Optical: systems and elements – Single channel simultaneously to or from plural channels – By partial reflection at beam splitting or combining surface
Reexamination Certificate
1999-10-29
2001-02-13
Epps, Georgia (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By partial reflection at beam splitting or combining surface
C359S636000, C359S639000
Reexamination Certificate
active
06188520
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to the field of light polarization converters, and more specifically to an improved polarization converter that uses beam splitting cubes.
2. Description of the Related Art
Naturally occurring light is not polarized, which is a state also known as unpolarized. Polarized light is desirable for various applications. Polarized light is derived from unpolarized light using a polarizer, as explained below.
Referring now to
FIG. 1
, a beam
22
of unpolarized light is incident upon polarizer
24
, which operates on the light. At least one beam
26
of polarized light exits polarizer
24
. The incident beam travels along a propagation direction
28
, and the exiting beam travels along a propagation direction
30
. In many applications, direction
30
is the same as direction
28
.
Referring now to
FIG. 2
, the polarization of beam
22
is explained in more detail. A point
40
is considered along direction
28
. What the human eye perceives as light is really traveling electric field vectors
44
,
46
. Although only two such vectors are shown, in fact there can be many. For unpolarized light, their points are distributed uniformly around circle
48
.
It is conventional to analyze the electric field in terms of its components along two orthogonal axes
52
,
54
, that are perpendicular to propagation direction
28
. It should be noted that the axes
52
,
54
can be translated along any point of direction
28
, such as point
40
. This analysis is useful for discussing polarizers. The directions of axes
52
,
54
, are also known as S and P directions.
It can be seen, therefore, how a single vector
46
relates to the two axes
52
,
54
. Each vector would have a component on axis
52
, and a component on axis
54
. In fact, all vectors with points on circle
48
can be similarly decomposed into components on the axes
52
,
54
. When they are all so decomposed, all the vectors for light beam
22
are added along each axis. This results in two vectors
72
,
74
, that represent the whole beam
22
, for polarization purposes. Due to the symmetry of circle
48
, the two vectors
72
,
74
are equal in intensity for unpolarized light.
Referring now to
FIG. 3
, the action of polarizer
24
of
FIG. 1
can be better appreciated. A light at a point
80
of beam
26
is considered. The point
80
is movable along direction
30
. The beam
26
is made from light that has an electric field vector
82
only along axis
52
. There is no component along axis
54
. This is called perfectly linear polarized light, and is polarized in the direction of axis
52
. While it appears the same to the human eye as unpolarized light, it has very useful properties, which makes polarizers desirable.
As such, a polarizer is a device or an arrangement that receives randomly polarized light, and permits to exit only linearly polarized light. Moreover, a polarization converter is a term in the art for a device that either rotates the polarization of received light, or converts randomly polarized light into linearly polarized light. In other words, the term polarization converter has come to be used also for a polarizer.
A useful prior polarizer is now described referring to
FIG. 4. A
polarizing beam splitting (PBS) cube
110
is transparent, and has a hypotenuse surface
112
. The cube
110
receives an incident beam of light
116
, traveling along an incident direction
117
. The cube
110
partially transmits a beam of light
118
along a transmission direction, which is typically identical to the incident direction
117
. The PBS cube
110
also partially reflects a beam of light
120
along a reflected direction
122
. Preferably the hypotenuse surface
112
is located at a 45° angle from the incident direction
117
, in which case reflected direction
122
is at right angles from the incident direction
117
.
The incident beam of light
116
is regarded as unpolarized, although that is not necessary. Specifically, one of its electric field vectors I
P
is in the same plane as the drawing. Vector I
P
is shown as an arrow, and corresponds to the P direction. The other electric field vector I
S
is perpendicular to the plane of the drawing, and thus also perpendicular to the paper. Vector I
S
is a shown as a circled dot, and corresponds to the S direction.
The transmitted beam of light
118
, that exits undeflected from the hypotenuse surface
112
of the cube
110
, has a transmission component T
P
in the P direction, and a transmission component T
S
in the S direction. Similarly, the beam of light
120
that is reflected by the hypotenuse surface
112
has a reflection component R
P
in the P direction, and a reflection component R
S
in the S direction.
The polarizing beam splitting cube
110
has a very useful property, which is why it is used for making polarizers. Theoretically, it splits the incident beam
116
into a P-polarized transmitted beam, and an S-polarized reflected beam. As such, the polarizing beam splitting cube
110
theoretically separates the incident light into two beams of equal intensity, each polarized only in its own direction.
In practice, the actual polarizing beam splitting cube
110
typically deviates from the above described theoretical performance, but not much. The transmission component T
S
is small. For example, if the illuminating beam is a f/2.5 white light, then T
S
=0.005×I
S
. As such, the transmitted beam
118
is light very highly polarized along that the P direction, with a small component in the S direction. Furthermore, the reflection component R
P
is relatively small, too. For example, for the same kind of illuminating light, R
P
=0.08×I
P
. As such, the reflected beam
120
is light mostly polarized along the S direction, with the diminished component in the P direction. In each case there is a dominant polarization component as prescribed by theory, but also a minor polarization component.
There are two criteria for gauging the performance of a polarizer. One criterion is how well the undesirable component has been extinguished. The other criterion is how much light intensity of the desirable polarization component is permitted to go through. Like all other real life optical devices, the PBS cube
110
introduces losses, too. For the same kind of illuminating light, T
P
=0.92×I
P
(a loss of 8%), and R
S
=0.995×I
S
(a small loss of 0.5%). A problem with using a PBS cube is that all the light of beam
120
is wasted.
A polarizer
140
in the prior art is now described with reference to FIG.
5
. The components of the polarizer
140
are shown separated from each other, i.e. not contacting each other, but that is only for purposes of illustration. It will be appreciated that the inclusion of the secondary cubes rescues a lot of the light that would have been otherwise wasted as beam
120
of FIG.
4
.
The polarizer
140
is made from stack of PBS cubes, with their hypotenuse surfaces parallel. Only six cubes
142
,
143
,
144
,
145
,
146
,
147
are shown. The front surfaces of odd-numbered PBS cubes
143
,
145
,
147
, are respectively obstructed by opaque shields
153
,
155
,
157
. The even numbered PBS cubes
142
,
144
,
146
have at their rear faces half-wave retarders
162
,
164
,
166
respectively. The whole stack has a polarizing filter
170
at the exit, to ensure that the undesirable polarization component has been extinguished.
Incident light
176
impinges upon the polarizer
140
along a direction
177
, which is shown as many parallel lines. A transmitted beam
178
emerges after the polarizing filter
170
, polarized in the desirable direction. A reflected beam
180
, polarized substantially only in the undesirable direction, emerges from the side along a side direction
182
and is discarded.
It should be noted that the emerging beam
178
emerges from the entire face of polarizing filter
170
. That is notwithstanding the fact that, due to the opaque shields, the incident light enters th
Epps Georgia
Marger & Johnson & McCollom, P.C.
Seyrafi Saeed
Sharp Laboratories of America Inc.
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