Download Optical Fibre Communication Systems

Optical Fibre Communication
Systems
Lecture 4 - Detectors & Receivers
Professor Z Ghassemlooy
Northumbria Communications Laboratory
Faculty of Engineering and
Environment
The University of Northumbria
U.K.
http://soe.unn.ac.uk/ocr
Prof. Z Ghassemlooy
1
Contents
 Properties and Characteristics
 Types of Photodiodes
 PIN
 APD
 Receivers
 Noise Sources
 Performance
 SNR
 BER
Prof. Z Ghassemlooy
2
Optical Transmission - Digital
• The design of optical receiver
is much more complicated
than that of
• optical
transmitter
because the receiver
must first detect weak,
distorted signals and
then make decisions on
what type of data was
sent.
• analogue receiver
• But offers much higher
quality
than
analogue
receiver.
Prof. Z Ghassemlooy
3
Optical Receiver – Block Diagram
Optical signal
(photons – hf)
Photodetection
Converting
optical
signal into
an electrical
signal
To recover the
information signal
Amplification
(Pre/post)
Filtering
Limiting the
bandwidth,
thus reducing
the noise
power
Prof. Z Ghassemlooy
Signal Processing
Information signal
4
Photodetection - Definition
It converts the optical energy into an electrical
current that is then processed by electronics to
recover the information.
Detection Techniques
• Thermal Effects
• Wave Interaction Effects
• Photon Effects
Prof. Z Ghassemlooy
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Photodiode - Characteristics
An electronics device, whose vi-characteristics is sensitive to the
intensity of an incident light wave.
I
Forward-biased
“Photovoltic”
operation
Dark current
V
Po
Reverse-biased 
“photoconductive”
operation
Short-circuit
“photoconductive”
operation
Prof. Z Ghassemlooy
• Small dark current due to:
• leakage
• thermal excitation
• Quantum efficiency
(electrons/photons)
• Responsivity
• Insensitive to temperature
variation
6
Photodetector - Types
The most commonly used photodetectors in optical
communications are:
– Positive-Intrinsic-Negative (PIN) 
•
•
•
•
•
No internal gain
Low bias voltage [10-50 V @ = 850 nm, 5-15 V @ = 1300 –1550 nm]
Highly linear
Low dark current
Most widely used
– Avalanche Photo-Detector (APD)
•
•
•
•
•
Internal gain (increased sensitivity)
Best for high speed and highly sensitive receivers
Strong temperature dependence
High bias voltage[250 V @  = 850 nm, 20-30 V @ = 1300 –1550 nm]
Costly
Prof. Z Ghassemlooy
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Photodiode (PIN) - Structure
Photons
Depletion
region
n
electron
I
Io
p
hole
Output
• No carriers in the I region
• No current flow
p
I
hole
Bias voltage
The power level at a distance x into
the material is:
Where  is the photon absorption
coefficient
n
electron
RL (load
resistor)
• Reverse-biased
• Photons generated electron-hole pair
• Photocurrent flow through the diode and in
the external circuitry
Prof. Z Ghassemlooy
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Photodiode (PIN) - Structure
Depletion region width
The capacitance of the
depletion layer Cj (F) is:
Prof. Z Ghassemlooy
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Photodetector - Reponsivity
PIN:
R = Io/Po
Io = Photocurrent;
G = APD gain;
Note: rp = Po/hf
APD:
RAPD = G R
A/W
Po = Incident (detected) optical power
 = Quantum efficiency = average number of
electron-hole pairs emitted re / average
number of incident photons rp
and re = Io/q
 = 99% ~ 1
l is the length of the photoactive region
Io = qPo/hf
Thus normally  is very low, therefore = 0.
So
Prof. Z Ghassemlooy
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Photodetector - Responsivity
 Silicon (Si)
– Least expensive
 Germanium (Ge)
– “Classic” detector
 Indium gallium
arsenide (InGaAs)
– Highest speed
G Keiser , 2000
Prof. Z Ghassemlooy
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Photodetector - Equivalent Circuit
Contact leads
Photodiode
Rs
Io
Cj
Rj
Amplifier
L
RL
Ramp
Camp
Output
L = Large, (i.e o/c)Rs = Small, (i.e s/c)
CT = Cj + Camp
RT = Rj || RL || Ramp
The transfer function is given by:
Prof. Z Ghassemlooy
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Photodetector - Equivalent Circuit
1
The detector behaves approximately like a first
f

B

order RC low-pas filter with a bandwidth of:
2CT RT
Prof. Z Ghassemlooy
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Photodiode Pulse Responses
Fast response time 
High bandwidth
• At low bias levels rise and fall times are different. Since photo
collection time becomes significant contributor to the rise time.
G Keiser , 2000
Prof. Z Ghassemlooy
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Photodiode Pulse Responses
Small area photodiode
Small area photodiode
Large area photodiode
w = depletion layer
s = absorption coefficient
Due to carrier generated in w
Due to diffusion of carrier from the edge of w
G Keiser , 2000
Prof. Z Ghassemlooy
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Photodetetor – Typical Characteristics
Si
Parameters
PIN
Wavelength range
Peak
(nm)
Ge
APD
400-1100
900
830
PIN
APD
800-1800
1550
1300
InGaAS
PIN
APD
900-1700
1300
1300
(1550)
(1550)
Responsivity
(A/W)
0.350.55
50-120
0.5-0.65
2.5-25
0.5-0.7
-
Quantum
Efficiency (%)
65-90
77
50-55
55-75
60-70
60-70
Bias voltage (-V)
45-100
220
6-10
20-35
5
<30
Dark current (nA)
1-10
0.1-1
50-500
10-500
-
1-5
Rise time (ns)
0.5-1
0.1-2
0.1-0.5
0.5-0.8
0.06-0.5
0.1-0.5
Capacitance (pF)
1.2-3
1.3-2
2-5
2-5
0.5-2
0.1-0.5
Source: R. J. Hoss
Prof. Z Ghassemlooy
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Minimum Received Power
• Is a measure of receiver sensitivity defined for a specific:
• Signal-to-noise ratio (SNR),
• Bit error Rate (BER),
• Bandwidth (bit rate),
at the receiver output.
Detector
Pr
Amplifier
Power loss
Po
MRP = Minimum Detected Power (MDP) – Coupling Loss
Prof. Z Ghassemlooy
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MRP Vs. Bandwidth
-20
SNR (dB)
50
MRP (-dBm)
-30
-40
30
-50
10
-60
 =1300
0
-70
1
2
5
10
20
50
100 200 500 1000
Bandwidth (MHz)
Prof. Z Ghassemlooy
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Selection Criteria and Task
Optical
 Optical Sensitivity for a given
BER and SNR
 Operating wavelength
 Dynamic range
 Simplicity
 Reliability and stability
Electrical
 Data rate
 Bit error rate (digital)
 Maximum Bandwidth
(analogue)
 Signal-to-noise ratio
(analogue)
Task:
•To extract the optical signal (low level) from various
noise disturbances
•To reconstruct the original information correctly
Prof. Z Ghassemlooy
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Receivers: Basics
The most important and complex section of an optical
fibre system
It sensitivity is design dependent, particularly the first
stage or front-end
Main source of major noise sources:
– Shot noise current
– Thermal noise: Due to biasing/amplifier input impedance
– Amplifier noise:
• Current
• Voltage
– Transimpedance noise
Prof. Z Ghassemlooy
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Receiver - Bandwidth
A range of frequencies that can be defined in terms of:
• Spectral profile of a signal
• Response of filter networks
• Equivalent bandwidth: Defines the amount of noise in a
system
Types of Bandwidth
• Ideal
• Baseband
• Passband
• Intermediate-Channel
• Transmission
• Noise
Prof. Z Ghassemlooy
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Ideal, Low-pass and Band-pass Filters
Band-pass filter
Low-pass filter
0 dB
-3
Higher order
filter
Ideal
Frequency
Bbp
Prof. Z Ghassemlooy
Blp
22
Noise Equivalent Bandwidth (NEB) B
0
NEB
-3 dB
Area under the response
cure
=
Area under the noise curve.
B3dB
B
Defines as the ideal
bandwidth
describing the point where:
Filter response
Prof. Z Ghassemlooy
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Optical System
P(t)
m(t)
Optical drive
circuit
Light
source
Fibre
Photodiode
ip(t)
Amplifier
P(t )  Pt (1  Mm(t ))
Photocurrent i p (t ) 
R  P (t )  R  Pt (1  Mm(t ))
Signal current
Average photocurrent
Photocurrent =
+
io(t)
(DC current) Io
Prof. Z Ghassemlooy
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Optical Receiver - Model
The received digital pulse stream incident at the photodetector is given by:
P(t ) 

b h
n  
n
p
(t  nTb )
where Tb is bit period, bn is an amplitude parameter of the nth message digit
and h p (t )is the received pulse shape which is positive for all t.
Prof. Z Ghassemlooy
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Optical Receiver - contd.
For m(t) = sin t
The mean square signal current is
is  io (t )
2
2
is  io (t )G
2
2
for PIN
2
for APD
For a digital signal
The mean square signal current is
is  io (t )  RP (t )
2
2
for PIN
is  io (t )G  RG P(t )
2
2
2
2
Prof. Z Ghassemlooy
for APD
26
Optical System - Noise
 Is a random process, which can’t be described as an
explicit function of time
 In the time domain – Can be characterized in
probabilistic terms as:
Mean - correspond to the signal that we are interested to recover
Variance (standard deviation) - represents the noise power at the
detector’s output
Can also be characterized in terms of the Root Mean Square (RMS)
value
Time average
Prof. Z Ghassemlooy
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Optical System - Noise
• The electric current in a photodetector circuit is composed of a
superposition of the electrical pulses associated with each
photoelectron
• The variation of this current is called shot noise
Prof. Z Ghassemlooy
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Optical System - Noise Sources
 At the receiver:
Additive
Signal dependent
 Modal noise
Due to interaction of (constructive & destructive)
multiple coherent modes, resulting in intensity
modulation.




Photodetector Noise 
Preamplifier (receiver) Noise 
Distortion due to Non-linearity
Crosstalk and Reflection in the Couplers
Prof. Z Ghassemlooy
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Noise - Source Noise - contd.
LED: Due to:
– In-coherent intensity fluctuation
– Beat frequencies between modes
LD: Due to:
–
–
–
–
Non-linearities
Quantum noise: In the photon generation
Mode hopping: Within the cavity
Reflection from the fibre back into the cavity, which reduces
coherence
– Difficult to measure, to isolate and to quantify
– Most problematic with multimode LD and multimode fibre
Prof. Z Ghassemlooy
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Noise Currents
Let noise current be defined as:
inoise(t) = i(t) - IDC
IDC = Photocurrent Io
(Amps)
Noise current from random current pulses is termed as shot-noise.
Shot-noise:
• Quantum
• Dark current
Prof. Z Ghassemlooy
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Quantum Shot Noise
The photons arrive randomly in a packet form, with no two
packets containing the same amount of photons.
Random generation of electron-hole pair, thus current.
Variation of the total current generated, about an average value.
This variation is best known as QUANTUM SHOT NOISE.
Prof. Z Ghassemlooy
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Quantum Shot Noise
 The average number of electronholes pairs per bits is:
Where  the time period.
The probability of detecting n photons in a
time period  is follows the Poisson
Distribution:
Incoherent light
Y Semenova, DIT, Ireland
Coherent light
Prof. Z Ghassemlooy
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Quantum Shot Noise
The rate of electron-hole pairs generated by incident photons is:
With an ideal receiver with no noise we have:
Note that, the minimum pulse energy of the quantum limit is:
Prof. Z Ghassemlooy
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Shot Noise - PIN
• The mean square quantum shot noise current on Io
ish  2qI o B
2
(A 2 )
• The mean square dark current noise (also classified as shot noise)
ids  2qI d B
2
(A 2 )
Where Id = surface leakage current, and B is the electrical bandwidth of the system
Q is the electron charge.
Total shot noise current ITs = Dark current + Photocurrent
The total mean square
shot noise
iTs  2q( I o  I d ) B
2
Prof. Z Ghassemlooy
(A 2 )
35
Noise Power Spectrum
Power spectrum
I2o
ITs2
Shot noise
0
B
Modulation
bandwidth
Prof. Z Ghassemlooy
Frequency
36
Shot Noise - APD
• The mean square photocurrent noise
iTs  2q[( I o  I d )G F ]B
2
2
where
F = The noise figure = Gx for 0<x<1
G = The optical gain
2
(A )
Bias voltage
hf
Av
RL
Prof. Z Ghassemlooy
Vo
Vi
37
Noise Currents - contd.
Thermal Noise
ith 
2
4 KTB
RL
RL = Total load seen at the input of the preamplifier
K = Boltzmann’s constant = 1.38x10-23 J/K
T = Temperature in degree Kelvin = Co + 273
Total Noise
PIN
iT  ish  ids  ith
APD
iT  ish  ids  ith
2
2
2
2
Prof. Z Ghassemlooy
2
2
2
2
38
Electrical Amplifier Noise
Amplifier type
- Voltage Noise
- Current Noise
BJT
va  2
JEFT
qI c
2
gm
2
va  2
2
B
Total amplifier noise
i
A
Prof. Z Ghassemlooy
B
ia  2qI g B
ia  2qIb B
1

B
gm
2
2
2
2
qI d
B
2
2
[
i

(
v
 a a / Z )] df
0
39
Receiver Signal-to-Noise Ratio (SNR)
hf
io
• PIN
SNR 
io
SNR 
2
iT
iT
Io
2
4 KTB 2
2qB( I o  I d ) 
i A
RL
2
• APD
iA
G2Io
SNR 
2qB[( I o  I d )G
Note: SNR cannot be improved be amplification
Prof. Z Ghassemlooy
2 x
2
4 KTB
]
F  i2 A
RL
40
SNR - Quantum Limit
The mean square quantum shot noise current on Io
ish  2qI o B
2
( Io ) Io
SNR)Q 
2qIoB
(A 2 )
RP oq / hf

2qB
RP o / hf

2B
nelectron
re nelectron / s
 

 N
B
bit / s
bit
Shot noise
Poisson
Prof. Z Ghassemlooy
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Type of Receivers - Low Impedance
Voltage Amplifier
+Bias
-
Simple
Low sensitivity
Limited dynamic range
It is prone to overload and saturation
Is
Output
RL
50 
Av
hf
CT
RL
Amplifier
Vo
Vi
1
• RC limited bandwidth B 
2CT RL
RL = Rdetector || Ramp.
Prof. Z Ghassemlooy
Ramp= High
42
Type of Receivers - High Impedance
Voltage Amplifier with Equaliser
+Bias
Is
• High sensitivity
• Low dynamic range
Output
RL
Ct
Amplifier
Equalizer
Equaliser
Av
hf
Vo
Vi
CT
RL
• Rdetector is large to reduce the effect of thermal noise
• Detector out put is integrated over a long time constant, and is
restored by differentiation
Prof. Z Ghassemlooy
43
Type of Receivers - Transimpedance
Feedback Amplifier
+Bias
• The most widely used 
Rf
Is
• Wide bandwidth
•High dynamic range
• No equalisation
• Greater dynamic range (same gain at all frequencies)
• Slightly higher noise figure than HIVA
Output
Ct
Amplifier
RF
Bandwidth
Av
hf
B
CT
RL
Vi
Prof. Z Ghassemlooy
Vo
Av
2CT RF
44
Transimpedance Feedback Amplifier
R
V*
F
F
V*
A
-A
I*
sh
RL
CT
SNR 
I*
Th
I*
A
Vi
Vi
G 2 I o2


V 
1
   R {1  3 (2BR C ) }B

 2qI G 2 F (G )  4kT  I *
o
A

RT

2
* 2
A
T
T
2

*
Where I . is the noise power spectral density, and RT = RL||RF
Prof. Z Ghassemlooy
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Optical Receiver - Analogue
Employ an analogue preamplifier stage, followed by
either an analogue output stage (depending on the type
of receiver).
Comms. Special. Inc.
Prof. Z Ghassemlooy
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Optical Receiver - Digital
 1st stage is a current-to-voltage converter.
 2nd stage is a voltage comparator, which produces a clean,
fast rise-time digital output signal. The trigger level may be
adjusted to produce a symmetrical digital signal.
Prof. Z Ghassemlooy
47
Optical Transmission - ISI
 Optical pulse spread
after traversing along
optical fiber
 Thus leading to ISI,
where some fraction of
energy remaining in
appropriate time slot,
whereas the rest of
energy is spread into
adjacent time slots.
Prof. Z Ghassemlooy
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Receiver Performance
Signal-to-Noise Ratio (SNR)
Bit Error Rate (BER)
Prof. Z Ghassemlooy
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SNR
 In analogue transmissions the
performance of the system is mainly
determined by SNR at the output of
the receiver.
 In case of amplitude modulation the
transmitted optical power P(t) is in
the form of:
P(t )  Pt [1  Mm(t )]
where M is modulation index, and m(t)
is the analogue signal.
 The photocurrent at receiver can be
expressed as:
is (t )  RGPr [1  Mm(t )]
Prof. Z Ghassemlooy
50
SNR
 The S/N can be written as
is2
S
(1 / 2)( RGPr ) 2
 2 
N
2q( RPr  I d )G 2 F (G ) B  (4k BTB / RT ) F
iN
(1 / 2)(GI P ) 2

2q ( I P  I d )G 2 F (G ) B  (4k BTB / RT ) F
Note, F is the amplifier noise figure.
For PIN: G = 1
So we have
S
(1 / 2)( I ) 2
2
2 2
(
1
/
2
)
G
R Pr
P


N (4k BTB / RT ) F (4k BTB / RT ) F
And for large signal level
Low input signal level
S RPr

N 4qB
Prof. Z Ghassemlooy
51
SNR Vs Receiver Sensitivity
Note: Io =RPo
G Keiser , 2000
Po(dBm)
Prof. Z Ghassemlooy
52
Bit Error Rate (BER)
Probability of Error = probability that the
output voltage is less than the threshold when a
1 is sent + probability that the output voltage is
more than the threshold when a 0 has been sent
bo
n
Variance
2on
1
vth
v
P1 (v) 
 p( y | 1)dy
probablity that the equalizer output vol tage is less than v, if 1 transmitt ed


P0 (v)   p ( y | 0)dy probablity that the equalizer output vol tage exceeds v, if 0 transmitt ed
v
Variance
2off
Pe  q1 P1 (vth )  q 0 P0 (vth )
 q1
0
boff

vth
 p( y | 1)dy  q  p( y | 1)dy
0

vth
where q1 and q0 are the probabilities that the transmitter sends 0 and 1 respectively.
Note, q0 = 1- q1.
Prof. Z Ghassemlooy
53
Bit Error Rate (BER)
BER = No. of error over a given time interval/Total no. of bits transmitted
P1 (vth ) 
vth
 p( y | 1)dy 


P0 (vth ) 
 p( y | 0)dy 
vth
1
2  on
1
2  off
 (v  bon ) 2 
  exp 
dv
2
2

on



vth

 (v  boff ) 2 
  exp 
dv
2
2

off


vth
If we assume that the probabilities of 0 and 1 pulses are equally likely
1
 Q 
BER  Pe  1  erf 

2
 2 
where
Prof. Z Ghassemlooy
 vth  boff
Q
 off
  bon  vth 


  on 
54
Bit Error Rate (BER) - contd.
For
• off = on =  RMS noise
• bon = V, and boff = 0
• Thus vth = V/2 and Q = V/2
1
 V 
Pe  1  erf 

2
 2 2 
In terms of power signal-to-noise ratio (S/N)
Therefore:


1
S

Pe  1  erf  0.345

2 
N


Prof. Z Ghassemlooy
55
BER Performance
 Minimum input power
depends on acceptable bit
error rate
 Many receivers designed
for 1E-12 or better BER
G Keiser , 2000
Prof. Z Ghassemlooy
56
Basic Receiver Design
Bias
Clock
Recovery
AGC
-g
Temperature
Control
Decision
Circuit
Monitors
& Alarms
 Optimized for one particular
– Sensitivity range
– Wavelength
0110
Remote
Control
 Can include circuits
for telemetry
Agilent Tech.
– Bit rate
Prof. Z Ghassemlooy
57
Optical Receivers - Commercial
Devices
28 GHz Monolithic InGaAs PIN Photodetector
100 kHz- 40 Gb/s
DC - 65 Gb/s InGaAs PIN Photodiodes
100 GHz Dual-Depletion InGaAs/InP Photodiode
Prof. Z Ghassemlooy
58
Wide-Band Optical Receiver (40 Gb/s)
• Operating current 75 mA
• Bandwidth: 100 KHz to 35 GHz
• Power dissipation: 400 mW
• Responsivity: 0.6 A/W
• Wavelength response: 800 - 1600 nm • Power gain: 8 dB
Linearity response
Sensitivity response
Typical eye diagram
Prof. Z Ghassemlooy
59
Wide-Band Optical Receiver (DC - 65
Gb/s)
 InGaAs PIN Photodiodes
 Reverse bias voltage: +3V
 Responsivity: 0.5 A/W at 1300 and 1550 nm wavelength.
 Opto-electronic Integrated Circuits (OEICs) which combine
optical, microwave, and digital functions on the same chip
 Application:
–
–
–
–
–
Ethernet fiber local area networks
Synchronized Optical Network SONET,
ISDN,
Telephony
Digital CATV).
Prof. Z Ghassemlooy
60
Regenerator (3R)
Receiver followed by a transmitter
– No add or drop of traffic
– Designed for one bit rate & wavelength
Signal regeneration
– Reshaping & timing of data stream
– Inserted every 30 to 80 km before optical amplifiers became
commercially available
– Today: reshaping necessary after about 600 km (at 2.5 Gb/s), often
done by SONET/SDH add/drop multiplexers or digital crossconnects
Fibre
Fibre
Prof. Z Ghassemlooy
61
Summary
 Photodiode characteristics
 Types of photodiode: PIN and APD
 Photodiode responsivity & equivalent circuit
 Minimum received power
 Optical receiver:
– Types
– Bandwidth
 Noise
 Signal-to-noise ratio
 Bit error rate
 Receiver design
 Regenerator
Prof. Z Ghassemlooy
62
Next Lecturer
Optical Devices
Prof. Z Ghassemlooy
63