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Optical Sources

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

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  1. Optical Sources By AsifSiddiq www.bzupages.com

  2. 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

  3. LED Drawbacks • Lower coupling efficiency • Lower modulation bandwidth • Harmonic distortion www.bzupages.com

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. Assignment Discuss Surface emitting LED and Edge emitting LED www.bzupages.com

  10. 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

  11. 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

  12. 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

  13. 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

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