Data communication systems are increasingly engrossing optical fiber communication system as the transmission paths for the information, the information is in the form of light pulses sending from one place to another through the optical fiber. Several types of optical amplifiers have been developed in optical fiber communication system to amplify the optical signals. The erbium doped fiber amplifier is one of the optical fiber amplifiers which are used for long distance communication. The most significant points in any optical amplifier design are gain and noise figure. They are connected to one another. The other optical amplifier, Raman amplifier has wide gain bandwidth. The EDFA gain spectrum has variations over 1536 to 1552 nm, therefore the gain flattening is a research issue in recent years with the development of high capacity DWDM. The gain variation becomes a problem as the number of channels increases. The gain of EDFA depends on large number of device parameters such as, Erbium ion concentration, amplifier length, core radius, pump power. Raman amplifiers can be combined with EDFAs to expand the optical gain flattened bandwidth. This paper focuses on different methods used for the gain flattening.

1. Int. Journal of Electrical & Electronics Engg. Vol. 2, Spl. Issue 1 (2015) e-ISSN: 1694-2310 | p-ISSN: 1694-2426
NITTTR, Chandigarh EDIT -2015 156
An Overview of EDFA Gain Flattening by
Using Hybrid Amplifier
1
ShivaniRadha Sharma, 2
TanviSood
1
M.Tech Student,
2
Assistant Professor
1,2
ECE Department, Chandigarh Engineering College, Landran, Mohali
1
379shivanisharma@gmail.com, 2
cecm.ece.ts@gmail.com
Abstract—Data communication systems are increasingly
engrossing optical fiber communication system as the
transmission paths for the information, the information is in
the form of light pulses sending from one place to another
through the optical fiber. Several types of optical amplifiers
have been developed in optical fiber communication system to
amplify the optical signals. The erbium doped fiber amplifier
is one of the optical fiber amplifiers which are used for long
distance communication. The most significant points in any
optical amplifier design are gain and noise figure. They are
connected to one another. The other optical amplifier, Raman
amplifier has wide gain bandwidth. The EDFA gain
spectrum has variations over 1536 to 1552 nm, therefore the
gain flattening is a research issue in recent years with the
development of high capacity DWDM. The gain variation
becomes a problem as the number of channels increases. The
gain of EDFA depends on large number of device parameters
such as, Erbium ion concentration, amplifier length, core
radius, pump power. Raman amplifiers can be combined with
EDFAs to expand the optical gain flattened bandwidth. This
paper focuses on different methods used for the gain
flattening.
Keywords— Optical amplifier, EDFA, Raman amplifier, Gain
flattening
I.INTRODUCTION
Optical fiber communication is seen as one of the most
reliable telecommunication technologies to achieve
consumer’s need for present and future applications. It is
reliable in handling and transmitting data through hundreds
of kilometers with an acceptable bit error rate. Today,
optical fiber communication has been established as one of
the most promising technologies within the area of
medium and long distance data transmissions. Optical
transmission systems are based on the principle that light
can carry more information over longer distances in a glass
medium, while the electrical signals can carry information
over copper or coaxial cable. Light is electromagnetic
waves and optical fiber is a wave-guide, in order to
compensate the loss of the wave-guide, an optical amplifier
is needed. Doped fiber amplifier (DFA) is an optical
amplifier which uses rare-earth doping material which are:
Erbium (Er3+), Praseodymium (Pr3+), Europium (Eu3+),
Neodymium (Nd3+), Terbium (Te3+), Lutetium (Lu3+),
Ytterbium (Yb3+), Holmium (Ho3+), Dysprosium (Dy3+),
Gadolinium (Gd3+), Samarium (Sm3+), Promethium
(Pm3+), Cerium (Ce3+), Lanthanum (La3+) and Thulium
(Tm3+) inside the fiber. Essentially, within a transmission
line the DFA is connected to a pump laser. It works on
principle of stimulated emission and pump laser is used to
provide energy and excite ions to an upper energy level
(Mohammed et al., 2011a, 2011b). Then, the ions are
stimulated by photons of the information signal and
brought down to lower levels of energy. Subsequently,
they emit photon energy exactly on the same wavelength
of the input signal.. In OFCS, the active medium of DFA
which has less attenuation is operating in the 1550 nm
window that is created by doping a silica fiber core with
the Erbium (Er3+). To date, research works are
concentrating more on the Erbium-doping, particularly in
silica based fibers. This is due to the emission of Er3+ ions
within a set of wavelength around 1550 nm where the
silica fiber exhibits the minimum attenuation on
information signal. Erbium doped fiber amplifiers (EDFA)
could provide gains as high as 40 dB associated with low
noise, as successfully demonstrated within a pumped
power range of 50 to 100 MW (Mears, 1987).
II.OPTICAL AMPLIFIERS
Optical amplifier is a device that boosts the light signals in
an optical fiber network. Unlike the generators which have
to convert light to electricity in order to amplify it, and
then convert it back again to light. The optical amplifier
amplifies the light signal itself. Optical amplifiers simply
strength the optical signal, optical amplifiers work without
having to convert an optical signal to electrical forms and
back. This feature has two great advantages over repeaters:
 They support any bit rate and signal format, because
they simply amplify the received signal. They are
transparent to the bit rate and signal format.
 They support not just the single wavelength but the
entire region of wavelength.
There are three most important types of optical
amplifiers: the erbium-doped fiber amplifier, the
semiconductor optical amplifier, and the fiber Raman
amplifier. We introduce each of these amplifiers in the
following subsections.
A. Erbium-Doped Fiber Amplifier
The amplifying medium is a glass optical fiber doped
with erbium ions. The erbium is pumped to a state of
population inversion with a separate optical input. The
erbium-doped glass optical gain medium amplifies light at
wavelengths that are in the neighborhood of 1550 nm – the
optical wavelengths that suffer minimum attenuation in
optical fibers. Erbium-doped optical fiber amplifiers
(EDFAs) have low noise and can amplify many
wavelengths simultaneously, making the EDFA the
amplifier of choice for most applications in optical
communications.

2. Int. Journal of Electrical & Electronics Engg. Vol. 2, Spl. Issue 1 (2015) e-ISSN: 1694-2310 | p-ISSN: 1694-2426
157 NITTTR, Chandigarh EDIT-2015
B. Semiconductor Optical Amplifier:
A semiconductor optical amplifier with gain medium is
undopedInGaAsP. This material can be tailored to provide
optical amplification at wavelengths near 1.3 μm or near
1.5 μm. A semiconductor optical amplifier (SOA) is
pumped with electrical current. SOAs are noisier than
EDFAs and generally handle less power. However, SOAs
are less expensive and are therefore suitable for use in local
area networks.
C. Raman amplifier:
In Raman amplifier the gain medium is undoped optical
fiber. Power is transferred to the optical signal by a
nonlinear optical process known as the Raman Effect.
Power to supply the optical gain is supplied by an optical
pump. The wavelengths that experience optical gain are
determined by the wavelength of the optical pump, so the
Raman amplifier can be tailored to amplify a given optical
wavelength by proper selection of the pump wavelength.
Its most popular configuration is a hybrid EDFA/Raman
amplifier, a device in which the Raman amplifier
compensates for the EDFA lack of gain.
D. Hybrid amplifiers
The combination of more than two optical amplifiers in
any configuration called the hybrid amplifier.
III.ERBIUM DOPED FIBER AMPLIFIERS
These fiber amplifiersare the ones normally deployed in
WDM fiber –optic communication systems today. EDFA
have the gift of nature that the give the output in the
vicinity of 1550nm, where silica fiber exhibits minimum
attenuation. This fortunate coincidence is why erbium
doped fiber amplifiers are so widely used. In EDFAs
pumping is done with a laser diode radiating powerful light
in a wavelength other than the information signals
wavelength. Specifically, an information signal is
transmitted around 1550 nm but pump laser radiate either
at 980nm or at 1480 nm, or both. The following figure
shows the basic diagram of EDFA.
Schematic diagram of EDFA [4]
A. EDFA amplification
The operation of EDFA is based on the stimulated
emission mechanism, the discussion of energy –level
diagram of an active medium as it pertains to this
amplifier. Free ions of erbium exhibit discrete energy
levels.The most important energy levels of erbium ions
incorporated into a silica fiber are shown in figure below.
Energy bands of erbium ions in silica fibers[4]
After study the figure, note in particular the width of
the energy bands, which determines the EDFA’s ability to
amplify the range of wavelengths from 1500nm to more
than 1600nm. Our goal is to achieve population inversion,
which means having more ions of erbium at the
intermediate level-2. To attain population inversion, we
need to pump erbium ions at the intermediate level [7].
There are two ways to do this: pumping them directly at
the 1480-nm wavelength or indirectly at the 980-nm
wavelength.
B. Gain of EDFA:
Gain of EDFA can be calculated as:
= (L, , )=
( )
( )
Where = pump wavelength, = signal wavelength,
- is the ratio of pump absorption and pump emission
( / ), r = the ratio of signal absorption and signal
emission( / ). The gain of EDFA is largely dependent
on the parameters explained above.The gain spectrum of
EDFA is shown below:
EDFA gain spectrum [5]
Here the gain variation takes place between 1530 and
1560nm.We need to have a flat gain over its range
operating wavelengths. This characteristic of and EDFA is
called gain flatness.Uniformity of gain involves two
aspects namely:
 Gain equalization: Gain equalization means achieving
identical gains for a discrete number of optical
channels.
 Gain flattening: Gain flattening means achieving a
spectrally uniform gain bandwidth.
IV.GAIN FLATTENING
Erbium doped fiber amplifiers have had a major impact in
the field of light wave communications. Optical amplifiers
have contributed to the growth of a fifth generation of
optical communication systems. But as the demands on the
Int. Journal of Electrical & Electronics Engg. Vol. 2, Spl. Issue 1 (2015) e-ISSN: 1694-2310 | p-ISSN: 1694-2426
157 NITTTR, Chandigarh EDIT-2015
B. Semiconductor Optical Amplifier:
A semiconductor optical amplifier with gain medium is
undopedInGaAsP. This material can be tailored to provide
optical amplification at wavelengths near 1.3 μm or near
1.5 μm. A semiconductor optical amplifier (SOA) is
pumped with electrical current. SOAs are noisier than
EDFAs and generally handle less power. However, SOAs
are less expensive and are therefore suitable for use in local
area networks.
C. Raman amplifier:
In Raman amplifier the gain medium is undoped optical
fiber. Power is transferred to the optical signal by a
nonlinear optical process known as the Raman Effect.
Power to supply the optical gain is supplied by an optical
pump. The wavelengths that experience optical gain are
determined by the wavelength of the optical pump, so the
Raman amplifier can be tailored to amplify a given optical
wavelength by proper selection of the pump wavelength.
Its most popular configuration is a hybrid EDFA/Raman
amplifier, a device in which the Raman amplifier
compensates for the EDFA lack of gain.
D. Hybrid amplifiers
The combination of more than two optical amplifiers in
any configuration called the hybrid amplifier.
III.ERBIUM DOPED FIBER AMPLIFIERS
These fiber amplifiersare the ones normally deployed in
WDM fiber –optic communication systems today. EDFA
have the gift of nature that the give the output in the
vicinity of 1550nm, where silica fiber exhibits minimum
attenuation. This fortunate coincidence is why erbium
doped fiber amplifiers are so widely used. In EDFAs
pumping is done with a laser diode radiating powerful light
in a wavelength other than the information signals
wavelength. Specifically, an information signal is
transmitted around 1550 nm but pump laser radiate either
at 980nm or at 1480 nm, or both. The following figure
shows the basic diagram of EDFA.
Schematic diagram of EDFA [4]
A. EDFA amplification
The operation of EDFA is based on the stimulated
emission mechanism, the discussion of energy –level
diagram of an active medium as it pertains to this
amplifier. Free ions of erbium exhibit discrete energy
levels.The most important energy levels of erbium ions
incorporated into a silica fiber are shown in figure below.
Energy bands of erbium ions in silica fibers[4]
After study the figure, note in particular the width of
the energy bands, which determines the EDFA’s ability to
amplify the range of wavelengths from 1500nm to more
than 1600nm. Our goal is to achieve population inversion,
which means having more ions of erbium at the
intermediate level-2. To attain population inversion, we
need to pump erbium ions at the intermediate level [7].
There are two ways to do this: pumping them directly at
the 1480-nm wavelength or indirectly at the 980-nm
wavelength.
B. Gain of EDFA:
Gain of EDFA can be calculated as:
= (L, , )=
( )
( )
Where = pump wavelength, = signal wavelength,
- is the ratio of pump absorption and pump emission
( / ), r = the ratio of signal absorption and signal
emission( / ). The gain of EDFA is largely dependent
on the parameters explained above.The gain spectrum of
EDFA is shown below:
EDFA gain spectrum [5]
Here the gain variation takes place between 1530 and
1560nm.We need to have a flat gain over its range
operating wavelengths. This characteristic of and EDFA is
called gain flatness.Uniformity of gain involves two
aspects namely:
 Gain equalization: Gain equalization means achieving
identical gains for a discrete number of optical
channels.
 Gain flattening: Gain flattening means achieving a
spectrally uniform gain bandwidth.
IV.GAIN FLATTENING
Erbium doped fiber amplifiers have had a major impact in
the field of light wave communications. Optical amplifiers
have contributed to the growth of a fifth generation of
optical communication systems. But as the demands on the
Int. Journal of Electrical & Electronics Engg. Vol. 2, Spl. Issue 1 (2015) e-ISSN: 1694-2310 | p-ISSN: 1694-2426
157 NITTTR, Chandigarh EDIT-2015
B. Semiconductor Optical Amplifier:
A semiconductor optical amplifier with gain medium is
undopedInGaAsP. This material can be tailored to provide
optical amplification at wavelengths near 1.3 μm or near
1.5 μm. A semiconductor optical amplifier (SOA) is
pumped with electrical current. SOAs are noisier than
EDFAs and generally handle less power. However, SOAs
are less expensive and are therefore suitable for use in local
area networks.
C. Raman amplifier:
In Raman amplifier the gain medium is undoped optical
fiber. Power is transferred to the optical signal by a
nonlinear optical process known as the Raman Effect.
Power to supply the optical gain is supplied by an optical
pump. The wavelengths that experience optical gain are
determined by the wavelength of the optical pump, so the
Raman amplifier can be tailored to amplify a given optical
wavelength by proper selection of the pump wavelength.
Its most popular configuration is a hybrid EDFA/Raman
amplifier, a device in which the Raman amplifier
compensates for the EDFA lack of gain.
D. Hybrid amplifiers
The combination of more than two optical amplifiers in
any configuration called the hybrid amplifier.
III.ERBIUM DOPED FIBER AMPLIFIERS
These fiber amplifiersare the ones normally deployed in
WDM fiber –optic communication systems today. EDFA
have the gift of nature that the give the output in the
vicinity of 1550nm, where silica fiber exhibits minimum
attenuation. This fortunate coincidence is why erbium
doped fiber amplifiers are so widely used. In EDFAs
pumping is done with a laser diode radiating powerful light
in a wavelength other than the information signals
wavelength. Specifically, an information signal is
transmitted around 1550 nm but pump laser radiate either
at 980nm or at 1480 nm, or both. The following figure
shows the basic diagram of EDFA.
Schematic diagram of EDFA [4]
A. EDFA amplification
The operation of EDFA is based on the stimulated
emission mechanism, the discussion of energy –level
diagram of an active medium as it pertains to this
amplifier. Free ions of erbium exhibit discrete energy
levels.The most important energy levels of erbium ions
incorporated into a silica fiber are shown in figure below.
Energy bands of erbium ions in silica fibers[4]
After study the figure, note in particular the width of
the energy bands, which determines the EDFA’s ability to
amplify the range of wavelengths from 1500nm to more
than 1600nm. Our goal is to achieve population inversion,
which means having more ions of erbium at the
intermediate level-2. To attain population inversion, we
need to pump erbium ions at the intermediate level [7].
There are two ways to do this: pumping them directly at
the 1480-nm wavelength or indirectly at the 980-nm
wavelength.
B. Gain of EDFA:
Gain of EDFA can be calculated as:
= (L, , )=
( )
( )
Where = pump wavelength, = signal wavelength,
- is the ratio of pump absorption and pump emission
( / ), r = the ratio of signal absorption and signal
emission( / ). The gain of EDFA is largely dependent
on the parameters explained above.The gain spectrum of
EDFA is shown below:
EDFA gain spectrum [5]
Here the gain variation takes place between 1530 and
1560nm.We need to have a flat gain over its range
operating wavelengths. This characteristic of and EDFA is
called gain flatness.Uniformity of gain involves two
aspects namely:
 Gain equalization: Gain equalization means achieving
identical gains for a discrete number of optical
channels.
 Gain flattening: Gain flattening means achieving a
spectrally uniform gain bandwidth.
IV.GAIN FLATTENING
Erbium doped fiber amplifiers have had a major impact in
the field of light wave communications. Optical amplifiers
have contributed to the growth of a fifth generation of
optical communication systems. But as the demands on the

3. Int. Journal of Electrical & Electronics Engg. Vol. 2, Spl. Issue 1 (2015) e-ISSN: 1694-2310 | p-ISSN: 1694-2426
NITTTR, Chandigarh EDIT -2015 158
networks increased techniques like Dense Wavelength
Division Multiplexing (DWDM) were developed. The
importance of EDFAs is due to their compatibility with the
fiber network, low insertion loss, polarization insensitivity,
high gain levels and near quantum limited noise
performance. Several methods for the gain flattening have
been presented for the last few years. Some of these are:
A. Gain flattening filters
1) Passive filters: include Fiber brag gratings,
Longperiod gratings (LPG) and Mach Zender
interferometers. Passive filters are very popular gain
equalizing devices, as they are very reliable. But there are
some limitations that they do not offer flexibility and that
changes in EDFA gain profile cannot be tracked. [7]. Wide
band fiber brag gratings have been shown to improve the
gain flatness of EDFA considerably by using optical
isolator followed by FBG [8]. MZ filter is placed after the
amplifier to remove amplified spontaneous emission
noise[9].
2) Active filters: As the gain spectrum of EDFA is
not constant but changes as EDFA conditions change, for
this purpose the need of tunable gain equalizing filters
takes place called the active filters. Active filters like
Acousto optic filter, Tunable MZIs, Tunable LPGs were
discussed [10-12].
B. Hybrid Optical Amplifier (HOA)
Cascading EDFAs with different gain spectra leads to
the formation of hybrid optical amplifier (HOA)[1]. Before
the formation of hybrid optical amplifier it is necessary to
evaluate the transmission performance. The transmission
performance is evaluated by transmission capacity.
Transmission capacity = bit rate × no. of wavelength
Where,no. of wavelengths= optical signal BW × channel
density[13].
C. Input signal power
Adjustment of input signal powers can also reduce the
gain variations[8].
IV. GAIN FLATTENING USING HYBRID
AMPLIFIERS
As mentioned earlier there are various techniques to reduce
the gain variations. The use of filters reduce the gain
variations to some extent but they do not offer flexibility
means they do adapt the changes which takes place in the
EDFA gain spectrum. One LPG along with other phase
shifted LPG reduced the gain ripple to approximately
0.9dB. But by the use of hybrid amplifiers, means the by
the use of dual stage EDFA with bidirectional pumping can
be used along with this the Raman amplifier with counter
pumping scheme is used. The use of Raman amplifier
reduces the automatic spontaneous emission.
V.CONCLUSION AND FUTURE SCOPE
This review paper presents the variations takes place is the
gain spectrum of EDFA and various techniques to reduce
the gain variations. These techniques include use of active
and passive filters, cascading EDFA with different optical
amplifier and adjustment of different parameters. These all
have some drawbacks, but by the use of Raman amplifier
which reduces the effective span loss can be combined
with EDFA to expand the optical gain flattened bandwidth.
After cascading these two amplifiers the use of a passive
filter can be considered as future work.
REFERENCES
[1] Sohn, Ik Bu, Jang Gi Kim, Hyo Sang, Seok Hyun Yun, HyangKyun
Kim, Namkyoo Park, Byoung Yoon Kim, “Actively gain-flattened
erbium-doped fiber amplifier over 35nm by using all-fiber
acoustooptic tunable filters,” 1998
[2] A. M. Vengsarkar, “Broad-band erbium-doped fiber
amplifierflattened beyond 40 nm using long-period grating filter,”
IEEEPhoton. Technol. Lett., vol. 9, pp. 1343–1345, Oct. 1997
[3] Gu, X. J. "Wavelength-division multiplexing isolation fiber filter and
light source using cascaded long-period fiber gratings,” Optics letters
vol. 23, no.7, pp. 509-510, 1998
[4] Masuda, Hiroji, “Review of wideband hybrid amplifiers,” In Optical
FiberCommunication Conference, vol. 1, pp. 2-4. Optical Society of
America, 2000
[5] Kim, Seung Kwan, Moo Jung Chu, and Jong Hyun Lee, “Wideband
multi wavelength erbium-doped fiber ring laser with frequency
shifted feedback,” Optics communications vol. 190, no. 1, pp. 291-
302, 2004
[6] Nam-Kwon Lee, Hyung Woo Kwon, Jae-Won Song, “Gain flattened
and improved EDFA using microbending long-period fibre gratings,”
Electronics Letters vol. 38, no. 22, pp. 1324-1325, 2002
[7] Surinder Singh, and R. S. Kaler, “Gain flattening approach to
physical EDFA for 16× 40 Gb/s NRZ-DPSK WDM Optical
Communication System,” Fiber and integrated optics vol. 25, no. 5,
pp. 363-374 2006
[8] Liaw, S. K., C. K. Huang, and Y. L. Hsiao, “Parallel-type C+ L band
hybrid amplifier pumped by 1480 nm laser diodes,” Laser Physics
Letters vol. 5, no.7, 2008
[9] Bae, Jun Kye, JinhoBae, Sang Hyuck Kim, Namkyoo Park, and Sang
Bae Lee, “Dynamic EDFA gain-flattening filter using two LPFGs
with divided coil heaters,” Photonics Technology Letters, IEEE
vol.17, no. 6 ,pp.1226-1228, 2005
[10]Martin Lopez, Sonia, Miguel Gonzalez Herraez, Pedro Corredera,
María Luisa Hernanz, A.Carrasco, “Gain-flattening of fiber Raman
amplifiers using non-linear pump spectral broadening,” Optics
communications vol. 242, no. 4, pp. 463-469, 2004
[11]Cho, Y.T Alahbabi, M.N, Brambilla, G.Newson, “Distributed Raman
amplification combined with a remotely pumped EDFA utilized to
enhance the performance of spontaneous Brillouin-based distributed
temperature sensors,” Photonics Technology Letters, IEEE , vol.17,
no.6, pp.1256-1258, June 2005
[12]Singh, Simranjit, and R. S. Kaler, “Flat-gain L-band Raman-EDFA
hybrid optical amplifier for dense wavelength division multiplexed
system,” Photonics Technology Letters, IEEE vol.25, no. 3, pp. 250-
252, 2013
[13]S. Singh, R.S. Kaler, “Investigation of hybrid optical amplifiers for
dense wavelength division multiplexed system with reduced spacings
at higher bit rates,” Fiber and Integrated Optics vol. 31, no. 3, pp.
208-220, 2012