NTT Technical Review
January 2016 Vol. 14 No. 1
View from the Top
Hiromichi Shinohara, Senior Executive Vice President and Head of R&D
Strategy Department, NTT

Front-line Researchers
Shinji Matsuo, Senior Distinguished Researcher, NTT Device Technology
Laboratories

Feature Articles: Photonics-electronics Convergence
Hardware Technology for Maximizing Network Performance
R&D Trends in Convergence of Photonic and Electronic Hardware
for Network Innovation
High-speed Electronic and Optical Device Technologies
for Ultralarge-capacity Optical Transmission
Advances in Phase Sensitive Amplifiers Based on PPLN Waveguides
for Optical Communication
Lateral Current-injection Membrane Lasers Fabricated on a Silicon Substrate
Photonics-electronics Convergence Design for Digital Mock-up
High-speed Optical Packet Switching for Photonic Datacenter Networks
Hardware/Software Co-design Technology for Network Virtualization

Regular Articles
Body-mind Sonification to Improve Players Actions in Sports
Development of Invisible Optical Fiber for Improved Aesthetic Appearance
Utilization of APIs for B2B2X Business Model

Global Standardization Activities
Latest TM Forum Developments

External Awards/Papers Published in Technical Journals
and Conference Proceedings
External Awards/Papers Published in Technical Journals and Conference
Proceedings

View from the Top

Creating New Value with a Spirit of
One for All, All for One—
Using Human Sensitivities toPopularize Services
Hiromichi Shinohara
Senior Executive Vice President andHead of R&D Strategy Department, NTT
Overview

The NTT Group has taken up the challenge of self-transformation toward
growth in profits as declared in its Medium-Term Management Strategy“Towards the Next Stage 2.0.” There are high expectations of the NTT
Group for technological development and social contributions in fields ofgreat global interest such as artificial intelligence, Internet of Things, andcybersecurity. At the dawn of a new year, we asked Senior Executive Vice
President Hiromichi Shinohara to tell us about NTT Group plans for 2016
and specific initiatives for creating new value.
Keywords: R&D, artificial intelligence, cybersecurity

2015: NTT Group makes a big turn from a main
player to a catalyst—Mr. Shinohara, what kind of results were obtainedlast year under the new Medium-Term Management
Strategy?
To begin with, the entire NTT Group has endeavored to change its mindset. We have abandoned oursense of being a “main player” and have put ourselveson the path to becoming a “catalyst.” The NTT Groupannounced this change in direction in its MediumTerm Management Strategy “Towards the Next Stage2.0” presented in May 2015. There are two main pillars to this strategy. One is self-transformation withthe aim of becoming a “value partner” and increasing
profits. The other is further promotion of the business-to-business-to-X (B2B2X) model and the open

ing up of new business fields and markets.
One example of self-transformation is the launch of
the Hikari Collaboration Model by NTT EAST and
NTT WEST. This model involves wholesaling of
optical-fiber access services and has enabled us tostart providing new value to our customers in cooperation with many partners other than telecom carriers. More developments are needed here, but we havegotten off to a good start.
In tandem with these efforts, NTT research andtechnology development has been focused on improving cost efficiencies by simplifying the network andpreparing technologies essential to new services in
the form of standardized “engines.” In addition, we
pursued collaboration and open innovation withgroup companies and various partners. One example
is the functional fabric called “hitoe” that can provide
biomedical information simply by wearing it. In
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December 2014, NTT DOCOMO launched a fitness
service using hitoe called “Runtastic for docomo,”
and not to be outdone, NTT Communications began
tests of a safety management system in which staff ofmajor corporations such as the construction firm
Obayashi and Japan Airlines are asked to wear hitoe
material in the field. NTT DATA, moreover, is working on using hitoe as a real-time tool for measuring
the condition of drivers in IndyCar racing in the
United States, and studies are also underway on theuse of hitoe for measuring the heart rate and myoelectric signals of pilots in the jet fighter test environmentof Japan’s Ministry of Defense. Indeed, the possible
applications of hitoe have expanded greatly this lastyear.
As a result of these activities, I think I can safely
say that this past year marked a major turn for theentire NTT Group in a new direction based on the
keyword “collaboration.”—A huge amount of energy would be needed to movesuch a massive organization as the NTT Group. How
do you keep everyone and everything in step?
When determining to move in the direction of newvalue, I think there are two ways to go about it. One
is to get everyone moving after giving instructions onwhat needs to be done, and the other is to get everyone moving by sharing an awareness of the problem
and an understanding of the objective. Sometimes I
feel that giving instructions is important, but I also
think it’s important that I get everyone to empathizewith the NTT Group objectives and way of thinking
and to think about the best way of dealing with problems that we will have to confront from here on. For
example, I have begun summoning those in charge oftechnology development at the various group companies to share each other’s research and development(R&D) plans and to discuss what should be developed in common. Here, with an eye to creating newservices and accelerating the generation of profits
while eliminating waste in technology development,
we looked for technologies that could be shared bygroup companies. These efforts have been producing
results. I believe that experiencing such a string of
successes leads to ongoing, self-motivated activities
within the NTT Group.

Vol. 14 No. 1 Jan. 2016

Linking people and technology to create
new value—How do you plan to expand upon last year’s
efforts?
One way is to make an all-out effort to create newvalue to drive the growth of the NTT Group. Here, I
would like to accelerate the development of a B2B2X
model. Moreover, from the viewpoint of R&D, stepping up the standardization of engines (technologies)
is important. Last year, for example, the NTT Groupannounced a variety of interactive, talking robots
typified by one named “Sota.” These robots incorporate advanced technologies for natural-language processing and other functions, but up to now, they havebeen independently developed and supported. Thisapproach, however, incurs unnecessary expenses andslows down the development process, so from here
on, I would like to further promote the standardization of technologies.
Another way is to accelerate “hybrid development.”
Up to now, we have been creating better services bypromoting joint development with NTT Group companies, but this year, I would like to accelerate thespeed of providing services to the market from theresearch stage. To this end, I will promote joint
activities from early on and target market needs fordevelopment. I would also like to envision those partsof a service that customers may not be aware of in thepresent and incorporate those parts in development
projects. Thus, in addition to simply doing something
because we are asked to do so by customers, I would
like to improve our competence at predicting futureneeds and responding accordingly. We need to look
into the future and move forward in the present. To
begin with, I would like all of us to develop thoseView from the Top

ways of thinking!
Regarding the network business, our goal was conventionally to achieve higher levels of quality.
However, considering today’s customer needs, thenetwork must be simplified, and the cost efficiency ofoperations and maintenance must be improved toincrease the profits of the entire NTT Group. To thisend, NTT announced the future network vision“NetroSphere” in February last year, and since then,we have been steadily forming business relationships,
so I would like to make this year one for implementing this concept in tangible ways.
I would also like to channel the results of development projects done at NTT Innovation Institute, Inc.
(NTT I3) to specific market needs in order to generate
profits in global cloud services. To push this along, I
would like to deepen the ties between NTT I3 andNTT laboratories on the Japan side.
—How do you approach the fields of AI, IoT, andcybersecurity, today’s top keywords?
Supercomputer performance, which at one time
was available to only a few people, is today achievedin smartphones, which many people now take for
granted as a tool they can use in the palm of theirhand. Advances in technology are truly amazing, butthey do not always translate into better lives for peo-

ple. As for artificial intelligence (AI), it has not yet
reached a level that can truly transform society. In
addition to making society more efficient and convenient for people, I would like to see AI play a role inachieving a prosperous society by helping to makethe lives of many people enriching and fulfilling in
the true sense of the words. Details on our AI initiatives will be announced at the NTT R&D Forum in
February 2016, but we can broadly divide them into
four areas. These are Agent AI that does things forpeople, Ambient AI based on massive amounts ofdata in relation to Internet of Things (IoT), Network
AI that links to networks, and Heart-Touching AI thatinteracts with people in a human-friendly way. Goingforward, we will continue to pursue ways in which AI
can be beneficial for our lives.
Next, as for IoT, there will be various requirements
depending on the application field. This year, I would
like to produce concrete examples of IoT applications
in collaboration with partners. Data will become ofprime importance in this endeavor. However, data
involves not only machines but also individuals andtheir personal attributes, so care must be taken in how
to obtain data from people, in how to safely use andcombine that data, and in how to otherwise apply thatdata in an appropriate manner. In addition, it’s not justtechnology that matters here but also the creation of
new value from collected data. I therefore believe thatgathering data in a divided “silo-type” manner is notenough, and that it will become increasingly important to coordinate different types of data in someway.
In terms of cybersecurity, we plan to build up ourtraining program for security personnel in the NTT
Group, and we aim to establish a security force on thescale of about 10,000 people within Japan by 2020.
Last year, we completed our first round of certifications for security qualifications and we will continue
to develop ourselves in this area. However, when
thinking ahead to 2020, it’s not only telecom carriers
but all operators involved with critical infrastructurethat must address the need for robust security measures. Today, just about everything including manufacturing plants and rail transport is connected to the
network, so security measures constitute an investment for future growth and a major management
issue. All companies and enterprises must identify
what needs to be protected within their operations
and must examine their ability to ensure security.
With this in mind, NTT established the “Cross-Sector
Cybersecurity Forum” in June of last year to contribute to the training of security personnel in Japan and
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to promote information sharing. This forum has so far
enabled more than 40 participating companies tointroduce their efforts in developing security personnel, present training case studies, exchange ideas andopinions, and talk about each other’s state of security.
In this way, each company can reexamine the problems they are facing and consider appropriate solutions. Even companies that had thought they were
doing okay when it comes to security have comeaway from such gatherings with a heightened senseof danger and a resolution to enhance their security
measures. This year, I would like to promote the sharing of security-related information with global telecom carriers and to deepen our interaction withthem.
Finally, as an example of activities toward 2020, wegave proof of concept (PoC) presentations on some of
our R&D results at last year’s NTT R&D Forum. At
this year’s forum, I would like to connect PoC toactual applications and specific ways of achieving
them together with attendees. To this end, the ability
to convey things to people with a richer level ofexpression will become necessary, so I think that collaboration with people who have highly developed
senses, have extensive experience, and are sensitivein nature will become important from here on. In
particular, when introducing foreigners to Japaneseculture, we must keep in mind that “every man knows
his own business best.” At NTT, technology is ourbusiness, but there are people that are considerably
more knowledgeable, experienced, and emotionallyequipped to convey the diverse aspects of culture.
Certainly a richer level of expression can be achievedby tapping the abilities of professionals and people
directly involved with culture.
Importance of resolution and effort in popularizing a new development—Mr. Shinohara, can you leave us with a message forall employees in the NTT Group?
I would be happy to. This is the year that the true
worth of collaboration will be put to the test.
Collaboration is a summation of individual strengths.
It is a natural tendency to do what we are capable ofdoing and to rely on others to do what we cannot. We
should have the courage to refrain from doing somethings even if we feel we can do them on our own.
There are also times when we need to discard somethings in our quest to create something strong andlasting. This is a matter of self-discipline. To this end,
Vol. 14 No. 1 Jan. 2016

I would like us to build relationships with partners inwhich conversations can be held on an equal footing.
This may sound easy, but it’s actually difficult; anequitable relationship is inherently unstable. But I
would like everyone to keep this goal in mind.
In addition, we need to remember that developing
something new must be accompanied with efforts to
spread and popularize that new technology or service.
Here, the resolution and effort required to popularize
something new is different from that needed to develop it. Leaving one’s mark on society is truly a goodfeeling. I would like to ask each and every employee
to think about the best way of popularizing a service
that we have developed that has something to offer
society. In a sense, the word “universal” is similar to“popular,” but giving an existing thing value to makeit universal is different from discovering somethingthat is universal in a new way. Wouldn’t this challenge of searching for universality be interesting?
Considering that we may spend about 30 years in
the workplace, it is impossible to keep up a winningstreak throughout that time. There are times when we
inevitably have to lose. Instead of being focused on
winning or losing, I would like everyone to think hardon what the company and society at large should be
like.
If we attempted 100 development projects andfailed 90, instead of dwelling on the 90 failures, weView from the Top

can say that this is a process that gave birth to 10 successes. Moreover, from the viewpoint of the entire
NTT Group, it can be said that a failure in one division sometimes leads to a success in another. Failure
is evidence of great effort. Let’s take up the challenges of R&D without fear.
By the way, rugby, which is famous for its “one forall, all for one” spirit, is growing in popularity thanks
to the success of the Japanese national team in the
Rugby World Cup last year. I believe that people felt
the power of a team in addition to the power of individual players. Let’s think of the entire NTT Group asa team and let’s venture into the new era together.

Interviewee profile
Career highlights
Hiromichi Shinohara joined Nippon Telegraphand Telephone Public Corporation (now NTT) in
April 1978. In 1998, he was promoted to Vice
President of the Information Sharing Laboratory
Group at Access Network Service Systems
Laboratories. He became Senior Vice Presidentof the Information Sharing Laboratory Group inJune 2007. In June 2009, he became a member ofthe Board of NTT and Head of the R&D Strategy
Department. In June 2012, he became Executive
Vice President and Head of the R&D Strategy
Department. He took up his present position inJune 2014.

NTT Technical Review

Front-line Researchers

Making Photonic Devices
Dramatically Larger in Scale andLower in Price
Shinji Matsuo
Senior Distinguished Researcher,
NTT Device Technology Laboratories
Overview

Shinji Matsuo, Senior Distinguished Researcher at NTT Device
Technology Laboratories, continues his challenging work on
achieving optoelectronic integrated circuits on silicon substrates
and making them into economical and low-power devices—key
issues in the worldwide information technology industry. He leadsa group dealing with a unique form of research, so we asked him
to give us some background on the inception of this groundbreaking technology. We also asked him to describe remaining issues
and to comment on his role as a researcher.
Keywords: photonic device, optoelectronic integrated circuit,laser on Si

Laser on Si: a long-standing challenge—Dr. Matsuo, please tell us about your current
research efforts.
In Japan, mobile data traffic is predicted to grow by
about 1000 times the 2010 level by 2025. At datacenters too, traffic volumes are growing at an amazing
rate. Facility investment is needed to deal with thisjump in traffic, but passing on this cost to customers
through usage fees must be prevented. The amount of
power used by datacenters is also a key issue. In the
United States, for example, it accounts for a substantial percentage of all power consumed in the country.
In this regard, 70% of datacenter traffic is made up ofcommunications within the datacenter, so “economical” and “low-power” are becoming important keywords when talking about short-range optical communications in datacenters.
In photonic devices too—my field of research—
there is considerable activity in solving these problems in optical communications.
In the field of electronic devices such as siliconbased complementary metal-oxide semiconductors(CMOS), innovation in integration technology has
enabled the number of transistors in an integrated
circuit to increase by 1 million times over a 40-year
period in accordance with Moore’s law while minimizing cost increases.
In the field of photonic devices, however, the modularization of individual components such as lasers,
light-receiving elements, and isolators is still just a
concept. To achieve photonic devices with large-scale
integration and low cost on a level equivalent to thatNTT Technical Review

Front-line Researchers

1P

Inter-chip,
intra-chip

Datacenter

AccessMulticore

100 T

1T

Commercialized
technology

TargetPhotonic
crystal laser

100 G

Digital coherent
technology

WDM
Membrane
DFB laser

WDM
Surface-emitting
laser

10 G

it

m
Li
ng
iri
lw
ca m
tri
/s
ec bit
el
G
of 00
1

Transmission capacity (bit/s)

10 T

Core/metro

1G

Waveguide
laser

100 M
10 M
1 cm 10 cm

1m

TDM
Waveguide laser

10 m 100 m 1 km 10 km 100 km 1000 km
Distance

DFB: distributed feedback
TDM: time division multiplexing
WDM: wavelength division multiplexing

Fig. 1. Expanded application of optical communications technology.

of electronic devices, we need to develop optoelectronic integrated circuits on a silicon platform andestablish silicon photonics technology that canincrease integration density by 100–1000 times thatof current levels. To this end, we have been looking
into the use of compound semiconductors such as
indium phosphide (InP) and gallium arsenide (GaAs)
in the field of optoelectronic integration in a scheme
similar to the use of silicon. If this could be achieved
and if semiconductor lasers could be manufactured
by a mechanism similar to that of a silicon fab*, thecost per bit could be decreased by 1/1000 in 20 years
by what we could call an optical version of Moore’slaw (Figs. 1–3).
—Has this research progressed smoothly?
There are two main technical issues in achieving
laser on Si (silicon). These have presented a challenge to me ever since I entered NTT laboratories
about 30 years ago.
The first is lattice misalignment. The atomic size ofsilicon (group IV in the periodic table) and that ofcompound semiconductors such as InP and GaAs
Vol. 14 No. 1 Jan. 2016

(group III-V) are different, so growing a compound
semiconductor directly on silicon is extremely difficult. To resolve this issue, we developed technology
for preparing silicon and a compound semiconductor
separately and then joining them directly at theatomic level.
The other issue is different thermal expansion coefficients. The temperature in the laser fabrication process increases from room temperature to 600°C, but
the thermal expansion coefficients of silicon and InP
differ greatly. As a result, the different amount ofthermal expansion in those materials as the processtemperature changes can cause strain and stress tooccur, which can lead to defects in the crystal and asignificant drop in device characteristics. In light of
this, conventional semiconductor lasers have been
fabricated with a thickness of several micrometers.
However, by making use of membrane lateral injection fabrication technology developed in conjunction*

Silicon fab: A plant for fabricating silicon CMOS circuits. Many
units of very expensive manufacturing equipment are needed tofabricate ultra-fine CMOS devices, so having more than one company use the same plant instead of each company having its own
can significantly lower manufacturing costs.

Front-line Researchers

n SiO

o
laser
brane
m
e
M
SSC
SiO2

y

rra

ra

AP

LD

LE
e

trat

ubs

Si s

/Si

2

Si

e
las

wa

e

uid

g
ve

Si

te

tra

s
ub

s

Laser on Si

Photonic-network chip

Large-diameterSi substrate

CMOS
One-chip photonics-electronics convergence circuit

LD: laser diode
LEAP: lambda-scale embedded active-region photonic-crystal
Si: silicon
SiO2: silicon dioxideFig. 2. Large-scale optoelectronic integrated circuits using “laser on Si” technology.

InP

Emitting layer
2.6 × 0.3 × 0.15 µm3

Fig. 3. Cross section of LEAP laser.

photonic integrated circuit with lasers using a siliconfab. Of course, the use of a silicon fab means that thiswould not be something that NTT could accomplish
on its own, so we would need to do this together withmany other parties. To this end, we will promote discussions at international conferences that up to now
have not been directly related to lasers and actively
pursue joint research with outside parties. We will
also work to persuade industry leaders, university
professors, etc., to support the direction of ourresearch.
—This research has been attracting attention in many
countries. What has been the response?

with a LEAP (lambda-scale embedded active-region
photonic-crystal) laser that we successfully developed in 2013, we achieved a breakthrough in laser
membrane thickness, reducing it to about 1/10 that of
the conventional level.
I believe that this achievement was a direct result of
the world-class expertise of NTT laboratories in epitaxial growth technology and optical semiconductor
processing technology (Fig. 4).
—What issues lie ahead?
I would like to try fabricating an actual large-scaleI have had many opportunities to present myresearch in the form of invited lectures in Europe, theUnited States, China, and other countries to people
involved in similar research, so I feel that interest is
growing in our technology.
I have also noticed a growing interest from otherfields in our technology for fabricating lasers on silicon. The questions that I receive have likewise beenchanging from general impressions such as “veryunique characteristics” to specific inquiries such as“Can this really be used?” and “Can this be incorporated in the silicon fab mechanism?” I think this isbecause of growing expectations for actual application
NTT Technical Review

Front-line Researchers

Conventional technology

100%

d = 850 nm

Proposed technology

100%

d = 250 nm
100 µm

100 µmSurface distribution 50%
of emitted intensity

Surface distribution 50%
of emitted intensity

p-InP
Active layern-InP
InP substrate

p-InP

Active layer

n-InP

SiO2
Large-diameter Si substrate

Fig. 4. Core technology for achieving laser on Si.

and commercialization of this technology.
Thus, while I think that this is a great accomplishment from a technical point of view, there are still
issues that need to be resolved in terms of reliability
and cost efficiency before we reach a practical stage.
This research is progressing steadily, but when considering the objectives to be attained, it is still in its
second stage.
“You evaluate yourself”—surrounding yourself
with brilliant people and recognizing yourstrong points—In what sort of environment and from what mindset
did these epoch-making research achievements originate?
We have abundant human resources at NTT laboratories, with specialists in a variety of fields, and I
enjoy having the opportunity to chat with such brightpeople over drinks whenever possible. Our discussions at such sit-downs are lively, and new projects
may get off the ground with declarations such as, “If
that’s the case, let’s work together and give it a try!”
Senior Distinguished Researcher Masaya Notomi,
known for his research in photonic crystals, is one
such colleague.
In the actual research process, ideas may come tosuch specialists in a variety of ways. Because such a
distinguished group of people have been purposely
brought together, it’s imperative that ways be foundfor making the most of their amazing abilities in myVol. 14 No. 1 Jan. 2016

research with the ultimate aim of producing results.
It was once said to me: “You know best as to who
has the technology that we need.” On hearing that, I
suddenly realized one of my strong points—knowing
where to find certain technologies. I also keep inmind that “you evaluate yourself,” and that I should
be careful not to be swayed by the judgments of others.
This mindset actually began when I succeeded forthe first time in achieving laser oscillation on a silicon
substrate. I immediately tried to announce this technology in a post-deadline paper at the Optical Fiber
Communication Conference, but the paper was notchosen. However, I persevered in announcing myresults in later papers and elsewhere, and I finallybegan to receive some recognition. In other words,
the position, background, viewpoint, etc., of an evaluator can influence the evaluation in question.
The same holds true for evaluations within the company. The staff of a company is not solely composedof specialists, so it cannot be said that those evaluations are always correct. For me, in all honesty, youlive or die by whether a device works or fails rather
than by the opinions of others. I take great pleasure in
research. By the way, to tell you something about mypersonal life, I put a lot of effort in tending the garden
at home. Similar to research, flowers may not bloom
as expected, and weeds that spring up daily must be
constantly pulled. However, unlike research, resultsin gardening can be obtained relatively quickly, andmost people will find the results of your time andeffort and care as something beautiful. This is whyFront-line Researchers

gardening is relaxing and enjoyable for me. In thecase of research, it’s not all praise; most feedback is
negative in nature.
—As an active researcher, what do you think is important to produce results?
I believe that a change in perspective can be very
helpful. It is also important to have motivation and tomerge information.
In 1997, exactly ten years after entering NTT, I was
assigned to the NTT laboratory in Yokosuka, where I
was given the opportunity to work on optical communications systems for about three years. This was
the first time that I had been involved in the development of systems-related technology, since I had
researched only components and devices up to then.
However, I am still using the mindset and outlook thatI developed there in my current research. In addition,
just hearing about the situation in the United Stateswith regard to the need to reduce power consumption
in optical semiconductor devices—which is being
researched there in the form of government-supported projects—has motivated me greatly in my work. In
particular, it has helped me to see the importance andsignificance of this research.
I have been invited to a variety of international conferences in recent years, and ideas sometimes cometo me from things that I hear at those events. At suchtimes, I promptly email my colleagues or young
researchers in Japan involved in photonic device
research. I am motivated not just by discussions withspecialists but by a variety of things including social
phenomena. I have a constant desire to create something new by combining such ideas with technologythat I am already knowledgeable about.
In a senior position, however, I must take on moremanagement activities, which limits the time that I
can allocate to real research. I therefore have theresponsibility to consider the extent to which I myself
should contribute to this research and how I should
navigate the researchers under me in the right direction.
Nevertheless, I think that it is very important for me
as an active researcher to be at the forefront of thisresearch. I would not be able to judge what is crucial
just listening to other people and not doing my own
research. If I were to lose my edge here, I would not,in the end, be able to use my research time efficiently,
and I would not know how to merge all the information I was taking in. However, being too absorbed in
my own research would prevent me from receiving

information from those around me or leave me withno time for management tasks. Thus, it is importantthat I strike a balance between what I should domyself and what I should entrust to others.
Establishing one’s core is essential toa researcher—By the way, why do you continue to be an active
researcher?
In my younger days, I experienced what it felt liketo be completely absorbed in research. Some things
can only be understood after making many attempts
to solve a problem. This experience involved not only
work on intricate and critical problems but everyday
failures and mishaps as well. Looking back, I cannow laugh at this incident, but once when trying toobserve a wafer with a microscope, I accidently damaged the wafer when adjusting the microscope’s
height. Even science textbooks in elementary school
describe the proper way of using a microscope. This
simple event demonstrates that sometimes there arethings that one should know how to do but can’t.
Doing even a simple task repeatedly can enhance
one’s power of judgment and enable one to better
understand cause-and-effect relationships or otherphenomena based on experience. I feel that the senseof “this is just what is needed” is backed by experience and that one should first become adept at any
task you are called to do.
There has recently been talk about the need forresearchers to gain experience in other fields such as
business or sales to broaden their perspective. However, I would like to have researchers become totallyabsorbed in their work for about ten years at first to
develop their core skills. I think that the number of
researchers without such core attributes have been
increasing in recent years. If there is no field in whicha researcher who has accumulated such a variety ofexperiences can apply all of them, such experiences
have no meaning in the end.
—Dr. Matsuo, please leave us with a few words for allof our young researchers.
When you first start out as a young researcher, failure is commonplace. But during that time, you will
unconsciously be gaining experience as to where toback off and where to devote all your energy.
Research is indeterminate, unpredictable work, so
no one knows just what the results might be. Please
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Front-line Researchers

take up the challenge that research presents with all
your heart! This also means doing work that you will
be responsible for above and beyond your duties as a
researcher and employee in an organization, although
it’s still okay to be practical about your work andmake time for yourself too.
In cooking, which is one of my hobbies, you usually don’t fail as long as you follow the recipe.
Research, however, is different; there are no set recipes. Therefore, you, the researcher, must add yourown creative touch. Do not lose sight of your original
objective and path. Even after a string of failures, you
should list your daily achievements within that timeand enjoy your research to the fullest.

Vol. 14 No. 1 Jan. 2016

Interviewee profile
Shinji Matsuo
Senior Distinguished Researcher, Materials andDevices Laboratory of NTT Device Technology
Laboratories and NTT Nanophotonics Center.
He received a B.E. and M.E. in electrical engineering from Hiroshima University in 1986 and1988, and a Ph.D. in electronics and applied
physics from Tokyo Institute of Technology in2008. He joined NTT Opto-electronics
Laboratories in 1988, where he researched on
photonic functional devices using multiple quantum well pin modulators and vertical cavity surface-emitting lasers. In 1997, he researched optical networks using WDM technologies at NTT
Network Innovation Laboratories. Since 2000, he
has been researching high-speed tunable opticalfilters and lasers for photonic packet switching at
NTT Photonics Laboratories. He is a member of
the Institute of Electrical and Electronics
Engineers Photonics Society, the Japan Societyof Applied Physics, and the Institute of Electronics,
Information and Communication Engineers.

Feature Articles: Photonics-electronics Convergence
Hardware Technology for Maximizing Network Performance

R&D Trends in Convergence of
Photonic and Electronic Hardware
for Network Innovation
Shinichiro Mutoh, Hirokazu Takenouchi,
Hideyuki Nosaka, Toshikazu Hashimoto,and Akihiko Miyazaki
Abstract

Network traffic will continue to increase exponentially as we get closer to deploying fifth generation
mobile networks, and as the Internet of Things and big data services expand through 2020 and beyond.
In these Feature Articles, we look at trends in the research and development of advanced hardware technology designed to achieve substantial increases in the capacity of communication networks with much
lower capital and operating expenditures in the next 5 to 10 years. We focus in particular on technology
based on new concepts and principles that involve the convergence of photonics and electronics.
Keywords: photonics-electronics convergence, networks, hardware

1. Introduction
The rapid spread of smartphones around the worldis increasing network traffic exponentially. In the next
five years, the amount of traffic is expected to increaseby a factor of 10 [1]. Development of the fifth-generation mobile network (5G) expected to be launched
beginning in 2020 is proceeding rapidly. Specifically,
efforts are underway to achieve substantially higher
capacity per square kilometer—by a factor of 1000 or
more—compared to the present LTE (Long-Term
Evolution) system [2].
The movement toward the Internet of Things is also
accelerating. One recent key term is Industry 4.0,
which refers to improving the efficiency of production and services by implementing a cycle of various
types of sensing, signal and knowledge processing,and optimization control. Ultimately, the astronomical amount of sensor data on human activity, objects,
and the environment will be distributed via the network, and such data will be a source for big dataservices driven by machine learning.


The virtualization of computations and networks
continues to progress, and this means that hugeamounts of network traffic will be processed in acloud environment by enormous datacenters. The
Internet protocol traffic of the world’s datacenters is
expected to triple over the five years from 2013 to2018, and three-fourths of that is expected to consistof traffic within the datacenters.
2. Convergence of photonic and electronic
technology: device technology for
network innovation
A photonic network is a huge system that combines
photonic and electronic technology. It comprises
optical transceivers, node equipment (electrical or
optical switches or routers), and servers. This equipment is implemented by a combination of electronic
and optical hardware. The concept of converging
photonic and electronic communication hardware isillustrated in Fig. 1. Photonic network communication has evolved through innovation of transmission
NTT Technical Review

Feature Articles

Ultimate communication performance

FusionDevice-level fusion
and unification

Mutualempowerment ofphotonics andelectronics

Digital coherent
schemeWDM/TDM scheme

Separation ofroles forphotonics andelectronics in thesystemElectronics

E.g., silicon photonics and design
techniques that fuse photonics
and electronics
E.g., compensation by digital signal processing,high symbol rate by photonics-electronics
interworking, and photonics-electronics fusion
packet processing

E.g., high performance foreach device

Photonics

Fig. 1. Fusion of photonic and electronic technology.

technology such as time division multiplexing (TDM)
and wavelength division multiplexing (WDM), andthrough innovation of optical and electrical devices.
Digital coherent technology [3] is a revolutionary
idea that is enabling communication over 100 Gbit/s
per wavelength and over distances of thousands ofkilometers. Digital signal processing LSIs (largescale integrated circuits) constructed using state-ofthe-art semiconductor process technology suppress
various types of fluctuation and noise in the communication channel, and they have achieved a level of
performance that has never been achieved before byindividual improvements in optical and electricaldevices alone. In the future, the synergistic effect andmutual compensation obtained by combining photonic and electronic technology including digital
signal processing may hold the key to even further
network innovation.
One direction for progress is device-level convergence. For example, silicon photonics technology is
expected to produce a paradigm shift in telecom hardware by increasing its functionality and reducing its
size, cost, and power consumption through a highlevel of integration of optical and electrical devices.
These Feature Articles introduce some of theresearch and development (R&D) on hardware being
Vol. 14 No. 1 Jan. 2016

conducted at NTT Device Technology Laboratories
to achieve network innovation. The first part focuseson cutting-edge technologies for converging photonic
and electronic devices, which are aimed at achieving
the ultimate in network capacity. The second partdescribes technology for achieving a radical decreasein cost and power consumption as well as higher
capacity. This technology is essential to sustain the
progress achieved in networks requiring a muchhigher level of economy and energy efficiency suchas datacenters. We also introduce research on hardware technology to achieve network virtualization in
the software-defined networking (SDN) era for
reducing capital expenditure (CAPEX) and operating
expenditure (OPEX). A mapping of the technology
for these two directions is shown in Fig. 2.
3. Hardware technology for ultimate highcapacity: challenge to achieve ultimate highsymbol rate
To increase the capacity of photonic networks,
technology must expand along three axes: the numberof multilevels, the number of subcarriers*1, and thesymbol rate*2. For example, recent digital coherent
optical communication systems have achieved aFeature Articles

High capacity and long distance

Maximum capacity,maximum distancetransmission

Economical
implementation andwidespread use ofhigh-capacitytransmission

Continuous
network provision(SDN/virtualization/
shallow hierarchy)

Noiseless
SDM devices
photonic
amplification
Ultralarge-scale
Fast broadband circuits,
integrated circuit
devices, and materials
designMultidimensional
multiplexing
devices and circuits

Heterogeneousmaterials anddevice integration Photonicselectronics
fusion design
Silicon photonics

Hardwareaccelerator
designHighcapacity
photonic
pathswitching–
Photonic
packetprocessing
devicesand circuits

Flexible
photonicselectronics
subsystems[Units of equipment][Entire system]

Economy andpower efficiency

Fig. 2. Hardware technology for the ultimate network.

transmission rate on the level of 400 Gbit/s by using
16QAM (16-level quadrature amplitude modulation)*3 and two-subcarrier multiplexing [4]. Of the
hardware research being done to expand the three
dimensions mentioned above, the research aimed atincreasing the symbol rate can increase the capacitywith the same number of subcarriers and multilevels,
so it is advantageous in that it simplifies ultrahighcapacity transceivers. The symbol rate for the current
100-Gbit/s digital coherent systems is 32 Gbaud, but
research is ongoing to achieve even higher speeds.
The article “High-speed Electronic and Optical
Device Technologies for Ultralarge-capacity Optical
Transmission” [5] in this issue describes an effort togreatly increase the symbol rate through conversionof ultrahigh speed indium phosphide (InP) integrated
circuits and OTDM (optical time-division multiplexing).
3.1 S
 DM technology for drastic increase in capacity
Extensive R&D is being done on space division

multiplexing (SDM), which targets a drastic increasein capacity that exceeds the physical limits of conventional single-mode optical fiber [6]. SDM canincrease the transmission capacity by a factor of several tens by making maximum use of the spatial
degrees of freedom in an optical fiber. This is done byusing multicore fiber, which has many cores in asingle fiber, or by using multiple waveguide modes
within a single core for multimode transmission. The
ongoing research toward implementation of SDM
includes work on erbium-doped fiber amplifiers(EDFAs) as optical amplifiers for long-distance*1	 Subcarrier: Multiplexed transmission in which a single ultrafast
communication channel is divided into optical signals of multiple
wavelengths (subcarriers).
*2 Symbol rate: A measurement of modulation speed, also known as‘baud rate.’ The unit is sps (symbols/second), but it is often referred to as baud in the field of communications. In multilevel
modulation, one symbol can carry multiple bits of information, so
the information bit rate is the product of the symbol rate and thenumber of levels.
*3	16QAM: Quadrature amplitude modulation, in which one symbol
is a 4-bit hexadecimal value.

NTT Technical Review

Feature Articles

multicore and/or multimode transmission, and connection devices that are required for branching andconverging with single-mode fibers, in addition to
many-core optical fiber. It also includes signal processing methods for compensating interference
between modes within a core [7].
3.2 C hallenging the Shannon limit: a noiselessoptical amplifier
According to Shannon’s theory, the capacity of acommunication channel is expressed by the following equation,

(

)

  
C = Blog2 1 + S ,N
where C is capacity, B is frequency bandwidth, and S
and N are the average signal power and the normal
distribution of noise power within B. From the equation, we can see that increasing the power of the signal light (S) will increase the transmission capacity.
However, the nonlinear effect of an optical fiber willcause signal distortion when the input optical power
is increased, so the signal-to-noise ratio (SNR) will
decrease. Therefore, reducing the noise (N) is an
important factor in increasing the transmission capacity. With the EDFAs that are currently used in opticaltelecommunications, however, the introduction ofexcess noise reduces the SNR of the input signal byhalf, even in the ideal case.
To cope with this situation, NTT is working on
research to improve the SNR by applying phase sensitive amplifier (PSA) technology. A PSA uses parametric amplification by a periodically polled lithium
niobate (PPLN)*4 device, which has a highly nonlinear optical effect. With the PSA, amplification withno degradation of the SNR is theoretically possible,
and a doubling of the transmission distance can be
expected in theory. This technology is explained in
the article “Advances in Phase Sensitive Amplifiers
Based on PPLN Waveguides for Optical Communication” [8] in this issue.
4. Hardware technology for economical network
innovation (towards smaller and moreenergy-efficient hardware)
4.1. D evice-level photonics-electronics convergence: silicon photonics
Silicon photonics is technology for integrating
electronic circuits and optical circuits/components ona silicon chip. One of its major advantages is that it
makes it possible to achieve ultra-compact opticalVol. 14 No. 1 Jan. 2016

circuits. The dimension of waveguide fabrication andradius of curvature are a few orders of magnitude
smaller than for the glass (SiO2) generally used in
optical circuits. This can reduce the optical circuit
size from centimeter- to millimeter-order. Furthermore, integration with large-scale complementary
metal-oxide semiconductor (CMOS) digital circuitswith high functionality will lead to an ideal photonics-electronics convergence system-on-a-chip with asmaller, less expensive form.
NTT has been leading the silicon photonics research
field and has achieved cutting-edge results for a Si
(silicon)-based multiparallel integrated WDM receiver and other advanced devices [9]. The article “Lateral Current-injection Membrane Lasers Fabricatedon a Silicon Substrate” [10] explains the cutting-edge
optical source integration technology used to integrate a low power consumption InP side-injectedthin-film laser on a silicon substrate. The greatest barrier to widespread use of silicon photonics is that thelight-emitting capability of silicon itself is very low.
We are working to overcome that problem by achieving true silicon photonics.
4.2. D evice-level photonics-electronics convergence: digital mock-up
Complexity in the convergence design is a serious
obstacle to radically reducing the hardware cost. The
optical transceiver shown in Fig. 3 comprises both
optical and electronic devices. In the conventional
design procedure, performance prediction and simple
modeling are done only for the optical circuit at first,
and the results are input to an electronic circuit simulator to estimate the overall performance. However,
performance prediction for a total system is difficult,so repeated test device fabrications are necessary.
That process increases the design and manufacturingcost. The article “Photonics-electronics Convergence
Design for Digital Mock-up” [11] in this issue
describes technology for implementing digital mockups that can estimate the convergence hardware performance with high accuracy and without making aseries of test devices. In the future, the boundary
between photonic and electronic circuits will become
blurred in a good sense, both physically and functionally in the silicon photonics era. This technology canbe expected to enable design that extracts maximum
performance with less cost for photonics-electronics
convergence devices.
*4 PPLN: Periodically poled lithium niobate, a non-linear opticalmaterial.

Feature Articles

Optical devices

Electronic circuit

To optical fiber

Optical passive

Optical sensor

TIA*

AD converter[Digital]

Modulator

Digitalcoherent
signal LSI
processor

Modulator
driverDA converter

Electronic device(server, switch, etc.)

LightsourceFrom optical fiber[Analog]Lightsource*Trans-impedance amplifier, an amplifier that converts the current generated by a light sensor(photodiode) to a voltage signal.
AD: analog-to-digital
DA: digital-to-analog

Fig. 3. Example of optical transceiver.

5. Ultrafast optical switching device technology
for extending the scalability of datacenters
Large-scale datacenters are used to support cloudcomputing and big data processing. These datacenters house hundreds of thousands of servers that areconnected hierarchically by a large number of network nodes (L2/L3 switches and routers). However,
the high latency and huge power consumption of thelarge numbers and layers of node units are serious
problems. Current routers and switches are electronicequipment that consists of integrated circuits. As weapproach the limit of Moore’s law*5, it is becoming
difficult to solve these problems only by increasing
the speed and reducing the power consumption of
circuits. The article “High-speed Optical Packet
Switching for Photonic Datacenter Networks” [12]describes the innovative research on a photonic
implementation of packet-by-packet switching,which has so far been done by conventional electronic routers and switches. Low-latency opticalpacket switching without opto-electrical conversion
is being developed as a solution to these problems.


NTT has been leading the world with its cutting-edge
research on optical packet switching technology, andhas proposed a hybrid opto-electric packet router thatcombines an ultrafast (nanosecond-order) opticalswitching device and an electronic buffer (integrated
circuit memory).
6. Hardware technology for advancing SDN
and virtualization
Studies on network virtualization are moving forward with the objective of reducing network CAPEXand OPEX. In network function virtualization (NFV),
network functions that have previously been implemented as dedicated hardware are implemented insoftware that runs on servers. The expected advantages of virtualization include flexible allocation of
resources and rapid service provision and modification, as well as reduction of hardware costs [13].
*5 Moore’s law: An index for predicting the evolution of integrated
circuits proposed by Gordon Moore, one of the founders of Intel
Corporation. The law states that the integration density of semiconductor devices will double every 18 to 24 months.

NTT Technical Review

Feature Articles

General-purpose servers use the most advanced
microprocessors, but the future rate of improvement
in semiconductor performance is expected to lag
behind the increase in network capacity. As we enter
the future terabit era, it is expected to be difficult toobtain speed and power efficiency through the flexibility of NFV. The article “Hardware/Software Codesign Technology for Network Virtualization” [14]in this issue describes hardware accelerator technology as one direction for research on hardware in theera of SDN and virtualization. In that approach, functions that are bottlenecks in improving performance
speed are carefully selected and implemented using amore hard-wired method with the objective of raising
the cost performance of NFV implemented on general-purpose servers. The acceleration effect wasverified by using the Lagopus software switch platform developed by NTT Network Innovation Laboratories [15].
References
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[2]
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[4]
[5]

[6]

Cisco, “Cisco Global Cloud Index: Forecast and Methodology,
2014–2019 White Paper.”http://www.cisco.com/c/en/us/solutions/collateral/service-provider/global-cloud-index-gci/Cloud_Index_White_Paper.html
NTT DOCOMO, “DOCOMO 5G White Paper,” 2014.
https://www.nttdocomo.co.jp/english/binary/pdf/corporate/technology/whitepaper_5g/DOCOMO_5G_White_Paper.pdf
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=ntr2011
08fa2.html
NTT press release issued on March 19, 2015.
http://www.ntt.co.jp/news2015/1503e/150319a.html
H. Yamazaki, M. Nagatani, T. Hashimoto, H. Nosaka, A. Sano, and Y.
Miyamoto, “High-speed Electronic and Optical Device Technologies
for Ultralarge-capacity Optical Transmission,” NTT Technical
Review, Vol. 14, No. 1, 2016.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2016
01fa2.html
Y. Miyamoto and H. Takenouchi, “Dense Space-division-multiplexing Optical Communications Technology for Petabit-per-second
Class Transmission,” NTT Technical Review, Vol. 12, No. 12, 2014.

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https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2014
12ra3.html[7] K. Shibahara, T. Mizuno, H. Takara, A. Sano, H. Kawakami, D. Lee,
Y. Miyamoto, H. Ono, M. Oguma, Y. Abe, T. Kobayashi, T. Matsui, R.
Fukumoto, Y. Amma, T. Hosokawa, S. Matsuo, K. Saitoh, H. Nasu,
and T. Morioka, “Dense SDM (12-core × 3-mode) Transmission over
527 km with 33.2-ns Mode-dispersion Employing Low-complexity
Parallel MIMO Frequency-domain Equalization,” OFC (Optical Fiber
Communication Conference and Exhibition) 2015, Th5C.3, Anaheim,
USA, 2015.
[8] T. Umeki, O. Tadanaga, T. Kazama, K. Enbutsu, Y. Miyamoto, and H.
Takenouchi, “Advances in Phase Sensitive Amplifiers Based on PPLN
Waveguides for Optical Communication,” NTT Technical Review,Vol. 14, No. 1, 2016.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2016
01fa3.html[9] C. Kachris, K. Kanonakis, and I. Tomkos, “Optical Interconnection
Networks in Data Centers: Recent Trends and Future Challenges,”
IEEE Communications Magazine, Vol. 51, No. 9, pp. 39–45, 2013.
[10] T. Kakitsuka, T. Fujii, K. Takeda, H. Nishi, T. Sato, K. Hasebe, T.
Tsuchizawa, T. Yamamoto, K. Yamada, and S. Matsuo, “Lateral Current-injection Membrane Lasers Fabricated on a Silicon Substrate,”
NTT Technical Review, Vol. 14, No. 1, 2016.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2016
01fa4.html[11] K. Takeda, K. Honda, H. Nosaka, H. Fukuda, and T. Yamamoto, “Photonics-electronics Convergence Design for Digital Mock-up,” NTT
Technical Review, Vol. 14, No. 1, 2016.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2016
01fa5.html[12] T. Segawa, Y. Muranaka, and R. Takahashi, “High-speed Optical
Packet Switching for Photonic Datacenter Networks,” NTT Technical
Review, Vol. 14, No. 1, 2016.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2016
01fa6.html[13] H. Arimitsu, K. Matsumoto, Y. Matsubayashi, M. Aihara, Y.
Makibayashi, H. Kojima, and T. Kosugi, “Current Status of Development of Network Virtualization Technologies,” NTT Technical
Review, Vol. 12, No. 8, 2014.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2014
08fa1.html[14] T. Hatano, K. Yamazaki, and A. Miyazaki, “Hardware/Software Codesign Technology for Network Virtualization,” NTT Technical
Review, Vol. 14, No. 1, 2016.
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01fa7.html[15] K. Obana, K. Yamada, Y. Nakajima, H. Masutani, K. Shimano, and M.
Fukui, “Fundamental Research Activities on Network Virtualization,”
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https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2014
08fa5.html

Feature Articles

Shinichiro Mutoh

Senior Manager, Photonics-Electronics Convergence Laboratory, NTT Device Technology
Laboratories.
He received a B.E. and M.E. in electronicengineering from Chiba University in 1986 and1988. He joined NTT in 1988 and initially conducted research on ultrahigh-speed BiCMOS
static random access memory (SRAM) and lowpower CMOS SRAM circuits. He was also
involved in R&D of low-power and low-voltage
digital circuit technologies for mobile phone/equipment applications, especially multiplethreshold-voltage CMOS circuits. He served asa program committee member of the Symposium
on VLSI Technology and Circuits from 2008 to
2011, a member of the International Solid-State
Circuits Conference (ISSCC) Technology Direction Subcommittee from 2013 to 2015, a program committee member of the IEEE (Instituteof Electrical and Electronics Engineers) Asian
Solid-State Circuits Conference (A-SSCC)
Emerging Technology and Applications from2013 to 2014, and as a member of the JSAP
(Japan Society of Applied Physics) Executive
Committee of the Symposia on VLSI Technology
and Circuits from 2012 to 2014. He was also the
Director, General Affairs of IEICE (Institute ofElectronics, Information and Communication
Engineers) Electronics Society from 2013 to2014. He is Deputy General Manager of theElectronics and Systems Business Unit at NTT
Electronics Corporation.

Hirokazu Takenouchi

Senior Research Engineer, Supervisor, Group
Leader of the Optoelectronic Subsystem ResearchGroup, Photonics-Electronics Convergence Laboratory, NTT Device Technology Laboratories.
Innovative Photonic Network Center, NTT
Network Innovation Laboratories.
He received a B.S. and M.S. in pure andapplied sciences from the University of Tokyo in
1992 and 1994, and a Ph.D. in ultrafast opticalsignal processing from Tokyo University of Agriculture and Technology in 2005. He joined NTT
Opto-electronics Laboratories in 1994, where hehas been researching such areas as optical signal
processing for high-capacity optical transport
networks. He is a member of IEEE, JSAP, and asenior member of IEICE.



Hideyuki Nosaka

Senior Research Engineer, 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 engaged in R&D of monolithic microwave ICs and frequency synthesizers.
He moved to NTT Photonics Laboratories in
1999, where he was involved in R&D of ultrahigh speed mixed-signal ICs for optical communications systems. He received the 2001 Young
Engineer Award and the 2012 Best Paper Award
presented by IEICE. He is a member of IEEE and
IEICE.

Toshikazu HashimotoGroup Leader, 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 (JPS).

Akihiko Miyazaki

Senior Research Engineer, Supervisor, MetroAccess Network Device Project, NTT Device
Innovation Center.
He received a B.S. and M.S. in pure andapplied sciences from the University of Tokyo in
1991 and 1993. He joined NTT LSI Laboratories
in 1993, where he initially engaged in R&D oflayout CAD (computer aided design) algorithms
and systems. In 2000, he moved to NTT Communications and worked on developing application-platform services for enterprise customers.
In 2004, he moved to NTT Microsystem Integration Laboratories and developed MAC (media
access control) LSIs for access networks. He is amember of IEICE and JPS.

NTT Technical Review

Feature Articles: Photonics-electronics Convergence
Hardware Technology for Maximizing Network Performance

High-speed Electronic and Optical
Device Technologies for Ultralargecapacity Optical Transmission
Hiroshi Yamazaki, Munehiko Nagatani,
Toshikazu Hashimoto, Hideyuki Nosaka, Akihide Sano,and Yutaka Miyamoto
Abstract

To increase the transmission capacity of optical fiber transmission systems, we have developed a multiplexer-digital-to-analogue converter and an optical time-division multiplexing modulator, which
respectively generate high-speed signals in the electronic and optical domains. With these devices, we
can significantly increase the capacity per wavelength channel.
Keywords: DAC, modulator, QAM

1. Introduction
The demand for data transmission capacity is continuing to increase, and to meet this demand, opticaltransmission systems with per-channel rates exceeding 400 Gbit/s are desired. Several advanced technologies such as multicarrier transmission, highorder multilevel modulation, and high-symbol-ratetransmission are expected to be key technologies forachieving a significant increase in capacity. In thisarticle, we describe a multiplexer-digital-to-analog
converter (MUX-DAC) and an optical time-divisionmultiplexing (OTDM) modulator, which respectively
double the symbol rates in the electronic and opticaldomains.
2. High-speed InP-HBT MUX-DAC
2.1 Circuit configuration
Multilevel modulation with a high baud rate is
essential for achieving a channel capacity larger than
400 Gbit/s. The MUX-DAC combines the functionsof a 2:1 multiplexer (MUX), which doubles the symbol rate, and a digital-to-analog converter (DAC),which converts the input binary (digital) signals to an
output multilevel (analog) signal [1]. A functionalblock diagram of the fabricated MUX-DAC with 6bit resolution is shown in Fig. 1. The MUX-DAC
operates as follows; first, the twelve input digital signals are synchronized with each other and reshapedby twelve D flip-flops (DFFs). Then, the outputs ofthe six pairs of DFFs are respectively multiplexed by
six 2:1 MUXs, which generates six digital signals
with a doubled symbol rate. Finally, the outputs of the
MUXs are converted to an analog signal with a resolution of 6 bits. Since the frequency of the clock signal input to the DFFs and MUXs is half the symbol
rate of the output analog signal, the MUX-DAC issuitable for high-symbol-rate applications. For the
DAC section, we employed the R-2R ladder configuration to achieve high-speed operation with low
power consumption.
2.2 Fabrication and characteristics
We have developed fabrication technologies forindium-phosphide (InP) heterojunction bipolar transistor (HBT) devices, which are promising for highspeed applications in photonic networks [2]. We used
NTT Technical Review

Feature Articles

T
DFF
2T

2:1 MUX

DFF
DFF

2:1 MUX

DFF
DFF
Digital input

2:1 MUX

DFF
DFF

2:1 MUX

DFF
2T

DFF

2:1 MUX

DFF
DFF

2:1 MUX

DFF
Clock

Analog output

DAC

Phase
shifterFig. 1. Block diagram of the MUX-DAC.

20 ps

13.3 ps(a)(b)

Fig. 2. Eye diagrams of output four-level signals with symbol rates of (a) 50 Gbaud and (b) 75 Gbaud.

our InP HBT process with a linewidth of 0.5 μm tofabricate the MUX-DAC. With this process, we canachieve a very high cutoff frequency (a metric of theoperation speed of a transistor) of 290 GHz. Powerconsumption of the MUX-DAC integrated circuit(IC) is 3.4 W, with roughly 2.9 W consumed in the
DFFs and MUXs and only 0.5 W used in the DAC
section. The differential output swing voltage is 1 V.
The 3-dB bandwidth of the frequency response of the
MUX-DAC module exceeds 40 GHz, which is currently the largest value in the world. Eye diagrams of
the output four-level (2-bit) signals with symbol rates
of 50 and 75 Gbaud are shown in Figs. 2(a) and 2(b),respectively. Clear eye openings were achieved at a
symbol rate as high as 75 Gbaud.

Vol. 14 No. 1 Jan. 2015

3. OTDM modulator with a silica-LN
hybrid configuration
3.1 Circuit configuration
OTDM technologies have been extensively studied
with the objective of breaking the limit of the speedof electronics and generating ultrahigh-speed opticalsignals. In an OTDM transmitter, each tributary signal (signal to be multiplexed) is shaped into a pulsed
waveform to avoid interference between the tributaries. Our modulator, which is designed for an OTDM
with two tributaries, combines the functions of apulse generator and two data modulators [3]. Eachdata modulator is a dual-polarization optical vector
modulator, which is the standard modulator in currently deployed 100-Gbit/s digital coherent systems.
Feature Articles

Data signals(Symbol rate = B/2)

1/B
B/4

Opticalfield
Polarization
beam
combiner
Outputsignal

A
CW
light

A

Time

0

Opticalfield
0
Polarization
rotator

B

B

Time

0

1/B
Pulse generator

Quad-parallel
IQ modulator

PM circuit

(a)

(b)

CW: continuous wave
IQ: in-phase and quadrature

Fig. 3. (a) Optical-circuit diagram and (b) principle of the OTDM modulator.

The OTDM modulator generates the output opticalsignal with a symbol rate that is double that of each
input electronic signal. Unlike conventional OTDM
setups, in which a pulsed light source and separate
modulators are connected via fiber, the integrated
OTDM modulator provides a much smaller footprint
and higher stability of the relative optical phasebetween the tributaries.
The OTDM modulator consists of a pulse generator, four optical vector modulators, and a polarization-multiplexing (PM) circuit, as shown in Fig. 3(a).
Each of the two outputs of the pulse generator isbifurcated and sent to two of the four vector modulators. The outputs of each pair of vector modulatorsconnected to different ports of the pulse generator arecoupled, and the two coupled pairs are finally multiplexed in the PM circuit with orthogonal polarizations. The pulse generator consists of two MachZehnder modulators (MZMs) connected in parallelwith an output 2 × 2 coupler. To obtain a final output
symbol rate of B, the two MZMs in the pulse generator are driven with sinusoidal clock signals with afrequency of B/4 and a relative delay of 1/B. Eachvector modulator also consists of two parallel MZMs,
which are driven with data signals with a symbol rateof B/2. As shown in Fig. 3(b), the intensity peak of
the output from one port of the pulse generator always
coincides with the extinction of the output from theother port and vice versa. Thus, we can transmit thetwo tributary signals without inter-symbol interference. Unlike the conventional OTDM with a pulsed
light source, which is complicated and provides low
spectral efficiency because of the broad spectral
bandwidth of the pulse train, our OTDM modulator
operates with a simple continuous-wave light source
and offers higher spectral efficiency.
3.2 Fabrication method and characteristics
To fabricate the OTDM modulator with the complex configuration shown in Fig. 3, we used a hybrid
integration of silica planar lightwave circuits (PLCs)
and a lithium niobate (LN) chip [4]. The hybrid integration enables us to obtain both high design flexibility for the silica PLCs and a broad electro-optic
modulation bandwidth of the LN chip at the sametime. The LN chip has ten push-pull pairs of straightphase modulators in an array, which corresponds tothe ten MZMs: two for the pulse generator and eight
for the four vector modulators (Fig. 4). All other passive components such as couplers, static phase shifters, and the PM circuit are fabricated in the PLCs.
The pulse generator and the vector modulators areconnected via U-turn waveguides to make the module
compact. The modulator’s static insertion loss forboth polarizations is 9.5 dB at a 1550-nm wavelength.
All the MZMs have electro-optic 3-dB bandwidths of
NTT Technical Review

Feature Articles

Silica PLC

LN

Silica PLC
CWlight
OutputsignalQuad-parallel
IQ modulator

Pulse generator

PM circuit

Phase shifter on silica PLC
RF: radio frequency

RF signal electrode on LN

Fig. 4. Configuration of the OTDM modulator.

Opticalamplifier
OTDM modulator
CW laser

PDM
emulator

MUXDAC1MUXDAC2
SSMF 80 km

BPG1

BPG2
Dual-polarization
coherent
receiverfrontend

31.25 GHz

63-GHzreal-time
oscilloscope

CW laserSSMF: standard single-mode fiber

Fig. 5. Experimental setup of signal generation.

~23 GHz. The half-wave voltage of each MZM is~3.5 V.
4. Large-capacity transmission experimentusing the MUX-DAC and OTDM modulator
Using the MUX-DACs and the OTDM modulator
in combination, we can generate an ultralarge-capacity optical signal [5]. The experimental setup for aVol. 14 No. 1 Jan. 2015

125-Gbaud polarization-division-multiplexed (PDM)
16-level quadrature amplitude modulation (16QAM)
signal generation using the two devices is shown in
Fig. 5. The MUX-DACs converted 31.25-Gbaud
binary signals (pseudo-random bit sequences) frombit-pattern generators (BPGs) into 62.5-Gbaud fourlevel electronic signals, which are used to drive the
OTDM modulator to generate the 125-Gbaud
16QAM optical signal. Since only two MUX-DACFeature Articles

BER

1e-1

X polarization

1e-2
Y polarization
Measured
Theory
FEC threshold

1e-3
OSNR, dB (0.1-nm resolution)

Fig. 6. Back-to-back BER curve and constellations of the 1-Tbit/s PDM-16QAM signal.

modules were available for the experiment, we drove
only one polarization channel of the OTDM modulator and generated the PDM signal by using an external emulator. The optical signal was received by acoherent receiver with a 63-GHz real-time oscilloscope and demodulated offline to calculate the biterror rate (BER).
The measured back-to-back BER versus opticalsignal-to-noise ratio (OSNR) curve is shown in
Fig. 6. We obtained BERs below 2.7 × 10−2, which is
the threshold of the forward error correction (FEC)
code with 20% overhead, with an OSNR larger than
28.4 dB. We also tested transmission over 80-km
standard single-mode fiber (SSMF) and obtained
BERs below the threshold of 2.7 × 10 −2 with alaunched power smaller than +14 dBm. Since thePDM-16QAM signal conveys 8 bits of information
per symbol, the gross bit rate of the 125-Gbaud PDM16QAM signal is 1 Tbit/s. Based on an assumed useof the FEC with 20% overhead, the net data rate is
800 Gbit/s. These values are records for single-carrier
optical transmission using a single modulator (withan external PDM emulator).
5. Summary
We developed a high-speed InP MUX-DAC and an
integrated OTDM modulator with a silica-LN hybrid

configuration, which double the respective symbol
rates in electronic and optical domains. Using thesedevices, we successfully demonstrated the first 1Tbit/s single-carrier optical transmission with a single
modulator. These device technologies are promisingfor future ultralarge-capacity optical transmission
systems.
References
[1]

[2]

[3]

[4]

[5]

M. Nagatani, H. Wakita, H. Nosaka, K. Kurishima, M. Ida, A. Sano,and Y. Miyamoto, “75 GBd InP-HBT MUX-DAC Module for Highsymbol-rate Optical Transmission,” Electron. Lett., Vol. 51, No. 9, pp.
710–712, 2015.
N. Kashio, K. Kurishima, Y. K. Fukai, and S. Yamahata, “High-speed,
High-reliability 0.5-μm-emitter InP-based Heterojunction Bipolar
Transistors,” NTT Technical Review, Vol. 7, No. 12, 2009.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2009
12le1.html
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 OFC (OpticalFiber Communication Conference and Exhibition) 2015, Th2A.18,
Los Angeles, USA, Mar. 2015.
S. Mino, H. Yamazaki, T. Goh, and T. Yamada, “Multilevel Optical
Modulator Utilizing PLC-LiNbO3 Hybrid-integration Technology,”
NTT Technical Review, Vol. 9, No. 3, 2011.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2011
03fa8.html
H. Yamazaki, A. Sano, M. Nagatani, and Y. Miyamoto, “Single-carrier
1-Tb/s PDM-16QAM Transmission Using High-speed InP MUXDACs and an Integrated OTDM Modulator,” Opt. Express, Vol. 23,
No. 10, pp. 12866–12873, 2015.

NTT Technical Review

Feature Articles

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. In
2005, he joined NTT Photonics Laboratories,
where he researched optical waveguide devices
for communication systems. He is currently withNTT Device Technology Laboratories and NTT
Network Innovation Laboratories, where he isinvolved in research on devices and systems for
optical transmission using advanced multilevel
modulation formats. Dr. Yamazaki is a memberof the Institute of Electronics, Information andCommunication Engineers (IEICE).

Munehiko Nagatani

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 worked on the research anddevelopment (R&D) of ultrahigh-speed mixed
signal ICs for optical communications systems.
He is a member of IEICE.

Toshikazu HashimotoGroup Leader, 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.

Vol. 14 No. 1 Jan. 2015

Hideyuki Nosaka

Senior Research Engineer, 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 electrical and electronics engineering from Tokyo Institute of Technology in
2003. He joined NTT Wireless System Laboratories in 1995, where he engaged in R&D of monolithic microwave ICs and frequency synthesizers.
He moved to NTT Photonics Laboratories in
1999, where he was involved in R&D of ultrahigh-speed mixed-signal ICs for optical communications systems. He received the 2001
Young Engineer Award and the 2012 Best Paper
Award presented by IEICE. Dr. Nosaka is amember of the Institute of Electrical and Electronics Engineers (IEEE) and IEICE.

Akihide Sano

Senior Research Engineer, NTT Network Innovation Laboratories.
He received a B.S. and M.S. in physics and a
Ph.D. in communication engineering from Kyoto
University in 1990, 1992, and 2007. Since joining NTT in 1992, he has been engaged in R&D
of high-speed, high-capacity, and long-haulfiber-optic communication systems. He received
the Young Engineer’s Award and the Achievement Award from IEICE in 1999 and 2010,
respectively, and the 56th Maejima Award fromthe Teishin Association in 2011. He is a memberof IEEE and IEICE.

Yutaka Miyamoto

Senior Distinguished Researcher, Director of
the Innovative Photonic Network Center, Group
Leader of the Lightwave Signal Processing
Research Group, Photonic Transport Network
Laboratory, NTT Network Innovation Laboratories.
He received a B.E. and M.E. in electrical engineering from Waseda University, Tokyo, in 1986
and 1988. In 1988, he joined NTT Transmission
Systems Laboratories, where he engaged inresearch and development of high-speed opticalcommunications systems including the 10-Gbit/s
first terrestrial optical transmission system using
EDFA inline repeaters. He was with NTT Electronics Technology Corporation between 1995
and 1997, where he was involved in the planningand product development of high-speed opticalmodules at data rates of 10 Gbit/s and beyond.
Since 1997, he has been with NTT Network
Innovation Labs, where he has been researchingand developing optical transport technologies
based on 40-Gbit/s and 100-Gbit/s channels. His
current research interests include high-capacity
optical transport systems with advanced modulation formats, digital signal processing, optical
pre-processing, and space division multiplexing.
He is a member of IEEE and a Fellow of IEICE.

Feature Articles: Photonics-electronics Convergence
Hardware Technology for Maximizing Network Performance

Advances in Phase Sensitive
Amplifiers Based on PPLN
Waveguides for OpticalCommunication
Takeshi Umeki, Osamu Tadanaga, Takushi Kazama,
Koji Enbutsu, Yutaka Miyamoto,and Hirokazu Takenouchi
Abstract

Drastic improvement of the signal-to-noise ratio (SNR) of optical communication systems is necessary in order to make rapid progress in digital-coherent optical communication technology based on
multilevel modulated signals. In this article, we describe the recent progress made in phase sensitive
amplifiers (PSAs) based on PPLN (periodically-poled lithium niobate) waveguides. These PSAs arecapable of low-noise amplification and both phase and amplitude regeneration, which improves the SNR
in optical transport systems.
Keywords: nonlinear optics, optical parametric amplification, optical signal processing

1. Introduction

2. PSA using highly efficient PPLN waveguides

The capacity of optical communication systems has
been increased in recent years by the improved frequency utilization efficiency achieved with advanced
digital-coherent technologies. According to Shannon’s theory, a higher signal-to-noise ratio (SNR) of
the signal will be required to achieve a higher frequency utilization efficiency of the system. However,
the achievable transmission distance and channel
capacity are now approaching their limits due to thedegradation of the SNR by the accumulation of noisefrom repeater optical amplifiers and inter-symbolinterference caused by nonlinear impairments in thetransmission optical fiber [1]. Therefore, a drastic
improvement in the SNR is necessary for future optical communication systems.

A signal-phase-dependent amplification characteristic can be achieved with optical parametric amplification (OPA), which transfers the energy of an intense
pump light to the signal light. It amplifies the inphase component of a signal relative to the phase of
the pump and de-amplifies the quadrature component. This phase sensitivity makes it possible to provide several unique features of the phase sensitive
amplifier (PSA).
The key feature is that the ideal noise figure (NF) ofa PSA is 0 dB [2]; in other words, optical amplification without SNR degradation is possible (feature (1)in Fig. 1(a)), whereas the NF of a phase insensitive
amplifier (PIA), for example, an erbium-doped fiber
amplifier (EDFA), cannot be reduced below the 3-dB
quantum limit. Furthermore, a PSA can reduce thephase noise by squeezing the phase because it deamplifies the quadrature component (feature (2) inNTT Technical Review

Feature Articles

Conventional optical amplifier(EDFA etc.)
Q Unavoidable excess noise(NF > 3 dB)

PSA
Quadrature phase component is

Q

Input

(2) attenuated (phase-noise reduction).
(1)

Input

OutputAmplification

Low excess noise(in principle, NF = 0)

Input(3) Reduction of intensity
noise by gain saturation

Output10

PSA output

PSA gain (dB)Fundamental(c) Appearance of PPLN module

Module for OPA(1.5-µm band)

PPLN waveguide

EDFA

FilterPump light(0.78-µm band)PZT

PPLN waveguide
Module for SHG
Phase-locked
loop (PLL) circuit

Output1.55-µm band(a) Features of PSA
Signal

0.78-µm
band

Feedback signal(b) Basic configuration of PPLN-based PSA

Calculation5

In-phase gain
(Amplification)0

Quadrature-phase gain
(Deamplification)

–5
–10
0

100
200
300
SHG pump power (mW)(d) Pump power dependency of in-phase gain
and quadrature-phase gain

PZT: piezoelectric transducer
SHG: second harmonic generation

Fig. 1. PSA using highly efficient PPLN waveguides.

Fig. 1(a)). A PSA also provides amplitude regeneration by means of gain saturation thanks to the highspeed response of the parametric process (feature (3)in Fig. 1(a)). Therefore, PSAs are now attracting agreat deal of interest for use in enhancing the SNR of
inline optical repeater systems because of their potential for low noise amplification and their signal regeneration capabilities.
The basic setup for our recent experimental workon periodically-poled lithium niobate (LiNbO 3)(PPLN)-based PSAs is shown in Fig. 1(b). Two
PPLN waveguides were used for second harmonic(SH) pump generation and OPA, respectively. Highconversion efficiency and high power tolerance areboth desirable in order to achieve a high parametricgain. We developed PPLN ridge waveguides using
the direct bonding method, which made the waveguides highly resistant to photorefractive damage [3].
We used the dry etching technique to obtain a ridge
waveguide with fine uniformity, which yielded highSH conversion efficiency. Recent advances in PPLN
waveguides and related module packaging technology have enabled us to explore χ(2)-based PSAs for
optical communication [4].
The PPLN waveguides were assembled into fiberVol. 14 No. 1 Jan. 2016

pigtail modules. The modules have four input/output
ports to allow SH pumping. To achieve a stable PSA
output, an optical phase-locked loop (PLL) is used tocompensate for the slow relative phase drifts between
the signal and SH-pump lights. A photo of the moduleis shown in Fig. 1(c). Polarization-maintaining fibers
for 1.55 and 0.78 μm were respectively used in thetwo signal ports and the two pump ports. The signal
and SH pump were combined with a dichromatic mirror and injected into the waveguide inside the module. The module packaging technology enabled us toachieve both low insertion loss and stable operation.
The in-phase gain and quadrature-phase gain as afunction of the SH pump power is shown in Fig. 1(d).
By increasing the SH pump power, the in-phase gain
was increased and the quadrature-phase gain was
decreased symmetrically. A gain over 10 dB was
obtained, and the pump power dependence agreed
well with the theoretical curve. Nearly perfect phasesensitive amplification characteristics were obtained
because the ridge waveguide provides a sufficientspatial mode overlap between the signal and SH
pump light, and temporal mismatching is prevented
thanks to continuous wave pumping.
Probably the most intriguing property of a PSA isFeature Articles

Input(signal)Local 1LD

Carrier recovery and phase locking
Circulator
EDFA
SHG/DFGPZTPPLN waveguide #3
Pump

EDFA (SF pump)

PPLN waveguide #1

PPLN waveguide #2

Wavelength
multiplexer

SFG

LD for injection

Local 2PLL
circuit

–10
–20

PD

Feedback
signalIntensity (dBm)

10% tap0

Output(PSA out)

OPA

External gain:
12.0 dB

Internal gain:
17.7 dB

Loss of unit:
5.7 dB

De-amp:9.3 dB–30–40–50
PSA-Amp
Input signal
Signal w/o pump
PSA-DeAmp–60(a) Configuration of in-line PSA–701535.4 1535.6 1535.8 1536.0 1536.2
Wavelength (nm)(c) Amplifying characteristics
EDFA: –5 dBm, after 2940 km

PSA: +7 dBm, after 2940 km
QQI(b) Appearance of PSA unitI(d) Comparison of transmission characteristics of
PSA and EDFA

Fig. 2. Multi-repeater transmission using amplitude and phase regeneration of in-line PSA.

the ‘0 dB’ quantum-limited NF. We evaluated the NF
of a degenerate PSA, where the signal and an idler
were the same wavelength. We confirmed the low
noise characteristics in which the NF values of the
PSA were 1.8 dB, which is below the standard quantum limit of 3 dB, by using an optical and an electrical spectrum analyzer [5]. We believe the measured
NF was largely due to the extrinsic coupling loss;
therefore, the intrinsic NF was very close to 0 dB.
3. Multi-repeater transmission using amplitude
and phase regeneration by in-line PSA
In addition to its capacity for low NF amplification,a PSA has a phase and amplitude regeneration capability that is achieved by using the phase squeezing
property and by operating the amplifier in the saturated regime. These capabilities mean that PSAs will
have a large impact on long-haul transmission if they
are used as multiple repeaters. For the repeater operation of the PSA, the phase of the pump should belocked to that of the input signal. The configuration of
an in-line PSA for a binary phase shift keying (BPSK)
signal using a carrier-recovery and phase-locking
system is shown in Fig. 2(a). The tapped signal isconverted to SH light in the PPLN waveguide. Doubling the signal phase makes it possible to recover the
carrier from a BPSK signal without a carrier compo

nent. The carrier phase is copied to the 1.5-μm-bandidler wave by the difference-frequency generation(DFG) process. Then the recovered carrier phase islocked by injecting the idler into a local laser. The
in-line PSA, which operates as a repeater amplifier, isachieved using the phase-locked local light.
The key components in Fig. 2(a), including thethree PPLN modules, an LD (laser diode) for injection, and a PZT (piezoelectric transducer) for the
PLL, were assembled into a PSA unit as shown in
Fig. 2(b) to obtain stable operation. We obtained ahigh external gain of 12 dB for a BPSK signal using
the in-line PSA as shown in Fig. 2(c).
We also examined the regeneration capability of thein-line PSA in a multi-span transmission. A comparison of the transmission performance for both the PSA
and an EDFA at the same link is shown in Fig. 2(d).
The left constellation is for the EDFA link after about
3000-km transmission in an optimal power condition.
The signal quality was degraded by amplified spontaneous emission (ASE) noise and fiber nonlinearity.
However, thanks to the phase and amplitude regeneration capabilities of the PSA, the inter-symbolinterference caused by the phase noise resulting fromfiber nonlinearity and the intensity noise caused by
the ASE of an optical amplifier are largely suppressed, as shown in the right constellation.

NTT Technical Review

Feature Articles

2ω ω + ω(Degenerate parametric process)

Input

PSA output
PBS

TM wavePump

2ω Frequency
Pump

PSA output

X-polarization

ω2 ω ω1Phase
Signalconjugate(a) Schematics of optical parametric process2ω Frequency
Pump

Input

TE wave

OPA

Polarization
rotator
OPA

PPLN

PPLN

To PLL

Polarization rotator
Pump

Y-polarizationω
Degeneratesignal2ω ω1 + ω2 (ω1 ω2)(Non-degenerate parametric process)To PLL(b) Configuration of polarization diversity loop(c) Phase sensitive amplification of PDM-QPSK signal

PBS: Polarization beam splitter

Fig. 3. Multilevel phase coded and PDM signal amplification.

4. PSA for multilevel phase coded andpolarization-division multiplexed signal
The in-line PSA using degenerate parametricamplification mentioned above can only handle abinary modulated signal. The high capacity communication of the future will most likely operate with
spectral efficiencies greater than 1 bit/s/Hz, thereby
requiring the use of multilevel phase coding andpolarization-division multiplexing. Extending theapplicability to a polarization-division multiplexed(PDM)-QAM (quadrature amplitude modulation)
signal is practically very important in order to install
the PSA in a practical system based on digital-coherent technologies. For the OPA process, energy istransferred from a pump light with a frequency of 2ω
to a signal light and an idler light with frequencies ofω1 and ω2. When ω1 = ω2, the process is said to bedegenerate, and when ω1 ≠ ω2, the process is said tobe non-degenerate (see Fig. 3(a)). For the non-degenerate process, a multilevel-modulation signal can behandled by using a phase conjugate signal as theidler.
Moreover, a polarization diversity scheme isrequired to handle the PDM signal because a secondorder nonlinear optical medium such as PPLN is usually polarization dependent. A schematic view of the
proposed polarization diversity loop configuration isshown in Fig. 3(b). In this setup, the two waveguides
Vol. 14 No. 1 Jan. 2016

are arranged in series, and a rotator is installed
between them. The configuration combines theadvantages of being free of delay alignment and having low reflection noise. Although the loop configuration was used to achieve polarization diversity without any delay adjustment, the TM (transverse magnetic) and TE (transverse electric) waves are independently amplified by two different waveguides.
The capability of amplification for multilevelmodulated and PDM signals was confirmed by using
the non-degenerate process and the diversity configuration. The transmitter generated a 20-Gbaud quadrature phase shift keying (QPSK) signal, and a phaseconjugated idler was generated by DFG. After the
DFG process, a PDM emulator was used to generate
an 80-Gbit/s PDM-QPSK signal with a phase conjugated idler. A LiNbO3 phase modulator was used to
add phase noise to confirm the phase regeneration
capability of the PSA. Constellation diagrams for an
80-Gbit/s PDM-QPSK input signal with phase noise
and the output signal after using the PSA are shownin Fig. 3(c). The constellation for the Y-polarizationof the input signal indicates that larger phase noise
was induced than that for the X-polarization becauseof the polarization dependence of the LiNbO3 phasemodulator. The constellation points were rotationallyspread due to the large phase noise. After using the
PSA, a major phase noise reduction was achieved
with the canceling effect. The phase sensitiveFeature Articles

amplification of the PDM-QPSK signal was confirmed from the inherent noise canceling characteristics of the PSA.

and Communications Technology of Japan.
References

5. Conclusion[1]

We introduced PSAs based on PPLN waveguides
that have capabilities of low noise amplification andboth phase and amplitude regeneration for improving
the SNR in optical transport systems. We will contribute to further expanding the capacity of future backbone optical communication systems by maximizing
the potential of digital-coherent technologies.
Part of this research uses the results of “R&D on
Optical Signal Transmission and Amplification with
Frequency/Phase Precisely Controlled Carrier” commissioned by the National Institute of Information

[2]



[3]

[4]
[5]

R. Essiambre, G. Kramer, P. J. Winzer, G. J. Foschini, and B. Goebel,“Capacity Limits of Optical Fiber Networks,” J. Lightwave 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 Photonics Technol. Lett., Vol. 17,
No. 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.
M. Asobe, T. Umeki, and O. Tadanaga, “Phase Sensitive Amplification with Noise Figure below the 3 dB Quantum Limit Using CW
Pumped PPLN Waveguide,” Opt. Express, Vol. 20, No. 12, pp.
13164–13172, 2012.

NTT Technical Review

Feature Articles

Takeshi Umeki

Senior Research Engineer, Optoelectronic
Subsystem Research Group, Photonics-Electronics Convergence Laboratory, NTT Device Technology Laboratories.
Innovative 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 a Ph.D. in nonlinear optics from the
University of Tokyo in 2004 and 2014. He joined
NTT Photonics Laboratories in 2004 and hassince been involved in research on nonlinear
optical devices based on PPLN waveguides. He
is a member of the Japan Society of Applied
Physics (JSAP) and the Institute of Electronics,
Information, and Communication Engineers(IEICE).

Osamu Tadanaga

Senior Research Engineer, Optoelectronic
Subsystem Research Group, Photonics-Electronics Convergence Laboratory, NTT Device Technology Laboratories.
He received a B.E. and M.E. in materials science and engineering from Kyoto University in
1993 and 1995, and a Ph.D. in the area of nonlinear optics from the University of Tokyo in 2009.
He joined NTT Opto-electronics Laboratories in
1995; since then, he has been involved inresearch on surface-normal modulators, verticalcavity surface emitting lasers, and quasiphasematched LiNbO3 wavelength converters.

Takushi Kazama

Researcher, Photonics-Electronics Convergence Laboratory, NTT Device Technology
Laboratories.
He received a B.S. and M.S. in electrical engineering from the University of Tokyo in 2009 and2011. In 2011, he joined NTT Photonics Laboratories, where he has been engaged in research on
PPLN waveguide devices. He is a member of
IEICE and JSAP.

Koji Enbutsu

Research Engineer, Optoelectronic Subsystem
Research Group, Photonics-Electronics Convergence Laboratory, NTT Device Technology
Laboratories.
He received a B.E. and M.E. in electronics
engineering from Hokkaido University in 1994
and 1996. In 1996, he joined NTT Opto-electronics Laboratories, where he conducted research onorganic optical waveguides for optical communication, 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.

Yutaka Miyamoto

Senior Distinguished Researcher, Director of
the Innovative Photonic Network Center, Group
Leader of the Lightwave Signal Processing
Research Group, Photonic Transport Network
Laboratory, NTT Network Innovation Laboratories.
He received a B.E. and M.E. in electrical engineering from Waseda University, Tokyo, in 1986
and 1988. In 1988, he joined NTT Transmission
Systems Laboratories, where he engaged inresearch and development of high-speed opticalcommunications systems including the 10-Gbit/s
first terrestrial optical transmission system using
EDFA inline repeaters. He was with NTT Electronics Technology Corporation between 1995
and 1997, where he was involved in the planningand product development of high-speed opticalmodules at data rates of 10 Gbit/s and beyond.
Since 1997, he has been with NTT Network
Innovation Labs, where he has been researchingand developing optical transport technologies
based on 40-Gbit/s and 100-Gbit/s channels. His
current research interests include high-capacity
optical transport systems with advanced modulation formats, digital signal processing, optical
pre-processing, and space division multiplexing.
He is a member of the Institute of Electrical andElectronics Engineers (IEEE) and a Fellow of
IEICE.

Hirokazu Takenouchi

Senior Research Engineer, Supervisor, Group
Leader of the Optoelectronic Subsystem ResearchGroup, Photonics-Electronics Convergence Laboratory, NTT Device Technology Laboratories.
Innovative Photonic Network Center, NTT
Network Innovation Laboratories.
He received a B.S. and M.S. in pure andapplied sciences from the University of Tokyo in
1992 and 1994, and a Ph.D. in ultrafast opticalsignal processing from Tokyo University of Agriculture and Technology in 2005. He joined NTT
Opto-electronics Laboratories in 1994, where hehas been researching such areas as optical signal
processing for high-capacity optical transport
networks. He is a member of IEEE, JSAP, and asenior member of IEICE.

Vol. 14 No. 1 Jan. 2016



Feature Articles: Photonics-electronics Convergence
Hardware Technology for Maximizing Network Performance

Lateral Current-injection Membrane
Lasers Fabricated on a Silicon
Substrate
Takaaki Kakitsuka, Takuro Fujii, Koji Takeda,
Hidetaka Nishi, Tomonari Sato, Koichi Hasebe,
Tai Tsuchizawa, Tsuyoshi Yamamoto, Koji Yamada,and Shinji Matsuo
Abstract

NTT has developed lateral current-injection membrane lasers fabricated on silicon for short-reach
optical interconnections. The lasers are fabricated using the crystal growth technique on a bonded substrate consisting of compound semiconductors and silicon. Integration of the lasers and silica-based
waveguides has also been achieved recently. In this article, we introduce directly modulated lateral-current injection membrane lasers and their integration with silica-based waveguides.
Keywords: semiconductor laser, silicon photonics, photonic integrated circuit

1. Introduction
The power consumption in information and communication technology devices has reached a criticallevel with the explosive increase in network traffic.
The power dissipation in large-scale datacenters is
estimated to exceed 1% of all power consumption. It
is therefore essential to cut the power consumption ofboth networks and devices. Data transmission within
datacenters consumes 70% of all network traffic
handled by datacenters [1]. Optical interconnection,in which the electrical data transmission is replacedwith optical data transmission, has been actively
developed to reduce datacenter power consumption.
Vertical cavity surface emitting lasers (VCSELs)
have already been commercialized as the transmittersfor intra-rack and inter-board data transmission. Furthermore, optical links for intra-chip data transmission are also attracting attention because the performance of chip-to-chip and intra-chip data transmission for large-scale integrated circuits is facing thelimit.
To introduce optical links into short-reach datatransmission, it is critical to reduce the power consumption and large-scale integration of opticaldevices, including transmitters and receivers. Wavelength division multiplexing (WDM) is a promisingtechnology to realize large-capacity and low-cost
optical links. However, the VCSELs currently used in
optical interconnections are inappropriate for singlemode transmission and the WDM network. It is thus
essential to achieve high-density integration of single-mode lasers such as distributed feedback (DFB)
lasers in order to introduce the WDM network into
short-reach optical interconnections.
Efforts to achieve high-density integration include
the development of silicon photonics to realize lowcost and high-density optical device integration onlarge-diameter silicon wafers [2]. The use of silicon
photonics is expanding from passive components
such as optical waveguides and filters to active components such as modulators and germanium-based
photodiodes. In addition, the fusion of optical andelectronic circuits is promising. Transceivers
NTT Technical Review

Feature Articles

III-V buried
heterostructure

SSC

SiO2
Si

LD

SiO

2

Si

Photonics-electronics
integrated circuit

Large-diameter silicon wafer
1. Photonic circuits on
silicon substrate

III-V compound layer(Template)
2. Membrane compound semiconductor
layers on photonic circuits

3. Device fabrication on III-V
membrane layers

LD: laser diode
SSC: spot-size converter

Fig. 1. Concept of laser integration on a silicon wafer and photonics-electronics integration.

consisting of silicon optical devices and CMOS
(complementary metal-oxide-semiconductor) transistors integrated on silicon wafers have already beencommercialized.
Meanwhile, the integration of lasers on siliconremains a challenge. Silicon is an indirect transition
semiconductor; it is not appropriate for light sources
because of its extremely low emission efficiency.
Some approaches have been proposed to achieve
emission of silicon-based materials. These approaches include adding impurities, enhancing the emission
rate by using fine structures, and using germaniumbased lasers.
However, commercialized lasers have not beenachieved yet. The standard approach is hybrid integration of compound semiconductor-based lasers andsilicon-based optical devices. The commercialized
transceivers still employ semiconductor lasers as
extra light sources. There are two ways of achieving
hybrid integration: laser bonding on silicon opticalcircuits, and laser fabrication on compound semiconductor active regions bonded on silicon waveguides.
The former approach requires fine alignment between
the lasers and silicon waveguides with submicron
order accuracy. This requirement will raise the fabriVol. 14 No. 1 Jan. 2016

cation cost as the integration density increases.
In the latter approach, laser waveguides are determined by the silicon waveguide; therefore, the alignment accuracy of bonding processes is eased. However, it is difficult to achieve compact lasers because
the optical confinement in the active layer is low;most optical power is confined in the silicon waveguides. Therefore, it is essential to make progress inboth the fabrication process and the development of
the laser structure in order to realize high-density
integration of lasers with low-power consumption on
silicon substrates.
2. Integration of compound semiconductors ona silicon substrate
NTT has developed novel techniques to integrate
semiconductor lasers on silicon. The concept of theintegration is shown in Fig. 1. First, silicon photonic
circuits are fabricated on a large-diameter silicon
dioxide and silicon (SiO2/Si) substrate. Next, membrane compound semiconductor layers are bonded on
the silicon photonic circuits. We call this bonded
compound semiconductor layer a template. The
lasers are fabricated on this template. The lasers canFeature Articles

InP s
ubst

Activ

rate

e co

re la

yer

SiO

SiO

Si

Si

2

2

SiO

2

Si
(a) Wafer bonding(b) Removal of InP substrate
Zn diffusion(c) Fabrication of
waveguide

Si-ionimplantation

SiO

SiO

SiO

Si

Si

Si

(d) Regrowth(e) Selective doping(f) Fabrication of gratingsand electrodes

2

2

2

Fig. 2. Membrane laser fabrication process.

be fabricated by using alignment marks made on asilicon substrate. It is easy to achieve fine alignment
between the silicon waveguides and the lasers. This
technique will enable high-density integration of all
optical devices on a large-diameter silicon wafer.
With further development of the integration of photonic and electronic circuits, it will be possible toachieve compact transceivers on a large-scale siliconwafer.
The process used to fabricate the membrane lasers
is shown in Fig. 2; it is the same as that described in
our previous reports [3–4]. An active layer is grownon an indium phosphide (InP) substrate to be bondedwith an SiO 2/Si substrate. The InP-substrate isremoved to form a membrane layer. The buried heterostructure (BH) is formed by means of waveguideetching and crystal regrowth on the template. The
lateral current injection structure is formed by selective impurity doping. Electrodes and Bragg gratings
are formed on the surface of the laser. This laser features high optical confinement in the active region
thanks to the large difference in the refractive index
between SiO2 and the thin compound semiconductor
layers. In addition, the BH provides strong carrierconfinement in the active region to enhance the carrier-photon interaction. This feature contributes toreducing the footprint and power consumption of

lasers [5]. We previously reported the energy cost of
171 fJ/bit, which was the smallest value of all DFB
lasers [6]. These approaches enable the high-density
integration of silicon optical circuits and high-performance lasers on a silicon wafer.
3. Membrane lasers on a silicon substrate
We fabricated the lasers using the fabrication process described in the previous section [7]. The mostchallenging step is growing the crystal on the bonded
wafer consisting of silicon, SiO2, and compound
semiconductors. The environmental temperature varies between room temperature and 600˚C during the
fabrication process. Meanwhile, the thermal expansion coefficients of the silicon, SiO2, and InP are different. This difference causes thermal stress in theepitaxial layer during the fabrication. The stresscould cause serious defects in the active region,
which would degrade the lasing characteristics. We
were able to suppress the effect of thermal stress byusing a thin InP-based template. The critical thickness of the InP layer was theoretically estimated to be
430 nm to achieve crystal growth under the temperature variation during the fabrication process. This
thickness is sufficient to realize optimized membranelasers. We used a 250-nm-thick InP-based template to
NTT Technical Review

1 µm

SiO2
InP
SiO2
MQW

Si

200 nm(a)Normalized PL intensity (arb. unit)

Feature Articles

1.2
1

on InP substrate

0.8
0.6
0.4
0.2

on SiO2/Si substrate

01350 1400 1450 1500 1550 1600
Wavelength (nm)(b)

PL: photoluminescenceFig. 3. (a) Cross section and (b) photoluminescence spectra of BH fabricated on SiO2/Si.

fabricate the lasers.
The template is fabricated using a 2-inch-diameter
thermally oxidized silicon wafer and an InP wafer.
The active layer, consisting of six InGaAsP (indium
gallium arsenide phosphide)-based quantum wells, isgrown on an InP substrate by metal-organic vapor
phase epitaxy (MOVPE). Covalent bonding assisted
by O2 plasma is used to bond the epitaxial layer andthe SiO2/Si wafer. After the bonding process, the BH
is fabricated by chemical etching and MOVPE
regrowth. The p-i-n diode structure is formed by Siion implantation for n-doped regions and Zn thermal
diffusion for p-doped regions. The surface grating ispatterned on the 130-nm-thick SiO2 layer above theactive region waveguide. A cross section of the fabricated BH is shown in Fig. 3(a). The active region was
successfully buried with the InP layer. No cracks or
dislocations are observed. The photoluminescence
spectrum of the fabricated BH on the SiO2/Si layer isshown in Fig. 3(b). The emission spectrum of BH
active layers grown on InP is superimposed. The
spectra are almost the same; no serious degradation isobserved after the laser fabrication. These results
verify that our approach is applicable to laser fabrication.
We fabricated the DFB laser by using this active
layer. The laser cavity is formed by the surface grating and etching mirrors at the facets. The coupling coefficient of the grating was designed to be 150 cm−1.
Single-mode lasing with a side mode suppression
ratio over 40 dB is observed. The current-output
power characteristics of the fabricated laser with acavity length of 120 μm are plotted in Fig. 4(a). The
Vol. 14 No. 1 Jan. 2016

threshold current was 1.8 mA. Lasing without kinks
is observed at temperatures up to 100˚C. An eye diagram of 40-Gbit/s direct modulation is shown in
Fig. 4(b). The bias current and modulation voltage
were 15 mA and 1.31 V, which corresponds to anenergy cost of 848 fJ/bit. The modulation efficiency
was 6.0 GHz/mA0.5. This is the first 40-Gbit/s directmodulation of membrane lasers.
The integration of the lasers and silicon/silicabased waveguide is a significant step forward towardrealizing photonic integrated circuits on silicon. A
distributed reflector (DR) laser integrated with a spotsize converter (SSC) and a SiOx waveguide [8] isshown in Fig. 5(a). The DR laser consists of a front
DFB section and a rear distributed Bragg reflector(DBR) section. The rear DBR mirror suppresses theoutput from the rear section, which can enhance theeffectiveness. A compact cavity can be achieved by
employing a DBR with high reflectivity. The BH
active region has almost the same structure as that ofthe previous DFB laser. The surface grating is patterned on the InP surface above the DFB and DBR
section. The coupling coefficient is designed to be1500 cm −1. The SSC section consists of an InP
tapered waveguide section covered with the output
SiOx waveguide and is formed in front of the DR
laser. The laser beam, strongly confined in the BH
active region, spreads in the tapered region to convert
into the propagation mode of the SiOx waveguide.
The DFB length and the SSC taper length are 50 μm
and 300 μm, respectively. The measured coupling
between the laser and optical fiber is 2.7 dB, which isalmost 6 dB smaller than the coupling between theFeature Articles

2.0

Output power (mW)

25ºC
1.5

40 Gbit/s

50ºC

1.0

75ºC
0.5
10 ps/div.

100ºC
0

0

5

10

15

20

25

30

35

40

Current (mA)
(a)

(b)

Fig. 4. (a) Current-output characteristics and (b) modulation waveform of membrane laser integrated on silicon.

DBR

2

InP tapered waveguide

Si

(a)

A 0.5

SSC-integrated DR laser: 50 µm
DFB laser: 73 µm

3

GH

z/m

20

cy

:9

.4

15
fic

ien

120 µm

5
0

ula

tio

n

ef

10
od

SiO

25

M

SiOx
waveguide

Relaxation oscillation frequency fr (GHz)DFB

00.5 1.0 1.5 2.0 2.5 3.0(I – Ith)1/2 (mA)1/2

3.5

(b)

Fig. 5. (a) Structure of SSC-integrated membrane laser and (b) modulation efficiency of membrane lasers.

laser without the SSC and a lensed fiber. The modulation efficiency of the fabricated lasers integrated on
silicon is plotted in Fig. 5(b). The DFB lasers withcavity lengths of 73 μm and 120 μm, and the DR laser
with a cavity length of 50 μm are compared. The
modulation efficiency of the DR laser was enhanced
to 9.43 GHz/mA0.5 thanks to the short cavity. Theenergy cost was also improved to 132 fJ/bit under themodulation speed of 25.8 Gbit/s.



4. Summary
We reported the development of semiconductor
lasers fabricated on a silicon substrate. The crystal
growth on the bonded wafer makes it possible toachieve membrane lasers operating with low power
consumption comparable to VCSELs. We also successfully integrated the laser and the silica-based
waveguide with high coupling efficiency. Our nextchallenges are to achieve high-density integration of
NTT Technical Review

Feature Articles

the lasers and a larger wafer size. The integration oflight sources and silicon optical circuits would greatly expand the application area of silicon photonics.
These technologies would pave the way to achieving
large-scale optical circuits for optical interconnections. The ultimate target in the future is the integration of electronics and optical circuits.

[4]

[5]

References
[6]
[1]

[2]

[3]

Cisco, “Cisco Global Cloud Index: Forecast and Methodology,
2014–2019 White Paper.”http://www.cisco.com/c/en/us/solutions/collateral/service-provider/global-cloud-index-gci/Cloud_Index_White_Paper.html
K. Yamada, T. Tsuchizawa, H. Nishi, R. Kou, T. Hiraki, K. Takeda, H.
Fukuda, M. Usui, K. Okazaki, Y. Ishikawa, K. Wada, and T. Yamamoto, “Silicon-germanium-silica Monolithic Photonic Integration
Platform for Telecommunications Device Applications,” NTT Technical Review, Vol. 12, No. 5, 2014.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2014
05ra1.html
T. Kakitsuka, K. Hasebe, T. Fujii, T. Sato, K. Takeda, and S. Matsuo,

Takaaki Kakitsuka

Senior Research Engineer, Materials andDevices Laboratory, NTT Device Technology
Laboratories; and NTT Nanophotonics Center.
He received a B.S. and M.S. in physics in 1994
and 1996 and a Dr. Eng. in 2012 from Kyushu
University, Fukuoka. In 1996, he joined NTT
Opto-electronics Laboratories, where he engaged
in research on semiconductor lasers and optical
functional devices. From 2009 to 2011, he waswith the Research and Development Planning
Department. He is a member of the Institute ofElectronics, Information and Communication
Engineers (IEICE), the Institute of Electrical andElectronics Engineers (IEEE) Photonics Society,
the Japan Society of Applied Physics (JSAP),
and the Physical Society of Japan.

Takuro Fujii

Researcher, Materials and Devices Laboratory,
NTT Device Technology Laboratories; and NTT
Nanophotonics Center.
He received a B.E. and M.E. in engineeringfrom Keio University, Kanagawa, in 2010 and2012. He joined NTT Photonics Laboratories in2012. He has been researching MOVPE growthof III-V semiconductors and the development of
III-V semiconductor lasers on Si for photonic
integrated circuits. He is a member of JSAP.

Vol. 14 No. 1 Jan. 2016

[7]

[8]

“InP-based Membrane Optical Devices for Large-scale Photonic
Integrated Circuits,” NTT Technical Review, Vol. 13, No. 5, 2015.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2015
05ra1.html
S. Matsuo, T. Fujii, K. Hasebe, K. Takeda, T. Sato, and T. Kakitsuka,“Directly Modulated Buried Heterostructure DFB Laser on SiO2/Si
Substrate Fabricated by Regrowth of InP Using Bonded Active
Layer,” Opt. Express, Vol. 22, No. 10, pp. 12139–12147, 2014.
T. Okamoto, N. Nunoya, Y. Onodera, T. Yamazaki, S. Tamura, and S.
Arai, “Optically Pumped Membrane BH-DFB Lasers for Low-threshold and Single-mode Operation,” IEEE J. Sel. Top. Quantum Electron., Vol. 9, No. 5, pp. 1361–1366, 2003.
S. Matsuo, T. Fujii, K. Hasebe, K. Takeda, T. Sato, and T. Kakitsuka,“Directly Modulated DFB Laser on SiO2/Si Substrate for Datacenter
Networks,” J. Lightwave Technol., Vol. 33, No. 6, pp. 1217–1222,
2015.
T. Fujii, T. Sato, K. Takeda, K. Hasebe, T. Kakitsuka, and S. Matsuo,“Epitaxial Growth of InP to Bury Directly Bonded Thin Active Layer
on SiO2/Si Substrate for Fabricating Distributed Feedback Lasers on
Silicon,” IET Optoelectronics, Vol. 9, No. 4, pp. 151–157, 2015.
H. Nishi, T. Fujii, K. Takeda, K. Hasebe, T. Kakitsuka, T. Tsuchizawa,T. Yamamoto, K. Yamada, and S. Matsuo, “Membrane Distributedreflector Laser Integrated with SiOx-based Spot-size Converter on Si
Platform,” Proc. of ECOC 2015 (the 41st European Conference on
Optical Communications), Valencia, Spain, 2015.

Koji Takeda

Research Engineer, Materials and Devices
Laboratory, NTT Device Technology Laboratories; and NTT Nanophotonics Center.
He received his B.S., M.S., and Ph.D. in electronics engineering from the University of Tokyoin 2005, 2007, and 2010. He received a research
fellowship for young scientists from the Japan
Society for the Promotion of Science for theyears 2008 to 2010. He joined NTT Photonics
Laboratories in 2010. His current research interests include ultralow-power optical interconnects, InP photonic integrated circuits, and photonic crystal lasers. He received the Best Student
Paper Award from the IEEE Photonics Society in
2009, and the Outstanding Student Presentation
Award from JSAP in 2010. He is a member of theIEEE Photonics Society, JSAP, and IEICE.

Hidetaka Nishi

Researcher, Materials and Devices Laboratory,
NTT Device Technology Laboratories; and NTT
Nanophotonics Center.
He received a B.S. and M.S. in mechanical science and engineering from Tokyo Institute ofTechnology in 2005 and 2007. In 2007, he joined
NTT Microsystem Integration Laboratories.
Since then, he has been conducting research on
integrated silicon photonic devices. He is a member of the Optical Society of America (OSA) and
JSAP.



Feature Articles

Tomonari SatoManager, IT Innovation Department, NTT
EAST Corporation.
He received his B.E., M.E., and Ph.D. in engineering from the University of Tsukuba, Ibaraki,in 2001, 2003, and 2009. In 2003, he joined NTT
Photonics Laboratories. He has been engaged inresearch on MOVPE growth of III-V semiconductors and the development of semiconductor
lasers for sensor applications and photonic crystal lasers. He moved to NTT EAST in July 2014.
He is a member of JSAP.

Koichi Hasebe

Research Engineer, Materials and Devices
Laboratory, NTT Device Technology Laboratories; and NTT Nanophotonics Center.
He received an M.E. and Ph.D. in electronics
and applied physics from Tokyo Institute ofTechnology in 2005 and 2008. He received a
research fellowship for young scientists from theJapan Society for the Promotion of Science forthe years 2006 to 2009. In 2008, he was a postdoctoral fellow and visiting researcher at Tokyo
Institute of Technology and the University ofCalifornia, Berkeley, USA. He has been withNTT Photonics Laboratories since 2009. His current research interests include next-generation
access systems, InP photonic functional devices,and nano-microcavity semiconductor lasers. He
is a member of IEICE and OSA.

Tai Tsuchizawa

Senior Research Engineer, Materials andDevices Laboratory, NTT Device Technology
Laboratories; and NTT Nanophotonics Center.
He received a B.S. and M.S. in physics from
Sophia University, Tokyo, in 1984 and 1986 anda Ph.D. from the University of Tokyo in 1990.
Since joining NTT in 1999, he has been studying
electron cyclotron resonance plasma technology
and its application to an etching process for
microfabrication. His current research interests
include fabrication technologies for siliconbased optoelectronics devices. He is a member of
JSAP.

Tsuyoshi Yamamoto

Senior Research Engineer, Supervisor, Group
Leader, Materials and Devices Laboratory, NTT
Device Technology Laboratories.
He received a B.E. in electrical engineeringfrom Kansai University, Osaka, in 1991. He
joined NTT Communication Switching Laboratories in 1991, where he engaged in R&D of
several optical interconnection systems using
free-space optics. He has recently been involved
in researching and developing several photonicintegrated devices based on silicon photonics and
III-V semiconductor technologies. During
1998–1999, he was a visiting research engineer
at the Department of Electrical and Computer
Engineering, McGill University, Quebec, Canada. During 2007–2011, he was the director of the
Optical Node System Department of the OpticalCommunication Systems Group at NTT Electronics Corp. He received Microoptics Conference Best Paper Awards in 2003 and 2009. He isa member of IEEE.

Koji Yamada

Senior Research Engineer, Supervisor, Materials and Devices Laboratory, NTT Device Technology Laboratories; and NTT Nanophotonics
Center*.
He received his B.E., M.E. and Ph.D. innuclear engineering from Kyushu University in
1986, 1988, and 2003. He joined NTT in 1988,
where he engaged in studies on accelerator physics and engineering for synchrotron light sources,
as well as silicon-based photonic platforms. He
was a member of the Photonics Electronics Technology Research Association (PETRA) from2010 to 2015 and served as a leader of the photonic wiring and waveguide research team for the
Photonics-Electronics Convergent System Technology (PECST) project. He is a member ofIEEE, IEICE, JSAP, the Atomic Energy Societyof Japan, and the Particle Accelerator Society ofJapan.
*He joined the National Institute of Advanced
Industrial Science and Technology (AIST) in
2015, where he continues his research on silicon-based photonic platforms.

Shinji Matsuo

Senior Distinguished Researcher, Materials
and Devices Laboratory, NTT Device Technology Laboratories; and NTT Nanophotonics Center.
He received a B.E. and M.E. in electrical engineering from Hiroshima University in 1986 and1988 and a Ph.D. in electronics and applied physics from Tokyo Institute of Technology in 2008.
In 1988, he joined NTT Opto-electronics Laboratories, where he researched photonic functional
devices using multiple quantum well pin modulators and VCSELs. In 1997, he researched optical
networks using WDM technologies at NTT Network Innovation Laboratories. Since 2000, he
has been researching high-speed tunable opticalfilters and lasers for photonic packet switching at
NTT Photonics Laboratories. He is a member of
the IEEE Photonics Society, JSAP, and IEICE.

NTT Technical Review

Feature Articles: Photonics-electronics Convergence
Hardware Technology for Maximizing Network Performance

Photonics-electronics Convergence
Design for Digital Mock-up
Kotaro Takeda, Kentaro Honda, Hideyuki Nosaka,
Hiroshi Fukuda, and Tsuyoshi Yamamoto
Abstract

We developed a design technique for an electronic and photonic integrated circuit that involves using
an equivalent circuit of optical devices in an electrical circuit simulator. Key features are the use of structural and physical parameters as calculation parameters in the equivalent circuit to provide a practicaldevice design and the use of an intermediate frequency to model the wavelength dependence of opticaldevices in an electrical circuit simulator. By combining these techniques, we simulated the bit error rates
and eye diagrams of a device that integrates an optical and electrical circuit. This technique turns anelectrical circuit simulator into a photonics-electronics convergence simulator.
Keywords: photonics-electronics convergence, digital mock-up, equivalent circuit

1. Introduction
1.1 Photonics-electronics convergence
Telecommunications network systems are now facing an explosive traffic increase, with a growth rate of
30% per year [1]. At this growth rate, the power consumption and capital/operating expenditures for network systems will increase by ten times or more. Oneof the breakthrough technologies for dealing with thisinformation explosion is photonics-electronics convergence. Photonics-electronics convergence devices,which comprise both optical and electrical components, can achieve performance that far exceeds thatof discrete devices. Moreover, their high integration
density and low power consumption make thempromising devices for reducing network costs [2–4].
Photonics-electronics convergence devices are
more than just combinations of electrical and opticalcircuits. They involve high-density integration thatprovides organized functions and high performance.
Thus, a photonics-electronics convergence device
must comprise integrated circuits. However, dense
and functional integration leads to complex designs
and high fabrication costs.
To reduce fabrication costs, we need to estimate theperformance of an integrated circuit before it is fabri

cated. For this purpose, NTT Device Technology
Laboratories has focused on developing a technique
to design photonics-electronics convergence. Conventionally, optical and electrical devices aredesigned individually to obtain the optimal performance of an integrated device, as shown in Fig. 1(a).
However, as mentioned above, the actual performance of integrated devices does not correspond to
the performance of the combined discrete optical andelectrical devices. For this reason, optical and electrical devices should be designed in a cooperative manner by converting and sharing data to maximize thetotal performance of integrated devices.
However, the potentially very large computational
cost of data conversion and sharing could become abottleneck in the overall design. Furthermore, in thedata sharing process, each design tool can only sharelimited information such as output waveforms, scattering parameters, and DC (direct current) and AC(alternating current) responses. This limitation
impedes the overall design, which increases the timeand cost. In addition, many software licenses are
required, which increases the capital cost for thedesign.

NTT Technical Review

Feature Articles

Electrical device• Total performance is unclear.

Integration
Optical device• Many design tools• Low design efficiency

Individually designed(a) Conventional design• Total design
Designed in unified platform • Single design tool
• Easy for optimizing design(b) Photonics-electronics convergent design

Fig. 1. Comparison of conventional design and photonics-electronics convergent design.

1.2 New design technique
NTT Device Technology Laboratories has deviseda photonics-electronics convergence design technique to solve these cost problems. The design
involves the use of a unified platform to design both
optical and electrical devices and simulate overall
performance (Fig. 1(b)). This approach eliminates
the need for data conversion/sharing and software
licenses and guarantees that information about devices is not lost. In other words, it reduces the computational and capital costs, as well as the required time.
This design technique, referred to as a digital mockup, uses precise design and simulation models that
reduce the number of trial fabrications to zero. As a
final objective of the photonics-electronics convergence design, we aimed to construct a digital mockup for a photonics-electronics convergence device.
The first step in providing the digital mock-up was
to use an electrical circuit simulator as a unified
design platform. This eliminates the need for an optical device simulator and enables us to design everything in one simulator. This unified platform is familiar to people in the electrical circuit field and iscompatible with electronic design automation. Sincea photonics-electronics convergence device has many
more electrical elements than optical ones, the use ofan electrical circuit simulator as a unified design platform is efficient from the viewpoint of computational
Vol. 14 No. 1 Jan. 2016

cost.
There are two important points for modeling opticaldevices in an electrical circuit simulator. One is thatthe models should be able to handle the physical andstructural parameters for designing them. The other isthat the electrical circuit simulator must handle characteristics that cannot be used as calculation parameters in a conventional electrical circuit simulator butthat are necessary for optical circuits. The necessary
parameters for simulating optical devices are wavelength, polarization, and waveguide mode, but anelectrical circuit simulator cannot handle them.
In this study, we used an equivalent circuit of an
optical device in an electrical circuit simulator forphotonics-electronics convergence design. Other
design techniques have used equivalent circuits of
optical devices in an electrical circuit simulator [5–7].
However, a conventional equivalent circuit uses several lumped elements to express only the frequency
response of optical devices. It cannot use device
structures or physical parameters as calculation
parameters. Thus, when an optical device is redesigned, the equivalent circuit has to be modeledagain. To design an optical device in an electricalcircuit simulator, the equivalent circuit should consistof lumped elements that have a one-to-one correspondence with the structural components and physicalparameters of an actual device. This allows us toFeature Articles

Parasitic
OutputcapacitanceG
Al

Dark current
shot noiseGTi/TiN
Photocurrent
Shot noise
Al
Responsivity

Carrierrunning time

Reflectivity

Input

Contact
resistance

n-Si
n-Ge

Al

Ge
(1 µm)

Depletion
layercapacitance
p-Si
BOX

Si substrate
Optical part

Electrical part

BOX: buried oxideFig. 2. Structure of Ge-PD equivalent circuit.

carry out structural and material redesigns of opticaldevices in an electrical circuit simulator by changing
the parameters of the lumped elements.
2. Design of optical equivalent circuits
2.1 M
 odel for designing optical devices in an electrical circuit simulator
In this study, we used a germanium photodiode(Ge-PD) for the optical device, as shown in Fig. 2. We
used lumped elements in our equivalent circuit anddivided it into electrical and optical parts. The electrical part consists of the contact resistances, the capacitance and resistance of the depletion layer, shot noisedue to dark current, and the parasitic capacitance
between electrodes. These parameters of the lumped
elements are independent of each other and have a
one-to-one correspondence with the structural components of an actual Ge-PD.
The optical part of the equivalent circuit includes
the optical parameters of the Ge-PD, which areresponsivity, carrier running time, shot noise due tophotocurrent, and reflectivity. The shot noise due tophotocurrent is a function of the input optical power.
The capacitance for carrier running time models thefrequency dependence due to carrier drift in a Ge
layer as a simple low-pass filter. The lumped elements reflect the structure and physical parameters of
the Ge-PD, and the circuit parameters depend on thedimensions and material parameters. Thus, we canachieve a simple structural design of an optical device
by using this equivalent circuit.
We fabricated, characterized, and simulated Ge

PDs with the same structure as in Fig. 2 to confirm thefeasibility of our equivalent circuit. The detailed
structure, fabrication process, and performance of the
Ge-PD are described in our previous reports [8–10].
The measured and simulated S21 parameters for GePDs with a heavily or lightly p-doped Si layer areplotted in Fig. 3(b). The doping concentration of Si
determines the contact and p-doped Si resistance asshown in Fig. 3(a), which mainly determines the total
resistance of the Ge-PD. In the measurement, the GePD with the lightly doped Si layer shows a lower 3dB cutoff frequency than that with the heavily doped
one, as shown in Fig. 3(b). In the simulation, wechanged the parameters of the two resistances, whose
values were estimated from a test element group. As
shown in Fig. 3, the agreement between the simulated
and measured frequency responses of the Ge-PD isvery good, which means that we can design opticaldevices by changing the parameters of the lumped
elements.
2.2 M
 odel for handling unique parameters of
optical devices in an electrical circuit simulator
In this study, we used as an example the wavelengthcharacteristic of optical devices. The method for handling wavelength in the electrical circuit simulator, in
this case for a wavelength filter, is shown in Fig. 4.
Details of the wavelength filter are described in ourprevious paper [9]. A modulated optical signal can beconsidered to be a modulation signal with a very highcarrier frequency. In the conventional method, thecarrier frequency, which is about 193 THz, is ignored
NTT Technical Review

Feature Articles

Highlyp-doped Si

0
Al
P-Si
p-Si
resistance

Lowly
p-doped Si–1
S21 (dB)Ge(1 µm)

Contact
resistanceSi substrate–2
1.4 GHz
–3
–4
0.01

Sim.
Meas.

18.3 GHz

0.1
1
Frequency(GHz)

(a)

10

40

(b)

Fig. 3. Example of changing parameters using Ge-PD equivalent circuit.

10–2

Filtering

Filtered∆λ = 0.3∆λ = 0.2∆λ = 0
10–4

Meas.
Sim.

BER

Input

Response of
w/o CF (no wavelength)
wavelength filter
Modulated
signal~193 THz
0 Hz
Multiply

10–6
Intermediate frequency (IF)~193 THz

10–8
10–10

Demodulation~193 THz(a) Signal processing with IF to handle wavelength
in electrical circuit simulator

10–12
–10

–5
0
5
Optical input power (dBm)

10(b) Measured and simulated BERs

CF: carrier frequency

Fig. 4. Method for handling wavelength in electrical circuit simulator.

because it is too high for an electrical circuit simulator to handle. In this study, we down-converted thecarrier frequency and the frequency response to anintermediate frequency (IF). We use the IF as a pseudo-carrier frequency to indicate the wavelength
dependence in an optical device. The signal processing with an IF to handle wavelength in an electricalcircuit simulator is shown in Fig. 4(a). An input signal
is multiplied by the IF, and the center frequency of the
wavelength filter is down-converted to it. Opticalsignal processing such as filtering and phase modulation is performed around the IF. Finally, the carrier
frequency is removed, and the envelope signal is
extracted.
The measured and calculated bit error rates (BERs)
of a signal after it has passed through a wavelength
filter are shown in Fig. 4(b). Here, Δλ is defined as theVol. 14 No. 1 Jan. 2016

difference between the IF and the center frequency of
the ring resonator response. The pseudo-random bit
sequence (PRBS) transmission word length and data
rate were 231–1 and 12 Gbit/s. As Δλ increases, theBERs shift to the high input power side because theinput signal should compensate for a power penalty
due to the wavelength filter. In the calculation, theBERs agree well, with the difference being at most
1 dB. The power penalty due to the wavelength shift
is properly depicted. We concluded from the simulation results that we have managed to use the IF tomodel the wavelength dependence in an electricalcircuit simulator.

Feature Articles

Transimpedance
amplifier (TIA)

m

10 m

5m

m

Magnified
image

Photodiodeinput
Wavelength filter(a) Appearance

Filter response
Input

Intermediate
frequency
sourceDemodulator

OutputEquivalentcircuit ofphotodiode
TIA

Wavelength filter(b) Block diagram
Transmitter(simplified)

Wavelength filter

PhotodiodeTIA(c) Equivalent circuit

Fig. 5. Optical receiver with integrated electrical circuit.

3. Measurement and simulation results for
optical receiver with integrated electrical circuit
We fabricated an actual silicon photonic opticalreceiver with our in-house transimpedance amplifier(TIA) as a test device for photonics-electronics convergence simulation, as shown in Fig. 5. A block diagram of the optical receiver is illustrated in Fig. 5(b).
It contains a wavelength filter, a Ge-PD, and the TIA.
As shown in Fig. 5(a), these devices are integrated
densely on one chip. Details of the optical receiverare described in our previous papers [11, 12]. An
equivalent circuit of the optical receiver is shown in
Fig. 5(c). We used a simplified TIA model to reducethe computational cost. The parameters of this sim

plified TIA block are the frequency response, input
referred noise, and transimpedance. These parameters
are determined from the discrete TIA chip. The simulated and measured eye diagrams and BERs of the
optical receiver for Δλ = 0, 0.2, 0.3, and 0.4 nm aregiven in Fig. 6. The PRBS word length and bit rate
were 231−1 and 25 Gbit/s. The IF was set at 200 GHz.
The measured and simulated BERs agree well, withan error less than 3 dB, and the simulations properly
depict the power penalty due to a wavelength shift.
Thus, we can conclude that we were able to successfully simulate a photonics-electronics convergencedevice. In other words, we can design photonics-electronics convergence devices using an electrical circuit
simulator as a unified design platform.
NTT Technical Review

Feature Articles

Measurement

Simulation10∆λ = 0 nm∆λ = 0.2 nm∆λ = 0.3 nm∆λ = 0.4 nm–225 Gbit/s
PRBS 231–1

BER: 10–12

10 ps

BER: 10–13

BER

10–4
10 ps

10–6
10–8

10 ps

10 ps

25 Gbit/s PRBS 231–1(a) Simulated and measured eye diagrams

10–10
10–12
–15

Sim.
Meas.
–10 –50
5Received power (dBm)10(b) Simulated and measured BERs

Fig. 6. Simulated and measured eye diagrams and BERs.

4. Summary
We developed a design technique for photonicselectronics convergence that uses equivalent circuitsof optical devices in an electrical circuit simulator.
The technique uses the physical parameters anddimensions of optical devices as calculation parameters. This allows us to design optical devices in anelectrical circuit simulator. We also use an intermediate frequency to model the wavelength dependence ofan optical device. We combined these techniques tosimulate the BER and eye patterns of a Si photonics
optical-electrical integrated receiver. The simulated
and measured BERs and eye diagrams showed good
agreement, and the power penalty due to the wavelength shift was properly depicted. This techniqueuses only the ordinary functions of the electrical circuit simulator. It does not require any other simulator
platform. Thus, the electrical circuit simulator
becomes a photonics-electronics convergence simulator by virtue of the equivalent circuit of opticaldevices. This is the first step to establishing a photonics-electronics convergence design for digital mockup.
References
[1]

[2]

Cisco, “Cisco Global Cloud Index: Forecast and Methodology,
2014–2019 White Paper.”http://www.cisco.com/c/en/us/solutions/collateral/service-provider/global-cloud-index-gci/Cloud_Index_White_Paper.html
C. Doerr, L. Chen, D. Vermeulen, T. Nielsen, S. Azemati, S. Stulz, G.

Vol. 14 No. 1 Jan. 2016

McBrien, X.-M. Xu, B. Mikkelsen, M. Givehchi, C. Rasmussen, andS.-Y. Park, “Single-chip Silicon Photonic 100-Gb/s Coherent Transceiver,” Proc. of OFC (Optical Fiber Communications Conference
and Exhibition) 2014, Th5C.1, San Francisco, USA, 2014.
[3] Y. Urino, N. Hatori, K. Mizutani, T. Usuki, J. Fujikata, K. Yamada, T.
Horikawa, T. Nakamura, and Y. Arakawa, “Athermal Silicon Optical
Interposers Operating Up to 125 ˚C,” Proc. of SPIE, Vol. 9390, 93900,
San Jose, USA, 2015.
[4] C. Gunn, “CMOS Photonics for High-speed Interconnects,” IEEE
Micro, Vol. 26, No. 2, pp. 58–66, 2006.
[5] F.-M. Kuo, M.-Z. Chou, and J.-W. Shi, “Linear-cascade Near-ballistic
Unitraveling-carrier Photodiodes with an Extremely High Saturation
Current-bandwidth Product,” J. Lightwave Technol., Vol. 29, No. 4,
pp. 432–438, 2011.
[6] J.-W. Shi, F.-M. Kuo, F.-C. Hong, and Y.-S. Wu, “Dynamic Analysis
of a Si/SiGe-based Impact Ionization Avalanche Transit Time Photodiode with an Ultrahigh Gain-bandwidth Product,” IEEE Elec. Dev.
Lett., Vol. 30, No. 11, pp. 1164–1166, 2009.
[7] J. Wu and G. Wang, “A Novel Equivalent Circuit Model for Separate
Absorption Grading Charge Multiplication Avalanche Photodiode(APD)-based Optical Receiver,” J. Lightwave Technol., Vol. 28, No.
5, pp. 784–790, 2010.
[8] S. Park, T. Tsuchizawa, T. Watanabe, H. Shinojima, H. Nishi, K.
Yamada, Y. Ishikawa, K. Wada, and S. Itabashi, “Monolithic Integration and Synchronous Operation of Germanium Photodetectors andSilicon Variable Optical Attenuators,” Opt. Express, Vol. 18, No. 8,
pp. 8412–8421, 2010.
[9] K. Takeda, K. Honda, T. Takeya, K. Okazaki, T. Hiraki, T. Tsuchizawa, H. Nishi, R. Kou, H. Fukuda, M. Usui, H. Nosaka, T. Yamamoto,and K. Yamada, “Comprehensive Photonics-electronics Convergence
Simulation and Its Application to High-speed Electronic Circuit Integration on a Si/Ge Photonic Chip,” Proc. of SPIE, Vol. 9388, 938806,
San Jose, USA, 2015.
[10] T. Tsuchizawa, K. Yamada, T. Watanabe, S. Park, H. Nishi, R. Kou, H.
Shinojima, and S. Itabashi, “Monolithic Integration of Silicon-, Germanium-, and Silica-based Optical Devices for Telecommunications
Applications,” IEEE J. Sel. Topics Quantum Electron., Vol. 17, No. 3,
pp. 516–525, 2011.
[11] K. Takeda, M. Usui, K. Okazaki, T. Hiraki, T. Tsuchizawa, H. Nishi,
R. Kou, K. Honda, H. Fukuda, Y. Ishikawa, K. Wada, K. Yamada, M.
Nogawa, and T. Yamamoto, “Flip-chip Integration of 4-channelFeature Articles

Transimpedance Amplifier Chip on a Si/Ge-based Photonic Circuit,”
Proc. of GFP 2014 (2014 IEEE 11th International Conference onGroup IV Photonics), ThA2, Paris, France, 2014.
[12] M. Usui, K. Takeda, H. Hirata, H. Fukuda, T. Tsuchizawa, H. Nishi,
R. Kou, T. Hiraki, K. Honda, M. Nogawa, K. Yamada, and T. Yama-

Kotaro Takeda

Hiroshi Fukuda

Researcher, NTT Device Innovation Center.
He received his B.E., M.E., and Ph.D. in materials science from Tokyo Institute of Technology
in 2007, 2009, and 2012. His current researchinterests focus on integrated silicon photonic
devices. He is a member of the Institute of Electronics, Information and Communication Engineers (IEICE).

Senior Research Engineer, NTT Device Innovation Center.
He received a B.E. and M.E. in nuclear engineering from Tohoku University, Miyagi, in 1993
and 1995 and a Ph.D. in materials engineeringfrom the University of Tokyo in 2014. He is currently engaged in R&D of micro-photonic
devices. He is a member of the Japan Society ofApplied Physics, IEICE, and IEEE.

Kentaro Honda

Tsuyoshi Yamamoto

Researcher, NTT Device Innovation Center.
He received his B.S. and M.S. in electronicengineering from the University of Tokyo in2010 and 2012. He is currently working on thedevelopment of transceivers for optical communications. He is a member of IEICE.

Hideyuki Nosaka

Senior Research Engineer, 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 electrical and electronics engineering from Tokyo Institute of Technology in
2003. He joined NTT Wireless System Laboratories in 1995, where he engaged in research anddevelopment (R&D) of MMICs (monolithic
microwave integrated circuits) and frequency
synthesizers. He moved to NTT Photonics Laboratories in 1999, where he was involved in R&D
of ultrahigh-speed mixed-signal ICs for opticalcommunications systems. He received the 2001
Young Engineer Award and the 2012 Best Paper
Award presented by IEICE. He is a member of
the Institute of Electrical and Electronics Engineers (IEEE) and IEICE.

moto, “Opto-electronic Hybrid Integrated Chip Packaging Technology for Silicon Photonic Platform Using Gold-stud Bump Bonding,”
Proc. of ICEP-IAAC 2015 (2015 International Conference on Electronics Packaging & iMAPS All Asia Conference), pp. 660–665,
Kyoto, Japan, 2015.

Senior Research Engineer, NTT Device Technology Laboratories.
He received a B.E. in electrical engineeringfrom Kansai University, Osaka, in 1991. He
joined NTT Communication Switching Laboratories in 1991, where he engaged in R&D of
several optical interconnection systems using
free-space optics. He has recently been involved
in R&D of several photonic-integrated devices
based on silicon photonics and III-V semiconductor technologies. During 1998–1999, he wasa visiting research engineer in the Department ofElectrical and Computer Engineering, McGill
University, Quebec, Canada. During 2007–2011,
he was the director of the Optical Node System
Department, Optical Communication Systems
Group at NTT Electronics Corp. He received theMicrooptics Conference Best Paper Awards in2003 and 2009. He is a member of IEEE.

NTT Technical Review

Feature Articles: Photonics-electronics Convergence
Hardware Technology for Maximizing Network Performance

High-speed Optical Packet Switchingfor Photonic Datacenter Networks
Toru Segawa, Yusuke Muranaka, and Ryo Takahashi
Abstract

Today’s large-scale datacenters have been constructed using a large number of electrical routers andswitches, and they are consequently facing problems such as high power consumption, high latency, andlow network throughput. Some of the research aimed at solving these problems focuses on photonic
datacenters based on optical technologies. In this article, we introduce the underlying optical packet
switching technologies of a hybrid optoelectronic router that researchers at NTT Device Technology
Laboratories are currently working on. We also explain a high-speed optical switch, which is a key component of the router.
Keywords: datacenter network, optical packet switching, optical switch

1. Introduction
The use of datacenters (DCs) is expanding rapidly
with the spread of cloud services such as SaaS (software as a service), PaaS (platform as a service), andIaaS (infrastructure as a service). DC Internet protocol traffic around the world is growing at an annual
rate of 23% and is expected to reach 8.6 zettabytes(ZB, 1021 bytes) in 2018 [1]. DCs are continuing to
expand in scale and are facing problems such as large
power consumption of the huge number of servers
and network equipment connecting them, high latency between servers, and insufficient throughput of the
network.
Efforts are underway to alleviate these problems,
and these efforts include implementing changes in
DC networks. Conventional DC networks have alarge number of electrical switches and routers thatare connected in a fat-tree hierarchical architecture,
as shown in Fig. 1(a). The spanning tree protocol(STP) used in these networks prevents multiple paths
from being set between an aggregation switch andcore switches. Such networks are powerful in handling client-server (north-south) traffic but have difficulty coping with server-server (east-west) traffic
due to limited throughput, lack of scalability and flexibility, high latency, and serious power issues. As a

result, DC networks have evolved from the traditional
fat-tree to fabric networks [2] as shown in Fig. 1(b).
In this type of network, the STP is not used, and it ispossible to have multiple paths between aggregation
switches and core switches by using a layer 3 protocol. This has improved the network throughput by
distributing the load by load balancing. However,some problems still remain such as the huge power
consumption due to the large amount of network
equipment, the need for extensive and complex wiring, and the limited network scalability. Research in
the field of photonic DCs based on optical switching
technology is actively being done in order to solvethese problems.
In this article, we present current research trends inphotonic DCs. We also introduce a new DC networkwith a multidimensional torus topology that combines hybrid optoelectronic routers (HOPRs) and an
OpenFlow controller. Finally, we highlight the highspeed optical switches that are one of the HOPR’s key
enablers used for forwarding incoming packets on apacket-by-packet basis.
2. Expectations for photonic DCs
Optical switching technologies used in photonic
networks can be divided into optical circuit switching
NTT Technical Review

Feature Articles

Core switches

Aggregation
switches
ToR switches
Servers(a)(b)

EPS: electrical packet switching
OCS: optical circuit switching
OPS: optical packet switching
ToR: top of rack

Fig. 1. Configuration of (a) traditional fat-tree DC network and (b) recent fabric DC network.

EPS

OCS

OPS

OCS

Core switches

Aggregation
switches
ToR switches
Servers(a)(b)

Fig. 2. Photonic DC networks: (a) EPS/OCS and (b) OPS/OCS.

(OCS), optical burst switching (OBS), and opticalpacket switching (OPS), depending on data granularity. OPS is promising for small-data transmission
where data are forwarded on a packet-by-packet basiswith low latency and high power efficiency, although
the possibility of packet loss is not completely avoidable. OPS is also capable of coping with large traffic
fluctuations while maximizing the bandwidth utilization, as a number of optical packets with different
destinations can be multiplexed in one wavelengthband, which is known as statistical multiplexing.
OCS is suitable for large-capacity transmission
because it provides reliable links with constant latency and without packet loss. However, it is not suitable
for networks in which quick changes occur in the
demanded connection paths, as it requires a long setVol. 14 No. 1 Jan. 2016ting time to establish a dedicated path at a given
wavelength.
The amount of research being done on photonic DC
networks based on such OCS/OPS technology has
recently increased. For example, a network that combines conventional electrical packet switching (EPS)and OCS has been proposed, as shown in Fig. 2(a)[3]. In this network, large-capacity data are forwarded
by OCS, making it possible to reduce the end-to-end
latency after the links are established and to improve
the network throughput by offloading the EPS traffic.
Introducing OPS instead of EPS, on the other hand,makes it possible to reduce the latency substantially,
even though the paths may be longer compared to
OCS, because no time is needed to establish links.
Other advantages include reduced power consumptionFeature Articles

Torus-datacenter network

HOPR
Label
processors

OpenFlow controller

Controller

100 Gbit/s(25 G × 4λ)OpticalswitchFiber delay line buffer
Optoelectronic shared buffer
100-Gbit/s
burst-mode
Tx
ToR
servers

ToR

CMOS

ToR

100-Gbit/s
burst-mode
Rx

10 GbE
ToR ToR

CMOS: complementary metal-oxide semiconductor

Fig. 3. Architecture of proposed torus-datacenter network with HOPR.

and improved network throughput. For example, a
network with a hybrid OPS/OCS configuration has
been proposed, as shown in Fig. 2(b) [4]. However,such a hybrid EPS/OCS or OPS/OCS configuration
requires separate hardware platforms with different
sets of controls.
We have proposed a DC network with a torus topology based on the combination of HOPRs and an
OpenFlow controller, as shown in Fig. 3. The new flat
network provides robust redundancy and superior
scalability. In addition, the combination of the OpenFlow-based centralized control and the label processor-based distributed control enables OPS, OCS, andvirtual OCS all on a single hardware platform to support a diversity of services in DCs [5]. The HOPR is
divided into two parts; one part consists of the opticalpacket switch that comprises the label processors,
optical switch, and optical fiber delay line, and theother part consists of the optoelectronic shared buffer.
The HOPR forwards burst-mode 100 Gbit/s (25 Gbit/s
× 4λ) optical packets as they are in the optical domain,that is, without performing optical to electrical conversion. However, it is difficult to process such highspeed burst-mode optical packets with the commercially available components and equipment. Therefore, we are developing novel optical and optoelectronic devices in our labs in order to construct a new
HOPR prototype that can handle 100-Gbit/s burst

mode optical packets.
3. High-speed optical switch technology
One of the key devices in the HOPR is an N×N optical switch that is used for forwarding the incoming
optical packets to their desired output ports. Because
the data rate of the optical packets is 100 Gbit/s, thelength of ordinary Ethernet packets that need to be
forwarded is as small as 120 ns. Thus, it is essential
to be able to perform switching at a higher speed
within the guard bands, that is, the time separating
successive packets, which are at least an order ofmagnitude shorter than the packet length. In addition,
the switch needs to be insensitive to bit rate (10–400
Gbit/s), packet format (coherent, wavelength divisionmultiplexing (WDM)), wavelength, and polarization,
besides fulfilling other basic demands such as low
power consumption, high extinction ratio, low crosstalk, ease of controllability, and compactness. To
meet these requirements, various optical switches
have been demonstrated so far such as the matrix
switches achieved by cascading 1×2 switches,
phased-array optical switches, wavelength-routing
switches, and broadcast-and-select switches.
Of these switches, we consider the wavelengthrouting switches and broadcast-and-select switches
to be the most promising. The structures of these
NTT Technical Review

Feature Articles

N×N AWG
λ1
λ2
λ3

λn

TWC

λn

λn

λn

λ3

λ1

λ1

1×N optical switchλ3

λn1×N optical switch

1×N optical switch

TWC
TWC

TWC

Optical gate
Tunable laser

Modulator

Optical gate
Optical gate

Photo detector

Optical gate

Input signal(a) Wavelength-routing switch

1×N splitter(b) Broadcast-and-select switch

TWC: tunable wavelength converter

Fig. 4. Schematic configuration of wavelength-routing switch and broadcast-and-select switch.

switches are shown in Fig. 4. The wavelength-routing
switch consists of a tunable wavelength converter thatchanges the wavelength of the input signal light, andan N×N arrayed-waveguide grating (AWG). Since
the output port of the N×N AWG is determined by the
input wavelength, the AWG enables the input packet
to be forwarded to a given output port by converting
the packet wavelength to the desired wavelength witha tunable wavelength converter. This kind of opticalswitch is characterized by high scalability, as it ispossible to increase the number of ports N by making
the wavelength-channel spacing narrower, without
increasing the AWG losses.
The broadcast-and-select switch, on the other hand,is an optical switch that consists of a 1×N optical
splitter and optical gates. The input signal is equallydivided by a 1×N optical splitter. Optical switching isdone by blocking the light or allowing it to pass using
the optical gate attached to the desired output port.
This optical switch suffers from limited scalability,
though, as the splitting losses increase with theincrease in the number of switch ports. It also operates with a simple control for blocking or unblocking
the optical gates. Thus, both the wavelength-routing
and broadcast-and-select switches have their own
merits, and we are conducting research on both ofthese optical switches.
We have proposed a tunable transmitter that relies
on a semiconductor ring resonator [6]. A photo of thisVol. 14 No. 1 Jan. 2016device is shown in Fig. 5. This device has a tunable
laser and optical modulator integrated on a single
chip. The tunable laser is promising as a high-speed
tunable light source where a low driving current isnecessary to change the output wavelength. Moreover, it can suppress the gradual drift in lasing wavelength caused by the Joule heating effect with current
injection. The tunable laser employs ring resonatorsplaced in parallel as a wavelength filter for selecting
the lasing wavelength and as a loop mirror that forms
the laser cavity. With this configuration, it is possible
to improve the wavelength selectivity while increasing and stabilizing the optical output. The opticalmodulator is an electro-absorption modulator (EAM)with InGaAlAs (indium gallium aluminum arsenic)based multiple quantum wells; it enables operation
with a high extinction ratio over a wide range of
wavelengths. Using this device and an AWG with
100-GHz channel spacing, we demonstrated errorfree wavelength-routed switching for 25-Gbit/s burstmode optical packets [6].
We have also proposed a broadcast-and-selectswitch based on monolithically integrated EAM gates[7, 8]. A two-array device of 1×8 optical switches isshown in Fig. 6. To compensate for splitting/coupling
loss, semiconductor optical amplifiers (SOAs) areusually deployed as optical gates in the broadcastand-select switches. However, SOAs characteristically experience nonlinear optical effects such asFeature Articles

Ring resonators

Size: 1.4 × 0.35 mm2
Optical output

Gain
Etched mirror
Tunable laser

1524-channel, 100-GHz spacing

5
Output power (dBm)

EAM

–5
–15
–25
–35
–45
–55
–65
1540

1545

1550

1555

1560

1565

1570

1575

Wavelength (nm)Fig. 5. Photo of tunable transmitter and output spectrum of the transmitter.

1

2

3

4

5

6

7

Size: 4.3 × 1.8 mm2

8Passive

EAM

OutputInput

1×8 splitter

Fig. 6. Photo of two-array 1×8 EAM-gate switches.

four-wave mixing and cross-gain modulation, andsignal-quality degradation for WDM signals is also
an issue. An SOA also requires an electrical currentof several hundred milliamperes for its operation,
which necessitates a driver circuit with high power
consumption. Since our alternative choice of an EAMis free from any associated nonlinear effects, andbecause the device in this case also operates with low
current, the power consumption of the driver circuit
can be reduced to an extremely low level. The EAMbased switch operates with a normally-on state. Thus,
without applying any control signals, the optical
NTT Technical Review

Feature Articles

Eight 1×8 switches

Eight 1×8 splitters
OUT1

IN1
IN2

OUT2
OUT3

IN3
IN4

OUT4

IN5
IN6

OUT5

IN7
IN8

OUT7

OUT6
OUT8
Size: 48 × 39 mm2Fig. 7. Schematic of 8×8 broadcast-and-select optical switch with photograph of the fabricated module.

signal will go through the bulk active layer that istransparent and polarization-insensitive for a wide
range of wavelengths even with a large change indevice temperature. In contrast, the SOA-based
switch operates with a normally-off state and relies
on carrier injection to achieve the on-state necessary
for signal transmission. The SOA gain-profile strongly depends on the energy distribution of carriers,
which causes its performance to exhibit undesirable
changes with wavelength, polarization, or temperature. Thus, in terms of wavelength dependent loss
(WDL), polarization dependent loss (PDL), and temperature dependent loss (TDL), the EAM-based
switch provides clear advantages when compared to
the SOA-based counterpart.
Our fabricated switch module exhibited a small
WDL and PDL of typically ± 0.5 dB in the wavelength range of 1540–1560 nm [7, 8]. We have developed a compact 8×8 broadcast-and-select switch
module that accommodates eight 1×8 optical switches, as shown in Fig. 7. It exhibits superior properties
such as a switching time of no more than 10 ns, a highextinction ratio of at least 40 dB, and power consumption of 3 W or less. We have also achieved errorfree forwarding for 25-Gbit/s optical packet signals[9]. This device has excellent low-power, high-speed,
and broadband capabilities and is very promising as
an optical switch for forwarding large-capacity optical packets since it is compatible with various opticalsignal formats.
4. Future work
We are currently considering scaling-up the highspeed optical switches (e.g., up to 16×16), and we areVol. 14 No. 1 Jan. 2016

also fabricating a pre-prototype HOPR that combines
the three basic functionalities of optical label processing, optical switching, and optical buffering. In
the future, we plan to build and demonstrate a DC
network with a torus topology using the pre-prototype under development, and we will continue ourresearch efforts aimed at realizing environmentallyfriendly DC networks.
Acknowledgment
This work was partially supported by the National
Institute of Information and Communications Technology (NICT).
References
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[2]
[3]

[4]

[5]

[6]

Cisco, “Cisco Global Cloud Index: Forecast and Methodology,
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Z. Kerravala, “A Data Center Fabric Is Critical to a Next-Generation
Unified Data Center,” 2011.
N. Farrington, G. Porter, S. Radhakrishnan, H. H. Bazzaz, V. Subramanya, Y. Fainman, G. Papen, and A. Vahdat, “Helios: A HybridElectrical/Optical Switch Architecture for Modular Data Centers,”
Proc. of ACM SIGCOMM (Special Interest Group on Data Communication) 2010, pp. 339–350, New Delhi, India, 2010.
J. Perello, S. Spadaro, S. Ricciardi, D. Careglio, S. Peng, R. Nejabati,
G. Zervas, D. Simeonidou, A. Predieri, M. Biancani, H. J. S. Dorren,
S. D. Lucente, J. Luo, N. Calabretta, G. Bernini, N. Ciulli, J. C. Sancho, S. Iordache, M. Farreras, Y. Becerra, C. Liou, I. Hussain, Y. Yin,
L. Liu, and R. Proietti, “All-optical Packet/Circuit Switching-based
Data Center Network for Enhanced Scalability, Latency, andThroughput,” IEEE Network, Vol. 27, No. 6, pp. 14–22, 2013.
R. Takahashi, T. Segawa, S. Ibrahim, T. Nakahara, H. Ishikawa, A.
Hiramatsu, Y. Huang, and K. Kitayama, “Torus Data Center Networkwith Smart Flow Control Enabled by Hybrid Optoelectronic Routers,”
J. of Optical Communications and Networking, Vol. 7, No. 12, pp.
B141–152, 2015.
T. Segawa, W. Kobayashi, T. Nakahara, and R. Takahashi,Feature Articles[7]“Wavelength-routed Switching for 25-Gbit/s Optical Packets Using a
Parallel-ring-resonator Tunable Laser Integrated with InGaAlAs
EAM,” IEICE Trans. Electron., Vol. E97-C, No.7, pp. 719–724,
2014.
T. Segawa, M. Nada, M. Nakamura, Y. Suzaki, and R. Takahashi, “An
8x8 Broadcast-and-Select Optical Switch Based on MonolithicallyIntegrated EAM-gate Array,” Proc. of ECOC 2013 (39th European
Conference and Exhibition on Optical Communication), We.4.b.1,
London, UK, 2013.

Toru Segawa

Senior Research Engineer, NTT Device Technology Laboratories.
He received a B.E. and M.E. in electrical engineering and a Ph.D. in integrated design engineering from Keio University, Yokohama, in
1999, 2001, and 2012. In 2001, he joined NTT
Photonics Laboratories, where he began conducting research on high-speed tunable lasers
and optical switches monolithically integrated on
InP substrates. His research interests lie in optoelectronic device and sub-system technologies
for optical packet switching. He is a member of
the Japan Society of Applied Physics (JSAP) andthe Institute of Electronics, Information, andCommunication Engineers (IEICE).

[8]

[9]

Y. Muranaka, T. Segawa, Y. Ogiso, T. Fujii, and R. Takahashi, “Performance Improvement of an EAM-based Broadcast-and-Select Optical
Switch Module,” Proc. of Photonics in Switching, ThII2.2, Florence,
Italy, 2015.
T. Segawa, S. Ibrahim, T. Nakahara, and R. Takahashi, “100-Gbps
Optical Packet Switching with Ultralow-power Label Processor andOptical Switch,” Proc. of ECOC (41st European Conference andExhibition on Optical Communication), Tu.3.5.5, Valencia, Spain,
2015.

Ryo Takahashi

Senior Research Engineer, Supervisor, NTT
Device Technology Laboratories.
He received a B.E., M.E., and D.E. in electronic engineering from the University of Tokyoin 1987, 1989, and 1992. In 1992, he joined NTT
Opto-electronics Laboratories. From 1998 to1999, he was a Visiting Scholar at the Ginzton
Laboratory, Stanford University, CA, USA. His
current research interests include ultrafast opticaldevices and optical signal processing technologies for optical packet switched networks. He isa member of JSAP and IEICE.

Yusuke Muranaka

NTT Device Technology Laboratories.
He received a B.S. and M.S. in applied physics
from Waseda University, Tokyo, in 2011 and2013. From 2011 to 2013, he was a Visiting
Researcher at the National Institute for Materials
Science (NIMS), Tsukuba, where he was engaged
in research on nonlinear optics and related materials. He joined NTT Photonics Laboratories in2013 and is now with NTT Device Technology
Laboratories, where he researches optical packet
switching technologies. He is a member of
JSAP.



NTT Technical Review

Feature Articles: Photonics-electronics Convergence
Hardware Technology for Maximizing Network Performance

Hardware/Software Co-design
Technology for Network
Virtualization
Takahiro Hatano, Koji Yamazaki, and Akihiko Miyazaki
Abstract

Network virtualization technology is a topic of great interest because of its ability to emulate networkfeatures in software. In this article, we present a technique for adding programmable hardware acceleration to network virtualization equipment, which is then used in coordination with software to preserve
the software’s flexibility while raising the limits on its performance.
Keywords: network virtualization, convergence of software and hardware, FPGA (field-programmablegate array)

1. Introduction
As the performance of commodity servers has
improved, we have set out to rapidly provide services
with reduced operating and capital expenditures by
employing network virtualization technology. Software-defined networking (SDN) is representative of
this; it separates the features that control a communication device from the device’s data transfer functionality, enabling network configurations and settings to be centralized in and automatically controlled
by software. To apply this SDN technology to carriers’ wide area networks, NTT has taken the lead in
the accelerated research, development, and opensource release of an SDN application to the world.
Our application has two components: the Ryu SDN
Framework [1] and Lagopus [2]. The Ryu SDN
Framework is an SDN controller that provides convenient tools and libraries for easily setting up an SDN.
Lagopus is a high-performance SDN software switch
with capabilities and features that have broad applications from datacenters to wide area networks.
2. Development of hardware accelerator for
network virtualization
Network features that have traditionally been provided by dedicated hardware can now be implemented in software by network virtualization technologies
on commodity servers. As the use of network virtualization becomes more common, we can expect commodity servers to be equipped with many networkfeatures that will increase the load on the servers’
central processing units (CPUs), which will be
responsible for handling network-related processing.
If we want to meet the demands for even moreadvanced network services and systems, we will needtechnology that can alleviate this additional CPU loadin order to increase the power/performance ratio ofprocessing on commodity servers and thus provide astable environment for running more software.
The NTT Device Innovation Center has been
involved in researching and developing hardware/
software co-design technologies to help hardware
and software work together. We developed a hardware accelerator (HWA) [3, 4] that implements theconcepts shown in Fig. 1, improving performanceand reliability while preserving the flexibility of thehighest-performance SDN software switch in theworld, Lagopus.
3. HWA properties
The system configuration of an SDN software
NTT Technical Review

Feature Articles

Commodity server

Performance

Network features
Conventional
SDN hardware
switchNFV applications

Higher performance
SDN switch

SDN software
switchHigher reliability

Software(x86 CPU)
processing

DPDK

API

Data plane

Flexibility

SDN software switch+
HWA

Availability

Hardware(FPGA)
processing

Configuration

HWA (FPGA)

Fig. 1. Conceptual diagram of hardware accelerator.
NIC
40G

switch with the HWA we developed is shown in
Fig. 2. The system is characterized by a reconfigurable HWA—specifically, a field-programmable gatearray (FPGA)—placed on a commodity server’s network interface card (NIC). Hardware ordinarily takes
much longer to develop than software. However, byusing an FPGA, which can be overwritten with features implemented in a program, we can not only
speed up the development process, but we can also
update network features and quickly make changes to
fix problems that arise later.
The SDN software switch system in Fig. 2 forwards
network packets through the FPGA on the NIC to thesoftware that will process them. In addition to the
NIC’s ordinary packet processing, the FPGA has
implemented preprocessing features to assist with thefirst part of the heavy processing in Lagopus’s packet
forwarding component (i.e., the data plane). The
HWA’s preprocessing cannot directly communicate
with—and is independent of—Lagopus’s packet processing. This allows Lagopus to retain its flexibility
while reducing the software load and further improving the performance of the SDN software switch.
Specifically, we have developed the following three
features.
(1) High-speed flow dispatcher
High-speed packet processing on a software switch
requires evenly distributing the work among the processing units (cores) in a multi-core processor (CPU),
which can process multiple instructions in parallel.
Vol. 14 No. 1 Jan. 2016

Network
API: application programming interface
DPDK: Intel® Data Plane Development Kit
NFV: network functions virtualization

Fig. 2. System configuration for SDN software switch.

There usually is no mechanism for distributing thepacket load in advance, so packets must be sent to a
designated CPU core to be divvied up in software.
This could cause packets to pile up on a single CPU
core, creating a bottleneck, and in the worst-case scenario leading to packet loss. The high-speed flow
dispatcher feature assigns packets to FPGA data
structures (queues) based on the packet flow data
received by the hardware, as shown in Fig. 3. Direct
memory access (DMA) is then used to transfer packets to the main memory regions used by the CPU
cores. In this way, the total processing load is distributed and smoothed out across all CPU cores without
wasting software cycles.
(2) Flow director
In conjunction with the high-speed flow dispatcher
feature, we need a way for software to freely specify
which flows to assign along with their corresponding
DMA queues. The flow director feature allows Lagopus to accomplish this by communicating with theFeature Articles

Flow #3
Incoming packets

Flow #2
Flow #1

Classifier

FPGA

Packets are assigned to
DMA queues in parallel.

Flow dispatcher

DMA queues

DMA transfer to main memory

Main
mem.
#1

Main
mem.
#2

Main
mem.
#3

Main
mem.
#4

Main
mem.
#5

Main
mem.
#6

Core
#1

Core
#2

Core
#3

Core
#4

Core
#5

Core
#6

Workload

Multi-core CPU (6-core case shown here)

#1

#2

#3

#4

#5

#6

CPU cores
CPU core load is distributed and smoothed out.

Fig. 3. High-speed flow dispatcher.

hardware through an application programming interface (API). The flow director feature can be used to
improve performance. Lagopus can figure out how
flows should be assigned to CPU cores, so flows do
not need to be identified twice, and the corresponding
processing can be skipped (Fig. 4). This gives Lagopus enough spare time to process short packets more
efficiently.

(3) Packet mirroring
We will need to make it easier to track down thecause of network problems in the future because we
expect them to become more complex with advancesin network virtualization. To that end, we have implemented packet mirroring in our HWA, enabling virtual networks to be monitored at all times without
placing any burden on the software. This feature copies incoming packets within the FPGA and forwards
NTT Technical Review

Feature Articles

Flow #3
Incoming packets

Flow #2
Flow #1

FPGA

ID tableClassifier

Flow dispatcher

ID

Destination

VLAN 1

Queue #1

VLAN 2

Queue #2

VLAN 32

Queue #32

DMA queues

DMA transfers

API

Configuration

Flow data

Flow #1

Inbound processing
Main mem.
Data plane processing
CPU core
Flow identification Unnecessary

Outbound processing

ID: identification
VLAN: virtual local area network

Fig. 4. Flow director.

them unmodified to an external device for analysis(Fig. 5). Problems can then be analyzed, and signs ofpossible problems can be detected without affecting
normal packet processing.
4. Trial and evaluation of systems withhardware acceleration
In collaboration with Xilinx Inc., we tested a prototype of our SDN software switch system equippedwith an HWA that had all of the aforementioned features. We then checked the performance gains
achieved through our hardware/software co-design.
Our tests used the latest FPGA devices and tools (i.e.,
SDNet*) capable of implementing cooperative hardware/software behavior. The results of evaluating theVol. 14 No. 1 Jan. 2016

packet-forwarding performance are plotted inFig. 6(a), and the measured power consumption is in
Fig. 6(b). The data show that short packets were forwarded more quickly when hardware acceleration
was enabled. Furthermore, when using eight CPU cores(the maximum), our system was able to process packets longer than 384 bytes at a wire rate of 40 Gbit/s.
We observed that the HWA only used 19 W—lessthan 5% of the 425 W used by an entire commodity
server—while processing packets at 40 Gbit/s, confirming that we can achieve this wire rate with only amodest increase in power consumption. By using
Lagopus to switch the HWA’s packet copying feature*

SDNet (Software Defined Specification Environment for Networking): Xilinx’s design tool for packet processing.

Feature Articles

Incoming packets

FPGA

Classifier

Flow dispatcher

Copy
DMA queues
Primary

Secondary
Forwarding

DMA
transfers

Packets

External device(Analytics)

Packets

Ordinary
packet processing
Support for analyzing anddetecting signs of problems

Fig. 5. Packet mirroring.

40

Throughput (Gbit/s)

35
30
Wire rate achieved for packets
longer than 384 bytes

25

CPU #2115 W (27%)

20

CPU #1
115 W (27%)

15

5
0

HWA 19 W (5%)

40 Gbit/s wire rate
8 CPU cores with HWA
4 CPU cores with HWA
4 CPU cores without HWA

10

0

200

400

600

800

1000

1200

1400

Other (RAM/HDD)
176 W (41%)
1600

Packet length (Bytes)(a) Packet forwarding performance(b) Power consumption at wire rate of 40 Gbit/s

Fig. 6. Results of prototype performance tests.



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Feature Articles

on and off via its API, we also confirmed that thefeature did not affect primary packet processing in
our prototype.
5. Future plans
In the future, we will work on improving efficiency
even further by tackling the problem of accepting andprocessing packets shorter than 384 bytes at the full
wire rate of 40 Gbit/s. To further improve reliability,
we will experiment with the use of packet mirroring
to implement network monitoring and analysis features and thus help improve the operational aspects of
network virtualization. We also plan to actually
implement network functions virtualization (NFV)
applications with all of these features and develop asystem for trial use.

Takahiro Hatano

Senior Research Engineer, Metro-Access Network Device Project, NTT Device Innovation
Center.
He received a B.E. and M.E. in electronics,
information and communication engineeringfrom Waseda University, Tokyo, in 1990 and
1992. He joined NTT LSI Laboratories in 1992.
In 2010, he moved to the Visual Communication
Division of NTT Bizlink, where he developed
and managed the video distribution services forlive viewing on the NGN (Next Generation Network). He has worked on developing hardware/
software co-design technology for future networks. He is a member of the Institute of Electronics, Information and Communication Engineers (IEICE).

In this way, we will continue to improve the performance and reliability of our network virtualization
technology, establish hardware/software co-design as
a fundamental technique for network virtualization,and commercialize our technology for practical use
on carrier networks.
References
[1]
[2]
[3]
[4]

Website of Ryu, http://osrg.github.io/ryu/
Website of Lagopus switch, https://lagopus.github.io/
Y. Nakajima, H. Takahashi, K. Ishiguro, and K. Yamazaki, “It’s Kindof Fun to Do the Impossible with DPDK,” DPDK Summit 2015, San
Francisco, USA, 2015.
K. Yamazaki, Y. Nakajima, T. Hatano, and A. Miyazaki, “Lagopus
FPGA - A Reprogrammable Data Plane for High-performance Software SDN Switches,” Proc. of HOT CHIPS 27, pp. 10–19, Cupertino,USA, 2015.

Akihiko Miyazaki

Senior Research Engineer, Supervisor, MetroAccess Network Device Project, NTT Device
Innovation Center.
He received a B.S. and M.S. in pure andapplied sciences from the University of Tokyo in
1991 and 1993. In 1993, he joined NTT LSILaboratories, where he initially engaged inresearch and development of layout CAD algorithms and systems. In 2000, he moved to NTT
Communications and worked on developing
application-platform services for enterprise customers. In 2004, he moved to NTT Microsystem
Integration Laboratories and developed MAC(media access control) LSIs for access networks.
He is a member of IEICE and the Physical Society of Japan.

Koji Yamazaki

Senior Research Engineer, Metro-Access Network Device Project, NTT Device Innovation
Center.
He received a B.S. in environmental information science from Keio University in 2000. Whilein school, he founded a venture company with hiscolleagues and after operating that, he received a
Master of Arts and Sciences (information studies) in interdisciplinary information studies fromthe University of Tokyo in 2004. Since joining
NTT, he has been researching and developing
large-scale integrated circuits (LSIs) and related
computer aided design (CAD) systems. In 2009,he was engaged in developing a large-scale distributed data processing platform for cloud computing. His current interests include SDN/NFV
and integration of high performance routers andswitches.

Vol. 14 No. 1 Jan. 2016

Regular Articles

Body-mind Sonification to Improve
Players Actions in Sports
Toshitaka Kimura, Takemi Mochida, Tetsuya Ijiri,
and Makio Kashino
Abstract

It is essential that we recognize both our own physical movements and mental condition if we are toachieve and improve goal-directed actions in sports. However, it is not easy to adequately understand
one’s own body-mind state during complicated sports actions. A potentially better way to overcome thisproblem is to use auditory feedback because auditory perception has higher temporal resolution thanvisual perception, and it interferes very little with the performance of an action. In this article, we
propose real-time auditory feedback techniques designed to sonify the temporal coordination of the bodyand the excessive activation caused by mental pressure when participating in sports based on surface
electromyography and acceleration signals. Our aim was to determine when and how the body-mind
behaves. We believe that these techniques will help players learn the skill needed for a desired action andwill reveal a player’s physical and mental condition when playing sports.
Keywords: auditory feedback, real time, motor learning

1. Introduction
It is essential that we understand our own movement if we are to achieve and improve goal-directedactions in sports. However, it is not easy to adequately recognize the state of one’s own body during complicated movements involving multiple segments ofthe body, as when a golfer needs to coordinate various
movements of the arms, trunk, and legs quickly andcorrectly when hitting a tee shot. In addition, the bodyis affected by a person’s mental condition; that is, thebody and mind are not independent but are closely
linked. Even a highly skilled player may not perform
well under mental pressure. Therefore, it is very
important to understand one’s own body-mind state
during sports actions so as to perform satisfactorily inan actual sports game.
NTT has long researched the neural mechanisms
behind motor control, sensory perception, and emotional processing in humans. We have incorporated
these findings and know-how into feedback techniques designed to convey certain key features of thebody-mind state during actual sports actions. Specifi

cally, we propose using auditory feedback to obtainthe temporal structure of segmental motions andmuscle activity using synthetic sound. Our aim is tomake a player intuitively aware of when and how
his/her body-mind behaves.
2. Use of auditory feedback
To sense one’s own motor state (e.g., posture,
motion, and muscle activity) during ongoing movement, we mainly depend on proprioception, which
originates in the sensors in muscles and joints. However, proprioception offers only rough spatiotemporal
resolution. Thus, many studies have proposed various
ways of providing the motor state with a multimodal
approach based on, for example, vision, audition, or
haptics, to compensate for poor proprioceptive information [1]. Visual feedback has been the most widelyused way of displaying body motion (e.g., looking at
snapshots and video). However, visual feedback isalso assumed to have some limitations. First, we cannot fully use vision during an ongoing action because
vision is commonly used for other goals (e.g., looking
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Regular Articles

at a target). Furthermore, vision is expected to belimited in terms of showing the temporal structure ofsegmental activities because of the insufficient temporal resolution of human visual perception, eventhough the very quick coordination of multiple bodysegments is a key feature in some sports.
A potentially better way to overcome the above
problems is to use auditory feedback. One reason forthis is that unlike vision, human auditory perception
is much less likely to interfere with performing anaction, which is a benefit of providing feedback inreal time. Auditory perception also has higher temporal resolution than vision. In fact, human beings often
utilize auditory feedback to understand the temporal
structure of motor behavior in daily life. A very familiar example is speech articulation. It is well known
that speech articulators (e.g., the lips and tongue)
have a very quick and complicated pattern, and auditory feedback plays a crucial role in controllingspeech articulation [2]. We designed techniques forreal-time sonification of the body-mind state toextend the human auditory system so that it can utilize motor control in the body in addition to speech.
3. Sonification of sports action
Although previous studies have already applied
auditory feedback to sports, most of them have sonified behavioral outcomes (e.g., ski displacement [3])or individual segmental activities (e.g., the timing ofwrist and ankle movements in karate [4]). However,these kinds of feedback for individual outcomes andactivities may be insufficient to utilize for motor recognition and learning because the spatiotemporal
coordination of the body appears to be critical interms of achieving a skillful movement in most
sports. In contrast, our idea is to sonify body coordination, namely when and how a player controls his/her body, based on muscle activity and acceleration
signals in multiple target segments. We expect our
artificially provided auditory feedback to be integrated with original proprioceptive information during sports actions and that this integration will facilitate improvements in sports skills.
3.1 Sonifying segmental coordination
For most sports, it is critical to coordinate multiple
body segments sequentially. For instance, when
pitching a baseball, a pitcher needs to operate his/herbody segments in sequence from the leg to the arm,
which is a well-known segmental kinetic chain [5],and a novice pitcher cannot perform this sequenceVol. 14 No. 1 Jan. 2016

well. Thus, we attempted to sonify such sequential
patterns during pitching using acceleration signals
recorded on the pitcher’s trunk, and then to apply it topitching practice for novice participants.
An example of the sonification of the pitchingsequence of the trunk in a single novice participant isshown in Fig. 1. We attached wireless acceleration
sensors (Trigno, Delsys) at three different positions(P1, P2, and P3) on the back of the trunk (Fig. 1(b))
and detected three axes of acceleration signals at a300-Hz sampling rate. We used analog acceleration
signals during pitching to calculate two acceleration
components in real time, namely, trunk rotation (A1)and tilting (A2); we calculated A1 and A2 from the P2
minus P1 signals and P3 minus P1 signals, respectively (top two panels in Fig. 1(c)). Then a sound (S)is synthesized as(1)
S(t) = A1(t) S1 + A2(t) S2,where S1 and S2 indicate band-limited pink noisewith peak frequencies of 100 and 500 Hz, respectively. The lowest panel in Fig. 1(c) shows a spectrogram of the generated synthesized sound. Although
this spectrogram contains two unclear frequency
components, if the player performed a good trunk
motion he/she would be able to hear the two components separately (see Fig. 1(d)). Time 0 approximately indicates the ball release timing of pitching.
The above sonification (auditory feedback) wasapplied while two novice participants engaged in
pitching practice that consisted of repeating 100 nondominant arm pitches with a fixed step length andmoderate effort (Fig. 1(a)). In particular, they focusedon trunk rotation and tilting, which are targeted in the
feedback information, because novices frequently
exhibit poor trunk motion [6]. They also performed
other practice trials using visual feedback (6-sec.delayed video streaming) in a separate session forcomparison with the effect of auditory feedback. The
order of auditory and visual practice sessions wascounterbalanced between the two participants. At the
beginning of each practice session, either the sound(Fig. 1(d)) or a video of an expert’s pitch was provided as a practice target. Before and after each practice session, we measured as a test session the chest,
shoulder, and wrist motions in 10 trials without any
feedback using a motion capture system (Oqus 300,
Qualisys).
The mean velocity waveforms of three segments(chest, shoulder, and wrist) in each test session are
shown in Fig. 2(a) for each participant. Both auditory(AF) and visual feedback (VF) practices improved
the body sequence; namely, the sequential patternRegular Articles(a) Pitching practice(b) Acceleration sensors
P3

P1

P2

(c) Novice’s profiles
Acc. (m/s2)

40
0
40

A1 (Rotation: P2 - P1)

A2 (Tilting: P3 - P1)

Freq. (kHz)Amplitude

0
Synthesized sound

1.00Sound spectrogram

–1.0

–0.5
0
Time from ball release (sec.)

0.5

Freq. (kHz)(d) Expert’s sound spectrogram
1.0

0

–1.0

–0.5
0
Time from ball release (sec.)

0.5

Fig. 1. Sonification of segmental coordination of the trunk during pitching.

from the chest to the wrist for peak velocity was
clearer than in pre-practice trials. The inter-trial
deviation of velocities for each test session is shownin Fig. 2(b). Interestingly, the auditory feedback
showed less velocity deviation across test trials thanthe visual feedback. This result suggests that our realtime sonification for segmental coordination enhances the inter-trial repeatability of movements, compared with standard visual feedback.
3.2 Sonifying coordination of muscle activity
The above sonification method expresses the kinematics of movement based on acceleration signals of
the body. We propose here another sonification method for body state that describes the dynamics ofmovement, that is, the muscle activity pattern. Even

if a player is made aware of a difference in motion by
the sonification of kinematics, it is probably difficult
to understand how to exert a force that will improvethe movement because it is a theoretically ill-posed
problem. Thus, we attempted to sonify the sequential
muscle activity patterns, which more directly reflect
the exerted force, based on multiple electromyography (EMG) signals from the trunk and the upper andlower limb muscles.
An example of the sonification of the muscle activity pattern of pitching is shown in Fig. 3. The generated sound consisted of multiple sound sources
whose fundamental frequencies were different forevery muscle and whose amplitudes were varied inproportion to the EMG activity (Fig. 3(a)). An expert(former professional) pitcher almost always exhibited
NTT Technical Review

Regular Articles

Velocity (m/s)(a) Mean velocity profiles in test sessions
12

Before practice

After VF
Wrist
Shoulder
Chest

8
4
0
– 0.5 – 0.4 – 0.3 – 0.2 – 0.1

Velocity (m/s)

After AF

Subject A

12

0 – 0.5 –0.4 –0.3 –0.2 –0.1

Before practice

0 –0.5 –0.4 –0.3 –0.2 –0.1

0After AF

After VF

Subject B

8
4
0
0 –0.5 –0.4 –0.3 –0.2 –0.1
0
– 0.5 – 0.4 – 0.3 – 0.2 – 0.1
0 – 0.5 –0.4 –0.3 –0.2 –0.1
Time from ball release (sec.)
Time from ball release (sec.)
Time from ball release (sec.)

2
1
0

2

Mean of SD (m/s)

Mean of SD (m/s)(b) Inter-trial variabilities of velocity profiles
3
Subject A

BeforeAF

VF

Subject B

1

0

BeforeVF

AF

Fig. 2. Effect of sonification of segmental coordination during pitching.

a clear kinetic chain of muscle activity from the leg
to the arm muscle (upper-left panel in Fig. 3(b)),
while an amateur pitcher did not (lower-left panel inFig. 3(b)), indicating unnecessary muscle activation.
Therefore, the expert’s sound is very rhythmical(upper-right panel in Fig. 3(b)), while the amateur’s isslurred (lower-right panel in Fig. 3(b)). This sonification can be described as an instrument type of sonification, and a player can hear the difference in themuscle activity pattern as a melody. Using this sonification, a player can move so as to make as purposeful a sound as possible.
3.3 Sonification of excessive muscle activation
Some sports players cannot perform well because
they try too hard, such as when putting while playing
golf. This corresponds to the unintentional excessive
activation of the muscles probably caused by mentalpressure. That is, such excessive muscle activationreflects the state of mind. This phenomenon is very
common and very serious in an actual sports match,but it is often hard to notice it. Therefore, it would be
useful to make a player aware of this potential forVol. 14 No. 1 Jan. 2016

excessive muscle activation.
An example of the sonification of overflow ofmuscle activity in the initial stages (address and takeback) of putting in golf is shown in Fig. 4, based on
EMG and acceleration signals of the wrist extensor
muscles. The sound signal (S) consisted of a periodic
waveform whose amplitude and fundamental frequency were varied in proportion to the short-term
power of the EMG activity (E) divided by the acceleration (A) as follows.
S(t) = E(t) / A(t).
(2)
The sound mainly reflects the EMG activity when
the acceleration was kept at almost zero. Therefore,
the golfer could hear annoying sounds during the
preparation (address) period when the muscle wasactivated more than necessary while the wrist moved
less (lower panels in Fig. 4). Conversely, the golfer
could not hear these sounds when the muscle wasrelaxed during preparation (upper panels in Fig. 4).
Little sound was produced when the wrist was moving (take-back period) because the muscle activity
and motion were coupled. This is a warning type ofsonification, unlike the above sonification, and theRegular Articles(a)Frequency

Each muscle activity assigns
each sound source witha different frequency.

Amplitude

Muscle activity pattern

0.6

Sound spectrogram(dB)
0

Clear kinetic chain(µV)
0Amateur

0

30

0.6

(dB)
0

–12

Freq. (Hz)Leg Trunk Arm(µV)
0

Freq. (Hz)ProfessionalLeg

Trunk Arm(b)

Unclear chain

0

30

–16

Fig. 3. Sonification of muscle activity pattern during pitching.

player aims to produce as little sound as possible.
4. Future developments
This article explained some novel examples ofsonification of the body-mind state in sports actions.
This sonification has the potential benefit of making
a player aware of the coordination of movements andof his/her condition under mental pressure. However,
the information regarding the body-mind state andauditory ways to provide them will vary according tothe objectives and the skill level of the player. For
example, the sonification of a segmental sequence(Fig. 1) would be more useful in helping a novice
player to acquire the basic temporal pattern of a target
action, while an instrument type of sonification ofmuscle activity (Fig. 3) may be more useful in helping an intermediate player to improve, rather thanacquire, a given movement. Furthermore, it is likelythat the sonification of excessive muscle activation
(Fig. 4) is beneficial in terms of recognizing one’s
own mental state in an actual sports game.
We should pay attention to other elements of sonification. For instance, it is critical to determine what
to provide in sonification, namely, the key essence or
the knack of a target action, before applying our sonification. It is also important to combine it with othertechniques to effectively improve a player’s performance. It would be useful, for example, to combine itwith visual feedback to characterize the spatial structure of an action, such as the form, while it seems
more appropriate to combine it with auditory feedback to characterize the temporal structure of anaction. Additionally, a wearable device such as thehitoe fabric bioelectrode developed by Toray and
NTT would make the sonification application easy touse for players and could potentially record EMG
signals during the sports action. Further research isneeded to develop effective ways to enable a player tolearn the skills needed for a desired action and to
NTT Technical Review

Regular Articles

Wrist profiles

Sound spectrogram
3.0

EMG
Good putt

–30
(dB)
0

0

0.5
Time (sec.)

1.0

0.05 mV

EMG

Excessive activity

Freq. (kHz)10 m/s2

Acc.

0
3.0

Bad putt(dB)
0

Freq. (kHz)Acc.

Preparation (address) Take back

0

0

0.5
Time (sec.)

1.0

–30

Fig. 4. Sonification of excessive muscle activation during golf putting.

reveal a player’s condition when playing sports.

[4]

References
[1]
[2]
[3]

R. Sigrist, G. Rauter, R. Riener, and P. Wolf, “Augmented Visual,
Auditory, Haptic, and Multimodal Feedback in Motor Learning: a
Review,” Psychon. Bull. Rev., Vol. 20, pp. 21–53, 2013.
J. Sasisekaran, “Effects of Delayed Auditory Feedback on Speech
Kinematics in Fluent Speakers,” Percept. Mot. Skills, Vol. 115, No. 3,
pp. 845–864, 2012.
R. Kirby, “Development of a Real-time Performance Measurement
and Feedback System for Alpine Skiers,” Sports Technology, Vol. 2,
No. 1–2, pp. 43–52, 2009.

Vol. 14 No. 1 Jan. 2016

[5]
[6]

G. Yamamoto, K. Shiraki, M. Takahata, Y. Sakane, and Y. Takebayashi,“Multimodal Knowledge for Designing New Sound Environments,”
Proc. of the 6th International Conference on Human Computer
Interaction with Mobile Devices and Services, Citeseer, Glasgow,
UK, 2004.
M. E. Feltner, “Three-dimensional Interactions in a Two-segment
Kinetic Chain. Part II. Application to the Throwing Arm in BaseballPitching,” Int. J. Sport Biomech., Vol. 5, No. 4, pp. 420–450, 1989.
S. Oyama, B. Yu, J. T. Blackburn, D. A. Padua, L. Li, and J. B. Myers,“Improper Trunk Rotation Sequence is Associated with Increased
Maximal Shoulder External Rotation Angle and Shoulder Joint Forcein High School Baseball Pitchers,” Am. J. Sports Med., Vol. 42, No. 9,
pp. 2089–2094, 2014.

Regular Articles

Toshitaka Kimura

Tetsuya Ijiri

Senior Research Scientist, Human Information
Science Laboratory, NTT Communication Science Laboratories.
He received his Ph.D. in human neurosciencefrom the Graduate School of Arts and Sciences,
the University of Tokyo, in 2003. He joined NTT
Communication Science Labs in 2003. He has
been engaged in research on sensorimotor control and action-perception interaction in humans.
He has recently been focusing on brain functionsin athletes and the use of ICT in sports. He is amember of the Society for Neuroscience (SfN),
the Japan Neuroscience Society (JNS), and theInstitute of Electronics, Information and Communication Engineers (IEICE).

Research associate, Human Information Science Laboratory, NTT Communication Science
Laboratories.
He received a B.S. in environmental engineering from the University of Tokyo in 2009 and an
M.S. and Ph.D. in human neuroscience in 2011
and 2014. He joined NTT Communication Science Laboratories in 2014 as a research associate. His research interests include the relationship between mental state and sensorimotor
control. He is a member of SfN, the European
College of Sports Science, and the Japanese
Neural Network Society.

Takemi Mochida

Senior Distinguished Scientist/Executive Manager of Human Information Science Laboratory,
NTT Communication Science Laboratories.
He received his B.A., M.A., and Ph.D. in psychophysics from the University of Tokyo in
1987, 1989, and 2000. He joined NTT in 1989.
From 1992 to 1993, he was a Visiting Scientist at
the University of Wisconsin. Currently, he is a
Visiting Professor in the Department of Information Processing, Tokyo Institute of Technology(2006–). He has been conducting psychophysicalexperiments and using neuroimaging, physiological recordings, and computational modeling toinvestigate the functional and neural mechanismsof human cognition, especially auditory perception, crossmodal and sensorimotor interaction,and interpersonal communication. His otherrecent interests include sports, music, andautism.

Makio Kashino

Research Scientist, Human Information Science Laboratory, NTT Communication Science
Laboratories.
He received a B.E. and M.E. in electrical engineering from Waseda University, Tokyo, in 1992
and 1994 and a Ph.D. in systems information
science from Future University-Hakodate, Hokkaido, in 2011. He joined NTT in 1994 and has
been conducting research on speech science. He
was involved in JST-CREST projects from 1998
to 2003 and again from 2014 to present. His
research interests include sensorimotor mechanisms of speech. He is a member of SfN, theAcoustical Society of America, JNS, the Acoustical Society of Japan, and IEICE.

NTT Technical Review

Regular Articles

Development of Invisible OpticalFiber for Improved Aesthetic
Appearance
Keisuke Yoneda, Jun Onishi, Tetsuya Iwadou,
Hironari Iino, Yoshitaka Enomoto,and Katsushi Nakayachi
Abstract

When optical fiber is installed on customer premises, there may be a delay in activating it if thecustomer complains about the conspicuous appearance of the exposed wiring. We have developed aninvisible optical fiber to improve the appearance of the installed cable and to reduce the amount of
unsuccessful activation work.
Keywords: invisible optical fiber, attachment, fixing component

1. Introduction
Exposed cable is conspicuous inside and outsidebuildings. There may be a delay in activating the fiber
if the customer dislikes the appearance of the exposed
wiring. Various measures have been implemented tocorrect this, for example, using cable coverings.
However, any covers or other components that areused must match the wall upon which the cable isinstalled. This can be difficult because there are various types of walls. Therefore, we have developed aninconspicuous transparent fiber with the aim ofachieving a more aesthetic appearance. The structure
and appearance of the fiber is suitable for various
types of walls.
2. New development concept
The objective in developing the new fiber was toimprove the outward appearance by using inconspicuous wiring. We aimed to develop wiring with thesmallest possible diameter as well as a transparent
color so that it would match any type of wall. Also, toimprove the aesthetic appearance, we investigated a

new distribution method using as few connection
points as possible. However, the thin cable may haveinferior impact resistance characteristics. Therefore,
we also investigated a new installation method thatuses the distribution method mentioned above alongwith peripheral components.
3. Product development
3.1 Invisible optical fiber
To improve the visible appearance of optical fiber
cabling, we developed a transparent cable sheath
instead of using the conventional black and whitecable. In addition, the structure is thinner than conventional cable, with a diameter of 0.9 mm (Fig. 1).
This product is suitable for use in restricted spaces; it
can be installed in the gap around a door, for example,
which is only about 2 mm wide. To install opticalcable in such a small gap, we employed single-modehole-assisted fiber (HAF) with bending-insensitive
characteristics. When wiring, we use a dedicated tube
in order to ensure the bending radius. The dedicatedtube is also transparent (Fig. 2).

NTT Technical Review

Regular Articles

1.6 mmSheathTension member
0.9 mm

2.0 mmNewly developed invisible optical fiber

Opticalfiber
SheathOptical fiber
Newly developed invisible optical fiber

Conventional indoor optical cable

Conventional indoor optical cable(a) Structure(b) Appearance

Fig. 1. Structure and appearance of new optical fiber and conventional optical cable.

Magnified photographFig. 2. Example of wiring in restricted space.

3.2 C onnection techniques for invisible opticalfiber
We developed techniques to connect the invisible
optical fiber using two different materials:(1) Fiber holder
We developed a 0.9 fiber holder to attach the field
assembly connector to the invisible optical fiber. The
cross-section size of the invisible optical fiber and theconventional indoor optical cable are 0.9 mm indiameter and 2.0 × 1.6 mm, respectively. Using the
0.9 fiber holder enables the invisible optical fiber tobe the same cross-section size as the conventional
indoor optical cable. The connector assembly procedure for an invisible optical fiber and the 0.9 fiber
holder is shown in Fig. 3.
(2) Solid refractive index matching material
We used single-mode HAF based on ITU-T* G.657
B3 for the invisible optical fiber. A liquid refractive
Vol. 14 No. 1 Jan. 2016index matching material is used to ensure that the
splicing characteristics in the field assembly connectors are sufficient. When we splice the HAF using theliquid refractive index matching material, the spliceloss characteristics are degraded. This is because thematching material penetrates the air holes in the HAF,
and a void is formed between the fiber end faces. This
problem can be solved by using a solid refractive
index matching material. Thus, we used the solid
refractive index matching material with the field
assembly connector for invisible optical fiber consisting of HAF. The HAF splice components with liquidand solid refractive index matching materials areillustrated in Fig. 4.
3.3 Wiring technique
The wiring route was in a corner of the wall. Cable
wiring is inconspicuous when installed in the corner(Fig. 5). To further improve the aesthetics of theroom, we used transparent fixing components. The
fixing components are available as a pin-point type orprotection type (Fig. 6), and both of these types arealso sorted into two styles. The pin-point type issorted into the simple and multi-types. The top priority with the simple type is to achieve a small size,
which makes it inconspicuous. This component isfixed in place with double-sided tape. The multi-type
can be fixed to different kinds of walls with doublesided tape, plaster pins, or screws. We also developed
two styles for the protection type fixing components,
as shown in the figure. As mentioned previously,
invisible optical fiber is inferior in impact resistancebecause it has a smaller diameter than that of theindoor optical fiber currently used. Consequently, we*

ITU-T: International Telecommunication Union, Telecommunication Standardization Sector

Regular Articles

Fixed cable sheath part

Field assembly connector

Invisible optical fiber

0.9 fiber holder

Fig. 3. Connector assembly procedure with invisible optical fiber and 0.9 fiber holder.

Solid refractive index matching material

Liquid refractive index matching material
Hole
Core
Hole

Fig. 4. HAF splice components with liquid and solid refractive index matching materials.

(a-1) Simple type(b-1) Single type

Lid

Fig. 5. Image of in-house wiring.
Basedeveloped a single-type fixing component of the protection type to be used in areas where it is likely to be
touched by customers. The protection-type of fixing
component is also available in a multiple type, providing greater flexibility.

(a-2) Multi-type(b-2) Multiple type(a) Pin-point type(b) Protection typeFig. 6. Fixing components.

4. Conclusion
We introduced in this article our newly developed
invisible optical fiber. We will continue to promotethe introduction of optical fiber in cooperation withbusiness companies and to support the business companies in deploying the fiber.

NTT Technical Review

Regular Articles

Keisuke Yoneda

Engineer, NTT Access Network Service Systems Laboratories.
He joined NTT in 2008 and moved to his current position in 2013. Since then, he has been
actively engaged in developmental research on
optical connector technologies.

Jun Onishi

Hironari Iino

Research Engineer, NTT Access Network Service Systems Laboratories.
He joined NTT in 1996 and moved to his current position in 2012. Since then, he has been
engaged in promoting optical wiring technologies for multiple dwelling units.

Yoshitaka Enomoto

Engineer, NTT Access Network Service Systems Laboratories.
He joined NTT in 2011 and moved to his current position in 2014. He has been engaged indeveloping optical wiring technologies for user
premises.

Senior Research Engineer, NTT Access Network Service Systems Laboratories.
He joined NTT in 1992. Since then, he has
been researching and developing an optical fiberline testing system and an optical fiber distribution system for central offices.

Tetsuya Iwadou

Katsushi Nakayachi

Engineer, NTT Access Network Service Systems Laboratories.
He joined NTT in 2012. He has recently been
involved in developing the optical fiber line system.

Vol. 14 No. 1 Jan. 2016

Senior Research Engineer, NTT Access Network Service Systems Laboratories.
He joined NTT in 1983 and moved to his current position in 2013. Since then, he has been
engaged in the development of optical wiring
technologies for user premises.

Regular Articles

Utilization of APIs for B2B2X
Business Model
Mayumi Takahashi
Abstract

The NetroSphere concept proposed by NTT laboratories aims to divide network functions into small
modular components and to flexibly assemble those components. This makes it possible to quickly form
proportional networks that can offer required functionality according to user needs. As a result, carrier
networks will achieve enhanced flexibility and elasticity while also drastically reducing costs. This
article introduces the architecture to provide network functions and resources via APIs (application
programming interfaces) to collaboration partners such as service developers.
Keywords: API, collaboration, NetroSphere

1. Introduction
The NetroSphere concept is a future network vision
proposed by NTT laboratories [1]. The concept
enables network functions to be divided into small
modular components, which can then be flexibly
assembled. This makes it possible to quickly form
proportional networks that can offer required functionality in accordance with the user’s needs for
B2B2X (business-to-business-to-X) business models
where various service partners and communication
providers collaborate to provide services. This will
increase the flexibility and elasticity of carrier networks while also drastically reducing costs.
Web-API*1 (API) is considered to be one of the key
technologies to achieve the NetroSphere concept. In
this article, the architecture that provides network
functions and resources via APIs and design rules for
the APIs are explained.
2. Trends in usage of APIs for collaboration
2.1 T rends of service providers and overseas
telecom operators
Well-known service providers such as Twitter,
Facebook, and Amazon Web Service expose APIs in
order to provide their own functions and information.
Some large overseas telecom operators have alsostarted providing network functions such as call control and payment services and have organized promotional programs such as ideathons and hackathons tobring business ideas into shape [2, 3].
Our view is that providing network functions via
APIs will bring two advantages to these service providers and telecom operators:
1)	Growth in revenue: When they provide functions and resources via APIs, collaboration
partners can create new services with thosefunctions and resources. Service providers andtelecom operators can earn revenue by charging
for the use of functions and resources that areprovided via APIs.
2)	Reduction in operating expenses (OPEX): Providing operational functions via APIs simplifies
operational cooperation with collaborationcompanies. It permits business process automation and reduces OPEX.
2.2 Trends in standardization
Mobile telecom operators provide network functions via APIs to their collaboration partners. The*1	 Web API: An API (application programming interface) is an interface to exchange information between software components. Web
API is one kind of API. Via an API, an application can call externalfunctions provided by other software components or applications.

NTT Technical Review

Regular Articles

Customers

Service developer / Service provider
APL

APL

APL

APL

Network (Internet)
API
API-GWAPI
API orchestrator
API
Mediator
API

UniqueIF

EnablerAPI

Enabler

Enabler

APL: application
IF: interface

GW: gateway

Fig. 1. API-providing architecture.

specifications of these APIs differ from operator tooperator. This forces collaboration partners to write
operator-specific programs. OneAPI is a set of APIs
supported by the GSM Association (GSMA)*2 thatdefines API specifications for elements such as SMS(Short Messaging Service)/MMS (Multimedia Messaging Service), device capabilities, and payments[4]. AT&T provides some APIs according to these
specifications.
The TeleManagement Forum (TM Forum)*3 has
started to define API specifications for the interface
between these business processes in order to facilitate
the automation of business processes of telecom
operators. TM Forum published some operational
specifications such as those for trouble tickets andproduct ordering. Further, in the Open Digital Project, one of TM Forum’s strategic programs, discussions have been initiated on the reference architecture
called the Digital Service Reference Architecture(DSRA), which allows collaboration partners to create applications with a number of digital services
provided via APIs [5].
As previously mentioned, some standardization
Vol. 14 No. 1 Jan. 2016

organizations have defined the specifications of APIs,
but their scope is limited. Thus, most operators provide APIs designed according to their own rules.
3. Technical considerations in providing
network functions via APIs
We consider that APIs represent one of the key
technologies to achieve the NetroSphere concept andthat they will be effective in providing the architecture for NetroSphere.
3.1 API-providing architecture
The architecture being studied to provide network
functions via APIs is shown in Fig. 1. This architecture consists of three functional units: the mediator*2 GSMA: GSMA is an association of mobile operators and relatedcompanies devoted to supporting the standardization, deployment, and promotion of the GSM mobile telephone system.
*3 TM Forum: TM Forum is a non-profit industry association forservice providers and their suppliers in the telecommunications
and entertainment industries. TM Forum defines reference models of interfaces between business processes and data models.

Regular Articles

API-GW
Interface transform
1-5
Error info.
transform

1-1
Formattransform

1-2
1-6
HeaderStatus codetransform
transform
1-3
Parametertransform

1-7
Masking
1-8
Routing

1-10

Protocoltransform(SOAP REST)

1-11
Cache

1-9

1-12

Abstraction

Mash-up

Traffic
control

Authentication/
Authorization
2-1

3-1
Flow control(Inbound)App auth.
2-2

3-2
Priority
control

App authz.

3-3Flow control(Outbound)

1-4
Methodtransform

System operation/
maintenance
4-1
4-5
API mgmt.

App mgmt.

4-2
Backendmgmt.

4-6
Backendmonitoring

4-3
Traffic log

4-7

1-13
Complexmash-up

5-1
Developer
mgmt.

6-1
Threatprotection

5-2
Developer
support

Encryption

6-2

4-4
Portal forbackend

System operation/
maintenance

1-14
Scenario
control

Security

CDR

API orchestrator
1-1~ 1-10

FW/WAF
Developer
portal

Our requirement
Requirement mentioned
in API broker

Mediator
1-1
~ 1-9

1-15
Protocol transform(except SOAP REST)

2-3

2-4

User auth.

User authz.

System operation/
maintenanceBackendValue
addfunction

2-3
~ 2-4

App: application
CDR: clock and data recovery
FW: firewall
SOAP: Simple Object Access Protocol
WAF: web application firewall

Fig. 2. Function deployment.

unit for protocol transformation, the API orchestrator
unit for scenario control and API mash-up, and the
API-GW (gateway) unit for authorization and traffic
control. The details of these functions are describedhere and shown in Fig. 2.
(1) Mediator unit
Many existing operational systems have a uniqueinterface to enable interconnection with other such
systems. Our view is that it would be advantageous
for network functions to be provided via representational state transfer (REST) APIs because the REST
API style is familiar to service developers. Thus, we
introduced the mediator unit to transform the uniqueinterface into the REST API style.
(2) API orchestrator unit
Telecom operators do not want to directly expose
their own backend systems such as their customer
management and billing systems from the viewpointof protecting the security of their systems and avoiding unnecessary complexity. The API orchestrator
unit provides API abstraction and scenario control
functions, which meet telecom operator requirements.
(3) API-GW unit
Service developers ordinarily have to call each endpoint that differs from system to system when they

use network functions provided by different systems.
Moreover, each system may provide different API
specifications such as those for the data format. This
is not helpful to service providers. Thus, we introduced the API-GW unit to centralize end-points andabsorb differences in API specifications. In addition,
the API-GW provides a common function to expose
APIs such as those for traffic control and authorization. The API-GW unit is divided into three functional units that have specific functions.
1)	API broker: API broker provides a security protection function, authentication and authorization function (API-key, OAuth), and a traffic
control function in accordance with the terms ofagreement and system state, and an interface
transformation function in accordance withdesign rules described in section 3.2.
2)	Developer portal: The developer portal provides service developers with a management
function and documents such as manuals andsample code.
3)	System operation/maintenance: The system
operation/maintenance function unit provides
operational functions for the API-GW operator
to manage enablers, APIs, and applications
developed by service developers and also to
NTT Technical Review

Regular Articles

manage the state of the API-GW.
3.2 API design rules
The design rules of REST APIs such as the API
naming rule and data format description rule are flexible. Thus, specifications of APIs provided by telecom operators may differ from operator to operator.
In addition, APIs may differ from vendor to vendor
even if the functions are the same because these APIs
are designed according to vendor-specific design
rules. This may cause difficulties when telecom
operators renew their equipment.
To simplify things for service developers, we
believe that telecom operators should provide APIs
designed according to a unified design rule based on
one of the two rules below.
1)	Function-independent rules: These rules define
common rules that apply regardless of the typeof functions; they include naming rules of
URLs, rules for determining data format, andauthentication methods of APIs.
2)	Function-dependent rules: These rules definefunctions themselves such as those for call control, service orders, and trouble tickets.
Our first step will be to focus on function-independent rules.

lowed by overseas telecom operators. An architecturethat provides network functions via APIs was also
discussed along with API design rules. Our aim is to
standardize the proposed architecture and describe
the API design rules at a TM Forum event. We willdefine the aforementioned architecture that allows
service developers to create applications with multiple functions provided by telecom operators.
In parallel, we will start providing an API testbed
called Atelier N [6], which is positioned as a collaborative program. We would like to use this program to
discuss issues related to creating services and providing functions with our collaboration partners such asservice developers and function providers. We alsoplan to evaluate the proposed architecture and design
rules with our collaboration partners.
References
[1]

[2]
[3]
[4]
[5]
[6]

R. Sumi, K. Matsumoto, I. Inoue, and Y. Kajiyama, “The NetroSphere
Concept and Network Architecture,” NTT Technical Review, Vol. 13,
No. 10, 2015.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2015
10fa1.html
AT&T, Developer Program, http://developer.att.com/
Website of Orange Partner, https://www.orangepartner.com/
GSMA, API Exchange, http://www.gsma.com/oneapi/
Website of TM Forum, https://www.tmforum.org/
Website of Atelier N, https://developer.api-trial.jp/en/index.html

4. Future prospects
This article reviewed the API-related trends fol-Mayumi Takahashi

Senior Research Engineer, Service Platform
Innovation & Development Project, Network
Service Innovation Project, NTT Network Service Systems Laboratories.
She received a B.Ed. and M.E. in mechanicalengineering from Saitama University in 1991 and1993. She joined NTT in 1993 and began
researching electro-magnetic compatibility forchassis of switching equipment and cables. She
is currently conducting research on an API-providing architecture for the B2B2X businessmodel.

Vol. 14 No. 1 Jan. 2016



Global Standardization Activities

Latest TM Forum Developments
Shingo Horiuchi
Abstract

As an established thought leader in telecom operations related standards, the TeleManagement (TM)
Forum is making good progress on three strategic programs in addition to its ongoing work in developing
operation systems. This article provides an overview of recent TM Forum initiatives concerning service
interaction between telecom operations and over-the-top content toward virtualization, the proof of concept Catalyst project, and overall TM Forum related developments.
Keywords: TM Forum, Frameworx, Catalyst project

1. Introduction
The TeleManagement (TM) Forum [1] was originally established in 1988 as the OSI (Open Source
Initiative)/Network Management Forum and today
has more than 950 corporate members and well over
90,000 technicians and engineers working to finetune telecom operations-related industry standards topromote improved interoperability. TM Forum focuses primarily on standards related to the developmentof operation systems (business processes, information models, applications). It produces Frameworx,
an updated suite of best practices and business metrics that define key performance indicators and key
quality indicators for frameworks and operations forautomatically generating interfaces. Frameworx is
released biannually; the current version is Frameworx
15.0. As of this writing, the upgrade to Frameworx
15.5 was scheduled for December 2015.
Investigative work in preparation for a new releaseof Frameworx begins with face-to-face meetings ofworking-level drafters during TM Forum Action
Week, which is held twice a year. These meetings arefollowed up with regular teleconferences and collaborative liaisons to complete the drafting work.
Frameworx specifications are deliberated on by themembers and then organized as a deliverable document divided into units called projects. Issues havetraditionally taken the form of projects as the basic
units making up the Frameworx standard. Over thelast few years, project discussions related to strategic
programs have become prominent (as discussed ingreater detail below), and the content of the various
projects is discussed in parallel and reflected in themain body of Frameworx.
2. Telecom operations standardization
Standardization of telecom operations involves
careful consideration of how equipment under management is linked with the business processes of theoperators. The equipment to be managed goes beyond
conventional equipment and systems that comply
with ITU-T* recommendations. The equipment dealt
with in recent years involves virtual technology, specifically, equipment for network function virtualization (NFV) specified by the European Telecommunications Standards Institute (ETSI) and network function virtualization/software-defined networking(NFV/SDN) dealt with by the Open Network Foundation (ONF). The standardization process involves
building liaison relationships with ETSI, ONF, andother organizations. We should also note that whereas
ETSI and ONF are primarily focused on management
at the network control level, TM Forum also studies
operations as a business process and is not justfocused on control alone.

*

ITU-T: International Telecommunication Union, Telecommunication Standardization Sector

NTT Technical Review

Global Standardization Activities

NGOSS(Four views)

Frameworx

Interfaces between
operations systems

BMS

MTOSI
(Shared interface study)
Fault
management

Integrates existing
interfaces

Servicemanagement

SMS

NMS

EMS
MTNM
(TMN model)

EMS

APIs

TIP

mTOP

Strategic programs• Agile & Virtualized• Open & Partner Effectively• Customer Centric

EMS

Changed conception ofoperations systemarchitecture

API: application programming interface
BMS: business management system
EMS: element management system
NGOSS: New Generation Operations Systems and Software
NMS: network management system
SMS: service management system
TMN: Telecommunications Management Network

Fig. 1. Transformation in concept of interfaces.

3. Frameworx and three strategic programs
3.1 Work on Frameworx
TM Forum has been working on the Frameworx
suite for some time and has recently placed particularemphasis on the following elements.
(1) Business Process Framework (eTOM)
The enhanced Telecom Operation Map (eTOM) isa business process organized based on phases andShared Information/Data Model (SID) domains.
(2) Information Framework (SID)
SID is organized not as a concrete data model of
how information is managed and distributed but
rather as an information model.
(3) Application Framework (TAM)
TAM (Telecom Application Map) is a standard
framework for organizing and mapping operation
support system (OSS) capabilities using relationships
with eTOM and SID.
Business processes involve more than operational
processes such as fulfillment, assurance, and billing.
They also focus attention on strategy (decision-making), readiness (migration, procurement), and otherfactors.
Vol. 14 No. 1 Jan. 2016

In terms of interface trends, we have already evaluated MTNM (Multi Technology Network Management) and MTOSI (Multi-Technology Operations
System Interface) as part of mTOP (multi Technology
Operations Program), and we are making headway inintegrating interfaces with other conventional interfaces through TIP (TM Forum Integration Program).
This reflects the changes in operations system architectures and the transformation in the concept ofinterfaces (Fig. 1). More specifically, we are now
pushing beyond modeling of network equipment
capabilities for each protocol by ITU-T and othertraditional organizations by integrating wireless andwireline network equipment models. As conventional
information models expand, there is growing momentum to develop models that incorporate virtual networks as well.
3.2 Strategic programs
The policy to expand the Frameworx suite waslaunched two years ago with the decision to pursue
three strategic programs. Efforts were focused on
dealing with the introduction of virtualization andbusiness-to-business-to-X (B2B2X) service. Morespecifically, efforts were focused on the followingGlobal Standardization Activities

Strategic
programs

Customer CentricOpen & Partner Effectively

Agile & Virtualized(Scope)• QoE management (Customer
experience management)(Scope)• Service interaction (DSRA)• B2B2X(Scope)• Virtualization (ZOOM)• APIsFindings
reflected in...

OSS building platform

I

ss

Information Framework(SID)
Information framework required in
the business processkIntegration Framework
at
plic
Ap

ion FrameworService group framework summarizing
flow of business processes and information

Integration
Frameworkwrame ork

Business Proc enF

Application
Framework
(TAM)

F

ma
t

r am

io

Business process framework

usiness Arc h
ed B
ite
rat
c tu
g
e
I
k
n
r
nt
o
f or
ew

re

Business Process
Framework(eTOM)

Integration framework to integrateother frameworks and implement
services

DSRA: Digital Service Reference Architecture
QoE: quality of experience
ZOOM: Zero-touch Orchestration, Operations and Management

Fig. 2. Three strategic programs.

three strategic programs (Fig. 2), which are introduced
here and described in more detail in the following
sections.
(1) Agile & Virtualized programThis program is centered on virtual operations andsecurity.
(2) Open & Partner Effectively programThis program is focused on scenarios through service interaction, for example, architecture and Internet of Things (IoT) to achieve an open digital ecosystem.
(3) Customer Centric program
The emphasis of this program is big-data analysis
to evaluate customer experience management andapplicable metrics based on criteria for assessing
customer experience.
4. Open & Partner Effectively programand DSRAThe Open and Partner Effectively program focuseson service interaction among telecom operators andschemes for quickly creating new value. More specifically, the program explores scenarios based on
B2B2X business best practices, a service interaction
architecture (i.e., Digital Service Reference Architecture (DSRA)) (Fig. 3), and concrete representationalstate transfer-based application programming interfaces (APIs) for promoting service interaction. The
DSRA is essentially a scheme in which digital services—private cloud, public cloud, NaaS (network as
a service)—provided by the service domain can offer
interactive services using an API broker based on aservice API catalog.
5. Agile & Virtualized program and ZOOMThe Agile & Virtualized program is committed to
the Zero-touch Orchestration, Operations and Management (ZOOM) project for exploiting NFV and
SDN in the operation of virtual networks. Primaryobjectives of the ZOOM project are to curtail capitaland operating expenditures through virtualization,
while creating service opportunities through rapid
deployment of new services. Not only has this promoted discussion of the OSS architecture and an
information model under management, it has also led
to the definition of a framework supporting procurement of VNF (Virtual Network Function). One challenge faced by the OSS architecture is that the sharp
demarcation between the network control layer andthe operation support layer that has existed until now
becomes rather blurry when dealing with virtual technology.
NTT Technical Review

Global Standardization Activities

Channels / Marketplace / Store
Download /Utilization

Development

Service API catalog

Consumption

Application catalog

Service composition/orchestration management

Servicedeployment

Service domain(Public cloud)Service domain(IaaS / PaaS)

Service domain(Private cloud)Exposure
On-boarding

Catalog lifecycle
MGMT & federation

InvoicingCharging

Assurance& traceability

Configuration& activation

Policy based
routing

Analytics

Profilemanagement

Identity
management

API broker/platform (Functional and management API)

Service domain(CaaS / NaaS)

Implementation

DSRA
CaaS: cloud as a service
IaaS: infrastructure as a service
MGMT: management
PaaS: platform as a service

Fig. 3. Architecture for service interaction.

6. Customer Centric program
The Customer Centric program deals with a framework for improving customer satisfaction that can be
adopted as a quality metric to assess customer enjoyment and experience using telecom services. This
involves defining the lifecycle of customers, deriving
a customer access channel metric for each phase of
the lifecycles, then developing a big data analysis
based reference architecture for calculating the metric.
7. Catalyst project
The goal of TM Forum’s Catalyst project is to verify the documented effectiveness of the Frameworx
suite as a standard and then continue to enhance andrefine the standard document. In the Catalyst project,dynamics using Frameworx standards prepared bymore than four software vendors are currently being
validated that will be applied to scenarios assembled
by more than two telecom operators. With this initiaVol. 14 No. 1 Jan. 2016

tive, new events including TM Forum Live! and Catalyst InFocus have largely superseded the biennial TM
Forum Management World that was held in earlier
years, and they provide representatives of corporate
TM Forum members an opportunity to provide feedback and offer comments that will be incorporated
into future releases of the Frameworx standard.
No less than 20 Catalysts were showcased at TM
Forum Live! 2015, which was held in Nice, France inthe first week of June. Note that half of these Catalysts involved virtualization, which clearly indicatesthe enormous interest among members in the operational aspects of virtualization.
8. Overall trends
Since it was first established, TM Forum has primarily been concerned with the development of telecom-oriented operations. Just a few years ago, most
discussions on the operation layer were centered on
network management, but today we find that upper
layer customer management and the businessGlobal Standardization Activities

management layer are also widely discussed. Indeed,
we find that even in lectures by TM Forum executives, the speakers tend to mention areas where sharing is achieved by standardizing telecom operator
network management and service management, andareas where differentiation is highlighted as a legitimate business strategy. Discussion of business metrics and other topics in Frameworx clearly reflects
these trends.
We have also recently begun to see more discussion
on operations geared toward IoT and 5G (fifth-generation mobile networks), which should open the way
to establishing cooperation with many OTT operators. We are thus making good headway in building
liaison relationships with other related standards
organizations. One initiative that clearly illustrates
these proactive collaborative efforts is the joint meetings with other standards-making bodies at TM
Forum Action Week, TM Forum Live!, and otherconferences organized by TM Forum.

9. Future prospects
The NTT Group plans to move forward with wholesale services based on virtual and B2B2X modelbased interactive services featuring open, competitive
architectures and interfaces. Upstream activities are
currently underway in the investigative projects of
TM Forum’s three strategic programs, with the ideathat these new insights will be reflected in future
Frameworx document releases.
The Forum also continues to be actively involved in
the proof of concept Catalyst project to verify theviability of operational scenarios by combining commercialized technologies. The findings obtained in
the Catalyst project were presented at Catalyst InFocus 2015 [2] held in Dallas, Texas in early November
2015.
References[1][2]

TM Forum site, http://www.tmforum.org/
TM Forum Catalyst InFocus Event site,
http://catalystinfocus.tmforum.org/

Shingo Horiuchi

Senior Research Engineer, Access Network
Operations Project, NTT Access Network Service Systems Laboratories.
He received a B.E. and M.E. in engineeringfrom the University of Tokyo in 1999 and 2001.
He joined NTT Access Network Service Systems
Laboratories in 2001. He has been researchingand developing access network operation systems. He has been involved in the standardization
work for operation support systems in TM Forumas a member of the ZOOM project since 2014.
He is a member of the Institute of Electronics,
Information and Communication Engineers.

NTT Technical Review

External Awards
Photonics in Switching 2015 Best Paper Award
Winner: Salah Ibrahim, Tatsushi Nakahara, Hiroshi Ishikawa, and
Ryo Takahashi, NTT Device Technology Laboratories
Date: September 24, 2015
Organization: Photonics in Switching 2015 Committee
For “An Ultralow-power Optical Label Processor for 100-Gbps
Optical Packet Switching.”
A novel label processor subsystem for 100-Gbps (25-Gbps × 4
Lambdas) burst-mode optical packets has been developed with asignificant reduction of power consumption and latency, based on anenhanced optoelectronic serial-to-parallel converter and a novel optical-trigger-pulse generator that consists of a semiconductor opticalamplifier and a single-chip optoelectronic integrated circuit highspeed current driver.
Published as: S. Ibrahim, T. Nakahara, H. Ishikawa, and R. Takahashi, “An Ultralow-power Optical Label Processor for 100-Gbps
Optical Packet Switching,” Proc. of Photonics in Switching 2015,
ThII2-4, Florence, Italy, Sept. 2015.
APCC2015 Best Paper Award
Winner: Kazuhisa Yamada, NTT Network Innovation Laboratories;
Akihiro Nakao, the University of Tokyo; Yasushi Kanada, Hitachi,
Ltd.; Yoshinori Saida, NEC Corporation; and Koichiro Amamiya,
Fujitsu Laboratories Ltd.
Date: October 15, 2015
Organization: APCC2015 (21st Asia-Pacific Conference on Communications) Award Committee
For “VNode Infrastructure Enhancement - Deeply Programmable
Network Virtualization.”
Published as: K. Yamada, A. Nakao, Y. Kanada, Y. Saida, and K.
Amamiya, “VNode Infrastructure Enhancement - Deeply Programmable Network Virtualization,” Proc. of APCC2015, Kyoto, Japan,
Oct. 2015.
WCSP 2015 Best Paper Award
Winner: Keisuke Sato, Yuichi Kawamoto, Hiroki Nishiyama, Nei
Kato, Graduate School of Information Sciences, Tohoku University;
and Yoshitaka Shimizu, NTT Network Innovation Laboratories
Date: October 16, 2015



Organization: WCSP 2015 (7th International Conference on Wireless Communications and Signal Processing)For “A Modeling Technique Utilizing Feedback Control Theory
for Performance Evaluation of IoT System in Real-time.”
In this paper, we focus on the modeling aspect for evaluating theperformance of a real-time Internet of Things (IoT) system. Specifically, our proposed model is constructed by utilizing feedback control theory.
Published as: K. Sato, Y. Kawamoto, H. Nishiyama, N. Kato, and Y.
Shimizu, “A Modeling Technique Utilizing Feedback Control Theory
for Performance Evaluation of IoT System in Real-time,” Proc. of
WCSP 2015, Nanjing, China, Oct. 2015.
IEEE Photonics Society Japan Chapter Young Scientist
Award
Winner: Satomi Katayose, NTT Device Technology Laboratories
Date: October 28, 2015
Organization: IEEE Photonics Society Japan Chapter
For “1×8 Silicon-silica Hybrid Thermo-optic Switch with Multichip Configuration Based on Optical Phased Array.”Published as: S. Katayose, Y. Hashizume, and M. Itoh, “1×8 Siliconsilica Hybrid Thermo-optic Switch with Multi-chip Configuration
Based on Optical Phased Array,” Proc. of MOC’15 (the 20th
Microoptics Conference), Fukuoka, Japan, Oct. 2015.
ISPACS 2015 Best Paper Award
Winner: Takayuki Nakachi and Tatsuya Fujii, NTT Network Innovation Laboratories
Date: November 11, 2015
Organization: ISPACS 2015 (The 2015 International Symposium onIntelligent Signal Processing and Communication Systems) Technical Program Committee
For “Layered Lossless Video Coding Based on Slepian-Wolf
Theorem.”Published as: T. Nakachi and T. Fujii, “Layered Lossless Video Coding Based on Slepian-Wolf Theorem,” Proc. of ISPACS 2015, pp.
652–657, Bali, Indonesia, Nov. 2015.

NTT Technical Review

Papers Published in Technical Journals andConference Proceedings
Verifying Power Distribution Network with ZDDs
T. Inoue, N. Yasuda, S. Kawano, Y. Takenobu, S. Minato, and Y.
HayashiProc. of ISMP 2015 (the 22nd International Symposium on Mathematical Programming), Pittsburg, USA, July 2015.
Power distribution networks should be restored by reconfiguringswitches automatically, given that several feeders are interrupted in asevere accident. The network’s design has to guarantee that it isrestorable under any possible failure, but it is a computationally hardtask to examine all possible failures. This paper proposes a novel
ZDD (zero-suppressed binary decision diagram) method to find all
the critical (unrestorable) line cuts with great efficiency to verify the
network design. The method includes a fast screening algorithm
based on hitting set enumeration, which is often used in data-mining.
Thorough experiments reveal that the proposed method can findthousands of unrestorable cuts from the trillions of possible cuts in alarge 432-Bus network.

Stable Load Balancing with Overlapping ID-space Management in Range-based Structured Overlay Networks
K. Mizutani, T. Inoue, T. Mano, O. Akashi, S. Matsuura, and K.
Fujikawa
Computer Software, Vol. 32, No. 3, pp. 101–110, August 2015.
This paper proposes a novel scheme that distributes, fairly, theloads without node migration and with little data reallocation, by
sharing some identification (ID)-space regions between neighboring
nodes. Our overlapping ID-space management scheme derives the
optimal overlap based on kernel density estimations; the query loads
based on the statistical theory are used to calculate the best overlap
regions. This calculation is executed in a distributed manner with nocentral coordinator. We conduct thorough computer simulations, andshow that our scheme alleviates the worst node load by 20–90%
against existing techniques without node migration and with the least
data reallocation.

Viewpoint Image Generation for Head Tracking 3D Display
Using Multi-camera and Approximate Depth Information
M. Date, H. Takada, and A. Kojima
Journal of SID, Vol. 23, No. 8, pp. 340–346, August 2015.
A simple and high image quality method for viewpoint image
synthesis from multi-camera images for a stereoscopic 3D display
using head tracking is proposed. In this method, slices of images for
depth layers are made using approximate depth information, theslices are linearly blended corresponding to the distance between the
viewpoint and cameras at each layer, and the layers are overlaid fromthe perspective of viewpoint. Because the linear blending automatically compensates for depth error because of the visual effects ofdepth-fused 3D (DFD), the resulting image is natural to the observer’s perception. Smooth motion parallax of wide depth range objects
induced by viewpoint movement for left-and-right and front-andback directions is achieved using multi-camera images and approximate depth information. Because the calculation algorithm is very
simple, it is suitable for real time 3D display applications.

Vol. 14 No. 1 Jan. 2016

Novel Applications of and Experiments on Programmable
Infrastructures
Y. Minami and K. Yamada
Proc. of MASONS (IEEE International Workshop on Manageability and Security of Network Function Virtualization and Software
Defined Network) 2015, Las Vegas, USA, August 2015.
We show novel applications as use cases and experiment results on
programmable infrastructures. We realized multicast streaming andadaptive bit rate streaming by automatically deploying video processing functions in a slice. Experimental results showed that we canenhance service functionalities and use resources efficiently with
programmable infrastructures.

Low Power Driving Techniques for 1-pixel Displays
H. Manabe, M. Date, H. Takada, and H. Inamura
Proc. of IAS 2015 (the 50th IEEE Industry Applications Society
Annual Meeting), 2015-ILDC-0339, Dallas, USA, October 2015.
Two techniques to reduce the power needed for driving 1-pixel
LCDs (liquid crystal displays) are proposed. The first one is the useof multiple capacitors, and the second is dividing the LCD into two
to lower the drive voltage. Simulations show that large capacitanceand many capacitors can reduce the power, and stacking two thin
LCDs driven by lower voltage maximizes the effect of the proposed
technique, especially for small LCDs. An experiment on the former
technique using actual PDLCDs (polymer dispersed liquid crystal
displays) confirms that it works as effectively as the simulation
implies. The overall energy consumption of large PDLCDs is reduced
more than 70%.

Development of a Handset for NGN Wideband Telephone
Service Trial
M. Okamoto, K. Noguchi, Y. Hiwazaki, and Y. Haneda
The Journal of the Acoustical Society of Japan, Vol. 71, No. 11, pp.
581–589, November 2015 (in Japanese).
Since 2008, a high-quality IP telephone service has been provided
over a Next Generation Network (NGN) developed by Nippon Telegraph and Telephone Corporation (NTT) Group of Japan. Prior to thecommencement of service provision, a wideband telephone handset
was developed on an experimental basis and tested in a field trial toverify service validity. The handset demonstrated improved characteristics over those of conventional telephone handsets.

Low-power Driving Technique for 1-pixel Display Using anExternal Capacitor
H. Manabe, M. Date, H. Takada, and H. Inamura
IEICE Transactions on Electonics, Vol. E98-C, No. 11, pp. 1015–
1022, November 2015.
Liquid crystal displays (LCDs) are suitable as elements underlying
wearable and ubiquitous computing thanks to their low power consumption. A technique that uses less power to drive 1-pixel LCDs is
proposed. It harvests the charges on the LCD and stores them in anPapers Published in Technical Journals and Conference Proceedings

external capacitor for reuse when the polarity changes. A simulationshows that the charge reduction depends on the ratio of the capacitance of the external capacitor to that of the LCD and can reach 50%.
An experiment on a prototype demonstrated an almost 30% reduction
with large 1-pixel LCDs. With a small 10 x 10 mm2 LCD, the overhead of the micro-controller matched the reduction, so no improvement could be measured. Though the technique requires longer time
for polarity reversal, we confirm that it does not significantly degrade
visual quality.

Network Clock System that Ensures a High Level of Frequency Accuracy
S. Fujikawa
IEICE Transactions on Communications, Vol. E98-B, No. 11, pp.
2212–2226, November 2015.
This paper proposes a network clock system that detects degradation in the frequency accuracy of network clocks distributed across a
network and finds the sources of the degradation. This system uses
two factors to identify degradation in frequency accuracy and analgorithm that finds degradation sources by integrating and analyzing
the evaluation results gathered from the entire network.

Differential Reliability Path Accommodation Design andReconfiguration in Virtualized Multi-layer Transport Network
A. Kadohata, T. Tanaka, A. Watanabe, A. Hirano, H. Hasegawa,
and K. Sato
IEICE Transactions on Communications, Vol. E98-B, No. 11, pp.
2151–2159, November 2015.
In this paper, we propose differentiated reconfiguration to address
the trade-off relationship between accommodation efficiency anddisruption risk in virtualized multi-layer transport networks. The
reconfiguration considers service classes defined as a combination ofincluding or excluding a secondary path and allowing or not allowing
reconfiguration. To implement the proposed network, we propose amulti-layer redundant path accommodation design and reconfiguration algorithm. A reliability evaluation algorithm is also introduced.
Numerical evaluations show that when all classes are divided equally,
the equipment cost can be reduced by approximately 6%. The proposed reconfigurable networks are shown to be a cost-effective solution that maintains reliability.

Real-time Robust Formant Estimation System Using a
Phase Equalization-based Autoregressive Exogenous
Model
H. Oohashi, S. Hiroya, and T. Mochida
Acoustical Science and Technology, Vol. 36, No. 6, pp. 478–488,
November 2015.
This paper presents a real-time robust formant tracking system forspeech using a real-time phase equalization-based autoregressive
exogenous model (PEAR) with electroglottography (EGG). Althoughlinear predictive coding (LPC) analysis is a popular method for estimating formant frequencies, it is known that the estimation accuracy
for speech with high fundamental frequency F0 would be degradedsince the harmonic structure of the glottal source spectrum deviates
more from the Gaussian noise assumption in LPC as its F0 increases.
In contrast, PEAR, which employs phase equalization and LPC with
an impulse train as the glottal source signals, estimates formant fre-

quencies robustly even for speech with high F0. However, PEAR
requires higher computational complexity than LPC. In this study, toreduce this computational complexity, a novel formulation of PEAR
was derived, which enabled us to implement PEAR for a real-timerobust formant tracking system. In addition, since PEAR requires
timings of glottal closures, a stable detection method using EGG wasdevised. We developed the real-time system on a digital signal processor and showed that for both the synthesized and natural vowels,
the proposed method can estimate formant frequencies more robustly
than LPC against a wider range of F0.

Mode-selective Coherent Detection Technique for Lowcomplexity Mode Division Multiplexing Systems
F. Hamaoka, S. Okamoto, K. Horikoshi, K. Yonenaga, A. Hirano,
and Y. Miyamoto
Electronics Letters, Vol. 51, No. 23, pp. 1899–1900, November
2015.
The mode division multiplexing (MDM) signal transmitted in an
optical multimode fibre link is usually received with multiple coherent receivers (RXs), resulting in an increase in RX system scale. A
mode-selective coherent detection technique is proposed based on
MDM-to-frequency division multiplexing (FDM) signal conversion
scheme using an MDM and FDM local oscillator. The proposed
technique uses only a single coherent RX to detect the MDM signal
to achieve low-complexity MDM systems. The proposed modeselective coherent detection technique is also experimentally shown.
The experimental results show that a single coherent RX successfullyreceives the MDM signal.

Impacts of Burst Disturbance on Throughput and Connection Quality of ADSL System
K. Ono, K. Okamoto, H. Tatemichi, and K. Takaya
Proc. of KJJC-2015 (2015 Korea-Japan EMT/EMC/BE Joint Conference), pp. 227–230, November 2015.
The relationship between duration of burst disturbances and thethroughput/connection quality of digital communication was investigated in order to evaluate the impact of burst disturbances. In thispaper, the impacts of the duration of burst disturbances with fixed
frequency on Receive Bit Rate (RBR) and applied levels of burst
disturbances causing Asymmetric Digital Subscriber Line (ADSL)
reconnection were evaluated experimentally. As a result, it has beenclarified that the RBR of an ADSL changes in accordance with theincreased number of disturbed ADSL frames in response to theincreased duration of burst disturbance, and ADSL reconnection
tends to occur even if the applied level of burst disturbance is small
because of the increased number of disturbed ADSL frames in
response to the increased duration of burst disturbance.

Optically Observed Imbibition and Drainage of Wetting
Fluid in Nanoporous Vycor Glass
S. Ogawa and J. Nakamura
Journal of the Optical Society of America A, Vol. 32, No. 12, pp.
2397–2406, December 2015.
The light scattering and absorption of nanoporous Vycor glass during imbibition and drainage of water exhibit the hysteresis of thewhite turbidity in the visible region and that of the absorbance peakin the near-infrared region. We analyzed the effect of changing
humidity on each of the hystereses. The effect of changing humidity

NTT Technical Review

Papers Published in Technical Journals and Conference Proceedings

up to various values of maximum attained humidity on both hystereses showed that the amount of water in pores determines unsaturated
optical hystereses in both regions.

Vol. 14 No. 1 Jan. 2016