Int. Journal of Electrical & Electronics Engg. Vol. 2, Spl. Issue 1 (2015) e-ISSN: 1694-2310 | p-ISSN: 1694-2426

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An Overview of EDFA Gain Flattening byUsing Hybrid Amplifier

1ShivaniRadha Sharma, 2TanviSood1M.Tech Student, 2Assistant Professor

1,2ECE Department, Chandigarh Engineering College, Landran, Mohali1379shivanisharma@gmail.com, 2cecm.ece.ts@gmail.com

Abstract—Data communication systems are increasinglyengrossing optical fiber communication system as thetransmission paths for the information, the information is inthe form of light pulses sending from one place to anotherthrough the optical fiber. Several types of optical amplifiershave been developed in optical fiber communication system toamplify the optical signals. The erbium doped fiber amplifieris one of the optical fiber amplifiers which are used for longdistance communication. The most significant points in anyoptical amplifier design are gain and noise figure. They areconnected to one another. The other optical amplifier, Ramanamplifier has wide gain bandwidth. The EDFA gainspectrum has variations over 1536 to 1552 nm, therefore thegain flattening is a research issue in recent years with thedevelopment of high capacity DWDM. The gain variationbecomes a problem as the number of channels increases. Thegain of EDFA depends on large number of device parameterssuch as, Erbium ion concentration, amplifier length, coreradius, pump power. Raman amplifiers can be combined withEDFAs to expand the optical gain flattened bandwidth. Thispaper focuses on different methods used for the gainflattening.

Keywords— Optical amplifier, EDFA, Raman amplifier, Gainflattening

I.INTRODUCTION

Optical fiber communication is seen as one of the mostreliable telecommunication technologies to achieveconsumer’s need for present and future applications. It isreliable in handling and transmitting data through hundredsof kilometers with an acceptable bit error rate. Today,optical fiber communication has been established as one ofthe most promising technologies within the area ofmedium and long distance data transmissions. Opticaltransmission systems are based on the principle that lightcan carry more information over longer distances in a glassmedium, while the electrical signals can carry informationover copper or coaxial cable. Light is electromagneticwaves and optical fiber is a wave-guide, in order tocompensate the loss of the wave-guide, an optical amplifieris needed. Doped fiber amplifier (DFA) is an opticalamplifier 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 transmissionline the DFA is connected to a pump laser. It works onprinciple of stimulated emission and pump laser is used toprovide energy and excite ions to an upper energy level

(Mohammed et al., 2011a, 2011b). Then, the ions arestimulated by photons of the information signal andbrought down to lower levels of energy. Subsequently,they emit photon energy exactly on the same wavelengthof the input signal.. In OFCS, the active medium of DFAwhich has less attenuation is operating in the 1550 nmwindow that is created by doping a silica fiber core withthe Erbium (Er3+). To date, research works areconcentrating more on the Erbium-doping, particularly insilica based fibers. This is due to the emission of Er3+ ionswithin a set of wavelength around 1550 nm where thesilica fiber exhibits the minimum attenuation oninformation signal. Erbium doped fiber amplifiers (EDFA)could provide gains as high as 40 dB associated with lownoise, as successfully demonstrated within a pumpedpower range of 50 to 100 MW (Mears, 1987).

II.OPTICAL AMPLIFIERS

Optical amplifier is a device that boosts the light signals inan optical fiber network. Unlike the generators which haveto convert light to electricity in order to amplify it, andthen convert it back again to light. The optical amplifieramplifies the light signal itself. Optical amplifiers simplystrength the optical signal, optical amplifiers work withouthaving to convert an optical signal to electrical forms andback. This feature has two great advantages over repeaters: They support any bit rate and signal format, because

they simply amplify the received signal. They aretransparent to the bit rate and signal format.

They support not just the single wavelength but theentire region of wavelength.

There are three most important types of opticalamplifiers: the erbium-doped fiber amplifier, thesemiconductor optical amplifier, and the fiber Ramanamplifier. We introduce each of these amplifiers in thefollowing subsections.

A. Erbium-Doped Fiber Amplifier

The amplifying medium is a glass optical fiber dopedwith erbium ions. The erbium is pumped to a state ofpopulation inversion with a separate optical input. Theerbium-doped glass optical gain medium amplifies light atwavelengths that are in the neighborhood of 1550 nm – theoptical wavelengths that suffer minimum attenuation inoptical fibers. Erbium-doped optical fiber amplifiers(EDFAs) have low noise and can amplify manywavelengths simultaneously, making the EDFA theamplifier of choice for most applications in opticalcommunications.

Int. Journal of Electrical & Electronics Engg. Vol. 2, Spl. Issue 1 (2015) e-ISSN: 1694-2310 | p-ISSN: 1694-2426

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B. Semiconductor Optical Amplifier:A semiconductor optical amplifier with gain medium is

undopedInGaAsP. This material can be tailored to provideoptical amplification at wavelengths near 1.3 μm or near1.5 μm. A semiconductor optical amplifier (SOA) ispumped with electrical current. SOAs are noisier thanEDFAs and generally handle less power. However, SOAsare less expensive and are therefore suitable for use in localarea networks.

C. Raman amplifier:

In Raman amplifier the gain medium is undoped opticalfiber. Power is transferred to the optical signal by anonlinear optical process known as the Raman Effect.Power to supply the optical gain is supplied by an opticalpump. The wavelengths that experience optical gain aredetermined by the wavelength of the optical pump, so theRaman amplifier can be tailored to amplify a given opticalwavelength by proper selection of the pump wavelength.Its most popular configuration is a hybrid EDFA/Ramanamplifier, a device in which the Raman amplifiercompensates for the EDFA lack of gain.

D. Hybrid amplifiers

The combination of more than two optical amplifiers inany configuration called the hybrid amplifier.

III.ERBIUM DOPED FIBER AMPLIFIERS

These fiber amplifiersare the ones normally deployed inWDM fiber –optic communication systems today. EDFAhave the gift of nature that the give the output in thevicinity of 1550nm, where silica fiber exhibits minimumattenuation. This fortunate coincidence is why erbiumdoped fiber amplifiers are so widely used. In EDFAspumping is done with a laser diode radiating powerful lightin a wavelength other than the information signalswavelength. Specifically, an information signal istransmitted around 1550 nm but pump laser radiate eitherat 980nm or at 1480 nm, or both. The following figureshows the basic diagram of EDFA.

Schematic diagram of EDFA [4]

A. EDFA amplification

The operation of EDFA is based on the stimulatedemission mechanism, the discussion of energy –leveldiagram of an active medium as it pertains to thisamplifier. Free ions of erbium exhibit discrete energylevels.The most important energy levels of erbium ionsincorporated 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 ofthe energy bands, which determines the EDFA’s ability toamplify the range of wavelengths from 1500nm to morethan 1600nm. Our goal is to achieve population inversion,which means having more ions of erbium at theintermediate level-2. To attain population inversion, weneed to pump erbium ions at the intermediate level [7].There are two ways to do this: pumping them directly atthe 1480-nm wavelength or indirectly at the 980-nmwavelength.

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 signalemission( / ). The gain of EDFA is largely dependenton the parameters explained above.The gain spectrum ofEDFA is shown below:

EDFA gain spectrum [5]

Here the gain variation takes place between 1530 and1560nm.We need to have a flat gain over its rangeoperating wavelengths. This characteristic of and EDFA iscalled gain flatness.Uniformity of gain involves twoaspects namely:

Gain equalization: Gain equalization means achievingidentical gains for a discrete number of opticalchannels.

Gain flattening: Gain flattening means achieving aspectrally uniform gain bandwidth.

IV.GAIN FLATTENING

Erbium doped fiber amplifiers have had a major impact inthe field of light wave communications. Optical amplifiershave contributed to the growth of a fifth generation ofoptical 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 provideoptical amplification at wavelengths near 1.3 μm or near1.5 μm. A semiconductor optical amplifier (SOA) ispumped with electrical current. SOAs are noisier thanEDFAs and generally handle less power. However, SOAsare less expensive and are therefore suitable for use in localarea networks.

C. Raman amplifier:

In Raman amplifier the gain medium is undoped opticalfiber. Power is transferred to the optical signal by anonlinear optical process known as the Raman Effect.Power to supply the optical gain is supplied by an opticalpump. The wavelengths that experience optical gain aredetermined by the wavelength of the optical pump, so theRaman amplifier can be tailored to amplify a given opticalwavelength by proper selection of the pump wavelength.Its most popular configuration is a hybrid EDFA/Ramanamplifier, a device in which the Raman amplifiercompensates for the EDFA lack of gain.

D. Hybrid amplifiers

The combination of more than two optical amplifiers inany configuration called the hybrid amplifier.

III.ERBIUM DOPED FIBER AMPLIFIERS

These fiber amplifiersare the ones normally deployed inWDM fiber –optic communication systems today. EDFAhave the gift of nature that the give the output in thevicinity of 1550nm, where silica fiber exhibits minimumattenuation. This fortunate coincidence is why erbiumdoped fiber amplifiers are so widely used. In EDFAspumping is done with a laser diode radiating powerful lightin a wavelength other than the information signalswavelength. Specifically, an information signal istransmitted around 1550 nm but pump laser radiate eitherat 980nm or at 1480 nm, or both. The following figureshows the basic diagram of EDFA.

Schematic diagram of EDFA [4]

A. EDFA amplification

The operation of EDFA is based on the stimulatedemission mechanism, the discussion of energy –leveldiagram of an active medium as it pertains to thisamplifier. Free ions of erbium exhibit discrete energylevels.The most important energy levels of erbium ionsincorporated 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 ofthe energy bands, which determines the EDFA’s ability toamplify the range of wavelengths from 1500nm to morethan 1600nm. Our goal is to achieve population inversion,which means having more ions of erbium at theintermediate level-2. To attain population inversion, weneed to pump erbium ions at the intermediate level [7].There are two ways to do this: pumping them directly atthe 1480-nm wavelength or indirectly at the 980-nmwavelength.

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 signalemission( / ). The gain of EDFA is largely dependenton the parameters explained above.The gain spectrum ofEDFA is shown below:

EDFA gain spectrum [5]

Here the gain variation takes place between 1530 and1560nm.We need to have a flat gain over its rangeoperating wavelengths. This characteristic of and EDFA iscalled gain flatness.Uniformity of gain involves twoaspects namely:

Gain equalization: Gain equalization means achievingidentical gains for a discrete number of opticalchannels.

Gain flattening: Gain flattening means achieving aspectrally uniform gain bandwidth.

IV.GAIN FLATTENING

Erbium doped fiber amplifiers have had a major impact inthe field of light wave communications. Optical amplifiershave contributed to the growth of a fifth generation ofoptical 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 provideoptical amplification at wavelengths near 1.3 μm or near1.5 μm. A semiconductor optical amplifier (SOA) ispumped with electrical current. SOAs are noisier thanEDFAs and generally handle less power. However, SOAsare less expensive and are therefore suitable for use in localarea networks.

C. Raman amplifier:

In Raman amplifier the gain medium is undoped opticalfiber. Power is transferred to the optical signal by anonlinear optical process known as the Raman Effect.Power to supply the optical gain is supplied by an opticalpump. The wavelengths that experience optical gain aredetermined by the wavelength of the optical pump, so theRaman amplifier can be tailored to amplify a given opticalwavelength by proper selection of the pump wavelength.Its most popular configuration is a hybrid EDFA/Ramanamplifier, a device in which the Raman amplifiercompensates for the EDFA lack of gain.

D. Hybrid amplifiers

The combination of more than two optical amplifiers inany configuration called the hybrid amplifier.

III.ERBIUM DOPED FIBER AMPLIFIERS

These fiber amplifiersare the ones normally deployed inWDM fiber –optic communication systems today. EDFAhave the gift of nature that the give the output in thevicinity of 1550nm, where silica fiber exhibits minimumattenuation. This fortunate coincidence is why erbiumdoped fiber amplifiers are so widely used. In EDFAspumping is done with a laser diode radiating powerful lightin a wavelength other than the information signalswavelength. Specifically, an information signal istransmitted around 1550 nm but pump laser radiate eitherat 980nm or at 1480 nm, or both. The following figureshows the basic diagram of EDFA.

Schematic diagram of EDFA [4]

A. EDFA amplification

The operation of EDFA is based on the stimulatedemission mechanism, the discussion of energy –leveldiagram of an active medium as it pertains to thisamplifier. Free ions of erbium exhibit discrete energylevels.The most important energy levels of erbium ionsincorporated 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 ofthe energy bands, which determines the EDFA’s ability toamplify the range of wavelengths from 1500nm to morethan 1600nm. Our goal is to achieve population inversion,which means having more ions of erbium at theintermediate level-2. To attain population inversion, weneed to pump erbium ions at the intermediate level [7].There are two ways to do this: pumping them directly atthe 1480-nm wavelength or indirectly at the 980-nmwavelength.

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 signalemission( / ). The gain of EDFA is largely dependenton the parameters explained above.The gain spectrum ofEDFA is shown below:

EDFA gain spectrum [5]

Here the gain variation takes place between 1530 and1560nm.We need to have a flat gain over its rangeoperating wavelengths. This characteristic of and EDFA iscalled gain flatness.Uniformity of gain involves twoaspects namely:

Gain equalization: Gain equalization means achievingidentical gains for a discrete number of opticalchannels.

Gain flattening: Gain flattening means achieving aspectrally uniform gain bandwidth.

IV.GAIN FLATTENING

Erbium doped fiber amplifiers have had a major impact inthe field of light wave communications. Optical amplifiershave contributed to the growth of a fifth generation ofoptical 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

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networks increased techniques like Dense WavelengthDivision Multiplexing (DWDM) were developed. Theimportance of EDFAs is due to their compatibility with thefiber network, low insertion loss, polarization insensitivity,high gain levels and near quantum limited noiseperformance. Several methods for the gain flattening havebeen presented for the last few years. Some of these are:

A. Gain flattening filters1) Passive filters: include Fiber brag gratings,

Longperiod gratings (LPG) and Mach Zenderinterferometers. Passive filters are very popular gainequalizing devices, as they are very reliable. But there aresome limitations that they do not offer flexibility and thatchanges in EDFA gain profile cannot be tracked. [7]. Wideband fiber brag gratings have been shown to improve thegain flatness of EDFA considerably by using opticalisolator followed by FBG [8]. MZ filter is placed after theamplifier to remove amplified spontaneous emissionnoise[9].

2) Active filters: As the gain spectrum of EDFA isnot constant but changes as EDFA conditions change, forthis purpose the need of tunable gain equalizing filterstakes place called the active filters. Active filters likeAcousto optic filter, Tunable MZIs, Tunable LPGs werediscussed [10-12].

B. Hybrid Optical Amplifier (HOA)Cascading EDFAs with different gain spectra leads to

the formation of hybrid optical amplifier (HOA)[1]. Beforethe formation of hybrid optical amplifier it is necessary toevaluate the transmission performance. The transmissionperformance is evaluated by transmission capacity.

Transmission capacity = bit rate × no. of wavelength

Where,no. of wavelengths= optical signal BW × channeldensity[13].

C. Input signal powerAdjustment of input signal powers can also reduce the

gain variations[8].

IV. GAIN FLATTENING USING HYBRIDAMPLIFIERS

As mentioned earlier there are various techniques to reducethe gain variations. The use of filters reduce the gainvariations to some extent but they do not offer flexibilitymeans they do adapt the changes which takes place in theEDFA gain spectrum. One LPG along with other phaseshifted LPG reduced the gain ripple to approximately0.9dB. But by the use of hybrid amplifiers, means the bythe use of dual stage EDFA with bidirectional pumping canbe used along with this the Raman amplifier with counterpumping scheme is used. The use of Raman amplifierreduces the automatic spontaneous emission.

V.CONCLUSION AND FUTURE SCOPE

This review paper presents the variations takes place is thegain spectrum of EDFA and various techniques to reducethe gain variations. These techniques include use of activeand passive filters, cascading EDFA with different optical

amplifier and adjustment of different parameters. These allhave some drawbacks, but by the use of Raman amplifierwhich reduces the effective span loss can be combinedwith EDFA to expand the optical gain flattened bandwidth.After cascading these two amplifiers the use of a passivefilter can be considered as future work.

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