Optical Communication.pdf

8/14/2019 Optical Communication.pdf

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By

K.Eugine Raj

Assistant Professor/ECE

SCAD Engg College



SOURCES AND DETECTORS

OPTICAL SOURCES

Principle light sources are,

(i) Hetero junction structured semiconductor LASER diode[injection laser diodes(ILD’s)]

(ii) Light Emitting Diodes

-Hetero junction consists of two adjoiningsemiconductor materials with different band gapenergies.



These devices are suitable for the fiber transmission

system for the following reason. They have adequate output power for a wide range

of application

Their optical power output can be directly

modulated by varying the input current to thedevice.

They have high efficiency.

The dimensional characteristics are compatible

with those are compatible with those of the opticalfiber.



Difference between LED & LASEER

LED

Optical output poweris incoherent

Has a broad spectral width

Has large beam

divergence.

LASER

Coherent

Highly monochromatic

Output beam is very directional.







Light Emitting Diode(LED’s)

LED is the best choice for the opticalcommunication system requiring bitrate less than 100 to 200Mbps together

with multimode fiber coupled opticalpower in tens of μ watts.

Requires less complex drive circuitry

Fabricated less expensively.



a)LED Structure To be useful in fiber transmission application an LED

must have

i) High radiance output

ii) A fast emission response time

iii) A high quantum efficiency

-Radiation shows the brightness of the optical radiation.

-Emission response time is the time delay between the

application of current pulse and the onset of optical.-Quantum efficiency is related to the fraction of injectedelectron hole pair that recombine radiatively.



To achieve a high radiance and a highquantum efficiency the LED structure mustprovide a means of configuring the charged

carrier and the stimulated optical emissionin the active region of the PN junction.

This can be attained by

i) Carrier Confinementii) Optical Confinement

a)LED Structure



Carrier Confinement

To achieve a high level of radiation recombination which yields a high quantum efficiency.

Optical Confinement

To achieve a high radiance for preventingabsorption of the radiation by the materialsurrounding the PN-Junction.

To achieve these confinement LED configuration

such as double hetero structure (or) hetero junction configuration is used.





Band gap difference of adjacent layer

confines the charge carrier Difference in refractive indices of adjacent layer

confines the optical field to the central active layer.

(A) Two basic LED structure are,

i) Surface Emitters(front emitters)

ii) Edge Emitters



Surface Emitters



In surface emitters the plane of active lightemitting region is oriented perpendicular tothe axis of the fiber as shown in figure.

A well is etched through the substrate into

which a fiber is cemented.Circular active area

.50μm in diameter and 2.5μm thick

Emission patternIsotropic with a 1200 half power beam

width.

Surface Emitters



In this pattern the source is equally bright when viewed from any direction but powerdimnished as cosѲ, where Ѳ is the anglebetween viewing direction and the normal

to the surface.

Surface Emitters



Edge Emitter It consists of active junction region and two

guiding layers.



The guiding layer both have a refractive index which is lower than that of active region buthigher than the surrounding material.

Contact strip50-70μm wide

Length of active region 100-150μm

Emission pattern More directional thansurface emitters

In plane parallel to junction, beam is lambertian,half power width of Ѳ11=1200 .

In the plane perpendicular to the junction , half

power beam width Ѳ=25-300

.

Edge Emitter



b)Light Source Material Semiconductor material used for active layer of an

optical source must have a direct band gapmaterial radiative recombination is sufficiently high.

That means in a direct bandgap semiconductor e-

and holes can recombine directly across thebandgap without needing a third particle toconserve momentum.

Normal single element semiconductor are notdirect bndgap material, but many binary compounds are direct bandgap material.



Most important of these are so called as III-V group material made from compound of a groupIII element(Al,Ga & Tn) & a group V elementeg(P,As & Sb).

For operation in 800 to 900nm spectrum, principlematerial is ternary alloy Ga1-x Alx As.

Where, x ratio of Aluminum Arsenic toGallium Arsenide.

Determines the bandgap,corresponding the wavelength of the peak emittedradiation.

b)Light Source Material






b)Light Source Material The emission spectrum of a Ga1-x Alx As LED with

x=0.08 is given by



Peak output power at 810nm.FWHM spectral width of the spectral path

at its half power point is known as full widthhalf maximum(FWHM) spectral width.

At alonger wavelength

primary material is Quaternary alloy,

In1-xGax As y P1-y .




By varying mole fraction x & y output peak power at wavelength between energy E and frequency υ is,E=hυ=hc/λ, the quick efficient wavelength λ in μm canbe expressed as a function of the bandgap energy Eg inelectron volts by the equation λ in μm as.

λ(μm)=1.240/(Eg(eV))

For ternary alloy Ga1-x Alx As the bandgap energy in eV is, Eg=1.424+1.266x+.266x2

For quaternary alloy In1-x Gax As y P1-y the bandgapenergy in eV is given by

Eg=1.35-.72y+.12y 2




FWHM spectral width in 800nm region is 35nm

in 1300nm to 1600nm region is 70 to 180nm.

The output spectral width of the surface emittingLED’s tend to be broader than those of edgeemitting LED’s because of different internalabsorption effect of the emitted light in the twodevice structure.




An excess of electrons and holes in p & ntype material respectively(refered asminority carriers) iscreated in a

semiconductor light source by carrierinjection at the device contacts.

The excess densities of electrons & holes areequal.

The excess carrier can recombine eitherradiatively or non radiatively.

c)Quantum efficiency & LED Power



In radiative recombination a photon of energy hυ which is approximately equal to bandgap energy which

is emitted and thee non rdiative recombination effectsinclude optical absorption in the active region etc.

When there is a constant current flow into a LED anequilibrium is established i.e., the excess density of e- s‘n’ & holes ‘p’ is equal since the injection carrier arecreated & recombined in pair such that chargenutrality is maintained within the device.



The total rate at which carrier are generated is the sumof externally supplied & the internally generated rates.

The externally supplied rate is J/qd. j Current density in A/cm2

q Electron charhe

d thickness of the recombination ragion

Thermally generated rate is n/τn Excess carrier density

τ Time constant, carrier life time.



Hence the rate equation for carrier recombination inan LED is

The equilibrium condition is frmed by setting aboveequation to zero. i.e.,

=>

This relation gives the steady state electron density in

the active region. When a constant current is flowing through it.

Internal quantum efficiency.

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Optical Communication.pdf