R&D Spirits
Research on Large-capacity Photonic Routers
toward Optical Packet-switched Networks
Ryo Takahashi
Senior Research Engineer, Supervisor
All-optical Processing Research Group
Advanced Optoelectronics Laboratory
NTT Photonics Laboratories

An optical packet-switched network using asynchronous bursts is often called the “ultimatenetwork.” To this end, the All-optical Processing Research Group of the Advanced Optoelectronics Laboratory at NTT Photonics Laboratories is researching large-capacity photonicrouters. To find out how this research is progressing and what significance it holds in the world
at large, we talked with senior research engineer Ryo Takahashi, the unifying force of the Alloptical Processing Research Group.

Achieving a 40-Gbit/s photonic RAM with
optical/electronic hybrid technology—Dr. Takahashi, please give us an overview of thisresearch.
Our ultimate goal is to construct a large-capacity
photonic packet router that can process high-speed
optical packets as needed. To deal with the explosive
growth of IP traffic, it is essential that link capacity beincreased by wavelength division multiplexing(WDM) while simultaneously increasing the packet
transfer speed at nodes. That is why R&D in largecapacity photonic networks using optical technology
has become quite active. Of particular interest here
are optical packet-switched (OPS) networks that we
expect to be the wave of the future. The key elementsof such a network are large-capacity photonic packet
routers. Their development, however, means solving
a number of technical problems, such as clock extraction from asynchronous-burst optical packets, highspeed optical label processing, large-capacity opticalbuffer memories, and bit-rate conversion. We certainly have a difficult journey ahead of us.
—How will the network change if OPS can beachieved?
6The current network uses optical fiber on links, but
at nodes, it uses ATM or other types of electronicswitches to switch paths. The next-generation network will set and switch optical paths by means of aG-MPLS (generalized multiprotocol label switching)router that treats wavelength as a label. This scheme
can reduce transmission delays and significantly
increase throughput. It does not, however, use wavelengths efficiently. To make more efficient use ofwavelengths, the successor to this network, the socalled burst-switched network, will switch a single
path at fixed time intervals. This network will in turnbe followed by the OPS network, the target of ourresearch. An OPS network transfers all packets inasynchronous burst mode making the above circuit
switching unnecessary. In other words, the OPS network can achieve maximum network efficiency,
which explains why it is called the “ultimate network.”—What are some important technical points of thisresearch?
While light can be used to perform simple processes at high speeds, it is not very adept at complex operations. In short, it would not be wrong to say thatusing only optical means to achieve the functions ofNTT Technical Review

R&D Spirits

a large-capacity photonic router is essentially impossible, at least for the present. Faced with this problem,
I considered the merits of using the beneficial features of electronics instead of pursuing all-opticalprocessing at all cost, and decided to use a complementary metal oxide semiconductor (CMOS) electronic circuit for advanced operations. CMOS technology can also be used for fabricating memory andfor creating compact, low-cost, and low-power-consumption devices. A CMOS circuit, though, cannot
be directly connected with high-speed optical packets, and for this reason, we developed an all-opticalserial-to-parallel converter (SPC) and a photonic parallel-to-serial converter (PSC). These converters andan optical clock-pulse generator make up a photonic
RAM (random access memory) (Fig. 1). They are
constructed using original devices that we developed.
Another important technical point here is a surfacenormal optical switch called LOTOS in the all-opticalSPC. LOTOS can achieve a switching time of 250 fsand has demonstrated ultrafast parallel conversion of
optical packets at 1 Tbit/s. No other surface-normal

switch in the world has yet achieved such high-speed
operation.
—How far do you think this technology will progressin three- or five-years time?
To date, we have successfully tested basic operations using prototype equipment, and while we have
not yet reached full-packet operation, we haveachieved a photonic RAM for 40-Gbit/s, 16-bit optical packets. We have also prepared basic tools that weplan to use to test various aspects of this equipment.
Assuming steady progress, I believe that, in three tofive years, we can achieve a router that can easily handle 40-Gbit/s optical packets, which would be of apractical level. Keep in mind, though, that, as I mentioned earlier, this router is targeted for use in theOPS network scheduled to follow the third-generation network. Whether or not this router will ever finduse remains to be seen. Nevertheless, my colleagues
and I would like to be the first in the world to achieve
this photonic packet router even if only by one day.

Photodetector array

40-Gbit/s 16-bit
input opticalpackets

All-optical
SPC

CMOS
RAMPhotonic
PSC

40-Gbit/s 16-bit
output opticalpackets

Photodetector

Opticalpulse source

OCG

SPC

OCG

PSC

CPUCMOSRAM

SPC: serial-to-parallel converter

PSC: parallel-to-serial converter

OCG: optical clock-pulse generator

Fig. 1. Structure of Photonic RAM.

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7R&D Spirits

From materials to devices and systems—astep-by-step broadening of research interests—Dr. Takahashi, what stimulated your interest inultrafast photonics?
At university, I majored in electronic engineering
thinking that “the age of light is coming.” At thattime, I had no particular research theme in mind, but
as the main keyword associated with light happened
to be “high speed,” I naturally turned my attention tothat aspect of optical systems. In time, I came to think
that, by using light, I would like to do something notsimply at high speeds but at ultrahigh speeds! Consequently, in graduate school, I conducted research on
short-optical-pulse generation and optical-pulse compression in semiconductor lasers, and on ultrafastmeasurement of electrical signals using light. Since
then, I have been engaged exclusively in the researchon ultrafast photonics.
—What kind of research in particular have you been
involved with up to now?

On entering NTT, I was initially occupied withfinding a means of achieving an ultrafast surface-normal all-optical switch. This type of device would
require high-speed materials, so I began my researchfrom a position of preparing materials. The materials
used in a surface-normal optical switch require
extremely large optical non-linearity, and I found thatusing exciton absorption saturation in multiple quantum wells (MQW) was a good way to meet thisrequirement. However, the lifetime of excited carriers
was long, making high-speed operation impossible. I
then changed the MQW growth temperature from theusual 500°C to 200°C and doped the material withberyllium (Be). With this approach, I succeeded in
shortening the carrier lifetime by four orders of magnitude, thereby achieving high-speed operation.
Expanding on this, I succeeded in fabricating theLOTOS planar optical-optical switch having anon/off ratio greater than 40 dB (Fig. 2). After theseachievements, I spent two years in the personal development department of NTT’s Tokai Office andreturned to NTT Laboratories in 1998. In just six
months, though, I was off again, this time to Stanford

SPCCMOScircuit

λ/4 plate
LOTOS

What is LOTOS?

PBS

Low-temperature-grown surface-reflection all-optical switch
Ultrafast operation High on/off ratio• 200°C-grown • 1% compressive• Be dopingstrain• AFP structure• Spin polarization
Switching time
< 300 fs

On/off ratio> 30 dB

AFP: asymmetric Fabry-Perot
PBS: polarizing beam splitter
PLC: planar lightwave circuit

AR coating
InP: Fe substrate

PacketFiberPump

InGaAsP/InP DBR
InGaAs/InAlAs
MQWs
InAlAs/InP SL
Au mirror

DBR: distributed bragg reflector
MQW: multiple quantum well
SL: superlattice
AR: anti-reflection

Two-dimensional
micro lens array
45-angled mirror

Side view
Top view

Optical packet
payload label Delaylines
Pump pulse

1: 16 splitter
Surface-emitting PLC

1.5-mm pitch
Hexagonal arrayof mirrors

Fig. 2. All-optical serial-to-parallel converter and LOTOS.

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R&D Spirits

University for a one-year stay as a guest researcher. I
returned to Japan in 1999 and soon began my present
line of research.
—Please tell us what got you started in your current
research?
Ever since I joined the All-optical Processing
Research Group, I have been researching ways ofachieving ultrafast control of optical signals by lightitself. At first, I felt that even optical packet processing could be performed using light alone, but this waseasier said than done. It was at that point that I wentto Stanford University and encountered the research
theme of converting ultrafast signals from analog todigital using light. It was Professor David A. B.
Miller’s idea that analog electrical signals could besampled by an ultrafast photoconductive sample-andhold circuit and then converted to digital signals by a
CMOS circuit. I did not fully understand the significance of this idea at first, but as my research continued, my thinking went through a radical change. “Theuse of light is not the objective, it is simply a means!”
I had to learn this obvious truth for a second time, but
it signaled the beginning of my current research inwhich I deal with more than just light.

Keeping one step ahead of a growing number ofcompeting researchers through original ideas—Tell us something about international and domestic
trends in this research.
The concept of OPS networks has been aroundsince the 1980s, but not much specific research has
actually been done. Recently, though, I have felt that
more and more research is focusing on optical packet
processing, which means that an increasing numberof competing researchers should be entering thisfield. While I am happy to see research in this field
becoming mainstream, I do not intend to give up ourleadership here so easily. This is because we are stillat the stage at which various ideas are being proposed, and it is still too early to tell which will dominate. In this environment, I believe our group is stillout in front. In optical label processing, for example,
we are implementing high-speed serial-bit label processing, which has been difficult to achieve in thepast, in a compact all-optical serial-to-parallel converter. In buffering, moreover, we are using CMOS
circuits as memory to construct buffers that satisfy all
the standard requirements including large capacity,Vol. 1 No. 7 Oct. 2003

long-term storage, and anytime access.
—Is your group engaged in joint activities with overseas research institutions or are you competing withthem?
Because our research involves many patents, I
guess I would have to say that all other research institutions are our competitors. It is therefore not easy toengage in joint research activities. We have actually
received some offers, but had to refuse in the end.
Personally, I have no hesitation about joint research if
I feel that something of benefit could be obtained. In
the past, when I was conducting research on my own,
I sometimes wanted to pursue collaborative researchsomewhere to speed things up. My group has sincegrown to six members, however, all of whom areexcellent researchers. At present, I am satisfied withthe present situation.
—What do you know of other researchers’ work?
Of course I survey other people’s research, and if
the circumstances warrant it, I would be inclined toevaluate certain work constructively. But as to imitating certain research, I think not. That is because I
have always thought that unoriginal research is hopeless. Refraining from copying other people’s researchis my research spirit and my most basic policy. At thesame time, I do not make it a point to hide my current
research activities at all times. While I do not usuallypartake in exchanges with overseas researchers
involved in similar research, I do interact with many
people here in Japan such as researchers that I meetthrough academic societies and friends from my university days. Whenever I have the opportunity to go
out for a drink with such people, our conversation
tends to be quite open. At any rate, my group’s
research is not something that can be easily imitatedfrom just conversation.
—Has your research to date had an influence?
After submitting our paper on experimental resultswith a prototype to Optical Fiber Communication(OFC), we received mail from a journalist at the Optical Society of America (OSA). It was agreed thatmaterials and information would be collected for astory in conjunction with the presentation that I
would be giving at OFC in March of this year. As luck
would have it, the outbreak of the Iraq war prevented
me from taking an overseas trip, and as the only otheralternative was to give a video presentation, we even9

R&D Spirits

tually decided to cancel. At that time, equipment had
already been forwarded with the aim of giving a
dynamic demonstration as well. But as we ourselves
could not go in the end, the demonstration could not
be given either. We therefore had no choice but tohave the container holding this equipment returned
without even being opened. In many aspects, but
especially in terms of not being able to experience
firsthand the response to our presentation, we were allquite disappointed. Since then, however, we havereceived two invitations to international conferences,
as well as many requests for papers. In fact, a paper
for next year’s OFC is already in the works. I therefore sense real interest in our research from the world
at large.

Contributing original and innovative devices
to science and business—In what direction would you like to take yourresearch in the years to come?
When I first entered NTT, I wanted to create usefuland innovative world-class devices, and that desire
hasn’t changed at all. My main goal is therefore to
take my research as far as practical, working devices.
I hope to create devices that can actually be used byNTT, a private company, and that can make a realcontribution to business. At the same time, I enjoy thechallenge of overcoming technical difficulties andachieving ultrafast processing. Of course, developing
practical devices operating at 40 Gbit/s is now thecorrect path to be on, but I still would like to hold aworld record at ultrahigh speeds on the Tbit/s level.
Looking a bit further into the future, though I havesaid that I am no longer obsessed about doing everything by only light, I am still deeply involved in optical research, and I would love to someday achieve alloptical ultrafast logic processing. While that may be
impossible without a major breakthrough, it has been
my dream for many years.

telecommunications carrier having a basic-researchgroup in components and materials, and NTT
researchers get due respect. There are also many
overseas researchers who would like to visit NTT. As
an organization, however, NTT and other companies
and institutions are naturally competitors, and I’m notsure what kind of expectations others might have.
NTT is certainly viewed as an important research
institution, at least in Japan. Our mission is naturally
to make NTT a leader in the field of communications,and others around NTT probably expect the same.
—Dr. Takahashi, what is it like working at NTT Laboratories?
I think it’s a great research complex covering a wide
range of fields on a large scale and assembling a greatgroup of brilliant researchers. As far as I know, NTT
Laboratories has the best working environment inJapan. While in university, I had the opportunity toconduct some experiments at what was then NTT’s
LSI Research Laboratory. I was very impressed withthe superb research equipment and facilities that Iwas able to use. If I was to pursue research in Japan,
I wanted to do it at the best research institution in thecountry, so I entered NTT Laboratories. Although
research among many high-caliber people can be
extremely competitive, such an environment is highly stimulating, providing ideal conditions for selfgrowth.

—Based on your experiences as a visiting scholar
and a participant in academic conferences, how is
NTT viewed by the outside world? What expectations
do people have of NTT?
NTT’s contributions at international conferences in
the field of communications are well known by justabout everyone. At such conferences, there is not
likely to be anyone that has not heard of the name“NTT.” It is widely known that NTT is the only
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Interviewee profile■ Career highlights
1987 B.E. from the University of Tokyo, Dept. ofElectronic Engineering.
1989 M.E. from the University of Tokyo, Dept. ofElectronic Engineering.
1992 D.E. from the University of Tokyo, Dept. ofElectronic Engineering.
Joined NTT Optoelectronics Laboratories,Kanagawa, Japan.
(1996-1998 Associate Manager, Personal Development Department, Tokai Regional CommunicationsSector)
1998-1999 Visiting scholar at Ginzton Laboratory,Stanford University, Stanford, CA.

Vol. 1 No. 7 Oct. 2003

1999- Currently with NTT Photonics Laboratories,
Kanagawa, Japan and engaged in ultrafast alloptical devices and optical signal processing
for photonic packet-switched networks. Dr.
Takahashi is a member of the Japan Society ofApplied Physics and the Institute of Electronics, Information and Communication Engineers of Japan.
■ Major awards
1994 NTT Optoelectronics Laboratories DirectorAward1995 NTT Optical Network Systems LaboratoriesDirector Award
1996 Japan Society of Applied Physics Award
2003 NTT Photonics Laboratories Director Award

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