Patent Publication Number: US-2013229712-A1

Title: Sandwiched diffractive optical combiner

Description:
TECHNICAL FIELD 
     This disclosure relates generally to the field of optics, and in particular but not exclusively, relates to diffractive elements. 
     BACKGROUND INFORMATION 
     In the field of optics, a combiner is an optical apparatus that combines two images together, from either the same side of the combiner (reflective/reflective, or transmissive/transmissive) or from the two different sides of the combiner (reflective/transmissive). Often times, optical combiners are used in heads up displays (“HUDs”), which allow a user to view a computer generated image (“CGI”) superimposed over an external view. The HUD enables the user to view the CGI without having to look away from his usual viewpoint. The term HUD originated from its use in avionics, which enabled a pilot to view information while looking forward with his head up, as opposed to looking down at an instrument panel. Conventional HUDs include tilted dichroic plates, holographic combiners, angled transparent substrates, and compound conjugate lenses. 
     Two version of combiners exist. The first version combines two fields without adding any lens prescription to either field (typically a tilted dichroic plate or compound conjugate lenses). The second version includes a lensing functionality in addition to the combining functionality, which is usually an off-axis aspheric lensing prescription for the field coming from the display. The field coming from the scenery is typically not changed with any lensing functionality. The lensing functionality is often used to form the virtual image of the display into the far field or at a specific distance from the combiner. 
     Holographic combiners are typically used in military applications, due to their significant costs, but do provide a high quality HUD. Holographic combiners can be fabricated by exposing a dichromated gelatin, silver halides, or photopolymers to a pair of intersecting laser beams (reference and object beams). The interference pattern between these beams is recorded into the holographic media thereby forming the holographic combiner after curing. The hologram can be fabricated as a complex mirror with optical power only for the reflected wave (the wave coming from the display), leaving the transmitted wave unperturbed. A hologram can also be fabricated to operate similarly in transmission mode. The complex mirror property reflects a given wavelength incident at a given angle in a desired direction, while the optical power property provides a lensing function, such as a concave reflector. This is the Bragg condition of a traditional volume hologram. However, holographic combiners have a number of drawbacks. They are expensive to fabricated, difficult to mass produce, and have limited life spans (e.g., begin to degrade due to temperature, humidity, pressure and other harsh environmental conditions). 
     Angled transparent substrate combiners have been used in automobiles to present the driver with HUD information on the windshield. These optical combiners are made of a clear see-through substrate upon which an external image source displays the CGI. However, since the clear see-through substrate is typically a flat substrate without optical power so as not to distort the external FOV, the clear substrate must be angled (e.g., near 45 degrees) and bulky external magnification lenses are used to expand the CGI over the display region. The bulky external lenses and angled nature of the clear see-through substrate combiners do not lend themselves well to compact arrangements, such as head mounted displays (“HMDs”). 
     Compound conjugate lens combiners are often used in scopes to display an image (e.g., gun sights) over an external view. These optical combiners include two lenses. The first lens is positioned nearer to the eye, relative to the second lens, and includes a partial reflective coating to project a virtual image of an object (laser reticle for instance) into the user&#39;s eye. The first lens also provides optical power to enlarge the image and virtually displace the image back from the eye to bring it into focus in the case of a near-to-eye display. The second lens is positioned in-line with the first lens opposite the user&#39;s eye and provides complementary optical power to the first lens to pre-distort the external view to offset the optical effects of the first lens on the external view. Compound lens combiners lend themselves well to the barrel configuration of a scope, but are otherwise bulky and rather heavy—thus not well suited for use in HMD configurations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles being described. 
         FIG. 1  is a cross sectional view of a sandwiched diffractive optical combiner, in accordance with an embodiment of the disclosure. 
         FIG. 2  is a plan view of a sandwiched diffractive optical combiner, in accordance with an embodiment of the disclosure. 
         FIG. 3  is a flow chart illustrating a process for fabricating a sandwiched diffractive optical combiner using lithography, in accordance with an embodiment of the disclosure. 
         FIGS. 4A-4F  illustrate fabrication steps for fabricating a sandwiched diffractive optical combiner using lithography, in accordance with an embodiment of the disclosure. 
         FIG. 5  is a top view of a binocular head mounted display using two sandwiched diffractive optical combiners, in accordance with an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of an apparatus, system, and methods of fabrication of a sandwiched diffractive optical combiner are described herein. In the following description numerous specific details are set forth to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
       FIGS. 1 and 2  illustrate a sandwiched diffractive optical combiner  100 , in accordance with an embodiment of the disclosure.  FIG. 1  is a cross-sectional view of optical combiner  100  while  FIG. 2  is a plan view of the same. The illustrated embodiment of optical combiner  100  includes a substrate  105 , a base sandwich layer  110 , a reflective diffraction grating  115 , a planarization sandwich layer  120 , an eye-ward side  125 , and an external scene side  130 . The illustrated embodiment reflective diffraction grating  115  is formed of a two-dimensional (“2D”) array of three-dimensional (“3D”) diffraction element shapes formed into base sandwich layer  110  with partially reflective elements  135  coated onto the 3D diffraction elements shapes and conforming thereto. 
     Optical combiner  100  is referred to as a sandwiched optical combiner since it sandwiches reflective diffraction grating  115  between two material layers (i.e., base sandwich layer  110  and planarization sandwich layer  120 ) having substantially equal, if not identical, indexes of refraction. By doing this, optical combiner  100  simultaneously operates in both reflection and transmission modes with each mode having different characteristics. In reflection, an image source  140  is positioned on the same side of optical combiner  100  as the user&#39;s eye  145  (i.e., eye-ward side  125 ). Since reflective diffraction grating  115  is composed of partially reflective elements  135 , a portion of image light  150  output from image source  140  is reflected back towards the user&#39;s eye  145 . In transmission, the diffractive effects of reflective diffraction grating  115  are annihilated by using the same or similar index of refraction material above and below partially reflective elements  135 . Since partially reflective elements  135  are also partially transmissive and sandwiched in substantially uniform index material(s), the portion of external scene light  155  that passes through reflective diffraction grating  115  is not diffracted, but rather passes to eye  145  substantially without optical distortion. By simultaneously operating optical combiner  100  in both reflective and transmissive modes, it can be used to overlay image light  150  onto external scene light  155  to provide a type of augmented reality to the user. 
     In some embodiments, the shape, size, orientation, and placement of the individual 3D diffraction element shapes formed into base sandwich layer  110  maybe designed to provide optical power for magnifying image light  150 . This magnifying configuration may be particularly useful in near-to-eye configurations, such as head mounted displays (“HMDs”) and some types of heads up displays (“HUDs”), such as scopes. The generic design of diffraction gratings that provide optical power is well known. For example, design of diffractive optics is discussed in “Applied Digital Optics: From Micro-optics to Nanophotonics” by Bernard Kress and Patrick Meyrueis, published by John Wiley and Sons in 2009. In particular, this book discusses how to design and subsequently carve out diffraction structures (microscopic grooves) and select their depth to maximize the amount of light diffracted in a specific diffraction order, while reducing the light diffracted in the zero and higher diffraction orders. 
     In one embodiment, reflective diffraction grating  115  is an off-axis lens, which is capable of receiving input light at incident angle A 1  and reflects the image light along a reflection path having an emission angle A 2  that is different from A 1 . Note, A 1  and A 2  are measured from the normal of the emission surface of optical combiner  100  out which the reflected image light  150  is emitted. In  FIG. 1 , the emission surface coincides with eye-ward side  125  of planarization sandwich layer  120 . In one embodiment, incident angle A 1  is greater or more oblique from normal than emission angle A 2 . This enables image source  140  to be positioned laterally to optical combiner  100  so as not to block external scene light  155 . In HMD configurations, off-axis lensing permits image source  140  to be positioned peripherally in the temple region of the user thereby not obstructing the user&#39;s forward vision. The off-axis lensing redirects the emission angle A 2  to be less oblique from normal than the incident angle A 1 , thereby directing the reflected image light into the user&#39;s eye at a closer to normal angle, versus overshooting the eye and illuminating the nose. Off-axis lensing using diffractive optics also provides a specific angular bandwidth to reflective diffraction grating  115 . This helps reduce distractions due to backside reflections and improve contrast of the reflected image light  150  over external scene light  155 . 
     In  FIG. 2 , the off-axis lensing is achieved by chirping the diffraction grating pattern and offsetting the center  160  of the pattern relative to the user&#39;s center of vision  165 . In the illustrated embodiment, the pattern center  160  is denoted as the center of the largest partially reflective element  135 . As the pattern extends out from center  160 , partially reflective elements  135  become gradually smaller. In  FIGS. 1 and 2 , the 3D diffraction element shapes have parabolic cross-sectional shapes (see  FIG. 1 ) and rotationally symmetric (circular or spherical lens) or non rotationally symmetric (aspheric lens) perimeter shapes (see  FIG. 2 ). However, other cross-sectional shapes and perimeter shapes (e.g., elliptical, etc.) may be used to create reflective diffraction grating  115 . The illustrated embodiment of  FIG. 2  is a 16 phase level off-axis diffractive lens; however, other number of phase levels may be used, the most effective lens having an infinite number of phase levels (quasi analog surface relief diffractive lens). 
     Reflective diffraction grating  115  is formed by overlaying each 3D diffraction element shape with a partially reflective element  135 . Partially reflective elements  135  each conformally coat a corresponding 3D diffraction element shape thereby creating a reflective structure that assumes the shape and orientation of the underlying 3D diffraction element shape. 
     Partially reflective elements  135  may be made of a variety of different materials. In one embodiment, partially reflective elements  135  are fabricated of a layer of conventional non-polarizing beam splitter material (e.g., thin silver layer, CrO2, etc.). The degree of reflectivity may be selected based upon the particular application (e.g., primarily indoor use, outdoor use, combination use, etc.). In one embodiment, partially reflective elements  135  comprise a 10% reflective 100 nm layer of CrO2. 
     In one embodiment, partially reflective elements  135  are fabricated of a multi-layer dichroic thin film structure. Dichroic films can be created to have a selectable reflectivity at a selectable wavelength. Additionally, the dichroic film can be designed to improve the angle selectivity of the reflective diffraction grating  115 . A dichroic film can be designed with high reflectivity to a specific wavelength or wavelength band that overlaps with image light  150  and to the angles of incidence of image light  150 , while being substantially more transparent to other visible spectrum wavelengths and to the normal incidence of external scene light  155 . In this manner, the efficiency of optical combiner  100  can be improved while also increasing the brightness of the transmitted external scene light  155 . 
     In one embodiment, partially reflective elements  135  are fabricated of polarizing beam splitter material that substantially reflects one linear polarization of incident light while substantially passing the orthogonal linear polarization. In this case, image source  140  could be designed to emit polarized image light matching the reflection characteristic of partially reflective elements  135 . Since ambient light typically has a random polarization, approximately 50% of external scene light  155  would pass through optical combiner  100  to eye  145 . 
     Image source  140  may be fabricated using a variety of compact image source technologies such as the various micro-displays used today in pico-projectors, liquid crystal on silicon (“LCOS”) displays, backlit liquid crystal displays, organic light emitting diode (“OLED”) displays, quantum dot array displays, light emitting diode (“LED”) arrays, or otherwise. CRT tubes are still used in HUDs today, but are less likely to be used in smaller devices such as see through Head Mounted Displays (HMDs). Optical combiner  100  may be fabricated of a variety of clear optically transmissive materials, including plastic (e.g., acrylic, thermo-plastics, poly-methyl-metha-crylate (PMMA), ZEONEX-E48R, glass, quartz, etc.). For example, in one embodiment, substrate  105 , base sandwich layer  110 , and planarization sandwich layer  120  are fabricated of plastic. In another embodiment, substrate  105  is glass while base sandwich layer  110  and planarization sandwich layer  120  are fabricated of silicon dioxide. Of course, other material combinations may be used. 
       FIG. 3  is a flow chart illustrating an example process  300  for fabricating one embodiment of sandwiched diffractive optical combiner  100  using lithography, in accordance with an embodiment of the disclosure. Process  300  describes one technique for fabricating an embodiment of optical combiner  100  using silicon dioxide on a glass substrate. Process  300  is described with reference to  FIGS. 4A-F . The order in which some or all of the process blocks appear in process  300  should not be deemed limiting. Rather, one of ordinary skill in the art having the benefit of the present disclosure will understand that some of the process blocks may be executed in a variety of orders not illustrated, or even in parallel. 
     In a process block  305 , base sandwich layer  110  is deposited onto substrate  105 , which is fabricated of a clear material such as glass, quartz, plastic, or otherwise. In this embodiment, base sandwich layer  110  is a layer of silicon dioxide approximately 1 μm thick. In a process block  310 , grayscale lithography and reactive ion etching is used to form the 2D array of 3D diffraction shapes  405  into base sandwich layer  110 . In a process block  315 , shapes  405  are overlaid via sputtering with a layer of partially reflective material. In one embodiment, the partially reflective material layer is approximately 10% reflective (other reflectivity percentages may be used). In one embodiment, the partially reflective material layer is approximately 100 nm thick of CrO2 material. In a process block  320 , planarization sandwich layer  120  is deposited onto of the partially reflective material layer. In one embodiment, planarization sandwich layer  120  is deposited to be approximately 1.5 μm thick. Of course, at this stage planarization sandwich layer  120  is not yet planar. In a process block  325 , a resist material  410  is coated over planarization sandwich layer  120 . Finally, in a process block  330 , resist material  410  is removed during planarization, which proceeds to a depth that results in a planar top to planarization sandwich layer  120 . Such a process can be implemented as a proportional reactive ion etching (RIE) process (or CAIBE process—Chemically Assisted Ion Beam Etching) where the resist etching rate and the underlying SiO2 etching rate are exactly similar. In one embodiment, chemical-mechanical polishing is used to remove resist layer  410  and planarize planarization sandwich layer  120 . In one embodiment, a proportional reactive ion etch with a 1:1 ratio that etches both resist material  410  and planarization sandwich layer  120  at the same rate is used. Other standard or custom planarization techniques may be used. 
     Mass production techniques may be used to fabricate various other embodiments of optical combiner  100 . For example, a master combiner may be fabricated to be used as a mold for plastic replication via injection molding or hot/UV embossing. Base sandwich layer  110  may be fabricated of thermo-plastic material that is injection molded. Partially reflective elements  135  may be overlaid or coated onto the 2D array of 3D diffraction shapes and planarization sandwich layer  120  laminated over the partially reflective material. Diamond turning with CNC machine-tools may be used in place of lithography to shape the various curved fringes making up the optical combiner. In other embodiments, base sandwich layer  110  may be fabricated using press molding into thermo-plastic or plastic embossing using a roller drum having a negative impression of the 2D array of 3D diffraction shapes disposed thereon. 
       FIG. 5  is a top view of a binocular HMD  500  using a pair of sandwiched diffractive optical combiners  501 , in accordance with an embodiment of the disclosure. Each optical combiner  501  may be implemented with an embodiment of optical combiner  100 . The optical combiners  501  are mounted to a frame assembly, which includes a nose bridge  505 , left ear arm  510 , and right ear arm  515 . Although  FIG. 5  illustrates a binocular embodiment, HMD  500  may also be implemented as a monocular HMD. 
     The two optical combiners  501  are secured into an eye glass arrangement that can be worn on the head of a user. The left and right ear arms  510  and  515  rest over the user&#39;s ears while nose assembly  505  rests over the user&#39;s nose. The frame assembly is shaped and sized to position each optical combiner  501  in front of a corresponding eye  145  of the user. Of course, other frame assemblies having other shapes may be used (e.g., a visor with ear arms and a nose bridge support, a single contiguous headset member, a headband, goggles type eyewear, etc.). 
     The illustrated embodiment of HMD  500  is capable of displaying an augmented reality to the user. Each optical combiner  501  permits the user to see a real world image via external scene light  155 . Left and right (binocular embodiment) image light  150  may be generated by image sources  140  mounted to left and right ear arms  510 . Image light  150  is seen by the user as a virtual image superimposed over the real world as an augmented reality. In some embodiments, external scene light  155  may be blocked or selectively blocked to provide sun shading characteristics and increase the contrast of image light  150 . 
     While the microscopic structures of the 2D array of 3D diffraction shapes along with the conforming partially reflective elements  135  produce the optical combiner effect, the macroscopic shape of optical combiners  501  (or  100 ) can include overall curvatures to include a corrective lensing prescription. For example, the external scene side of substrate  105  and/or base sandwich layer  110  may include a first curvature that imparts a corrective lensing prescription. Additionally (or alternatively), the eye-ward side surface of planarization sandwich layer  120  may include a second curvature that imparts a corrective lensing prescription. The first and second curvatures may be different, and in one embodiment, one of the two curvature may be flat while the other is curved. 
     The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
     These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.