Information & Communication Technology Modul e 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 07/04/22 1 TTC Riyadh, ICT–BS-2.3/2
Jan 11, 2016
Information & Communication TechnologyModule 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
04/21/23 1TTC Riyadh, ICT–BS-2.3/2
Optical Fiber Communications
CodeModules L P ∑
ICT-BS-2.3/2 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
04/21/23 2TTC Riyadh, ICT–BS-2.3/2
Learning Content:
• Optical sources
- Light emitting diode (LED)
- Laser diode (LD)
• Optical power coupling
• Optical detection
• Optical modulation and demodulation
• Optical signal amplification
ICT–BS–2.3/2 Optical Signals: Attenuation and Amplification
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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
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ICT–BS–2.3/2 Optical Signals: Attenuation and Amplification
Review – Optical Fiber Communication System
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Electrical Signal Input
ModulatorOptical Source Output Signal
DemodulatorOptical
Detector
Transmission path (Optical Fiber)
Transmitter Receiver
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)
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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
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Conduction of Electrons
• 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.
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Conduction band
Valance band
Movement of electronsCurrent
flow
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.
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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.
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Electrons
Holes
n-typep-type
The pn Junction Diode (Contd.)
• Even without applying any voltage, a barrier is formed at the boundary. This is
called ad the depletion region.
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n-typep-type
Potential barrier/ depletion region
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.
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n-typep-type
Increased depletion region
• 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.
Forward Biased - pn Junction
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Reduced depletion region
n-typep-type
Optical Fiber Communications
CodeModules L P ∑
ICT-BS-2.3/2 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
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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.
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V
IR
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)
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• 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.
LED Operation
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1 3 2
Conduction band
Valance band
Band gap energy
( )
( )
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.
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3 1
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)
• Then, the internal optical power produced due to the recombination process is
h – Planck’s constant I – current
c – velocity of the light in the vacuum
e – charge of an electron
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non-rad
intrad non-rad
.E
E E
int inthc
P Ie
191 602 10( . C)
8 13 10( ms )
• 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.
Types of Band gap Transitions
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Energy
Conduction band
Valance band
Conduction band
Valance band
Momentum
(Direct transition) (Indirect transition)
- - - -- - - - - - -
- - -
+ + + ++ + +
+ + + ++ + +
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.
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Composition of the Semi-conductor (Contd.)
• Different material compositions have different bandgap energies.
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Composition of the Semi-conductor (Contd.)
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
• 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)
LED Physical Structure
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(Homojunction structure)
(Heterojunction structure)
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.
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01200120
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.
number of photons coming from the device at an angle of per second.
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0 ( ) cos (W/steradian)I I
( )I
• 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.
Edge Emitting LED
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0120
030
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
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optical receiver
optical receiver
Optical Fiber Communications
CodeModules L P ∑
ICT-BS-2.3/2 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
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• The spectral width (line width) of the laser is much narrower than the LED.
• All lasers must have the following characteristics.
– Pumping threshold
– Output spectrum
– Radiation pattern
Laser Principles
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LED
Laser
• 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.
Laser Principles (Contd.)
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LED
Laser
Op
tica
l po
we
r /
(mW
)
Current / (mA)
LED region Laser region
(Spontaneous) (Stimulated)
• 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.
Laser Principles (Contd.)
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• LASER – Light Amplification by Stimulated Emission of Radiation
• The laser operation differs from other optical sources because of it is resulted from
stimulated emissions.
Laser Operation
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e
s
Conduction band
Valance band
ee s External
photonStimulated
photon
( )
( )
• 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.
Laser Operation (Contd.)
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• 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.
Population Inversion
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Conduction band
Valance band
External photon
Two photons
(Before) (After)
Conduction band
Valance band
(Stimulated emission)
(Stimulated absorption)
• Once the population inversion is achieved, the multiplication of photons is done by
keeping two reflected mirrors at two ends.
Positive Feedback
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Conduction band
Valance band
( )
( )
A
BC
D
E
• 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.
Positive Feedback (Contd.)
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• 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.
Laser Output Mode Structure
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Frequency
Laser output gain
Mode spacing
Longitudinal modes
• 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
• In decibels,
Laser Output Mode Structure (Contd.)
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Power in the main modeMSR =
Power in the most dominant secondary mode
10
MSR log .m
s
P
P
• 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
Physical Structure – Laser Diode
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Active layer
Active layer
Cleaved surface (mirror)
Metallic layer
Cladding layer
• 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
Types of Laser Diodes
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• 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.
Quantum Well Laser
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Active region
n-layer
p-layer
Quantum wells(InAs dots in
the well)
Quantum dot
• 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.
Distributed Feedback Laser (DFL)
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Mirror
Active region
Grating
Distributed Feedback Laser (DFL)
• 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.
DFL (Contd.)
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Mirror
Active region
Grating
Distributed Baggy Reflector (DBR)
• 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.
External Cavity Laser (ECL)
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Active region
External mirror
Lens
• 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.
Vertical Cavity Surface-Emitting Laser (VCSEL)
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Active region
DBR mirror
• 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.
Advantages of LD over LED
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Optical Fiber Communications
CodeModules L P ∑
ICT-BS-2.3/2 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
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Review – Optical Fiber Communication System
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Electrical Signal Input
ModulatorOptical Source Output Signal
DemodulatorOptical
Detector
Transmission path (Optical Fiber)
Transmitter Receiver
Attenuation and losses
Attenuation and losses
Coupling lossestransmitter-fibre
Coupling lossesfibre-fibre
Coupling lossesfibre-receiver
Transmission level
Fibre attenuation
Min. requiredreception 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
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Data conversion Laser Driver
Laser controlModulation
and bias
Temperature control
TE
Bias monitor
Data
Disable laser
Current monitor Temperature monitor
Optical power monitor
TE = Thermoelectric cooler
• 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.
Optical Transmitter (Contd.)
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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.
Optical detectors
• 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
Source-to-Fiber Coupling
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• Lens coupling
– Approximately 100% efficiency is achievable by using lens coupling
– Sometimes suffers from lens mounting problems
Source-to-Fiber Coupling (Contd.)
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SourceCylindrical lens
Fiber
SourceCylindrical
lens FiberSpherical
lens
• Direct coupling
– Makes the fiber close as much as possible to the source and then the source is
epoxied into fiber.
• By fiber pigtailing with integrated transmitter module, the efficiency of the direct
coupling can be improved.
Source-to-Fiber Coupling (Contd.)
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Source Fiber
Rubber boot
Fiber pigtail
FerruleOptical IsolatorSource
• 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 bi-
directionality and isolation.
• Star coupler
– Number of inputs are coupled to number of outputs
Fiber Optic Couplers
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• 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.
Fiber Optic Couplers (Contd.)
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• 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.
Fiber Optic Couplers (Contd.)
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Optical Fiber Communications
CodeModules L P ∑
ICT-BS-2.3/2 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
04/21/23 70TTC Riyadh, ICT–BS-2.3/2
Review – Optical Fiber Communication System
04/21/23 71TTC Riyadh, ICT–BS-2.3/2
Electrical Signal Input
ModulatorOptical Source Output Signal
DemodulatorOptical
Detector
Transmission path (Optical Fiber)
Transmitter Receiver
• 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.
• Type of optical detectors
– pn-junction photodiode
– Positive-intrinsic-negative (PIN) photodiode
– Avalanche (AP) photodiode
– Metal-semiconductor-metal (MSM) photodiode
Optical Detectors
04/21/23 72TTC Riyadh, ICT–BS-2.3/2
(AP)
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 AP-diodes).
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
Optical detectors
.
Iph= Photo Current
C= Barrier layer capacitance
G= Barrier layer conductance
R= Path resistance
RL= Load resistor
A= Amplification
Diode equivalent circuit
• 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.
Optical Absorption
04/21/23 79TTC Riyadh, ICT–BS-2.3/2
Incident photons
+
-
Semiconductor
Generated photocurrent
Reverse biased voltage
• 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.
Optical Absorption (Contd.)
04/21/23 80TTC Riyadh, ICT–BS-2.3/2
1
3
2Conduction band
Valance band
Bandgap energy
( )
( )
• 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).
- Optical power incident on the semiconductor material
- Fresnel reflection
- Absorption coefficient
Optical Absorption (Contd.)
04/21/23 81TTC Riyadh, ICT–BS-2.3/2
1 1 ( ) xiP P R e
iP
R
• Penetration depth defines as the depth at which the power level falls of
initial power.
Optical Absorption (Contd.)
04/21/23 82TTC Riyadh, ICT–BS-2.3/2
1( ) 1( )e
SemiconductorIncident power
x
Radiative power
Distance into the semiconductor
Power loss due to Fresnel reflection
Incident power level
Penetration depth
• 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.
pn-junction Photodiode
04/21/23 83TTC Riyadh, ICT–BS-2.3/2
p-region n-region
Depletion region
+ -
I
• 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.
pn-junction Photodiode (Contd.)
04/21/23 84TTC Riyadh, ICT–BS-2.3/2
Photodiode voltage
Photodiode current
Dark current
Forward bias
Reverse bias
Reverse breakdown
voltage
• 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.
PIN Photodiode
04/21/23 85TTC Riyadh, ICT–BS-2.3/2
p-region n-regionIntrinsic region
+ -
I
• 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.
PIN Photodiode (Contd.)
04/21/23 86TTC Riyadh, ICT–BS-2.3/2
• 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.
Avalanche Photodiode (AP Diode)
04/21/23 87TTC Riyadh, ICT–BS-2.3/2
p+ n+Lightly doped
+ -
I
p
Optical Fiber Communications
CodeModules L P ∑
ICT-BS-2.3/2 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
04/21/23 88TTC Riyadh, ICT–BS-2.3/2
Signal Encoding & Decoding
04/21/23 89TTC Riyadh, ICT–BS-2.3/2
Information Transmission signal type in the optical fiber
Analog
Analog signals Modulation
Digital signals Encoding
Digital
Analog signals Modulation
Digital signals Encoding
• 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.
04/21/23 90TTC Riyadh, ICT–BS-2.3/2
Encoder Decoder
Analog optical
data
( )m t( )m t
( )m t
Fiber cableAnalog
optical data
Signal Encoding & Decoding (Contd.)
optical receiver
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.
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
Control of the transmitter diode
Signal Encoding & Decoding (Contd.)
04/21/23 94TTC Riyadh, ICT–BS-2.3/2
• Analog signals are digitized by using pulse code modulation (PCM).
Sampler
LPF
Analog optical input
Analog optical output
Quantizer Encoder
Decoder Quantizer
PAM
Quantized PAM
PCM
PCM
PAM
Quantized PAM
Fiber cable
Advantages of Digital Transmission
04/21/23 95TTC Riyadh, ICT–BS-2.3/2
• 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.
Sampling
04/21/23 96TTC Riyadh, ICT–BS-2.3/2
• 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
PAM signal
Sampling pulses
t
t
t
Quantizing
04/21/23 97TTC Riyadh, ICT–BS-2.3/2
• 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.
01234567
Code levels
Encoding
04/21/23 98TTC Riyadh, ICT–BS-2.3/2
• 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
Decimal Number
Binary EquivalentPulse Code Waveform22 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
Multiplexing & Demultiplexing
04/21/23 99TTC Riyadh, ICT–BS-2.3/2
• 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.
PCM Encoding
Analog input 1
PCM Encoding
Analog input 2
PCM Encoding
Analog input 3
PCM Decoding
PCM Decoding
Analog output 1
PCM Decoding
Analog output 2
Analog output 3
(Multiplexing) (Demultiplexing)
Rotary switch
Fiber cable
04/21/23 TTC Riyadh, ICT–BVF–4 /1/1 100
Multiplexing & Demultiplexing
Optical Fiber Communications
CodeModules L P ∑
ICT-BS-2.3/2 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
04/21/23 101TTC Riyadh, ICT–BS-2.3/2
Review – Optical Fiber Communication System
04/21/23 102TTC Riyadh, ICT–BS-2.3/2
Electrical Signal Input
ModulatorOptical Source Output Signal
DemodulatorOptical
Detector
Transmission path (Optical Fiber)
Transmitter Receiver
Signal Modulation & Demodulation
04/21/23 103TTC Riyadh, ICT–BS-2.3/2
Information Transmission signal type in the optical fiber
Analog
Analog signals Modulation (Analog)
Digital signals Encoding
Digital
Analog signals Modulation (Digital)
Digital signals Encoding
Modulator Types
04/21/23 104TTC Riyadh, ICT–BS-2.3/2
• 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).
Direct Modulation
04/21/23 105TTC Riyadh, ICT–BS-2.3/2
• In direct modulation, the modulated electrical signal is input directly to the source
and obtained the modulated optical signal 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.
Modulated electrical input
Optical Source
Modulated optical output
Indirect Modulation
04/21/23 106TTC Riyadh, ICT–BS-2.3/2
• 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
Analog (Intensity) Modulation
04/21/23 107TTC Riyadh, ICT–BS-2.3/2
• 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
Amplifier
Baseband output
LPF
Optical source
Optical detector
Fiber cable
LED Intensity Modulation
04/21/23 108TTC Riyadh, ICT–BS-2.3/2
• 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
Current
tdcP
dcI
spP
spI
t
(Resulting output power)
(LED driving current)
LED Intensity Modulation (Contd.)
04/21/23 109TTC Riyadh, ICT–BS-2.3/2
• - dc bias current
• - signal current
• - average power
• - peak amplitude of the modulated portion of the output power
• Therefore, the total diode current is and the corresponding output
power is
dcI
spI
dcP
spP
sindc spI I I t
sin .dc spP P P t
04/21/23 110TTC Riyadh, ICT–BS-2.3/2
• The modulation index in terms of current can be defined as
• Similarly, the modulation index related to the power is
• Thus,
LED Intensity Modulation (Contd.)
' .sp
dc
IIm
.sp
dc
PPm
sin .dc spP P P t
1 ( sin )sp
dc
Pdc PP t
1 ( sin )dcP m t Same as amplitude modulation (AM)
Optical carrier intensity
t
Baseband signal
LED Intensity Modulator
04/21/23 111TTC Riyadh, ICT–BS-2.3/2
• The modulator circuit operates with the help of a bipolar junction transistor (BJT).
cR ERBR
aR
dcV
spV
CI
CEV
BI
Q
OFF
ON
Load line for BJT
LED
Laser Intensity Modulation
04/21/23 112TTC Riyadh, ICT–BS-2.3/2
• 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
Current
tdcP
dcI
spP
spI
t
(Resulting output power)
(Laser driving current)
Subcarrier Intensity Modulation
04/21/23 113TTC Riyadh, ICT–BS-2.3/2
• 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.
Modulators (two level)
Analog baseband signal s
Optical source
Fiber cable
Modulator & (drive circuit)
Demodulators (drive circuit)
Optical detector
RF subcarriers
Amplifier
Analog baseband
signals
• 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.
• The other methods used for digital modulation of optical fiber transmission are pulse
position modulation (PPM) and pulse width modulation (PWM).
Digital Modulation
04/21/23 114TTC Riyadh, ICT–BS-2.3/2
1R
R
dcV
spV
LEDC
2R
(Transistor switched LED digital modulator)
Demodulation Circuits
04/21/23 115TTC Riyadh, ICT–BS-2.3/2
• 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.
Output Output
LR
R
ccV
sV
DDV
R
LR
PIN photodiode
PIN photodiode
sV
G D
S
(BJT amplifier) (FET amplifier)
Optical Fiber Communications
CodeModules L P ∑
ICT-BS-2.3/2 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
04/21/23 116TTC Riyadh, ICT–BS-2.3/2
Review – Optical Fiber Communication System
04/21/23 117TTC Riyadh, ICT–BS-2.3/2
Electrical Signal Input
ModulatorOptical Source Output Signal
DemodulatorOptical
Detector
Transmission path (Optical Fiber)
Transmitter Receiver
Receiver Operation
04/21/23 118TTC Riyadh, ICT–BS-2.3/2
• 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.
Analog Receiver
04/21/23 119TTC Riyadh, ICT–BS-2.3/2
• 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.
Optical Signal
Output
AmplifierPre-
amplifier
Photodiode
PowerSupply
Filter
Automatic Gain Control
(Data Recovery)(Main Amplifier)(Front End)
Analog Receiver (Contd.)
04/21/23 120TTC Riyadh, ICT–BS-2.3/2
• 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.
Digital Receiver
04/21/23 121TTC Riyadh, ICT–BS-2.3/2
• 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.
Optical Signal
Output
AmplifierPre-
amplifier
Photodiode
PowerSupply
Filter
Automatic Gain Control
(Data Recovery)(Main Amplifier)
(Front End)
DecisionCircuit
Clock Recovery
Signal Recovery in a Digital receiver
04/21/23 122TTC Riyadh, ICT–BS-2.3/2
• 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.
Receiver Performance
04/21/23 123TTC Riyadh, ICT–BS-2.3/2
• 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.
Receiver Performance (Contd.)
04/21/23 124TTC Riyadh, ICT–BS-2.3/2
• 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.
Receiver packaging
04/21/23 125TTC Riyadh, ICT–BS-2.3/2
• 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.
Transceiver
04/21/23 126TTC Riyadh, ICT–BS-2.3/2
• By combining the transmitter and the receiver also can increase the performance of
the transmission.
Fiber Connector
LaserDiode
Photodiode
(Transmitter)
(Receiver)
(Connector)
Pre-amp
AmplifierWith AGC
Data Recovery
CircuitFilter
Control Electronics
Electro-absorptionModulator
Laser Diode Drive
Data
In
Out
Data Fiber Connector
Transceiver = Transmitter + Receiver
Power Supply
Optical Fiber Communications
CodeModules L P ∑
ICT-BS-2.3/2 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
04/21/23 127TTC Riyadh, ICT–BS-2.3/2
Amplifiers
04/21/23 128TTC Riyadh, ICT–BS-2.3/2
• 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.
Type of Optical Amplifiers
04/21/23 129TTC Riyadh, ICT–BS-2.3/2
• 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.
Optical Tx
Fiber cable
Optical RxG
In-line amplifier
Type of Optical Amplifiers (Contd.)
04/21/23 130TTC Riyadh, ICT–BS-2.3/2
• 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.
Optical Tx
Fiber cable
Optical RxG
Pre- amplifier
Type of Optical Amplifiers (Contd.)
04/21/23 131TTC Riyadh, ICT–BS-2.3/2
• 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.
Optical Tx
Long fiber link
Optical RxG
Power amplifier
Type of Optical Amplifiers (Contd.)
04/21/23 132TTC Riyadh, ICT–BS-2.3/2
– Power amplifier can be used to compensate coupler-insertion loss and power-
splitting loss in a local area network.
Optical Tx
Fiber cable
G
LAN booster amplifier
Star coupler
Receiver stations
High-Impedance Amplifier
04/21/23 133TTC Riyadh, ICT–BS-2.3/2
• 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.
LPFOutput
High-Impedance Amplifier
Photocurrent
Photodiode
Bias Voltag
e
Optical Signal
1 MR
inZ
Transimpedance Amplifier
04/21/23 134TTC Riyadh, ICT–BS-2.3/2
• 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
Output
Transimpedance Amplifier
Photodiode
Bias Voltage
Optical Signal
Zf
Feedback Impedance
inZ
Semiconductor Optical Amplifier
04/21/23 135TTC Riyadh, ICT–BS-2.3/2
• 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.
Input FiberSemiconductor
Optical Amplifier Output Fiber
Active layer
Antireflectioncoating
Repeaters and Regenerators
04/21/23 136TTC Riyadh, ICT–BS-2.3/2
• 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.
Types of Regenerators
04/21/23 137TTC Riyadh, ICT–BS-2.3/2
• Three regenerator types.
– 1R device : Amplifying only
– 2R device : Amplifying and reshaping
– 3R device : Amplifying, reshaping and retiming
OutputInput
R
2R
3R
Re-amplify
Re-amplify, Re-shape
Re-amplify, Re-shape, Re-time