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PHOTO
DETECTORS
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Module III
INTRODUCTION &
CHARACTERISTICS
Converts intensity of light energy into electric current
Since optical power will be very feeble at the ends of the
fiber, photo detector should show very high performance
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It should be insensitive to temperature variations, be
compatible with physical dimensions of fiber, have
reasonable cost and high operating life.
Should have high response & sufficient bandwidth to
handle the desired data rate.
Types: Photomultipliers, Pyroelectric detectors,
semiconductors
Most extensively used is the semiconductor photodiode
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PHOTODIODE
CHARACTERISTICS &
TYPES
Small size
Suitable material
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High sensitivity
Fast response time
Durability
Types:
 PIN Photo detector
 Avalanche Photodiode (APD)
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PIN DIODE
Most common semiconductor photo detector
P & N region separated by very lightly doped intrinsic (i)
region
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In normal operation a sufficiently large reverse bias
voltage is applied so that the intrinsic region is fully
depleted of carriers
When a photon is incident in this junction, and if it has
an energy greater than the band gap energy, the photon
will give up the energy and excite electrons from the
valance band to conduction band
This generates electron-hole pairs known as photo
carriers
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PIN DIODE WORKING
Normally the PIN diode is made in such a way that this
electron hole pairs normally gets generated in the
depletion region
The high electric field in the depletion region causes
carriers to separate and to be collected across the
reverse biased junction
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This gives rise to a current flow in the external circuit
which is known as photocurrent
Some of the carriers may disappear in this process as
they gets recombined on the way
On an average the carriers may travel a distance of Ln
and Lp for electrons and holes respectively. Diffusion
Length,
D is diffusion coefficient,
t is carrier
lifetime
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PIN DIODE – ENERGY
BAND DIAGRAM
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PIN DIODE –
ABSORBED POWER
Optical radiation is absorbed in the semiconductor
material according to the exponential law,
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is the absorption coefficient at wavelength
P0 is the incident optical power level
P(x) is the optical power absorbed in a distance
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AVALANCHE
PHOTODIODE (APD)
Multiplies internal photon current before giving to amplifier
This increases receiver sensitivity as no receiver noise is
added
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HOW?
In order to happen multiplication, carriers must pass
through a region where a strong electric field is present.
In this region, the carriers obtain so much energy that it
ionizes the bound electrons in the valance band upon
colliding with them
This carrier multiplication mechanism is called impact
ionization
The newly created carriers are also accelerated by the high
electric field and cause further impact ionization –
Avalanche effect
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AVALANCHE
PHOTODIODE (APD)
Below the breakdown region, a finite total number
of carriers are created, but after breakdown, this
number can be infinite.
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Commonly used structure for carrier multiplication:
reach through construction
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RAPD-REACH THROUGH
AVALANCHE PHOTO DIODE
CONSTRUCTION
Highly resistive P type material deposited as an
epitaxial layer on a p+ substrate
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A p type diffusion or ion implant is then made in
the resistive material followed by construction of an
n+ layer
If silicon is used as substrate, boron and
phosphorous are dopants respectively
This configuration is termed as
through structure
reach
layer is basically an intrinsic material that has
some p doping due to imperfect purification
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RAPD-WORKING
Term reach through arises from RAPD operation
When reverse bias is applied, most of the potential
drop is across the pn+ junction
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Depletion region widens with increasing bias until a
certain voltage is reached at which the peak electric
field at the pn junction is about 5-10% below that
needed to cause avalanche breakdown
At this points, the depletion region just reaches
through the nearly intrinsic pi region
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RAPD-WORKING
In normal usage, the RAPD is operated in fully depleted
mode
Light enters the device through the p+ region and is
absorbed in the material which acts as collection region
for the photo generated carriers
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Upon being absorbed, the photon gives up its energy
creating electron – hole pairs which are then separated by
the electric field in the region
The electrons drift via the region to the pn+ junction
where a high electric field exists
It is in this high field region that carrier multiplication
takes place
This process is known by the name impact ionization
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NOISE IN DETECTORS
Photo detector detects very weak signals
For optimized detection, the SNR should be maintained
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2 kind of noises may be present in detectors
 Statistical nature of photon to electron conversion process
 Thermal noises in amplification circuitry
To achieve high SNR, following conditions must be met
 Photodetector must have high quantum efficiency to generate
high signal power
 Noises must be kept as low as possible
Sensitivity of a photodetector in an OFC – Minimum detectable Optical
Power
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NOISE SOURCES
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Rs : Series Resistance
Cd: Total Capacitance, junction & packaging
capacitances
RL: Load Resistance
Ra & Ca: Amplifier’s input resistance & capacitance
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OPTICAL RECEIVERS
Photo detector + Amplifier + Signal Processing
Circuitry
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Consideration must be given in suppressing the
noise
Current generated by detector is weak and affected
by random noises in the photo detection process
Additional noises from amplifier will also be present
Mathematical model based testing and simulation is
necessary
Average error probability
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FUNDAMENTAL
RECEIVER
More complicated than OPERATION
transmitter.
Actual signal need to be extracted from distorted
Signal
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Tb – Bit Period
ASK
Threshold Level
Comparison
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QUANTUM LIMIT OF
DETECTION
Consider an ideal photo detector with unity
quantum efficiency
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Here, there is no dark current
In this situation, the minimum received optical
power required for a specific bit error rate
performance in the digital system is known as
quantum limit of detection
Here all system parameters are ideal and
performance is only limited by photo detection
statistics.
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QUANTUM LIMIT OF
DETECTION
Consider an ideal receiver that consists of a device that is able to
detect even a single receiver photon.
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Consider the interval (0,T) and the reliability of decision a0 on the
transmitted radio wave a0.
According to the Gaussian process, assume a received optical power
of the form,
PR(t)= 2PR
When ao = 0, the received power is zero and no photons are
detected.
•The conditional bit- error probability,
Pe(0)=P[a=1| ao = 0] is zero
•Average Pe = Pe(1)/2
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OPTICAL RECEIVER
EQUIVALENT CIRCUIT
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PERFORMANCE
PARAMETERS
QUANTUM EFFICIENCY,
RESPONSIVITY
& MULTIPLICATION
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PROBLEMS
MODULE III
QUANTUM EFFICIENCY,
RESPONSIVITY
MULTIPLICATION FACTOR
3 important characteristics of a photodiode
Depends on material bandgap, operating wavelength,
and doping thickness of P, I, N regions
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The efficiency of a photodiode in converting photos to
electron – hole pairs is known by the term Quantum
Efficiency
Relation between optical power given to the photodiode
and the photocurrent generated – Responsivity
Relation between Multiplied avalanche photo current to
primary photo current – Multiplication Factor
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QUANTUM
EFFICIENCY
Photo Current
1.6 * 10
Number of carrier pairs generated per incident
photon energy
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Output Power Level
6.625 * 10-34
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-19
C
6.625 * 10-34
Quantum Efficiency (%)
RESPONSIVITY
1.6 * 10
-19
The performance of a photodiode is characterized
by responsivity. This is related to the quantum
efficiency by:
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C
MULTIPLICATION
FACTOR
Only applicable in RAPD
Multiplied current
Primary photocurrent
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