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LASER
Light Amplification byStimulated Emission ofRadiation
BY
DR. SRIDHARBABU YARRAMANENIASSISTANT PROFESSOR
DEPARTMENT OF PHYSICS
SCHOOL OF HUMANITIES AND APPLIED SCIENCES
LINGAYAS UNIVERSITY
FARIDABAD - 121002
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LASER: CONTENTS
Spontaneous emission Stimulated emission
Laser actionPumping
Population inversion Characteristics of laser beamConcepts of coherence
Spatial coherence
Temporal coherence
Solid state laser: ruby laser
Gas laser : He-Ne laser
Applications
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Laser: Spontaneous and Stimulated Emissions
absorption Spontaneous
emission
Stimulatedemission
h h
h
E1
E2
E1
E2
h =E2-E
1
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Laser: Laser ActionA schematic illustration of the steps leading to laser action.
(a) The Boltzmann population of states, with more atoms in the ground state.(b) When the initial state absorbs, the populations are inverted (the atoms are
pumped to the excited state).
(c) A cascade of radiation then occurs, as one emitted photon stimulates
another atom to emit, and soon. The radiation is coherent (phases in step).
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Laser: Laser Action
The excited state is populated while the cavity is non resonant. Then the
resonance characteristics are suddenly restored, and the stimulatedemission emerges in a giant pulse.
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Laser: Laser ActionA summary of the features needed for efficient laser action.
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Laser: Characteristics of Laser BeamLaser light has three unique characteristics, that make it different
than "ordinary" light. It is:Monochromatic
Directional
Coherent
Monochromatic means that it consists of one single color orwavelength. Even through some lasers can generate more thanone wavelength, the light is extremely pure and consists of a verynarrow spectral range.
Directional means that the beam is well collimated (very Directional means that the beam is well collimated (veryparallel) and travels over long distances with very little spread.parallel) and travels over long distances with very little spread.
Coherent means that all the individual waves of light are movingprecisely together through time and space, i.e. they are in phase.
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Laser: CoherenceCoherence is the degree to which electromagnetic radiation maintains a near-constant phase relationship, both temporally and spatially. The time over whichthe phase relationship remains nearly constant is called the Coherence time (tc).
Temporal Coherence
Temporal coherence is a measure of the correlation between the phases of alight wave at different points along the direction of propagation.
Temporal coherence tells us how monochromatic a source is.
Let the source emits waves with wavelength d. Waves with wavelength and +d, which at some point in space constructively interfere, willdestructively interfere after some optical path length lc. lc is called the
coherence length.
The wave pattern travels through space with speed c.The coherence time t
c
= lc
/c. also tc
= 1/d.
Spatial Coherence
Spatial coherence is a measure of the correlation between the phases of alight wave at different points transverse to the direction of propagation.Spatial coherence tells us how uniform the phase of the wave front is.
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Laser: Solid State Laser (Ruby Laser)
Discovered in 60-ies of the
20th century.
Ruby (Al2O
3) monocrystal,
Cr doped.
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Laser: Solid State Laser (Ruby Laser)
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Laser: Solid State Laser (Ruby Laser)
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Laser: Solid State Laser (Ruby Laser)
Ene
rgy
4A2
4F2
4F1
2E
LASING
Al2O3Cr+
6943 0A
Optical pumping: 510 600 nm and 360 450 nm.Fast transition on 2E.
Lasing: 2E on 4A2
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Laser: Gas laser (He-Ne laser)
The pumping (of the neon) depends on acoincidental matching of the helium and neonenergy separations, so excited He atoms cantransfer their excess energy to Ne atoms during a
collision.The pumping (of the neon) depends on a
coincidental matching of the helium and neon
energy separations, so excited He atoms cantransfer their excess energy to Ne atoms during acollision.
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Laser: Gas laser (He-Ne laser)
The transitions involved in a helium-neon laser.
