lasers 2.71/2.710 optics (laser lecture) 12/12/01-1
TRANSCRIPT
Lasers
2.71/2.710 Optics (Laser lecture)12/12/01-1
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Semi-classical view of atom excitationsEnergy
Energy
Atom in ground state
Atom in excited state
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Light generation
Energyexcited state
equilibrium: most atomsin ground state
ground state
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Energyexcited state
ground state
Light generation
A pump mechanism (e.g. thermalexcitation or gas discharge) ejectssome atoms to the excited state
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Light generation
The excited atoms radiativelydecay, emitting one photon each
ground state
excited stateEnergy
hν
hν
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equilibrium: most atomsin ground state; note the existenceof a third, “super-excited” state
ground state
Light amplification: 3-level system
excited state
Energy super-excited state
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Light amplification: 3-level system
excited state
ground stateUtilizing the super-excited stateas a short-lived “pivot point,” thepump creates a population inversion
Energy super-excited state
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Light amplification: 3-level systemEnergy
excited state
ground stateWhen a photon enters, ...
hν
super-excited state
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hνhν
hν
super-excited state
ground state
excited state
Energy
Light amplification: 3-level system
When a photon enters, it “knocks”an electron from the inverted populationdown to the ground state, thus creatinga new photon. This amplification processis called stimulated emission
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Light amplifier
Gain medium(e.g. 3-level system
w population inversion)
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Gain medium(e.g. 3-level system
w population inversion)
Light amplifier w positive feedback
When the gain exceeds the roundtrip losses, the system goesinto oscillation
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Gain medium(e.g. 3-level system
w population inversion)
Laserinitial photon
Partiallyreflectingmirror
LightAmplification throughStimulatedEmission ofRadiation
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Laser
initial photon
PartiallyreflectingmirrorLight
Amplification throughStimulatedEmission ofRadiation
amplified once
Gain medium(e.g. 3-level system
w population inversion)
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Laser
initial photon
Partiallyreflectingmirror
LightAmplification throughStimulatedEmission ofRadiation
amplified once
reflected Gain medium(e.g. 3-level system
w population inversion)
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amplified twice
amplified twice
reflected
LightAmplification throughStimulatedEmission ofRadiation
Partiallyreflectingmirror
initial photon
Laser
Gain medium(e.g. 3-level system
w population inversion)
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reflected
output
Gain medium(e.g. 3-level system
w population inversion)
Laser
initial photon
Partiallyreflectingmirror
LightAmplification throughStimulatedEmission ofRadiation
amplified once
reflected
amplified twice
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reflected
amplified twice
reflected Gain medium(e.g. 3-level system
w population inversion)
Laser
initial photon
PartiallyreflectingmirrorLight
Amplification throughStimulatedEmission ofRadiation
amplified twice
amplified againetc.
output
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Overview of light sourcesnon-Laser Laser
Thermal: polychromatic,spatially incoherent(e.g. light bulb)
Gas discharge: monochromatic,spatially incoherent(e.g. Na lamp)
Light emitting diodes (LEDs):monochromatic, spatiallyincoherent
mono/poly-chromatic = single/multi color
Continuous wave (or cw):strictly monochromatic,spatially coherent
(e.g. HeNe, Ar+, laser diodes)
Pulsed: quasi-monochromatic,spatially coherent(e.g. Q-switched, mode-locked)
~nsec ~psec to few fsec
pulse duration
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Monochromatic, spatially coherentlight
• nice, regular sinusoid• λ, ν well defined• stabilized HeNe lasergood approximation• most other cw lasersrough approximation• pulsed lasers & nonlasersources needmore complicateddescription
Incoherent: random, irregular waveform