optical sourcesss (1)

8/10/2019 Optical Sourcesss (1)

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Energy-Bands

In a pure Gp. IV material, equal number of holes and electrons

exist at different energy levels.


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n-type material

Adding group V impurity will create an n- type material


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p-type material

Adding group III impurity will create a p-type material


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The Light Emitting Diode (LED)

For fiber-optics, the LED should have a

high radiance (light intensity), fast response

timeand a high quantum efficiency

Double or single hetero-structure devices

Emitted wavelength depends on bandgap

energy

/hchEg


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Heterojunction

Heterojunction is the advanced junction

design to reduce diffraction loss in theoptical cavity.

This is accomplished by modification of the

laser material to control the index of

refraction of the cavity and the width of the

junction.


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Homojunction

The p-n jun ct ion of the basic GaAs

LED/laser descr ibed before is called a

homojunc t ion because only one type of

semiconducto r mater ial is used in the

junction w ith d if feren t dopants to produce

the junct ion i tself .

The index of refract ion o f the mater ial

depends upon the impu ri ty used and the

doping level .


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The Heterojunc t ionregion is actually lightlydoped with p-type material and has the highest

index of refraction. The n-type material and the more heavily doped p-

type material both have lower indices of refraction.

In the homojunction, however, this index difference

is low and much light is lost.


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Gallium Arsenide-Aluminum Gallium

Arsenide Heterojunction Structure and index of refraction n for various types of junctions in

gallium arsenide with a junction width d.

(a) is for a homojunction.

(b) is for a gallium arsenide-aluminum gallium arsenide singleheterojunction.

(c) is for a gallium arsenide-aluminum gallium arsenide double

heterojunction with improved optical confinement.

(d) is for a double heterojunction with a large optical cavity of width w.


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LED Wavelength

(eV)

2399.1m)(

E

= hc/E(eV)

= wavelength in microns

H = Planks constantC = speed of light

E = Photon energy in eV


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Bandgap Energy and Possible WavelengthRanges in Various Materials


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SEMICONDUCTOR LIGHT-

EMITTING DIODES

Semiconductor LEDs emit incoherent

light. Spontaneous emission of light in

semiconductor LEDs produces light

waves that lack a fixed-phase

relationship. Light waves that lack a

fixed-phase relationship arereferred to

as incoherent light


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SEMICONDUCTOR LIGHT-EMITTING DIODESCont

The use of LEDs in single mode systems isseverely limited because they emit

unfocused incoherent light. Even LEDs developed for single mode

systems are unable to launch sufficientoptical power into single mode fibers for

many applications. LEDs are the preferred optical source for

multimode systems because they canlaunch sufficient power at a lower cost than

semiconductor Laser Diodes.


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Semiconductor LDs

Semiconductor LDs emit coherentlight.

LDs produce light waves with a fixed-phase relationship between points onthe electromagnetic wave.

Light waves having a fixed-phaserelationship are referred to ascoherent light.


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Semiconductor LDs Cont..

Semiconductor LDs emit morefocused light than LEDs, they are able

to launch optical power into bothsingle mode and multimode opticalfibers.

LDs are usually used only in singlemode fiber systems because theyrequire more complex driver circuitryand cost more than LEDs.


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Spontaneous Emission

Spontaneous emission is the random

generation of photons within the a layer of

the LED. The emitted photons move inrandom directions. Only a certain

percentage of the photons exit the

semiconductor and are coupled into the

fiber. Many of the photons are absorbed by

the LED materials and the energy

dissipated as heat.


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LIGHT-EMITTING DIODES

A light-emitting diode (LED) is asemiconductor device that emits

incoherent light, throughspontaneous emission, when acurrent is passed through it. Typically

LEDs for the 850-nm regionarefabricated using GaAs and AlGaAs.LEDs for the 1300-nm and 1550-nmregionsare fabricated using InGaAsP

and InP.


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Types of LED

The basic LED types used for

fiber optic communicationsystems are

Surface-emitting LED (SLED),

Edge-emitting LED (ELED), and


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LED performance differences(1)

LED performance differences help linkdesigners decide which device is appropriate

for the intended application. For short-distance (0 to 3 km), low-data-rate

fiber optic systems, SLEDs and ELEDs are thepreferred optical source.

Typically, SLEDs operate efficiently for bitrates up to 250 megabits per second (Mb/s).Because SLEDs emit light over a wide area(wide far-field angle), they are almost

exclusively used in multimode systems.


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LED performance differences (2)

For medium-distance, medium-data-ratesystems, ELEDs are preferred.

ELEDs may be modulated at rates up to 400Mb/s. ELEDs may be used for both singlemode and multimode fiber systems.

Both SLDs and ELEDs are used in long-

distance, high-data-rate systems. SLDs areELED-based diodes designed to operate in thesuperluminescence mode.

SLDs may be modulated at bit rates of over400 Mb/s.


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Surface-Emitting LEDs

The surface-emitting LED is also known as the Burrus

LED in honor of C. A. Burrus, its developer.

In SLEDs, the size of the primary active region is limited

to a small circular area of 20 m to 50 m in diameter.

The active region is the portion of the LED where

photons are emitted. The primary active region is belowthe surface of the semiconductor substrate perpendicular

to the axis of the fiber.


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Surface-emitting LED


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Edge-emitting LED


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LED Spectral Width

Edge emitting LEDs have slightly narrow line width


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The internal Quantum efficiency of LEDs

decrease exponentially with increasing

temperature.

Internal quantum efficiency is the ratiobetween the radiative recombination rate and

the sum of radiative and nonradiativerecombination rates

The light emitted from these devices decreases

as the p-n junction temperature increases.


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The internal Quantum efficiency of LEDsdecrease exponentially with increasing

temperature. Internal quantum efficiency is the ratio

between the radiative recombination rate andthe sum of radiative and nonradiative

recombination rates

The light emitted from these devicesdecreases as the p-n junction temperature

increases.

)/(int nrrr RRR


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It can be observed from the above figure that

the edge-emitting device exhibits a greater

temperature dependence than the surfaceemitter.

And the output of SLD is strongly dependent

on the temperature.


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The line width increase due to increased

doping levels.


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The Spectra of ELED is narrow than that of

SLED.


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The output spectra also broaden at a rate of 0.1

and 0.3 nm / C with the increase in

temperature due to greater energy spread incarrier distributions at higher temperatures.


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3-dB bandwidths

Optical Power I(f); Electrical Power I2(f)

2)2(1/)( fPfP o

Electrical Loss = 2 x Optical Loss


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LED Spectral Width

Edge emitting LEDs have slightly narrow line width

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