R&D Spirits
Developing New Photonic Semiconductor Devices
for Photonic Networks
Hiroshi Yasaka
Senior Research Engineer, Supervisor
Group Leader of High-speed Semiconductor Photonic Device
Research Group
Photonics Device Laboratory, NTT Photonics Laboratories

At NTT Photonics Laboratories, the research and development of modulators, the core deviceof photonic networks, is giving birth to a novel product: the semiconductor Mach-Zehnder modulator. What changes will this new device bring to optical communications, and what problems
must still be solved before actual implementation? We put these questions to Senior ResearchEngineer, Hiroshi Yasaka, the leader of the High-speed Semiconductor Photonic Device
Research Group.

Development of optical modulators to meetthe demand for long-distance, large-capacity
communications—Dr. Yasaka, please describe your current R&D
efforts for us.
Our group researches and develops semiconductor
optical components for use in optical communication
systems. The spread of the Internet is generating a
dramatic increase in the amount of information flowing through the network, and to support this increase,
we must find ways of increasing network capacity
and speed. Of particular importance here is the development of a high-capacity core network (Fig. 1).
There is therefore a strong need for high-speed, lowloss photonic devices, and making devices of thistype is foremost in our minds in our daily R&D activities.
Specific products include semiconductor lasers andmodulators as well as integrated devices that combine
these products. At present, we are putting much of
our energy into modulators that use semiconductors.
Let me point out here that modulators come in various types, from those used for short-distance applications such as interconnecting equipment in the home6

to those used for interconnecting long-distance nodes
in the core network. The type that we are focusing onis the latter. Modulators used for long-distance transmission on the order of several tens of kilometers
require modulation control to negate the effects ofnonlinearity in optical fiber. And these modulators
must, of course, be able to support wavelength division multiplexing (WDM). To meet these needs, weare developing a semiconductor Mach-Zehnder (MZ)
modulator [1].
—In what ways do you think this R&D work is useful?
First of all, it will enable modulation appropriate
for longer transmission distances. In a semiconductor
MZ modulator (Fig. 2), input light is divided in two
optical waveguide branches each paired with a signal
electrode. Applying a voltage to a waveguide changes
its refractive index, thereby changing the phase of thelight passing through it. The two optical signals are
finally recombined before output. As a result, light is
output if these two optical signals are in phase, and nolight is output if their phases are out of phase. This isthe basic operating principle of the device. Comparedwith electroabsorption type semiconductor modulators that are now in common use in Metro area netNTT Technical Review

R&D Spirits• WDM technology• Optical router etc.

GatewayCore node(Pbit/s)Core network

To other networks

Optical network basis
Edge node(Tbit/s)(Gbit/s)
Optical access
LAN

Opticalaccess

OpticalaccessWireless
accessDSL access

TV

Enterprise network Home network
Core network

Node

Node

Transmission between nodes
To another node

Opticalrouter- Long reach, WDM+SDM- Modulation format control

Compact, low-chirp semiconductor external modulator

NodeEquipment

DSL: digital subscriber line
LAN: local area network
SDM: space division multiplexing

Fig. 1. Photonic network systems. Optical access is up to 1 Gbit/s, an edge node handles several hundred terabits per
second, and a core node handles several petabits per second.

Output waveguide
Phase modulation waveguides

Outputsignal light

Input waveguide

Optical coupler & combiner
Input CW light
Signalelectrodes

Input electrical signal
4.5 mm

CW: continuous wave

Fig. 2. Semiconductor MZ modulator.

works, this semiconductor MZ modulator can inhibit
spectrum broadening by using special modulation
formats, so the output has a low chirp characteristic
suitable for long-distance transmission. Another benVol. 4 No. 11 Nov. 2006

efit of our R&D work here is cost reduction. With achip size of 0.8 × 4.5 mm2 and driving voltage of lessthan 1.5 Vpp, we have achieved a compact device thatsaves space and consumes less power.
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R&D Spirits—What are the key technical features of the semiconductor MZ modulator?
The most outstanding feature is the use of semiconductor material. Most MZ modulators available on
the market use a dielectric material, usually lithium
niobate (LiNbO3; hereafter, LN). Instead, we use
indium phosphide (InP), a semiconductor material,
which enables us to make a smaller device with alower driving voltage. Moreover, since it’s the samematerial as used in lasers, there are good prospects for
optical integrated circuits in the future.
Another major feature is the introduction of a new“n-i-n” (negative-insulator-negative) cross-sectional
structure in the optical waveguides. Existing semiconductor MZ modulators use a “p-i-n” (positiveinsulator-negative) structure in which a p-InP
cladding layer, an i-MQW (multiple quantum well)
optical core layer, a semi-insulating InP layer, and an
n-InP cladding layer are grown on an InP substrate inthat order. This structure, however, suffers from large
loss in both electrical and optical signals due to the pInP cladding layer. In response to this problem, we
decided to form an n-type semiconductor layer,
whose signal loss is about 1/20 that of a p-type semiconductor layer, on both the signal-electrode andground-electrode sides. We also inserted an Fe-doped
semi-insulating InP layer between the optical corelayer and upper n-InP cladding layer to complete thisnew n-i-n waveguide structure. This structure enables
us to make a device having low wavelength dependency, low temperature dependency, low loss, andhigh breakdown voltage.

—How is development progressing?
In terms of characteristics, our semiconductor MZ
modulator has reached the working level. However, atransponder that actually incorporates the device has
not yet been commercialized, and we are very eager
to develop and market the world’s first transponder ofthis type ahead of our competitors. But there is notmuch time. Competition has intensified over the last
two years or so, and we have been focusing ourefforts on developing smaller modules because module size has been an obstacle to commercialization.
We have reduced the module size from about 100 mm
to almost 20 mm (Fig. 3). And regarding performance, we have already confirmed through experiments that the device generates good eye patterns,
indicating good signal band characteristics. All in all,I am confident that our approach will lead to a device
suitable for practical applications.
—What issues must be addressed toward commercialization?
We must ensure that all characteristic parameters
are equivalent to or better than those of the LN-typeof MZ modulator. Although our semiconductor MZ
modulator is definitely superior to the LN MZ modulator in terms of major requirements including a compact configuration, we cannot say that our device is
not inferior in other respects. Being superior in someways while having worse characteristics in otherrespects is not what I would call advanced technology development. To reach a true stage of practical

Electrical output
Phase-shift region 50-Ω termination
L = 3.0 mm
OpticaloutputOpticalinput

0.8 × 4.5 mm

21 mm
DC ports

Electrical input

17 mm

40-Gbit/s eye pattern

10 ps/div.
PRBS 231-1
1.3-Vpp push-pull drive

PRBS: pseudo-random bit stream

V-connectors

Fig. 3. Semiconductor MZ modulator module.

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R&D Spirits

deployment and make inroads into the market, wemust produce a device that is superior in all aspects.

—How do you think NTT is ranked in this field?

Receiving international praise amidst
difficult R&D trends

Over the last two to three years, our papers havebeen accepted annually at the Optical Fiber Communication Conference & Exposition (OFC), the leading
conference in this field. And at last year’s conference,
we gave an invited talk. So I believe we are gettingsome degree of recognition on the international level.
Actually, we get lots of questions every time we present our research results. In addition to making formal replies to inquiries during the presentation session, it is not uncommon for us to get approached
w i t h q u e s t i o n s b y i n d iv i d u a l s a f t e r w a r d s .
Researchers sometimes ask point-blank questions
like “How can that result be achieved?” This, ofcourse, tells us that there is interest in our work, andwhile we cannot answer every question due to prohibitions on releasing know-how, we greatly enjoy the
chance to hold energetic discussions with otherresearchers on our work.

—Dr. Yasaka, what international trends can be seenin this R&D field?

—Are you collaborating with other research institutions or companies?

In the world of photonic devices, whether it belasers or modulators, fierce competition in development is now taking place at research institutions anddevice makers throughout the world. There is also amove by some makers to buy out other makers thathave developed novel photonic devices. Such shrewd
purchases are made because photonic devices have a
direct effect on the performance of photonic transmission systems and buyouts can greatly speed up
device development.
As for the type of modulator that we are currently
working on, there is no doubt that size reduction has
become a worldwide development theme. The current objective is a module of the size of a businesscard, and it is commonly recognized that this cannot
be done without the use of semiconductor material
because there is a limit to how small the LN MZ modulator can be made. At standardization meetings, discussions have begun on the feasibility of achieving a
module that integrates a laser and modulator. At theacademic level, our group and other companies havebeen presenting research results, but a practical levelof operation has yet to be reached. From here on, weplan to accelerate our research and devote our efforts
to achieving, as soon as possible, an integrated module that can stand the test of practical application.

Not especially at present, but we have received
offers from foreign companies. For an offer from a
university in Japan that wishes to use our modulator,
discussions are under way. In device development, it
is essential that a new device be tried out at as manysites as possible. There is always the possibility that
another party can uncover problems that we had notconsidered or think up new applications. I therefore
think that it is important for us to expand our work to
include such cooperative relationships.

—Where do you think this technology will be threeand five years from now?
Our objective at present is to integrate a laser andmodulator on the same semiconductor substrate. Thisis our target for the next one to two years. Until then,I cannot talk about specific product details, but I
believe we will see much more functionality than ispossible with our current devices. At the same time,enabling device control to be performed directly by
optical signals instead of having to first convert optical signals to electrical signals as is done now will
mark a great leap in performance. We have therefore
made that a target as well.

Vol. 4 No. 11 Nov. 2006

From basic materials research to thedevelopment of semiconductor devices—Dr. Yasaka, please tell us about your technicalbackground.
At university, I was enrolled in the physics department and studied material properties. My special
field of interest was not semiconductors but rather
insulators, and I researched the structural phase transitions that take place in ferroelectric materials at different temperatures with an optical probe. Since mycurrent R&D theme concerns the control of lightthrough appropriate semiconductor properties, I
would say that the only direct connection between
this work and my university research is the use oflight. Nevertheless, I can view this as a beneficial pro9

R&D Spirits

gression from a basic research field dealing withmaterials to an application field both centered aboutlight.
—Why did you select NTT Laboratories as a site to
conduct R&D?

up all kinds of possibilities. It is also a promising
device for modularization together with the tunable
laser that our laboratory had developed. For these reasons, this was something that I felt should beachieved with great urgency, and we began development work.

I felt that NTT Laboratories had more of an academic atmosphere than a private research laboratory
and that it could be a place where I could settle down
and do some serious research. I was first assigned to
Atsugi Electrical Communications Laboratories, andI became a member of the semiconductor laser group.
I initially wanted to take on a more basic theme dealing with material properties, but I soon saw the positive aspects of progressing to application-oriented
research as I just mentioned. Looking back, I probably could have done basic research too by coming up
with some clever way of establishing my research
themes, but at that time, I was hardly that resourcefuland the thought of attempting such a thing nevercrossed my mind. At that time, moreover, semiconductor laser research was on the brink of producing
practical, working products, and that made my work
extremely interesting. Since then, my research
themes at NTT have dealt with light exclusively, andthat brings me to the present.

For me, this work of mine is practically a hobby. I
think about work at the laboratory, of course, butquite frequently at home too. When I was single, I
was completely immersed in my work, almost forgetting to eat and sleep. If it’s just work, it can be suffocating, but if it’s something that you truly enjoy, thenstress and fatigue are minimal, I believe. Asking
myself “Why?” and coming up with an explanation
one step at a time is a very enjoyable process for me.
For example, if a device that we are researching fails
to work according to principle, nothing can please me
more than using a trial-and-error approach to find an
answer. I also find it very interesting to examine aphenomenon that is not yet understood and try tocome up with a theory to explain it. This kind of challenge is very enjoyable and no doubt the reason I
entered the world of R&D.

—What specific research themes have you been
involved with up to now?

Recommending original and independent
R&D whatever the target

My work has involved two key themes: the researchof semiconductor laser control and the developmentof highly functional photonic semiconductor devices.
One example of a commercial product coming out of
this research is a distributed-feedback laser diode(DFB-LD) integrated with an electroabsorption (EA)
modulator. This product is still on the market.

—In what direction would you like to take yourresearch from here on?

—What gave you the idea of taking on your current
research theme?
It all began with attempts to overcome the drawbacks of existing modulators. In particular, the factthat existing modulators are limited in distance prevents them from being applied to optical communication systems that need to support ultralong distances.
Thus, on starting out to investigate what type of modulator could meet the needs of future systems, theidea that popped into my head was an MZ modulator
using semiconductor material. This modulator has, inprinciple, no wavelength dependency, which opens
10—What has been your prime goal in your R&D life up
to now?

Whenever I’m asked what our research on photonic semiconductor devices will be like in the future, I
have to honestly say that I don’t know. This is such a
fast-moving field that there’s always the possibility
that a completely new form of technology will appear
and shake the very foundation of existing techniques.
Because our group is an R&D team that aims to produce commercial products, we usually think in termsof a one-to-three-year time span. With that said, I
would say that improving the functionality and integration of our photonic semiconductor devices is one
direction that we plan to take. And though this is
product-oriented research, it goes without saying that
we have to keep in mind future ways of linking ourtechnology to other technologies and devices. In thisregard, we are thinking of establishing the principle
technology behind our semiconductor MZ modulator
as an elemental technology that can be applied to
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R&D Spirits

other devices. We would also like to link this technology with various future devices by applying it topassive devices and switches, for example.
—What is your ultimate dream as a researcher anddeveloper?
Well, to begin with, I would love to discover a theory that overturns what is considered to be common
knowledge as established by others in the past. Even
if one finds a way to quantify a phenomenon of somecomplexity, it cannot be called a breakthrough overturning the work of your predecessors if your findings
conform to an existing theory. I would say that you
are performing truly advanced R&D when you discover a phenomenon that contradicts existing principles and come up with an original theory to explain it.
In a similar vein, though closer to the commercialization aspect of our work, I would like to leave behindsome kind of achievement that no one else had a handin. In this sense, I am an amanojaku (a mythicalJapanese demon known for its mischievous and contrarian nature), but sometimes “thinking outside thebox” is necessary. Of course, going with the flow canalso be important, but I myself would rather pursue
R&D without following anyone else as much as possible.

—Dr. Yasaka, what would you say to young
researchers?
I would say find something that you can immerse
yourself in to the point of forgetting about life’s basic
comforts. It is difficult to exert 100% of your energy
when simply going with the flow, but if you findsomething for which you can say “this is it,” you will
put out 200 or 300%. In short, find a topic that you
can totally immerse yourself in and focus all of yourconcentration. It doesn’t matter if your initial target isa small one. If you can decide on one target, even if
small, the next one will come naturally. If you follow
this path, your research cannot help but become
increasingly interesting.
Reference[1]

E. Yamada, K. Tsuzuki, N. Kikuchi, and H. Yasaka, “Compact Lowpower-consumption Optical Modulator,” NTT Technical Review,Vol. 3, No. 3, pp. 41-45, 2005.

—What value does NTT Laboratories hold for you asa place to work?
It’s a wonderful think tank! For me, it’s like having
a living encyclopedia at my disposal. Whenever there
is something that I don’t understand, there is always
a “professional” in that area that I can go to withquestions. Being able to ask questions whenever one
has a problem enables a researcher to give 100% to
his or her field of specialization without having toexert more effort than needed. And while I often hearfrom others that they would like to travel abroad to dowork at other research institutions, I believe that there
is no other research site in the world that covers sucha wide range of research fields as NTT Laboratories.
It is because of this wealth of technology that NTT
can provide such a wide array of services as a carrier.
This is why there is a great need to inherit andimprove technology.

Vol. 4 No. 11 Nov. 2006

Interviewee profile■ Career highlights
Hiroshi Yasaka received the B.S. and M.S. degrees
in physics from Kyushu University, Kyushu, in 1983
and 1985, respectively, and Ph.D. degree in electronics engineering from Hokkaido University, Hokkaido,
in 1993. In 1985, he joined NTT Atsugi ElectricalCommunications Laboratories. Since then, he has
been engaged in R&D of semiconductor photonic
devices for optical fiber communication systems.
From 1996 to 1998 he worked in NTT Optical Network Systems Laboratories, where he engaged in
R&D of photonic transport systems. He is now a
Senior Research Engineer, Supervisor in NTT Photonics Laboratories and is engaged in R&D of semiconductor photonic devices and their monolithically integrated devices.

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