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Polarization Components 12.2
1
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O E M
Polarization Components
w w w . m e l l e s g r i o t . c o m
Singlets
Multielement Lenses
Cylindrical Optics
Mirr
or
s
Prisms and Retror
eflector
s
Beamsplitter
s
,
W
indows
,
Optical Flats
P
olarization Components
Filter
s
Polarization
Components
unpolarized
input beam
birefringent
material
ordinary
ray
extraordinary
ray
linearly polarized
output beam A
unpolarized output beam
linearly polarized
output beam B
Double refraction in a birefringent crystal
Polarized light carries valuable information about the various
physical parameters that have been acting on it. Magnetic fields,
chemical interactions, molecular structures, and mechanical stress
all affect optical polarization. Applications relying on these polar-
ization changes include astrophysics, agricultural production, elec-
tric power generation, and molecular biology.
Polarization states are linear, circular, or elliptical according to
the paths traced by electric field vectors in a propagating wave train.
Unpolarized light (such as from an incandescent bulb) is a combi-
nation of all linear, circular, and elliptical states. Randomly polar-
ized light, in reference to laser output, is composed of two
orthogonally linearly polarized collinear beams whose power ran-
domly varies over time. Although random, this radiation is always
linearly polarized.
Depolarized light is usually linearly polarized light that has been
randomized by either temporal or spatial retardation variations
across or along the beam. If the various retardations are integrated
enough, the beam will appear to be depolarized. The randomiza-
tion process usually varies the linear polarization in a fairly smooth
and predictable manner.
BIREFRINGENCE
A birefringent crystal, such as calcite, will divide an entering
beam of monochromatic light into two beams having opposite
polarization. The beams usually propagate in different directions
and will have different speeds. There will be only one or two opti-
cal axis directions within the crystal in which the beam will remain
collinear and continue at the same speed, depending on whether the
birefringent crystal is uniaxial or biaxial.
If the crystal is a plane-parallel plate, and the optical axis direc-
tions are not collinear with the beam, radiation will emerge as two
separate, orthogonally polarized beams. The beam will be unpo-
larized where the beams overlap upon emergence. The two new
beams within the material are distinguished from each other by
more than just polarization and velocity. The rays are referred to
as extraordinary (E) and ordinary (O). These rays need not be con-
fined to the plane of incidence. Furthermore, the velocity of these
rays changes with direction. Thus, the index of refraction for extra-
ordinary rays is also a continuous function of direction. The index
of refraction for the ordinary ray is constant and is independent of
direction.
The two indexes of refraction are equal only in the direction of
an optical axis within the crystal. The dispersion curve for ordi-
nary rays is a single, unique curve when the index of refraction is
plotted against wavelength. The dispersion curve for the extraor-
dinary ray is a family of curves with different curves for different
directions. Unless it is in a particular polarization state, or the crys-
talline surface is perpendicular to an optical axis, a ray normally inci-
dent on a birefringent surface will be divided in two at the boundary.
The extraordinary ray will be deviated; the ordinary ray will not.
The ordinary ray index n, and the most extreme (whether greater
or smaller) extraordinary ray index n
e
, are together known as the
principal indices of refraction of the material.
If a beam of linearly polarized monochromatic light enters a
birefringent crystal along a direction not parallel to the optical axis
of the crystal, the beam will be divided into two separate beams. Each
will be polarized at right angles to the other and will travel in dif-
ferent directions. The original beam energy, which will be divided
between the new beams, depends on the original orientation of the
vector to the crystal.
12ch_PolaizationComponents_f_v2.qxd 6/8/2005 10:20 AM Page 12.2 1
12.3
ASK ABOUT OUR CUSTOM CAPABILITIES
O E M
Polarization Components
Singlets
Multielement Lenses
Cylindrical Optics
Mirr
or
s
Prisms and
Retr
or
eflector
s
Beamsplitter
s
,

W
indows
,
Optical Flats
P
olarization
Components
Filter
s
The energy ratio between the two orthogonally polarized beams
can be any value. It is also possible that all energy will go into one
of the new beams. If the crystal is cut as a plane-parallel plate, these
beams will recombine upon emergence to form an elliptically polar-
ized beam.
The difference between the ordinary and extraordinary ray may
be used to create birefringent crystal polarization devices. In
some cases, the difference in refractive index is used primarily to sep-
arate rays and eliminate one of the polarization planes, for exam-
ple, in Glan-type polarizers. In other cases, such as Wollaston and
Thompson beamsplitting prisms, changes in propagation direc-
tion are optimized to separate an incoming beam into two orthog-
onally polarized beams.
QUARTZ WAVEPLATES:
OPTICAL ACTIVITY vs BIREFRINGENCE
Birefringence is applicable to nonactive crystals, such as calcite,
that have a specific direction and index of refraction exactly equal
for the ordinary and extraordinary rays. Active crystals, such as
quartz, have no such axis, so there is no direction within the crys-
tal in which the indices of refraction are equal. These types of mate-
rials exhibit a phenomenon known as optical activity, whereby the
axis of incident, linearly polarized light appears to rotate as it prop-
agates along the optic axis. The optic axis for active crystals is the
direction in which the index difference between the O and E indices
is minimum.
Melles Griot quartz retardation plates do not use the principle
of optical activity to create a phase retardation. Instead, the crys-
tals are cut with the optic axis parallel to the surfaces of the plate.
When quartz is used in this manner, the retardation is caused by the
birefringence of the quartz, not the optical activity of the material.
DICHROISM
Dichroism is selective absorption of one polarization plane
over the other during transmission through a material. Sheet-type
polarizers are manufactured with organic materials imbedded
into a plastic sheet. The sheet is stretched, aligning molecules and
causing them to be birefringent, and then dyed. The dye molecules
selectively attach themselves to aligned polymer molecules, so
that absorption is high in one plane and weak in the other. The
transmitted beam is linearly polarized. Polarizers made of such
material are very useful for low-power and visual applications. The
usable field of view is large (up to grazing incidence), and diame-
ters in excess of 100 mm are available.
POLARIZATION BY REFLECTION
When a beam of ordinary light is incident at the polarizing angle
on a transmissive dielectric such as glass, the emerging refracted
ray is partially linearly polarized. For a single surface (with n
=1.50)
at Brewsters angle, 100 percent of the light whose electric vector oscil-
lates parallel to the plane of incidence is transmitted. Only 85 per-
cent of the perpendicular light is transmitted (the other 15 percent
is reflected). The degree of polarization from a single-surface reflec-
tion is small.
If a number of plates are stacked parallel and oriented at the
polarizing angle, some vibrations perpendicular to the plane of
incidence will be reflected at each surface, and all those parallel to
it will be refracted. By making the number of plates within the stack
large (more than 25), high degrees of linear polarization may be
achieved. This polarization method is utilized in Melles Griot polar-
izing beamsplitter cubes which are coated with many layers of quar-
ter-wave dielectric thin films on the interior prism angle. This
beamsplitter separates an incident laser beam into two perpendic-
ular and orthogonally polarized beams.
THIN METAL FILM POLARIZERS
Optical radiation incident on small, elongated metal particles
will be preferentially absorbed when the polarization vector is
aligned with the long axis of the particle. Melles Griot infrared
polarizers utilize this effect to make polarizers for the near-infrared.
These polarizers