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

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Kenneth Golden
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1 12/12/01-1 Lasers

2 Semi-classical view of atom excitations Energy e - Ze + Atom in ground state Energy Atom in excited state 12/12/01-2

3 Light generation Energy excited state equilibrium: most atoms in ground state ground state 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 12/12/01-4

5 Light generation Energy excited state hν hν The excited atoms radiatively decay, emitting one photon each ground state 12/12/01-5

6 Light amplification: 3-level system Energy super-excited state excited state equilibrium: most atoms in ground state; note the existence of a third, super-excited state ground state 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 12/12/01-7

8 Light amplification: 3-level system Energy super-excited state excited state hν When a photon enters,... ground state 12/12/01-8

9 Light amplification: 3-level system Energy super-excited state excited state hν hν hν 12/12/01-9 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

10 Light amplifier P in Gain medium (e.g. 3-level system w population inversion) P out =gp in 12/12/01-10

11 Light amplifier w positive feedback P in Gain medium (e.g. 3-level system w population inversion) P out =gp in When the gain exceeds the roundtrip losses, the system goes into oscillation + Σ + g 12/12/01-11

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

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

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

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

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

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

18 12/12/01-18 Overview of light sources Laser non-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) ~nsec pulse duration mono/poly-chromatic = single/multi color ~psec to few fsec

19 Monochromatic, spatially coherent light 1/ν nice, regular sinusoid λ 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 12/12/01-19

Unit I Light and Optics

Unit I Light and Optics Outline By the time you finish this, you should understand the following aspects of our experiment: 1) Why you produce a grating pattern when you cross two laser beams. 2) What