Optic Fibers(2)

8/8/2019 Optic Fibers(2)

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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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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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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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plastic jacketglass or plasticcladdingfiber core

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 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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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

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