Basic Laser Principles_pdf.

This includes basic laser principles which are required to understand working of any laser system. It consists of the terms like excitation, de-excitation, population inversion, optical resonator, pumping source, active system and basic laser action.

2. Contents:
•Basic laser principles
•Laser Types: Semiconductor Laser diode and Nd-YAG laser
•Applications of Lasers: Laser printer, drilling and cutting

3. LASER:
•LASERis a device which produces highly intense light through the
process of Stimulated Emission
•Properties:
•Highly monochromatic
•Highly coherent (Temporal as well as Spatial)
•High intensity (more bright)
•High unidirectionality (low beam divergence)
Light Amplification by Stimulated Emission of Radiation

4. Basic Principles of LASER :
Stimulated Absorption (Excitation)
Itistheprocessoftransitionofanelectronfromnormalstate
(groundstate)toexcitedstateduetoabsorptionofenergy
Before During After
hυ
. .
.
E1
E2

5. Basic Principles of LASER :
Spontaneous Emission
•Inthisprocesselectronundergotransitionfromhigherstatetolowerstate
spontaneouslybyemittingaphotonofenergyhυ
•Thisisnaturalandinstantaneousprocess,independentofoutsidecircumstances
•Photonsemittedbyspontaneousemissionprocessarerandomindirectionandhence
theyhaverandomphase.Thus,theradiationisincoherentandhasabroadspectrum

6. Basic Principles of LASER :
Stimulated Emission
•Theforcedtransitionofelectronfromhigherstatetolowerstateisreferredtobeasstimulated
emission
•Inthisprocess,anatominexcitedstateistriggeredorstimulatedbyincidentenergyofsuitable
value
•Aphoton(havingsameenergytothatphotonwhichwouldbeemittedduetode-excitation)
appliedtoexcitedsystemstimulatesde-excitationprocess.
•Duringthisprocessaphotonisemittedhavingenergyhυ=(E
2-E
1)alongwithonemorephotonof
sameenergy

7. Features of Stimulated Emission
•Photonsemittedareidenticalwitheachother
•Photonshavesameenergy,frequency,wavelength,direction
•Thus,radiationproducedbystimulatedemissionresultsintoa
monochromatic,coherentandunidirectionalbeamoflight.
•Also,astimulatedemissioninitiatedbyaphotonproducestwosame
photons,thesetwophotonsde-excitestwomoreelectronsandgive
outfourphotons.
•Stimulatedemissioncausedbythesefourphotonsgivesawayeight
photonsandsoon.
•Thiswaynumberofphotonsincreaseswhichinturnincreasesthe
intensityoflight

8. Basic Principles of LASER :
•Metastable state: An excited state having lifetime more than 10
-8
sec
•Pumping Source: It is a type of energy required to transfer electrons from
lower to excited state. e.g. electrical or optical depending upon type of
laser.
•Population Inversion: An artificial situation created in which we get N
2>N
1
•Optical Resonator: It is a pair of reflecting surfaces, one completely
reflecting and other partially reflecting

9. Basic Principles of LASER :
•Active system or Active medium:
collection of atoms, ions or molecules
e.g. He-Ne laser: mixture of Helium and Neon gases
Ruby Laser: Ruby crystal in the form of rod
•In general, active system may be gas, liquid or solid and its
physical state decides the type of laser

10. Basic Principles of LASER :
Basic Laser action
Due to optical resonator, these photons are reflected back and forth millions of times. As a result of this,
amplification takes place (i.e. intensity increases)
Finally, through partially reflecting surface beam emerges out in the form om of laser beam