c10 optical amplifiers intro

7/24/2019 C10 Optical Amplifiers Intro

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Optical Amplifiers

Chapter 10


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Optical Amplifiers: amplifies in optical domain without O-E or E-O.E.g. SOA, EDFA, Raman Amplifier

Regenerators regenerate by converting O-E, re-time, re-shape

and E-O conversion.

As signal propagates through fiber channel, it gets attenuated due

to absorption, scattering, etc. and gets broadened due to dispersion.

Optical Amplifiers

Advantages of Optical Amplifiers:1. Insensitive to data rate

2. Large gain bandwidths

Disadvantages:

1. Do not regenerate,2. amplifies noise,3. no dispersion compensation


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Semiconductor Optical Amplifiers (SOA)

Semiconductor optical amplifiers are similar in constructionto semiconductor lasers.

The main difference is that SOAs aremade with layers of antireflection coatings

to prevent light from reflecting back intothe circuit.

semiconductor material such as indium phosphide.

Optical gain occurs as excited electrons in the semiconductormaterial are stimulated by incoming light signals; when current is

applied across the p-n junction the process causes the photons to

replicate, producing signal gain.


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SOA operation

The DC current applied to the device results in electrons being pumped intothe (normally empty) conduction band and removed from the (normally full)valence band. This creates the population inversion which is a pre-cursor tooptical gain. When signal photons travel through the device they cause

stimulated emission to occur when an electron and hole recombine.


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By adjusting the chemical composition of III-V semiconductors (typicallyGaInAsP) the band gap can be adjusted to give optical gain in the

telecommunications windows of interest.

The devices are typically 250mm long although devices of up to1mm have been made. In general the longer devices canachieve higher gain and wider bandwidths.

be as large as 100 nm and the fact that the energy source is a DC

electrical current makes these devices look promising as opticalamplifiers.

A major advantage of SOAs is that they can be integrated with othercomponents on a single planar substrate. For example, a WDMtransmitter device may be constructed including perhaps 10 lasersand a coupler all on the same substrate. In this case an SOA could beintegrated into the output to overcome some of the coupling losses.


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SOA: Semiconductor Optical Amplifier

The configuration of a SOA uses the familiar Double-Hetero (DH) Structure.Requirements are different than ILD.

1. Band-gap of active layer < surrounding layers2. RI of active layer > surrounding layers3. Efficient coupling of signal photons4. Optical feedback to be suppressed (AR coatings)

Optical Gain

The optical power P propagating through amplifiers is described as

PgPdzdP

eff=

where, g is gain coefficient and eff is effective loss coefficient.


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( )trg NNg =

If1. N is carrier concentration per unit volume,

2. Ntr is carrier conc. at transparency (when gain is unity),3. g is the gain cross-section (differential gain coefficient dg/dN)4. is the confinement factor,the gain coefficient is written as,

The carrier population rate change with injection current I and signal power P.

Ah

gPN

eV

I

dt

dN

c =

Ah

gP

eV

IN cc

=

Total number of carriersper unit volume Carrier loss per unit volumedue to non-radiative processesc- carrier life time

Carrier loss due to

stimulated emission

Under-steady state, dN/dt = 0,


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+

=

cg

trcg

Ah

P

Nev

I

g

1

Hence, The Gain Coefficient,

If P is so small that may be neglected,cg

sat

AhP

=&,

= trc

g NevIg 0

+

=

satP

Pgg

1

0

Then,

&,


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PP

P

P

g

dz

dPeff

sat

+

=

1

0

==

+

L

z

P

Pdzg

P

P

dPout

in 0

0

1

Substituting value of g,

Neglecting eff, & integrating,

satP

=

=+L

z

P

Psat

P

P

dzgdPPP

dP out

in

out

in 0

0

1

[ ] [ ] LgPPP

PP inoutsat

inout 0

1lnln =+

=

sat

inout

in

out

P

PPLg

P

P0ln

or,

or,

or,


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== sat

inout

P

PPLg

in

out eP

PG

0

inout PP >> Lg

eG 00=&,

sat

out

PPGG =

0lnln

Thus, The amplifier Gain, G is

If,

Then a good approximation is,

-

( )

sat

dBsat

P

PG

G = 30

0 ln2

ln

( )cg

satdBsatAhPP

== )2ln()2ln(3

,

or,


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Travelling Wave

Semiconductor Laser

Amplifier (SLA)

Angled-facet ortilted-stripe the

reflected beam atthe facet isphysicallyseparated from theforward beam

Mirror

Effect of optical reflections

Fabry-Perot

Semiconductor

Laser Amplifier

(SLA)

-

window facet theoptical beamspreads in thetransparentwindow


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( )( )

( ) )(sin4111

2

21

2

21

21

RRGRRG

GRR

P

PG

ss

s

in

out

+

==

Ln )(20

=

where, G is measured gain, Gs is single pass gain,& phase shift the wave goes on traversing length L.

Effect of optical reflections

==

2/1

21

211

0

)4(

1sin

2

)(2

RRG

RRG

nL

c

s

s

2

21

21

1

1

+=

RRG

RRGG

s

s

where, n is RI of Active region material, is incident signal frequency,0 is frequency of resonant mode.

3 dB spectral Bandwidth,

Peak trough ratio of passband ripple,


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Limitations of SOA:

The carrier lifetime is of order of 0.1 ns, hence gain recovery timeis short w.r.t to GHz data rate

Therefore, different levels of signal intensity will experience different gains,

leading to signal distortion.

This becomes significant when SOA is operating near saturation.This sets an upper limit on maximum amplifier output power.

Semiconductor Layers are sensitive to polarization.

Non-linearities leads to inter-channel cross talk.


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What is EDFA

An optical fiber heavily doped with Er3+ at core that serves asan optical amplifier.

EDFA Stands for Erbium Doped Fiber Amplifier Used to boost the intensity of optical signals being carried

through a fiber optic communications system

Works on the concept of stimulated emission Operates near 1550 nm

Other Rare-earth elements:1. Holmium (Ho),

2. Neodymium (Nd)3. Samarium (Sm)4. Ytterbium (Yb)


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Why Erbium?

Erbium has several important properties that make it anexcellent choice for an optical amplifier

Erbium ions (Er3+) have quantum levels that allows them to bestimulated to emit in the 1540nm band, which is the band that

has the least power loss in most silica-based fiber.

r um s quan um eve s a so a ow o e exc e y a s gna

at either 980nm or 1480nm, both of which silica-based fiber

can carry without great losses


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Basic block diagram of EDFA


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EDFA Configuration

Co-propogating (Forward) Pump EDFA

Counterpropogating (Backward) Pump EDFA


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EDFA Configuration

Forward pump= lower noise but lower output power

Backward pump = higher output power but higher noise


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Amplification between 1.53 and 1.56 um.


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Advantages Insensitivity to light polarization state High gain

Low noise figure: 4.5 dB to 6dB No distortion at high bit rates Simultaneous amplification of wavelength division multiplexed

Immunity to cross talk among wavelength multiplexedchannels Do not require high speed electronics

Independent of bit rate (Bit rate transparency)

Commercially available in C-band & L-band.


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Drawbacks Pump laser necessary Need to use a gain equalizer for multistage amplification

Difficult to integrate with other components Dropping channels can give rise to errors in surviving channels

EDFA: Mature technology1. New materials (Fluoride, Tellurite)

2. New dopant (Pr, Tm) ~PDFA, TDFA to exhibit

broader and flatter gain


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Band Name Wavelengths Description

O-band 1260 1360 nm Original band, PON upstream

E-band 1360 1460 nm Water peak band

S-band 1460 1530 nm PON downstream

C-band 1530 1565 nm

Lowest attenuation, original DWDMband, compatible with fiber amplifiers,CATV

L-band 1565 1625 nmLow attenuation, expanded DWDMband

U-band 1625 1675 nm Ultra-long wavelength


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1660 nm1640162016001580156015401520150014601440 1480

Fluoride EDFA 62 nm

EDFA 52 nm

EDFA ~47 nm

Tellurite EDFA 76 nm]

TDFA 37 nm

Rare earth-Doped Fiber Amplifiers

Erbium-Doped Fiber Amplifiers (EDFA) : C, L-BandThulium-Doped Fiber Amplifiers (TDFA) : S-BandPraseodymium-Doped Fiber Amplifiers (PDFA) : O-Band

1660 nm1640162016001580156015401520150014601440 1480

TDFA 35 nm

Raman + Fluoride EDFA 80 nm

Dist. Raman + Fluoride EDFA 83 nm

Raman + TDFA 53 nm

Raman 18 nm

Raman 40 nm

Raman 100 nm

Raman 132 nm

C-Band L-BandS-Band U-BandE-Band


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Traditional Optical

Communication SystemLoss compensation: Repeaters at every 20-50 km


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Optically Amplified SystemsEDFA = Erbium Doped Fibre Amplifier


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Typical Packaged EDFA


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Erbium

Pump laserWDM Fibre coupler

Interior of an Erbium Doped Fibre Amplfier (EDFA)

Fibreinput/output

doped fibre

loop

Source: Master 7_5


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Erbium doped aluminiumoxide spiral waveguide

1 mm square waveguide

Pum ed at 1480 nm

Miniature Optical Fibre Amp

Low pump power of 10 mW

Gain only 2.3 dB at present

20 dB gain possibleFOM Institute Amsterdam and

University of Holland at Delft


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Raman

Source: Master 7_5


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Raman Fibre Amplifiers (RFAs) rely on an intrinsic non-

linearity in silica fibre

Variable wavelength amplification:

Raman Amplifiers

For example pumping at 1500 nm produces gain at about 1560-1570nm

RFAs can be used as a standalone amplifier or as a

distributed amplifier in conjunction with an EDFA

Source: Master 7_5


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Raman Effect AmplifiersStimulatedStimulated RamanRaman ScatteringScattering (SRS)(SRS) causes a new signal (a Stokeswave) to be generated in the same direction as the pump wave down-

shifted in frequency by 13.2 THz (due to molecular vibrations) providedthat the pump signal is of sufficient strength.In addition SRS causes the amplification of a signal if it's lower in

.

difference in wavelengths is around 13.2 THz.The signal to be amplified must be lower in frequency (longer inwavelength) than the pump.It is easy to build a Raman amplifier, but there is a big problem:we just can't build very high power (around half a watt or more) pumplasers at any wavelength we desire! Laser wavelengths are veryspecific and high power lasers are quite hard to build.


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Distributed Raman Amplification (I)

Raman pumping takes place backwards over the fibre

Gain is a maximum close to the receiver and decreases in the

transmitter direction

Long Fibre Span

Source: Master 7_5

TransmitterOptical

ReceiverEDFA

RamanPumpLaser


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With only an EDFA at the transmit end the optical power level decreases

over the fibre length

With an EDFA and Raman the minimum optical power level occurs towardthe middle, not the end, of the fibre.

Distributed Raman Amplification (II)

Source: Master 7_5

Distance

OpticalPower

+

Raman

EDFA

only

Animation


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Raman amplification can provides very broadbandamplification

Multiple high-power "pump" lasers are used to producevery high gain over a range of wavelengths.

Broadband Amplification using Raman

Amplifiers

93 nm bandwidth has been demonstrated with just twopumps sources

400 nm bandwidth possible?

Source: Master 7_5


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Advantages

Variable wavelength amplification possible

Compatible with installed SM fibre

Can be used to "extend" EDFAs

Advantages and Disadvantages of Raman

Amplification

an resu t n a ower average power over a span, goo or ower crossta

Very broadband operation may be possible

Disadvantages

High pump power requirements, high pump power lasers have only recently

arrived

Sophisticated gain control needed

Noise is also an issue

Source: Master 7_5


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The Need of Optical Amplification

Erbium-Doped Fiber Amplifiers (EDFAs) application in long haul. Todays

amplifier of choice. Erbium-Doped Waveguide Amplifiers (EDWAs) application in metro andaccess networks

Raman Amplifiers application in DWDM

Why? Extend distance light signal can travelwithout regeneration

Semiconductor Optical Amplifiers (SOA) not fiber based type, application inmetro and access networks

Standard FiberAmplifier

Pump Lasers


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General Application of OpticalAmplification

In-line amplifierPreamplifier

LAN booster amplifier

Standard FiberAmplifier

Pump Lasers


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In-lineAmplifier

Fibre Link

Transmitter

Optical Amplifiers

Fibre Link

OpticalReceiver

Optical Amplifier Applications

Amplifier

Preamplifier

Transmitter

Transmitter

Optical Amplifier

Optical Amplifier

Fibre Link

Optical

Receiver

OpticalReceiver

Source: Master 7_5


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60 mW

30

35

40 Boosteramplifier

preamplifier

Amplifier OperationPoints

30 mW

10

15

20

-10 -5 0 5 10 15

Output power[dBm]

n- ne

amplifier

c10 optical amplifiers intro

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