# Patent

## Title
Ultra-compact HUD utilizing waveguide pupil expander with surface relief gratings in high refractive index materials

## Abstract

A head up display include an image source, and a waveguide. The waveguide has a first diffraction grating and a first end and a second diffraction grating at a second end. The waveguide is positioned as a combiner and allows viewing of an outside scene and information from the image source. The first and second diffraction gratings are surface relief gratings having a high index of modulation.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
The present application is related to: U.S. patent application Ser. No. 13/250,940, entitled, “Head Up Display (HUD) Utilizing Diffractive Gratings Having Graded Efficiency,” filed on an even date herewith, incorporated herein by reference, and assigned to the assignee of the present application; U.S. patent application Ser. No. 13/251,087, now U.S. Pat. No. 8,903,207, entitled, “System For and Method of Extending Vertical Field of View in Head Up Display Utilizing a Waveguide Combiner,” filed on an even date herewith, incorporated herein by reference in its entirety, and assigned to the assignee of the present application; U.S. patent application Ser. No. 13/250,970, now U.S. Pat. No. 8,937,772, entitled, “System for and Method of Stowing HUD Combiners,” filed on an even date herewith, incorporated herein by reference in its entirety, and assigned to the assignee of the present application; U.S. patent application Ser. No. 13/250,994, now U.S. Pat. No. 8,749,890, entitled, “Compact Head Up Display (HUD) for Cockpits with Constrained Space Envelopes,” filed on an even date herewith, incorporated herein by reference in its entirety, and assigned to the assignee of the present application; and U.S. patent application Ser. No. 13/250,621, now U.S. Pat. No. 8,634,139, entitled, “System for and Method of Catadioptric Collimation in a Compact Head Up Display (HUD),” filed on an even date herewith, incorporated herein by reference herein in its entirety and assigned to the assignee of the present application.
BACKGROUND OF THE INVENTION
The present specification relates to displays. More particularly, the present specification relates to head up displays (HUDs).
Conventional HUDs are generally large, expensive and difficult to fit into small airplanes. Often, conventional HUDs rely on large lenses to form adequate field of view and viewing eye box. Compact HUDs are needed for small business jets and other aircraft where space is constrained in the cockpit.
Substrate guided HUDs have been proposed which use waveguide technology with diffraction gratings to preserve eye box size while reducing lens size. U.S. Pat. No. 4,309,070 issued St. Leger Searle and U.S. Pat. No. 4,711,512 issued to Upatnieks disclose substrate guided HUDS. However, such systems have faced difficulties in design. For example, diffraction gratings based upon holographic materials can be difficult to process consistently. Holograms are generally extremely angle- and wavelength-sensitive because they rely on low index modulation throughout a thick volume (Δn<0.05), where the required phase shift for diffraction can only be met for a small set of wavelengths and angles. Diffraction gratings fabricated using embossing and casting processes are more repeatable but can be limited to organic low refractive index materials, thereby limiting the field of view and spectral range. Diffraction gratings that have mechanically reproduced gratings (ruled gratings) often do not achieve the required resolution for HUD applications.
Therefore, there is a need for diffraction gratings optimized for HUD applications. Yet further, there is need a substrate waveguide HUD including diffraction gratings that meet performance and cost requirements. Still further, there is a need for a HUD including substrate waveguide with diffraction gratings that can be fabricated with sufficient repeatability on a substrate material with acceptable optical characteristics. Yet further, there is a need for a HUD substantive including high index of refraction substantive waveguide with diffraction ratings that enables wide field of view and acceptable luminance.
SUMMARY OF THE INVENTION
An exemplary embodiment relates to a head up display including an image source, and a waveguide. The waveguide has a first diffraction grating at a first end and a second diffraction grating at a second end. The waveguide is positioned as a combiner and allows viewing of an outside scene and information from the image source. The first diffraction grating and the second diffraction grating are surface relief gratings having a high refractive index modulation.
Another exemplary embodiment relates to a method of providing information to a pilot. The method includes providing light associated with the information through a first lens to an input of a substrate waveguide. The method also includes diffracting the light at the input of the waveguide into the waveguide using a surface relief grating and diffracting the light out of the waveguide at an output of the waveguide using a second surface relief grating. The waveguide has a high index of refraction.
Another embodiment relates to an optical system for a head up display including an image source. The optical system includes a combiner. The combiner is disposed to receive light from the image source. The combiner is a waveguide including surface relief diffraction gratings etched into inorganic high index of refraction material of the substrate waveguide.



BRIEF DESCRIPTION OF THE DRAWINGS
Exemplary embodiments are hereafter described with reference to the accompanying drawings, wherein like numerals denote like elements; and:
FIG. 1 is a general block diagram of a head up display (HUD) system in accordance with an exemplary embodiment;
FIG. 2 is a general block diagram of a HUD system in accordance with another exemplary embodiment;
FIG. 3 is a top view schematic drawing of a waveguide for the system illustrated in FIG. 1 in accordance with yet another exemplary embodiment;
FIG. 4 is a top view schematic drawing of a waveguide for the system illustrated in FIG. 1 in accordance with still another exemplary embodiment;
FIG. 5 is a cross sectional view schematic drawing of the waveguide illustrated in FIG. 4 along line 5-5;
FIG. 6 is a side view schematic drawing of collimating optics for the system illustrated in FIG. 1 in accordance with another exemplary embodiment; and
FIG. 7 is a perspective view schematic illustration of an embodiment of the HUD system illustrated in FIG. 1 and attached to a bracket in accordance with another exemplary embodiment.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Before describing in detail the particular improved system and method, it should be observed that the invention includes, but is not limited to, a novel structural combination of optical components and not in the particular detailed configurations thereof. Accordingly, the structure, methods, functions, control and arrangement of components have been illustrated in the drawings by readily understandable block representations and schematic drawings, in order not to obscure the disclosure with structural details which will be readily apparent to those skilled in the art, having the benefit of the description herein. Further, the invention is not limited to the particular embodiments depicted in the exemplary diagrams, but should be construed in accordance with the language in the claims.
With reference to FIG. 1, head up display (HUD) system 10 can be utilized in various applications, including aviation, medical, naval, targeting, ground based, military, etc. HUD system 10 is preferably configured for use in smaller cockpit environments and yet provides an appropriate field of view and eye box for avionic applications.
HUD system 10 preferably includes an image source 20 and a substrate waveguide 40. Image source 20 can be any device for providing an image including but not limited to a CRT display, an LED display, an active matrix liquid crystal display (LCD), etc. In a preferred embodiment, image source 20 is a micro LCD assembly and can provide linearly polarized light.
In addition, system 10 can include collimating optics 30 disposed between substrate waveguide 40 and image source 20. Collimating optics 30 can be a single optical component, such as a lens, or include multiple optical components. In one embodiment, collimating optics 30 are configured as a catadioptric collimator as described with reference to FIG. 6. Collimating optics 30 can be any optical component or configuration of optical components that provide light (preferably collimated light) from image source 20 to substrate waveguide 40. Collimating optics 30 can be integrated with or spaced apart from image source 20 and/or substrate waveguide 40.
In operation, system 10 provides images from image source 20 to a pilot or other operator so that the pilot can simultaneously view the images and a real world scene. The images can include graphic and/or text information (e.g., flight path vector, etc.) related to avionic information in one embodiment. In addition, the images can include synthetic or enhanced vision images. In one embodiment, collimated light representing the image from image source 20 is provided on substrate waveguide 40 so that the pilot can view the image conformally on the real world scene through substrate waveguide 40. Waveguide 40 is preferably transparent for viewing the real world scene through main surfaces or sides 84 and 88.
With reference to FIGS. 1 and 3, waveguide 40 includes an input diffraction grating 42 and an output diffraction grating 44. Gratings 42 and 44 can be a gradient output coupling grating that provides excellent image quality and acceptable brightness in a preferred embodiment. Gratings 42 and 44 are preferably implemented as surface relief gratings in a high refractive index (e.g., N>1.6) dielectric materials, thereby enabling wider field of view with acceptable luminance. Gratings 42 and 44 can be implemented according to a number of techniques. In a preferred embodiment, gratings 42 and 44 are surface relief gratings fabricated using lithographic mastering in a wafer foundry.
Applicants have found that surface relief gratings formed by lithographic mastering can have better performance in avionic HUD applications over holographic gratings. Surface relief gratings can be formed in high refractive index materials, such as, inorganic glass materials, thereby enabling wide field of view with acceptable luminescence. Holographic gratings can have disadvantages related to angle dependency and wavelength sensitivity because such gratings often rely on low index modulation throughout a thick volume (Δ N is less than 0.05). In contrast to holographic gratings, surface relief gratings have much broader angular and spectral acceptance because the surface relief gratings can be extremely thin and use very high index modulations (Δ N equal to approximately 0.6-0.7), thereby satisfying the phase shift over a broad spectrum and angular range.
In a preferred embodiment, gratings 42 and 44 are etched directly in an inorganic high index material (e.g., glass material having refractive index of diffraction, N≧1.5) using reactive ion etching (RIE). This replication can utilize a step and repeat process with less than 100 nanometers repeatability.
Substrate waveguide 40 can be a single glass plate 78 or can be made from two or more fixed glass plates. Substrate waveguide 40 can have a variety of shapes including generally rectangular, oval, circular, tear drop-shaped, hexagonal, rectangular with rounded corners, square-shaped, etc.
In operation, substrate waveguide 40 advantageously receives light from image source 20 provided through collimating optics 30 at an input 72 and provides light to a user at its output 74. Image source 20 provides information using a single color of light (e.g., green light with a approximately between 500 and 550 nanometers (nm)). Light provided to substrate waveguide 40 is preferably linearly or S polarized and collimated. Alternatively, other polarization, multiple colors, or other colors at different wavelengths can be utilized without departing from the scope of the invention.
Substrate waveguide 40 preferably performs two operations in a preferred embodiment. First, substrate waveguide 40 is disposed to provide a medium for transporting light by total internal reflection from input 72 to output 74. Light is reflected multiple times off of opposing main sides 84 and 88 of substrate 40 as it travels from input 72 to output 74. Second, substrate waveguide 40 operates as a combiner allowing the user to view the light from image source 20 at output 74 and light from the real world scene through sides 84 and 88.
Light from collimating optics 30 first strikes diffraction grating 42 at input 72 on side 84 of substrate waveguide 40. Grating 40 diffracts light toward the length of substrate 40 so that it travels by total internal reflection to output 74 on side 84. At output 74, diffraction grating 44 diffracts the light toward the user and out of the substrate waveguide 40. Diffraction grating 42 at input 72 preferably has a greater efficiency than diffraction grating 44 at output 74. In one example, grating 42 has an efficiency of as high as possible (e.g., 50 percent or greater) and grating 44 has an efficiency low enough to provide a uniform image across output 74.
With reference to FIG. 1, diffraction gratings 42 and 44 are disposed on respective opposing sides 84 and 88 of substrate waveguide 40 in one embodiment. With reference to FIG. 2, gratings 42 and 44 can also be formed on the same side 84 of waveguide 40 in one alternative embodiment.
With reference to FIG. 3, a single glass plate 78 of inorganic glass material is utilized for substrate waveguide 40. The thickness of glass material is dependent upon field of view and parameters associated with collimating optics 30. Gratings 42 and 44 are surface relief gratings directly formed on respective sides 84 and 88 according to one embodiment.
With reference to FIG. 3, diffraction gratings 42 and 44 are preferably disposed in respective areas that are rectangular in shape and have the same width as each other in one embodiment. Alternatively, gratings 42 and 44 can have different widths. Grating 44 has a greater height than grating 42 in one embodiment.
Gratings 42 and 44 preferably have a period of 330 nm (plus or minus 20 percent) nanometers. Grating 42 preferably has a trench depth of 100-150 nm, and grating 44 has a trench depth of 50-100 nm in one embodiment. Both gratings 44 and 42 preferably have a 40-70% duty cycle.
With reference to FIGS. 4 and 5, substrate waveguide 40 can be made from two equal sized glass plates 91 and 92 adhered together by optical adhesive or contact bond in one embodiment. Glass plates 78, 91 and 92 can be rectangular in cross-sectional area.
With reference to FIGS. 4 and 5, diffraction gratings 42 and 44 are preferably disposed in respective areas that are rectangular in shape with the same width as each other in one embodiment. Grating 44 preferably has a pitch of 330 nanometers and a trench depth of 60 nanometers in one embodiment. Both gratings 40 and 42 preferably have a 50% duty cycle and are disposed on wafers 94 and 98. Plates 91 and 92 can be approximately 2-10 millimeters thick.
With reference to FIG. 5, diffraction gratings 42 and 44 can be formed on fused silica wafers 94 and 98 that are adhered to respective outside surfaces 85 and 87 of plates 84 and 88 using optical adhesive or contact bond according to one embodiment. In one embodiment, a beam splitting coating 48 is disposed between plates 91 and 92 (FIG. 5) and is parallel to the major external surfaces 84 and 88 of plates 91 and 92. Beam splitting coating 48 increases the number of rays propagating toward output 74 in a preferred embodiment. Alternatively, wafers 94 and 98 can be attached to a single glass plate, such as, plate 78 (FIG. 3). Alternatively, the gratings can be formed directly on surfaces 88 and 85.
In one preferred embodiment, system 10 is configured to expand the pupil of system 10 in a single axis (e.g., in the vertical direction). In one embodiment, substrate waveguide 40 provides an approximately 100 mm vertical×75 mm horizontal exit pupil. Waveguide 40 can effect the single axis pupil expansion. The single axis expansion can be on the order of 3 to 8 times (e.g, approximately 5.8 times in one preferred embodiment). Other orders of pupil expansion are possible depending upon performance criteria, design parameters, and optical components utilized without departing from the scope of the invention.
With reference to FIG. 6, collimating optics 30 can be an assembly 31 disposed adjacent to image source 20 in accordance with one embodiment. Assembly 31 of collimating optics 30 is preferably a catadioptric folded collimator system and includes a fold prism 54, a field lens 56, a beam splitter 59, a curved mirror 58 and a corrective lens 60. Corrective lens 60 is disposed to provide collimated light to diffraction grating 42 (FIG. 1). Fold prism 54 receives polarized light from image source 20 at a face 600. An element 52 is disposed next to the face 600.
The light received at face 600 from image source 20 is bounced by total internal reflection off a surface 602 of prism 54 to an exit surface 604. Exit surface 604 is disposed to provide light to field lens 56. Field lens 56 provides light to an input surface 606 of beam splitter 59. Field lens 56 is preferably configured as a field flattener lens, such as a plano-convex spherical lens. Alternatively, fold prism 54 can be a mirror or include a mirrored surface. In alternative embodiment, fold prism 54 is not required for assembly 51 and lens 64 system. Corrective lens 60 is preferably an aspheric lens.
In one embodiment, collimating optics 30 can provide a 30 degree field of view from image source 20 embodied as a 1.3 inch diagonal LCD which translates into a focal length of approximately 2 inches. Exit pupil 612 is preferably wide enough to allow biocular viewing (e.g., approximately 3 inches which forces the F ratio to be approximately 0.67 or 2⁄3). In one embodiment, optics 30 provide a field of view of 30 degrees horizontally by 18 degrees vertically. An exemplary exit aperture for optics 30 is rectangular having dimensions of 4 inches×1 inch which can be extended to be 4 inches by 4 inches by waveguide 40. Assembly 31 of collimating optics 30 advantageously provides excellent performance, meeting requirements for efficiency, color correction and collimation accuracy.
In one embodiment, exit pupil 612 from lens 60 is truncated to 17 millimeters vertical by 75 millimeters horizontal. This truncation allows system 10 to be folded into a very compact volume. Advantageously, substrate waveguide 40 provides pupil expansion in one direction to achieve a 100 millimeter vertical by 75 millimeter horizontal pupil in one embodiment. Assembly 31 preferably has a cross section that is only approximately 50 millimeters×70 millimeters in one embodiment.
With reference to FIG. 7, HUD system 10 can be packaged as a compact HUD system 702 including substrate waveguide 40 and a fixed bracket 700. Bracket 700 includes a portion including image source 20, and optical components of collimating optics 30. Bracket 700 also includes a portion 702 including the remaining optical components in collimating optics 30. Image source 20 receives data from a HUD computer via wiring 704 associated with bracket 700. Bracket 700 can be coupled to the frame of a cockpit. The specific shape and structure of system 702 is not shown in a limiting fashion.
It is understood that while the detailed drawings, specific examples, material types, thicknesses, dimensions, and particular values given provide a preferred exemplary embodiment of the present invention, the preferred exemplary embodiment is for the purpose of illustration only. The method and apparatus of the invention is not limited to the precise details and conditions disclosed. For example, although specific types of optical component, dimensions and angles are mentioned, other components, dimensions and angles can be utilized. Various changes may be made to the details disclosed without departing from the spirit of the invention which is defined by the following claims.

## Claims

1. A head up display, comprising:
an image source;
a folded catadioptric collimator comprising a fold prism receiving light from the image source, a field lens receiving light from the fold prism, and a beam splitter receiving light from the field lens and redirecting light to a curved reflector, the curved reflector providing collimated light through the beam splitter to an exit surface on the beam splitter, wherein the folded catadioptric collimator is configured to a have a light path from the image source to the exit surface on the beam splitter, the light path having a single bounce within the fold prism as the light travels from the image source to the field lens, wherein the fold prism receives the light directly from the image source;
a bracket; and
a waveguide attached to a planar surface of the bracket, the waveguide having a first diffraction grating at a first end for receiving the collimated light at the exit surface and a second diffraction grating at a second end, the first diffraction grating being an input diffraction grating and the second diffraction grating being an output diffraction grating and the waveguide being positioned as a combiner and allowing viewing of an outside scene and information from the image source, the waveguide having a high index of refraction of greater than 1.6, wherein the first diffraction grating and the second diffraction grating have a period of 330 nm plus or minus 20 percent, wherein the waveguide provides single axis pupil expansion, the single axis pupil expansion provided by the waveguide being on the order of 3 to 8 times, wherein the waveguide comprises a first plate and a second plate and a beam splitting coating disposed between the first plate and the second plate along a line defined by the first plate and the second plate, the line disposed between the first diffraction grating and the second diffraction grating and parallel to main surfaces of the first diffraction grating and the second diffraction grating, the beam splitting coating being configured to cause light entering the waveguide from the image source and traveling from the first diffraction grating to the second diffraction grating via the first and second plates by total internal reflection between the main surfaces to have an increased number of rays propagating to the second diffraction grating.

2. The head up display of claim 1, wherein the folded catadioptric collimator is disposed adjacent the image source and provides the collimated light to the first diffraction grating.

3. The head up display of claim 2, wherein the waveguide is a non-organic material.

4. The head up display of claim 1, wherein the image source is an active matrix liquid crystal display.

5. The head up display of claim 1, wherein the first and second diffraction gratings are etched into the waveguide.

6. The head up display of claim 1, wherein the waveguide has an index of refraction of 1.7 or more.

7. The head up display of claim 1, the first diffraction grating has an area smaller than an area of the second diffraction grating and being located at a position to receive light from the image source.

8. The head up display of claim 7, wherein the first plate and the second plate are equal sized.

9. The head up display of claim 7 wherein the bracket has a same width as the waveguide.

10. The head up display of claim 1, wherein the first diffraction grating has a greater efficiency than the second diffraction grating.

11. A method of providing information to a pilot, the method comprising:
providing light associated with the information from an image source using a folded catadioptric collimator comprising a fold prism receiving light from the image source, a field lens receiving light from the fold prism, a beam splitter receiving light from the field lens and redirecting light to a curved reflector, the curved reflector providing collimated light through the beam splitter to an exit surface on the beam splitter, wherein the substrate waveguide is part of a fixed head up display configuration, wherein the folded catadioptric collimator is configured to a have a light path from the image source to the exit surface on the beam splitter, the light path having a single bounce within the fold prism as the light travels from the image source to the field lens, wherein the fold prism receives the light directly from the image source; and
diffracting the collimated light at an input of the substrate waveguide using a first surface relief grating into the waveguide and diffracting the light out of the waveguide using a second surface relief grating at an output of the waveguide, wherein the substrate waveguide has a high index of refraction greater than 1.6, and wherein the substrate waveguide comprises a first plate and a second plate, wherein the first plate and the second plate are of equal size.

12. The method of claim 11, wherein the first surface relief grating is etched into the waveguide.

13. The method of claim 11, wherein the surface relief gratings are lithographically formed.

14. The method of claim 11, wherein the waveguide is a combiner.

15. The method of claim 11, wherein the input and output are on the same side of the waveguide.

16. An optical system for a head up display including an image source, the optical system comprising:
a combiner, wherein the combiner is positioned to receive light from an image source, wherein the combiner is a waveguide including surface relief diffraction gratings etched into high index of refraction inorganic material, the waveguide having a high index of refraction of greater than 1.6, a first grating of the surface relief diffraction gratings being an input grating and a second grating of the surface relief diffraction gratings being an output grating; and
a folded catadioptric collimator disposed adjacent the first grating, wherein the folded catadioptric collimator provides the light to the first grating, the folded catadioptric collimator having a rectangular exit pupil having dimensions of 4 units by 1 unit, the rectangular exit pupil being extended to dimensions of 4 units by 4 units by the combiner, wherein the folded catadioptric collimator comprises a fold prism receiving light from the image source, a field lens receiving light from the fold prism, a beam splitter receiving light from the field lens and redirecting light to a curved reflector, the curved reflector providing collimated light through the beam splitter to an exit surface of the beam splitter for receipt by the input grating, wherein the field lens is disposed at an output face of the fold prism, wherein the folded catadioptric collimator is configured to a have a light path from the image source to the exit surface on the beam splitter, the light path having a single bounce within the fold prism as the light travels from the image source to the field lens, wherein the fold prism receives the light directly from the image source.

17. The system of claim 16, wherein the combiner is a substrate waveguide.

18. The system of claim 17, wherein a period of the surface relief gratings is 330 nm plus or minus 20 percent and the duty cycle of the surface relief gratings is between 40 and 70 percent.

19. The system of claim 16, wherein the folded catadioptric collimator is disposed in a bracket attached to the waveguide.

20. The system of claim 16, wherein the surface relief gratings are provided on wafers adhered to the combiner.