CT 1 KH sir (GCE 4129) Advance ceramic
Optical fiber
What is Optical Fiber?
Optical fiber refers to the technology that transmits information as light
pulses which is usually made of plastic or glass. Optical fibers are highly
significant due to their ability to transmit data signals over long distances
with high speed and bandwidth.
They are commonly utilized for internet, telephone, and TV services.
Unlike other mediums, optical fibers, made of glass and plastic, are
immune to electromagnetic interference during data transmission.
Design Of An Optical Fiber
1. The Transmitter – It produces the light signals and encodes them
to fit to transmit.
2. The Optical Fibre – The medium for transmitting the light pulse
(signal).
3. The Optical Receiver – It receives the transmitted light pulse
(signal) and decodes them to be fit to use.
4. The Optical Regenerator – Necessary for long-distance data
transmission.
The information carrying capacity of a communication system is directly
proportional to it's bandwidth. The wider bandwidth the greater is it's
information carrying capacity.
Structure of optical fiber
Core( diameter can vary from about 5um-100 um)
✓ Glass or plastic with a higher index of refraction than the Cladding
✓ Carries the signal
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Cladding
✓ Glass or plastic with a lower index of refraction than the core
✓ keep the light within the core
Buffer
✓ Protects the fiber from damage and moisture
Jacket
✓ Holds one or more fibers in a cable
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Snell”s Law
Refraction of light
As a light ray passes from one transparent medium to another, it changes
direction; this phenomenon is called refraction of light. How much that
light ray changes its direction depends on the refractive index of the
mediums.
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Refractive Index
Refractive index is the speed of light in a vacuum (abbreviated c,
c=299,792A58km/second) divided by the speed of light in a material
(abbreviated v). Refractive index measures how much a material refracts
light Refractive index of a material, abbreviated as n, is defined as
n=c/v
Total internal reflection
when light traveling within a medium strikes the boundary with another
medium at an angle greater than the critical angle, causing the light to be
completely reflected back into the original medium instead of being
transmitted.
This has an interesting implication: at some angle, known as the critical
angle θc, light traveling from a higher refractive index medium to a lower
refractive index medium will be refracted at 90'; in other words, refracted
along the interface.
If the light hits the interface at any angle larger than this critical angle, it
will not pass through to the second medium at all. Instead, all of it will be
reflected back into the first medium, a process known as total internal
reflection.
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Basic principle
Optical fibers are based entirely on the principle of total internal
reflection. This is explained in the following picture
Working principal of optical fiber
✓ Optical fiber utilizes total internal reflection, where light is
completely reflected when it encounters a boundary at a steep
angle. This is called total internal reflection.
✓ Total internal reflection is employed in optical fibers to confine light
within the core by bouncing it back and forth at the core-cladding
boundary.
✓ The acceptance cone determines the range of angles at which light
can enter the fiber without leaking out, and its size depends on the
refractive index difference between the core and cladding.
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✓ Light travels through fiber-optic cables by repeatedly bouncing off
the walls, similar to a bobsleigh on an ice run, due to total internal
reflection.
✓ Total internal reflection occurs when light hits the glass at a shallow
angle (<42 degrees), causing it to reflect back into the fiber and
preventing it from leaking out, effectively keeping the light confined
inside the cable.
✓ The fiber optic cable structure consists of a core in the middle,
where light travels, and a cladding layer wrapped around it. The
cladding, made of different glass with a lower refractive index,
keeps the light signals confined within the core, ensuring their
transmission.
✓ Light bounces off the walls of a fiber optic cable, traveling down the
core. The core is the middle glass structure, while the cladding is an
outer layer of glass wrapped around it. The cladding's purpose is to
confine the light signals within the core.
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Explain the signal transfer mechanism of optical fiber.
✓ Encoding: The information is encoded onto the light beam using
techniques like intensity or phase modulation.
✓ Core and cladding: The optical fiber consists of a core (carries the
light signal) surrounded by cladding (confines the light).
✓ Total internal reflection: occurs at the core-cladding interface,
where light bounces back and forth due to the core's higher
refractive index.
✓ Single-mode or Multimode: Optical fibers can be single-mode (thin
core for long-distance) or multimode (larger core for shorter
distances).
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✓ Propagation: The light signal propagates through the fiber by
repeatedly reflecting off the core-cladding interface.
✓ Signal Reception: At the receiving end of the optical fiber, a
photodetector or receiver is used to detect the light signal. The
photodetector converts the incoming light into electrical signals,
which can then be processed and decoded to retrieve the original
information.
✓ Amplification and Regeneration: Optical amplifiers may be used to
boost the signal strength without converting it back to electrical
form.
Types of optical fiber
a. Based on material
1. Plastic core with plastic cladding
2. Plastic core with glass cladding
3. Glass core with glass cladding
b. Based on mode of transmission
1. Single mode fiber
2. Multimode fiber
c. Index profile
1. Step index fiber
2. Graded index fiber
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Based on material
Plastic core with plastic cladding
✓ This type of fiber cable have same material so it is easy in
production
✓ less expensive and easy to install
✓ Mostly use in short distance and have capability of 6Mbps
Glass core with plastic cladding (PCS)
✓ It is having low signal lost
✓ Less affected by radiation
✓ Suitable for military application
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Glass core with glass cladding (SCS)
✓ It is having lower signal lost than PCS
✓ More susceptible in radiation areas and losses Signal
Based on mode of transmission
Single mode fiber
✓ it is having only one path for light to pass.
✓ Very small diameter of core (7 to 10 pm)
✓ It have bandwidth up to 40Ghz.
✓ Mostly use in long distance and low cost circuit like T.V. cable.
Multimode fiber
✓ Light takes more than one path to travel
✓ Core is having diameter of 20 to 100pm.
✓ Usually use for medium distance and high bandwidth.
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Index profile
Step index fiber
✓ Step index have uniform reflective index of core.
✓ Core have bigger refractive index than cladding.
✓ Graph of radial distance vs. refractive index is seems like a step-
index
Graded index fiber
✓ Core of graded index fiber have non-uniform
Refractive index
✓ Refractive index is highest at canter and decrease till end of core
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Three Methods
1. I Modified Chemical Vapor Deposition (MCVD)
2. Outside Vapor Deposition (OVD)
3. Vapor Axial Deposition (VAD)
Describe the manufacturing process of optical fiber
There are two main steps in the process of converting raw materials
into optical fiber ready to be shipped:
1. manufacturing of the pure glass preform and
2. drawing of the preform
Manufacturing the preform
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The first step in manufacturing glass optical fibers is to make a solid glass
rod, known as a preform. Ultra-pure chemicals: primarily silicon
tetrachloride (SiCl4) and germanium tetrachloride (GeCl4) – are
converted into glass during preform manufacturing. These chemicals are
used in varying proportions to fabricate the core regions for the different
types of preforms.
SiCl4 (gas) + A2 > SiO2 (solid) + 2Cl2 (in the presence of heat)
GeCl4 (gas) + 02 > GeO2 (solid) + 2Cl2 (in the presence of heat)
✓ varying amounts of Germania added to increase the fiber's
refractive index
✓ Single-mode fibers typically have only small amounts of germania
✓ Multimode fibers typically have a much
✓ higher refractive index, and therefore much higher germania
content.
Manufacturing the preform
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Test and Measurement
The drawn fiber is next tested, where all optical and geometrical
parameters are checked to ensure that they meet stringent
requirements.
First, the tensile strength of the fiber is tested. Each spool of drawn fiber
is wound through a series of Capstans and subjected to loads to ensure
that the fiber has the minimal tensile strength specified. The fiber is then
spooled onto shipping reels and cut to specified lengths.
The fiber is tested for point defects with an optical time domain
reflectometer (OTDR), which uses scattered light to indicate the location
of any anomalies along the length of the fiber.
The spooled fiber is automatically tested for transmission parameters
including:
attenuation: decrease in signal strength over distance
bandwith: information-carrying capacity; an important measurement for
multimode fiber
numerical aperture: the measurement of the light of a fiber
cut-offwavelength: in single-mode fiber the wave length above which
only a single mode propagates
mode field diameter: in single-mode fiber the radial width of the light
pulse in the fiber; important for interconnecting
chromatic dispersion: the spreading of pulses of light due to rays of
different wavelengths traveling at different speeds through the core; in
single-mode fiber this is the limiting factor for information carrying
capacity
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geometrical parameters including :
✓ cladding diameter
✓ cladding non-circularity
✓ coating outer diameter
✓ coating outer non-circularity
✓ coating concentricity error
✓ core-clad concentricity error
✓ core hon-circularity
✓ core diameter
Environmental and mechanical testing is also performed periodically to
ensure that the product maintains its optical and mechanical integrity
and complies with customer requirements. These tests include:
✓ coating strip force
✓ operating temperature range
✓ temperature dependence of attenuation
✓ temperature-humidity cycling
✓ accelerated aging
✓ water immersion
Applications of optical fiber
(1 ) Communication
✓ Optical fiber is mostly use in communication
✓ It is use in Wi-Fi router,
✓ Landline phone and server- connector.
✓ A single optical fiber can carry over 3,000,000 full-duplex voice
calls or 90,000 TV channels So it is use in Broad bandwidth.
(2) Military
✓ Optical fiber is use to make military equipment, and weapons.
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✓ It is also use to make antenna to communicate in far areas.
(3) Sensor
✓ Most of sensors are made from optical fiber.
✓ Optical is also use to make detectors - Metal detector
(4) Other application
✓ Optical fiber is use to make lamps, decorative application, art,
toys, micro scope and outer body of devices.
✓ Many medical devices are made from optical fiber.
Advantages of Optical Fibres Over Wires
There are many advantages of using optical fibres over traditional wires.
Some of the advantages are listed below:
✓ Lower cost in the long run
✓ Lower loss of signal, typically less than 0.3 db/km). So, repeater-less
transmission over a long distance is possible
✓ Large data-carrying capacity (thousands of times greater, reaching
a speed of up to 1.6 Tb/s in the field-deployed system and up to 10
Tb/s in lab systems)
✓ No electromagnetic radiation; difficult to eavesdrop
✓ High electrical resistance. So, safe to use near high-voltage
equipment or between areas with different earth potentials
✓ Low weight
✓ Signals contain very little power
✓ No cross-talk between cables
✓ No sparks (e.g. in automobile applications)
✓ Difficult to place a tap or listening device on the line, providing
better physical network security
Fiber performance
Attenuation
✓ Attenuation is the loss of the optical power.
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✓ Attenuation in optical fiber take place due to elements like
coupler, splices, connector and fiber itself.
Dispersion
✓ causes the digital waveform to be “smeared” Rise/fall time
expands over the length of the fiber
✓ Modal dispersion only present in multi-mode fibers
✓ Chromatic dispersion arises from spectral width
1. Critical Angle (θc)
At core-cladding interface, if θc = θ, then
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2. Acceptance Angle (θa)
The value of maximum angle of incidence with the axis of fibre in the
air for which all the incident light is totally reflected is known as
acceptance angle.
If θa = Acceptance angle, µ1= refractive index of core and µ2=refractive
index of cladding, then
3. Numerical Aperture
Light gathering capability of an optical fibre is related to its numerical
aperture. This is defined as the sine of its acceptance angle. That is,
Sample problem 1:
A step-index fiber has a core index of refraction of n1 = 1.425. The cut-
off angle for light entering the fiber from air is found to be
8.50o. (a) What is the numerical aperture of the fiber? (b) What is the
index of refraction of the cladding of this fiber? (c) If the fiber were
submersed in water, what would be the new numerical aperture and
cut-off angle?
Solution:
(a)From the Table of indices of refraction, we see that n0 = nair
=1.0003. The numerical aperture is therefore
(b)The index of refraction of the cladding can be found from the
numerical aperture:
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(c)From the Table, we see that the n0 = nwater = 1.33. Since the
numerical aperture is a property of the fiber and only depends upon n1
and n2, it will not change when the medium outside the fiber changes.
The cut-off angle, however, will have to change if the numerical aperture
is to be unaffected by a change in n0:
The refractive indices of core and cladding materials of a step index
fibre are 1.48 and 1.45, respectively. Calculate: (i) numerical aperture,
(ii) acceptance angle, and (iii) the critical angle at the core-cladding
interface and (iv) fractional refractive indices change.
Given: n₁ (refractive index of the core) = 1.48 n₂ (refractive index of the
cladding) = 1.45
(i) Numerical Aperture: NA = √(1.48² - 1.45²) ≈ 0.296
(ii) Acceptance Angle: θ = sin-1(NA) = sin-1(0.296) ≈ 17.215 degrees
(iii) Critical Angle: θc = sin-1(n2/n1) =sin-1(1.45/1.48) ≈ 78.23 degrees
(iv) Fractional Refractive Index Change: Δ=(n1-n2)/n1= (1.48 - 1.45) /
1.48 ≈ 0.020
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