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Light Waves andPolarization

Xavier FernandoRyerson Communications Lab

http://www.ee.ryerson.ca/~fernando

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The Nature of Light

There are three theories explain the nature oflight:

Quantum Theory Light consists of small

particles (photons) better explains lightdetection and generation

Wave Theory Light travels as atransverseelectromagnetic wave

Ray Theory Light travels along a straight lineand obeys laws ofgeometrical optics usefultool when the objects are much larger than the

wavelength of light

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Wavelength Ranges

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Wave Theory of Light

Electromagnetic light signal has electric andmagnetic fields orthogonal to each other.

The frequency of this EM wave is in the order ofTHz. Therefore, it is convenient to measure it in

terms of wavelength.

where, c - speed of light 3 X 108 m/s in air,

- frequencyand- wavelength Ex: Find the when = 1550 nm.

Answer: 193.5 THz

=c

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Quantum Theory of Light

Optical Signal is consists of discrete unitscalled photons. The energy in a photon

Eg = h, whereh=6.6256X10(-34)J .s is theplancksconstant and is the frequency.

Ex1: Find the energy of a photon travelling

with 200 THz frequency

Ex2: Show eV)(

24.1

mEg

=

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Plane Waves

Most light waves are plane waves For plane waves the transverse condition

requires that theelectric and magnetic field

be perpendicular to the direction ofpropagation and to each other.

The electric field vector of a plane wave

may be arbitrarily divided into twoperpendicular components labeled x and y(with z indicating the direction of travel).

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Field distributions in plane E&M waves

Electric and magneticfields are orthogonalto each other and tothe direction ofpropagation Z

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Basics about Plane Waves

/ propagation constant

The combined wave

)cos( kztEE o =

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Phase Velocity

vp

n

c

k

vp ==

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Group of Waves

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Carrier and Envelope

vp vg

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Group Velocity

The function (k) is known as the dispersion

relation. If is directly proportional to k, then the group

velocity is exactly equal to the phase velocity.

Otherwise, the envelope of the wave will becomedistorted as it propagates. This "group velocitydispersion" is an important effect in thepropagation of signals through optical fibers.

m/sg

gn

c

kv =

=

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Polarization

Polarizationof a plane wave is theorientation of the oscillations of theE field

For a simple harmonic wave, amplitude of

the electric vector may have

Different amplitude

Different phase

The resulting wave is

Linearly, elliptically or circularlypolarized

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Polarization states

If the electric field is oscillating along astraight line, it is called a linearlypolarized(LP) or plane polarizedwave

If theE field rotates in a circle (constantmagnitude) or in an ellipse then it iscalled acircular or elliptically polarized

wave

Natural light hasrandom polarization

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Adding two linearly polarized waves with aphase shift will produce an elliptically

polarized light

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Elliptical Polarization

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Adding two linearlypolarized waves with zero

phase shift will generateanother linearly polarizedwave

)cos( kztEE o =

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Linear Polarization

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Adding two linearlypolarized waves with

equal amplitude and90ophase shift resultsin circular polarized

wave

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Circular Polarization

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Unpolarized Light

In most natural light sources, the orientationof the electric fields may not be correlated.

This light is said to beunpolarizedor

partially polarized. Partially polarized light can be described as

a superposition of a completely unpolarized

component, and a completely polarized one.

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Polarized Light

In practice, if the electric field is aligned inone direction, that light is called the

polarized light Horizontally polarized light

Vertically polarized light

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Polarization Control

Polarisers are used toblock reflected light inoptical fibres Optical Isolators

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Polarized Sunglasses

These providebetter view by

blackingreflected(glare) light

that hashorizontalpolarization