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Georgia tech ece 3080 dr alan doolittle ee 332 spring

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Unformatted text preview: tion. Georgia Tech ECE 3080 - Dr. Alan Doolittle EE 332 Spring 2013 Semiconductor LED vLEDASER Semiconductor s L vs LASER? •Light Emitting Diode •Light is mostly monochromatic (narrow energy spread comparable to the distribution of electrons/hole populations in the band edges) •Light is from spontaneous emission (random events in time and thus phase). •Light diverges significantly •LASER •Light is essentially single wavelength (highly monochromatic) •Light is from “stimulated emission” (timed to be in phase with other photons •Light has significantly lower divergence (Semiconductor versions have more than gas EE 332 Spring 2013 lasers though). Semiconductor Laser EE 332 Spring 2013 Spontaneous Light Emission Light Material Interaction • • • • We can add to our understanding of absorption and spontaneous radiation due to random recombination another form of radiation – Stimulated emission. Stimulated emission can occur when we have a “population inversion”, i.e. when we have injected so many minority carriers that in some regions there are more “excited carriers” (electrons) than “ground state” carriers (holes). Given an incident photon of the bandgap energy, a second photon will be “stimulated” by the first photon resulting in two photons with the same energy (wavelength) and phase. This phase coherence results in minimal divergence of the optical beam resulting in a directed light source. EE 332 Spring 2013 Spontaneous vs Stimulated Light Emission Emission Spontaneous vs Stimulated Light Emission The power-current curve of a laser diode. Below threshold, the diode is anan LED. Above threshold, the The power-current curve of a laser diode. Below threshold, the diode is LED. Above threshold, the population is inverted and the light output increases rapidly. light output increases rapidly. population is inverted Georgia Tech Georgia Tech ECE 3080 - Dr.-AlanAlan Doolitt ECE 3080 Dr. Doolittle EE 332 Spring 2013 Semiconductor Laser LED vs LASER? Semiconductor •Light Emitting Diode •Light is mostly monochromatic (narrow energy spread comparable to the distribution of electrons/hole populations in the band edges) •Light...
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