1. Fiber-Optic Communications
James N. Downing

2. Optical Detectors and Receivers
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 Shot 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 Transimpedance 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 Components
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 Sensitivity BER
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