Optical Fiber - Wilson College

OPTICAL FIBERS

USPH303 unit 1

1 Mahesh Shetti, Wilson College

Image courtesy of TE Connectivity SubCom

Refraction of light

Snell’s law of

refraction:

Incident ray, refracted

ray and normal to the

surface at the point of

incidence are in the

same plane.

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Mahesh Shetti, Wilson College

Incident ray Reflected ray

Refracted ray

i

r

Table of typical refractive indices ref: wikipedia.org

Vacuum 1

Gases at 0 °C and 1 atm

Air 1.000293

Helium 1.000036

Hydrogen 1.000132

Carbon dioxide 1.00045

Liquids at 20 °C

Water 1.333

Ethanol 1.36

Olive oil 1.47

Solids

Ice 1.31

Fused silica (quartz) 1.46

Window glass 1.52

Polycarbonate (Lexan™) 1.58

Flint glass (typical) 1.69

Sapphire 1.77

Cubic zirconia 2.15

Diamond 2.42

Moissanite 2.65

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When light is incident from denser medium

on the boundary to rarer medium:

in this case, angle of refraction is larger than angle of incidence.

For angle of refraction to be 90, angle of incidence is

This is called critical angle.

If the angle of incidence is greater than this, then there is total internal reflection.

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Image Credit: https://physics.stackexchange.com

Real life examples 5


Highway Mirage Fiber toys – total internal reflection along with optical leakage

Basic structure of optical fiber cable

Optical parts are core and cladding.

Core has greater refractive index than cladding.

Outer jackets are mainly to give mechanical strength to the optical fiber.

It also protects fiber from chemical erosion.

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Mahesh Shetti, Wilson College Image: https://www.pinterest.com

Numerical aperture

There is a maximum angle of incidence max that ensures light

to remain inside the optical fiber.

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Calculation


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Find the numerical aperture for an optical fiber with core

refractive index of 1.500 and cladding refractive index of

1.480. Hence find the angle of acceptance.

n1 = 1.500

n2 = 1.480

N.A. = 0.1628 angle of acceptance is 922’

Multimode Fibers 9


Modes of transmission

Different rays travel making different angles with axis of the core.

Each different angle is different transmission mode.

The time required for each mode is different as it has to travel different

path lengths.

The colours shown here are only to differentiate them (they are not actual

colours of light passing through them.

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cladding

cladding

core

Modal dispersion 11


cladding

cladding

core

L

This time difference for different modes causes signal to spread in time. It is called

modal dispersion.

Graded Index Optical Fiber

In the graded index, the refractive index of the core gradually reduces from axis to cladding.

Ray travelling closer to axis, travels less physical path, but since it is travelling through denser medium, it takes more time.

Ray travelling at larger angle to axis has to travel greater physical distance, but while passing near cladding, it passes through rarer medium and hence travel faster.

Hence the net time difference between different modes reduces.

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Core

Cladding

V number


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A fibre is frequently characterised by V number given by: V = 2a(N.A.)/0

Where 0 is wavelength, a is radius of core, N.A. is numerical aperture

V is dimensionless and is known as normalised frequency.

If V is large, the total number of modes in step index fibre is given by Ms = V2/2

For graded index optical fibre the total number of modes are V = /(+2) x V2/2

For parabolic profile = 2, hence modes are V2/4.

Modal dispersion 14


For Step Index:

For Graded index:

Modal dispersion is smaller by factor of

100 or more than step-index fiber.

Single mode fibers 15


Single mode optical Fiber


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If there is just one mode = no modal dispersion!!

When optical fiber is very thin, it does not allow

different modes!

When wavelength of light is comparable to the

diameter of fiber, it acts as a waveguide and wave

travels with constraint of single mode.

Typical wavelength used is around 1.31 or 1.55

and diameter of single mode fiber is 8-10.

All international sub-sea optical fiber cables are

single mode.

Attenuation in Optical Fiber


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Ref: https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=3255

Sizes of optical fiber


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Why glass fiber?


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Its viscosity is variable for a wide range of temperatures, and can be well controlled. Hence it can be easily drawn into a thin fiber.

Highly pure silica has very low loss. For most commercially available silica fibers 96% power gets transmitted through 1km of fiber.

Glass is intrinsically strong. Its strength is 13.8GPa i.e. 125 thick fiber can support load of 17kg wt.

The exposure of glass with external atmosphere leads to formation of cracks and fracture. While manufacturing it is drawn in extremely clean environment and coated with polymer so it does not come in contact with atmosphere.

ILLUMINATION

Application 1 20


Advantage of fiber in illumination


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It uses multimode optical fiber

Core diameter is large and refractive index

difference can be large

Claddingless plastic fibers can be used

Inexpensive LED can be used

Heat from the source can be avoided with filter at

the source end, hence it is called cold light

UV light is absorbed in plastic core fibers

Glow cables

These are plastic fibers without

cladding.

Reason for side glow is scattering

inside the core.

Features of Plastic Fiber Optic:

Very low attenuation with

<0.25dB/m (That is actually not quite

low. It implies power will be half in

4m or longer.)

Excellent toughness

Excellent fiber diameter uniform

distribution

Nice surface

Excellent heat resistance

Excellent anti-UV performance

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http://www.wiedamark.com/images/products/detail/sideglo

wanilarge.gif

Interior light decorations with cold light 23

Mahesh Shetti, Wilson College Ref://www.fibercreations.com/gallery/

Guiding Light


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The light-detecting rods

and cones in the eye

(pencil-shaped cells

near the bottom) are

obscured beneath

several layers of cells

in the retina, shown in

this artist’s conception.

Computer simulations

suggest that a specific

set of cells can act like

optical fibers to

channel light through

these layers.

https://physics.aps.org/story/v25/st15

K. Franze et al., “Müller Cells are Living Optical Fibers in the Vertebrate Retina,” Proc. Natl. Acad. Sci. 104, 8287

(2007)

Communication

Application 2: 25


Some features


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For short distance communication multi-mode fibers

are used.

For long distance communication single mode fiber

is the only choice.

The frequency for single mode fiber is selected such

that minimum absorption takes place in the core

medium.

Small core diameter and smaller acceptance angle,

single mode fibers use laser source.

Medical Applications

Application 3: 27


Endoscope parts


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Endoscope is an optical device to see and operate inside any system.

There are typically two sets of the fibers.

The outer fiber supplies light, it is incoherent bundle. It is enclosed with sleeves to

protect it from water or moisture.

The inner coherent ring serves to transmit the image. The tiny lens connected to

the end of this bundle allows for light to be effectively focused so that reflected

light from the object of interest can be collected and transmitted for viewing.

Water pipes - The pipes serve to carry water which is used to wash the lens thereby

maintaining a clear view.

The operational channel - This is an opening on the device that is used to move

various accessories to the distil end (of the endoscope) for surgery purposes.

Control cables - This is used to control the direction that the distil end will bend as it

moves through body cavities.

A commercial Endoscope features

The scopes have high resolution

sensors with LED illumination

The Tele-View USB Endoscopes

come in 1.0m, 1.5m, 3.0m and

3.5m lengths with 8.7mm, 9.8mm

or 12.6mm diameters.

The insertion tube includes:

high resolution camera sensor,

super-bright LED lights

a working channel

an irrigation channel to clean the

lens

an air-water channel for lavage

or insufflation.

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http://www.veteldiagnostics.com/products/endoscope

-gastroscope

Optical Fiber - Wilson College

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