LASER MRU With Diode Laser
October 6, 2022 | Author: Anonymous | Category: N/A
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LASER
Dr.T.VIJAY KUMAR M.Sc., Ph.D DEPARTMENT OF PHYSICS SCHOOL OF SCIENCES
Acronym AcronymofLASE ofLASER R
L - LIGHT A - AMPLIFICATION S - STIMULATED E - EMISSION R - RADIATION The LASER beam was invented by the physicist MAIMAN in 1960
ModuleI-A:LASER Characteristics of lasers
Absorption, Spontaneous and Stimulated transitions Einstein’ Einstein’ss coefficients and relations between them Population inversion Pumping – Optical and Electrical Meta stable state
Three and four level pumping schemes
Ruby LASER Helium-Neon LASER Semiconductor Diode LASER
WhatistheIm WhatistheImportanceof portanceofLaserinda Laserindailylife? ilylife?
NORMALLIGHTVs.LASERLIGHT
Characteristics of lasers • •
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Monochromatic: ForOrdinarywhitelight,∆λ=300nm, Formonochromaticlight,∆λ=5-10nm ForLaserlight,∆λ=0.2nm
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Directionality:
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laserwillspreadtolessthan1cmdiameteratadistanceof10m fromthelaser.
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• •
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Coherence: Highintensity: Highintensity : I=P/A=E/At P-POWER,E-ENERGY,t-TIME
INTERACTION OF RADIATION WITH MATTER
E2
E2
E1
E1
Band gap (Eg)
Every atom or molecule have different energy levels Atoms in their ground state level stays for long period When external energy is provided, atom in the ground state
absorbs energy and move to excited state The external energy should satisfy the condition, [hν ≥ Eg]
ABSORPTION An atom or molecule in the ground state E 1 can absorb a photon of energy hν and go to the higher energy state E2, this process is known as absorption.
E2
E2
E1
E1
Let, 12 – upward transition from E1 to E2 R 12 N1 – number of atoms in ground state (E1) ρ ν – energy density of radiation
B12 – probability of absorption per unit time
SPONT SPONTANEOUS ANEOUS EMISSION An atom or molecule in the higher energy state E 2 returns to the ground state E1 by emitting their excess energy spontaneously, spontaneously, this process is known as spontaneous emission E2 E2
E1
E1
Let, R 21 (SP) – downward transition from E 2 to E1 N2 – Number of atoms in excited state (E 2) ρ ν – Energy density of radiation A21 – Probability of spontaneous emission per unit time
STIMULATED EMI SSION STIMULATED EMISSION In this type of emission, a photon stimulate an atom in the higher state E2 to make a transition to lower state E 1with the creation of second photon E2
E2
E1
E1
Let, R 21 – Downward transition from E 2 to E1 N2 – Number of atoms in excited state (E 2) ρ ν – Energy density of radiation B21 – Probability of stimulated emission per unit time
WHY WE NEED STIMULATED EMISSION FOR LASER? E
E2
E2
2
E1
E1 ABSORPTION
SPONTANEOUS EMISSION
STIMULATED EMISSION
It is a natural process
It is stimulated process
One radiation or photon is
Two radiation or photon is
emitted in each transation emitted in each transation Emitted photon is in random Emitted photons are in direction single direction For multiple transitions, the emitted photons are out of
For multiple transitions, the emitted photons are in phase
phase It is observed in ordinary
It is observed in LASER
light source
source
Einstein's coefcient coefcient and Einstein's relation Let N1 & N2 be the no. of atoms in the ground state and excited state and is the energy density per unit volume Rate of absorption α N1 Rate of absorption α ρ(r) Then the rate of absorption per unit volume Rate of spontaneous emission α N2 Rate of spontaneous emission per unit volume Rate of stimulated emission α N2 Rate of stimulated emission α ρ(r) Rate of stimulated emission per unit volume Where B12,B21 and A21 are Einstein's coefficient under thermal equilibrium, the rate of absorption = rate of emission
N0.
N0.
1
In general,
POPULATION INVERSION
Number of atoms or molecules in the ground state (N1) is greater than the number atoms or molecules in the excited state (N2) i.e., N1 > N2
In ordinary source, spontaneous emission dominates (N 1 > N2) To obtain lasing action, stimulated emission should be dominate (N2 > N1) Inverse of this condition is said to be population inversion. i.e.,
N2 > N1
PUMPING: The process of creating population inversion is called as pumping
Mainly two types of pumping
1. Opti Optica call pump pumpin ing g 2. El Elec ectri trica call Pum Pumpi ping ng Other pumping mechanisms are
X-ray pumping, e-beam pumping, chemical pumping and gas dynamic pumping
http://www.fizik.itu.edu.tr/akturks/file
s/L&P%2011%20pumping.pdf
Optical pumping: The pump is a powerful lamp, emitting light in all directions, over a
broad spectrum
Lamps are commonly used for many solid state lasers When the lamp and laser rod is close together the pumping will be more uniform
Electrical pumping:
Free electrons and ions are initially created through the high voltage.
These charges are then accelerated in the same electric field. Free electrons then generate excited atoms through collisions e + X → X* + e Where, e – electron, X – atom before excited, X* - atom after excited
Metastable state: It is an excited state of state of an atom that has a longer lifetime than the ordinary excited states and shorter lifetime than the ground state. state.
It is an temporary trapmay or stable stage of a system, the energyenergy of which be lostintermediate in discrete amounts. Thus creating metastable state, population inversion can be achieved and lasing action takes place
1. If the collectio collection n of atoms atoms is intensely intensely pumped, pumped, a large large number of atoms are excited through stimulated absorption 2. If the excited excited energy energy state state has very very short lifetim lifetime, e, the atoms atoms decay decay fast to ground level. 3. If we can able able to make make one sub level level or metastabl metastablee where the atoms can stay for long period when excited than the population inversion is easily achieved. E populat 4. There are mainly mainly two two E types of schemes schemes to achieve achieve population ion inversion E3 E 1. Thr hree ee le leve vell E2 Fast decay Fast decay 2. Fou ourr le level E2 E2 E1 Pum ping Laser actio E1 Pumping
Laser action E1 Fast decay E0
E0 N
N After pumping
E0
After pumpi
Before pumping
Four level um i
Components of LASER Activemed Activemedium ium
ExcitationMechanismorEnergySource forPumping Resonantcavity
The cavity or resonator is composed of several mirrors that bounce the beam back and forth through the amplifying medium. There are two different types: linear cavities (light is reflected back and forth) and ring cavities (light circulates round and round).
u y
Three major parts
1. Ruby rod 2. Re Reso sona nati ting ng ca cavi vity ty 1. Ruby rod3:. Xen enon on fl flas ash h lam lamp p It is a combination of 99.95% aluminum oxide doped with 0.05% of chromium oxide Chromium ions replaces aluminum atoms in the crystal lat 2. Resonating cavity: It consist of 4 cm in length and 0.5 cm diameter of ruby (Pink) rod The two ends of the rods are strictly grounded so they are parallel to each other The end faces are silvered in such a way that one end is fully reflecting and the other is partially reflecting 3. Xenon flash tube: It is used for optical pumping It is helical shaped surrounds the ruby rod It provides pumping light to raise the chromium
:
ions to the u
er levels
APPLICATIONSOFRUBYLA ONSOFRUBYLASER SER APPLICATI •
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Usingwhererequiredshortpulsesofred Usingwhererequiredshor tpulsesofredlight light Holographicportraitswith Holograph icportraitswithrubylasers,in rubylasers,insizes sizes uptoametersquared. Manynondestructive Manynon destructivetestinglabsu testinglabsuseruby seruby laserstocreatehologra laserstocrea tehologramsoflargeo msoflargeobjective bjective suchasaircrafttirestoloo suchasaircra fttirestolookforweaknes kforweaknessesin sesin thelining. Rubylaserswereusedexte Rubylaserswe reusedextensivelyintat nsivelyintattoo too andhairremoval.
He-Ne Laser •
Thislaserconsists oftwogases He-Ne. two gases do not interact from a molecule.
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This is one type of atomic gas LASER . And four level laser.
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Gasdischargetubeof length30cmanddiameter 1.5cm.Thistubeismade upquartzandisfilledwith mixtureofneonunderthe pressureof0.1mmof Hg.heliumunderthe pressureof1mmofHg.is 10:1bothgasses
• Laser produced by atomic transitions within the neon atom.the atom.the helium does not directl produce laser light but acts as a buffer gas, this purpose of which is to assist/help the of the other gas to produce prod uce lasing action. • atoms He-three active energy levels & Ne-six active energy energy levels Ne-metastable levels Ne-metastable state are E and E6 • Excited He atoms collision with Ne atoms results in energy transfer to He to Ne . • Stimulated emission takes place b/w E6-E3-----6328A
•• Stimulated emission takes place b/w E6-E5-----33910A Stimulated emission takes place b/wE4-E3----1 b/wE4-E3----11520A 1520A
• Optical elements placed insides the laser system are absorb the IR rays hence output
APPLICATIONS OF
He-Ne LASER
1.The narrow red beam is used in supermarkets to read bar codes 2.Used in holography in producing the 3D image of objects. 3.Used in industrial and scientific laborites ….
Semiconductor Lasers •
Semiconductordiode(the firstlaserdiode)was demonstratedin1962by twoUSgroupsledby RobertN.Hall •
Thesemiconductorlaser ismadeinmass quantitiesfromwafersof galliumarsenideorsimilar crystals.
SemiconductorLaser Construction: 1.Active Medium:
PN junction acts as active medium in Semiconductor Laser Laser.. GaAs semiconductor is doped with Al or Se for P-type and N-type Thickness of junction is about 0.1µm
2.Pumping or Excitation mechanism:
PN diode must be connected in forward bias to produce the laser When p-region is connected to positive terminal of the battery and
n region is connected to negative terminal of the battery, then said to b in forward bias. The majority carriers in P-type material are holes and these are in the valence band of P-type material. The majority carriers in N-type material are electrons and these
are in the conduction band of N-type material.
3.Optical Resonator:
Both sides of junction are polished.
One side is 100% polished and the side is partially polished
Remaining sides of pn diode are metal shielded to avoid the loss
radiation.
Working:
Principle: Recombination When a pn junction is forward biased, e-’s from n region and holes from p region cross the junction & recombine with each other This is called Recombination process. In the Recombination process photons are emitted. In the pn diode, n region is highly doped as compared to p-type. Hence the no. of e-’s in conduction band of n-region greater than
the no. of holes in valence band of p-region.
When the junction is forward biased, e-’s from n-region and holes from p-region are injected into junction. No. of electrons are greater than no. of holes and hence Population inversion condition is achieved. Then immediately electrons are recombined with holes in the junction with the help of potential from battery. battery. In the recombination process, photons are emitted These photons are emitted through the process stimulation emission. Hence this radiation is coherent light i.e. Laser light.
The wavelength of laser is 8300A to 8500A
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cooling
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