Yasuharu Suematsu

Yasuharu Suematsu
Portrait of Yasuharu SUEMATSU-2006
Born	September 22, 1932 (1932-09-22) (age 92) Gifu, Japan
Nationality	Japanese
Alma mater	Tokyo Institute of Technology
Known for	Contributing to the development of optical fiber communication
Awards	2015 The Order of Culture, from the Emperor of Japan. 2014 Japan Prize 2003 IEEE James H. Mulligan Jr. Education Medal 1996 Medal of Honour with Purple Ribbon 1994 C&C Prize 1994 John Tyndall Award 1986 IEEE David Sarnoff Award
Scientific career
Fields	Optical communications
Notable students	Yoshihisa Yamamoto

Yasuharu Suematsu (末松 安晴, Suematsu Yasuharu, born September 22, 1932) is a Japanese electrical engineer renowned for his foundational contributions to fiber-optic communication technology, especially his pioneering work on dynamic single-mode lasers. He is currently a professor emeritus at the Institute of Science Tokyo.

Life

Yasuharu Suematsu was born on September 22, 1932, in Gifu, Japan. He received his B.S. (1955) and Ph.D. (1960) from the Tokyo Institute of Technology. Afterwards, he joined the faculty of the Tokyo Institute of Technology as a professor. He became its president in 1989 and then left it in 1993 and became the inaugural president of the Kochi University of Technology and Director General of the National Institute of Informatics. During the same year, he was elected a member of the National Academy of Engineering (US) with the award citation "For contributions to the understanding and development of optical fibers, high-performance semiconductor lasers, and integrated optoelectronics".

He was awarded the IEEE Edison Medal (2006) and Japan Prize (2008) for his breakthroughs in photonics.

He is a Foreign Associate of the National Academy of Engineering, a Foreign Member of the National Academy of Engineering of Korea, a fellow of the Optica Fellow and a fellow of the Institute of Electrical and Electronics Engineers.

He has authored at least 19 books and more than 260 scientific papers.

Research

Professor Suematsu is best known for his contributions to the development of optical fiber communication. He developed semiconductor lasers which even under high-speed modulation produce light at a stable wavelength that coincides with the wavelength region where the optical losses of fibers reach their minimum.

Fig.1. Replica of the earliest demonstration of optical fiber communication experiment, on May 26, 1963, restored in 2008-7. (Registered as Future Technology Heritage, at the National Museum of Science, Japan). By courtesy of the Museum of Tokyo Institute of Technology.

The earliest demonstration of an optical fiber communication experiment

The earliest demonstration of optical fiber communication was performed by Suematsu and his students, on May 26, 1963, on the occasion of the open house of the Tokyo Institute of Technology (Fig.1).
The light source was a helium-neon gas laser, the modulator was hand made modulator by use of ADP crystal, applied signal voice voltage of 1.200 volts, for polarization rotation in response to the voice signal, the optical bundle glass fiber for the transmission medium, and the photomultiplier tube for the detector. The original ADP reserved in the desiccator as well as the replica of that experiment, restored in 2008-7 as shown in Fig.1, was registered as a Future Technology Heritage at the National Museum of Science, Japan, in 2019.

Fig.2. Principle of Single-mode resonator consisted of two distrusted reflectors connected with phase shift of integer multiple of Π/2, for Dynamic Single Mode (DSM) Lasers, in 1974.

Fig.3. Laser tip on the mount of the first demonstration of dynamic single mode laser at a wavelength of 1.5 micrometers, in October 1980. By courtesy of the Museum of Tokyo Institute of Technology.

Fig.4. Single-mode property and schematic structure of the first demonstration of dynamic single mode laser at wavelength of 1.5 micrometers, in October 1980.

Creation of Dynamic Single Mode lasers

Light (specifically visible/near-infrared wavelengths) is the highest-frequency electromagnetic radiation that humans can precisely control. It far outperforms radio waves in information-carrying capacity, enabling the transmission of vastly larger data blocks. Optical communication research advanced significantly in the United States, Japan and England, driven by the understanding that fiber-optic systems could transmit high-capacity information over global distances with minimal loss. To demonstrate this physically, researchers developed the Dynamic Single-Mode laser (DSM laser) (Fig. 2), which exhibits three key characteristics:

1. It operates in the 1.5-micrometer wavelength band, which minimizes optical fiber attenuation, enabling efficient long-distance transmission. This ideal wavelength was identified through subsequent research.
2. It operates stably at a single wavelength to surmount the problem of transmission capacity reduction due to dispersion on the propagation constant in single-mode optical fiber.
3. It allows the wavelength to be tuned to adapt to communication in multiple wavelengths.

In 1972–1974, Suematsu and his students proposed a single-mode resonator design comprising a refractive index waveguide for transverse mode control and two distributed reflectors joined with a phase shift of an odd multiple of π/2 (half pi) to enable axial single-mode operation (Fig. 2). Concurrently, Suematsu pioneered the development of mixed-crystal GaInAsP/InP semiconductor lasers operating in the 1.5-micrometer wavelength band of a region, proven by Donald Keck and collaborators in 1973, to minimize optical fiber loss. By July 1979, his team achieved continuous room-temperature operation of these lasers, enabling the first generation of practical fiber-optic systems.

Building on these advances, Suematsu and his team successfully developed an integrated laser with built-in distributed reflectors, using 1.5-micrometer-band materials. By October 1980, they achieved a breakthrough: a dynamic single-mode laser (DSM laser) that maintained stable single-mode operation even under rapid direct modulation (Figs. 3–4) and continuous room-temperature performance. Notably, this laser retained stable operation across temperature variations, enabling thermal wavelength tuning within the 1.5-micrometer band. This invention—the world’s first thermo-tunable dynamic single-mode laser—directly enabled high-speed 1.5-micrometer fiber-optic systems. Its impact was recognized by awards such as the 1983 Valdemar Poulsen Gold Medal and the 1986 IEEE David Sarnoff Award.

Further studies rigorously characterized its spectral behavior to ensure full single-mode operation. Meanwhile, research and development accelerated across industries, driving advances in optical fibers, photonic circuits, devices, modulation techniques, and system desisgns. The DSM laser’s commercialization in the late 1980s became the cornerstone of high-capacity, long-distance optical communications.

Fig.5 Schematic structure of Phase-Shift Distribute Feedback laser, in October 1983 ~Thermo-tunable Dynamic Singlr Mode Laser ~.

Fig.6. A commercial phase shift distributed reflector laser array, with 100\ coin for size reference. By courtesy of Furukawa Electric Co.

Fig.7. Schematic structure of Wavelength Tunable Laser ~ Electro, in 1980 ~Electro-tunable Dynamic Single Mode Laser ~.

Phase-shift distributed feedback laser

Among these, the phase-shift distributed feedback (DFB) laser that Suematsu and his students proposed in 1974 and demonstrated with Kazuhito Furuya in November 1983 (Fig.5) is a thermo-tunable dynamic single-mode laser which had a high rate of production yield, as cited by the 1985 Electronics Letter Premium Award, IEE, UK. Since the beginning of the 1990s, it had been consistently and widely used commercially as a standard laser for long-distance use, as awarded the 1994 C&C Prize. Often, a laser array is used to cover wide wavelength regions (Fig.6).

Wavelength tunable laser

On the other hand, the electro-tunable dynamic single-mode laser, which would be a goal of the Dynamic Single Mode Laser, is, a so-called, wavelength tunable laser that was proposed by Suematsu and his students in 1980 (Fig.7) and demonstrated in 1983. Later, the tuning wavelength range was increased by the introduction of distributed reflectors with multi-grating pitches by Yuichi Tohmori and Yuhzou Yoshikuni, and Larry Coldren. The electro-tunable dynamic single-mode laser is especially important because it could be finely tunable and also monolithically integrable together with other photonic devices which need the specific thermal tuning separately in the form of PICs (Photonic Integrated Circuits). It was around 2004, through the efforts of those involved, that this wavelength tuneable laser was developed and used commercially in dense wavelength division multiplexing (D-WDM) systems and optical coherent systems. It became utilized around 2010.

Fig.8. International submarine cables around the world. By courtesy of KDDI.

Fig.9. Transmission performance of communication fiber. Prime data by Courtesy of NTT & KDDI.

Social contribution by research

High-capacity and long-distance optical fiber communications in the lowest loss wavelength band of 1.5 micrometres use dynamic single mode lasers (DSM lasers), such as phase shift distributed feedback lasers and wavelength tunable lasers, as their light sources, and have progressed along with research and development of optical fiber, optical devices, modulation schemes, and the like. Phase shift distributed feedback lasers developed by this research have been commercially applied for long distances—for overland trunk systems (1987) and for intercontinental submarine cables (1992) (Fig. 8) —and continue to support the progress of the Internet to this day.Later, since around 2004, wavelength tunable lasers are being used as the light source to advance dense wavelength division multiplexing (D-WDM) systems and optical coherent fiber systems for multi-level modulation schemes.

Optical fiber communications make up a highly dense communications network circling the globe tens of thousands of times and are also used in applications such as middle-distance Ethernets. Additionally, DSM lasers in the band of 1.5 micrometers are used for optical lines from the exchange centre to the home in FTTH. The transmission performance of fiber represented a byproduct of the transmission capacity, and the distance has been increased yearly exponentially, as shown in Fig. 9.

In such ways, the information transmission capability of optical fiber has reached several hundred thousand times as much as the coaxial cables preceding them and has significantly lowered the cost of transmitting the information. Reflecting this, the mid-1990s saw the network industry such as Yahoo, Google, and Rakuten appear one after the other. Optical fiber communications have progressed and the Internet has developed, and instantaneous transmission of a large volume of knowledge is now a daily occurrence.

In the electrical communication era of the 1960s, large volumes of data, such as documents on which civilization depend, were circulated slowly in forms such as books. In contrast, the proliferation of high-capacity and long-distance optical fiber communications has allowed for large-volume information such as books to become used interactively in an instant. The research of optical fiber communications contributed to the rapid transition to a civilization based on the information and communications technology.

See also

• Distributed-feedback laser  – Critical structure Suematsu refined for stable single-mode operation.
• Charles K. Kao – "Father of fiber optics". Suematsu’s work realized practical lasers for Kao’s vision.
• History of telecommunication – Suematsu’s role in the fiber-optic revolution.