Optical Fiber

Basic Structure of an Optical Fiber
 An optical fiber is a flexible, transparent fiber made by glass (silica) or plastic
to a diameter slightly thicker than that of a human hair.
 Optical fiber is a dielectric waveguide and ideally has a cylindrical shape.
 It consists of a core made up of a dielectric material which is surrounded by a
cladding made up of a dielectric material of lower refractive index than core.

Principles of Light Transmission in a Fiber
 Fiber optics deals with the transmission of light energy
through transparent fibers.
 How an optical fiber guides light depends on the nature of
light and structure of the optical fiber.
 A light wave is a form of energy that is moved by wave
motion.
 In fiber optics, wave motion is the movement of light
energy through an optical fiber.

Properties of Light
 When light waves strike an object, some of the waves
are absorbed by the object, some are reflected by it,
and some might pass through it.
 When light strikes an object it is:
1) Reflected
2) Transmitted
3) Absorbed

Properties of Light
 What happens to the light depends on the material
 Transparent (clear) materials- transmit light.
 Translucent (see through) materials- scatters the
transmitted light.
 Opaque (not see through) materials- absorbs and
reflects.

Properties of Light
Transparent (clear) materials--
 Those materials that transmit almost all the light waves falling upon
them are said to be transparent materials.
 you can clearly see other objects through materials such as glass and
clear plastic that allow nearly all the light that strikes them to pass
through.

Properties of Light
Translucent (see through) materials--
 The materials through which some light rays can pass but the objects can not
seen clearly because the rays are diffused, are known as translucent materials.
 Although objects behind these materials are visible, they are not clear.
 A frosted glass or a piece of oiled paper are the examples of translucent
materials.

Properties of Light
Opaque (not see through) materials--
 Those materials that are unable to transmit light waves falling upon
them are said Opaque materials.
 You cannot see other objects through opaque materials.
 Example: walls of a room etc.

Properties of LightReflection--
 Reflected waves are those waves that are neither transmitted nor absorbed but
are reflected from the surface of the medium.
 When a wave approaches a reflecting surface such as a mirror, the wave that
strikes the surface is called incident wave and the wave that bounces back is
called the reflected wave.
The amount of incident energy that is reflected from a surface depend on:
 The nature of the surface
 The angle at which the wave strikes the surface

Properties of LightRefraction—
 When a light wave passes from one medium to another medium having
different velocity of propagation, a change in the direction of the wave will
occur.
 This change of direction as the wave enters the second medium is called
refraction.
 Refraction- bending of light waves due to a change in speed
 Lens, curved glass or transparent material that Refracts light

Properties of LightDiffusion—
 When light wave is reflected from a piece of white paper, the reflected
beam is scattered or diffused.
 Since the surface of paper is not smooth, the reflected light is broken up
into many light beams that are reflected in all directions.

Properties of LightAbsorption—
 A light wave is reflected and diffused from a piece of white paper.
 But if the light beam falls upon a piece of black paper, the black paper
absorbs most of the light and very small amount of light is reflected
from the paper.
 If the surface upon which the light beam falls is perfectly black, then
there is no reflection; the light is totally absorbed.

Ray Theory Transmission
Ray Optics: basic laws
 Light rays in homogeneous media are straight lines
 Law of Reflection – Reflection from a mirror or at the boundary
betweem two media of different refractive index: the angle of reflection
equals to the angle of incidence i.e. qr = qi
 Snell’s law of Refraction – At the boundary between two media of
different refractive index n the angle of refraction qt is related to the
angle of incidence qi by
ni sin θi = nt sin θt

Refractive index
 The index of refraction of a material is the ratio of the speed
of light in a vaccum to the speed of light in the material
n = c/v
The factor n is the index of refraction (or refractive index) of
the medium.
For air and gases n ~ 1. At optic frequencies, the refractive
index of water is 1.33.
 Glass has many compositions, each with a slightly different
n. An approximate refractive index of 1.5 is representative
for the silica glasses used in fibers; more precise values for
these glasses lie between ~1.45 and ~1.48.

Refractive Index for Some Materials
 Air -----------------------------------------------------------------------1.0
 Water -------------------------------------------------------------------1.33
 Magnesium fluoride -------------------------------------------------1.38
 Fused silica (SiO2)----------------------------------------------------1.46
 Sapphire (Al2O3)------------------------------------------------------1.8
 Lithium niobate (LiNbO3)-------------------------------------------2.25
 Indium phosphide (InP)----------------------------------------------3.21
 Gallium arsenide (GaAs) ---------------------------------------------3.35
 Silicon (Si)---------------------------------------------------------------3.48
 Indium gallium arsenide phosphide (InGaAsP)-----------------3.51
 Aluminum gallium arsenide (AlGaAs)-----------------------------3.6
 Germanium (Ge) -------------------------------------------------------4.0
The index varies with a number of parameters, such as wavelength and
temperature.

Critical Angle (qc)
 The angle at which total internal reflection occurs is called the critical angle of
incidence.
 At any angle of incidence (q1) greater than the critical angle, light is totally
reflected back to the glass medium.
 For n1 > n2, the angle of refraction q2 is always greater than the angle of
incidence q1.
 When the angle of refraction q2 is 90o the refracted ray emerges parallel to the
interface between the media.
The critical angle is determined
by using Snell’s Law. The critical
angle is given by :

Total internal reflection
 At angles of incidence q1 > qc the light is totally reflected back into the
incidence higher refractive index medium. This is known as total
internal reflection.

Acceptance Angle
 The acceptance angle of an optical fiber is defined as the maximum angle of a
ray (against the fiber axis) hitting the fiber core which allows the incident light to
be guided by the core.
 The sine of that acceptable angle is called the numerical aperture, and it is
essentially determined by the refractive index contrast between core and cladding
of the fiber, assuming that the incident beam comes from air or vacuum.

Numerical Aperture
 The numerical aperture is a measurement of the ability of an optical fiber to
capture light. The NA is also used to define the acceptance cone of an optical
fiber. Mathematically it is defined as:
Where, n1 is the refractive index of core and n2 is refractive index of cladding.

Geometrical Optics Description
Optical fibers based on modes or mode types :
 Mode is the one which describes the nature of propagation of
electromagnetic waves in a wave guide.
 it is the allowed direction whose associated angles satisfy the
conditions for total internal reflection and constructive
interference.
 Based on the number of modes that propagates through the
optical fiber, they are classified as:
 Single Mode fibers can propagate only the fundamental mode.
 Multimode fibers can propagate hundreds of modes.

Single mode fibers:
In a fiber, if only one mode is transmitted through it, then it is said to be a
single mode fiber.
A typical single mode fiber may have a core radius of 3 μm and a
numerical aperture of 0.1 at a wavelength of 0.8 μm.

Characteristics of Single Mode Fiber
 The single mode fiber has the following characteristics:
 Only one path is available.
 Core diameter is small
 No dispersion
 Higher band width (1000 MHz)
 Used for long haul communication
 Fabrication is difficult and costly

Multi mode fibers :
 If more than one mode is transmitted through optical fiber, then it is
said to be a multimode fiber.
 The larger core radius of multimode fibers make it easier to launch
optical power into the fiber and facilitate the end to end connection of
similar powers.

Characteristics of Multimode Fiber
The Multimode fibers has the following characteristics:
 More than one path is available
 Core diameter is higher
 Higher dispersion
 Lower bandwidth (50MHz)
 Used for short distance communication
 Fabrication is less difficult and not costly

Types of Optical Fibers
Optical fibers based on refractive index profile :
Based on the refractive index profile of the core and cladding,
the optical fibers are classified into two types:
 Step index fiber
 Graded index fiber

Step index fiber :
 In a step index fiber, the refractive index changes in a step fashion,
from the centre of the fiber, the core, to the outer shell, the cladding.
 It is high in the core and lower in the cladding. The light in the fiber
propagates by bouncing back and forth from core-cladding interface.
 The step index fibers propagate both single and multimode signals
within the fiber core.
 The light rays propagating through it are in the form of meridinal
rays which will cross the fiber core axis during every reflection at
the core – cladding boundary and are propagating in a zig – zag
manner.

Step Index Single Mode & Multimode Fibers

Graded index fibers :
 In graded index fibers, the refractive index of the core varies
gradually as a function of radial distance from the fiber center.
 The refractive index of the core decreases as we move away from
the centre.
 The refractive index of the core is made to vary in the form of
parabolic manner such that the maximum refractive index is
present at the centre of the core.

Different Structures of Optical Fiber

Electromagnetic Optics
 Electromagnetic radiation propagates in the form of two mutually
coupled vector waves, an electric field wave & a magnetic field wave.
Both are vector functions of position & time.
 In a source-free, linear, homogeneous, isotropic & non-dispersive
media, such as free space, these electric & magnetic fields satisfy the
following partial differential equations, known as Maxwell’ equations:
0
0








H
E
t
H
E
t
E
H









Electromagnetic Optics
 In Maxwell’s equations, E is the electric field expressed in [V/m], H is
the magnetic field expressed in [A/m].
 The solution of Maxwell’s equations in free space, through the wave
equation, can be easily obtained for monochromatic electromagnetic
wave. All electric & magnetic fields are harmonic functions of time of the
same frequency. Electric & magnetic fields are perpendicular to each other
& both perpendicular to the direction of propagation, k, known as
transverse wave (TEM). E, H & k form a set of orthogonal vectors.
typermeabiliMagnetic:[H/m]
typermittiviElectric:[F/m]


operationcurlis:
operationdivergenceis:



Electromagnetic Plane wave in Free space
Ex
z
Direction of propagation
By
z
x
y
k
An Electromagnetic wave is a travelling wave which has
time varying electric and magnetic fields which are
perpendicular to each other and the direction of
propagation Z.

TE modes and Electric fields distribution

Dispersion Effect in Optical Fiber
 In communication, dispersion is used to describe any process by which any
electromagnetic signal propagating in a physical medium is degraded because the
various wave characteristics (i.e., frequencies) of the signal have different
propagation velocities within the physical medium.
 The dispersion cause that optical pulses to broaden as they travel along a fiber, the
overlap between neighboring pulses, creating errors in the receiver output,
resulting in the limitation of information-carrying capacity of a fiber.

Dispersion and Bit Rate
 The higher dispersion the longer the bit interval which
must be used
 A longer the bit interval means fewer bits can be
transmitted per unit of time
 A longer bit interval means a lower bit rate

Types of Dispersion
 Intermodal dispersion: Different modes propagate at different
group velocities.
 Intramodal or Chromatic Dispersion
 Material dispersion: The index of refraction of the medium
changes with wavelength.
 Waveguide dispersion: The index change across waveguide
means that different wavelengths have different delays.
 Polarization mode dispersion: If waveguide is birefringent.
Birefringent is a optical property of a material having a refractive
index that depends on the polarization and propagation direction
of light.

Intermodal Dispersion
 In a multimode fiber different modes travel at different velocities.
 If a pulse is constituted from different modes then intermodal dispersion
occurs.
 Modal dispersion is greatest in multimode step index fibers.
 The more modes the greater the modal dispersion.
 Typical bandwidth of a step index fiber may be as low as 10 MHz over 1 km.

Intramodal or Chromatic Dispersion
 Intramodal or Chromatic dispersion (CD) is caused by the fact that single mode glass fibers
transmit light of different wavelengths at different speeds. The ratio of the speed of light in a
medium to the speed in a vacuum defines the index of refraction or refractive index of the
material.
 Material Dispersion
 This is due to intrinsic properties of the material, glass.
 Glass is a dispersive medium. We can recall from our high school physics that glass has
different refractive index for different colors.
 Different colors (wavelengths) have different velocity in glass.
 A type of dispersion that occurs in optical fiber due to the interaction of various wavelengths
with the physical matter in the crystalline structure of the glass.
 The refractive index of the glass varies according to the wavelength of the optical signal.
 Material dispersion is the phenomena whereby materials cause a “bundle” of light to spread
out as it propagates.

Intramodal or Chromatic Dispersion
 Waveguide Dispersion
 This is due dispersive nature of the bound medium. In a bound medium like the
optical fiber, the velocity is a function of frequency.
 Waveguide dispersion is chromatic dispersion which arises from waveguide
effects: the dispersive phase shifts for a wave in a waveguide differ from those
which the wave would experience in a homogeneous medium. Waveguide
dispersion is important in waveguides with small effective mode areas. But for
fibers with large mode areas, waveguide dispersion is normally negligible, and
material dispersion is dominant.

Polarization mode dispersion:
 The polarization mode dispersion is due unequal velocities of two
orthogonal states of polarization.
 The PMD puts the ultimate restriction on the data rate on the long haul
single mode optical fiber.
 The pulse slowly broadens due to the statistical fluctuation of the velocities
of the two orthogonal polarizations.

Optical Fiber Losses
Attenuation in Optical Fibers
 Attenuation limits the optical power which can reach the receiver, limiting the
operating span of a system.
 Once the power of an optical pulse is reduced to a point where the receiver is unable
to detect the pulse, an error occurs.
 Attenuation is mainly a result of:
 Light Absorption
 Scattering of light
 Bending losses
 Attenuation is defined as the ratio of optical input power (Pi) to the optical output
power (Po).
 The following equation defines signal attenuation as a unit of length :
Attenuation

 Types of Attenuation
Absorption Loss:
 Caused by the fiber itself or by impurities in the fiber, such
as water and metals.
Scattering Loss:
 Intrinsic loss mechanism caused by the interaction of
photons with the glass itself.
Bending loss:
 Loss induced by physical stress on the fiber.

Material Absorption Losses
 Material absorption is caused by absorption of photons within the fiber.
 – When a material is illuminated, photons can make the valence electrons
of an atom transition to higher energy levels
 – Photon is destroyed, and the radiant energy is transformed into electric
potential energy. This energy can then
 • Be re-emitted (scattering)
 • Frees the electron (photoelectric effects) (not in fibers)
 • Dissipated to the rest of the material (transformed into heat)
 In an optical fiber Material Absorption is the optical power that is effectively
converted to heat dissipation within the fiber.
 • Two types of absorption exist:
 – Intrinsic Absorption, caused by interaction with one or more of the
components of the glass.
 – Extrinsic Absorption, caused by impurities within the glass.

Material Absorption Losses
 Intrinsic Absorption is caused by basic fiber material properties. If an
optical fiber is absolutely pure, with no imperfections or impurities, ten all
absorption will be intrinsic. Intrinsic absorption in the ultraviolet region is
caused by electronic absorption bands. Intrinsic Absorption occurs when a
light particle (photon) interacts with an electron and excites it to a higher
energy level.
 Extrinsic Absorption is caused by impurities caused by impurities
introduced into the fiber material. The metal impurities such as iron, nickel
and chromium are introduced into the fiber during fabrication. Extrinsic
Absorption is caused by the electronic transition of these metal ions from
one energy level to another energy level.

Fiber Bend Losses
Bending loss is classified according to the bend radius of curvature :
1. Microbend Loss 2. Macrobend Loss
 Microbend Loss are caused by small discontinuities or imperfections in the
fiber. Uneven coating applications and improper cabling procedure increases
micro bend loss. External forces are also a source of micro bends.

Fiber Bend Losses
Bending loss is classified according to the bend radius of curvature :
1. Microbend Loss 2. Macrobend Loss
 Macrobend Losses are observed when a fiber bend’s radius of curvature is
large compared to the fiber diameter. These bends are a great source of loss
when the radius of curvature is less than several centimeters.

Linear Scattering Losses
 Light scattering is a form of scattering in which light in the form of propagating
energy is scattered.
 Light scattering can be thought of as the deflection of a ray from a straight path, for
example by irregularities in the propagation medium, particles, or in the interface
between two media.
 Deviations from the law of reflection due to irregularities on a surface are also
usually considered to be a form of scattering.
 When these irregularities are considered to be random and dense enough that their
individual effects average out, this kind of scattered reflection is commonly referred
to as diffuse reflection.
Linear Scattering may be of two types
 Rayleigh Scattering
 Mie Scattering

Rayleigh Scattering
 The scattering losses are caused by the interaction of light with density fluctuations
within a fiber.
 Density changes are produced when optical fibers are manufactured.
 During manufacturing, regions of higher and lower molecular density areas, relative to
the average density of the fiber, are created.
 Light travelling through the fiber interacts with the density areas then partially scattered
in all directions.
 In commercial Fibers operating 700nm and 1600nm wavelength, the main source of loss
is called Rayleigh Scattering (named after the British physicist Lord Rayleigh).
 Rayleigh Scattering is the main loss mechanism between the ultraviolet and infrared
regions.
 Rayleigh scattering occurs when the size of density fluctuations (Fiber defect) is less
than one-tenth of the operating wavelength of light.
 As the wavelength increases, the loss caused by Rayleigh Scattering decreases.

Mie Scattering
 If the size of the defect is greater than one-tenth of the wavelength of light, the scattering
mechanism is called Mie Scattering (named after Gustav Mie).
 It is caused by these large defects in the fiber core, scatters light out of the fiber core.
 However, in commercial fibers, the defects of Mie Scattering are insignificant.
 Optical fibers are manufactured with less defects.
 Linear scattering may also occur at inhomogeneties and they are comparable in size to
the guided wavelength. This type of scattering is because of fiber imperfections such as:
 Irregularities in the core-cladding interface.
 Core-cladding refractive index differences along the fiber.
 Diameter fluctuation.
 Stains and bubbles.
 Scattering intensity can be very large if the scattering inhomogeneties size is greater
than one-tenth of the operating wavelength of the light. Such inhomogeneties creates
scattering in forward direction and is known as Mie Scattering.

Nonlinear Optical Effects
 Optical waveguides do not always behave as linear channels where optical output
power is equal to optical input power.
 Several nonlinear effects occurs which causes scattering.
 Nonlinear Scattering is the transfer of optical power from one mode to be
transferred in either the forward or backward direction or other modes at
different frequency.
 The types of nonlinearities are:
1. Stimulated Raman Scattering
2. Stimulated Brillouin Scattering
3. Self Phase Modulation
4. Cross Phase Modulation
5. Four Wave Mixing

Stimulated Raman Scattering
 In Stimulated Raman Scattering (SRS) a high frequency optical photon is generated.
 The Stimulated Raman Scattering (SRS) process is initiated by noise, thermally induced
fluctuations in the optical fields and active vibrational modes.
 An incident pump field (ωP) interacts with the vibrational fluctuations, losing a photon
which is down shifted in frequency by the vibrational frequency () to produce a Stokes
wave (ωS,) and also an optical phonon (quantum of vibrational energy ).
 The pump decays with propagation distance and both the phonon population and Stokes
wave grow together.
 If the generation rate of Stokes light exceeds the loss, stimulated emission occurs and the
Stokes beam grows exponentially. Threshold Power PR is given by:

Stimulated Brillouin Scattering
 In Stimulated Brillouin Scattering (SBS) a high frequency acoustic phonon is generated.
 The Stimulated Brillouin Scattering (SBS) is the modulation of light through thermal
molecular vibrations within the fiber.
 The scattered light appears as upper and lower sidebands which are separated from the
incident light by the modulation frequency.
 Stimulated Brillouin Scattering (SBS) is significant above a threshold power density.
Threshold power PB is given by:
 Where;
 d = Fiber core diameter
 l  Operating wavelength
 adB = Fiber Attenuation
 v = Source Bandwidth

The Kerr Effect
 The Kerr effect is due to the non-linear response of the material. It means that the
index of the silica is now depending on the optical field propagation through it.
 The power dependence of the refractive index is responsible for the Kerr-effect.
 Depending upon the type of input signal, the Kerr-nonlinearity has three different
effects such as Self-Phase Modulation (SPM), Cross-Phase Modulation (CPM) and
Four-Wave Mixing (FWM).









effA
P
nnn 2The nonlinearity in refractive
index is known as Kerr nonlinearity.
This nonlinearity produces a
carrier induced phase modulation of
the propagating signal, which is
called Kerr Effect.
Where:
n = Ordinary refractive index of material
n2 = Nonlinear index coefficient
P = Optical Power
Aeff = Effective mode area
The numerical value of n2=2.6 X10-20 m2/W

(55)
 Self Phase Modulation (SPM) : If an intensity modulated signal propagates
in the fibre, the intensity modulation induces an index modulation of the fibre
and in return a phase modulation to the signal.
 The signal modulates itself
 The SPM induced phase modulation broadens the signal spectrum.
Self-phase modulation (SPM) is a fiber nonlinearity caused by the
nonlinear index of refraction of glass. The index of refraction varies with
optical power level causing a frequency chirp which interacts with the fiber’s
dispersion to broaden the pulse.
Nonlinear Optical Effects due to Kerr Effect

SELF-PHASE MODULATION
Spectral broadening of the pulse
The non-linear phase follows exactly the power shape of the
optical pulses. The frequency chirp is then proportional to the
derivative of the optical power. If pulses propagate under the
non-linear regime :
 the optical frequency will decrease on the pulse leading edge.
 the optical frequency will increase on the pulse trailing edge.

 Cross Phase Modulation (XPM) : In the case of a
multi–channel propagation, the index modulation
induced by the Kerr–effect modulates the other
channels and vice-versa.

Cross Phase Modulation (XPM)
 In the case of multi-channel propagation at various wavelength,
the different channels modulate themselves via SPM but also
each other via the fibre index modulation.
 The efficiency of Cross Phase Modulation (XPM) depends on :
 The fibre chromatic dispersion
 Channel spacing
 Channel power
 XPM induces non-linear crosstalk.

 Four Wave Mixing (FWM) : In the case of a multi–channel
propagation and under phase matching conditions, new
frequencies are generated in the fibre causing crosstalk and
power depletion.

Four Wave Mixing (FWM)
 Under specific phase and wave vectors matching conditions, four
different waves will interact in the fibre in a non-linear way.
 The easiest way to obtain FWM in a fibre is to propagate two waves
at angular frequencies w1 and w2 that will create new waves at
frequencies w3 and w4 such as:
 The phase matching condition is :
 This phenomenon is strongly dependent on channel spacing and
chromatic dispersion.
 The generated waves may cause crosstalk if they are at the same
wavelength as incident channels.
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Some solutions for Kerr effect in fibres
 Decrease the field intensity by increasing the effective area. e.g.
by using G.652 or G.655 LEAF (Large Effective Area Fibre) fibres.
 In the case of single channel transmission, the increase of the
chromatic dispersion will automatically lower the SPM. But the
problem is reported to the chromatic dispersion compensation if
DCF is used as SPM may be high in such fibres.
 In the case of multi-channel transmission, the increase of the
channel spacing and /or chromatic dispersion will decrease XPM
effects.

Optical fibers consist of:
1. A core, having high
refractive index.
2. Cladding.
3. Buffer, protective polymer
layer.
4. Jacket, protective polymer
layer.
Manufacturing Optical Fibres

Two methods to manufacture optical glass fiber
1. Draw the fiber from molten glasses, which are placed in two concentric
crucibles (Direct melt methods)
2. Draw from a glass rod called preform (Vapor-phase oxidation process)
 Direct Melt Methods
-Optical fibers are made directly from the molten state of purified
components of silica glasses.
 Vapor –phase Oxidation Process
-Highly pure vapors of metal halides react with O2 to form white powder
of SiO2 particles.
-The particles are then collected on the surface of a bulk glass and are
sintered to form a glass rod.
-This rod or tube is called a preform.
-Typically 10-25mm dia and 60-120cm long.

 Vapor-phase oxidation process
1. Outside vapor phase oxidation
2. Vapor phase axial deposition
3. Modified chemical vapor deposition
 Direct Melt Methods
1. Drawing the fibre
2. Double Crucible Method
3. Rod-in-Tube method
Types of Manufacturing Optical Fibres

1. Drawing the fibre
The tip of the preform is heated to about
2000°C in a furnace. As the glass softens, a
thin strand of softened glass falls by gravity
and cools down.
The fibre diameter is constantly monitored
as it is drawn.
A plastic coating is then applied to the fibre,
before it touches any components. The coating
protects the fibre from dust and moisture.
The fibre is then wrapped around a spool.
Direct Melt Methods

2. Double crucible method
• The molten core glass is placed in the inner
crucible.
• The molten cladding glass is placed in the
outer crucible.
• The two glasses come together at the base of
the outer crucible and a fibre is drawn.
• Long fibres can be produced (providing you
don't let the content of the crucibles run dry!).
• Step-index fibres and graded-index fibres can
be drawn with this method.
Direct Melt Methods

3. Rod-in-Tube method
• A rod of core glass is placed inside a tube of
cladding glass. The end of this assembly is
heated; both glass are softened and a fibre is
drawn.
• Rod and tube are usually 1 m long. The core
rod has typically a 30 mm diameter. The core
glass and the cladding glass must have similar
softening temperatures.
• However, one must be very careful not to
introduce impurities between the core and the
cladding.
Direct Melt Methods

1. Outside Vapor Deposition (OVD)
 This process is also called the “soot process”.
 Halogens and O2 react in a hot flame to form hot glass soot, which is deposited
layer by layer on an aluminium oxide or graphite mandrel.
 The central mandrel is removed after deposition.
 In the last step, called sintering, a hollow porous preform is dehydrated and
collapsed in controlled atmosphere, (e. g. helium) to form desired preform.
Vapor-phase oxidation process

1. In VAD method, the preform can be fabricated
continuously.
2. Starting chemicals are carried from the bottom
into oxygen-hydrogen burner flame to produce glass
soot which is deposited on the end of a rotating silica
rod.
3. A porous preform is then grown in the axial
direction.
4. The starting rod is pulled upward and rotated in
the same way as that used to grow single crystals.
5. Finally the preform is dehydrated and vitrified in
ring heaters.
6. This process is preferred for the mass production.
2. Vapor phase axial deposition

3. Modified Chemical Vapor Deposition (MCVD)
 The gaseous mixture of reactants is fed at the end of a rotating silica tube.
 This tube is heated by a traversing oxygen-hydrogen burner.
 As a result of chemical reactions glass particles, called soot, are formed.
 These particles are deposited on internal wall of the tube.
 The soot is then vitrified by the traversing burner to provide a thin glass layer.
 The process is repeated many times as the cladding layers and core layers are formed.
 When the deposition is finished, the temperature of the burner is increased to collapse the tube
into a solid preform.
 The entire process is highly automated and all process parameters are precisely controlled.

Fiber Optic Connectors and Splices
Fiber Joints
Fibers must be joined when
You need more length than you can get on a single roll
Connecting distribution cable to backbone
Connecting to electronic source and transmitter
Repairing a broken cable
Splices v/s Connectors
A permanent join is a splice
Connectors are used at patch panels, and can be disconnected

Requirements of a Good Connector
1. At connector joint, it should offer low coupling losses.
2. Connectors of the same type must be compatible from one
manufacturer to another.
3. In the fiber link, the connector design should be simple so that it can
be easily installed.
4. Connector joint should not be affected by temperature, dust and
moisture. That is, it should have low environmental sensitivity.
5. It should be available at a lower cost and have a precision suitable to
the application.
Fiber Optic Connectors

Butt-Joint Alignment type connectors are:
Straight Sleeve
Tapered Sleeve
In the straight sleeve connector, there is a metal,
ceramic or molded plastic ferrule for each fiber and the
ferrule fits into the sleeve.
The fiber is epoxied into the drilled hole of the
ferrule.
In the straight sleeve connector or tapered sleeve
connector the length of the sleeve and a guide ring on
the ferrules determine the end separation of the fibers.
In the tapered sleeve connector, the ferrules and
sleeves are tapered.
Butt-Joint Alignment Mechanism
Straight Sleeve Mechanism
Tapered Sleeve Mechanism

The expanded beam connector employing collimating lens at the end of the
transmitting fiber and focusing lens at the entrance end of the receiving fiber.
The collimating lens converts the light from the fiber into a parallel beam of light
and the focusing lens converts the parallel beam of light into a focused beam of
light on to the core of the receiving fiber.
The fiber-to-lens distance is equal to the focal length of the lens.
The lenses are antireflection coated spherical micro lenses.
Expanded Beam Connector

Fiber Optic Splices
A Fiber Optic Splice is a permanent fiber joint whose purpose is to
establish an optical connection between two individual optical fibers.
There are two techniques used for fiber splicing:
Mechanical Splicing: A mechanical splice has Mechanical Fixtures
and materials that are used to fiber alignment and connection.
Fusion Splicing: A Fusion Splice is a fiber joint which is done by
Heat Fuses or by melting the ends of two optical fibers together.

Mechanical Splicing may involve the use of a glass or ceramic alignment tube.
The inner diameter of this glass or ceramic tube is slightly larger than the outer diameter of
the fiber.
A transparent adhesive is injected into tube and bonds the two fibers together.
The adhesive also provides index matching between the optical fibers.
This technique relies on the inner diameter of the fiber.
If the inner diameter is too large, splice loss will increase because of fiber misalignment.
If the inner diameter is too small, it is impossible to insert the fiber into the tube.
Glass or Ceramic Alignment Tube Splices
Mechanical Splicing

Mechanical Splicing
V-Grooved Splicing
V-Grooved Splicing may involve sandwiching the butted ends of two prepared
fibers between a V-Grooved substrate.
When inserting the fibers into the grooved substrate, the V-Groove aligns the
cladding surface of each fiber end.
A transparent adhesive makes the splice permanent by securing the fiber ends to the
grooved substrate.
Open V-Grooved Splice Spring V-Grooved Splice

In a Rotary Splice, the fibers are mounted into a glass ferrule and secured with
adhesives.
The fiber ends retain their original orientation and have added mechanical stability
since each fiber is mounted into a glass ferrule and alignment sleeve.
The Rotary splice may use index matching gel within the alignment sleeve to
produce low-loss splices.
Rotary Splices
Mechanical Splicing

The process of Fusion Splicing
involves using heat to melt or fuse the
ends of two optical fibers together.
The splicing process begins by
preparing each fiber end for fusion.
Fusion splicing requires that all
protective coatings be removed from
the ends of each fiber.
The basic fusion splicing apparatus
consists of two fixtures on which the
fibers are mounted and two
electrodes.
Fusion Splicing

Optical Loss
Intrinsic Loss
 Problems the splicer cannot fix
 Core diameter mismatch
 Concentricity of fiber core or connector
ferrules
 Core ellipticity
 Numerical Aperture mismatch

Optical Loss
Extrinsic Loss
 Problems the person doing the
splicing can avoid
 Misalignment
 Bad cleaves
 Air gaps
 Contamination: Dirt, dust, oil, etc.
 Reflectance

Optical Fiber

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