Information & Communication Technology ModuleICT–BS–2.3 Optical Fiber Communications Unit...

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

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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


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


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.


• Optical Fiber Communications, 4th Edition, Gerd Keiser

Published by Tata McGraw-Hill.



ICT–BS–2.3/2 Optical Signals: Attenuation and Amplification

Review – Optical Fiber Communication System


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)


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


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.


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.


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.


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.


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.


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


Reduced depletion region

n-typep-type


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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.


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)


• 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


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.


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


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


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.



Composition of the Semi-conductor (Contd.)

• Different material compositions have different bandgap energies.


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


(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.


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.


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


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


optical receiver


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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


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.)


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.)


• 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


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.)


• 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


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


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.)


• 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


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.)


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


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


• 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


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)


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.)


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)


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)


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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DemodulatorOptical

Detector



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


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


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.)


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


• Lens coupling

– Approximately 100% efficiency is achievable by using lens coupling

– Sometimes suffers from lens mounting problems

Source-to-Fiber Coupling (Contd.)


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.)


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


• 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.)


• 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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DemodulatorOptical

Detector



• 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


(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


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


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


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.)


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



1 1 ( ) xiP P R e

iP

R

• Penetration depth defines as the depth at which the power level falls of

initial power.



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


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.)


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


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.)


• 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)


p+ n+Lightly doped

+ -

I

p


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Signal Encoding & Decoding


Information Transmission signal type in the optical fiber

Analog

Analog signals Modulation

Digital signals Encoding

Digital

Analog signals Modulation


• 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.


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.


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).

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


• 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


• 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


• 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


• 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


• 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

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Multiplexing & Demultiplexing


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DemodulatorOptical

Detector



Signal Modulation & Demodulation

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Information Transmission signal type in the optical fiber

Analog

Analog signals Modulation (Analog)


Digital

Analog signals Modulation (Digital)


Modulator Types

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• 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

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• 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

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• 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

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• 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

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• 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.)

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• - 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

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• 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

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• 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

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• 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

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• 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

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1R

R

dcV

spV

LEDC

2R

(Transistor switched LED digital modulator)

Demodulation Circuits

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• 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)


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DemodulatorOptical

Detector



Receiver Operation

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• 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

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• 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.)

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• 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

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• 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

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• 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

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• 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.)

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• 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

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• 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

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• 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


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Amplifiers

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• 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

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• 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.)

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• 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


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• 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


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– 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

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• 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

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• 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

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• 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

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• 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

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• 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

Information & Communication Technology ModuleICT–BS–2.3 Optical Fiber Communications Unit...

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Transcript of Information & Communication Technology ModuleICT–BS–2.3 Optical Fiber Communications Unit...