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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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