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Transcript of 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