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Optic fibers
Lecture 3
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Early optical communication
The French used semaphores to transmit
messages in the 1790s
Later systems also sent optical signals throughthe air
But clouds, rain, and other atmospheric
disturbances can disrupt optical signals sent
through the air
Electric signals through wires avoid that problem
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Semaphores
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Guiding Light With Water
Light in a stream of water stays inside the
water and bends with it
This was first demonstrated in the 1840s
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Total Internal Reflection
There is a critical angle at which no light canbe refracted at all, so 100% of the light isreflected
Light is trapped in the water and cannot escapeinto the air
This works with any dense medium, such as plasticor glass, the same way it works with water
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Total Internal Reflection
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What are Fiber Optics?
Fibers ofglass
Usually 120 micrometers in diameter
Used to carry signals in the form of light overdistances up to 50 km.
No repeaters needed.
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What are Fiber Optics?
If you look closely at a single optical fiber, youwill see that it has the following parts:
Core - Thin glass center of the fiber where the
light travels Cladding - Outer optical material surrounding the
core that reflects the light back into the core
Buffer coating - Plastic coating that protects the
fiber from damage and moisture
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Fiber Optic Cables
SOURCE: SURFNET.NL
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What are Fiber Optics?
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What are Fiber Optics?
plastic jacketglass or plasticcladdingfiber core
TOTAL INTERNAL REFLECTION
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Total Internal Reflection
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How Does an Optical Fiber Transmit
Light? The light in a fiber-optic cable travels through the
core (hallway) by constantly bouncing from thecladding (mirror-lined walls), a principle calledtotal internal reflection.
Because the cladding does not absorb any lightfrom the core, the light wave can travel greatdistances.
However, some of the light signal degradeswithin the fiber, mostly due to impurities in theglass. The extent that the signal degradesdepends on the purity of the glass and thewavelength of the transmitted light
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Total Internal Reflection
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Total Internal Reflection
Total internal reflection is an opticalphenomenon that happens when a rayof light strikes a medium boundary at an angle
larger than a particular critical angle withrespect to the normal to the surface. Ifthe refractive index is lower on the other sideof the boundary, no light can pass through andall of the light is reflected. The critical angle isthe angle of incidence above which the totalinternal reflection occurs.
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Total Internal Reflection
When light crosses a boundary between materials withdifferent refractive indices, the light beam will bepartially refracted at the boundary surface, and partiallyreflected. However, if the angle of incidence is greater than
the critical angle the angle of incidence at which light isrefracted such that it travels along the boundary then thelight will stop crossing the boundary altogether and insteadbe totally reflected back internally. This can only occurwhere light travels from a medium with a higher [n1=higherrefractive index] to one with a lower refractive index
[n2=lower refractive index]. For example, it will occur whenpassing from glass to air, but not when passing from air toglass.
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Critical Angle
The critical angle is the angle of
incidence above which total internal reflection
occurs. The angle of incidence is measured
with respect to the normal at the refractive
boundary
The critical angle c
is given by:
= sin(n1/n2)
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Refractive Index
The refractive index or index of refraction ofa
substance is a measure of the speed of light in
that substance
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Refractive Index
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Types Of Optic Fiber
Single-mode fibers used to transmit one
signal per fiber (used in telephone and cable
TV). They have small cores(9 microns in
diameter) .
Multi-mode fibers used to transmit many
signals per fiber (used in computer networks).
They have larger cores(62.5 microns indiameter).
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Types Of Optic Fiber
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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
Repairinga broken cable
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Splices v. Connectors
A permanent join is asplice
Connectors are used at patch panels, and can
be disconnected
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Fusion or MeltingSplicing
Fusion splicing is the act of joining two optical
fibers end-to-end using heat. The goal is to
fuse the two fibers together in such a way that
light passing through the fibers is
not scattered or reflected back by the splice,
and so that the splice and the region
surrounding it are almost as strongas thevirgin fiber itself
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Fusion or MeltingSplicing
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Mechanical Splicing
A mechanical splice is a junction of two or
more optical fibers that are aligned and held
in place by a self-contained assembly (usually
the size ofa large carpenter's nail). The fibers
aren't permanently joined, just precisely held
together so that light can pass from one to
another.
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Mechanical Splicing
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Mechanical Splicing
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Mechanical Splicing