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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
– 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
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)
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
6.2 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
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
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
6.2 Receiver Photodiodes
• MSM Photodiode
–
–
–
–
–
Metal-semiconductor-metal
Based on Schottky diodes
Extremely fast response time
High responsivity
Efficiencies near 90%
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
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
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.
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
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
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
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
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
6.5 Receivers
• Receiver Packaging
– All components must be protected from
environmental conditions
– Transmission of correct signal
– Elimination of loss
6.5 Receivers
• Transceiver
– Transmitter and receiver in one unit