Edfa Seminar

Erbium doped fiber amplifier

Contents

1. Introduction 2

2. EDFA- An Overview 3

3. Basics of Optical Amplification 5

3.1 Population Inversion 5

Boltzmann distributions and thermal equilibrium 5

3.2 The interaction of light with matter 6

Absorption 6

Spontaneous Emission 6

Stimulated Emission 7

3.3 Creating a Population Inversion 7

Three Level Lasers 8

Four Level Lasers 9

3.4 Frequency ranges used in Optical Amplification 11

4. Optical Amplifier 12

4.1 Laser Amplifier 12

4.2 Doped Fiber Amplifier 12

4.3 Erbium Doped Fiber Amplifier 13

4.4 Doped Fiber Amplifier for the other wavelength ranges 13

4.5 Limitations On Practical Bandwidths 15

5. What is an EDFA 16

5.1 Physical Model 16

5.2 How to think of an EDFA 16

5.3 Setup and Operation Principle 18

5.4 Gain Spectrum 20

6. Applications 21

7. Conclusion 23

References

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INTRODUCTION

An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. An optical amplifier may be thought of as a laser without an optical cavity, or one in which feedback from the cavity is suppressed. Stimulated emission in the amplifier's gain medium causes amplification of incoming light. Optical amplifiers are important in optical communication and laser physics.

In early days in place of optical amplifiers repeaters are used. Repeaters converts optical signal to electrical signal and amplifies and again reproduces the optical signals. This causes noise and speed loss. Optical amplifiers are used overcome the above. Most widely used optical amplifiers are EDFA, TDFA, PDFA, Raman amplifiers.

Erbium-Doped Fiber Amplifier is device that boosts the signal in an optical fiber. Introduced in the later 1980s, the EDFA was the first successful optical amplifier. It was a major factor in the rapid development of fiber-optic networks in the 1990s, because it extended the distance between costly regenerators. In addition, an EDFA amplifies all the channels in a WDM signal simultaneously, whereas regenerators require optical to electrical conversion for each channel.

Erbium-doped fiber amplifiers are the by far most important fiber amplifier in the context of long range optical communications. Functioning like a laser without mirrors, the EDFA uses a semiconductor pump laser to introduce a powerful beam at a shorter wavelength into a section of erbium-doped fiber several meters long. The pump light excites the erbium atoms to higher orbits, and the input signal stimulates them to release excess energy as photons in phase and at the same wavelength. EDFAs boost wavelengths in the 1550 nm range, and the pump light is typically 1480 nm or 980 nm.

Over the past years, erbium-doped fiber amplifiers (EDFAs) have received great attention due to their characteristics of high gains, bandwidths, low noises and high efficiencies. As a key device, EDFA configures wavelength division multiplexing systems (WDMs) in optical telecommunications, finding a variety of applications in traveling-wave fiber amplifiers, nonlinear optical devices and optical switches.

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EDFA- An Overview

Erbium-doped fiber amplifier (EDFA) Invented by a group of Davidpayne, R.Mears & L. Reekie from the university of southampton and AT&T Lab. Commercially available since the early 1990’s Works best in the range 1530 to 1565 nm, Gain up to 30 dB .An erbium amplifier, also called optical amplifier or an erbium-doped fiber amplifier or EDFA, is an optical or IR repeater that amplifies a modulated laser beam directly, without opto-electronic and electro-optical conversion. The device uses a short length of optical fiber doped with the rare-earth element erbium. When the signal-carrying laser beams pass through this fiber, external energy is applied, usually at IR wavelengths. This so-called pumping excites the atoms in the erbium-doped section of optical fiber, increasing the intensity of the laser beams passing through. The beams emerging from the EDFA retain all of their original modulation characteristics, but are brighter than the input beams.

In fiber optic communications systems, problems arise from the fact that no fiber material is perfectly transparent. The visible-light or infrared (IR) beams carried by a fiber are attenuated as they travel through the material. This necessitates the use of repeaters in spans of optical fiber longer than about 100 kilometers.

A conventional repeater puts a modulated optical signal through three stages: (1) optical-to-electronic conversion, (2) electronic signal amplification, and (3) electronic-to-optical conversion. (The term optical encompasses IR as well as visible-light energy in this context.) Repeaters of this type limit the bandwidth of the signals that can be transmitted in long spans of fiber optic cable. This is because, even if a laser beam can transmit several gigabits per second (Gbps) of data, the electronic circuits of a conventional repeater cannot.

Besides eliminating complex and inefficient conversion and electronic amplification stages, the EDFA allows the transmission of signals that employ wavelength-division multiplexing (WDM). This increases the realizable bandwidth relative to conventional repeaters still further (1000 photons out per photon in).

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BASICS OF OPTICAL AMPLIFICATION

Population Inversion

In physics, specifically statistical mechanics, a population inversion occurs when a system (such as a group of atoms or molecules) exists in state with more members in an excited state than in lower energy states. The concept is of fundamental importance in laser science because the production of a population inversion is a necessary step in the workings of a laser (Light Amplification by Stimulated Emission of Radiation").

Boltzmann distributions and thermal equilibrium

To understand the concept of a population inversion, it is necessary to understand some thermodynamics and the way that light interacts with matter. To do so, it is useful to consider a very simple assembly of atoms forming a laser medium.

Assume there are a group of N atoms, each of which is capable of being in one of two energy states, either

1. The ground state, with energy E1; or2. The excited state, with energy E2, with E2>E1.

The number of these atoms which are in the ground state is given by N1, and the number in the excited state N2. Since there are N atoms in total,

N1 + N2 = N

The energy difference between the two states is given by

ΔE = E2 − E1

determines the characteristic frequency ν21 of light which will interact with the atoms; This is given by the relation

E2 − E1 = ΔE = hν21, h being Planck's constant.

If the group of atoms is in thermal equilibrium, it can be shown from thermodynamics that the ratio of the number of atoms in each state is given by a Boltzmann distribution:

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,

where T is the temperature of the group of atoms, and k is Boltzmann's constant. T must be given in kelvins or degrees Rankine, not degrees Celsius or degrees Fahrenheit.

We may calculate the ratio of the populations of the two states at room temperature (T≈300 K) for an energy difference ΔE that corresponds to light of a frequency corresponding to visible light (ν≈5*1014 Hz). In this case ΔE = E2 - E1 ≈ 2.07 eV, and kT ≈ 0.026 eV. Since E2 - E1 >> kT, it follows that the argument of the exponential in the equation above is a large negative number, and as such N2/N1 is vanishingly small; i.e., there are almost no atoms in the excited state. When in thermal equilibrium, then, it is seen that the lower energy state is more populated than the higher energy state, and this is the normal state of the system. As T increases, the number of electrons in the high-energy state (N2) increases, but N2 never exceeds N1 for a system at thermal equilibrium; rather, at infinite temperature, the populations N2 and N1 become equal. In other words, a population inversion (N2/N1 > 1) can never exist for a system at thermal equilibrium. To achieve population inversion therefore requires pushing the system into a non-equilibrated state.

The interaction of light with matter

There are three types of possible interactions between a system of atoms and light that are of interest:

Absorption

If light (photons) of frequency ν21 pass through the group of atoms, there is a possibility of the light being absorbed by atoms which are in the ground state, which will cause them to be excited to the higher energy state. The probability of absorption is proportional to the radiation intensity of the light, and also to the number of atoms currently in the ground state, N1.

Spontaneous emission

If a collection of atoms are in the excited state, spontaneous decay events to the ground state will occur at a rate proportional to N2, the number of atoms in the excited state. The energy difference between the two states ΔE is emitted from the atom as a photon of frequency ν21 as given by the frequency-energy relation above.

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The photons are emitted stochastically, and there is no fixed phase relationship between photons emitted from a group of excited atoms; in other words, spontaneous emission is incoherent. In the absence of other processes, the number of atoms in the excited state at time t, is given by

,

where N2(0) is the number of excited atoms at time t=0, and τ21 is the lifetime of the transition between the two states.

Stimulated emission

If an atom is already in the excited state, it may be perturbed by the passage of a photon which has a frequency ν21 corresponding to the energy gap ΔE of the excited state to ground state transition. In this case, the excited atom relaxes to the ground state, and is induced to produce a second photon of frequency ν21. The original photon is not absorbed by the atom, and so the result is two photons of the same frequency. This process is known as stimulated emission. The rate at which stimulated emission occurs is proportional to the number of atoms N2 in the excited state, and the radiation density of the light. Therefore, when the numbers of atoms in the ground and excited states are equal, the rate of stimulated emission is equal to the rate of absorption for a given radiation density.

The critical detail of stimulated emission is that the induced photon has the same frequency and phase as the inducing photon. In other words, the two photons are coherent. It is this property that allows optical amplification, and the production of a laser system. During the operation of a laser, all three light-matter interactions described above are happening. Initially, atoms are energised from the ground state to the excited state by a process called pumping. Some of these atoms decay via spontaneous emission, releasing incoherent light as photons of frequency ν. These photons are fed back into the laser medium, usually by an optical resonator. Some of these photons are absorbed by the atoms in the ground state, and the photons are lost to the laser process. However, some photons cause stimulated emission in excited-state atoms, releasing another coherent photon. In effect, this results in optical amplification.

Creating a population inversion

As described above, a population inversion is required for laser operation, but cannot be achieved in our theoretical group of atoms with two energy-levels when they are in thermal equilibrium. In fact, any method by which the atoms

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are directly and continuously excited from the ground state to the excited state (such as optical absorption) will eventually reach equilibrium with the de-exciting processes of spontaneous and stimulated emission. At best, an equal population of the two states, N1 = N2 = N/2, can be achieved, resulting in optical transparency but no net optical gain.

Three-level lasers

A three-level laser energy diagram.

To achieve non-equilibrium conditions, an indirect method of populating the excited state must be used. To understand how this is done, we may use a slightly more realistic model, that of a three-level laser. Again consider a group of N atoms, this time with each atom is able to exist in any of three energy states, levels 1, 2 and 3, with energies E1,E2 and E3, and populations N1, N2 and N3, respectively.

Note that E1 < E2 < E3; that is, the energy of level 2 lies between that of the ground state and level 3.

Initially, the system of atoms is at thermal equilibrium, and the majority of the atoms will be in the ground state: i.e. N1 ≈ N, N2 ≈ N3 ≈ 0. If we now subject the atoms to light of a frequency ν31, where E3-E1 = hν31, the process of optical absorption will excite the atoms from the ground state to level 3. This process is called pumping, and in general does not always directly involve light absorption; other methods of exciting the laser medium, such as electrical discharge or chemical reactions may be used. The level 3 is sometimes referred to as the pump level or pump band, and the energy transition E1 → E3 as the pump transition, which is shown as the arrow marked P in the diagram above.

If we continue pumping the atoms, we will excite an appreciable number of them into level 3, such that N3 > 0. In a medium suitable for laser operation, we

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require these excited atoms to quickly decay to level 2. The energy released in this transition may be emitted as a photon (spontaneous emission), however in practice the 3→2 transition (labeled R in the diagram) is usually radiation less, with the energy being transferred to vibrational motion (heat) of the host material surrounding the atoms, without the generation of a photon.

An atom in level 2 may decay by spontaneous emission to the ground state, releasing a photon of frequency ν21 (given by E2-E1 = hν21), which is shown as the transition L, called the laser transition in the diagram. If the lifetime of this transition, τ21 is much longer than the lifetime of the radiationless 3→2 transition τ32 (if τ21 >> τ32, known as a favourable lifetime ratio), the population of the E3 will be essentially zero (N3 ≈ 0) and a population of excited state atoms will accumulate in level 2 (N2 > 0). If over half the N atoms can be accumulated in this state, this will exceed the population of the ground state N1. A population inversion (N2 > N1 ) has thus been achieved between level 1 and 2, and optical amplification at the frequency ν21 can be obtained.

Because at least half the population of atoms must be excited from the ground state to obtain a population inversion, the laser medium must be very strongly pumped. This makes three-level lasers rather inefficient, despite being the first type of laser to be discovered (based on a ruby laser medium, by Theodore Maiman in 1960. A three-level system could also have a radiative transition between level 3 and 2, and a nonradiative transition between 2 and 1. In this case, the pumping requirements are weaker. In practice, most lasers are four-level lasers, described below.

Four-level lasers

A four-level laser energy diagram.

Here, there are four energy levels, energies E1, E2, E3, E4, and populations N1, N2, N3, N4, respectively. The energies of each level are such that E1 < E2 < E3 < E4.

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http://en.wikipedia.org/wiki/Image:Population-inversion-4level.png


In this system, the pumping transition P excites the atoms in the ground state (level 1) into the pump band (level 4). From level 4, the atoms again decay by a fast, non-radiative transition Ra into the level 3. Since the lifetime of the laser transition L is long compared to that of Ra (τ32 >> τ43), a population accumulates in level 3 (the upper laser level), which may relax by spontaneous or stimulated emission into level 2 (the lower laser level). This level likewise has a fast, non-radiative decay Rb into the ground state.

As before, the presence of a fast, radiationless decay transitions result in population of the pump band being quickly depleted (N4 ≈ 0). In a four-level system, any atom in the lower laser level E2 is also quickly de-excited, leading to a negligible population in that state (N2 ≈ 0). This is important, since any appreciable population accumulating in level 3, the upper laser level, will form a population inversion with respect to level 2. That is, as long as N3 > 0, then N3 > N2 and a population inversion is achieved. Thus optical amplification, and laser operation, can take place at a frequency of ν32 (E3-E2 = hν32).

Since only a few atoms must be excited into the upper laser level to form a population inversion, a four-level laser is much more efficient than a three-level one, and most practical lasers are of this type. In reality, many more than four energy levels may be involved in the laser process, with complex excitation and relaxation processes involved between these levels. In particular, the pump band may consist of several distinct energy levels, or a continuum of levels, which allow optical pumping of the medium over a wide range of wavelengths.

Note that in both three- and four-level lasers, the energy of the pumping transition is greater than that of the laser transition. This means that, if the laser is optically pumped, the frequency of the pumping light must be greater than that of the resulting laser light. In other words, the pump wavelength is shorter than the laser wavelength. It is possible in some media to use multiple photon absorptions between multiple lower-energy transitions to reach the pump level; such lasers are called up-conversion lasers.

In some media it is possible, by imposing an additional optical or microwave field, to use quantum coherence effects to reduce the likelihood of an excited-state to ground-state transition. This technique, known as lasing without inversion, allows optical amplification to take place without producing a population inversion between the two states.

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Frequency ranges used in Optical Amplification

Table below shows different wave length bands used in optical fiber Amplification.

Band Description Wavelength Range

O band original 1260 to 1360 nm

E band extended 1360 to 1460 nm

S band short wavelengths 1460 to 1530 nm

C band conventional ("erbium window") 1530 to 1565 nm

L band long wavelengths 1565 to 1625 nm

U band ultra long wavelengths 1625 to 1675 nm

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OPTICAL AMPLIFIER

An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. An optical amplifier may be thought of as a laser without an optical cavity, or one in which feedback from the cavity is suppressed. Stimulated emission in the amplifier's gain medium causes amplification of incoming light. Optical amplifiers are important in optical communication and laser physics. Direct optical amplification using erbium doped fiber amplifiers (EDFAs) is now preferred over optoelectronic repeaters as the primary means of restoring the signal power in long distance fiber optic links and branched networks. These amplifiers naturally provide gain at very high bit rates and at many wavelengths in a broad band stretching from 1530nm to1580nm.Without them high data rate wavelength division multiplexed communications systems would not exist. Different types of Optical Amplifiers are explained below.

Laser amplifiers

Almost any laser active gain medium can be pumped to produce gain for light at the wavelength of a laser made with the same material as its gain medium. Such amplifiers are commonly used to produce high power laser systems. Special types such as regenerative amplifiers and chirped-pulse amplifiers are used to amplify ultra short pulses.

Doped fibre amplifiers

Schematic diagram of a simple Doped Fibre Amplifier

Doped fibre amplifiers (DFAs) are optical amplifiers that use a doped optical fibre as a gain medium to amplify an optical signal. They are related to fibre lasers. The signal to be amplified and a pump laser are multiplexed into the doped fibre, and the signal is amplified through interaction with the doping ions. The most common example is the Erbium Doped Fiber Amplifier (EDFA), where the core of a silica fiber is doped with trivalent Erbium ions (Er+3) and can be efficiently pumped with a laser at 980 nm or at 1,480 nm, and exhibits gain in the 1,550 nm region.

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http://en.wikipedia.org/wiki/Image:Doped_fibre_amplifier.svg


Amplification is achieved by stimulated emission of photons from dopant ions in the doped fibre. The pump laser excites ions into a higher energy from where they can decay via stimulated emission of a photon at the signal wavelength back to a lower energy level. The excited ions can also decay spontaneously (spontaneous emission) or even through nonradiative processes involving interactions with phonons of the glass matrix. These last two decay mechanisms compete with stimulated emission reducing the efficiency of light amplification.

The amplification window of an optical amplifier is the range of optical wavelengths for which the amplifier yields a usable gain. The amplification window is determined by the spectroscopic properties of the dopant ions, the glass structure of the optical fibre, and the wavelength and power of the pump laser.

Erbium-doped fibre amplifiers

The erbium-doped fibre amplifier (EDFA) is the most deployed fibre amplifier as its amplification window coincides with the third transmission window of silica-based optical fibre. Two bands have developed in the third transmission window - the Conventional, or C-band, from approximately 1525 nm - 1565 nm, and the Long, or L-band, from approximately 1570 nm to 1610 nm. Both of these bands can be amplified by EDFAs, but it is normal to use two different amplifiers, each optimized for one of the bands.It is explained in detail in the following chapter.

Doped fibre amplifiers for other wavelength ranges

Thulium doped fibre amplifiers have been used in the S-band (1450-1490 nm) and Praseodymium doped amplifiers in the 1300 nm region. However, those regions have not seen any significant commercial use so far and so those amplifiers have not been the subject of as much development as the EDFA. However, Ytterbium doped fiber lasers and amplifiers, operating near 1 micrometre wavelength, have many applications in industrial processing of materials, as these devices can be made with extremely high output power (tens of kilowatts).

Semiconductor Optical Amplifier (SOA)

Semiconductor optical amplifiers are amplifiers which use a semiconductor to provide the gain medium. These amplifiers have a similar structure to Fabry-Perot laser diodes but with anti-reflection design elements at the endfaces. Recent designs include anti-reflective coatings and tilted waveguide and window regions which can reduce endface reflection to less than 0.001%. Since

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this creates a loss of power from the cavity which is greater than the gain it prevents the amplifier from acting as a laser.The semiconductor optical amplifier is of small size and electrically pumped. It can be potentially less expensive than the EDFA and can be integrated with semiconductor lasers, modulators, etc. However, the performance is still not comparable with the EDFA. The SOA has higher noise, lower gain, moderate polarization dependence and high nonlinearity with fast transient time.

Raman amplifier

In a Raman amplifier, the signal is intensified by Raman amplification. Unlike the EDFA and SOA the amplification effect is achieved by a nonlinear interaction between the signal and a pump laser within an optical fibre. There are two types of Raman amplifier: distributed and lumped. A distributed Raman amplifier is one in which the transmission fibre is utilised as the gain medium by multiplexing a pump wavelength with signal wavelength, while a lumped Raman amplifier utilises a dedicated, shorter length of fibre to provide amplification. In the case of a lumped Raman amplifier highly nonlinear fibre with a small core is utilised to increase the interaction between signal and pump wavelengths and thereby reduce the length of fibre required.

Optical parametric amplifier

An optical parametric amplifier allows the amplification of a weak Signal-Impulse in a noncentrosymmetric nonlinear medium (e.g. BBO). In contrast to the previously mentioned amplifiers, which are mostly used in telecommunication environments, this type finds its main application in expanding the frequency tunability of ultrafast solid-state lasers (e.g. Ti:sapphire). By using a noncollinear interaction geometry Optical Parametric Amplifiers are capable of extreme broad amplification bandwidths.

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Optical Amplifier: Limitations on Practical Bandwidths

EDFAs popular in C-band Raman: proposed for S-bandGain-shifted EFDA for L-band

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WHAT IS AN EDFA?

PHYSICAL MODEL:

An Erbium Doped Fiber Amplifier (EDFA) consists of a piece of fiber of length L, whose core is uniformly doped with Erbium ions. Such ions can be thought of as simple two-level systems, i.e., they can have only two energy states: 1) a fundamental state and 2) an excited state.

A strong ``pump'' laser light at the proper wavelength (usually 980 nm or 1480 nm) is propagated into the core of the fiber in order to excite its ions.

Excited ions, when ``hit'' by an input ``signal'' photon, have a certain probability (depending on the wavelength of the input photon) of releasing by stimulated emission a photon identical to the hitting one. The release of the energy of the stimulated photon brings the excited ion to its fundamental state.

Hence from one input ``signal'' photon we can obtain, by an avalanche process along the fiber core, an average of G photons at the fiber output, i.e., the signal photon has been amplified by a factor G, known as the Gain. Since the probability of stimulated emission depends on the photon wavelength, so does the Gain. The Gain clearly also depends on the total number of excited ions in the fiber, since the more the ions, the larger the average number of stimulated emissions per input photon.

In Optical Communication systems based on Wavelength Division Multiplexing, N signal laser beams at N different wavelengths (each carrying modulated user information) are coupled into the EDFA and propagate down the fiber along with the pump. While the pump ``loses'' photons to excite the ions as it propagates, the signals stimulate emission from the excited ions and ``gain'' photons.

HOW TO THINK OF AN EDFA:

The figure shows a simple but correct way of thinking about the amplification process in the EDFA.

The EDFA is a sort of ``hydraulic system'', composed of a tank containing a ``reservoir'' of charges (the white balls in the figure, physically corresponding to the excited ions in the fiber) ready to be converted into output photons by stimulated emission. The reservoir is limited to a maximum value, i.e., to the total number of ions in the doped fiber.

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Consider now an input red photon (i.e., one at a specific wavelength, or color). As it enters the EDFA it draws from the reservoir on the average Gr-1 charges, which get colored the same color as the input photon and give at the output Gr signal photons (r stands for red, as the gain depends on wavelength). The same is true for a green input photon, which draws an average of Gg-1 charges, or a blue input photon, drawing Gb-1 charges.

That is, all input signal photons draw charges from the same reservoir, which is therefore subject to oscillations when the signal photon fluxes (average number of input photons per second) abruptly vary in time due to the sudden addition of new channels or to the removal of existing channels. Such oscillations of the reservoir level occur like in any water basin.

The fact that all signals draw from the same reservoir is a consequence of what is known as the ``homogeneous broadening''. Since a lower reservoir level corresponds to a lower gain for all signals, the variations of the reservoir level correspond to variations of the gain and hence of the output signal photon fluxes, a phenomenon known as cross-gain modulation.

The tank in the figure is shown to have leakage of charges. This physically corresponds to fluorescence, i.e., the process of natural relaxation of ions from the excited state to the fundamental state, generating non-stimulated photons

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with wavelengths spread over a wide spectral range. The amplification of such photons along the amplifier generates the so-called amplified spontaneous emission noise (ASE). The figure, for simplicity of drawing, neglects the fact that the leakage flux actually enters the amplifier for amplification.

Finally, it is shown that the purple pump photons feed the reservoir. i.e., the purple photons enter the tank and lose their color as they get converted into excited ions. Some purple pump photons are also shown in the leakage flux, as not all pump photons are absorbed by the dopant ions.

The erbium-doped fibre amplifier (EDFA) is the most deployed fibre amplifier as its amplification window coincides with the third transmission window of silica-based optical fibre.Two bands have developed in the third transmission window - the Conventional, or C-band, from approximately 1525 nm - 1565 nm, and the Long, or L-band, from approximately 1570 nm to 1610 nm. Both of these bands can be amplified by EDFAs, but it is normal to use two different amplifiers, each optimized for one of the bands.

The principal difference between C- and L-band amplifiers is that a longer length of doped fibre is used in L-band amplifiers. The longer length of fibre allows a lower inversion level to be used, thereby giving at longer wavelengths (due to the band-structure of Erbium in silica) while still providing a useful amount of gain.

EDFAs have two commonly-used pumping bands - 980 nm and 1480 nm. The 980 nm band has a higher absorption cross-section and is generally used where low-noise performance is required. The absorption band is relatively narrow and so wavelength stabilised laser sources are typically needed. The 1480 nm band has a lower, but broader, absorption cross-section and is generally used for higher power amplifiers. A combination of 980 nm and 1480 nm pumping is generally utilised in amplifiers.

Setup and Operation Principle

A typical setup of a simple erbium-doped fiber amplifier (EDFA) is shown in the figure below. Its core is the erbium-doped optical fiber, which is typically a single-mode fiber. In the shown case, the active fiber is "pumped" with light from two laser diodes (bidirectional pumping), although unidirectional pumping in forward or backward direction (co-directional and counter-directional pumping) is also very common. The pump light, which most often has a wavelength around 980 nm and sometimes around 1450 nm, excites the erbium ions (Er3+) into the 4I13/2 state (in case of 980-nm pumping via 4I11/2), from where

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they can amplify light in the 1.5-μm wavelength region via stimulated emission back to the ground-state manifold 4I15/2.

Fig.: Schematic setup of a simple erbium-doped fiber amplifier. Two laser diodes (LDs) provide the pump power for the erbium-doped fiber.

The shown setup also contains two "pig-tailed" (fiber-coupled) optical isolators. The one at the input prevents light originating from amplified spontaneous emission from disturbing any previous stages, while the isolator at the output avoids lasing (or possibly even destruction) in case that output light is reflected back to the amplifier. Without isolators, fiber amplifiers can be quite sensitive to back reflections.Very high signal gains, as used e.g. for the amplification of

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An EDFA amplifier consists of an erbium-doped silica fiber, an optical pump, a coupler and isolators at both ends.


ultrashort pulses to high energies, are usually realized with amplifier chains, consisting of several amplifier stages with additional optical elements (e.g. isolators, filters, or modulators) in between.

Gain Spectrum

The shape of the erbium gain spectrum depends on the host glass as well as on the excitation level, because the erbium ion represents a quasi-three-level gain medium. The figure below shows data for a common type of glass, which is some variant of silica with additional dopants e.g. to avoid clustering of erbium ions.

Fig.: Gain and absorption (negative gain) of erbium (Er3+) ions in germano-alumino-silicate glass for excitation levels from 0 to 100% in steps of 10%.

Strong three-level behavior (with transparency reached only for >50% excitation) occurs at 1535 nm, where the unpumped fiber exhibits substantial losses, while the high emission cross section allows for a high gain for strong excitation. At longer wavelengths (e.g. 1580 nm), a lower excitation level is required for obtaining gain, but the maximum gain is smaller.

The maximum gain typically occurs in the wavelength region around 1530-1560 nm, with the 1530-nm peak being most pronounced for high excitation levels. The local excitation level depends on the emission and absorption cross sections and on the pump and signal intensity (apart from that of ASE light). The average excitation level over the whole fiber length depends on pump and signal powers, but also on the fiber length and the erbium concentration. Such

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parameters are used to optimize EDFAs for a particular wavelength region, such as e.g. the telecom C or L band.

APPLICATIONS

EDFAs can serve various functions in systems for optical fiber communications; the most important applications are described in the following:

The power of a data transmitter may be boosted with a high power EDFA before entering a long fiber span, or a device with large losses, such as e.g. a fiber splitter. Such splitters are widely used e.g. in cable-TV systems, where a single transmitter is used to deliver signals into many fibers.

A fiber amplifier may also be used in front of a data receiver, if the arriving signal is weak. Despite the introduction of amplifier noise, this can improve the signal-to-noise ratio and thus the possible data transmission rate, since the amplifier noise may be weaker than the input noise of the receiver. It is more common, however, to use avalanche photodiodes, which have some built-in signal amplification.

In-line EDFAs are used between long spans of passive transmission fiber. Using multiple amplifiers in a long fiber-optic link has the advantage that large transmission losses can be compensated without (a) letting the optical power drop to too low levels, which would spoil the signal-to-noise ratio, and (b) without transmitting excessive optical powers at other locations, which would cause detrimental nonlinear effects due to the unavoidable fiber nonlinearities. Many of these in-line EDFAs are operated even under difficult conditions, e.g. at the ground of oceans, where maintenance would be hardly possible.

While data transmitters are normally not based on erbium-doped devices, EDFAs are often part of equipment for testing transmission hardware. They are also used in the context of optical signal processing.

These functions can be realized in the telecom C band and L band. Other types of fiber amplifiers, e.g. based on praseodymium, have been considered for other bands, but none can compete with erbium-based devices in terms of gain and gain efficiency.

A particular attraction of EDFAs is their large gain bandwidth, which is typically tens of nanometers and thus actually more than enough to amplify data channels with highest data rates without introducing any effects of gain narrowing. One may even use a single EDFA to simultaneously amplify many data channels at different wavelengths within the gain region; this technique is

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called wavelength division multiplexing. Before such fiber amplifiers were available, there was no practical method to amplify all channels e.g. between long fiber spans of a fiber-optic link: one had to separate all data channels, detect and amplify them electronically, optically resubmit and again combine them. Particularly for operation at the ground of an ocean, the introduction of fiber amplifiers offered an enormous reduction of the complexity, along with a corresponding increase of reliability. Particularly by using redundant down-rated pump diodes, one can make EDFAs with very long lifetimes.

The only competitors to erbium-doped fiber amplifiers in the 1.5-μm region are Raman amplifiers, which profit from the development of higher power pump lasers. Raman amplification can also be done in the transmission fiber. Nevertheless, EDFAs stay very dominant.

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CONCLUSION

In conclusion we have studied about EDFA, its working principles and Applications. Erbium-doped fiber amplifiers are the by far most important fiber amplifiers in the context of long-range optical fiber communications. They can efficiently amplify light in the 1.5-μm wavelength region, where telecom fibers have their loss minimum. . Direct optical amplification using erbium doped fiber amplifiers (EDFAs) is now preferred over optoelectronic repeaters as the primary means of restoring the signal power in long distance fiber optic links and branched networks. The Erbium Doped Fiber Amplifier (EDFA) has now replaced optoelectronic repeaters as the primary design option for extending the range and capacity of the World’s fiber optic telecommunications systems. Over the past years, erbium-doped fiber amplifiers (EDFAs) have received great attention due to their characteristics of high gains, bandwidths, low noises and high efficiencies. As a key device, EDFA configures wavelength divisionmultiplexing systems (WDMs) in optical telecommunications, including a variety of applications in traveling-wave fiber amplifiers, nonlinear optical devices and optical switches.

References

1) www.wikipedia.org2) www.ieeexplore.org

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Documents

Edfa Seminar