Development of Er/Yb Co-doped Fiberfor High-Power Optical Amplifiers
by Keiichi Aiso *, Yoshio Tashiro *2, Tsuneo Suzuki *3 and Takeshi Yagi *The increasing capacity of wavelength-division multiplexing (WDM) transmission
systems requires higher optical output power of erbium doped fiber amplifiers(EDFAs). Higher output causes in the EDFA adverse nonlinear effects --four-wave mixing (FWM)
and cross-phase modulation (XPM) etc.-- that result in signal distortion. In the past fiber parameters of Er-doped fibers (EDFs) have been optimized from the standpoint of improving amplification efficiency, but designs that improve efficiency increase nonlinearity. This makes it difficult toachieve low nonlinearity with EDF. Here the authors have developed fibers that are co-dopedwith erbium and ytterbium. This Er, Yb co-doped fiber (EYDF) successfully realizes low nonlinearity without degrading amplification efficiency, and when pumped using a 1480-nm laser diode,EYDFAs achieve both lower nonlinearity and higher output power.

ABSTRACT1.

INTRODUCTION

There has been a rush to expand the capacity of telecommunications services resulting from the increased demanddue mainly to the Internet. This has been achieved bothby increasing the number of channels and by expanding
the transmission band. Expansion of WDM systems thus
makes the capacity and bandwidth of fiber-optic amplifiers
key aspects of the system.
Increasing the number of channels of a WDM transmission system requires an increase in total input signal
power, resulting in a need for higher output power from
EDFAs, which provide batch amplification of 1550-nm
band WDM signals.
EDFA output power has grown in step with greater EDF
efficiency and increased pumping laser diode power.
Furukawa Electric, in an effort to achieve good conversion
efficiency under highly pumped conditions, is working to
optimize EDF fiber parameters and has achieved a quantum conversion efficiency from pumping power to signal
power of 90% or more 1). The output power of 1480-nm
laser diodes has been increased 2), and the technologies
of wavelength multiplexing and polarization multiplexing of
the laser diodes have been applied to obtain increased
pumping power 3). By these means it has been possible todesign EDFAs with signal power of 1.5 W 4).
Several high-power pump sources have also been proposed for laser diodes of other than 1480 nm. There is, for* Optical Fiber Gr., Research Dept., FITEL-Photonics Lab.
*2 WP Team, FITEL-Photonics Lab.
*3 TF Team, Chiba Optical Fiber Production Dept., OpticalFiber Div.

example, a report of a 1480-nm cascaded Raman resonator (CRR) being used as a pump source 5).
In addition to 1480 nm, 980 nm is also in general use as
the pumping wavelength for EDF, but the pump-to-signal
conversion efficiency is lower than for 1480 nm and theabsorption spectrum is also narrower. This makes wavelength multiplexing difficult, as well as disadvantageous interms of increasing the output power of EDF.
To broaden the range of pumping power wavelength,
consideration was given to EYDF, in which Yb is co-dopedwith Er. In EYDF, energy transfer from the excited state of
Yb to that of Er is utilized to form population inversion
between lasing levels of Er and the signal is amplified
through stimulated emission. Yb has a wide absorption
band from 800 to 1100 nm, and the use of a 1064-nm
Nd:YAG laser, and more recently of an Nd-doped doubleclad laser, as the pumping light source 6), 7) has beenreported.
As can be seen from the above, the output power of the
EDFA is largely dependent on the power of the pumping
light source. EDF core diameter, however, ordinarily
ranges from a few micrometers to 10 µm at the most, andthis restricted cross-section places an inherent limitationon the pumping power that can be coupled. To relax thislimitation a double-clad amplifier has been proposed, inwhich the signal light propagates in single mode in thecore and the pumping light propagates in the first cladding
around the core in multimode 8), 9). Such an amplifier, whichis "side-pumped" configuration, is less efficient than a configuration in which the pumping light is directly coupled tothe core, but, since it can use Watt-class multimode laser
diodes as the pumping source, gives promise of realizing
Watt-class amplifier power with comparative ease.
The realization of such Watt-class fiber-optic amplifiers

41raises a new problem of generating nonlinear effects in
EDF, which in the past was negligible. Accordingly theauthors embarked on a study of how to minimize nonlinearity in EDFs for high-power amplifiers without suffering
degradation in amplifier efficiency. This paper reports ontechniques for reducing nonlinearity in Er-doped fibers forhigh-power optical amplifiers.

2.

REDUCING NONLINEARITY IN EDF

With the increasing number of WDM channels and thehigher optical power per channel, there has been a conspicuous increase in such nonlinear optical effects asfour-wave mixing (FWM) 10) and cross-phase modulation(XPM) in EDFA 11). L-band EDFA is also coming to be
used in addition to C-band, but because of its smaller gain
coefficient it requires a greater length of EDF. This meansthat nonlinear effects arise more readily in L-band EDFAs
than in C-band 12), 13). The degree of signal distortion due tononlinear phenomena in EDF may be estimated roughly
using Equation (1) 14).

κP0Leff =κP0
2π n2 P0
[exp(gL) - 1] =[exp(gL) - 1]g
λ Aeff g(1)

where: κ is the nonlinear coefficient; Leff is the effective
length; n2 is the nonlinear refractive index; Aeff is the effective core area; P0 is the averaged input power per channel; g is the averaged gain coefficient; and L is the lengthof the EDF.
As will be readily understood from Equation (1), theappearance of nonlinear phenomena can be inhibited byincreasing Aeff and g. Accordingly the values of P0 and G(= exp(gL)) are selected in accordance with the EDFA
application.
Typically the value of Aeff is about an order of magnitude
smaller for an EDF than for a conventional single-mode
fiber, because relative refractive index difference ∆ of EDF
is set up higher to improve gain efficiency. Conversely,
increasing the Aeff of the EDF will result in degradation in
the power conversion efficiency (PCE) from pumping
power to signal. Accordingly A eff must be optimallydesigned in accordance with pumping power, so that there
is no degradation in PCE.
In order to increase the gain coefficient g in an EDF, theamount of doped Er ion must be increased, but higher Erconcentration leads to concentration quenching and aconsequent degradation in amplification efficiency. It is
generally held that limitation on Er concentration for theoccurrence of concentration quenching is reported to be
several hundred ppm by weight in the case of a pure SiO2
host, and about 1,000 wt ppm even for an Al2O3-SiO2
host, which has been co-doped with aluminum to inhibit
concentration quenching 15). For this reason there are limits
to the improvement in gain coefficient g that can be madeby increasing Er concentration.
Thus when it is desired to use EDF of conventional
design in a high-power amplifier, it is very difficult to42

achieve both low nonlinearity and high conversion efficiency, and it was therefore necessary to adopt a novel design
technique.

3.

INHIBITING CONCENTRATIONQUENCHING IN EDF

If the doping concentration of Er in EDF is increased, thepumping efficiency of the EDFA is reduced because ofconcentration quenching. This is because when the distance between Er ions decreases, any two adjacent ions
excited at the 4I13/2 state interact in a process known ascooperative upconversion, whereby energy is transmitted
from one ion, which makes a transition to the 4 I 15/2
(ground) state, while the other ion is excited to the 4I9/2
state.
The ion excited to the 4 I 9/2 state decays, through aprocess of nonradiative transition (or multiphonon decay),
to the 4I13/2 state, with a resultant decrease in quantum
conversion efficiency (QCE) 16).
Cooperative upconversion is related to the way in which
the Er ions are introduced into the SiO2 glass network. Er
ions have a low level of solubility to SiO2 glass so thatwhen they are doped to a pure silica core they form clusters, maintaining the charge balance and dissolving into
the silica network 17). This clustering reduces the distance
between ions, resulting in cooperative upconversion.
Concentration quenching caused by cluster formation is
known as pair-induced quenching (PIQ) 18), and efforts to
reduce PIQ involve changing the composition or fabrication method of the core. The most commonly used methodof accomplishing this is by co-doping with Al 2 O 3 .
Aluminum ions surround the Er ions to form a solvent
shell, thereby adjusting the charge balance and improving
the solubility of Er ions to the host. This inhibits clusteringof the Er ions. It is known that with a pure SiO2 host concentration quenching occurs at an Er concentration of several hundred ppm by weight, but that when co-doped withaluminum, concentration quenching is inhibited up to Er
concentrations of about 1,000 wt ppm 15). Recently a report
has also appeared of a case in which concentrationquenching was inhibited by changing the core host glass
to increase Er solubility 19).

4.

CO-DOPING WITH Yb TO INHIBIT PIQ

EYDF, which is fiber co-doped with Yb as well as Er, canbe pumped by a high-power laser--Nd:YAG, etc.--placing
it in contention for use in high-power optical fiber amplifiers 6). High-power laser diodes are available for both
1480 and 980 nm wavelengths, so that EDF is used in virtually all the optical amplifiers presently used for telecommunications applications. Recently, however, attention isreturning to EYDF as a fiber for use in double-clad amplifiers 8).
Figure 1 shows a model of energy levels in an Er-Yb
system. In an EYDF, Yb ions are excited by pumping light

Furukawa Review, No. 20

2001

400

Energy Transfer

Pumping0.8-1.1 µm

4I

11/2

4I

13/2

Signal Power (mW)EYDF L=7 m

2F
5/2

Stimulated
Emission

1.55 µm

4I

2F
7/2

300

200

100

0

15/2

0

Er3+

Yb3+

100

200

300

400

500

Pump Power (mW)

Figure 1

Model of energy levels in an Er-Yb system
Figure 3

Pumping power dependence of output signal powerfor EYDF pumped at 1480 nm

12.5
EDF (Er:650wtppm)

12

EDF (Er:1000wtppm)

Lifetime (ms)11.5

EYDF (Er:2000wtppm)

11
10.5
10
9.5
9
8.5
8
1

2

3

4

5

6

7

Pump power (mW avg.)

Figure 2

Pumping power dependence of fluorescence lifetimeof 800-1,000 nm to the 2F5/2 state, after which Er ions are
excited to the 4I11/2 state through energy transfer from theYb ions, while the Yb ions revert to the 2F7/2 state.
The Er ions that were excited to the 4I11/2 then make atransition to 4I13/2 by a nonradiative process, forming a
population inversion between the 4I13/2 and 4I15/2 states
and amplifying the incident optical signal by stimulated
emission.
Yb ions as well as Er ions have low solubility to a silica
host, and since both have about the same ionic radius,
they cluster together. Thus the distance between Yb ionsand Er ions is reduced and energy transfer proceeds withgood efficiency 20).
This means that if a number of Yb ions surround Er ionsto form a cluster, the distance separating the Er ions fromeach other would increase, and this should produce a
reduction in the PIQ occurring between Er ions.
Recognizing this the authors conducted a study of theinhibiting effect of co-doping with Yb.
The EYDF evaluated in this study had an Er concentration that had been raised to 2,000 wt ppm. To determine
whether or not concentration quenching was taking place,measurements of the fluorescence lifetime at the 4I13/2
state were made for the EYDF and for two types of conventional EDF, with Er concentrations of 650 and 1,000 wt
ppm. When concentration quenching occurred, it wasobserved that the fluorescence lifetime was shortened.
Figure 2 shows the experimental results. For the EDF withan Er concentration of 1,000 wt ppm it was observed thatas pumping power increased the fluorescence lifetime
became shorter. This is attributed to an increase in thenumber of Er ions in the excited state as the pumping
power increased, resulting in cooperative upconversion. It
may therefore be concluded that in the EDF with an Erconcentration of 1,000 wt ppm, concentration quenching
was already occurring. In the EYDF, by contrast, where Er
concentration was 2,000 wt ppm, there was no correlationof fluorescence lifetime with pumping power, and it isthought that no marked cooperative upconversion
occurred. This result makes it obvious that Yb co-doping
makes it possible to increase the limit on Er doping concentration (up to 1,000 wt ppm) that was imposed by concentration quenching in EDF.

5.

1480-nm PUMPED EYDF

As we have pointed out, EYDF has the advantage that it
can make use of the broad absorption band of Yb, thereby
broadening the range of pumping wavelength. The Er-Yb
co-doped amplifiers (EYDFAs) that have heretofore beenreported have, obviously enough, been pumped by using
a light source with a wavelength matching the absorption
band of Yb. In such cases the energy transfer efficiency
from the 2F5/2 state of the Yb ions to the 4I11/12 state of the
Er ions is largely dependent on such factors as the concentration ratio of Er to Yb ions and the core material, andwe may say that to improve the conversion efficiency frompumping light to signal light, optimization of the composition of the core is indispensable 21). When the EYDF is
excited at 980 nm, it passes through a complex energy
transition process involving back-transfer from the 4I11/2
state to the 2F5/2 state and emissions between the energy
states of the Yb, so that the conversion efficiency will
inherently be lower than that of EDF.
Accordingly the authors carried out experiments on thepumping of EYDF using a 1480-nm laser diode, which isunaffected by Yb absorption. It is thought that with 1480nm pumping the Yb ion is virtually inactive optically, andwe can expect that it will play a role as a spacer for Er
ions, that is to say a PIQ inhibitor.
Figure 3 shows the results of measurements of thepumping power dependence of the output signal power for
an EYDF pumped at 1480 nm. The occurrence of concentration quenching brings about a degradation in the power

431480 nm/1550 nm WDM coupler

EDF or EYDF
Isolator

WDM coupler

1480 nm band LD

Figure 4

Type of fiberCore composition
Er concentration (wt%)
Peak absorption(dB/m)@1530 nm
MFD (um)@1550 nm
Chromatic dispersion(ps/nm/km)@1550 nm

Characteristics of Fibers

29.5

EYDF

EDF

Er2O3-Yb2O3-

Er2O3-Al2O3-

Al2O3-GeO2-SiO2

GeO2-SiO2

0.2

0.065

22.7

2.4

7.0

5.8

-5

-25

Total signal output power (dBm)

Table 1

Schematic diagram of WDM amplifier

6.

WDM SIGNAL AMPLIFICATION CHARACTERISTICS

Figure 4 shows a schematic diagram of the amplifier used
to evaluate the WDM signal amplification characteristics,
and Table 1 shows the characteristics of the fibers (EYDF
and EDF) evaluated in this study.
The amplifier was of a two-stage configuration, pumped
by wavelength-multiplexed 1480-mn laser diodes. The
two-stage EDF / EYDF was pumped with a total launched
power of 1.56 W from eight high-power 1480-nm laser
diodes. Fiber lengths were 30 m (1st stage) and 51 m
(2nd stage) for the EDF, and 4.5 m (1st stage) and 7 m(2nd stage) for the EYDF.
Using the configuration described above, 8-channel
WDM signals with 2-nm spacing were applied and the

44

28.5
28
27.5
27

Test EDF

26.5

Test EYDF

26
- 20

- 15

- 10

-5

0

5

10

15

Total signal input power (dBm)

Figure 5

conversion efficiency (PCE) from pumping power to signal
power. Thus measurements of the value of PCE may betaken as one indicator of whether conversion quenching isoccurring or not.
The measurements were carried out using a singlestage amplifier configuration bi-directionally pumped at
1480-nm, at a signal wavelength of 1560 nm and a signal
power of 0 dBm. Optimum length of EYDF was 7 m. PCE
in the EYDF calculated from these results was 76% -comparable to conventional EDF. Again it can be seenthat there is no marked concentration quenching in this
EYDF.

29

Signal input power dependence of output signal
power

nonlinear effects under high-power operating conditions
were evaluated. Figure 5 is a graph plotting the signal
input power dependence of output signal power.
Maximum total output power reached 28.8 dBm (760mW), or approximately 20 dBm per channel. This is anextremely high power characteristic to be achieved with
only eight laser diodes, and was only made possible bymaking use of high-efficiency EDF, high-power diodes andhighly efficient wavelength multiplexing technology.
Figures 6 and 7 show the output spectra of the EDF and
EYDF, and a comparison of the two reveals a clear difference in the generation of nonlinear phenomena. Although
clear evidence of four-wave mixing is apparent in the caseof the EDF, virtually none shows in the EYDF spectrum.
The total length of fiber used in the amplifier was 81 m for
EDF and 11.5 m for EYDF, and this length difference had
a major impact on the generation of FWM. That is to say,by raising the limitation on Er doping concentration
through co-doping with Yb, it was possible to reduce
effective length Leff (or in other words to increase gain
coefficient g), thereby suppressing nonlinearity below thelevel in the conventional EDF.
This demonstrates that EYDF has great possibilities asa fiber for high-power amplifiers that also provide low nonlinearity.

Furukawa Review, No. 20

2001

30

20

20

10

10

Power (dBm)

Power (dBm)

30

0
-10

-10

-20

-20

-30

-30

-40

-40

1544 1546 1548 1550 1552 1554 1556 1558 1560 1562

1544 1546 1548 1550 1552 1554 1556 1558 1560 1562

Wavelength (nm)Wavelength (nm)Figure 67.

0

Output spectrum of EDF

CONCLUSION

It has been found possible, by co-doping EDF with Yb, toinhibit the degradation in conversion efficiency associatedwith concentration quenching, while at the same time raising the limitation on Er doping concentration for conventional EDF. Use of the EYDF developed in the present
work makes it possible to achieve a high-power amplifierthat combines both low nonlinearity and high conversion
efficiency--characteristics that have conventionally been ina trade-off relationship.
Studies are already under way on the possibility of
applications to L-band, on optimization of core composition for even higher Er doping concentrations, and onlower nonlinearity, including amplifier fiber parameters.

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Manuscript received on November 2, 2000.

45