lasers 2.71/2.710 optics (laser lecture) 12/12/01-1

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Lasers 2.71/2.710 Optics (Laser lecture) 12/12/01-1

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Page 1: Lasers 2.71/2.710 Optics (Laser lecture) 12/12/01-1

Lasers

2.71/2.710 Optics (Laser lecture)12/12/01-1

Page 2: Lasers 2.71/2.710 Optics (Laser lecture) 12/12/01-1

2.71/2.710 Optics (Laser lecture)12/12/01-2

Semi-classical view of atom excitationsEnergy

Energy

Atom in ground state

Atom in excited state

Page 3: Lasers 2.71/2.710 Optics (Laser lecture) 12/12/01-1

2.71/2.710 Optics (Laser lecture)12/12/01-3

Light generation

Energyexcited state

equilibrium: most atomsin ground state

ground state

Page 4: Lasers 2.71/2.710 Optics (Laser lecture) 12/12/01-1

2.71/2.710 Optics (Laser lecture)12/12/01-4

Energyexcited state

ground state

Light generation

A pump mechanism (e.g. thermalexcitation or gas discharge) ejectssome atoms to the excited state

Page 5: Lasers 2.71/2.710 Optics (Laser lecture) 12/12/01-1

2.71/2.710 Optics (Laser lecture)12/12/01-5

Light generation

The excited atoms radiativelydecay, emitting one photon each

ground state

excited stateEnergy

Page 6: Lasers 2.71/2.710 Optics (Laser lecture) 12/12/01-1

2.71/2.710 Optics (Laser lecture)12/12/01-6

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

Page 7: Lasers 2.71/2.710 Optics (Laser lecture) 12/12/01-1

2.71/2.710 Optics (Laser lecture)12/12/01-7

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

Page 8: Lasers 2.71/2.710 Optics (Laser lecture) 12/12/01-1

2.71/2.710 Optics (Laser lecture)12/12/01-8

Light amplification: 3-level systemEnergy

excited state

ground stateWhen a photon enters, ...

super-excited state

Page 9: Lasers 2.71/2.710 Optics (Laser lecture) 12/12/01-1

2.71/2.710 Optics (Laser lecture)12/12/01-9

hνhν

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

Page 10: Lasers 2.71/2.710 Optics (Laser lecture) 12/12/01-1

2.71/2.710 Optics (Laser lecture)12/12/01-10

Light amplifier

Gain medium(e.g. 3-level system

w population inversion)

Page 11: Lasers 2.71/2.710 Optics (Laser lecture) 12/12/01-1

2.71/2.710 Optics (Laser lecture)12/12/01-11

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

Page 12: Lasers 2.71/2.710 Optics (Laser lecture) 12/12/01-1

2.71/2.710 Optics (Laser lecture)12/12/01-12

Gain medium(e.g. 3-level system

w population inversion)

Laserinitial photon

Partiallyreflectingmirror

LightAmplification throughStimulatedEmission ofRadiation

Page 13: Lasers 2.71/2.710 Optics (Laser lecture) 12/12/01-1

2.71/2.710 Optics (Laser lecture)12/12/01-13

Laser

initial photon

PartiallyreflectingmirrorLight

Amplification throughStimulatedEmission ofRadiation

amplified once

Gain medium(e.g. 3-level system

w population inversion)

Page 14: Lasers 2.71/2.710 Optics (Laser lecture) 12/12/01-1

2.71/2.710 Optics (Laser lecture)12/12/01-14

Laser

initial photon

Partiallyreflectingmirror

LightAmplification throughStimulatedEmission ofRadiation

amplified once

reflected Gain medium(e.g. 3-level system

w population inversion)

Page 15: Lasers 2.71/2.710 Optics (Laser lecture) 12/12/01-1

2.71/2.710 Optics (Laser lecture)12/12/01-15

amplified twice

amplified twice

reflected

LightAmplification throughStimulatedEmission ofRadiation

Partiallyreflectingmirror

initial photon

Laser

Gain medium(e.g. 3-level system

w population inversion)

Page 16: Lasers 2.71/2.710 Optics (Laser lecture) 12/12/01-1

2.71/2.710 Optics (Laser lecture)12/12/01-16

reflected

output

Gain medium(e.g. 3-level system

w population inversion)

Laser

initial photon

Partiallyreflectingmirror

LightAmplification throughStimulatedEmission ofRadiation

amplified once

reflected

amplified twice

Page 17: Lasers 2.71/2.710 Optics (Laser lecture) 12/12/01-1

2.71/2.710 Optics (Laser lecture)12/12/01-17

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

Page 18: Lasers 2.71/2.710 Optics (Laser lecture) 12/12/01-1

2.71/2.710 Optics (Laser lecture)12/12/01-18

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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2.71/2.710 Optics (Laser lecture)12/12/01-19

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