Development of High-Power Optical Amplifier
by Yoshio Tashiro *, Satoshi Koyanagi *2, Keiichi Aiso *3,
Kanji Tanaka * and Shu Namiki *With the development of the D-WDM (Dense Wavelength Division-Multiplex)
transmission technology together with its practical applications in recent years,
optical amplifiers --in particular EDFA (Erbium-Doped Fiber Amplifier)-- used in these transmission systems are required to have ever higher performance. In this report, a high-power EDFA ispresented which has achieved a high signal output of 1.5 W. The EDFA uses a set of high power
pumping light sources in which the outputs of semiconductor laser diodes in the 1480-nm range
are multiplexed over three wavelengths.

ABSTRACT1.

INTRODUCTION

With the development of the D-WDM transmission technology together with its practical applications in recent
years, optical amplifiers --in particular the EDFA which isfinding wide applications- used in these transmission systems are required to have higher power. This is because,as the total power of optical signal increases due to theincrease in multiplexing signals, so does the power of the
EDFA necessary to obtain the same level of optical gain.
Apart from this application, high-power EDFAs have suchapplications as repeater-less transmission system 1), intersatellite optical links 2), and optical signal processing whichuses nonlinear effects in the fiber 3).
The realization of high-power EDFAs of the watt class
may be classified into two broad categories according to
the pumping wavelength used: the 980-nm range or the1480-nm range. The former is characterized by a narrowabsorption bandwidth, corresponding to the excited-stateenergy levels of Er ion, of 16 nm in FWHM (Full Width atHalf Maximum) centering at 978 nm. Furthermore, thequantum conversion efficiency from pumping light to signal light is low with around 60% because of the large difference in the energy levels between the pumping wavelength and the signal wavelength spanning the 1530-1580
nm range. Higher power in pumping light sources is therefore needed to compensate for the low efficiency.
EDF (Erbium Doped Fiber) co-doped with Yb(Ytterbium) is used to solve this problem. Yb co-doping
enables indirect stimulation of Er ions by the interaction
between the ions, since Yb ion has a broader absorption
range spanning 800-1100 nm in which energy transfer
takes place from excited Yb ions to Er ions. The broader
range of pumping wavelength permits the use of Nd-YAG*

WP Team, Opto-Technology Lab., R&D Div.
2nd Sec., Optical Device Dept., FITEL Div.
*3 Optical Fiber Development Center, Optical Fiber Div.
*2

lasers 5) of high-power at 1064 nm for example --which is
pumped by GaAs semiconductor lasers at 800 nm-- as a
pumping light source, thereby compensating for the lowquantum conversion efficiency. What is more, intensive
research 6) is going on to obtain higher outputs, in which
high-power semiconductor lasers 4) at 980 nm in multimode are employed to pump a double-clad fiber. The fiber
has a core co-doped with Er and Yb together with the firstand second-clad layers, such that the first clad supports
the propagation of the pumping light in multi-mode, while
the core the propagation of the signal light in single-mode.
On the other hand, the use of pumping lights in the1480-nm range presents the following advantages. (1) Ahigh quantum conversion efficiency of as high as 90% is
obtained since the pumping wavelength is close to the signal wavelength. (2) A broad pumping wavelength range of
50 nm between 1450-1500 nm is available, permitting
wavelength multiplexing of pumping lights. The disadvantage is that their noise characteristics are inferior to thoseusing 980-nm pumping because a satisfactory population
inversion is difficult to be generated due to the stimulated
emission of Er ions in the 1480-nm range. It is possible toovercome this problem, however, by constructing a twostaged EDFA in which a preamplifier with an EDFA of
980-nm pumping is used 7) .
Methods of upgrading the output power of pumping lightsources in the 1480-nm range are known to include thoseof wavelength-multiplexing and polarization-multiplexing 8)of the 1480-nm semiconductor lasers. Moreover, a cascaded Raman laser 9) has been recently reported which
incorporates Raman amplification with a fiber laser. The
authors have been carrying out research and development of high-power EDFAs which use the 1480-nm semiconductor lasers as pumping light source, and haverecently succeeded in developing a high-power EDFA of
1.5 W signal output power. In this report, upgrading technologies of EDFAs which use the 1480-nm semiconductor
lasers as pumping light source will be presented.

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2.

FBG (Fiber Bragg Grating) is used as external cavity tostabilize the output wavelength 12) . Figure 1 shows aschematic of the module. The output of the laser is coupled into a pigtail fiber through collimating lens and focusing lens. Polarization maintaining fiber was selected as
the pigtail fiber for the purpose of polarization combiningof laser outputs. The FBG used for the developed laser
module measures 50 mm in length and 2.7 mm in diameter, and is placed 700 mm to 800 mm apart from the module.
Figure 2 shows the output vs. driving current characteristics together with the output spectrum of the module driven at a sub-mount temperature of 25°C. Data for boththe laser modules with (Fig.2 (b)) and without (Fig.2 (a))
FBG for wavelength stabilization are shown for comparison. The driving current characteristics in the upper half of
Figure 2 shows that the output has no kinks, irrespectiveof being with or without FBG, up to 800 mA in driving current; that an output power of 160 mW is obtained at a driving current of 600 mA; and that the insertion of FBG
reduces outputs only slightly. The lower half of Figure 2
gives the results of output spectrum measurement at a driving current of 500 mA, showing that the output spectrum
has successfully been reduced from 5 nm to 1.5 nm
FWHM with the developed laser module due to the use of
the FBG.
Figure 3 shows the measured results of the driving current dependence of center wavelength. The center wavelength of the developed laser module has a small dependence on the driving current, so that more stabilized characteristics in output combining are expected to come compared to conventional laser modules in which no FBG is
used.

CHARACTERISTICS OF 1480-NM SEMICONDUCTOR LASER FOR PUMPING

In order to realize high-power EDFAs, the authors areinvestigating the technology of high-power injection inwhich pumping lights are combined by wavelength multiplexing prior to injection into EDFs. While ordinary semiconductor lasers in the 1480-nm range have a center
wavelength located between 1460 nm and 1490 nm, they
conventionally have a wide output spectrum of around 5
nm FWHM since they oscillate in a Fabry-Perot mode.
This wide output spectrum leads to imposing limitations on
the wavelength spacing at wavelength multiplexing. In
terms of loss also, if the output spectrum of the pumping
light is wider than the transmission spectrum of the wavelength-multiplexing device, the loss in effective pumping
power will become significant. Furthermore, the combining
efficiency disadvantageously depends on the driving current of the laser since the current causes shifts in the center frequency.
In an effort to solve these problems, the authors havedeveloped a semiconductor laser module, in which an

Fiber length~2.0 m
Fiber

2nd Lens 1st Lens
LD
TEC

FBG

Package

50.0
module~FBG:700 mm~800 mm

Schematic of stabilized laser diode module

300

300

250

250

Output Power (mW)

Output Power (mW)

Figure 1

200
150
100
50

200
150
100
50

0

0
0

100

200

300

400

500

600

700

0

800

200

300

400

500

600

700

800

LD Current (mA)

1440 1450 1460 1470 1480 1490 1500 1510 1520 1530

1440 1450 1460 1470 1480 1490 1500 1510 1520 1530

Wavelength (nm)Wavelength (nm)(a) Without FBG(b) With FBG

Figure 2

12100Power (10 dB/DIV)Power (10 dB/DIV)LD Current (mA)

Comparison of IL characteristics and output spectra of laser modules

Furukawa Review, No. 19. 2000

quently by wavelength multiplexing using the two-channel
and three-channel WDM couplers for the first-stage andsecond-stage EDFAs respectively. In all, 20 laser modules
were used. The center wavelengths of pumping lasers
were 1465 nm and 1490 nm for the two-channel combination, and 1463 nm, 1478 nm, and 1493 nm for the threechannel combination. The pumping light power into the
EDF was 880 mW at the three-channel combination inwhich six laser modules were used. To deal with this highpower, the FC connector with an angled end face wasused at the output port of EDFA, and an optical isolator
was inserted into the midpoint of EDFs, thereby suppressing oscillation due to multiple reflections. The measurement method for the output was such that a calorimeter
was used for power measurement, and an optical spectrum analyzer was used for simultaneous measurement of
the output spectrum branched through an 18-dB coupler.
The optimization of EDF is another important element indesign technology. In this development, an EDF was
designed to maximize its amplification efficiency at 1480nm pumping, and a new EDF with a numerical aperture(NA) of 0.23 and a core diameter of 5.5 µm has beendeveloped. In terms of conversion efficiency from pumping
light to signal light using this EDF, a single-stage EDFA
with 1480-nm pumping has achieved a quantum conversion efficiency of 91% and a power conversion efficiencyof 86%.

Center Wavelength (nm, RMS)

1500
without FBG
with FBG

1495

1490

1485

1480

1475
0

100

200

300

400

500

600

700

800

LD current (mA)

Figure 33.

Driving current dependence of center wavelength

STRUCTURE AND CHARACTERISTICS
OF HIGH-POWER OPTICAL FIBERAMPLIFIER

The developed EDFA uses semiconductor lasers
described above, in which the FBG is employed to stabilize the wavelength. EDFAs of bi-directional pumping arecascaded to constitute a two-stage configuration, as
shown in Figure 4.
The pumping lights were combined first by polarization
combining the outputs from polarization maintaining fibers
using PBC (Polarization Beam Combiner), and subseWDM coupler

Isolator
EDF

18-dB coupler

EDF

Input port

Output port
PBC CalorimeterTwo-channel WDM-LD unit

Optical spectrum
analyzerThree-channel WDM-LD unit
WDM coupler
WDM coupler
PBC
1463 nm LD

WDM coupler
PBC
1465 nm LD

1478 nm LD

1490 nm LD

1493 nm LD

Two-channel WDM-LD unit

Schematic diagram of high-power EDFA

40
30
20
10
0
-10
-20
-30
-40
1550

1555

1560

1565

Output Power (dBm)

35

1.6
1.5
1.4
1.3
1.2
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4

Power (dBm)

Output Signal Power (W)Figure 4

Three-channel WDM-LD unit

1570

Wavelength (nm)

0

0.5

1

1.5

2

2.5

3

33

31

29

27

25
1540

Injected pumping power at EDF ends (W)Figure 5

Output characteristics of high-power EDFA

1550

1560

1570

1580

Wavelength (nm)Figure 6

Wavelength dependence of the output power

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The length of the EDF was also optimized to have amaximum output, and it was found that the optimum
lengths for the first-stage and second-stage EDFAs were
93 m and 34 m respectively. Figure 5 shows the output
characteristics of the EDFA vs. pumping power injected at
the EDF ends together with the output spectrum. The
input optical signal is 1560 nm in wavelength and +7.2
dBm in power. The ratio of optical signal power to total
optical output power was calculated as 0.99 or more --ahigh value, indicating that the EDFA was operating at ahighly saturated state. The power of the optical signal output was found to increase linearly in proportion to theincrease of the pumping light power, and 1.5 W was
attained at a pumping light power of 2.56 W. The power
conversion efficiency was 59% --from the pumping light tothe signal light measured at the input and output ports ofthe EDFA-- and the slope efficiency 72%. Compared withthe single-stage configuration, the developed EDFA has alower power conversion efficiency. This is mainly due tothe insertion loss of the optical devices inserted at the
midpoint between the EDFAs, in addition to the fact that -since conversion efficiency depends on the wavelength ofthe pumping light-- the conversion efficiency is lower in
the near 1465-nm range than in the 1480-nm range.
Figure 6 shows the wavelength dependence of the signal
light power at an input signal power of -3 dBm. It is seenthat a signal light power of 1 W (30 dBm) is obtained in a
bandwidth of 30 nm at least, with its peak at 1560 nm.

4.

TASKS FOR THE FUTURE

We will briefly discuss the importance of both high-power
pumping lasers and the wavelength multiplexing technology of pumping light, which are necessary to realize highoutput powers of several watts with the EDFAs pumped in
the 1480-nm range.
4.1 High-Power Pumping Laser
It is difficult to upgrade the conversion efficiency with the1495 nm1465 nm
1480 nm
1450 nm1557.5 nm
1487.5 nm
1472.5 nm
1502.5 nm

EDF, which is approaching the theoretical limits. Highpower laser modules are therefore needed to obtain higher output powers. At present, modules with wavelengthstabilized semiconductor lasers having an output power of
180 mW --fiber output-- at a driving current of 600 mA arecommercially available. To have higher outputs in the
future, the present technology has to be examined interms of the cooling efficiency of the laser module and thereliability assurance at large current driving.
4.2

Wavelength Multiplexing Technology of
Pumping Light
So far, WDM couplers based on dielectric filters havebeen used as wavelength multiplexing device. With thistype of WDM couplers, however, the required number ofcouplers increases as the number of multiplexing increases --two or more WDM couplers are required for multiplexing three or more waves. The loss of pumping light before
being injected into EDF will become significant, and thegreater the number of multiplexing, the larger the loss willbe. What is more, from the standpoint of downsizing
EDFAs, it is desirable to use an optical circuit which
includes fewer fiber joints enabling circuit integration. In
an effort to develop a new device, the authors are investigating the application of PLC (Planar Lightwave Circuit)
formed on silica substrates, which allows higher degree ofmultiplexing of pumping lights while maintaining small
combining losses. A prototype of the PLC device has
been fabricated, which combines eight waves by wavelength multiplexing using Mach-Zehnder interferometers 13).
Figure 7 shows the schematic diagram of the new PLC
device. The device aims to combine eight outputs ofwavelength stabilized pumping lasers so as to obtain apumping output spanning from 1450 nm to 1502.5 nm with
7.5-nm spacing. The difference in relative refractive index
of TiO2-doped waveguide is 0.4%, and the dimensions are
3.4 mm in width, 74.0 mm in length, and 2.0 mm in height.
Figure 8 shows an appearance of the wavelength multiplexing device based on the Mach-Zehnder interferometer. Figure 9 gives the transmission spectrum showing aminimum insertion loss of 0.9 dB for every port. The effective combining loss of pumping power due to the transmission wavelength limitation at each port was found to be
1.2 dB or less.
0LD

Figure 7

FBG

Schematic diagram of PLC Mach-Zehnder interferometric wavelength multiplexer

Input Port

PLC

Output Port

Transmittance (dB)-1

PLC-MZI type optical combiner

-2

-3

-4

-5
1450.0 1457.5 1465.0 1472.5 1480.0 1487.5 1495.0 1502.5

Wavelength (nm)Figure 9
Figure 8

14

Appearance of PLC wavelength multiplexer

Transmission spectrum of PLC Mach-Zehnder interferometric multiplexer

Furukawa Review, No. 19. 2000

5.

CONCLUSIONS

A high-power fiber-optic amplifier with an output of 1.5 W
has been developed based on the pumping technology
which combines 1480-nm semiconductor lasers throughwavelength multiplexing. The amplifier has achieved an
output of 1 W or greater at a wavelength bandwidth ofover 30 nm, thus demonstrating improvements in thenumber of waves to be multiplexed together with the combining efficiency of pumping lights based on the use of
wavelength stabilized 1480-nm pumping lasers.
Furthermore, a prototype PLC combiner based on MachZehnder interferometer has been fabricated, which is
expected to improve the efficiency in wavelength multiplexing. In the future, we plan to develop pumping units
and high-power fiber-optic amplifiers using the PLC technology based on the Mach-Zehnder interferometer.
We would like to thank Mr. Masaru Fukushima for his
helpful suggestions in preparing this paper.

REFERENCES

1) Oshima, et al.: Proc. Suboptic'97, (1997), p. 898.
2) T. Araki et al.: Technical Report of IEICE, SAT96-7, (1996), p.
43. (in Japanese)
3) T. Yamamoto et al.: Trans. IEICE, J81-C-I, (1998), p.148. (inJapanese)
4) L. Benjamin et al.: Proc. Optical Fiber Communication
Conference (1997), FC3.
5) V. P. Gapontsev et al.: Optics and Laser Technol., August,(1982), p. 189.
6) J. D. Minelly et al.: IEEE Photon. Technol. Lett., vol. 5, (1993),
p. 301.
7) Y. Tashiro et al. : Proc. Optical Fiber Communication
Conference (1997), WA5.
8) S. Shikii and Y. Tamura: Proc. IEICE, Spring Conference, C623,(1989), p. 412. (in Japanese)
9) G. Grubb et al.: Proc. Optical Amplifier and their Applications(1995), SaA4, p. 197.
10) M. Fukushima et al.: Proc. Optical Amplifier and their
Applications (1997), TuD3.
11) Y. Tashiro et al.: Proc. Optical Amplifier and their Applications(1998), WC2.
12) S. Koyanagi et al.: Proc. Optical Amplifier and their
Applications (1998), MC2.
13) K. Tanaka et al.: Proc. Optical Fiber Communication
Conference (1999), TuH5.
Manuscript received on June 28, 1999.

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