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Optical Sources. By Asif Siddiq. LED. Electron from the conduction band recombines with a hole in the valance band of a semiconductor to produce a PHOTON Stimulated emission is not encouraged By not adding a cavity and mirrors Operates at lower current densities
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Optical Sources By AsifSiddiq www.bzupages.com
LED • Electron from the conduction band recombines with a hole in the valance band of a semiconductor to produce a PHOTON • Stimulated emission is not encouraged • By not adding a cavity and mirrors • Operates at lower current densities • Emitted photons have random phases • Incoherent light source www.bzupages.com
LED Drawbacks • Lower coupling efficiency • Lower modulation bandwidth • Harmonic distortion www.bzupages.com
LED Advantages • Simpler fabrication • No mirror facets • Lower cost • Because of simpler fabrication • Higher reliability • no catastrophe degradation • Less sensitive to gradual degradation • Less temperature dependent • Output against current xteristics less affected by temp www.bzupages.com
LED Advantages • Simpler drive circuitry • Due to lower derive current and reduced temperature dependence • We don’t need temperature compensation circuits • Linearity • Linear output against current characteristics www.bzupages.com
LED Power & Efficiency • LEDs tend to limit internal quantum efficiency ηint • (Ratio of photons generated to injected electrons) • Reliance on spontaneous emission allows non-radiative recombination • Crystalline imperfections and impurities • ηintat the best is 50% as compared to 60% to 80% for ILD • Internal quantum efficiency ηintof LED can be defined as • Ratio of the radiative recombination rate to the total recombination rate www.bzupages.com
LED Power & Efficiency • ηint = rr/rt = rr/(rr+ rnr) = Rr/RtWhere Rt = i/e under equilibrium conditions • Rearranging we get Rr= ηinti/e • Power is then given by • Pint = ηint (i/e)hf = ηint (nci/eλ) www.bzupages.com
LED Structures • Five major types • Planar LED and Dome LED • Used in applications as plastic encapsulated visible devices • Infrared version also available • Surface emitter, Edge emitter and Superluminescent LEDs • First two extensively used in optical fiber communication while the third is becoming of increasing interest www.bzupages.com
Assignment Discuss Surface emitting LED and Edge emitting LED www.bzupages.com
LED Characteristics • Optical output power • Intrinsically a very linear device • More suitable for analog transmission • May exhibit significant nonlinearities depending upon its utilization • Linearization techniques such as negative feedback used in such conditions • Internal quantum efficiency decreases with increase in temperature www.bzupages.com
LED Characteristics • Output spectrum • At room temperature the spectral line width is 25-40 nm for 0.8 -0.9 µm wavelength band • Output spectra tends to broaden with temperature at a rate of 0.1 to 0.3 nmoc-1 • Heat sinks are therefore used to control the broadening www.bzupages.com
LED Characteristics • Modulation Bandwidth • Electrical bandwidth • Optical bandwidth • Electrical Bandwidth is defined as • Electrical signal power drops to half of its constant value • Corresponds to electrical 3dB point www.bzupages.com
LED Characteristics • Reliability • More reliable since no catastrophic degradation • Exhibit gradual degradation which may take a form of rapid degradation • Rapid degradation is due to • Growth of dislocations • Precipitate type defects • Injection current densities • Temperature • Impurity concentration • Life of LED at room temperature is • 106 to 107 hours for AlGaAs devices (100 to 1000 years) • In excess of 109 hours for Surface emitting InGaAsP LED www.bzupages.com