Vadodara Institute of Engineering.

Subject: Engineering Physics.

Mechanical F.Y. (shift 2)

Kotambi

Vadodara Institute of Engineering.

Credits: Kothari Vrajesh 130800119517 Ojha Anandvardhan 130800119518 Pandya Kaushal 130800119519 Pandya Param 130800119520

Under the guidance of:

Professor Anand M Panchal.

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FIBRE OPTICS

Topic:

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Contents : Introduction.. Advantages.. Total Internal Reflection.. Numerical Aperture.. Acceptance Angle.. Types of Fibre..

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Introduction

An optical fiber is essentially a waveguide for light.

It consists of a core and cladding that surrounds 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 refracted back into the core.

A light-emitting diode (LED) or laser diode (LD) can be used for the source.

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What’s inside a fiber cable ?

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Layers surrounding Fiber cables Primary coating

Gives additional mechanical strength Protects fiber from stress Protection against chemical attacks

Buffer jacket Avoids the formation of microbends Bending stress is relieved Fiber losses are reduced

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Layers surrounding Fiber cables…

Outer Jacket To reduce abrasion and to provide extra protection

against external mechanical strength Filler materials

To avoid moisture absorption by fibers Prevents corrosion

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Layers of Fiber cables

Core 8µm

Cladding125µm

Buffer250 µm

Jacket400 µm

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Advantages of Optical fibre

Less expensive. Thinner. Higher carrying capacity. Less signal degradation Light signals Low

power . Digital signals Non-flammable . Lightweight . Flexible Medical imaging.

Mechanical imaging - inspecting mechanical welds in pipes and engines (in airplanes, rockets, space shuttles, cars)

Plumbing - to inspect sewer lines

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The Basic principle :Total Internal Reflection.

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° i.e. 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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The angle of refraction at the interface between two media is governed by Snell’s law:

2211 sinsin nn

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Numerical Aperture

The numerical aperture of the fiber is closely related to the critical angle and is often used in the specification for optical fiber and the components that work with it

The numerical aperture is given by the formula:

The angle of acceptance is twice that given by the numerical aperture.

22

21.. nnAN

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Acceptance Angle.. Acceptance angle of the fibre Φmax is defined

as the maximum value of the angle of incidence at the entrance end of the fibre, at which the angle of incidence at the core-cladding interface is equal to critical angle of the core medium.

Mathematically given as:

sin Φmax = (n12 – n2

2 )0.5

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Vadodara Institute of Engineering.

Types of Optical Fiber.

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Single mode Step-Index Fibre (SMSI Fibre)

A single mode step index fibre has a central core material of uniform refractive index value.

The diameter of the core is (8 – 10 µm) sufficiently small so that, there is essentially only one path that light may take as it propagates down the cable.

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Multi mode Step-Index Fibre. In comparison to the single-mode fibre, multimode

fibre has a relatively large core of diameter 50 – 25 µm and a high numerical aperture.

The diameter of the cladding usually lie between 100-250µm.

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Multimode Graded Index Fibre. In an effort to reduce the pulse dispersion,

multimode graded index fibre was developed. A graded index index fibre is characterized by

a central core that has a refractive index which is not uniform.

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Reference : Engineering Physics by G. Vijayakumari. Engineering Physics by V Rajendran. http://en.wikipedia.org/wiki/Optical_fiber

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