R&D Spirits
Contributing to the Evolution of Wavelength Division
Multiplexing through Silica-based Optical WaveguideTechnologyHiroshi Takahashi
Group Leader, Lightwave Circuit Research Group
Photonics Integration Laboratory, NTT Photonics Laboratories

The silica-based optical waveguide is expected to be a key technology for rapidly expanding
optical communication networks. To respond to advanced network needs predicted for the near
future, NTT has been actively researching the practical application of devices using opticalwaveguide technology. We sat down with Hiroshi Takahashi, group leader of the Lightwave
Circuit Research Group, to ask him about the state of optical waveguide technology, current
issues, and international trends in this field.

Promoting research on the application ofoptical waveguides centered on AWG
technology

to communication systems based on wavelength division multiplexing (WDM). In addition to research on
the AWG itself, we are involved in the developmentof optical switches and other optical devices and in
the integration of these circuits and devices. And
recently, we have begun to research new optical circuits using NTT’s original wavefront matching—Dr. Takahashi, what is your current area ofresearch?
In the Lightwave Circuit
Research Group that I am currently leading, we are researching anddeveloping planar lightwave circuits (PLCs) that use silica-based
optical waveguides. This fieldencompasses a wide range ofthemes from fundamental toapplied technologies, but the areathat I am particularly involved in
is arrayed waveguide grating(AWG) technology. An AWG iscomposed of a lot of waveguides
created in a silica-glass cladding
layer formed on the surface of asilicon substrate. It can be used toconstruct a wavelength multiplexer/demultiplexer that uses an optical interference effect to break up
light like a prism (Fig. 1). This
device is becoming indispensable
6Cross sectional view
Waveguide core
Cladding layer

AWG
Output slab waveguideSi substrate

λ1,λ2,

WDM light

λ3,.

..,λN

Input fiber
Input waveguide

Output fiber

Substrate
λ1
λ2
λ3

Input slab waveguide
Output waveguide
λN

Fig. 1. AWG-type wavelength multiplexer/demultiplexer using PLC. Input lightis demultiplexed into different outputs thank to the prism-like dispersion
function of the AWG.

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method for optical waveguide design.
Now, it may sound like all of this is being handled
by just our group, but actually the research on silicabased optical waveguides at NTT is being pursued bythree groups including ourselves. These three groups
work closely with each other with some areas thateven overlap. This makes for a relatively flexible
R&D system, which is one of the features of NTT
Laboratories.
—What are some key technical points in opticalwaveguide research?
Silica-based optical waveguides exhibit excellent
optical characteristics and reliability, and they can beused in practical applications. But I don’t think they
are a spectacular breakthrough as such. What I reallyfind significant here is the selection by seniorresearchers of silica glass as the material for opticalwaveguides. Optical circuits are based on technologythat forms a thin glass film on top of a silicon substrate. Silica is extremely reliable, as it has to be,given the need for optical fiber in actual communication networks to last more than 20 years without
degradation. The use of the same material for both
optical fiber and optical waveguides also leads to

Fundamental equation

superb coupling efficiency. Needless to say, thesefeatures support the expansion of optical-waveguideresearch.
But looking to the future, if anything is to be considered a breakthrough, perhaps it’s the wavefront
matching method that I just mentioned [1]. Thismethod was proposed by a young researcher in ourlaboratory. It’s a computational scheme for creating
the optimum shape for waveguides to enable light topass in the most efficient manner possible. In the past,
the only way to create waveguide patterns was bytrial and error. The wavefront matching method, incontrast, enables an optimal core pattern to be automatically generated by first setting conditions andrequirements and then performing inverse calculations. Because of this capability, the wavefront
matching method has been attracting attention evenat academic societies as a promising waveguide technology for the future (Fig. 2).
—What form do you think this technology will takethree and five years down the road?
Our waveguide technology has already been
deployed in optical-fiber communication networks
where it is serving a very useful purpose for all of us.

η = ∫ ψ *M (x )φ M (x )dx 2= ∫ ψ *M ( x )(ABM A)(ABM−1 A) • • • • • • • • • (AB1 A)(AB0 A)φ0 (x )dx 2xψM

Design model

φ0η

0m

M

Simulation results

Photograph of sample

Fig. 2. Generation of waveguide pattern by wavefront matching method. A waveguide pattern is automatically designed
by inverse-calculation for maximum η. The pattern shown in the picture can separate 1.3- and 1.5-µm lights.

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R&D Spirits

For example, whenever you connect to the Internetand click on a link on the screen, the information thatyou request will pass through an AWG or opticalswitch that we created. But some technical issues still
remain such as how to improve the operating characteristics. There is also an urgent need to lower costs
because the range of applications of waveguide technology is expanding rapidly, from the trunk network
to regional and access networks. It is therefore highly likely that research on enhancing this technology
will make great strides in three or five years’ time.
And as I mentioned before, we are also moving forward on the R&D of new technology. I see no reason
why we should not be able to develop and deploy
novel optical circuits using the wavefront matching
method.
—What do you see as future issues and how do you
plan to deal with them?
The biggest issue, I believe, is integration. It isextremely important that various waveguide elements and devices be integrated so as to provide
value-added products. This is because it is cheaper toprepare waveguides and optical switches together
than to fabricate them separately. In addition, separately prepared devices require more space if they are
to be interconnected by optical fiber. Integrating
devices eliminates this requirement while making iteasier to achieve a more compact product.
The problem here, however, is that the conditions
for fabricating an optical waveguide differ slightly
from one device to another. In other words, if wewere to focus on the fabrication conditions that suitthe characteristics of a certain circuit, the characteristics of another circuit might be hampered. Preparing
multiple circuits on one silicon substrate therefore
requires the control of very difficult fabrication conditions. Finding a way to overcome this hurdle is amajor technical issue. To this end, we are working on
solutions from both the fabrication and design perspectives.

Experiencing all phases of implementationfrom theoretical research to technology sales—How did you become involved in your current lineof research?
Well, AWG was the first research theme that I was
assigned to on entering NTT, which was 18 years ago.
At that time, there were great expectations for WDM,
which was called a “dream transmission system.” Buton the other hand, user needs had not yet reached a8point where such a system was actually needed.
Against this background, I happened to receive anovel idea from one of my superiors, and I began toresearch it. To tell the truth, I did not know muchabout optics at the time, so I began to study opticalprinciples and learn measurement methods, fabrication methods, and other techniques one by one withlots of hands-on experience. This is how my research
progressed.
—What other research themes have you taken up?
For the first several years, I performed basic
research on waveguides, as I just mentioned. I did nothave access to sufficient facilities at the time, and I
would sometimes construct measurement equipment
on my own. “Ah, so this is research,” I thought, and I
became happily engaged in this work. In the end, I
became one of the three researchers in the world credited with creating the AWG, a development that
attracted much interest from around the world. The
other two researchers worked at Bell Laboratories
and Delft University of Technology in the Netherlands. Although we were all working on the same
type of AWG, each of our prototypes was slightly different from the others, and I think that we were not somuch rivals as researchers with a mutually respectfulrelationship. After completing my work with AWGs,
I was assigned for two years to NTT Transmission
Laboratories (now NTT Network Innovation Laboratories) for more training, and it was there that I began
my research on WDM in earnest. I performed varioussystem experiments using the AWG that I developed,
and the experience that I gained there proved to beimmensely useful in my later research.
Next, in 1995, with my research on AWG completed for the time being, it was time to tackle a newresearch theme. I became involved in research onlasers with external resonators, working alongside
young researchers new to the company. At firstglance, one would think that AWGs and external resonators have nothing in common, but that is not thecase. Their basic substrate structures are actually thesame. Consequently, thinking that I could study what
for me was a new technology while, at the same time,making use of my AWG know-how, I selected thistype of laser as my next research theme. However,just as I began to make progress in this research,
WDM began to be deployed on a large scale, and I
was rushed off to NTT Electronics (NEL), an NTT
group company, where plans were being made tomass produce the AWG-type wavelength multiplexer/demultiplexer. There, I took on a variety of tasks tohelp put those plans into action.
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I returned to NTT Laboratories in 2001 and again
took up research on PLC-related topics. Although thetheme was the same, various aspects of AWGs had, ofcourse, evolved. In particular, the Mach-Zehnderinterferometer type of optical switch was underdevelopment. An optical switch, though using basically the same fabrication techniques as an AWG, canswitch optical paths in a non-mechanical manner byutilizing the optical property that the transmission
speed changes with temperature. While a mechanicaloptical switch can deteriorate due to defective parts
or a buildup of dust, the PLC-based optical switch iscompletely immune to such problems—it is a highly
reliable device. In addition, a matrix switch, which
integrates many optical switches, holds muchpromise for the future as a device that can performlarge-scale switching (Fig. 3). We have so far reachedthe 16 × 16 level of integration [2], but we areresearching ways of integrating an even larger number of switches.
—Is there anything in your R&D career that has lefta lasting impression on you?
In the sense of an invaluable experience, yes. My
stay at NTT Electronics left a big impression on me.

At that time, I did just about everything including
product development, implementation of fabrication
techniques, design, and technology sales. For example, in technology sales, I made the rounds of systemvendors in North America in a rental car along with arepresentative from a partner trading company. I
came to experience what it must be like to be a sales
representative for a start-up company. Also, in connection with the actual delivery of products, I became
involved with quality-control matters such as theinspection of packaging materials. This was, ofcourse, somewhat different from my main line ofwork, but being able to experience with my own eyes
the process of taking a product from the R&D stage
to actual delivery was extremely worthwhile.
—Dr. Takahashi, what motivates you in R&D?
I would say “making something useful.” I’ve had
an interest in engineering-related things ever since I
was a child, and I was a tech-oriented teenager thatenjoyed building his own transistor radios. This interest expanded to electrical communication engineering, and at university, I researched measurement
methods using ultrasound. This research was quite
interesting, but in reality, I had a very strong desire toout 1

in 1
in 2

out 2
out 3

8 × 8 optical matirix switch

in 3
in 4
out 4
out 5
in 5
in 6out 6
out 7
in 7
in 8out 8

Mach-Zehnder interferometer

Electric supply
Directional
coupler

Input

Thin film heater
AWhen heater is onOutput

BWhen heater is offFig. 3. Structure of a Mach-Zehnder-interferometer optical switch (8 × 8). The device divides input light along two opticalwaveguides and switches the output port according to the phase difference between the two signals after merging.
A phase difference is generated by changing the temperature of a waveguide using a heater (on/off operation).
Integrating a large number of these switches (as shown by the matrix switch in the upper left) leads to large-scale
switching.

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9R&D Spirits

make things that could find widespread use in the outside world. Thus, when it came time to look for work,
I decided on NTT, which was setting out to construct
a new communication network using light, an electromagnetic wave expressed by the wave equation
just like ultrasound, my major at university. Throughall these years at NTT, I have held on to this desire to
make optics-related things that are useful to society.

equipment maker in the NTT Group, it goes without
saying that we have a close working relationship. We
also engage in joint research with universities fromtime to time, and as they are academic institutions,the content of that research often deals with thesearch for future research themes dealing, for example, with design methods and materials as opposed tofabrication techniques.

Participating in academic societies and otherinternational activities as an authority on
AWG—What kind of response have you received to yourresearch achievements to date?

—What is the worldwide trend in waveguideresearch?
If anything, waveguide research is a field dominated by know-how as opposed to theory. At the sametime, there has been little discussion at academic
societies on fabrication techniques. Most research
institutions, moreover, are conducting research onpractical application of optical waveguides while
simultaneously searching for future devices.
But with that said, I would like to point out here thatwaveguide research is not that busy at present compared with the past when many research institutions
and communication-equipment and optical-component makers were actively researching optical waveguides in addition to NTT. Why is this? One reason isthat the prices of optical devices dropped across theboard after the bursting of the telecom bubble. In
other words, to survive the resulting low-cost competition, it was better for a company to purchase finished products than to pursue R&D. At the same time,surviving makers have manufacturing departments inaddition to research departments and it is easy forthem to commercialize such devices. Furthermore,
the large-scale makers carry out production in China,
Taiwan, and elsewhere where labor costs are much
lower than in Japan, so that it is rather difficult to beatthem in a price competition. However, NTT’s waveguide technology, if I may say so, is the mostadvanced in the world at the fundamental level, and I
can feel a shift toward R&D that is oriented to winning through superb functions and characteristics
rather than competing in the war of attrition for costreduction.
—Are you collaborating with other companies orresearch institutions?
As far as makers are concerned, there are hardly
any cooperative projects since they are basically ourcompetitors. But with NTT Electronics, the major
10Well, first of all, as one of the first researchers tocreate an AWG, my name is known in the field ofwaveguide research. In fact, some people familiar
with my papers knew my name long before meetingme. As a result, I am sometimes invited to speak. For
example, last year, I attended meetings of the IEEE
Lasers and Electro-Optics Society (LEOS) and Optical Fiber Communications (OFC). And several years
ago, I was asked by a university professor to discuss
this field with his students. There are also overseas
students that have sent me their doctoral theses. Thiskind of response is a real pleasure for me, and holding discussions with a variety of people is quite stimulating. Talking with many researchers and students
in this manner is one thing that I truly look forward towhen traveling overseas.
—Could you tell us about your activities at academic
societies and elsewhere?
In Japan, I served as secretary of the TechnicalCommittee on Opto-Electronics at the Institute ofElectronics, Information and Communication Engineers up until May of this year. And overseas, I am
serving on the 2007 program committee at OFC, thelargest international conference in the field of opticalcommunications.
—Based on your international experiences, how doyou think NTT is viewed from overseas?
There is a true sense of NTT as a top runner in the
optical-device field. For example, there are many
NTT researchers serving as directors of academic
societies, and there are many member of societies thatsay things like “NTT always makes top-qualitythings.” But this praise includes expectations of further progress, and it won’t do to simply be satisfied
with words of praise. I want to meet those expectations by making full use of NTT’s strengths to develop novel products like a totally new type of circuit.

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Asking young researchers to think creatively
and challenge themselves daily—In what direction would you like your own research
to go in the years to come?
Competition in the AWG market is severe, and nomatter how you look at it, current research in the field
is leaning toward fabrication techniques. I cannot
deny the feeling that there is a slight disparity
between this and the true meaning of “research.” Of
course, R&D with the aim of beating the competition
is a prime mission of a research laboratory that is partof a private enterprise. But charging ahead on thatmission alone hinders creativity in research. It istherefore my desire from here on to create a balance
between research on fabrication techniques and basicresearch. I would like to concentrate my efforts on
new circuits and new design techniques. I also thinkthat the cultivation of application fields for our technology is very important. It’s too early to discussspecifics, but in broad terms, I am thinking about
using waveguides to achieve devices that are conventionally configured with elements other than waveguides.
—What is your ultimate dream as a researcher?
I really don’t think that far ahead. I would rather
devote all my energies to what I see in front of me in
the present!
—What is it like working at NTT Laboratories for you
personally?
NTT Laboratories is a place where I can put myskills to work. It’s a great environment from both ahuman and material perspective. It has a deep understanding of research that enables researchers toengage in world-class research. Applying my own
ideas to produce something that the world has neverseen before is what makes R&D interesting. And a
researcher is certainly blessed if that something
proves to be useful to society. NTT Laboratories canmake me feel that way. It’s a workplace that can makea researcher feel very worthwhile to society.
—Dr. Takahashi, could you leave us with a message
for young researchers?
Of course. First of all, please get into the habit ofthinking freely about all kinds of things. Practice“brainstorming” inside your head and check out for
yourself anything that attracts your interest. Then, if
Vol. 4 No. 7 July 2006

you believe it to be promising idea, try testing it withan experiment. In short, create small challenges foryourself on an everyday basis. I’ve recently noticed
that young people, for some reason, are not very
aggressive in their pursuits. When I ask university
professors that I meet at research meetings about this,they tell me that students are quite serious about thework assigned to them but rarely challenge themselves to achieve bigger and better things. If you only
take up themes that you are given, you will prevent
any possibilities that you encounter from taking rootand blossoming. That is a big danger. Perhaps thisproblem can be traced to the present trend in societyof relying on sure results to avoid failure. Nevertheless, I would ask you to challenge yourself to come up
with new ideas. You’ll never know what research
theme is the right one for you if you don’t follow up
on your ideas. If your ideas are regarded as simply adelusion, don’t let that bother you. By all means,
think for yourself and challenge yourself often, evenin small ways. Give yourself great expectations!
References
[1]

[2]

H. Takahashi, T. Saida, Y. Sakamaki, and T. Hashimoto, “Novel
Optical Waveguide Design Based on Wavefront Matching Method,”
NTT Technical Review, Vol. 4, No. 2, pp. 54-59, 2006.
H. Takahashi, T. Watanabe, T. Goh, S. Sohma, and T. Takahashi,“PLC Optical Switch that Enhances the Optical Communication Network,” NTT Technical Review, Vol. 3, No. 7, pp. 17-21, 2005.

Interviewee profile■ Career highlights
Hiroshi Takahashi received the B.E. and M.E.
degrees in electrical engineering and the Ph.D. degree
for research on arrayed waveguide grating wavelength
multiplexers from Tohoku University, Miyagi, in1986, 1988, and 1997, respectively. Since joining
NTT Laboratories in 1988, he has been engaged inresearch on the design and fabrication of silica-based
optical waveguide devices including wavelength multiplexers, optical switches, and WDM transmission
systems. He is interested in the creation of new functional optical devices based on PLC technology. He
received the NTT President’s Award from NTT in
2000, National Invention Award from the Ministry ofEconomy, Trade and Industry in 2000, and the Electronics Letter Premium Award from the Institution ofElectrical Engineers in 1997. He wrote parts of theEncyclopedic Handbook of Integrated Optics (Taylor& Francis). He is a member of IEEE, the Institute ofElectronics, Information and Communication Engineers of Japan, and the Japan Society of Applied
Physics.

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