Optical Sources

							Optical Sources

        By
   Asif Siddiq



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                           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


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             LED Drawbacks
• Lower coupling efficiency
• Lower modulation bandwidth
• Harmonic distortion




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                 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

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                 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




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          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
   • ηint at the best is 50% as compared to 60% to 80% for ILD
• Internal quantum efficiency ηint of LED can be defined
  as
   • Ratio of the radiative recombination rate to the total
     recombination rate

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            LED Power & Efficiency
   • ηint = rr/rt    = rr/(rr + rnr )
          = Rr/Rt Where Rt = i/e under equilibrium conditions
   • Rearranging we get
      Rr = ηint i/e
• Power is then given by
   • Pint = ηint (i/e)hf   = ηint (nci/eλ)




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                  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



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     Assignment

Discuss Surface emitting LED
    and Edge emitting LED



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              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



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              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




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              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




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                   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

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