NTT Technical Review

August 2022 Vol. 20 No. 8
View from the TopAkira Shimada, President and Chief Executive Officer, NTT

Front-line ResearchersMitsuaki Akiyama, Senior Distinguished Researcher, NTT Social Informatics Laboratories

Rising Researchers


Hiroshi Yamazaki, Distinguished Researcher, NTT Device Technology Laboratories

Feature Articles: Device Technologies for Enabling the All-Photonics NetworkHigh-output Optical Transmitter and High-sensitivity Optical Receiver for 400-Gbit/s 40-km
Fiber-amplifier-less Transmission



C+L-band Colorless, Directionless, Contentionless Reconfigurable Optical Add/Drop
Multiplexing for High-capacity Network Flexibility



Development of Membrane Optical Modulators for IOWN



Observation of Exceptional Point Degeneracy with Current-injected Photonic Crystal Lasers

Feature Articles: Optical Device Technologies for Next-generation Computing Using Light

Optical Device Technology for Next-generation Computing Using Light

Broadband Continuous-wave Optical Quadrature Squeezer for Ultra-fast Optical Quantum
Computers



Optical Circuit Technologies for Next-generation Computing Using Light

Photonic Implementation of Reservoir Computing

Regular Articles
Business Design Support Technology to Promote Digital Transformation

Global Standardization Activities


Report on WTSA-20 (World Telecommunication Standardization Assembly)

Practical Field Information about Telecommunication Technologies


Development of the Hikari Denwa Fault-isolation Tool for Voice Over IP Services

External Awards/Papers Published in Technical Journals and Conference Proceedings

View from the Top

People Are the Starting Point for All
New Services and Products
Akira Shimada
President and Chief Executive Officer, NTT
Abstract

With people at its core, the NTT Group defines the capabilities it aims toachieve as service expertise, technical expertise, and intelligence. All
330,000 NTT Group employees—working in approximately 90 countries
and regions—are accelerating the group’s self-transformation into a new
NTT that is open, global, and innovative to be valuable to all stakeholders,
including customers, shareholders, local communities, and employees. We
asked Akira Shimada, president and chief executive officer of NTT, whotook up the post in June 2022, about the details of the guidelines that heannounced at his inaugural press conference and his thoughts on what itmeans to be a top executive.
Keywords: customer experience, employee experience, carbon neutrality

Accelerating our self-transformation anddelivering new experiences and value toour customers—Congratulations on your new position. You must be
busy every day with press conferences and interviews.
First, could you tell us about your vision for NTT?
As I mentioned at my inaugural press conference, I
will first work on creating better CX (customer experience) through better EX (employee experience) in
the current business environment. All 330,000
employees of the NTT Group worldwide are required
to focus on our customers and create new experiencesand value for them. Therefore, the top management
needs to create an environment in which all employees are motivated to meet this requirement.
Although our business started in the field of infrastructure, we are now expanding into a wide range ofbusinesses such as solutions and applications. For ourcustomers, being constantly connected via telecommunications is something they take for granted that
goes unnoticed, like the air we breathe. However, our
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Vol. 20 No. 8 Aug. 2022

employees understand the importance of their work,
introduce new technologies and mechanisms, andmaintain and inspect equipment and facilities daily toensure stable connections. I recognize that this steadyeffort is one of NTT’s strengths.
I believe that people are the starting point for all theservices and products that NTT creates. Services andproducts do not arise spontaneously; they are createdby employees in charge of research and development(R&D), maintenance and inspection, and customer
service at call centers and stores who strive to satisfy
customers through their originality and ingenuity
when carrying out their assigned missions. In otherwords, people are the source of our success.
—There are high expectations for the services andproducts that will be created by the increased satisfaction of your employees, right?
Naturally, we face competition. If, despite our best
efforts, we cannot satisfy our customers, other companies will replace us. To prevent this, we need toconstantly check whether our customers are satisfied
1View from the Top

with the services and products that we provide forthem. We must always understand the viewpoint ofour customers and work to improve what can be
improved—even if only slightly.
We must also impress customers with new services
and products, so we will attempt to provide services
that give them a glimpse of the future. It is importantto challenge the status quo by creating something
new to adapt to an ever-changing world.
To build a foundation to support such efforts andboost employee morale, I want to create a system thatrecognizes employees who work hard and acquire
new skills, improves the expertise of all employees,
and reforms working styles. As I mentioned at myinaugural press conference, due to the COVID-19
pandemic, many of our employees have experienced
remote working. Employee surveys have confirmed
some positive effects of remote working, including
reduced commuting burdens and improved satisfaction among employees raising children.
Remote working is allowed according to the desireof the individual employee because of the variable
nature of an employee’s job, experience and skills,
and living environment. However, much work is completed by a team rather than one person. I, therefore,
want our employees to work in a hybrid fashion combining remote and office-based working in accordance with each employee’s situation. That said, I
think it is essential for new employees to learn how to
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do their jobs, and for that reason, I hope that theycome to the office to receive guidance from theirsupervisors and peers in person and try to build relationships with colleagues.
Reducing electricity usage through IOWN andachieving carbon neutrality—Social contribution is an important objective for
the NTT Group. Could you tell us about some of theinitiatives that you will be focusing on?
The NTT Group is promoting various initiatives tocreate a sustainable society, and we consider energy aparticularly important social issue. To address thisissue, the NTT Group aims to achieve carbon neutrality by 2040. As the amount of communication continues to increase with the spread of the Internet andsmartphones, the information and telecommunications industry currently consumes nearly 1% ofJapan’s commercial electricity consumption, andmuch of its greenhouse-gas emissions stem fromelectricity consumption.
Against that backdrop, we have set our carbonneutrality goal ten years ahead of the one set by theJapanese government. I hope people will consider our
declaration of achieving carbon neutrality by 2040 as
our determination to make a concerted effort by all
our employees. We will achieve this goal by reducing
2

View from the Top

power consumption using photonics-based technology in everything from networks to terminals in the
Innovative Optical and Wireless Network (IOWN)
and developing renewable energy.
—Society also has high expectations for NTT’s R&D.
Would you tell us about some of NTT’s world-leadingor unique achievements, including IOWN?
A representative example of our achievements isphotonics-electronics convergence technology,
which is a component of IOWN. Currently, opticalfiber is used for transmission lines and wiring
between devices. However, photonics-electronics
convergence technology takes this material a stepfurther and uses photonics instead of electronics toconnect circuit boards within a device, between chips
and other parts on the board, and ultimately even
within chips. This technology lowers power consumption and increases processing speed. It is a longterm goal, and we first plan to achieve the board-to-board optical interconnection by 2025.
We have also achieved a variety of world-classresearch results concerning, for example, artificial
intelligence and quantum computers, which arefamiliar to everyone these days. One of our uniqueachievements is our research in the field of sports science, which contributed to the Japanese women’s
softball team winning the gold medal at a major international sporting event held in Tokyo in 2021. Bymaking it possible to “train the brain and win thecompetition,” our researchers took a practical
approach in conjunction with academic research. In
particular, they developed a batting-training systemthat combines videos of pitchers with a pitchingmachine that can analyze the pitch quality of opposing pitchers and throw a variety of pitches based on
the analysis results and provided the system at training camps for the team.
We regularly present many of our achievements at
the NTT R&D Forum, which is currently held only
once a year and is by invitation only, so only a limited
number of people can attend. Since these researchachievements involve technologies that are not widespread, it may be difficult for the general public tounderstand them. Therefore, I believe it is necessary
to explain our research results in an easy-to-understand manner to disseminate them in society. To that
end, we are considering shooting and distributing
videos of my visits to our laboratories to talk withexperts and researchers in various fields.
Realizing that “what goes around comesaround” while in the US—What path did you take from the time you joined thecompany until you were appointed president? Did
you set your sights on your current position?
When many people my age became working adults,
they probably joined companies intending to reachthe top. However, even though they had such a passionate aspiration, I don’t think it is realistic. That
was the case for me. After joining Nippon Telegraphand Telephone Public Corporation in 1981, I started
working in sales at a telephone exchange. The following year, I completed training in outdoor and indoor
maintenance and then training in a department handling data communications, which were the main
businesses of NTT at the time. After that, I was incharge of planning budgets at the Planning Departmentof the head office. Next, I became assistant managerof the Sales Section at the Makuhari Telegram and

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View from the Top

Telephone Office, seconded to the then Economic
Planning Agency of Japan. I then was appointed
assistant manager of the Human Resources Planning
Section at the head office. Afterwards, I was appointed manager of the Planning Section of the Labor
Department of the Tokai Branch Office before beingtransferred to NTT Europe in London in 1995.
When I was in London, NTT was preparing to enter
the international telecommunications market. Our
first line was between Tokyo and London, and it
started operating as an international data-communication line for the then Sakura Bank (now Sumitomo
Mitsui Banking Corporation) on September 18, 1997.
That was NTT’s first global network business. As
someone who has been involved in the global business from its very beginning, I am proud when I think
about the fact that it has grown from almost zero sales
to over a two trillion yen annual sales. I have worked
for about 13 different companies within the NTT
Group, both domestically and internationally, before
assuming my current position. I have moved house 13
times.
My 40-plus years in the profession have not all
been good ones, and I’ve had some tough times along
the way. An example was when a US company weacquired was on the verge of financial collapse due tothe bursting of the Internet bubble in the late 1990s
and we had to streamline its operations to rebuild. Atthat time, I was working at the company branch in theUS, and my views on streamlining policies did notmatch those of the company’s top management, so I
asked for suggestions from the head office in Japan.
However, others discovered that I had created the
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Vol. 20 No. 8 Aug. 2022

original draft concerning those suggestions, and myrelationship with the top management deteriorated
further. That situation made it harder to do my job,
but I had no choice but to carry on. From then on, I
worked on increasing sales by supplying products toJapan as an OEM (original equipment manufacturer),and gradually gained the trust of several department
heads, making my job easier. That experience mademe keenly aware of the challenges of corporate communication and the fact that even if a company is awholly owned subsidiary, employees of the company
do not always listen to the opinions of the parent company. It also made me realize the truth in the saying“what goes around comes around.”—Thank you for sharing your experiences with us.
Could you tell us about your beliefs as a top executive
based on those experiences?
I believe it is essential for an organization involving
many people to have good communication so thatpeople can move in a positive direction. As a leader
who works with the 330,000 employees of the NTT
Group, I want to value people. A leader cannot doanything alone. The appeal of the existence of a company is that it is an organization with many people. In
an organization, together we can overcome what one
cannot overcome alone.
The role of the leader is to energize the organization
to generate new ideas, and to fulfill that role, should
not take an arrogant and formal attitude. If you takesuch an attitude, no one will want to talk to you. I
want to be open-minded to encourage anyone to talk4

View from the Top

with me.
For those at the top management, it is our job to“discover” people. There may be people in our group
who have a variety of ideas but go unnoticed. It is
important to seek out such people. We must alsotransform our group by combining completely different organizations and people with various skills to
promote innovation.

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Interviewee profile
Career highlights
Akira Shimada joined Nippon Telegraph andTelephone Public Corporation (now NTT) in
1981. He became vice president of the Corporate
Strategy Planning Department of NTT in 2007;
senior vice president and member of the Board of
NTT EAST in 2011; senior vice president, headof the General Affairs Department, and memberof the Board of NTT in 2012; executive vicepresident, head of the General Affairs Department,
and member of the Board of NTT in 2015; andsenior executive vice president and representative
member of the Board of NTT in 2018. He has
held his current position since June 2022.

5

Front-line Researchers

I Would Like to See a World in WhichAll Engineers Have a Cybersecurity
Background
Mitsuaki Akiyama
Senior Distinguished Researcher,
NTT Social Informatics Laboratories
Abstract
NTT laboratories have a long history of research and development (R&D) in the field of computer science, including theworld’s most-advanced cryptography and cybersecurity. SeniorDistinguished Researcher Mitsuaki Akiyama is conducting R&D
on cybersecurity by incorporating social science, such as social
value, people’s happiness, legal systems, and social acceptability,
to transform and develop society. We asked him about the progress of his research and attitude as a researcher.
Keywords: socio-technical system, usable security, research ethics

Pursuing a socio-technical system that is based onrelationships and interactions among
technology, people, and society—It has been three years since your last interview in2019. How have your research activities been goingsince then?
Continuing from our last interview, I have been
researching cybersecurity to protect the safety andsecurity of users from cyber-attacks. I’m focusing onfour key themes: (i) research on analyzing the characteristics of cyber-attacks, accumulating such information (i.e., intelligence for cyber-attack countermeasures), and using this information to prevent
similar attacks in the future; (ii) research on offensive
security to stop attacks before they happen by finding
and dealing with potential security and privacy
threats and flaws (such as bugs in systems and serNTT Technical Review

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vices) from the attacker’s viewpoint; (iii) activities
related to ethics in research on cybersecurity toensure that the results of advanced research properly
benefit society, including experimental methods fordiscovering security and privacy threats and methods
for disclosing discovered threats; and (iv) research on
usable security to design systems that prioritize security threats and help users recognize such threats andmake safer decisions by quantifying these threatsaccording to users’ security and privacy awarenessand behavior toward systems and services.
In July 2021, NTT laboratories were reorganized,
and NTT Social Informatics Laboratories was established. Considering it necessary to proceed withresearch and development (R&D) in various fields ina more composite manner, NTT Social Informatics
Laboratories is engaged in a variety of research
projects. These include (i) well-being research forhuman happiness; (ii) innovation technology for6Front-line Researcherssocial systems through the fusion of information andcommunication technology (ICT) technology andsocial science; (iii) establishment of new technologies to eliminate threats such as cyber-attacks; (iv)establishment of technologies to realize innovations
to create safe social systems through the analysis andprediction of social information; (v) creation of highvalue-added social systems through data distribution
and utilization that balances usability and security;(vi) new data protection technology that utilizes cryptography as well as physical properties; and (vii)
creation of fundamental next-generation cryptography theories that will lead global expansion [1].
Cybersecurity has often been regarded as a cost factor. At NTT Social Informatics Laboratories, we are
breaking away from this mindset. That is, we areengaged in R&D on cybersecurity technology thatacts as a “prime mover” to drive and develop people
and society by maintaining security and privacy.
Accordingly, I’m taking advantage of the opportunity
to pursue my research activities at this lab andincreasing the weight of interdisciplinary aspects,
such as human behavior and social aspects, in myresearch compared with the past. As people areforced to use new technologies without having sufficient knowledge of their security and privacy, newsecurity and privacy threats are emerging. Under
these circumstances, I believe there is an emerging
need to establish a scientific foundation for cybersecurity and an approach to solving problems concerning cybersecurity from an interdisciplinary perspective.
In the industrial sector, for example, cybersecurity
technology has been dominated by symptomatic andempirical measures and operations against daily
cyber-attacks. Such reactive measures require an endless amount of operations, which results in increased
personnel costs, slow response, and security fatigue
and makes it difficult to maintain a reliable ICT system against increasingly sophisticated cyber-attacks.
To create a fundamental solution that eliminates thisproblem, I believe it is important to create cybersecurity technologies that are transparent, reproducible,
and verifiable through means such as theoreticallysound mathematical and scientific methods, comprehensive theory of cybersecurity, principled systemdesign methods, modeling at various layers andscales for complex and dynamic systems, and creating indices to evaluate the effectiveness of cybersecurity technologies.
From the perspective of an interdisciplinary
approach, I’m also pursuing a socio-technical system
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that is based on the relationships and interactions
among technology, people, and society. Even if atechnology is secure, it may not be secure if people
use it incorrectly, and if it does not become widelyused in society, people will not be able to fully benefit
from it. Although many security technologies havebeen devised, many have never been fully used. There
are many cases in which end-users are deceived byphishing scams or a technology was not properly
used because the reasons for issues, such as the difficulty of implementing security-by-design in development projects, had not been clarified.
In view of these issues, I’ve been adopting an interdisciplinary approach that incorporates fields, such associal science and social psychology, in addition tocomputer science to clarify the root causes of cybersecurity problems from the viewpoints of humans andsociety and review ICT systems from their designphase. To achieve usable security, I’m aiming to solve
security and privacy problems that depend on people’s perceptions and decision-making so they cannot
be solved simply by focusing on systems. Solving
such problems involves the following process: (i)observing and analyzing human behavior, mental
models, and decision-making processes regarding
security and privacy, (ii) feeding the findings of theobservation and analysis back to system design,
implementation, and operation, and (iii) enabling
people to make appropriate decisions on the basis ofcorrect perceptions about security and privacy.
To make this process feasible, I believe it is essential to properly observe and analyze people’s behavior and perceptions, and I am researching measures toprevent the spread of false information. False information includes misinformation, which is spread
unintentionally, and disinformation, which is spreadwith the intent to cause harm or deceive. The spreadof misinformation and disinformation through social
media represents the next generation of cyber-attacks
that threaten correct human cognition and judgment.
It is not limited to individual problems such as falling
victim to phishing or being deceived by hoaxes; it canhave a major impact on democracy, as in the case of
the 2016 US presidential election.
Our papers highlighting social issues were
accepted by prestigious conferences—You have achieved significant academic results
through these research activities.
Our paper on offensive security was accepted at the7Front-line ResearchersAB

a.example

b.example

Web rehosting
service(Web proxy,
web translation,
web archive)

B
EffectDiscovered attacks
1) Persistent man-in-the-middle
2) Privilege abuse
3) Credential theft
4) History theft
5) Session hijacking and injection

rehosted.example

evil.example(

A

Origin boundaries)

Fig. 1. Potential threats to web rehosting services.

27th Annual Network and Distributed System Security Symposium (NDSS 2020), one of the top cybersecurity conferences, and received the Distinguished
Paper Award [2]. We named a service that rehosts
web content on another web site a web rehosting service, developed multiple threat models for web
rehosting services, and clarified, through verification,
the conditions under which the threats are actually
manifested (Fig. 1).
We discovered a critical phenomenon concerning
web rehosting services; namely, the mechanisms
underlying web security fail to work when web content from different origins (i.e., attributes defined by
URL protocol, host, port pairs) are merged into thesame origin. This discovery indicates that a variety of
attacks are possible in web rehosting services, andappropriate design guidelines for web services toavoid such attacks can be provided.
This research has made it possible to detect threats
before they become apparent and take countermeasures against them at the design phase. It is thus possible to avoid service operators having to redesign aservice as a whole. Our goal is not to simply findvulnerabilities but to establish a more-versatile verification method and theoretical foundation for finding such security and privacy threats. With that goalin mind, we have identified many threats on the web,where various types of communication are currently
aggregated, and established methods for verifying
them. I hope to summarize our findings in about five
years.
Regarding usable security, our research aimed atenabling secure software development focusing on
developers and development projects was selected forpresentation at the 37th Annual Computer Security
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Applications Conference (ACSAC 2021) [3]. This
research was based on a large-scale online survey ofprofessional software developers in Japan and theUnited States to identify organizational issues such aswhether a decision-making authority exists and difficulties in decision-making (Fig. 2).
Our paper on explaining the principles of humandeception and creating appropriate support technologies was accepted at the 17th Symposium on Usable
Privacy and Security (SOUPS 2021) [4]. While previous research implicitly studied perceptions andbehaviors of users when confronted with phishingemails in their native language, we focused on nonnative English speakers, who make up the majority of
the world’s population, confronting English phishingemails. For the first time in a large-scale study, weclarified the relationship between language and handling phishing emails and proposed support techniques specific to non-native English speakers.
At European Symposium on Usable Security(EuroUSEC) 2021, our paper indicating problems inuser-study methods regarding why people are
deceived with respect to phishing emails received theBest Paper Award [5]. The study revealed for the firsttime that several methods used for screening participants in user studies are highly inconsistent because
they exclude careless participants who are easily
deceived by phishing emails, namely, those whose
data are most needed. Another paper of ours, presenting research on countermeasures against the spreadof false information in social media, was accepted at
the 7th International Winter School and Conference
on Network Science (NetSci-X 2022) [6], and someof the results of that research were exhibited andreported at the NTT R&D Forum.
8

Front-line Researchers

Characteristics of thedevelopment project anddevelopment product

Security awarenessand behavior

Developer

Are there any gaps between developers and managers
regarding security behavior and awareness? If so, how
do they impact the security of products?

Manager

How do software development characteristics impact developers’ security
behavior and awareness?

Fig. 2. Analysis of factors that hinder secure software development.

You become good at what you like doing—What do you keep in mind and consciously implement when looking for research themes?
First, I try to be interested in new technologies andservices while constantly updating my knowledge.
Rather than thinking about a technology alone, I consider how such technology is used from the perspective of the people, organizations, and society involved
with the technology as well as laws and ethics.
I also value having discussions with experts infields different from mine. Since it is common to go
beyond one’s field of expertise when attempting toachieve something, it is essential to collaborate withresearchers in other fields and adopt an interdisciplinary approach. In this sense, I think that NTT laboratories have abundant human resources.
I believe that the phrase “you become good at what
you like doing” is an important attitude to have as a
researcher. I don’t think I have special research talents that are superior to those of others, but I do think
I have a stronger interest in cybersecurity issues.
I have seen many people who have stopped doing
research despite their very high technical ability or
did not take a research job because they said they did
not have the confidence to do research. However,research does not produce immediate results; in fact,
it can take several years, maybe even 10 or 20 years,
to produce a single result.
In such a situation, you may want to quit by thinking that you have no talent; however, if a research
theme is something you are interested in, you may beable to continue to work on it with high motivationand interest, regardless of whether it produces results.
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If you can continue to work on it, the likelihood ofachieving results will certainly increase.
For this reason, I first consider whether I really likethe research theme I intend to work on. If I really likea research theme, I can only accept the results of myresearch, whether successful or not, and that thoseresults will not change my liking of the research
theme.
—How will you advance your research activities in
the future?
R&D on cybersecurity had often considered individual threats in isolation and in a retrospective manner, so it has been difficult to solve fundamental
problems. I’d like to see a world in which not only
cybersecurity experts but also all engineers have abackground in cybersecurity and take cybersecurity
into account when manufacturing products. In otherwords, I want all engineers to share the mindset of acybersecurity expert. I believe it is my job to help create cybersecurity technology that is available toeveryone. In that case, the meaning of the term cybersecurity expert would change.
I’m also working on research ethics. In the cybersecurity-research community in Japan, I’ve been continuously raising awareness of cybersecurity-research
ethics since around 2016, and the concept has spreadto a certain level (Photo 1). In fact, a consultation
service on research ethics has been available in Japan
since 2018 at the Computer Security Symposium, oneof the largest computer security symposiums inJapan, even before measures regarding research ethics were taken at the IEEE (Institute of Electrical andElectronics Engineers) Symposium on Security and9Front-line Researchers

tant for researchers to work together as members ofthe development community.
It is important that cybersecurity researchersrespect software developers and cooperate with themon better technology development on the basis ofthese three arguments, and I intend to continue tofocus my efforts on raising awareness of cybersecurity-research ethics.
References[1][2]

Photo 1. P anel discussion at a symposium in the Japanesecybersecurity community.

[3]

[4]

Privacy. The Computer Security Research Group ofInformation Processing Society of Japan has also
released a checklist of research ethics in a form thatcan be used by various research groups.
In 2021, the conduct of a researcher at a US university became controversial. The researcher implemented patches with vulnerabilities to the Linuxkernel to evaluate the code review process in the
open-source software community and wrote a paper
about it. In light of this, a panel discussion was heldin the Japanese cybersecurity community to discuss
the relationship between cybersecurity researchers
and the development community. In this discussion, I
argued that (i) experiments on people and their communities and organizations must be designed andconducted with full consideration of their impact on
those people, communities, and organizations, (ii) nodivisions should exist between the development community and research community, and (iii) it is impor-

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[5]

[6]

Website of NTT R&D, NTT Social Informatics Laboratories, https://www.rd.ntt/e/sil/overview/index.html#introduction
T. Watanabe, E. Shioji, M. Akiyama, and T. Mori, “Melting Pot ofOrigins: Compromising the Intermediary Web Services that Rehost
Websites,” Proc. of NDSS 2020, San Diego, USA, Feb. 2020.
F. Kanei, A. A. Hasegawa, E. Shioji, and M. Akiyama, “A Cross-role
and Bi-national Analysis on Security Efforts and Constraints of Software Development Projects,” Proc. of ACSAC 2021, pp. 349–364,
Austin, USA, Dec. 2021.
A. A. Hasegawa, N. Yamashita, M. Akiyama, and T. Mori, “Why They
Ignore English Emails: The Challenges of Non-native Speakers in
Identifying Phishing Emails,” Proc. of SOUPS 2021, Online, Aug.
2021.
T. Matsuura, A. A. Hasegawa, M. Akiyama, and T. Mori, “CarelessParticipants Are Essential for Our Phishing Study: Understanding the
Impact of Screening Methods,” Proc. of EuroUSEC 2021, pp. 36–47,
Online, Oct. 2021.
S. Furutani, T. Shibahara, M. Akiyama, and M. Aida, “Competitive
Information Spreading on Modular Networks,” NetSci-X 2022, Porto,
Portugal, Feb. 2022.

Interviewee profile
Mitsuaki Akiyama received an M.E. and Ph.D.
in information science from Nara Institute of
Science and Technology in 2007 and 2013. Sincejoining NTT in 2007, he has been engaged inresearch and development on cybersecurity. His
research interests include cybersecurity measurement, offensive security, and usable security.

10

Rising Researchers

Bandwidth-doubler Technology
Eliminates Bandwidth Bottlenecks
Hiroshi Yamazaki
Distinguished Researcher, NTT Device
Technology Laboratories
Abstract
In communication systems that involve signal processing using
digital circuits, the bandwidth at the interface between digital andanalogue is a bottleneck to improving communication speed. We
spoke to Distinguished Researcher Hiroshi Yamazaki, who developed the principles of the bandwidth-doubler technology for solving this bottleneck.
Keywords: bandwidth doubler, digital signal processor, analogue
multiplexer

Bandwidth-doubler technology for doubling
optical communication speed—What are the ways to increase speed in opticalcommunication?
The speed at which information is transmitted incommunication is determined by the product of spectral efficiency and bandwidth.
Spectral efficiency refers to the amount of information that can be transmitted per unit frequency andtime. For example, compared with a binary systemthat represents “0” when the light is off and “1” when
the light is on, a quaternary system that represents
four values “00,” “01,” “10,” and “11” with four different states of light can transmit information twice as
efficiently. Spectral efficiency in optical communication has been dramatically improved in recent years
due to performance improvement in digital signal
processors (DSPs) and the evolution of algorithms.
Meanwhile, bandwidth refers to the range of available frequencies. The pace of bandwidth expansion,
however, has been relatively slow. A particular chalNTT Technical Review

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lenge lies in the analogue bandwidth of the digital-toanalogue converter (DAC) and analogue-to-digitalconverter (ADC), which convert digital and analogue
signals at the input/output unit of the DSP. The DAC
and ADC are monolithically integrated with the DSP
using complementary metal oxide semiconductor(CMOS) technology. However, while CMOS isalmost the only technology in the construction oflarge-scale digital circuits, it is not necessarily thebest technology from an analogue bandwidth perspective. As a result, the bandwidth limitations of
DACs and ADCs formed using CMOS technology
are becoming a bottleneck in the bandwidth for theentire communication system.
Bandwidth-doubler technology can be used toeliminate this bandwidth bottleneck in order toincrease the transmission capacity per transceiver.
—What is the principle of the bandwidth-doubler
technology?
Figure 1 shows the principle of the bandwidthdoubler technology. Two DACs (sub-DACs) are
11

Rising Researchers

Digital pre-processing

AMUX

Target signal

+1

+r

×1/r

fc

CLK

fc/2

fc

Band splitting

f

fc/2

fc

Emphasis ofhigh-frequency
component

f

fc/2

fc

f

Folding andaddition ofhigh-frequency
component

fc/2

fc

f

fc/2

subDACfc

f

OutputTarget signal

AMUX
subDAC

f

fc/2

fc

f

Residual
image
CLK
+1

fc/2

fc

fc

f

f
−r

Fig. 1. Principle of bandwidth-doubler technology.

arranged in parallel, and their outputs are multiplexedwith an analogue multiplexer (AMUX) to transmit asignal with twice the bandwidth of the signal output
by each sub-DAC. Currently, we are focusing mainlyon the DAC side (transmitter side), so I will focus myexplanation on that.
In the middle of the diagram, the yellow section
with two sub-DACs is the bottleneck area. The figure
shows many light blue and pink graphs on the rightand left of the bottleneck. The “f” on the horizontal
axis of the graphs refers to frequency, wherein a wide
bandwidth means a wide range extending along thehorizontal axis, and a narrow bandwidth means a narrow range along the horizontal axis.
The “target signal” on the left-hand side of the figure is the signal you want to output and has a bandwidth of fc. However, the sub-DAC only has a bandwidth of fc/2, i.e., half of fc. In other words, the entire
signal cannot be transmitted in its original state, but
the signal must not be partly cut off. Therefore, it isnecessary to think about how to send the entire target
signal through the sub-DAC.
In the bandwidth-doubler technology, digital preprocessing is first carried out in the DSP, where thetarget signal is separated into low and high frequencies at fc/2 as the threshold. The high-frequency components are then emphasized; these are the pink parts.
Two signals are then created, namely, signals withpositively and negatively folded high-frequency components. Since these signals have a width of only fc/2,
they can pass through the sub-DACs.
The signals then enter the AMUX. The AMUX is a
NTT Technical Review

Vol. 20 No. 8 Aug. 2022

simple electronic circuit in which two input signals
are alternately connected to an output at high speed,
without distorting the waveform. Switching at a fixedtiming is equivalent to making a copy of the input
signal (called an image) around the switching frequency fc on the frequency axis. This widens the
bandwidth of the signal. Further, by combining thepositive and negative relationship of the above two
signals with the positive and negative relationship of
the image, the light-blue-colored low-frequency
component and the pink-colored high-frequency
component reinforce each other in the original frequency domain and cancel each other out in the restof the frequency domain. This enables the target signal to eventually be transmitted.
The method shown in the figure is the initial version, and we have now validated a method thatenables the clock frequency to be halved.
—How is the progress of the research?
Bandwidth-doubler technology was first applied toa relatively simple short-range intensity-modulation
direct detection (IMDD) transmission system, where
it broke the world record in IMDD optical transmission capacity at 250 Gbit/s per wavelength at thattime (2016). Since then, we have widened thedevice’s bandwidth and improved the folding method, increasing the capacity to 400 Gbit/s per wavelength. Also, experiments are underway for digital
coherent transmission systems for medium- to longrange applications, and long-distance transmission of
12

Rising Researchers

high-capacity signals over 1 Tbit/s per wavelength
has also been achieved. These results have been published and gained wide attention as top-scoring
papers and post-deadline papers by the Optical Fiber
Communication Conference (OFC) and the European
Conference on Optical Communication (ECOC), themost prestigious academic societies in the field of
optical communication.
Various approaches have been proposed in attempts
to increase capacity, including by other companies
and other organizations. We are also in the midst ofcompetition among the top players.
Investigation of improvements in both device
technology and signal processing—What are the future prospects?
Bandwidth-doubler technology is basically intended for application in optical communication systems.
The demand for larger capacities for the devices thatmake up the optical fiber communication system,
especially optical transceivers, is expected to continue in the future. Therefore, we will continue to workon bandwidth expansion technology as one of theapproaches to meet these demands. One of our missions is to deploy and promote the innovation of scientifically and academically superior, world-leading
technologies, so we will continue our research in thisarea.
From a technical perspective, there is still room for
improvement in the bandwidth doubler, so we are
pursuing research both from the device technology
and signal processing aspects. For example, developing an implementation technology for compact inte-

NTT Technical Review

Vol. 20 No. 8 Aug. 2022

gration of AMUX and optical modulators is one of
the important issues to address. Furthermore, since
the idea of bandwidth expansion is not tied to a specific device configuration, we will also aim to develop technologies that enable optical circuits to achieve
the same functions as AMUX.
—What led you to the bandwidth-doubler principle?
There were various hints that led to conceiving the
bandwidth-doubler principle. I had been conducting
research primarily on optical devices until around2015. In a world where all optical devices were analogue, increasing the capacity of communicationentailed various problems. I had also been studying
about digital and other technologies, and it wasaround the time I realized that the improvements incommunication capacity in the analogue world would
soon reach their limits and high-speed AMUX technology was developed, which led to the idea of the
bandwidth doubler, where analogue and digital work
together to expand bandwidth.
Although at that time the digital part was outside
my specialization, it helped that the environment wasin place for easily carrying out various experiments.
—Could you give a message to students, young
researchers, and future business partners?
When I talk to researchers from other companies orother institutions at academic conferences, I hear
about their problems, e.g., that although they wereable to fabricate the device, it is difficult for them toconduct system experiments, and that they do nothave access to the devices they want to use for system
experiments.
NTT has laboratories that are conducting researchacross a wide range of fields from devices to systems
and that are collaborating in various ways. I think thisis a major advantage for us. I am a member of both
NTT Device Technology Laboratories and NTT Network Innovation Laboratories. And various achievements have come about, including the bandwidthdoubler technology described above, in collaborations between the two laboratories and with the NTT
Device Innovation Center.
I would like to say to students and young researchers that having a different combination of “drawers”
from those of other people is an advantage. As adevice researcher, I also learned about digital signal
processing, which is not my specialization, leading to
the development of the bandwidth-doubler technology.
13

Rising Researchers

Please do not hesitate simply because it is outside
your specialization or because you are a beginner, if
it is necessary, take the bold step to learn about theknowledge and technologies in other fields.

NTT Technical Review

Vol. 20 No. 8 Aug. 2022

Interviewee profile
Hiroshi Yamazaki joined Nippon Telegraph andTelephone Corporation in 2005 as a member of
NTT Photonics Laboratories. He is engaged inresearch and development of integrated photonic
devices. Currently, he holds concurrent research
positions at NTT Device Technology Laboratories
and NTT Network Innovation Laboratories. He
has a Doctor in Engineering, and is currently the
Deputy Editor of Journal of Lightwave
Technology.

14

Feature Articles: Device Technologies for Enabling the
All-Photonics Network

High-output Optical Transmitter andHigh-sensitivity Optical Receiver for
400-Gbit/s 40-km Fiber-amplifier-less
Transmission
Shigeru Kanazawa, Takahiko Shindo,
Yasuhiko Nakanishi, Masahiro Nada, Koichi Hadama,
Mingchen Chen, Shoko Tatsumi, Atsushi Kanda,
and Hirotaka Nakamura
Abstract

We developed a high-output optical transmitter and a high-sensitivity optical receiver for long-distancetransmission in the All-Photonics Network that is being promoted under IOWN (the Innovative Opticaland Wireless Network). This article introduces the 400-Gbit/s optical transmitter with a semiconductor
optical amplifier-assisted extended-reach electro-absorption modulator integrated distributed feedback
laser called AXEL, a key device for higher optical output, and the 400-Gbit/s optical receiver with anavalanche photodiode, a key device for higher sensitivity.
Keywords: intensity-modulation direct-detection (IM-DD), AXEL, APD

1. Introduction
The amount of data handled by communication
networks has been increasing dramatically with thedevelopment of cloud services and other new services, and traffic volume has also been increasing
sharply. In response, NTT proposed the Innovative
Optical and Wireless Network (IOWN) to increasetransmission capacity, decrease latency, reduce
power consumption, and provide excellent flexibility
in the communication network.
The All-Photonics Network (APN), one of the threemain components of IOWN, will introduce photonics
technology everywhere, from the network to the terminal and from short distances to long distances, toachieve overwhelmingly lower power consumption,higher quality, greater capacity, and lower latency innetwork-data transmission.
NTT Technical Review

Vol. 20 No. 8 Aug. 2022

At the NTT Device Innovation Center, we developed an optical transmitter and optical receiver for
intensity modulation format to fabricate a compact
and low-power-consumption transceiver and increasetransmission capacity. To fabricate the transmitterand receiver for long-distance transmission, a higher
output optical transmitter and higher sensitivity optical receiver are needed. We therefore conducted
research and development on a semiconductor opticalamplifier (SOA)-assisted, extended-reach electroabsorption modulator integrated distributed feedback(EADFB) laser called AXEL [1], which is a key
device for high optical output, and an avalanche photodiode (APD) [2], which is a key device for highsensitivity. This work resulted in the development ofan optical transmitter and receiver that are capable oftransmitting a 400-Gbit/s signal over a distance of 40
km [3, 4]. The developed transmitter and receiver15

Feature Articles

100-Gbit/s
electrical signal
input

Current
injection

Current
injection

100-Gbit/s
optical signal

Amplification

Conversion to
optical signal

SOA

EA modulator

Single-wavelength
light generation

DFB laser

Fig. 1. Schematic of AXEL chip.

extend the region in which intensity modulation canbe applied, contributing to reducing network-interface device size and power consumption.
2. AXELThe key device of the high-output optical transmitter is a high-speed, high-output optical transmission
device, which is generally an EADFB laser that generates an optical signal of 100 Gbit/s per chip. Thislaser comprises a DFB laser that emits single-wavelength light and an EA modulator that converts anelectrical signal to a high-speed, high-quality opticalsignal. However, conventional EADFB lasers havehigh optical loss in the EA modulator and inhibit theincrease in higher output power.
To address this issue, we have been developing
AXEL as a key transmitter that can achieve highspeed optical transmission with high output (Fig. 1).
An SOA is driven by current injection, and light incident to the SOA is amplified as it propagates. In conventional EADFB lasers, the intensity of the light
output by the DFB laser is decreased by modulationin the EA modulator, but in AXEL, the light is amplified by the SOA to obtain a high-output optical signal. The SOA is fabricated from the same semiconductor materials as the DFB laser; therefore small,
integrated AXEL chips can be fabricated with thesame process, enabling low-cost mass production.
Because each SOA chip must amplify a high-speed
100-Gbit/s optical signal, this SOA uses a new opticalamplification layer that efficiently converts electrical
NTT Technical Review

Vol. 20 No. 8 Aug. 2022

current into light with less degradation in the optical
waveform, thus achieving output of a 100-Gbit/s signal with no loss of quality. The AXEL chip we developed is thus capable of outputting a 100-Gbit/s optical signal with an intensity of +8.0 dBm or more [1].
An optical transmitter that generates a 400-Gbit/s
optical signal can be achieved with an array of four
discrete AXEL chips.
3. APDThe key device of the high-sensitivity photoreceiver is a high-speed, high-sensitivity PD. The PDs thatare generally used in receivers have a theoretical
opto-electrical conversion efficiency of 100%, but, inmost cases, the actual efficiency is only a few tens ofpercent because of loss at light incidence and incomplete absorbance of light within the PD.
We developed an APD, a special PD designed toserve as a high-speed, high-sensitivity receiver. A
strong electric field is induced within the APD, andmore electrons and holes are generated by collisionsof ionized photoelectrons in the field. The result is an
opto-electrical conversion efficiency that can exceed
100%, enabling the fabrication of a high-sensitivity
optical receiver. The main components of the APD
are light-absorption layers and an avalanche amplification layer (Fig. 2). The optical signal is converted
to an electrical signal in the absorption layers, and theresulting electrical signal is amplified in the amplification layer.
The APD converts a 100-Gbit/s optical signal to an
16

Feature Articles

N-type conductor layer
Edge-relaxation layer
Avalanche amplification layer

Electrons

Hybrid light-absorption layer
Holes

P-type conductor layer
Incident signal light
InP substrate

InP: indium phosphide

Fig. 2. Schematic of APD chip.

electrical signal. PDs generally must sacrifice sensitivity to received light to achieve higher speed. They
also require a waveguide that makes photonic coupling of the incident optical signal to them difficult.
For this APD, we adopted a hybrid two-layer lightabsorption structure that combines one absorption
layer that uses electron diffusion as the main carriertransport mechanism and another absorption layerthat uses hole drift as the main carrier-diffusion
mechanism. This makes it possible for the opticalreceiver to handle a 100-Gbit/s signal while maintaining high sensitivity and facilitating optical couplingwith a vertical injection structure [2]. An opticalreceiver that converts a 400-Gbit/s optical signal to anelectrical signal can be fabricated by mounting four
APD chips.
4. 400-Gbit/s high-output optical transmitter
Our four-channel, 400-Gbit/s optical transmitterachieves high output power with four AXEL chipsthat are mounted inside the transmitter on four subassemblies (Fig. 3(a)) [3]. Each chip outputs a
100-Gbit/s optical signal. The light wavelengths output from the subassemblies, 1295.5 nm (Lane 0),1300 nm (Lane 1), 1304.5 nm (Lane 2), and 1309.1
nm (Lane 3), are determined using a local area network-wavelength division multiplexing grid. The
NTT Technical Review

Vol. 20 No. 8 Aug. 2022

output light is collimated after passing through thefirst lens and input to an optical multiplexer that comprises a mirror, glass block, and wavelength filters.
The 400-Gbit/s optical signal from the multiplexer ispassed through an isolator and the second lens andthen coupled to an internal optical waveguide in a
Lucent connector (LC) receptacle. A photograph ofthe fabricated compact 4-channel optical transmitteris shown in Fig. 3(b). The device is 18.2 mm long, 6.2
mm wide, and 5.4 mm high, small enough to be
mounted in a quad small-form-factor pluggable,
double density (QSFP-DD) optical transceiver.
We measured the waveform eye diagrams and theoutput power for operation at 400 Gbit/s. The AXEL
chip temperature was 50°C. The laser diode and SOA
currents were 80 and 40 mA, respectively. The measured electrical signal was 53.125 Gbaud, and theamplitude voltage of the four-level pulse amplitude
modulation signal was 0.75 Vpp. The eye diagrams
were observed with a sampling oscilloscope after thesignal was passed through a 26.6-GHz low-pass filterand equalization with a transmitter dispersion eye
closure quaternary (TDECQ) filter. Clear eye openings were obtained for all lanes, with TDECQ (anindicator of waveform quality) of 2.4 dB or less(Fig. 4). Optical output power (optical modulation
amplitude (OMA)) of at least +4.7 dBm was also
confirmed for all lanes, indicating the high output
17

Feature Articles(a) Schematic of 400-Gbit/s optical transmitter(b) Photograph of optical transmitter

18.2 mm

Lane 06.2 mmLane 1h = 5.4 mm
Lane 2

Lane 3

AXEL mounting subassembly

LC receptacle
First lens

Optical multiplexer

Isolator/second lensFig. 3. 400-Gbit/s optical transmitter.

Lane 0Lane 1

Lane 2

Lane 3

TDECQ (dB)

1.8

2.4

1.2

1.8

Extinction ratio (dB)

3.9

4.2

3.8

4.3

OMA (dBm)

4.7

5.8

5.8

6.3

Eye diagram

Fig. 4. Eye diagrams for operation at 400 Gbit/s.

characteristic of the AXEL chip.
5. 400-Gbit/s high-sensitivity optical receiver
The optical receiver comprises a collimating lensfor the light output from the LC receptacle, opticaldemultiplexer for demultiplexing the four-wavelength optical signal from the transmitter, four-channel lens array, four-channel APD array, and fourchannel trans-impedance amplifier (TIA). The optical signal input to the receiver is separated into
wavelengths of 1295.5, 1300, 1304.5, and 1309.1 nm
by the optical demultiplexer (Fig. 5(a)) [4]. The
demultiplexed optical signals pass through the lensarray and input to the four-channel APD array forconversion to amplified electrical signals. The four
NTT Technical Review

Vol. 20 No. 8 Aug. 2022

100-Gbit/s electrical signals are output from thereceiver via the TIA. A photograph of the fabricatedfour-channel optical receiver is shown in Fig. 5(b).
The device is 16.7 mm long, 6.2 mm wide, and 5.3
mm high, small enough to be mounted in a QSFP-DD
optical transceiver in the same manner as the transmitter described above.
We measured the bit-error-rate (BER) of the fabricated four-channel optical receiver with the system
configuration illustrated in Fig. 6. The four-channel
optical transmitter outputs one 100-Gbit/s signal per
channel to generate a total signal-transmission rate of
400 Gbit/s. The extinction ratio for the optical signal
ranges from 3.8 to 4.3 dB. The signal is input to thefour-channel receiver via the variable attenuator, converted to an electrical signal, then output. The output
18

Feature Articles

Flexible printed
circuit

Optical demultiplexer
LC receptacle

4-channel
lens arrayLens

4-channel
APD array

4-channel TIA(a) Schematic of 400-Gbit/s optical receiver

6.2 mm16.7 mm

5.3 mm(b) Photograph of optical receiverFig. 5. 400-Gbit/s optical receiver.

DriverDriverDriver4-channel
opticaltransmitter

Back-toBack or
40 km

Opticalvariable
attenuator

4-channel
opticalreceiver

Driver

400-Gbit/s digital signal processor

Optical signal
Electrical signal

Fig. 6. Evaluation system configuration.

signal is demodulated by the 400-Gbit/s digital signal
processor and the BER is calculated under discrete
operation.
The BER for each channel is shown in Fig. 7. In a
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back-to-back (BtoB) configuration, where the opticaltransmitter and optical receiver are connected almost
directly, the minimum receiving sensitivity for a BER
of 2.4 × 10–4 ranged from –13.5 to –14.0 dBm for19

Feature Articles

Lane 1

1E−3

1E−5
1E−6
1E−7
−20

2.4E−04

BER

BER

2.4E−04

1E−4

−15

−10

−5

1E−5
1E−6
1E−7
−20

1E−3

1E−3

2.4E−04

Lane 1 (BtoB)
Lane 1 (40 km)

−10

−15

−5

1E−5
1E−6
1E−7
−20

2.4E−04

1E−4

1E−4

1E−4

Lane 0 (BtoB)
Lane 0 (40 km)

Lane 3
1E−2

BER

1E−3

BER

Lane 2
1E−2

1E−2

BER

Lane 01E−2Lane 2 (BtoB)
Lane 2 (40 km)

−10

−15

−5

1E−5
1E−6
1E−7
−20

Lane 3 (BtoB)
Lane 3 (40 km)

−15

−10

−5

Received power, OMA (dBm) Received power, OMA (dBm) Received power, OMA (dBm) Received power, OMA (dBm)

Receiver sensitivity BtoB
(dBm)
40 km

−14.0
−13.4

−13.5
−12.3

−13.7
−13.4

−13.8
−14.1

Fig. 7. BERs for operation at 400 Gbit/s.

OMA. When the receiver was connected to a 40-km
standard single-mode fiber as a transmission line, theminimum receiving sensitivity for OMA ranged from
–12.3 to –14.1 dBm. These results indicate that thetransmission distance in a 400-Gbit/s transmission
service can be extended to 40 km by combining afour-channel optical transmitter assembled with
AXEL chips and a four-channel optical receiver
assembled with APD chips and using intensity modulation.

APD receiver are promising technology for highcapacity and energy-efficient networks.
References
[1]

[2]

6. Conclusion
We fabricated a compact high-output four-channel
AXEL transmitter and compact high-sensitivity fourchannel APD receiver for 400-Gbit/s application. The
transmitter and receiver are capable of transmitting a
400-Gbit/s optical signal to a distance of 40 km overoptical fiber by intensity modulation, with the expectation of a reduction in power consumption. These
indicate that the fabricated AXEL transmitter andShigeru Kanazawa

Senior Research Engineer, Metro-Access Network Device Project, NTT Device Innovation
Center.
He received a B.E., M.E., and Ph.D. in electronic engineering from the Tokyo Institute ofTechnology in 2005, 2007, and 2016. He joined
NTT Photonics Laboratories (now NTT Device
Innovation Center) in 2007. He is engaged in the
research and development of optical semiconductor devices and integrated devices for opticalcommunications systems. He is a senior memberof the Institute of Electrical and Electronics
Engineers (IEEE)/Photonics Society and a member of the Japan Society of Applied Physics(JSAP) and the Institute of Electronics, Information and Communication Engineers (IEICE) of
Japan.

NTT Technical Review

Vol. 20 No. 8 Aug. 2022

[3]

[4]

T. Shindo, N. Fujiwara, S. Kanazawa, M. Nada, Y. Nakanishi, T.
Yoshimatsu, A. Kanda, M. Chen, Y. Ohiso, K. Sano, and H. Matsuzaki, “High Power and High Speed SOA Assisted Extended Reach
EADFB Laser (AXEL) for 53-Gbaud PAM4 Fiber-amplifier-less
60-km Optical Link,” J. Lightw. Technol., Vol. 38, No. 11, pp. 2984–
2991, 2020.
M. Nada, T. Yoshimatsu, F. Nakajima, K. Sano, and H. Matsuzaki, “A
42-GHz Bandwidth Avalanche Photodiodes Based on III-V Compounds for 106-Gbit/s PAM4 Applications,” J. Lightw. Technol., Vol.
37, No. 2, pp. 260–265, 2019.
S. Kanazawa, T. Shindo, M. Chen, N. Fujiwara, M. Nada, T. Yoshimatsu, A. Kanda, Y. Nakanishi, F. Nakajima, K. Sano, Y. Ishikawa, K.
Mizuno, and H. Matsuzaki, “High Output Power SOA Assisted
Extended Reach EADFB Laser (AXEL) TOSA for 400-Gbit/s 40-km
Fiber-amplifier-less Transmission,” J. Lightw. Technol., Vol. 39, No.
4, pp. 1089–1095, 2021.
A. Kanda, T. Yoshimatsu, M. Nada, S. Kanazawa, Y. Nakanishi, F.
Nakajima, H. Matsuzaki, and K. Sano, “400-Gbit/s High-sensitivity
APD-ROSA for 4λ LAN-WDM 40-km Optical Link,” Proc. of the25th Opto-Electronics and Communications Conference (OECC2020), T4-5.4, Taipei, Taiwan, Oct. 2020.

Takahiko Shindo

Senior Research Engineer, Metro-Access Network Device Project, NTT Device Innovation
Center.
He received a B.E., M.E., and Ph.D. in electrical and electronic engineering from the Tokyo
Institute of Technology in 2008, 2010, and 2012.
He received a research fellowship for young scientists from the Japan Society for the Promotionof Science from 2010 to 2012. After obtaining his
Ph.D., he worked as a research fellow at theJapan Society for the Promotion of Science. In
April 2013, he joined NTT Photonics Laboratories (now NTT Device Innovation Center), where
he is currently engaged in research on opticalsemiconductor devices. He is a member of theIEEE Photonics Society, JSAP, and IEICE.

20

Feature Articles

Yasuhiko Nakanishi

Senior Research Engineer, Metro-Access Network Device Project, NTT Device Innovation
Center.
He received a B.E. and M.E. from Hokkaido
University in 2000 and 2002. He joined NTT
Access Network Service Systems Laboratories in
2002, where he has been involved in the researchand development of optical communication systems and devices. He is a member of IEICE.

Masahiro Nada

Senior Research Engineer, Metro-Access Network Device Project, NTT Device Innovation
Center.
He received an M.E. in applied physics fromthe University of Electro-Communications,Tokyo, in 2009 and Ph.D. in engineering from the
University of Tokyo in 2017. He joined NTT
Photonics Laboratories in 2009 and has worked
on high-speed, high-responsivity APDs and theirapplication to optical-fiber communications systems. He is currently with the NTT Device
Innovation Center. He was the recipient of theYoung Researcher’s Award from IEICE in 2014
and the Electronics Society Award from IEICEElectronics Society in 2021. He is a senior member of IEICE and IEEE. He is also a member of
Optica.

Koichi Hadama

Senior Research Engineer, Metro-Access Network Device Project, NTT Device Innovation
Center.
He received a B.E. and M.S. in applied physics
and advanced materials science from the University of Tokyo in 1999 and 2001. He joined NTT
Telecommunication Energy Laboratories in2001. Since then, he has worked on research anddevelopment of free-space optical modules for
WDM network systems. He is currently with the
NTT Device Innovation Center, where he isengaged in the development of optical and electrical modules for metro and access networks. He
is a member of IEICE.

Mingchen Chen

Shoko Tatsumi

Engineer, Metro-Access Network Device Project, NTT Device Innovation Center.
She received a B.E. and M.E. in engineeringfrom Osaka University in 2011 and 2013. She
joined NTT Photonics Laboratories (now NTT
Device Innovation Center) in 2013 and worked
on optics related to optical coherence tomography and microscopy. She is currently engaged in
the development of high-speed, high-responsivity APDs and their application to the optical communication applications.

Atsushi Kanda

Engineer, Metro-Access Network Device Project, NTT Device Innovation Center.
He received a B.E. from Waseda University,
Tokyo, M.E. from the Tokyo Institute of Technology, and Ph.D. from University of Tsukuba,
Ibaraki, in 1988, 1990, and 2017. In 1990, hejoined NTT Radio Communication Systems
Laboratories, where he was engaged in researchon microwave and millimeter-wave integrated
circuits for wireless communication. He moved
to NTT Photonics Laboratories in 1999 and is
now at the NTT Device Innovation Center, where
he is involved in the research and development ofelectronic and optical devices for optical networks. He is a member of IEEE.

Hirotaka Nakamura

Senior Research Engineer, Supervisor, MetroAccess Network Device Project, NTT Device
Innovation Center.
He received a B.E. in applied physics from the
University of Tokyo in 1999 and Ph.D. from
Hokkaido University in 2011. In 1999, he joined
NTT Access Network Service Systems Laboratories, where he was engaged in research on opticaltechnologies of high-speed passive optical networks. Since 2020, he has been with the NTT
Device Innovation Center, where he is engaged
in the research and development of laser diodes,
photodiodes, and analog integrated circuits for
optical metro and access networks. He is a member of IEICE. He has been participating in ITU-T(International Telecommunication Union - Telecommunication Standardization Sector) and
FSAN (Full Service Access Network) since2009.

Research Engineer, Metro-Access Network
Device Project, NTT Device Innovation Center.
He received a B.E. and M.E. in electronicengineering from the University of ElectroCommunications, Tokyo, in 2010 and 2012 andPh.D. in engineering from the University ofElectro-Communications in 2015. He joined the
NTT Device Innovation Center in 2015 andworked on the development of high-speed wavelength swept source for the optical coherence
tomography applications. He is currently
engaged in the development of a semiconductor
laser device for optical communication applications. He is a member of IEICE.

NTT Technical Review

Vol. 20 No. 8 Aug. 2022

21

Feature Articles: Device Technologies for Enabling the
All-Photonics Network

C+L-band Colorless, Directionless,Contentionless Reconfigurable
Optical Add/Drop Multiplexing forHigh-capacity Network Flexibility
Kenya Suzuki, Koichi Hadama, Shuto Yamamoto,
Hiroki Taniguchi, and Yoshiaki Kisaka
Abstract

We investigated the possibility of expanding the operating wavelength range of one of the most flexible optical networks, i.e., colorless, directionless, contentionless reconfigurable optical add/drop multiplexing (CDC-ROADM), to the C+L-band. By expanding the operating wavelength range of multicast
switches, which are indispensable for CDC-ROADM, to the two bands of C+L, a seamless optical node
can be achieved. This concept reduces the complexity in network equipment as well as network operation because it enables operation without operators having to be aware of the bands.
Keywords: C+L-band optical network node, C+L-band multicast switch, CDC-ROADM (colorless,directionless, contentionless reconfigurable optical add/drop multiplexing)

1. Introduction
The All-Photonics Network (APN), one of the threetechnologies that comprise the Innovative Opticaland Wireless Network (IOWN), is expected to leverage photonics technology to achieve a significant
increase in the potential of the information-processing infrastructure, something that is difficult toachieve with current electronics technology [1]. The
APN is expected to achieve a 125-fold increase intransmission capacity and maximum end-to-end
adoption of optical technology from the network to
the terminal. For high-capacity optical transmission,
it is important to expand the use of wavelength-division multiplexing (WDM), which is currently used in
optical networks, in addition to the application oftechnologies that have not yet been commercialized
such as spatial multiplexing. In other words, thewavelength bandwidth used for optical fiber communications will be expanded to achieve higher capacity.
Expanding the wavelength bandwidth of WDM is
NTT Technical Review

Vol. 20 No. 8 Aug. 2022

also effective in the end-to-end application of opticaltechnology [2], which requires an increase in thenumber of optical paths that can be established. In
this case, the expansion of the wavelength bandwidthin WDM is also an important issue.
2. Multiband ROADM network
In optical networks, optical switches are essential
for routing light as it is. Reconfigurable optical add/
drop multiplexing (ROADM) systems have been
introduced for optical networks using optical switches, which enable optical signals to be added anddropped at each node. By enabling optical-signaltransmission between multiple rings without electrical regeneration, ROADM systems can flexibly reconfigure the network and reduce operation and maintenance costs. The conventional single-ring network
has been extended to a more economical multi-ring
network, as shown in Fig. 1 [3]. The optical-node configuration called colorless, directionless, contentionless22Feature Articles

Four-degree node

Ring A
Degree 2

Degree 3

Ring B

Degree 4

Degree 1

Two-degree nodeFig. 1. Example of multi-ring network.

Bit rate

100 Gbit/s

500 Gbit/s class

1 Tbit/s class

Baud rate

32 Gbaud

64 Gbaud

130 Gbaud

Channel spacing

50 GHz>75 GHz>150 GHz

Channel countper band

~90

× 2/3

~60

× 1/2

~32

Signal spectrum

Fig. 2. Relationship between signal baud rate and channel count per band.

(CDC)-ROADM is one of the most flexible opticalnode configurations for efficient data communicationin multi-ring and mesh networks. It is not only effective for efficient operation of communication
resources [4] but also expected to contribute to rapid
restoration in the event of optical-transmission-line
breakdown during a disaster [5].
Another trend in optical transmission technology isthe discussion about increasing the baud rate of optical signals [6]. High baud rates are suitable for transmitting large signals over long distances. This isbecause a high-baud-rate signal contributes to a
reduction in the level of multiplicity when comparedwith a signal of the same bit rate, thus expected to
improve the signal-to-noise ratio. However, highbaud-rate signals occupy a wider signal bandwidth,
which reduces the number of wavelengths available
NTT Technical Review

Vol. 20 No. 8 Aug. 2022

in a single-band ROADM system. As shown in Fig. 2,
for example, a 100-Gbit/s signal occupies about 32
GHz, and about 90 wavelength channels can be
deployed in the currently used C-band (1530 to 1565nm) or L-band (1565 to 1625 nm) [7]. For a 500-Gbit/sclass signal with an occupied bandwidth of 64 GHz
or 1 Tbit/s-class signal with an occupied bandwidthof 130 GHz, only about 60 or 30 waves can be placedat most, respectively. The use of both C- and L-bands
is an effective solution to this problem.
3. C+L-band CDC-ROADM
CDC-ROADM is an optical network node configuration that makes one of the most efficient use of the
optical transmitters and receivers (transponders)installed in a system. Because an optical node must23

Feature Articles

North
C/L-band WDM coupler

North
West

Optical crossconnect

West
East

Optical crossconnect
Optical crossconnect

North
West

East

Transponder
aggregator
Transponder
aggregator

Transponder
aggregator

Optical crossconnect

East

Transponder
aggregator

Optical transponder (L)Optical transponder

Optical transponder

Optical transponder (C)(a) Single-band CDC-ROADM node(b) Multiband CDC-ROADM node withconventional switch devices for single band(c) Multiband CDC-ROADM node with
C+L-band optical cross-connect andmulticast switches

Fig. 3. Configuration of multiband CDC-ROADM node.

be able to communicate with optical nodes in otherdirections, a CDC-ROADM node can use opticaltransponders installed in the node for communication
with any direction with the fewest restrictions, thus
effectively using communication resources.
Figure 3(a) shows a typical CDC-ROADM configuration, which consists of an optical cross-connect
block and optical transponder aggregation blocks.
The optical cross-connect switches optical signals
from different nodes directly to other paths (e.g.,
from West to East) or uses these signals for communication between the node and others. The opticaltransponder aggregator controls the connection
between the optical cross-connect and optical transponder for the optical signals handled by the node.
In conventional ROADM systems, optical transponders can only be used for communication with a specific direction (directioned), or only one opticaltransponder of the same wavelength can be used(contensioned). CDC-ROADM is a highly flexible
optical node configuration that enables any opticaltransponder to be used for communication with any
path as long as that path’s wavelength is not alreadyin use. The NTT Device Innovation Center was thefirst in the world to successfully implement multicast
switches [8].
With the shift to multiband optical transmission,
CDC-ROADM, which has been configured with asingle C- or L-band, must now also be multiband. If a
CDC-ROADM is configured using conventional optical switch devices that operate only in the C- or
L-band, it will have a complex configuration, as
NTT Technical Review

Vol. 20 No. 8 Aug. 2022

shown in Fig. 3(b). Both the optical transponder
aggregator and optical cross-connect must be prepared for the C-band and L-band, and the opticalsignals handled by each must be combined anddivided by a C-/L-band (de)multiplexer to communicate with the transmission fiber, doubling the amountof equipment compared with the conventional singleband system. This results in complexity in the nodeconfiguration as well as operation because operators
have to be aware of the bands. For example, opticaltransponders are optimized for only the C- or L-band,so that the operator needs to be careful into which
system he/she should plug the transponders. In contrast, by using a multicast switch with an extended
operating wavelength range to the C+L-band, a CDCROADM node with a simple configuration can be
implemented, as shown in Fig. 3(c). This is thought
to contribute to the reduction in errors during operation.
Multicast switches have intrinsic loss due to theirprinciple. Figure 4(a) shows the circuit topology of amulticast switch that accommodates M-degree × N
transponder ports. The arrow indicates the directionof optical-signal propagation where the figure shows
the case of signal drop use. Figure 4(b) shows theappearance of the optical circuit chip of the fabricated
C+L-band multicast switch. The multicast switch
contains a splitter that splits the input signal into N
branches. This is the principal loss factor and cannot
be avoided. Therefore, it is preferable to reduce thenumber of branches as much as possible for opticaltransmission characteristics, but reducing the number
24

Feature Articles

From optical cross-connect
#1

#2

#M

#1

#2

#N

1 × N splitterM × 1 switches

To transponder(a) Configuration of multicast switch(b) Appearance of C+L-band multicast switch

Fig. 4. Multicast switch.

Table 1. Dependence of add/drop ratio on the number of branches of multicast switch and signal baud rate.
Number of transponder ports of multicast switch
4

8

12

16

24

Single-band node with 32-Gbaud signals

6.8%

13.5%

20.3%

27.1%

40.6%

C+L-band node with 130-Gbaud signals

13.0%

26.0%

39.1%

52.1%

78.1%

of branches also impacts the achievable add-drop
ratio. However, when dealing with high-baud-ratetransmission, the add-drop ratio can be kept at thesame level as the conventional single-band version of
the node even if the number of branches is reduced.
In a high-baud-rate system, such as 130 Gbaud, thenumber of wavelengths that can be allocated in aband is also reduced, as mentioned above. Therefore,
the reduction in the add-drop ratio due to the reduced
number of branches can be maintained at the samelevel compared with a conventional 32-Gbaud system. Table 1 summarizes the add-drop ratio for asingle-band system with a conventional 32-Gbaud
signal and a C+L-band configuration of 130-Gbaud
signals. The former is assumed to have a channel
spacing of 50 GHz and 96 signals in the C-band,
while the latter has a spacing of 150 GHz and 64 signals in both the C- and L-bands. The add-drop ratio
depends on the size of the wavelength selective
switch (WSS) in the optical cross-connect block. In
this article, we assume a 1 × 20 WSS, which wasavailable when the single-band system was develNTT Technical Review

Vol. 20 No. 8 Aug. 2022

oped, for the conventional single-band-only system,
and 1 × 32 WSS, which has been put into practicaluse, for the C+L-band ROADM system. As shown in
Table 1, a C-band-only system using a multicast
switch with 8-degree ports and 16 transponder ports
has an add-drop ratio of 27%, while a multicast
switch with 8 optical transponder ports has an adddrop ratio of 26% and can be obtained for a 130Gbaud signal. Therefore, even if the number of multicast-switch branches is halved from 16 to 8, thesame level of operability can be secured as with theconventional single-band configuration. In practice,
it is reasonable to estimate the required average adddrop ratio by dividing the total number of wavelengths by the number of nodes in the ROADM system. Thus, even a network with 10 or so nodes
requires an average add-drop ratio of only 10%. The
cases shown in Table 1 clearly satisfy this requirement.

25

Feature Articles

4. Summary
We described the feasibility of a CDC-ROADM
configuration with an optical transponder aggregatorthat operates in the C+L-band. We also successfully
conducted feasibility verification experiments of a
C+L-band CDC-ROADM node using the aforementioned C+L-band multicast switch [9].
Multiband technology not only increases capacity
but also increases the degree of freedom in ROADM
systems by expanding the number of transmission
channels. Combined with the increase in transmission distance due to higher baud rates, multiband
technology contributes to the advancement of optical
networks. NTT is currently conducting research anddevelopment of the APN for IOWN. To implement
the APN, we will continue our research and development to dramatically expand the transmission capacity and improve optical transmission systems byincreasing the speed to 1 Tbit/s by using a wider
wavelength band such as the S-band as well as using
spatial multiplexing technology.
References[1]

Website of NTT R&D, IOWN, “Peripheral Technology for the Scalable and Flexible IOWN Network,” https://www.rd.ntt/e/iown/0008.
html

NTT Technical Review

Vol. 20 No. 8 Aug. 2022

[2]

[3]

[4]

[5]
[6]

[7]
[8]

[9]

T. Sogawa, M. Tomizawa, A. Okada, and H. Gotoh, “All-Photonics
Network and Photonics-electronics Convergence Technologies as a
Vision of the Future,” NTT Technical Review, Vol. 18, No. 10, pp.
12–15, 2020.
h t t p s : / / n t t - r ev i ew. j p / a r c h ive / n t t t e c h n i c a l . p h p ? c o n t e n t s =ntr202010fa1.html
S. Matsuoka, “Ultrahigh-speed Ultrahigh-capacity Transport Network Technology for Cost-effective Core and Metro Networks,” NTT
Technical Review, Vol. 9, No. 8, 2011.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201108fa1.html
Y. Sakamaki, T. Kawai, and M. Fukutoku, “Next-generation Optical
Switch Technologies for Realizing ROADM with More Flexible
Functions,” NTT Technical Review, Vol. 12, No. 1, 2014.
h t t p s : / / n t t - r ev i ew. j p / a r c h ive / n t t t e c h n i c a l . p h p ? c o n t e n t s =ntr201401fa6.html
A. Maeda, H. Emina, R. Morisawa, and S. Takashina, “Improving the
Reliability of Optical Transmission Networks with CDC Technology,”
NTT DOCOMO Technical Journal, Vol. 21, No. 4, pp. 52–60, 2020.
Y. Ogiso, J. Ozaki, Y. Ueda, H. Wakita, S. Kanazawa, and M. Ishikawa, “Ultra-high Bandwidth and Low Drive Voltage InP-based IQ
Optical Modulator for 100-GBd Class Optical Transmitter,” IEICE
Trans. Electron. (JPN Edition), Vol. J103-C, No. 1, pp. 61–68, 2019.
W. Kawasaki, “Standardization Activities in Fourth Quarter of
FY2015—Optical Fiber Working Group,” TTC Report, Vol. 30, No. 3,
pp. 35–39, 2016 (in Japanese).
T. Watanabe, K. Suzuki, and T. Takahashi, “Multicast Switch Technology that Enhances ROADM Operability,” NTT Technical Review,Vol. 12, No. 1, 2014.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201401fa8.html
S. Yamamoto, H. Taniguchi, Y. Kisaka, S. Camatel, Y. Ma, D. Ogawa,
K. Hadama, M. Fukutoku, T. Goh, and K. Suzuki, “First Demonstration of a C + L Band CDC-ROADM with a Simple Node Configuration Using Multiband Switching Devices,” Opt. Express, Vol. 29, No.
22, pp. 36353–36365, 2021.

26

Feature Articles

Kenya Suzuki

Distinguished Researcher, Project Leader,
NTT Device Innovation Center.
He received a B.E. and M.E. in electrical engineering and Dr. Eng. in electronics engineeringfrom the University of Tokyo in 1995, 1997, and2000. In 2000, he joined NTT. From September
2004 to September 2005, he was a visiting scientist with the Research Laboratory of Electronics,
Massachusetts Institute of Technology, Cambridge, MA, USA. From 2008 to 2010, he waswith NTT Electronics Corporation, where he wasinvolved in the development and commercialization of silica-based waveguide devices. In 2014,
he was a guest chair professor at the Tokyo Institute of Technology. He served as secretary of the
Optoelectronics Research Technical Committeeof the Institute of Electronics, Information andCommunication Engineers (IEICE) of Japan
from 2014 to 2016. He was a member of thesubcommittee of Passive Optical Devices for
Switching and Filtering of the Optical Fiber
Communication Conference (OFC) from 2019 to
2021, and served as the chair of the subcommittee in 2022. He is currently a distinguishedresearcher and project leader at NTT Device
Innovation Center. He is also the director of the
New Energy and Industrial Technology Development Organization (NEDO) project on the photonic and electronic hybrid switch for futuredatacenter networking of Photonics Electronics
Technology Research Association (PETRA). His
research interests include optical functional
devices and optical signal processing. He was therecipient of the Young Engineer Award and Electronics Society Activity Testimonial from IEICE
in 2003 and 2016. He is a member of the Instituteof Electrical and Electronics Engineers (IEEE),
Optica, IEICE, and the Physical Society of Japan(JPS).

Koichi Hadama

Senior Research Engineer, Metro-Access Network Device Project, NTT Device Innovation
Center.
He received a B.E. and M.S. in applied physics
and advanced materials science from the University of Tokyo in 1999 and 2001. He joined NTT
Telecommunication Energy Laboratories in2001. Since then, he has been engaged in the
research and development of free-space opticalmodules for WDM network systems. He is currently with NTT Device Innovation Center,
where he is engaged in development of opticaland electrical modules for metro and access networks. He is a member of IEICE.

NTT Technical Review

Vol. 20 No. 8 Aug. 2022

Shuto Yamamoto

Senior Research Engineer, NTT Network Innovation Laboratories.
He received a B.S. and M.S. in physics andPh.D. in electronic engineering from the Tokyo
Institute of Technology in 2003, 2005, and 2018.
His research interests include optical signaltransmission and optical transport network systems. He received the Best Paper Award at the
16th OptoElectoronics and Communications
Conference (OECC), the Young Researcher’sAward from IEICE, and the Achievement Award
from IEICE in 2011, 2012, and 2016, respectively. He also received the NTT Presidential
Award and the TELECOM System Technology
Award from the Telecommunications Advancement Foundation in 2014 and 2019, respectively.

Hiroki Taniguchi

Research Engineer, NTT Network Innovation
Laboratories.
He received a B.E. and M.E. in electrical engineering from the Tokyo Institute of Technology
in 2014 and 2016. His research interest includes
photonic transport network systems. He was therecipient of the 2019 IEICE Communications
Society Optical Communication Systems Young
Researchers Award and the Young Researcher’sAward from IEICE in 2020. He is a member of
IEICE.

Yoshiaki Kisaka

Senior Research Engineer, Supervisor, Photonic Transport Network Laboratory, NTT Network Innovation Laboratories.
He received a B.S. and M.S. in physics from
Ritsumeikan University, Kyoto, in 1996 and1998. In 1998, he joined NTT Optical Network
Systems Laboratories, where he was engaged inresearch and development on high-speed opticalcommunication systems including 40-Gbit/s/ch
WDM transmission systems and the mapping/
multiplexing scheme in the optical transport network (OTN). From 2007 to 2010, he was withNTT Electronics Technology Corporation, where
he was engaged in the development of
40/100-Gbit/s OTN framer large-scale integration. His current research interests are in highspeed and high-capacity optical transport networks using digital coherent technology. Since
2010, he has contributed to the research anddevelopment of digital coherent signal processer
for 100 Gbit/s and beyond. He is a member of
IEICE.

27

Feature Articles: Device Technologies for Enabling the
All-Photonics Network

Development of Membrane Optical
Modulators for IOWN
Tatsurou Hiraki, Takuma Aihara, Takuro Fujii,
Koji Takeda, Toru Segawa, and Shinji Matsuo
Abstract

To develop a high-speed, large-capacity optical network with low power consumption, which is important for IOWN (the Innovative Optical and Wireless Network), we are conducting research and development on membrane optical devices that integrate III-V compound semiconductors on silicon-photonics
circuits. Membrane optical devices are expected to be used for high-modulation efficient lasers andoptical modulators because they can achieve high optical confinement in the core layer. This article
introduces the current state of research and development focusing on optical modulators and the integration of such modulators and lasers on the basis of membrane-device technologies.
Keywords: silicon photonics, optical interconnection, optical modulator

1. Introduction
Optical interconnection is a technology that is moreadvantageous than data communication using electricity as the transmission capacity and transmission
distance increase due to the characteristics of wideband and low loss of optical fiber. This technology
has been used in long-distance communication networks since the 1980s. Due to the increase in Internet
traffic, the application area where optical fiber canshow its superiority has been expanded to shorterdistance transmission, and optical interconnection is
widely used in datacenter networks. As the number ofdevices connected to the Internet and the amount oftraffic from devices increase, it is expected that theincrease in traffic will continue. Therefore, researchinterests are to increase the transmission capacity
where optical interconnections have already been
applied and apply optical interconnection to shorterdistance transmission, as shown in Fig. 1. Specifically, optical interconnections inside the board andchip are expected to become important for improving
the throughput of electronic devices such as routersand servers.
For this purpose, it is important to reduce the power
consumption and cost of optical transmitters. To
NTT Technical Review

Vol. 20 No. 8 Aug. 2022

reduce power consumption, it is important to modulate optical devices at high speed. To achieve this, itis necessary to increase the optical-confinement factor in the core region as much as possible. A large costis required for assembly and testing, so cost reduction
is necessary by integrating multiple optical devices inthe same substrate. It is therefore important to use
silicon (Si) photonics technology, which can be used
to fabricate optical waveguides with low loss andoptical circuits with high performance by using fabrication technology used in Si electronic circuits. Since
it is not possible to fabricate a laser or a highly efficient optical modulator using Si, a method for integrating III-V compound semiconductors, which are
the materials for lasers and modulators, is an issue forlarge-scale integration.
In this article, we describe membrane optical devices on Si substrates. Such devices are thin-film opticaldevices fabricated on low-refractive materials. We
previously fabricated and demonstrated high-efficiency lasers and modulators using indium phosphide(InP)-based compound semiconductors with a typical
thickness of about 250 nm on silicon dioxide (SiO2)/
Si substrates [1–5]. High optical confinement of thecore layer can be obtained by sandwiching a device
between low-refractive index materials such as SiO2
28

Feature Articles

< km

<m

~mm

Board-to-board

Inside board

Inside chipFig. 1. Development of optical interconnection.

and air. We specifically describe the optical modulators in detail.
2. Membrane optical modulators
Optical modulators can be classified into two types:
phase modulators that mainly modulate the refractive
index and intensity modulators that modulate theabsorption coefficient. A phase modulator is used asa Mach-Zehnder (MZ) modulator in combination
with an MZ interferometer, and by combining MZmodulators and modulating the phase and intensity, it
is possible to obtain a large-capacity transmission
exceeding 1 Tbit/s per wavelength and long-distancetransmission. Typical materials used are lithium niobate (LiNbO3), Si, and InP. Due to the increase intraffic, optical interconnects increase their importance in a datacenter. Thus, it is important to reduce
the transmitter cost by reducing the device size andintegrating many devices including lasers on thesame chip. For this purpose, InP-based and germanium silicon (GeSi) intensity modulators, which are
one order of magnitude more efficient than LiNbO3and Si, are considered key devices. Since intensity
modulators use only the intensity change as a signal,
the transmission capacity is generally smaller thanthat of MZ modulators, but their simple and compact
configuration is important when a large number oftransmitters are required at shorter distances. Considering their use in the 1.3-μm wavelength used indatacenters, there is currently a problem in the growthof GeSi; thus, InP compound semiconductors havebecome advantageous in addition to integrate lasers.
Figure 2(a) shows a schematic of a photonic-integrated circuit using an MZ modulator consisting of a
Si MZ interferometer and InP phase modulators, andmembrane lasers [6]. By integrating the spot size
converters (SSCs), it is possible to assemble a device
with optical fiber with low-coupling loss by buttcoupling the device to the optical fiber without using
NTT Technical Review

Vol. 20 No. 8 Aug. 2022

a lens. Figures 2(b) and (c) show the cross-sectionalview of a membrane laser and membrane phasemodulator. Reduction in the optical-confinement factor in the laser core layer is important to suppress
internal loss and spatial hole-burning because thelaser for biasing the modulator requires high output
power and stable single-mode lasing. Therefore, theSi waveguide is placed under the laser core layer.
Since the effective refractive index of the membranelaser structure is similar to that of the Si waveguide,
the optical-confinement factor in the laser core layer
can be controlled by adjusting the width of the Si
waveguide. In a phase modulator, the Si waveguide iseliminated to maximize the optical-confinement factor. Therefore, the membrane optical device canfreely design the confinement of light between the Si
waveguide and membrane optical device.
The fabrication process is shown in Fig. 3. (a) A Si
waveguide is fabricated using a Si on insulator (SOI)
substrate, and after the entire waveguide is covered
with SiO2, it is planarized using chemical mechanicalpolishing, and the multiple-quantum-well (MQW)
layer, which is the core layer of the laser grown on the
InP substrate, is directly bonded on the planarized
SiO2. (b) The MQW layer is removed by selective
etching except for the area that includes the laser corelayer, and the InP layer is exposed. (c) An n-type
indium gallium arsenide phosphide (InGaAsP) layer,
which is the core layer of the phase modulator, isgrown on the InP layer by using metal organic chemical vapor deposition. (d) The laser and phase modulator core regions are formed using selective etching.
(e) Core regions are embedded with an undoped InP
layer by using selective regrowth. (f) Selective doping is carried out to form n- and p-type doping
regions. (g) The laser and phase modulator are separated and a grating is formed in the top surface layerof the laser region. (h) Finally, an SSC and electrodes
are formed. The advantage of this fabrication method
involves the regrowth of compound semiconductors
29

Feature Articles

n-InGaAs

n-InGaAs

p-InGaAs

p-InGaAs

SiN grating
Phase
modulator

SSC

n-InP

p-InP

MQW

InGaAsP

n-InP

p-InP

Si
Membranelaser

Si MMI

SiO2

SiO2

(a)

(b)

(c)

InGaAs: indium gallium arsenide
MMI: multimode interferometer
SiN: silicon nitride

Fig. 2. (a) Schematic diagram of photonic integrated circuit including MZ modulator and membrane laser. (b) Crosssectional view of membrane laser. (c) Cross-sectional view of phase modulator.

(a)

MQW

Si

InGaAs

Undoped InP

Si taper tip(b)

LD area

InP template(c)

InP cap

n-InGaAsP bulk

LD coreMZM core

(d)

(e)

(f)

Si

(g)

DFB grating

(h)

Zn

Si

Zn

SiOx core

DFB: distributed feedback
LD: laser diodeZn: zincFig. 3. Fabrication procedure.

NTT Technical Review

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30

Feature Articles

2.5

0
−10
25°C
Output power (dBm)

Output power (mW)

2.0

1.5

1.0
80°C
0.5

−20

25°C
80°C

−30
−40
−50
−60
−70

0.0

0

10

20

30

40

50

60

70

80

−80
1550

1555

1565

1560

LD current (mA)

Wavelength (nm)

(a)

(b)
80°C

25°C

5 ps/div(c)

5 ps/div(d)Fig. 4. C haracteristics of fabricated device. (a) I-L characteristics at 25 and 80°C. (b) Lasing spectra. (c), (d) Eye diagramswith 50-Gbit/s NRZ signals at 25 and 80°C.

on Si substrates. Since the total thickness of the membrane optical device is about 230 nm, regrowth on the
Si substrate becomes possible due to the resistance tothe strain caused by the thermal-expansion coefficient difference between Si and InP. Thus, InP-based
semiconductors with different bandgaps, such as
laser-active layers and phase-modulator layers, canbe fabricated on Si on a wafer scale. Since the marker
for the stepper on the Si substrate is used for thealignment of the III-V core regions, the Si waveguideand membrane optical device can be integrated withthe alignment accuracy of the stepper.
The characteristics of the fabricated device areshown in Fig. 4. The length of the laser-active region
and phase modulator is 500 μm. Thanks to the highmodulation efficiency of the n-InGaAsP layer, thelength of the phase modulator is much shorter thanthe Si phase modulator, which has a length of several
millimeters. Figure 4(a) shows the current dependence of the output light intensity when the MZ optiNTT Technical Review

Vol. 20 No. 8 Aug. 2022

cal modulator is set to the OFF state. The output lightis received by the lensed fiber at measurement temperatures of 25 and 80°C. In this experiment, reflection occurred at the end facet of the device, resulting
in mode hopping, as shown in the figure. It is possible
to suppress the reflection by directly connecting
fibers, but we use lensed fiber to simplify the experiment. The threshold current was about 6 mA at 25°C
and about 8 mA at 80°C. The maximum fiber output
power was 2 mW at 25°C and about 1 mW at 80°C.
The fiber coupling loss was 3 dB. Figure 4(b) shows
the oscillation spectra at 25 and 80°C with bias currents of 76.0 and 50.6 mA, respectively. Single-mode
oscillation was obtained, and the side mode suppression ratio at 80°C was 59 dB. Figure 4(c) shows the
dynamic characteristics of the MZ modulator whenmodulated with a 50-Gbit/s non-return-to-zero(NRZ) signal at operating temperatures of 25 and
80°C. An electrical signal with a peak voltage of 2.5
V was input into the phase modulator, and the signal
31

Feature Articles

1
0
−1

−5

E-O response (dB)Normalized transmittance (dB)

0

−10

−15

−2
−3
−4
−5
−6
−7
−8

−20
0

1

2

3

4

5

6

0

10

20

30

40

DC bias (V)Frequency (GHz)

(a)

(b)

50

60

Extinction ratio: 4.6 dB

(c)

(d)

DC: direct current
E-O response: electro-optical response

Fig. 5. C haracteristics of fabricated EAM. (a) Voltage dependence of transmittance at operating wavelength of 1280 nm.
(b) Small signal response at bias current of 2 V. (c) Input electrical and (d) output optical eye diagrams with
100-Gbit/s NRZ signals.

was terminated by 50 ohms. Although the extinction
ratio was as low as 3 dB, we confirmed a clear eyeopening at both temperatures. The extinction ratio
can be improved by differential operation and applying
an inverted signal to the other electrode of the phasemodulator. The core layer of the phase modulator isrelatively long (500 μm) for lumped electrodes, but it
can operate on 50-Gbit/s NRZ signals thanks to the
low capacitance of the membrane structure.
We also developed an electro-absorption modulator(EAM) using InP-based MQWs [7]. Since we canobtain large absorption change in the EAM, there isno need to use an interferometer such as an MZ
modulator, which results in a very simple configuration. Therefore, EAMs with a core-layer length ofabout less than 300 μm on InP substrates are inteNTT Technical Review

Vol. 20 No. 8 Aug. 2022

grated with lasers and are widely used in datacenters.
However, the operation speed of the EAM on InP
substrate is limited by the capacitance, and it is necessary to use the 50-ohm termination and apply thetraveling wave-type electrode to increase the speed.
Therefore, the EAM using a membrane structure has
attracted attention due to its low capacitance. Since it
is important to increase the optical-confinement factor to the core layer as well as the phase modulator,
we fabricated a device without placing a Si waveguide under the EAM core layer. A nine-layer MQWwith a photoluminescence peak of 1230 nm was usedas the core layer. The voltage dependence of transmittance at an operating wavelength of 1280 nm isshown in Fig. 5(a). For a device with a core-layer
length of 200 μm, an extinction ratio of 8.5 dB can be
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Feature Articles

obtained by changing the bias voltage from 0 to 3 V.
The insertion loss of the EAM excluding the fiber
coupling loss is estimated to be 3 dB compared witha Si waveguide. Figure 5(b) shows the small-signal
response. The operating wavelength was set to 1280
nm, and the bias voltage was set to 2 V. We did not use
a 50-ohm termination. As shown in the figure, the
3-dB band was 59 GHz. This clearly shows theadvantage of a membrane photonic device.
Next, we measured the eye diagrams of the
100-Gbit/s NRZ signal. Figures 5(c) and (d) show
the input electrical signal and optical output signal,
respectively. The bias voltage of the EAM was set to
1.2 V. An electrical signal with a peak voltage of 0.12
V from the pulse pattern generator was amplified
with an electrical linear amplifier of 22 dB andapplied to the EAM. The eye diagram is comparable
to the input electrical signal, and the extinction ratio
was 4.6 dB. Since laser integration is possible withthe same manufacturing process as a MZ-modulatorintegrated distributed feedback laser, it is expected tobe applied to the optical interconnection of shortdistances such as inside racks and boards.
3. Summary
This article argued that membrane optical modulators are suitable for fabricating modulators with highspeed and low power consumption because of theirhigh optical-confinement factor and low capacitance.
An SSC is also integrated so that it can be easily connected to fiber arrays. Because an SCC can be integrated on the Si photonic circuit, it is also possible tointegrate it with multiplexer/demultiplexer components using the Si waveguide. Therefore, it is expected to be a key device for short-distance optical inter-

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connections, where it is important to integrate opticaldevices at high density and maximize throughput perunit length.
References
[1]

[2]

[3]

[4]

[5]

[6]

[7]

K. Takeda, T. Fujii, T. Kishi, K. Shikama, H. Wakita, H. Nishi, T. Sato,
T. Tsuchizawa, T. Segawa, N. Sato, and S. Matsuo, “Device Technology for Short-range Optical Interconnections with High Density and
Low Power Consumption,” NTT Technical Review, Vol. 18, No. 10,
pp. 21–30, 2020.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr202010fa3.html
T. Fujii, K. Takeda, N-P. Diamantopoulos, E. Kanno, K. Hasebe, H.
Nishi, R. Nakao, T. Kakitsuka, and S. Matsuo, “Heterogeneously
Integrated Membrane Lasers on Si Substrate for Low Operating
Energy Optical Links,” IEEE J. Sel. Top. Quantum Electron., Vol. 24,
No. 1, 1500408, 2016.
H. Nishi, T. Fujii, N-P. Diamantopoulos, K. Takeda, E. Kanno, T.
Kakitsuka, T. Tsuchizawa, H. Fukuda, and S. Matsuo, “Monolithic
Integration of an 8-channel Directly Modulated Membrane-laser
Array and a SiN AWG Filter on Si,” Proc. of, Optical Fiber Communication Conference and Exhibition 2018, Paper Th3B.2, San Diego,
USA, Mar. 2018.
T. Kishi, H. Wakita, K. Shikama, M. Nagatani, S. Kanazawa, T. Fujii,
H. Nishi, H. Ishikawa, Y. Kawajiri, A. Aratake, H. Nosaka, H. Fukuda,
and S. Matsuo, “A 25-Gbps x 4ch, Low-power Compact Wire-bondfree 3D-stacked Transmitter Module with 1.3-μm LD-array-on-Si for
On-board Optics,” Optical Fiber Communications Conference andExhibition (OFC) 2019, Paper Tu2l.1, San Diego, USA, Mar. 2019.
K. Takeda, T. Sato, A. Shinya, K. Nozaki, W. Kobayashi, H. Taniyama, M. Notomi, K. Hasebe, T. Kakitsuka, and S. Matsuo, “Few-fJ/bit
Data Transmissions Using Directly Modulated Lambda-scale Embedded Active Region Photonic-crystal Lasers,” Nat. Photonics, Vol. 7,
pp. 569–575, 2013.
T. Hiraki, T. Aihara, T. Fujii, K. Takeda, Y. Maeda, T. Kakitsuka, T.
Tsuchizawa, and S. Matsuo, “Integration of a High-efficiency MachZehnder Modulator with a DFB Laser Using Membrane InP-based
Devices on a Si Photonics Platform,” Opt. Express, Vol. 29, pp.
2431–2441, 2021.
T. Aihara, T. Hiraki, Y. Maeda, T. Fujii, T. Tsuchizawa, K. Takahata,
T. Kakitsuka, and S. Matsuo, “60-GHz-bandwidth O-band Membrane
InGaAlAs Electro-absorption Modulator on Si Platform,” Proc. of
the17th IEEE International Conference on Group IV Photonics (GFP
2021), Malaga, Spain, Dec. 2021. https://doi.org/10.1109/
GFP51802.2021.9674001

33

Feature Articles

Tatsurou Hiraki

Distinguished Researcher, Materials andDevices Laboratory, NTT Device Technology
Laboratories; and NTT Nanophotonics Center.
He received a B.E., M.E., and Ph.D. fromTohoku University, Miyagi, in 2009, 2011, and2017. In 2011, he joined NTT Microsystem Integration Laboratories. His current research interests include heterogeneously integrated III-V
semiconductor optical modulators, opticalamplifiers, and laser diodes on Si photonics circuits. He is a member of the Institute of Electricaland Electronics Engineers (IEEE), Institute ofElectronics, Information and Communication
Engineers (IEICE), and Japan Society of Applied
Physics (JSAP).

Takuma Aihara

Research Engineer, Materials and Devices
Laboratory, NTT Device Technology Laboratories.
He received a B.E., M.E., and Ph.D. in electrical and electronic information engineering from
Toyohashi University of Technology, Aichi, in
2010, 2012, and 2015. He joined NTT Device
Technology Laboratories in 2015. He has been
researching III-V semiconductor lasers on Si
photonic integrated circuits. He is a member ofIEEE and JSAP.

Takuro Fujii

Research Engineer, Materials and Devices
Laboratory, NTT Device Technology Laboratories.
He received a B.E. and M.E. in system design
engineering from Keio University, Kanagawa, in2010 and 2012. In 2012, he joined NTT Photonics Laboratories, where he has been researching
the metalorganic vapor phase epitaxy (MOVPE)
growth of III-V semiconductors and development of III-V semiconductor lasers on Si forphotonic integrated circuits. He was the recipientof the Young Scientist Presentation Award from
JSAP in 2014. He is a member of IEEE, IEICE,
and JSAP.

NTT Technical Review

Vol. 20 No. 8 Aug. 2022

Koji Takeda

Senior Research Engineer, Materials andDevices Laboratory, NTT Device Technology
Laboratories.
He received a B.S., M.S., and Ph.D. in electronics engineering from the University of Tokyoin 2005, 2007, and 2010. From 2008 to 2010, he
received a research fellowship for young scientists from Japan Society for the Promotion of
Science. In 2010, he joined NTT Photonics Laboratories. In 2018, he also joined IHP microelectronics GmbH as a visiting researcher. His current research interests include ultralow-power
optical interconnects, photonic integrated circuits, and photonic crystal lasers. He is therecipient of the Best Student Paper Award fromthe IEEE Photonics Society in 2009, Outstanding
Student Presentation Award from JSAP in 2010,
and Best Paper Award from IEICE in 2012. He isa member of IEEE, IEICE, and JSAP.

Toru Segawa

Group Leader, Materials and Devices Laboratory, NTT Device Technology Laboratories.
He received a B.E. and M.E. in electrical engineering and Ph.D. in integrated design engineering from Keio University, Kanagawa, in 1999,
2001, and 2012. In 2001, he joined NTT Photonics Laboratories and has been engaged inresearch on high-speed tunable lasers and opticalswitches monolithically integrated on InP substrates. His research interests include opticaldevice and sub-system technologies for opticalcommunications systems. He is a member ofIEEE, JSAP, and IEICE.

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 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 MQW pin modulators and verticalcavity surface emitting lasers. In 1997, heresearched optical networks using wavelength
division multiplexing technologies at NTT Network Innovation Laboratories. Since 2000, he
has been researching high-speed tunable opticalfilters and lasers at NTT Photonics Laboratories
and NTT Device Technology Laboratories. He isa member of JSAP and IEICE and a Fellow ofIEEE and Optica.

34

Feature Articles: Device Technologies for Enabling the
All-Photonics Network

Observation of Exceptional Point
Degeneracy with Current-injected
Photonic Crystal Lasers
Kenta Takata, Kengo Nozaki, Eiichi Kuramochi,
Shinji Matsuo, Koji Takeda, Takuro Fujii, Shota Kita,
Akihiko Shinya, and Masaya Notomi
Abstract

Coupled resonators and coupled waveguides with contrast of their gain and loss are non-Hermitian
optical systems, which are based on energy-non-conserving processes. They have been reported to
exhibit a number of unconventional phenomena that are expected to contribute to practical applications.
We developed a coupled system of two current-injected photonic crystal lasers that enable precise control of gain and loss, thus establishing a promising non-Hermitian nanophotonic platform. We introduce
our demonstration of a peculiar non-Hermitian mode degeneracy called an exceptional point (EP) andthe EP-based enhancement of spontaneous emission with this system.
Keywords: non-Hermitian photonics, exceptional point, photonic crystal lasers

1. Non-Hermitian photonics
To generate and control optical signals, devices
such as lasers, modulators, and photodetectors are
required. They vary the intensity of light by amplifying it via stimulated emission, releasing it from optical signal channels, or absorbing it with the generation of electric carriers. In the Maxwell equation ofelectromagnetism, optical amplification gain andabsorption loss in a medium are expressed as theimaginary part of its refractive index. When resonators and waveguides comprise media with finite
imaginary parts of refractive indices, possible statesof electromagnetic waves confined in them (eigenmodes) have generally complex frequencies andpropagation constants (eigenvalues). The imaginary
part of the eigenvalue corresponds to the rate of optical gain or loss per unit time for a resonator and perunit propagation distance for a waveguide.
A system is called Hermitian when the energy ofthe physical quantity of interest (light waves here) is
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conserved. In contrast, a system based on energynon-conserving processes is non-Hermitian. Since
the mode eigenvalues of a non-Hermitian opticalsystem are complex numbers, as mentioned above,
light waves there may continue to be amplified orabsorbed until the carrier response is saturated or may
be totally radiated out. Unlike Hermitian systems,
eigenmodes of non-Hermitian systems are generally
not orthogonal to each other. Given that it is impossible to measure a completely closed system without
energy dissipation, all the states we observe are thoseof non-Hermitian systems. This means that every
state of optical systems may not be independent but
more or less mixed with other ones. Responses ofsuch non-Hermitian systems seem to be much morediverse and difficult to understand compared with
Hermitian systems that are assumed as approximations in many cases. In addition, gain and loss had
been considered to only control the intensity of lightin photonics. Thus, the research for other applicationsof them had been limited.
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Coupling 
Gain
−



T

Loss
>0

+

  −

+−−Central axis−  

P

Back to original state(a) PT symmetry of a system of two coupled cavities.  : coupling rate.
 : loss rate in a single cavity mode (a negative value represents gain).

Anti-symmetricmode

SymmetricmodeReal partEP

Loss and gain magnitude  (unit:  )Net gain and loss (unit:  )Eigenmode frequency (unit:  )Complex eigen-detuning:  −  0 = ±  2 −  2Imaginary partEP

Loss and gain magnitude  (unit:  )Localized modewith loss

Localized modewith gain(b) EP transition of the eigenmode frequency and net gain and loss. The red and blue ellipses in the mode
schematics represent antiphase fields.  0: resonance frequency of each cavity mode.

Fig. 1. PT symmetry and EP phase transition of two coupled resonators.

1.1 PT symmetry
A turning point in these circumstances was theintroduction of the concept of parity-time (PT) symmetry to photonics [1] by the group headed by Professor Christodoulides at the University of Central
Florida. Parity in PT symmetry means spatial inversion. This corresponds to setting a central axis orplane of symmetry (double-sided mirror) in spacethen inverting the system (or jumping to the world on
the other side of the mirror). Time in PT symmetry
stands for time reversal, which means turning gain toloss and loss to gain. A system is said to have PT symmetry when both of these operations make it get backto its original shape. While PT symmetry has attracted attention in quantum mechanics, Makris et al.
showed that the analogy of PT symmetry in theabove-mentioned sense held in the refractive index
distributions of optical systems. They also showedthat such a system could have a peculiar mode degeneracy called an exceptional point (EP).
We now explain the characteristics of a coupled
two-cavity system with PT symmetry shown in
Fig. 1(a). We assume that each resonator has an
eigenmode (cavity mode) with an identical resonance
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frequency when isolated. The cavities are placed in
proximity with an interval of the order of the mode
wavelength. Thus, a small number of fields of one
cavity mode seeps into the other via the evanescentwave*1 so that they are coupled with each other. The
system is PT-symmetric when two cavities have loss
and gain with the same magnitude γ. Note that thegain rate is denoted as a negative value of –γ, whichis contrasted to that of the loss γ > 0.
1.2 EP phase transition
Figure 1(b) shows how the system response changes when the coupling rate is fixed to κ and the γ of thebalanced gain and loss is varied. The left and rightplots respectively show the eigenmode frequencies(the real part of the complex eigenfrequency) withreference to the single cavity mode frequency ω0 andtheir net loss rates (the imaginary part of the complex
eigenfrequency) for which negative values mean
*1 Evanescent wave: A weak light wave that enters a medium withexponential spatial attenuation. It appears when the light with acertain wavelength is reflected from a medium that has no propagation mode for that wavelength, such as a photonic crystal or
mirror.

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gain. When γ = 0, which corresponds to the Hermitiansystem, the two eigenmode frequencies are split by
2κ by the coupling. The eigenmode distributions are
two orthogonal states termed symmetric and antisymmetric. As γ increases, the frequency splitting
decreases then changes abruptly toward zero near γ =κ. In this case, because both eigenmodes distribute anequal field intensity for the two cavities with balanced gain and loss, their net loss or gain rates areexactly zero. When γ > κ, the two mode frequencies
are the same. In contrast, the eigenmodes’ loss or gain
rates split sharply into positive and negative values.
Despite the fact that the cavities are supposed to becoupled, the eigenmodes become localized by theincrease in γ ; one concentrates in the cavity withgain, hence amplified, and the other is in the lossy
cavity, thus damped. When the gain, loss, and coupling rates are equal (γ = κ), the eigenmode frequencies coincide, and the net loss or gain rates remain
zero just before the bifurcation. What happens is not
the accidental degeneracy of two different states withthe same complex frequency; the distributions of thetwo modes rather become completely the same. Thus,
there is only a single state. The degeneracy in thissense is a phenomenon peculiar to non-Hermitiansystems and termed an EP. It can also be said that an
EP is a phase-transition point from modes spreadingover the entire system to those localized in cavities
with gain or loss.
To date, a number of unconventional phenomena
based on this EP phase transition have been reported,
and readers can refer to a previous review article [2].
Examples include non-reciprocal propagation*2, single-mode oscillation, and the enhancement of frequency perturbation effect, each of which has a goodpotential for practical use. Non-reciprocal propagation is a necessary condition for an optical isolator
that protects a laser against instability by undesired
optical feedback. Although integrating an optical isolator into a photonic circuit is technically difficult, it
is an important element to complete optical information processing within a chip. Unfortunately, the nonreciprocal property near an EP is valid only for acertain range of the input light intensity because it isbased on gain saturation. Thus, further studies areneeded to pave the way for achieving a practical isolator based on an EP.
2. Observation of EP degeneracy by coupled
photonic crystal laser
There are very few examples of observing EP
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degeneracy in photonics because of certain technicalproblems. One issue is the need for precise control ofgain and loss. An EP is actually a point in the gain andloss parameter space, around which the complex
eigenfrequencies vary greatly. Therefore, to observe
EP degeneracy, it is necessary to control the gain andloss of each cavity independently and continuously.
Another issue is the change in individual cavity frequency in controlling gain and loss. Regarding coupled cavities with the above-mentioned controllability, integrated systems of multiple semiconductor
lasers are considered first. However, since driving
them is accompanied by the generation of heat andfree carriers, the real part of the refractive index ofeach cavity medium also changes when the gain and/or loss difference is introduced. As a result, the resonant frequencies of the two cavity modes diverge.
Because this frequency detuning is known to lift EP
degeneracy, it is necessary to use lasers with highexcitation efficiency as much as possible to minimalize the impact of such an effect.
2.1 Current-injected photonic crystal laser
We fabricated the coupled system of two currentinjected photonic crystal lasers [3] shown in Fig. 2(a).
The system is structured on an indium phosphide(InP) thin film suspended in air. It has a two-dimensional photonic crystal with an array of air holes witha diameter of about 200 nm. Two wavelength-scale
gain media*3 with quantum wells (red rectangles in
the figure) are embedded in line defects without airholes and work as nanocavities. In addition, p-doped
layers (purple parts) and n-doped layers (green parts)
are formed in the oblique directions from the fourelectrodes (yellow parts), constituting two in-planecurrent-injection channels. Because this structure
suppresses the leakage current between the electricchannels, the gain and loss of each nanocavity can becontrolled independently by current injection. To
date, by using such a photonic crystal laser on thebasis of a buried heterostructure nanocavity, we demonstrated the smallest laser threshold current in the*2 Non-reciprocal propagation: Propagation under the condition thatthe scattering matrix representing the input and output relation
for an optical component is asymmetric because of the magnetooptical, nonlinear optical, or time-modulation effects.
*3 Gain medium: A medium that can amplify light in the spontaneous emission wavelength range by stimulated emission based on
the population inversion of electric carriers. Its activation requires
short-wavelength light irradiation (optical pumping) or current
injection (electric pumping). Our experiment exploited the property that a gain medium with no or small injection current acts as
an optical absorber.

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Wavelength (nm)I2 = 100 µA

I2 (µA)

Near EP(Peak coalescence)

I1 (µA)
Small(a) False-color microscope image of thecurrent-injected photonic crystal lasers

Photon count

Channel 1, Current I1 (µA)
Large
Cavity 1, loss  1(b) I1 dependence of the emission spectrum for I2 = 100 µA.
Black points: theoretical eigen-wavelengths.

Fig. 2. EP phase transition of current-injected photonic crystal lasers.

world [4]. The low current drive of our system minimalizes the detrimental effect of the cavity-frequency
detuning due to heat and carriers.
In Fig. 2(a), the right and left cavities and current
channels are numbered 1 and 2, and the inter-electrode current values for channel 1 and 2 are denotedas I1 and I2, respectively. The emission spectrum of
the system was measured while I1 and I2 were varied.
As a result, when I2 was fixed at 100 μA and I1 wasdecreased, a clear EP phase transition was observed.
Figure 2(b) shows the color plot of such emission
spectra for different I1. Here, I2 = 100 μA is the condition to compensate for the loss γ2 due to absorption
and radiation in cavity 2 (γ2 ~ 0). However, a finite
loss γ1 of resonator 1 remains. Consequently, theentire system does not oscillate, and weak spontaneous emission of photons is hence observed. As I1
diminishes, the two spectral peaks indicating coupledmodes approach each other and eventually coalesce.
This is because the difference in the loss of the twocavities is enhanced by the increase in γ1, resulting in
the variation of the eigenmode frequencies similar tothat in Fig. 1(b). By fitting the spectra using a theoretical model, we can estimate the resonance frequency of each cavity mode, cavity coupling κ, andloss γ1. We found that the system was very near theexact EP when I1 = 1.4 μA. The detrimental frequency detuning of the two cavities is also very small. Thiscan be seen from the fact that the theoretical eigenwavelengths (black points in Fig. 2(b)) calculated
from the fitting result are aligned with a point in therelevant region. If I1 is further decreased, the peak
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photon number increases in spite of the magnified
loss. This is thought to be because one of the eigenmodes is localized in resonator 2, which has a large
injection current, becoming less susceptible to theloss of resonator 1. This intensity recovery also supports the EP phase transition illustrated in Fig. 1(b).
2.2 Spontaneous emission spectrum at the EP
We also clarified that the spectrum near theobserved EP mentioned above indicated the peculiar
enhancement of spontaneous emission. Even though
the loss of one cavity is compensated and the otherhas a loss, the system with a large coupling exhibits aspontaneous emission spectrum that is expressed as
the sum of two split Lorentzian functions*4 (bluespectrum in Fig. 3(a)). In contrast, the emission spectrum of the EP state takes the shape of the squared
Lorentzian function (red spectrum in Fig. 3(a)) [5].
When the system can be approximated as a Hermitian
one, the two modes are almost independent. Thus,
even if their peaks coincide accidentally, the emission
spectrum will only be the sum of them, that is, aLorentzian function with twice their peak intensity. In
stark contrast, the two modes degenerate into exactly*4 Lorentzian function: The name of a function that represents theshape of the power spectrum of a general resonator response thatexhibits exponential energy decay (in the present case, radiation
based on spontaneous emission). If the system suffers from significant noise, the spectrum is disrupted and its shape becomes a
Gaussian function with a broader linewidth. The reliable spectrum analysis for our experiment was enabled by both low-noise
current drive of the device and low-noise spectroscopic measurement by long-time integration.

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Feature Articles

Spectral power



0

+1

EP:  = 1/2Squared Lorentzian >> 1/2
LorentzianSingle cavity frequency  0

Number of detected photons

Linear×4

Experiment
Analysis
Lorentzian(Lorentzian)2Lorentzian curve:
deviates fromexperimental
data

Major structure issquared LorentzianFrequency(a) Change in the emission spectrum at the EP.
The EP condition for  2 = 0 is  =  1/2.

Semi-logarithmic

Frequency (THz)

Frequency (THz)(b) Spontaneous emission spectrum very near the EP (I1 = 1.4 µA, I2 = 100 µA).
Left: Linear scale; Right: Semi-logarithmic scale.

Fig. 3. Enhancement of the system’s spontaneous emission due to EP degeneracy.

the same state at the EP, exhibiting constructive interference in the spectral domain. Suppose we vary only
the coupling κ that always preserves the energy sothat the system is dragged to the EP condition while
keeping the other parameters constant. In this case,
we can measure the impact of the spectral change
induced purely by EP degeneracy. Because of the
maximum intensity change due to the interference,the peak intensity of the EP spectrum (colored red) is
four times (= (1 + 1)2) larger than those of the split
Lorentzian peaks (blue). From the energy conservation, the total integrated powers for the two spectra in
Fig. 3(a) are equal, and the loss of the eigenmodes forthem is also the same. Despite this, the peak intensityof the EP is enhanced via the spectral change inwhich the shape becomes squared-Lorentzian; thus,
the linewidth becomes narrower than the Lorentzianpeaks for the large coupling.
Our system can control the gain and loss of eachlaser but not the coupling between the lasers. In our
experiment, unfortunately, the effect of reducing thepeak intensity by increasing γ1 was superimposed on
the measurement results. Thus, the directly observed
ratio of the peak-intensity enhancement by the EP isabout 30%. However, as shown in Fig. 3(b), we also
observed that the emission spectrum at I1 = 1.4 μA,
which was closest to the EP, was a squared Lorentzian function. This result indicates that the system isunder an almost exact EP degeneracy. This meansthat the effect of EP degeneracy can be clearlyobserved if the system approaches the states at which
its complex-eigenfrequency splitting is small enoughcompared with the linewidths of the modes.

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3. Future outlook
In this article, we introduced the observation of EP
degeneracy with our current-injected photonic crystal
lasers. We are now conducting experimental demonstrations of our other theoretical proposals, such as
the control of group velocity using an EP [6] and thatof photonic topology by gain and loss [7], by using
more large-scale photonic crystal laser arrays. We arealso exploring new principles and phenomena in nonHermitian nanophotonics.
References
[1]
[2]
[3]

[4]

[5]

[6]
[7]

K. G. Makris, R. El-Ganainy, D. N. Christodoulides, and Z. H. Musslimani, “Beam Dynamics in PT Symmetric Optical Lattices,” Phys.
Rev. Lett., Vol. 100, No. 10, 103904, 2008.
. K. Özdemir, S. Rotter, F. Nori, and L. Yang, “Parity-time Symmetry
and Exceptional Points in Photonics,” Nat. Mater., Vol. 18, No. 8, pp.
783–798, 2019.
K. Takata, K. Nozaki, E. Kuramochi, S. Matsuo, K. Takeda, T. Fujii,
S. Kita, A. Shinya, and M. Notomi, “Observing Exceptional Point
Degeneracy of Radiation with Electrically Pumped Photonic Crystal
Coupled-nanocavity Lasers,” Optica, Vol. 8, No. 2, pp. 184–192,
2021.
K. Takeda, T. Sato, A. Shinya, K. Nozaki, W. Kobayashi, H. Taniyama, M. Notomi, K. Hasebe, T. Kakitsuka, and S. Matsuo, “Few-fJ/bit
Data Transmissions Using Directly Modulated Lambda-scale Embedded Active Region Photonic-crystal Lasers,” Nat. Photon., Vol. 7, No.
7, pp. 569–575, 2013.
A. Pick, B. Zhen, O. D. Miller, C. W. Hsu, F. Hernandez, A. W. Rodriguez, M. Solja i , and S. G. Johnson, “General Theory of Spontaneous Emission Near Exceptional Points,” Opt. Express, Vol. 25, No. 11,
pp. 12325–12348, 2017.
K. Takata and M. Notomi, “PT-symmetric Coupled-resonator Waveguide Based on Buried Heterostructure Nanocavities,” Phys. Rev.
Appl., Vol. 7, No. 5, 054023, 2017.
K. Takata and M. Notomi, “Photonic Topological Insulating Phase
Induced Solely by Gain and Loss,” Phys. Rev. Lett., Vol. 121, No. 21,213902, 2018.

39

Feature Articles

Kenta Takata

Research Scientist, Photonic Nanostructure
Research Group of NTT Basic Research Laboratories and NTT Nanophotonics Center.
He received a B.S. in engineering in 2010 fromthe University of Tokyo and M.S. and Ph.D. in
information science and technology in 2012 and2015 from the same university. He was a visiting
student researcher at Stanford University in2013. He joined NTT Basic Research Laboratories in 2015. Since then, his research interest has
been to introduce emergent concepts in quantum
and condensed matter physics, such as EPs andtopology, to nanophotonics and explore theirapplications. He received the Young Scientist
Presentation Award from the Japanese Society ofApplied Physics (JSAP) in 2017. He was selected
as an Outstanding Referee of the American
Physical Society (APS) in 2021. He is a memberof APS, JSAP, Optica, and the Physical Societyof Japan.

Kengo NozakiDistinguished Researcher, Photonic Nanostructure Research Group of NTT Basic ResearchLaboratories and NTT Nanophotonics Center.
He received a B.E., M.E., and Ph.D. in electrical and computer engineering from Yokohama
National University, Kanagawa, in 2003, 2005,
and 2007. He joined NTT Basic Research Laboratories in 2008. His current interests are alloptical switches, memories, and electro-optic
devices based on photonic crystals and related
photonic nanostructures. He received the Best
Paper Award from Photonics in Switching (PS) in
2012, Institute of Electronics, Information andCommunication Engineers (IEICE) Electronics
Society Young Researchers Award in 2014, andthe Best Paper Award from OptoElectronics andCommunications Conference (OECC)/PS in
2016. He is a member of JSAP.

Eiichi Kuramochi

Senior Research Engineer, Photonic Nanostructure Research Group of NTT Basic ResearchLaboratories and NTT Nanophotonics Center.
He received a B.E., M.E., and Ph.D. in electrical engineering from Waseda University, Tokyo,
in 1989, 1991, and 2004. In 1991, he joined NTT
Opto-electronics Laboratories, where he wasengaged in research on semiconductor nanostructures for photonic devices. In 1998, hemoved to NTT Basic Research Laboratories. He
has been engaged in research on photonic crystals. He is a member of the Institute of Electricaland Electronics Engineers (IEEE) Photonics
Society and JSAP.

Shinji Matsuo

Senior Distinguished Researcher, Materials
and Devices 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 Ph.D. in electronics and applied physics from Tokyo Institute of Technology in 2008.
In 1988, he joined NTT Opto-electronics Laboratories, where he was engaged in research on
photonic functional devices using multiple
quantum well-pin modulators and vertical cavity
surface-emitting lasers. In 1997, he researched
optical networks using wavelength-division multiplexing technologies at NTT Network Innovation Laboratories. Since 2000, he has been
researching high-speed tunable optical filters andlasers for photonic packet switching. He is amember of JSAP and IEICE and a Fellow ofIEEE and Optica.

NTT Technical Review

Vol. 20 No. 8 Aug. 2022

Koji Takeda

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

Takuro Fujii

Research Engineer, Materials and Devices
Laboratory of NTT Device Technology Laboratories and NTT Nanophotonics Center.
He received a B.E. and M.E. in system design
engineering from Keio University, Kanagawa, in2010 and 2012. He joined NTT Photonics Laboratories in 2012. He has been researching metalorganic vapor phase epitaxy growth of III-V
semiconductors and the development of III-V
semiconductor lasers on Si for photonic integrated circuits. He received the Young Scientist
Presentation Award from JSAP in 2014. He is amember of JSAP.

Shota Kita

Senior Research Scientist, Photonic Nanostructure Research Group of NTT Basic ResearchLaboratories and NTT Nanophotonics Center.
He received a Ph.D. in engineering from Yokohama National University, Kanagawa, in 2012.
He was a postdoc researcher in Lon ar’s laboratory at Harvard University until 2015. He thenjoined the current laboratory as a research associate. He investigates nanophotonic devices andcircuits for photonic processors. He received theYoung Scientist Presentation Award from JSAPin 2010. He is a member of JSAP and IEICE.

Akihiko ShinyaGroup Leader, Senior Research Scientist,
Supervisor, Photonic Nanostructure ResearchGroup of NTT Basic Research Laboratories and
NTT Nanophotonics CenterHe received a B.E., M.E., and Ph.D. in electrical engineering from Tokushima University in
1994, 1996, and 1999. In 1999, he joined NTT
Basic Research Laboratories. He has been
engaged in research and development of photonic crystal devices. He is a member of JSAPand the Laser Society of Japan.

40

Feature Articles

Masaya Notomi

Senior Distinguished Researcher, Photonic
Nanostructure Research Group of NTT Basic
Research Laboratories. Project Leader of NTT
Nanophotonics Center.
He received a B.E., M.E., and Ph.D. in applied
physics from the University of Tokyo in 1986,
1988, and 1997. In 1988, he joined NTT. Since
then, his research interest has been to control the
optical properties of materials/devices by artificial nanostructures (quantum wires/dots andphotonic crystals). He is also a professor atTokyo Institute of Technology. He received theIEEE Laser and Electro-Optics Society Distinguished Lecturer Award in 2006, JSPS prize in2009, Japan Academy Medal in 2009, and theCommendation by Japanese Minister of Education, Culture, Sports, Science and Technology in2010. He is serving as a member of the National
University Corporation Evaluation Committee in
the Japanese government. He is an IEEE Fellow
and a member of JSAP, APS, and Optica.

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Vol. 20 No. 8 Aug. 2022

41

Feature Articles: Optical Device Technologies for Nextgeneration Computing Using Light

Optical Device Technology for Nextgeneration Computing Using Light
Akira Okada and Toshikazu Hashimoto
Abstract

The performance of conventional computers, which have supported the infrastructure of our digitalsociety and developed in accordance with Moore’s law, seems to be approaching its limits. To solve
many social issues and achieve a safe, secure, and prosperous society, next-generation computers withperformance far beyond that of conventional computers, such as quantum computers, are expected. This
article outlines the possibilities of next-generation computers that compute using light and optical device
technologies introduced in the Feature Articles of this issue.
Keywords: next-generation computing, optical computing, optical devices

1. Introduction
Digital technologies used in all aspects of society,including smartphones, consumer electronics, automobiles, the Internet, public infrastructure information systems, finance, and commercial transactions,have become a social infrastructure. In addition, communication technologies that facilitate the coordination of distributed digital information and new digitaltechnologies such as artificial intelligence (AI) andblockchain are having a significant impact on society.
Sustainable development of digital technology,
which is the foundation of society, is essential forsolving many social problems, including energy
problems, and for achieving a safe, secure, and prosperous society. Digital technology has continued todevelop by increasing the integration of semiconductor integrated circuits that execute calculations,achieving both high performance and low power
consumption. Moore’s law, which states that the
number of transistors in a semiconductor integrated
circuit doubles every two years, is widely known as asymbol of this trend. However, the miniaturization ofsemiconductor integrated circuits is said to beapproaching its physical limits, pointing to the limitsof Moore’s law, which has been maintained for half acentury. In addition, the explosive growth of AI,
which executes machine learning using more than
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Vol. 20 No. 8 Aug. 2022

one trillion parameters, is driving the need forincreased computing power beyond Moore’s law [1].
For example, a high-performance AI language model
called GPT-3, published by OpenAI, consumes 1287
MWh of power when trained on the largest model.
This is equivalent to the power consumption of an
average Japanese household for approximately 300
years [2].
To overcome these challenges, there is a strong
demand for the emergence of next-generation computers that can achieve performance far beyond thatof conventional computers with low power consumption, not only by developing semiconductor integrated circuit technology but also using other methods.
The Feature Articles in this issue focus on computing technology using light, which has been attracting
attention as a next-generation computing technology,
and introduce the optical device technologies used forit. In light-based computing, it is necessary to consider how to represent information in the state of lightand how to execute computing using the information
represented using light. In the following sections, we
review the advantages of representing information asa state of light and computing in such a state as information processing for next-generation computing aswell as the optical device technologies required forlight-based computing.

42

Feature Articles

2. Information processing using light for
next-generation computing
A term similar to information processing is signal
processing. Using the analogy of manufacturing at afactory, information corresponds to parts, signals correspond to packages containing parts, signal processing corresponds to transportation such as sorting andtransshipment, and information processing corresponds to the manufacturing process by using thecontents of the package to make something new.
Just as the upper limit of the production capacity ofa factory can be roughly estimated by the upper limitof the flow of goods, we first consider how much light
can handle signals and the information they contain,
including from the viewpoint of signal processing.
Let us consider how information processing can be
developed by incorporating light, which has beenused in signal processing, in the same manner that
manufacturing technology has greatly developed
through the line-production system that integrates the
manufacturing process and transport of goods.
Optical fiber communication is a typical example
of superimposing information on light. In such communication, signal processing is executed to convert
information into light intensity, frequency, and phase
to transmit information. To provide an optical communication line, optical signals are distributed using
an optical splitter, wavelength-multiplexed signals
are separated or bundled using an optical multiplexer/
demultiplexer filter, or processed in accordance withthe frequency of the optical signal using an opticalfrequency filter. These can be regarded as a type ofsignal processing using an optical functional device.
The signal superimposed on the light is a digital signal, and the information extracted from the light isprocessed using a digital filter to remove the noisecontained in the phase, frequency, and amplitude.
Therefore, signal processing and information processing are limited by the processing speed of electronic circuits.
In optical fiber communication, ultrahigh-speedtransmission experiments at 100 Gbit/s-class modulation rate frequencies have already been demonstrated. In contrast, more information can be superimposed on light. For example, light with a wavelength of 1.5 μm is an electromagnetic wave with afrequency of about 200 THz, and light with an intensity of 1 mW can be thought of as a stream of about
7.55 × 1015 photons per second, leaving a great manydegrees of freedom. However, to process information
beyond the processing speed of electronic circuits, it
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is essential to compute with light because it is necessary to manipulate lightwaves and photons beyond
the processing speed of electronic circuits. Figure 1
shows the relationship between signal processing andinformation processing with respect to the granularity
of information processed using light. When manipulating a digital-signal sequence or a single symbol ofa digital signal, light is mainly used as a carrier wave
processed using an optical filter as an element for
signal processing that is superimposed on light. In
information processing using light, however, light istreated as an analog signal wave, or the quantum stateof light is used to superimpose information on it andmanipulate the state of light to process information.
In optical signal processing, optical circuits are usedin cooperation with electronic circuits, whereas in
optical information processing, optical operations are
executed in a closed manner within an optical circuit.
Optical operations for information processing are
executed by precisely controlling optical interference, homodyne detection, and autocorrelation to
obtain amplitude and phase information, then combining them. As described in the Feature Articles inthis issue, the basic operations of optical neural networks [3] and optical quantum information processing [4, 5] are beginning to be demonstrated in actual
optical circuits. Optical information processing is
considered a promising candidate for next-generation
computing technology.
3. Optical device technologies for computing
using light
By superimposing information on the enormous
degrees of freedom of the state of light, we can see thepotential of optical computing technology beyond
that of conventional computers. In this section, we
consider what type of computing is possible withinformation represented using light and what kind of
devices will be needed to achieve this.
Digital technology has mainly used Neumann-typecomputing, which executes sequential logical operations on data based on programs. High-speed logic
operations, including information transmission
between processors and peripheral devices, areimportant for the development of digital technology.
Non-Neumann-type computing, on the other hand,uses a method different from conventional sequential
logic operations. When using light, as mentioned
above, non-Neumann-type operations can be executed instead of sequential logic operations using theenormous amount of light states, enabling large-scale,43

Feature Articles

Signal processing using light

Information processing using light

Digital filter
Intradyne detection~ operation for signal frequency
Splitter, wavelength filter

Optical analog circuit
Homodyne detection, autocorrelation~ operation for optical wave frequency

Optical digital filter

Optical neuralnetwork
Optical communications
Continuous signals(superimposed on light)

Symbols indigital signal

Optical circuit forneural network

Optical circuit for quantum
information processing

Optical reservoir
computing
Optical quantum computing
Analogue wave
(lightwave)

Photons

Granularity of information processed using light

Fig. 1. R elationship between signal processing and information processing with respect to the granularity of information
processed using light.

high-speed computing that is not limited by the processing speed of electronic circuits. Figure 2 shows
the correspondence between Neumann-type and nonNeumann-type computing and optical device technologies. For optical devices, lightwaves and photons
must be handled well to achieve computing using
light, while high-speed and broadband performanceis needed to accelerate logic operations in computing.
In the direction of high-speed and broadband, optoelectronic convergence technology [6] is expected tocontribute to improving the performance of current
computing systems, including optical interconnection, in response to high-capacity transmission technology used for optical fiber transmission. Whenconsidering wide-area networks, conventional opticaldevices such as linear optical circuits also support
this area of technology.
From the viewpoint of handling lightwaves andphotons, nonlinear optical elements for generatingcorrelation of lightwaves superimposed with information, optical circuit technology for generatingcomplicated correlation and operating calculation
using light as a system, and systematization technology including electronic control are important. When
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Vol. 20 No. 8 Aug. 2022

using photons, a quantum light source, corresponding
to a laser light source in optical communications, isrequired to generate quantum states of light. These
devices enable non-Neumann-type computers such as
coherent Ising machines [7] and optical reservoir
computers/optical neural networks [3]. Quantum
light sources and optical circuits for optical quantum
information processing are also considered essential
devices for implementing optical quantum computers, and their basic operation for optical quantum
information processing has been demonstrated[4, 5, 8].
Neumann-type and non-Neumann-type computation technologies are not mutually contradictory, but
are combined or complemented in accordance withthe computation target and computation purpose, andthe equipment-integration technology that integrates
them as hardware is important for constructing practical systems [9].
In the following three Feature Articles in this issue,
we introduce optical device technologies for opticalquantum computers, optical neural networks, andoptical reservoir computers, which can be called
computing using light in the next generation of44

Feature Articles

ns

Neumann-typecomputing

Application of optics/photonics technology for computing

Fa
st

er

log

ic o

pe

rat
io

High-speed, low-latencytransmission
Optical switch
Non-Neumann-type computing
Optical logic operation
Coherent Ising machine
Optical reservoir computing
Optical neural network
Optical quantum computing
Computing using light

Photonics-electronics
device integration tech.

Photonics-electronics
convergence tech.

Linear optical circuit tech.
Nonlinear optical circuit tech.

pe

ed

,b

roa

db

an

do

pe

rat

ion

Optical device technologies

Hig

h-s

Optical/photon information
processing tech.
Quantum light source
Handling lightwaves and photons

Fig. 2. Correspondence between computing and optical device technologies.

computing. The first article, “Broadband Continuouswave Optical Quadrature Squeezer for Ultra-fast
Optical Quantum Computers” [10], introduces theperiodically poled lithium niobate (PPLN) waveguide technology, an optical nonlinear device, and its
application to quantum light source technology. In
the next article entitled “Optical Circuit Technologies
for Next-generation Computing Using Light” [11],linear optical circuits and their applications for optical quantum computing are introduced. In the third
article “Photonic Implementation of Reservoir Computing” [12], we introduce optical reservoir computing technology as an optical neural network technology that combines optical device technologies into asystem and achieves large-scale sum-of-products
operations.
These technologies enable cutting-edge researchthat incorporates a variety of technologies and are
expected to become core technologies for advancing
next-generation computing.

References
[1]
[2]

[3]
[4]

[5]

[6]

[7]

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Vol. 20 No. 8 Aug. 2022

N. C. Thompson, K. Greenewald, K. Lee, and G. F. Manso, “TheComputational Limits of Deep Learning,” arXiv preprint,
arXiv:2007.05558, 2020. https://doi.org/10.48550/arXiv.2007.05558
D. Patterson, J. Gonzalez, Q. Le, C. Liang, L. M. Munguia, D. Rothchild, D. So, M. Texier, and J. Dean, “Carbon Emissions and Large
Neural Network Training,” arXiv preprint, arXiv:2104.10350, 2021.
https://doi.org/10.48550/arXiv.2104.10350
M. Nakajima, K. Tanaka, and T. Hashimoto, “Scalable Reservoir
Computing on Coherent Linear Photonic Processor,” Commun. Phys.,Vol. 4, No. 20, 2021.
G. Masada, K. Miyata, A. Politi, T. Hashimoto, J. L. O’Brien, and A.
Furusawa, “Continuous-variable Entanglement on a Chip,” Nat. Photonics, Vol. 9, No. 5, pp. 316–319, 2015. https://doi.org/10.1038/
nphoton.2015.42
J. Carolan, C. Harrold, C. Sparrow, E. Martín-López, N. J. Russell, J.
W. Silverstone, P. J. Shadbolt, N. Matsuda, M. Oguma, M. Itoh, G. D.
Marshall, M. G. Thompson, J. C. F. Matthews, T. Hashimoto, J. L.
O’Brienand, and A. Laing, “Universal Linear Optics,” Science, Vol.
349, No. 6249, pp. 711–716, 2015. https://doi.org/10.1126/science.
aab3642
T. Sogawa, M. Tomizawa, A. Okada, and H. Gotoh, “All-Photonics
Network and Photonics-electronics Convergence Technologies as a
Vision of the Future,” NTT Technical Review, Vol. 18, No. 10, pp.
12–15, 2020.
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K. Horikawa, “LASOLVTM Computing System: Hybrid Platform for
Efficient Combinatorial Optimization,” NTT Technical Review, Vol.
18, No. 1, pp. 35–40, 2020.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr202001fa5.html[8] T. Kashiwazaki, T. Yamashima, N. Takanashi, A. Inoue, T. Umeki, andA. Furusawa, “Fabrication of Low-loss Quasi-single-mode PPLN
Waveguide and Its Application to a Modularized Broadband Highlevel Squeezer,” Appl. Phys. Lett., Vol. 119, 251104, 2021. https://doi.
org/10.1063/5.0063118[9] T. Haga, A. Okada, and S. Matsuo, “On Si Platform Opens a New
World in AI-IoT Service Era,” NTT Technical Review, Vol. 15, No. 1,
2017.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201701fa1.html[10] T. Kashiwazaki, A. Inoue, and T. Umeki, “Broadband Continuous-

Akira Okada

Senior Vice President, Head of NTT Science
and Core Technology Laboratory Group.
He received a B.S. and M.S. in physics in 1988
and 1990, and Ph.D. in materials science in 1993
from Keio University. He joined NTT in 1993
and conducted research on polymer-based waveguide devices, full-mesh wavelength divisionmultiplexing networks, optical packet switching,and optical modules for access networks. From
October 1997 to October 1998, he was a visiting
scholar at Stanford University, CA, USA. He is amember of the Institute of Electrical and Electronics Engineers (IEEE), the Institute of Electronics, Information and Communication Engineers (IEICE), and the Japan Society of Applied
Physics (JSAP).

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Vol. 20 No. 8 Aug. 2022

wave Optical Quadrature Squeezer for Ultra-fast Optical Quantum
Computers,” NTT Technical Review, Vol. 20, No. 8, pp. 47–51, Aug.
2022.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr202208fa6.html[11] T. Hashimoto, J. Sakamoto, and S. Konisho, “Optical Circuit Technologies for Next-generation Computing Using Light,” NTT Technical Review, Vol. 20, No. 8, pp. 52–57, Aug. 2022.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr202208fa7.html[12] M. Nakajima, T. Tsurugaya, K. Tanaka, and T. Hashimoto, “Photonic
Implementation of Reservoir Computing,” NTT Technical Review,Vol. 20, No. 8, pp. 58–63, Aug. 2022.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr202208fa8.html

Toshikazu Hashimoto

Senior Distinguished Researcher, Group
Leader, Optoelectronics Integration ResearchGroup, NTT Device Technology Laboratories.
He received a B.S. and M.S. in physics from
Hokkaido University in 1991 and 1993 and Ph.D.
in engineering from Kyushu University in 2022.
He joined NTT Photonics Laboratories in 1993
and has been researching hybrid integration ofsemiconductor lasers and photodiodes on silicabased planar lightwave circuits and conducting
theoretical research and primary experiments on
the wavefront-matching method. He is a memberof IEICE, the Physical Society of Japan, and
JSAP.

46

Feature Articles: Optical Device Technologies for Nextgeneration Computing Using Light

Broadband Continuous-wave Optical
Quadrature Squeezer for Ultra-fast
Optical Quantum Computers
Takahiro Kashiwazaki, Asuka Inoue, and Takeshi Umeki
Abstract

An optical quadrature squeezer is expected to be a key device for high-speed optical quantum computing. We developed a high-gain and broadband optical quadrature squeezer by using a periodically poledlithium niobate (PPLN) waveguide. The squeezer is assembled as a pump-combiner-integrated modulewith four optical-fiber input/output ports. By improving the fabrication processes of the PPLN waveguide, we achieved a low-loss module that exhibits about 1-dB optical loss. By using this module, we
successfully obtained continuous-wave 6-dB-squeezed light at over-6-THz sideband frequency even ina fiber-closed optical system with commercially available optical components. This fiber-compatible
broadband quadrature squeezer will accelerate the development of optical quantum computers.
Keywords: periodically poled lithium niobate, optical quadrature squeezer, optical quantum computer

1. Introduction
Research and development of universal quantum
computers has accelerated worldwide, and various
computing methods have been proposed using superconducting circuits, trapped ions, photons, etc. [1, 2].
We are developing an optical quantum computer thatuses the amplitude and phase of propagating light as
an information carrier, similar to the current opticalcommunication technology [3]. This is because an
optical quantum computer has the potential of ultrafast clock-rate computing thanks to the high frequency of light. We are aiming to develop an overwhelmingly large-scale and high-speed universal quantum
computer using time-domain multiplexing and measurement-induced quantum manipulation [4]. The
most important element of this optical quantum computer is a squeezed light source. In this article, we
introduce the research and development of a squeezed
light source, i.e., a squeezer, which is a key device forhigh-speed, large-scale, universal quantum computing.

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2. Squeezed state of light and its application toquantum computers
Squeezed states are non-classical states in which
the quantum noise in a non-commutative quantity(momentum and position, energy and time, etc.) iscompressed. Due to the uncertainty relation, the noisefor the other quantities is amplified. The quadratureamplitude-phase squeezed vacuum state of light (orsimply squeezed light) has compressed quantum
noise of the sinusoidal or cosine component with awave image, as shown in Fig. 1(a). With a particle
image, the squeezed light is an even-number photon
state. By interfering two squeezed states with a half
beamsplitter, we can deterministically generate aquantum entangled state (with 100% probability) [5].
Using multiple squeezed light sources and delay line
interferometers, we are able to generate a timedomain multiplexed large-scale quantum entangledstate (Fig. 1(b)) [4]. This state is called an opticalcluster state and used as a resource for measurementbased quantum computation. By measuring one qubitof the optical cluster state, we can execute somequantum gates to other qubits with a methodology
47

Feature Articles(a)Coherent light
Sin

Squeezing

Squeezing
Electric field

Electric field

Squeezed lightt

Cos
Quantum noise on one quadrature amplitude
is “squeezed,” while the other is amplified.

Sin
t

Cos
Quantum noise on both quadrature amplitudes
has same magnitudes.

(b)
Squeezed vacuum state
Squeezer
Squeezer

50/50 beamsplitter

Optical delay line

Time-domain-multiplexed entangled stateTime
One-dimensional optical cluster stateFig. 1. (a) Image of quantum noise in squeezed light. While coherent light has quantum noise equal to the sinusoidal andcosine components, squeezed light has compressed one side of the noise. The quantum noise of the other side
becomes larger due to the requirement of the uncertainty principle. (b) A method for generating multiplexed largescale quantum entangled states in the time domain using squeezed light.

such as quantum teleportation [6].
To develop a high-speed, large-scale, universal
quantum computer, continuous-wave broadband
high-level squeezed light is required. This is because
the continuous wave and broadband nature ofsqueezed light maximizes time resources and enables
high-speed and large-scale computational performance that cannot be achieved with other methods[7]. High-level squeezed light has quantum superposition states with even-photon-number components,
which is important for quantum error correction [8].
3. PPLN-based quadrature squeezer module
Squeezed light can be generated from a nonlinear
optical phenomenon. In 1985, the first generation ofsqueezed light was demonstrated using the thirdorder nonlinear optical effect of natrium vapor [9].
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Vol. 20 No. 8 Aug. 2022

Since then, squeezed light has been generated in avariety of ways [10], and second- and third-ordernonlinear optical effects in solids have become mainstream. Waveguide-type optical parametric amplifiers based on second-order nonlinear optical crystals
are expected to exhibit broadband characteristics inprinciple [7]. However, nonlinear optical devices are
generally difficult to fabricate, and it has been difficult to generate high-level squeezed light. To generate a high level of squeezed light, the optical waveguide must have low loss, exhibit high nonlinear
optical properties, thus be resistant to strong excitation light. Low loss is required because squeezed
light is easily degraded by contamination of the vacuum field associated with optical loss. We have beendeveloping direct-bonded periodically poled lithium
niobate (PPLN) waveguides, as shown in Fig. 2(a)[11]. Lithium niobate is a ferroelectric material with48

Feature Articles(a)
PPLN waveguide
Poling direction

Substrate (LiTaO3)(b)

VacuumLensesPPLN waveguide (43 mm)

Wavelength division mirrors
Pump light
Squeezed light

Fig. 2. ( a) Schematic view of a PPLN waveguide. The waveguide core is directly bonded to the substrate. (b) Fiber-coupled
optical parametric amplifier. Wavelength-division mirrors and lenses are installed inside so that the pump light andsqueezed light are separated and injected to each output optical fiber [14].

a wide transmission bandwidth and high secondorder nonlinear optical coefficient. Thanks to its
periodical poling, it is possible to enhance the nonlinear interaction of light passing through the waveguide. By bonding the waveguide directly to thesubstrate without using any adhesive, we fabricateddevices that can operate even under pump light withwatt-level intensity. Our fabrication techniques haverecently enabled low-loss, high-gain optical parametric amplifiers with high pump-light tolerance.
Our group is also developing modular optical parametric amplifiers that can be easily coupled with fiber
optics [12–14]. This is to enable the construction ofmaintenance-free optical systems in anticipation ofpractical applications. Quantum optics experiments
had been conducted on an optical table with many
mirrors and lenses arranged with high precision to
verify the proof-of-principles. These optical systems
require constant precise adjustment, and this has beena problem that needs to be solved for practical applications. As shown in Fig. 2(b), our module has astructure in which the pump light and squeezed light
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Vol. 20 No. 8 Aug. 2022

are separated inside the module, and each is efficiently coupled to an optical fiber [12–14]. This fiberpigtailed module will greatly advance the development of practical-use optical quantum computers
because we can combine various highly reliable andhigh-performance optical components, which arecultivated in the field of optical communications.
4. Squeezing-level measurement
The squeezing levels have conventionally beenmeasured by balanced homodyne detection [15].
With this method, the squeezed light interferes with alocal oscillator, the frequency of which is the same as
the center frequency of the squeezed light, by using ahalf beamsplitter. The difference between the output
intensities of the two paths is then extracted as anelectrical signal by using a balanced photodetector.
Accordingly, the bandwidth of the squeezed lightmeasured using balanced homodyne detection is limited by the bandwidth of the electrical circuit up to a
few gigahertz. Therefore, we developed a method for49

Feature Articles

6-THz sideband (1595 nm)
−25

−20

Light intensity (dBm/nm)

Light intensity (dBm/nm)

−10

−30
−40
−50

,

−60
1490

Shot noise
Squeezed noise

Shot noise, Squeezed noise (phase lock)
Squeezed noise (phase scan)

−30
−35
−40
−45
−50
−55

1510

1530

1550

1570

0

1590

Wavelength (nm)0.5
Sweep time (s)1Fig. 3. Q uantum-noise-intensity measurement results for squeezed light. The noise level was more than 6 dB lower than
the shot noise level. The squeezing level was maintained over the broadband component above 6 THz [14].

measuring quantum-noise intensity up to THz-classbroadband components by further optical parametricamplification of the squeezed light [16]. This meansthat the quantum information is converted directly
into classical light, whereas previous methods (balanced homodyne detection) convert it onto the classical state of an electrical signal. Figure 3 shows theresults of noise-intensity-level measurements. The
experimental results indicate that the noise of thesqueezed light is suppressed by more than 6 dB compared with the shot-noise level, even for THz-order
sideband components [14]. This value is more than
the 4.5 dB required to generate two-dimensional
optical-cluster states, which enables multi-qubit
quantum computation. This is the world’s highest
value for squeezing using a waveguide-type opticalparametric amplifier with an all-optical-fiber-closed
system.
5. Conclusion
We reported on the generation of continuous-wave,
broadband squeezed light for the development of ahigh-speed, large-scale, universal quantum computer.
The squeezing of 6 THz and more than 6 dB was
achieved using an optical parametric amplifier consisting of a PPLN waveguide, which was developed
by NTT. The device can be implemented as a fibercoupled module to enhance compatibility with optical communication components and is being fabricated in anticipation of the future development of
quantum computers. We aim to achieve a higher levelby improving the device-fabrication technique and
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optimizing the design and are attempting more than
10-dB squeezing for fault-tolerant quantum computation.
References[1]

M. A. Nielsen and I. L. Chuang, “Quantum Computation and Quantum Information,” Cambridge University Press, 2010.
[2] T. D. Ladd, F. Jelezko, R. Laflamme, Y. Nakamura, C. Monroe, and J.
L. O’Brien, “Quantum Computers,” Nature, Vol. 464, pp. 45–53,
2010.
[3] S. Takeda and A. Furusawa, “Toward Large-scale Fault-tolerant Universal Photonic Quantum Computing,” APL Photonics, Vol. 4, No. 6,60902, 2019.
[4] W. Asavanant, Y. Shiozawa, S. Yokoyama, B. Charoensombutamon,
H. Emura, R. N. Alexander, S. Takeda, J. Yoshikawa, N. C. Menicucci,
H. Yonezawa, and A. Furusawa, “Generation of Time-domain-multiplexed Two-dimensional Cluster State,” Science, Vol. 366, pp. 373–
376, 2019.
[5] A. Furusawa, J. L. Sørensen, S. L. Braunstein, C. A. Fuchs, H. J.
Kimble, and E. S. Polsik, “Unconditional Quantum Teleportation,”
Science, Vol. 282, pp. 706–709, 1998.
[6] W. Asavanant, B. Charoensombutamon, S. Yokoyama, T. Ebihara, T.
Nakamura, R. N. Alexander, M. Endo, J. Yoshikawa, N. C. Menicucci,
H. Yonezawa, and A. Furusawa, “Time-domain-multiplexed Measurement-based Quantum Operations with 25-MHz Clock Frequency,”
Phys. Rev. Applied, Vol. 16, 034005, 2021.
[7] T. Kashiwazaki, N. Takanashi, T. Yamashima, T. Kazama, K. Enbutsu,
R. Kasahara, T. Umeki, and A. Furusawa, “Continuous-wave 6-dBsqueezed Light with 2.5-THz-bandwidth from Single-mode PPLN
Waveguide,” APL Photonics, Vol. 5, No. 3, 036104, 2020.
[8] D. Gottesman, A. Kitaev, and J. Preskill, “Encoding a Qubit in an
Oscillator,” Phys. Rev. A, Vol. 64, 012310, 2001.
[9] R. E. Slusher, L. W. Hollberg, B. Yurke, J. C. Mertz, and J. F. Valley,“Observation of Squeezed States Generated by Four-wave Mixing in
an Optical Cavity,” Phys. Rev. Lett., Vol. 55, 2409, 1985.
[10] U. L. Andersen, T. Gehring, C. Marquardt, and G. Leuchs, “30 Years
of Squeezed Light Generation,” Phys. Scr., Vol. 91, 053001, 2016.
[11] 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.
[12] N. Takanashi, T. Kashiwazaki, T. Kazama, K. Enbutsu, R. Kasahara,

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T. Umeki, and A. Furusawa, “4-dB Quadrature Squeezing with Fibercoupled PPLN Ridge Waveguide Module,” J. Quantum Electron., Vol.
56, No. 3, 6000100, 2020.
[13] T. Kazama, T. Umeki, S. Shimizu, T. Kashiwazaki, K. Enbutsu, R.
Kasahara, Y. Miyamoto, and K. Watanabe, “Over-30-dB Gain and
1-dB Noise Figure Phase-sensitive Amplification Using a Pumpcombiner-integrated Fiber I/O PPLN Module,” Opt. Express, Vol. 29,
No. 18, pp. 28824–28834, 2021.
[14] T. Kashiwazaki, T. Yamashima, N. Takanashi, A. Inoue, T. Umeki, andTakahiro Kashiwazaki

Researcher, Heterogeneous Materials andDevices Research Group, NTT Device Technology Laboratories.
He received a B.E. and M.E. in materials science from Keio University, Kanagawa, in 2013
and 2015. In 2015, he joined NTT Device Technology Laboratories, where he has been engaged
in research on nonlinear optical devices for optical communication and quantum computation.
He is a member of the Institute of Electronics,
Information, and Communication Engineers(IEICE) and the Japan Society of Applied Physics (JSAP).

Asuka Inoue

A. Furusawa, “Fabrication of Low-loss Quasi-single-mode PPLN
Waveguide and Its Application to a Modularized Broadband Highlevel Squeezer,” Appl. Phys. Lett., Vol. 119, 251104, 2021.
[15] H. P. Yuen and V. W. S. Chan, “Noise in Homodyne and Heterodyne
Detection,” Opt. Lett., Vol. 8, No. 3, pp. 177–179, 1983.
[16] N. Takanashi, A. Inoue, T. Kashiwazaki, T. Kazama, K. Enbutsu, R.
Kasahara, T. Umeki, and A. Furusawa, “All-optical Phase-sensitive
Detection for Ultra-fast Quantum Computation,” Opt. Express, Vol.
28, No. 23, pp. 34916–34926, 2020.

Takeshi Umeki

Distinguished Researcher, HeterogeneousMaterials and Devices Research Group, NTT
Device Technology Laboratories.
He received a B.S. in physics from Gakushuin
University, Tokyo, in 2002 and M.S. in physics
and Ph.D. in nonlinear optics from the Universityof Tokyo, in 2004 and 2014. He joined NTT
Photonics Laboratories in 2004 and has been
involved in research on nonlinear optical devices
based on periodically poled lithium-niobate
waveguides. He was a visiting researcher at the
National Institute of Standards and Technology(NIST), Boulder, CO, USA in 2020. He is amember of JSAP, IEICE, and the Institute ofElectrical and Electronics Engineers (IEEE)
Photonics Society.

Researcher, Heterogeneous Materials andDevices Research Group, NTT Device Technology Laboratories.
She received a B.E., M.E., and Ph.D. in electrical and electronic engineering from Kobe University, Hyogo, in 2015, 2016, and 2019. In 2019,she joined NTT Device Technology Laboratories, where she has been engaged in research on
waveguide devices for optical quantum computation. She is a member of JSAP.

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51

Feature Articles: Optical Device Technologies for Nextgeneration Computing Using Light

Optical Circuit Technologies for Nextgeneration Computing Using Light
Toshikazu Hashimoto, Junji Sakamoto,and Shiori Konisho
Abstract

To sustainably achieve a society in which digital technologies have penetrated in various forms suchas smartphones and cloud services, computing technologies that greatly exceed the current performances are becoming important. NTT is advancing research and development of computing technology using
lightwaves/photons by applying device technology for communications using optical waveguides. In thisarticle, we introduce optical circuit technology mainly for optical quantum computing as an example.
Keywords: optical circuits, planar lightwave circuits, optical quantum computers

1. Optical circuit technology for computing
using lightwaves
A variety of optical devices have been developed
for optical fiber communications by using planar
lightwave circuit (PLC)* technology (Fig. 1(a)), in
which the circuit is fabricated by combining opticalfiber and semiconductor manufacturing technologies[1]. PLC technology is considered a promising device
technology for computing using light as an opticalcircuit technology capable of integrating linear optical elements.
Electronic devices, such as transistors, three-pole
vacuum tubes, and three-terminal regulators, can beclassified as three-terminal devices. They have inputs
and outputs, and the desired output can be obtainedby reflecting the control signals from the control terminals to the input state. A computer is composed ofa very large number of transistors, which are threeterminal devices. In a vacuum tube (Fig. 2(a)), forexample, the operation of the three-terminal device iscontrolled by the voltage applied to the intermediate
terminal called the grid. Regarding optical devices,
there are optical switches and optical modulators thatdrive control terminals from electronic devices, butfrom the viewpoint of computing using light, theinput to the control terminals must also be light.
One such device that can be considered a threeNTT Technical Review

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terminal device is an optical interferometer. An optical interferometer outputs the interference light frommultiple optical inputs. It is considered a three-terminal device by regarding some of the optical inputs as
ports of light for control. Figure 1(b) shows the basic
operation of a directional coupler, a type of interferometer often used in PLC technology. When two
optical waveguides (paths of light) are brought close
together, light is transferred from one optical waveguide to the other. By separating the optical waveguide at half the length of the complete transition, we
can split the input light into two lights with phasedifference by π/2. The input from the other opticalwaveguide is also divided into two lights with thephase difference of π/2. When these are input andsuperposed at the same time, and one is considered an
input port and the other a control port, and if the output from the two waveguides on the output side isconsidered one output state, it becomes a three-terminal optical circuit that controls light by the phase difference between the light at the input and control
ports.
An example of a device using this interferometer isan optical circuit called an optical 90-degree hybrid*

Planar lightwave circuit (PLC): An optical circuit that provides
pathways for light using a silica microstructure on a silicon wafer
based on optical-fiber fabrication technology and semiconductor
manufacturing technology.

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Feature Articles

Optical waveguides(optical pathways)

 = 180°

 = 0°

 = 90° = 90°~7 µm
Cladding

 = 0°

 = 90°Superimposing input and control
Output = 90°~ 10 cm

Silica (SiO2) on Si wafer

Input

Optical waveguides(optical pathways) = 0°
 = 90°Control(a) Basic structure of PLC(b) Directional coupler and its basic operation

SiO2: silicon dioxideFig. 1. PLC and basic operation of directional coupler.

Plate

Ix
A

ElectronY

Path difference: 0·

Objectimage

Input signal
Path difference:  ⁄4Heater

Referencelight

Reproducing

Reproduced
image

Local oscillator(a) Electrical 3-terminal circuit (vacuum tube)

RecordingAI

Grid
Cathode

Referencelight(b) Optical 90-degree hybrid circuit(c) Holography

Fig. 2. Example of electrical/optical 3-terminal circuit.

circuit [1] (Fig. 2(b)), which can measure both thephase difference and amplitude of the input signal
with respect to the local oscillator light (local coherent light) used in coherent transmission. In coherent
transmission, local oscillator light with a constantfrequency is used. In optical quantum information
processing, which is introduced later, local oscillator
light is combined with the entangled state as a control
signal to enable quantum teleportation to obtain adesired output. For another example, the holographic
recording process can be regarded as a three-terminal
device for obtaining a Fourier transform image if theimage light is used as an input and the reference light
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is regarded as a control light (Fig. 2(c)).
If an optical circuit in which multiple inputs interfere with each other can be regarded as a three-terminal device, it is expected to enable new information
processing using light, similar to processing by acomputer composed of vacuum tubes and transistors.
Quantum information processing using optical circuits is introduced as a concrete example.
2. Quantum information processing using
optical circuits
Quantum computers use superposition and53

Feature Articles(a) Optical circuit by spatial optics with mirrors and lenses (left), and PLC for universal (right)

Entangled photon state
Qubit 1
Qubit 2
Ancilla photons(reflecting successful generationof quantum states)
HeaterPhase adj.

HeaterBranchratio adj.

(b) Configuration of generating entangled photon state with optical circuit by PLC

Fig. 3. Mode-type quantum-information-processing optical circuit.

entanglement of quantum states to solve problems
that cannot be solved using a conventional computerin practical terms. Various physical quantum states
and their implementations have been proposed foruse in quantum computers, including superconducting circuits and ion traps. Optical quantum computers
using photons are promising candidates of feasible
quantum computers because they can operate at room
temperature and at high speed. Optical quantum
information processing using optical circuits is mainly divided into two types, mode (or path qubit) andcontinuous-variable, which are introduced below.
2.1 M
 ode-type optical quantum information processing
Mode-type optical quantum information processing uses photon states (mode) in optical waveguides.
There are two methods of expressing a qubit. One
provides a qubit as superposition of a state with andwithout a photon in one optical waveguide (singlerail representation), and the other expresses a qubit bysuperposition of a state with a photon in either waveguide using two optical waveguides (dual-rail repreNTT Technical Review

Vol. 20 No. 8 Aug. 2022

sentation). To achieve the nonlinear operation
required for quantum information processing in a
linear circuit, it is necessary to achieve it stochastically in combination with measurement. In the single-rail representation, it is difficult to distinguishwhether a photon has disappeared due to loss or achange in the quantum state. However, the dual-rail
representation always manipulates the state in whicha photon is present, so it is easy to detect whether aquantum operation has succeeded by combining itwith an auxiliary photon (ancilla photon). Using
dual-rail representation and posterior selective processing of successful quantum states, research is
being conducted to achieve larger-scale quantum
computing.
NTT has fabricated the world’s first 6 × 6 universal
quantum optical circuit using PLC technology(Fig. 3(a)). Conventionally, it had difficult to ensurestability because of the extremely large size of the
optical system constructed with spatial optics on an
optical bench. However, by making it a chip, a compact and stable optical system was achieved. The
configuration of the optical circuit is shown in
54

Feature Articles

OpticalcircuitInput
xin, pin

Bell measurementxFeedforward
xout
pout

p
Directional
coupler

Squeezed light

Directional
coupler

Entanglement
generation

xin
pin

Displace- Displacement(p) ment(x)Output(a) Configuration of optical circuit for quantum teleportation(b) Quantum teleportation circuit by PLC technology

Fig. 4. Optical circuit for continuous-variable-type optical quantum information processing.

Fig. 3(b). The part where the optical waveguides
appear to intersect is equipped with a heater that
locally changes the refractive index by the thermooptic effect, and the phase and branching ratio can be
adjusted. Although it may look like a complex circuit,it combines the basic operations of interfering photons in a directional coupler, allowing variable, arbitrary linear operation (6 × 6 unitary operation). In
collaboration with the University of Bristol, we haveconfirmed that various types of quantum information
processing can be executed with high precision [2].
Figure 3(b) shows the configuration of an opticalcircuit that generates an entangled state from two
qubits in a dual-rail representation. To implement apractically useful quantum computer, it is necessary
to further increase the scale, but in addition toincreasing the number of waveguides, it is also necessary to develop a more advanced circuit configuration, such as post-selecting only successful quantum
states reflecting the states of ancilla photons andtransmitting them to the subsequent stages.
2.2 C ontinuous-variable-type optical quantum
information processing
A quantum computer using a continuous variable
system has also been proposed. With a continuous
variable system, the quantum state is determined bymeasuring the interference state with the reference
light by putting information on the phase and amplitude of the lightwave as in the case of radio or opticalfiber communications. This is called projection measurement because the base of the state to be measured
can be specified by changing the way the referencelight state interferes with the observed state. For
example, a 90-degree hybrid circuit, as a measurement of the classical state, is, from a quantum
mechanical point of view, a projective measurement(bell measurement) that detects photons in the mode
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corresponding to the entangled state (bell basis) of
the input and reference light. By using the result of
the bell measurement of one of the entangled states,
quantum teleportation can be achieved by phaseshifting another one of the entangled states (Fig. 4(a)).
This circuit is extremely important as a basic operation of measurement-type quantum computation,
which executes quantum information processing bymeasurement with feedforwarding via a classicalpath. In cooperation with the University of Tokyo, wefabricated a PLC circuit that operates in the 850-nm
band, as shown in Fig. 4(b). As a preliminary study,
we evaluated the squeezed light output from the circuit and confirmed a squeezing level of approximately 3.2 dB, which enables non-classical operation [3,4]. To achieve a level sufficient for quantum computation, including quantum teleportation, we are now
shifting to research on quantum information processing in the 1.5-μm band, where device technology fortelecommunications can be widely applied, and arevigorously improving the characteristics, such astransmittance of optical circuits.
3. Future technology of optical circuit
technology for computation using light
As we have seen, multiple inputs interfere witheach other in a way that corresponds to a three-terminal element in an electronic circuit, enabling new
information processing with light. In addition to theresearch introduced in this article, research on neuralnetworks using optical circuits has been vigorously
studied. However, how to scale a computational circuit using light is a major issue for its practical use as
a computing technology in the future. When opticalchips are connected and scaled, as with electronic
devices, optical interference is lost or delayed whendata transmission is converted to digital data. Even
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Feature Articles

Large scale based on opticalwaveguide chips

Monolithic integrationof optical circuits

Design of monolithically integrated circuit= Optical circuits as neural networks

Optical/electrical/optical converteror optical fiber connection

Fig. 5. Scalability and monolithic integration of optical circuits for computing using light.

when optical chips are connected with optical fibers,
loss and phase shift become unavoidable (Fig. 5,left). Therefore, we need a different approach than
combining elements such as electronic circuits andscaling them up. One method is to use the time
domain to compute by using the entanglement/correlation between light pulses. Examples using thetime domain include continuous quantum opticalinformation processing using multiple pulses
described in this article and optical reservoir computing introduced in the article “Photonic Implementation of Reservoir Computing” [5] in this issue.
Another means of scaling up is to increase the spatial
density. As long as optical interference is used, thesize of the components of the optical circuit is limited
to the size of the wavelength of light. However, in
mode-type optical quantum information processing,for example, a method using multi-value states(qudits) with multiple waveguide modes instead ofbinary (qubit) states has been proposed. As an
extreme case of infinitely increasing the number ofmodes on the basis of this idea, we can use anapproach called monolithic integration of optical circuits, in which information is carried on the wavefront instead of the mode, and computing is executedby interference. The wavefront-matching method has
been proposed as an optical-circuit design methodthat uses the entire refractive index distribution of an
optical circuit, and it has been shown that the opticalcircuit corresponds to a neural network when theentire refractive-index distribution is considered an
optical circuit [6] (Fig. 5, right). A neural network
based on the evolution equation of the Schrödinger

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network [7] has also been proposed.
It is expected that optical circuit technology, whichenables computing using light, will continue to be
developed toward the next generation of computing
technology in conjunction with the development of
new technologies such as optical quantum computersand neural networks.
References
[1]
[2]

[3]

[4]

[5]

[6]
[7]

“Special Feature: Silica-based Planar Lightwave Circuits for Photonic
Networks,” NTT Technical Review, Vol. 3, No. 7, pp. 12–41, 2005.
https://ntt-review.jp/archive/2005/200507.html
J. Carolan, C. Harrold, C. Sparrow, E. Martín-López, N. J. Russell, J.
W. Silverstone, P. J. Shadbolt, N. Matsuda, M. Oguma, M. Itoh, G. D.
Marshall, M. G. Thompson, J. C. F. Matthews, T. Hashimoto, J. L.
O’Brien, and A. Laing, “Universal Linear Optics,” Science, Vol. 349,
No. 6249, pp. 711–716, 2015. https://doi.org/10.1126/science.aab3642
G. Masada, K. Miyata, A. Politi, T. Hashimoto, J. L. O’Brien, and A.
Furusawa, “Continuous-variable entanglement on a chip,” Nat. Photonics, Vol. 9, No. 5, pp. 316–319, 2015. https://doi.org/10.1038/
nphoton.2015.42
N. Takanashi, T. Serikawa, W. Inokuchi, Y. Sakamaki, J. Sakamoto, R.
Kasahara, T. Hashimoto, and A. Furusawa, “Quantum Entanglement
Swapping on a Waveguide Element,” Proc. of the 73rd Annual Meeting of the Physical Society of Japan, 25aK203-7, Tokyo, Japan, Mar.
2018 (in Japanese).
M. Nakajima, T. Tsurugaya, K. Tanaka, and T. Hashimoto, “Photonic
Implementation of Reservoir Computing,” NTT Technical Review,Vol. 20, No. 8, Aug. 2022.
h t t p s : / / n t t - r ev i ew. j p / a r c h ive / n t t t e c h n i c a l . p h p ? c o n t e n t s =ntr202208fa8.html
T. Hashimoto, “Wavefront Matching Method as a Deep Neural Network and Mutual Use of Their Techniques,” Opt. Commun., Vol. 498,
127216, 2021. https://doi.org/10.1016/j.optcom.2021.127216
M. Nakajima, K. Tanaka, and T. Hashimoto, “Neural Schrödinger
Equation: Physical Law as Deep Neural Network,” IEEE Trans. Neural Netw. Learn. Syst., Vol. 33, No. 6, pp. 2686–2700, 2021. https://doi.org/10.1109/TNNLS.2021.3120472

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Feature Articles

Toshikazu Hashimoto

Senior Distinguished Researcher, Group
Leader, Optoelectronics Integration ResearchGroup, NTT Device Technology Laboratories.
He received a B.S. and M.S. in physics from
Hokkaido University in 1991 and 1993 and Ph.D.
in engineering from Kyushu University in 2022.
He joined NTT Photonics Laboratories in 1993
and has been researching hybrid integration ofsemiconductor lasers and photodiodes on silicabased PLCs and conducting theoretical researchand primary experiments on the wavefrontmatching method. He is a member of the Instituteof Electronics, Information and Communication
Engineers (IEICE), the Physical Society ofJapan, and the Japan Society of Applied Physics(JSAP).

Shiori Konisho

Researcher, Optoelectronics Integration
Research Group, NTT Device Technology Laboratories.
She received a B.S. in physics from Ochanomizu University, Tokyo, in 2013 and M.S. inphysics from the University of Tokyo in 2015.
She joined NTT Device Technology Laboratories
in 2015, where she has been researching silicabased PLCs. She is a member of IEICE and the
Physical Society of Japan.

Junji Sakamoto

Research Engineer, Optoelectronics Integration Research Group, NTT Device Technology
Laboratories.
He received a B.S., M.S., and Ph.D. in applied
physics from Osaka Prefecture University in2010, 2012, and 2019. He joined NTT Device
Technology Laboratories in 2012, where he has
been researching silica-based PLCs. He is amember of IEICE.

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57

Feature Articles: Optical Device Technologies for Nextgeneration Computing Using Light

Photonic Implementation ofReservoir Computing
Mitsumasa Nakajima, Takuma Tsurugaya, Kenji Tanaka,
and Toshikazu Hashimoto
Abstract

Today’s computing for machine learning is carried out with digital electric processors, which haveissues regarding energy consumption and processing speed, by emulating brain-inspired information
processing. To address these issues, photonic devices, comprising a photonic neural network, are gaining
much attention for achieving high-speed and energy efficient brain-inspired computations. We report on
the photonic implementation of a simplified recurrent neural network called reservoir computing.
Keywords: photonic computing, machine learning, reservoir computing

1. Introduction
Recent large-scale information-transportation technology has been driven by the development of photonic devices, such as photonic transceivers, receivers, and switches. Due to these technological developments, the barrier between photonics and electronics has been lowered, which enables the integration ofphotonic and electric circuits into a single module.
Such optoelectronic integration technology is now
advancing for much larger-scale and energy-efficient
optical transmission [1]. Optoelectronic integration
has been gaining attention for applying photonic circuits as computation units, such as logic gate [2],matrix operation [3] and/or optimization solver [4],beyond the simple optical link function. These studies revived the development of optical computing,which was intensively studied during the 1980s. This
revival is also related to the explosive evolution in
artificial intelligence (AI).
Information processing in AI involves braininspired computational algorithms called artificial
neural networks (ANNs). Their computation is basedon a huge amount of matrix operations and nonlinear
processing, which requires energy-hungry large-scalecomputational resources. As the demand for suchcomputation is explosively increasing, the development of special-purpose AI hardware providing much
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faster and more energy-efficient computational
resources has been intensively studied. The photonic
implementations of ANNs are attracting interest
because they have potential to reduce operational
power, increase speed, and reduce latency beyond
what is possible in electronic computing. Unlike
digital calculation on conventional electronic circuits,
photonic computing uses analog values, ​​such as theintensity and phase of optical signals, as information.
Its propagation and interference are considered as thecomputation. For example, when we input an opticalsignal to an optical interferometer, as shown in
Fig. 1(a), the optical signals interfere with each other.
The output intensities can be considered the results ofthe matrix product of the input signals and parametersin the interferometer. This calculation is conducted
only by high-speed light propagation and interference
without principle energy consumption. More largescale computation is possible by using the opticalsignal multiplexing technologies developed for telecom application, such as time, wavelength, and space
division multiplexing. We can construct various
ANNs by designing the configuration of an opticalinterference system, as shown in Fig. 1(b).

58Feature Articles

(a)

(b)
Convolutional neural network

O/E conversion(nonlinear activation)

Original data

Feature data

Electric
signal

Y1
Y2

E/O
conversion

Opticalpulse

O/E
conversion

Optical circuit

RNN
X1
Matrix operation by interferometer
Electric
signal

( )=(
Y1
Y2

X2

~pico seconds
E/O conversion(nonlinear activation)

W11
W21

W12
W22

Feature data

Original data

)( )
X1
X2

E/O
conversion

O/E
conversion

Optical circuit

E/O: electrical to optical
O/E: optical to electrical
RNN: recurrent neural network

Fig. 1. (a) Schematic of matrix product operation using photonic interferometer. (b) Examples of ANN emulations using
photonic circuits.

(a)

(b)

Input layer

Output layer

Input electronic
signal, u(t)Fixedrandomly

Input layer
Parameter setting

Input PLC

Reservoir layer

Trainonly here

Opticalpulse
sourceInputsopticalpulse

Reservoir layer
Optical fiber array Parameter setting
Optical pulse
from input
layer, mu(t)Reservoir PLC
Optical pulse
from reservoir PDs
layer, x(t)Digital processingE/O
modulator
Input layerTime multiplexed neuronsSpace
multiplexedneurons : duration of pulses
F in: nonlinearity of modulator

Reservoir layer

Fout(x)

mu(t)
u(t)

F (u)
in

Output layer

K × M reservoir neuronsy(t)F out: nonlinearity of PDs

Fig. 2. (a) Overview of RC framework. (b) Schematic of our on-chip photonic RC architecture.

2. Reservoir computing and its
photonic implementation
Reservoir computing (RC) is a type of recurrent
neural network having recursive connections, as
shown in Fig. 2(a). In the RC framework, the weightsof the input and reservoir layers are randomly fixedand not trained. Only the output weights are trainedby linear regression such as the least squares method,which is much simpler than the training method usedin standard ANNs such as backpropagation. In spiteof the simple configuration, RC has shown excellent
performance comparable to that of standard ANNs on
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Vol. 20 No. 8 Aug. 2022

a series of benchmark tasks such as speech recognition, economic forecasting, and action detection invideo data. RC also has advantages from the viewpoint of photonic implementation. Standard deep
ANNs require fine tuning of each weight through theuse of the error back propagation algorithm, which
requires highly accurate and uniform large-scale integration of tunable optical elements, which is a challenging issue for fabrication. The training time of aphotonic ANN is generally much longer than thatwith electrical devices because the reconfiguration of
optical weights takes milliseconds. In the RC framework, however, there is no need for any fine tuning of59

Feature Articles

(a)

(b)
Tx

Rx

Optical fiber

Tx

Rx

Wavelength divisionmultiplexing

Tx

time

Input

Im

Re

Im

Filter
anxn

A/D

O/E

FPGA

Fiber

n datatime

Optical reservoir

RC

Intensity
detection

Re

A/D: analog to digital
D/A: digital to analog

n-1 data

CompensatedsignalEx. 2: Phase recovery from intensity information
Beforetransmission

Generating thetime-multiplexed neurons

D/A
Z−1

CPU

After
transmission

Fixed RNN

E/O

time

Feed forward
to next layer
OutputEx. 1: Nonlinear compensation
Beforetransmission

Digital NN

Mask

Data

Rx

DSP

Photonic RC

RandommatrixCtrl- Coding- System control

Phase
recovery- Pre/post processing- AD/DA

Photonic
computationRC
Rx: receiver
Tx: transmitter

Fig. 3. (a) Application of photonic RC for coherent fiber communication. (b) FPGA-assisted photonic RC platform toward
general AI application.

the optical system under training. As the training timeis determined by forward propagation in RC, it can be
accelerated using photonics. Thus, we focused on thisconfiguration at the first stage of photonic implementation of an ANN.
Figure 2(b) shows our previously proposed device
architecture for photonic RC [5]. In the input opticalcircuit (left side in figure), the optical signal isweighted along the time and space directions, whichemulate the input layer of RC. In the circuit for the
reservoir layer (right side in figure), a ring-shaped
optical cavity array represents a recursive coupling of
neurons, emulating the reservoir layer of RC. The
output signals from the reservoir optics are converted
to an electric signal by using a photodetector (PD).
By weighting the output signals on the electric digital
circuit, we can obtain a final output signal of RC. We
implemented our RC architecture using a silica-based
optical waveguide technology called planar lightwave circuit (PLC), which was originally developed
for optical telecom devices. Our PLC acts as photonic RC with 512 reservoir neurons, which is over 30
times larger than in previous on-chip photonic implementations. The PLC can execute the multiply and
NTT Technical Review

Vol. 20 No. 8 Aug. 2022

accumulate operations beyond 20 tera times per second for each wavelength and reach peta-scale computation speed on a single photonic chip by using wavelength division multiplexing. This value is beyond
recent electronic computation. We experimentallyconfirmed the performance of our PLC using standard benchmark tasks such as hand-written-digitclassification. We could classify hand-written-digits
images with an accuracy of 91.7% and ultrafast processing speed of 17.1 ns per image. This accuracy andspeed is the highest for on-chip photonic RC implementation.
3. Application of photonic RC
As one of the features of photonic computation is
its high-speed processing, we believe that it is suitable for applications that can use this advantage. An
example of such an application is signal processing
for optical communication, as shown in Fig. 3(a). In
recent optical communication, the distortions of thetransmitted optical signals are compensated for using
a digital signal processor (DSP). Its computationalcosts, however, becomes an issue to increase the60Feature Articles

(a)

(b)
Cross-section

Nonlinearity

n-InP

SiO2

MQW p-InPSiz (x, z = z1)
 (x, z)Diffractive index = weight in ANN
High nOutput power

SiO2

x

Low n

Beam propagation

= H
j
z

Input electricfield, Diffractive index, nTraining using physical simulator

Input power

Forward (x, z) 

SiOx SiSOA (x, z + 1) 

Equilibrium matrix, W 

Si SiOx

H( 2, V,  )Intensity

Backward
Update nPhase

InP: indium phosphide
MQW: multiple-quantum-well
SiO2: silicon dioxide
SiOx: silicon oxideFig. 4. ( a) All-optical implementation of nonlinear functionality by an SOA on silicon. (b) Schematic of wave-equation-basedneural network.

communication capacity. When we use the photonic
ANN including RC as the signal processor for suchapplication, we can outsource the computationalcosts of a DSP to the photonic processor [6]. We arealso studying a method for drastically simplifying thecoherent receiver configuration by using photonic RC[7].
We are also considering the application of photonic
RC to more general machine learning tasks. For suchapplication, it is important to develop the hardware/
software interface between photonic RC and a standard computation device such as a central processing
unit (CPU). Therefore, we built a test platform ofphotonic RC using a field-programmable gate array(FPGA)-based hardware interface and Python-based
software stack (Fig. 3(b)). The user can drive theoptical RC system like standard computation hardware using a standard programming language(Python/Pytorch). We have also experimentally confirmed its superior performance [8].
4. Toward improving performance ofphotonic RC
To further improve the processing performance ofphotonic RC, it is essential to improve and harness
the functionalities of photonic devices. For example,
nonlinear optical effects should be more positively
exploited for optical implementation of neurons,
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Vol. 20 No. 8 Aug. 2022

while in conventional optical communication they
have been intentionally suppressed due to undesirable signal distortion. However, it is difficult for silica-based PLCs to implement such nonlinear functionalities; thus, in the above-explained physicalimplementation, the nonlinear functionality was
implemented opto-electronically, necessitating optoelectronic conversion.
Photonic devices based on III-V semiconductors,
such as an optical amplifier, can be used as all-opticalnonlinear elements. By heterogeneously integratingsuch III-V devices on silicon photonics, nonlinear
activation can be implemented all-optically even on
photonic integrated circuits (PICs). To this end, weused our recently developed III-V semiconductor
optical amplifier (SOA) on silicon shown in Fig. 4(a)to implement an all-optical nonlinear reservoir. We
then experimentally evaluated its processing performance via a nonlinear benchmark task, and it was
shown that the obtained performance was as good as
those of typical RC with optoelectronic nonlinearity[9]. On the basis of this achievement, we are considering integrating such nonlinear functionalities on
PICs toward further performance improvement ofphotonic RC.
In addition to the optical nonlinear implementation,
large-scale optical integration of neurons is essential.
Although the state-of-the-art ANN requires about 10
billion weight parameters, it is currently difficult to61

Feature Articles

implement such a large-scale model with an optical
ANN or RC system. The scalability of a photonic
ANN is basically limited by the size of optical gate
element shown in Fig. 1(a). As its typical size is about
100 μm square, it is difficult to implement the largescale state-of-the-art ANN into standard optical circuits with a realistic footprint. We previously proposed a framework that uses lightwave propagation
as a neural network, as shown in Fig. 4(b) [10]. In thisframework, we can consider the distribution of therefractive index in the optical circuit as the weight of
the neural network. Since the refractive-index distribution can be controlled to 1-μm2 pixel size, we canachieve large-scale implementation of an optical
ANN and RC about 1 million times that of the conventional method. We confirmed that the performance of the proposed wave-equation-based neuralnetwork is comparable to one for the state-of-the-art
ANN. By using such new processing methods, we are
now developing a photonic computation platform
toward future ultrafast information processing.
5. Summary
We gave an overview of recent developments in our
photonic implementation of a special type of neuralnetwork called RC. By using the nature of light, photonic RC can achieve high-speed, low-power parallel
computing beyond the conventional electronic processor. We also introduced its potential applicationand performance improvement. In spite of the attractive features of photonic computation, the re-invention of the computer using photonic technology is
still not easy. We will continue to further conduct
research on a future photonic computing platform byconsidering the entire computing system and algorithms beyond device-level approaches.

NTT Technical Review

Vol. 20 No. 8 Aug. 2022

References[1]

T. Sakamoto, N. Sato, and T. Segawa, “Photonics-electronics Convergence Technologies for Disaggregated Computing,” NTT Technical
Review, Vol. 19, No. 7, pp. 58–64, 2021.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr202107fa8.html[2] K. Nozaki, A. Shinya, and M. Notomi, “Nanophotonic Technologies
toward Opto-electronic Integrated Accelerators,” NTT Technical
Review, Vol. 18, No. 10, pp. 31–35, 2020.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr202010fa4.html[3] B. J. Shastri, A. N. Tait, T. Ferreira de Lima, W. H. P. Pernice, H.
Bhaskaran, C. D. Wright, and P. R. Prucnal, “Photonics for Artificial
Intelligence and Neuromorphic Computing,” Nat. Photonics, Vol. 15,
pp. 102–114, 2021. https://doi.org/10.1038/s41566-020-00754-y[4] T. Inagaki, Y. Haribara, K. Igarashi, T. Sonobe, S. Tamate, T. Honjo,
A. Marandi, P. L. McMahon, T. Umeki, K. Enbutsu, O. Tadanaga, H.
Takenouchi, K. Aihara, K. Kawarabayashi, K. Inoue, S. Utsunomiya,and H. Takesue, “A Coherent Ising Machine for 2000-node Optimization Problems,” Science, Vol. 354, No. 6312, pp. 603–606, 2016.
https://doi.org/10.1126/science.aah4243[5] M. Nakajima, K. Tanaka, and T. Hashimoto, “Scalable Reservoir
Computing on Coherent Linear Photonic Processor,” Commun. Phys.,Vol. 4, 20, 2021. https://doi.org/10.1038/s42005-021-00519-1[6] A. Argyris, “Photonic Neuromorphic Technologies in Optical Communications,” Nanophotonics, Vol. 11, No. 5, pp. 897–916, 2022.
https://doi.org/10.1515/nanoph-2021-0578[7] M. Nakajima and T. Hashimoto, “Phase-retrieval Coherent Detection
with On-chip Photonic Linear Processing,” Proc. of Frontiers in
Optics, FW5E.2, Washington, D.C., USA, Nov. 2021. https://doi.
org/10.1364/FIO.2021.FW5E.2[8] M. Nakajima, K. Inoue, K. Tanaka, Y. Kuniyoshi, T. Hashimoto, and
K. Nakajima, “Physical Deep Learning with Biologically Plausible
Training,” arXiv: 2204.13991.
https://doi.org/10.48550/arXiv.2204.13991[9] T. Tsurugaya, T. Hiraki, M. Nakajima, T. Aihara, N.-P. Diamantopoulos, T. Fujii, T. Segawa, and S. Matsuo, “Reservoir Computing with
Low-power-consumption All-optical Nonlinear Activation Using
Membrane SOA on Si,” Proc. of CLEO: Science and Innovation 2021,
AW2E.5, San Jose, USA, May 2021. https://doi.org/10.1364/CLEO_
SI.2021.AW2E.5[10] M. Nakajima, K. Tanaka, and T. Hashimoto, “Neural Schrödinger
Equation: Physical Law as Deep Neural Network,” IEEE Trans. Neural Netw. Learn. Syst., Vol. 33, No. 6, pp. 2686–2700, 2021. https://doi.org/10.1109/TNNLS.2021.3120472

62

Feature Articles

Mitsumasa Nakajima

Senior Research Engineer, Photonics-Electronics Convergence Laboratory, NTT Device
Technology Laboratories.
He received an M.E. and Ph.D. in material science from Tokyo Institute of Technology in 2010
and 2015. In 2010, he joined NTT Device Technology Laboratories, where he was involved in
the development of large-scale optical switches.
His research interests are optical switches andtheir applications including neuromorphic photonics and optical signal processing for telecommunications. He was a recipient of eight researchawards including the Young Engineer Award
from the Institute of Electronics, Information andCommunication Engineers (IEICE) of Japan. He
is a member of the Japan Society of Applied
Physics (JSAP), IEICE, and the Institute of Electrical and Electronics Engineers (IEEE).

Takuma Tsurugaya

Researcher, Materials and Devices Laboratory,
NTT Device Technology Laboratories.
He received a B.E. and M.E. in electrical andelectronic engineering from the University ofTokyo in 2016 and 2018. He joined NTT Device
Technology Laboratories in 2018, where he has
been engaged in research on photonic-crystal
lasers and photonic neural networks based on
III-V semiconductor optical devices. He received
the Young Scientist Presentation Award from
JSAP in 2021 and the Young Researcher’s Award
from IEICE in 2022. He is a member of JSAP
and IEICE.

NTT Technical Review

Vol. 20 No. 8 Aug. 2022

Kenji Tanaka

Researcher, Photonics-Electronics Convergence Laboratory, NTT Device Technology
Laboratories.
He received an M.S. in complexity science andengineering from the University of Tokyo in
2015. In 2016, he joined NTT Device Technology Laboratories, where he researched ultrahighspeed integrated circuits and systems for opticaltransmission using FPGAs. His current researchis on adding programmability to network switches and network interface cards and finding newuse cases for newly available programmablehardware. He is a member of IEICE.

Toshikazu Hashimoto

Senior Distinguished Researcher, PhotonicsElectronics Convergence Laboratory, NTT
Device Technology Laboratories.
He received a B.S. and M.S. in physics from
Hokkaido University in 1991 and 1993 and Ph.D.
in engineering from Kyushu University in 2022.
Since joining NTT in 1993, he has been involvedin the research on hybrid integration of opticalsemiconductor devices on silica-based PLCs andtheoretical research on the wavefront matching
method. He is a member of the Physical Societyof Japan, IEICE, and JSAP.

63

Regular Articles

Business Design Support Technology
to Promote Digital Transformation
Taisuke Wakasugi, Fumihiro Yokose, Ryou Uchida,
Kimio Tsuchikawa, Haruo Oishi, Sayaka Yagi,
and Yuki Urabe
Abstract

Business processes for providing services and products are becoming increasingly complicated tomeet the diverse needs of customers brought on by dramatic changes in the social environment. To make
improvements to these complicated processes, it is important to accurately understand and analyze
business processes on the basis of comprehensive and objective data. This article introduces the businessdesign support technology we developed that can be used immediately on-site to effectively understandand analyze the current state of business processes and the NTT technologies at the core of thistechnology.
Keywords: DX promotion, business analysis, operations

1. Objective understanding of businessprocesses to promote digital transformation
Digital transformation (DX) has been attracting
attention as a means of making business more efficient through digital technology. In the corporate andgovernment sectors, there are two approaches toachieving DX in a variety of business processes. The
first is system-oriented DX to increase the efficiencyof business operations required to process a large
volume of items with clear requirements through systemization. The other is site-oriented DX to increase
the efficiency of infrequently occurring business
operations with wide-ranging requirements. A typicalexample of site-oriented DX is robotic process automation (RPA)*1 to automate deskwork. Plans forachieving site-oriented DX are becoming common in
many corporate and government operations throughthe use of RPA such as WinActor®*2 [1].
However, determining exactly where to apply RPA
to gain effective improvements is extremely difficult
regarding complicated business processes, even for
veteran operators well acquainted with business
operations.
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Vol. 20 No. 8 Aug. 2022

The corporate and government sectors have been
searching for clues to achieving business improvements by using commercially available tools in addition to conducting on-site interviews with operators,
measuring work times, etc. However, techniques suchas interviews and time measurements are highly subjective, and if using commercially available tools,
there is a need for detailed settings and analysis support by a specialist, which makes the hurdle to adopting such tools quite high.
2. Business design support technology
To solve the problems described above, NTT Network Innovation Center has developed businessdesign support technology on the basis of technology
established by NTT Access Network Service Systems
Laboratories that can be put to use immediately onsite.
*1 RPA: Generic name for software technology that automates user
operations.
*2 WinActor® is a registered trademark of NTT Advanced Technology Corporation.

64

Regular Articles(1) Obtain log
Viz

Logging tool (VizLogger)[ Business improvement steps ](2) Divide/classify log(1) Understandcurrent situation

Viz

Logging tool
(VizLogger)

Work B(2) Analyze
On-site manager(3) Make
improvement plans

Work A

Supports deep digging
for problems using thistechnology(3) Analyze/visualizeVizAnalysis tool
(VizAnalyzer)(4) Execute plans
Start(4) Link with RPA
Improvement!

Viz

End

10:00

11:00

Editing tool for automatically generated scenarios(VizRPADesigner)

Fig. 1. Business design support technology.

Working to make business improvements requires
the following steps: understand the current situation,
analyze problems and make improvement plans, andexecute plans.
Business design support technology provides thefollowing tools to put the above steps into practice
immediately at the user’s site: a logging tool (VizLogger) that outputs the operations executed on a personal computer (PC) as a log file to support understanding of the current business situation; an analysis
tool (VizAnalyzer) that classifies and visualizes theoutput log file and supports the analysis of problems
and making of improvement plans; and a scenario
editing tool (VizRPADesigner) that enables the editing of automatically generated scenarios of operational procedures from classification and visualization results and supports the generation of RPA scenario files and execution of improvement plans [2](Fig. 1).
With these tools, an analyst can obtain objective
and comprehensive information on the work carriedout on a PC and visualize that information, enabling
analysis from a variety of viewpoints. This makes it
possible to extract business problems in a more effective manner than conventional techniques such as
NTT Technical Review

Vol. 20 No. 8 Aug. 2022

interviews and commercially available tools. These
tools can be applied to not only RPA in site-oriented
DX but also general business-improvement activities
such as business process management*3.
2.1 L
 ogging tool for objective and comprehensive
collection of information
The logging tool automatically records operations
carried out by the user to support the understand thecurrent situation step.
As a platform supporting Windows 10 and Windows 7, this tool can be executed by simply opening
it on the PC targeted for logging. A variety of browser environments such as Microsoft Edge*4, Google
Chrome*5, Firefox*6, and Internet Explorer*7, are also
supported.
*3 Business process management: The discipline of understanding
the current state of business processes in a company or other organization to improve and optimize them on an ongoing basis.
*4 Microsoft Edge is a registered trademark or trademark of Microsoft Corporation in the United States and other countries.
*5 Google Chrome is a registered trademark of Google LLC.
*6 Firefox is a registered trademark of Mozilla Foundation in the
United States and other countries.
*7 Internet Explorer is a registered trademark or trademark of Microsoft Corporation in the United States and other countries.

65Regular Articles

Timeline visualization
Settingeamme
p n r na e
t
e
ou
r
s
U
Da
G

e
me
am
na tle
tn
ti
on
ee
i
t
h
ca dow el s
pli
c
in
Ap
W
Ex

Outside group
11/11/2021 (Thu.)

O’Reilly Japan – Python tutorial
Python tutorial – Python
Python tutorial – Google search
Python tutorial 4th edition
Easy JavaScript [ECMAScript
Easy Python: Book guide | Tech
Easy JavaScript – Google search
Easy Python – Google search
Science of Diversity – Google sea
Science of Diversity | Discover…
New tab – Google Chrome
Nurture science, Peer-review tec
Nurture science, Peer-review tec

Fig. 2. Analysis tool (timeline visualization).

The logging tool detects user input operations,applications on the terminal screen, and changes in awindow’s status. An operation carried out by the user
is obtained as an operation event. The above content
can be recorded and saved as a log file (in text format)
in any folder. The work screen can also be saved as acaptured screen when detecting an operation event toassist the analyst in understanding business operations.
2.2 Analysis tool for making improvement plans
The analysis tool analyzes the log file stored by thelogging tool using automatic work-classification
technology described later to support the analyze
problems and make improvement plans step. This tool
can use three types of visualization methods to visualize the log file: timeline visualization, process visualization, and operation-sequence visualization.
(1) Timeline visualization
Timeline visualization is a visualization method foranalyzing usage conditions at the window level
NTT Technical Review

Vol. 20 No. 8 Aug. 2022

(Fig. 2).
This method can display what someone is working
on in a time-series manner. It can be used to intuitively discover work trends or distinctive work (workdifferent from the norm, work taking longer thanusual, etc.). This makes it possible, for example, todetermine the daily operations of a certain operator
and the content of that work and to compare that
operator with other operators doing the same work to
uncover efficient operators. In short, analyses can be
conducted while switching viewpoints.
(2) Process visualization
Process visualization is a visualization method foranalyzing process flow at a more detailed operationlevel such as clicks and keyboard input.
This method enables overall work flow to be understood through nodes (boxes) and edges (arrows). It
can be used to visualize the flow of business operations, work flow, etc. It displays the captured screen
and operation location at the instant of an actual
operation in a node (box), which makes it even easier
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Process visualization
Setting

Business ID: All

Work ID: Work 1

Display the parent and child relationshipTransfer guide/time table
Station search
Transfer guide/time table
Station search

Transfer guide/time table

Transfer guide/time table

Station search

Station search

Number of items: 3

Number of items: 3

Number of items: 3

Number of items: 3

Transfer guide/time table
Station search

Transfer guide/time table

Number of items: 2

Station search

Number of items: 1

Fig. 3. Analysis tool (process visualization).

to understand the work procedure.
With this method, an analyst can compare the work
procedure of a certain operator with that of otheroperators and analyze the differences or problems inthose procedures (Fig. 3).
The main-flow extraction technology describedlater makes it possible to extract and highlight main
operation flows and repeated operations from theoverall work flow to support effective improvements
to business.
(3) Operation-sequence visualization
Operation-sequence visualization is a visualization
method for analyzing operation sequences for eachspecified unit (business, work, item, log, user, anddate) in detail.
This method can display rectangular objects vertically for each specified display unit. It can be used for
accurately tracing and analyzing individual operations or an item’s operation order, which can be difficult to understand by timeline visualization or process visualization, or for making detailed studies ofmanual or RPA scenarios.

3. Editing tool for automatically generated
scenarios supporting the execution of
improvement plans
The editing tool for automatically generated scenarios supports the execute plans step by making thecreation and editing of RPA scenarios on the basis of
the results of the analysis tool easy to understandthrough visual means.
A key feature of this tool is that it can link with theanalysis and visualization results of the analysis tool
so that the results of the analyze problems and make
improvement plans step can be accurately reflected in
RPA scenario creation and editing. Therefore, asmooth transition can be made from analyze problems and make improvement plans to execute plans(Fig. 4). This tool can also automatically generate
repeated operations extracted and highlighted using
main-flow extraction technology in the analysis toolas RPA scenarios.
4. NTT technology features
The following describes two key NTT technologies

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67

Regular Articles

Editing screen
Scenario-testing file

Flow chart
Edit

RPA

Add control node

Node Search Subroutine Library[CHROME Value change]
Dec. 3 (Fri.)

Node
Flow
Group
BranchMulti-branch
Loop
Post-test loop
Exit from loop
Next iteration
Exception handlingCall subroutine
Subroutine group
Exit subroutine
ActionImage matching
Contour matching
Wait for window status
Wait for time
Send character string
Execute command
Run script[CHROME Click]
10:00

Start loop

Start

Start loop
1

Exit fromloop 1End

UMS file output

VizRPADesigner

Excel operation
Clipboard
User
Waiting box
Input box
Select box
Sound
Variable
Set variable value
Copy variable value
Obtain date and time
Obtain user name
Four arithmetic operations
Count up
Full/half-width conversion

Main
Scenario
Start

Loop 1Loop[CHROME Value change] Dec.

[CHROME Click] 19:00[CHROME Click] 5 min.

End

WinActor®

UMS: unified management support system

Fig. 4. Editing tool for automatically generated scenarios.

making up the core of the business design support
technology [3–5].
4.1 Automatic work-classification technology
Automatic work-classification technology is incorporated in the analysis tool. It automatically classifies
operation events for each type of work while absorbing fluctuations in operations that may occur such as
re-ordering, reworking, and interruptions (Fig. 5).
An operation event is recorded by associating amouse-click or keyboard-input event with the graphical-user-interface component (button, textbox, etc.)
targeted by that operation. It corresponds, for example, to inputting the day of a business trip and pressing an approval button. With this technology, “work”
is a set of operation events, and in this example, itwould specifically correspond to preparing a voucher
or order slip for travel expenses.
This technology focuses on the co-occurrence ofconsecutive operation events. It counts the number oftimes that operation events mutually co-occur frommultiple operation events of any length and expresses
each operation event as a co-occurrence vector. It also
calculates the degree of relevance between operation
events using cosine similarity on the basis of thoseco-occurrence vectors. In short, the technology canclassify a log into different work groups by treating
operation events with close cosine similarity as agroup. A work group is defined as a set of multiple
operation events corresponding to the same type of
NTT Technical Review

Vol. 20 No. 8 Aug. 2022

work.
With this technology, it becomes possible to appropriately classify work into work groups even if fluctuations in operations should occur, such as re-ordering, reworking, and interruptions, that are bound tooccur in actual work settings.
4.2 Main-flow extraction technology
Main-flow extraction technology is incorporated inprocess visualization of the analysis tool. It identifies
main flows in which operation frequency in the processing carried out by each work group is high andfor which an improvement effect can therefore be
expected.
This technology applies sequence-alignment technology used in genome analysis and other fields toalign multiple character strings. It can take a workgroup classified with the automatic work-classification technology described above and align the same
operation events into the same column. The technology can also extract operation events with a highfrequency of appearance in common and connect theextracted operation events in a time series, thus identifying a main flow with a high improvement effect(Fig. 6).
5. Future work
Our business design support technology provides
tools that can support the steps of understand the68Regular Articles

OperationWork 2Work 1

Operation events including fluctuations
a b c d a c b d a b c b c a bStandard flow
Re-ordering
ReworkingFluctuations(1) Division by workx y z c d x y z
Interrupted
Standard flowby other workab

Process (1)-1
Express relevance between operation eventsusing co-occurrence vectors.

a c b da b x y z

a c b da b c b c

a b

x y z

c d

i =1 Xi Yicos(X · Y) =

|V |

|V |

i =1 X2i i =1 Y2i

c d x y z

Breaks in work flow
Process (2)Automatically classify work by grouping similar
operations.

(2) Automaticclassificationa b c da b c b c

Co-occurrence vectors|V |Process (1)-2
Calculate degree of relevance between
individual operation events using cosine
similarity and treat similar events as agroup.
a b c da b c d0 6 5 16 0 5 2

Cosinesimilarity

x y zWork group 2Work group 1

Fig. 5. Automatic work-classification technology.

Process (1)Use sequence-alignment technology to align work-group operation
sequences into operation-event types.
a b c da x b za b c da x y b

e
a
c d e
a x
1. Align
f
a
a x y
c d

b
b z
b
b

c d e
c d e
c d
f
c d

Work group
2. Identify main flow
Process (2)Extract the operation events with high frequency of appearance andconnect in a time sequence to determine the main flow.
x
a

y
c

b
z

Example of displaying
operation processdef: Main flow

Fig. 6. Main-flow extraction technology.

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current situation, analyze problems and make
improvement plans, and execute plans. The commercial development of this technology has been completed, making it available for use in a variety of sites
and settings. Going forward, we plan to promote its
application in actual business operations through its
deployment in the private sector and develop it into abusiness for more generalized markets.

[3]

References

[5]

[1]
[2]

WinActor®, https://winactor.biz/en/
F. Yokose, Y. Urabe, S. Yagi, K. Tsuchikawa, T. Masuda, and H. Oishi,

NTT Technical Review

Vol. 20 No. 8 Aug. 2022

[4]

“Operation-visualization Technology to Support Digital
Transformation,” NTT Technical Review, Vol. 18, No. 5, pp. 43–48,
2020.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr202005ra2.html
S. Yagi, K. Tsuchikawa, F. Yokose, Y. Urabe, and T. Masuda, “AnInteractive Grouping Method for Timeline-based Operational Log
Visualization,” IEICE Technical Report, Vol. 119, No. 111, pp. 41–46,
2019 (in Japanese).
Y. Urabe, S. Yagi, K. Tsuchikawa, and T. Masuda, “Study of Business
Process Visualization Method Using Operation Log,” IEICE
Technical Report, Vol. 118, No. 483, pp. 83–88, 2019 (in Japanese).
Y. Urabe, S. Yagi, K. Tsuchikawa, and H. Oishi, “Task Clustering
Method Using User Interaction Logs to Plan RPA Introduction,” Proc.
of the 19th International Conference on Business Process Management(BPM 2021), LNCS 12875, pp. 273–288, 2021.

70

Regular Articles

Taisuke Wakasugi

Senior Research Engineer, Navigation Technology Group, Access Network Operation Project, NTT Access Network Service Systems Laboratories.
He received an M.E. in materials science fromJapan Advanced Institute of Science and Technology, Ishikawa, in 1998. He joined NTT thesame year and has been engaged in research anddevelopment (R&D) on the IPv6 domain namesystem server and navigation technology forbusiness operation.

Fumihiro Yokose

Senior Research Engineer, Navigation Technology Group, Access Network Operation Project, NTT Access Network Service Systems Laboratories.
He received an M.E. in electrical engineeringfrom the University of Electro-Communications,Tokyo, in 2007 and joined NTT Access Network
Service Systems Laboratories the same year. He
has been engaged in R&D of navigation technology for business operation.

Haruo Oishi

Senior Research Engineer, Leader of Navigation Technology Group, Access Network Operation Project, NTT Access Network Service Systems Laboratories.
He received a B.E. and M.E. in electrical engineering from Tokyo Institute of Technology in1995 and 1997. He received a Ph.D. in computerscience and communications engineering fromWaseda University, Tokyo, in 2022. He joined
NTT in 1997 and he has been engaged in R&D
on educational technology, mobile PC middleware technology, business Ethernet service
management system development, network failure estimation technology, and business navigation technology. He is a senior member of IEICE
and a vice chair of Technical Committee on
Information and Communication Management(ICM).

Sayaka Yagi

Research Engineer, Planning Section, NTT
Access Network Service Systems Laboratories
She received a B.S., M.S., and Ph.D. in information sciences from Ochanomizu University,
Tokyo, in 2011, 2013, and 2016. She joined NTT
Access Network Service Systems Laboratories in2016. Her research currently focuses on information visualization and visual analytics for supporting business operation analysis. She is amember of the Visualization Society of Japan.

Ryou Uchida

Researcher, Navigation Technology Group,
Access Network Operation Project, NTT AccessNetwork Service Systems Laboratories.
He received a B.E. and M.E. from Chuo University, Tokyo, in 2017 and 2019. He joined NTT
EAST in 2019 and was engaged in communications construction. He joined NTT Access Network Service Systems Laboratories in 2021 andhas been researching business operation analysis
methods. He is a member of the Institute of Electronics, Information and Communication Engineers (IEICE).

Yuki Urabe

Business Developer, NTT EAST Corporation.
She received a B.E., M.S., and Ph.D. in information science from Hokkaido University in2013, 2015, and 2021. She was engaged in R&D
of business operation analysis technology from2015 to 2021 at NTT Access Network Service
Systems Laboratories. She currently designs
business operations for Video Analytics Service
at NTT EAST.

Kimio Tsuchikawa

Senior Research Engineer, Navigation Technology Group, Access Network Operation Project, NTT Access Network Service Systems Laboratories.
He received a B.E. and M.E. in applied physics
from Nagoya University, Aichi, in 2000 and
2002. He joined NTT in 2002 and is engaged in
R&D of navigation technology for business
operation. He is a member of IEICE.

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Vol. 20 No. 8 Aug. 2022

71

Global Standardization Activities

Report on WTSA-20 (World
Telecommunication Standardization
Assembly)
Kazuhisa Yamagishi, Yoshinori Goto, Kazuhiro Takaya,
and Noriyuki Araki
Abstract

Due to the impact of COVID-19, the World Telecommunication Standardization Assembly (WTSA20), which was scheduled to be held in 2020, was held locally and online in Geneva, Switzerland, fromMarch 1 to 9, 2022. This article provides an overview of WTSA-20 and the main deliberations.
Keywords: WTSA, ITU-T, Study Group

1. Position of WTSA
The World Telecommunication Standardization
Assembly (WTSA) is a general assembly of the International Telecommunication Union - Telecommunication Standardization Sector (ITU-T), which meetsonce every four years. The Plenipotentiary Conference makes decisions for the ITU as a whole, as
shown in Fig. 1. As shown in Fig. 2, there are fivecommittees (COM1–5) under the Plenary session(PL), which is responsible for decision-making as theGeneral Assembly. Under COM3 and COM4, there
are two working groups (WG3A, WG3B, WG4A,
and WG4B) for detailed discussions. Since eachcommittee has limited time for deliberations, ad-hoc
meetings are held as needed for additional discussions, and drafting meetings are held as needed torevise resolutions. In WTSA-20, chairs and vicechairs were elected, as shown in Fig. 2, and the PL
vice-chair and COM2 vice-chair were elected fromJapan.
In addition to deliberating on resolutions that establish guidelines such as ITU-T activities and working
methods, WTSA deliberates on the reorganization of
Study Groups (SGs) and elects chairs and vice-chairsof the Telecommunication Standardization Advisory
Group (TSAG) and WTSA’s committees. Many disNTT Technical Review

Vol. 20 No. 8 Aug. 2022

cussions at WTSA are related to policy issues and the
management policies of the organization, unlike SG
meetings. Therefore, a large number of government
officials participate in WTSA. The participation of
the private sector also plays an important role, as
individual deliberations include technical content.
WTSA has six regions: Asia-Pacific Telecommunity(APT), European Conference of Postal and Telecommunications Administrations (CEPT), Inter-American Telecommunications Commission (CITEL),
African Telecommunications Union (ATU), Councilof Arab Ministers of Telecommunication and Information represented by the Secretariat-General of the
League of Arab States (LAS), and Regional Commonwealth in the field of Communications (RCC).
2. Impact of COVID-19 on WTSA-20
WTSA-20 was originally scheduled to be held inHyderabad, India, but was repeatedly postponed dueto the global pandemic (COVID-19) starting in early
2020, and was finally held as a hybrid local andonline meeting in Geneva, Switzerland.
In March 2022, the effects of the global pandemic
had not subsided, and many companies placed travel
restrictions on participants, forcing many to abandon
their on-site participation. The conference was held72

Global Standardization Activities

ITU

Plenipotentiary
ConferenceWRC(WorldRadiocommunication
Conference)RA(Radiocommunication
Assembly)

BR(Radiocommunication Bureau)

Council

WCIT(World Conference on
International
Telecommunications)

WTSA

WTDC
(World Telecommunication
Development Conference)

TSB(Telecommunication
Standardization
Bureau)

BDT(Telecommunication
Development
Bureau)RAG(Radiocommunication
Advisory Group)

TSAG(Telecommunication
Standardization
Advisory Group)

TDAG(Telecommunication
Development
Advisory Group)

ITU-R (Radiocommunication Sector)

ITU-T (Telecommunication
Standardization Sector)

ITU-D (Telecommunication
Development Sector)

Fig. 1. Relationship between ITU and WTSA.

under unusual circumstances, as travelers wererequired to be vaccinated in advance, undergo PCR(polymerase chain reaction) testing, and wear masks
at the venue (Photos 1 and 2).
During deliberations, with the exception of somediscussions, online participation was made possible,
allowing for remote participation. The disciplined
behavior of the participants allowed us to complete
the deliberations of WTSA-20 without any major
problems. WTSA-24 has been announced to behosted by India.
3. SG reorganization
SG reorganization is an important deliberation item
handled by WTSA. However, as WTSA-20 was postponed due to COVID-19, it was agreed to basicallymaintain the SG structure for the 2017–2020 study
period, and its operation is already underway from2021. There was not much discussion and the SG
structure was approved in Resolution 2 (ITU Telecommunication Standardization Sector study groupresponsibility and mandates). In Resolution 2, it wasadded that SG11 and SG17 should cooperate in
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security-related studies, and other minor revisions
were made and approved. The Question proposed byeach SG was also approved. The new study period
was successfully started.
4. Election of chairs and vice-chairs
Although the appointment of chairs and vice-chairs
was provided in Resolution 35, the new Resolution
208 (Appointment and maximum term of office forchairmen and vice-chairmen of the ITU) wasapproved at the 2018 ITU Plenipotentiary Conference (PP-18). Therefore, Resolution 35 was suppressed. PP Resolution 208 includes a provision thateach SG should have no more than three vice-chairsfrom each region to ensure regional balance. The
chairs and vice-chairs elected by WTSA-20 in accordance with PP Resolution 208 are listed in Table 1(* in the table indicates a second term). New chairs
were elected in TSAG, SG3, 5, 11, 12, 13, 15, 17, and
20. Regional balance was emphasized in the selectionof chairs, with the exception of Japan (SG9 andSG13) and Korea (SG17 and SG20), which elected
two chairs, and no other country elected more than73

Global Standardization Activities

Plenary
Chair: Mr Bruce Gracie (Canada)
Vice-chairs: Mr Yoichi MAEDA (APT, Japan), Mr Tobias KAUFMANN (CEPT, Germany), Mr Mohamed AL RAMSI(LAS, United Arab Emirates), Mr Kwame BAAH-ACHEAMFUOR (ATU, Ghana), Ms Tania VILLA (CITEL, Mexico)

COM1 (Steeringcommittee)
Plenary chair and vicechair
COM chair and vice-chair

COM2 (Budget control
committee)
Chair: Mr Bahtiyar
MAMMADOV (RCC,
Republic of Azerbaijan)Vice-chairs: Mr Yoshiaki
NAGAYA (APT, Japan),Ms Seynabou SECK (ATU,
Senegal), Mr Santiago
REYES-BORDA (CITEL,
Canada), Mr Fayçal
BAYOULI (LAS, Tunisia)

COM3 (Working method)
Chair: Mr SteveTROWBRIDGE (CITEL,
USA)
Vice-chairs: Ms Umida R.
MUSAEVA (RCC, Republicof Uzbekistan), Ms
Rebecca MUKITE (ATU,
Uganda), Mr Zhaoji LIN(APT, People’s Republicof China)

COM4 (ITU-T work
program/organizationcommittee)
Chair: Mr Philip RUSHTON(CEPT, United Kingdom)
Vice-chairs: Mr Maksud
AZIMOV (RCC, Republic of
Uzbekistan), Mr Mohamed
ELHAJ (ATU, Sudan), MrAbraão BALBINO E SILVA(CITEL, Brazil), Mr Jasim AL
ALI (LAS, United ArabEmirates)

COM5 (Editorial committee)
Chair: Ms Rim BELHAJ (ATU,
LAS, Tunisia)
Vice-chairs: Ms Samira
BELAL (LAS, Kuwait (Stateof)), Mr Tong WU (APT,
People’s Republic of China),
Mr Greg RATTA (CITEL,USA), Mr Jesús MARCOS
MORELL (CEPT, SPAIN)

WG3A
Chair: Ms Basma TAWFIK(LAS, Egypt)
Vice-chair: Mr Heung-Youl
YOUM (APT, Republic ofKorea)

WG4A
Chair: Mr Hyoung Jun KIM(APT, Republic of Korea)
Vice-chair: Ms Hend
BENHAJI (LAS, Tunisia)

WG3B
Chair: Mr Arnaud TADDEI(CEPT, United Kingdom)
Vice-chair: Mr Wakil BAKO
(ATU, Nigeria)

WG4B
Chair: Mr João Alexandre
Moncaio ZANON (CITEL,
Brazil)Vice-chair: Mr Guy Michel
KOUAKOU (ATU, Coted’Ivoire)

Fig. 2. Structure of WTSA-20.

Photo 1. International Conference Center Geneva.

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Vol. 20 No. 8 Aug. 2022

Photo 2. Plenary of WTSA-20.

74

Global Standardization Activities

Table 1. Chairs and vice-chairs.
SG

ChairsVice-chairs

TSAG

Mr Abdurahma AL HASSAN (Saudi Arabia)

10 incl. Ms Miho NAGANUMA (NEC, Japan)

SG2

Mr Philip Mark RUSHTON (United Kingdom)*

9

SG3

Mr Ahmed SAID (Egypt)

12 incl. Ms Eriko HONDO (KDDI, Japan)

SG5

Mr Dominique WÜRGES (France)

9 incl. Mr Kazuhiro TAKAYA (NTT, Japan)*

SG9

Mr Satoshi MIYAJI (KDDI, Japan)*

4

SG11

Mr Sh. Ritu Ranjan MITTAR (India)

9

SG12

Ms Tania VILLA TRAPALA (Mexico)

11 incl. Mr Kazuhisa YAMAGISHI (NTT, Japan)

SG13

Mr Kazunori TANIKAWA (NICT, Japan)

13

SG15

Mr Glenn Wilson PARSONS (Canada)

7

SG16

Mr Zhong (Noah) LUO (China (P.R.))*

8 incl. Mr Hideki YAMAMOTO (OKI, Japan)*

SG17

Mr Heung Youl YOUM (Korea (Rep. of))13 incl. Mr Yutaka MIYAKE (KDDI, Japan)*

SG20

Mr Hyoung Jun KIM (Korea (Rep. of))11 incl. Mr Toru YAMADA (NEC, Japan)* indicates a second term.

two. Seven vice-chairs were elected from Japan
including two from NTT.
5. Major resolutions discussed at WTSA-20
WTSA-20 approved two new resolutions and suppressed four resolutions (Resolutions 35, 45, 59, and66). Other resolutions were approved with revisions
or no changes. The following is a summary of themain deliberations.
5.1 Resolutions 72/73/79
In response to the progress in wireless communication services such as the 5th-generation mobile communication system (5G), and the growing concern
about climate change, the committee discussed the
proposed revisions to human exposure to electromagnetic fields (Resolution 72), climate change (Resolution 73), and the circular economy (Resolution 79).
For Resolution 72 (Measurement and assessment
concerns related to human exposure to electromagnetic fields), all regions (APT, CEPT, CITEL, ATU,
LAS, RCC) proposed revisions. At COM4, the proposals from each region were compiled into a Working Document for ad-hoc discussion. The first of the
proposed revisions was to rationalize the content of
the description in accordance with the rationalization
guidance approved at PP-18, and the second was tocomply with the International Commission on NonIonizing Radiation Protection (ICNIRP) guidelines(revised in March 2020), which had been revised in
anticipation of the use of new frequency bands in the
5G mobile network. The third was support for develNTT Technical Review

Vol. 20 No. 8 Aug. 2022

oping countries. The ad-hoc meeting agreed to update
and delete redundant parts of the preamble and add
and update the content such as promotion of ITU-T
recommendations referring to the ICNIRP guidelines
and support for developing countries.
For Resolution 73 (Information and communication technologies, environment, and climate change),
five regions other than LAS proposed revisions,which were discussed in an ad-hoc meeting. There
were two main points of revision: streamlining thedescription and adding the circular economy to thescope. The ad-hoc meeting reflected the proposals to
remove redundant parts, add the circular economy toclimate-change measures, clarify the various conditions (geographical, industrial structure, etc.) andchallenges faced by developing countries, and add
ecosystem monitoring, which was proposed by APT.
For Resolution 79 (The role of telecommunications/information and communication technologies
in handling and controlling e-waste from telecommunication and information technology equipment
and methods of treating it), ATU and APT submitted
a proposal for revision, and an informal draftinggroup edited the draft. In addition to updating thedescription, it was agreed to add a section on regulations against counterfeiting of electronic waste andthe importance of sustainable management.
5.2 N ew resolution on considering reorganizationof SG structure
As mentioned in the previous section, it was agreed
that the SG structure for the next study period will
maintain the structure of the 2017–2020 study period,75

Global Standardization Activities

and that the reorganization of the SG structure in
Resolution 2 will be implemented at WTSA-24. Prior
to this, TSAG has been studying the analysis ofevaluation indicators for the optimal allocation of the
SG structure, and an action plan for the restructuring
analysis was agreed upon at the TSAG meeting heldin January 2022. This action plan aims for a thoroughreview of potential restructuring options for ITU-T
based on the empirical analysis and was input by
TSAG to WTSA-20 with a view to approving the
proposed SG restructuring at WTSA-24. A new resolution on the consideration of ITU-T restructuring forthe purpose of implementing this action plan on theanalysis of the ITU-T SG restructuring prepared by
TSAG and for the TSAG to provide each SG with aprogress report on the restructuring analysis and submit a report with recommendations for consideration
at the next WTSA-24 was proposed. The draft proposal (Consideration of organizational reform of the
ITU Telecommunication Standardization Sector
study group) was approved.
5.3 New resolution on pandemics
Due to the impact of COVID-19, proposals weremade regarding global pandemics and remote participation.
First, APT, LAS, and ATU proposed and drafted a
new resolution on the role of telecommunications andinformation and communication technology (ICT) in
mitigating global pandemics. The new draft resolution was agreed upon in an ad-hoc meeting on thebasis of three proposals, but the United States proposed that the new resolution should be reported at
the next Plenipotentiary Conference (PP-22) since
the issue of a global pandemic is not an ITU-T issue
alone and to inform the Radiocommunication Bureau(BR) and the Telecommunication Development
Bureau (BDT) to avoid duplication of activities. The
action was approved.
Second, a new resolution on the position of face-toface and remote participation was proposed becauseof the increase in remote participation due to COVID19. However, since there are issues such as the inability to give voting rights to remote participants, the
new resolution could not be agreed upon and wastabled due to the need for further discussion.

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5.4 New resolution on artificial intelligence
LAS proposed and drafted a new resolution on artificial intelligence (AI) technology, as it plays animportant role in telecommunications and ICT. However, the following issues were raised during thedrafting process: (1) Many SGs and Focus Groups in
ITU-T are already discussing AI-based technologies,and (2) the description of machine learning is included in Resolution 2. Therefore, it was proposed by theUnited States, Europe, and Japan that a new resolution not be drafted. Discussions were unable to reachan agreement on a new resolution, and it was decided
to drop it.
5.5 New resolution on smart cables
CEPT proposed a new resolution because smart
cables are useful for detecting natural disasters suchas tsunamis and earthquakes, as well as climatechange. However, the United States and Japan raised
the issue that it is not appropriate to establish resolutions on individual technologies. Since the importance of the technology was recognized, it was proposed to continue its consideration in SG15. Discussions were parallel, and it was not possible to agree
on a new resolution. However, the importance of the
Joint Task Force on SMART cable systems was recognized and WTSA action was approved to avoid
duplicate discussions on smart cables in cooperation
with other standardization organizations.
6. Future developments
At WTSA-20, developed and developing countries
could not reach an agreement on AI and other issues.
Many developing countries proposed resolutions
calling for ITU-T to solve their own problems, whiledeveloped countries proposed avoiding the inclusionof specific problems in resolutions. This trend is
expected to continue. However, cooperation between
developed and developing countries has resulted in
the revision of many resolutions, making the policy
more suitable for the times.
NTT has secured two vice-chair positions and willcontinue to work on future activities related to telecommunications and ICT in accordance with thepolicies of the approved resolutions.

76

Global Standardization Activities

Kazuhisa Yamagishi

Senior Research Engineer, NTT Network Service Systems Laboratories.
He received a B.E. in electrical engineeringfrom Tokyo University of Science in 2001 and an
M.E. and Ph.D. in electronics, information, andcommunication engineering from Waseda University, Tokyo, in 2003 and 2013. In 2003, hejoined NTT, where he has been engaged in thedevelopment of objective quality-estimation
models for multimedia telecommunications. He
has been contributing to ITU-T SG12 since 2006.
He was a co-rapporteur of Question 13/12 for the
2017–2020 study period.

Yoshinori Goto

Senior Research Engineer, NTT Network Service Systems Laboratories.
He received a B.E. and M.E. in applied physics
from Tohoku University, Miyagi, in 1992 and1994. He joined NTT Basic Research Laboratories in 1994 and has been researching and developing cable television systems, Internet protocol
television (IPTV), and machine-to-machinetechnology. He has been engaged in the standardization of IPTV in ITU-T as a member of the
IPTV Focus Group and Global Standards Initiative since 2006. He has also served as Rapporteurof Question 11 of ITU-T SG9, Questions 5 and
25 of ITU-T SG13, and Question 21 of ITU-T
SG16. He has been a vice-chair of ITU-T SG13
since 2013. He is a member of the Institute ofElectronics, Information and Communication
Engineers (IEICE).

NTT Technical Review

Vol. 20 No. 8 Aug. 2022

Kazuhiro Takaya

Senior Research Engineer, Supervisor, NTT
Space Environment and Energy Laboratories.
He received a B.E. and M.E. in electrical andelectronic engineering from Okayama University
in 1993 and 1995 and Ph.D. in electrical engineering from Kyoto University in 2019. He has
been engaged in research on carbon-dioxide
conversion technologies using electrochemistry
and biology.

Noriyuki Araki

Director, Standardization Office, Research andDevelopment Planning Department, NTT.
He received a B.E. and M.E. in electrical andelectronic engineering from Sophia University,
Tokyo, in 1993 and 1995. He joined NTT AccessNetwork Service Systems Laboratories in 1995,
where he researched and developed operationand maintenance systems for optical fiber cable
networks. He has been contributing to standardization efforts in ITU-T SG6 since 2006. He was
the rapporteur of Question 6 of ITU-T SG6 from2006 to 2008 and the rapporteur of Question 17
of ITU-T SG15 from 2008 to 2012. He also
served as the chairman of the ITU-T Focus
Group on Disaster Relief Systems and Network
Resilience and Recovery. He was the vice-chairman of ITU-T SG15 from 2013 to 2022. He also
contributes to the activities of International Electrotechnical Commission (IEC) Technical Committee 86 (fiber optic systems). He received the
ITU-AJ award from the ITU Association ofJapan in 2017. He is a member of IEICE.

77

Practical Field Information about
Telecommunication Technologies
Practical Field Information about Telecommunication Technologies

Development of the Hikari Denwa
Fault-isolation Tool for Voice Over IP
ServicesTechnical Assistance and Support Center, NTT EAST
Abstract

Internet Protocol (IP)-based communication systems have become more sophisticated, and the details/causes of problems are becoming more complex and diverse. This article introduces a tool developed by
the Technical Assistance and Support Center, NTT EAST that is efficient in identifying the faults in voiceover IP services and connected equipment. This is the seventy-first article in a series on telecommunication technologies.
Keywords: voice over IP service, packet capture, fault-isolation tool

1. Introduction
The Technical Assistance and Support Center(TASC), NTT EAST provides technical support tosolve highly technical problems, the causes of which
are difficult to identify. The Network Interface Engineering Group of TASC provides technical support
for telecommunication services such as public
switched telephone networks and leased lines, broadband internet access services called FLET’S HIKARI
NEXT, etc., closed network connections using virtualprivate networks, Internet Protocol (IP)-based data
communications, and voice over IP services, as wellas terminal equipment connected to these lines.
Fixed telephone services have recently evolved andprovide high-value-added services such as cloudbased private branch exchange (PBX) or the use ofsmartphones as PBX extensions. As such services
have become more sophisticated, the details/causesof communication problems are also becoming more
complex and diverse. This article introduces a tool
developed by TASC that efficiently identifies thefaults in voice over IP services such as Hikari Denwa.

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2. Background and initiatives concerning
problems with voice over IP services
Conventionally, when a trouble occurs in a voiceover IP service, the fault point can be isolated bychecking the status of the equipment, measuring theoptical power of the communication signal, or replacing that equipment with alternative equipment. However, as networks of enterprise customers havebecome larger and more complex and the amount ofequipment has increased, the time required to identify
the cause of problems has increased. Detailed analysis is also required when customers report problems
with the quality of voice over IP services, such as
voices being interrupted or heard in one direction
only (one-way calls).
To speed up fault isolation and identify the causesof quality problems, it is effective to capture packets
between equipment and analyze them. For example,
the packet-data flow from the optical network unit
(ONU), which terminates the optical signal to the
Hikari Denwa router (which enables voice services),
telephones, etc., includes the voice data as Real Time
Protocol (RTP) packets that contain the sequence78

Practical Field Information about Telecommunication Technologies

FLET’SHIKARI
NEXT

Opticalfiber cable

OGW(master unit)

ONU
LAN cable

Mirrorhub
Mirror port(monitor port)

OGW(extension unit)
Mirrorhub

Metal cable

TelephoneNoise

Mirror port(monitor port)

HDfi-toolHDfi-tool

Monitoring PC

Monitoring PC

Fig. 1. Configuration diagram for isolating a fault by using HDfi-tool.

number. It is possible to identify the equipment causing the problem through analysis of the sequencenumber of packets or voice data.
However, it is necessary to install equipment tocapture packets on site, and such packet analysis
requires a high level of skill to understand and analyze the network sequence. Therefore, we developed
a tool that enables on-site maintenance personnel tocheck the analysis results and quickly and easily
identify the equipment and sections where anomalies
are suspected to exist.
3. Hikari Denwa fault-isolation tool
3.1 Overview
The Hikari Denwa fault-isolation tool (hereafter,“HDfi-tool”) is software that enables users to checkthe real-time call status and statistical analysis resultof voice data on IP packets. It can operate on a personal computer (PC) or tablet PC that runs on the
Windows operating system and has a wired local area
network (LAN) interface. As shown in Fig. 1, thepacket-capture function of HDfi-tool uses a generalswitching hub with a mirror function (hereafter, “mirror hub”) or network tapping device (TAP) to interrupt a section of the LAN cable between equipment
and analyze the data collected from each section.
Process flows for isolating a fault by using HDfitool are shown in Fig. 2 and compared with those of
the conventional method of replacing failed equipment with alternative equipment. To check the condition of voice packets by using HDfi-tool, the mirror
hub is inserted between equipment; no need tochange equipment (ONU or office gateway (OGW))
and no need for reconfiguration. In other words, theNTT Technical Review

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failed equipment can be identified through a simple
operation, such as backing up the configurations andreconfiguration, backing up the reconfiguration process, that only needs to be done once to identify thecause of the fault. Therefore, the fault-isolation process is significantly shortened.
3.2 Functions and measurement procedureHDfi-tool analyzes the Session Initiation Protocol(SIP) sequence of Hikari Denwa from packet data
between equipment and automatically detects andindicates the existence of calls and their anomalies.
The packet data contain telephone numbers of theoutgoing and incoming terminals, and voice quality
can be evaluated. It is possible to determine that a callis busy from the packet sequence and measure theindexes that affect communication quality such as
packet loss and delay. HDfi-tool offers two modes ofanalysis: real-time analysis and statistical analysis.
The packet-capture procedure is introduced and eachfunction is explained in detail as follows.
3.2.1 Procedure
The process from packet capture to data analysis
using HDfi-tool for the hypothetical configuration
shown in Fig. 1 is as follows.
1)	Insert the mirror hub between equipment at the
point to be analyzed.
2)	Connect the LAN cable from the mirror port of
the hub to the analysis (monitor) PC and startHDfi-tool.
3)	When a phone call is detected, the call-list tableshows the call start time, telephone number, etc.
(Fig. 3).
4)	On the call-list table (3), click the target call forwhich the voice status is to be checked to display
79

Practical Field Information about Telecommunication Technologies

Fault isolation by sequentiallyreplacing equipment

Using HDfi-tool + replacing
failed equipment(1) Fault isolation: 10 minutes• Insert TAP• Monitoring(1) Time to replace: 20 minutes• Re-wiring• ONU-registration(2) Fault isolation: 10 minutes• Insert TAP• Monitoring(2) Time to replace: 30 minutes• Re-wiring• Configuration(3) Time to replace: 30 minutes• Re-wiring• Setup data

(3) Time to replace: 30 minutes• Re-wiring• Setup data

Fig. 2. Process flows of fault isolation with conventional method and using HDfi-tool.

Hikari Denwa - Call selection
Setting

Call monitoring
Number of calls
Call start time

Call duration

Caller (from)

Callee (to)Statusactive call
active call

Display call-monitoring
results after the tool isstartedFig. 3. Call-list table (after a call starts).

call details and statistics (Fig. 4).
5)	Determine whether an anomaly call exists inaccordance with the content shown in the table.
3.2.2 Details of functions
1) Real-time-analysis function
The real-time-analysis function displays the voicepacket conditions of the current call flowing throughNTT Technical Review

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the Hikari Denwa line by analyzing the RTP packets
flowing in the LAN-cable section. The call-list tableis shown in Fig. 3. It displays start time, call duration,
caller, callee, status of incoming and outgoing calls,
and total number of simultaneous calls. It is possible
to see if the maximum number of calls has beenreached.
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Practical Field Information about Telecommunication Technologies

Hikari Denwa – Call details
Caller (from)
Callee (to)Basicinformation

Call duration
Status

Detailed information(1) Detect fault due tounreachable
packet(2) Detect fault
due to packet
re-order(3) Detect fault due
to packets containing
silent data

<Call Evaluation>
CallerCalleeCalleeCaller
•••

(1)

None

•••

(2)

None

•••

(3)

Number of packet losses
Detection of re-order

None

Detection of completely silent call

None

Number of packets excessing the packet-arrival interval
Packet-arrival interval  80 ms

Fig. 4. Call-details window.

2) Statistical-information-analysis function
When a call is selected from the call list in the realtime-analysis function described above, the “CallEvaluation” section displayed in the call-details window (Fig. 4) allows the user to determine the presenceor absence of the following anomalies in the voice
data sent via RTP.
If the “Number of packet losses” increases, it isdetermined that the voice data are abnormal in thesection in question because the increase in packet
losses may cause voice interruption or silence. The
relative position of the anomaly to the section where
the packets are captured can be identified since thisfunction allows the user to recognize the direction of
the lost packets.
•	“Number of packets excessing the packet-arrival
interval” indicates the jitter of the packet-arrival
time. The number of packets with an arrival
interval of 80 ms or more is displayed.
•	“Detection of re-order” indicates whether RTP
packets arrive in the order in which they weresent. If the arrival interval of RTP packets fluctuates or packet order is reversed due to congestionin the network, etc., voice interruption may
occur.
•	For “Detection of completely silent call,” thevoice data contained in the RTP packets arechecked, and the presence or absence of packets
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that contain continuous silent voice data is displayed. If silent data flow continuously, a malfunction of the equipment that builds RTP packets from voice data or that of the telephone issuspected.
Conventional analysis tools can list these statistics
for each packet, but using them requires a high levelof analysis skills and effort, such as sequentiallychecking sequence numbers in a huge log or calculating delays from captured packet timestamps. HDfitool enables simple and quick analysis of fault factors
for problems with voice over IP services, and we
expect it to significantly improve the workability offault isolation.
4. How to obtain HDfi-tool
HDfi-tool has been available for download from the
TASC’s website* since April 2022. We believe that
HDfi-tool makes on-site investigation into the causesof problems with the Hikari Denwa service faster andeasier and prevents unnecessary replacement ofequipment and devices.

*

Due to software platform limitations, HDfi-tool is currently offered only within the NTT Group.

81Practical Field Information about Telecommunication Technologies

5. Concluding remarks
This article introduced HDfi-tool, which was developed as one of the technical-support initiatives targetingproblems with IP-based systems in the field. By
acquiring and analyzing data using a variety of tools,

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the Network Interface Engineering Group of TASC
provides technical support for the early resolution ofproblems with equipment, terminals, and networks.
We will continue to disseminate technologies by providing technical support, developing tools, holding
technical seminars, and other means.

82External Awards
IPSJ Fellow
Winner: Makoto Iwamura, NTT Social Informatics Laboratories/
NTT Security (Japan) KK
Date: March 28, 2022
Organization: Information Processing Society of Japan (IPSJ)
For his contribution to the development of pioneering anti-cyberattack technologies and revitalization of the security field throughindustry-academia cooperation.
Specially Selected Paper
Winners: Yukako Iimura, NTT Computer and Data Science Laboratories; Shinobu Saito, NTT Computer and Data Science Laboratories
Date: April 18, 2022
Organization: IPSJ
For “Industrial Practice and Evaluation of Microtask Programmingfor Achieving Working Style Diversity.”
Published as: Y. Iimura and S. Saito, “Industrial Practice and Evaluation of Microtask Programming for Achieving Working Style Diversity,” IPSJ Journal, Vol. 63, No. 4, pp. 999–1007, Apr. 2022.
Best Paper Award
Winners: Takafumi Tanaka, NTT Network Innovation Laboratories;
Seiki Kuwabara, NTT Network Innovation Laboratories/NTT Communications; Tetsuro Inui, NTT Network Innovation Laboratories
Date: May 11, 2022
Organization: The Institute of Electronics, Information and Communication Engineers (IEICE) Communications Society
For “Demonstration of Flex Ethernet over OTN Link State Monitoring Method.”
Published as: T. Tanaka, S. Kuwabara, and T. Inui, “Demonstrationof Flex Ethernet over OTN Link State Monitoring Method,” IEICE
Trans. Commun. (JPN Edition), Vol. J103-B, No. 11, pp. 595–604,
Nov. 2020.
Kenneth C. Smith Early Career Award in Microelectronics
Winner: Akira Ito, NTT Social Informatics Laboratories
Date: May 21, 2022
Organization: The Institute of Electrical and Electronics Engineers

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Vol. 20 No. 8 Aug. 2022

(IEEE) Computer Society, Technical Community on Multiple-Valued
Logic
For “A Formal Approach to Identifying Hardware Trojans in Cryptographic Hardware.”Published as: A. Ito, R. Ueno, and N. Homma, “A Formal Approach
to Identifying Hardware Trojans in Cryptographic Hardware,” 2021
IEEE 51st International Symposium on Multiple-Valued Logic(ISMVL), pp. 154–159, Nur-Sultan, Kazakhstan, May 2021.
Standardization Achievement Award
Winner: Seishi Takamura, NTT Computer and Data Science Laboratories
Date: May 24, 2022
Organization: IPSJ/Information Technology Standards Commissionof Japan
For his long-term contribution to the standardization activities inInternational Organization for Standardization/International Electrotechnical Commission (ISO/IEC) Joint Technical Committee 1 Subcommittee 29 since 1998.
IPSJ Fellow
Winner: Seishi Takamura, NTT Computer and Data Science Laboratories
Date: June 7, 2022
Organization: IPSJ
For his achievement concerning the research and development ofvideo coding and contribution to its standardization and dissemination.
IACR Fellow
Winner: Masayuki Abe, NTT Social Informatics Laboratories
Date: June 27, 2022
Organization: The International Association for Cryptologic
Research (IACR)
For influential contributions to practical cryptosystems and for
exemplary service to the IACR and the Asia-Pacific cryptography
community.

83

Papers Published in Technical Journals andConference Proceedings
Sumcheck-based Delegation of Quantum Computing toRational Server
Y. Takeuchi, T. Morimae, and S. Tani
Theoretical Computer Science, Vol. 924, No. 1, pp. 46–67, April
2022.
Delegated quantum computing enables a client with weak computational power to delegate quantum computing to a remote quantum
server in such a way that the integrity of the server can be efficiently
verified by the client. Recently, a new model of delegated quantum
computing has been proposed, namely, rational delegated quantum
computing. In this model, after the client interacts with the server, theclient pays a reward to the server depending on the server’s messages
and the client’s random bits. The rational server sends messages thatmaximize the expected value of the reward. It is known that the classical client can delegate universal quantum computing to the rational
quantum server in one round. In this paper, we propose novel oneround rational delegated quantum computing protocols by generalizing the classical rational sumcheck protocol. An advantage of our
protocols is that they are gate-set independent: the construction of theprevious rational protocols depends on gate sets, while our sumchecktechnique can be easily realized with any local gate set (each ofwhose elementary gates can be specified with a polynomial numberof bits). Furthermore, as with the previous protocols, our reward
function satisfies natural requirements (the reward is non-negative,
upper-bounded by a constant, and its maximum expected value islower-bounded by a constant). We also discuss the reward gap. Simply speaking, the reward gap is a minimum loss on the expected valueof the server’s reward incurred by the server’s behavior that makes
the client accept an incorrect answer. The reward gap should therefore be large enough to incentivize the server to behave optimally.
Although our sumcheck-based protocols have only exponentiallysmall reward gaps as in the previous protocols, we show that a con-

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Vol. 20 No. 8 Aug. 2022

stant reward gap can be achieved if two noncommunicating but
entangled rational servers are allowed. We also discuss whether asingle rational server is sufficient under the (widely believed)
assumption that the learning-with-errors problem is hard for polynomial-time quantum computing. Apart from these results, we show,
under a certain condition, the equivalence between rational andordinary delegated quantum computing protocols. This equivalence
then serves as a basis for a reward-gap amplification method.

Rewindable Quantum Computation and Its Equivalence to
Cloning and Adaptive Postselection
R. Hiromasa, A. Mizutani, Y. Takeuchi, and S. TaniarXiv:2206.05434, June 2022.
We define rewinding operators that invert quantum measurements.
Then, we define complexity classes RwBQP, CBQP, and AdPostBQP
as sets of decision problems solvable by polynomial-size quantum
circuits with a polynomial number of rewinding operators, cloning
operators, and (adaptive) postselections, respectively. Our main
result is that BPPPP ⊆ RwBQP = CBQP = AdPostBQP ⊆ PSPACE.
As a byproduct of this result, we show that any problem in PostBQP
can be solved with only postselections of outputs whose probabilities
are at least some constant. Under the strongly believed assumptionthat the shortest independent vectors problem cannot be efficiently
solved with quantum computers, we also show that a single rewinding operator is sufficient to achieve a task that is intractable for quantum computation. In addition, we consider rewindable Clifford andinstantaneous quantum polynomial time circuits.

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