Optical Amplifiers

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Optical Amplifiers

• Why is there a requirement for optical
  amplifiers?
• gt Light signal over long distances become
  attenuated.
• gt Regeneration of the light signal is
  necessary especially over distances of perhaps
  several thousand kilometers.
• gt The overhead of photodetection to
  electrical conversions and back to light via a
  lsaer diode. Contributes to high cost and speed.
• Optical Amplifiers EDFA, Raman, Laser
  amplifier.

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Erbium Doped Fibre Amplifier

• Fibre amplifier
• Core region is doped with ions.
• Another rare earth ion dopant that is used is the
  neodynium ion ( )
• Host fibre is glass based.( ) with
  other glass forming oxides ( )
• Important factor It is possible to have
  relatively high concentrations of erbium in the
  core (up to 1000 ppm).

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Erbium Doped Fibre Amplifier

• Refer to energy diagram.
• Erbium ions are optically pumped, typically by a
  980 nm laser diode. is raised to .
• The erbium ions decay rapidly to . They
  experience an atomically long lifetime here (10
  ms).
• Decays from
  
• Therefore there is an accumulation of erbium ions
  at sitting 0.80 eV above ground.
• This accumulation of erbium ions lead to a
  population inversion between .
• Photons at 1550 nm have an energy of 0.80eV (
  ).

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EDFA

• This triggers stimulated emission of erbium ions
  from
• Erbium ions left at E1 will absorb the incoming
  1550 nm photons and rise to E2.
• Stimulated emission must exceed light absorption
  to achieve light amplification.
• Therefore there must be more erbium ions at E2
  then at E1.

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EDFA

• Assume N2 and N1 are the number if erbium ions at
  E2 and E1.
• The difference between stimulated emission (E2
  toE1) and absorption (E1 to E2) rate controls the
  optical gain.
• where K is a constant that depends on
  pumping intensity.

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EDFA

• Considerations
• Spontaneous decay of erbium ions from E2 to E1
  will generate noise in the amplified light
  signal.
• If the EDFA is not pumped then it presents itself
  as an attenuator. 1550 nm photons will be
  absorbed by ions which will rise from E1
  to E2.

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EDFA

• The range of stimulated transistions from E2 to
  E1 correspond to a wavelength range of 1525 to
  1565 nm that can be amplified.
• This delivers an optical bandwidth of 40 nm.
• This permits usage with a WDM.
• Problem gt Gain is not uniform across bandwidth.
• Techniques must be implemented to flatten the
  response.

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EDFA

• Factors controlling the degree of gain
  uniformity
• Concentrations of the active ion (erbium).
• Optical gain flattening filter.
• Additional (second ) pump laser at each end of
  the fibre.
• One pump beam propagates with signal
  beam while the other propagates against it.
  Ensures that population inversion and gain
  remains constant along the fiber.

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EDFA

• Physical Components of EDFA
• Biconical fused fibre couplers.
• One or two (if high output required) laser pumps.
• Polarization-insensitive optical isolators front
  and back. Allows only 1550 nm signals to pass.
  Pump radiation should not enter main fibre as
  well as optical feedback from reflections.
• Optical filter for gain flattening.
• Phgotodetector system to monitor pump power or
  EDFA output power.