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 Emitted photons have random phases Incoherent light source

LED Drawbacks Lower coupling efficiency Lower modulation bandwidth Harmonic distortion

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

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

LED Power & Efficiency LEDs tend to limit internal quantum efficiency η int (Ratio of photons generated to injected electrons) Reliance on spontaneous emission allows non-radiative recombination Crystalline imperfections and impurities η int at the best is 50% as compared to 60% to 80% for ILD Internal quantum efficiency η int of LED can be defined as Ratio of the radiative recombination rate to the total recombination rate

LED Power & Efficiency η int = r r /r t = r r /(r r + r nr ) = R r /R t Where R t = i/e under equilibrium conditions Rearranging we get R r = η int i/e Power is then given by P int = η int (i/e)hf = η int (nci/eλ)

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

Assignment Discuss Surface emitting LED and Edge emitting LED

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

LED Characteristics Output spectrum At room temperature the spectral line width is nm for µm wavelength band Output spectra tends to broaden with temperature at a rate of 0.1 to 0.3 nm o c -1 Heat sinks are therefore used to control the broadening

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

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 10 6 to 10 7 hours for AlGaAs devices (100 to 1000 years) In excess of 10 9 hours for Surface emitting InGaAsP LED