lasers - dspace.mit.edu · 2.71/2.710 optics (laser lecture) 12/12/01-17 laser gain medium (e.g....
TRANSCRIPT
2.71/2.710 Optics (Laser lecture)12/12/01-1
Lasers
2.71/2.710 Optics (Laser lecture)12/12/01-2
Semi-classical view of atom excitationsEnergy
Energy
Atom in ground stateAtom in ground state
Atom in excited stateAtom in excited state
Ze+e-
2.71/2.710 Optics (Laser lecture)12/12/01-3
Light generation
ground state
excited state
equilibrium: most atomsin ground state
Energy
2.71/2.710 Optics (Laser lecture)12/12/01-4
Light generation
ground state
excited state
A pump mechanism (e.g. thermal excitation or gas discharge) ejects some atoms to the excited state
Energy
2.71/2.710 Optics (Laser lecture)12/12/01-5
Light generation
ground state
excited state
The excited atoms radiativelydecay, emitting one photon each
Energy
hν
hν
2.71/2.710 Optics (Laser lecture)12/12/01-6
Light amplification: 3-level system
ground stateequilibrium: most atomsin ground state; note the existenceof a third, “super-excited” state
Energy super-excited state
excited state
2.71/2.710 Optics (Laser lecture)12/12/01-7
Light amplification: 3-level system
ground stateUtilizing the super-excited stateas a short-lived “pivot point,” thepump creates a population inversion
Energy super-excited state
excited state
2.71/2.710 Optics (Laser lecture)12/12/01-8
Light amplification: 3-level system
ground stateWhen a photon enters, ...
Energy super-excited state
excited state
hν
2.71/2.710 Optics (Laser lecture)12/12/01-9
Light amplification: 3-level system
ground state
Energy super-excited state
excited state
hν
When a photon enters, it “knocks” an electron from the inverted population down to the ground state, thus creatinga new photon. This amplification process is called stimulated emission
hν
hν
2.71/2.710 Optics (Laser lecture)12/12/01-10
Light amplifier
Gain medium(e.g. 3-level system
w population inversion)
Pin
Pout =gPin
2.71/2.710 Optics (Laser lecture)12/12/01-11
Light amplifier w positive feedback
Gain medium(e.g. 3-level system
w population inversion)
Pin
Pout =gPin
gΣ+
+
When the gain exceeds the roundtrip losses, the system goes into oscillation
2.71/2.710 Optics (Laser lecture)12/12/01-12
Laser
Gain medium(e.g. 3-level system
w population inversion)
LightAmplification throughStimulatedEmission ofRadiation
initial photon
Partiallyreflecting
mirror
2.71/2.710 Optics (Laser lecture)12/12/01-13
Laser
Gain medium(e.g. 3-level system
w population inversion)
LightAmplification throughStimulatedEmission ofRadiation
initial photonamplified once
Partiallyreflecting
mirror
2.71/2.710 Optics (Laser lecture)12/12/01-14
Laser
Gain medium(e.g. 3-level system
w population inversion)
LightAmplification throughStimulatedEmission ofRadiation
initial photonamplified once
reflected
Partiallyreflecting
mirror
2.71/2.710 Optics (Laser lecture)12/12/01-15
Laser
Gain medium(e.g. 3-level system
w population inversion)
LightAmplification throughStimulatedEmission ofRadiation
initial photonamplified once
reflected
amplified twice
Partiallyreflecting
mirror
2.71/2.710 Optics (Laser lecture)12/12/01-16
Laser
Gain medium(e.g. 3-level system
w population inversion)
LightAmplification throughStimulatedEmission ofRadiation
initial photonamplified once
reflected
amplified twice
Partiallyreflecting
mirror
reflected
output
2.71/2.710 Optics (Laser lecture)12/12/01-17
Laser
Gain medium(e.g. 3-level system
w population inversion)
LightAmplification throughStimulatedEmission ofRadiation
initial photonamplified once
reflected
amplified twice
Partiallyreflecting
mirror
reflected
output
amplified againetc.
2.71/2.710 Optics (Laser lecture)12/12/01-18
Overview of light sourcesLasernon-Laser
Thermal: polychromatic,spatially incoherent(e.g. light bulb)
Gas discharge: monochromatic,spatially incoherent(e.g. Na lamp)
Light emitting diodes (LEDs):monochromatic, spatially incoherent
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)
mono/poly-chromatic = single/multi color
pulse duration
~nsec ~psec to few fsec
2.71/2.710 Optics (Laser lecture)12/12/01-19
λ1/ν • nice, regular sinusoid• λ, ν well defined• stabilized HeNe laser good approximation• most other cw lasers rough approximation• pulsed lasers & non-laser sources need more complicated description
Incoherent: random, irregular waveform
Monochromatic, spatially coherent light