Gain-Flattening Filters with Autonomous Temperature
Stabilization of EDFA Gainby You Mimura *, Kazuyou Mizuno *, Syu Namiki *, Yoshio Tashiro *2,
Yoshihiro Emori *2, Mariko Miyazawa *, Toshiaki Tsuda *2,
Kazuhiko Nishiyama *2,Yasuo Uemura *2 and Toshihiko Ohta *2With the steadily increasing demand for fiber-optic communications, hope is more
and more being placed in the method known as wavelength division multiplexing,or WDM. The key device supporting WDM technology is the erbium-doped fiber amplifier (EDFA),and such amplifiers need to have a flat gain spectrum. A problem arises, however, in the fact thatthe loss profile and gain spectra of erbium-doped fiber are temperature dependent, resulting intemperature-dependent changes in the EDFA gain spectra. The authors took note of long-period
fiber gratings having a large wavelength shift temperature dependence, and have combined thegrating with a conventional gain-flattening filter (GFF) in a compound GFF that provides stabilization of EDFA gain spectra over a wide range of temperatures.

ABSTRACT1.

INTRODUCTION

With widespread use of Internet services, major attention
has focused in recent years on wavelength division multiplexing (WDM), a technique whereby optical signals of anumber of different wavelengths are multiplexed for transmission. The erbium-doped amplifier (EDFA) is a key
device in WDM systems. Its comparatively wide wavelength range of amplification allows it to provide batch
amplification of the signals within the wavelength range,
making it essential as an amplifier of transmission in WDM
systems.
In WDM systems the deviation in power level of the signals causes a decrease in transmission distance andwavelength range, requiring that the EDFA gain spectra
be flat within the signal range. EDFAs, however, makeuse of input signal amplification by stimulated emission
from erbium ions, so that, reflecting the fine structure ofion energy levels, the gain spectra have an asymmetricaltwin peaks within the amplification range. The usual way
to flatten gain is to use a gain-flattening filter (GFF), andthe EDFA gain is flattened by inserting into it a fiber having a loss profile that is the reverse of the gain spectra of
the EDF.
There is, however, a temperature-dependent wavelength shift in the loss profile of the GFF, and there is also
a temperature dependence in the EDF gain spectra. Thismeans that for EDFAs, temperature dependence is partic-* WP Team, FITEL-Photonics Lab.
*2 No.1 Development Gr., Optical Components Dept., FITELProducts Div.

ularly large in the region of 1530 to 1535 nm 1). Currently
steps are being taken to stabilize gain by controlling thetemperature of the EDFA as a whole, but there are problems in terms of unit size and power consumption, raisingcalls for a technological breakthrough.
In this study the authors took note of long-period fiber
gratings, which have a large wavelength shift temperaturedependence 2), and have combined the grating with a conventional gain-flattening filter in a compound GFF that provides stabilization of EDFA gain spectra over a wide rangeof temperatures 3). This paper discusses on the principle oftemperature stabilization of gain, and reports on the fabrication of a prototype.

2.

TEMPERATURE-DEPENDENCE OF
GAIN IN CONVENTIONAL EDFAs

Normally, the flattening of EDFA gain is accomplished byintroducing an optical GFF having a loss profile that is thereverse of the gain spectra of EDF. A number of types of
optical GFF have been proposed --using etalon filters 4)-6),dielectric filters 7) , long-period fiber gratings 8) , MachZehnder silica waveguides 9), split-beam Fourier filters 10),and so on.
In all of them, there normally occurs a wavelength shift
in the loss profile due to changes in the temperature of themedium. And while the gain spectra of EDF is also dependent on temperature, it accords with changes in ion energy levels, so that the shape of the gain spectra changes
with temperature, but does not manifest itself in the formof a wavelength shift.
Figure 1 shows a typical example of the temperature

21

EDF

Gain (dB)

17
16
15
14
1530

1540

1550

1560

0.5
0-25°C65-25°C

0.0

e)

- 0.5
1530

1540

Wavelength (nm)Conventional GFF

c)

- 2.0
- 3.0
0°C
25°C
65°C

- 4.0

1540

1550

1560

Wavelength (nm)d)
0-25°C65-25°C

0-25°C65-25°C

0.5

0.0

- 0.5
1530

1540

1550

1560

Wavelength (nm)

0.0

- 0.5
1530

EDF+
Conventional GFF

1540

1550

1560

Wavelength (nm)Figure 1

Temperature dependence of EDF and GFF

dependence of the gain spectra of an EDF and the loss
profile of an optical gain-flattening filter (GFF). Here theamount of increase in EDFA gain due to temperature riseis defined as the temperature coefficient, and is given a
positive value when gain increases with a rise in temperature, and a negative value when it decreases. The plus
signs (+) in the figure show the wavelength range in which
the temperature coefficient is positive, and the minus
signs (-) show the range in which it is negative. The temperature coefficient of the gain spectrum of the EDF is
positive below 1540 nm and negative above that wavelength, whereas a temperature-dependent wavelength
shift toward the positive direction can be observed for the
optical GFF, so that the temperature coefficient is positiveat wavelengths of 1535 nm and below, negative at from1535 to 1540 nm, and positive above 1540 nm. Thus in an
EDFA combining EDF gain and an optical GFF, the temperature dependence of the EDF gain spectra and of theloss profile of the optical GFF cancel each other out at
wavelengths above 1535 nm, whereas at wavelengths
below 1535 nm, their temperature dependences are mutually reinforcing (see Figure 1-e).
Recently a dielectric filter type of GFF has been developed in which temperature dependence of the wavelength
shift is made virtually negligible by adjusting the stressbetween the dielectric membrane and the substrate, butsince in this design there is a residual temperature dependence of EDF gain, no complete solution can be achieved.
Attempts have also been made to stabilize gain by controlling the temperature of the entire EDFA, but this haspresented problems in terms of unit size and power consumption.

22

1560

0.5

Transmittance variationfrom 25°C (dB)

Transmittance (dB)

- 1.0

- 5.0
1530

1550

Wavelength (nm)Gain variation from 25°C (dB)

0°C
25°C
65°C

b)
Gain variation from 25°C (dB)

a)
18

3.

TEMPERATURE STABILIZATION USING
LONG-PERIOD GRATINGS

3.1 Temperature Dependence
The temperature dependence of filters other than longperiod fiber filters may be expressed as the wavelength
shift due to temperature T at a given wavelength λ using
the interference width l and the refractive index n
dλ
=λ
dT

(

1
l

.

dl
1
+
dT
n

. dn
dT

)

The first term relates to the coefficient of linear expansion and the second to the temperature dependence ofthe refractive index, and both are positive for ordinary
glass. For ordinary GFFs, the wavelength shift due to temperature is about 0.010 nm/°C at most. In dielectric multilayers it is necessary to add another term for the temperature dependence of the stress between the multilayer andthe substrate, and this term can be made negative by controlling the linear expansion coefficient of the substrate sothat the temperature coefficient can be made to approachzero indefinitely. But even if the temperature coefficient of
the GFF is zero, there will be a residual temperaturedependence of EDF gain, making it impossible to achieve
an EDFA with a low overall temperature dependence ofgain.
The temperature dependence of a long-period fiber
grating, on the other hand, may be expressed as a wavelength shift at center wavelength λc caused by temperature T, using grating pitch Λ, the effective refractive index
of the fiber in guided mode ncore, and the effective refractive index in the mth clad mode ncladm, according to the following Equation 11)

Furukawa Review, No. 20

2001

19
Gain (dB)

- 2.0
- 4.0
0°C
25°C
65°C

- 6.0
- 8.0
1510

1520

1530

1540

1550

1560

Wavelength (nm)Transmittance (dB)

Transmittance variationfrom 25°C (dB)

0.5
0-25°C65-25°C
0.0

1716

Erbium gain

-1
GFF

-2
-3
-4

Design value for GFF as a whole

-5
0
-1
-2
-3
-4
1530

Long-period grating

1540

1550

1560

Wavelength (nm)
- 0.5
1530

1540

1550

1560

Wavelength (nm)Figure 2

Figure 3

Characteristics of a GFF with long-period fiber grating

Temperature dependence of long-period fiber grating

dncore dncladmdλcdΛ= (ncore - ncladm)
+Λ
dT
dT
dT
dT

(

)

The first term, relating to the coefficient of linear expansion, is about the same magnitude as for an ordinary GFF,
but the second term, which relates to the temperaturedependence of the effective refractive index in guided andclad mode, Λ will be about 100 µm, as compared to λ/n ofabout 1 µm for an ordinary GFF. This means that for longperiod fiber gratings, the temperature coefficient is larger.
When a grating is made using ordinary fiber, the wavelength shift due to temperature for long-period fiber gratings is about 0.050 nm/°C, which is larger than for ordinary GFFs. Reducing the temperature coefficient would
normally entail addition of a dopant to the fiber 11) or theuse of a temperature-compensating package, but in thepresent work a long-period fiber grating with a large temperature coefficient was used without modification to stabilize the temperature dependence of gain for the EDFA
as a whole.
3.2

18

15
Tramsmittance (dB)

Transmittance (dB)0.0

Principle of Temperature Stabilization in a LongPeriod Fiber Grating
Temperature stabilization of EDFA gain requires a filter
having a negative temperature coefficient in the range of
wavelengths below 1535 nm, where the temperaturedependence of gain presents a problem. Since the wavelength shift in characteristics due to filter temperature riseis normally in the long-wavelength direction, we need a filter having characteristics with a slope that is the reverseof the loss profile of the GFF at wavelengths below 1535
nm. It is possible to compensate for the temperaturedependence of the EDFA gain spectra by combining along-period fiber grating having a large temperature coefficient, using it as a reverse-slope filter, with an ordinary
GFF having a small temperature coefficient.
When a filter having characteristics with a slope thereverse of the loss profile of an optical GFF is to be used
for temperature stabilization, gain flattening must be carried out so as to include a temperature-stabilized component. Since the temperature coefficient of the long-period
fiber grating is large, compensation for temperaturedependence can be effected by a filter with a small
reverse slope, and the amount of flattening GFF gain issmaller. Figure 2 shows typical temperature dependenceof a long-period fiber grating. It can be seen that toachieve a temperature stabilizing component of 0.4 dB
from 25 to 65°C at 1530 nm, a transmittance loss of about
1 dB is ample.
Figure 3 shows the characteristics of the components ofa temperature-stabilized EDFA with long-period fiber grating. To flatten EDFA gain, the GFF must also be provided
with a loss profile obtained by subtracting the loss component of the long-period fiber grating from loss that is thereverse of the wavelength dependence of EDF gain. The
long-period fiber grating used to compensate for the temperature dependence of the EDFA gain spectra has a loss
at 1530 nm of about 1 dB, so that the loss profile of the
GFF is not substantially different from loss profile required
when the GFF stands alone.
Since the temperature dependence of the loss profile ofthe long-period fiber grating increases on the short-wavelength side at 1540 nm or less, temperature stabilization
for EDFA gain can be effected at wavelengths below 1535
nm, a range that presented problems in the past. Further,at wavelengths above 1540 nm, the loss profile of thelong-period fiber grating is less temperature-dependent.
Thus conventionally there was no effect in the range inwhich the temperature dependences of the EDF gain
spectra and of the GFF loss profile cancel each other,while it is desirable that, for the canceling effect, that there
still be a certain amount of wavelength shift in the GFF.

23

4.

4.3 Prototype Characteristics
Figure 6 shows the characteristics of the prototype. The
period of the fiber grating was 400 µm, and the fourth cladmode was used. The temperature dependence of wavelength shift was 0.006 nm/°C for the GFF and 0.050
nm/°C for the long-period fiber grating.
It can be seen that the GFF of Figure 6 exhibits the loss
profile of a compound GFF consisting of a GFF plus longperiod fiber grating, and that it has a temperature dependence that is the reverse of EDF gain. There is a particular improvement at wavelengths below 1535 nm, where
EDFA gain temperature dependence has constituted aproblem, and the temperature dependence of gain
between 0 and 65°C was 0.25 dB, in contrast to 0.79 dB
before stabilization. Further, gain flatness was 0.60 dB orless for all temperatures from 0 to 65°C.
A prototype was also fabricated for an EDFA with a gainof 26 dB, and between 0 and 65°C, a result of 0.45 dB
was obtained for the temperature dependence of gain for
the EDFA as a whole.

PROTOTYPE

4.1 Appearance
Figure 4 is a photograph of a long-period fiber grating
module used in a temperature-stabilized GFF. It measures
47 mm in length by 3.5 mm in diameter. The GFF is thesame as the grating module in appearance and size.
4.2 EDFA Configuration
Figure 5 shows the configuration of the EDFA withautonomous temperature stabilization used in the prototype. Normally, gain flattening is accomplished by connecting the optical GFF in series with the EDF. In the present work a GFF and long-period grating were series connected in the center portion of a two-stage EDFA. The
prototype EDFA had a gain of 13 dB.

GFF

Long Period Grating

Isolator
EDF
WDM Coupler
980 nm LD 1

Photograph of long-period fiber grating module

18.0

- 2.0

17.0

0°C
25°C
65°C

16.0

1550

1560

0∞C
25°C
65°C

- 4.0

1540

1550

1560

11.0
1530

0.0

0-25°C65-25°C
1550

Wavelength (nm)Figure 6

1560

1550

1560

0.5

0.0

0-25°C65-25°C
- 0.5
1530

1540

Wavelength (nm)

0.5

1540

0°C
25°C
65°C

Wavelength(nm)Transmittance variationfrom 25°C (dB)Gain variation from 25°C (dB)

13.0

12.0

- 5.0
1530

0.5

24

14.0

- 3.0

Wavelength (nm)

- 0.5
1530

EDFA
15.0

Gain (dB)- 1.0Transmittance (dB)Gain (dB)

19.0

1540

Configuration of EDFA with autonomous temperaturestabilization

GFF(conventional GFF + long-period fiber grating)

EDF

15.0
1530

Figure 5

Gain variation from 25°C (dB)Figure 4980 nm LD 2

1540

1550

1560

0.25 dB

0.0

0-25°C65-25°C- 0.5
1530

Wavelength (nm)

1540

1550

1560

Wavelength (nm)Characteristics of EDFA used in a prototype GFF withautonomous temperature stabilization of Erbium gain

Furukawa Review, No. 20

2001

5.

CONCLUSION

In the past the problem of EDFA gain temperature dependence, arising from the temperature dependence of thegain spectra of EDF and of the loss profile of gain flattening filters, has presented a problem. By incorporating an
ordinary gain flattening filter (GFF) and a long-period fiber
grating having a large temperature coefficient, a compound GFF has been realized that stabilizes the temperature dependence of EDFA gain over a wide range of temperatures.
Prototypes of compound GFFs have been fabricated for
two types of EDFA, and the temperature stabilizing effectof long-period fiber gratings has been confirmed. The temperature dependence of EDFA gain between 0 and 65°C
was 0.25 dB for the EDFA with a gain of 13 dB, and 0.45
dB for the EDFA with a 26-dB gain, confirming the feasibility of fabricating EDFAs having gain of low temperaturedependence.

REFERENCES
1) N. Kagi et al.: J. Lightwave Tech. Vol. 9, 2 (1991), 261.
2) M. Vengsarkar et al.: J. Lightwave Tech. Vol. 14, 1 (1996), 58.
3) Tsuda et al.: Gain-flattenig filters with with autonomous temperature stabilization of Erbium gain, OAA '2000, IWA 4-1.
4) Takeda et al.: Gain flattening of optical amplifiers using etalon filter, Proc. Autumn 1995, IEICE, (1995), B-759. (in Japanese)5) Okuno et al.: Optical fiber amplifier and optical gain equalizer forwavelength division multiplexing transmission, Technical Reportsof IEICE, EMD96-42 (1996), 19-24. (in Japanese)
6) T. Naito et al.: Gain equalizer in long-haul WDM transmission
system, IEICE Trans. Electron., Vol.E81-C, 8 (1998), 1293-1300.
7) N. Shimojoh et al.: New gain equalization scheme in WDM opticalamplifier repeated transmission systems, OECC'96, Technicaldigest, 17B3-3 (1996), 120-121.
8) P. F. Wysocki et al.: Erbium-doped fiber amplifier flattened
beyond 40 nm using long-period grating, OFC'97, PD2 (1997).
9) H. Toba et al.: Demonstration of optical FDM based self healing
ring network employing arrayed-waveguide-grating ADM filters
and EDFAs, ECOC'94, (1994), 263-266.
10) J. W. Arkwright et al.: Custom designed gain-flattning filters withhighly reproducible spectral characteristics, OAA'99, ThD18-1(1999).
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Manuscript received on November 15, 2000.

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