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 excitations Energy Atom in ground state Atom in excited state

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

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

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

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

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

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

2.71/2.710 Optics (Laser lecture) 12/12/01-9 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 population down to the ground state, thus creating a new photon. This amplification process is called stimulated emission

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

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 goes into oscillation

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

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

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

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

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 Partially reflecting mirror Light Amplification through Stimulated Emission of Radiation amplified once reflected amplified twice

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 Partially reflecting mirror Light Amplification through Stimulated Emission of Radiation amplified twice amplified again etc. output

2.71/2.710 Optics (Laser lecture) 12/12/01-18 Overview of light sources non-LaserLaser Thermal: polychromatic, spatially incoherent (e.g. light bulb) Gas discharge: monochromatic, spatially incoherent (e.g. Na lamp) Light emitting diodes (LEDs): monochromatic, spatially incoherent 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

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