Polarization converter

Abstract: A polarization device comprising: a light path; a micro lens array (32) disposed in the light path for separating light into a plurality of individual beams of light; a polarization filter (38) disposed in the light path to receive a plurality of the beams of light and to transmit the received light of a desired polarization state and reflect the received light of a perpendicular polarization state that is perpendicular to the desired polarization state; and a reflecting polarization converter (40) disposed to receive and reflect the reflected light of the perpendicular polarization state and to convert the perpendicular polarization state to the desired polarization state

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to optical systems, and more particularly, to a high efficiency polarization device for converting substantially all incoming light into a single polarization. 2. Description of the Related Art Applications that use liquid crystal cells to render an image usually require linearly polarized input light to function properly. Conventional light sources are typically unpolarized. In order to polarize this light, it is passed through a polarizer, which transmits the desired polarization state and absorbs or deflects the unusable, perpendicular polarization state. Thus, approximately half of the incoming source light has to be discarded, resulting in reduced system efficiency and brightness. Solutions to the foregoing efficiency problem have been developed to transform the light from the unusable polarization state into the desired state. One solution developed by 3M Company is a dual brightness enhancement film which is used in laptop LCD displays to increase screen brightness. The film transmits the desired polarization state and reflects the perpendicular polarization state back to the light source. Due to scatter and reflection, part of this light is reflected back in the desired polarization state and passes to the screen. However, due to high brightness requirements and light absorption in the source, this approach is of limited efficiency for use in LCD projection applications. Another solution in the art is the use of a polarizing beam splitter of the type shown in Fig. 1. Unpolarized light 10 is directed to a polarizing beam splitter 12. A desired light polarization is transmitted by the polarizing beam splitter 12 and is represented by the light beam 14. The polarizing beam splitter 12 reflects the perpendicular state of polarization. This reflected perpendicular state of polarization is then directed by a mirror 16 through a half wave plate 18, which functions to change the polarization state of the reflected light into light 20 with the desired state of polarization. In theory, the total amount of unpolarized incoming light is transformed into linearly polarized light. However, the resulting light consists of two distinct optical beams, which are difficult to utilize in an optical system. Large and more expensive optics would be required in order to facilitate the utilization of both beams, and the optical efficiency of such a complex system would typically not be optimum. Accordingly, there continues to be an efficiency problem in the conversion of light to a single polarization. Documents WO 99/61942, US-A-5,751,480, US-A-5,650,873 and EP-A-0 957 387 disclose related prior art. SUMMARY OF THE INVENTION Briefly, the resent invention comprises, in one embodiment, a polarization device comprising: a light path; a micro lens array disposed in said light path for separating light into a plurality of individual beams of light; a polarization filter disposed in said light path to receive a plurality of said beams of light and to transmit said received light of a desired polarization state and reflect said received light of a perpendicular polarization state that is perpendicular to said desired polarization state, said polarization filter having a plurality of sawtooth or pyramid shape features; and a reflecting polarization converter disposed to receive and reflect said reflected light of said perpendicular polarization state and to convert said perpendicular polarization state to said desired polarization state, said polarization converter being disposed in said light path between said micro lens array and said polarization filter and including a plurality of transmissive regions to transmit said plurality of individual beams of light, characterised in that each sawtooth or pyramid feature of the polarization filter has its apex centerline aligned with the centerline of a different transmissive region of the polarization converter and the centerline of a different micro lens in the micro lens array. In a further aspect of the present invention, each of a plurality of the transmissive regions is aligned with a different one of the plurality of beams of light from the micro lens array to transmit the beam of light aligned therewith. In a further aspect of the present invention, the polarization filter is shaped to reflect the light of the perpendicular polarization state of each of a plurality of the individual beams of light from the micro lens array at an angle so that the beam is not substantially reflected straight back through the transmissive region aligned therewith and so that each of the reflected individual beams of light impinges on an area on the reflecting polarization converter adjacent to the transmissive region. In yet a further aspect of the present invention, the transmissive regions are formed by holes in the reflecting polarization converter. In a further aspect of the present invention, the reflecting polarization converter comprises a quarter wave plate with a reflective surface disposed thereon with the transmissive regions comprising openings defined in the reflective surface. In a further aspect of the present invention, the polarization filter has a plurality of features therein, each shaped to reflect light from a different one of the individual beams of light to a different particular area on the reflecting polarization converter. In a further aspect of the present invention, the reflecting polarization converter includes a reflective surface disposed on one face of a quarter wave plate. In a further aspect of the present invention, a shape and angle of the polarization filter and a shape and angle of the reflecting polarization converter cause light reflected from the reflecting polarization filter to have substantially a same range of emission angles as the light of the desired polarization state transmitted by the polarization filter. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic block diagram of an embodiment of the prior art. Fig. 2 is a schematic block diagram of a first embodiment of the present invention. Fig. 3 is a schematic block diagram of a preferred embodiment of the present invention. Fig. 4 is a schematic diagram of a reflector or mirror surface that may be used in forming the polarization filter. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to Fig. 2, there is shown a first embodiment of the present invention. Incoming unpolarized light 30 is directed to a micro lens array 32 that operates to separate the incoming unpolarized light 30 into a multitude of individual beams 34. A variety of different types of micro lens array 32 are available for implementing the present invention, and example parameters for such arrays will be discussed below. As described by fundamental optics, the divergence of each of the individual beams created by the individual lenses in the micro lens array 32 will be larger than the divergence of the original beam 30. A polarization filter 38 is disposed in the light path of the light beams 34. The polarization filter 38 transmits with a given range of emission angles received light 37 of a desired polarization state, while substantially reflecting that received light 39 that has a polarization state that is perpendicular to the desired polarization state. By way of example, this polarization filter 38 may be comprised of a formable film for separating the perpendicular polarizations. Dual brightness enhancement film made by 3M Company, or alternatively any device which performs polarization separation filtering may be utilized to implement the polarization filter 38. In a preferred embodiment, the polarization filter 38 is formed to include a plurality of features 44 shaped so as not to reflect the perpendicular polarization of an individual beam straight back in the direction from which it has come, but rather to a designated region on a reflecting polarization converter 40. In a preferred embodiment, the individual beams 34 propagate through the reflecting polarization converter 40 in order to reach the polarization filter 38. To accomplish this propagation, the reflecting polarization element 40 includes an array of transmissive regions 36, with each transmissive region aligned with a different beam 34. The size of each of the transmissive regions 36 in the reflecting polarization converter 40 approximates the waist of the individual beams 34 in order to ensure maximum transmission of the individual beams 34 by the transmissive regions 36. Accordingly, the transmissive regions 36 in the reflecting polarization converter 40 are preferably on the same scale as the micro lenses in the micro lens array 32, with the transmissive region centerline to centerline spacing being substantially the same as the centerline to centerline spacing between micro lenses in the micro lens array 32. The transmissive regions 36 may be formed simply by making holes extending through the reflecting polarization converter 40 of the appropriate diameter aligned with the individual micro lenses. This implementation of the transmissive regions 36 is shown in Fig. 2. Alternatively, in a preferred embodiment, the transmissive regions 36 may be formed in the reflecting polarization converter 40 by forming regions of the appropriate size with no reflective backing aligned with individual micro lenses for transmitting the individual beams to the polarization filter 38. This embodiment of the transmissive regions 36 is shown in Fig. 3. As noted above, in the preferred embodiment, the polarization filter 38 functions to reflect light of the perpendicular polarization so that it does not propagate straight back through the transmissive region 36 from which the light beam has come, but rather to a particular area on the reflecting polarization converter 40. In a preferred embodiment, the features 44 shown in Fig. 2 are two-sided sawtooths or four-sided pyramids to reflect the perpendicular polarization of the light beam to a surface of the reflecting polarization converter 40 to the sides adjacent to the transmissive region 36 through which that beam 34 had initially propagated. It can be seen that there are a plurality of sawtooths or pyramids shown in Fig. 2, each sawtooth or pyramid with its apex centerline aligned with the centerline of a different transmissive region 36 and the centerline of a different micro lens in the micro lens array 32. The feature 44 is constructed on the scale of the micro lens, with the centerline to centerline distance being substantially the same as the centerline to centerline spacing of the micro lens array 32. Fig. 4 shows a top view of the features 44 on the polarization filter 38 configured as a matrix of four-sided pyramids. Note that although the feature 44 is illustrated in the top view in Fig. 4 as a pyramid, the present invention could be implemented with any feature shape that appropriately reflects the perpendicular polarization to a desired surface on the reflecting polarization converter 40 including cone shapes, hemispherical-type shapes, aspherical shapes and asymmetric shapes. Note that cone, hemispherical, aspherical and asymmetrical features would require more complex shapes for the surface. Also, note the scale of Fig. 4 is not identical in scale to Fig. 3 for ease of illustration. The reflecting polarization converter 40 is disposed to re-reflect the reflected light of the perpendicular polarization state and to convert that perpendicular polarization state to the desired polarization state. In a preferred embodiment, the reflecting polarization converter 40 is implemented simply by using a quarter wave plate element 43 with a mirror backing 46 as the base structure, with the transmissive regions 36 formed by not having a mirror backing in those regions. The preferred embodiment with the transmissive regions 36 formed in this manner is shown in Fig. 3. The mirror backing 46 for the reflecting polarization converter 40 may be deposited on a quarter wave plate with appropriate masking techniques used to define the transmissive regions 36 and keep them clear of the mirror material. Alternatively, the mirror or other reflecting material 46 may be formed as a sheet with openings therein defining the transmissive regions 36. This mirror sheet 46 may then be affixed or disposed adjacent to and parallel to the quarter wave plate to form the reflecting polarization converter 40 with the transmissive regions 36 formed therein. Fig. 3 illustrates a mirror surface 46, formed either as a deposition on a surface of a quarter wave plate or a separate sheet affixed or supported parallel to one surface of the quarter wave plate 43, with transmissive regions 36 formed therein that contain no reflective material. In operation, light beam 39 reflected back by the polarization filter 38 impinges upon the quarter wave plate 43 in the reflecting polarization converter 40, propagates through the quarter wave plate 43 to the reflective mirror backing 46 deposited, affixed, or supported adjacent to the quarter wave plate 43, and is reflected back through the quarter wave plate 43 by the mirror backing. Accordingly, light beam 39 reflected by the polarization filter 38 passes twice through the quarter wave plate 43, thereby converting a perpendicular polarization state to light rays 42 having the desired polarization state. Note that the polarization filter 38 will not be an impediment to the re-reflected light rays 42 because the reflected light rays have now been converted to a polarization state that will be passed through the filter 38. Subsequent optics (not shown) are then provided to gather the light beams 37 and 42 and refocus them in a well-known manner. It should be noted that the range of emission angles of the beam 42 reflected by the mirror surface 46 of the reflecting polarization converter 40 depends on the geometries, for example the shape and angle, of the features 44 of the polarization filter 38, and on the geometries of the areas around the transmissive regions 36 in the reflecting polarization converter 40. Note that each instance of an area around a transmissive region 36 in the reflecting polarization converter 40 may take a spherical shape, an aspherical shape, or an asymmetric shape. Accordingly, the geometries of the polarization filter 38 and the reflecting polarization converter 40 can be optimized to cause the resulting converted light beams 42 to have a range of emission angles very close to the range of emission angles of the original transmitted light 37 with the desired polarization, i.e., they have a substantially similar angular distribution of rays. This optimization can be accomplished by shaping the elements 38 and/or 40 and empirically determining the optimum shapes. Alternatively, software design programs such as ZEMAX or ASAP may be utilized in this optimization process. Thus, this system can convert a beam of unpolarized light with high efficiency into a beam of polarized light with a size close to the size of the original beam. It should be noted that the drawn shapes of the elements 38 and 40 are for ease of illustration only and are not to scale. It should also be noted that the angles shown for the light beams 37 and 42 are for ease of illustration only. In a preferred embodiment of the present invention, each micro lens in the micro lens array 32 may be implemented to have a focal length of on the order of 1 mm with a diameter of the micro lens of on the order of 100 microns, and with a spacing between lenses of on the order of 100 microns. It should be noted that the micro lens parameters comprising the focal length, the diameter of the lenses, and the number of lenses per unit length may be optimized empirically to achieve a maximum light polarization efficiency. As a general rule, the spacing of the centers of the individual micro lenses in the micro lens array 32 will be substantially identical to and coincide with the spacing and location of the centers of the transmissive regions 36 in the reflecting polarization converter 40, with a different transmissive region 36 being provided for each of a plurality of the micro lenses in the micro lens array 32. Note that the diameter of each of the transmissive regions 36 in the reflecting polarization converter 40 preferably will be the diameter of the focused beam 34, and thus will depend on the optical characteristics of the lenses used in the micro lens array 32. By way of example, micro lens arrays designed by 3M or by NEC Optics may be utilized to implement the present invention. As noted above, in order to optimize the polarization device of the present invention, the polarization filter 38 or its functional equivalent and/or the reflecting polarization converter 40 should be shaped so as to guide the reflected light beams 42 along paths that are substantially parallel to the unreflected light beams 37, i.e., having a substantially similar angular distribution of rays, but offset from the light beam 37 by ½ the lens array spacing. The number of lenses in the micro lens array 32 must be large enough so that the resulting beam comprising the unreflected light bundles 37 and the reflected/converted light bundles 42 fills with sufficient uniformity the etendue of the optical transport system that follows the device. This optimization will depend on the characteristics of the subsequent optical transport system, and therefore cannot be specified in advance. The present invention provides a method of transforming unpolarized light into polarized light with high efficiency. The polarized light resulting from the present invention is contained in a single beam with a size close to the original unpolarized beam. This contrasts with the prior art where the original beam is expanded to twice its original size in order to obtain all of the light in a single polarization. Thus, the present invention provides light which can be easily utilized in optical systems resulting in higher system efficiency and less complexity than would otherwise be required to improve beam properties. The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiment was chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.

Claims: A polarization device comprising: a light path; a micro lens array (32) disposed in said light path for separating light into a plurality of individual beams of light; a polarization filter (38) disposed in said light path to receive a plurality of said beams of light and to transmit said received light of a desired polarization state and reflect said received light of a perpendicular polarization state that is perpendicular to said desired polarization state, said polarization filter having a plurality of sawtooth or pyramid shape features (44); and a reflecting polarization converter (40) disposed to receive and reflect said reflected light of said perpendicular polarization state and to convert said perpendicular polarization state to said desired polarization state, said polarization converter being disposed in said light path between said micro lens array and said polarization filter and including a plurality of transmissive regions (36) to transmit said plurality of individual beams of light, characterised in that each sawtooth or pyramid feature (44) of the polarization filter has its apex centerline aligned with the centerline of a different transmissive region (36) of the polarization converter and the centerline of a different micro lens in the micro lens array. A device as defined in claim 1, wherein each of a plurality of said transmissive regions of said reflecting polarization converter is aligned with a different one of said plurality of beams of light from said micro lens array to transmit said beam of light aligned therewith. A device as defined in claim 2, wherein said polarization filter is shaped (44) to reflect said light of said perpendicular polarization state of each of a plurality of said individual beams of light from said micro lens array at an angle so that it is not substantially reflected straight back through said transmissive region aligned therewith and so that each of said reflected individual beams of light impinges on an area on said reflecting polarization converter adjacent said transmissive region. A device as defined in claim 2 or 3, wherein said transmissive regions (36) are formed by holes in said reflecting polarization converter. A device as defined in one of claims 2 to 4, wherein said reflecting polarization converter comprises a quarter wave plate (43) and a reflective surface (46) disposed parallel thereto with said transmissive regions (36) comprising openings defined in said reflective surface. A device as defined in claim 5, wherein said reflective surface is disposed on said quarter wave plate. A device as defined in one of the preceding claims, wherein each of said features (44) of said polarization filter is shaped to reflect light from a different one of said individual beams of light to a particular area on said reflecting polarization converter adjacent to said transmissive region. A device as defined in claim 1 or claim 2, wherein said reflecting polarization converter includes a reflective surface (46) disposed on one face of a quarter wave plate (43). A device as defined in one of the proceeding claims, wherein a geometry of said polarization filter and a geometry of said reflecting polarization converter cause light reflected from said reflecting polarization converter to have substantially a same range of emission angles as said light of said desired polarization state transmitted by said polarization filter.

Inventors: HARDCASTLE IAN, ROBRISH PETER, WEBER ANDREAS, HARDCASTLE, IAN, ROBRISH, PETER, WEBER, ANDREAS
Applicants: AGILENT TECHNOLOGIES INC, AGILENT TECHNOLOGIES, INC. (A DELAWARE CORPORATION)
Application year: 2001
Main classification: G02B27
IPC codes: G02B5, G02F1, G02B27