56588412 Optical Sources

8/11/2019 56588412 Optical Sources

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F IBER OPTICS AND

OPTO-ELECTRONICS

BY

M. RAJARAO

Optical sources


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General block diagram of optical communication system


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OPTICALSOURCES: INTRODUCTION

Fundamental function:

To convert the electrical energy in the form currentinto optical energy (light) in an efficient manner

Requirements : The size and shape of the source should be compatible with the size

of the fiber so that it can couple max. power into the fiber.

The responseof the source should be linear.

It should emit monochromatic radiation at the wavelength where thefiber has low losses and low dispersion.

It should provide sufficient optical power so that it overcomes thetransmission losses down the link.

Should have a very narrow spectral width in order to minimize thedispersion.

Must be capable of maintaining a stable optical output which isunaffected by changes in ambient conditions.

It must be reliable and cheapas far as possible.


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Monochromatic coherentsources

The optical energy is producedin optical resonant cavity (the

formation of an electromagneticstanding wave within a cavity)

It provides mono-chromatichighly coherent radiation and theoutput beam is very directional

It can be coupled into either

single mode or multimode fibers

Monochromatic incoherentsources

No optical cavity exists forwavelength selectivity

The output radiation has a broadspectral width since the emittedphoton energies range is 1 to2kBT

LEDs can only be coupled intomultimode fibers.

Some applications have usedspecially fabricated LEDs withSMFs for data transmission at

bit rates up to 1.2Gb/s overseveral Km.

Optical sources

Injection LASER diodes Light Emitting Diodes (LEDs)


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BASICCONCEPTS

To understand the basic operation of light sources, it isnecessary to study about

Properties of semiconductor materials (Energy band structure

of these materials (intrinsic and extrinsic))

pnjunction

Emission of radiation by recombination

Direct and indirect band gaps semiconductors


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Intrinsic semiconductors

)2

exp(Tk

Enpn

B

g

i


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Extrinsic semiconductors: n-type

Fig . (a) Donor level in an n-type semiconductor.

Fig. (b) The ionization of donor impurities creates an increased electron

concentration distribution.


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p-typesemiconductor

Fig (a) Acceptor level in anp-type semiconductor.

Fig (b) The ionization of acceptor impurities creates an increased hole

concentration distribution


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P-NJUNCTION

The p-n junction is formed by

adjoining the p and n type

semiconductor layers

A thin depletion region is

formed at the junction through

carrier recombination (diffusion

of holes and electrons).

This establishes a potentialbarrier between the p and n type

regions which restricts the

diffusion of majority carriers.

In the absence of an external

applied voltage no current flows

as the potential barrier preventsthe net flow of carriers from one

region to another.

electronhole


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If the p-n junction is forward biased, the majority carriers from

both sides cross the junction and enter the opposite sides. This

results in an increase in the minority carrier concentration on thetwo sides. This process is known as minority carrier injection.

The injected carriers diffuse away from the junction, recombining

with majority carriers.

This recombination of electrons and holes may beEither radiatively(in which case a photon energy is emitted)

Or non-radiatively (where the recombination energy is

dissipated in the form of heat)

The phenomenon of emission of radiation (photon) by therecombination of injected minority carriers with majority carriers

is called as injection luminescence orelectroluminescence. This is

the mechanism by which light is emitted in LED.

Mechanism behind photon emission in LEDs


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In Si and Ge, the greater percentage is given up in the

form of heat and the emitted light is insignificant.

In other materials, such as gallium arsenide phosphide

(GaAsP) or gallium phosphide (GaP), the number of

photons of light energy emitted is sufficient to create a

very visible light source.

Here the photon energy is equal to the energy of band

gap

Emission of radiation in

p-n junction


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DIRECTANDINDIRECTBANDGAP

SEMICONDUCTORS

Electron transitions to or from the conduction band with

the absorption or emission of a photon respectively.

Here both energy and momentum must be conserved.

Semiconductors are classified either as direct or indirect

band gap materials depending on the shape of band gap

as a function of the momentum (k).

In order to encourage the electroluminescence it is

necessary to select an appropriate semiconductor

material.

The most useful material for electroluminescence

purpose are direct bad gap semiconductors


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Direct band gap

For direct band gap materials, the minimum energy levels of conduction

band and the maximum energy levels of valence band occur at same valuesof momentum

The direct transition of an electron across the energy gap provides an

efficient mechanism for photon emission

Ex: GaAs, GaSb,

InAs


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Indirect band gap

For indirect band gap materials, the minimum conduction

band and the maximum valence band energy levels occur at

different values of momentum

Here, the electron and hole recombination requires the

simultaneous emission of a photon in order to conserve the

momentum

Ex: Si, Ge, GaP


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Direct band gap semiconductors in general have a much

higher internal quantum efficiency .The ratio of the number of radiative recombinations (photons

produced within the structure) to the number of injected

carriers is known as internal quantum efficiency.


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OTHERRADIATIVERECOMBINATIONPROCESSES

Energy levels may be introduced into the band gap by

adding impurities, which may greatly increase theelectron-hole recombination (effectively reduces the

carrier life time)

An indirect band gap semiconductors may be made into a

more useful electroluminescence material by addingimpurities, which will effectively convert it into a direct

band gap materials.

Types radiative recombination processes:

Conduction to valence band transition (band to band) Conduction band to acceptor impurity transition

Donor impurity to valence band transition

Donor impurity to acceptor impurity transition


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A homo-junction is a semiconductor interface that occurs

between the layers of similar semiconductor material, these

materials have equalband gapsbut typically have different doping.

In most practical cases a homo-junction occurs at the interface

between an n-type (donor doped) and p-type (acceptor doped)

semiconductor such as silicon, this is called ap-n junction.

A hetero-junction is the interface that occurs between two

layers of dissimilar crystalline semiconductors. Thesesemiconducting materials have unequalband gapsas opposed to a

homo-junction.

The radiative properties of a junction may be improved by the

use hetero-junctions.

A double hetero-junction is formed when a layer of narrowband gap material (ex: GaAs) is sandwiched between the layers of

wide band gap materials (ex:GaAl As).
http://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Band_gaphttp://en.wikipedia.org/wiki/Doping_(semiconductor)http://en.wikipedia.org/wiki/N-type_semiconductorhttp://en.wikipedia.org/wiki/Donor_(semiconductors)http://en.wikipedia.org/wiki/P-type_semiconductorhttp://en.wikipedia.org/wiki/Acceptor_(semiconductors)http://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Pn_junctionhttp://en.wikipedia.org/wiki/Band_gaphttp://en.wikipedia.org/wiki/Homojunctionhttp://en.wikipedia.org/wiki/Homojunctionhttp://en.wikipedia.org/wiki/Homojunctionhttp://en.wikipedia.org/wiki/Homojunctionhttp://en.wikipedia.org/wiki/Band_gaphttp://en.wikipedia.org/wiki/Band_gaphttp://en.wikipedia.org/wiki/Band_gaphttp://en.wikipedia.org/wiki/Pn_junctionhttp://en.wikipedia.org/wiki/Pn_junctionhttp://en.wikipedia.org/wiki/Pn_junctionhttp://en.wikipedia.org/wiki/Pn_junctionhttp://en.wikipedia.org/wiki/Pn_junctionhttp://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Acceptor_(semiconductors)http://en.wikipedia.org/wiki/P-type_semiconductorhttp://en.wikipedia.org/wiki/P-type_semiconductorhttp://en.wikipedia.org/wiki/P-type_semiconductorhttp://en.wikipedia.org/wiki/Donor_(semiconductors)http://en.wikipedia.org/wiki/N-type_semiconductorhttp://en.wikipedia.org/wiki/N-type_semiconductorhttp://en.wikipedia.org/wiki/N-type_semiconductorhttp://en.wikipedia.org/wiki/Doping_(semiconductor)http://en.wikipedia.org/wiki/Band_gaphttp://en.wikipedia.org/wiki/Band_gaphttp://en.wikipedia.org/wiki/Band_gaphttp://en.wikipedia.org/wiki/Semiconductor


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Double hetero-junction

When forward bias is

applied, the holes from

p-GaAlAs are injected

into n-GaAs and

electrons from n-

GaAlAs are injected into

n-GaAs.

A large number ofcarriers are confined in

the central layer of n-

GaAs (active layer),

where they recombine

and produced opticalenergy equal to band gap

of n-GaAs.


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LIGHTEMITTINGDIODE(LED)

For optical comm. systems requiring bit rates less than 100-200Mbpswith multimode fibers

coupling optical power is in tens of microwatts

Semiconductor LED is the best choice for this, because

Require less complex drive circuitry than LASER diodes

No thermal or optical stabilization circuits are needed

They can be fabricated less expensively


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LED STRUCTURE

In fiber transmission applications, the source must have

high radiance output (to couple sufficient high optical powerlevels into the fiber)

fast emission response time (the time delay between the

application of a current pulse and the onset of optical emission)

high quantum efficiency (related to the fraction of injected

electron-hole pairs that recombine radiatively)

To obtain the necessary high radiance and high quantum

efficiency, LEDs may be fabricated with DH structure.


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( )( )

hcnm

E eV


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LED STRUCTURE(CONT)

There are two basic LED structures

Surface (Burrus or front) emitting LED

Edge emitting LED


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SURFACEEMITTINGLED

In surface emitting LED, the plane of the active light

emitting region is oriented perpendicular to the axis of the

fiber.

A well is etched through the substrate of the device in

order to prevent heavy absorption of the emitted radiation

and physically to accommodate the fiber.

Into which a fiber is then cemented in order to accept the

emitted light.

The emission pattern from active layer is essentially

isotropic (Lambertian pattern) with a 120 degrees halfpower beam width.

The circular active region in practical surface emitting

LEDs is 50m and up to 2.5m thick.


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Schematic of high-radiance surface-emitting LED. The active region is

limited to a circular cross section that has an area compatible with the

fiber-core end face.


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EDGEEMITTINGLED

It consists of an active junction region and two guiding layers,

both have a R.I less than that of the active region, but higherthan the index of the surrounding material.

This structure forms a waveguide channel that directs the

optical radiation towards the fiber core.

To match the typical fiber core diameters, the contact strips are

50-70m.

Range of length of the active regions is usually from 100 to

150m

The emission pattern is more directional than that of surface

emitters. In the plane parallel to the junction the emitted beam is

Lambertian with half power with of 120 degrees. In

perpendicular to the junction, the half power beam with is 25-

35 degrees


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Schematic of an edge-emitting double hetero-junction

LED


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LIGHTSOURCEMATERIALS

The semiconductor material that is used for the activelayer of an optical source must have a direct band gap.

Most of the light sources contain III-V ternary &

quaternary compounds.

In Ga1-xAlxAs, by varying x it is possible to control theband-gap energy and thereby the emission wavelength

over the range of 800 nm to 900 nm. The spectral width

is around 20 to 40 nm.

In In1-x

GaxAs

yP

1-yby changing 0


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Band gap energy and output wavelength as a function of

aluminum mole fraction x


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SPECTRALWIDTHOFLED TYPES


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QUANTUMEFFICIENCYANDLED POWER

When there is no external carrier injection, the excess

density decays exponentially due to electron-holerecombination.

Where n0 is the initial injected excess electron densityand is carrier life time

The total rate at which carriers are generated is the sum ofthe externally supplied and the thermally generated rates.

/

0

tenn

regionionrecombinatofthickness:electron;theofcharge:

)(

dq

n

qd

J

dt

tdn


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When there is a constant current flow into LED, an

equilibrium condition is established.

In equilibrium condition:dn/dt=0

Internal quantum efficiency:

For homo-junction LEDs the internal quantum efficiency

is about 50%, for double hetero-junction LEDs is about

60-80%

qd

Jn

rnrr

nr

nrr

r

RR

R

int

nrr

rnr

Where is Bulk recombination life

time


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Optical powergenerated internally in the active region in

the LED is

q

hcIh

q

IP intintint

regionactivecurrent toInjected:

power,opticalInternal:int

I

P


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dTc

)sin2()(4

1

0

ext

2

21

21

)(

4)0(tCoefficienonTransmissiFresnel:)(

nn

nnTT

2

11

ext2)1(

11If

nnn

2

11

intintext

)1(powr,opticalemittedLED

nn

PPP

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56588412 Optical Sources