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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