6.0 Introduction
 Light detectors convert optical signal to electrical signal
 semiconductor photodiode is the most common detector
 LED - light energy emitted during electron-hole recombination
 Photodiode - opposite - light striking photodiode creates electron
flow
Light
I
1
OPTICAL DETECTORS
Photodetector
An photodetector is used at the front end of every optical receiver to generate a
photocurrent proportional to the incident light intensity.
The characteristics of photodetectors useful for fiber optic communication are:
• High sensitivity at the operating wavelength
• Sufficient bandwidth or speed of response to accommodate the information rate
• Very low noise
• Low power consumption and low operating voltage
• Less sensitive to changes in ambient temperature and in operating voltage
• Compatibility with the fiber parameters
• Small size
• Low cost
• High reliability
Photodiodes are the primary type used in optical communication systems.
There are two types of photodiodes commonly used: PIN (P-type, intrinsic, Ntype) diodes and avalanche photodiodes (APDs).
2
6.1 Important Photodetector Parameters
 Responsivity - ratio of current output to light input




varies with wavelength
theoretical maximum resposivity: 1.05A/W at 1300nm
typical responsivity: 0.8 - 0.9 A/W at 1300nm
formula for theoretical maximum responsivity (quantum efficiency = 100%)
n
R
1240
where:
R = theoretical maximum responitivity in Amps/Watt
 = quantum efficiency
 = wavelength in nanometers
R = ηeλ
hc
e=1.6e-19, h=6.63e-34, c=3e8
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6.1 Important Photodetector Parameters
 Quantum Efficiency ratio of primary
electron-hole pairs
created by incident
photons to the
photons incident
on the diode material
Figure 6.1 Typical Spectral Response of
Various Detector Materials
(Illustration courtesy of Force, Inc.)
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6.1 Important Photodetector Parameters
 Capacitance of a detector
 dependent upon the active area
of the device and the reverse
voltage across the device.
 A smaller active diameter
makes it harder to align the
fiber to the detector.
 Also, only the center should
be illuminated
 photodiode response is slow
at the edges
 edge jitter
Figure 6.2 Capacitance versus Reverse
Voltage
(Illustration courtesy of Force, Inc.)
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6.2 Response Time
 Time needed for the photodiode to
respond to optical input and
produce an external current
 Dependent on
Vout
 photodiode capacitance
 load resistance
 design of photodiode
90%
10%
Time
Rise
Time
 Measured between 10% and 90%
of amplitude
Fall
Time
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6.2 Response Time
 Approximate -3 dB frequency formula:
f 3dB
where:
R = Impedance that the detector operates into
C = Capacitance of the detector
 Rise or fall time formula:
1

2RC
  2.2RC
 Formula for  and f-3dB
0.35

f 3dB
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6.2 Response Time
 Dark Current
 Current that flows in the absence of light because of the intrinsic resistance of the detector and
the applied reverse voltage
 Very temperature sensitive - may double every 5°C to 10°C
 Contributes to detector noise
 Edge Effect
 Only detector center provides fast response
 Outer regions exhibit edge effect
 Detector edge has higher responsivity – can cause problems in alignment, important to use square
wave (> 1 MHz) instead of continuous source
 Detector edge has slower response
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6.2 Response Time
 Linearity and Backreflection
 All PIN diodes are inherently linear
 Some applications, such as CATV links,
require that distortion be reduced to very
low levels
 Coupling the fiber at an angle to the detector
will produce low backreflections
 Noise - any electrical or optical energy other
than the signal itself
 Noise appears in all elements of a
communication system; however, it is usuallly
most critical to the receiver
 Shot noise - occurs because the process of
creating a current is a discrete process
 Thermal noise - arises from fluctuations in the
load resistance
 Signal quality can be expressed as a signal-tonoise ratio (SNR)
Figure 6.4 Low Backreflection Detector
Alignment
(Illustration courtesy of Force, Inc.)
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6.3 PIN Photodiode
 A regular PN diode has a limited
depletion area which makes it
inefficient for converting light to
current
 A PIN photodiode has a large
depletion area and current is more
easily generated
 A lightly-doped intrinsic layer
separates the more heavily doped ptype and n-type regions
 PIN - Positive, Intrinsic, Negative
Figure 6.3a Pigtailed and Connectorized
PIN Photodiodes
(Illustration courtesy of Hewlett-Packard.)
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OPTICAL DETECTORS
Physical Principles of Photodiodes
The simplest photodiode is a PN junction operated under reversed-bias.
Diode current,
I  I 0 exp qV kT   1  I op
Dark current
photocurrent
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6.3 PIN Photodiode
 With no light, PIN Photodiodes
behave electrically like an
ordinary rectifier diode. If
forward biased, they conduct
large amounts of current.
 Two Operating Modes
 Photovoltaic - no bias is
applied, logarithmic output.
(not used in real-world
applications)
 Photoconductive - a reverse
bias is applied. The output
current is very linear with the
light input power.
Figure 6.5 Cross-Section and Operation of
a PIN Photodiode
(Illustration courtesy of Force, Inc.)
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6.4 IDP Detectors
 IDP - Integrated
Detector/Preamplifier
 Noise can occur between the diode
and the first receiver stage
 A transimpedance amplifier
(current to voltage) is combined
with the detector in an integrated
circuit to reduce noise.
 Responsivity is measured in
Volts/Watt
Figure 6.5a. Fiber Optic "Light to Logic"
TM Receiver
(Illustration courtesy of Hewlett-Packard.)
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6.5 Avalanche Photodiode
 In APD’s , free electrons and holes
created by absorbed photons accelerate
and collide with neutral atoms and create
more free electron-hole pairs.
 Collision Ionization, Photomultiplication
 Typical multiplication ranges in the tens
and hundreds
 Disadvantages
 High-voltage power supplies (20 - 300
volts)
 Temperature sensitive
 Less reliable
 PIN detectors are usually used at lower
data rates because they can almost match
the performance of an APD.
Figure 6.5b Avalanche Photodiode Module
with Preamplifier IC
(Illustration courtesy of Fujitsu)
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Problems
 Calculate the theoretical maximum responsivity of a detector at 1550nm.
 Calculate the theoretical maximum responsivity of a detector at 820nm.
 Calculate the -3dB frequency and rise time of a detector with a capacitance of 0.5pF
operating into an impedance of 50W.
Answers: 1.25 Amps/Watt, 0.661
Amps/Watt, 6.4 GHz
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OPTICAL DETECTORS
P-i-N Photodiode
• the thickness of the depletion region is controlled by i-layer, not by reverse voltage
• most of the incident photons absorbed in the thick i-layer - high 
• large electric field across the i-layer - efficient separation of generated electrons & holes
• the P and N layers are extremely thin compare to i-layer - diffusion current is very small
• The increase in the iwidth reduces the
speed of a photodiode.
• The speed of response
of the photodiode is
limited by
- the time it takes to
collect the carriers
(drift time)
- the capacitance of the
depletion layer (RC
time constant of the
detector circuit).
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OPTICAL DETECTORS
Avalanche Photodiodes (APDs) - photodiodes with internal gain
• Internally multplied the primary photocurrent before it enters the input circuitry of
the following amplifier.
• In the high field region of an APD, photogenerated electrons and holes can acquire
sufficient energy to create new electron-hole pairs through impact ionization process.
These secondary carriers gain enough energy to ionize other carriers, causing the
avalanche process of creating new carriers.
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OPTICAL DETECTORS
Receiver sensivitiy comparison of P-I-N photodiode and APD devices at BER of 10-9.
InGaAs
 = 1.55mm
Si
 = 0.82mm
Drawbacks of APD
• fabrication difficulties due to their more complex structure and hence increased cost.
• the random nature of the gain mechanism which gives an additional noise contribution.
• the often high bias voltage required which are wavelength dependent.
• the variation of gain with temperature.
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Photodiode construction
 Silicon photodiodes are constructed from single crystal silicon wafers similar to those
used in the manufacture of integrated circuits
 The major difference is that photodiodes require higher purity silicon
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Photodiode Responsivity
 Is a measure of the current produced per unit power received
Responsivity( A/W) = Current(A) / Power (W)
 At specified bias voltage responsivity depends on the operating wavelength
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Quantum Efficiency (Q.E)
 A photodiode's capability to convert light energy to electrical energy, expressed as a
percentage, is its Quantum Efficiency
 The QE is related to the photodiode's responsivity over operating wavelength
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Responsivity vs Wavelength at 100% Q.E.
Operating wavelength (nm)
Responsivity (A/W)
400
0.323
500
0.403
600
0.484
700
0.565
800
0.645
900
0.726
1000
0.806
1100
0.887
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Temperature Effects
 Increasing the operating temperature of a photodiode device results in two distinct
changes in operating characteristics
 a shift in the Quantum Efficiency (Q.E.) due to changes in the radiation absorption of the device
 exponential increases in the thermally excited electron-hole pairs resulting in increasing dark
current
 Q.E. values shift lower in the UV region and higher in the IR region
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QE/oC vs Operating Wavelength
24
Dark Current, Id vs Temperature
25
Noise Equivalent Power (NEP)
 The minimum incident power required on a photodiode to generate a photocurrent equal
to the total photodiode noise current
 dependent on the bandwidth of the measuring system
 Since the photodiode light power to current conversion depends on the radiation
wavelength, the NEP power is quoted at a particular wavelength
 non-linear over the wavelength range
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Photodiode Noise
 noise generated by a silicon photodiode, operating under reverse bias, is a combination
of shot noise, due to dark leakage current, and Johnson noise due to the shunt resistance
of the device and the ambient temperature
 Shot noise is the dominant component of the noise current of a reverse-biased
photodiode
 If devices are operated in a photovoltaic mode with zero bias, the Johnson noise
dominates, as dark current approaches zero
 operating in the zero bias mode the noise current is reduced such that the NEP, and
hence the minimum detectable signal, is reduced in spite of some loss of absolute
sensitivity
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Shot Noise
 Proportional to the total dark current and system bandwidth
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Johnson noise, Ij
 Johnson noise contribution is provided by the shunt resistance of the device, series
resistance and the load resistance
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The equivalent circuit of a photodiode
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PIN Photodiode Specification
31
APD Photodiode Specification
32
DIRECT DETECTION RECEIVERS
An optical receiver consists of a photodetector, an amplifier, and signal processing
circuitry.
It first converting the optical energy emerging from the end of a fiber into an electric signal,
and then amplifying this signal to a large enough level so that it can be processed by signal
processing circuits for reducing the noise and improving the output pulse shape.
Noise sources and disturbances in the optical pulse detection mechanism
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DIRECT DETECTION RECEIVERS
Signal-to-Noise Ratio (SNR)
The power signal-to-noise ratio at the output of an optical receiver is defined by
S
Signal power from photocurrent

N Photodetector noise power+amplifier noise power
For both signal power and noise power are released at the same load resistance,
2
Ip
S
 2
N inoise
Ip 
average photocurrent
inoise  root mean square value of the noise
induced current
Noise Equivalent Power (NEP)
NEP is the minimum optical signal power that produces SNR = 1.
This is the optical power necessary to produce a photocurrent of the same magnitude
as total noise current.
NEP determines the weakest optical signal that can be detected in the presence of
noise.
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DIRECT DETECTION RECEIVERS
Quantum Noise
•The detection of light by a photodiode is a discrete process - an electron-hole pair is
generated from the absorption of a photon.
•The photocurrent generated is dictated by the statistics of photon arrivals.
• When the detector is illuminated by an optical signal P0, the
P 
average number of electron-hole pairs generated in a time  is z m  re  0
hf
•The actual number of electron-hole pairs z that are generated fluctuates from the
average according to the Poisson distribution, where the probability that z electrons
are generated in an interval  is
zmz exp zm 
P( z ) 
z!
quantum noise - it is not
possible to predict exactly
how many electron-hole
pairs are generated by a
known optical power
incident on the detector.
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DIRECT DETECTION RECEIVERS
Digital Signaling Quantum Noise
• For an ideal receiver (Idark= 0, =1 and able to detect an individual photon), the
probability of no electron-hole pairs (z = 0) being generated when an optical pulse
of energy E falls on the photodetector in the time interval  is
P0 1  exp zm 
•This error probability represents the bit-error-rate of digital system, [ P(0/1)=10-9,
on the average, one error occurs for every billion pulses sent].
•The minimum optical power (or pulse energy) required to maintain a specific bitz hf
error-rate performance in a digital system is known as the quantum limit. Pmin  m

Analog Transmission Quantum Noise
•In analog optical receiver quantum limit manifests itself as a shot noise which has
Poisson statistics. The shot noise current is on the photocurrent Ip is given by
is2  2qBI p
2
Ip
S Ip
 2 
•Neglecting other sources of noise the SNR at the receiver is
N is 2qB
P0 q
P0
•The minimum incident optical power
 S  2hfB


necessary to achieve a specific S/N is Pmin   N  
hf 2qB 2hfB
 
• In term of the absolute optical power requirements analog transmission compares
unfavorably with digital signaling.
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DIRECT DETECTION RECEIVERS
Quantum / Shot Noise
•The detector average current Ip exhibits a random fluctuation about it mean value as a
result of the statistical nature of the quantum detection process.
•The number of electrons producing photocurrent will vary because of their random
absorption and recombination.
• Deviation of an instantaneous number of electrons from their average value is known
as shot noise and its current mean square value is is2  2qBI p
B = post-detection bandwidth
Dark Current Noise
• A small leakage current flows from the device terminals when there is no optical power
incident on the photodiode.
• This current contribute to the random fluctuations about the average particle flow of the
photocurrent and manifests itself as shot noise.
• The mean square value of dark current noise is id2  2qBI d
Thermal Noise
• Electron motion due to temperature (external thermal energy) occurs in a random way.
• The number of electrons flowing through a given circuit at any instance is a random
variable.
• The mean square value of thermal-noise current in a resistor R,
kB = Boltzmann’s constant
4k BTB
2
it 
T = absolute temperature
R
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DIRECT DETECTION RECEIVERS
Noise in a P-I-N Photodiode
• Three sources of noise:
Quantum noise,
Dark current noise,
Noise due to background radiation
2
• The total shot noise, iTS  2qB  I P  I d  I b 
Ib = background radiation
induced current
• For photodiode without internal gain, thermal noise from the detector load resistor
and from active elements in the amplifier tends to dominate.
Noise in an APD
• Due to avalanche multiplication gain in an APD, the amount of noise is higher than
that in a P-I-N photodiode
• An excess noise in the output photocurrent due to gain fluctuation
Quantum noise
is2  2qBI p M 2 F ( M )
Dark current noise
id2  2qBI d M 2 F ( M )
Background noise
ib2  2qBI b M 2 F ( M )
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DIRECT DETECTION RECEIVERS
Receiver Noise
The equivalent circuit for the front end of an optical fiber receiver, including the
effective input capacitance Ca and resistance Ra.
Noise sources within an amplifier can be represented by a series voltage noise
2
source va and a shunt current noise source ia2 .
The total noise associated with the amplifier is
B


2
iamp
 0 ia2  va2 Y df
2
where Y is the shunt admittance and f is frequency. i 2 may be reduced with
amp
low detector and amplifier capacitance.
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DIRECT DETECTION RECEIVERS
SNR of P-i-N Photodiode Receiver
The SNR at the output of the P-i-N photodiode receiver is
I p2
S

N 2qBI  I   4k BTB  i 2
p
d
amp
RL
2
When the noise associated with the amplifier iamp is referred to the load resistance
RL the noise figure Fn of the amplifier may be obtained. This allows i 2
to be
amp
2
combined with the thermal noise it from the load resistance to give
2
it2  iamp

Then the SNR can be written as
4k BTBFn
RL
I p2
S

N 2qB( I  I )  4k BTBFn
p
d
RL
The thermal noise contribution may be reduced by increasing the value of the load
resistor RL, however this will decrease the post detection bandwidth
B
1
2RL Cd  Ca 
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DIRECT DETECTION RECEIVERS
SNR of APD Receiver
The total shot noise current multiplied through impact ionization is given by
2
iSA
 2qB I P  I d M 2 x where F ( M )  M x , x ~0.3 to 0.5 for Si APDs
x ~ 0.7 to 1.0 for Ge or III-VAPDs
The SNR at the output of the APD receiver is
I 2p M 2
I 2p
S


4
k
TBF
N 2qB I  I M 2  x  B
x 4k BTBF n
n
2
qB
I

I
M

p
d
p
d
RL
RL M 2
For low M the combined thermal
and amplifier noise term dominates
and giving an improved SNR.
For large M the SNR decreases with
increasing M at the rate of Mx.
For the maximum SNR,
x
2qB( I p  I d ) M op
2

2
x
4k BTBFn RL M op

and
2 x
M op


4k BTFn
xqRL I p  I d




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