EE 4392 Introduction To Optical Systems Instructor: Dr Mohammed Zamshed Ali Optical Detectors Fall 2

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WELCOME
EE 4392Introduction To Optical
SystemsInstructor Dr Mohammed Zamshed
AliOptical Detectors Fall 2007Department of
Electrical EngineeringUniversity of Texas at
Dallas
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Optical Detectors

• Light Detectors
• PN Photodiodes
• PIN Photodiode
• Avalanche Photodiode

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Photo Detection Mechanisms

• Detectors convert light signals into electrical
  signals
• Photo Detection Mechanisms
• Internal Photoelectric Effect - Generation of
  mobile charge carriers in semiconductors by
  absorption of photons.
• Devices PN photodiode, PIN photodiode,
  Avalanche photodiode
• External Photoelectric Effect Generation of
  free electrons when photons strike the surface of
  a metal. Electrons are emitted from the surface.
  
• Devices Vacuum photodiode, Photomultiplier tube

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Photo Detection Mechanisms
Important Detector Properties 1. Responsivity
P
i
Optical Power
Electrical Current
Photodetector
(7.1)
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Photo Detection Mechanisms
2. Spectral Response Range of optical
wavelengths over which the detector is useful.
It is often displayed as a curve of responsivity
versus wavelength. Example Silicon photodiode
response
0.5
0
0.5 0.7 0.9 1.1
? (?m)
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Photo Detection Mechanisms
3. Speed of Response Range of modulation
frequencies over which the detector is useful.
As before, if tr is the rise time, the bandwidth
is (approximately)
(7.2)
90
P
i
10
Input
Output
tr
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Photo Detection Mechanisms
Other Important Properties Size, temperature
sensitivity, gain, lifetime, circuit complexity,
and cost.
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Vacuum Photodiode and Photomultiplier
Vacuum Photodiode
Cathode
Anode
hf
-
Electrons
i
-

RL
V
v
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Vacuum Photodiode and Photomultiplier
The work function ? is defined to be Energy
required to liberate an electron from the metal
cathode. Units of energy Joules In order to free
an electron the photon energy must equal, or
exceed, the work function.
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Vacuum Photodiode and Photomultiplier
Thus, the incoming photon frequency must satisfy
(7.3)
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Vacuum Photodiode and Photomultiplier
The longest wavelength that can be detected is
called the cutoff wavelength. It is given by
or
(7.4)
where ?c is in ?m, and ? in eV. For detection we
require
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Vacuum Photodiode and Photomultiplier
Current flows through the load resistor RL during
the time that electrons travel from the cathode
to the anode. At the anode, the electrons and
positive charges drawn there through the circuit
neutralize each other. The current stops when
the electrons reach the anode.
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Vacuum Photodiode and Photomultiplier
Example Consider Cesium, where ? 1.9 eV.
Find the cutoff wavelength.
This wavelength is shorter than the wavelengths
commonly used for glass fiber systems. Thus,
this detector will not work for fiber systems.
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Vacuum Photodiode and Photomultiplier
Calculation Responsivity of the Vacuum
Photodiode Let h be the quantum efficiency,
defined by
It is the fraction of incident photons that
results in emitted electrons.
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Vacuum Photodiode and Photomultiplier
P
i
Optical Power
Electrical Current
Detector
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Vacuum Photodiode and Photomultiplier
The current is thus
(7.6)
And the responsivity is
(7.7)
This result is valid for all photodetectors.
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Vacuum Photodiode and Photomultiplier
In general then, we have shown that
From the photodiode circuit, the output voltage is
or
(7.8)
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Vacuum Photodiode and Photomultiplier
Conclusions 1. The detected current is
proportional to the optical power, which itself
is proportional to the message current. Thus,
the receiver current is proportional to the input
message current as required. 2. The detector acts
like a current source for the receiver.
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Vacuum Photodiode and Photomultiplier
P
i
Detector
Receiver
Receiver
Detector
i
i ? P
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Vacuum Photodiode and Photomultiplier

• Example Compute the responsivity if ? 1 at
• 0.8 ?m.
• From (7.7)

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Vacuum Photodiode and Photomultiplier
Compute the output voltage if the input power is
1 ?W. Let RL 50 ?.
Change RL to 50,000 ? and recompute the voltage.
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Vacuum Photodiode and Photomultiplier
Photomultiplier (PMT)
Cathode
Anode

-
100V
400V
1000 V
200V
300V
Dynodes
Secondary Electrons
i
-

RL
V
v
( V 1000 volts )
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Vacuum Photodiode and Photomultiplier
Fast moving electrons hit the metal dynodes
causing the release of additional (secondary)
electrons. Gains of 2 to 6 per dynode are
typical. Let ? gain per dynode, and N
number of dynodes The total gain is M
?N (7.9)
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Vacuum Photodiode and Photomultiplier
The current is then
(7.10)
Example
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Semiconductor (pn) Photodiode
hf
i
-
v
VB
RL

Reverse Bias
hf
p
n
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Semiconductor Photodiode
Reverse Biased Photodiode
p
n
hf
E
-
- - - - - - - - - - -
hf
Electron Energy
Wg


Junction Region
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Semiconductor Photodiode
An incident photon absorbed in the junction
gives up its energy, creating a free electron and
a free hole in the junction.
The generated free charges move due to the strong
electric field E in the junction. Recall that
the electric field is generated by the change in
electric potential V given by
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Semiconductor Photodiode
As before
A condition for detection is that the photon
energy be greater than the bandgap energy
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Semiconductor Photodiode
At cutoff, then
or
(7.11)
where Wg is in eV and ?c is in ?m. Only
wavelengths where ? ? ?c will be detected.
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Semiconductor Photodiode
Example Compute the cutoff wavelength for
Silicon. The bandgap energy for silicon is Wg
1.1 eV
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Semiconductor Photodiode
Thus, silicon detectors are useful only in the
first window 0.8 0.9 ?m for glass fiber
systems.
0.5
?
0.5 0.7 0.9 1.1
? ( ?m )
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Semiconductor Photodiode
Example Repeat the previous calculation for
germanium. Wg 0.67 eV
Germanium detectors are useful in all fiber
windows.
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Semiconductor Photodiode
Response Time The pn photodiode responds slowly
because many photons are absorbed in the n and p
regions close to the junction.
E
Diffusion
p
-
n
hf
Electron Energy

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Semiconductor Photodiode
The free charge carriers diffuse slowly (yellow
arrow) into the junction where they are
accelerated (green arrow) by the large electric
field there. This produces a delayed current in
the external circuit, causing pulse spreading,
lowering the data rates that can be received, and
reducing the analog 3-dB frequency bandwidth of
the receiver.
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Semiconductor Photodiode
The analog 3-dB frequency bandwidth is typically
about tr 1 ?s for pn junction photodetectors.
The solution to this problem is the pin diode.
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PIN Photodiode
PIN Photodiode
i
hf
Intrinsic Layer
-
E
p
n

Thin Layer
-
v
V
RL

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PIN Photodiode
The intrinsic layer is on an insulator. Most of
the photons are absorbed in that layer because it
is long. Most of the voltage drop is across the
intrinsic layer. This creates a high electric
field in the intrinsic layer. Now there is no
delay caused by diffusion and the response time
is much faster that that of a pn photodiode.
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Cutoff Wavelength
PIN Photodiode
A condition for detection is that the photon
energy be greater than the bandgap energy
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PIN Photodiode
At cutoff, then
or
(7.11)
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PIN Photodiode
Example Compute the cutoff wavelength for
Silicon. The bandgap energy for silicon is Wg
1.1 eV
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PIN Photodiode
Materials
Wavelength Range (?m)
Peak Response l (?m)
Peak Responsivity r (A/W)
Material
Silicon 0.3 1.1 0.8 0.5 Germanium 0.5
1.8 1.55 0.7 InGaAs 1.0 1.7 1.7 1.1
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PIN Photodiode
Photodetector Circuit
vd
-
p
n
id
-
v
V
RL

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PIN Photodiode
Current-Voltage Characteristic
Silicon Photodiode
Diode Voltage vd (volts)
-20 -10 0 0.5
ID
P 10 ?W
-5 -10 -15 -20
Diode Current id (?A)
20
30
40
Photovoltaic Region
Photoconductive Region
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PIN Photodiode
ID is called the dark current. This is the
current that flows when no photons are incident
(this is just the diode reverse leakage current).
It is due to thermal generation of minority
charge carriers. Small optical signals are masked
by the dark current. For good signal reception,
the received power must greatly exceed levels of
power which generate currents on the order of the
dark current.
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PIN Photodiode
Typical Dark Current Values Silicon 2
nA InGaAs 50 nA Germanium
500 nA
Example Compute the responsivity from the vd
id curve given several slides back. Solution
id -10 ?A when P 20 ?W, so that
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PIN Photodiode
Example Let ID 2 nA in the previous example.
What is the minimum detectable optical power if
we can detect a signal current equal to (or
greater than) the dark current? Solution We
want i 2 nA as a minimum.
We can detect powers as low as 4 nW.
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PIN Photodiode
Graphical Circuit Analysis
vd
-
id
-

v
VB
RL

-
VB 20 V, RL 106 ? Load Line Equation VB
vd id RL 0
(7.12)
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PIN Photodiode
Silicon Photodiode
Diode Voltage vd (volts)
-20 -10 0 0.5
P 10 ?W
-5
-10
20
Diode Current id (?A)
-15
30
-20
40
-25
50
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PIN Photodiode
From the load line equation (7.12), if
The operating point is at the intersection of the
load line and the characteristic curve. In this
example, the optical power can range from 0 to 40
?W. The receiver is saturated beyond this power
level. The response becomes non-linear (yielding
distortion) and the response time increases.
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PIN Photodiode
The output characteristic looks like the
following
Linear Region
Output Voltage v (volts)
20 10 0
Saturation Region
20 40 60
Input Power (?W)
Saturation occurs when vd 0, that is when
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PIN Photodiode
The corresponding optical power is
(7.13)

• This is the maximum optical power that can be
  received before saturation is reached.
• Conclusions
• Make RL large to increase the output voltage

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PIN Photodiode
2. Make RL small to increase the receivers
dynamic range. As seen from (7.13), the load
resistance is limited to
3. Make RL small to reduce the response time as
indicated on the following slides.
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PIN Photodiode
Speed of Response Transit time Time for free
charges to move across the depletion region (the
intrinsic layer in a pin photodiode). The speed
of response is ultimately limited by the transit
time. Example d depletion width 50 ?m
v carrier velocity 5 x 104 m/s Then the
transit time is
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PIN Photodiode
The detectors rise time is on the order of its
transit time. The bandwidth is limited to In
addition to the transit time, the detectors
capacitance and load resistance also limit the
response speed.
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PIN Photodiode
Detector Equivalent Circuit
id (?P)
v
Cd
RL
Cd Diode Capacitance
The rise time of this circuit is
(7.15)
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PIN Photodiode
The 3-dB bandwidth is
(7.16)
This establishes the relationship between the
load resistance and the bandwidth. The larger the
load resistance, the smaller the bandwidth.
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PIN Photodiode
Current-to-Voltage Converter
(Transimpedance Amplifier)
A Operational amplifier
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PIN Photodiode
vd (volts)
-VB
0
Diode Current id (?A)
P 10 ?W
- 5
Load Line
20
-10
-15
30
-20
40
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PIN Photodiode
Since vA ? 0, circuit equation is VB vd 0 so
that the load line equation becomes vd
-VB This circuit yields a higher dynamic range
than the conventional receiver circuit.
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PIN Photodiode
Packaging

• These packages are similar to those of the LED
  and laser diode. Possibilities
• Mounted on a standard transistor header with a
  clear glass cover or lens.
• The package may include a fiber pigtail.
• Photodiodes may be mounted on printed circuit
  boards.
• Plus others.

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Avalanche Photodiode
The avalanche photodiode (APD) is a pin
photodiode having internal gain. The circuit is
the same as that for a conventional pin
photodiode.
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Avalanche Photodiode
The supply voltage is on the order of a few
hundred volts to produce a high electric field in
the junction. When a photon is absorbed in the
junction, free charges are released as we
described for the pin photodiode. These charges
are accelerated by the force of the high electric
field. The charges gain kinetic energy and
collide with neutral atoms.
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Avalanche Photodiode
The kinetic energy causes the release of new free
charges from the neutral atoms (these atoms are
ionized). These new free charges go through the
same process, ionizing even more atoms, creating
even more free charges. An avalanche of new
charges flow in the junction increasing the total
circuit current. In this way, there is an
increase (gain) in the current.
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Avalanche Photodiode

• Gains up to a few hundred are possible.
• The APD is the solid state equivalent of the
  photomultiplier tube (PMT).
• Unfortunately, it does not have as much gain as
  the PMT.
• Fortunately, it is a solid state device and is
  compatible with fiber-optic system requirements.
• The APD improves the receiver sensitivity.