Overlapping facets

Abstract

To reduce non-uniformity in an image out-coupled toward a nominal point of observation on each item by specifically managing a configuration of overlap of facets.SOLUTION: A waveguide including at least two parallel surfaces, first, middle, and last partially reflecting facets are configured such that in a geometrical projection of the facets onto one of the surfaces the facets overlap, preferably, adjacent facets overlap, and non-adjacent facets starts and ends coincide along at least a portion of the waveguide.SELECTED DRAWING: Figure 16B

Description

[Technical field]

[0001] This application claims the benefit of Provisional Patent Application (PPA) No. 62 / 474,614, filed March 22, 2017 by the present inventors, which is incorporated herein by reference in its entirety.

[0002] The present invention relates generally to optical systems, and more particularly to uniform reflection. [Background technology]

[0003] One important application for compact optical elements comprises head-mounted displays (HMDs) where the optical module acts as both an imaging lens and a combiner, whereby a two-dimensional representation is imaged at infinity and reflected to the observer's eye. The representation is obtained directly from a spatial light modulator (SLM), such as a cathode ray tube (CRT), a liquid crystal display (LCD), an organic light-emitting diode array (OLED), a scanning source or similar device, or indirectly by a relay lens or a fiber optic bundle. The representation comprises an array of elements (pixels) imaged at infinity by a collimator lens and transmitted to the viewer's eye by a reflective or partially reflective surface acting as a combiner for non-see-through or see-through applications, respectively. Typically, conventional free-space optical modules are used for this purpose. As the desired field of view (FOV) of the system expands, these conventional optical modules necessarily become larger, heavier and bulkier, making the device impractical even for moderate performance. These are major drawbacks for all types of displays, but especially for head-mounted applications where the system must be as lightweight and compact as possible.

[0004] The effort to be compact has resulted in several different complex optical solutions. All of these, on the one hand, are still not compact enough for most practical applications, and on the other hand, are difficult to manufacture. Furthermore, the eye-motion-box (EMB) of the optical viewing angle resulting from these designs is usually very small, typically less than 8 mm. The performance of the optical system is therefore very sensitive to even small movements relative to the viewer's eyes, and does not allow sufficient pupil movement to conveniently read the displayed text. Summary of the Invention

[0005] According to the teachings of the present embodiment, an optical device is provided that includes a waveguide, the waveguide having a first surface of at least one pair of surfaces parallel to one another, a first region where light is coupled into the waveguide, and a first array of facets, the facets including: a first facet disposed proximal to the first region and having a first width in a direction between the first pair of surfaces; a last facet at a distal end of the first array of facets from the first region and having a third width in a direction between the first pair of surfaces; and one or more intermediate facets between the first and last facets and having a second width in a direction between the first pair of surfaces, each of the facets having a width in a plane of the facet, the facet being at least partially reflective and at an oblique angle to the first pair of surfaces, the width of the facet being at least partially reflective and at least partially reflective with respect to the first pair of surfaces. a facet start on a proximal side of the width of the facet and a facet end on a distal side of the width of the facet, and wherein the geometric projection is made onto one of the first pair of surfaces in the direction of a nominal light ray outcoupled from the waveguide, the nominal light ray being a central ray of light coupled out of the waveguide, the geometric projection of the last facet and each of the one or more intermediate facets overlaps with respective geometric projections of the one or more intermediate facets and adjacent first facet, and the geometric projection of the facet start of the last facet and each of the one or more intermediate facets coincides with respective geometric projections of the one or more intermediate facets and non-adjacent facet ends of the first facet, the coincidence being along at least a portion of the waveguide.

[0006] In any embodiment, the first width of the first facet is less than the second width of one or more of the intermediate facets. In another optional embodiment, many facets intersect with a nominal ray coupled out of the waveguide, and the number of facets is constant for all of the first array. In another optional embodiment, the light corresponds to an image, and the central ray is a central ray from the center of the image. In another optional embodiment, the light corresponds to an image, and the central ray corresponds to a central pixel of the image. In another optional embodiment, the last facet has a reflectivity that is substantially 100% of the nominal reflectivity, the nominal reflectivity being the total reflection required at a particular position of the waveguide. In another optional embodiment, the third width is less than the second width. In another optional embodiment, the third width is substantially half the second width. In another optional embodiment, the one or more intermediate facets are selected from the group consisting of: 1; 2; 3; 4; 5; and more. In another optional embodiment, a certain number of the facets overlap in a line of sight toward a nominal observation point of the light coupling from the waveguide by one of the surfaces of the first pair. In another optional embodiment, the width of one facet of the plurality of facets of the first array of facets varies monotonically with the width of an adjacent one of the plurality of facets of the first array of facets. In another optional embodiment, the spacing between one pair of adjacent facets of the first array of facets varies monotonically with the adjacent spacing between another pair of adjacent facets of the first array of facets. In another optional embodiment, the light from the first region is such that at least a portion of the light strikes the first facet before striking one of the one or more intermediate facets. In another optional embodiment, the spacing between adjacent facets is greater than the coherence length of the light coupling into the waveguide.

[0007] In any embodiment, the first width is substantially equal to the second width, and the first facet has a first portion corresponding to a geometric projection of the first facet that does not overlap with a geometric projection of an adjacent, intermediate facet. In another optional embodiment, the first portion is transparent to light. In another optional embodiment, the first portion has a reflectivity that is substantially twice the reflectivity of an adjacent facet. In another optional embodiment, the facet has a uniform partial reflectivity across the facet.

[0008] In any embodiment, the waveguide further has a second pair of surfaces that are parallel to each other and non-parallel to the first pair of surfaces, and the facets are configured such that when an image is coupled into the waveguide in the first region with an original direction of propagation at an oblique coupling angle to both the first and second pair of surfaces, the image travels along the waveguide for four intra-fold reflections. In another optional embodiment, the second pair of surfaces is perpendicular to the first pair of surfaces. In another optional embodiment, the facets are at an oblique angle to the second pair of surfaces.

[0009] In any embodiment, a first width of the first facet is substantially equal to a second width of the intermediate facet, a first reflectivity of the first facet is greater than 50% of the nominal reflectivity, a second facet adjacent to the first facet has a second reflectivity, the second reflectivity and the first reflectivity being substantially the nominal reflectivity, a third facet adjacent to the second facet has a third reflectivity that is greater than 50% of the nominal reflectivity and less than the first reflectivity, and a fourth facet adjacent to the third facet has a fourth reflectivity, the fourth reflectivity and the third reflectivity being substantially the nominal reflectivity.

[0010] In another optional embodiment, a first width of the first facet is substantially equal to a second width of the intermediate facet, the first array of odd facets includes the first facet and any number of every other facet from the first facet, the first array of even facets includes a second facet adjacent the first facet and any number of every other facet from the second facet, the first set of facets includes a first odd facet from the first array of odd facets and a corresponding first even facet from the first array of odd facets, and the first odd facet has a first reflectance greater than 50% of the nominal reflectance. a first even facet has a reflectivity of 1, a first even facet has a second reflectivity, the second reflectivity and the first reflectivity being substantially the nominal reflectivity, each subsequent set of facets including the next odd and even facet from the respective array of the first odd and even facets, each odd facet from the subsequent set having an odd reflectivity that is greater than 50% of the nominal reflectivity and less than the reflectivity of the odd facet from the previous set, and each even facet from the subsequent set having an even reflectivity such that the odd reflectivity plus the even reflectivity of the corresponding even facet is substantially the nominal reflectivity. [Brief description of the drawings]

[0011] Embodiments are herein described, by way of example only, with reference to the accompanying drawings.

[0012] [Figure 1] FIG. 1 is a side view of a prior art foldable optical device. [Diagram 2] FIG. 2 is a side view of a typical light guide optical element. [Diagram 3] A and B illustrate the desired reflectance and transmittance characteristics of a selectively reflective surface for two ranges of angles of incidence. [Figure 4] 1 illustrates a typical structure of a light guide optical element. [Diagram 5] 13A-13C are diagrams illustrating alternative configurations of light guide optical elements. [Figure 6]FIG. 13 illustrates a detailed cross-sectional view of a one-dimensional waveguide of a lateral pupil dilation section having a symmetrical structure. [Figure 7] FIG. 1 illustrates how a dual LOE structure can be used to expand a beam along two axes. [Figure 8] FIG. 13 illustrates another method of expanding a beam along two axes using a dual LOE structure. [Figure 9] 1 illustrates an exemplary embodiment of a LOE embedded in a standard eyeglass frame. [Figure 10] 1A and 1B are schematic diagrams of a waveguide with non-overlapping facets illustrating the effect of variations on image uniformity; [Figure 11] AC is a typical alternative structure for the implementation of overlapping facets with various angle propagation structures. [Figure 12] AB are schematic side and front depictions, respectively, of a two-dimensional optical aperture multiplier. CD is a schematic diagram illustrating two possible geometric arrangements of image rays propagating from the optical aperture multipliers of A and B to internal facets that are partially reflected within the waveguide. [Figure 13] FIG. 13 is a schematic isometric view illustrating an implementation of a 2D waveguide in which the internal partially reflective facets are angled at an angle to both sets of elongated parallel exterior surfaces. [Figure 14] 1A and 1B are schematic side and front representations, respectively, of an optical aperture multiplier constructed with overlapping facets. [Figure 15] 14A and 14B are schematic side and front depictions, respectively, of an optical aperture multiplier that is modified from the structure of FIG. 14A and B to provide expansion in a free-space optical configuration. [Figure 16A] 1 is a schematic diagram of a typical facet implementation. [Figure 16B] 1 is a schematic diagram of a typical facet implementation. [Figure 16C] 1 is a schematic diagram of a typical facet implementation. [Figure 17]A is a rough schematic of a dual facet, B is a rough schematic of varying facet spacing, C is a rough schematic of decreasing facet spacing from the proximal to distal end of the waveguide, and D is a rough schematic of varying facet width. [Figure 18] 1 is a rough schematic diagram of applying overlapping facets to a symmetric structure. [Figure 19A] 1 is a graph of the total nominal reflectance in a double overlap structure. [Figure 19B] 1 is an exemplary graph of total nominal reflectance in a dual overlap structure with alternative facet reflectance variations. [Figure 20] 1A illustrates a process that can be used to create a waveguide with overlapping facets. FIG. 1B illustrates a typical procedure for attachment of a coupling prism. [Figure 21] 1A-D are further details of a typical procedure for producing a waveguide with overlapping facets.

[0013] <Abbreviations and definitions> For ease of reference, this section contains a concise list of abbreviations, acronyms, and short definitions used in this document. This section should not be considered as limiting. More detailed descriptions may be found below and in the applicable standards. 1D - one dimension 2D - two dimensions CRT - cathode ray tube EMB - Eye Motion Box FOV - field of view HMD - Head Mounted Display HUD - Head Up Display LCD - Liquid Crystal Display LOE - light-guide optical element OLED - Organic Light Emitting Diode Array SLM - Spatial Light Modulator TIR - total internal reflection DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The principles and operation of the present system may be better understood with reference to the drawings and the accompanying description.The present invention is an optical device that produces a uniform reflection to an observer.

[0015] The configuration of the facet overlap is specifically controlled to reduce non-uniformity in the image outcoupled to a nominal point of observation. A waveguide including at least two surfaces, i.e., a first, middle, and a last partially reflective facet, is configured such that, upon geometric projection of the facets onto one of the surfaces, the facets overlap, preferably adjacent facets overlap, and the beginning and end of non-adjacent facets coincide along at least a portion of the waveguide.

[0016] <Basic Technology - Figures 1 to 9> Figure 1 illustrates a conventional prior art folded optical system configuration, where a substrate (2) is illuminated by a display source (4). The display is collimated by a collimating optic (6), e.g. a lens. Light from the display source (4) is coupled to the substrate (2) by a first reflective surface (8) such that the chief ray (11) is parallel to the substrate plane. A second reflective surface (12) couples the light from the substrate to the eye of a viewer (14). Although this configuration is compact, it has significant drawbacks. In particular, only a very limited FOV can be achieved.

[0017] Please refer now to FIG. 2, which is a side view of an exemplary light guide optical element (LOE). To alleviate the above limitations, the present embodiment utilizes an array of selectively reflective surfaces fabricated in the light guide optical element (LOE). A first reflective surface (16) is illuminated by a collimated viewing light beam (18) emanating from a light source (not shown) located behind the device. In the current figure, for simplicity, generally only one light beam is shown, which is the incident light beam (38) (also called the "beam" or "incident light beam"). Other rays of incident light, such as beams (18A) and (18B), may be used to specify the edges of the pupil where the light enters, such as the left and right edges of the pupil where the light enters. In general, it should be noted that wherever an image is represented herein by a light beam, the beam is a sample beam of the image, typically formed by multiple beams, at slightly different angles, each corresponding to a point or pixel of the image. Except when specifically referred to as the tip of the image, the illustrated beam is typically the centroid of the image.

[0018] The reflective surface (16) reflects the incident light from the light source so that the light is trapped inside the waveguide (20) by total internal reflection. The waveguide (20) is also referred to as a "planar substrate" and an "optically transmissive substrate." The waveguide (20) includes at least two (major) surfaces that are parallel to each other, which are shown in the drawings as a lower (major) surface (26) and an upper (major) surface (26A).

[0019] An incident light ray (38) enters the substrate at the proximal end of the substrate (right side of the figure). The light propagates through the waveguide and one or more facets, usually at least a number of facets, and typically several facets, toward the distal end of the waveguide (left side of the figure). The light propagates through the waveguide in both the initial direction of propagation (28) and the alternative direction of propagation (30).

[0020] After several reflections from the surface of the substrate (20), the trapped waves reach an array of selectively reflective surfaces (22) that couple the light from the substrate to the viewer's eye (24). In an alternative configuration, the selectively reflective surfaces (22) are present immediately after the light beam (18) enters the substrate (20) without first reflecting off the surface of the substrate (20).

[0021] An internal partially reflective surface, such as the selectively reflective surface (22), is generally referred to as a "facet" in the context of this specification. In the limit, the facet can also be fully reflective (100% reflectivity, or mirror, e.g., the last facet at the distal end of the substrate), or minimally reflective. In an augmented reality application, the facet is partially reflective, allowing light from the real world to enter through the top surface (26A), traverse the substrate containing the facet, and exit the substrate through the bottom surface (26) to the viewer's eye (24). In a virtual reality application, the facet may have an alternative reflectivity, such as a first internally connected mirror with 100% reflectivity, since image light from the real world does not need to traverse this mirror. The internal partially reflective surface (22) generally at least partially traverses the waveguide (20) at an oblique angle (i.e., neither parallel nor perpendicular) to the elongation direction of the waveguide (20).

[0022] References to reflectivity are generally to nominal reflectivity. Nominal reflectivity is the total reflectivity required at a particular location in the substrate. For example, if a facet is said to have a reflectivity of 50%, this generally refers to a nominal reflectivity of 50%. If the nominal reflectivity is 10%, then a reflectivity of 50% would result in a facet reflectivity of 5%. Those skilled in the art will understand the use of reflectivity percentages from the context of use. Partial reflectivity can be achieved by a variety of techniques, including but not limited to the use of a percentage of light transmission or polarization.

[0023] Figures 3A and 3B illustrate the desired reflective behavior of a selectively reflective surface. In Figure 3A, light ray (32) is partially reflected from facet (34) and coupled out (38B) from substrate (20). In Figure 3B, light ray (36) is transmitted through facet (34) without any significant reflection.

[0024] 4 is a detailed cross-sectional view of an array of selectively reflective surfaces that couple light into a substrate and then out to the viewer's eye. As can be seen, a light ray (38) from a light source (4) hits a first partially reflective surface. A portion of the light ray (41) continues in the original direction and is coupled out of the substrate. The other part of the light beam (42) is coupled into the substrate by total internal reflection. The captured light beam is gradually coupled out of the substrate by the other two partially reflective surfaces (22) at point (44). The coating properties of the first reflective surface (16) do not necessarily have to be similar to those of the other reflective surfaces (22), (46). This coating could be a simpler beam splitter, either metallic, dichroic or a hybrid metallic-dichroic. Similarly, in the case of a non-see-through system, the last reflective surface (46) could be a simple mirror.

[0025] Figure 5 is a detailed cross-sectional view of a device including an array of reflective surfaces, the last reflective surface (46) being a fully reflective mirror. In such a case, it is true that the leftmost portion of the last reflective surface (46) cannot be optically active, and the fringe rays (48) cannot be coupled out of the substrate. Thus, the output aperture of the device will be slightly smaller. However, the optical efficiency can be very high, greatly simplifying the manufacturing process of the LOE.

[0026] It is important to note that, unlike the configuration illustrated in FIG. 2, there are constraints on the orientation of the reflective surfaces (16), (22). In the former configuration, all light is coupled into the substrate by the reflective surface (16). Thus, the reflective surface (16) does not need to be parallel to the reflective surface (22). Furthermore, the reflective surface may be oriented such that light is coupled out of the substrate in a direction opposite to the direction of the input wave. However, for the configuration illustrated in FIG. 4, a portion of the input light is not reflected by the reflective surface (16), but continues in the original direction of the input light (38), and is immediately coupled out of the substrate as the output light (41). Thus, to ensure that the output direction of all light rays originating from the same plane wave is the same, it is not enough for all the reflective surfaces (22) to be parallel to each other, the reflective surface (16) must be parallel to these surfaces as well.

[0027] Referring again to FIG. 4, a system is described having two reflective surfaces for coupling the light out of the substrate, however any number of reflective surfaces may be used according to the required output aperture of the optical system and the thickness of the substrate. Of course, there are cases where only one outcoupling surface is required. In this case, the output aperture would be effectively twice the size of the input aperture of the system. The only reflective surfaces required in the final configuration are simple beam splitters and mirrors.

[0028] 4, the light from the display source is coupled into the substrate at the edge of the substrate, however, there are systems in which it is preferable to have a symmetrical system, i.e. the input light should be coupled into the substrate at the central portion of the substrate.

[0029] FIG. 6 illustrates a detailed cross-sectional view of a one-dimensional waveguide with a symmetrical structure of lateral pupil dilation. This figure illustrates how to combine two identical substrates to produce a symmetrical optical module. As can be seen, part of the light from the display source (4) leaves the substrate directly through the partially reflective surface. Another part of the light is coupled into the right side of the substrate (20R) and into the left side of the substrate (20L) by the partially reflective surfaces (16R) and (16L), respectively. The captured light is then gradually outcoupled by the reflective surfaces (22R) and (22L), respectively. Obviously, the output aperture is three times the size of the input aperture of the system, with the same magnification as described in FIG. 8. However, unlike that system, the present system is symmetric about the joint surface (29) of the right and left substrates.

[0030] Please refer now to Figures 7 and 8, which are exemplary implementations of Figures 5 and 6 on a waveguide. The configurations of Figures 5 and 6 expand the incident image laterally. The apparatus of Figure 5 can be used to implement the first LOE (20a) of Figure 7, the apparatus of Figure 6 can be used to implement the first LOE (20a') of Figure 8, and the apparatus of Figure 2 can be used to implement the second LOE (20b).

[0031] FIG. 7 illustrates an alternative method for expanding a beam along two axes using a dual LOE configuration. An input wave (90) is coupled by a first reflective surface (16) into a first LOE (20a) having an asymmetric structure similar to that illustrated in FIG. 5, and then propagates along the η axis. A partially reflective surface (22a) couples the light out of the first LOE (20a), where it is then coupled into a second asymmetric LOE (20b) by a reflective surface (16b). The light propagates along the ξ axis, where it is then outcoupled by a selectively reflective surface (22b). As shown, the original beam (90) is expanded along both axes, where the overall expansion is determined by the ratio between the lateral dimensions of elements (16a) and (22b). The configuration provided in FIG. 7 is merely an example of a dual LOE scheme. Other configurations are possible in which two or more LOEs are combined together to form a complex optical system.

[0032] Please refer now to FIG. 8, which illustrates another method for expanding the beam along two axes using a dual LOE configuration. Usually, the area where the light is coupled to the second LOE (20b) by the partially reflective surface (16b) cannot be transparent to the outside light and is not part of the see-through area. Therefore, the first LOE (20a) does not need to be transparent itself. As a result, even in a see-through system, it is usually possible to design the first LOE (20a) to have a symmetric structure, as seen in this drawing. The second LOE (20b) has an asymmetric structure that allows the user to see the outside view. In this configuration, a portion of the input beam (90) continues along its original direction (92) to the in-coupling mirror (16b) of the second LOE (20b), while another portion (94) is coupled by a partially reflective surface (16a) into the first LOE (20a'), propagates along the η-axis, and is then coupled by a selectively reflective surface (22a) into the second LOE (20b). Both portions are then coupled by a reflective surface (16b) into the second asymmetric LOE (20b), propagates along the ξ-axis, and is then out-coupled by a selectively reflective surface (22b).

[0033] FIG. 9 illustrates an embodiment of LOEs (20a) / (20a') and (20b) embedded in a standard eyeglass frame (107). The display source (4) and folding and collimating optics (6) are assembled inside the arm portion (112) of the eyeglass frame, immediately adjacent to the LOE (20a) / (20a'), which is located at the edge of the second LOE (20b). If the display source is an electronic device such as a small CRT, LCD, or OLED, the driving electronics (114) for the display source may be assembled inside the rear of the arm (112). A power and data interface (116) may be connected to the arm (112) by leads (118) or other communication devices, including wireless or optical transmission. Alternatively, a battery and small data link electronics may be integrated into the eyeglass frame. The embodiment illustrated in FIG. 9 is merely an example. Other possible head mounted display configurations can be constructed, including assemblies in which the display source is mounted parallel to the plane of the LOE or mounted on top of the LOE.

[0034] Further details of this underlying technology can be found in US Pat. No. 7,643,214.

[0035] <First embodiment - Figs. 10A to 21D> Refer now to FIG. 10A, which is a schematic diagram of a waveguide with non-overlapping facets, illustrating the effect of variations on image uniformity. A source of perceived non-uniformity is related to the angle of overlap of the internal facets at different fields of view. In the region of the waveguide (10) or (20) illustrated in this drawing (see FIG. 12A and FIG. 12B), the waveguide includes internal facets (two are shown as the last facet (2515) and the first facet (2517)). Most of the outcoupled light is reflected from a single internal facet. However, at the edges of the facets, there is a non-uniformity in the off-axis angles. For the region of the FOV pointing to the left (marked with a solid arrow), at this angle there is an effective gap between the light reflected by the last facet (2515) and the first facet (2517), so the conventional gap area (2520) (also commonly called the "underlapping area", "black line" area or "dark strip" area) does not reflect light, resulting in the perception of a dark strip. On the other hand, light outcoupled to the right (marked with a dashed line) has a conventional bright area (2525) (also commonly called the "partially overlapping" area or "intense" area), in which there is an overlap of the light reflected from the facets (2515) and (2517), resulting in the waveguide reflecting almost twice as much light. Thus, the non-uniformity in FIG. 10A will vary between approximately 200% and 0% mean image intensity across the elongated aperture in various regions of the FOV and eye positions.

[0036] Reference is now made to FIG. 10B, which is a schematic diagram of a waveguide with overlapping facets illustrating the effect of variation on image uniformity. As illustrated in this figure, a significant overlap is introduced between the facets. In this case, the spacing between adjacent facets is halved, resulting in a large portion of the FOV at most eye positions being illuminated from the image via overlapping reflections from two facets. In this typical case, a single intermediate facet (2535) is configured between the last facet (2515) and the first facet (2517). At the angular extremities of the image and near the end of the facets, there will still be variation in the number of overlapping facets contributing to a particular region of the image, as illustrated by the overlap region (2540) resulting from only one facet (the intermediate facet (2535)) and the bright region (2545) contributed by three adjacent facets (2517), (2535), and (2515). Therefore, the output non-uniformity varies between 50% and 150% of the mean reflectance.

[0037] Light from the first half of facet (2517) (light propagating from the right) couples out as reduced energy (ray / output beam (2546)) because at this location there is no overlap of the next facet (2535) i.e. there is only one facet reflecting light to the observer. The same reduced power occurs in the second half of facet (2515) (ray / output beam (2547)). In these areas the reflectance is 50% of the intermediate reflectance.

[0038] A feature of this embodiment is the management of the configuration of facet overlap, specifically optimizing the overlap to obtain a constant number of facets (more than one) that reflect light onto the observer, in other words at least two facets reflect light into the observer's FOV.

[0039] See now Fig. 11A-C, which are exemplary alternative configurations for implementing overlapping facets for different reflection (angular propagation) configurations (these configurations are also described in Fig. 12C and Fig. 12D). For simplicity in this figure, only one ray is shown, the incident ray (38) (also called "beam"), along with the corresponding out-coupled ray (38B). For simplicity, cross-coupling is not shown. Some of the rays (38B1) pass through the internal facet (2560), and some out-coupled rays (38B2) couple directly out.

[0040] In the configuration of FIG. 11A, the incident light ray (38) intersects the internal facet (2560) from both sides of the internal facet (2560). The first intersection is from the rear side of the facet (2562), where the coating on this rear side of the facet should be transmissive due to the shallow angle. The beam also intersects the facet (2564) from another side, on the front side, opposite the rear side, where at this typically steeper angle the coating on the facet should be partially reflective so that some of the light is directed out of the waveguide. (A similar single facet is described in U.S. Pat. No. 7,391,573 B2.)

[0041] In the configuration shown in Figure 11B and C, the angle of the internal facet (2560) and the direction of light propagation are set so that the beam (incident ray (38)) always passes through the internal facet (2560) from the same side of the facet. Coatings on the facet can be used to set the reflectivity and transmission so that the appropriate beam (38B) is reflected out.

[0042] In Figure 11B, the beam (38) intersects the first internal facet (2560) near perpendicular, as shown at point (2568) where the facet's coating is designed to be transmissive. The second intersection is at a shallower angle, as shown at point (2570) (further from perpendicular), where the coating is designed to be a partial reflector so that some of the light is outcoupled (38B).

[0043] In FIG. 11C, the facet coating is set to be a partial reflector at angles near normal, as shown at point (2574), and to be transmissive at angles away from normal, as shown at point (2576).

[0044] Referring now to the drawings, Figures 12A-12D illustrate overlapping facets in one-dimensional (1D) and two-dimensional (2D) waveguides of an optical aperture multiplier. In general, the term optical aperture multiplier according to an embodiment of the present invention includes a first optical waveguide (10) having a direction of elongation arbitrarily illustrated herein as corresponding to the "X-axis". The first optical waveguide (10) has first and second pairs of parallel faces (12a), (12b), (14a), (14b) that form a rectangular cross-section. A plurality of internal partially reflective surfaces (40) at least partially traverse the first optical waveguide (10) at an oblique angle (neither parallel nor perpendicular) to the direction of elongation. The optical aperture multiplier also includes a second optical waveguide (20) optically coupled to the first optical waveguide (10), preferably having a third pair of parallel surfaces (22a), (22b) forming a slab-type waveguide, i.e., the other two dimensions of the waveguide (20) are at least an order of magnitude greater than the distance between the third pair of parallel surfaces (22a), (22b). Also herein, the plurality of partially reflective surfaces (45) preferably at least partially traverse the second optical waveguide (20) at an oblique angle to the third pair of parallel surfaces.

[0045] The optical coupling between the waveguides, and the arrangement and configuration of the partially reflective surfaces (40), (45) are such that when an image is coupled into the first optical waveguide (10) in an initial direction (28) of propagation (e.g., light ray (38)) at an oblique coupling angle to both the first and second pairs of parallel surfaces (12a), (12b), (14a), (14b), the percentage of intensity of the image reflected at the partially reflective surface (40) to be coupled into the second optical waveguide (20) is (10) by internal reflection of four folds (images (a1), (a2), (a3) ​​and (a4)) and then propagates by reflection of two folds (images (b1), (b2)) in the second optical waveguide (20) with a proportion of the intensity of the image reflected at the partially reflective surface (45) being oriented outwardly from one of the parallel surfaces as a visible image c seen by the user's eye (47).

[0046] 12A and 12B, which are schematic side and front depictions, respectively, of a two-dimensional optical aperture multiplier, show a first example of the above described implementation. The first waveguide (10) is referred to herein as a two-dimensional (2D) waveguide in the sense that it guides the injected image in two dimensions by reflection between two sets of parallel planes (in this case a first and second pair of parallel planes (12a), (12b), (14a), (14b)), while the second waveguide (20) is referred to as a one-dimensional (1D) waveguide in the sense that it guides the injected image in only one dimension between one pair of parallel planes (in this case a third pair of parallel planes (22a), (22b)).

[0047] Further improvement to the reduction in non-uniformity can result from the introduction of a "multi-path" image produced by overlapping internal facets, as shown in FIG. 12B. In general, there is a similar process in the implementation of overlapping facets. Light propagating within the 2D waveguide (10) (marked as a solid arrow and designated "a") is out-coupled (designated "b"), but some of the light from b is back-coupled to a' (marked as a dashed arrow) before being out-coupled as "b'" (marked as a dashed arrow). This back-and-forth coupling between "a'" and "b'" causes an averaging of the intensity across the aperture while maintaining parallelism, thereby further improving the uniformity of the light. This improvement can also be implemented in other waveguides in a similar process using overlapping facets, such as shown in FIG. 12A for the 1D waveguide (20). Light propagating within the 1D waveguide (20) (marked as solid arrows and shown as beams "b1" and "b2") is outcoupled (shown as beam "c"), however, some of the light from beam (c) is coupled back into (b2') (marked as dashed arrow) before being outcoupled as beams (c3) and (c4) (marked as dashed arrows).

[0048] A light ray (38) from an optical image generator (not shown) is injected into the first waveguide (10) at an angle. Eventually, the light propagates along the waveguide (10) while being reflected from all four exterior faces of the waveguide as shown in the side view of Figure 12A. In this process, four conjugate beam vectors (a1), (a2), (a3), and (a4) are generated, which represent the same image as that internally reflected by the faces.

[0049] The angle of the beam (38) injected into the waveguide (10) is set so that it reflects from all four outer faces of the waveguide. The ray should reflect from the bottom face (12b) of the first waveguide (10), i.e., the face adjacent to the second waveguide (20), at a shallow angle (line of sight angle), and preferably transmit from (10) to (20) at a steep angle. This property can be achieved by total internal reflection (TIR) ​​or optical coatings. Diffraction patterns can also implement this optical property by combining diffraction with transmission on the same surface. Reflection from the other three faces (12a), (14a), and (14b) of the first waveguide (10) can be effected in the same way or by the use of reflective coatings.

[0050] A portion of the guided light beams in the first waveguide (10), e.g., beam (a1) and beam (a2), are reflected downwards by an internal parallel partial reflector (facet) (40) onto the input coupling surface of the second waveguide (20). In the second waveguide (20), these beams are defined as exemplary beams (b1) and (b2). In this process, the overlapping configuration causes cross-coupling, thereby improving uniformity without degradation of image quality (as described).

[0051] Beams (b1) and (b2) are reflected by the exterior surfaces and become conjugate, i.e., beam (b1) is reflected to become beam (b2) (as shown in FIG. 12A) and vice versa. The exterior front and back surfaces (14a), (14b) of the first waveguide (10) should be parallel to each other and to the corresponding exterior surfaces (22a), (22b) of the second waveguide (20) in this implementation. Any deviation from parallelism will cause the images coupled from beams (b1) and (b2) to not be exact conjugate images, resulting in poor image quality.

[0052] An internal facet (45) in the second waveguide (20) reflects the beam (b2) out of the waveguide to the eye of the observer (47). The internal facets (45) may also overlap, which further improves image uniformity as described for facet (40).

[0053] The reflection process by internal facets in waveguides (10) and (20) is further explained in Figures 12C and 12D. Two basic configurations are shown, which differ by the relative angles of the rays and facets. In this schematic illustration, beams (a1), (a2) and (b1) are shown as the same vector (only (b1) is referenced), since the same geometric considerations apply to each as observed from a side view of the corresponding waveguide. Beams (a3), (a4) and (b2) are also shown as the same vector (only (b2) is referenced).

[0054] Ray (b2) is actually a bundle of rays propagating in the same direction as shown by two vectors in FIG. 12C. In this case, one vector is reflected by the outer surface to become beam (b1) and onto the internal facet (40) (or (45)), where a portion of the one vector is reflected as beam (c1). The other beam (b2) vector is reflected by the facet as direct vector beam (c2). Vector beams (c1) and (c2) represent the normal and ghost images, not necessarily in that order. In this configuration, beams (b1) and (b2) strike the facet (45) from the same side.

[0055] FIG. 12D describes essentially the same process, but the geometry is such that beams (b1) and (b2) strike facet (40) (or (45)) from opposite sides.

[0056] In both cases, the magnitude of the reflections for images (c1) and (c2) in S and P polarizations is determined by the coatings on these facets. Preferably, one reflection is the image and the other reflection is suppressed, since the other image corresponds to an unwanted "ghost" image. It will be appreciated that coatings suitable for adjusting what range of incident beam angles are reflected and what range of incident beam angles are transmitted are known in the art and are described in detail in U.S. Pat. Nos. 7,391,573 and 7,457,040, which are co-assigned herewith.

[0057] 13 illustrates an alternative implementation in which a partially reflective surface of the first waveguide (10), designated herein as (155), is at an oblique angle to both surfaces (12a) and (14a). (The dashed lines are intended to facilitate visualization of the slope of the facet by showing one plane perpendicular to both outer surfaces and another plane that is tilted relative to only one surface.)

[0058] Refer now to Figures 14A and 14B, which are schematic side and front depictions, respectively, of an optical aperture multiplier constructed with overlapping facets. The general operation of this figure is described above with respect to Figures 12A and 12B. Facet overlap is applied in 2D waveguides (10) as well as 1D waveguides (20). In this example, in Figure 14B, the 2D waveguide (10) expands its optical aperture laterally (from right to left in this figure) and the 1D waveguide (20) expands its optical aperture vertically (from top to bottom in this figure) before transmitting light to the eye of the observer (47).

[0059] In FIG. 14A, light (shown as incident ray (38)) is coupled into a 2D waveguide (10). The waveguide includes overlapping facets (40). Dashed lines are used in this figure to show the alignment of the facets (40), which are shown as double lines. In this implementation, the first facet (40a) and the last facet (40b) have smaller areas than the facets in between the internal facets (40). This allows the coupled-out light ("b") of the 2D waveguide (10) to be substantially uniform, because the coupled-out light "b" results from a constant number of facets, including the beginning and end of the 2D waveguide (10). For example, output rays (b10) and (b20) (which actually overlap when output from waveguide (10), but are shown slightly separated in the figure for clarity) produce a combined output caused by two facets (first facet (40a) and adjacent facets of interior facet (40)). Similarly, output rays (b30) and (b40) produce outputs from two facets.

[0060] For comparison, see FIG. 10B, where the light output beam (2546) from the first complete facet (2517) and the light output beam (2547) from the last complete facet are outcoupled as reduced energy. Using partial first and last facets (40a, 40b), this reduction in energy is avoided because the partial first facet (40a) and partial last facet (40b) overlap adjacent facets (40) by being shorter. Note that if the last facet to be illuminated is designed to have 100% reflectivity (nominal reflectivity of 100% when used for enhanced visibility), then the last facet will perform similarly to the full facet (2515).

[0061] The configuration of overlapping facets described for the 2D waveguide (10) works similarly to the 1D waveguide (20). The internal facets (45) reflect light to the observer (47). The internal facets (45) of the 1D waveguide overlap as described for the internal facets (40) of the 2D waveguide. Similar to the partial first and last facets (40a) and (40b), the first and last facets (45a) and (45b) have reduced areas to maintain uniform illumination as described for the 2D waveguide (10).

[0062] Now, see Figures 15A and 15B, where the basic structure of Figures 14A and 14B is modified by replacing the 2D waveguide (10) to perform lateral expansion with a free-space optical arrangement (11) (e.g. as described in Figures 5 and 6). An innovative overlapping structure of 1D waveguides (20) is also used to perform longitudinal expansion.

[0063] Please refer now to Figures 16A to 16C, which show schematic diagrams of typical facet implementations. The facets can be arranged in various overlap configurations, including but not limited to the amount of overlap, the angle of the facet (major edges, such as the paired lower surface (26) and upper surface (26A)) with respect to the parallel plane of the waveguide substrate, and the reflectivity. The overlap of the facets can be implemented in what is referred to in the context of this document as single (non-overlapping), double, and triple (overlapping) facets. In general, the overlap of two or more facets (with the definition starting with "double facet") is referred to as "multiple facets" or "multiple overlaps". As will be evident from this description and the non-limiting examples, partial overlaps as well as additional overlaps beyond triple are possible. For clarity in this figure, the propagation from the incident ray (38) to the out-coupled ray (38B) is not shown.

[0064] As described above in connection with FIG. 2, FIG. 16A shows, for reference, a conventional implementation with a single facet or no overlap. The waveguide (20) includes a facet (22), shown as a double line, between the first two surfaces (26, 26A). The first region (54) is the region where light (shown as ray (38)) is coupled into the substrate. The solid arrow indicates an out-coupled ray (38B) that intersects only one facet (single facet crossing). Note that references to the number of "intersecting" and "intersected" facets include counting the facet from which the out-coupled ray originates. The dashed lines are used to indicate the alignment of the facets (22). In this single facet configuration, the facets (22) do not overlap, specifically consisting of the end of one facet aligned with the beginning of an adjacent facet.

[0065] Reference to alignment will be clear to one of skill in the art with respect to the geometric projection of a facet onto one of the surfaces. For example, the facet start of exemplary facet (F1) has a geometric projection onto the lower surface (26) at point (P1). The facet end of exemplary facet (F2) also has a geometric projection onto the lower surface (26) at point (P1). The facet start of exemplary facet (F2) has a geometric projection onto the lower surface (26) at point (P2). The facet end of exemplary facet (F3) also has a geometric projection onto the lower surface (26) at point (P2).

[0066] FIG. 16B is a schematic diagram of a double facet (double facet intersection, double overlap). This is a preferred implementation that minimizes the increase in manufacturing complexity (compared to higher level intersections) while experiments have shown it produces excellent results. A non-limiting example of a double facet overlap is generally used in this description. A waveguide (light transmitting substrate, waveguide (20)) includes an overlapping internal facet (40) between its first (two) surfaces (26, 26A), shown as a double line. A solid arrow indicates the incident light ray (38). Another solid arrow indicates a nominal light ray that intersects two facets and is then outcoupled from the substrate (arrow outcoupling light ray (38B)). The intersection of these two facets (facet (F11) and facet (F12)) is a double facet intersection. As in similar figures, dashed lines are used to indicate the alignment of the facets (40). In this example, a single initial partial facet (40a) and a single final partial facet (40b) are shown.

[0067] The waveguide includes at least one pair of surfaces that are parallel to one another (the lower surface (26) and the upper surface (26A), referred to as the "first surfaces"). The substrate width (52) is the distance between the first surfaces. The first region (54) is the region where light (shown as light ray (38)) is coupled into the substrate.

[0068] The waveguide includes an array of facets (56). The array of facets (56) includes a first facet (40a), a last facet (40b), and one or more intermediate facets (40c). The first facet (40a) is positioned proximate to a first region (54), where proximate is the nearest portion of the array of facets (56). The first facet has a first width (52a) in a direction between the first surfaces (26, 26a).

[0069] The final facet (40b) is at a distal end (55) of the array of facets (56) in the first region (54). The final facet (40b) has a third width (52b) in a direction between the first surfaces (26, 26a).

[0070] One or more intermediate facets (40c) are between the first facet (40a) and the last facet (40b). The intermediate facets (each of which) have a second width (52c) in a direction between the first surfaces (26, 26a). For clarity, only one second width (52c) is shown. In a typical implementation, the widths of all of the intermediate facets are equal. However, this implementation is not limiting, and the widths of each facet may differ from one another, as described below. The number of intermediate facets may vary depending on the application. Exemplary numbers of the one or more intermediate facets include 1, 2, 3, 4, 5, and more.

[0071] Each facet in the array of facets (56) is typically at least partially reflective, oblique to the surfaces (26, 26a), and has a facet start on a proximal side of the facet width and a facet end on a distal side of the facet width. An exemplary facet start is shown for the first facet (40a) as point (57a); for the intermediate facet adjacent to the first facet (40a) as point (57m), for the next intermediate facet as point (57n), and for the last facet (40b) as point (57b). Similarly, an exemplary facet end is shown for the first facet (40a) as point (58a); for the intermediate facet adjacent to the first facet (40a) as point (58m), for the next intermediate facet as point (58n), and for the last facet (40b) as point (58b).

[0072] We now turn to the alignment of the overlapping facets. First, define a geometric projection onto one of the surfaces (in this case, using the bottom surface (26)) in the direction of the nominal ray (38B) that is coupled out of the substrate (20). The nominal ray (38B) is typically the central ray of light that is substantially coupled out of the substrate (20). Usually, the nominal ray (38B) is the ray that the designer desires to perform optimally in the light field. The nominal ray (38B) may also be the optimal ray for a particular location on the substrate (20). In certain particularly preferred embodiments, the nominal ray is designed to be perpendicular to the parallel plane of light guiding the optical element, but depending on various design considerations, the nominal ray may be tilted relative to the perpendicular to such parallel plane in one or two dimensions. It should be noted that if the nominal ray (38B) is not perpendicular to the parallel surface (e.g., (26)) of the substrate (20), the nominal ray (38B) will be at an angle to the surface, and when the nominal ray (38B) is coupled out of the substrate (20), it will refract and be at a different angle from the substrate (20). In the context of this document, reference will generally be made to a nominal ray (38B) inside the substrate (20). Typically, the nominal ray corresponds to a ray from or near the center of the incident image. In some implementations, the nominal ray is the chief ray of the incident image. Typically, the incident light (38) corresponds to an image, and the central ray is the central ray from the center of the image. Additionally or alternatively, the incident light (38) corresponds to an image and the central ray corresponds to a central pixel of the image.

[0073] The geometric projection of the last facet (40b) and each of the one or more intermediate facets (40c) then overlaps the geometric projection of the adjacent one or more intermediate facets (40c) and the first facet (40a), respectively. In other words, adjacent facets overlap. For example, the last facet (40b) at the distal end overlaps the adjacent left-most (in the figure) intermediate facet, each of the intermediate facets (40c) overlaps the adjacent intermediate facet, and the right-most intermediate facet at the proximal end overlaps the first facet (40a).

[0074] In addition, the geometric projection of the facet start of the last facet (57b) and each of the one or more intermediate facets (57n, 57m, etc.) preferably substantially coincides with the geometric projection of each of the one or more intermediate facets (58n, 58m, etc.) and non-adjacent facet ends of the first facet (58a). In essence, each facet start aligns with, or is preferably close to, a non-adjacent facet end in the direction of nominal ray outcoupling (with the notable exception of the first facet (40a) since there is no facet end with which it is aligned). The coincidence is along at least a portion of the substrate.

[0075] Alternatively, facet overlap can be described as a constant number of facets overlapping in a line of sight to a nominal observation point for light coupling out of the substrate through one of the surfaces. In essence, the nominal point is the typical location of a user's eye (47), most likely the location of the pupil of the observer's eye. In some applications, the nominal point is the center of the observer's eyeball. The internal facets are optimized to produce a uniform reflection to the observer by having a constant number of facets overlapping in a line of sight to the nominal point of observation.

[0076] A feature of this embodiment is the specific management of the configuration of the overlapping facets, where the facet ends of the first and all of the intermediate facets of the intersection of the double facets are in line with the center of the adjacent intermediate or last facet. Similarly, the facet starts of the last and all of the intermediate facets are in line with the center of the adjacent intermediate or last facet. In this case, the following exemplary facets have a geometric projection onto the lower surface (26) in the following respects: Facet end of facet (F11) at point (P11); midway between facets (F11) at point (P12); facet start of facet (F11) at point (P14); Facet end of facet (F12) at point (P12); midway between facets (F12) at point (P14); the facet end of facet (F13) at point (P14); and Ray (38B) intersects facets (F11) and (F12) at point (P13).

[0077] Thus, the proximal end of the middle facet (F11) overlaps the distal end of the adjacent middle facet (F12), and the facet beginning of the middle facet (F11) aligns with the facet ending of the non-adjacent middle facet (F13).

[0078] The facets are typically parallel to each other and at regular intervals, i.e., the spacing between one pair of adjacent facets in the array of facets is the same as the spacing between another pair of adjacent facets in the array of facets. For example, the spacing (59a) between facet (F11) and facet (F12) is substantially the same as the spacing (59b) between facet (F12) and facet (F13). The spacing between adjacent facets is typically greater than the coherence length of the light coupled into the substrate. The coherence length is the propagation distance over which a coherent wave (e.g., an electromagnetic wave) maintains a specified degree of coherence. Typically, the coherence length is the wavelength squared and divided by the spectral width. If the facet spacing varies along the waveguide, the overlap condition must be preserved.

[0079] In a preferred embodiment, the first width of the first facet is less than the second width of one or more of the intermediate facets. In other words, the first facet is a partial facet. In a typical implementation, the first width is approximately half the second width.

[0080] In another option, the third width is less than the second width. In essence, the last facet is a partial facet, preferably half the width of the middle facet (the third width is approximately half the second width). In another option, half of the last facet has a reflectivity that is substantially 100% nominal reflectivity. For example, if the nominal reflectivity is 50% (as in the case of a double overlap), then at the distal half of the waveguide, the last facet has a reflectivity of 50%. For example, in FIG. 10B, if half of the facets (2517) have a nominal reflectivity of 100%, then ray (2546) will have the same intensity as the rest of the outcoupled light. Similarly, if half of the facets (2515) have a reflectivity of 100%, then ray (2547) will also have the same intensity as the rest of the outcoupled light.

[0081] The propagation of light from the first region is such that at least a portion of the light encounters the initial facet before encountering one of the one or more intermediate facets.

[0082] Referring now to FIG. 16C, there is a schematic diagram of a triple facet (triple facet intersection, triple overlap). As in the other examples, the waveguide (waveguide (20)) includes an overlapping internal facet (40) between first surfaces (26, 26A), shown as a double line. A solid arrow indicates a nominal ray that intersects three facets and is then outcoupled from the substrate (arrow outcoupling ray (38B)). As in similar figures, dashed lines are used to indicate the alignment of the facets (40). In this example, a number (specifically two) of initial partial facets and a number (two) of final partial facets are shown.

[0083] Typically, a number of facets are crossed by a nominal ray that is coupled out of the waveguide substrate. In an example of a double-facet crossing, the number of crossed facets is two. Similarly, in an example of a triple-facet crossing, the number of crossed facets is three. Typically, the number of crossed facets is constant for all of the facet arrangements. The construction of a waveguide in which a constant number of facets are crossed can be implemented in various configurations. For example, as described with respect to FIG. 16B, the first width (52a) of the first facet (40a) can be approximately half the second width (52c) of the adjacent middle facet (one facet of the middle facets (40c)). In another example, ¼ of the first facet and ¾ of the adjacent facet can be used. In another example, referring to FIG. 16C, both the first facet (40a) and the first adjacent facet (F14) are a fraction of the width of the next adjacent facet (F15).

[0084] Based on the current description using a typical 1D waveguide (e.g., 1D waveguide (20)) for implementing overlapping facets, one skilled in the art can implement overlapping facets for 2D waveguides (e.g., in 2D waveguide (10)) and other waveguide configurations.

[0085] Returning to FIG. 14A, FIG. 14B and reference to FIG. 13, typically in a 2D waveguide, the waveguide comprises a first surface (26, 26a or 12b, 12a) and a second pair of surfaces (14a, 14b). The second surfaces (14a, 14b) are parallel to each other and not parallel to the first surface (12b, 12a). As well as the width of the facets relative to the first surface, the first facet has a fourth width in the direction between the second surfaces, the last facet has a sixth width in the direction between the second surfaces, and one or more intermediate facets have a fifth width in the direction between the second surfaces. A feature of a 2D waveguide is that the facets are configured such that when an image is coupled into the waveguide in a first region with a first direction of propagation at an oblique coupling angle to both the first and second surfaces, the image travels by four internal fold reflections along the waveguide.

[0086] In an alternative embodiment, the second surface is perpendicular to the first surface. In another alternative embodiment, each of the facets is at an oblique angle to the second surface.

[0087] 17A-D, there is a rough schematic diagram of an alternative facet configuration. In this illustration, the facets are parallel.

[0088] Referring now to FIG. 17A, for comparison, there is a rough schematic of a dual facet, as detailed in FIG. 16B. As the waveguide projects an image to the user's eye (47), different light rays propagate at different angles, thereby creating overlaps and underlaps that reduce uniformity (introduce non-uniformity), as discussed above with respect to FIG. 10A. The overlapping configuration of FIG. 17A (also FIG. 16B, and FIG. 10B) reduces the effect of this non-uniformity compared to the non-overlapping configuration of FIG. 10A (also FIG. 16A). For many applications, this dual facet configuration is sufficient, and the overlap is sufficient to suppress the non-uniformity.

[0089] Referring now to FIG. 17B, there is a rough schematic of different facet spacings. A further reduction in non-uniformity from the dual facet configuration of FIG. 17A is shown in FIG. 17B, where the spacing between one pair of adjacent facets of the array of facets is different than the adjacent spacing between another pair of adjacent facets of the array of facets. In a preferred embodiment, the change in spacing is monotonic between one pair of adjacent facets and another pair of adjacent facets. For example, spacing (59d) is greater than spacing (59c), and spacing (59e) is greater than spacing (59d). Note that the change in facet spacing can be reduced by refraction of output beam (38B) as it bends vertically as it exits the substrate (not depicted for simplicity). In this configuration, different angles of the nominal wave are addressed and the overlap is constant relative to the observer (user's eye (47)). In this non-limiting example, a nominal output ray will always pass through two facets. Note that in this figure, the nominal ray (38B) at the center of the substrate (20) is at a different angle than the nominal rays at the edges (e.g., proximal and distal) of the substrate (20).

[0090] Referring now to FIG. 17C, there is a rough schematic of the decrease in facet spacing from the proximal end to the distal end of the waveguide. The waveguide (20) includes a first spacing in a first portion of the waveguide (20) and at least a second spacing in a second portion of the waveguide (20). In this non-limiting example, the first portion (61a) includes non-overlapping facets. The second portion (61c) includes a double overlapping facet, and another portion (61e) includes a triple overlapping facet. The portions (61b) and (61d) are transition portions, or regions of transition from one distinct overlap to another distinct overlap. In alternative embodiments, the overlap of the portions may be non-discrete, continuously varying, or another spacing configuration designed to manage the effective output intensity from the waveguide.

[0091] To maintain a constant reflection intensity along the waveguide, all facets must have a higher reflection coefficient, starting from the proximal end and increasing the reflectivity in the direction of the distal end. This management of reflection intensity improves the uniformity of the output (uniformity of intensity) to the observer. In the current illustration, the reflectivity of all facets can be kept constant, while the spacing between facets varies according to the required reflectivity. The light is introduced into the waveguide from the proximal end (right side of the current illustration), and therefore has the highest intensity at the proximal end. At the proximal end, the spacing between facets is maximum, and the overlap between facets is minimum. As the light propagates along the waveguide (not shown), the power of the light decreases, and the higher degree of overlap of the facets compensates for this decrease in power. Thus, the overall power output is maintained along the waveguide.

[0092] Where no discontinuity in overlap is observed, at close spacing of facets, continuity may be maintained along the waveguide by either a discontinuous change in the overlap integer or a continuous change (non-integer).

[0093] If facet spacing and height are kept constant across the waveguide, the optimization procedure should consider the effects of facet overlap versus underlap. Overlapping facets result in more output power and more mixing of non-uniformity. Furthermore, overlap results in an intensity change of 100% to 150% (or 100% ± 20%), while underlap results in an intensity change of 50% to 100% (or 100% ± 33%). In overlap, the relative intensity change is lower. Thus, the reflectivity of one or more facets varies from the reflectivity of another one or more facets in the array of facets.

[0094] Also note that the observer's eye does not respond linearly to intensity changes, but rather has a logarithmic response. This also suggests that underlap has more of an effect on the observer's perception. From the above, one should consider more reduction in underlap at the cost of increasing overlap.

[0095] Reference is now made to FIG. 17D, a rough schematic illustration of varying facet width. A further reduction in non-uniformity from the dual facet configuration of FIG. 17A is shown in the present illustration, where the width of one facet of the array of facets varies in proportion to the width of an adjacent one of the array of facets. In a preferred embodiment, the change in width is monotonic between one facet and an adjacent facet for the entire array of facets. In the present illustration, the width narrows from the bottom to the top of the waveguide as the array of facets traverses from the proximal to the distal end. An alternative implementation is to reduce the width of the facets from both sides (from the top and bottom of each facet, toward the center). In the present example, width (59e) is greater than width (59d), and width (59f) is greater than width (59e).

[0096] In both implementations of Figure 17B, which varies the facet spacing, and Figure 17D, which varies the facet width, an observer (user's eye 47) at the nominal convergence of the light beam (output beam (38B)) will see no overlap or underlap. However, any change in eye position will cause some overlap / underlap to occur that is suppressed by the dual facet configuration.

[0097] Reference is now made to FIG. 18, a rough schematic diagram of the application of overlapping facets to a symmetrical structure as described with respect to FIGS. 6 and 8. In the present figure, only the top lateral waveguide is shown, which is similar to the overlapping configuration of FIG. 17B. In this symmetrical configuration, the first region (similar to the first region (54) of FIG. 16B) is the region where light (shown as ray (38)) is coupled to the substrate, in this case the region in the middle of the waveguide. The symmetrical left and right sides of the waveguide each have a region where light is coupled to the respective side of the substrate (alternatively referred to as adjacent first and second regions), with the left and right facets being equal and oppositely tilted sides. This symmetrical structure can also be implemented with the side-by-side configuration of facets in FIG. 17A, and the varying width configuration of FIG. 17D.

[0098] Refer again to FIG. 10B, where in the overlap configuration, the (first complete) facet (2517) and the (last complete) facet (2515) are partially non-overlapping. As noted above, clearly the beginning of facet (2517) does not overlap facet (2535), and the end of facet (2515) does not overlap facet (2535). Thus, the intensity of the outcoupled light ((2546) and (2547)) is not as strong in these non-overlapping portions. For example, in a double overlap configuration, half of the first complete facet is non-overlapping, and half of its power will be outcoupled from the non-overlapping portions.

[0099] Several techniques can be used to overcome the problem of less intensity at the non-overlapping beginning and ending portions.

[0100] 1. Using shorter facets at the beginning and end, as described above with respect to FIG. 14B, elements (40a) and (40b), and FIG. 16B.

[0101] 2. Coating the non-overlapped portion with a highly reflective coating that increases the reflectivity of the non-overlapped portion in proportion to the nominal reflectivity of the other (middle) facet.

[0102] 3. Gradual change in the characteristic reflectance of the facets from the non-overlapping to the overlapping areas, as follows:

[0103] Here, the technique for gradual change in characteristic reflectance is described using a double overlap configuration for simplicity, but the technique can be applied to more advanced overlap configurations.

[0104] Reference is now made to FIG. 19A, a graph of all the nominal reflectivities in a double overlap configuration. The x-axis shows the facets starting with the facet numbered "1" (one), which is the first facet near the first region 54 where light is coupled into the waveguide (substrate). The increasing numbered facets are the facets following facet "1" toward the distal end (55) of the waveguide. The y-axis shows the reflectivity as a percentage of the nominal reflectivity. The thin line black boxes are the reflectivity of each individual facet (percent of nominal reflectivity) and the thick black lines are the characteristic reflectivities-effective reflectivities that the coupled out light rays would experience. Each facet is shown to have a certain nominal reflectivity, for example, a nominal required reflectivity of 50%.

[0105] The characteristic reflectance is the sum of the individual reflectances for the portion of the waveguide into which a ray is outcoupled. Thus, the characteristic reflectance outcoupling from the non-overlapping portion from facet "1" in the current example is 50% (nominal reflectance), which may be shown as ray (2546) (or ray (2547)) in FIG. 10B. The characteristic reflectance outcoupling from the overlap of facet "1" with facet "2" (overlap of two adjacent facets) achieves 100% (nominal reflectance). Thus, there is a discontinuity between the beginning and the next portion of the waveguide (50% vs. 100%).

[0106] Reference is now made to FIG. 19B, an exemplary graph of total nominal reflectance in a dual overlap configuration with alternating changes in facet reflectance. In the current figure, facet "1" (the first facet) is designed to approach no overlap (i.e., have approximately 100% nominal reflectance) and facet "2" (the second facet) is designed to have minimal reflected light. Thus, when combined, the first and second facets have approximately a non-overlapping characteristic reflectance. Similarly, facet "3" (the third facet) has almost the same but reduced reflectance as facet "1" and facet "4" has almost the same but increased reflectance as facet "2". In the current figure, facets "7" and "8" have a (nominal) reflectance of 50%, resulting in the characteristic reflectance as described in FIG. 19A in a dual overlap configuration. The dashed and dotted lines represent the reflectance of the odd numbered facets (starting with the non-overlapping coating parameters), while the dashed lines represent the reflectance of the even numbered facets (representing the increasing overlap characteristic). The thick black line (solid line) represents the characteristic reflectance that sums the reflectance of two adjacent facets (as caused by a double overlap and a non-overlapping first facet), showing that there is also no half reflectance of the first half facet, compared to the 50% characteristic reflectance of facet "1" in Figure 19A.

[0107] While the configuration of FIG. 19A has a discontinuity between the beginning and the next portion of the waveguide (50% vs. 100%), the innovative configuration of FIG. 19B reduces this discontinuity. The remaining discontinuity depends on the convergence rate, e.g., after six facets, the discontinuity would be approximately 10% with respect to convergence, thus overcoming the problem of less intensity at the beginning and end portions that do not overlap. The current configuration can be repeated on the other side, at the distal end of the waveguide. The slope of the change in reflectivity (dotted and dashed lines) can be altered to vary the resulting effect and characteristic reflectivity of the array of facets. The waveguide can have various combinations of overlapped and non-overlapping characteristic reflectivities over the length of the waveguide. For example, facets "4" and "5" in the current figure can be repeated over at least a portion of the array of facets without converging to a configuration of facets "7" and "8".

[0108] Figure 20A illustrates a non-limiting but preferred process that may be used to initially produce a 1D waveguide 10. For clarity, the internal facets are not shown to scale or density in the drawing.

[0109] A set of coated transparent parallel plates are mounted together as a stack (400). The stack is cut (402) diagonally to produce slices (404). If required, cover transparent plates (405) can be attached on the top and / or bottom (not shown) of the slices (404). The slices (404) are then cut either perpendicular to the facet edges (dashed lines on (404)) if 1D facet tilt is required, or diagonally (dashed lines on (404)) if 2D facet tilt is required to produce 2D waveguides (406).

[0110] Figures 20B-20E are an exemplary procedure for mounting a coupling prism. A sliced ​​2D waveguide (406) is shown in Figure 20B with overlapping facets (two reflecting facets per line of sight). This is just a non-limiting example, non-overlapping facets are also possible.

[0111] As illustrated in FIG. 20B, the 2D waveguide (406) (shown opaque for clarity) is cut, for example along the dotted line (420A) as illustrated. The cut can be in any orientation, but a perpendicular cut relaxes the stringent index matching requirement. If overlapping facets are present (see the cut ends in FIG. 20C) to maintain uniformity of illumination, the cut is preferably performed as seen in FIG. 20C. Otherwise, the first facet would reflect without overlap, resulting in weakened illumination. If necessary, a transparent extension (413) can be added, and a prism (414) is attached to the waveguide (406) to create a 2D waveguide (416) with the extension and connecting prism (as shown in FIG. 20D). In cases where an extension is not required, it may be attached directly to the waveguide (406) to create an assembled waveguide (417) (as shown in FIG. 20D). The distal end of the waveguide (406) may be left to allow residual light to scatter therefrom and may optionally be painted with a light absorbing material (e.g., black paint) to minimize stray reflections.

[0112] 21A-21D are a typical procedure for making a waveguide with overlapping facets. In FIG. 21A, a sliced ​​2D waveguide (406) may be cut vertically along both sides, for example, along dotted line (420A) to become the proximal end of the waveguide and along dotted line (420B) to become the distal end of the waveguide. This results in a waveguide (420) with partial facets (40a) and (40b) at each proximal and distal end. In FIG. 21B in this example, waveguides (426) and (424) are attached to the proximal and distal ends of waveguide (420), respectively, and the combination (of (420), (424), and (426)) is polished to produce a smooth outer surface of the combined waveguide (428), FIG. 21C. In this combined waveguide (428), the attached waveguides (426) and (424) do not need to have as precise an index of refraction as waveguide (406).

[0113] This fabrication method can also be applied to waveguides without overlapping facets, obviating the need for precise index matching.

[0114] In FIG. 21D, optionally, the smoothness and optical properties of the external facets of the combined waveguide (428) can be improved by attaching an index-matching outer surface (427) to result in waveguide (429).

[0115] Note that although FIG. 21C and FIG. 21D illustrate separate component margins, for light (such as incoming light, ray (38)), the margins are transparent; only the outer surfaces and the angled coated internal facets reflect light.

[0116] The present method (FIGS. 20A-21D) can be applied to 1D as well as 2D waveguides.

[0117] Various embodiments in this description, such as varying facet spacing, width, and reflectivity, have been described separately for clarity. Those skilled in the art will understand that these embodiments can be combined. For example, one could vary the width of the facets from the proximal to distal end of the waveguide while narrowing the spacing of the varying facets.

[0118] Please note that the above examples, numbers used, and typical calculations are intended to aid in the explanation of this embodiment. Unintentional typographical errors, mathematical errors, and / or use of simplified calculations do not detract from the utility and fundamental benefits of the present invention.

[0119] To the extent the appended claims have been drafted without multiple dependencies, the invention has been implemented merely to accommodate formal requirements in jurisdictions that do not permit such multiple dependencies. It should be noted that all possible combinations of features that would be implied by multiple dependency of the claims are expressly contemplated and should be considered part of the invention.

[0120] It will be understood that the above description is intended to serve as an example only, and that many other embodiments are possible within the scope of the invention as defined in the appended claims.

Claims

1. 1. An optical device comprising: The optical device is (a) a waveguide, the waveguide comprising: (i) a first surface of at least a pair of surfaces that are parallel to one another; (ii) a first region where light is coupled into the waveguide; (iii) having a first array of facets; The facet is (A) a first facet (I) disposed proximate the first region and (II) having a first width in a direction between the first pair of surfaces; (B) (I) a final facet at a distal end of the first array of facets from the first region and (II) having a third width in a direction between the first pair of surfaces; and (C) (I) one or more intermediate facets between the first facet and the last facet, and (II) having a second width in a direction between the first pair of surfaces; (b) each of the facets comprises: (i) a width in the plane of the facet; (ii) an at least partially reflective surface; (iii) at an oblique angle to the first pair of surfaces; (iv) having a facet beginning proximal to the width of the facet; and (v) having a facet termination distal to the width of the facet; and (c) a geometric projection is onto one of the first pair of surfaces in a direction of a nominal ray of light coupled out of the waveguide, the nominal ray being a central ray of light coupled out of the waveguide; (d) a geometric projection of each of the final facet and the one or more intermediate facets overlaps a geometric projection of each of the one or more intermediate facets and adjacent first facets; (e) a geometric projection of the facet start of the last facet and each of the one or more intermediate facets corresponds to a geometric projection of a non-adjacent facet end of the one or more intermediate facets and the first facet, (f) the coincidence is along at least a portion of said waveguide.

2. The optical device of claim 1 , wherein the first width of the initial facet is less than a second width of one or more intermediate facets.

3. 2. The optical device of claim 1, wherein many of the facets intersect with the nominal ray coupled out of the waveguide, the number of facets being constant for all of the first array.

4. The optical device of claim 1 , wherein the light corresponds to an image and the central ray is a central ray from a center of the image.

5. The optical device of claim 1 , wherein the light corresponds to an image and the central ray corresponds to a central pixel of the image.

6. 10. The optical device of claim 1, wherein the final facet has a reflectivity that is substantially 100% of a nominal reflectivity, the nominal reflectivity being the total internal reflection required at a particular position of the waveguide.

7. The optical device of claim 1 , wherein the third width is less than the second width.

8. The optical device of claim 7 , wherein the third width is substantially half the second width.

9. One or more intermediate facets (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, and (f) a plurality of The optical device of claim 1 selected from the group consisting of:

10. The optical device of claim 1 , wherein a number of facets overlap in a line of sight toward a nominal observation point of optical coupling from the waveguide by one of the first pair of surfaces.

11. 2. The optical device of claim 1, wherein a width of a facet of a plurality of the facets of the first array of facets varies monotonically with respect to a width of an adjacent facet of the plurality of facets of the first array of facets.

12. 10. The optical device of claim 1, wherein a spacing between one pair of adjacent facets of the first array of facets varies monotonically with respect to an adjacent spacing between another pair of adjacent facets of the first array of facets.

13. The optical device of claim 12 , wherein the intervals vary monotonically.

14. 10. The optical device of claim 1, wherein light from the first region is such that at least a portion of the light strikes the initial facet before striking one of the one or more intermediate facets.

15. The optical device of claim 1 , wherein the spacing between adjacent facets is greater than a coherence length of light coupled into the waveguide.

16. (a) the first width is substantially equal to the second width; and 2. The optical device of claim 1, wherein (b) the initial facet has a first portion corresponding to the geometric projection of the initial facet that does not overlap with the geometric projection of an adjacent intermediate facet.

17. The optical device of claim 15 , wherein the first portion is transparent to light.

18. 16. The optical device of claim 15, wherein the first portion has a reflectivity substantially twice that of an adjacent facet.

19. The optical device of claim 1 , wherein each of the facets has a uniform partial reflectivity across the facet.

20. The optical device of claim 1 , wherein the reflectivity of one or more of the facets is different from another reflectivity of one or more other facets.

21. The waveguide further comprises: (a) a second pair of surfaces that are parallel to one another and non-parallel to the first pair of surfaces; 2. The optical device of claim 1, wherein (b) the facets are configured such that when an image is coupled into the waveguide in the first region with an original direction of propagation at an oblique coupling angle to both a first pair and a second pair of surfaces, the image travels along the waveguide for four intra-fold reflections.

22. 20. The optical device of claim 19, wherein the second pair of surfaces is perpendicular to the first pair of surfaces.

23. 20. The optical device of claim 19, wherein each of the facets is at an oblique angle to the second pair of surfaces.

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