The present application is related generally to polarizing beam splitters, especially wire grid polarizers disposed inside of a cube.
Wire grid polarizers can be fastened inside of a cube. A cube polarizer can be better than a plate polarizer to (1) reduce astigmatism; (2) provide a mechanical structure, which can allow attachment of other devices (e.g. other polarizers or an LCOS imager); and (3) reduce wavefront distortion.
As shown in
An unpolarized light beam U can enter one side (outer faceA) of prismA 135 and can be polarized into a reflected beam R and a transmitted beam T. The reflected beam R can reflect off the wires 131W of the wire grid polarizer 131, continue through prismA 135, and exit through another side (outer sideA) of prismA 135. The transmitted beam T can transmit through the polarizer 131 and prismB 136, and exit through a side (outer faceB) of prismB 136.
The reflected beam R has an optical path length OPLR and the transmitted beam T has an optical path length OPLT. The optical path length OPL is defined as the actual physical distance the light travels through the cube polarizer times an index of refraction n of the material(s) through which the light travels.
In some cube polarizer designs, there is a substantial difference in optical path length between the reflected and transmitted beams due to a thickness t of the substrate 131S (see
This difference in optical path length
can cause problems in some applications. Methods have been proposed to solve such problems, some of which may be impractical due to high manufacturing cost.
Curvature of a wire grid polarizer 131 in a cube can cause problems. The wire grid polarizer can curve due to stresses induced by the wires or other thin films adjacent to the wires. This curvature can result in a reflected light beam reflected off of one region of the polarizer having a different optical path length than a reflected light beam reflected off of another region of the polarizer, thus causing wavefront distortion. There can be a similar problem with the transmitted beam.
Information relevant to wire grid polarizers and polarizing cubes can be found in U.S. Pat. No. 8,467,128; U.S. Pat. No. 7,570,424; U.S. Pat. No. 7,085,050; U.S. Pat. No. 6,288,840; U.S. Patent Publication Number 2007/0297052; and in the publication “A new type of beam splitting polarizer cube,” Meadowlark Optics, Thomas Baur, 2005, pages 1-9.
It has been recognized that it would be advantageous to have a cube polarizer with minimal difference in optical path length between reflected and transmitted beams. The present invention is directed to various embodiments of cube polarizers that satisfy this need.
The cube polarizer can comprise a first prism and a second prism. The first prism can include two triangular faces linked by an inner face, an outer face, and an outer side; a junction of the inner face of the first prism and the outer side of the first prism defining a first edge of the first prism; a junction of the outer face of the first prism and the outer side of the first prism defining a second edge of the first prism; a junction of the inner face of the first prism and the outer face of the first prism defining a third edge of the first prism; and a distance from the first edge of the first prism to the second edge of the first prism defining LOS1. The second prism can include two triangular faces linked by an inner face, an outer face, and an outer side; a junction of the inner face of the second prism and the outer side of the second prism defining a first edge of the second prism; a junction of the outer face of the second prism and the outer side of the second prism defining a second edge of the second prism; a junction of the inner face of the second prism and the outer face of the second prism defining a third edge of the second prism; and a distance from the first edge of the second prism to the second edge of the second prism defining LOS2.
The cube polarizer can also comprise a wire grid polarizer. The wire grid polarizer can include a substrate with a first surface and an opposite second surface substantially parallel to the first surface and an array of parallel, elongated, separated wires disposed over the first surface of the substrate. The wire grid polarizer can be sandwiched between the first prism and the second prism such that: the second surface of the substrate is attached to and faces the inner face of the second prism; the wires are attached to and face the inner face of the first prism; the outer face of the first prism is opposite to the outer face of the second prism; and the outer side of the first prism is opposite to the outer side of the second prism.
In one embodiment, LOS1>LOS2. In another embodiment, d11 can define a distance between a plane of the outer face of the second prism and the first edge of the first prism. d11 can be less than 400 micrometers.
In another embodiment, a beam of light entering through the outer face of the first prism can be polarized at the wire grid polarizer, forming a transmitted beam of light transmitting through the wire grid polarizer and exiting through the outer face of the second prism and a reflected beam of light reflecting off of the wire grid polarizer and exiting through the outer side of the first prism. The cube polarizer can satisfy the equation:
where: an optical path length is a distance of light travel through a material times an index of refraction of the material; OPLT is an optical path length of the transmitted beam; OPLR is an optical path length of the reflected beam; t is a thickness of the substrate between the first surface and the second surface of the substrate; and nP is an index of refraction of the first prism.
As used herein, “cube” means a solid that is bounded by six faces. Each face need not be square, rectangle, or parallelogram. At least one of the faces can have a curved surface, such as a parabolic shape for example.
As used herein, the term “light” can mean light or electromagnetic radiation in the x-ray, ultraviolet, visible, and/or infrared, or other regions of the electromagnetic spectrum.
As used herein “thin film” means a substantially continuous or unbroken film of material having a thickness not larger than three times a maximum wavelength in the light spectrum of interest. “Substantially continuous” in this definition means that there may be some discontinuity, such as pinholes, but no major discontinuity, such as a division into a grid or separate wires.
Various cube polarizer and wire grid polarizer designs will be described and shown in the figures. These cube polarizers and wire grid polarizers are not necessarily drawn to scale. Due to a relatively large size of prisms of the cubes, smaller size of wire grid polarizer substrates, and very small size of wires or thin films, it would be impractical to draw to scale. Some dimensions of these components are specified below and others are known in the art.
As illustrated in
The first prism 15 can include two triangular faces linked by an inner face (inner face1), an outer face (outer face1), and an outer side (outer side1). A junction of the inner face1 and the outer side1 defines a first edge (first edge1). A junction of the outer face1 and the outer side1 defines a second edge (second edge1). A junction of the inner face1 and the outer face1 defines a third edge (third edge1). A distance from the first edge1 to the second edge1 defines an outer side length (LOS1). A distance from the second edge1 to the third edge1 defines an outer face length1 (LOF1).
The second prism 16 can include two triangular faces linked by an inner face (inner face2), an outer face (outer face2), and an outer side (outer side2). A junction of the inner face2 and the outer side2 defines a first edge (first edge2). A junction of the outer face2 and the outer side2 defines a second edge (second edge2). A junction of the inner face2 and the outer face2 defines a third edge (third edge2). A distance from the first edge2 to the second edge2 defines an outer side length2 (LOS2). A distance from the second edge2 to the third edge2 defines an outer face length2 (LOF2).
The cube polarizer 10 can include a wire grid polarizer 11. The wire grid polarizer can be any wire grid polarizer or can be made according to one of the various embodiments of wire grid polarizers 90, 41, 110, and 120 shown in
An unpolarized light beam U can enter through the outer face1. The unpolarized light beam U can be polarized at the wire grid polarizer 11, forming (1) a transmitted beam T of light transmitting through the wire grid polarizer 11 and exiting through the outer face2; and (2) a reflected beam R of light reflecting off of the wire grid polarizer 11 and exiting through the outer side1. The cube polarizer 10 can be designed for equal, or nearly equal, optical path lengths of the reflected beam R and the transmitted beam T. Optical path length is a distance of light travel through a material times an index of refraction of the material.
One way of equalizing, or nearly equalizing, the optical path lengths of the reflected beam R and the transmitted beam T is to align a plane (face plane2) of the outer face2 with the first edge1. Exact alignment can be optimal, but considerable benefit can be gained by substantial alignment. Imperfections in manufacturing may make exact alignment too difficult. This alignment can be quantified by a distance d11 between the face plane2 and the first edge1. For exact alignment, d11=0. Substantial alignment can be d11<500 micrometers in one aspect, d11<450 micrometers in another aspect, d11<400 micrometers in another aspect, d11<250 micrometers in another aspect, d11<100 micrometers in another aspect, or d11<10 micrometers in another aspect. Such alignment can equalize, or nearly equalize, optical path lengths of the reflected beam R and the transmitted beam T.
This alignment can be done by shifting the second prism 16 down and to the left (based on view of
In some designs, it can be desirable to have LOS1>LOS2 and LOF1>LOF2. Having LOS1>LOS2 and LOF1>LOF2 may be desirable to form a square end of the cube polarizer 10 where the triangular faces of the prisms 15 and 16 join, to allow the cube polarizer 10 to fit into a structure where the cube polarizer 10 will be used, or to avoid an edge of a prism sticking out beyond the rest of the cube where it could be damaged. Having LOS1>LOS2 and LOF1>LOF2 may be desirable if the cube polarizer 10 is designed to allow unpolarized light to enter through the outer side1 and it is important for reflected and transmitted beams from this light to also have equal, or nearly equal optical path lengths.
Thus, in addition to aligning the face plane2 with the first edge1, a plane (side plane2) of the outer side2 can be substantially aligned with the third edge1, thus minimizing a distance d12 between the side plane2 and the third edge1. d12 can be less than 500 micrometers in one aspect, less than 450 micrometers in another aspect, less than 400 micrometers in another aspect, less than 250 micrometers in another aspect, less than 100 micrometers in another aspect, or less than 10 micrometers in another aspect.
Examples of possible relationships between LOF1 and LOF2 include: LOF1−LOF2>1 micrometer, LOF1−LOF2>10 micrometers, LOF1−LOF2>50 micrometers, LOF1−LOF2>100 micrometers, LOF1−LOF2>500 micrometer, LOF1−LOF2<250 micrometers, LOF1−LOF2<500 micrometers, LOF1−LOF2<600 micrometers, LOF1−LOF2<750 micrometers, LOF1−LOF2<1000 micrometers. The actual desired difference between LOF1 and LOF2 can depend on a thickness t of the substrate 92.
By minimizing the distances d11 and/or d12, optical path lengths of the reflected beam R and the transmitted beam T can be equal or substantially equal. Thus, the cube polarizer 10 can satisfy the equation
in one aspect, can satisfy the equation
in another aspect, can satisfy the equation
in another aspect, or can satisfy the equation
in another aspect, wherein:
Illustrated in
Illustrated in
The first prism 45 can include two triangular faces linked by an inner face (inner face1), an outer face (outer face1), and an outer side (outer side1). A junction of the inner face1 and the outer side1 defines a first edge (first edge1). A junction of the inner face1 and the outer face1 defines a third edge (third edge1).
The second prism 46 can include two triangular faces linked by an inner face (inner face2), an outer face (outer face2), and an outer side (outer side2). A junction of the inner face2 and the outer side2 defines a first edge (first edge2). A junction of the inner face2 and the outer face2 defines a third edge (third edge2).
The cube polarizer 40 can be designed for equal, or nearly equal, optical path lengths of a reflected beam R and a transmitted beam T. Such equality of optical path lengths can be achieved by wire grid polarizer symmetry and prism symmetry.
For wire grid polarizer symmetry, the wire grid polarizer 41 can include an array of parallel, elongated, separated wires 91 (separated by gaps G) sandwiched between a first substrate 92 and a second substrate 104, as shown in
The substrates 92 and 104 can be thick in an optical sense (e.g. not thin films) in order to provide structural support for the wire grid polarizer 41. A thickness th92 of the first substrate 92 and a thickness th104 of the second substrate 104 can both be greater than 0.4 millimeters in one aspect, greater than 0.5 millimeters in another aspect, or between 0.4 and 1.4 millimeters in another aspect. For wire grid polarizer symmetry, the thickness th92 of the first substrate 92 can equal or substantially equal the thickness th104 of the second substrate 104.
A wire grid polarizer 110, as shown in
For prism symmetry, a size of the two triangular faces of the first prism 45 can equal or substantially equal a size of the two triangular faces of the second prism 46. For equal or substantially equal optical path lengths, the first prism 45 can be made of substantially the same material as the second prism 46. Alternatively, there can be differences of materials and index of refraction between the prisms, and such differences can be compensated for by differences in size between the prisms, but such a design can be complex. Symmetry of both material and size can be a simple way to obtain equivalent optical path lengths.
In addition to equal or similar prism 45 and 46 size and material, alignment of the prisms 45 and 46 can also be important for symmetry of the cube polarizer 40. The following description, and
Cube symmetry, due to combined wire grid polarizer symmetry and prism symmetry, can allow equal, or substantially equal optical path lengths as described below and shown in
Curvature of a wire grid polarizer in a cube can cause problems. The wire grid polarizer can curve due to stresses induced by the wires, or other thin films adjacent to the wires. This curvature can result in a reflected light beam from one region of the polarizer having a different optical path length than a reflected light beam from another region of the polarizer, thus causing wavefront distortion. There can be a similar problem with the transmitted beam.
This curvature problem can be solved or improved as shown on cube polarizer 80 and wire grid polarizer 90 in
The cube polarizer 80 can include a first prism 85 and a second prism 86. The first prism 85 can include two triangular faces linked by an inner face, an outer face (outer face1), and an outer side (outer side1). The second prism 86 can include two triangular faces linked by an inner face, an outer face (outer face2), and an outer side (outer side2).
A wire grid polarizer 90 can be sandwiched between the inner faces of the prisms 85 and 86. The wire grid polarizer 90 can include a substrate 92 having a first surface 92f and an opposite second surface 92S substantially parallel to the first surface 92f. There can be a material (material1) 96 disposed over the first surface 92f of the substrate 92. The material1 96 can include an array of parallel, elongated, separated wires 91 (separated by gaps G). Material1 96 can also include other thin films 93 and/or 94 as will be described below. There can be a thin film (thin film2) 95 disposed over the second surface 92S. The thin film2 95 can balance stresses caused by material1 96, thus reducing curvature of the wire grid polarizer 90 and reducing wavefront distortion.
This reduced wavefront distortion can be demonstrated by minimal variation of optical path lengths of light beams, such as for example light beams 82-84. Light beams, including light beams 82-84, can enter through the outer face1, can reflect off of portions of the wire grid polarizer 90 within the cube polarizer 80, then can exit through the outer side1. Light beams can reflect off of all portions of the wires 91 of the wire grid polarizer 90 within the cube polarizer 80. These light beams can include (1) a light beam having a shortest optical path length (OPLS) and (2) a light beam having a longest optical path length (OPLL). Optical path length is a distance of light travel through a material times an index of refraction of the material. A difference between the OPLL and the OPLS defines a peak to valley (PTV). In other words, |OPLL−OPLS|=PTV. The thin film2 95 can include a material and a thickness to reduce a curvature of the wire grid polarizer 90 such that the PTV is less than λ/2 in one aspect, less than λ/4 in another aspect, less than λ/8 in another aspect, less than 500 nanometers in another aspect, less than 350 nanometers in another aspect, or less than 100 nanometers in another aspect.
One way for the thin film2 95 to balance stresses caused by the material1 96 is for the thin film2 to include a same material as in the material1 96. For example, if the material1 includes silicon dioxide, then the thin film2 can also include silicon dioxide; or if the material1 includes titanium dioxide, then the thin film2 can also include titanium dioxide. Another way for the thin film2 95 to balance stresses caused by the material1 96 is to have similar thicknesses between the thin film2 95 and the material1 96.
Use of the thin film2 95 for reduction of wavefront distortion can also be used in cube polarizers 10 and 40. Thus, the benefits of reduced wavefront distortion can be combined with the benefits of equalizing optical path lengths of reflected and transmitted beams.
Shown in
In some applications of cube polarizers, both the reflected beam R and the transmitted beam T are used and it may be desirable to reflect one polarization as much as possible. In other applications, it can be beneficial to suppress or absorb the reflected beam R. For example, the reflected beam R may interfere with other devices in the system where the cube polarizer is used. Shown in FIG.
12 is a wire grid polarizer 120 which can be used in the various cube polarizers 10, 40, and 80 described above. For cube polarizer 10, 40, or 80, designed to polarize light including a wavelength λ, the wires 91 can include a layer of metal 121 and a layer of a material (absorptive layer) 122 that is substantially absorptive of light having the wavelength λ. The cube polarizer 10, 40, or 80 can polarize an incoming beam of light having the wavelength λ into a first beam that is primarily reflected or absorbed by the wires 91 and a second beam that is primarily transmitted through the wires 91. At least 75% of the first beam can be absorbed by the wires in one aspect, at least 85% in another aspect, or at least 92% in another aspect.
The prisms in the cube polarizers 10, 40, and 80 described herein can be triangular prisms. The inner faces, outer sides, and outer faces of the prisms can have a parallelogram shape, can be rectangular, can be square, but need not be such shapes. The two triangular faces of each prism can be parallel or substantially parallel to each other, but such relationship is not required. The prisms and the wire grid polarizer substrates can be made of a material that is substantially transparent of the desired light wavelength band (e.g. glass for visible light). In one embodiment, the wires 91 can extend longitudinally in the direction of one triangular face to the other triangular face of each prism (into the page in the figures).
This application is a continuation-in-part of U.S. patent application Ser. No. 14/699,803, filed on Apr. 29, 2015, which claims priority to U.S. Provisional Patent Application No. 62/004,010, filed May 28, 2014, which are hereby incorporated herein by reference in their entirety.
Number | Date | Country | |
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62004010 | May 2014 | US |
Number | Date | Country | |
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Parent | 14699803 | Apr 2015 | US |
Child | 15357356 | US |