Methods and intermediate structures for producing lightguides with internal reflectors for near-eye displays

Abstract

A method is provided for fabricating lightguide components (32) with internal partially-reflecting surfaces ("facets") for use in near-eye displays. A stack (10) is formed by bonding a plurality of optical plates (12) using optical adhesive, with at least one surface at each adhesive interface carrying a partially reflective coating. The inter-plate spacing is controlled using spacer components, which may include transparent, index- matched solid inclusions (20) dispersed within the adhesive, or peripheral spacer elements (22, 24) positioned at the plate margins. The adhesive is cured to form a bonded stack, which is sliced along planes non-parallel to the plates to form lightguide components (32) containing internal facets at controlled orientations. These components may be combined into an assembled lightguide structure (34) including a coupling-in region (38), a redirection region (32) for in-plane propagation (36), and a coupling-out region (33) directing light toward a viewer (40), enabling two-dimensional aperture expansion.

Description

[0001] Methods and Intermediate Structures for Producing Lightguides with Internal Reflectors for Near-Eye Displays

[0002] FIELD AND BACKGROUND OF THE INVENTION

[0003] The present disclosure relates to the manufacture of lightguide components for use in near-eye displays. More particularly, it concerns methods and intermediate structures for assembling and bonding plate-based structures to form internal partially-reflecting surfaces within a lightguide.

[0004] Lightguides used in near-eye display systems often incorporate internal partially- reflective surfaces to redirect image light along defined optical paths. These surfaces, sometimes referred to as facets, may serve functions such as expanding the optical aperture, coupling image light into or out of a lightguide, or modifying the direction of guided propagation.

[0005] One technique for forming such internal facets involves assembling a plurality of plates with partially reflective coatings and bonding them using optical adhesive layers. The resulting stack may be sliced along predetermined planes to yield lightguide components containing the desired internal surfaces. The structural and optical properties of the adhesive layers — such as thickness uniformity, index homogeneity, and alignment — can strongly affect the performance of the resulting lightguide.

[0006] SUMMARY OF THE INVENTION

[0007] The present invention provides a lightguide structure and related production methods and intermediate work products.

[0008] According to the teachings of an embodiment of the present invention there is provided, a lightguide structure comprising: a pair of mutually parallel major surfaces for supporting propagation of light by internal reflection at the major surfaces, wherein at least one region of the lightguide structure comprises a set of mutually-parallel partially- reflective surfaces located between, and non-parallel to, the major surfaces, the set of partially-reflective surfaces being formed at interfaces between a plurality of successive plates, the partially-reflective internal surfaces being configured to partially reflect light propagating within the lightguide, wherein at least one of the interfaces includes a cured optical adhesive layer disposed between adjacent plates, the optical adhesive layer comprising a plurality of index-matched inclusions embedded within the adhesive material, the index-matched inclusions being configured to define a minimum height when disposed between two planar surfaces, wherein a thickness dimension of the adhesive layer corresponds substantially to the minimum height of the index-matched inclusions.

[0009] According to a further feature of an embodiment of the present invention, each interface between successive plates in the region comprises a cured optical adhesive layer containing the plurality of index-matched inclusions.

[0010] According to a further feature of an embodiment of the present invention, the inclusions are spherical. According to an alternative feature of an embodiment of the present invention, the inclusions are cylindrical rods with circular cross-section.

[0011] According to a further feature of an embodiment of the present invention, the region is configured to redirect light propagating in a first direction within the lightguide so as to propagate in a second direction by internal reflection at the major surfaces of the lightguide.

[0012] According to a further feature of an embodiment of the present invention, the region is configured to couple light propagating within the lightguide out of the lightguide toward an eye of a viewer.

[0013] According to a further feature of an embodiment of the present invention, the inclusions comprise solid glass having a refractive index substantially matched to that of the adhesive material.

[0014] According to a further feature of an embodiment of the present invention, the inclusions have a minimum height of between 3 pm and 10 pm.

[0015] According to a further feature of an embodiment of the present invention, the inclusions are distributed within the adhesive layer at a numerical density sufficient to define the adhesive thickness substantially uniformly across the interface.

[0016] There is also provided according to the teachings of an embodiment of the present invention, a method of producing an optical assembly for use in manufacturing a lightguide, the method comprising: (a) assembling a stack including a first plate, a second plate, an optical adhesive interposed between the first and second plates, and one or more spacer components configured to define a uniform minimum spacing between the plates; (b) pressing the plates together to distribute the adhesive between the plates while maintaining the uniform minimum spacing defined by the one or more spacer components; and (c) curing the adhesive.

[0017] According to a further feature of an embodiment of the present invention, the one or more spacer components comprise a plurality of transparent, index-matched solid inclusions dispersed within the optical adhesive.

[0018] According to a further feature of an embodiment of the present invention, the one or more spacer components comprise peripheral spacer elements applied to one or more edge regions of at least one of the plates.

[0019] According to a further feature of an embodiment of the present invention, the one or more spacer components comprise one or more regions of a coating material selectively deposited on a surface of one of the plates with a coating thickness that defines the uniform minimum spacing.

[0020] According to a further feature of an embodiment of the present invention, the one or more spacer components comprise preformed adhesive elements disposed on one or more marginal region of at least one of the plates.

[0021] According to a further feature of an embodiment of the present invention, the stack further comprises a third plate, and a second optical adhesive layer and one or more additional spacer components disposed between the second plate and the third plate, the steps of pressing and curing being performed simultaneously on the stack.

[0022] According to a further feature of an embodiment of the present invention, subsequent to the curing: (a) to the stack is added a third plate, and a second optical adhesive layer and one or more additional spacer components disposed between the second plate and the third plate, thereby forming an incremented stack; (b) the incremented stack is pressed to distribute the second optical adhesive layer between the second and third plates while maintaining a uniform minimum spacing defined by the one or more additional spacer components; and (c) the second optical adhesive layer is cured. According to a further feature of an embodiment of the present invention, a surface of one of the plates facing each layer of optical adhesive is provided with a partially reflective coating.

[0023] According to a further feature of an embodiment of the present invention, the cured stack is sliced along a plurality of parallel slicing planes to form a plurality of lightguide components each having a plurality of internal partially-reflective surfaces at interfaces between the plates.

[0024] There is also provided according to the teachings of an embodiment of the present invention, an intermediate work product sliceable to produce a plurality of lightguide components each including a plurality of mutually-parallel partially-reflecting internal surfaces, the intermediate work product comprising a stack formed from: (a) a plurality of plates, each bounded by a pair of mutually parallel major surfaces; (b) for each pair of adjacent plates of the plurality of plates having major surfaces in facing relation, one or more spacer components at least partially interposed between the major surfaces in facing relation, the one or more spacer components being configured to define a uniform minimum spacing between the major surfaces; and (c) a cured optical adhesive substantially filling a space between the adjacent plates defined by the one or more spacer components, wherein one of the major surfaces in facing relation between each pair of adjacent plates has a partially-reflecting coating.

[0025] According to a further feature of an embodiment of the present invention, the one or more spacer components comprise a plurality of transparent, index-matched solid inclusions dispersed within the cured optical adhesive.

[0026] According to a further feature of an embodiment of the present invention, the one or more spacer components comprise peripheral spacer elements disposed on marginal regions of at least one of each pair of adjacent plates.

[0027] BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: FIGS. 1A and IB are schematic side views comparing molecular orientation effects in a thick adhesive layer and a thin adhesive layer, respectively, sandwiched between two layers of glass.

[0029] FIG. 2 A is a schematic side view of a stack of coated plates joined by layers of adhesive in the absence of thickness control elements.

[0030] FIG. 2B is a schematic side view similar to FIG. 2A in which transparent, index- matched inclusions are dispersed within the adhesive to define the inter-plate spacing according to an aspect of the present invention.

[0031] FIGS. 3A and 3B are a schematic side view and a schematic plan view, respectively, of a stacked assembly incorporating edge-positioned spacer frames or sheets to define adhesive thickness.

[0032] FIG. 4A is a schematic side view of a stacked assembly incorporating edge shims formed by a deposition process as spacer elements.

[0033] FIGS. 4B-4D are sequential plan views of an individual plate from the stack of FIG. 4A illustrating steps in the use of removable masking to form edge shims by a precoating process.

[0034] FIGS. 5 A and 5B are schematic illustrations of a process for forming edge shims (in this case, corner shims) by dipping plate edges in adhesive.

[0035] FIG. 5C is a side view of a plate pre-treated according to the process of FIGS. 5A and 5B.

[0036] FIG. 5D is a plan view of a plate pre-treated according to the process of FIGS. 5 A and 5B implemented along the entire edges of the plate.

[0037] FIGS. 6A-6C are schematic side views showing stages of a sequential assembly and curing process illustrating, respectively: (a) pressing and curing of a first pair of plates; (b) pressing and curing of a third plate to the stack; and (c) pressing and curing of a fourth plate to the stack to form a final stack structure.

[0038] FIG. 7 is a schematic isometric view showing stages of a production process from: (a) a stack according to the present invention through (b) slicing of the stack and (c) cutting a lightguide component from the slice, culminating in (d) integration of the lightguide component into an assembled lightguide structure. DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] The present disclosure relates to techniques for fabricating lightguide components used in near-eye display systems. These lightguide components typically include one or more regions containing partially-reflecting internal surfaces, also referred to as “facets,” arranged to progressively redirect image light along a desired optical path. For example, such facets may be configured to progressively redirect light propagating in a first direction within the lightguide to propagate in a second direction while expanding the effective image aperture in the first direction, and / or to progressively couple light propagating in a second direction out of the lightguide towards the eye of a user while expanding the effective image aperture in the second direction.

[0040] A preferred method for forming these internal facets involves assembling a plurality of parallel-sided optical plates in a stack, bonded together by interposing a layer of optical adhesive at the interfaces, and with one major surface at each interface provided with a partially-reflecting optical coating. This can be through single-sided coating of each plate, or by double-sided coating of alternate plates. The stack is then sliced along parallel slicing planes, non-parallel to the major surfaces of the plates, to form a plurality of lightguide components, each containing a corresponding plurality of internal partially- reflecting surfaces formed at the original plate interfaces.

[0041] As shown in FIGS. 1 A and IB, it has been observed that the optical properties of the interfaces between plates can be affected by the behavior of the optical adhesive layer which, in turn, may vary according to the residual thickness of the adhesive layer between the plates. In particular, when the adhesive layer is thin (FIG. IB), its optical properties can be dominated by surface-induced effects such as molecular alignment at the adhesiveglass interface and / or surface inhomogeneity due to particles, scratches or the like. These effects may result in optical anisotropy or birefringence, as well as local variation in refractive index and / or internal stress. In contrast, a thicker adhesive layer (FIG. 1A) tends to exhibit optical properties dominated by the bulk material, which may be more uniform and isotropic under suitable processing conditions. Under typical production methods, parallelism and effective bonding are achieved by inserting a quantity of liquid adhesive between the plates and then pressing the plates together in a clamp so as to squeeze the adhesive between the plates, spreading it thinly and ejecting excess from the sides. This typically results in a very thin layer of adhesive, which is then typically cured by UV exposure in situ, resulting in the pronounced surface-induced effects of FIG. IB.

[0042] The present disclosure provides, in various aspects, methods and intermediate structures for assembling plate stacks in a way that reliably controls the spacing between adjacent plates. This spacing control serves to define the physical thickness of each adhesive layer, and to promote uniformity and reproducibility across the stack. According to a preferred but non-limiting approach, the method includes assembling a pair of optical plates with adhesive interposed between them, and one or more spacer components configured to define a uniform minimum spacing between the plates. The plates are pressed together so as to distribute the adhesive while maintaining the defined spacing, and the adhesive is cured to fix the interface geometry. These steps may be repeated sequentially or applied to a larger set of plates assembled in a single operation.

[0043] Also disclosed are intermediate work product structures comprising a plurality of plates bonded by cured optical adhesive layers, wherein each layer exhibits a defined and uniform thickness established by the presence of one or more spacer components. In preferred embodiments, at least one surface at each adhesive interface carries a partially- reflecting coating. These intermediate stacks are structured and dimensioned to be sliced along planes non-parallel to the major surfaces of the plates, thereby producing lightguide components containing internal reflective facets of precise geometry.

[0044] Adhesive Thickness Control Using Embedded Inclusions

[0045] In a first preferred group of embodiments, the inter-plate spacing at each adhesive interface is defined by the inclusion of transparent, index-matched solid elements dispersed within the adhesive layer. These elements are dimensioned to establish a minimum spacing between the opposing plate surfaces during the pressing stage of assembly and remain embedded in the cured adhesive layer as permanent structural components.

[0046] FIG. 2A shows a schematic side view of a conventional stacked assembly of coated plates 12 joined by adhesive layers 16. In the absence of positive thickness control elements, the adhesive layers 16 may vary in thickness across the interface and between layers, depending on local surface features, particulates, and the uniformity of pressure applied during bonding, and the final thickness of the adhesive layer may be less than a desired value. Such variations and / or reduction in the adhesive layer thickness may lead to optical nonuniformities in the final lightguide component.

[0047] FIG. 2B shows a corresponding assembly according to an aspect of the present invention. Each adhesive layer 16 contains a plurality of solid inclusions 20 dispersed within the liquid adhesive prior to assembly. The solid inclusions are configured to define a minimum height when disposed between two planar surfaces. As used herein, the term “configured to define a minimum height when disposed between two planar surfaces” refers to a physical property of each inclusion such that, when placed between two substantially rigid and planar plates, the inclusion resists compression beyond a defined dimension along the normal direction to the plates. This minimum height may correspond, for example, to the diameter of a sphere, the shortest axis of an ellipsoid, or the smallest face-to-face distance of a polyhedral inclusion such as a cube. The inclusions may have uniform or non-uniform geometry, provided that they each present a stable height dimension that constrains the separation between adjacent plates during adhesive curing. This minimum height is preferably selected according to the desired optical thickness of the adhesive layer.

[0048] In the non-limiting preferred exemplary embodiment illustrated here, the inclusions 20 are spherical or cylindrical in shape, i.e., with circular symmetry, with uniform diameters, and are fabricated from a material having a refractive index closely matched to that of the adhesive. Suitable materials for the inclusions include optical glass, plastic or solidified adhesive of the same composition as the liquid adhesive layer. In the case of inclusions with circular symmetry, the minimum height generally corresponds to the diameter. Where inclusions with elliptical form are used, such as a spheroid or other ellipsoid, or an elliptical rod, the minimum height generally corresponds to the shorter axis of the ellipse.

[0049] The inclusion diameter is selected according to the desired inter-plate spacing and the desired optical behavior of the cured interface. In some cases, the use of such inclusions may be desirable even when implementing thin adhesive layers, from 50 nm upwards, to help ensure a uniform layer thickness and a high degree of parallelism between the plates. In certain preferred but non-limiting implementations, the inclusion diameter (or more generally, minimum height) lies within the range of about 1 pm to 20 pm, and most preferably between about 3 pm and 10 pm, thereby serving to define an adhesive layer in which the properties are defined primarily by the bulk adhesive properties rather than by surface effects. Here and elsewhere in this document, wherever the term “about” is used to qualify a value, it should be taken to include all adjacent values which achieve a generally similar result and / or, where a numerical limit is required, a range of ±10% from the stated value, unless otherwise stated.

[0050] The numerical density or volumetric proportion of inclusions is selected to provide sufficient distribution across the adhesive interface to maintain a substantially uniform adhesive thickness. Additionally, in certain cases, where the optical uniformity of the inclusions is generally superior to that of the adhesive, it may be preferable to increase the volumetric proportion of the inclusions within the adhesive so that the inclusions contribute a correspondingly larger proportion of the optical properties of the adhesive / inclusion layer. Volumetric proportions of at least 10 percent may make a contribution to the optical properties in this way, while larger proportions of 20-40 percent, and in some cases higher, may be beneficial to further reduce index inhomogeneity of the layer.

[0051] During the pressing step, the inclusions limit the extent to which the plates can be brought together, thereby defining a controlled adhesive thickness corresponding approximately to the inclusion’s minimum height dimension (e.g., diameter). Once cured, the adhesive layer retains the inclusions in place, forming a robust and optically stable interface with improved homogeneity.

[0052] The inclusions are referred to herein as “index-matched” to the adhesive. The matching of the refractive index between the adhesive and the inclusions is defined functionally as sufficient to avoid scattering effects sufficient to noticeably impact the optical quality of an image propagating along the final lightguide product. This, in turn, is highly dependent upon the volumetric proportion of the inclusions within the adhesive layer. Where the inclusions make up a small proportion of the volume, any scattering effects from the inclusions will occur at a correspondingly small number of highly localized locations at the interface, so that rough index matching is sufficient to avoid noticeable deterioration of image quality. Where a high volumetric proportion of inclusions is used, the sensitivity to index matching increases. Numerically, the implications of the above functional definition will be clear to one ordinarily skilled in the art, but in certain typical cases, the index matching of the inclusions is preferably within a few percent of that of the adhesive, more preferably within 1 percent, and in high volumetric implementations, preferably within a tenth of a percent.

[0053] This approach enables a precise, self-limiting adhesive thickness to be defined throughout the stack, while reducing the sensitivity of the interface properties to external pressure conditions and to surface particulates or irregularities. Moreover, the ability to generate adhesive layers of well-defined thickness and parallelism provides enhanced design control which may facilitate designs with improved optical performance by avoiding or minimizing birefringence arising from ultra-thin layers and / or ensuring a thickness of the adhesive layer greater than the coherence length of the light source to be used in conjunction with the waveguide.

[0054] Adhesive Thickness Control Using Peripheral Spacer Elements

[0055] In a second preferred group of embodiments, the inter-plate spacing is defined by peripheral spacer elements positioned at or near the edges of the plates during assembly. Unlike the previously described embedded inclusions, these spacer elements are deployed in regions which lie outside the regions that are sliced and cut to form the lightguide components, and are typically not present in the final lightguide assembly. Their presence during pressing and curing still serves to define a minimum separation between the plates during pressing, which is subsequently fixed by the curing of the adhesive.

[0056] FIGS. 3A and 3B illustrate one example of this approach. FIG. 3A is a schematic side view of a stacked assembly of plates 12, where adhesive layers 16 are interposed between adjacent plates and the inter-plate spacing is defined by peripheral spacer elements 22. The spacer elements 22 are placed at multiple edge locations around the plate periphery to ensure that a uniform minimum spacing is maintained during pressing. FIG. 3B is a corresponding plan view showing the location of the peripheral spacer elements 22 near the corners or along the margins of the plates 12.

[0057] These peripheral spacer elements 22 may take various forms, including mechanical shims, polymeric or adhesive tapes, metal foils or other physical structures of predetermined thickness. In preferred but non-limiting embodiments, the spacer elements are applied before the adhesive is introduced, such that when the plates are pressed together, the adhesive is spread across the interface and limited in thickness by the height of the spacer elements. Gaps are preferably left between regions of the spacer at the periphery of the plates to allow escape of excess adhesive when pressed.

[0058] Depending on the mechanical properties of the spacer elements 22 and the extent of overlap with the plates 12, an external support structure may be provided to keep the spacer elements correctly positioned. This is illustrated in FIGS. 3A and 3B as an arrangement of holder frames 21, one for each adhesive interface layer, mounted on alignment poles 23.

[0059] FIG. 4A illustrates an alternative approach in which edge shims are formed directly on the plate surface by a pre-deposition process. FIGS. 4B through 4D show a sequence of plan views illustrating one example of this pre-deposition process. In FIG. 4B, a masking element 25 (e.g., a Kapton™ frame) is applied to the plate to leave a defined exposed region of the plate 12 along the edge, shown here as L-shaped corner regions. In FIG. 4C, a uniform coating 26 of spacer material, such as a layer of SiCh, is deposited over the plate. The thickness of the coating is chosen according to the thickness of the desired layer of adhesive which, as before, may be any thickness, typically from 50 nm upwards, serving to improve the precision, uniformity and parallelism of the desired adhesive interface. The coating may also be implemented as a thicker coating, such as in the range of 3-10 pm, for reducing surface effects within the adhesive and / or to ensure thickness of the adhesive layer greater than the coherence length of the light source to be used in conjunction with the waveguide. The masking element is then removed, as shown in FIG. 4D, leaving a raised peripheral spacer region 24 that defines the minimum plate spacing during assembly.

[0060] FIGS. 5 A through 5D illustrate yet another variation, in which edge or corner shims are formed by dipping the edges of each plate 12 into a bath 27 of adhesive or curable polymer. FIGS. 5 A and 5B illustrate a dipping and curing process applied to the corners of a plate. FIG. 5C shows a side view of a pre-treated plate with localized corner shims 22 formed in this way. FIG. 5D illustrates a corresponding plate in which adhesive shims 22 have been formed continuously along the plate edges. These edge-formed spacers may be cured prior to stack assembly and retained in place during the bonding process.

[0061] In all of these frame-based embodiments, the spacer elements are positioned to maintain a target adhesive thickness during pressing, while allowing the adhesive to fill the central bonding region between plates. The spacer elements are preferably confined to marginal regions of the plates that will later be removed or excluded from the final lightguide component during the slicing or cutting steps, and thus need not remain optically functional or visible in the finished product.

[0062] This class of spacer structures provides a flexible and mechanically simple way to control inter-plate spacing without introducing foreign particulate material into the central optical region of the adhesive layer. The approach is particularly suited to manufacturing environments where adhesive uniformity and process repeatability are prioritized, and where the presence of visible inclusions in the final optical aperture may be undesirable.

[0063] Sequential Assembly and Curing to Reduce Cumulative Shrinkage

[0064] In a further preferred aspect of the present disclosure, a method is provided for reducing cumulative deformation and shrinkage effects during adhesive curing by assembling and curing the stack in multiple stages. This approach is particularly relevant when using UV-curable adhesives that exhibit volumetric shrinkage upon curing.

[0065] FIGS. 6A-6C illustrate a sequence of stages in such a process. In a first stage (FIG. 6A), a first pair of plates 12 is assembled with an adhesive layer 16 interposed between their major surfaces, and the adhesive is cured to form a bonded pair. A third plate 12 is then added to this bonded pair with a second adhesive layer 16 interposed between it and the previously cured plate (FIG. 6B). After pressing to distribute the adhesive to a uniform thickness — preferably controlled using spacer components 22 of one of the types described above — the adhesive layer is cured to form a three-plate stack. This process is repeated to add a fourth plate (FIG. 6C), and may be continued as needed to complete the stack.

[0066] Each adhesive interface is therefore formed and cured independently, rather than curing all layers simultaneously. This method limits the total amount of adhesive being cured at any given time, thereby reducing the aggregate volume change and the internal stresses that can arise from simultaneous curing of multiple layers. As a result, the final stack structure exhibits improved flatness, reduced curvature, and better optical uniformity across the interfaces.

[0067] The sequential assembly method is compatible with both inclusion-based and edge-based spacer techniques for defining adhesive thickness, and may be applied in single-plate increments or in small sub-stacks assembled in parallel and then combined. The selection of sequencing strategy may depend on equipment constraints, throughput requirements, or tolerance sensitivities in the final application.

[0068] Slicing and Integration into a Lightguide Structure

[0069] After assembly and curing, the bonded stack is processed to form functional lightguide components. FIG. 7 illustrates a schematic sequence of steps in this process.

[0070] At 7(a), a completed stack 10 is shown comprising a plurality of plates 12 bonded by cured adhesive layers (not separately visible at this scale). One major surface at each adhesive interface carries a partially reflective coating, thereby defining partially- reflecting internal surfaces (“facets”).

[0071] Stack 10 is then sliced along a set of parallel slicing planes non-parallel to the facets to generate a plurality of slices 30, only one of which is shown here at 7(b), each including a set of facets. Depending on the required component functionality, the slicing planes may be oblique to the plane of the plates, resulting in internal facets which are obliquely angled to the sliced surfaces of the lightguide component, or the slicing may be perpendicular to the plates, resulting in orthogonal internal facets. The slicing process may be performed using precision dicing saws or other planar cutting methods suitable for brittle materials.

[0072] At 7(c), a single lightguide component 32 is cut from the slice 30 according to the illustrated outline, resulting in the separate component 32 shown at 7(d). The internal partially reflective coatings at each interface now form a corresponding set of internal facets oriented at a defined angle to the major surfaces and outer profile of the component. Because the slicing planes are aligned with the desired direction of image propagation, and the coatings are oriented accordingly during stack assembly, the resulting facets are positioned and angled to control the flow of light within the lightguide. Step 7(e) illustrates integration of lightguide component 32 into an assembled lightguide structure 34 configured for two-dimensional aperture expansion. A typical example includes a coupling-in element 38, such as a prism or mirror, that injects image light into the lightguide along a first direction. A first region of the lightguide structure comprises one or more components 32 with internal facets arranged to progressively redirect the light along a second propagation direction within the lightguide (path 36), thereby expanding the effective image aperture in the first direction. A second region of the structure comprises one or more additional components 33 with internal facets configured to progressively couple the image light out of the lightguide toward the eye of a viewer (exit path 40), thereby expanding the effective image aperture in the second direction. Component 33 may also be implemented according to one or other of the above-discussed approaches to producing the stack.

[0073] In certain preferred implementations, the lightguide components 32 formed as described above retain structural features derived from the adhesive interfaces in the original stack. In particular, when index-matched inclusions 20 are used as described with reference to FIG. 2B, these inclusions remain embedded within the adhesive layer of the sliced lightguide component and are present in the final assembled lightguide structure 34. The presence of these inclusions in the final product may contribute positively to the optical properties of the adhesive layer, as discussed above.

[0074] This slicing and integration process allows precise control over the position, angle, and optical function of internal reflective facets within each lightguide component. It also enables modular fabrication of two-dimensional aperture expanding lightguides by combining independently fabricated regions with distinct facet orientations and functions.

[0075] It will be appreciated that the above descriptions are intended only to serve as examples, and that many other embodiments are possible within the scope of the present invention as defined in the appended claims.

Claims

1. WHAT IS CLAIMED IS:

1. A lightguide structure comprising: a pair of mutually parallel major surfaces for supporting propagation of light by internal reflection at the major surfaces, wherein at least one region of the lightguide structure comprises a set of mutually - parallel partially-reflective surfaces located between, and non- parallel to, the major surfaces, the set of partially-reflective surfaces being formed at interfaces between a plurality of successive plates, the partially-reflective internal surfaces being configured to partially reflect light propagating within the lightguide, wherein at least one of the interfaces includes a cured optical adhesive layer disposed between adjacent plates, the optical adhesive layer comprising a plurality of index-matched inclusions embedded within the adhesive material, the index-matched inclusions being configured to define a minimum height when disposed between two planar surfaces, wherein a thickness dimension of the adhesive layer corresponds substantially to the minimum height of the index-matched inclusions.

2. The lightguide structure of claim 1, wherein each interface between successive plates in the region comprises a cured optical adhesive layer containing the plurality of index-matched inclusions.

3. The lightguide structure of claim 1, wherein the inclusions are spherical.

4. The lightguide structure of claim 1 , wherein the inclusions are cylindrical rods with circular cross-section.

5. The lightguide structure of claim 1 , wherein the region is configured to redirect light propagating in a first direction within the lightguide so as to propagate in a second direction by internal reflection at the major surfaces of the lightguide.

6. The lightguide structure of claim 1 , wherein the region is configured to couple light propagating within the lightguide out of the lightguide toward an eye of a viewer.

7. The lightguide structure of claim 1 , wherein the inclusions comprise solid glass having a refractive index substantially matched to that of the adhesive material.

8. The lightguide structure of claim 1, wherein the inclusions have a minimum height of between 3 pm and 10 pm.

9. The lightguide structure of claim 1, wherein the inclusions are distributed within the adhesive layer at a numerical density sufficient to define the adhesive thickness substantially uniformly across the interface.

10. A method of producing an optical assembly for use in manufacturing a lightguide, the method comprising:(a) assembling a stack including a first plate, a second plate, an optical adhesive interposed between the first and second plates, and one or more spacer components configured to define a uniform minimum spacing between the plates;(b) pressing the plates together to distribute the adhesive between the plates while maintaining the uniform minimum spacing defined by the one or more spacer components; and(c) curing the adhesive.

11. The method of claim 10, wherein the one or more spacer components comprise a plurality of transparent, index-matched solid inclusions dispersed within the optical adhesive.

12. The method of claim 10, wherein the one or more spacer components comprise peripheral spacer elements applied to one or more edge regions of at least one of the plates.

13. The method of claim 10, wherein the one or more spacer components comprise one or more regions of a coating material selectively deposited on a surface of one of the plates with a coating thickness that defines the uniform minimum spacing.

14. The method of claim 10, wherein the one or more spacer components comprise preformed adhesive elements disposed on one or more marginal region of at least one of the plates.

15. The method of claim 10, wherein the stack further comprises a third plate, and a second optical adhesive layer and one or more additional spacer components disposed between the second plate and the third plate, the steps of pressing and curing being performed simultaneously on the stack.

16. The method of claim 10, further comprising, subsequent to the curing:(a) adding to the stack a third plate, and a second optical adhesive layer and one or more additional spacer components disposed between the second plate and the third plate, thereby forming an incremented stack;(b) pressing the incremented stack to distribute the second optical adhesive layer between the second and third plates while maintaining a uniform minimum spacing defined by the one or more additional spacer components; and(c) curing the second optical adhesive layer.

17. The method of claim 15 or claim 16, wherein a surface of one of the plates facing each layer of optical adhesive is provided with a partially reflective coating.

18. The method of claim 17, further comprising slicing the cured stack along a plurality of parallel slicing planes to form a plurality of lightguide components each having a plurality of internal partially-reflective surfaces at interfaces between the plates.

19. An intermediate work product sliceable to produce a plurality of lightguide components each including a plurality of mutually-parallel partially-reflecting internal surfaces, the intermediate work product comprising a stack formed from:(a) a plurality of plates, each bounded by a pair of mutually parallel major surfaces;(b) for each pair of adjacent plates of the plurality of plates having major surfaces in facing relation, one or more spacer components at least partially interposed between the major surfaces in facing relation, the one or more spacer components being configured to define a uniform minimum spacing between the major surfaces; and(c) a cured optical adhesive substantially filling a space between the adjacent plates defined by the one or more spacer components, wherein one of the major surfaces in facing relation between each pair of adjacent plates has a partially-reflecting coating.

20. The intermediate work product of claim 19, wherein the one or more spacer components comprise a plurality of transparent, index-matched solid inclusions dispersed within the cured optical adhesive.

21. The intermediate work product of claim 19, wherein the one or more spacer components comprise peripheral spacer elements disposed on marginal regions of at least one of each pair of adjacent plates.

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