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Lasers

Lasers. 2.71/2.710 Optics (Laser lecture) 12/12/01-1. Semi-classical view of atom excitations. Energy. Atom in ground state. Energy. Atom in excited state. 2.71/2.710 Optics (Laser lecture) 12/12/01-2. Light generation. Energy. excited state. equilibrium: most atoms in ground state.

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Lasers

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

  2. Semi-classical view of atom excitations Energy Atom in ground state Energy Atom in excited state 2.71/2.710 Optics (Laser lecture) 12/12/01-2

  3. Light generation Energy excited state equilibrium: most atoms in ground state ground state 2.71/2.710 Optics (Laser lecture) 12/12/01-3

  4. Light generation Energy excited state A pump mechanism (e.g. thermal excitation or gas discharge) ejects some atoms to the excited state ground state 2.71/2.710 Optics (Laser lecture) 12/12/01-4

  5. Light generation Energy excited state hν hν The excited atoms radiatively decay, emitting one photon each ground state 2.71/2.710 Optics (Laser lecture) 12/12/01-5

  6. Light amplification: 3-level system Energy super-excited state excited state ground state equilibrium: most atoms in ground state; note the existence of a third, “super-excited” state 2.71/2.710 Optics (Laser lecture) 12/12/01-6

  7. Light amplification: 3-level system Energy super-excited state excited state Utilizing the super-excited state as a short-lived “pivot point,” the pump creates a population inversion ground state 2.71/2.710 Optics (Laser lecture) 12/12/01-7

  8. Light amplification: 3-level system Energy super-excited state excited state hν When a photon enters, ... ground state 2.71/2.710 Optics (Laser lecture) 12/12/01-8

  9. Light amplification: 3-level system Energy super-excited state excited state hν hν hν When a photon enters, it “knocks” an electron from the inverted population down to the ground state, thus creating a new photon. This amplification process is called stimulated emission ground state 2.71/2.710 Optics (Laser lecture) 12/12/01-9

  10. Light amplifier Gain medium (e.g. 3-level system w population inversion) 2.71/2.710 Optics (Laser lecture) 12/12/01-10

  11. Light amplifier w positive feedback Gain medium (e.g. 3-level system w population inversion) When the gain exceeds the roundtrip losses, the system goes into oscillation 2.71/2.710 Optics (Laser lecture) 12/12/01-11

  12. Laser initial photon Gain medium (e.g. 3-level system w population inversion) Partially reflecting mirror Light Amplification through Stimulated Emission of Radiation 2.71/2.710 Optics (Laser lecture) 12/12/01-12

  13. Laser amplified once initial photon Gain medium (e.g. 3-level system w population inversion) Partially reflecting mirror Light Amplification through Stimulated Emission of Radiation 2.71/2.710 Optics (Laser lecture) 12/12/01-13

  14. Laser amplified once initial photon reflected Gain medium (e.g. 3-level system w population inversion) Partially reflecting mirror Light Amplification through Stimulated Emission of Radiation 2.71/2.710 Optics (Laser lecture) 12/12/01-14

  15. Laser amplified twice initial photon reflected Gain medium (e.g. 3-level system w population inversion) amplified twice Partially reflecting mirror Light Amplification through Stimulated Emission of Radiation 2.71/2.710 Optics (Laser lecture) 12/12/01-15

  16. Laser amplified once initial photon reflected Gain medium (e.g. 3-level system w population inversion) output amplified twice reflected Partially reflecting mirror Light Amplification through Stimulated Emission of Radiation 2.71/2.710 Optics (Laser lecture) 12/12/01-16

  17. Laser amplified twice initial photon reflected Gain medium (e.g. 3-level system w population inversion) output amplified twice reflected amplified again etc. Partially reflecting mirror Light Amplification through Stimulated Emission of Radiation 2.71/2.710 Optics (Laser lecture) 12/12/01-17

  18. Overview of light sources non-Laser Laser Continuous wave (or cw): strictly monochromatic, spatially coherent Thermal: polychromatic, spatially incoherent (e.g. light bulb) (e.g. HeNe, Ar+, laser diodes) Gas discharge: monochromatic, spatially incoherent (e.g. Na lamp) Pulsed: quasi-monochromatic, spatially coherent (e.g. Q-switched, mode-locked) Light emitting diodes (LEDs): monochromatic, spatially incoherent ~psec to few fsec ~nsec pulse duration mono/poly-chromatic = single/multi color 2.71/2.710 Optics (Laser lecture) 12/12/01-18

  19. Monochromatic, spatially coherent light • nice, regular sinusoid • λ, ν well defined • stabilized HeNe laser good approximation • most other cw lasers rough approximation • pulsed lasers & nonlaser sources need more complicated description Incoherent: random, irregular waveform 2.71/2.710 Optics (Laser lecture) 12/12/01-19

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