Cube polarizer with minimal optical path length difference

The cube polarizer can have modified prism dimensions to satisfy the following 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.

FIELD OF THE INVENTION

The present application is related generally to polarizing beam splitters, especially wire grid polarizers disposed inside of a cube.

BACKGROUND

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 inFIG. 13, a cube polarizer130can include a wire grid polarizer131sandwiched between two prisms—prismA135and prismB136. The wire grid polarizer131can include wires131wdisposed over a substrate131s. The cube polarizer130is not drawn to scale. In one example of a cube polarizer, the cube can be 10 millimeters (mm) wide, the substrate131scan be 0.7 mm thick, and the wires131wcan be about 0.0003 mm thick. Thus, in order to show all components of the cube polarizer130, the drawings have not been drawn to scale.

An unpolarized light beam U can enter one side (outer faceA) of prismA135and can be polarized into a reflected beam R and a transmitted beam T. The reflected beam R can reflect off the wires131wof the wire grid polarizer131, continue through prismA135, and exit through another side (outer sideA) of prismA135. The transmitted beam T can transmit through the polarizer131and prismB136, and exit through a side (outer faceB) of prismB136.

The reflected beam R has an optical path length OPLRand 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 substrate131s(seeFIGS. 13 and 14). For example, both prisms135and136can have the same size, and can be combined such that edges137of the prisms135and136align with edges of the wire grid polarizer131. The wire grid polarizer131can be disposed at a 45° angle between the prisms135and136, such that light entering perpendicularly to the outer faceAwill meet the wire grid polarizer131at a 45° angle. The wire grid polarizer131can have wires131won one face of the wire grid polarizer131. This cube may be physically symmetric based on outer dimensions, but not optically symmetrical due to the effect of the thickness t of the substrate131s. Following are calculations showing this lack of optical symmetry. See reference variables inFIGS. 13 and 14and definitions below. Note thatFIG. 14shows only the substrate131sof the wire grid polarizer131without the wires131w.

⁢1.⁢⁢d42=t2+t2,d4=2*t.⁢⁢2.⁢⁢OPLR=d1*np+d3*np-d4*np2.⁢3.⁢⁢OPLR=d1*np+d2*np-t*np2,(d2=d3⁢⁢and⁢⁢d4=2*t)⁢4.⁢⁢OPLT=d1*np+d2*np-2*t*np+2*t*ns.⁢5.⁢⁢Δ⁢⁢OPL=OPLT-OPLR=-2*t*np+2*t*ns+t*np2=t*(2*ns-np)2.⁢⁢6.⁢⁢If⁢⁢ns=np,then⁢⁢Δ⁢⁢OPL=t*np2.
Reference variable definitions:d1is a distance from the outer faceAto a center of the polarizer131.d2is a distance from the outer faceBto a center of the polarizer131.d3is a distance from where the light is polarized to the outer sideA. Due to structural symmetry of the cube, d2can equal d3.d4is a distance of travel of the transmitted beam104through the polarizer131.npis an index of refraction of the prisms (assuming both prisms have the same index).nsis an index of refraction of the substrate. Any thin films on the substrate131sare ignored as they are negligible relative to a thickness of the substrate131s.t is a thickness of the substrate. t is also a third leg of a triangle formed by d4and t for a light beam U meeting the polarizer at a 45° angle.ΔOPL is an absolute difference in optical path length between the transmitted beam T and the reflected beam R.

This difference in optical path length

Δ⁢⁢OPL=t*np2
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 polarizer131in 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.

SUMMARY

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. It has been recognized that it would be advantageous to have a cube polarizer with flat wire grid polarizer and minimal wavefront distortion. The present invention is directed to various embodiments of cube polarizers that satisfy these needs. Each embodiment may satisfy one or both of these needs.

In one embodiment, 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 (inner face1), an outer face (outer face1), and an outer side (outer side1). There can be a junction of the inner face and the outer side defining a first edge (first edge1). The second prism can include two triangular faces linked by an inner face (inner face2), an outer face (outer face2), and an outer side (outer side2). The cube polarizer can comprise a wire grid polarizer. The wire grid polarizer can include a substrate having a first surface and an opposite second surface substantially parallel to the first surface. An array of parallel, elongated, separated wires can be 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: (1) the second surface of the substrate is attached to and faces the inner face2; (2) the wires are attached to and face the inner face1; the outer face1is opposite to the outer face2; and the outer side1is opposite to the outer side2. A plane of the outer face2(face plane2) can be substantially aligned with the first edge1.

In another embodiment, 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 (inner face1), an outer face (outer face1), and an outer side (outer side1). The second prism can include two triangular faces linked by an inner face (inner face2), an outer face (outer face2), and an outer side (outer side2). The cube polarizer can comprise a wire grid polarizer. The wire grid polarizer can include an array of parallel, elongated, separated wires sandwiched between a first substrate and a second substrate. Each substrate can have a thickness of greater than 0.4 millimeters. The wire grid polarizer can be sandwiched between the inner faces of the prisms.

In another embodiment, a cube polarizer can be designed for polarization of light including a wavelength λ. The cube polarizer can comprise a first prism, a second prism, and a wire grid polarizer sandwiched between inner faces of the prisms. The wire grid polarizer can include a substrate having a first surface and an opposite second surface substantially parallel to the first surface. There can be a material (material) disposed over the first surface of the substrate. The material1can include an array of parallel, elongated, separated wires. There can be a thin film (thin film2) disposed over the second surface. The thin film2can include a material and a thickness to reduce a curvature of the first surface in order to minimize wavefront distortion.

DEFINITIONS

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.

DETAILED DESCRIPTION

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 inFIGS. 1 and 2, a cube polarizer10is shown comprising a first prism15and a second prism16. This cube polarizer10can be designed for equal, or nearly equal, optical path lengths of a reflected beam R and a transmitted beam T of light. It has been recognized that it can be important to keep these two optical path lengths as close to equal as possible. Examples of applications which require equal (or near equal) optical path lengths are interferometry and 3D projection displays.

The first prism15can 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 face and the outer side1defines a first edge (first edge1). A junction of the outer face1and the outer side1defines a second edge (second edge1). A junction of the inner face and the outer face defines a third edge (third edge1). A distance from the first edge1to the second edge defines an outer side length (LOS1). A distance from the second edge1to the third edge defines an outer face length1(LOF1).

The second prism16can 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 face2and the outer side2defines a first edge (first edge2). A junction of the outer face2and the outer side2defines a second edge (second edge2). A junction of the inner face2and the outer face2defines a third edge (third edge2). A distance from the first edge2to the second edge2defines an outer side length2(LOS2). A distance from the second edge2to the third edge2defines an outer face length2(LOF2).

The cube polarizer10can include a wire grid polarizer11. The wire grid polarizer can be made according to one of the various embodiments of wire grid polarizers90,41,110, and120shown inFIGS. 9-12. The wire grid polarizer11can include a substrate92having a first surface92fand an opposite second surface92ssubstantially parallel to the first surface92f. An array of parallel, elongated, separated wires91(separated by gaps G) can be disposed over the first surface92fof the substrate92. The wire grid polarizer11can be sandwiched between the first prism15and the second prism16such that: (1) the second surface92sof the substrate92is attached to and faces the inner face2; (2) the wires91are attached to and face the inner face1; (3) the outer face1is opposite to the outer face2; (4) and the outer side1is opposite to the outer side2.

An unpolarized light beam U can enter through the outer face1. The unpolarized light beam U can be polarized at the wire grid polarizer11, forming (1) a transmitted beam T of light transmitting through the wire grid polarizer11and exiting through the outer face2; and (2) a reflected beam R of light reflecting off of the wire grid polarizer11and exiting through the outer side1. The cube polarizer10can 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 face2with 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 d11between the face plane2and the first edge1. For exact alignment, d11=0. Substantial alignment can be d11<500 micrometers in one 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 prism16down and to the left (based on view ofFIGS. 1-2) or by decreasing LOS2relative to LOS1. LOS1minus LOS2can be between 500 micrometers and 1000 micrometers in one aspect. The actual desired difference between LOS1and LOS2can depend on a thickness t of the substrate92.

In some designs, it can be desirable to have LOS1>LOS2and LOF1>LOF2. Having LOS1>LOS2and LOF1>LOF2may be desirable to form a square end of the cube polarizer10where the triangular faces of the prisms15and16join, to allow the cube polarizer10to fit into a structure where the cube polarizer10will 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>LOS2and LOF1>LOF2may be desirable if the cube polarizer10is designed to allow unpolarized light to enter through the outer side1and 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 plane2with the first edge1, a plane (side plane2) of the outer side2can be substantially aligned with the third edge1, thus minimizing a distance d12between the side plane2and the third edge1. d12can be less than 500 micrometers in one aspect, less than 100 micrometers in another aspect, or less than 10 micrometers in another aspect. LOF1minus LOF2can be between 500 micrometers and 1000 micrometers in one aspect. The actual desired difference between LOF1and LOF2can depend on a thickness t of the substrate92.

By minimizing the distances d11and/or d12, optical path lengths of the reflected beam R and the transmitted beam T can be equal or substantially equal. Thus, the cube polarizer10can satisfy the equation

OPLT-OPLR<0.5*t*np2
in one aspect, can satisfy the equation

OPLT-OPLR<0.5*t*np2
in another aspect, can satisfy the equation

OPLT-OPLR<0.01*t*np2
another aspect, or can satisfy the equation

OPLT-OPLR<0.001*t*np2
in another aspect, wherein:OPLTis an optical path length of the transmitted beam T;OPLRis an optical path length of the reflected beam R;t is a thickness of the substrate92between the first surface92fand the second surface92s; andnpis an Index of refraction of the first prism15.
|OPLT−OPLR| can be less than 500 micrometers in one aspect, less than 100 micrometers in another aspect, less than 10 micrometers in another aspect, or less than 1 micrometer in another aspect.

Illustrated inFIG. 3are a first prism35and a second prism36in schematic perspective view, to more clearly show the sides, faces, and edges of the prisms. The first prism35can include two triangular faces31and32linked by an inner face (inner faces), an outer face (outer faces), and an outer side (outer sides). The second prism36can include two triangular faces33and34linked by an inner face (inner face2), an outer face (outer face2), and an outer side (outer side2). These prisms35and36are applicable to all embodiments described herein.

Illustrated inFIGS. 4-7is another embodiment of a cube polarizer40. The cube polarizer40can include a wire grid polarizer41sandwiched between the inner faces of a first prism45and a second prism46.

The first prism45can Include two triangular faces linked by an inner face (inner faces), an outer face (outer faces), and an outer side (outer sides). A junction of the inner faces and the outer sides defines a first edge (first edges). A junction of the inner faces and the outer faces defines a third edge (third edges).

The second prism46can 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 face2and the outer side2defines a first edge (first edge2). A junction of the Inner face2and the outer face2defines a third edge (third edge2).

The cube polarizer40can 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 polarizer41can include an array of parallel, elongated, separated wires91(separated by gaps G) sandwiched between a first substrate92and a second substrate104, as shown inFIGS. 10-11. The wires91can be added to the first substrate92by standard thin film deposition and patterning techniques. The second substrate104can be attached on top of the wires91by an adhesive.

The substrates92and104can be thick in an optical sense (e.g. not thin films) in order to provide structural support for the wire grid polarizer41. A thickness th92of the first substrate92and a thickness th104of the second substrate104can 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 th92of the first substrate92can equal or substantially equal the thickness th104of the second substrate104.

A wire grid polarizer110, as shown inFIG. 11, can be used in the cube polarizer40instead of wire grid polarizer41. Wire grid polarizer110includes a first thin film93, such as silicon dioxide for example, disposed above the wires91. The first thin film93can also be disposed in and can substantially fill the gaps G between the wires. For symmetry, a thickness th92of the first substrate92can equal or substantially equal a combined thickness th104+93of the second substrate104and the first thin film93above the wires91. Due to the small thickness of the first thin film93, however, its thickness might be ignored, and a cube polarizer designer might only consider thicknesses th92and th104of the substrates92and104. Wire grid polarizers90and120, as shown inFIGS. 9 and 12, can also be used in cube polarizer80, with the addition of a second substrate104.

For prism symmetry, a size of the two triangular faces of the first prism45can equal or substantially equal a size of the two triangular faces of the second prism46. For equal or substantially equal optical path lengths, the first prism45can be made of substantially the same material as the second prism46. 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 prism45and46size and material, alignment of the prisms45and46can also be important for symmetry of the cube polarizer40. The following description, andFIGS. 5 and 6, describe and show such alignment. As shown inFIG. 5, a plane of the outer face2(face plane2) can cross perpendicularly a plane of the outer side1(side plane1) at a first junction42. A distance d14between the first edge and the first junction42can equal, or substantially equal, a distance d13between the third edge2and the first junction42. As shown inFIG. 6, a plane of the outer face (face plane1) can cross perpendicularly a plane of the outer side2(side plane2) at a second junction43. A distance d16between the first edge2and the second junction43can equal, or substantially equal, a distance d15between the third edge1and the second junction43.

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 inFIG. 7. An unpolarized beam of light U, having a wavelength λ, can enter the cube polarizer40through the outer face1and be polarized at the wire grid polarizer41, forming (1) a reflected beam R of light reflecting off of the wire grid polarizer41and exiting through the outer side1and (2) a transmitted beam T of light transmitting through the wire grid polarizer41and exiting through the outer face2. An absolute value of a difference between an optical path length (OPLT) of the transmitted beam T minus an optical path length (OPLR) of the reflected beam R (|OPLT−OPLR|) can be less than 100*A in one aspect, less than 10*A in another aspect, less than 500 micrometers in another aspect, less than 100 micrometers in another aspect, less than 10 micrometers in another aspect, or less than 1 micrometer in another aspect. The optical path length (OPL) is a distance of light travel through a material times an index of refraction of the material.

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 polarizer80and wire grid polarizer90inFIGS. 8 and 9and as described below. The cube polarizer80can be designed for polarization of light including a wavelength λ.

The cube polarizer80can include a first prism85and a second prism86. The first prism85can include two triangular faces linked by an inner face, an outer face (outer face1), and an outer side (outer side1). The second prism86can include two triangular faces linked by an inner face, an outer face (outer face2), and an outer side (outer side2).

A wire grid polarizer90can be sandwiched between the inner faces of the prisms85and86. The wire grid polarizer90can include a substrate92having a first surface92fand an opposite second surface92ssubstantially parallel to the first surface92f. There can be a material (material1)96disposed over the first surface92fof the substrate92. The material196can include an array of parallel, elongated, separated wires91(separated by gaps G). Material196can also include other thin films93and/or94as will be described below. There can be a thin film (thin film2)95disposed over the second surface92s. The thin film295can balance stresses caused by material196, thus reducing curvature of the wire grid polarizer90and 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 beams82-84. Light beams, including light beams82-84, can enter through the outer face1, can reflect off of portions of the wire grid polarizer90within the cube polarizer80, then can exit through the outer side1. Light beams can reflect off of all portions of the wires91of the wire grid polarizer90within the cube polarizer80. 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 OPLLand the OPLSdefines a peak to valley (PTV). In other words, |OPLL−OPLS|=PTV. The thin film295can include a material and a thickness to reduce a curvature of the wire grid polarizer90such 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 film295to balance stresses caused by the material196is for the thin film2to include a same material as in the material196. For example, if the material1includes silicon dioxide, then the thin film2can also include silicon dioxide; or if the material includes titanium dioxide, then the thin flim2can also include titanium dioxide. Another way for the thin film295to balance stresses caused by the material196is to have similar thicknesses between the thin film295and the material196.

Use of the thin film295for reduction of wavefront distortion can also be used in cube polarizers10and40. Thus, the benefits of reduced wavefront distortion can be combined with the benefits of equalizing optical path lengths of reflected and transmitted beams.

Shown inFIG. 9is a wire grid polarizer90which can be used in the various cube polarizers10,40, and80described above. Specifics of this design, including materials and thicknesses, can improve overall cube polarizer performance. A first thin film93can fill gaps G between the wires91and can extend above the wires91. The first thin film93can comprise silicon dioxide. The first thin film93can extend above the wires91for a thickness th93of between 40 and 120 nanometers. A second thin film94can be disposed over the first thin film93. The second thin film94can comprise titanium dioxide. The second thin film94can have a thickness th94of between 50 and 150 nanometers. If a thin film295is disposed over the second surface92sof the substrate, for reduced wire grid polarizer90curvature, then this thin film295can include silicon dioxide having a thickness th95of between 80 and 300 nanometers.

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 inFIG. 12is a wire grid polarizer120which can be used in the various cube polarizers10,40, and80described above. For cube polarizer10,40, or80, designed to polarize light including a wavelength λ, the wires91can include a layer of metal121and a layer of a material (absorptive layer)122that is substantially absorptive of light having the wavelength λ. The cube polarizer10,40, or80can polarize an incoming beam of light having the wavelength λ into a first beam that is primarily reflected or absorbed by the wires91and a second beam that is primarily transmitted through the wires91. 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 polarizers10,40, and80described 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 wires91can extend longitudinally in the direction of one triangular face to the other triangular face of each prism (into the page in the figures).