fiber optic communication lec 13 by engr.muhammad ashraf bhutta
TRANSCRIPT
Fiber Optic Communication Lec 13By
Engr.Muhammad Ashraf Bhutta
Optical Receiver
Optical Receivers
The purpose of the receiver is:
i) to convert the optical signal to electrical
ii) recover the data
Photo detectors
The most critical component is the photodetector. It should have:
• high sensitivity
• fast response time
• low noise
• size compatible with fibers
• high reliability
This means that semiconductor materials are exclusively used in light wave systems. In
these, photons are absorbed to generate electron-hole (e-h) pairs producing a photo-current.
A basic requirement is that the detector material bandgap energy (Eg) must be smaller than the photon energy (h).
Light Detectors
These are Opto-electric devices i.e. to convert the optical signal back into electrical impulses.
The light detectors are commonly made up of semiconductor material.
When the light strikes the light detector a current is produced in the external circuit proportional to the intensity of the incident light.
Important Detector Parameters
Responsivity
Quantum Efficiency
Capacitance
Response Time
Dark Current
Edge Effect
Noise
Photodiodes: Responsivity
The detector responsivity R is defined as the photo-current/incident optical power:
η
Responsivity
Theoretically maximum responsivity is about 1.05A/W at a wavelength of 1310nm.
Commercial InGaAs detectors provide typical responsivity of 0.8 to 0.9 A/W at a wavelength of 1310nm.
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Important Detector Parameters Responsivity Quantum
Efficiency
Capacitance
Response Time
Dark Current
Edge Effect
Noise
Quantum Efficiency
It is the ratio of primary electron-hole pairs created by incident photon to the photon incident on the diode material.
A 100% quantum efficiency mean that every absorbed photon creates an electron-hole pair.
Commercial detectors usually have 70 to 90% quantum efficiency.
Important Detector Parameters
Responsivity
Quantum Efficiency Capacitance
Response Time
Dark Current
Edge Effect
Noise
Capacitance
Capacitance of a detector depends upon the active area of the device and the reverse voltage across it.
A small active diameter allows for lower capacitance but as the active area decreases, it become harder to align the fiber to the detector.
Capacitance
If the edges of the detector are illuminated, it results in slow response and increased edge jitter.
It is important to illuminate the central region of the detector.
The capacitance decreases with the increasing reverse voltage before it acquires a constant value.
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Capacitance Vs. Reverse Voltage
Important Detector Parameters
Responsivity
Quantum Efficiency Capacitance
Response Time
Dark Current
Edge Effect
Noise
Response Time
Response time represents the time needed for the photodiode to respond to optical inputs and produce an external current.
Response time for emitters and detectors is specified as rise time or fall time and it determines useful bandwidth of the diode.
Response Time
= 2.2 x R x C
The combination of the diode’s capacitance and load resistance, along with the design of the diode, determines the response time.
Response time is usually measured between 10% and 90% amplitude point.
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Important Detector Parameters Responsivity
Quantum Efficiency
Capacitance
Response Time
Dark Current
Edge Effect
Noise
Dark Current The term “Dark Current” implies that
the detector some how manages to put out a current when there is no light.
This current produced in the absence of light is caused by the intrinsic resistance of the detector and the applied reverse voltage.
Dark current is temperature sensitive and is doubled after every 50C to 100C rise.
Dark Current
Dark current is temperature sensitive and is doubled after every 50C to 100C rise in temperature.
Dark current contribute to the detector noise and also creates difficulties for DC coupled amplifier stages.
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Important Detector Parameters Responsivity
Quantum Efficiency
Capacitance
Response Time
Dark Current
Edge Effect
Noise
Edge Effect Detectors provide fast response in
their center region and the outer regions exhibit a phenomenon known as edge effect.
The detector edge as a higher responsivity compared to the center.
The operator may be fooled in to aligning the fiber to the edge of the detector, not center, because of higher responsivity at the edges
Edge Effect
The response at the edges is significantly slower than the central region.
To align detector to the fiber a high frequency square wave light source and an oscilloscope is used to look for the cleanest edges.
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Important Detector Parameters
Responsivity Quantum
Efficiency Capacitance
Response Time
Dark Current
Edge Effect
Noise
Noise
Noise is the unwanted electrical or optical energy other than the signal itself.
Noise is more critical phenomenon at the receiver end as the signal is already weak when it reaches the receiver.
Noise
In optical communication shot noise occurs because the process of creating the current is a set of discrete occurrences rather than a continuous flow.
Shot noise occurs when no light falls on the detector in the form of dark current.
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Noise
A second type of noise, thermal noise arises from fluctuations in the load resistance of the detector.
The signal power should be 10 times that of the noise or signal will not be detected adequately.
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Types of Light Detectors
PIN Photodiode
Avalanche Photodiode
PIN Photodiode
A p-n junction photodiode has a small depletion region.
If this depletion region is made large by adding a lightly doped intrinsic semiconductor between highly doped p-type and n-type layers, its called PIN photodiode.
PIN Photodiode
A p-n junction photodiode has a small depletion region.
If this depletion region is made large by adding a lightly doped intrinsic semiconductor between highly doped p-type and n-type layers, its called PIN photodiode.
p-i-n diodes
PIN Photodiode
PIN photodiode has two operating modes i.e photovoltaic and photoconductive.
In photovoltaic mode no biasing voltage is applied, so the detector is slow and the detector output is approximately logarithmic to the input light.
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PIN Photodiode
The real world fiber optic receivers use photoconductive mode where the detector is reverse biased and the output is a current that is very linear with the input light.
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Cross section and operation of a PIN Photodiode
Types of Light Detectors
PIN Photodiode
Avalanche Photodiode
Avalanche Photodiode
In PIN photodiode one photon absorbed produces one electron-hole pair that sets one electron flowing in the external circuit.
In Avalanche photodiode the primary carriers, the free electron and hole created by absorbed photons, accelerate gaining several electron volts of kinetic energy.
Avalanche Photodiode
The fast moving carriers collide with neutral atoms and use some of their energy to help the bound electrons break out of the valence shell. .
The secondary carrier then produce another stream of electron hole pairs.
This process of photo multiplication ultimately results in the production of a large current.
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Avalanche Photodiode
APD’s are expensive than PIN diodes.
APD require high voltage power supply for their operation i.e 30 or 70 volts in InGaAs APD’s and over 300 volts for Silicon APD’s.
APD’s temperature sensitive and can generally for digital system because of their poor linearity.
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Receiver components
The digital receiver consist of three blocks
• front end (photodetector, transimpedance amplifier)
• linear channel (amplifier, low-pass filter)
• data recovery (clock recovery, decision circuit)
The front end consists of a photodiode and a preamplifier. It is a trade off between sensitivity (low thermal noise) and high bandwidth.
A high-impedance front end suffers from low bandwidth.
A low-impedance front end suffers from high thermal noise. Usually a transimpedance design is used:
Linear Channel
The linear channel consists of:
• a high-gain (limiting) amplifier, which provides a constant output voltage irrespective of the average input power. (within certain limits)
• a low-pass filter with bandwidth chosen to minimize noise (proportional to filter bandwidth) while not introducing much inter-symbol-interference (ISI). The best situation is when the filter (and not other components) limits the overall bandwidth of the receiver.