Letters

Ultra-wideband Amplification
Technologies for Optical Fiber Amplifiers
Makoto Yamada† and Makoto Shimizu
Abstract
To make a large-scale optical communication system that exhibits high performance, we must expand
the range of operating wavelengths of optical fiber amplifiers. In this report, we describe S-band amplification with a Tm3+-doped fluoride fiber amplifier and an Er3+-doped fiber amplifier and broadband
fiber Raman amplification with a tellurite fiber.

1. Importance of broadband optical fiberamplifiers
Optical fiber amplifiers play an important role intelecommunication networks because they compensate for the insertion loss of the optical fiber used fortransmission lines and for the loss due to the optical† NTT Photonics Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: myamada@aecl.ntt.co.jp

0.6

O band(1260–1360)

components used at optical nodes. Their amplification band determines the operating wavelength
region of the network.
2. Wideband amplification techniques in NTT Labs.
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. Figure 1 shows their amplification bandwidths

ITU-T definition of spectral bands
E bandS bandC band
L bandU band(1360–1460)(1460–1530) (1530–1565) (1565–1625) (1625–1675)

Loss of transmission fiber (dB/km)

Rare-earth-doped fiber amplifiers
PDFA

C-band and L-band
EDFAs

The amplification band that
we are developing
for practical use

0.51.6-µm-band
TDFAfiber Raman amplifier(FRA)
0.4FRA can amplify over whole wavelength region.
Raman amplification band: ~100 nm

Expansion of Raman
amplification band

0.3
OH absorption
Rayleighscattering
0.2

1300

1350

Bending

OH-free fiber
1400

1450
1500
Wavelength (nm)

1550

1600

1650

1700

Fig. 1. Amplification bandwidth of each type of amplifier and loss spectrum of transmission fiber.

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Letters

and the transmission fiber loss.
NTT Photonics Laboratories have been expanding
the amplification bands of both types. An S-bandrare-earth fiber amplifier is being developed as a successor to the C-band EDFA (amplification bandw i d t h : 1 5 3 0 – 1 5 6 5 n m ) a n d L - b a n d E D FA
(1565–1625 nm). And a broadband FRA is being
developed using fiber made of tellurite glass, which
has excellent Raman amplification band characteristics.
3. Principle and characteristics of S-band rareearth fiber amplifiersS-band amplification can be achieved with two
types of rare-earth fiber amplifier: the thulium-doped
fiber amplifier (TDFA) and the S-band EDFA, as
shown in Fig. 2.
3.1 TDFAThe TDFA uses Tm3+-doped fiber (TDF) as theamplification medium. As shown in the energy level

diagram of Tm3+ in Fig. 2, S-band amplification utilizes stimulated emission between the 3H4 and 3F4energy levels. Fluoride glass is used to obtain efficient S-band amplification. (S-band amplification
using Tm3+-doped silica fiber has been reported, but
the stimulation efficiency was low.) The fluorescence
lifetime of the upper level (3H4) is short comparedwith that of the lower level (3F4), so a populationinversion between the 3H4 and 3F4 levels is created by
the two-step excitation process in which i) Tm3+ ionsare excited from the ground level (3H6) to the lowerlevel (3F4) and ii) the accumulated Tm3+ populationin the lower level (3F4) is excited to the upper level(3H4) [3]-[4]. If the population inversion is high, aTDF gain peak appears at about 1460 nm in the gainspectrum. By choosing a population inversion rate ofabout 40%, we can get an amplification bandwidthwith a peak in the S-band. For this reason, a TDFA
that operates in the S-band is called a gain-shifted
TDFA. At present, this population inversion isachieved by adding a high dose of Tm3+ (6000 ppm)
to a fluoride fiber and increasing the number of Tm3+

Basic configuration of S-bandrare-earth doped fiber amplifier

Opticalisolator WDM

Using Tm3+-doped fluoride fiber (TDF)

Amplification medium(TDE, EDF)
Optical
WDM isolator

Input

3F
3 2F
3 3
H4OutputPump
source1.4 µmpumpingPump
sourceUsing Er3+-doped fiber (EDF)

Signal LightC-band ASE C-band ASE C-band ASE

33Output
Tm3+

F4

H6

Tm3+

Tm3+ energy level(amplifiedsignal)

Pump light
Rejection of C-band ASE

EDF

64
20-2-4-6
1400

High population inversion(>60-70%)
1530–1565 nm
Medium population
1530–1620 nm
inversion(about 50%)
1570-1620(1630) nm
Low population inversion(about 40%)

TDF gain coefficient

S-band amplification technique using EDF

EDF gain coefficient (m-1)

3
H5
Cross relaxation

S-band
1.4 µm amplification
pumpingPopulation inversion:
100%
40% (for S-bandamplification)

0%
C-band L-band
S-band
1450
1500
1550
1600
Wavelength (nm)
EDF gain spectrum

1350
1650

S-band1480–1510
nm
1400
1450
Wavelength (nm)1500

TDF gain spectrum

Fig. 2. S-band rare-earth doped fiber amplifiers.

Vol. 1 No. 3 June 2003

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Letters

ions in the lower level (3F4) by utilizing cross relaxation among Tm3+ ions.
Figure 3(a) shows the gain spectrum of the S-band
TDFA [5] obtained with the above technique and 1.4µm semiconductor laser pumping for both excitation
processes. It shows that practical high-gain and lownoise amplification was achieved in the S-band. Furthermore, excellent flat gain characteristics wereobtained in the 1480–1510-nm band by using a gainequalizer.

EDFA) and amplification characteristics below 1590
nm. On the other hand, the S-band EDFA can bemade using basically the same optical components asfor conventional C- and L-band EDFAs. We intend tocontinue improving the characteristics of both amplifiers for practical use.
4. Broadband Raman amplification using
tellurite fiber
Compared with a rare-earth-doped fiber amplifier,an FRA has two advantages:• It can use the optical fiber that is transmitting thedata signals as a Raman amplification medium(distributed amplification).
• It can amplify a signal at an arbitrary wavelengthby using a pump light with a wavelength shorter
than the signal (approx. 100 to 170 nm shorter forthe 1.5 µm wavelength region).
Distributed amplification is particularly effective
for reducing the signal deterioration caused by thenonlinearity of the transmission fiber, which was aproblem in high-speed communication systems suchas 40-Gbit/s transmission systems, and similar applications where a high signal-to-noise ratio is required.
At present, the FRA is being studied not only for distributed amplification, but also for discrete amplification such as that provided by a rare-earth-doped fiber
amplifier. The goals are to expand the amplification
band and increase the operating efficiency. An FRA
using tellurite fiber has a higher operating efficiency
and superior bandwidth characteristics to a conventional FRA using silica fiber.
The FRA amplifies light signals via stimulated

3.2 S-band EDFAAs shown in Fig. 2, an S-band EDFA can be madeby utilizing the S-band amplification bandwidth thatappears together with the C-band when the excitationof the Er3+-doped fiber (EDF) is increased to achieve
greater population inversion (at least 60 to 70%) andby suppressing the amplified spontaneous emission(ASE) in the C-band that occurs to maintain the highpopulation inversion [6]-[7]. Figure 3(b) shows thegain spectrum of the S-band EDFA [7]. The gain andnoise figure were measured by scanning the gain
bandwidth with a low-power probe signal (inputprobe power: –30 dBm) while inputting five WDMchannels (wavelengths: 1493.7, 1499.2, 1504.8,1511.9, and 1517.6 nm, input signal power: –12dBm/ch). We obtained an S-band EDFA with a highgain of over 21 dB (gain excursion was less than 1.9dB) and a low noise figure of less than 6.7 dB in the
1491–1518-nm wavelength region by inserting an
ASE elimination filter in the EDF.
At present, the S-band TDFA has advantages inefficiency (power conversion efficiency is about 40%for TDFA compared with about 10% for S-band
354530402525
Input signal power: –7 dBm
Without gain equalizer
With gain equalizer

15102015

5015151010

NF=6.7 dB1014701480

1490 1500 1510
Wavelength (nm)

1520

51530

Noise figure (dB)2020
Gain (dB)30

Noise figure (dB)

Signal gain (dB)2520

Gain=21 dB

35501480

1490

1510
1500
Wavelength (nm)1520

Fig. 3. (a) Amplification characteristics of TDFA,(b) Amplification characteristics of S-band EDFA.

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LettersBasic configuration of FRA (discrete type)

WDM

Input

Output...

Raman gain coefficient(1/W•km)

λN

λ2

λ1

Multi-wavelength
pumping
Pump sources∆n: 2.5%
Loss: 0.6 dB/km
Pumping: 1460 nm

32
10λN

λ2 λ1
Wavelength

60

Tellurite fiber

50
40∆n: 2.2%
Loss: 20 dB/km
Pumping: 1460 nm

302010170 nm
0050 100 150 200 250 300
50100 150 200 250
Raman Stokes shift (nm)Raman Stokes shift (nm)Raman gain spectrum (single-wavelength pumping)

100 nm0Gain profile
by multiwavelength
pumping

Multi-wavelength pumpingSilica fiber (DCF)

4

Gain profile
by pump: λ1...

WDM coupler

Raman gain coefficient(1/W•km)

Silica fiber
Tellurite fiber

Gain profile
by pump: λ2

Pump light

Optical isolator

Intensity

Optical isolator

Gain profile
by pump: λN

Fig. 4. Configuration of broadband FRA.

Signal gain and noise figure (dB)Raman scattering, which is a nonlinear
30
optical effect, when intense excitation light
Amplification band: 160 nm
is passed through an optical fiber. The
25
Signal gain: > 10 dB, noise figure: < 10 dB
resulting gain spectrum (Raman gain spectrum) depends strongly on the composition
20of the glass used in the fiber. As shown in
Signal gain
Fig. 4, with dispersion-compensated fiber,
15
which is a type of silica fiber, the Raman
shift in the 1.5-µm band is about 100 nm
10
and the full width at half maximum is about
30 nm. In contrast, the tellurite fiber spec5
trum has twin peaks, but the gain coefficient
Noise figure
is 16 times that of silica fiber and the0Raman shift is 170 nm (1.7 times that of sil1480
1520
1560
1600
1640
ica fiber) [8].
Wavelength (nm)Furthermore, the FRA employs a multiFig. 5. Amplification characteristics of tellurite FRA.
wavelength pump technique [9] for broadband amplification. This utilizes the Ramangain spectrum superimposition that occurs
when pump lights of various wavelengths are combecause of its larger Raman shift. It has now been
bined and passed through an optical fiber. Thus,
confirmed that an FRA using a tellurite fiber canbroadband operation is achieved. In addition, byachieve amplification over a wide bandwidth up toadjusting the pump intensity at each wavelength, we
160 nm (see Fig. 5) [8]. However, in terms of effican obtain flat gain characteristics. The limit to theciency, the tellurite fiber still has slightly high loss, sogain bandwidth expansion that can be achieved using
we cannot make optimum use of its high efficiency.
multi-wavelength pumping is determined by theFor this reason, determined efforts are being made to
Raman shift. Whereas the maximum bandwidth of an
reduce the loss of this fiber to establish tellurite fiber
FRA using a silica fiber is about 100 nm, a tellurite
Raman amplification technology that permits highly
fiber can inherently achieve broadband amplification
efficient broadband operation.
Vol. 1 No. 3 June 2003

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Letters

5. Application of broadband amplification
technology
By efficiently combining the already developed Sband rare-earth-doped fiber amplification, broadband
tellurite fiber Raman amplification, C- and L-bandrare-earth fiber amplification, 1300-nm bandwidth
Pr-doped fiber amplification (another type of rareearth-doped fiber amplification), the 1650-nm-band
TDFA, and silica fiber Raman fiber amplification, we
expect to achieve amplification over a wide waveband extending from 1300 to 1650 nm. We hope that
all of these optical fiber technologies can be put toeffective use to help make a large-scale, highly functional photonic network.
References
[1]
[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

84

M. Shimizu, “Optical fiber amplifiers for next generation networks,”
Material Stage, Vol. 12, 2001 (in Japanese).
H. Masuda, “Recent progress in broadband optical fiber amplifiers,”
J.J. Optics (Kougaku), Vol. 29, No. 3, pp. 136(18)-142(24), 2000 (inJapanese).
T. Komukai, T. Yamamoto, T. Sugawa, and Y. Miyajima, “Upconversion pumped thulium-doped fluoride fiber amplifier and laser
operating at 1.47 µm,” IEEE J. Quantum Electron., Vol. 31, No. 11,
pp. 1880-1889, 1995.
T. Sakamoto, M. Shimizu, T. Kanamori, Y. Terunuma, Y. Ohishi, M.
Yamada, and S. Sudo, “1.4-µm-band gain characteristics of a Tm-Hodoped ZBLYAN fiber amplifier pumped in the 0.8-µm band,” IEEE
Photonics Technol. Lett., Vol. 7, No. 9, pp. 983-985, 1995.
S. Aozasa, H. Masuda, H. Ono, T. Sakamoto, T. Kanamori, Y. Ohishi,and M. Shimizu, “1480-1510 nm-band Tm doped fiber amplifier(TDFA) with a high power conversion efficiency of 42 %,” in Proc.
OFC’01, PD1, 2001.
E. Ishikawa, M. Nishihara, Y. Sato, C. Ohshima, Y. Sugaya, and J.
Kumasaka, “Novel 1500 nm-band EDFA with discrete Raman amplifier,” in Proc. ECOC’01, Vol. 6, pp. 48-49, 2001.
H. Ono, M. Yamada, and M. Shimizu, “S-band erbium-doped silica
fibre amplifier with flattened-gain of over 21 dB,” Electron. Lett.,Vol. 33, No. 19, pp. 1084-1086, 2002.
A. Mori, H. Masuda, K. Shikano, K. Oikawa, K. Kato, and M.
Shimizu, “Ultra-wideband tellurite-based Raman fiber amplifier,”
Electron. Lett., Vol. 37, No. 24, pp. 1442-1443, 2001.
S. Namiki, “Recent advances in optical fiber Raman amplifiers,” J.
Japan Soc. of Applied Physics (Oyo Butsuri), Vol. 70, No. 11, pp.
1299-1303, 2001 (in Japanese).

Makoto Yamada
Senior Research Engineer, Supervisor Fiber
Amplifiers Research Group, Hyper-photonic
Component Laboratory, NTT Photonics Laboratories.
He received B.E. and M.E. degrees in electricalengineering from the Technical University ofNagaoka, Niigata, Japan 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 in the areaof optical amplifiers from the Technical University of Nagaoka.
Dr. Yamada is a menber of the IEEE, the Institute of Electronics, IEICE (Information andCommunication Engineers of Japan), and Japan
Society of Applied Physics.

Makoto Shimizu
Group Leader, Fiber Amplifiers ResearchGroup, Hyper-photonic Component Laboratory,NTT Photonics Laboratories.
He received the B.E. and M.E. degrees in electrical engineering from the University of ElectroCommunications, Tokyo, Japan in 1981 and1983, respectively. Since joining the ElectricalCommunication Laboratories, Nippon Telegraphand Telephone Public Corporation (now NTTCo.) in 1983, He has been engaged mainly inresearch on silica-based optical fiber fabrication
technology and optical fiber amplifier technology. He is a menber of the IEEE, IEICE, and Japan
Society of Applied Physics.

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