“Electronics, Information and Communication” field
Achievement : Pioneering research on semiconductor
lasers for high-capacity, long-distance
optical fiber communication

Dr. Yasuharu Suematsu

Born: September 22, 1932 (age 81)
Honorary Professor, Tokyo Institute of Technology

In the early 1960s, Dr. Yasuharu Suematsu, Honorary Professor ofTokyo Institute of Technology, undertook the development of a semiconductor laser which oscillates light used for optical transmission.
In 1981, he succeeded in developing a semiconductor laser thatuses a wavelength band that minimizes loss of the light signal inan optical fiber (essential to long-distance communication), while
maintaining a stable wavelength during a high-speed modulation(essential to large-capacity transmission). This technology greatly
contributed to realizing a high-capacity, long distance optical fibernetwork.

Summary
Optical communication network using optical fiber is the pillarof present information society. Dr. Yasuharu Suematsu, Honorary
Professor of Tokyo Institute of Technology, has been undertaking
the study of optical communication since the early 1960s, the dawnof the optical electronics age. Dr. Suematsu was also a forerunner intaking a “problem-solving approach” in research. In this approach,levels of performance required by society are projected first, andtheory and experiments are combined to achieve the goal. In the early
1980s, Dr. Suematsu gave shape to his idea of dynamic single-modelaser, which emits light in the wavelength range where the minimum
loss is achieved and has a stable wavelength even with high-speed
light modulation when transmitting information. His research on
semiconductor lasers has greatly contributed to the realization of ahigh-capacity, long-distance optical fiber communication.

Semiconductor laser innovation: The driving force behind
the achievement of the optical fiber network
The term “information society” was coined in the 1960s when
the world was entering a new phase after World War II. The term
refers to a society where information technology permeates and revolutionizes various facets of our lives such as economic activity, culture,education and daily lives. In order to realize this information society,scientists and engineers have taken on the challenge of innovation incommunication technology. For example, owing to the development inwireless communication technology using microwaves and millimeterwaves, Japan and the United States succeeded in launching satellite
transmission in 1963. In 1979, a car phone service, which was theforerunner of mobile phones, started in the Tokyo Metropolitan area.
Another technology which contributed to the sophistication of theinformation society is optical communication using optical fiber.
Optical fibers enabled transmission of a large amount of data bysending laser light modulated with high speed to carry information
over light waves through optical fibers.

Surpassing millimeter waves in one leap, taking on the challengeof the new frontier of optical communication technology
Dr. Suematsu was born in Gifu Prefecture, central Japan, in 1932.
From childhood, he was one of the so-called “Radio Boys,” creating
vacuum-tube radios with his own hands. He went on to go to Tokyo
Institute of Technology, heeding his uncle’s advice, “If you want tostudy technology, TIT is the place to go.”At the university, he was deeply impressed by his mentor Professor
Kiyoshi Morita’s experiments with microwave communication,
and was spurred on to carry out studies in communication himself.
However, as he proceeded with his postgraduate research, he came towonder if microwaves and millimeter waves can handle high-capacity
communication. He thought: “According to Shannon’s theorem, withelectromagnetic waves, the limit in information volume is half thenumber of frequencies. However, using a shorter wavelength, it may bepossible to transmit several thousand times the information millimeterwaves can carry.” When he became an assistant professor in 1961 after
completing his postgraduate studies, he surpassed millimeter waves andchose optical communication as his research theme.
However, it was not an easy decision for him to make as prevailingopinions in the university at that time were that one should tackle
the task at hand, rather than engaging in research on a technologythat seemed to be far away from practical use. Yet what served asa tailwind was that element technologies that led to the realizationof optical communication sprang up one after another. In 1964,
Professor Junichi Nishizawa of Tohoku University proposed a“self-focusing optical fiber” which made possible a wide-area signaltransfer. Then, in 1966, Charles Kao (1996 Japan Prize Laureate)
theoretically forecasted that low-loss optical fiber is feasible. Thus,
expectations mounted toward a digital communication network using
optical fiber.

Figure 1: Basic principle of a semiconductor laser
A semiconductor laser has
structure in which mattercalled the active layer issandwiched between two
types of semiconductors,
namely the n type and thep type. When voltage isapplied to these twosemiconductors,
oscillation of light withwavelength according to
the changes in energy
takes place.

electroden typeactive layerp typeelectrodeLaser beam

Figure 3: Period structure reflector used for the integrated lasern eqΛintermediate spacephase of intermediate space βl = mπ+π/2
waveguide2saintermediate
space
L1I
L2

laser
period structurereflector

2a

The period structurereflector used for theproposed single-modelaser by Dr. Suematsu.
By creating anintermediate region
between the two reflectors
place in the waveguide,
light with a more uniform
wavelength was achieved.

The main challenge was to develop an optimum laser for opticalFigure 2: Light wave and the period structure reflectorn typeA boundary surface becomes
the diffraction grating

active layerp typewavelength( twice the width of the diffraction grating )laser
The wavelength of the specificlight is amplified
n typewidth of the diffraction gratingn typeThe light with certain wavelength
gets amplified
and suppress the lightwith other wavelength

When a “zigzag cut” diffraction grating is inserted onto the surface ofthe semiconductor at half the interval of the wavelength, light with afixed wavelength is amplified, and the other wavelengths are cancelled.
Thus, the wavelength of the light oscillated becomes uniform.

fiber communication. In order for information to be passed without
attenuation through such scales as several tens of kilometers or several
hundreds of kilometers of fiber, a laser with adequate wavelength anddirection was necessary. At the time, various types of lasers had beendeveloped; however, with optical fiber digital communication whereinformation is transmitted in combination of 0s and 1s, information
could not be transmitted long distance with precision when various
types of lights are mixed. A high-precision laser with stable oscillationof a fixed wavelength was required.
Dr. Suematsu’s choice was semiconductor lasers, which wereinvented in 1962. A semiconductor laser has a structure in whichmatter called an “active layer” is sandwiched between two types ofsemiconductors, namely the n type and the p type. By applying voltage to the semiconductor, the electrons are transferred between both
semiconductors, light with wavelength corresponding to changes inenergy (photon) results. The resultant light continues to be amplified
within the active layer in order to induce the transfer of the next
electron, and when it exceeds a certain strength, it oscillates as a laser
light (Figure 1).
However, the issue with semiconductor lasers is that the wavelengthchanges with high-speed modulation, and specialists use such
expression as “oscillation mode hopping” or “multimode oscillation”
to describe this phenomenon. In order to transmit digital data, an
unchanging mode (single mode) even with high-speed modulation oflight to carry high-capacity information was required.

Dynamic single-mode laser achieved long-distance optical fiber communication becomes a reality
Dr. Suematsu’s research style of tackling a challenge was uniquefor a university professor. His research stance was to achieve theperformance demanded by the society and university researchers wereto thoroughly pursue a basic theory to arrive at an “optimumsolution.”
For example, there exists a technology in which a period structureof a light reflector is used in order to align the light wavelength.
When inserting a “zigzag cutting” into the surface or inside thesemiconductor at half the interval of the wavelength, the target light of
the wavelength strengthens one another and cancels the others (Figure2). Dr. Suematsu not only pursued the theory of this technology, butalso applied this technology by introducing a device which could
actually be used for optical communication, thereby stabilizing thelaser motion.
Afterwards, he undertook research of an integrated laser. In asemiconductor laser, there is an “active region” which strengthens thelaser light, a “waveguide” which leads the light in a fixed direction,
and a “reflective part” which reflects light. By using a period
structure reflector on the reflective part, and integrating it with theother parts, Dr. Suematsu anticipated that this would become thefoundation of next-generation technology such as the “opticalintegrated circuit.” Thereafter, in 1974, 13 years after beginning his
research at the university, he proposed a single-mode laser which
oscillates light with uniform wavelength by placing two periodic
structure reflectors within the integrated “waveguide,” and creating anintermediate region between the respective reflectors so that half of
the wavelength phase is deviated (Figure 3).
Furthermore, when it was discovered in the 1970s that light loss
within the optical fiber is minimal when the wavelength band is 1.5
micrometers, Dr. Suematsu began an independent study of semiconductors and succeeded in the room-temperature continuous-wave
operation of 1.5 micrometer wavelength using an InGaAsP laser. In
the fall of 1980, by means of an integrated laser using with a uniqueperiod structure reflector, he succeeded in creating a prototype for a
1.5 micrometer wavelength band laser. It was verified that lightoscillation was stable even under high-speed modulation in order totransmit data, and in the following year (1981), at the academic
conference in Europe, this invention was presented as a “dynamicsingle-mode laser.”
The dynamic single-mode laser proved to be an indispensable

technology in the field of optical fiber communication in the years that
followed. In the mid-1980s, technology using optical fiber for long
distance communication was established and began to be used asinter-city and international communication infrastructure. It can besaid that the Internet, which has become widespread to the generalpublic from 1995 onwards, would not have been possible without thistechnology.
Dr. Suematsu has also contributed toward the sophistication of theintegrated laser technology. In 1983, Dr. Suematsu et al. became thefirst in the world to achieve a variable wavelength semiconductor
laser which can electrically control the oscillation wavelength.
Coming into the 2000s, a new technology, namely, wavelength
division multiplexing was introduced into the optical fiber field toachieve an even faster communication network, and the technology
pioneered by Dr. Suematsu has greatly contributed to this development.
The integrated laser technology pioneered by Dr. Suematsu willcontinue to evolve our information society in the future.