Optical Light Sources

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Optical Light Sources

• Introduction Light sources for optical fiber
• communication
• Difference between LED used as a lamp and LED
• used for optical fiber communication
• Requirements of an LED used for optical
• fiber communication
• Configurations of LED the Double-
• Hetrojunction structure of LED
• Types of LEDs used for optical fiber
• communication

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Principle Light Sources used for optical Fiber
Communication

• LED
• Laser Diode

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Reasons for choosing the devices

• They have adequate output power for wide range of
  applications
• Their optical power output can be directly
  modulated by varying the input current to the
  device
• They have high efficiency
• Their dimensional characteristics are compatible
  with those of the optical fiber

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Construction and Working in general

• Light-emitting regions of both LEDs and laser
  diodes consist of a pn-junction.
• When this junction is forward biased, electrons
  and holes are injected into the p-region and
  n-region, respectively.
• These injected minority carriers can recombine
  either radiatively (in this case, a photon of
  energy h? is emitted) or non-radiatively (in
  this case the energy during recombination is
  dissipated in the form of heat)

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Optical Energy from the two types of Optical
Sources

• The optical energy from a laser diode is
  coherent.
• That is, optical energy is produced in an optical
  resonant cavity and this optical energy when
  released is highly monochromatic
• Also, the output beam is highly directional.

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Optical Energy from the two types of Optical
Sources

• Optical energy from an LED is incoherent.
• That is, no optical cavity exists for wavelength
  selectivity.
• The output radiation has a broad spectral width
  since the emitted photon energies range over the
  energy distribution of the recombining electrons
  and holes.
• Incoherent optical energy has a large beam
  divergence.

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Factors to be considered while choosing an
optical source compatible with the optical
waveguide

• Geometry of the fiber
• The optical fiber may be a single mode fiber or a
  multimode fiber
• In general, LEDs are used only with multimode
  fibers since it is only the multimode fiber that
  the incoherent optical power from an LED can be
  coupled in sufficient quantity.
• Laser diodes are used in single mode fibers.

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Factors to be considered while choosing an
optical source compatible with the optical
waveguide

• Attenuation as a function of wavelength
• Different fiber cables have different operating
  wavelengths like 850nm, 1310nm, 1550nm.
• The wavelength of the type of optical source used
  must be compatible with the operating wavelength
  of the optical fiber. Maximizing system bandwidth
  involves designing optical fibers and sources
  that minimize dispersion (distortion in the
  optical signal as it propagates along the fiber)
  at the intended operational wavelength.

Edmond Fernandes
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Factors to be considered while choosing an
optical source compatible with the optical
waveguide

• Group Delay Distortion (Bandwidth)
• Each mode contains all the spectral components in
  the wavelength band over which the source emits.
• As the signal propagates along the fiber, each
  spectral component can be assumed to travel
  independently and undergo a time delay or a group
  delay per unit length in the direction of the
  propagation.
• Different spectral components have different time
  delays.
• As a result of difference in time delays, the
  optical signal pulse spreads out with time over
  the fiber.

Edmond Fernandes
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Factors to be considered as far as the optical
source is concerned

• Be compatible in size to low-loss optical fibers
  by having a small light-emitting area capable of
  launching light into fiber
• Launch sufficient optical power into the optical
  fiber to overcome fiber attenuation and
  connection losses allowing for signal detection
  at the receiver
• Emit light at wavelengths that minimize optical
  fiber loss and dispersion.
• Optical sources should have a narrow spectral
  width to minimize dispersion

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Factors to be considered as far as the optical
source is concerned

• Allow for direct modulation of optical output
  power (The optical source should be able to
  properly modulate the light intensity according
  to the variations in the input electrical signal)

Edmond Fernandes
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LIGHT EMITTING DIODES