Liquid crystal cells – elements and systems – Particular structure – Lens or prism separate from projection system
Reexamination Certificate
1998-01-27
2002-02-26
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Lens or prism separate from projection system
C349S158000, C349S160000
Reexamination Certificate
active
06351296
ABSTRACT:
FIELD OF THE INVENTION
The present invention is in the field of polarization sensitive beam splitting techniques and relates to a beam polarizer device based on the effect of double refraction of light.
BACKGROUND OF THE INVENTION
Beam polarizers are well known optical devices that are widely used as filters for allowing the passage of light polarized in one direction only, or for image separation. Beam polarizer devices of the kind specified above, employing the effect of double refraction of light, are used in applications which need two spatially separated light components of the different polarizations to be produced from an unpolarized light beam. Such a device typically comprises two spaced-apart glass wedges and a polarization sensitive media therebetween. The term “different polarizations” signifies different orientations of the electric fields of a light wave, which are mutually perpendicular, each being perpendicular to the direction of propagation of an unpolarized beam impinging onto a polarization sensitive media.
The polarization sensitive media is typically in the form of either a plane-parallel, solid-state, birefringent plate, or a multi-layered dielectric structure. The production of the plate-like birefringent media requires the use of natural crystals of high optical quality such as, for example, calcite. Unfortunately, the natural crystals of large sizes cannot be easily obtained. The use of a multi-layered structure provides high polarization purity in one of the channels, unless a very complicated structure up to forty layers is employed. However, even employing such a complicated multi-layered structure, the beam polarizer suffers from a drawback consisting in an unavoidable requirement of a substantially small ‘acceptance angle’, i.e. the angle of incidence of a beam of radiation to be split onto a beam polarizer device. This is caused by the fact that the operation of the multi-layered structure (which is typically formed of different dielectric materials) is based on an interference phenomenon which allows for an acceptance angle not exceeding 3°.
Beam polarizer devices employing a liquid crystal (LC) cell as a birefringent medium have been developed and disclosed, for example, in the article “Using the Interface Between Glass and a Nematic Liquid Crystal for Optical-Radiation Polarization Over a Broad Spectral Range”, A. A. Karetnikov, Opt. Spectrosk. (USSR), 67, 324-326, August 1989. Such a device is schematically illustrated in
FIG. 1
being generally designated
1
. The device
1
comprises a conventional LC cell
2
located between parallel sides
4
a
and
6
a
of a pair of glass prisms
4
and
6
. The LC cell
2
typically comprises a layer
8
formed of a nematic liquid crystal material (NLC), which is enclosed between two so-called ‘orienting layers’
10
a
and
10
b
formed on the sides
4
a
and
6
a
. The orienting layers
10
a
and
10
b
, which are in the form of thin polymer films, provide a homogeneous orientation of the long axes, generally at AX, of rod-like molecules
12
of the NLC, defining thereby the orientation of an optical axis of the layer
8
. The molecules
12
are oriented at a certain so-called “pre-tilt angle” &PSgr;(0°<&PSgr;<90°) relative to the surface
2
a
of the LC cell
2
.
The device
1
operates in the following manner. An unpolarized light wave
14
impinges from the glass
4
onto the surface
2
a
at an angle &phgr;. The surface
2
a
of the LC cell represents an interface on which two different light components contained in the unpolarized wave
14
are spatially separated into so-called “ordinary” and “extraordinary” beams
16
and
18
, respectively.
The terms “ordinary beam” and “extraordinary beam” used herewith signify the beams of different polarizations produced by the passage of an unpolarized light beam through a crystal. The “ordinary beam” is that which obeys Snell's Law and gives a constant refraction index for all angles of incidence, while the “extraordinary beam” is that which does not obey Snell\s Law. The different polarizations are defined by different orientations of the electric fields of a light wave relative to a plane of polarization. The plane of polarization, generally designated
20
, is such a plane that contains beams impinging onto and reflected from the birefringent cell, i.e. beams
14
and
16
, and a normal ON to the cell's surface.
Thus, the beam
16
contains a light component having one of the two orientations of the electric field, i.e. one polarization, while the beam
18
contains a light component of the other orientation of the electric field, i.e. the other polarization. As indicated above, these different orientations of the electric field are mutually perpendicular, each being perpendicular to the direction of propagation of the beam
14
impinging onto the LC cell's surface
2
a.
The beam
18
propagates inside the prism
6
defining an angle &thgr; between the direction of its propagation and the optical axis AX of the NLC layer
8
. The NLC layer
8
, similar to uniaxial crystal plate, is characterized by refraction indices n
or
and n
ex
for ordinary and extraordinary beams
16
and
18
, respectively. It is known that the refraction index n
ex
represents a function of the angle &thgr; and is associated with the refraction index n
or
, as follows:
n
ex
(&thgr;)=N
pr
·N
ex
(n
or
2
sin
2
&thgr;+N
ex
2
cos
2
&thgr;)
−½
wherein N
pr
is the refraction index of the glass prisms; N
ex
is the fundamental value of the refraction index for an extraordinary beam, that is:
N
ex
=n
ex
(90°)
The refraction index N
pr
is chosen to be as follows:
N
pr
≈N
ex
N
pr
≈n
ex
(&thgr;)
If the angle of incidence &phgr; satisfies the following condition:
&phgr;>&phgr;
cr
wherein &phgr;
cr
is a critical angle defined by Snell's Law, then the ordinary beam
16
undergoes a total internal reflection (TIR), while the extraordinary beam
18
propagates inside the LC cell
2
with a divergence angle &phgr;. The angle of orientation &thgr; of the direction of propagation of the beam
18
relative to the optical axis AX is associated with the divergence angle &phgr;′ and with the angle of orientation &psgr; of the optical axes AX relative to the surface
2
a
as follows:
&thgr;=90°+&phgr;′−&psgr;
It is thus understood that the existence of the pre-tilt angle &psgr; significantly influences the above conditions related to the refraction index n
ex
. Obviously, if a beam polarizer is a so-called “active device”, a certain desired value of a pre-tilt angle can be obtained by means of an electric field appropriately applied across the LC layer. However, the case may be such that a beam polarizer is a so-called “passive device” and, therefore, such an application of the electric field is either undesirable or ineffective.
It is often the case that a beam polarizer device is the constructional part of a complicated polarization sensitive optical system such as, for example, projection display. This requires the maximum purity of two different polarizations, the preset orientations thereof relative to the plane of polarization and a substantially wide range of the acceptance angle.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a novel beam polarizer device and a method for its manufacturing for splitting an unpolarized radiation into a pair of spatially separated radiation components of different polarizations.
It is a further object of the present invention to provide such a beam polarizer device in which the different polarizations are substantially fully separated from each other.
There is thus provided, according to the present invention, a beam polarizer device for splitting an unpolarized beam of incident radiation into first and second beams of different polarizations, said beam polarizer comprising:
a birefringent cell interposed between a pair of parallel sides of first and second prisms made of an optically transparent material;
wherein the biref
Pilossof Nissim
Shariv Isaac
Vinokur Klara
Chowdhury Tarifur R.
Fabia Engineering (1992) Ltd.
Rosenman & Colin LLP
Sikes William L.
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