Subject – Optical
Communication
Unit-1
Introducti
on
▪ Optical
fiber technology is mainly implemented to achieve rapid transmission of
data or signals over a long distance through glass fiber.
▪ The size of the glass fiber is very thin which can be comparable with size of the
human hair.
What is optical
Fiber
▪ Physical optical fiber is a very thin and flexible medium having cylindrical shape. The
structure of the fiber mainly consisting of 3 sections are follows 1. Core 2. Cladding 3.
Jacket
▪ Core is the innermost section of the structure. It has the property of conducting
optical beam. It is made of glass or plastic.
▪ Through the core, the signal is first converted into a light beam and then passed in
between the boundaries and propagated as a result of multiple internal reflections.
▪ Cladding is also a glass or a plastic material whose optical properties are different to
core material. Actually the core is surrounded by the cladding material.
▪ Jacket is the outermost part of the structure. It is made of a plastic or polymer and other
materials and it is mainly used to provide protection against various environmental
factors like moisture, absorption etc.
2
Basic structure of the
fiber
Advantages/
Importance
• Less attenuation
• Smaller size and lighter
weight
• Electromagnetic isolation
• Reliability
• More bandwidth
• Isolation
coating
• High data
rate
• Economical
3
Generation of the optical
fiber
First Generation
• Operating wavelength of 820 nm
• Data rate of 90 Mbps over 8 – 12
Km
• Attenuation 3 – 8 dB/Km
• It has been implemented
multimode fiber
Second Generation
• Operating wavelength of 1300 nm
• Data rate of 565 Mbps over 45 Km
• Attenuation 0.5 dB/Km
• It has been implemented single
mode fiber
4
Third Generation
• Operating wavelength of 1550 nm
• Data rate of 1.3 Gbps over 45 Km
• Attenuation 0.3 dB/Km
• It has been implemented
singlemode fiber
Fourth Generation
• Operating wavelength of 1550 nm.
• Data rate of 2 Gbps over a distance of
1330 Km
• Attenuation is less than of 0.3 dB/Km
• It is also used singlemode fiber.
Ray Theory
Transmission
Propagation of light in an
optical Fiber
Mechanism
the ray of light enters at one end of the fiber, it normally propagates along the length of
•a) When
•
the fiber and comes out at the outer end of the fiber.
During the propagation some loss may be occurred because of the small fraction leakage
through the side walls.
5
•
•
As the light wave enters the fiber at one end with an angel to the axis of the fiber usually it
follows a Zig-Zag path due to series of reflections by the inside surface of the fiber.
The propagation of light through the fiber obeys the laws of reflection and refraction of the
light waves. The confining of the light wave inside the fiber is due to the result of the total
internal reflection of the light waves by the inside surface of the fiber.
b)
Conditions
Reflection and
Refraction
6
•
•
When light ray propagates from an optically more denser medium to the less denser medium,
the refracted ray is moved away from the normal which is known as internal reflection.
The bending of the light ray at the interference is due to the difference in the speed of light in two
different materials.
7
8
Acceptance
Angle
Acceptance angel usually describes the light gathering capacity or light acceptance ability of the
fiber structure. It is also can be defined as the maximum angel at which the light may enter to the
fiber in order to propagate along the fiber core.
So that, it is said that the critical angel is the minimum angel of incidence to achieve total internal
reflection, whereas the acceptance angel is the maximum angel of incidence to gather maximum
light to propagate along the fiber.
9
•
•
•
•
Numerical
Aperture
Numerical aperture (NA) describes the ability of the fiber to capture the light. It is also define the
acceptance cone of the optical fiber.
It is used to measure the fiber coupling efficiencies between the source and fiber structure.
For shorter distance typical range of NA is 0.4 to 0.5.
For long distance communication NA is 0.1 to 0.3.
10
Optical Fiber Modes and
Configuration
11
12
13
Subject – Optical
Communication
Unit-2
Absorpti
on
Absorption is usually caused by 3 different
mechanisms such as
1. Atomic defects in the composition
2. Extrinsic absorption by impurity atoms
3. Intrinsic absorption by basic constituent atom of the fiber material
Atomic Defects
• Atomic defect is the imperfections in the atomic structure of the material. For example missing of
molecule, high density cluster etc.
• Usually the effect due to the atomic defects is very negligible in comparison to extrinsic and
intrinsic absorption.
• It is very significant when the fiber is exposed to ionizing radiation.
Intrinsic Absorption
• Intrinsic absorption is occurred in the material when it is in perfect state with no density variation,
impurities, material inhomogeneities and so on.
• Intrinsic absorption is considered as the lower limit absorption of a fiber material.
• Absorption occurs when a photon interacts with the electron in the valence band and excites into a
higher energy level.
• Usually the absorption results from the electronic absorption band in ultraviolet region and from
the atomic vibration bands in the near infrared region.
• The electronic absorption bands are associated with the band gaps of the amorphous (non
crystalline material) glass material.
15
x is the mole fraction of
Geo2
UV loss is small as compare with the scattering loss in the near infrared region. For the
Infrared region the attenuation is given as
16
Scattering Loss
•
•
•
Scattering loss in fiber occurs due to different reasons such as variation in material density,
compositional fluctuations and structural indifferences.
These effects cause variation in refractive index occur within the glass. This variation in
refractive index causes Rayleigh scattering when light signal is launched into the fiber.
Attenuation due to scattering is given as,
17
18
•
Bending
Losswhen the fiber undergoes a bend.
Bending loss is usually occurred
•
Usually, there are two types of bending are occurred in the fiber.
•
Firstly, macroscopic bends having larger radii in comparison to fiber diameter.
•
Secondly random microscopic bends of the fiber axis that can occur when fibers are
connected into cables.
•
Bending loss is also known as the radiative loss. Number of modes supported by a curved
multimode fiber
Number of modes in a
straight fiber
19
20
Core and Cladding
Losses
21
Refractive index variation of the graded index fiber
is given as
Numerical aperture of the graded index fiber
is given as
Number of the modes supported by
the fiber
22
Q. Compute the microbend loss at operating wavelength 850 nm of a graded index fiber with
index profile 2 which is bent into a curve of radius 2 cm. The core refractive index is 1.45, core
diameter 85 micrometer, numerical aperture 0.21 and relative refractive index difference is
1.26%.
Q. The optical power after propagating through a fiber that is 450 m long is reduced to 30% of
its original value. Calculate the fiber loss in dB/Km.
Q. A fiber has 150 m length and is fed with an optical power of 10 micro-watt. The optical
power is found to be 8 micro-watt. Calculate the loss in dB/Km.
Q. A 100 km fiber is used in a communication system with a loss of 3.0 dB/Km. Determine the
output power when it is fed with 500 micro-watt in input.
23
•
Dispersion in Optical
Fiberdue to the effect of attenuation and broadens due to the
An optical signal weakens
distortion effects as it travels along a fiber. Distortion mainly occurs due to the
dispersion.
•
Due to dispersion, two adjacent optical pulses overlap with each other so that, the
receiver is not able to distinguish the adjacent pulses and error occurs at the
received signal.
24
•
Overlapping of two adjacent pulses usually occur due to pulse broadening or
spreading of the pulses. Dispersion is responsible for such broadening of the
pulses.
•
Dispersion limits the maximum possible bandwidth of the channel. Limiting
of the bandwidth also affect the channel capacity which means that more
data can’t be transmitted over the channel.
•
Multimode step index fiber usually having highest level of dispersion,
whereas multimode graded index fiber having improved performance
against the dispersion.
•
In case of a single mode optical fiber usually produces minimum pulse
broadening leads to less dispersion.
•
Different kinds of dispersions are such as Intermodal dispersion (Modal
delay), Intramodal dispersion or chromatic dispersion, polarization mode
dispersion and higher order distortion effects.
•
Distortions can be studied by analyzing the nature of the group velocities of
the guided modes. Group velocity is the speed at which energy in a certain
25
Intermodal
Dispersion
26
27
•
Pulse broadening is directly proportional to the relative refractive index
difference.
•
As the index difference decreases, numerical aperture decreases which
indicates that ability to launch optical power to the fiber decreases.
•
Here it is observed that pulse broadening also directly proportional to the
length of the fiber, so that at higher length of the fiber leads to increase the
broadening of pulse.
•
So that, at increase of the fiber length bandwidth is limited which is one of the
major limitation for long distance communication.
•
At higher order mode, more field will occur at cladding and less field will occur
at lower order mode for cladding region.
•
It means that, more power is concentrated at lower order mode and less power
at higher order mode.
28
•
As the signal propagates along the fiber, each spectral component can be
assumed to be travel independently produces a time delay or group delay per
unit length.
29
•
D indicates the dispersion which indicates
the pulse spread as a function of
wavelength.
30
Intramodal
Dispersion
31
Material
Dispersion
•
•
•
•
•
It causes due to the variation of the refractive index of the core material as a function of
wavelength.
Variation of the refractive index causes dependance of group velocities upon
wavelength of a given mode. So that pulse spreading occurs even when different
wavelength follows the same path.
Since the group velocity of a mode is a function of the refractive index, the various
spectral components of a given mode will propagate at different speeds depending upon
the wavelength.
Its effect is very significant in LED source, since it has more spectral width in
comparison to LASER source.
The modal propagation constant of the fiber is given as
32
Waveguide
Dispersion
Considering small values
of the index difference, b
can be expressed as
33
As we know that the normalized frequency parameter
is given as
34
The main reason for waveguide dispersion is that follows, as the light
pulse is launched into fiber it is distributed among many guided modes.
These various modes arrive at the fiber end at different times depending
on their group delay leads to result of the pulse spreading.
35
Polarization mode Dispersion
(PMD)
36
Fiber
Materials
Plastic Fibers
• Plastic fiber means the core of the fiber is made up of a plastic material. It is
usually manufactured from a polymer.
• Various important characteristics of a plastic materials are High light
gathering capacity and Larger core area.
• Low cost components like fiber, cables, data link and LEDs.
• Users visible LED, which means testing of the fiber is very easy.
• Easy for connections.
Type
Bandwidth (MHz)
Numerical
Aperture
Operating
Temperature
(Degree
centigrade)
SK or EH
50
0.5
-40 to 75
EK or EH
120
0.47
-40 to 85
DK or DH
50
0.54
-40 to 115
FH
100
0.75
-40 to 125
37
Major limitations of the plastic
fiber are
• Higher attenuation
• Less bandwidth
• Poor mechanical strength
• Low operating temperature.
Chalcogenide
glass
38
Halide
Fiber
• Halide fiber are intended for transmission of light from 3 to 15 micro-meter.
• These are having low losses are able to transmit light from high power, long
wave length lasers.
• These are very much flexible and much mor convenient.
• This fiber compromise a core poly crystalline silver halide surrounded by an
opaque tube.
• The tube is necessary to prevent UV light from reaching the core.
• Due to higher refractive index of the material, total internal reflection takes
place at the boundary with air.
Important characteristics
• Numerical aperture is less than that of 0.7.
• Outer diameter 0.36 – 1.5 mm.
• Attenuation at 10.6 micro meter is 0.5 – 1.5 dB/m.
• Usable wavelength range 3 – 15 micrometer.
• Maximum length is less than that of 15 m.
• Radius of elastic bending greater than that of 0.4 cm.
• Operating temperature is less than that of 100 degree centigrade.
39
Glass
Fiber
40
•
•
•
•
•
Active Glass
Fiber
Active glass fiber is produced by incorporating rare earth elements into a
normally passive glass elements.
Rare earth elements having the atomic number 57 – 71.
Active glass fiber exhibits new optical and magnetic properties.
These new properties allow the material to perform amplification,
attenuation and phase retardation on the light passing through it.
Active glass fibers are usually used for the design of optical amplifiers.
41
Subject – Optical
Communication
Unit-3
Power Launching and
Coupling
43
Radiance
•
•
•
Optical power that can be coupled into the fiber depends upon the radiance.
Radiance is the optical power radiated into a unit solid angel per unit emitting surface
area. It is usually expressed in Watts/square cm/steradian.
It is also considered as the spatial distribution of the optical power. It usually indicates
the brightness of the source.
Source output pattern
•
•
It is required to know the power accepting capability of the optical power.
The radiation pattern of the source is usually described by a spherical coordinate system.
44
The radiance of an edge emitting LED and laser
source the approximated radiance is given as
Where T and L are the transverse and lateral
distribution coefficients.
Power coupling calculation
Lambertian
pattern
Light source coupled to an
optical fiber
45
•
•
In this power calculation, first the radiance of an individual point source on the
emitting surface is integrated over the solid acceptance angel of the fiber which
is related to numerical aperture is given inside the square bracket.
Then the total coupled power is the sum of the each individual emitting point
source of incremental area and that is integrated over the emitting area.
46
Total power, that is emitted from the source into a
hemisphere
Final expression of the power for a step indexed fiber launched from
a LED source
47
Calculation of power coupled into a Graded
Index Fiber
For a graded index fiber, the numerical aperture (NA) is varying with the
radial distance r from the center of the core is given as
So that amount of the power coupling is can be computed by substituting the
NA in the expression of the power, then the coupled power can be expressed
as
48
49
50
Equilibrium NA is significant in long multimode fibers after
the launched modes have come to equilibrium. Usually, This
equilibrium is occur at 50 m.
Laser Diode to Fiber coupling
• For edge emitting laser diode have an emission pattern has a FWHM (Full
width at half maximum) of 30 to 50 degree in the plane perpendicular to the
active area junction
• FWHM of 5 – 10 degree in the plane parallel to the active area junction.
• Since the angular output distribution of the laser is greater than the fiber
acceptance angel and the laser emitting area is much smaller than the fiber
core can also be used to improve the coupling efficiency between edge
emitting laser and optical fiber.
51
The measured FWHM values of the laser output becomes
• Between 3 and 9 micrometer for the near field parallel to
the junction.
• Between 30 and 60 degree for the field perpendicular to
the junction.
• Between 15 to 55 degree for the field parallel to the
junction.
For these cases the coupling efficiency lies between 50 to 80 %
52
Fiber to Fiber Joints
Optical Fiber
Connectors
• Interconnection of fibers in a low loss manner is a significant
requirement in any fiber optic system installation.
• These interconnections occur at the optical source, at the detector and
at the intermediate points within a cable where two fibers are joined.
• It is also needed at intermediate points in a link where two cables are
connected.
• The techniques for the joining of fibers are of two types are 1. splice, 2.
connector.
• Splice is considered as a permanent bond, whereas the connector is
known as a demountable joint.
• Each of the techniques of joining two fibers is subjected to cause
various amount of power loss at the joint.
• These loss depends upon the different parameters such as 1. input
power distribution to the joint, 2. length of the fiber between the optical
source and joint, 3. fiber end face qualities.
53
54
Different Modal distributions of the
optical beam
(a
)
(b
)
55
• Usually, the loss depends upon the power distribution among the
modes in the fiber.
• Consider the first case in which all the propagating modes are
equally excited. So that the emerging optical beam fills the entire
exit numerical aperture of this emitting fiber.
• In this case the receiving optical fiber to accept all the optical
power emitted by the first fiber, there must be a perfect
mechanical alignment between two optical waveguides and their
geometrical and waveguide characteristics must be properly
matched.
• On the other hand if the steady state modal equilibrium has been
established in the emitting fiber, most of the energy is concentrated
in the lower order fiber modes. This means optical power is
concentrated near the center of the fiber core.
• In this case the emerging optical beam only fills the equilibrium
numerical aperture. Here the input NA is larger than the
equilibrium NA of the emitting fiber, slightly mechanical
misalignment and small variation in geometric characteristics do
not contribute significantly to joint loss.
56
Fiber
Splicing
• Fiber splicing is a permanent or semipermanent joint usually used to
establishing a long optical link where frequent connections and
disconnections are not needed.
• For establishing such splices, the important considerations are to be required
are 1. Geometrical differences in two fibers 2. Fiber misalignment at the joint
3. Mechanical strength of the splice.
Splicing Techniques
• Different splicing techniques are 1. Fusion splice 2. V-groove mechanical
splice 3. Elastic tube splice.
• Out of all the techniques, Fusion splicing technique is a permanent joint
whereas the other two methods are can be disassembled if necessary.
Fusion Splice
• At the beginning, the fiber ends are prealigned and butted together in this
method. This is done either in a grooved fiber holder or under a microscope
with micromanipulators.
• The butt joint is then heated with an electric arc or a laser pulse in order to
bonded together.
• This produce a very low splice losses less than that of 0.06 dB.
57
• In this method care must be taken because of 1. surface defect during
heating 2. Residual stresses induced near the joint during melting.
V-groove Mechanical Splicing
• In this technique, initially the prepared fiber grooves are butted together in
a V-shaped groove.
• Then they are bonded together with an adhesive or are held in place by
means of a cover plate.
• The V-shaped groove can be either a grooved silicon, plastic, ceramic or
metal substrate.
• The splice loss in this method depends upon 1. Fiber size 2. Eccentricity (the
position of the core relative to the center of the fiber).
Elastic-Tube splicing
• It splices the multimode fibers to provide the loss in the same range as
commercial fusion splices.
• It is basically a tube made of an elastic material. The central hole diameter
is slightly smaller than that of the fiber to be spliced and it is tapered on
each end for easy insertion.
• When a fiber is inserted it expands the hole diameter so that the elastic
material produces a symmetrical force on the fiber.
• This symmetry feature allows an accurate and automatic alignment of the
58
Fusion
splicing
V-groove mechanical
splicing
Elastic tube
splicing
59
Splicing Single-Mode
Fibers
The lateral offset misalignment presents the most serious loss in splicing of
singlemode fibers. The loss usually depends upon the shape of the propagating
mode. For Gaussian shaped beams the loss between identical fibers is
60
Optical Fiber Connectors
Principal requirement of the good fiber connectors are 1. Low coupling losses 2.
Interchangeability 3. Ease of assemble 4. Low environment sensitivity 5. Low
cost and reliable construction 6. Ease of connection.
Connector Types
• Different types of connectors are 1. Screw on 2. Twist on 3. Snap on. The most
commonly used connectors are the twist on and snap on design.
• These include both single channel and multichannel assemblies for cable to
cable and cable to circuit card connections.
• The basic coupling mechanisms for these connectors are 1. butt joint 2.
expanded beam classes.
Butt joint connectors
• It consists of a metal, ceramic or molded plastic ferrule for each fiber and a
precession sleeve into which the ferrule fit.
• The fiber is coated into a precession hole which has been drilled into the
ferrule.
• There are two popular butt joint alignment designs used in both multimode
and singlemode fiber systems given as 1. straight sleeve 2. tapered sleeve.
61
• In the straight sleeve connector the length of the sleeve and guide ring on the
ferrules determine the end separation of the fibers.
• The tapered sleeve or biconical connector uses a tapered sleeve to accept and
guide tapered ferrules. The sleeve length and the guide rings maintain a given
fiber end separation.
Expanded beam connector
• It employs lenses on the ends of the fibers. These lenses collimate the light
emerging from the transmitting fiber and focus the expanded beam onto the core
of the receiving fiber.
• The distance of the fiber to lens is equal to the focal length of the lens.
• Since the beam is collimated, separation of the fiber ends takes place within the
connector. So that the connector is less dependent on the lateral displacement.
• Another advantage of this connector is that, the different optical processing
elements like beam splitters and switches can be easily inserted into the
expanded beam between the fiber ends.
Butt
joint
Expanded
beam
62
Connector return loss
• A connection point on the optical link can be categorized into 4 interface types.
These are 1. Perpendicular 2. Angel end face 3. Direct physical contact 4. Index
matching material.
• Each of the methods have a basic application for which it is suited. For example
direct physical contact type connectors are useful where frequent reconnections
are required, such as within a building premises.
• Index matching connectors are suitable in outside cable plants where
reconnections are infrequent, but need to have low loss.
• In each case, these connectors require high return loss or low reflection levels
and low insertion loss.
• Without achieving low reflection levels or high return loss affects the optical
frequency response, the linewidth and internal noise of the laser which causes
degradation of system performance.
63
64
65
Subject – Optical
Communication
Unit-4
Introduction
Optical
Sources
Two principal light sources are used for the fiber optics communications are
• Heterojunction structured semiconductor laser diodes or injection laser diodes (ILD)
• Light emitting Diode (LED)
Heterojunction structure
• Heterojunction structure means two adjoining semiconductor materials having
different band gap energies.
These structure are useful or suitable for optical communication because
1. Having adequate amount of power for a wide range of application.
2. Optical output power can be directly modulated by varying the input current to the
device.
3. Having high efficiency.
4. The dimensional characteristics are very compactible with the optical fiber.
PN junction in LED and LASER
• Light emitting region of both LED and LASER consisting a PN junction is constructed
of direct band gap III-V semiconductor materials.
67
• When this junction is forward biased, the electron and holes are injected to the p
and n regions respectively.
• These injected minority charge carriers can recombine either radiatively or non
radiatively.
• At the time of recombine radiatively, recombination energy emitted and the time
of recombine non radiatively, recombination energy is dissipated inform of heat.
• So that the PN junction present in the sources is known as the active or
recombination region.
Major differences between LED and LASER
• The optical output from an LED is incoherent, whereas in case of the laser diode
the optical output is coherent.
• For the laser diode, there is an optical resonant cavity is exist. Due to the optical
resonant cavity the optical output from this source is highly monochromatic and
directional.
• For the case of an LED source, there is not any existence of optical resonant
cavity. The output radiation of the LED source has a broad spectral width.
• The spatially directed coherent optical power from the laser diode can be
coupled into either single or multimode fibers.
• The incoherent optical output power from the LED can be coupled with
68
multimode fibers.
• In spectral splicing a passive device such as a waveguide grating array is used to
split the broad spectral emission of the LED into narrow spectral splices.
Direct and Indirect Band gap materials
• Semiconductors are also classified into two categories 1. Direct band gap
materials 2. Indirect band gap materials
• This classification basically depend upon the shape of the band gap as a function
of the momentum k.
• In a recombination process where the electron and hole have the same
momentum value is known as the direct band gap material.
• In case of a indirect band gap material, the conduction band having the
minimum energy levels, whereas the valence band having the maximum energy
levels. The electron and holes having the different values of the momentum.
Light Emitting Diodes (LEDs)
Why LEDs
• Usually the optical communication requires a bit rate of less than approximately
100 – 200 Mbps together with multimode fiber coupled optical power in tens of
microwatts.
• For the above purpose LED is a suitable choice and it also requires less complex
circuitry. Along with this it doesn’t require any thermal or optical stabilization
circuit.
69
• It also can be fabricated less expensively than the laser circuits.
Desirable characteristics of LED
• To be suitable for the fiber optics transmission LED must have
1. High radiance output
2. Fast emission response time
3. High quantum efficiency
• Radiance is the brightness is a measure in watts of the optical power in a unit of solid
angel per unit area of the emitting surface.
• Emission response time is the time delay between the application of a current pulse
and the onset of the optical emission.
How to Achieve high radiance and quantum efficiency
• LED structure must confine the charge carriers to the active region of the pn junction.
• LED structure must stimulate the optical emission to the active region of the pn
junction.
• In the pn junction radiative recombination takes place.
• Carrier confinement is used to high level of radiative recombination to achieve high
quantum efficiency.
• Stimulate optical emission is used to prevent the absorption of the emitted radiation
by the material.
70
LED Configuration
There are basically two types of LED configuration are
available such as
1. Surface emitting LED (Also known as Burrus or front
emitters)
2. Edge emitting LED
Surface emitting
LED
71
Edge emitting
LED
72
73
Quantum Efficiency and LED
Power
74
75
76
77
Only light falling within a core defined by the critical angel will be emitted
from the source
Modulation of an LED
• Response time or frequency response of an optical source indicates how fast
an electrical input drive the signal to vary the output light level.
• Response time mainly depends upon the 3 factors 1. Doping level in the
active region 2. Injected carrier life time in the recombination region.
• Parasitic capacitance of the LED.
78
79
LASER
DIODES
Basic working principles of the Laser
Diodes
80
• In fiber optics, the laser diodes are usually made of semiconductor
diodes. The output radiations from the laser are highly
monochromatic and directional.
• Basic working principle of the laser diode is based upon 3 process
are known as 1. Photon absorption 2. Spontaneous emission 3.
Stimulated emission.
• As per the Plank’s law, the transition between two states occurs due
to absorption or emission of a photon (energy).
• Normally the system is in ground state. When a photon of energy
impinges, the electron in the ground state absorbs energy and
excited to the excited state.
• After reaching at the excited state due to absorption, the electron is
again return back to the ground state due to emission of a photon of
energy. This is happened only due to instability.
• Since this kind of emission is occurred without application of any
external stimulation is known as spontaneous emission.
• The spontaneous emission is basically isotropic, random in phase
and have a narrowband gaussian shape.
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• The stimulated emission is achieved by an external stimulation.
• In this process, transition of electron from excited state to
ground state is achieved with induction of external electrons.
• Here the nature of the emission is in phase not in random as in
the case of spontaneous emission.
• The resultant emission is known as the stimulated emission.
Population Inversion
•
It is the condition to achieve the stimulated emission. It is
achieved when the population of the excited state is greater than
the population of the ground state.
• Population inversion is not in equilibrium condition and it is
achieved by different pumping techniques.
• In a semiconductor laser, population inversion is achieved by
injecting electrons into the materials at the device.
82
LASER
Diodes
Fabry – Perot resonator cavity for a
laser Diode
83
Two parallel light reflecting
mirrored surfaces define a
Fabry-Perot resonator cavity
• The construction of the laser diodes is more
complicated because of the requirement of
confining current in a small lasing cavity.
• One type of cavity used to generate
radiation is known as Fabry-Perot cavity.
• It has a longitudinal length of 250 – 500
micrometer, lateral length of 5 – 15
micrometer wide and transverse length of
0.1 – 0.2 micrometer thickness.
• The cavity encloses two parallel partially
reflecting mirrors are directed towards each
other.
• The main purpose of these mirrors are to
establish a strong optical feedback in the
longitudinal direction.
• The feedback mechanism convert it into an
oscillator which produces a certain
resonant frequency.
84
• The sides of the cavity are formed by roughing the edges of the
device to reduce unwanted emission in the lateral direction.
• The frequency at which constructive interference occurs are called
as the optical resonant frequency of the cavity.
• The resonant wavelength are called as the longitudinal modes of
the cavity, since they resonate along the length of the cavity.
Distributed Feedback laser
diode
85
• In this structure, the cleaved facets are not required for the optical
feedback. The fabrication is very similar to that of Fabry-Perot cavity
expect the lasing action.
• The lasing action in this structure is obtained from the grating
reflectors or periodic variations of the refractive index.
• This is known as the principle of distributed feedback corrugations
which are employed into a multilayer structure along the length of the
diode.
• In this configuration, dielectric reflector can implemented to reduce 1.
optical loss in the cavity 2. reduce the threshold current density 3.
Increase the external quantum efficiency.
• Threshold current density is the point at which lasing starts towards
the given optical fiber.
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Avalanche Photodiode
(APD)
Basic function of APD
• APD internally multiply (increases) the incoming photocurrent before it
enters to the input of the amplifier present in the receiver circuit.
• Due to the multiplication of the photocurrent before the amplification, it
increases the sensitivity of the receiver at presence of the thermal noise.
Impact Ionization
• It is a process is used to achieve carrier multiplication.
• In a high electric field region, a photogenerated charge carrier can gain
enough energy to ionize the electrons in the valence band by colliding with
them.
• This type of carrier multiplication mechanism is known as the impact
ionization.
Avalanche Effect
• The newly created carriers due to the impact ionization gains more energy
due to the acceleration of high electric field causes more impact ionization is
known as the avalanche effect.
• Below the breakdown voltage finite numbers of carriers are generated
whereas at above the breakdown voltage infinite numbers of charge carriers 87
Structu
re
• It describes a reach through avalanche
photodiode (RAPD).
• p+ is the heavily dopped p-type layer.
• n+ is the heavily dopped n-type layer.
• p region is a highly resistive region.
• Pi layer is basically an intrinsic region.
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Reach through condition
• It is a certain voltage at which the peak of the electric field at the pn+
junction is about 5 – 10 % below that is required to cause the avalanche
breakdown.
• At this point the depletion layer reaches through to the nearly intrinsic pi
region.
Operation of APD
• Normally RAPD is operated in fully depleted mode.
• The pi region is the collection region of the photogenerated carriers. When
light enters to the p+ region and it absorbed in the pi region.
• As it absorbs, photon gives up the energy and leading to create electronhole pairs. Then these pairs are separated by the electric field present in
the pi region.
• The photogenerated electrons drift through pi region in the pn+ region
where a high electric field exists. In this high electric field region carrier
multiplication takes place.
Ionization Rate
• The average numbers of electron – hole pairs are created by the carrier per
unit distance travelled is known as the ionization rate.
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Subject – Optical
Communication
Unit-5
Optical
Receiver
Basic function
• The optical receiver first converting the optical energy into an electrical signal
and then amplifying it to a large level so that it can be processed successfully.
• The different components present in an optical receiver are 1. A photodetector
2. An amplifier 3. A signal processing system.
Operation of the receiver
1. Digital Signal Transmission
• The block diagram presents the shape of the digital pulses at different points
along the optical link.
• The transmitted signal is consists of a two level binary pulses (digital signal)
either 0 or 1. The duration of the pulses are Tb. The pulse duration is also
known as the duration of a bit or bit period. The logic 1 represents a +V volt
and the logic 0 represents a no pulse or zero volt.
• The optical transmitter LED or Laser is there to convert the electrical signal
into light signal. This is done by modulating the light source drive current with
variation of optical power p(t).
• The output of the transmitter is a varying optical signal. Here the logic 1
represents the pulse of optical power of duration Tb and logic 0 represents
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An optical data
link
94
• The output of the optical transmitter is given to the optical fiber. As it
propagates along the fiber waveguide, the signal becomes attenuated
and distorted.
• As it propagated through the optical fiber it is given to a PIN or APD
detector circuit to convert into the electrical signal from the optical
signal. The PIN or APD is the first element of the optical receiver. The
output current from the detector is very weak.
• In order to boost the signal to a certain level is amplified through a front
end amplifier and it is passed through a low pass filter to reduce the
noise that is occurred at the outside of the signal bandwidth. So that the
low pass filter defines the bandwidth of the receiver circuit.
• In order to minimize the effect of the inter symbol interference, the
filter can reshape the pulses that have distorted through the fiber
during transmission. Reshaping the pulses to minimize the ISI effect is
can be achieved through equalizing or cancels the pulse spreading
effects.
• Finally, a decision circuit or pulse regenerator is used. The decision
circuit samples the signal level at the mid point of each time slot and
compares it within a certain references voltage is known is known as
threshold level.
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• To ensure the interpretation of the bits correctly, a clock signal is used
whose period is the time period of the pulses equal to the bit duration.
This is known as the clock recovery or time recovery operation.
Error Sources
• Error occurred in the reception process in the receiving system due to
mainly the noise.
• Noise is the unwanted component which interfere with the signal
internally and disturb the transmission and processing of the signal in a
system. It is very difficult to handle because it varies randomly.
• Noise occurs internally as well as externally in a given communication
system. Usually the internal noise is taken only for the consideration.
• The internal noise is generated due to the spontaneous fluctuations of
the current or voltage in the electric circuits. For example shot noise and
thermal noise.
• Shot noise arises in the electronic devices because of the discrete nature
of the current flow into the device. Thermal noise arises from the
random motion of the electrons in a conductor.
• Dark current and leakage current are responsible for the additional
photodetector noise.
• Thermal noise is generated from the detector load resistor and amplifier
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Basic sections of the optical
receiver
97
98
• Because of this thermal noise is also increased, but it also
can be tolerated.
High impedance
amplifier
Spreading of
pulse
Transimpedance
amplifier
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Attenuation
Measurement
• Attenuation results in a waveguide due to absorption, scattering and
waveguide effects.
• There are three basic methods are used in optical fiber to measure attenuation
such as a. Cutback technique b. Insertion loss method c. OTDR technique.
Cutback Technique
• It is a method which requires to access both the ends of the fiber.
Measurements may be made at one or more wavelength or over a range of
wavelength.
• To measure the transmission loss, the optical power is first measured at the
output (Far end) of the fiber.
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101
Cable attenuation
measurement
102
103
Dispersion
Measurement
Time Domain intermodal dispersion
measurements
• For the measurement of dispersion in time domain, it is required to inject a
narrow optical pulse into one end and detect broadened pulse at the other end.
• The output of the laser sources are coupled through a mode scrambler into a
test fiber. Then the output of the fiber is measured with a sampling oscilloscope.
The oscilloscope is already built in the receiver.
• The shape of the input pulse is measured in the same way by replacing the test
fiber with a short reference fiber. The length of the fiber is just less than 1
percent of the test fiber length.
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Frequency Domain intermodal dispersion
measurements
105
106
Chromatic Dispersion Measurement
set up
107
Polarization mode dispersion
measurement
• This technique is known as the fixed analyzer method. In this method, the mean
differential group delay is evaluated statistically in the optical signal power
through a polarizer and scanned as a function of wavelength.
• Then it is processed through a test fiber and polarizer and given to a optical
analyzer. It shows the transmitted power level as a function of wavelength.
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Eye Diagram
Tests
109