Fields of Electronics, Information, and Communication
Achievement
Distinguished contributions to global long-distance, high-capacity optical fiber network through the developmentof semiconductor laser pumped optical amplifier

Prof. Masataka Nakazawa (Japan)

Born: September 17, 1952 (Age: 70)
Distinguished Professor, Specially Appointed Professor,
Tohoku University

Mr. Kazuo Hagimoto (Japan)

Born: January 8, 1955 (Age: 68)
Principal Researcher,
National Institute of Information and Communications Technology

The optical transmission system connecting the world

Principles of optical amplifiers and their advantages

Submarine cables are used to bridge the distances between continents and connect the world, and the term optical communicationrefers to the transmission of information as signals that travel along
optical fibers within those cables. Information is sent using flashinglight that represents the binary 0s and 1s in digital data, whichallows information to be sent faster and over longer distances.
Optical fibers are made of high-purity glass and other materials
in order to minimize signal loss, but when transmitting over long
distances, there will always be some loss of signal strength. For
example, it is impossible to deliver a signal that crosses the Pacific
Ocean from Tokyo to San Francisco (roughly 8,300 km) with asingle transmission of an optical signal. To resolve this issue,
optical repeaters are installed on submarine cables anywherefrom a few dozen kilometers to one hundred kilometers apart,and the optical amplifiers inside boost signals that pass throughto compensate for signal loss. The erbium-doped fiber amplifiers(EDFA) developed by Nakazawa and Hagimoto to amplify signals
in this way are now being used worldwide.

Single-mode optical fibers were put into practical use in the1980s. However, at that time, signal amplification was achieved
using electrical amplifiers, which converted optical signals into
electrical signals which were then amplified. Such devices werelarge and required a large amount of power, so there was demand
for a compact, highly efficient, broadband optical amplifiers that
could amplify the original signals without having to convert themfirst.
Nakazawa was the first person to propose using a 1.48 µm
InGaAsP (Indium, Gallium, Arsenic, Phosphorus) laser diodeto excite erbium-doped fibers. The principle under which suchan optical amplifier works is shown in Fig. 2. A diode emits anexcitation signal at 1.48 µm, which gives energy to an electron in
the ground state in an erbium atom (4I15/2) and raises it to an excitedstate (4I13/2). Energy is released from that electron in a phenomenon
called stimulated emission, and it is used to amplify the 1.55 µm
optical signal.
Currently, optical communication is carried out in the 1.5 µm
band (the minimum loss wavelength), which has the lowest possible
optical attenuation at 0.2 dB per kilometer. Energy produced bystimulated emission in erbium atoms occurs in this band precisely,
and it provides a gain of 12.5 dB (corresponding to the ratio of input
to output strength). Furthermore, it was shown that amplification
wasn’t limited to a single wavelength; this stimulated emission
could be used to amplify signals within a range of 40 nm.

Figure 1: The intercontinental optical submarine cable network
北極海

北極海

Figure 2: Principles of optical amplifiers
北太平洋

Thermal transition
Excited state

南大西洋

13/2Stimulated emission

南太平洋
南極海

4

南極海

Outer sheath

Insulator

Transmitted
optical signal
1.55µmFeed line

submarine optical repeater

Optical fiber

Map source: Submarine Cable Map（https://www.submarinecablemap.com/）

Excitation light
1.48µm4

Electron

Ground state

15/2

Driving greater increases in capacity

Nakazawa’s proposal showed that it was possible to build abattery-powered optical amplifier much more compact than existing
amplifiers, which required light sources as big as 1.5 square meters.
Moreover, they were broadband amplifiers capable of amplifying1.5 µm signals over a 40 nm band, which illustrated their potential
to help increase optical communication capacity.
Hagimoto learned of Nakazawa’s proposal and immediatelybegan building a practical optical communication system based on
its principles. He used an EDFA measuring roughly 10 cm square,a device that had only recently been developed, and harnessed
intensity modulation and direct detection to show that a 1.8 Gbit/s
signal could be delivered over as long a distance as 212 km. This
was the first successful demonstration of the practicality of opticalamplifiers.
This technology was so exceptional that within only five years,
it was being used in the long-distance transmission networks thatconnect the world.

Figure 3: Semiconductor laser pumped EDFAs andthe 212km repeaterless transmission experiment at 1.8 Gbps
Low-loss, low-dispersion optical fiber (A = 1.55 µm)

212km

Transmitter

90m

60m

Er -doped
fiber
3+

Opticalamplifier

1.48-µm excitation laser
1.48-μm励起レーザー

Er3+-doped
fiberBand-passfilterError
detector

Figure 4：Practical application of long-distance large-capacity
optical fiber communication（bit/s）
10 P

Space-division multiplexing (SDM)

1P

Conventional type

Source : https://www.youtube.com/watch?v=v_Xkn14XWcQTransmission capacity per fiber

Pulse wave
generator

Transmitter

DM

After optical amplification was introduced in the 1990s, it became
absolutely indispensable to optical communications. Other opticalamplifiers have been developed, but EDFAs remain the most commonly used around the world. The work of Hagimoto and his fellow
researchers in standardizing the technology internationally also helped
demonstrate its superiority.
EDFAs are capable of amplifying multiple optical signals at
different wavelengths simultaneously, so they can be combined withwavelength-division multiplexing, a technology that allows multiple
signals at different wavelengths to be transmitted together and separated again at the detection end. Since the mid-1990s, that combination
has driven greater increases in optical communication capacity, andopened the door to terabit-scale (1012 bit), large-capacity optical datatransmission.
In addition, new technologies appear with regularity – technologies
such as multi-level modulation transmission and digital coherent
transmission – and both Nakazawa and Hagimoto continue toresearch these latest developments in transmission technology.
The world has entered the Data Era with the use of IoT, big data,and other developments, and the demand for increased communication capacity continues to rise. Experiments have shown thatpetabit-scale (1015 bit) transmission is possible, surpassing the
100 Tb physical limitations of single-mode fiber, thus illustratingthat the evolution of optical communication – a technology that isinterwoven into modern life – continues unabated.

100 T
10 T
1T
100 G

Physical limitations ofsingle-mode fibers (SMF)
Digital coherent transmission

tems
ar l sy
ye cia1/ mer4.
m1
X T’s coNT

Wavelength-divisionmultiplexing (WDM)
technology and opticalamplifiers（bit/s）

1T
100 G

Time-divisionmultiplexing (TDM)

10 G

10 G

1G

1G

100 M

100 M

10 M

1980

1990

2000

2010

2020

2030

Channel capacity

An urgent need for practical applications