Special Feature

Wideband Rare-earth-doped Fiber Amplification Technologies—Gain Bandwidth Expansion in the C and L bands
Tadashi Sakamoto†, Atsushi Mori, Hiroji Masuda,and Hirotaka Ono
Abstract
We are expanding the gain bandwidth of Er3+-doped fiber amplifiers (EDFAs) by using tellurite fiber
instead of silica fiber. This article describes EDFAs that cover a wide wavelength region (1535–1605 nm)
in the C and L bands and EDFAs with a 50-nm amplification range (1560–1610 nm) in the L band. We
also present transmission experiment results for a remotely pumped hybrid inline-repeater system thatuses a tellurite-based EDFA in conjunction with distributed Raman amplification.

1. Importance of expanding the amplification
bandwidth of EDFAs
Medium- and long-haul optical transmission mainly uses signals with wavelengths in the vicinity of1550 nm because optical fiber has its minimum loss
in this region. The erbium-doped fiber amplifier(EDFA), which uses Er3+ as the active ion, has been
widely used for 1550-nm signal amplification. The
EDFA features high-gain and low-noise amplification in either the 1530–1560-nm wavelength region
within the C band (1530–1565 nm) or the 1570–
1600-nm wavelength region within the L band
(1565–1625 nm). The signal wavelengths available
for practical optical-communication systems havebeen limited by the EDFA amplification-wavelength
regions (only 30 nm wide at most). Therefore, if we
can expand the EDFA amplification bandwidth, weshould be able to increase the transmission capacity
by increasing the number of wavelength divisionmultiplexing (WDM) channels, which will spur thedevelopment of large-scale high-performance photonic networks.
One effective way of expanding the amplifier bandwidth is to find an appropriate optical fiber material
that best exploits the amplification capability of Er3+
† NTT Photonics Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: tsaka@aecl.ntt.co.jp

38

ions; we chose a tellurite glass for this purpose as areplacement for conventional silica glass fiber. In thisarticle, we introduce two important types of amplifiers that we have made in this way: a (C+L)-band
Er3+-doped tellurite fiber amplifier (EDTFA) and a
wideband L-band EDTFA. We also present the resultsof a transmission experiment that applies an EDTFA
along with a distributed Raman amplifier to a remotely pumped hybrid inline-repeater system.
2. Bandwidth expansion using tellurite fiber
Extending the EDFA bandwidth requires a large
Er3+ stimulated emission rate across a wide range of
wavelengths. The wavelength dependence of thestimulated emission rate differs according to the typeof glass material into which the Er3+ ions are doped
because the stimulated emission rate of Er3+ is affected to some extent by the electric field (ligand field)
generated by the atoms of the glass. It is important tofind a glass material that can extract the latent capacity of Er3+ for wideband amplification. Our measurements of the basic characteristics of various types ofglass material doped with Er3+ showed that tellurite
glass is an effective material for expanding the bandwidth [1], [2].
Figure 1 shows the stimulated emission rate (solidlines) and excited state absorption (ESA) (brokenlines) for tellurite glass and silica glass doped with
Er3+. Tellurite glass has a higher stimulated emission
NTT Technical Review

Stimulated emission probability and excited-state absorption

Special Feature

1.5
1530–
1480 nm 1620 nm

Tellurite glass<1620 nmAmplification

980 nmhνPump Pump Absorp- Emission ESA
tion
Energy diagram of Er3+
Excited state
absorption (loss)

Silica glass
1
Stimulated
emissionprobability(amplification)

Large emission
probability and low ESA
enable high gain in the Lband for tellurite glass.

0.5

0
1450

1500

1550

1600

1650

Wavelength (nm)Fig. 1. Optical properties of Er3+ ions in tellurite glass.

material for wideband amplification with an EDFA.
3. EDTFA providing (C+L)-band amplification
It is known that the EDFA gain dependence on
wavelength changes if the population inversion stateis altered by pumping. Figure 2(a) shows an example
in the case of an EDTFA. The population inversion
factor α is defined as the Er3+ density in the excitedstate divided by the total density of doped Er3+. When

40
1
C bandUsable gain bandwidth willchange according to the
population inversion state.

C+L band0.5

α = 1.0
0.8
0.6
0.5
0.4
L bandHigh gain is obtained throughout
the C+L band in EDTFA.
30
Gain (dB)Gain per unit EDTF length (arb. unit)

rate especially in the 1580–1630 nm wavelength
region, which is advantageous for obtaining high gainin the L band. ESA, which corresponds to signal loss,
arises at wavelengths longer than 1610 nm with silica
glass but longer than 1620 nm with tellurite glass, so
tellurite glass is also advantageous in terms of ESA,
especially when we expand the amplification bandwidth of the EDFA to wavelengths above 1610 nm in
the L band. The basic characteristics described aboveshow that tellurite glass is a promising optical fiber

20

10
EDTFA
EDSFA

0
1520

1540

1560

1580
1600
Wavelength (nm)

1620

1640

(a) Gain bandwidth for various population inversion states
EDTF: erbium-doped tellurite fiber

0

1540

1560
1580
1600
Wavelength (nm)1620(b) Comparison between EDTFA and EDSFA for(C+L)-band amplification

Fig. 2. (C+L)-band amplification in EDTFA.

Vol. 2 No. 12 Dec. 2004

39

Special Feature

30
14
25
EDTF
GEQ
#1

EDTF

SignalGEQ#2

12Signal input (dBm)–5 dBm
–7 dBm
–9 dBm
–11 dBm–13 dBm–15 dBm

20
15
10

10
8

Noise figure (dB)

EDTF

Gain (dB)

Signal6
5980 nm1480 nm 1480 nm 1480 nm 1480 nm

40(a) Amplifier configuration

1540

1560
1580
1600
Wavelength (nm)(b) Gain characteristics

1620

Fig. 3. Gain-equalized (C+L)-band EDTFA.

α is in the range 0.8 to 1, we can obtain an EDTFA
with a large gain in the C band. For α = 0.4, we get an
EDTFA with gain in the L band. (This requires a long
Er3+-doped fiber because the gain per unit length issmall.) For α = 0.5–0.6, we obtain an EDTFA with
C+L amplification with gain across both these bands.
Figure 2(b) shows the amplification characteristics
of Er3+-doped tellurite and silica fiber amplifiers(EDTFA and EDSFA, respectively) calculated fromthe basic optical properties, including emission probability and ESA shown in Fig. 1, when the population
inversion factor and peak gain near 1560 nm were set
to 0.5 and 30 dB, respectively. The EDTFA gain isrelatively flat above 15 dB in the 1580–1610-nm
wavelength range, while the gain of the EDSFA continues to fall as the wavelength increases in thisrange. Referring again to Fig. 1, this difference arises
from the stimulated emission rate for Er3+-doped tellurite glass being higher than that of Er3+-doped silica glass in the 1580–1630-nm range. These results
demonstrate that tellurite glass is an effective material for amplifying the broad C+L band.
By applying gain equalization to this (C+L)-band
EDTFA, we developed an amplifier that can beapplied to WDM transmission systems [3]. As shownin Fig. 3(a), this amplifier had a 3-stage configuration. Because of the extremely large deviation in gainthat occurred during wideband amplification in the
C+L band (Fig. 2(b)), a gain equalizer (GEQ) with alarge loss was needed to flatten the gain. If the amplifier had been configured in two stages, the large GEQ
loss would have caused the noise figure of the amplifier to increase. In contrast, a 3-stage configuration
distributed the GEQ loss, which kept the noise figure
40

low. The erbium-doped tellurite fiber (EDTF) in thefirst stage was forward pumped with a 980-nm laser
diode (LD) and those in the second and third stages
were bidirectionally pumped with 1480-nm LDs(total pump power: 674 mW, consisting of 116 mW
from the 980-nm LD and 151+158+151+98 mW
from the four 1480-nm LDs).
Figure 3(b) shows experimental results for theamplification characteristics when the total power ofa WDM signal input to the amplifier was varied
between –15 and –5 dBm. Good characteristics consisting of a flat gain of 24.3 dB, gain deviation of 1.5
dB, noise figure of less than 6 dB, and output powerof 19.5 dBm were obtained over a wide wavelength
range (70.8 nm) from 1532.7 to 1603.5 nm. Here, thewavelength dependence of the gain was the sameeven when we varied the signal input power, whichshows that we can easily control the gain spectrum.
4. Wideband L-band EDTFA
Figure 4 shows the amplification characteristics of
the L-band EDTFA and L-band EDSFA with maximum gain of 28 dB and relatively flat gain [4]. The
amplifier performed bidirectional pumping using a1480-nm LD. The EDTFA had a wider “3-dB-down
gain bandwidth” than the EDSFA: 50 nm (from 1560
to 1610 nm) compared with 38 nm (from 1568 to
1606 nm). The noise figure was less than 6.5 dB andthe output power was 18 dBm. The power conversion
efficiency (= output signal power ÷ pump power) was
nearly the same (about 50%) for both the EDTFA andEDSFA.
The noise figure rose at longer wavelengths, startNTT Technical Review

Special Feature

30
50 nm
3 dB
down

30

25

20

Gain (dB)

EDTFA
15
10
EDSFA
10

Noise figure (dB)

38 nm
20

0
5

–10

1540

1560

1580

1600

1620

0

Wavelength (nm)Fig. 4. Gain characteristics of L-band EDTFA.

ing from 1610 nm for the EDSFA and from 1620 nm
for the EDTFA due to ESA effects. Furthermore, for
the EDTFA, the gain remained above 10 dB even at
wavelengths above 1620 nm. These results indicatethat the EDTFA is preferable to the EDSFA whenusing wavelengths longer than 1610 nm. In fact,
efforts are being made to shift this EDTFA gain intoa longer-wavelength region from 1583 to 1617 nm. It
has been reported that if WDM transmission were tobe performed in this band through a dispersion-shifted fiber, the four-wave mixing* effect would be lessof a problem than with transmission in the EDSFA
amplification band of 1570–1600 nm in the L band[5]. This would increase the tolerance in L-band
WDM transmission even when the zero-dispersion
wavelength of the dispersion-shifted fiber fluctuates.
5. Application of EDTFA to R-EDFA/DRA
hybrid repeater system
We conducted a transmission experiment where weused the EDTFA along with a distributed Raman
amplifier to make a remotely pumped hybrid inline* Four-wave mixing (FWM) is a phenomenon of nonlinear optics.
In FWM, three signal lights of different wavelengths generate
light of yet another wavelength as a result of nonlinear effects.
This newly generated light greatly degrades the optical signal of
the same wavelength due to cross talk. Therefore, to achieve highquality WDM transmission, one must take care to avoid generating FWM.

Vol. 2 No. 12 Dec. 2004

repeater system (R-EDFA/DRA hybrid repeater system) [6]. We inserted an erbium-doped tellurite fiber
at a (remote) point along the transmission path in asystem that uses a DRA and operated the fiber as an
EDFA using residual light from the pump light used
for the DRA. Even though this scheme does not
involve a power supply along the transmission path, it
can improve the optical signal-to-noise ratio (OSNR)(or alternatively increase the tolerable loss per span)compared with the use of the DRA alone. When an
EDSFA is used as a remotely pumped EDFA (REDFA), the signal band is about 30 nm wide for the C
or L band, but when an EDTFA is used as an REDFA, the signal band is about 80 nm wide (about 2.7
times as wide).
The configuration of an experimental system using
a single-mode fiber as the transmission path is shownin Fig. 5(a). The figure shows the configuration of
only one repeater (one span) in a multi-repeater transmission. This span consisted of a 180-km transmission path and an inline amplifier ((C+L)-band amplifier). An R-EDTFA was located 120 km from one
end, and DRA pump light was input at both ends ofthe transmission path (1500 nm into the shorter segment and both 1455 and 1500 nm into the longer segment).
The configuration of the R-EDTFA module isshown in Fig. 5(b). It has two stages: the first stage isa single pass with forward pumping and the second
stage is a double pass. Here, 1500-nm DRA residual41

Special Feature

40

Single-mode fiber
60 km
SignalGain (dB)

SignalR-EDTFA
modulePump

Pump(C+L)-band
EDFA

P

P

1455 nm
1500 nm

1500 nm

1535–1615 nm
R-EDTFADRA
R-EDTFA+DRA

30
20
10
0

1540

(a)

Gain (dB)

EDTF
EDTF

Signal

WDM

WDM
Circulator

1620

12

MirrorSignal1560 1580 1600
Wavelength (nm)
(c)

Pump

30

R-EDTFA/DRA hybrid system
DRA system (without R-EDTFA) 10
OSNR difference

25

8
6

20
4
15
10

(b)

∆OSNR (dB)Single-mode fiber
120 km2
1535–1615 nm
1540

1560 1580 1600
Wavelength (nm)
(d)

0
1620

Fig. 5. Transmission experiment with R-EDTFA/DRA hybrid system.
(a) Configuration of transmission experiment (one span). (b) Configuration of R-EDTFA module.
(c) Gain characteristics of R-EDTFA/DRA hybrid amplification.
(d) Comparison of OSNR with and without R-EDTFA.

light input backward on the transmission path isintroduced into the module and used as a pump lightfor the R-EDTFA. The individual gains and the total
gain for the R-EDTFA and DRA are shown in Fig.
5 ( c ) . T h e R - E D T FA h a d a h i g h g a i n i n t h e
1535–1565-nm range, while the DRA gain peaked at1610 nm. Combining the two gains resulted in a total
gain of no less than 19 dB in the 1535–1615-nm
range (80-nm bandwidth). Figure 5(d) compares the
R-EDTFA/DRA hybrid amplification output from the
180-km transmission path with DRA-only amplification in terms of OSNR for the same pump power. The
addition of the R-EDTFA improved the OSNR by atleast 1.9 dB in the 1535–1615-nm wavelength region.
WDM signals modulated by a 43-Gbit/s carrier-suppressed return-to-zero (CS-RZ) code were transmitted over a five-repeater span (total length: 900 km (=5 × 180-km spans)).
In conclusion, an EDFA with tellurite fiber
achieves wideband amplification in the C+L and L
bands, which is impossible using a silica-fiber EDFA.
Therefore, we can expect to reduce the transmission
cost per channel by increasing the number of WDM
42transmission channels by using this amplifier.
References
[1]

[2]

[3]

[4]

[5]

[6]

M. Yamada and M. Shimizu, “Ultra-wideband Amplification Technologies for Optical Fiber Amplifiers,” NTT Technical Review, Vol.
1, No. 3, pp. 80-84, 2003.
A. Mori, Y. Ohishi, M. Yamada, H. Ono, and S. Sudo, “Broadband
Amplification Characteristics of Tellurite-based EDFAs,” ECOC’97,
Vol. 3, Paper We2C.4, pp. 135-138, 1997.
H. Ono, A. Mori, K. Shikano, and M. Shimizu, “A Low-noise andBroadband Erbium-doped Tellurite Fiber Amplifier with a Seamless
Amplification Band in the C- and L-Bands,” IEEE Photon. Technol.
Lett., Vol. 15, No. 8, pp. 1073-1075, 2002.
A. Mori, T. Sakamoto, K. Kobayashi, K. Shikano, K. Oikawa, K.
Hoshino, T. Kanamori, Y. Ohishi, and M. Shimizu, “1.58-µm Broadband Erbium-doped Tellurite Fiber Amplifier,” J. Lightwave Technol., Vol. 20, No. 5, pp. 822-827, 2002.
H. Ono, T. Sakamoto, A. Mori, J. Kani, and M. Fukutoku, “AnErbium-Doped Tellurite Fiber Amplifier for WDM Systems with Dispersion-Shifted Fibers,” IEEE Photon. Technol. Lett., Vol. 14, No. 8,
pp. 1070-1072, 2002.
H. Masuda, S. Kuwahara, H. Kawakami, A. Hirano, Y. Miyamoto, A.
Mori, and T. Sakamoto, “Ultra-wideband remotely-pumped
EDFA/DRA hybrid inline-repeater system using tellurite-based EDFs
and 1500-nm pumping method,” OAA04, paper OWB2, 2004.

NTT Technical Review

Special Feature

Tadashi Sakamoto

Hiroji Masuda

Senior Research Engineer, Photonics Integration Laboratory, NTT Photonics Laboratories.
He received the B.S. and M.S. degrees in electronic engineering from Waseda University,
Tokyo in 1990 and 1992, respectively. In 1992,
he joined NTT and has been working on opticalfiber amplifiers and their application to the optical transmission systems. He received an IEICE
Young Researchers’ Award in 1995, two OECCBest Paper Awards in 1997 and 1998, and a JSAP
Young Scientist Award for the Presentation of an
Excellent Paper in 1998. He is a member ofIEEE, the Institute of Electronics, Information
and Communication Engineers (IEICE) of Japan,
and the Japan Society of Applied Physics (JSAP).

Senior Research Engineer, High-speed Lightwave Transport Systems Research Group, Photonic Transport Network Laboratory, NTT Network Innovation Laboratories.
He received the M.Sc. degree in physics from
Tokyo Institute of Technology, Tokyo in 1988.
He joined NTT Laboratories, Kanagawa, Japan,
in 1988. He has been engaged in research on optical amplification technologies and high-speed
and large-capacity transmission systems. He is amember of IEICE and IEEE.

Atsushi Mori

Hirotaka Ono

Senior Research Engineer, Photonic Integration Laboratory, NTT Photonics Laboratories.
He received the B.S. and M.S. degrees inphysics from Tohoku University, Sendai, Miyagi
in 1989 and 1991, respectively. He joined NTT
Opto-Electronics Laboratories in 1991, where he
was engaged in research on multicomponent
glass fiber including oxide, fluoride and chalcogenide glass fiber for optical fiber amplifiers.
Since 1995, he has mainly been engaged inresearch on wideband optical fiber amplifiers
using Er3+-doped tellurite fibers and on tellurite
fiber as a fiber Raman amplifier medium. He
received the 1996 JSAP Young Scientist Awardfor the Presentation of an Excellent Paper and the2000 IEICE Achievement Award. He is a member of IEICE, JSAP, and the Physical Society ofJapan.

Research Engineer, Photonics Integration Laboratory, NTT Photonics Laboratories.
He received the B.S., M.S., and Ph.D. degrees
in applied physics from Tohoku University,
Sendai, Miyagi in 1993, 1995, and 2004, respectively. In 1995, he joined NTT and has been
engaged in R&D of optical fiber amplifiers usedin WDM transmission and access systems. He
also worked on research into WDM transmission
systems. He is a member of IEICE, IEEE, and the
Optical Society of America.

Vol. 2 No. 12 Dec. 2004

43