Optical Sources

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Optical Sources
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History of Lasers

• In 1917, Einstein predicted the existence of
  spontaneous and stimulated emission by which an
  atom can emit radiation.
• To make use of the stimulated emission for the
  construction of coherent optical sources Townes
  and Schawlow in the US and Basov and Prochorov in
  the USSR.
• In 1960- Maiman demonstrated the first laser.

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The Einstein Coefficients

N2
E2
hw
E1
N1
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The Einstein Coefficients
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The Einstein Coefficients
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The Einstein Coefficients
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Example 1
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Population inversion

• Light amplification can take place only if Rstgt
  Rabs or
• N2 gt N1.
• Under thermal equilibrium, it is N1 gt N2 .
  Therefore, N2
• should be increased by external means. The
  condition N2 gt N1
• is referred as population inversion.

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Review of Semiconductor Physics
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Review of Semiconductor Physics
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Example 2
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Direct and Indirect Band Gaps
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P-N Junctions
p-type
n-type
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P-N Junctions
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Non-radiative recombination
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Non-radiative recombination
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Light emitting diode (LED)

• LED is a forward-biased p-n homojunction or
  heterojunction.
• Radiative recombination of electron-hole pairs in
  the depletion region generates light.
• For LEDs, radiative recombination of holes and
  electrons is dominated by spontaneous emission
  and stimulated emission is negligible.
• The emitted light is incoherent with a relatively
  wide spectral width (30-60 nm) and a relatively
  large angular spread.
• For bit rates less than 100-200 Mb/s together
  with multimode fibers, LEDs are usually the best
  light source.
• LEDs can not be used for long haul WDM systems
  because of their large spectral width.

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LED Light-Current Characteristics
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LED Light-Current Characteristics
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LED Light-Current Characteristics
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Example 3
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Example 3
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Laser diode (LD)

• Laser diodes emit light through stimulated
  emission while LEDs emit light through
  spontaneous emission.
• Laser diodes can emit high powers (100 mW) and
  also it is coherent.
• A relatively narrow angular spread of the output
  beam compared with LEDs permit high coupling
  efficiency.
• A relatively narrow spectral width of LD makes it
  a suitable candidate for wavelength division
  multiplexing (WDM) applications.

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Laser diode (LD)

• When a photon of energy hf impinges on the
  system, electrons in the valence band can be
  excited to conduction band. This is called
  absorption.
• Electrons in the conduction band are also
  stimulated to make a transition to valence band
  in the presence of a photon of energy hf which is
  equal to the band gap energy. This is called
  stimulated emission.
• Electrons in the conduction band could emit a
  photon of energy hf without any external
  stimulation and go to valence band. This is
  spontaneous emission.
• The stimulated emission exceeds absorption only
  if the number of electrons in the excited state
  exceeds that in ground state. This condition is
  known as population inversion.
• Population inversion is achieved by various
  pumping techniques. In semiconductor lasers,
  population inversion is accomplished by injecting
  electrons into the semiconductor material.

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Light Amplification by Stimulated Emission for
Two Level Systems
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Light Amplification by Stimulated Emission
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Optical Feedback and Laser Threshold
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Optical Feedback and Laser Threshold
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Optical Feedback and Laser Threshold
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Example 4
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Optical Feedback and Laser Threshold
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DFB and DBR Semiconductor lasers
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Laser Diode Rate Equations
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Laser Diode Rate Equations
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Laser Diode Rate Equations
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Example 5