unit 9 - optical amplifier

8/12/2019 Unit 9 - Optical Amplifier

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Unit 9

Optical Amplifiers


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Necessity of Optical

amplifiers?

To Transmit a signals over long

distances (>100km), to compensateattenuation losses.


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Amplification:

OPTOELECTRONIC

MODULE Initially this wasaccomplished

with an optoelectronic module

Light to electron conversion

Electrical amplification

Pulse shaping

Electrical to optical conversion


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Limitations of Optoelectronic

amplifiers

Limits the speed of the system

Makes the system expensive andcomplex.


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Optical Amplifiers

Replace the optoelectronic amplifiers

OA eliminates the need of optical to

electron and electron to opticalconversion.

OA: Directly boosts the optical signal.


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Generic optical amplifier

Continuous Wave

(Constant)

Energy is transferred from the pump to signal


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Introduction: Optical Amplifier

OA is an essential element for

High capacity, Long lifespan,

Multiple connection of optical for

communication network

applications.

I t d ti OPTICAL


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Introduction-OPTICAL

AMPLIFIER

An optical amplifieris a device which amplifies the opticalsignal directly without ever changing it to electricity. The light

itself is amplified.

Reasons to use the optical amplifiers:

Reliability (long life and stable operation)

Flexibility (easy maintenance)

Easily incorporated in Wavelength Division Multiplexing

(WDM)

Low CostExtends the system margin

Lessens the effect of dispersion and attenuation

Therefore gives improved performance for long haul

communication.


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Application of OA

To compensate for losses in

Long distance point to point optical

fiber links

Multi-access networks


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Applications of optical amplifiers


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Applicaitions

Inline amplifier Compensate the fiber attenuation

Dispersion in fiber is less.

Does not need complete regeneration ofsignal.

Just amplification is needed

OA compensates for transmission lossesand increases the distance between

regenerative repeaters


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Application of OA

Preamplifier Used in front end preamplifier for an

optical receiver.

Weak optical signal is amplified beforephoto detection.

Signal to noise ratiodegradation caused

due to thermal noise in receiver

electronics is reduced.

OA provides larger gain, high sensitivity,

broader bandwidth.


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Application of OA

Power amplifier Power or booster amplifier

Amplifier device immediately after

transmitter to boost up the transmittedpower.

Increases the transmission distance

Power boosters allows repeater less

transmission (Ex: undersea transmission

distance of 200-250km0


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Types of Optical Amplifier

Semiconductor Optical Amplifier

(SOA)

Rare Earth doped Fiber Amplifier

(EDFA)

Raman Amplifier


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Basic Concepts

Optical gain is realized when the

amplifier is pumped optically (or

electrically) to achievepopulationinversion

Gain depends on wavelength,internal light intensity and amplifier

medium


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Semiconductor Optical

Amplifier

SOA is a InGaAsP/InP laser that is

operating below the threshold point.

External pumping (current injection) isused to create population inversion.

Population inversion is done to

achieve the gain mechanism in SOA

(amplification through stimulated

emission).


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Structure of SOA


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Condition for Amplification

by Stimulated Emission

Population Inversion:

More Electrons in higher energy level

Pumping:

Process to achieve population inversion

usually through external energy source


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Working of SOA

Stimulated emission creates in

phase photons


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Coherence

Spontaneous emission Stimulated emission

Incoherent light waves Coherent light waves

All the inphase photons add up

to create a much stronger light

signal.


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Working of SOA

Pump current is given

Semiconductor absorbs the pumpenergy to create population inversion.

Population inverison: electrons movefrom low energy band to high energy

band.


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Working of SOA

The incoming optical signal (which isto be amplified) causes stimulated

emission.

The incoming photons strikes theexcited electrons to drop down to the

lower level.

This causes the new photons of equalenergy and in phase to the incident

photons.

This leads to amplified optical signal


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Advantages of SOA

Compact Can be easily integrated with other

optical devices.

Consumes less electrical power Used in O band (1310 nm)

Can operate at 800 , 1300, 1500 nm

wavelength Relatively large gain


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Limitation of SOA

High noise figure and cross talk.

Limited in operation below 10Gbps.

High data rate is possible but withlower gain.


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EDFA : Eribium doped Fiber

amplifier EDFA is used in long distance

communication.

Uses Silica fiber doped with Eribium

Eribium doped silica

Operating range is 1530 to 1560nm

Operating region of EDFA depends on

the host material and the dopingelement,


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EDFA working principle


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EDFA

The active medium consists of 10 m to 30 m length optical fiber

Lightly doped by rare earth elements such

as Eribium (Er)

Ytterbium (Yb)

Thulium (Tm)

Praseodymium (Pr)

Doping is 1000 parts in million (1000ppm)


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Erbium energy-level diagram


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EDFA

Energy Level Diagram

Process of Amplification

Transition Process

Spontaneous Emission Process

Stimulated Emission Process


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Energy band diagram of a

EDFA EDFA has following energy levels

Pump band Designated as 4L(11/2)

Exists at 1.27eV from the ground state band 4L

(15/2) .

The 1.27eV corresponds to a wavelength of

980nm.

Top metastable band 4L(13/2) Seperated from ground state band 4L(15/2) by

0.841eV.

0.841eV corresponds to a wavelength of

1480nm.


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EDFA

EDFA has following energy levels

Bottom metastable band

Seperated from the ground state band by0.814eV

0.814 eV corresponds to 1530nm wavelength.

Bottom of the metastable band seperated from

top of ground state by 0.775eV. 0.775eV corresponds to 1600nm wavelength.


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Basic Principle

EDFA: Power is transferred from PUMP

source to WEAK signal.

Amplification occurs through

STIMULATED EMISSION PROCESS.

B i P i i l


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Basic Principle

EDFA works similar to laser without thereflectors.

Amplification occurs through

STIMULATED EMISSION PROCESS. Medium is pumped with pump signal until

POPULATION INVERSION is achieved.

Gives high power transfer efficiency frompump source to the signal power.

EDFA: Power is transferred from PUMP

source to WEAK signal.


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EDFA Transition Process: Optical Pump

Pumping is done optically with primary pump

wavelengths at 980nm or 1480 nm.

Input photons of 980 nm exites the ions of ground state

to the pump level.

These excietd electrons relax very quickly from pump

level to meta stable level. (Transnsiton process)

During this decay of electrons from pump level to

metastable level excess energy is released as

photons.

Another possible pump wavelength is 1480 nm.

The absorption of 1480nm pump photons excites the

electons from ground level to metastable level(Transition process)


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EDFA Spontaneous Emission Process:

Some of the electrons present at the metastable state

will decay to the ground state generating photons of

1550nm.

Stimulated Emission

The weak light signal to be amplified when

passing through the amplifier causes the

electrons in the metastable state to drop downto the ground state,

Thereby generating new photons of same

energy, phase, polarization as the incoming

photons.


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Advantages of EDFA

High power transfer efficiency from pumpto signal power (>50%).

Wide spectral band amplification with

relatively flat gain useful for WDM

applications,

Large dynamic range

Suitable for long distance communication

Low noise figure.

Polarization independent


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Device size is large (km length of fiber)

Not easily integrable with other devices.

Limitation of EDFA


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~1550 nm

980 nm

RadiationlessDecay

~1550 nm

Pump

Signal

Output

Optical Pumping to Higher Energy levels Rapid Relaxation to "metastable" State

Stimulated Emission and Amplification

N1

N2

N3

N1

N2

N3

N1

N2

N3

Amplification Process of EDFA


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Gain versus EDFA length

There is an

optimum length

that gives the

highest gain Negative gain if

too long


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Fig. : Gain-flattened EDFA-B


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Raman Amplifier

Optical Amplifier has non linear scattering effects.

SBS

SRS

Photon when incident on scatter produces a

new photon in forward and backward direction

with a frequency shift and energy transfer. SRS progressively transfers power from shorter

wavelength to longer wavelength.


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Raman Fiber Amplifiers (RFAs) rely on an

intrinsic non-linearity in silica fiber (SRS

Scattering)

This amplifier is used at the receiver end

to boost up the signal attenuated over theentire fiber length.

Raman Amplifiers


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Features of Raman Amplifier

Simpler to design

Uses intrinsic optical nonlinearity of fiber

Amplification takes place throughout the length of

fiber

Hence also known as Distributed Amplifier


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Stimulated Raman ScatteringStimulated Raman Scatter ing (SRS)

causes a new signal (a Stokes wave) to be

generated in the same direction as thepump wave down-shifted in frequency by

13.2 THz (due to molecular vibrations)

provided that the pump signal is of

sufficient strength.

R A lifi i


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Raman Amplification

Raman pumping takes place backwards over the fiber

(Backward Pumping) Gain is a maximum close to the receiver and

decreases in the transmitter direction

Useful for compensating the losses.

TransmitterOptical

ReceiverEDFA

Raman

Pump

Laser

Long Fiber Span

W ki i i l f R


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Working principle of Raman

amplifier

Over the length of the fiber attenuation takes place.

To overcome the attenuation at receiver end of fiber we

introduce a high energy pump signal of lower frequency .

Physics behind is called Stimulated Raman Scattering

Fpump = fsignal 13THz

This pump signal while traveling through the fiber towards

transmitter will get scattered and produce a new signal (stokessignal)

Stokes signal is in backward direction with a frequency shift of

13THz (stokes signal is a low wavelength signal)


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Raman Amplifier

Raman gain depends on the

pump power

frequency offset between pump and

signal.


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With only an EDFA at the transmit end the optical powerlevel decreases over the fiber length

With an EDFA and Raman the minimum optical power

level occurs toward the middle, not the end, of the fiber.

Distance

OpticalPower

EDFA

+

Raman

EDFAonly

Distributed Raman Amplification (II)


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Advantages

Variable wavelength amplification possible (pump freq. has to be

adjusted)

Compatible with installed SM fiber

Can be used to "extend" EDFAs Very broadband operation may be possible

Disadvantages

High pump power requirements, high pump power lasers have

only recently arrived

Sophisticated gain control needed (laser freq. control circuitry

required)

Noise is also an issue

Source: Master 7_5

Advantages and Disadvantages of

Raman Amplification

C i i f EDFA d


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Comparision of EDFA and

Raman amplifier.


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Conclusion

Optical amplifiers perform a critical

function in modern optical networks,

enabling the transmission of many

terabits of data over long distances of

up to thousands of kilometers.

unit 9 - optical amplifier

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