Development of Etalon-Type Gain-Flattening Filterby Kazuyou Mizuno *, Yasuhiro Nishi *, You Mimura *, Yoshitaka Iida *,Hiroshi Matsuura *, Daeyoul Yoon *, Osamu Aso *, Toshiro Yamamoto *2,
Tomoaki Toratani *2, Yoshimi Ono *2 and An Yo *2

As a result of rapid increase of amount of traffic data and transmission rates intelecommunication, the wavelength division multiplexing (WDM) system has
been studied. An erbium-doped fiber amplifier (EDFA) which is a key component in this systemneeds flat gain spectra. One of the gain-flattening technologies is usage of an optical gain-flattening filter that has a reverse loss profile against the gain profile of EDFA. We have developed adesign method using nonlinear fitting upon an etalon-type gain-flattening filter, so we can design
a gain-flattening filter that has less error against any gain spectra than conventional methods.
Thereupon we have manufactured modules designed by the method, and we can get excellent
performance upon gain-flattening filters. In this paper, we demonstrate the design method andreport performances of several types of gain-flattening filters for optical amplifiers that have different gain spectra.

ABSTRACT1.

INTRODUCTION

As the Internet is spreading all over the world, telecommunication technologies to meet the rapid increase in traffic
capacity and rate are urgently needed. Conventional
means involves increasing the bit rate to expand traffic
capacity. Recently WDM system using different wavelength optical signals that has been investigated forenlargement of traffic capacity has been reported year
after year. One of the key devices supporting WDM system is EDFA, which amplifies light signals with stimulated
emission by pumping erbium ion doped in optical fiberwith pumping light. EDFA, which can amplify different signals spreading in a wide range, is needed as the amplifierof transmission in the system.
The gain spectrum of EDFA has asymmetrical twin
peaks, due to a luminescent spectrum caused by finestructure of the energy levels. The gain spectrum isn't flat,so there is power deviation between amplified signals. In
the long haul optical transmission systems, optical signals
are transmitted by a multi-amplifier system, so differences
between optical signal powers are accumulated. When
dividing optical signals for various wavelengths at thereceiver, other optical signals affect the signal as noise. If
there are differences between optical signals transmitted
by amplifiers, optical signals with low signal to noise ratio
are more and more deteriorated at WDM as that are transmitted by EDFA. So the transmission distance becomesshorter and the wavelength range of transmittance and**2

WP-Project Team, Opto-Technology Lab., R & D Div.
Production Technology Development Center, Plant andFacilities Div.

the number of signal channels decrease. Therefore, it isneeded that the gain of EDFA be flat in the range of signals for getting an adequate signal to noise ratio at eachwavelength. One of the methods to flatten gain of EDFA isusing an optical gain-flattening filter. This is the method toflatten the gain of EDFA by using a filter with reverse loss
spectrum against the gain spectrum. This filter is called
the optical gain-flattening filter.
We established a method to design an optical gain-flattening filter with Fabry-Perot etalon by nonlinear fitting, so
we can manufacture optical gain-flattening filters with loss
profiles accommodating to various gain spectra. As theresult of evaluation of sample modules, we confirmed thatcharacteristics of these samples correspond to the design.
And gain characteristics of EDFA with the optical gain-flattening filter is excellent. Moreover, we made sure thatthese filters are reliable as optical devices for telecommunication by reliability tests.

2.

OPTICAL GAIN-FLATTENING FILTERAs the method of flattening a gain spectrum of EDFA,
methods using optical gain-flattening filter with reverse
loss spectrum against gain spectrum have been investigated. The principles of optical gain-flattening filter are
shown in Figure 1. It is shown that EDFA has gain dependence on wavelength before flattening. After flattening ofgain by using the optical gain-flattening filter, it is shownthat dependence of EDFA on wavelength is flattened anddeviation of signal power is improved. In Figure 2 sample
configuration of EDFA with optical gain-flattening filter isshown. Normally optical gain-flattening filter is used in53Transmittance of gain-flattening filter

15
10

+
5
0
1530

1540

1550

Transmittance (dB)Gain (dB)

20

20

5

15

0

=
-5
-10
1530

1560

Gain after equalization

10

Gain (dB)Gain before equalization

Wavelength (nm)

1540

1550

Wavelength (nm)Flattening of optical
output level

-10

1555

Optical output (dBm)

Optical output (dBm)

0

1545

WDM

1565

0

-10

-20
1525

1535

1545

1555

1565

Function of optical gain-flattening filter

EDF
Optical gainflattening filter WDM

1560

Wavelength (nm)Figure 1

Input signal

1550

10

Wavelength (nm)

EDF

1540

Wavelength (nm)Transmission characteristics
after equalization

10

1535

5
0
1530

1560

Transmission characteristics
before equalization

-20
1525

10

Incident light
Output signal
WDM

Reflecting film
Incident angle

Pumping LD

Figure 2

Pumping LD Pumping LD

Configuration of a typical EDFA
Substrate
Exit light

Figure 3

series with EDFA as shown in Figure 2, where the filter isinserted in the middle stage of EDFA between two erbium
doped fibers (EDFs).
The gain-flattening using passive components has been
investigated. Optical filters such as an etalon filter 1)~3), a
dielectric filter 4), and a long period fiber Bragg grating filter 5),
a Mach-Zehnder type filter 6), and the split-beam Fourier filter 7) are proposed. The above-mentioned intrinsic EDFA
gain-wavelength characteristics have asymmetrical twin
peaks and the loss-wavelength characteristics of the optical gain-flattening filter to compensate the EDFA gainwavelength characteristics have counterbalanced characteristics. The EDFA gain-wavelength characteristics
depend on power characteristics of pumping lasers, theconcentration of erbium ion in EDF, the kind of co-doped
ion and its volume, and the length of EDF as well.
The gain-flattening filter requires flexibility of design forvarious kinds of EDFA gain-wavelength characteristics tominimize errors between the target and design. Moreover
it is required to have low polarization dependence loss
(PDL) and capability of producing highly reproducible
spectral profiles.

54

3.

Schematic diagram of the Fabry-Perot etalon filter

ETALON-TYPE GAIN-FLATTENING FILTER

The intrinsic EDFA gain-wavelength characteristics haveasymmetrical twin peaks and the required loss-wavelength characteristics of an optical gain-flattening filter tocompensate the EDFA gain-wavelength characteristics
depend on the signal wavelength bandwidth and therequired gain of EDFA. We can design optical gain-flattening filters using several kinds of etalon filters. In this section characteristics of Fabry-Perot etalon filter used in theetalon-type gain-flattening filter and the design are
described.
3.1 Characteristics of Etalon Filter
The Fabry-Perot etalon filter named after its inventors canbe considered as archetype of the optical resonator and it
is a one-dimensional resonator. It uses interference phenomena of multiple reflections between mirrors paralleledin medium. Figure 3 shows the etalon filter that we use in
the gain-flattening filter. It is produced by grinding twoplane-parallel faces on a substrate with thickness d andindex n and then evaporating a dielectric layer of

Furukawa Review, No. 19. 2000

1

18
Wavelength band for use:

17

Transmittance (dB)016Gain (dB)

-1
-2
-3

15
14
Offset13
12

Incident angle

-4
-5
1500

1520

1540

1560

0 deg
5 deg

1580

Typical gain
characteristics
Line (a)Line (b)

11

1600

10
1520

1530

Wavelength (nm)Figure 4

Etalon filter (2)

1550

1560

1570

Wavelength (nm)Characteristics of a Fabry-Perot etalon filter

Etalon filter (1)1540

Etalon filter (3)Figure 6

Typical gain spectrum of the EDFA

Etalon filter (4)Transmittance (dB)

0
-1
-2

+

+

+

-3
-4
-5
1530

1540

1550

1560

Wavelength (nm)

1530

1540

1550

1560

1530

Wavelength (nm)

1540

1550

1560

Wavelength (nm)

1530

1540

1550

1560

Wavelength (nm)Transmittance (dB)

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

1540

1550

1560

Wavelength (nm)Figure 5

Principles of the etalon-type gain-flattening filter

reflectance R on the surface.
Taking the case of incidence angle θ, we obtain the following expression of transmission characteristics of theetalon filter in air 8):T( λ )=10log(1-R) 2(1-R) 2 +4Rsin 2 ( δ /2)(1)

where δ is the phase delay given byδ =amplitudes and phases the loss-wavelength compensating EDFA gain characteristics is obtained. The fine controlof thickness and reflectance of each etalon filter is needed
to obtain each amplitude and phase of etalon filters.

4 π nd 1-sin 2 θ /n 2
λ3.3 Design of Etalon-type Gain-flattening Filter
Figure 6 shows sample gain-wavelength characteristics of
EDFA. In this case the signal wavelength bandwidth isfrom 1530 nm to 1560 nm. We can obtain flat gain characteristics in the bandwidth by compensating gain-wavelength characteristics over the basis of line (a) that shows
minimum gain in this area or line (b) that shows a gain
less than minimum. The design of the gain-flattening filter
depends on deciding the basis of gain and it is one ofparameters of the design. There is a difference between
the minimum gain level and the basis level used in thedesign, which is named difference offset. In the design
process, the number of etalon filters composing gain-flattening filter and thickness and phase of each etalon filter,in other words reflectance and thickness of etalon filters
and incident angle, are acquired.
Design methods of etalon-type gain-flattening filter suchas methods based on the loss peak 3) and Fourier series
expansion 2) are proposed. We established a method using
nonlinear fitting. For instance, transmittance of etalon-typegain-flattening filter composed of m sheets of etalon filters
is expressed bym

Figure 4 shows the sample characteristics of an etalon
filter. It has periodic characteristics similar to a sine wave.
We can obtain any amplitude of transmission characteristics by optimizing reflectance R and any phase by optimizing thickness d. As shown in Figure 4 by changing theincidence angle θ we can control the phase.
3.2 Principles of Etalon-type Gain-flattening Filter
The etalon-type gain-flattening filter consists of several
etalon filters with different amplitudes and phases to compensate asymmetric EDFA gain-wavelength characteristics for flattening gain shapes. Figure 5 shows the principles of the etalon-type gain-flattening filter. In this case it
is shown that by using four etalon filters with differentT( λ )=

Σ 10log (1-R ) +4R sin ( δ /2)(1-R j ) 2

j

j=1

2

j

2

(2)

j

where δ j is phase delay of each etalon filter,δj =4 π nd j 1-sin 2 θ j /n 2
λOur design method is looking for the number of etalon
filters and characteristics of each etalon filer, amplitude
and phase, in other words reflectance and thickness, byusing nonlinear fitting method to minimize the least-square
between target loss-wavelength characteristics and T(λ).
By using this method we can design an etalon-type gainflattening filter with superior characteristics for flattening
EDFA gain-wavelength characteristics.

55

0.1

Error;
0.17 dB

-1

-0.1

-2

-0.2
-0.3

-3
Design target-4

Designed value

Transmittance (dB)

0.0

Error;
0.2 dB

Error (dB)

Transmittance (dB)

0

0.1

1

0

0.0

-1

-0.1

-2

Designed value
Error

Error

-5
-0.5
1520 1525 1530 1535 1540 1545 1550 1555 1560 1565

-3
1545

Wavelength (nm)Figure 7-0.2

Design target-0.4

Error (dB)

1

1550

1555

1560

-0.3
1565

Wavelength (nm)Designed characteristics of the etalon-type gain-flattening filter (1)Figure 9

Designed characteristics of the etalon type gain-flattening filter (2)5Error (dB)4

Number of filter
:1
:2
:3

:4

3
2
1
0
0.0

0.5

1.0

1.5

2.0

Offset (dB)Figure 8

Figure 7 shows a sample design of an etalon-type gainflattening filter with four etalon filters. In this figure, the target profile is loss-wavelength characteristics with offset. In
this case, offset is 0.8 dB, and the error deviation between
target and design characteristics is 0.2 dB. The error deviation mainly depends on the number of etalon filters andthe offset. Figure 8 shows the relationship between offset
and error deviation with the number of etalon filters as
parameter. In this figure, using nonlinear fitting methodwith fixed offset we can get the error deviations. The error
deviation is less when the offset and the number of etalon
filters are increased. In this case, minimum error deviation
is 0.1 dB with the four etalon filters and an offset of 1.2
dB. Increasing the offset, however, is increasing of theinsertion loss of optical gain-flattening filter and decreasing the gain of EDFA. The relationship between offset anderror deviation is the trade-off, and it is not guaranteedthat the offset with least error deviation is optimum for
EDFA. In the case of Figure 6, design with an offset of 0.8
dB is used for the above-mentioned reason.
The above-mentioned offset and number of etalon filters
depend on target characteristics of the gain-flattening filter. Figure 9 shows another sample design of etalon-typegain-flattening filter with one etalon filter. In this case, thesame as the above-named design shown in Figure 8, the56

Figure 10 Appearance of etalon-type gain-flattening filter

Dependence of error deviation on the number of
etalon filters and offset for the etalon type gainflattening filter (1)design is suitable for EDFA use by the relationshipbetween error deviations and offset as the parameter ofthe number of etalon filters. Design with an offset of 0 dB
and an error deviation of 0.17 dB is used.
As mentioned above, target characteristics of an optical
gain-flattening filter depend on the EDFA that employ thefilter, since the gain characteristics of EDFA depend on
the characteristics of optical components and configurations, such as EDF, pumping lasers, and other opticalpassive components. We develop the design method of
etalon-type gain-flattening filter suitable for EDFA by using
nonlinear fitting, so the gain-flattening filter has best configuration and characteristics of etalon filters. By thismethod we can design the optical gain-flattening filter for
any type of EDFA.

4.

EXPERIMENT

4.1 Configurations and Appearance
Figure 10 shows an experimental etalon-type gain-flattening filter. Its dimensions are 48 mm x φ 5.5 mm including
the sleeve of the optical fiber. It is composed of nonspherical lenses for collimating optical beam and etalon filters between input and output lenses.

Furukawa Review, No. 19. 2000

01
Target characteristics
No.3
No.4
No.5

Target characteristics

Transmittance (dB)

Transmittance (dB)

No.1

-1

No.2

-2

-3

0

-1

-2

-4

-5
1525

-3
1535

1545

1555

1565

1545

1550

1555

Wavelength (nm)(a) Transmittance

1560

1565

Wavelength (nm)(a) Transmittance
0.20

0.20

No.3

No.1
No.2

No.4

0.15

PDL (dB)

PDL (dB)

0.15

0.10

0.10

0.05

0.05

0.00
1525

No.5

0.00
1535

1545

1555

1545

1565

1550

1555

(b) PDL

Figure 11 Characteristics of prototype gain-flattening filter ofthe etalon type (1)4.3 Results
Figure 11 and Figure 12 show experimental results basedon designs shown in Figure 7 and Figure 9. In each figure,(a) shows transmittances of composed etalon filters and(b) shows polarization dependence losses (PDL).
Practical loss-wavelength characteristics of the gain-flattening filter include these losses and losses caused by

1565

Figure 12 Characteristics of prototype gain-flattening filter ofthe etalon type (2)Table 1

4.2 Attachment of Etalon Filters into the Optical Path
In order to realize amplitude and phase of each designed
etalon filter, it is necessary to control the thickness andreflectance of each, but there are practical errors.
Especially accuracy of thickness that determines thephase of the filter is needed, but the real thickness does
not fit the designed thickness because of the limit of manufacture. Tuning the insertion angle of etalon filters isneeded to cancel the error of phase caused by the thickness error. So we developed a method optimizing the
combination of the insertion angle of etalon filters for minimum error deviation by using nonlinear fitting with characteristics of etalon filters put to use, so the etalon-type gainflattening filter becomes highly reproducible.
Problem of return loss and multi-reflection between
etalon filters is eliminated by attention to the designed
insertion angle of etalon filters and considering the optimized insertion angle during the attachment process.

1560

Wavelength (nm)(b) PDL

Wavelength (nm)Results of reliability tests
Results

Test item

Damp

Conditions

Insertion loss

Error deviation

change (dB)(dB)

85°C, 85%RH

Avg: 0.06

Avg: 0.08

336 hours

Worst: 0.09

Worst: 0.09

10~55 Hz,
Vibration

1.52 mmp-p

Avg: 0.06

Avg: 0.02

3 axesWorst: 0.07

Worst: 0.06

2 hours/axis
ImpactLow

1.8 mp-p,

Avg: 0.10

Avg: -0.08

3 axes, 8 cycle

Worst: 0.13

Worst: -0.10

0.04

0.05

-40°C

temperature

336 hours

Heat cycle

42 cycles/2 weeks,

21~76°C
10~80%RH

Avg: 0.09

Avg: 0.01

Worst: 0.14

Worst: 0.05

optical collimating components. In the case of Figure 11,
the average of error deviation between loss characteristics
shown in (a) and target is 0.27 dB, and in the case of
Figure 12, the average of error deviation between loss
characteristics shown in (a) and target is 0.2 dB. Each
PDL shown in (b) is low, and it is sure that each backreflection is over 45 dB.

57

17

Gain (dB)16

7.

With gain-equalizer
Without gain-equalizer

15
14

Gain deviationin the band in use;
0.32 dB

13
12
11
1500

1510

1520

1530

1540

1550

1560

1570

Wavelength (nm)Figure 13 Gain spectrum of the EDFA with etalon-type gain-flattening filter

5.

RELIABILITY TEST

In order to make sure of the reliability of the gain-flattening
filter for optical passive components, reliability tests suchas damp test, vibration test, impact test, low temperature
test, and heat cycle test were carried out. Table 1 shows
results of reliability tests. In every test, changes of error
are less than 0.2 dB, so it is confirmed that etalon-typegain-flattening filter performs satisfactorily.

6.

EDFA CHARACTERISTICS WITHETALON-TYPE GAIN-FLATTENING FILTER

Figure 13 shows characteristics of EDFA with etalon-typegain-flattening filter. In the signal wavelength bandwidthfrom 1530 nm to 1560 nm, the gain deviation is 0.32 dB;
and this flatness extends to 1565 nm, thus realizing an
excellent flatness in gain characteristics.

CONCLUSION

We established the design method of etalon-type gain-flattening filter by using nonlinear fitting and analyzed therelationship between the number of etalon filters and offset for compensating any type of EDFA gain-wavelengthcharacteristics and made it possible to design thickness
and reflectance of etalon filters. It is possible to minimize
error deviation caused in the assembly process by adjusting the insertion angle of etalon filters to cancel fabrication
errors, thus we demonstrated the etalon-type gain-flattening filter is easy to fabricate and is capable of producing
highly reproducible profiles suitable for any type of EDFA.
Its PDL and back reflection are low for practical use. We
also demonstrated EDFA with flat gain characteristics
over a wide bandwidth by using the etalon-type gain-flattening filter. We have confirmed that this filter is reliable
for optical passive components in telecommunications.

REFERENCES

1) Takeda et al.; "Gain equalizing of optical amplifiers using etalon
filters", Proc. IEICE, Autumn 1995. (in Japanese)2) Okuno et al.;"Optical fiber amplifier for multi-channel transmission and gain equalizer",Technical report of IEICE, EMD9642,CPM96-65, OPE96-64, LQE96-66, pp.19-24,1996. (inJapanese)
3) T. Naito et al.;"Gain equalizer in long-haul WDM transmission
system," IEICE Trans. Electron., Vol.E81-C, No.8, pp.12931300, 1998.
4) N. Shimojoh et al.,;"New gain equalization scheme in WDM
optical amplifier repeated transmission systems," OECC'96,
Technical digest, 17B3-3, pp.120-121, 1996.
5) P. F. Wysocki et al.,"Erbium-doped fiber amplifier flattened
beyond 40 nm using long-period grating," OFC'97, PD2, 1997.
6) H. Toba et al.,;"Demonstration of optical FDM based self healing ring network employing arrayed-waveguide-grating ADM
filters and EDFAs," ECOC'94, pp.263-266, 1994.
7) J. W. Arkwright et al.,;"Custom designed gain-flattning filterswith highly reproducible spectral characteristics," OAA'99,
ThD18-1, 1999.
8) Yariv,;"Optical Electronics in Modern Communications,"
Oxford University Press, 1997.
Manuscript received on December 20, 1999.

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Furukawa Review, No. 19. 2000