixed-signal integrated circuits (ICs) for optical communications systems.
He is currently with NTT Device Technology
Laboratories and NTT Network Innovation Laboratories, where he is involved in R&D of ultrahigh-speed ICs and systems for optical transmission using advanced modulation formats. He wasa recipient of the 2011 Young Researchers Award
by IEICE. He served as a technical program committee member for IEEE Compound Semiconductor Integrated Circuits Symposium (CSICS)
from 2014 to 2017. He has been serving as atechnical program committee member for IEEE
BiCMOS and Compound Semiconductor Integrated Circuits and Technology Symposium(BCICTS) since 2018 and IEEE International
Solid-State Circuits Conference (ISSCC) since2019. He is a member of IEICE and IEEE.

Yutaka Miyamoto

Masanori Nakamura

Research Engineer, Photonics Transport Network Laboratory, NTT Network Innovation
Laboratories.
He received a B.S. and M.S. in applied physics
from Waseda University, Tokyo, in 2011 and2013. He joined NTT Network Innovation Laboratories in 2013, where he has been engaged inresearch on high-capacity optical transport networks. His research interests include the modulation format optimization for optical transmission
and digital signal processing to calibrate imperfections of analog devices. He was a recipient of
the 2016 IEICE Communications Society Optical Communication Systems Young ResearchersAward. He is a member of IEICE.

NTT Technical Review

Vol. 17 No. 5 May 2019

Senior Distinguished Researcher, Director,
Innovative Photonic Network Research Center,
NTT Network Innovation Laboratories.
He received a B.E. and M.E. in electrical engineering from Waseda University, Tokyo, in 1986
and 1988, and a Dr. Eng. from the University ofTokyo. He joined NTT Transmission Systems
Laboratories in 1988, where he engaged in R&D
of high-speed optical communications systems
including the first 10-Gbit/s terrestrial opticaltransmission system (FA-10G) using EDFA
(erbium-doped optical fiber amplifier) inline
repeaters. He was with NTT Electronics Technology Corporation between 1995 and 1997,
where he engaged in the planning and productdevelopment of high-speed optical modules atdata rates of 10 Gbit/s and beyond. Since 1997,
he has been with NTT Network Innovation Labs,
where he has been researching and developing
optical transport technologies based on
40/100/400-Gbit/s channels and beyond. He has
been investigating and promoting scalable optical transport networks with Pbit/s-class capacity
based on innovative optical transport technologies such as digital signal processing, space
division multiplexing, and cutting-edge integrated devices for photonic pre-processing. He is amember of IEEE and a Fellow of IEICE.

19

Feature Articles: Ultrahigh-speed Transmission Technology to
Support High-capacity Communications Infrastructure of the FutureLow-noise Amplification and
Nonlinearity Mitigation Based on
Parametric Repeater Technology
Takeshi Umeki, Takushi Kazama, Takayuki Kobayashi,
Koji Enbutsu, Ryoichi Kasahara, and Yutaka Miyamoto
Abstract

Digital coherent technology has advanced greatly in recent years. Further enhancement of its potential
will require a breakthrough in both electrical digital signal processing and innovative technologies that
exploit the coherency of light. In this article, we describe repeater technology that uses optical parametric amplification (OPA) to minimize degradation in the signal-to-noise ratio in optical transport systems.
OPA can potentially achieve low-noise amplification and mitigate nonlinear impairments.
Keywords: nonlinear optics, optical parametric amplification, optical phase conjugation

1. Introduction
The recent advances in digital coherent technology
have increased the capacity of optical communications systems by improving spectral efficiency.
According to Shannon’s Theory, a signal must have ahigh signal-to-noise ratio (SNR) if we are to achievea system with high spectral efficiency. However, it
has been pointed out that any attempt to improve the
SNR is limited due to the accumulation of noise arising from optical amplifiers and signal waveform distortion (nonlinear impairments) caused by the nonlinear effect in the optical transmission fiber itself(Fig. 1(a)) [1]. New technologies designed to overcome this limit are now required if we are to improve
the SNR in optical transport systems.
2. Optical parametric amplification technology
based on periodically poled lithium niobate(PPLN) waveguides
With conventional optical amplifiers such as the
widely used erbium-doped fiber amplifier (EDFA),the noise figure cannot be reduced below the 3-dB
quantum limit, which means that a multi-repeater
NTT Technical Review

Vol. 17 No. 5 May 2019transmission accumulates excess noise (Fig. 1(b)).
Phase sensitive amplification (PSA), which has aphase-dependent amplification property, provides anideal noise figure of 0 dB [2]. In other words, it makes
optical amplification without SNR degradation possible (Fig. 1(b)).
Current digital coherent systems achieve long-haultransmission by using electrical digital signal processing in the transmitter/receiver to compensate forvarious types of signal waveform distortion that arepresent after signals are transmitted through opticalfiber. In the future, as transmission capacity is further
increased, signal waveform distortion in optical fiber(nonlinear impairment) will become apparent andwill limit the extension of the transmission distance(Fig. 1(c)).
Optical phase conjugation (OPC)*1 has the potential*1 OPC: Light has the same sort of wave-like properties as radio
waves, and the timing of a wave’s vibrations is called its phase. A
wave in which the positive and negative phases have been reversed is called a phase conjugate wave, and the process of phasereversal is called optical phase conjugation. A phase conjugate
wave is transmitted just as if it were traveling backwards in time,
like a movie being played backwards, and is therefore sometimes
called a time reversal wave.

20

Feature Articles(a) Factors limiting transmission distanceCentraloffices

Fiber

OpticalamplifierTransmitter

Receiver~100 km
Factor limiting transmission distance (2):signal distortion in transmission fiber itself

Factor limiting transmission distance (1):accumulation of excess noise of amplifiers

Q

Excessnoise

Conventional
amplifierOutputI

Input
Q
No excess 1/G
noise
OutputPSA(c) Optical phase conjugation (OPC): traveling backwards
in time, like a movie being played backwards
Signal distortion(b) Phase sensitive amplification (PSA): low noise
amplification without SNR degradation

Currenttechnology
Our technology

ulatesn accum
Distortio
Time reversal operation
ted. Distance(Invert the signal
is mitiga
istortionDdistortion)

I
Signal distortion

Input

Mitigation bytime reversal(d) Key device: PPLN

SignalAmplified signal
Phase conjugated signal

Pump

SignalPump

Nonlinear material(e.g., PPLN)

AmplifiedsignalPhase
conjugatedsignalFig. 1. Optical parametric amplification technology based on periodically poled lithium niobate (PPLN) waveguides.

to mitigate the waveform distortion that occurs during optical fiber transmission by generating a time
reversal wave. This technique can be likened to playing a movie backwards. If OPC is performed at apoint in the middle of the optical fiber transmission
path, the signal distortion that occurred in the firsthalf of the transmission path can be compensated forin the second half. This makes it possible to improve
the SNR by increasing the power of the optical signals. It also offers the possibility of simultaneously
processing wavelength division multiplexed channels
with a single optical phase conjugator. This is expected to greatly reduce both the amount of digital signal
processing needed for distortion compensation andthe electrical power consumption.
NTT Technical Review

Vol. 17 No. 5 May 2019

The PSA/OPC is achieved by optical parametricamplification (OPA), which transfers the energy of an
intense pump to the signal (Fig. 1(d)). PSA/OPC canbe implemented by inputting the signal and the pump,which is approximately twice the frequency of thesignal light, into a nonlinear optical medium while
appropriately adjusting the wavelength assignments
and the phase relation of the signal light and pumplight.
Our research group has developed PPLN*2 as anonlinear optical medium. We have developed aridge-shaped waveguide structure that is highly resistant to photorefractive damage [3], and high-precision waveguide fabrication technology [4], whichenables us to both utilize a high-power pump and21

Feature Articles(a) Single channel, single pol., binary signal(b) Gain of our PPLN module2   + 
(Degenerate OPA)Q
I

Image ofdegenerate OPA

Amplification20

In-phasesignalQuadraturephase signal

2 Frequency
Pump

Single pol. configuration

Singlepol.

Intensity (dBm)
Degenerate signal

10

/2

Attenuation

Pump

−20
−30
−40

Pump

−50
−60
1553.11553.21553.31553.41553.5 1553.6

Wavelength (nm)(c) WDM, PDM, QAM signal

I

OutputInput

Gain:>25 dB−10

OPA
Expansion of
applicability

Q0(d) Experimental result for PSA of PDM- and WDM-16QAM signals using both
nondegenerate parametric amplification and the polarization diversity configuration
Signals2  1 + 2 (1 2)(Nondegenerate OPA)

IdlersPSA (Q = 9.8 dB)

2Idlers1 Signals
Y-pol.

2 Frequency
Pump

Pol. diversity configuration
PBS

X-pol.
Pol. rotator

OPA
OPA
Pump

PBS: polarization beam splitter
PDM: polarization division multiplexing
Pol.: polarization

Intensity (dBm)

−10

−20
X-pol.

Y-pol.
EDFA (Q = 8.4 dB)−30
PSA out
EDFA out
Input

−40
1540

1545

1550

1555

1560

1565

X-pol.

Y-pol.

Wavelength (nm)QAM: quadrature amplitude modulation
WDM: wavelength division multiplexing

Fig. 2. Increased gain and expanded applicability of phase sensitive amplifier.

achieve high conversion efficiency. We have also
developed a fiber-pigtailed module that integrates a
PPLN waveguide and couplers that split/multiplex
pump light and signal light that have widely different
wavelengths and thus achieve fiber input and output
easily and stably (Fig. 1(d)). The module has four
input/output fibers suitable for the respective wavelengths of the signal and pump light beams, anddielectric multilayer mirrors for multiplexing or splitting the signal and pump light beams. This structure
achieves both high stability and low-loss optical coupling between the optical fiber and a PPLN waveguide.
3. Increased gain and expanded applicability ofphase sensitive amplifier
In the OPA process, energy is transferred from apump with a frequency of 2ω to a signal with a freNTT Technical Review

Vol. 17 No. 5 May 2019quency of ω1 and an idler with a frequency of ω2. If
ω1 = ω2, the process is called degenerate OPA, and a
PSA can be obtained with gain for the in-phase component and attenuation for the quadrature phase component with reference to the pump phase (Fig. 2(a)).
At the start of this research, the gain of a PPLN module was around 6 dB (quadruple). Now, having
improved the efficiency of the PPLN waveguide andgreatly reduced the coupling loss of the module,
we can achieve a gain exceeding 25 dB (a 320fold increase) (Fig. 2(b)), which can sufficiently*2 PPLN: Lithium niobate (LiNbO3) is a crystalline material thatexhibits nonlinear optical effects that enable light waves of different wavelengths to interact with each other. PPLN is an artificial crystal in which the orientation of positive and negative
charges in the crystal is forcibly inverted with a fixed period inside the crystal by spontaneous polarization. With PPLN, it ispossible to achieve an overwhelmingly high nonlinear optical effect compared with the original lithium niobate crystal.

22

Feature Articles

compensate for the loss in the fiber transmission
span.
It is also extremely important to ensure compatibility
with the wavelength division multiplexing (WDM)
and digital coherent technology used in existing optical communications systems. We have not only
improved the performance of PPLN devices but also
expanded the applicability of PSA because the abovementioned degenerate OPA was only able to handle abinary modulation signal and a single wavelength. In
addition, it could only amplify a single polarizedwave since a second-order nonlinear optical medium
such as PPLN is usually polarization dependent(Fig. 2(a)).
We have achieved the amplification of a quadratureamplitude modulation (QAM) signal and the simultaneous amplification of WDM signals (Fig. 2(c))
using nondegenerate OPA (ω1 ≠ ω2) in a parametric
process. We have also achieved PSA of a polarization
division multiplexing (PDM) signal by using a polarization diversity configuration. In this configuration,
the input signal is split with a polarization beam splitter (PBS), independently amplified with two OPA
devices, and recombined using a PBS.
An experimental result for PSA of PDM- andWDM-16QAM signals using both nondegenerateparametric amplification and the polarization diversity configuration is shown in Fig. 2(d) [5]. We
placed 16 carrier waves at intervals of 100 GHz at thetransmitter and generated 20-Gbaud PDM-16QAM
signals with pairs of phase-conjugated light beams.
An 80-km dispersion-compensated fiber was used as
the transmission line. At the receiver, one of the
WDM signals was extracted using an optical filter,received by a digital coherent receiver, and demodulated using off-line signal processing. As shown in
the input and output optical spectra in Fig. 2(d), all 16
WDM signals were simultaneously amplified with again of over 20 dB.
This figure shows the output spectrum of an EDFA
with the same gain for comparison with the PSA.
This comparison indicates that the optical SNR(OSNR) of the PSA was about 5 dB higher than thatof the EDFA. The constellations of the PSA and the
EDFA that were received and demodulated by acoherent receiver are also shown in Fig. 2(d). Distinct
symbol separation with the demodulated signal for
the PSA was clearly obtained as a result of low-noise
amplification with a difference in signal quality (Qfactor) of about 1 dB, which corresponded to the difference in OSNR.

NTT Technical Review

Vol. 17 No. 5 May 2019

4. Proposal for complementary spectral
inversion OPC and mitigation of
nonlinear impairments
Digital signal processing can compensate not only
for linear signal distortion but also for a nonlinear
signal. However, further enhancement of the compensation performance requires an increase in the size ofthe signal processing circuit, which results in an
increase in power consumption. OPC has long beenstudied with the aim of compensating for signal distortion in the optical domain without electrical signal
processing. It was difficult to implement thisapproach with a high-capacity optical transmission
system because conventional OPC occupies twice thebandwidth due to wavelength conversion, thereby
reducing the spectral efficiency to less than half. To
overcome this problem, we have developed a new
optical signal processing circuit that spatially separates WDM signals into long-wavelength and shortwavelength signal channel groups and then applies
OPC to each group using high-efficiency PPLN
waveguide devices (Fig. 3(a)). Thus, we haveachieved complementary spectrally inverted OPC
that simultaneously compensates for the signal distortions of WDM signals without sacrificing the spectral efficiency.
An experimental result for the simultaneous phaseconjugation of WDM signals is shown in Fig. 3(b).
We used 22.5-GBaud polarization multiplexed
16QAM signals with 92 WDM channels with a
25-GHz spacing. The optical spectrum at the transmitter and that after transmission over 3840 km (12times recirculating loop transmission through a 320km line) are also shown in Fig. 3(b). A comparison of
the optical spectra indicates that the original signal
bandwidth was retained, although the short- andlong-wavelength bands, each with 46 channels, were
exchanged 12 times. In addition, guard-band-lessconversion was achieved except for one channel inthe middle between the short- and long-wavelength
bands, which is the pump bandwidth. We thenachieved the highest-level transmission experiment
yet reported in terms of both capacity (13.6 Tbit/s)and spectral efficiency (5.84 bit/s/Hz) using OPC [6].
We also conducted a transmission experiment using
96-Gbaud polarization multiplexed 8QAM signals inorder to verify the applicability of this system to signals of 400 Gbit/s per channel. The transmission distances with and without OPC for respective optimal
input powers of +6 dBm and +2 dBm are compared inFig. 3(c). This means that the use of phase conjugation
23

Feature Articles(a) New optical signal processing circuit

Pump attelecom band

SignalE1E2E3…

Input

H/LPF

Complementary spectrally
inverted OPC

Conventional OPC

Complementary spectrally inverted OPC
Sub-OPC 1(PPLN waveguide) Combine
OutputHigh-to-low conversion
L/HPF

SignalSub-OPC 2(PPLN waveguide)

Conjugated signal

Low-to-high conversion

Conversion
Conjugated signal
En*…E*E*E*
3

Unnecessary
band afterconversion

2

1

Wide guard band

…E*E*E*
3 2 1

Narrow guard band(c) Transmission distance with andwithout OPC(b) Experimental result for simultaneous phase conjugation of
WDM signals

9
12 times complementary spectral inversion(25 GHz × 92 ch)
Pump bandwidth:
25 GHz92 WDM channel input signals
After 3840-km transmission(After 12 optical phase conjugations)
1530

1535

With OPC @ +6 dBm

8Signal bandwidth: 2.3 THz

Q-factor (dB)Intensity (10 dB/div)(Our
proposal)
En

Reservedband

Conversion

High/lowfrequency
separation

Second harmonic

E1E2E3… (SH) pump

1540

7

Without OPC@+2 dBm

6

5

1545

Wavelength (nm)
4

0

2000

4000

6000

8000

10,000Distance (km)Fig. 3. Complementary spectrally inverted OPC.

enables a higher power optical signal to be input intoa transmission fiber. Thus, it was demonstrated thatthe maximum transmission distance (when the forward error correction threshold value Q = 5 dB) canbe extended from 7040 km to 9600 km. This was thefirst demonstration showing that the use of phaseconjugation to compensate for signal distortion canbe applied to an ultrahigh-speed baud signal at a levelof 400 Gbit/s [7].
5. Future prospects
This article introduced the research and development (R&D) of OPA for optical communications
with a view to improving the SNR in optical transportsystems. This technology provides low-noise amplification and compensation for optical signal distortionand is also expected to lead to the generation/amplification of coherent light with various wavelengths
NTT Technical Review

Vol. 17 No. 5 May 2019using wavelength conversion techniques, and toquantum information processing such as squeezed
light and photon pair generation. By further exploring
this technology, we aim to create innovative technology that fully exploits optical coherence. Part of thisresearch uses the results of “R&D on Optical Signal
Transmission and Amplification with Frequency/
Phase Precisely Controlled Carrier” commissionedby the National Institute of Information and Communications Technology of Japan.
References
[1]
[2]
[3]

R.-J. Essiambre, G. Kramer, P. J. Winzer, G. J. Foschini, and B. Goebel, “Capacity Limits of Optical Fiber Networks,” J. Lightw. Technol.,Vol. 28, No. 4, pp. 662–701, 2010.
C. M. Caves, “Quantum Limits on Noise in Linear Amplifiers,” Phys.
Rev. D, Vol. 26, No. 8, pp. 1817–1839, 1982.
Y. Nishida, H. Miyazawa, M. Asobe, O. Tadanaga, and H. Suzuki,“0-dB Wavelength Conversion Using Direct-bonded QPM-Zn:
LiNbO3 Ridge Waveguide,” IEEE Photon. Technol. Lett., Vol. 17, No.

24

Feature Articles

[4]
[5]

5, pp. 1049–1051, 2005.
T. Umeki, O. Tadanaga, and M. Asobe, “Highly Efficient Wavelength
Converter Using Direct-bonded PPZnLN Ridge Waveguide,” IEEE J.
Quantum Electron., Vol. 46, No. 8, pp. 1206–1213, 2010.
T. Umeki, T. Kazama, T. Kobayashi, S. Takasaka, Y. Okamura, K.
Enbutsu, O. Tadanaga, H. Takenouchi, R. Sugizaki, A. Takada, R.
Kasahara, and Y. Miyamoto, “Polarization-diversity In-line Phase
Sensitive Amplifier for Simultaneous Amplification of Fiber-transmitted WDM PDM-16QAM Signals,” Proc. of the 41st Optical Fiber
Communication Conference and Exhibition (OFC 2018), paper
M3E.4, San Diego, CA, USA, Mar. 2018.

NTT Technical Review

Vol. 17 No. 5 May 2019

[6]

[7]

T. Umeki, T. Kazama, A. Sano, K. Shibahara, K. Suzuki, M. Abe, H.
Takenouchi, and Y. Miyamoto, “Simultaneous Nonlinearity Mitigation in 92 × 180-Gbit/s PDM-16QAM Transmission over 3840 km
Using PPLN-based Guard-band-less Optical Phase Conjugation,”Opt. Exp., Vol. 24, No. 15, pp. 16945–16951, 2016.
T. Kobayashi, T. Umeki, R. Kasahara, H. Yamazaki, M. Nagatani, H.
Wakita, H. Takenouchi, and Y. Miyamoto, “96-Gbaud PDM-8QAM
Single Channel Transmission over 9,600 km by Nonlinear Tolerance
Enhancement Using PPLN-based Optical Phase Conjugation,” Proc.
of OFC 2018, paper Th3E．4, San Diego, CA, USA, Mar. 2018.

25

Feature Articles

Takeshi Umeki

Distinguished Researcher, HeterogeneousMaterials and Devices Research Group, Materials and Devices Laboratory, NTT Device Technology Laboratories, Innovation Photonic Network Center, NTT Network Innovation Laboratories.
He received a B.S. in physics from Gakushuin
University, Tokyo, in 2002 and an M.S. in physics and Ph.D. in the area of nonlinear optics fromthe University of Tokyo in 2004 and 2014. He
joined NTT Photonics Laboratories in 2004,
where he has since been researching nonlinear
optical devices based on PPLN waveguides. He
is a member of the Japan Society of Applied
Physics (JSAP), the Institute of Electronics,
Information and Communication Engineers(IEICE), and the Institute of Electrical and Electronics Engineers (IEEE)/Photonics Society.

Takushi Kazama

Researcher, Heterogeneous Materials andDevices Research Group, Materials and Devices
Laboratory, NTT Device Technology Laboratories.
He received a B.S. and M.S. in electrical engineering from the University of Tokyo in 2009 and2011. He joined NTT Device Technology Laboratories in 2011, where he has been researching
PPLN waveguide devices. He is a member of
IEICE and JSAP.

Takayuki Kobayashi

Distinguished Researcher, Photonics Transport
Network Laboratory, NTT Network Innovation
Laboratories.
He received a B.E. and M.E. in communications engineering from Waseda University,
Tokyo, in 2004 and 2006. In 2006, he joined NTT
Network Innovation Laboratories, where heresearched high-speed and high-capacity digital
coherent transmission systems. He moved to
NTT Access Network Service Systems Laboratories in 2014 and studied fifth-generation mobile
optical network systems. He moved back to NTT
Network Innovation Laboratories in 2016 andhas been working on high-capacity optical transmission systems. His current research interests
are long-haul optical transmission systems
employing spectrally efficient modulation formats enhanced by digital signal processing. He
served as Technical Program Committee (TPC)
Member in the electrical subsystems category of
the Optical Fiber Communication Conference
from 2016 to 2018. He has been a TPC Memberin the point-to-point optical transmission category of the European Conference on Optical Communication since 2018. He is a member of IEICE
and IEEE/Photonics Society.

NTT Technical Review

Vol. 17 No. 5 May 2019

Koji Enbutsu

Research Engineer, Heterogeneous Materials
and Devices Research Group, Materials andDevices Laboratory, NTT Device Technology
Laboratories.
He received a B.E. and M.E. in electronics
engineering from Hokkaido University in 1994
and 1996. He joined NTT Opto-electronics Laboratories in 1996, where he conducted researchon organic optical waveguides for optical communications and electro-optic crystals and theirdevices. In 2007, he moved to NTT Access Network Service Systems Laboratories, where heengaged in research on optical fiber testing andmonitoring. He is a member of IEICE and JSAP.

Ryoichi Kasahara

Senior Research Engineer, Supervisor, Heterogeneous Materials and Devices Research Group,
Materials and Devices Laboratory, NTT Device
Technology Laboratories.
He received a B.S. from the University ofElectro-Communications, Tokyo, in 1995 and an
M.S. from Tohoku University, Miyagi, in 1997.
He joined NTT Opto-electronics Laboratories in
1997, where he was involved in research on silica-based planar lightwave circuits, including
thermo-optic switches, arrayed-waveguide grating multiplexers, and integrated optoelectricalreceiver modules. He is currently with NTT
Device Technology Laboratories, where he has
been involved in R&D of the fabrication technologies of optical dielectric waveguide devices.
He is a senior member of IEICE and a member of
JSAP.

Yutaka Miyamoto

Senior Distinguished Researcher, Director,
Innovative Photonic Network Research Center,
NTT Network Innovation Laboratories.
He received a B.E. and M.E. in electrical engineering from Waseda University, Tokyo, in 1986
and 1988 and a Dr. Eng. from the University ofTokyo. He joined NTT Transmission Systems
Laboratories in 1988, where he engaged in R&D
of high-speed optical communications systems
including the first 10-Gbit/s terrestrial opticaltransmission system (FA-10G) using EDFA
inline repeaters. He was with NTT Electronics
Technology Corporation between 1995 and1997, where he engaged in the planning andproduct development of high-speed optical modules at data rates of 10 Gbit/s and beyond. He has
been with NTT Network Innovation Labs since1997, where he has been researching and developing optical transport technologies based on
40/100/400-Gbit/s channels and beyond. He has
been investigating and promoting scalable optical transport networks with Pbit/s-class capacity
based on innovative optical transport technologies such as digital signal processing, space
division multiplexing, and cutting-edge integrated devices for photonic pre-processing. He is amember of IEEE and a Fellow of IEICE.

26

Feature Articles: Ultrahigh-speed Transmission Technology to
Support High-capacity Communications Infrastructure of the Future

Ultrahigh-speed Optical Front-end
Device Technology for Beyond-100GBaud Optical Transmission Systems
Munehiko Nagatani, Hitoshi Wakita, Yoshihiro Ogiso,
Hiroshi Yamazaki, Minoru Ida, and Hideyuki Nosaka
Abstract

To support sustainable progress of optical communications, intense research and development (R&D)
is being conducted to expand the transmission capacity per channel (transmission capacity per wavelength). Beyond-100-GBaud high-symbol-rate optical transmission technology is now attracting a great
deal of attention for its use in constructing future cost-effective optical transport networks. This article
introduces recent trends and challenges in optical communications and presents NTT’s recent R&D inultrahigh-speed optical front-end device technology for beyond-100-GBaud systems.
Keywords: high-symbol-rate optical transmission, bandwidth doubler, optical front-end device

1. Trends and challenges in
optical communications
The amount of data traffic in optical communications networks continues to grow exponentially due
to the spread of broadband applications and services
such as video streaming, cloud computing, and IoT(Internet of Things). In particular huge-capacity andlong-haul transmission technology is required in thecore network in order to accommodate client data andto link metropolitan areas. Novel digital coherent
technology, which combines coherent detection anddigital signal processing, has been deployed to copewith such rapid growth in communications traffic [1].
To date, 100-Gbit/s-per-channel (wavelength) systems based on 32-GBaud polarization division multiplexing (PDM) quadrature phase-shift keying and
400-Gbit/s-per-channel systems based on two-subcarrier 32-GBaud PDM 16-ary quadrature amplitude
modulation (16QAM) have been put into practicaluse.
In the future, transmission capacity per channel is
expected to exceed 1 Tbit/s to handle the ever-growing communications traffic. The transmission capacNTT Technical Review

Vol. 17 No. 5 May 2019ity can be increased by increasing the symbol rate,increasing the modulation order, or adding more subcarriers. Increasing the symbol rate is the most advantageous approach from the viewpoint of ensuring
both cost effectiveness and transmission distance.
Therefore, high-symbol-rate beyond-100-GBaud
optical transmission technology is now attracting agreat deal of attention for its use in constructing
future optical transport systems with capacities
exceeding 1 Tbit/s per channel.
Researchers face several challenges in constructing
an optical transceiver enabling beyond-100-GBaud
systems. A block diagram of a conventional opticaltransceiver for digital coherent systems is shown in
Fig. 1. To construct a 100-GBaud system, each building block in the transceiver needs to have at least a
50-GHz analog bandwidth, which is the Nyquist frequency of 100 GBaud. One of the biggest challenges
is finding a way to overcome the analog-bandwidth
limitation of digital-to-analog converters (DACs) andanalog-to-digital converters (ADCs), which are fabricated using Si (silicon) complementary metal oxide
semiconductor (CMOS) technology.
The analog-bandwidth performance of cutting-edge
27

Feature Articles

Receiver (Rx)

Transmitter (Tx)

DRV

DAC

DRV

DAC

LD

DSP
DAC

DRV

DAC

DRV

X-pol.
I-ch.

Optical90
degreehybridX-pol.
Q-ch.
Optical I/Q
modulator
Optical I/Q
modulator

PBS

PBC

Y-pol.
Q-ch.

ADC: analog-to-digital converter
Ch.: channel
DAC: digital-to-analog converter
DRV: optical modulator driver

TIA

ADC

TIA

ADC
DSP

PD
Y-pol.
I-ch.
Y-pol.
Q-ch.

TIA

ADC

TIA

ADC

Optical front-end (Rx)

Optical front-end (Tx)Tx-bandwidthbottleneck

X-pol.
Q-ch.

PD

LD
Optical90
degreehybridY-pol.
I-ch.

X-pol.
I-ch.

DSP: digital signal processor
I/Q: in-phase and quadrature
LD: laser diode
PBC: polarization beam combiner

Rx-bandwidthbottleneck

PBS: polarization beam splitter
PD: photodiodePol.: polarization
TIA: trans-impedance amplifierFig. 1. Block diagram of conventional optical transceiver for digital coherent systems.

Analog bandwidth (GHz)

Required bandwidth for
100-GBaud systems
50
40
30

Fujitsu(2012)

Expected performance

K. Kull et al.,
(ISSCC2014)

T. Drenski et al.,
(OFC2018)

20
10

J. Cao et al.,
(ISSCC2017)
Fujitsu H. Huang et al.,(2012)(RFIC2014)

45/40

32/28

20

K. Kull et al.,
(ISSCC2018)

16/14

ADC
DAC

10/7

5

3

CMOS technology (process node) (nm)Fig. 2. Cutting-edge CMOS-based DACs and ADCs (analog bandwidth performance vs. CMOS technology).

CMOS-based DACs and ADCs is shown in Fig. 2.
This graph indicates that it is very hard for CMOSbased DACs and ADCs to satisfy the target analogbandwidth of over 50 GHz. One more unavoidable
challenge is determining how to integrate and assemble optical front-end devices—modulator drivers(DRVs) and optical modulators on the transmitterside and trans-impedance amplifiers and photodiodeson the receiver side—into a packaged module to
avoid a degradation in quality of beyond-100-GBaud
signals due to extra loss derived from packaging.
NTT Technical Review

Vol. 17 No. 5 May 2019

Hence, integration and packaging technology
becomes much more important in beyond-100GBaud systems.
2. Bandwidth doubler technology
We have devised novel bandwidth doubling technology to overcome the analog-bandwidth limitationof CMOS-based DACs and ADCs [2]. A block diagram of an optical transceiver applying the bandwidth doubler is shown in Fig. 3. On the transmitter
28

Feature Articles

Transmitter (Tx)

Sub-DAC
Sub-DAC
Sub-DAC
Sub-DAC

AMUX

DRV

AMUX

DRV

Sub-DAC
Sub-DAC
Sub-DAC
Sub-DAC

AMUX

DRV

AMUX

X-pol.
I-ch.

DRV

PBS

PBC

Y-pol.
Q-ch.

Sub-ADC

TIA

ADE
MUX

Sub-ADC

TIA

ADE
MUX

Sub-ADC

TIA

ADE
MUX

Sub-ADCPD

LD

B

Y-pol.
I-ch.
Y-pol.
Q-ch.

ADEMUX

2B

To DRVClock

ADEMUX: analog demultiplexer
AMUX: analog multiplexer

Sub-ADC

Sub-ADC
Sub-ADCf
B

SubADC

f
B

2B

DSP

f

From TIA

B

B

CMOS

Sub-ADC

DSP

PD

Doubling
ff

SubDAC

Doubling

B

DSP

ADE
MUX

Rx bandwidth doubler
InP HBTInP HBT

SubDAC

TIA

Optical front-end (Rx)

Tx bandwidth doubler

AMUX

X-pol.
Q-ch.

Optical90
degreehybridY-pol.
I-ch.

Optical front-end (Tx)fX-pol.
I-ch.

Optical90
degreehybridX-pol.
Q-ch.
Optical I/Q
modulator
Optical I/Q
modulator

LD

DSP

Receiver (Rx)

SubADC

Clock

CMOS

InP HBT: indium phosphide heterojunction bipolar transistor

Fig. 3. Block diagram of optical transceiver applying bandwidth doubler technology.

side, two pre-processed analog signals from CMOSsub-DACs are multiplexed into one double-bandwidth signal by the analog multiplexer (AMUX). On
the receiver side, one broadband signal is demultiplexed into two half-bandwidth signals by the analog
demultiplexer (ADEMUX), and they are digitizedand post-processed by the following CMOS-subADCs and digital signal processor. Using this bandwidth doubler technology, we can expand the usablebaseband signal bandwidth twice and achieve twice
the symbol rate compared with a conventional transceiver (which is why we call this technology bandwidth doubler).
The AMUX and ADEMUX integrated circuits(ICs) for the bandwidth doubler were designed andfabricated using our in-house indium phosphide heterojunction bipolar transistor (InP HBT) [3]. We havealready succeeded in conducting a proof-of-principle
experiment and beyond-100-GBaud optical transmission [4]. In addition, we recently demonstrated theworld’s first 1-Tbit/s-per-channel long-haul WDM
NTT Technical Review

Vol. 17 No. 5 May 2019(wavelength division multiplexing) optical transmission using 120-GBaud probabilistically shaped PDM
64QAM [5]. These results confirmed that the bandwidth doubler is promising for future beyond-100GBaud systems.
3. The latest AMUX IC and 160-GBaud
signal generation
We are now developing faster AMUX and ADEMUX ICs to further improve optical transmission
performance. Using newly developed in-house 0.25μm InP HBT technology [6], we succeeded in developing an AMUX IC with a bandwidth over 110 GHz(a world record) in 2018 [7]. The performance of the0.25-μm InP HBTs and AMUX IC is summarized inFig. 4. The fabricated HBTs have a peak fT (cutoff
frequency) and fmax (maximum oscillation frequency)of 460 and 480 GHz, respectively. The AMUX IC
consists of two input buffers, a clock buffer, an
AMUX core, and an output buffer. It is designed to29

Feature Articles

0.25-µm InP HBTAMUX IC diagram (left) and photo (right)
Multiplexed
analog output
Analog input
1
Input
bufferMultiplexed
analog output

AMUX
core

Analog input
2

Output buffer

Analog input
1

Input
bufferClock input
Clock buffer

Through response (dB)

(2 x 2 mm2)

Radio frequency performance of AMUX IC
Clock path

Analog path
20
15
10
5
0

Measurement
Simulation

−5
−10
0

Clock input

10 20 30 40 50 60 70 80 90 100 110

Through response (dB)

Cutoff frequency fT: 460 GHz
Maximum oscillation frequency fmax:
480 GHz

Analog input
2

30
25
20
15
10

Measurement
Simulation

500

10 20 30 40 50 60 70 80 90 100 110

Frequency (GHz)

Frequency (GHz)

Fig. 4. Images of 0.25-μm InP HBT and AMUX IC and radio frequency performance of AMUX IC.

Measurement setupf

SubDAC40 GHz160-GBaud PAM-4 eye diagram

AMUX
f
80 GHz

OfflineDSP

f

SubDAC40 GHzArbitrary waveformgenerator

Clock

2 ps

Fig. 5. Demonstration of 160-GBaud PAM-4 (4-level pulse amplitude modulation) signal generation.

have broad peaking characteristics in its frequency
response to compensate for packaging loss.
The measured bandwidths for the analog and clock
paths were both over 110 GHz. These measurement
results indicate that this AMUX IC can potentially be
used in constructing a 110-GHz-bandwidth 220-GS/s
DAC subsystem and to generate 200-GBaud-classmodulated signals. We have already demonstrated
signal generation at ultrahigh symbol rates by applying this AMUX IC to the bandwidth doubler. The
NTT Technical Review

Vol. 17 No. 5 May 2019

measurement setup and results are shown in Fig. 5. In
this demonstration, we succeeded in generating a
160-GBaud PAM-4 (4-level pulse amplitude modulation) signal with two 40-GHz-bandwidth sub-DACs
and the AMUX IC and demonstrating the further
scalability of the bandwidth doubler technology [8].
Digital coherent optical transmission with a capacityof over 1-Tbit/s per channel will be achievable byusing this AMUX IC.

30

Feature Articles

Concept of integrated optical front-end module
Co-packaged module
Monolithic IC

AMUX

DRV
Optical I/Q
modulator

Monolithic IC

AMUX

DRV

LD

Analog input
2

Analog input
1

Clock input

InP MZM

24

n
i
p
n
Semi-insulating InP substrate

BCB

18
12
6
0

0

20 40 60 80
Frequency (GHz)EO response (dB)DRVfunction

Through response (dB)

AMUX-DRV IC
Multiplexed
analog output

0
−3
−6
−9

−12

100

0

20 40 60 80
Frequency (GHz)100

BCB: benzocyclobutene

Fig. 6. Conceptual block diagram of integrated optical front-end module and performance summary of AMUX-DRV IC andInP MZM.

4. Concept of ultrahigh-speed integrated optical
front-end module
One more important issue for beyond-100-GBaud
systems is integration and packaging technology, as
mentioned in the first section. This is especially true
regarding the transmitter, where the AMUXs, DRVs,
and optical modulators have to be placed as close toeach other as possible. In addition, all these devices
have to be assembled into one integrated packaged
module in order to ensure the quality of ultrabroadband modulated signals.
We have developed an AMUX IC equipped with a
DRV function (AMUX-DRV IC) and studied adesign incorporating the AMUX-DRV IC and opticalmodulators for an ultrahigh-speed integrated optical
front-end module. Because monolithic integration of
the AMUX function and DRV function into one chipis advantageous for ensuring the signal quality andreducing power consumption, we replaced an output
buffer with a DRV function block consisting of ahigh-gain, high-linearity, and large-output-swing
amplifier in the AMUX IC, and designed the AMUXDRV IC. For the optical front-end, we used in-house
InP MZM (Mach-Zehnder modulator)-based optical
IQ (in-phase and quadrature) modulators, which havean electro-optical (EO) modulation bandwidth of 80
GHz. The AMUX-DRV IC was designed to have
NTT Technical Review

Vol. 17 No. 5 May 2019broad peaking characteristics to compensate for thefrequency response of the following modulator and tohave optimum output impedance and driving voltage
for the modulator.
A conceptual block diagram of the integrated optical front-end module is shown in Fig. 6, which also
summarizes the performance of the AMUX-DRV IC
and InP MZM. The AMUX-DRV IC has broad peaking characteristics and ultrabroad bandwidth of over
110 GHz as expected. With this AMUX-DRV IC, theoptical front-end module could be expected to havean ultrabroad EO bandwidth of 80 GHz. We haveactually already fabricated a sub-assembly that contains the AMUX-DRV IC and InP MZM and havesucceeded in demonstrating 400-Gbit/s-per-channel
DMT (discrete multi-tone) signal optical transmission [9]. This is a record for IMDD (intensity modulation and direct detection) optical transmission so far
and indicates the capability of over-1-Tbit/s-perchannel digital coherent optical transmission. In thenext step, we will complete the optical front-end
module and apply it to digital coherent optical transmission.
5. Summary
In this article, we introduced recent trends and challenges in optical communications and our research
31Feature Articles

and development (R&D) of ultrahigh-speed optical
front-end device technology for future beyond-100GBaud systems. We will continue to promote further
speed improvements and continue with our R&D sothat this technology can ensure sustainable progressof optical communications.

[5]

[6]

References
[1]

[2]

[3]

[4]

Y. Miyamoto, A. Sano, E. Yoshida, and T. Sakano, “Ultrahigh-capacity Digital Coherent Optical Transmission Technology,” NTT Technical Review, Vol. 9, No. 8, 2011.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201108fa2.html
M. Nagatani, H. Yamazaki, F. Hamaoka, H. Nosaka, and Y. Miyamoto, “Bandwidth Doubler Technology for Increasing the Bandwidth of
an Optical Transmitter,” NTT Technical Journal, Vol. 29, No. 3, pp.
62–66, 2017 (in Japanese).
S. Yamahata, K. Kurishima, Y. Fukai, H. Fukuyama, and M. Hirata,“Fabrication Technology of Highly-reliable InP-HBT Integrated Circuits,” NTT Technical Journal, Vol. 19, No. 11, pp. 62–65, 2007 (inJapanese).
H. Yamazaki, M. Nagatani, S. Kanazawa, H. Nosaka, T. Hashimoto,
A. Sano, and Y. Miyamoto, “Digital-preprocessed Analog-multiplexed DAC for Ultrawideband Multilevel Transmission,” J. Lightw.

NTT Technical Review

Vol. 17 No. 5 May 2019

[7]

[8]

[9]

Technol., Vol. 34, No. 7, pp. 1579–1584, 2016.
M. Nakamura, F. Hamaoka, M. Nagatani, H. Yamazaki, T. Kobayashi,
A. Matsushita, S. Okamoto, H. Wakita, H. Nosaka, and Y. Miyamoto,“1.04 Tbps/carrier Probabilistically Shaped PDM-64QAM WDM
Transmission over 240 km Based on Electrical Spectrum Synthesis,”
Proc. of the 42nd Optical Fiber Communication Conference andExhibition (OFC 2019), paper M4I.4., San Diego, CA, USA, Mar.
2019.
N. Kashio, K. Kurishima, M. Ida, and H. Matsuzaki, “Over 450-GHz
fT and fmax InP/InGaAs DHBTs with a Passivation Ledge Fabricatedby Utilizing SiN/SiO2 Sidewall Spacers,” IEEE Trans. Electron
Devices, Vol. 61, No. 10, pp. 3423–3428, 2014.
M. Nagatani, H. Wakita, H. Yamazaki, M. Mutoh, M. Ida, Y. Miyamoto, and H. Nosaka, “An Over-110-GHz-bandwidth 2:1 Analog
Multiplexer in 0.25-μm InP DHBT Technology,” Proc. of the 2018
IEEE/MTT-S International Microwave Symposium (IMS2018), Philadelphia, PA, USA, June 2018.
Y. Ogiso, J. Ozaki, Y. Ueda, N. Kashio, N. Kikuchi, E. Yamada, H.
Tanobe, S. Kanazawa, H. Yamazaki, Y. Ohiso, T. Fujii, and M. Kotoku,“Over 67 GHz Bandwidth and 1.5-V Vπ InP-based Optical IQ Modulator with n-i-p-n Heterostructure,” J. Lightw. Technol., Vol. 35, No.
8, pp. 1450–1455, 2017.
H. Yamazaki, M. Nagatani, H. Wakita, Y. Ogiso, M. Nakamura, M.
Ida, T. Hashimoto, H. Nosaka, and Y. Miyamoto, “Transmission of
400-Gbps Discrete Multi-tone Signal Using > 100-GHz-bandwidth
Analog Multiplexer and InP Mach-Zehnder Modulator,” Proc. of the44th European Conference on Optical Communication (ECOC
2018), pp. 1–3, Rome, Italy, Sept. 2018.

32

Feature Articles

Munehiko Nagatani

Distinguished Researcher, NTT Device Technology Laboratories and NTT Network Innovation Laboratories.
He received an M.S. in electrical and electronics engineering from Sophia University, Tokyo,
in 2007. He joined NTT Photonics Laboratories
in 2007, where he engaged in the research anddevelopment (R&D) of ultrahigh-speed analog
and mixed-signal ICs for optical communications systems. He is concurrently with NTT
Device Technology Laboratories and NTT Network Innovation Laboratories, where he isinvolved in R&D of ultrahigh-speed ICs andsystems for optical transmission using advanced
modulation formats. He was a recipient of the2011 Young Researchers Award by the Instituteof Electronics, Information and Communication
Engineers (IEICE). He served as a technicalprogram committee member for the Institute ofElectrical and Electronics Engineers (IEEE)
Compound Semiconductor Integrated Circuits
Symposium (CSICS) from 2014 to 2017. He has
been serving as a technical program committee
member for IEEE BiCMOS and Compound
Semiconductor Integrated Circuits and Technology Symposium (BCICTS) since 2018 and IEEE
International Solid-State Circuits Conference(ISSCC) since 2019. He is a member of IEICE
and IEEE.

Hitoshi Wakita

Researcher, NTT Device Technology Laboratories.
He received a B.S. in organic and polymericmaterials in 2008 and an M.S. in chemistry andmaterials science in 2010, both from Tokyo Institute of Technology. In 2010, he joined NTT
Photonics Laboratories, where he has been
engaged in R&D of ultrahigh-speed mixed signal
ICs and modules for optical communications
systems. He is currently with NTT Device Technology Laboratories. He is a member of IEICE
and IEEE.

Yoshihiro Ogiso

Researcher, NTT Device Innovation Center.
He received a B.E. and M.E. in applied physics
in optoelectronics from Waseda University,
Tokyo, in 2008 and 2010. He joined NTT Photonics Laboratories in 2010. He is currently withNTT Device Innovation Center, where he isinvolved in R&D of ultrahigh-speed opticalmodulators. He is a member of IEICE.

NTT Technical Review

Vol. 17 No. 5 May 2019

Hiroshi Yamazaki

Distinguished Researcher, NTT Device Technology Laboratories and NTT Network Innovation Laboratories.
He received a B.S. in integrated human studies
in 2003 and an M.S. in human and environmental
studies in 2005, both from Kyoto University, anda Dr. Eng. in electronics and applied physics
from Tokyo Institute of Technology in 2015. He
joined NTT Photonics Laboratories in 2005,
where he has been involved in research on opticalwaveguide devices for communications systems.
He is concurrently with NTT Network Innovation Laboratories and NTT Device Technology
Laboratories, where he is involved in research on
devices and systems for optical transmission
using advanced multilevel modulation formats.
He has been an Associate Editor of IEEE Journalof Lightwave Technology since 2016. He is amember of IEICE.

Minoru Ida

Senior Researcher, NTT Device Technology
Laboratories.
He received a B.S. and M.S. in electrical engineering from Keio University, Kanagawa, in
1989 and 1991, and a Ph.D. in physical electronics from Tokyo Institute of Technology in 2005.
He joined NTT LSI Laboratories in 1991, where
he conducted research on metalorganic vapor
phase epitaxy growth and InP-based heterojunction bipolar transistors (HBTs). From 1996 to
1998, he worked with NTT Wireless System
Laboratories on GaAs monolithic microwave
integrated circuits (MMICs) for wireless applications. He is currently researching high-speed
InP-based HBT devices and developing fabrication processes of ICs for optical networks at NTT
Device Technology Laboratories.

Hideyuki Nosaka

Senior Research Engineer, Group Leader ofHigh-Speed Analog Circuit Research Group,
NTT Device Technology Laboratories.
He received a B.S. and M.S. in physics from
Keio University, Kanagawa, in 1993 and 1995,
and a Dr. Eng. in electronics and electrical engineering from Tokyo Institute of Technology in
2003. He joined NTT Wireless System Laboratories in 1995, where he was engaged in R&D of
MMICs and frequency synthesizers. Since 1999,
he has been involved in R&D of ultrahigh-speed
mixed-signal ICs for optical communications
systems at NTT Photonics Laboratories. He
received the 2001 Young Engineer Award and the2012 Best Paper Award presented by IEICE. He
served as a technical program committee member for IEEE CSICS from 2011 to 2013 and IEEE
ISSCC from 2013 to 2017. He is a member ofIEEE Microwave Theory and Techniques Society(MTT-S) Technical Committee on Digital Signal
Processing (MTT-9). He is a member of IEICE
and IEEE.

33

Feature Articles: Ultrahigh-speed Transmission Technology to
Support High-capacity Communications Infrastructure of the Future

Toward Terabit-class WirelessTransmission: OAM Multiplexing
Technology
Doohwan Lee, Hirofumi Sasaki, Yasunori Yagi,
Takayuki Yamada, Takana Kaho, and Hiroshi Hamada
Abstract

NTT is researching and developing technology with the aim of achieving terabit-class wireless transmission that achieves the next generation of 5G (fifth generation). In particular, we are investigating
orbital angular momentum (OAM) multiplexing transmission technology utilizing the OAM of electromagnetic waves as a new spatial multiplexing technology capable of transmitting multiple radio waves
at the same time using the same frequency band. In an experiment on OAM multiplexing transmission
using the 28-GHz band, we successfully demonstrated the world’s first 120-Gbit/s wireless transmission.
This article introduces the details of this technology.
Keywords: high-capacity wireless transmission technology, OAM multiplexing, OAM-MIMO multiplexing

1. Introduction
The coming 5G (fifth generation) will accelerate
the use of wireless communications in all fields
including connected cars, virtual reality/augmented
reality, and high-definition video transmission.
Mobile traffic is expected to increase by 1.5 times
annually, and from such a trend, it is expected thatwireless transmission at the hundreds-of-gigabits tothe terabit class will be necessary in the 2030s.
To provide for the ever-increasing demand for wireless communications, NTT is researching and developing technology with the aim of achieving terabitclass wireless transmission. There are three directions
that can be taken to increase capacity in wirelesscommunications: increasing the spatial multiplexing*1 order, broadening the transmission bandwidth,and increasing the modulation level (Fig. 1).
Of these, the direction involving increasing themodulation level is already reaching the limit. For
example, to obtain twice the capacity of 1024QAM(quadrature amplitude modulation), which transmits
NTT Technical Review

Vol. 17 No. 5 May 201910 bits of information at a time, it is necessary to carryout one million QAM modulation to transmit 20 bitsof information at a time. Consequently, it is difficult
to achieve large capacity with this approach.
NTT is taking the approach of broadening the transmission bandwidth using the quasi-millimeter wave*2
band and increasing the spatial multiplexing orderthrough the use of a new principle based on electromagnetic waves having orbital angular momentum (OAM)*3.
*1 Spatial multiplexing: A signal multiplexing method that sends
and receives multiple data streams in parallel by using multiple
spatially independent electromagnetic waves.
*2 Millimeter waves, quasi-millimeter waves: Millimeter waves
refer to the radio waves with a wavelength of 1–10 mm, in thefrequency band of 30–300 GHz and that feature strong rectilinear
propagation similar to that of microwaves. Quasi-millimeterwaves refer to radio waves with a wavelength that, at 1–3 cm, areclose to those of millimeter waves in the frequency band of
10–30 GHz.
*3 OAM: The nature of radio waves is such that the angular momentum of radio waves is represented by the product of position coordinates and the conjugate momentum; radio waves with different OAM are not correlated, so they can be separated independently even if they overlap.

34

Feature Articles

40

Increase spatial
multiplexing order

Spatial multiplexing order
New research area
Current research achievement

11

e
as

mo

cre

In

e

nl

tio

la
du

l
ve

256QAM

Bro

ade

nb

2 GHz

and

wid

th

1024QAM

10 GHzModulation level

Transmission bandwidth

QAM: quadrature amplitude modulation

Fig. 1. Research directions toward large-capacity wireless transmission.

OAM mode 1

OAM multiplexing technology

OAM mode 2

OAM mode 3(a) Trace of the same phase(b) Concurrent transmission

Fig. 2. Principle of OAM multiplexing transmission technology.

2. Principles of OAM multiplexing transmission
and NTT’s new transmission technology
In this section, we explain OAM multiplexing
transmission and introduce a new wireless transmission technology developed by NTT.
2.1 OAM multiplexing transmission
OAM multiplexing transmission is a technique toincrease the number (multiplexing number) of data
signals to be transmitted at the same time by transmitNTT Technical Review

Vol. 17 No. 5 May 2019ting different signals using radio waves having different OAM modes [1, 2]. OAM is a property of electromagnetic waves expressing phase rotation on thevertical plane in the propagation direction. The number of phase rotations is called an OAM mode. Whenan electromagnetic wave has this OAM property, thetrace of the same phase takes on a helical shape in the
direction of propagation (Fig. 2). For example, thetrace of the same phase in OAM mode 1, OAM mode2, and OAM mode 3 is shown in Fig. 2(a).
Radio waves having this OAM property cannot be
35

Feature Articles

UCA #4
UCA #3
UCA #2
UCA #1

Transmitting 11 streams(UCA #0, UCA#1, UCA #4)

UCA #0
−2

−1

0

1

2

OAM modes
UCA: uniform circular array antenna

Fig. 3. Example of configuring OAM-MIMO multiplexing transmission.

received without a receiver having the same numberof phase rotations at the time of transmission. For thisreason, if multiple radio waves having different OAM
modes are superposed, they can eventually be separated without mutual interference as long as there is areceiver that can receive radio beams with the same
phase rotations corresponding to each OAM mode.
For instance, an example is shown in Fig. 2(b) where
different signals are put on OAM modes 1, 2, and 3
and transmitted simultaneously. Technology fortransmitting multiple data signals using this feature iscalled OAM multiplexing transmission technology.
Research on radio wave OAM dates back to thebeginning of the 20th century. More recently, with thematuring of large-capacity transmission technologies
using high-frequency bands in the 2010s, OAM multiplexing transmission in the millimeter-wave bandhas been attracting attention. An example of a recent
achievement is a demonstration at the University ofSouthern California in the United States of 32-Gbit/s
transmission using the 28-GHz band in 2014 and the60-GHz band in 2016 [3, 4].
2.2 O
 AM-MIMO (multiple-input multiple-output) wireless multiplexing transmission technology
In theory, the OAM mode order (number of phaserotations) can be infinitely increased, so the spatial
multiplexing order can be infinitely increased by
OAM multiplexing transmission. However, radio
waves with the property of OAM have the characteristic such that the electric energy diverges spatially as
the propagation distance becomes larger as the OAM
mode becomes higher. Thus, there is a practical limiNTT Technical Review

Vol. 17 No. 5 May 2019

tation to the use of the higher order OAM modes toincrease the spatial multiplexing order.
NTT has devised OAM-MIMO multiplexing transmission technology that integrates the widely used
MIMO*4 technology in order to efficiently increase
the spatial multiplexing order while overcoming thepractical limitation [5]. The devised OAM-MIMO
multiplexing transmission technology allows theusage of the same OAM modes. In particular, multiple sets of low OAM mode signals are simultaneously transmitted to address the divergence issue ofhigher OAM modes. The nature of OAM multiplexing that generates multiple independent channels
among different OAM modes is still applicable.
At the same time, the MIMO concept is exploited toincrease the number of concurrently transmitted data
streams. By properly exploiting MIMO technology, it
is possible to separate signals between multiple setsof the same OAM modes while maintaining noninterfering properties between different OAM modes.
A configuration example of OAM-MIMO multiplexing transmission technology is illustrated inFig. 3. This figure shows an example in which a totalof 11 multiplexing transmissions to be described laterare performed. The left figure represents four uniform
circular array antennas (UCAs) concentric with one
in the center. UCAs #1 to 4 can generate OAM modes–2, –1, 0, 1, and 2, respectively, and five OAM modes*4 MIMO: Radio signal processing technology for improving thequality of communications by using multiple antennas at both thetransmitter and receiver. It has attracted the attention of the wireless communications industry since it can greatly improvethroughput and transmission distance without having to increase
bandwidth or power.

36

Feature Articles

can be generated and transmitted. The antenna in thecenter (described as UCA #0 for notation convenience) is used for the axis alignment and/or transmission of OAM mode 0. The signals multiplexed in
the same mode are separated by MIMO technology at
the receiver.
3. World-first transmission of over 100 Gbit/s
using 28-GHz band
To demonstrate the effectiveness of the OAMMIMO multiplexing transmission technology, NTT
has developed a transmitter and receiver that uses a
bandwidth of 2 GHz in the 28-GHz band (Fig. 4). The
developed transmitter/receiver consists of four concentric UCAs with different radii and one antenna in
the center. Each UCA is composed of 16 antenna elements and can transmit and receive radio waves in 5
OAM modes. Simultaneous transmission of a total of
21 data signals is possible using these antenna elements. Although frequency bands under 6 GHz havebeen used in the past, the development of equipmentthat can simultaneously transmit up to 21 signals inquasi-millimeter-wave and higher frequency bands isa world first.
The configuration of this transmitter/receiver andthe experimental setup are shown in Fig. 5. On thetransmitter side, signals are generated and converted
into analog intermediate frequency (IF) band signals
using arbitrary waveform generators that work as
digital-to-analog converters. IF signals are inserted
into the developed transmitter. The input signal istransmitted by each UCA via an OAM mode generation circuit after up-converting IF signals to the
28-GHz band. On the receiver side, the wireless signals received by each UCA are separated into each
OAM mode by OAM mode separation circuits, andthen separated signals are down-converted into IF
band signals. Those IF band signals are converted todigital signals by the digital oscilloscope, which isused as an analog-to-digital converter. Finally, thedigital signals are processed to extract each transmitted signal. Here, the OAM mode generation circuitand the OAM mode separation circuit were respectively achieved by designing a 5 × 16 and a 16 × 5
wideband Butler matrix circuit. Five OAM modes
can be simultaneously generated and separated by the
wideband Butler matrix circuits.
Using this transmitter/receiver, we conducted atransmission experiment at a distance of 10 m in ashield room. Multiple data signals were wirelesslytransmitted using multiple OAM multiplexed radio
NTT Technical Review

Vol. 17 No. 5 May 2019

Fig. 4. Developed transceiver.

waves. We confirmed that wireless transmission was
possible according to the OAM principle. In addition,
we succeeded in the world-first large-capacity wireless transmission of 120 Gbit/s [6] by multiplexing 11
data signals of 9.6 to 13.3 Gbit/s. We also successfully implemented the corresponding signal processing technology that can handle such multiplexed signal transmission and reception.
4. Expectation for broadband technology
Technology to utilize wide frequency bands is currently being studied. For example, the mixer integrated circuit (IC) and module in the 300-GHz bandshown in Fig. 6 make it possible to both expand thetransmission bandwidth and improve the signal-tonoise ratio, which had been a problem in the conventional 300-GHz band radio front end [7]. We used thisdevice to carry out 100-Gbit/s transmission in one
stream using a wide bandwidth of 25 GHz. By combining such wideband IC technology and OAM multiplexing transmission technology, we can expect toachieve even larger capacity.
5. Future work
In order to use OAM multiplexing technology in areal environment, it is necessary to address practicalissues that occur in the real environment and to conduct experimental evaluations in various environments. We are now preparing to verify the feasibilityof OAM multiplexing in the field by conducting outdoor wireless transmission experiments. In addition,
we will utilize frequency bands that can use wider
37

Feature Articles

OAM
separation

OAM
generation

UCA #3

UCA #3OAM
separation

OAM
generation

UCA #2

UCA #2OAM
separation

OAM
generation

UCA #1

UCA #1

OAM
separation

UCA #0

UCA #0Rx signal processing

UCA #4IF band signal

UCA #4

Digital oscilloscope

OAM
generation

SwitchReceiver (Rx)

Down-conversion

Up-conversion

Arbitrary waveform generator

IF band signal

Tx signal processing

Transmitter (Tx)

IF: intermediate frequency

Fig. 5. Configuration of transmitter and receiver.

300-GHz band mixer

300-GHz band mixer module
LO port(waveguide)

LO

IF

RF

IF portRF port(waveguide)

Chip size 1 mm × 1 mm

LO: local oscillation
RF: radio frequency

Fig. 6. 300-GHz band mixer and IC module.

bandwidths and lead research and development
toward realizing terabit-class wireless communications.

[4]

References
[1]
[2]

[3]

A. E. Willner, “Communication with a Twist,” IEEE Spectrum, Vol.
53, No. 8, pp. 34–39, Aug. 2016.
D. Lee, H. Sasaki, H. Fukumoto, K. Hiraga, and T. Nakagawa,“Orbital Angular Momentum (OAM) Multiplexing: An Enabler of a
New Era of Wireless Communications,” IEICE Trans. Commun., Vol.
E100-B, No. 7, pp. 1044–1063, July 2017.
Y. Yan, G. Xie, M. P. J. Lavery, H. Huang, N. Ahmed, C. Bao, Y. Ren,
Y. Cao, L. Li, Z. Zhao, A. F. Molisch, M. Tur, M. J. Padgett, and A. E.

NTT Technical Review

Vol. 17 No. 5 May 2019

[5]

[6]

Willner, “High-capacity Millimeter-wave Communications with
Orbital Angular Momentum Multiplexing,” Nature Commun., Vol. 5,
p. 4876, Sept. 2014.
Y. Ren, L. Li, G. Xie, Y. Yan, Y. Cao, H. Huang, N. Ahmed, Z. Zhao,
P. Liao, C. Zhang, G. Caire, A. F. Molisch, M. Tur, and A. E. Willner,“Line-of-sight Millimeter-wave Communications Using Orbital
Angular Momentum Multiplexing Combined with Conventional Spatial Multiplexing,” IEEE Trans. Wireless Commun., Vol. 16, No. 5, pp.
3151–3161, May 2017.
D. Lee, H. Sasaki, H. Fukumoto, Y. Yagi, T. Kaho, H. Shiba, and T.
Shimizu, “An Experimental Demonstration of 28 GHz Band Wireless
OAM-MIMO (Orbital Angular Momentum Multi-input and Multioutput) Multiplexing,” Proc. of the 2018 IEEE 87th Vehicular Technology Conference (VTC 2018-Spring), Porto, Portugal, June 2018.
H. Sasaki, D. Lee, H. Fukumoto, Y. Yagi, T. Kaho, H. Shiba, and T.
Shimizu, “Experiment on Over-100-Gbps Wireless Transmission with38

Feature Articles[7]

OAM-MIMO Multiplexing System in 28-GHz Band,” Proc. of 2018
IEEE Global Communications Conference (GLOBECOM), Abu
Dhabi, UAE, Dec. 2018.
H. Hamada, T. Fujimura, I. Abdo, K. Okada, H. Song, H. Sugiyama,

Doohwan Lee

Distinguished Researcher, Wireless Systems
Innovation Laboratory, NTT Network Innovation
Laboratories.
He received a B.S. (Hons.) and completed an
M.S. course in electrical engineering from Seoul
National University, South Korea, in 2004 and2006. He received a Ph.D. in electrical engineering and information systems from the Universityof Tokyo in 2009. He has been with NTT Network Innovation Laboratories since 2009. From2012 to 2014, he was an assistant professor at the
Research Center for Advanced Science and Technology, the University of Tokyo. From 2016 to
2018, he was a part-time lecturer at Kanagawa
University. His research interests include compressed sensing, software/cognitive radio, signal
processing, and OAM multiplexing. He received
the Best Paper Award and Best Technical Exhibition Award from the Institute of Electronics,
Information and Communication Engineers(IEICE) Software Radio in 2011, the IEICE
Communications Society Excellent Paper Awardin 2012, the IEICE Short Range Wireless Communications Young Researcher’s Award in 2016,
the Best Technical Exhibition Award from SmartCom 2014, the Best Paper Award from SmartCom 2016, and the Best Paper Award and Special
Technical Award in Smart Radio from IEICE
Smart Radio in 2017. He served as a General
Co-Chair in the Institute of Electrical and Electronics Engineers (IEEE) ICC 2019 Workshop on
Electromagnetic Wave Transmission with Orbital Angular Momentum.

Hirofumi Sasaki

Researcher, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received a B.E. and M.E. in engineeringfrom Osaka University in 2011 and 2013. He
joined NTT Network Innovation Laboratories in
2013, where he researched a heterogeneous wireless network system in spectrum sharing scenarios. Since 2015, he has been engaged in researchon super high-speed wireless communication in
millimeter-wave regions. He received the Best
Paper Award from IEICE SmartCom 2016 andthe Best Paper Award and Special TechnicalAward in Smart Radio from IEICE Smart Radio
in 2017.

Yasunori Yagi

Wireless Systems Innovation Laboratory, NTT
Network Innovation Laboratories.
He received a B.E. and M.E. in electrical engineering from Tokyo University of Science in2015 and 2017. He joined NTT Network Innovation Laboratories in 2017. Since then, he has
been researching high-capacity wireless systems.

NTT Technical Review

Vol. 17 No. 5 May 2019

H. Matsuzaki, and H. Nosaka, “300-GHz, 100-Gb/s InP-HEMT Wireless Transceiver Using a 300-GHz Fundamental Mixer,” Proc. of 2018
IEEE/MTT-S International Microwave Symposium, Philadelphia, PA,
USA, June 2018.

Takayuki Yamada

Research Engineer, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received a B.E. in electronics from Doshisha University, Kyoto, in 2005 and an M.E. incommunications and computer engineering fromthe Graduate School of Informatics, Kyoto University, in 2007. Since joining NTT Network
Innovation Laboratories in 2007, he has been
engaged in research and development (R&D) of
flexible wireless systems, high-capacity wirelesssystems, and wireless signal processing. He
received the Best Paper Award from the IEICE
Communications Society in 2012 and the Young
Researcher’s Award from IEICE in 2013. He also
received the Best Technical Exhibition Award at
SmartCom2014 and the Best Paper Award fromIEICE in 2015.

Takana Kaho

Senior Research Engineer, Wireless Systems
Innovation Laboratory, NTT Network Innovation
Laboratories.
She received a B.S. and M.S. in physics from
Tokyo Metropolitan University in 1994 and1996, and a Dr. Eng. in communication engineering from Tokyo Institute of Technology in 2007.
She joined NTT Radio Communication Systems
Laboratories in 1996 and has since been engaged
in R&D of monolithic microwave integrated circuits (MMICs). Since 2009, she has been an
Expert Committee Member on the Information
and Communications Council of the Ministry ofInternal Affairs and Communications, Japan.
From 2010 to 2012, she was a Visiting Associate
Professor at the Research Institute of ElectricalCommunication, Tohoku University, Miyagi.
From 2014 to 2019, she was a Visiting Associate
Professor at the Graduate School and Faculty ofInformation Science and Electrical Engineering,
Kyushu University, Fukuoka. Since November
2016, she has also been a Visiting Professor at theGraduate School of Information Science andTechnology, Osaka University. Dr. Kaho is asenior member of IEICE. She received the Japan
Microwave Prize at the 1998 Asia Pacific Microwave Conference and the Young Researchers’Award in 2004 presented by IEICE. She received
a Best Paper Award from IEICE in 2015. She wasa treasurer of the Japan Chapter of the IEEE
Microwave Theory and Techniques Society from2011 to 2012, and from 2012 to 2014, she was a
Committee Member of Tokyo Section, IEICE.

Hiroshi Hamada

Researcher, Photonics-Electronics Convergence Laboratory, NTT Device Technology
Laboratories.
He received a B.E. and M.E. in electrical engineering from Tokyo Institute of Technology in2009 and 2011. He joined NTT Photonics Laboratories in 2011 and studied millimeter-wave/
THz MMICs for wireless communications. He iscurrently involved in R&D of THz MMICs forvery high-speed wireless communications. He
received the 2016 IEEE International Microwave
Symposium (IMS) Best Industry Paper Award,
2016 URSI Asia Pacific Radio Science Conference (URSI AP-RASC) Young Scientist Award,
and 2018 IEICE Asia-Pacific Microwave Conference (APMC) APMC Prize.

39

Regular Articles

Recent Progress in Applications of
Optical Multimode Devices Using
Planar Lightwave Circuits
Junji Sakamoto and Toshikazu Hashimoto
Abstract

The optical propagation mode—the electromagnetic field distribution of light propagating through a
waveguide—is attracting attention as a new degree of freedom of light and is expected to provide newfunctions and improved optical characteristics in optical devices and systems. In this article, we introduce
optical device technologies that utilize the optical propagation mode in an integrated optical waveguidecomponent called a planar lightwave circuit.
Keywords: multimode, waveguide, PLC

1. Introduction
In optical communications, the degrees of freedomof light such as its intensity, wavelength, phase, andpolarization have been used for multiplexing andhigher-order modulation [1]. In an optical waveguide,
the optical field propagates in specific distribution
patterns called modes. Single-mode transmission
with only the fundamental mode has mainly beenused in optical communications. In recent years,
however, large-capacity transmission using mode
multiplexing has been reported [2]. This is called
spatial division multiplexing (SDM) because it uses
the degree of freedom of the spatial optical field distribution.
Optical propagation modes are also expected to be
utilized for the next generation of technology for
optical signal processing and sensing, as well as forSDM transmission. For stable optical performance,
optical communications systems are generally composed of devices for single-mode transmission, which
include a light source, modulator, and multiplexer.
Therefore, an important component to handle themodes is a mode converter (MC) that can convert thefundamental mode and higher-order modes.
A technology map of MCs is shown in Fig. 1. There
are MCs using optical fiber, spatial modulators such
NTT Technical Review

Vol. 17 No. 5 May 2019as liquid crystal on silicon, and waveguides. Sincemode utilization has been limited to SDM transmission, the focus has mainly been on signal multiplexing. For other functions such as wavelength multiplexing and sensing, precise mode conversion isnecessary.
We are now researching silica planar lightwave
circuit (PLC) technology and various devices using it
as a means of controlling modes. In this article, we
introduce a wavelength division multiplexer using
optical propagation modes and an ultralow-crosstalk
MC for an optical time domain reflectometer(OTDR).
2. Structure of optical multimode waveguide
A PLC is a waveguide circuit composed of a silica
core and cladding on a substrate such as Si (silicon)
or SiO2 (silicon dioxide), as shown in Fig. 2(a). A
waveguide pattern is fabricated using standard wafer
processes such as photolithography and reactive ion
etching. The core structure of single-mode and multimode waveguides is shown in Fig. 2(b); the intensityof optical field distribution with several modes is also
shown in a cross-sectional view. The core width andheight of the single-mode waveguide enable only thefundamental mode to exist in a lateral direction. The
40

Regular Articles

Region of focus
in optical communications

Unexplored fields

Application

SDM coupler

RGB coupler

Sensing coupler

Function

Power coupler

Wavelength
division
multiplexing

Mode divisionmultiplexing

Multiplicity

Wavelengthcharacteristic

Extinction ratio

Fiber

SilicaCharacteristic

Fabrication

Waveguide

Polymer

Semiconductor

Spatialmodulator

RGB: red, green, and blue color modelFig. 1. Technology map of MCs.

Core (cross-sectional view)

Lateral modes (cross-sectional
optical field distribution)

Propagation direction
Core

Single mode

Order: Fundamental = 0th

Multimode

0th

Cladding
Substrate
1st

2nd

(b)

(a)

Fig. 2. (a) Structure of waveguide. (b) Cross-sectional view of cores and optical field distributions in single and multimodewaveguides.

intensity of the optical field distribution is shown at
the upper right of Fig. 2(b). A multimode waveguide
has a core size in which first or higher-order modes
can coexist with the fundamental mode. The numberof modes that can exist is determined by the core size.
The intensity of optical field distribution is shownat the lower right of Fig. 2(b). In the waveguide, themode should be precisely controlled for various
applications by changing the width of the core, since
the characteristics change when the modes propagating through the waveguide change. PLC technology
could be the most promising approach for providing
such highly accurate fabrication of waveguides, since
it has been developed for application to optical comNTT Technical Review

Vol. 17 No. 5 May 2019

munications for many years [3]. In the following sections, we introduce several applications using multimode PLC technology.
3. RGB coupler
A compact red (R)-green (G)-blue (B) multiplexer
for laser diodes is attractive for use in glass-based
displays, head-mounted displays, and compact projectors. Such laser imaging applications use multiple
laser sources and multiplex them into a single beamthat is then scanned onto a screen by a beam scannersuch as a microelectromechanical system mirror.
RGB multiplexers have conventionally been
41

Regular Articles

W0

B

W0

W2

G
B
(a)

8.2 mm
2.6 mmGin
Bin

127 µm127 µmRin

Bin-RGBout
Gin-RGBout
Rin-RGBout
Transmittance (dB)

G

MC1
B+G

MC2

RGBout

0
−2
−4
−6
−8
−10
−12
−14
−16
420

B

470

520
570
620
670
G
R
Wavelength (nm)

(B+G)+R(c)(b)

Fig. 3. (a) Wavelength multiplexer using MCs. (b) Optical circuit of the RGB coupler. (c) Spectra of the RGB coupler.

constructed by using mirror prisms, multilayer films,
and optical fibers. An RGB multiplexer (RGB coupler) based on a PLC is promising for downsizing toa single chip integrating the functions used in a conventional RGB multiplexer [4, 5].
An RGB coupler typically uses directional couplers(DCs) to multiplex light with different wavelengths.
The wavelength dependence of the coupling length isused to implement the wavelength multiplexing function because the higher the wavelength is, the shorter
the coupling length becomes. For example, a DC
structure is designed to match the coupling length ofblue light and the recoupling length of green light.
Then the blue and green lights are multiplexed by the
DC. It is difficult to multiplex more than three wavelengths this way, and even in the case of two wavelengths, the DC tends to become long due to the needfor period-matching between the coupling and recoupling.
To solve this problem, we developed an RGB coupler using MCs, each of which includes an additional
multimode waveguide that is wider than the singlemode waveguides. The added multimode waveguideis sandwiched between the single-mode waveguides
of a conventional DC, as shown in Fig. 3(a), where
the black line represents the core of the optical circuit. The additional multimode waveguide works as
an MC that converts between 0th-order and higherorder modes. Because the MC converts only wavelengths satisfying the conversion conditions, it ispossible to selectively multiplex desired wavelengths.
NTT Technical Review

Vol. 17 No. 5 May 2019For example, when the MC is designed to satisfy theconversion conditions at the green wavelength, thegreen light can move to the next single-mode waveguide by means of mode conversion.
In contrast, almost all the blue light goes throughthe MC, as shown in Fig. 3(a). Since this coupling
structure can multiplex green and blue light as if itwere wavelength-independent thanks to suppressionof the transition of unwanted-wavelength light, thecoupler length does not need to be the lowest common multiple of the coupling lengths of the colors. As
a result, the coupler can be shorter than conventional
DCs.
Our RGB coupler using MCs is shown in Fig. 3(b).
Because MC1 and MC2 were designed to satisfy the
coupling conditions at the green and red wavelengths,
the coupler can multiplex R, G, and B into the same
waveguide. The total length of MC1 and MC2 is 2.6mm, which is less than half as long as the conventional RGB coupler using DCs.
The measurement results for the RGB coupler fabricated using PLC technology is shown in Fig. 3(c).
The transmittances are −0.3 dB for blue, −1.2 dB for
green, and −0.3 dB for red. Although the transmittance of green light is relatively low, the results confirmed that we succeeded in making a very small
RGB coupler with low loss (better than 1.5 dB) using
MCs.

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Regular Articles

Port 3ModecrosstalkPort 2

Port 40Transmittance (dB)Port 1−2−4

Port 1 4−6−8

1-µm OTDR

−10
1

(a)

Port 3

Port 2

1.04
1.06
Wavelength (µm)

1.08

1.1

(c)

Port 4−20
Transmittance (dB)Port 1

1.02

−22
−24

Port 4 1

−26
−28
−30
−32

−34
−0.5 −0.4 −0.3 −0.2 −0.1 0 0.1 0.2 0.3 0.4 0.5
Distance from an alignment position in lateral
direction (µm)

(b)
(d)

Fig. 4. (a) Conventional structure and operation of mode coupler. (b) Structure of mode coupler with low mode crosstalk.
Measured (c) conversion efficiency and (d) mode crosstalk.

4. Sensing coupler for sensitive OTDRAn OTDR is used to diagnose optical fiber links. A
1-μm-band mode-detection OTDR (1-μm OTDR)
system has been reported [6, 7] that uses 1-μm-bandprobe pulses to generate not only the fundamental
mode but also the first-order mode as backscattered
light in typical single-mode fibers compliant with
ITU-T G.652*. A mode coupler is used to separate thebackscattered light into individual modes and toselect the mode of the probe pulses. The first-ordermode at the wavelength in the 1-μm band is moresensitive to trouble spots in optical fiber links, such as
macro/micro-bends, than the fundamental mode in
the communications wavelength band. Therefore, the
1-μm OTDR is very promising for finding potential
trouble points before they have a serious impact on an
optical communications system. The 1-μm OTDR
requires an MC with low mode crosstalk to ensuresensitive detection. Here, we introduce a mode coupler that achieves high conversion efficiency and low
mode crosstalk by using a multistage mode converNTT Technical Review

Vol. 17 No. 5 May 2019sion method.
A schematic of the structure and operation of aconventional mode coupler is shown in Fig. 4(a). The
input fundamental mode from port 1 is converted to
the first-order mode and output from port 4. Almost
all of the fundamental mode from port 2 passes
through the coupler. The fundamental mode and firstorder mode are generated as backscattered light. The
fundamental mode and first-order mode from port 4
can be detected separately from the mode-mixed lightat ports 1 and 2. The dashed line shows the modecrosstalk path. Part of the fundamental mode fromport 4 is coupled to the thin waveguide with the fundamental mode and output from port 1 as a noise
component.
To solve this noise component problem, we developed a coupler with high conversion efficiency and*

ITU-T G.652: Recommendation issued by the International Telecommunication Union - Telecommunication Standardization
Sector (ITU-T) concerning characteristics of a single-mode optical fiber and cable.

43Regular Articles

low mode crosstalk, as shown in Fig. 4(b). An additional mode conversion waveguide placed between
the single-mode waveguide and multimode waveguide of the mode coupler indirectly converts themode into a desired one via an intermediate mode. In
this work, we chose the second-order mode as theintermediate mode. Adding the second-order modeenhances the mode isolation and reduces mode crosstalk. Setting the waveguide width correctly ensuresthat no additional loss occurs due to adding the waveguide.
The measured conversion efficiency of the coupler
fabricated using PLC technology is plotted inFig. 4(c). The conversion efficiency is −0.8 dB at the
wavelength of 1.05 μm, which is used for the 1-μm
OTDR. The measured mode crosstalk is shown in
Fig. 4(d). The mode crosstalk is under –30 dB. These
results confirm that we succeeded in making a mode
coupler with high conversion efficiency and low
mode crosstalk by using a multistage mode conversion method.
5. Future prospects
This article introduced optical multimode devices
that use PLC technology. We showed that by using
the mode as a new degree of freedom of light, we canminiaturize waveguide devices and achieve high performance. Establishing design, manufacturing, andevaluation technologies for handling modes will pave

Junji Sakamoto

Researcher, Optoelectronics Integration
Research Group, NTT Device Technology Laboratories.
He received a B.S. and M.S. in applied physics
from Osaka Prefecture University in 2010 and2012. He joined NTT Device Technology Laboratories in 2012, where he has been researchingsilica-based planar lightwave circuits. He is amember of the Institute of Electronics, Information and Communication Engineers (IEICE).

NTT Technical Review

Vol. 17 No. 5 May 2019the way for the implementation of unprecedented
optical devices with new functions and unique properties.
References
[1]

[2]

[3]
[4]

[5]

[6]

[7]

H. Yamazaki, T. Goh, T. Hashimoto, A. Sano, and Y. Miyamoto,“Generation of 448-Gbps OTDM-PDM-16QAM Signal with an
Integrated Modulator Using Orthogonal CSRZ Pulses,” Proc. of the
38th Optical Fiber Communication Conference and Exhibition (OFC2015), Paper Th2A.18, Los Angeles, CA, USA, Mar. 2015.
T. Sakamoto, T. Matsui, K. Saitoh, S. Saitoh, K. Takenaga, T. Mizuno,
Y. Abe, K. Shibahara, Y. Tobita, S. Matsuo, K. Aikawa, S. Aozasa, K.
Nakajima, and Y. Miyamoto, “Low-loss and Low-DMD 6-mode
19-core Fiber with Cladding Diameter of Less Than 250 μm,” J.
Lightw. Technol., Vol. 35, No. 3, pp. 443–449, 2017.
A. Himeno, K. Kato, and T. Miya, “Silica-based Planar Lightwave
Circuits,” IEEE J. Sel. Top. Quantum Electron., Vol. 4, No. 6, pp.
913–924, 1998.
J. Sakamoto, S. Katayose, K. Watanabe, M. Itoh, and T. Hashimoto,“High-efficiency Multiple-light-source Red-green-blue Power
Combiner with Optical Waveguide Mode Coupling Technique,” Proc.
of SPIE, Vol. 10126, pp. M1–M7, San Francisco, CA, USA, Feb.
2017.
J. Sakamoto, T. Goh, S. Katayose, K. Watanabe, M. Itoh, and T.
Hashimoto, “Compact and Low-loss RGB Coupler Using Modeconversion Waveguides,” Opt. Commun., Vol. 420, No. 1, pp. 46–51,
2018.
A. Nakamura, K. Okamoto, Y. Koshikiya, T. Manabe, M. Oguma, T.
Hashimoto, and M. Itoh, “High-sensitivity Detection of Fiber Bends:
1-μm-band Mode-detection OTDR,” J. Lightw. Technol., Vol. 33, No.
23, pp. 4862–4869, 2015.
A. Nakamura, K. Okamoto, Y. Koshikiya, T. Manabe, M. Oguma, T.
Hashimoto, and M. Itoh, “Loss Cause Identification by Evaluating
Backscattered Modal Loss Ratio Obtained with 1-μm-band Modedetection OTDR,” J. Lightw. Technol., Vol. 34, No. 15, pp. 3568–
3576, 2016.

Toshikazu HashimotoGroup Leader, Optoelectronics Integration
Research Group, NTT Device Technology Laboratories.
He received a B.S. and M.S. in physics from
Hokkaido University in 1991 and 1993. He
joined NTT Photonics Laboratories in 1993. He
has been researching hybrid integration of semiconductor lasers and photodiodes on silica-based
planar lightwave circuits and conducting theoretical research and primary experiments on the
wavefront matching method. He is a member of
IEICE and the Physical Society of Japan.

44

Global Standardization Activities

Standardization Efforts ofInternational ElectrotechnicalCommission Related to Surge
Protection Components, and Receipt
of METI Minister’s Award
Hidetaka Sato
Abstract

An International Electrotechnical Commission (IEC) international standard for surge isolation transformers (SITs) for communications use has been established based on a proposal from Japan. SITs arecomponents featuring high insulation-breakdown voltage and impedance. They reduce lightning surges
to a small voltage level so there is no effect on protected equipment, while simultaneously blocking acommon mode noise loop. They provide excellent protection for the DC (direct current) SELV (safety
extra-low voltage) circuit on the secondary side in information and communication technology equipment. This article introduces international standardization activities related to SITs at IEC and discusses
associated technology.
Keywords: lightning protection, self-contained isolation system (SIS), surge isolation transformer

1. Introduction
There is an ongoing need to protect electronicequipment from damage caused by lightning surges,
and therefore, research on technology to achieve thiscontinues. In this section, issues that have become
newly apparent are introduced.
1.1 Emergence of new issues
Lightning protection measures up to now havemainly focused on preventing permanent damage orfires caused by burning, scorching, or melting as aresult of an insulation breakdown within informationand communication technology (ICT) equipment andelectric/electronic appliances triggered by overvoltage or overcurrent surges caused by lightning. Today,
advances in lightning protection technology have
reduced equipment damage due to overvoltage breakdown in hardware, but data errors have recently
NTT Technical Review

Vol. 17 No. 5 May 2019

become evident as a new form of lightning-related
damage. This is a phenomenon in which errors occur
in data being processed even if the hardware itself has
suffered absolutely no damage. It can give rise to
problems such as corruption of original data, equipment freezing, system hang-ups, malfunctions, outof-control operation, and system downtime. In theevent of such a problem, it is usually possible torestore operation by resetting the equipment, turning
power OFF/ON (rebooting), reloading the operating
system, or restoring (salvaging) corrupted data. Nevertheless, such lightning-related data errors run therisk of lost opportunities, disappearance of immediately previous data deliverables, and failure to noticethat data errors lay hidden within memory. While data
errors that occur during data reading or transfer canusually be handled by error-correcting code, the needhas arisen for more robust measures against data
errors that occur at the time of a lightning strike.
45

Global Standardization Activities

Lightning-surge protection method: IEC 62305• IEC 61643-11, -12, -21, -22: SPDs
• IEC 61643-311, -312: GDTs
• IEC 61643-331: MOVs
• IEC 61643-321: ABDs
• IEC 61643-341: TSSs

Conventional method
Equipotential

Recent method
Isolation• IEC 61643-351, -352: SIT for com.
• IEC 62305-4: Isolating interface• IEEE PC62.69TM/D1: Surge parametersof isolating transformers (ITs)• ITU-T K.95: Surge parameters for ITs• ITU-T K.126: SPC application guide –high frequency signal ITs

Com.: communications

Fig. 1. Equipotential and isolation lightning-surge protection methods and their international standards.

1.2 P roblem with Japan’s isolated earth environment
Surge isolation transformers (SITs) have been
widely used in Japan’s power supply system for sometime. They are known as lightning-resistance transformers, and they have rarely been used outside
Japan. To give some background, while Japanfocused on isolated-earth systems, the trend overseas
was integrated-earth systems and lightning protection
by equipotential methods such as using the earth as areference point. For example, grounding for communications in Japan has been achieved by installing
subscriber arresters for communications use in all
telephone subscriber homes, but this method involves
the independent grounding of subscriber arresters in
isolation from the grounding of power systems, buildings, and other elements. As a result, the following
problem arises with Japan’s isolated earth environment: if a lightning surge flows in one such earth
connection, an earth potential rise occurs due to earth
resistance, resulting in a new overvoltage surge
source.
The International Electrotechnical Commission(IEC) has been active in developing standards forsurge protective devices (SPDs) and surge protective
components (SPCs) used in equipotential methods,
and the development of standards for isolation methods has been progressing at the Institute of Electricaland Electronics Engineers (IEEE) and International
Telecommunication Union - Telecommunication
Standardization Sector (ITU-T).

NTT Technical Review

Vol. 17 No. 5 May 2019

2. Overview of IEC committee organization
The IEC committee and Working Group addressing
the standardization of technology in this area are
introduced in this section.
2.1 IEC/SC 37B organization
IEC Technical Committee (TC) 37 (surge arresters)/ Subcommittee (SC) 37B (components for lowvoltage surge protection) is a group working to establish IEC international standards on discrete surgeprotection components that reduce lightning surges toa sufficiently small level to protect the human body,equipment, and appliances. Four international standards have been established so far: IEC 61643-311:
GDTs (gas discharge tubes); IEC 61643-321: ABDs
(avalanche breakdown diodes); IEC 61643-331:
MOVs (metal oxide varistors); and IEC 61643-341:
TSSs (thyristor surge suppressors) (Fig. 1). All ofthese standards concern components for achieving
overvoltage protection through lightning equipotential methods using the voltage non-linear resistancecharacteristics of these SPCs. In relation to the above,
Japan submitted a new work item proposal (NP) on
SITs as a fifth type of SPC using an isolation methodcompletely different from the above equipotential
methods.
2.2 O
 verview of Working Group (WG) 3 project
activities
The initial 2010 NP and CD (committee draft) wasapproved by international vote, but since the participation of at least four overseas experts required for46

Normal condition

Global Standardization Activities

Signal / SurgeSPDSMC (SIT)

SignalNormally off

Normally on

Differential mode
GND isolation
Surge

Surge condition

ICT
equipment

Block
DivertCommonmode
Common modenoise loop

Non-existent
Common modenoise loop

GND: groundFig. 2. Lightning-surge overvoltage and common mode noise protection using SIT and SPD.

establishing a project could not be obtained from the
14 participating members (P-members), Japan’s proposal for a new IEC project was rejected. We attributed this to the low international awareness of SITs.
We therefore conducted an information campaign
over the next two years to heighten worldwide awareness, and by resubmitting the NP in 2012, we succeeded in establishing an IEC 61643-351/352 projectthrough the participation of experts from the four
countries of China, Germany, the United Kingdom,
and Japan. IEC/SC 37B/WG 3 (surge isolation transformers) was then established with the author as
convenor, and standardization work began.
At the same time, France and the United States
opposed this project for the reason that SITs wereoutside the scope of SC 37B. They argued that thescope of IEC SC 37B, the parent subcommittee, iscomponents for use as SPDs, and the definition of an
SPD was “a device incorporating one or more nonlinear elements having the purpose of limiting surge
voltage and diverting surge current,” so SITs, having
no non-linear elements, were outside the scope.
A revision to the scope of SC 37B was therefore
proposed by Japan at the 2013 plenary meeting anddiscussions, and the result of an international voteproduced the following new scope: “To prepare international standards for components for low-voltage
surge protection. These SPCs are used in power, telecommunication and/or signaling networks with voltNTT Technical Review

Vol. 17 No. 5 May 2019ages up to 1000 V a.c. and 1500 V d.c.”In other words, all components that reduce surge, or
SMCs (surge mitigation components), were now to
be handled by SC 37B, and on the basis of this revision, France and the United States became involved
as experts. Additionally, an IEC NP in relation to
SITs proposed by Japan was introduced at the ITU-T
SG5 (Study Group 5: environment and circular economy) meeting in Geneva held from April 26 to May
3, 2011. As a result, even ITU-T came to recognize
the need for SITs, and thus, SIT-related activities
progressed with a liaison member.
3. Lightning protection of ICT equipment by
SITs for communications
An example of a countermeasure to lightning overvoltage surge protection and common mode noise
combining an SPD (equipotential method) and SIT(isolation method) is shown in Fig. 2 using equivalentswitch circuits. Here, the SPD and SIT operate in an
exclusive and complementary manner. The SPD
requires grounding to achieve equipotential surge
protection. In the event of a surge, it immediately
turns the switch ON to form a path that diverts thesurge current to earth. At this time, a large-area common mode noise loop including the earth forms,
resulting in a noise source that can generate data
errors, and an earth potential rise occurs due to earth47

Global Standardization Activities

Antenna port

Separation distance(Clearance, creepage
distance)

1/4
short stubfor
antenna
port

Optical fiberCom. port

SIT
for
com.

SPD

DC
SELV
circuit

DC/AC

SIT
for
power

Reinforced
insulation

EM shield

Quasi built-in
power system

AC power port

SPD

Basic
insulation

Earth port
SPD
Lightning equipotential binding bar

AC: alternating current
DC: direct current
EM: electromagnetic

Fig. 3. Self-contained isolation system for DC SELV (direct current safety extra-low voltage) circuit in ICT and Internet ofThings equipment.

resistance. These are weak points of this device.
However, the SIT turns OFF under a surge condition, thereby blocking a common mode surge current
and dramatically improving noise resistance from anelectromagnetic compatibility perspective. Thanks to
its high insulation-breakdown voltage, a SIT can treat
its primary-side and secondary-side ground (GND)
potentials as different, thereby achieving GND isolation.
An example of the application of a SIT to ICT
equipment is shown in Fig. 3 and to Ethernet communications in combination with a direct current(DC) power feed is shown in Fig. 4. The ICT equipment on the secondary side usually features a DC
safety extra-low voltage (SELV; not exceeding a peakvalue of 42.4 V or DC 60 V) circuit that requires nogrounding when used in combination with insulation,
enabling it to be set at a floating potential. In conventional SPD multistage protection, the let-through current that flows due to SPD coordination becomes a
powerful common mode noise source that has been
the cause of data errors in ICT equipment. The combined SPD and SIT block common mode noise by
virtue of the SIT and drastically reduce surge voltage
migration to 1/100–1/1000 of the SPD peak voltage.
It is therefore an excellent countermeasure to bothlightning overvoltage surges and data errors.

NTT Technical Review

Vol. 17 No. 5 May 2019

4. METI Minister’s Award
The author received the 2018 Ministry of Economy,
Trade and Industry (METI) Minister’s Award inIndustrial Standardization in recognition of the following activities: submitting Japan’s proposal for
IEC/TC 37/SC 37B/WG 3 and serving as convenor(WG 3 international chairman); contributing to theformulation of IEC 61643-351 (SIT performancerequirements and test methods) and IEC 61643-352(SIT selection and application principles); and in
anticipation of artificial intelligence (AI), Internet ofThings (IoT), and big data connections that are
expected to expand on a global scale, formulatinginternational standards for lightning protection measures in SELV ICT equipment connected to the Internet, thereby making Japan a forerunner in thisendeavor and enhancing the country’s international
industrial competitiveness.
5. Future plans
In recent years, studies have been accelerating on
Industry 4.0 (transformation of the manufacturingindustry through IoT and AI technologies) and theinternational standardization of ISO (International
Organization for Standardization) 8000 (data quality)with an eye to achieving the Society 5.0 vision [1].
48

Global Standardization Activities

PD

4
5

4
5

1
2

1
2

SIT
PoE
Power
supplyEthernet
PHY
SIT

SIT
RJ-45

RJ-45

3
6

3
6
7
8

7
8

+
−
Ethernet
PHY

SIT
A power
power
Mode aPHY: physical layer(a) PSE (power sourcing equipment) side(b) PD (power device) side

Fig. 4. SIT application example for PoE (Power over Ethernet) communication.

From here on, as the use of big data expands and IoT
and AI spread throughout society, we can expect data
quality to become all the more important in industrial, service, and medical-care fields and in databases
released by government and public institutions. In
addition to simultaneously supporting lightning overvoltage surge protection and data quality, the SIT is aleading component embodying the idea of a selfcontained isolation system [2], similar to that of airplanes, having a proven record in withstanding direct
lightning strikes and lightning electromagnetic

NTT Technical Review

Vol. 17 No. 5 May 2019waves. With this in mind, we plan to proceed with the
IEC international standardization of SITs for power
supply systems.
References[1][2]

Government of Japan, “The 5th Science and Technology Basic Plan,”
Jan. 2016.
http://www8.cao.go.jp/cstp/english/basic/5thbasicplan.pdf
H. Sato, “Self-contained Isolation System Using Surge Mitigation
Components against Lightning Surge and EM Wave for ICT and IoT
Equipment,” Proc. of the 34th International Conference on Lightning
Protection (ICLP 2018), Rzeszow, Poland, Sept. 2018.

49

Global Standardization Activities

Hidetaka Sato

Senior Manager, NTT Facilities Inc.
He received a B.E. from Iwate University in1981 and a Ph.D. from Tohoku University in
1995. In 1981, he joined Musashino ElectricalCommunications Laboratories of the Nippon
Telegraph and Telephone Public Corporation(now NTT), in Musahino-shi, Tokyo. He was avisiting professor at the University Technology
Malaysia (UTM) in 1996 and returned to the
NTT laboratories in 1997. He is currently affiliated with the research and development (R&D)
division of NTT Facilities, where he has been
engaged in R&D of lightning protection technologies and their standardization as chairman of
IEC SC37A and SC37B Japanese National Committee. He received the METI Minister’s Awardin 2018, and IEC 1906 Awards in 2013 and 2018.
He is a Professional Engineer Jp, IntPE(Jp), and
APEC Engineer (Electrical). Dr. Sato is a senior
member of IEEE and the Institute of Electronics,
Information and Communication Engineers(IEICE), and a member of the Institute of Electrical Installation Engineers of Japan (IEIE) and theInstitute of Electrical Engineers of Japan (IEEJ).

NTT Technical Review

Vol. 17 No. 5 May 201950

Short Reports

NTT Develops Non-rigid Object
Recognition Technology that Achieves
Recognition of Database Images—
Not Only Rigid 3D Objects but also
Non-rigid Objects—at Low Cost
1. Introduction
NTT has developed a new technology called anglefree rigid and non-rigid object information retrieval,
which recognizes and searches deformable objects
with a small number of database images (Fig. 1). This
technology makes it possible to recognize rigid threedimensional (3D) objects as well as non-rigid objects
such as soft packaging or cloth products with highaccuracy.
Conventionally, image recognition technologies
including deep learning based ones require the preparation of many database images capturing multiple
deformation patterns in order to recognize deformed
objects. If an unknown deformation is added to theobject, many more database images are needed torecognize it. Our new rigid and non-rigid object
image matching technology recognizes an object
with an unknown deformation as the same object as
the non-deformed object image in the database.
With this technology, the cost of preparing database
images is greatly reduced, which makes it possible todevelop a product management system for a retailindustry in which new products are released frequently. This is also applicable to product recognition,
which saves labor in check-out operations, and product information presentation, which enables customers to browse product descriptions in multiple languages.
2. Backgroundcommunication technology, it is essential to establish
recognition and search technologies that can performthese functions using images taken by smartphones
and cameras in place of the human eye.
However, non-rigid objects such as soft packaging
products and cloth products have different patterns ofdeformation than rigid 3D objects. Therefore, theappearance of the images changes greatly, resulting
in lower recognition accuracy. To maintain recognition accuracy, it is necessary to register the image
corresponding to the deformation of the target object
as a reference image. However, if the pattern of deformation of the target object is arbitrary, it is necessary
to prepare a large number of images, which substantially increases the cost.
3. Research and development achievements
In the angle-free rigid and non-rigid object information retrieval technology developed by NTT, ageometric constraint is applied for each of multiple
partial areas rather than for the entire object. The
rigid and non-rigid object image matching accurately
specifies the correct correspondence from the correspondence relationship of the image features. Thus,
the correct correspondence between the input image
and the reference image can be specified even if theobject is deformed. As a result, it is possible to recognize and search with high accuracy not only rigid 3D
objects, but also non-rigid objects in which arbitrary
deformations occur.

To realize a digital transformation that improves
our lives by connecting people with information and
NTT Technical Review

Vol. 17 No. 5 May 201951

Short Reports• The conventional angle-free object information retrieval can recognize/retrieve 3D objects with high accuracy using a few registeredimages. However, the recognition accuracy for deformed objects decreases, as this technology assumes that the object has afixed shape.
• Angle-free rigid and non-rigid object information retrieval can recognize/retrieve with high accuracy objects having
arbitrary deformations, with just a few registered images.

Conventional technologies(marketed technologies)
ViewpointchangeAngle-free object information retrieval

Deformations

Requires manyreference images

ViewpointchangeDeformations

Requires few
reference images

Angle-free rigid and non-rigid
object information retrieval
ViewpointchangeDeformations

Requires few
reference images

Fig. 1. Features of angle-free rigid and non-rigid object information retrieval.

4. Technical features
Angle-free rigid and non-rigid object information
retrieval is an NTT corevo® artificial intelligence
technology that was developed based on angle-freeobject information retrieval, a technique for recognizing and searching 3D objects only by registering
images from a few directions. The features of thistechnology are as follows.
In the conventional angle-free object information
retrieval, the recognition accuracy deteriorates for
deformed objects because the constraint condition on
the same object is derived from the projective geometric examination based on the assumption that theobject has a fixed shape. Furthermore, with existinglearning based methods such as those based on deeplearning, if a non-rigid object is used as a target, therecognition accuracy also deteriorates, since there arecountless deformation patterns, unless a large numberof reference images observing various deformations
are prepared in advance.
Therefore, rigid and non-rigid object image match-

NTT Technical Review

Vol. 17 No. 5 May 2019ing has been developed, which accurately specifies
the correct correspondence from the correspondenceof the image features by applying the geometric constraint to multiple partial regions instead of to theentire object. This makes it possible to specify thecorrect correspondence even if the object is deformed,by clustering using geometric features such as distances and rotation angles between multiple correspondence relationships based on the correspondence
result of image features between the input image andthe reference image. NTT’s angle-free rigid and nonrigid object information retrieval expands this technology, making it possible to achieve recognition andretrieval with high accuracy by only registering asmall number of images even if the pattern of deformation is arbitrary.
For Inquiries
Public Relations,
NTT Service Innovation Laboratory Group
http://www.ntt.co.jp/news2018/1811e/181126b.html

52

External Awards
Best Paper Award
Winner: Yuiko Tsunomori, NTT DOCOMO, INC.; Ryuichiro
Higashinaka, NTT Media Intelligence Laboratories; and Takeshi
Yoshimura, NTT DOCOMO, INC.
Date: February 21, 2019
Organization: The Institute of Electronics, Information and Communication Engineers (IEICE) Technical Committee on Natural
Language Understanding and Models of Communication (NLC)
For “A Long-term Evaluation of a Chat-oriented Dialogue Systemthat Utilizes User Information Acquired through Dialogue.”
Published as: Y. Tsunomori, R. Higashinaka, and T. Yoshimura, “ALong-term Evaluation of a Chat-oriented Dialogue System that Utilizes User Information Acquired through Dialogue,” IEICE Tech.
Rep., Vol. 118, No. 122, NLC2018-4, pp. 29–34, July 2018.
Maejima Hisoka Award
Winner: Masayuki Abe, NTT Secure Platform Laboratories
Date: February 26, 2019
Organization: Tsushinbunka Association
For his pioneering research on secure and user-friendly electronic
signatures and cryptographic protocols.
IEICE Information Networks Best Research Award
Winner: Yasuhiro Ikeda, Keisuke Ishibashi, Yuusuke Nakano,
Keishiro Watanabe, and Ryoichi Kawahara, NTT Network Technology Laboratories
Date: March 4, 2019
Organization: IEICE Technical Committee on Information Networks (IN)For “Retraining Anomaly Detection Model Using Autoencoder.”Published as: Y. Ikeda, K. Ishibashi, Y. Nakano, K. Watanabe, and R.
Kawahara, “Retraining Anomaly Detection Model Using Autoencoder,” IEICE Tech. Rep., Vol. 117, No. 397, IN2017-84, pp. 77–82,
Jan. 2018.
IOP Publishing Outstanding Reviewer Award 2018
Winner: Xuejun Xu, NTT Basic Research Laboratories
Date: March 13, 2019
Organization: IOP Publishing Limited
For his outstanding work as a reviewer of semiconductor scienceand technology in 2018.
TELECOM System Technology Award
Winner: Hideki Maeda, Kohei Saito, Takashi Kotanigawa, Takeshi
Seki, NTT Network Service Systems Laboratories; Shuto Yamamoto,
Fukutaro Hamaoka, NTT Network Innovation Laboratories; andNTT Technical Review

Vol. 17 No. 5 May 2019

Mitsuteru Yoshida, NTT Electronics Corporation
Date: March 20, 2019
Organization: The Telecommunications Advancement Foundation
For “Field Trial of 400-Gbps Transmission Using Advanced Digital Coherent Technologies.”
Published as: H. Maeda, K. Saito, T. Kotanigawa, T. Seki, S. Yamamoto, F. Hamaoka, and M. Yoshida, “Field Trial of 400-Gbps Transmission Using Advanced Digital Coherent Technologies,” J. Lightw.
Technol., Vol. 35, No. 12, June 2017.
Young Researcher’s Award
Winner: Toshiro Nakahira, NTT Access Network Service Systems
Laboratories
Date: March 21, 2019
Organization: IEICE
For “802.11ax Based Multi-RF Control Method for WiSMA” and“Spatial Resource Optimization under Multi-interface Control
Scheme on Strategy Management Architecture for Wireless Resource
Optimization (WiSMA).”
Published as: T. Nakahira, T. Murakami, H. Abeysekera, K. Ishihara,
A. Inoki, K. Wakao, Y. Takatori, and T. Hayashi, “802.11ax BasedMulti-RF Control Method for WiSMA,” Proc. of the 2018 IEICE
General Conference, B-5-135, Tokyo, Japan, Mar. 2018 (in Japanese). T. Nakahira, H. Abeysekera, T. Murakami, K. Ishihara, and T.
Hayashi, “Spatial Resource Optimization under Multi-interface Control Scheme on Strategy Management Architecture for WirelessResource Optimization (WiSMA),” Proc. of the 2018 IEICE Society
Conference, B-5-96, Kanazawa, Ishikawa, Japan, Sept. 2018 (inJapanese).
Young Researcher’s Award
Winner: Akito Suzuki, NTT Network Technology Laboratories
Date: March 21, 2019
Organization: IEICE
For “Evaluation of Routing Control Method Utilizing External
Information on Large-volume Traffic Generation” and “Applicability
Evaluation of NFV-integrated Control Method Using Reinforcement
Learning.”
Published as: A. Suzuki, M. Kobayashi, S. Harada, and R. Kawahara, “Evaluation of Routing Control Method Utilizing External
Information on Large-volume Traffic Generation,” Proc. of the 2018
IEICE General Conference, B-7-22, Tokyo, Japan, Mar. 2018 (inJapanese). A. Suzuki, M. Kobayashi, S. Harada, and R. Kawahara,“Applicability Evaluation of NFV-integrated Control Method Using
Reinforcement Learning,” Proc. of the 2018 IEICE Society Conference, B-7-12, Kanazawa, Ishikawa, Japan, Sept. 2018 (in Japanese).

53Papers Published in Technical Journals andConference Proceedings
Live Line Aging Estimation of AC Adapters
F. Ishiyama and Y. Toriumi
Proc. of the 18th IEEE International Symposium on Signal Processing and Information Technology (ISSPIT 2018), pp. 175–178,
Louisville, KY, USA, December 2018.
There is a risk that highly deteriorated alternating current (AC)adapters may ignite and catch fire. If we could estimate the extent ofaging using live line measurement, we would be able to find deteriorated AC adapters without interrupting the operation of electricalequipment. As we know that deteriorated AC adapters cause electromagnetic noise on power lines, we conducted a study to ascertain if
we could estimate the extent of aging from the intensity of the noise.
We found that this was possible using our method of time series
analysis, which has extremely high time-frequency resolution. In thispaper, we describe our method and present the results of the analysis.

Intelligent Monitoring of Optical Fiber Bend Using Artificial Neural Networks Trained with Constellation Data
T. Tanaka, W. Kawakami, S. Kuwabara, S. Kobayashi, and A.
HiranoIEEE Networking Letters, DOI: 10.1109/LNET.2019.2897295,
February 2019.
We demonstrate, for the first time, the highly accurate detection of
the physical condition of optical fiber by using an artificial neuralnetwork (ANN). The ANN takes constellation data as its input, andoutputs estimation data indicating the presence of optical fiber bending. This technique can estimate optical fiber condition without
additional testers. We verify that the trained ANN can precisely detect
optical fiber bends from test data as well as validation data in transmission lines with no optical amplifiers. The proposal is effective if
optical amplifiers are used provided the amplitude spontaneous emission noise is small.

Economical Speed Adjustment Scheme Based on Deep
Reinforcement Learning
M. Yoshida, K. Mizutani, T. Hata, I. Shake, and T. KashiwaiTransactions of Society of Automotive Engineers of Japan, Vol. 50,
No. 2, pp. 622–628, March 2019 (in Japanese).

NTT Technical Review

Vol. 17 No. 5 May 2019

In this paper, we report on a deep reinforcement learning techniquethat outputs the optimum speed acceleration to improve fuel economy while taking into account traffic flow on a public highway. The
deep reinforcement learning utilizes the distance of spatiotemporalfeatures of traffic signal conditions. From the simulation evaluationof our scheme, we confirmed that fuel efficiency can be improved byabout 10% while maintaining the overall traffic flow with the proposed method, compared to the rule-base speed adjustment scheme.

Development of Resilient Information and Communications Technology for Relief against Natural Disasters
H. Kumagai, H. Sakurauchi, S. Koitabashi, T. Uchiyama, S. Sasaki, K. Noda, M. Ishizaki, S. Kotabe, A. Yamamoto, Y. Shimizu, Y.
Suzuki, Y. Owada, K. Temma, G. Sato, T. Miyazaki, P. Li, Y. Kawamoto, N. Kato, and H. Nishiyama
Journal of Disaster Research, Vol. 14, No. 2, pp. 348–362, March
2019.
The study focused on the research and development of informationand communication technology (ICT) for disaster preparedness andresponse with respect to two categories, namely, the delivery of alert
messages to a wider group of residents and providing quick relief
communications in affected areas. In the former category, the development focused on two targets, one involving the delivery of alert
messages to indoor residents with a V-Low broadcasting service andthe other involving the delivery of an alert message to individuals
with disabilities and difficulties in understanding Japanese. In thelatter category, a portable ICT unit was developed for rapid relief
communications, and mesh network technology enabling robust
information sharing among base stations in the affected area wasdeveloped. Furthermore, a related development focused on a resilient
information management system to collect information in areas that
do not have access to the Internet. Furthermore, device relay technology was developed to expand access network coverage areas. Afterthe development of individual technology, activities for the societal
implementation of the development results were conducted throughfield experiments and disaster drills in which the developed technologies were integrated and utilized.

54