Download File - NETWORKING SYSTEM

Information & Communication Technology
Module
ICT–BS–2.3 Optical Fiber Communications
Unit
ICT–BS–2.3/2 Optical Signals: Attenuation and Amplification
ICT–BS–2.3/2
Optical Signals:
Attenuation and Amplification
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
1
Optical Fiber Communications
L
P
∑
Optical Signals: Attenuation and Amplification
12
0
12
ICT-BS-2.3/2/1
Optical Sources
1
ICT-BS-2.3/2/2
Structures and Characteristics of Light-Emitting Diodes LED
1
ICT-BS-2.3/2/3
Semiconductor Laser Structures
1
ICT-BS-2.3/2/4
Power Launching and Coupling
1
ICT-BS-2.3/2/5
Optical Detectors
2
ICT-BS-2.3/2/6
Signal Encoding/Decoding
2
ICT-BS-2.3/2/7
Modulation and Demodulation Formats
2
ICT-BS-2.3/2/8
Receiver Sensitivities
1
ICT-BS-2.3/2/9
Optical Amplifiers
1
Modules
Code
ICT-BS-2.3/2
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
2
ICT–BS–2.3/2 Optical Signals: Attenuation and Amplification
Learning Content:
• Optical sources
- Light emitting diode (LED)
- Laser diode (LD)
• Optical power coupling
•
Optical detection
• Optical modulation and demodulation
• Optical signal amplification
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
3
ICT–BS–2.3/2 Optical Signals: Attenuation and Amplification
Recommended Books:
•
Fiber Optic Communications, James N. Dowing,
Published by Thomson Delmar Learning.
Copyright 2005, Pages: 378
•
Optical Fiber Communications: Principles and Practice, 3rd Edition
John M. Senior and M. Yousif Jamro, Published by Prentice Hall.
Copyright 2009, Pages: 1075
•
Optical Fiber Communications, 4th Edition, Gerd Keiser
Published by Tata McGraw-Hill.
Copyright 2008, Pages: 580
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
4
Review – Optical Fiber Communication System
Electrical Signal
Input
Optical
Source
Optical
Detector
Modulator
Demodulator
Output Signal
Transmission path
(Optical Fiber)
Transmitter
22/05/2017
Receiver
TTC Riyadh, ICT–BS-2.3/2
5
Course contents
•Introduction to the principles of optical telecommunications: Conversion of electrical
signals into optical signals
•Introduction to the most important optical telecommunication components
•Examining the advantages and disadvantages of optical transmission links
•Recording an infrared transmitter diode's characteristic and frequency response
•Controlling a transmitter diode
•Measuring a transmitter diode's frequency response
•Measurement-based examination of various modulation techniques for analog and TTL
signals
•Investigating transmission paths for infrared light of various wavelengths
•Configuring an optical waveguide
•Measuring a receiver diode's frequency response
•Examining a receiver diode's influence on signal recovery
•Determining an optical transmission link's bandwidth
•Examining the influence of an optical transmission link's input capacity on bandwidth
•Measurement-based examination of attenuation along an optical transmission link
•Measurement-based examination of the influence of longitudinal and transverse offset at
splice points
•Comparing the properties of step-index and graded-index fibres
•Examining the influence of wavelength on attenuation
optical transmitter
optical receiver
Light Sources
•
Optical sources are used to convert electrical signals into optic beams
thus enables information carrying facility though the fiber core.
•
Generally, the information is put into the beam by modulating the source
input current.
•
Two basic types which rely on semiconductor principles of operation are
– Light emitting diodes (LEDs)
– Laser diodes (LDs)
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
10
Light Sources Considerations
•
The light source must be matched with the fiber in terms of
– Size
– Modal characteristics
– Numerical aperture
– Line width
– Fiber-window wavelength range
– Transmitted power
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
11
Conduction of Electrons
Conduction band
Current
flow
Movement of electrons
Valance band
•
When a small voltage is placed across the conductor, electrons in the outermost
shell move from the valance band to conductor band.
•
This results positively charged “holes’ in the valance band.
•
Then, the holes are appeared to be moved to the negative source terminal and
electrons are to the positive terminal.
•
Therefore, it said the a current flows through the circuit in the opposite direction of
electrons flow.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
12
Conduction of Electrons (Contd.)
•
Good conductors have few electrons on the valance band.
•
On the otherhand, insulators (poor conductors) have a full valence band thus it
requires more energy to make current flowing (actually they are not).
•
In addition, there are semiconductor materials, which requires more energy to allow
current flowing than in a conductor.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
13
The pn Junction Diode
•
A semiconductor source consists of a pn junction diode.
•
To create a pn junction diode, p-material and n-material are fabricated next to each
other. (e.g.; silicon an gallium arsenide)
•
To alter the localized charges at the material boundary, a small amount of impurities
is added. This process is called as doping.
•
However, the total net charge is equal to zero.
p-type
n-type
Electrons
Holes
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
14
The pn Junction Diode (Contd.)
•
Even without applying any voltage, a barrier is formed at the boundary. This is
called ad the depletion region.
p-type
n-type
Potential barrier/
depletion region
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
15
Reverse Biased - pn Junction
•
When an external voltage is applied with the positive voltage to the n-side and
negative voltage to the p-side, the barrier becomes larger.
•
Therefore, a very small current is flown through the circuit.
•
This is happened due to the surplus electrons are moved for p-to-n.
•
This is called as reverse current and the circuit is called as in reverse biased.
p-type
n-type
Increased depletion region
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
16
Forward Biased - pn Junction
•
However, once the external voltage is applied such that positive voltage for p-side
and negative for n-side, then the depletion region becomes shrink.
•
Now, it is possible to move more electrons, thus a larger current is produced.
•
This is the forward biased current.
p-type
n-type
Reduced depletion
region
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
17
Optical Fiber Communications
L
P
∑
Optical Signals: Attenuation and Amplification
12
0
12
ICT-BS-2.3/2/1
Optical Sources
1
ICT-BS-2.3/2/2
Structures and Characteristics of Light-Emitting Diodes LED
1
ICT-BS-2.3/2/3
Semiconductor Laser Structures
1
ICT-BS-2.3/2/4
Power Launching and Coupling
1
ICT-BS-2.3/2/5
Optical Detectors
2
ICT-BS-2.3/2/6
Signal Encoding/Decoding
2
ICT-BS-2.3/2/7
Modulation and Demodulation Formats
2
ICT-BS-2.3/2/8
Receiver Sensitivities
1
ICT-BS-2.3/2/9
Optical Amplifiers
1
Modules
Code
ICT-BS-2.3/2
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
18
Light Emitting Diode (LED)
•
A light emitting diode (LED) is a p-n junction semi-conductor that emits light when it
is in forward biased.
I
R


V
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
19
LED (Contd.)
•
Eventhough LED has a less attraction with optical systems, it can be still used
because of
– Simple fabrication
– Cost
– Reliability (no catastrophic degradation, immune to modal noise)
– Less temperature dependency
– Simpler drive circuitry (lower drive currents)
– Linearity (linear light output versus current)
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
20
LED Operation
•
When a conduction band electron falls back to the valence band, this electron gets
recombined with a hole, thus creates a photon (electron + hole)
•
As a result this photon creation, light gets emitted.
•
This is a spontaneous process according to the Planck’s law.
( )
Conduction band
1
Valance band
22/05/2017
3
2
Band gap
energy
()
TTC Riyadh, ICT–BS-2.3/2
21
LED Operation (Contd.)
•
The light is emitted in all directions and does not depends on other (incoherent).
•
Band gap energy = Energy difference between excited state (conduction band) and
ground state (valance band).
•
The energy of the photon emission should be at least slightly larger than the band
gap energy.
•
The spread in the energy of light emissions is defined as line width of the LED.
  3  1
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
22
LED Operation (Contd.)
•
All the photon creations do not emit radiation. Some are non-radiative, thus be the
causes of vibrational effects and heat dissipations.
•
Therefore, the internal quantum efficiency of the LED can be defined as (which is
photon producing process or the lifetime)  
int
•
Enon-rad
.
Erad  Enon-rad
Then, the internal optical power produced due to the recombination process is
 hc 
Pint  int 
I

e


h – Planck’s constant
I – current
c – velocity of the light in the vacuum (3  108 ms1 )
e – charge of an electron (1.602  1019 C)
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
23
Types of Band gap Transitions
•
There is no changes in the momentum (direction) in direct band gap transition.
•
However, some energy must be used for momentum changes in indirect band gap
transition.
Therefore, direct transition acquires more efficiency than the indirect transition.
------
Conduction
band
------
Conduction
band
Energy
Momentum
Valance
band
++++
+++
(Direct transition)
22/05/2017
++++
+++
•
Valance
band
(Indirect transition)
TTC Riyadh, ICT–BS-2.3/2
24
Composition of the Semi-conductor
•
Eventhough many semi-conductor materials can be induced to emit light, an
appropriate composition can enhance the efficiency of the system by minimizing the
waste of energy.
•
The primary target is to reduce the band gap energy.
•
Normally, two elements are compounded from
Group III materials (Aluminum, Gallium, Indium) and
Group V materials (Phosphorous, Arsenic)
in the periodic table.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
25
Composition of the Semi-conductor (Contd.)
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
26
Composition of the Semi-conductor (Contd.)
•
Different material compositions have different bandgap energies.
22/05/2017
Material
Band gap Energy
Si
1.11
Ge
0.66
GaAs
1.43
Al As
2.16
GaP
2.21
InAs
0.36
InP
1.35
In.53Ga.47As
0.74
AlxGa1-xAs
1.424+1.247x
AIxIn1-xP
1.351+2.23x
TTC Riyadh, ICT–BS-2.3/2
27
LED Physical Structure
•
Basically a fabricated LED structure can be
– either a homojunction structure
(when p- and n-side have same base material).
– or a heterojunction structure
(when p- and n-side have different base materials so that it is formed a
waveguide at the junction)
(Heterojunction
structure)
(Homojunction
structure)
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
28
Surface Emitting Diode
•
When refractive indices of both p- and n-type materials are same, light is free to
come out from all sides of the semi-conductor device because there is no
confinement.
•
However, only the active region near (but not on) the surface will emit a significant
amount of light while reabsorbing from the other parts. Therefore, this is called as
surface emitting LED.
1200
1200

22/05/2017
TTC Riyadh, ICT–BS-2.3/2
29
Surface Emitting Diode (Contd.)
•
However, a large amount of power generated by the LED get wasted.
•
To increase the output power, only allowing the light be exit from the surface can be
done while confining from others.
•
The output beam makes a Lambertian shape.
I ( )  I0 cos
(W/steradian)
I ( )  number of photons coming from the device at an angle of  per second.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
30
Edge Emitting LED
•
When the refractive indices differ from each other, it can be confined the light to exit
only from one edge of the device (i.e. plane parallel to the junction). This is called as
edge emitting LED.
•
When the light is come out from one edge and the plane is perpendicular to the
junction, the elliptical beam nature gives some problems in fiber launching
applications.
1200
300
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
31
Overview - LED
•
Not expensive.
•
Operates at low power (1.5 V to 2.5 V and 50 mA to 300 mA)
•
Can be coupled to approximately 10 to 100 µW of optical power to a fiber.
•
Drive circuitry is not very complex.
•
LEDs are capable of cover the entire fiber window from 850 to 1550 nm with a line
width 15 to 60nm.
•
Do not require any temperature or current control.
Applications
-
Used in low cost applications with data rates of 100 Mbps
-
Used in LANs coupled to multimode fiber
-
Local area WDM (wavelength division multiplexing) networks
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
32
optical receiver
optical receiver
Optical Fiber Communications
L
P
∑
Optical Signals: Attenuation and Amplification
12
0
12
ICT-BS-2.3/2/1
Optical Sources
1
ICT-BS-2.3/2/2
Structures and Characteristics of Light-Emitting Diodes LED
1
ICT-BS-2.3/2/3
Semiconductor Laser Structures
1
ICT-BS-2.3/2/4
Power Launching and Coupling
1
ICT-BS-2.3/2/5
Optical Detectors
2
ICT-BS-2.3/2/6
Signal Encoding/Decoding
2
ICT-BS-2.3/2/7
Modulation and Demodulation Formats
2
ICT-BS-2.3/2/8
Receiver Sensitivities
1
ICT-BS-2.3/2/9
Optical Amplifiers
1
Modules
Code
ICT-BS-2.3/2
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
35
Laser Principles
•
The spectral width (line width) of the laser is much narrower than the LED.
LED
Laser
•
All lasers must have the following characteristics.
– Pumping threshold
– Output spectrum
– Radiation pattern
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
36
Laser Principles (Contd.)
Pumping threshold
– The input power to a laser must be above than a threshold level to make it acts
as an emitter whereas an LED radiates even at low levels of input current.
– The device behaves like an LED, before it is reached to the threshold.
Optical power / (mW)
•
LED
Laser
Current / (mA)
LED region
(Spontaneous)
22/05/2017
Laser region
(Stimulated)
TTC Riyadh, ICT–BS-2.3/2
37
Laser Principles (Contd.)
•
Output spectrum
– The laser output power is not at a single frequency but is spread over a range
of frequencies. Therefore, power profile is not very smoothed and has a series
of peaks and valleys.
•
Radiation pattern
– Laser light emission angles are depend on the size of the emitting area and on
the modes of oscillations within the layer.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
38
Laser Operation
•
LASER – Light Amplification by Stimulated Emission of Radiation
•
The laser operation differs from other optical sources because of it is resulted from
stimulated emissions.
Conduction band
( )
e
e
External
photon
Valance band
22/05/2017
Stimulated
s photon
e  s
()
TTC Riyadh, ICT–BS-2.3/2
39
Laser Operation (Contd.)
•
When this external photon (injected photon) hits with the excited electron at the
valence band, it is forced to create a stimulated photon and light is emitted with the
same wavelength and the same linewidth as the external photon. They are also in
phase.
•
Once these photons are travelled through the same direction, it will result further
stimulated emissions to support the directionality of the beam.
•
This causes to deplete the conduction band electrons very quickly, but generates a
large current to sustain the laser operation. The number of spontaneous emissions
are proportional to the number of injected photons.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
40
Population Inversion
•
To sustain the laser operation, it requires more electrons in the excited state
(conduction band) than the ground state.
•
Then only the stimulated emissions get higher than the stimulated absorptions.
•
Therefore, a high-density injected current (upto 150 mA) is fed across a small active
area.
(After)
(Before)
(Stimulated
emission)
(Stimulated
absorption)
Conduction
band
External
photon
Valance
band
Two photons
Conduction
band
Valance
band
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
41
Positive Feedback
•
Once the population inversion is achieved, the multiplication of photons is done by
keeping two reflected mirrors at two ends.
( )
A
Conduction band
C
D
B
E
Valance band
()
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
42
Positive Feedback (Contd.)
•
First, a stimulated photon is produced at point A and both photons are
continue towards the end of cavity (right hand side).
•
Then, they are reflected back at point B and continue the other direction.
•
When they are reach at point C, more stimulated emitting occurs.
•
Now the number of photons are doubled.
•
At point D, again they are reflected back due to the left hand side mirror.
•
The process is continued back and forth.
•
Normally, two ends are cleaved to act as mirrors and a Fabry-Perot cavity
configuration is used for optical confinement in a semi-conductor structure.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
43
Laser Output Mode Structure
•
Generally, the laser produces a finite number of radiative recombinations due to the
use of Fabry-Perot cavity structure thus creates many longitudinal modes.
•
Therefore, in each case the resulting gain is the superposition of two processes.
Longitudinal
modes
Laser output
gain
Mode
spacing
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
Frequency
44
Laser Output Mode Structure (Contd.)
•
Normally, the device can be tuned to in favor of single longitudinal mode (main
lobe).
•
Therefore, a measure called mode-suppression ratio (MSR) is introduced as
MSR =
•
In decibels,
22/05/2017
Power in the main mode
Power in the most dominant secondary mode
P 
MSR  10 log  m  .
 Ps 
TTC Riyadh, ICT–BS-2.3/2
45
Physical Structure – Laser Diode
•
Laser diodes has a similar structure to edge-emitting LED.
•
However, it has a thinner active region (gain-guided).
•
In addition, it consists of
- strip contacts to high density current injection
- cladding thickness variations to fabricate a ridge waveguide
Cleaved
surface
(mirror)
Active layer
Cladding
layer
Active layer
Metallic
layer
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
46
Types of Laser Diodes
•
At the beginning, Fabric-Perot cavity configuration is used with two directions
optical confinement. This makes broader- area semiconductor lasers.
•
With highly elliptical spatial output pattern, several improvements were followed to
obtain better performances.
– Gain-guided semiconductor lasers
Limits the current injection to a small stripe to provide lateral optical confinement
– Index-guided semiconductor lasers
Confinement is achieved with index steps in the lateral direction
– Buried hetrostructure lasers
Obtains single mode output by controlling the width and thickness of the active layer
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
47
Quantum Well Laser
•
The single quantum well (SQW) laser offers better efficiency and wavelength by
using a thick active region of 5 to 20 nm.
•
Small cavity size makes easy confinement.
•
Used in lightwave communication systems.
p-layer
Quantum
dot
Active region
Quantum wells
(InAs dots in
n-layer
the well)
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
48
Distributed Feedback Laser (DFL)
•
A Braggy grating inside the heterostructure is used to select one reflective
wavelength.
•
Slopes of the grating generate a distributed reflection which couples both forward
and backward travelling waves and a single wavelength is supported.
•
Therefore, a powerful output can be obtained with even a smaller linewidth.
Grating
Mirror
Active region
Distributed Feedback Laser (DFL)
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
49
DFL (Contd.)
•
A separate Braggy reflector is used externally to the active region.
•
With this preparation, it is possible to select main mode wavelength outside the
cavity with an MSR > 30 dB.
Mirror
Grating
Active region
Distributed Baggy Reflector (DBR)
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
50
External Cavity Laser (ECL)
•
One cavity mirror is moved outside the active region.
•
Therefore, the second set of cavity parameters has to be coupled with the first but,
loss is occurred inside the cavity.
•
However, minimum loss is occurred at the peak while the maximum is at the
nearest secondary mode.
•
Consequently, a higher MSR can be obtained.
Active region
Lens
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
External
mirror
51
Vertical Cavity Surface-Emitting Laser (VCSEL)
•
This produces a single mode, narrower linewidth and circular output which can be
easily coupled into fibers for LAN applications.
•
Emissions exit from the surface rather than the edge.
•
Attractive in communication applications because of low power consumption and
relatively high switching speeds.
Active region
DBR mirror
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
52
Advantages of LD over LED
•
A higher radiance due to amplifying effect of the stimulated emission.
– Optical output power in mW
•
Narrower linewidth minimizes the effect of material dispersion.
– Order of 1 nm or less
•
Extension of modulation capabilities upto GHz range.
•
Applicability of heterodyne (coherent) detection in high capacity systems.
•
Good spatial coherent allows efficient coupling into the fibers even with low
numerical apertures thus results a higher efficiency.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
53
Optical Fiber Communications
L
P
∑
Optical Signals: Attenuation and Amplification
12
0
12
ICT-BS-2.3/2/1
Optical Sources
1
ICT-BS-2.3/2/2
Structures and Characteristics of Light-Emitting Diodes LED
1
ICT-BS-2.3/2/3
Semiconductor Laser Structures
1
ICT-BS-2.3/2/4
Power Launching and Coupling
1
ICT-BS-2.3/2/5
Optical Detectors
2
ICT-BS-2.3/2/6
Signal Encoding/Decoding
2
ICT-BS-2.3/2/7
Modulation and Demodulation Formats
2
ICT-BS-2.3/2/8
Receiver Sensitivities
1
ICT-BS-2.3/2/9
Optical Amplifiers
1
Modules
Code
ICT-BS-2.3/2
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
54
Review – Optical Fiber Communication System
Electrical Signal
Input
Optical
Source
Optical
Detector
Modulator
Demodulator
Output Signal
Transmission path
(Optical Fiber)
Transmitter
22/05/2017
Receiver
TTC Riyadh, ICT–BS-2.3/2
55
Attenuation and losses
Attenuation and losses
Coupling losses
transmitter-fibre
Transmission level
Coupling losses
fibre-fibre
Fibre
attenuation
Coupling losses
fibre-receiver
Min. required
reception level
Coupling losses in a fibre-optic transmission system
Attenuation and losses
In optical telecommunications systems, the method of coupling the glass fibres
is of prime importance.
Low-attenuation couplings are essential, not only between the fibre-optic cable
sections themselves, but also between them and the transmitter / receiver
elements.
The low light intensities employed cause small additional attenuations due to
coupling losses in the light junctions between transmitter & fibre, fibre & fibre,
and fibre & receiver.
The extremely small dimensions of the fibre-optic cables require accurate
alignment of the coupling elements, fibres being coupled permanently (spliced
joints) or with detachable elements (connectors).
Optical Transmitter
Bias monitor
Data
Disable laser
Data
conversion
Laser control
Laser Driver
Modulation
and bias
Temperature
control
Current monitor
Temperature
monitor
TE
Optical power
monitor
TE = Thermoelectric cooler
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
59
Optical Transmitter (Contd.)
•
In a optical fiber communication system, the transmitter is responsible of
– generating an optical signal (source)
– modulating the signal (modulator)
– coupling the signal into the fiber (coupling mechanism).
•
In addition, there may be a photodiode monitor, a temperature sensor, cooling
devices and feedback mechanisms.
•
It is useful to monitor the transmitter performance to make sure that there is a
stable output with minimal noise effect.
•
Generally, to maintain constant transmitter power output, laser diode transmitters
requires feedback monitoring mechanism.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
60
Optical detectors
Irrespective of the field of application, photo-detectors must exhibit the
following properties:
• High sensitivity to the light received in the range of wavelengths from the
source of optical radiation
• Short response times
• Low noise
• Insensitive to temperature changes
• Reasonably priced
• Long service life
• Good coupling possibilities for fibre-optic cables
Optical detectors
Semiconductor photodiodes function on the direct internal photo-electric
effect. This occurs at the p-n junction of the semiconductor material when
light energy strikes the junction.
This in turn, causes the charge-carriers to be separated, thus producing
diffusion and drift currents that result in a photoelectric current.
The charge-carriers pass through the space charge region and induce a
photocurrent signal in the external circuit.
Optical detectors
The frequency response of the photodiode is influenced by the electrical
equivalent circuit of the diode, taking into account the external load circuit
(input of amplifier).
Typical path resistance values R for an AP-diode are in the region of a few
ohms to a few tens of ohms.
The conductance of the barrier layer G can usually be ignored. The figure
shows the equivalent circuit for avalanche (AP) and PlN photodiodes with
junction capacitance C and the other parasitic elements.
In high-frequency diodes, the value of C is about 1 pF, assuming the reverse
voltage is not too small, and the diode surfaces are 100...300 nm diameter.
A load resistor RL of 50 Ω therefore results in an RC limit frequency of 2...4
GHz.
Source-to-Fiber Coupling
•
The main objective of the coupling mechanism is to couple much light into the fiber.
•
However, several losses may arise due to reflection loss, area mismatch, packing
fraction loss and numerical aperture mismatch.
•
Two basic types
– Lens coupling
– Direct coupling
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
64
Source-to-Fiber Coupling (Contd.)
•
Lens coupling
– Approximately 100% efficiency is achievable by using lens coupling
– Sometimes suffers from lens mounting problems
Cylindrical lens
Source
Source
22/05/2017
Cylindrical
lens
Spherical
lens
Fiber
Fiber
TTC Riyadh, ICT–BS-2.3/2
65
Source-to-Fiber Coupling (Contd.)
•
Direct coupling
– Makes the fiber close as much as possible to the source and then the source is
epoxied into fiber.
Source
•
Fiber
By fiber pigtailing with integrated transmitter module, the efficiency of the direct
coupling can be improved.
Rubber boot
Fiber pigtail
Source
22/05/2017
Optical Isolator
Ferrule
TTC Riyadh, ICT–BS-2.3/2
66
Fiber Optic Couplers
•
Fiber optic couplers transmit one or more fiber inputs to one or more fiber outputs.
•
Therefore, it is possible to transmit the same signal to two places or to provide bidirectionality and isolation.
•
Star coupler
– Number of inputs are coupled to number of outputs
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
67
Fiber Optic Couplers (Contd.)
•
Tree coupler
– Distributes incoming light to several outputs evenly.
•
Tee (tap) coupler
– Three ports, one input and two outputs and third port can be used for
monitoring purposes by taking out a portion of the output signal.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
68
Fiber Optic Couplers (Contd.)
•
Four-port directional coupler
– Two bare fibers are twisted together and then pulling and melting together.
•
The losses involved in coupling include insertion loss, excess loss and splitting or
directional loss.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
69
Optical Fiber Communications
L
P
∑
Optical Signals: Attenuation and Amplification
12
0
12
ICT-BS-2.3/2/1
Optical Sources
1
ICT-BS-2.3/2/2
Structures and Characteristics of Light-Emitting Diodes LED
1
ICT-BS-2.3/2/3
Semiconductor Laser Structures
1
ICT-BS-2.3/2/4
Power Launching and Coupling
1
ICT-BS-2.3/2/5
Optical Detectors
2
ICT-BS-2.3/2/6
Signal Encoding/Decoding
2
ICT-BS-2.3/2/7
Modulation and Demodulation Formats
2
ICT-BS-2.3/2/8
Receiver Sensitivities
1
ICT-BS-2.3/2/9
Optical Amplifiers
1
Modules
Code
ICT-BS-2.3/2
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
70
Review – Optical Fiber Communication System
Electrical Signal
Input
Optical
Source
Optical
Detector
Modulator
Demodulator
Output Signal
Transmission path
(Optical Fiber)
Transmitter
22/05/2017
Receiver
TTC Riyadh, ICT–BS-2.3/2
71
Optical Detectors
•
Photodetection process is used to convert the optical signal back to the
electrical signal at the receiver.
•
The common light detectors are semiconductor junction devices.
•
The basic principle used for detection is optical absorption.
(AP)
•
Type of optical detectors
– pn-junction photodiode
– Positive-intrinsic-negative (PIN) photodiode
– Avalanche (AP) photodiode
– Metal-semiconductor-metal (MSM) photodiode
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
72
Optical detectors
Optical detectors convert light intensity back into an electrical variable, the
current.
In modern optical transmission lines, the detector components are usually
silicon
PIN-diodes
for short distances and low-cost systems.
These diodes have an intrinsic (neutral) range between the P and N ranges.
AP-diodes
(avalanche photodiodes) are used in systems with larger bandwidths, where
the cost of the detector is not of prime importance.
Optical detectors
Irrespective of the field of application, photo-detectors must exhibit the
following properties:
• High sensitivity to the light received in the range of wavelengths from the
source of optical radiation
• Short response times
• Low noise
• Insensitive to temperature changes
• Reasonably priced
• Long service life
• Good coupling possibilities for fibre-optic cables
Optical detectors
Semiconductor photodiodes function on the direct internal photo-electric
effect. This occurs at the p-n junction of the semiconductor material when
light energy strikes the junction.
This in turn, causes the charge-carriers to be separated, thus producing
diffusion and drift currents that result in a photoelectric current.
The charge-carriers pass through the space charge region and induce a
photocurrent signal in the external circuit.
Optical detectors
The frequency response of the photodiode is influenced by the electrical
equivalent circuit of the diode, taking into account the external load circuit
(input of amplifier).
In a receiver for low light intensity (or photon flux), the photodiode is
operated in the reverse (non-conducting) direction.
The value of the load resistance determines whether the circuit is to be used
for a large output signal (= large load resistance) or a high limit frequency (=
smaller load resistance).
Further influencing factors are the internal diffusion processes, the charge
transit time and timing effects (in time-division multiplex processes in APdiodes).
The equivalent circuit of a PIN- and an AP-diode are shown below.
Optical detectors
The frequency response of the photodiode is influenced by the electrical
equivalent circuit of the diode, taking into account the external load circuit
(input of amplifier).
Typical path resistance values R for an AP-diode are in the region of a few
ohms to a few tens of ohms.
The conductance of the barrier layer G can usually be ignored. The figure
shows the equivalent circuit for avalanche (AP) and PlN photodiodes with
junction capacitance C and the other parasitic elements.
In high-frequency diodes, the value of C is about 1 pF, assuming the reverse
voltage is not too small, and the diode surfaces are 100...300 nm diameter.
A load resistor RL of 50 Ω therefore results in an RC limit frequency of 2...4
GHz.
Optical detectors
.
Iph= Photo Current
C= Barrier layer capacitance
G= Barrier layer conductance
R= Path resistance
RL= Load resistor
A= Amplification
Diode equivalent circuit
Optical Absorption
•
When a photon strike the semiconductor material with more than the bandgap
energy, it is absorbed and an electron-hole pair is generated.
•
Thus an electric field applied across the semiconductor creates a current flow due
to the attraction of positive and negative charges to the electron and the hole
respectively.
+
Reverse
biased voltage Generated
photocurrent
Incident
photons
Semiconductor
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
79
Optical Absorption (Contd.)
•
Once a incoming photon is detected by the semiconductor material over a range of
wavelength, it converts the photon energy greater than the bandgap energy into an
electron-hole pair.
( )
Conduction band
Valance band
1
2
Bandgap
energy
3
()
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
80
Optical Absorption (Contd.)
•
Although the process of optical absorption is available while the light reaches at the
semiconductor, not all the incident photos are converted back to the electric current
(includes in Fresnel reflection).
•
The total power absorbed depends on the Fresnel reflection and absorption
coefficient (absorption length).

P  Pi (1  R ) 1  e x

Pi - Optical power incident on the semiconductor material
R - Fresnel reflection
 - Absorption coefficient
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
81
Optical Absorption (Contd.)
Incident
power
Semiconductor
x
Radiative power
Incident power
level
Power loss due to Fresnel
reflection
Penetration
depth
•
Distance into the
semiconductor
Penetration depth ( 1 ) defines as the depth at which the power level falls ( 1 e ) of
initial power.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
82
pn-junction Photodiode
•
Performs almost the reverse function of an LED.
•
When light is applied to the p-region, photon energy is absorbed by an electron.
Therefore, the absorbed energy raises a bound electron across the bandgap from
the valance band to the conduction band.
•
This separated electron and hole is attracted to the positive and negative potentials
in the depletion region and a current is produced.
•
However, the pn-junction photodiode responsivity is low and rise time is large.
p-region
n-region
+
-
Depletion
region
I
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
83
pn-junction Photodiode (Contd.)
•
When pn-junction is reverse biased no current flows.
•
Even without the presence of light, a small current can be flown through the circuit
and it is called as the dark current.
Photodiode
current
Forward
bias
Reverse
breakdown
voltage
Dark
current
Photodiode
voltage
Reverse
bias
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
84
PIN Photodiode
•
A lightly n-doped intrinsic layer is included between p- and n- regions and it acts as
the depletion layer.
•
The absorption is taken place inside the thick intrinsic layer thus most of the
photons can be converted into electron-hole pairs.
•
Hence the quantum efficiency (efficiency of photon-to-electron conversion) is
increased.
p-region
Intrinsic
region
n-region
+
-
I
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
85
PIN Photodiode (Contd.)
•
Because of depletion region is inside the intrinsic region, charge carriers can be
moved with a higher velocity.
•
Therefore, this performs better than the pn-junction photodiode in reverse biased
mode.
•
Also the rise time is increased relative to pn-junction photodiode.
•
The wider depletion region decreases the junction capacitance and consequently
increases the bandwidth.
•
On the other hand, increased transmit time within the layer decreases the
bandwidth.
•
Therefore, selecting the width and the area of the intrinsic region have to done
carefully.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
86
Avalanche Photodiode (AP Diode)
•
APD is also a semiconductor junction detector which aquires more photodiode gain
thus increases the responsivity over PIN diode (range of 20-80 A/W).
•
Hence, this is capable of allowing longer fiber lengths between repeaters.
•
Consists of lightly doped intrinsic and p-regions are packed between p+- and n+regions.
p+
Lightly
doped
p
n+
+
-
I
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
87
Optical Fiber Communications
L
P
∑
Optical Signals: Attenuation and Amplification
12
0
12
ICT-BS-2.3/2/1
Optical Sources
1
ICT-BS-2.3/2/2
Structures and Characteristics of Light-Emitting Diodes LED
1
ICT-BS-2.3/2/3
Semiconductor Laser Structures
1
ICT-BS-2.3/2/4
Power Launching and Coupling
1
ICT-BS-2.3/2/5
Optical Detectors
2
ICT-BS-2.3/2/6
Signal Encoding/Decoding
2
ICT-BS-2.3/2/7
Modulation and Demodulation Formats
2
ICT-BS-2.3/2/8
Receiver Sensitivities
1
ICT-BS-2.3/2/9
Optical Amplifiers
1
Modules
Code
ICT-BS-2.3/2
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
88
Signal Encoding & Decoding
Information
Transmission signal type in the optical fiber
Analog signals
Modulation
Digital signals
Encoding
Analog signals
Modulation
Digital signals
Encoding
Analog
Digital
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
89
Signal Encoding & Decoding (Contd.)
•
Hence, encoding in optical fiber transmission means the transmission of analog
optical information through fiber optics digitally.
•
This improves the acceptable signal-to-noise ratio (SNR) by 20 to 30 dB over
analog transmission.
m(t )
m(t )
m(t )
Encoder
Fiber cable
Analog
optical
data
Analog
optical
data
22/05/2017
Decoder
TTC Riyadh, ICT–BS-2.3/2
90
Control of the transmitter diode
Basically, selecting the method of modulation depends greatly on the types
of signal to be transmitted; these can either be analog or digital. It is also
necessary to determine which field of telecommunication applications the
optical waveguide system is intended for, in order to establish the bandwidth
required and the length of the transmission path. Involved here might be
broad-band transmission as in cable TV, cross-connections in telephone and
data networks, wide-area networks (WAN), submarine cables, etc., or
transmission with narrow and medium bandwidths and data rates, such as
data and signal transmission in buildings, ships, aircraft, computer systems,
studios, between studios, etc.
With some limitations, the characteristics of LED and laser diodes permit a
direct modulation of intensity for transmitting analog signals. This means
that the intensity of the light source is directly varied in relation to the applied
analog or digital signal. This form of modulation however, assumes that the
characteristic is linear.
optical receiver
Control of the transmitter diode
Pulse modulation, with the possibility of time-division multiplex operation,
requires a large and sometimes, complex circuit. In optical transmission, the
pulses directly drive the LED or laser diodes functioning as an optical
transmitter. If analog signals are to be transmitted using pulse modulation,
the signals must be modulated using a known method (e.g. pulse code
modulation).
Improvement in the quality of transmission and immunity to interference with
pulse modulation however, requires larger bandwidths which are gaining in
importance, particularly in long-distance telephony.
With direct pulse modulation of the transmitter diode, however, it is
necessary to note the turn-on delay which occurs when the diode is
switched from the zero state. An advantage therefore, is to adapt the pulse
to the characteristic. This is achieved by applying a biasing current and
matching the pulse amplitude to the characteristic.
Control of the transmitter diode
Before examining the various methods of modulation, however, it is
necessary to know the characteristics of the infrared transmitter diodes, so
that the biasing current can be set correctly for linear transmission of the
signals.
The aim of modulation is to convert the signals, usually in the form of a
voltage varying as a function of time, into a luminous flux as a function of
time, without any loss of information. However, two non-linear factors are
present: The non-linear characteristic of the diode I = f(U) and a saturation
area in the upper section of the characteristic of light intensity as a function
of the diode current
Φ = f(I),
i.e. the outer quantum efficiency drops as the current increases
Signal Encoding & Decoding (Contd.)
•
Analog signals are digitized by using pulse code modulation (PCM).
Analog optical
input
Sampler
Quantized
PAM
PAM
Quantizer
Encoder
PCM
Fiber cable
PCM
Decoder
Quantizer
LPF
PAM
Quantized
PAM
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
Analog optical
output
94
Advantages of Digital Transmission
•
There are several benefits of digital transmission over analog transmission.
– Produces fewer errors than analog transmission.
– Permits higher maximum transmission rates.
– More data transmission through a given circuit (more efficient).
– More secure because it is easier to encrypt.
– Integrating voice, video and data on the same circuit much simpler.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
95
Sampling
•
The analog signal is first sampled at a rate greater than the Nyquist sampling rate
(greater than twice the maximum signal frequency).
•
Thus the pulse amplitude modulated (PAM) signal is obtained where the amplitude
for constant width sampling pulses.
Analog signal
Sampling pulses
PAM signal
22/05/2017
t
t
t
TTC Riyadh, ICT–BS-2.3/2
96
Quantizing
•
The PAM signal is then quantized to into a number of discrete levels so that each of
the distinct binary codeword represents a pulse code modulated (PCM) signal.
Code levels
22/05/2017
7
6
5
4
3
2
1
0
TTC Riyadh, ICT–BS-2.3/2
97
Encoding
•
Afterthat, different discrete amplitude values are encoded by using binary patterns.
–
8 levels
PAM is encoded into 3 bits
– 16 levels
PAM is encoded into 4 bits
Binary Equivalent
Decimal
Number
22
21
20
0
0
0
0
1
0
0
1
2
0
1
0
3
0
1
1
4
1
0
0
5
1
0
1
6
1
1
0
7
1
1
1
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
Pulse Code
Waveform
98
Multiplexing & Demultiplexing
•
Conversion of analog signal to a discrete PCM signal allows number of analog
channels to be transmitted through a single optical fiber link.
•
This is called as time-division multiplexing.
•
Multiplexing improves the information transfer rate.
Analog
input 1
PCM
Encoding
Rotary
switch
PCM
Decoding
Analog
output 1
Fiber cable
Analog
input 2
PCM
Encoding
PCM
Decoding
Analog
output 2
Analog
input 3
PCM
Encoding
PCM
Decoding
Analog
output 3
(Multiplexing)
22/05/2017
(Demultiplexing)
TTC Riyadh, ICT–BS-2.3/2
99
Multiplexing & Demultiplexing
22/05/2017
TTC Riyadh, ICT–BVF–4 /1/1
100
Optical Fiber Communications
L
P
∑
Optical Signals: Attenuation and Amplification
12
0
12
ICT-BS-2.3/2/1
Optical Sources
1
ICT-BS-2.3/2/2
Structures and Characteristics of Light-Emitting Diodes LED
1
ICT-BS-2.3/2/3
Semiconductor Laser Structures
1
ICT-BS-2.3/2/4
Power Launching and Coupling
1
ICT-BS-2.3/2/5
Optical Detectors
2
ICT-BS-2.3/2/6
Signal Encoding/Decoding
2
ICT-BS-2.3/2/7
Modulation and Demodulation Formats
2
ICT-BS-2.3/2/8
Receiver Sensitivities
1
ICT-BS-2.3/2/9
Optical Amplifiers
1
Modules
Code
ICT-BS-2.3/2
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
101
Review – Optical Fiber Communication System
Electrical Signal
Input
Optical
Source
Optical
Detector
Modulator
Demodulator
Output Signal
Transmission path
(Optical Fiber)
Transmitter
22/05/2017
Receiver
TTC Riyadh, ICT–BS-2.3/2
102
Signal Modulation & Demodulation
Information
Transmission signal type in the optical fiber
Analog signals
Modulation (Analog)
Digital signals
Encoding
Analog signals
Modulation (Digital)
Digital signals
Encoding
Analog
Digital
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
103
Modulator Types
•
In optical fiber communication can be achieved in two ways.
– Direct modulation
– Indirect modulation
•
Further, it can categorized as
– Analog modulation (Intensity modulation)
Primary modulation method is amplitude modulation.
– Digital modulation
Commonly used technique is on-off keying (OOK).
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
104
Direct Modulation
•
In direct modulation, the modulated electrical signal is input directly to the source
and obtained the modulated optical signal output.
Modulated electrical
input
Optical
Source
Modulated optical
output
•
This introduces transient changes (chirps) in the wavelength.
•
Chirps are caused for dispersion on the waveform thus limit the distance and also
the bandwidth capabilities of the transmitter.
•
Not suitable for high speed transmitters.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
105
Indirect Modulation
•
The modulation is achieved externally.
•
Used for higher data rate transmitters (greater than 10 Gbits/s).
Optical
Source
Modulator
Modulated optical
output
Modulated electrical
input
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
106
Analog (Intensity) Modulation
•
In fiber optic signal modulation, the intensity of the light source is varied according
to some electrical input signal (baseband signal). Thus it is called as intensity
modulation (analog modulation).
•
This method is inexpensive and easy to implement.
Source drive circuit
(Optical modulator)
Baseband
input
22/05/2017
Amplifier
LPF
Fiber cable
Optical
detector
Optical
source
TTC Riyadh, ICT–BS-2.3/2
Baseband
output
107
LED Intensity Modulation
•
The diode output power is modulated by a current source which simply turns the
LED on or off.
•
Requires a dc bias to keep the total current in the forward direction at all times.
•
Without the dc current, a negative swing in the signal current would reverse the
direction thus shutting the diode off.
Output
power
Psp
Pdc
Idc
t
(Resulting output
power)
Current
Isp
(LED driving
current)
t
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
108
LED Intensity Modulation (Contd.)
• Idc - dc bias current
• Isp - signal current
• Pdc - average power
• Psp - peak amplitude of the modulated portion of the output power
•
Therefore, the total diode current is I  Idc  Isp sin t and the corresponding output
power is P  Pdc  Psp sin t .
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
109
LED Intensity Modulation (Contd.)
•
I
The modulation index in terms of current can be defined as m' 
•
P
Similarly, the modulation index related to the power is m 
•
Thus,
sp
sp
Idc .
Pdc .
P  Pdc  Psp sin t .
 Pdc (1 
Psp
Pdc
sin t )
 Pdc (1  m sin t )
Optical carrier
intensity
Same as amplitude modulation (AM)
Baseband signal
t
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
110
LED Intensity Modulator
•
The modulator circuit operates with the help of a bipolar junction transistor (BJT).
Vdc
IC
IB
ON
Ra
LED
Q
Vsp
Rc
RB
Load line
for BJT
RE
OFF VCE
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
111
Laser Intensity Modulation
•
The analog circuit used for LED is suitable for analog modulation of a laser diode.
•
A heat sink has to be used to cool the temperature dependency effects of laser
diode.
Output
power
Psp
Pdc
t
(Resulting output
power)
Current
Idc
Isp
(Laser driving
current)
t
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
112
Subcarrier Intensity Modulation
•
Although the direct intensity modulation is suitable for transmitting a baseband
analog signal though a single fiber.
•
But, for a wideband fiber, number of baseband channels have to be used the same
fiber for efficient utilization.
•
Therefore, subcarrier intensity modulation can be applied by multiplexing
(frequency division) composite electrical signal prior to the intensity modulation.
Analog
baseband
signal s
Modulators
(two level)
Modulator &
(drive circuit)
Optical
source
Fiber cable
RF subcarriers
Optical
detector
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
Amplifier
Analog
baseband
signals
Demodulators
(drive circuit)
113
Digital Modulation
•
The most common digital modulation technique used is on-off keying (OOK).
•
When binary value “1” used for optic power pulse is ON and binary value “0” for
optic power pulse is OFF.
•
Transistor provides the switching and current amplification.
Vdc
LED
C
R
(Transistor switched LED
digital modulator)
Vsp
R1
•
R2
The other methods used for digital modulation of optical fiber transmission are
pulse position modulation (PPM) and pulse width modulation (PWM).
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
114
Demodulation Circuits
•
Demodulation circuits are operated by using either a bipolar junction transistor
(BJT) or a field effect transistor (FET).
•
For higher data rates (larger bandwidths), the bipolar transistor introduces less
noise than the field effect transistor.
VDD
Vcc
R
R
Output
Output
G
PIN
photodiode
PIN
photodiode
RL
S
RL
Vs
Vs
(BJT amplifier)
22/05/2017
D
TTC Riyadh, ICT–BS-2.3/2
(FET amplifier)
115
Optical Fiber Communications
L
P
∑
Optical Signals: Attenuation and Amplification
12
0
12
ICT-BS-2.3/2/1
Optical Sources
1
ICT-BS-2.3/2/2
Structures and Characteristics of Light-Emitting Diodes LED
1
ICT-BS-2.3/2/3
Semiconductor Laser Structures
1
ICT-BS-2.3/2/4
Power Launching and Coupling
1
ICT-BS-2.3/2/5
Optical Detectors
2
ICT-BS-2.3/2/6
Signal Encoding/Decoding
2
ICT-BS-2.3/2/7
Modulation and Demodulation Formats
2
ICT-BS-2.3/2/8
Receiver Sensitivities
1
ICT-BS-2.3/2/9
Optical Amplifiers
1
Modules
Code
ICT-BS-2.3/2
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
116
Review – Optical Fiber Communication System
Electrical Signal
Input
Optical
Source
Optical
Detector
Modulator
Demodulator
Output Signal
Transmission path
(Optical Fiber)
Transmitter
22/05/2017
Receiver
TTC Riyadh, ICT–BS-2.3/2
117
Receiver Operation
•
Receiver is responsible for converting the optical signal back to the original
information set by the transmitter.
•
However, interfacing from fiber to photodiode has to be done carefully to increase
the amount of light entering to the detector circuit.
•
Lens coupling, using anti-reflection coatings, applying index-matching gel and using
pigtail packaging are some solution to that.
•
The basic subsections in the receiver are the photodiode, low noise pre-amplifier,
main amplifier section and the data recovery stage.
•
The receivers can be categorized as
– analog receivers and
– digital receivers.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
118
Analog Receiver
•
Eventhough digital signal transmission is preferred in optical communication, there
are many potential applications for analog transmission.
•
It ranges from individual 4 kHz voice channels to multi-GHz microwave links.
Preamplifier
Amplifier
Photodiode
Filter
Output
Optical
Signal
Power
Supply
(Front End)
22/05/2017
Automatic Gain
Control
(Main Amplifier)
TTC Riyadh, ICT–BS-2.3/2
(Data Recovery)
119
Analog Receiver (Contd.)
•
The optical signal coupled from the light source to the fiber gets attenuated and
distorted during the transmission through the fiber cable.
•
Once it is detected and converted back to the electrical form by using a
photodetector, the produced electrical current is typically very weak.
•
Therefore, to boost its level, the main amplifier is used.
•
To minimize the effect of intersymbol interference (ISI), a lowpass filter is used
remove the parts outside the signal bandwidth.
•
Then, the demodulator is used to recover original data sent by the transmitter.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
120
Digital Receiver
•
The notable difference in the digital receiver is the data recovery subsection
compared to the analog receiver because the analog receiver data recovery can be
done directly by using the demodulator.
•
However, the digital one requires further signal processing.
•
It consists of a decision circuit and a clock recovery circuit.
Preamplifier
Amplifier
Photodiode
Filter
Decision
Circuit
Output
Optical
Signal
Power
Supply
(Front End)
22/05/2017
Automatic
Gain Control
(Main Amplifier)
TTC Riyadh, ICT–BS-2.3/2
Clock
Recovery
(Data Recovery)
121
Signal Recovery in a Digital receiver
•
This is responsible of checking the validity of the received information.
•
The decision circuit is used to separate bits (to either ones or zeros) of the received
data. The data is compared with a threshold level.
– If the received voltage is more than the threshold will results bit “1”.
– Otherwise bit “0”.
•
To accomplish this bit interpretation, the receiver should be able to understand the
bit boundaries.
•
The clock recovery circuit measures the bit interval and regenerates a new clock
pulse to the decision circuit.
•
However, to minimize the bit error rate, the receiver should be capable of detecting
and correcting the errors of the received data stream.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
122
Receiver Performance
•
Receiver performance is determined by transforming the received optical signal to
meaningful data.
•
To evaluate the receiver performance, dynamic range, sensitivity, SNR and bit
error rate can be used.
•
Dynamic range
– The amount of signal level can be detected with a linear response.
– Sometimes at high powers, the receivers may become nonlinear thus
anomalies can be occurred.
– Typical range is 30 to 40 dB.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
123
Receiver Performance (Contd.)
•
Sensitivity
– The minimum optical input power can be detected by the receiver.
– This determines the quality of the service, i. e., for a given SNR, the minimum
input optical power needed.
•
Signal-to-noise ratio (SNR)
– This determines detectability of the signal with the addition of noise.
•
Bit error rate (BER)
– The average probability of incorrect bit identification.
– If there is one error bit for every 109 bits, then BER is 10-9.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
124
Receiver packaging
•
Receiver packaging is useful for high data rate systems to protect from installation
environment effects such as mismatching of connecting devices.
•
As an example by keeping shorter photodiode connections will amplify less noise to
the data recovery section.
•
Thus, the detector performance can be significantly enhanced by integrating
packages.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
125
Transceiver
•
By combining the transmitter and the receiver also can increase the performance of
the transmission.
Transceiver = Transmitter + Receiver
(Connector)
Laser
Diode
Control
Electronics
Data
(Transmitter)
Electroabsorption
Modulator
Laser Diode Drive
Out
Power Supply
Amplifier
With
AGC
Data
In
Fiber
Connector
Data
Recovery
Circuit
Photodiode
Filter
Preamp
Fiber
Connector
(Receiver)
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
126
Optical Fiber Communications
L
P
∑
Optical Signals: Attenuation and Amplification
12
0
12
ICT-BS-2.3/2/1
Optical Sources
1
ICT-BS-2.3/2/2
Structures and Characteristics of Light-Emitting Diodes LED
1
ICT-BS-2.3/2/3
Semiconductor Laser Structures
1
ICT-BS-2.3/2/4
Power Launching and Coupling
1
ICT-BS-2.3/2/5
Optical Detectors
2
ICT-BS-2.3/2/6
Signal Encoding/Decoding
2
ICT-BS-2.3/2/7
Modulation and Demodulation Formats
2
ICT-BS-2.3/2/8
Receiver Sensitivities
1
ICT-BS-2.3/2/9
Optical Amplifiers
1
Modules
Code
ICT-BS-2.3/2
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
127
Amplifiers
•
Amplifiers are needed to increase the amplitude of the detected signal.
•
However, the bandwidth should remain unchanged and also the amplification of the
noise part has to be minimized for a proper communication.
•
Amplifiers are consist of transistors, resistors and other components.
•
In fiber optic transmission, number of amplification stages are used especially in
long distance communication.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
128
Type of Optical Amplifiers
•
In-line optical amplifier
– In single-mode fiber transmission, the effect of signal dispersion is very less.
– Therefore, the transmission can be done by regenerating the signal without
using repeaters in between.
– Thus, the main purpose of in-line amplifier is compensating for transmission
loss and increasing the distance between repeaters.
Fiber cable
Optical Tx
G
Optical Rx
In-line
amplifier
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
129
Type of Optical Amplifiers (Contd.)
•
Pre-amplifier
– Used to amplify the weak optical signal before the photodetection.
– Thus SNR reduced because of the thermal noise effect can be suppressed.
– Provides a larger gain factor and also increases the bandwidth.
Fiber cable
Optical Tx
G
Optical Rx
Pre- amplifier
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
130
Type of Optical Amplifiers (Contd.)
•
Power amplifier
– Used to boost the transmitted power thus to increase the transmission distance
by 10-100 km.
– Placed immediately after the optical transmitter.
– This techniques is used with pre-amplifier in undersea transmission where
repeaters can not be installed.
Long fiber
link
Optical Tx
G
Optical Rx
Power amplifier
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
131
Type of Optical Amplifiers (Contd.)
– Power amplifier can be used to compensate coupler-insertion loss and powersplitting loss in a local area network.
Receiver
stations
Fiber cable
Optical Tx
G
LAN booster
amplifier
22/05/2017
Star
coupler
TTC Riyadh, ICT–BS-2.3/2
132
High-Impedance Amplifier
•
Used in early communication systems as a pre-amplifier.
•
Thermal noise generated due to the output resistance and reflecting back to the
input is minimized by using the high input impedance.
•
The main drawback of this amplifier is reduced bandwidth.
Photocurrent
High-Impedance
Amplifier
Av
Optical
Signal
Photodiode
Bias
Voltage
22/05/2017
LPF
Output
Zin  
R  1 M
TTC Riyadh, ICT–BS-2.3/2
133
Transimpedance Amplifier
•
A higher sensitivity and a relatively wide bandwidth can be obtained.
•
The difference of this amplifier compared to high-impedance amplifier is feedback
impedance enables converting the input current into a voltage output.
•
This can be used with a second amplifier to achieve the required gain.
Photocurrent
Feedback
Impedance
Zf
Transimpedance
Amplifier
Av
Output
Optical
Signal
Photodiode
Zin  
Bias
Voltage
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
134
Semiconductor Optical Amplifier
•
Amplification is done by using a semiconductor laser placed between two fibers.
•
Active region of both ends are cleaved an coated with anti-reflective coating.
•
Advantage are wide spectral range and easiness of integrating with other
semiconductor devices and planar optical waveguide components.
•
But, suffers from fiber coupling difficulties.
Semiconductor
Optical Amplifier
Input Fiber
Output Fiber
Active layer
Antireflection
coating
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
135
Repeaters and Regenerators
•
A repeater consists of an optical receiver, an amplifier and an optical transmitter.
•
An optical signal is first converted into electrical signal, then amplified and next
converted back to the optical mode (optical-electrical-optical conversion).
•
Regenerator is required to remove the noise and generate a clean signal for further
transmission.
•
Discriminator is used to separate the noise from the signal and retiming is required
to make sure that the pulse timing is in order.
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
136
Types of Regenerators
•
Three regenerator types.
– 1R device : Amplifying only
– 2R device : Amplifying and reshaping
– 3R device : Amplifying, reshaping and retiming
Output
Input
R
Re-amplify
2R
Re-amplify, Re-shape
3R
Re-amplify, Re-shape, Re-time
22/05/2017
TTC Riyadh, ICT–BS-2.3/2
137