This article provides a rudimentary understanding of the basic concepts of optical fibres and optical fibre communication, the manufacturing techniques of optical fibres and different terms and terminologies related to optical fibres.

1. OPTOMETRY – Part XI
OPTICAL FIBRE
ER. FARUK BIN POYEN
DEPT. OF AEIE, UIT, BU, BURDWAN, WB, INDIA
FARUK.POYEN@GMAIL.COM

2. Contents:
1. Optical Fibre Communication System
2. Pros & Cons of Optical Fibres
3. Optical Fibre Light Source & Architecture
4. LED & ILD
5. Light Detectors – PIN & Avalanche PD
6. Optical Fibre Construction
7. Optical Fibre Manufacture
8. Silica Optical Fibre Types
9. Optical Fibre Communication Phenomenon – Total Internal Reflection, NA
10. Waveguide Calculation
11. Fibre Types – Step Index, Graded Index, Single Mode, Multi Mode
12. Optical Rays – Meridional Rays & Skew Rays
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3. Optical Fibre Communication Systems:
 An optical fiber is essentially a waveguide for light.
 It consists of a core, cladding and jacket that surround the core.
 The index of refraction of the cladding is less than that of the core, causing rays of light
leaving the core to be reflected back into the core.
 Optical fiber is made from thin strands of either glass or plastic.
 It has little mechanical strength, so it must be enclosed in a protective jacket.
 Often, two or more fibers are enclosed in the same cable for increased bandwidth and
redundancy in case one of the fibers breaks.
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4. Optical Fibre Communication Systems:
 It is also easier to build a full-duplex system using two fibers, one for transmission in
each direction.
 The higher core refractive index (~ 0.3% higher) is typically achieved by doping the
silica core with germanium dioxide (GeO2).
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5. Optical Fibre: Pros & Cons
Advantages Disadvantages
High Capacity: much wider bandwidth (10 GHz least) Higher initial cost in installation
Crosstalk immunity Interfacing cost
Immunity to static interference like lightening Lower tensile strength
Higher Environmental Immunity Remote electric power
Non explosive & non choking More expensive to repair/maintain
Higher Data Security (Tapping is difficult) Costly Tools: Specialized and sophisticated
Economic – Low Transmission loss
Fewer Repeaters required
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6. Optical Fibre: Light Source & Architecture
 Amount of light emitted is proportional to the drive current
 Two common types:
 LED (Light Emitting Diode)
 ILD (Injection Laser Diode)
 Optical Fibre Architecture:
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7. Optical Fibre: Light Source LED
 Made from material such as AlGaAs or GaAsP.
 Light is emitted when electrons and holes recombine.
 Either surface emitting or edge emitting.
 An LED is a form of junction diode that is operated with forward bias.
 Instead of generating heat at the PN junction, light is generated and passes through an
opening or lens.
 LEDs can be visible spectrum or infrared.
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8. Optical Fibre: Light Source ILD
 Injection Laser Diode abbreviated as ILD.
 Similar in construction as LED except ends are highly polished to reflect photons back
& forth
 Laser diodes generate coherent, intense light of a very narrow bandwidth
 A laser diode has an emission line width of about 2 nm, compared to 50 nm for a
common LED
 Laser diodes are constructed much like LEDs but operate at higher current levels
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9. Optical Fibre: Light Source - ILD vs. LEDILD
 Advantages:
 more focused radiation pattern; smaller Fiber
 much higher radiant power; longer span
 faster ON, OFF time; higher bit rates possible
 monochromatic light; reduces dispersion
 Disadvantages:
 much more expensive
 higher temperature; shorter lifespan
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10. Optical Fibre: Light Detector
 PIN (p-type-intrinsic-n-type)
 APD (avalanche photo diode)
 Both convert light energy into current
 Source–to-fiber-coupler (similar to a lens): A mechanical interface to couple the light
emitted by the source into the optical fiber
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11. Optical Fibre: Light Detector – PIN
 Photons are absorbed in the intrinsic layer.
 Sufficient energy is added to generate carriers in the depletion layer for current to flow
through the device.
 The most common optical detector used with fiber-optic systems is
the PIN diode.
 The PIN diode is operated in the reverse-bias mode.
 As a photo detector, the PIN diode takes advantage of its wide depletion region, in
which electrons can create electron-hole pairs.
 The low junction capacitance of the PIN diode allows for very fast switching.
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12. Optical Fibre: Light Detector – APD
 Avalanche Photodiodes (APD)
 Photo generated electrons are accelerated by relatively large reverse voltage and collide
with other atoms to produce more free electrons.
 Avalanche multiplication effect makes APD more sensitive but also more noisy than
PIN diodes.
 The avalanche photodiode (APD) is also operated in the reverse-bias mode.
 The creation of electron-hole pairs due to the absorption of a photon of incoming light
may set off avalanche breakdown, creating up to 100 more pairs.
 This multiplying effect gives an APD very high sensitivity.
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13. Optical Fiber Construction:
 Core – thin glass center of the fiber where light travels.
 Cladding – outer optical material surrounding the core.
 Buffer Coating – plastic coating that protects the fiber.
 Advantages of Cladding:
1. It adds mechanical strength to the fibre and protects the fibre from absorbing surface
contaminants with which it may come in contact.
2. The cladding is capable of reducing the scattering loss of light resulting from
dielectric discontinuities at the core surface.
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14. Optical Fiber Construction: Manufacture
 There are a number of processes fro producing optical fibres.
 The Outside Vapour Deposition (OVD) is one such which is widely used and it
produces fibre with low loss.
 It is also known as Outside Vapour Phase Oxidation process.
 The first step is to prepare a preform which is a glass rod that has the right refractive
index profile across its cross section and the right glass properties i.e. negligible
impurities.
 The rod is typically 10-30 mm in diameter and about 2 metres in length.
 The optical fibre is drawn from this preform.
 The preform rod is slowly fed into a hot furnace that has a hot zone around 1900-2000
ᵒC where the glass flows like a viscous melt.
 On reaching the hot zone its tip is pulled with the right amount of tension as it comes
out as a fibre and is spooled on a rotating drum.
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15. Optical Fiber Construction: Manufacture
 The diameter of fibre is to be properly controlled during this process.
 As soon as the fibre is drawn, it is coated with polymer layer (urethane acrylate) to
mechanically and chemically protect the fibre surface from microcracks.
 Cladding is typically 125-150 μm and the overall diameter with polymer coating is 250-
500 μm.
 There is a thick polymer buffer jacket to protwct the fibre against mechanical pressure.
 OVD technique is used to produce the rod preform used in fibre drawing.
 The first laydown stage involves using a fused silica glass rod as a target rod.
 This acts as a mandrel and is rotated.
 The required glass material with right composition is grown on the outside surface of
the target rod by depositing glass soot particles.
 The deposition is obtained by burning various gases in an oxy-hydrogen burner flame.
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16. Optical Fiber Construction: Manufacture
 Suppose we need a preform with a core that has germania (GeO2) in silica glass so that
the core has a higher refractive index.
 The required gases SiCl4 (Silicon Tetrachloride), GeCl4 (Germanium Tetrachloride) and
the fuel in the form of oxygen O2 and hydrogen H2 are burnt in a burner flame over the
target rod surface.
SiCl4 (gas) + O2 (gas) = SiO2 (solid) + 2Cl2 (gas)
GeCl4 (gas) + O2 (gas) = GeO2 (solid) + 2Cl2 (gas)
 These reactions produce fine glass particles silica and Germania called “soot” that
deposit on the outside surface of the target rod and form a porous glass layer as the
burner travels along the mandrel.
 First the layers for the core region are deposited and then gas composition is adjusted
for the cladding layer.
 Any refractive index profile can be obtained by controlling the layer composition.
 The second consolidation stage involves sintering this porous glass rod.
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17. Optical Fiber Construction: Manufacture
 The porous perform is fed through a consolidation furnace (1400-1600 ᵒ C) in which the
high temperature sinters (fuses) the fine glass particles into a dense, clear solid, the glass
perform.
 At the same time, drying gases (such as chlorine or thionyl chloride) are forced through
to remove water vapours and hydroxyl impurities that otherwise would result in
unacceptably high attenuation.
 This clear glass perform is then fed into a draw furnace to draw the fibre.
 The central hollow simply collapses and fuses at the high temperature of the draw
process.
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18. Splices and Connectors:
 The interconnections are needed at the optical sources in the transmitter, at the photo
detector, in the receiver and at intermediate points within a cable where two fibres are
joined together.
 The particular technique for joining two fibres depends on whether a permanent bond oe
easily remountable connection is desired.
 The permanent bond is known as a Splice where as the demountable joint is referred to
as a Connector.
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19. Silica Optical Fibre Types:
 Plastic core and cladding
 Glass core with plastic cladding PCS (Plastic-Clad Silicon)
 Glass core and glass cladding SCS: Silica-clad silica
 Under research: non silicate: Zinc-chloride
 1000 time as efficient as glass
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20. Optical Fibre Communication Phenomenon:
 Optical fibres work on the principle of total internal refraction.
 Refraction is the change in direction of a wave due to a change in its speed. Any type of
wave can refract when it interacts with a medium
 Refraction is described by Snell's law, which states that the angle of incidence is related
to the angle of refraction by:
sin 𝜃1
sin 𝜃2
=
𝑣1
𝑣2
=
𝑛2
𝑛1
 The index of refraction is defined as the speed of light in vacuum divided by the speed
of light in the medium: 𝑛 = 𝑐/𝑣.
 As the angle of incidence is increased more than the critical angle, no light enters into
the Cladding layer i.e. no refraction takes place and the light reflects back into the core.
This is called “Total Internal Reflection”.
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21. Optical Fibre Communication Phenomenon:
 Acceptance angle: Acceptance angle, 𝑞 𝑐, is the maximum angle in which external light
rays may strike the air/Fiber interface and still propagate down the Fiber with <10 dB
loss.
 The angle of acceptance is twice that given by the numerical aperture (N.A).
𝑞 𝑐 = 2 ∗ 𝑁. 𝐴
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22. Optical Fibre Communication Phenomenon:
 Numerical Aperture: The measurement of the acceptance angle of an optical fibre which
is the maximum angle at which the core of the fibre will take in light that will be
contained within the core.
 Taken from the fibre core axis (centre of core), the measurement is the square root of the
squared refractive index of the core minus the squared refractive index of the cladding.
 Cut off Wavelength: The cut off wavelength for any mode is defined as the maximum
wavelength at which that mode propagates. It is the value of λ that corresponds to 𝑉𝐶 for
the mode concerns.
 For each Linearly Polarized (LP) mode, the two parameters are related
 𝜆 𝐶 =
2𝜋𝑎
𝑉 𝐶 𝑙𝑚
(𝑛1
2 − 𝑛2
2)
1
2
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23. Waveguide calculation of Fiber Mode:
 V number determines the numbers of guided modes.
 When V number is smaller than 2.405, only one mode can be guided by the fiber, this is
called single mode fiber. Therefore for single mode fibre, 𝜆 > 1214 𝑛𝑚
 When V Numer is larger than 2.405 severals modes can be guided by the fiber. This is
called Multimode Fiber.
 Higher the V number, larger is the number of modes.
𝑽 =
𝟐𝝅𝒂
𝝀
(𝒏 𝟏
𝟐 − 𝒏 𝟐
𝟐)
𝒃 = (𝜷 𝟐 − 𝒏 𝟐
𝟐)/(𝒏 𝟏
𝟐 − 𝒏 𝟐
𝟐)
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24. Fibre Types:
 Modes of operation (the path which the light is traveling on)
1. Single Mode
2. Multi Mode
 Index profile
1. Step
2. Graded
 Variation in the composition of the core material gives rise to two types of optical fibres
viz. step index fibre and graded index fibre.
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25. Fibre Types:
 Step Index: In step index fibre, the refractive index n1 of the core material is uniform
through-out and undergoes an abrupt change in step at the core cladding boundary.
 Graded Index: In graded index fibre, the refractive index of the core is made to vary as a
function of radial distance from the centre of the fibre.
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26. Fibre Types:
 Single Mode Fibre: A single mode fibre can sustain only one mode of propagation. In
“Single Fiber”, the core is so tiny that only one light ray which is perpendicular to the
cable may propagate along.
 Multi Mode Fibre: Multi mode fibres can contain a large number of modes. Having a
bigger core diameter, multiple rays of light can propagate along.
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27. Fibre Types:
Single-Mode Multimode
1. Small core
2. Less dispersion
3. Carry a single ray of light, usually generated
from a laser.
4. Employ for long distance applications
(100Km)
5. Uses as Backbone and distances of several
thousands of meters.
1. Larger core than single mode cable.
2. Allows greater dispersion and therefore, loss of
signal.
3. Used for shorter distance application, but
shorter than single-mode (up to 2Km)
4. It uses LED source that generates differed
angles along cable.
5. Often uses in LANs or small distances such as
campus networks.
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28. Fibre Types:
 Single - Mode Step - Index Fibre:
 Advantages:
1.Minimum dispersion: all rays take same path, same time to travel down the cable. A
pulse can be reproduced at the receiver very accurately.
2.Less attenuation and therefore can run over longer distance without repeaters.
3.Larger bandwidth and higher information rate.
 Disadvantages:
1.Difficult to couple light in and out of the tiny core.
2.Highly directive light source (laser) is required.
3.Interfacing modules are more expensive.
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29. Fibre Types:
 Multi Mode:
 Multimode step-index Fibers:
1. Inexpensive
2. Easy to couple light into Fiber
3. Result in higher signal distortion
4. Lower TX rate
 Multimode graded-index Fiber:
1. Intermediate between the other two types of Fibers
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30. Optical Rays: Meridional Rays
 Meridional Rays: Meridional rays are those that have no φ component – they pass
through the z axis, and are thus in direct analogy to slab guide rays.
 These are confined to the meridional planes of the fiber.
 These planes are planes which contain the axis of symmetry of the fibre (core axis).
 A given meridional ray propagates in a single plane and hence it is easy to track its path
as it propagates along the fiber axis.
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31. Optical Rays: Meridional Rays
 Meridional rays can be further classified into two categories.
 Bound Rays: These rays are trapped in the core of the fibre and they travel along the
fibre axis according to Snell’s laws of reflection and refraction.
 Unbound Rays: These rays are refracted out of the fibre core according to the Snell’s
laws of refraction and cannot be trapped in the core of the fibre.
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32. Optical Rays: Skew Rays
 Skewed Rays: The skew rays can propagate without passing through the core of the
fibre.
 They are not confined to a single plane, but they follow a helical path along the fibre.
 It is difficult to track the rays as they travel along the fibre and don’t lie on a single
plane.
 The point of emergence of the skew rays from the fibre in air depends on the number of
reflections they undergo rather than the input conditions of the optical fibre.
 Such a ray exhibits a spiral-like path down the core, never crossing the z axis.
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33. Optical Rays: Skew Rays
 With the light input in the fibre not being uniform, the skew rays give a smoothing
effect to the distribution of transmission resulting in uniform output for non-uniform
inputs.
 Another advantage is that their numerical aperture (N.A) is greater than that of the
meridional rays.
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