LASER And Fibre Optics

Biomedical physics :- Physiotherapy (stream)
Chapter :- laser and fiber optics
#biomedical
#physiotherapy

1. TheacronymLASERstandsforLightAmplificationby
StimulatedEmissionofRadiation.
Alaserisadevicethatgenerateslightbyaprocesscalled
STIMULATEDEMISSION.
Thetheoreticalexplanationforlaseroscillationwasgivenby
A.LSchawlowandC.H.Townesintheyear1958.The1
st
laser,
namely,RubyLaser,wasdemonstratedbyT.H.Maimaninthe
year1960.
Introduction
Unit-4 –LASER And Fibre Optics

2. Thelaserlightexhibitssomepeculiarpropertiesthancomparewiththeconvectionallight.
Thoseare
•Highlydirectionality
•Highlymonochromatic
•Highlyintense
•Highlycoherence
HighlydirectionalityThelightray
comingordinarylightsource
travelsinalldirections,butlaser
lighttravelsinsingledirection.
CharacteristicsofLASER
Highly directionality

3. Thelightfromanormalmonochromaticsourcespreadsovera
rangeofwavelengthoftheorderof100nm.Butthespreadis
oftheorderof1nmforlaser
Sinceanordinarylightspreadsinalldirections,the
intensityreachingthetargetisveryless.Butinthecase
oflaser,duetohighdirectionality,theintensityoflaser
beamreachingthetargetisofhighintensebeam.For
example,1millwattpowerofHe-Nelaserappearsto
bebrighterthanthesunlight.
CharacteristicsofLASER...
Highly intense
Highly monochromatic
Hence laser is highly monochromatic, that is, it can emit light
of single wavelength

4. Thewavetrainswhichareidenticalinphaseanddirectionare
calledcoherentwaves.
CharacteristicsofLASER...
Highly coherent
Sincealltheconstituentphotonsoflaserbeampossessthe
sameenergy,momentumandpropagateinsamedirection,
thelaserbeamissaidtobehighlycoherent.
Differences between ordinary light and Laser beam

5. Ifaphotonofenergyhvisincidentontheatom.Inthelower
state,theatomabsorbstheincidentphotonandgetsexcited,
tojumpthehigherenergystate.Thisprocessiscalled
absorption.
where B
12is the proportionality const. known as prob. of absorption of radiation per unit time.
SpontaneousandStimulatedemission
The rate of absorption R
12is proportional to the population of lower energy level N
1and to the
density of incident radiation ρ. Hence
Absorption
Atom + photon = Atom*
E
1+ hv= E
2
hv= E
2-E
1
R
12αN
1ρ
or R
12= B
12N
1ρ

6. SpontaneousandStimulatedemission...
Spontaneous emission
Itisaprocessinwhichthereisanemissionofaphoton
wheneveranatomtransitsfromahighertolowerenergystate
withouttheaidofanyexternalagency.
Forthisprocesstotakeplace,theatomhastobeintheexcitedstate.
Sincethehigherenergylevelisanunstableone,theexcitedatominthehigherlevelE
2
spontaneouslyreturnstothelowerenergylevelE
1withtheemissionofphotonofenergy
hν=E
2-E
1
Atom*=Atom+photon
whereA
21istheproportionalityconstantknownasprob.ofspontaneousemissionperunittime.
TherateofspontaneousemissionofradiationR
21(sp)isproportionaltothepopulationN
2at
thehigherenergylevelE
2.Hence
R
21(sp)αN
2
or R
21(sp) = A
21N
2

7. SpontaneousandStimulatedemission...
Stimulated emission
Itisaemissionofphotonwheneveranatomtransitsfroma
highertolowerenergystateundertheinfluenceofanexternal
agencyi.e.,anexternalphoton.
Forthisprocesstotakeplace,theatomhastobeintheexcitedstate.
Letaphotonhavinganenergyhνinteractwiththeatomintheexcitedstate.Thisincident
photontriggerstheexcitedatominthehigherlevelE
2totransittolowerlevelE
1,resultingin
theemissionofanotherphotonofenergyhν.
Boththeinducing(incident)andtheemittedphotonhavethesamephase,energyand
directionofmovement.
whereB
21istheproportionalityconstantknownasprob.ofstimulatedemissionperunittime.
Thiskindofemissionisresponsibleforlaseraction.
R
21(st)αN
2ρ
or R
21(st) = B
21N
2ρ
TherateofstimulatedemissionofradiationR
21(st)isproportionaltothepopulationN
2atthe
higherenergylevelE
2andtothedensityρoftheinducingphoton.

8. SpontaneousandStimulatedemission...
Differences between Spontaneous and Stimulated emission

9. WorkingofLASER
Population Inversion
Itisastateofachievingmore
numberofatomsinexcitedstate
comparedtogroundstate.Itisan
essentialconditionforproducing
laserbeam.
Populationinversioncanbe
achievedbyaprocesscalled
pumping.
Pumping Mechanism
Itisthemechanismofexcitingatomsfromthelowerenergystateto
ahigherenergystatebysupplyingenergyfromanexternalsource.
Themostcommonlyusedpumpingmechanismare:optical,
electricalanddirect.

10. WorkingofLASER...Pumping Mechanism . . .
Optical Pumping
Inthistypeadirectconversionofelectricenergyintolighttakesplace.Thistechniqueis
adoptedinsemiconductorlaser.
Inadditiontoabovethree,theothertypesofpumpingareinelasticcollisionbetweenatomsand
chemicalmethodswhicharerespectivelyadoptedinHe-Negaslaserandindyeandchemical
lasers.
Inthistypeofpumping,atomsareexcitedbymeansofan
externalopticalsource.Thistypeisadoptedinsolidstatelasers
suchasrubyandNd:YAGlaser.
Electrical Pumping
Inthistypeofpumpingtheelectronsareacceleratedtoahighvelocitybyastrongelectric
field.Thesemovingelectronscollidewiththeneutralgasatomsandionizethemedium.Thus,
duetoionizationtheygetraisedtoahigherenergylevel.Thistechniqueisadoptedingas
laserssuchasCO
2laser.
Direct Conversion

11. WorkingofLASER...
Life time
Thelimitedtimeforwhichanatomremainsintheexcitedisknownaslifetime.
Metastable state
Itisanenergylevelinanatomicsystemwhere
thelifetimeofatomsisverylarge(oftheorder
10
-3
to10
-2
seconds).Ithelpsinachievingthe
populationinversion.

12. WorkingofLASER...
Components of LASER

13. WorkingofLASER...
Components of LASER . . .
Itisapairofreflectingsurfaces(mirrors)ofwhichoneisa
perfectreflectorandtheotherisapartialreflector.Itisused
foramplificationofphotonstherebyproducinganintense
andhighlycoherentoutput.
OR

14. ApplicationsofLASER
Themostsignificantapplicationsoflasersinclude:
1.Lasersinmedicine
2.Lasersincommunications
3.Lasersinindustries
4.Lasersinscienceandtechnology
5.Lasersinmilitary
1. Lasers in Medicine
Lasers are used for bloodless surgery.
Lasers are used to destroy kidney stones.
Lasers are used in cancer diagnosis and therapy.
Lasers are used for eye lens curvature corrections.
Lasers are used in fiber-optic endoscope to detect ulcers in the intestines.
The liver and lung diseases could be treated by using lasers.
Lasers are used to study the internal structure of microorganisms and cells.
Lasers are used to create plasma.

15. ApplicationsofLASER...
1. Lasers in Medicine . . .
Lasers are used to remove tumorssuccessfully.
Lasers are used to remove the caries or decayed portion of the teeth.
Lasers are used in cosmetic treatments such as acne treatment, cellulite and hair removal.
2.LasersinCommunications
Laserlightisusedinopticalfibercommunicationstosendinformationoverlargedistances
withlowloss.
Laserlightisusedinunderwatercommunicationnetworks.
Lasersareusedinspacecommunication,radarsandsatellites.
3.LasersinIndustries
Forcutting,welding,meltinganddrilling.
Totestthequalityofthematerial.
Fortheheattreatment.

16. ApplicationsofLASER...
4.LasersinScienceandTechnology
AlaserhelpsinstudyingtheBrownianmotionofparticles.
Withthehelpofahelium-neonlaser,itwasprovedthatthevelocityoflightissameinall
directions.
Withthehelpofalaser,itispossibletocountthenumberofatomsinasubstance.
LasersareusedincomputerstoretrievestoredinformationfromaCompactDisc(CD).
LasersareusedtostorelargeamountofinformationordatainCD-ROM.
Lasershelpsindeterminingtherateofrotationoftheearthaccurately.
Lasersareusedincomputerprinters.
Lasersareusedforproducingthree-dimensionalpicturesinspacewithouttheuseoflens.
Lasersareusedfordetectingearthquakesandunderwaternuclearblasts.
5.LasersinMilitary
Laserrangefindersareusedtodeterminethedistancetoanobject.
Theringlasergyroscopeisusedforsensingandmeasuringverysmallangleofrotationofthe
movingobjects.

17. Introduction
FIBER OPTICS

18. OpticalFibreConstruction
Refractive index??????=
??????
??????
Theratioofthespeedoflightinavacuumtoits
speedinaspecificmedium.
Therefractiveindexofglassngis1.52andrefractiveindexofwaternwis1.33.Sincethe
refractiveindexofglassishigherthanthewater,thespeedoflightinwaterisfasterthanthe
speedoflightthroughglass.

19. OpticalFibreConstruction...
Refractive
index

20. WorkingPrincipleofOpticalFibre
LetusconsiderapointsourceOinopticallydenser
medium(Waterormediumwithhighrefractiveindex).
Astheangleofincidenceincreases,theangleofrefractionalsoincreases.
Total Internal Reflection
LetXYbetheboundaryseparatingtheopticallydenser
medium.
Ataparticularangleofincidencei
c,calledasCriticalangle,theangleofrefractionis90°
andhencetherefractedraymovesalongthesurfaceofwateri.e.alongXY.
Iftheangleofincidenceismorethani
c,thereisno
refractedray,theincidentrayiscompletelyreflected
backinthewater.Thisphenomenonisknownastotal
internalreflection.
Opticalfiberworksontheprincipleoftotalinternal
reflection.

21. WorkingPrincipleofOpticalFibre...
Explanation

22. WorkingPrincipleofOpticalFibre...
Explanation . . .

23. AcceptanceAngle:
NumericalAperture:

24. TypesofOpticalFibers:

25. Attenuation(Powerloss)inOpticalFibers:

26. OpticalFiberinCommunicationSystem:
Information signal source:
•The information signal to be transmitted
may be voice, video or computer data
(analog signals).
•In order to communicate through
optical fiber, the analog signals are
converted into electrical signals.( by
Analog to Digital converter)
•The converted electrical signals are
passed through the transmitter.

27. OpticalFiberinCommunicationSystem:
Transmitter:
•The transmitter is a modulator device used
to receive electrical input signal, and then
modulate it into digital pulses for
propagation into an optical fiber.
•The modulator consists of a driver and a
light source as shown in fig. The driver
receives the electrical signals and then
converts into the digital pulses. These digital
pulses are converted into optical signals after
passing through a light source, generally
either light emitting diodes (LED’s) or a
semi conductor laser is used as light source.
•The optical signals are then focused into the
optical fiber as shown in fig.

28. OpticalFiberinCommunicationSystem:
OpticalFiber(or)Transmission
medium:
•Theopticalfiberisusedastransmission
mediumbetweenthetransmitterandthe
receiver.
•Theopticalsignalsarethenfedintoan
opticalfibercablewheretheyare
transmittedoverlongdistancesusingthe
principleoftotalinternalreflection.

29. OpticalFiberinCommunicationSystem:
Receiver:
•The receiver is a demodulator device used to receive the
optical signals from the optical fiber and then convert
into electrical signals.
•The demodulator consists of a photodetector, an
amplifier and a signal restorer.
•The optical signals which are emerging from the optical
fiber are received by photo detector.
•The photodetectorconverts the optical signals into
electrical signals.
•Theelectricalsignalsarethenamplifiedbytheamplifier
andtheamplifiedelectricalsignalsareconvertedinto
digitalform.
•The amplified electrical signals are fed to a signal
restorer where the original voice is recovered.

30. AdvantagesofOpticalFiber:

31. AdvantagesofOpticalFiber:

32. ApplicationsofOpticalFibre: