R&D Spirits
Pursuing the Limits of Measurement Technology
for the 40-Gbit/s Optical Network Era
Fumihiko Ito
Senior Researcher, Group Leader, Media Facilities Operations
Access Media ProjectNTT Access Network Service Systems Laboratories

NTT Access Network Service Systems Laboratories is researching and developing opticalline measurement technology to achieve smooth and efficient operations and quality control inultrahigh-speed optical fiber networks. We asked Senior Researcher and Group Leader Fumihiko Ito to tell us how optical communications in the 40-Gbit/s era can be supported and what
technologies will be necessary to provide that support.

Development of signal measurement technology in response to faster optical networks—Could you describe some technical features of yourR&D?

—Dr. Ito, please tell us about your current research
theme.

I think the main technical feature is the use of linear sampling for our oscilloscope (Fig. 1). Thismethod utilizes the optical interference phenomenon
that occurs when a laser-generated sampling pulse isapplied to an optical signal and measures signal
amplitude using a semiconductor optical sensor in thedetector. Another measurement system based on optical interference uses a nonlinear optical crystal in thedetector, but that system, which has been known forsome time, switches the light, causing interferencewith other light and resulting in insufficient sensitivity. Our method, in contrast, measures optical signals
directly, resulting in an exceptionally sensitive system. We also use two channels to offset the time ofsampling pulses so that the state of phase modulation
can also be observed. These features are not possible
in existing systems.
Another feature is the use of an optical 90° hybrid
element in our prototype equipment (Fig. 2). Settingthe relative optical path difference between channel 1and 2 to be Δτ makes it possible to simultaneously
measure the amplitude and phase of an interference
pattern as opposed to simply generating interference
in a plane. Optical 90° hybrid technology itself is wellknown and is not our original design, but we are theTransmission rates as high as 10 Gbit/s are now
being achieved, and we expect next-generation networks to operate at the 40-Gbit/s level. At that level,
it will no longer be possible to detect the optical signals traveling through fiber cables by electricalmeans. Against this background, our research groupis undertaking the development of high-performanceand high-sensitivity tools for next-generation networks. Our present work is mainly devoted to thedevelopment of an oscilloscope that can handle 40Gbit/s optical signals. We are also researching an
optical time domain reflectometer (OTDR) for determining the location of an optical fiber’s fracture
plane, but we are not yet at a stage where we can formally present this product. I’d like to point out herethat the research of our group is future oriented compared with other areas of the Access Network Service
Systems Laboratories. We are free to pursue newideas outside the present framework. The members of
this group, by the way, are all young researchers, andmy role as group leader is to put them on a path to full
personal development with an eye to the future.
6NTT Technical Review

R&D Spirits

Interference
pattern

Optical sampling
τ1
pulse

Channel 1

τ2
Optical sampling
pulseΔττ1

Channel 2

Opticalsignal

OpticalsignalAmplitude of interference pattern ⇔amplitude of signal light at time τ1 ortime τ2

Phase difference between interference
patterns of channels 1 and 2 ⇔ phaseof signal light altered at time Δτ

Fig. 1. Principle of linear optical sampling.

Signalsource

PBS
λ/4

I1

HM

Q1PBS

λ/2

A/D converter

Samplingsource

PBS
HM: (half mirror) 3-dB splitter
PBS: polarization beam splitter
λ/2: half-wave plate
λ/4: quarter-wave plate
A/D: analog-to-digital
τ:delayI:
in-phase
Q: quadrature

τ

λ/4

I2

HM

Q2
Optical 90°
hybridλ/2

PBSTrigger

Fig. 2. Equipment configuration.

first to apply it to linear optical sampling.
—How do you think this research will contribute toNTT and society on the whole?
The most straightforward contribution will be theability to quickly find the location of a fault that has
occurred in the optical fiber network. This will obviously make network maintenance and management
more efficient and reduce the cost of maintenance,
but it will also help stabilize service quality. Opticalline measurement technology will also be indispensVol. 5 No. 2 Feb. 2007

able for checking the performance of existing facilities and for assessing quality. Incidentally, phasemodulation systems are slated to be the main kind ofmodulation for high-speed transmission at rates of 40
Gbit/s and higher, and for this reason, we believe that
this technology in the form of a next-generation oscilloscope will be very influential.
—What is the current state of progress and what
problems must be solved?
We have checked the validity of the basic principle
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R&D Spirits

(a)

(b)

Intensity

50
Relative opticalfrequency0(GHz)

Frequency
decrease

Frequency
increase–50050Time (ps)100050100Time (ps)Fig. 3. Experimental results for linear optical sampling.

behind this technology and its general operation. We
achieved a first by clearly determining how amplitude
information and frequency change during the application of a pulse (Fig. 3). That was a big turning pointfor us.
At the same time, our R&D goal here is a practical,
working system, but there are still a number of problems that must be solved before we get to that point.
For example, there is the problem of transforming ourexperimental system into real equipment. Performing
measurements with various components connectedby fiber cords does not constitute the use of “equipment.” Until these components are neatly installed ina cabinet and operating stably, we can’t really call thesystem an “oscilloscope.” I’m afraid that we are still
two to three years away from that stage.
—How do you see this technology developing in theyears to come?
Another factor in achieving a practical, working
system is downsizing. Needless to say, the system
must be as compact as possible if it is to be used as aneffective tool. And still another factor is the development of applications. Various applications can beconsidered, but right now we are looking at the possibility of analyzing wavelength dispersion and polarization dispersion in detail by exploiting our measurement technology’s key feature of allowing phaseand amplitude to be observed simultaneously. Wavelength dispersion and polarization dispersion of an
optical signal can only be understood by observing
signal phase. And analyzing optical dispersion
enables the user to not only assess the quality of sig8

nals traveling through an optical fiber cable, but also
determine why a signal is bad and where the problem
is located. This makes it easier to control quality andto pinpoint and correct a problem. I think that accumulating application examples like this one should
lead to further development of our technology.

Taking on diverse themes through afoundation in optical communications
technology—Dr. Ito, what was your major as a university student
and what motivated you to enter NTT Laboratories?
I was researching optical conducting devices using
gallium arsenide (GaAs), and thinking that I wanted
to research topics related to optics after entering thework force, I visited several corporate research laboratories including those of manufacturers and airlines. But the company that I thought had the mostactivity in this area was NTT. I also heard from a fellow student that NTT was a place where one could
pursue long-term research without being pressured
by corporate product strategies and other factors. In
the end, I decided on NTT with no misgivings.
—What research themes have you been involved withat NTT up to now?
Immediately after entering NTT Laboratories, I
was assigned to a group conducting research on transmission lines. As it turned out, this was a period inwhich the research on optical fiber cables was entering a mature stage, compelling NTT to look for theNTT Technical Review

R&D Spirits

next research target. Consequently, as I was known tohave been researching optical conducting devices, Iwas given the theme of researching optical switches
that were to be incorporated into the network. Unfortunately, this research came to an end after threeyears.
I next took on technology related to optical computing, a big topic of discussion at that time, andresearched, in particular, optical memory through theuse of photoreactive crystals. A photoreactive crystal
can act as a medium for storing the spatial information of light as a hologram by having two waves oflight interfere with each other. My initial objective
here was to apply this optical memory to image operations in optical computing. But eventually I came tofeel that this was not something that could be easily
achieved. Academically speaking, it was a topic that
I found very interesting, and papers that I wrote on
this subject were well received at academic societies.
Nevertheless, I found that it was too difficult toachieve output in a useful form, and I decided to makea small change in my direction. Specifically, the idea
came to me that I could use the same hologram torecord the temporal waveform of light instead of a
spatial image. If I could capture a signal having various frequencies that change over time and record in aphotoreactive crystal the amplitude and phase corresponding to each frequency, I should be able to record
not only two-dimensional image information but
temporal changes as well. I was involved with thisresearch from about 1990 to 1995, during which time
I wrote many papers and even obtained a degree
through them. It was also at this time that I first gotthe idea of applying the results of this research tosampling, which is what I am researching now.
After that, I left research for a four-year period
starting in 1996 to work at the NTT main office on
facility investment and a next-generation network
vision. I returned to NTT Laboratories in 2000 andbecame involved in operations research with the aimof developing home network operations. Then, in2004, NTT Laboratories announced a policy offocused research on optical measurement systems,
and as I had experience in optical-interference
research, I was appointed group leader in this area.
And that brings me to the present.

own ideas. In today’s research, there are very few
examples of completely original work. Even though
research work may take on various forms, there isusually one researcher who is out ahead of the others.
The only way to outstrip those that have come before
you is to establish originality. For this reason, I havealways tried to incorporate my own ideas into myresearch endeavors.

—What has been your common goal in all of theresearch projects you’ve been involved with?

I first presented a paper on measuring waveforms
through the use of optical interference in the 1990s.
After the presentation, a number of researchers cameup to me saying things like “That was a wonderfulpresentation!” and generally praising my work. That

Whatever the theme, I always challenge myself tocome up with something original on the basis of myVol. 5 No. 2 Feb. 2007

Receiving high praise for original work in theworld of optical communications technology
research—Dr. Ito, what are the R&D trends in interferencemeasurement technology in Japan and overseas?
Research and development in the field of opticalinterference measurement actually takes on variousforms, but within this range, I think we can say thatthe optical sampling method is NTT-created technology. There are consequently no competitors at all inthis regard, at least not at present. But in terms ofapplications, there are, I admit, competing researchgroups, which means that we cannot become overconfident in our achievements.
But if we look around with a broader perspective,
one field that deserves attention is research into thecharacterization of laser pulses, which are the basis
for the linear sampling method. Though this research
is not in direct competition with our current research,both types of research deal with the universal phenomenon of optical interference. We are keeping aconstant watch on this field. There is also much activity in detection technology, especially in the researchon coherent detection. From the viewpoint of signal
processing such as when making optical-dispersion
guarantees, coherent detection enables phase to beobserved just like the linear optical sampling method.
This capability is prompting a reevaluation of thistechnology. There are research groups on coherent
detection in Japanese universities, leading electricalequipment makers, and elsewhere, and their knowledge has much in common with ours.
—What kind of reaction have you received at academic presentations and elsewhere?

9R&D Spirits

was a happy occasion for me. Now, after about ten
years of many twists and turns in my research, I havefinally come to the point where I can make a contribution to society using the principle I introduced backthen, and I feel even more emotion thinking back tothat first presentation.

OTDR as well. Of course, it is also important to produce results that are useful for NTT’s business and forsociety, but if a system’s potential can be understood,
I believe that demands at the practical level can beefficiently met.
—What is your goal as a researcher?

—How do you evaluate NTT based on your personal
experiences?
Interference measurements and OTDR technologythat I am now researching are part of the field of optical communications. NTT has a long history ofoptics-related research that has garnered much praise.
For example, in terms of very fundamental technology, it was NTT Laboratories that proposed the manufacturing method for optical fiber. And in Japan, NTT
Laboratories is unrivaled in transmission equipmentthat connects both ends of optical fiber. I don’t think
it would be an exaggeration to say that NTT built up
Japan’s optical-communications industry. In thefuture, I believe that our research will be part of thecontinuing growth in this industry.
—Are you involved in any collaborative research withother research institutions? How about international
activities?
At present, we are not involved in any collaborative
research either in Japan or overseas. But as forinternational activities, I am seeing to it that ourgroup is actively involved in academic societies. I,
personally, have not presented any papers over thelast one or two years because I want as many young
members in the group as possible to do this for me to
get training as next-generation researchers. We are
not involved in standardization activities at present.

Pursuing the limits of possibility as selfactualization while making a contribution tobusiness and society—Looking to the future, where do you think yourR&D work is headed?
In terms of research on the whole, I would like topursue the limits of possibility. In my current researchusing the linear optical sampling method, my ultimate objective is not to create a measuring instrument
but to clarify the full potential of this system by determining how far measurements can be performed
based on scientific principles. This holds true for
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First and foremost, I want to establish the technology related to my current area of research. Whether itbe the linear sampling method or OTDR, I want togive concrete form to technology that will impact
society to a degree that people will say that “the world
with this technology is vastly different than the world
without it.”
Next, as another major objective, I want to activelydevelop next-generation researchers. Though this isjust my personal opinion, there seem to have beenfewer aggressive young researchers in recent years.
One reason for this, I believe, is that since its privatization, NTT has been requiring even R&D to output
immediate results in the face of a competitive market.
At any rate, I sense a scaling back of true research.
However, if NTT intends to continue its role as aleader of Japan’s information-communications industry, young people that have a good understanding ofcommunication and measurement systems and thatcan perform creative research will be indispensable.
—Then what would you say to young researchers?
When I was a young researcher, a more experienced
member of the group that I belonged to would often
recite interesting sayings. One was “when starting out
on some research, everyone is a latecomer, and theonly way to keep up or overtake the groups ofresearchers ahead of you is to have an original idea.”
He also said that “you should feel mortified if yoursuperior, who is probably more inflexible than you,comes up with a good idea first” and “always endeavor to be original.” These sayings reflect the free andbroadminded culture of NTT Laboratories that motivates me to continue my research. I would ask allyoung researchers to keep these words in mind. Neverbe satisfied with the status quo and always keep moving forward!
—Dr. Ito, what is it like for you personally to work at
NTT Laboratories?
It’s an amazing place where you can do what has
never been done before while also providing someNTT Technical Review

R&D Spirits

thing very useful for society. Although a researcher
should not be allowed to pursue research simply for
self-satisfaction, NTT enables you to undertake relatively long-term research without being at the mercyof the company’s business strategies. This may also
be true at other research institutions, but I believe that
this quality is particularly strong at NTT Laboratories. It’s a unique environment where scientists andengineers can experience self-actualization.

Vol. 5 No. 2 Feb. 2007

Interviewee profile■ Career highlights
Fumihiko Ito received the B.E. and Ph.D. degrees inelectrical engineering from the University of Tokyo,
Tokyo, in 1985 and 1999, respectively. Since joining
NTT Ibaraki Electrical Communications Laboratories
in 1985, he has been engaged in research on coherent
optical signal processing for ultrafast communication.
Since 2000, he has been in NTT Access Network Systems Laboratories and has worked in the field of ontology-based network operation. His current work isdeveloping future optical measurement technology forfiber network management and evaluation.

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