Fiber-Optic Communications

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Fiber-Optic Communications

• James N. Downing

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Chapter 6

• Optical Detectors and Receivers

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6.1 The Photodetection Process

• Optical Absorption
• Condition in which light striking an electron
  will create enough energy to exceed the bandgap
  energy and the photon is absorbed
• Absorption coefficient The length of time that
  the photon energy in a material takes to decay
  exponentially
• Penetration depth Depth to which the photon
  energy falls in the material

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6.1 The Photodetection Process

• Quantum Efficiency
• The efficiency with which the light energy is
  converted to electrical energy
• Typical efficiencies range from 50 to 90
• Responsivity
• The efficiency with which the photodetector
  converts the light energy to electrical energy
  (the transfer function)

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6.1 The Photodetection Process

• Response Time
• The amount of time that a photodiode takes to
  respond to an optical input (in other words, the
  amount of time needed for the input of the
  photodiode to produce an output)
• Cutoff Frequency
• The maximum frequency that a device can transfer

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6.2 Receiver Photodiodes

• A photodiode is a photodetector that uses a pn
  junction to detect light.
• When light strikes the pn junction, current is
  caused to flow in reverse bias.
• Dark current Current that flows in the absence
  of light

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6.2 Receiver Photodiodes

• pin Photodiode
• The pn junction is separated by a slice of
  intrinsic material.
• Most absorption takes place in the intrinsic and
  depletion layers,
• Increased quantum efficiency (near 100) is due
  to wider depletion layer.
• Increase in response time

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6.2 Receiver Photodiodes

• Avalanche Photodiode
• Makes use of an extra intrinsic p junction to
  increase photodiode gain
• Impact ionization
• Collision of accelerated charge carriers with
  other carriers causing them to ionize
• Avalanche breakdown
• The tremendous reverse voltage causing huge
  amounts of current to flow

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6.2 Receiver Photodiodes

• MSM Photodiode
• Metal-semiconductor-metal
• Based on Schottky diodes
• Extremely fast response time
• High responsivity
• Efficiencies near 90

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6.3 Noise Factors

• Thermal Noise
• Other names Johnson or Nyquist noise
• Due to random motion of electrons and dissipation
  of heat within the device
• Shot Noise
• The noise due to the small amount of current
  produced from the random light to electrical
  energy conversion

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6.3 Noise Factors

• Dark Current Noise
• The noise due to the small amount of current that
  flows in the absence of light
• Increases with temperature and applied voltage

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6.3 Noise Factors

• Signal-to-Noise Ratio
• The ratio of the communications signal to the
  amount of noise present
• The noise should be much smaller than the signal.
• Noise equivalent power is the minimum detectable
  power level at which the signal equals the noise
  in a 1-Hz system.

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6.4 Amplifiers

• High Impedance Amplifier
• High input impedance minimized thermal noise
  generated by the feedback of the amplifier
• Not suitable for wide bandwidths
• Transimpedance Amplifier
• Optimizes the tradeoffs between speed and
  sensitivity
• Improved dynamic range

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6.4 Amplifiers

• Main Amplifier
• A second amplifier that is added after the front
  end amplifier to maximize the gain and bandwidth
• Contains the automatic gain control (AGC)
• Uses a low-pass filter to shape the output pulse
• Reduces noise

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6.5 Receivers

• The Receiver
• Receives the incoming optical signal
• Converts an optical signal to an electrical
  signal
• Amplifies the electrical signal
• Components
• Optical input signal, photodiode, low-noise
  preamp, main amp, data recovery stage, and
  electrical output

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6.5 Receivers

• Signal Recovery
• This circuit makes sure that the correct
  information is received
• Decision Circuit
• Compares the incoming signal to a threshold level
  to determine ones and zeros
• Clock Recovery Circuit
• Measures the bit slot and generates the clock
  pulse for the decision circuit

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6.5 Receivers

• Receiver Performance
• Dynamic Range
• The range of detectable signal levels with linear
  response
• Sensitivity
• Minimum input optical power that can be detected
• BER
• Average probability of incorrect bit
  identification

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6.5 Receivers

• Receiver Packaging
• All components must be protected from
  environmental conditions
• Transmission of correct signal
• Elimination of loss

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6.5 Receivers

• Transceiver
• Transmitter and receiver in one unit