Downside emission repopulates the ground state making

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Unformatted text preview: t; hf/ln(2) 3 For hf in the visible (2 eV), what is T for RsFm=Rspon? T = 33,500 K (i.e. happens in stars, not on earth) 2 1 0.5 1.0 1.5 2.0 hf ￿kT 15 Route to populaFon inversion •  Need a way to get around equilibrium (that isn’t at 35,000 K) •  One method: use 3 levels instead of 2 E3 Fast, spontaneous transiFon pump transiFon E2 “metastable” state, long lifeFme SFmulated emission E1 How do we achieve a metastable state? Down ­side: emission re ­populates the ground state making it harder to maintain populaFon inversion. Need very high pumping. 16 One more level E4 Fast, spontaneous transiFon pump transiFon E3 “metastable” state SFmulated emission E2 E1 rapid relaxaFon to ground state RelaxaFon from E4 to E3 is rapid: efficient populaFon of E3. Emission from E3 to E2 and E1 forbidden. TransiFon from E2 to E1 rapid, to empty lower lasing state 17 Elements of a laser •  Light amplificaFon by sFmulated emission of radiaFon: LASER •  Three things needed to make a laser: –  Pump (energy source to produce populaFon inversion. Can be opFcal, electrical, chemical or thermal) –  Amplifying medium: gas, liquid or solid. Must support populaFon inversion. –  Resonator: Cavity to couple the light back into the gain medium. 18 Pumping schemes •  Last Fme we looked at a 4 level system as a good method of obtaining popula&on inversion. •  How do we pump the electrons into the upper energy state? E4 Fast, spontaneous transiFon pump transiFon E3 “metastable” state SFmulated emission E2 E1 rap...
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This document was uploaded on 02/04/2014.

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