Special Feature: Optical Fiber Amplification Technologies

Overview of Wideband Optical Fiber
Amplification Technologies
Makoto Yamada†
Abstract
Optical fiber amplifiers are essential for increasing the scale and performance of communication systems. They play a major role in determining the operating wavelength region of a network, especially in
long-haul transmission systems. This article gives an overview of wideband technology for rare-earthdoped fiber amplifiers and fiber Raman amplifiers as an introduction to the other articles in this specialfeature.

1. Importance of wideband optical fiber
amplifiersWith the use of personal computers and other information terminals spreading and multimedia technologies including the Internet and electronic commerce
becoming more popular, the communication network
is growing in scale and complexity. To handle thisgrowing traffic, photonic networks are being developed for long-haul communication network systems[1]. Photonic networks can increase the transmission
capacity by handling optical signals of different
wavelengths in an optical fiber by dense wavelength
division multiplexing (DWDM), and optical nodes
that function as optical cross-connects and add/dropmultiplexers etc. improve system flexibility and functionality. Optical fiber amplifiers are important system components in the construction of photonic networks based on DWDM transmission. They can compensate for loss in transmission optical fiber and in
optical devices (such as optical add/drop multiplexers
and optical switches) that make up a network andenable greater network scale and transmission distances. It is no exaggeration to say that the amplification bandwidth or operating wavelength region of theoptical fiber amplifiers determines the operating
wavelength region of the network. Thus, to create
large-capacity photonic networks, we must expand† NTT Photonics Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: myamada@aecl.ntt.co.jp

34

the bandwidth of optical fiber amplifiers. Efforts toincrease network capacity are being combined withefforts to expand the optical fiber amplification bandwidth to the C band (1530–1565 nm), L band (1565–1625 nm), and S band (1460–1530 nm) [2]-[3]. Figure 1 shows the development of optical-fiber transmission capacity over time [3]. Large-volume transmission of 10.92 Tbit/s has been demonstrated using
a combination of the S, C, and L bands.
In recent years, we have also seen the introductionof coarse wavelength division multiplexing (CWDM)
as a transmission scheme for constructing low-cost
networks (compared with DWDM) for short- andmedium-distance transmission systems and metropolitan networks. Adding optical fiber amplifiers tothis type of transmission system should also lead togreater transmission capacities and longer transmission distances.
In addition to studying optical fiber amplifiers for
DWDM or CWDM using the 1500-nm band, we are
studying the introduction of optical amplification
technologies and ways of improving network characteristics in the 1300-nm band (O band: 1260–1360nm) for short- and medium-distance transmission
systems and in the U band (1625–1675 nm), a wavelength region used for supervision. Last year, thisjournal published a letter from a co-worker and I
describing ultra-wideband amplification technologies for optical fiber amplifiers [2]. This issue
expands upon that theme with a special feature offour articles.

NTT Technical Review

and Their Applications

30,00010,000ExperimentSingle-channel transmission
DWDM transmission
Using S, C, and L bands
Using C and L bands
Using C band

3000

Transmission capacity (Gbit/s)

1000

300
100

30
103
Commercial
Single-channeltransmission
DWDM transmission

10.3
0.1
80

82

84

86

88

90
92
Year

94

96

98

00

02Fig. 1. Development of optical-fiber transmission capacity over time.

2. Overview of wideband efforts
There are two types of optical fiber amplifier: i)rare-earth-doped fiber amplifiers, such as the erbiumdoped fiber amplifier (EDFA) and ii) the fiber Raman
amplifier (FRA), which uses stimulated Raman scattering. For rare-earth-doped fiber amplifiers, thebands for which amplification can be performed
depend on the type of rare-earth ion used. For example, Er3+ (erbium) corresponds to optical amplification in the S, C, and L bands, Tm3+ (thulium) to the S
and U bands, and Pr3+ (praseodymium) to the O band.
Erbium- and thulium-doped fiber amplifiers areknown as EDFAs and TDFAs, respectively. The fiber
Raman amplifier, on the other hand, while inferior torare-earth-doped fiber amplifiers in terms of excitation efficiency, can amplify a signal of any wavelength by using a pump light on the short-wavelengthside of the wavelength band targeted for amplification (for the 1500-nm band, a wavelength about 100
nm shorter than that of the signal is used).
Figure 2 shows the amplification bandwidth ofrare-earth-doped fiber amplifiers and the fiber Raman
amplifier developed so far and the loss spectrum ofVol. 2 No. 12 Dec. 2004

transmission fiber. Table 1 summarizes wideband
concepts using fiber amplifiers and gives the requirements and associated amplifier configurations [4].
Wideband technologies are being developed to
expand the WDM transmission bandwidth based on
the following two methods:
1) Connecting optical fiber amplifiers having different amplification bandwidths in parallel and2) Expanding the amplification bandwidth itself.
In particular, we are developing practical C-band
and L-band EDFAs as wideband, parallel-type rareearth-doped fiber amplifiers and TDFAs and S-band
EDFAs as S-band rare-earth-doped fiber amplifiers.
Furthermore, to expand the bandwidth of the amplifiers themselves, we have replaced the silica fiberused in EDFAs with a new glass fiber such as tellurite
or bismuth fiber, and have developed C+L dual-band
amplification technology that extends the C-band
EDFA amplification bandwidth into the L-band andS+C dual-band amplification technology that extends
the S-band TDFA amplification bandwidth into the Cband.
As for the fiber Raman amplifier, we are working to
expand its bandwidth through wavelength multiplex35

Special Feature

ITU-T definitions of spectral bandsE band
S bandC band
L band(1360–1460)(1460–1530) (1530–1565) (1565–1625)

O band(1260–1360)

Loss of transmission fiber (dB/km)

Transmission wavelength region
O-band system (short reach access)18ch CWDM system

Next-generation
DWDM system

Supervision

Er 3+
C-bandEDFA L-band EDFA (silica EDF)

Tm3+

Amplification region of fiber amplifiers

TDFA TDFA

C-, L-, S-band amplification techniques

0.5

Present
DWDM system

Present 8ch CWDM system

Amplification region ofeach rare-earth ion
Pr3+
0.6

U band(1625–1675)

S-band EDFA

Tm3+

L-band EDFA (tellurite EDF)
C+L (silica EDF)

Dual-band amplification techniques

C+L (tellurite EDF)

S+C
Raman amplification technique

0.4

Rare-earth-doped fiber amplification+ Raman amplification

TDFA+FRA

C+L band collective EDFA+FRA
1650-nm-band
TDFA

1300- and 1650-nm-band rare-earth-doped fiber amplification technique
0.3
OH absorption

Rayleighscattering
0.2

1300

OH-free fiber
1350

1400

Bending
1450

1500
Wavelength (nm)

1550

1600

1650

1700

Fig. 2. Amplification bandwidth of rare-earth-doped fiber amplifiers and the fiber Raman amplifier developed so far andthe loss spectrum of transmission fiber.

Table 1. Ways of achieving wideband amplifiers.
Concept
Parallel configuration
Gain

New bandNew bandConventional
band
Wavelength

Expansion of theamplification band itselfTechniqueA• C-, L-, S-band rare-earth doped fiber
AamplificationD• C-band EDFAA
• L-band EDFA
• S-band amplification (TDFA, S-band EDFA)F C• Fiber Raman amplification (FRA)P• New fiber glass host (tellurite, bismuth)
for rare-earth-doped fiber amplifier and FRA

Gain• Dual-band amplification• C+L bands• S+C bands
Expansion Expansion
Conventionalband

C(C+L) band
AS band C band
A AF C• Multi-wavelength pumping for FRA
P1 P2 P3 PN

Wavelength• Combination of a rare-earth-doped fiberamplifier and FRA

F CAP

A: Rare-earth-doped fiber amplifier, F: optical fiber for Raman amplification
C: multiplexer, D: demultiplexer, P: pump source for Raman

36NTT Technical Review

Special Feature

ing excitation technology and are investigating further bandwidth expansion through the use of tellurite
glass fiber as a Raman amplification medium in thesame technology. We have also proposed technology
for connecting rare-earth-doped fiber amplificationand fiber Raman amplification devices in series tosupplement the amplification bandwidth of rareearth-doped fiber amplifiers with Raman amplification and have shown this to be effective in expanding
bandwidth. Using parallel amplification technology,
bandwidth can also be expanded by combining an
FRA with a rare-earth-doped fiber amplifier.
At present, we can perform amplification throughout the whole wavelength region (1300–1650 nm)
usable by transmission fiber by combining various
technologies: C-band, L-band, S-band, C+L dualband and S+C dual-band rare-earth fiber amplification, wideband-fiber-Raman amplification that canamplify optical signals in any band, series-connected
rare-earth-doped fiber amplification and fiber Raman
amplification, 1300-nm-band Pr3+-doped fiber amplification, and 1650-nm-band TDFA amplification. In
the future, we expect to see large-scale, broadbandphotonic networks or optical communication systems
that make effective use of all of the above opticalfiber amplification bandwidths.
The next three articles in this special feature give
more details about the following technologies thatNTT Photonics Laboratories has been developing: 1)
wideband rare-earth-doped fiber amplification to
expand the gain bandwidth in the L and C bands, 2)
S-band amplification through TDFAs and S-band
TDFAs and O-band amplification through Pr3+doped fiber amplifiers, and 3) wideband Raman
amplification using tellurite fiber. We also introduce
CWDM optical fiber amplifiers that we recently
made using these wideband technologies.

Makoto Yamada
Senior Research Engineer, Supervisor, Photonic Processing Devices Research Group, Photonics Integration Laboratory, NTT Photonics
Laboratories.
He received the B.E. and M.E. degrees in electrical engineering from the Technical Universityof Nagaoka, Nagaoka, Niigata in 1983 and 1985,
respectively. He joined NTT Laboratories in1985, where he was engaged in research on guided-wave optical devices. Since 1989, he has been
engaged in research on optical fiber amplifiers.
In 1998, he received the D.E. degree for work on
optical amplifiers from the Technical Universityof Nagaoka. He received the 1994 Paper Award
from the Institute of Electronics, Information andCommunication Engineers (IEICE) of Japan. He
is a member of IEEE, IEICE, and the Japan Society of Applied Physics.

References
[1]
[2]

[3]

[4]

T. Aoyama, “Outlook for Photonic Networks,” Journal of IEICE, Vol.
82, No. 7, pp. 704-712, 1999 (in Japanese).
M. Yamada and M. Shimizu, “Ultra-wideband Amplification Technologies for Optical Fiber Amplifiers,” NTT Technical Review, Vol.
1, No. 3, pp. 80-84, 2003.
M. Koga, N. Takachio, and Y. Miyamoto, “Current State and Futureof Wavelength Multiplexed Optical Transmission Systems,” Journalof IEICE, Vol. 83, No. 7, pp. 569-575, 2000 (in Japanese).
M. Yamada, S. Aozasa, H. Ono, A. Mori, T. Sakamoto, K. Shikano,and M. Shimizu, “Novel fiber amplifier technology,” SPIE, Vol.
5246, pp. 216-223, 2003.

Vol. 2 No. 12 Dec. 2004

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