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Fiber-Optic Communications

Fiber-Optic Communications. James N. Downing. Chapter 6. Optical Detectors and Receivers. 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

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Fiber-Optic Communications

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  1. Fiber-Optic Communications James N. Downing

  2. Chapter 6 Optical Detectors and Receivers

  3. 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

  4. 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)

  5. 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

  6. 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

  7. 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

  8. 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

  9. 6.2 Receiver Photodiodes • MSM Photodiode • Metal-semiconductor-metal • Based on Schottky diodes • Extremely fast response time • High responsivity • Efficiencies near 90%

  10. 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

  11. 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

  12. 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.

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. 6.5 Receivers • Receiver Packaging • All components must be protected from environmental conditions • Transmission of correct signal • Elimination of loss

  19. 6.5 Receivers • Transceiver • Transmitter and receiver in one unit

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