Methods and arrangements for bonding transparent plates and for fabricating light-guide optical elements

Abstract

Methods for bonding parallel-faced transparent plates are provided. The plates are arranged in a stack with a lateral offset between one or more pairs of adjacent plates to define plate steps. Adhesive is provided at the interfaces between adjacent plates of the stack, and a coating is provided at one face at each of the interfaces. In one set of methods, the stack is placed between a pair of pressing members, and compensation members are provided in a stepped configuration that corresponds to the plate steps. Pressure is then applied to the plates via the pressing members. In another set of methods, the stack is placed in a flexible container that has an opening, and gas is removed from the container via the opening to cause the container to deform around the stack which applies pressure on sides of the stack and causes redistribution of excess adhesive from the interfaces.

Description

[0001] APPLICATION FOR PATENT

[0002] TITLE METHODS AND ARRANGEMENTS FOR BONDING TRANSPARENT

[0003] PLATES AND FOR FABRICATING LIGHT-GUIDE OPTICAL ELEMENTS

[0004] CROSS-REFERENCE TO RELATED APPLICATIONS

[0005] This application claims priority from US Provisional Patent Application No. 63 / 621,601, filed January 17, 2024 and US Provisional Patent Application No. 63 / 636,889, filed April 22, 2024, the disclosures of which are incorporated by reference in their entirety herein. TECHNICAL FIELD

[0006] The present disclosure relates to optical systems, and in particular, it concerns methods and arrangements for bonding transparent plates, and methods for fabricating light-guide optical elements (LOEs) from the bonded transparent plates.

[0007] BACKGROUND OF THE INVENTION

[0008] Various types of display, such as near eye displays, require large aperture to cover the area where the observer’s (i.e., user’s, viewer’s) eye is located (commonly referred to as the eye-motion box - or EMB). In order to implement a compact device, the image that is to be projected into the observer’s eye is generated by a small optical image generator (projector) having a small optical aperture. Optical arrangements for displays may employ a light-guide optical element (LOE) to expand an input image in one or more dimensions. Of particular relevance to the present disclosure are reflective LOE’s, where the LOE is implemented as a transparent substrate bounded by two parallel major external surfaces configured to support propagation of light therebetween via (total) internal reflection, and where the image expansion is performed by a set of mutually-parallel partially-reflecting internal surfaces (or “facets”) located between the major external surfaces. A collimated image propagating within the LOE is progressively partially deflected by the set of facets, thereby achieving aperture expansion.

[0009] Conventional LOE production methods rely on stacking and bonding parallel-faced transparent plates that are provided with an at least partially reflective coating at interfaces between the plates. The bonded stack is cut along parallel cutting planes that are obliquely inclined relative to the faces of the transparent plates, such that the interfaces form the facets. The bonding of the stack is achieved by optical adhesive provided between adjacent plates. FIG. 1 illustrates a conventional stack 1 of transparent plates, showing parallel cutting planes 15 obliquely inclined to the plate faces. The stack 1 is shown as being topped off (at the top and bottom) with additional transparent plates having a thickness of several times that of the other plates. In order for the final LOE product to have optimal mechanical and optical properties, the layer of adhesive between the plates should be as thin as possible. This can be achieved using press mechanisms to apply pressure to the stack via the top and bottom plates of the stack. However, conventional pressing techniques may lead to warping or deformation of the plates if uniform pressure is not applied. In addition, in order to reduce the amount of scrap, the stack is typically arranged as a staggered stack 1’, as shown in FIG. 2. However, optimal mechanical and optical properties may not be achievable using conventional pressing techniques on a staggered stack.

[0010] SUMMARY OF THE INVENTION

[0011] The present disclosure provides methods and arrangements for bonding transparent plates and for fabricating light-guide optical elements (LOEs) from the bonded transparent plates.

[0012] According to the teachings of an embodiment of the present disclosure, there is provided a method that comprises: arranging a plurality of parallel-faced transparent plates in a stack with a lateral offset between one or more pairs of adjacent transparent plates to define one or more plate steps, optical adhesive being provided at interfaces between adjacent transparent plates of the stack, and a coating being provided at one face at each of the interfaces, the stack being placed between first and second pressing members; providing a plurality of compensation members between the first and second pressing members in a stepped configuration, the one or more plate steps and the stepped configuration being correspondingly configured such that the compensation members compensate for the offset between the one or more pairs of adjacent transparent plates; and applying pressure to the plurality of transparent plates via the first and second pressing members.

[0013] Optionally, the stepped configuration of the compensation members is provided at least in part by dimensions of the compensation members.

[0014] Optionally, at least some of the compensation members have adjustable height, the height is measured along a dimension perpendicular to the parallel faces of the transparent plates.

[0015] Optionally, the plurality of compensation members includes a first set of compensation members associated with the first pressing member and a second set of compensation members associated with the second pressing member.

[0016] Optionally, the plurality of compensation members includes a first set of compensation members and a second set of compensation members, the first set of compensation members associated with a first end region of the stack and compensating for the offset between one or more pairs of adjacent transparent plates at the first end region of the stack, the second set of compensation members associated with a second end region of the stack and compensating for the offset between one or more pairs of adjacent transparent plates at the second end region of the stack. Optionally, the stack is placed between first and second pressing member such that a first block member is positioned between the first pressing member and a first transparent plate at a top end of the stack, and such that a second block member is positioned between the second pressing member and a second transparent plate at a bottom end of the stack.

[0017] Optionally, the method further comprises: deploying a first insulating member between the first block member and the first pressing member; and deploying a second insulating member between the second block member and the second pressing member.

[0018] Optionally, the coating provides partially-reflecting optical properties.

[0019] Optionally, the method of further comprises: solidifying the adhesive so that the stack forms a bonded stack; and cutting the bonded stack along at least two parallel cutting planes obliquely inclined relative to the faces of the transparent plates to extract one or more parallelfaced substrates having a plurality of mutually-parallel partially-reflective internal surfaces formed from the interfaces.

[0020] Optionally, the method further comprises: placing the stack in a flexible container having an opening; and removing gas from the container via the opening so as to cause the container to deform around the stack thereby applying an applied pressure on sides of the stack and creating pressure differentials at regions around the stack within the container, the applied pressure and pressure differentials causing excess adhesive from the interfaces to be redistributed to the regions.

[0021] There is also provided according to the teachings of an embodiment of the present disclosure a press arrangement that comprises: a first pressing member; a second pressing member opposite the first pressing member, the first and second pressing members configured to be spaced apart to receive a stack of parallel-faced transparent plates, the transparent plates arranged in the stack with a lateral offset between one or more pairs of adjacent transparent plates to define one or more plate steps, optical adhesive being provided at interfaces between adjacent transparent plates of the stack, and a coating being provided at one face at each of the interfaces; a plurality of compensation members provided between the first and second pressing members in a stepped configuration, the one or more plate steps and the stepped configuration being correspondingly configured such that the compensation members compensate for the offset between the one or more pairs of adjacent transparent plates; and an actuator associated with at least one of the first or second pressing members and configured to move at least one of the first or second pressing members to apply pressure to the plurality of transparent plates.

[0022] There is also provided according to the teachings of an embodiment of the present disclosure a method that comprises: arranging a plurality of parallel-faced transparent plates in a staggered stack, optical adhesive being provided at interfaces between adjacent transparent plates of the stack, and a coating being provided at one face at each of the interfaces; placing the stack in a flexible container having an opening; and removing gas from the container via the opening so as to cause the container to deform around the stack thereby applying an applied pressure on sides of the stack and creating pressure differentials at regions around the stack within the container, the applied pressure and pressure differentials causing excess adhesive from the interfaces to be redistributed to the regions.

[0023] Optionally, the method further comprises: inflating the container; and removing at least some of the redistributed excess adhesive.

[0024] Optionally, the method further comprises: repeatedly removing gas from the container, inflating the container, and removing at least some redistributed excess adhesive, until a stopping condition is satisfied.

[0025] Optionally, the method further comprises: thinning the adhesive provided at the interfaces by diluting the adhesive with an additive so as to reduce the viscosity of the adhesive.

[0026] Optionally, the additive includes a solvent.

[0027] Optionally, the method further comprises: cutting the stack along at least two parallel cutting planes obliquely inclined relative to the faces of the transparent plates to extract at least one parallel-faced substrate having a plurality of mutually-parallel internal surfaces formed from the interfaces; and placing one or more of the at least one substrate in a chamber to expedite evaporation of the solvent.

[0028] Optionally, the method further comprises: solidifying the adhesive so that the stack forms a bonded stack.

[0029] Optionally, the solidifying the adhesive is performed while the container is deformed around the stack.

[0030] Optionally, the coating provides partially-reflecting optical properties.

[0031] Optionally, the method further comprises: cutting the stack along at least two parallel cutting planes obliquely inclined relative to the faces of the transparent plates to extract at least one parallel-faced substrate having a plurality of mutually-parallel partially-reflective internal surfaces formed from the interfaces.

[0032] Optionally, the applied pressure is substantially uniform on all sides of the stack thereby mitigating distortion at edges of the transparent plates.

[0033] Optionally, the stack is topped off at the top and bottom with high-stiffness plates having stiffness that is sufficient to oppose the bending of the remaining plates of the stack under the applied pressure.

[0034] Optionally, the method further comprises: heating the container while the container is deformed around the stack. Optionally, the method further comprises: applying external pressure to the container while the container is undergoing the heating.

[0035] Optionally, the staggered stack is such that there is a lateral offset between one or more pairs of adjacent transparent plates defining one or more plate steps, and the method further comprises: placing the stack between first and second pressing members; providing a plurality of compensation members between the first and second pressing members in a stepped configuration, the one or more plate steps and the stepped configuration being correspondingly configured such that the compensation members compensate for the offset between the one or more pairs of adjacent transparent plates; and applying pressure to the plurality of transparent plates via the first and second pressing members.

[0036] Unless otherwise defined herein, all technical and / or scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. Although methods and materials similar or equivalent to those described herein may be used in the practice or testing of embodiments of the invention, exemplary methods and / or materials are described below. In case of conflict, the patent specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and are not intended to be necessarily limiting.

[0037] BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Some embodiments of the present disclosure are herein described, by way of example only, with reference to the accompanying drawings. With specific reference to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of embodiments of the disclosure. In this regard, the description taken with the drawings makes apparent to those skilled in the art how embodiments of the disclosure may be practiced.

[0039] Attention is now directed to the drawings, where like reference numerals or characters indicate corresponding or like components. In the drawings:

[0040] FIG. 1 is a schematic representation of a stack of parallel-faced transparent plates that can be cut along parallel cutting planes to produce one or more LOEs;

[0041] FIG. 2 is a schematic representation of a staggered stack of parallel-faced transparent plates that can be cut along parallel cutting planes to produce one or more LOEs;

[0042] FIG. 3 is a schematic representation of a staggered stack of parallel-faced transparent plates, similar to the staggered stack illustrated in FIG. 2, showing interfaces between pairs of adjacent transparent plates and plate steps defined by lateral offset between one or more pairs of adjacent transparent plates; FIG. 4 is a schematic representation of an exemplary one of the interfaces between transparent plates of the staggered stack of FIG. 3, showing optical adhesive provided at the interface;

[0043] FIG. 5A is a schematic representation of a press arrangement having a pair of oppositely disposed pressing members and a compensation arrangement having independently adjustable compensation members arranged in a stepped configuration that corresponds to the plate steps of the staggered stack of FIG. 3, according to embodiments of the present disclosure, and showing the staggered stack of FIG. 3 placed between the pressing members and interfaced with the compensation arrangement;

[0044] FIG. 5B is an enlarged view of the region of FIG. 5A designated V.

[0045] FIG. 6 is a schematic representation of a compensation arrangement and a staggered stack, according to embodiments of the present disclosure, showing the compensation arrangement and the staggered stack spatially separated from each other to more clearly illustrate the stepped configuration and the correspondence between the stepped configuration and the plate steps;

[0046] FIG. 7 is a schematic representation of an embodiment similar to the embodiment illustrated in FIGS. 5A and 5B, but with insulation members provided at the pressing members, and without independent control of the compensation members;

[0047] FIG. 8 is a schematic representation of an LOE, having a pair of mutually-parallel major external surfaces and a set of mutually-parallel partially-reflecting internal surfaces obliquely inclined relative to the major external surfaces, that may be fabricated from a staggered stack of plates that are bonded together according to embodiments of the present disclosure;

[0048] FIG. 9 is an alternative example of a staggered stack that can be processed using the press arrangement according to embodiments of the present disclosure;

[0049] FIG. 10 is a further alternative example of a staggered stack that can be processed using the press arrangement according to embodiments of the present disclosure;

[0050] FIG. 11 is yet another alternative example of a staggered stack that can be processed using the press arrangement according to embodiments of the present disclosure;

[0051] FIG. 12 is a flow diagram of a method for bonding transparent plates and optionally fabricating one or more LOEs from the bonded plates, according to embodiments of the present disclosure;

[0052] FIG. 13 is a schematic representation of a staggered stack of transparent plates, showing the interfaces between the transparent plates and optical adhesive provided at the interfaces;

[0053] FIG. 14 is a schematic representation of the staggered stack of FIG. 13 placed within a flexible container having an opening and containing a quantity of gas, according to embodiments of the present disclosure; FIG. 15 is a schematic representation of the flexible container with staggered stack of FIG. 14, showing that the removal of the gas from within the container causes the container to deform around the stack so as to thin-out the adhesive at the interfaces between the transparent plates and redistribute excess adhesive to pocket regions of the container, according to embodiments of the present disclosure;

[0054] FIG. 16 is a schematic representation of the flexible container with staggered stack of FIGS. 14 and 15, showing the container inflated and the redistributed excess adhesive at exposed regions of the staggered stack, according to embodiments of the present disclosure;

[0055] FIG. 17 is a schematic representation of the staggered stack of FIG. 16 with the redistributed excess adhesive removed;

[0056] FIG. 18 is a schematic representation of the staggered stack of FIG. 17, showing a pair of parallel cutting planes along which the staggered stack can be cut to produce an LOE;

[0057] FIG. 19 is a schematic representation of multiples LOEs, which can be extracted from the staggered stack of FIG. 17, showing the LOEs deployed within a vacuum chamber to expedite evaporation of an additive that is added to the optical adhesive, according to embodiments of the present disclosure;

[0058] FIG. 20 is a schematic representation of a flexible container with a staggered stack similar to FIG. 14, but in which the stack is topped off at the top and bottom with high-stiffness plates to provide structural support to the stack under pressure; and

[0059] FIG. 21 is a flow diagram of a method for bonding transparent plates and optionally fabricating one or more LOEs from the bonded plates, according to embodiments of the present disclosure.

[0060] DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0061] Embodiments of the present disclosure provide methods and arrangements for bonding transparent plates and for fabricating light-guide optical elements (LOEs) from the bonded transparent plates.

[0062] The principles of the methods and arrangements according to present disclosure may be better understood with reference to the drawings accompanying the description.

[0063] Before explaining at least one embodiment of the disclosure in detail, it is to be understood that the disclosure is not necessarily limited in its application to the details of construction and the arrangement of the components and / or methods set forth in the following description and / or illustrated in the drawings and / or the examples. The embodiments of the disclosure are capable of other embodiments or of being practiced or carried out in various ways. Initially, throughout this document, references are made to directions, such as, for example, left and right, top and bottom, upper and lower, and the like. These directional references are exemplary only, and are used only for ease of presentation and refer to the arbitrary orientations as illustrated in the drawings.

[0064] Embodiments according to a first aspect of the present disclosure provide a method for bonding transparent plates arranged in a staggered stack, which can be used for light-guide optical element (LOE) fabrication. The method includes various steps (stages), at least some of which are illustrated schematically in FIGS. 3 - 7, and shown in the flow diagram of FIG. 12. An arrangement, and parts thereof, for executing at least part of some of the method stages is also illustrated schematically in FIGS. 5A - 7.

[0065] Looking first at FIG. 3, a plurality of parallel-faced transparent plates 12 is arranged in a staggered stack 10, similar to as in FIG. 2. Only some of the plates 12 are designated in FIG. 3, for clarity of presentation. The plates can be made from any suitable optically transparent material, typically glass (such as BK7), but other transparent materials, such as plastic, may also be used. The staggering of the transparent plates 12 is such that there is a lateral offset between one or more pairs of adjacent transparent plates. The lateral offset between the plates defines one or more plate steps 14 that are formed from exposed parts of the parallel faces and the edges the plates. The staggering of the plates may be performed on groups of plates, for example as shown in FIG. 3, where groups of four plates are aligned (not staggered) and laterally offset from adjacent groups of four aligned plates. The stack 10 illustrated in FIG. 3 includes five such aligned groups of thin plates, and is also shown as being topped off at the top and bottom with transparent plates 12T and 12B, having a thickness of several times that of the other plates 12. Within the context of this document, the transparent plates 12 are alternatively referred to as “thin plates”, and the transparent plates 12T and 12B are alternatively referred to as “thick plates”.

[0066] In FIG. 3, the thick plates 12T and 12B are also laterally offset from adjacent thin plates 12. As a result of the offsets of the thick plates, coupled with the offsets of the five groups of aligned thin plates, the stack 10 shown in FIG. 3 has a plurality of steps 14, in particular six steps 14 at each end (left end and right end) of the stack 10.

[0067] Parenthetically, the offset between adjacent plates is typically along a first dimension that is along a direction of elongation of the thin plates 12 (which in the drawings is the horizontal dimension). In a second dimension, that is perpendicular to the first dimension and is in a plane that is parallel to the plane of the faces of the plates (i.e., going in to / coming out of the drawing), there may be no offset (i.e., the plates may be aligned in the second dimension).

[0068] Each of the steps 14 has a height dimension and a width dimension. For most of the steps, the height dimension, which is in the vertical direction in the figure, generally corresponds to the combined thickness of the plates in an aligned group, and the width dimension, which is in the horizontal direction in the figure, generally corresponds to the amount of lateral offset between the two groups of aligned plates that form the step. For the top-right and bottom-left steps, the height dimension corresponds to the thickness of the thick plate 12T and 12B, respectively, optionally in combination with some or all of the thickness of a support surface upon which the thick plate 12T and 12B rests. For the top-left and bottom-right steps, the width dimension corresponds to the amount of lateral offset between the thick plates 12T and 12B and their adjacent group of thin plates.

[0069] In general, the staggering between adjacent plates may or may not be uniform. For example, each pair of laterally offset plates need not necessarily be offset by the same amount, and the number of plates in each group of aligned plates need not necessarily be the same. Accordingly, the steps 14 may have uniform dimensions (uniform width and / or uniform height) across the steps, or variable dimensions (variable width and / or variable height) that differ from step to step. The uniformity or variability of the step dimensions is a function of various parameters, including the dimensions of the thin plates 12 (and thick plates 12T and 12B), and the amount of lateral offset between pairs of adjacent plates and / or the number of aligned plates in a group.

[0070] An optical adhesive, which may be a liquid optical curable adhesive, is provided at each of the interfaces 17 between adjacent thin plates 12 of the stack 10. The interfaces 17 are between adjacent faces (of the parallel faces) of adjacent plates. Only some of the interfaces 17 are designated in the figure, for clarity of presentation. Furthermore, the optical adhesive at the interfaces 17 is not shown in FIG. 3, but is shown schematically in FIG. 4 for an exemplary one of the interfaces 17. In FIG. 4, the optical adhesive is exaggerated for clarity and is represented by an elongated oblong dotted-pattern-filled shape, designated 13. As shown in FIG. 4, the interface 17 is between the lower face 11L of the upper plate and the upper face 11U of the lower plate.

[0071] Adhesive is also provided at the interface between the top thick transparent plate 12T and its adjacent thin plate 12, and the interface between the bottom thick transparent plate 12B and its adjacent thin plate 12.

[0072] An optical coating is provided at one face (but in certain cases both faces) at each of the interfaces 17 between the thin plates 12. In embodiments in which the stacked plates are used to fabricate LOEs, the optical coating is an at least partially-reflecting coating that provides partially- reflecting optical properties. It is noted, however, that the methods according to the embodiments described herein may be used to produce other types of optical devices aside from LOEs, including optical elements having fully-reflecting internal surfaces. Therefore, other types of coating, such as fully-reflecting coating, can be provided at the interfaces 17 between the plates 12.

[0073] Typically, the stack may be built-up by alternately layering plates and adhesive. For example, the thick plate 12B may be placed on a surface, such as a carrier surface or carrier plate, and then optical adhesive may be applied to the exposed face of the thick plate 12B. Then, a first thin plate 12 may be placed on top of the thick plate 12B. Optical adhesive may then be applied to the exposed face of the first thin plate 12, and then a second thin plate 12 may be placed on top of the first thin plate 12 such that the faces of the two thin plates contacting the adhesive form the interface 17. This process may be repeated until the stack 10 is fully built-up, optionally topping off the stack with thick plate 12T. The plates may be placed one on top of the other with offset as needed, in order to achieve staggering of the plates.

[0074] Referring now to FIGS. 5 A and 5B, the stack 10 is placed (positioned) between oppositely disposed upper and lower pressing members 22 and 24, which are shown as being part of a press apparatus 20. As shown, the stack 10 is positioned in a space 26 between the pressing members 22 and 24. The stack 10 is placed so that the thin plate 12 toward the upper (top) part of the stack 10 is associated with the upper pressing member 22 and the thin plate 12 toward the lower (bottom) part of the stack 10 is associated with the lower pressing member 24. In embodiments in which the stack is topped off with thick plates 12T and 12B, this placement of the stack 10 is such that the transparent plate 12T at the top part of the stack 10 is proximate to the upper pressing member 22 and the transparent plate 12B at the bottom part of the stack 10 is proximate to the lower pressing member 24.

[0075] Actuation of the press apparatus 20 may be achieved via actuator 28, which may be associated with the upper press member 22 and / or the lower press member 24. The actuator 28 is configured to drive the upper pressing member 22 and / or the lower pressing member 24 so as to relatively move the press members 22 and 24 towards one another so as to apply pressure to the transparent plates of the stack. The actuator 28 may also be configured to move the upper press member 22 and / or the lower press member 24 in order to separate the pressing members 22 and 24 from each other to create the space 26 so that the stack can be received in the space 26. The actuator 28 may be, for example, a pneumatic actuator, hydraulic actuator, or any other suitable actuation device. Alternatively, the actuator 28 can be a user-operated mechanical actuator, such as a hand crank or lever.

[0076] In the illustrated embodiment, the stack 10 is positioned between block members 34 and 36, which are configured to contact the upper and lower pressing members 22 and 24, respectively. The block members 34 and 36 are solid and dense members, for example dense metal members (e.g., dense metal blocks or anvils). The lower block member 36 is positioned on the exposed pressure-applying surface 25 of the lower pressing member 24. The thick transparent plate 12B is positioned, for example in alignment, on the exposed (upper) surface of the block member 36. The upper block member 34 is positioned, for example in alignment, on the exposed face of the thick transparent plate 12T. In certain embodiments, the stack 10 may be built-up on a carrier surface that is positioned in association with the lower pressing member 24 of the press apparatus 20. One example of such a carrier surface is the block member 36. In other embodiments, the stack 10 may be built-up on a carrier surface away from the press apparatus 20, and the carrier surface may be transferred to the lower pressing member 24 after the stack 10 is built-up. In yet other embodiments, the stack 10 may be built-up on a carrier surface away from the press apparatus and then transferred to the block member 36.

[0077] As mentioned in the background section, the layer(s) of adhesive between the plates 12 should be as thin as possible. For example, for LOEs, the thickness of the adhesive layer(s) is preferably in a range between approximately 0.3 microns and 2 microns. However, the thickness of the adhesive layer(s) may vary depending on the final optical product and / or the optical requirements of the final optical product. The desired thickness of the adhesive layer(s) can be achieved by applying pressure to the plates 12 via the upper and lower pressing members. Typically, at least 95% of the adhesive is squeezed / drained from the interfaces using the pressing technique. However, conventional presses cannot create uniform pressure over all of the adhesive layers (interfaces), resulting in non-uniformity of the adhesive thickness, which has a negative effect on the optical performance of the final optical product (e.g., LOE). In order to create uniform pressure on each adhesive layer, all of the areas of the plates need to be pressed.

[0078] According to certain embodiments of the first aspect of the present disclosure, uniform (or approximately uniform) pressure is achieved by using compensation members 42. The compensation members 42 are part of a compensation arrangement 40, which together with the press apparatus 20 form a press arrangement. The compensation members 42 are provided between the upper and lower pressing members 22 and 24 in a stepped configuration 44. The plate steps 14 and the stepped configuration 44 of the compensation members 42 have complementary features so as to be correspondingly configured, such that the compensation members 42 compensate for the offset(s) between the one or more pairs of adjacent transparent plates 12, 12T, 12B. This correspondence between the plate steps 14 and the stepped configuration 44 of the compensation members 42 allows contact surfaces of the compensation members 42 to contact (preferably at a direct abutment) the exposed parts of the parallel faces (and preferably also the edges) of the plates 12, 12T, 12B that form the steps 14, in particular when the press apparatus 20 is actuated (i.e., when the press members 22 and 24 apply pressure to the stack 10). In other terms, the complementary features between the plate steps 14 and the stepped configuration of the compensation members 42 promotes interlocking or mating between the compensation members 42 and the stack 10, which allows the compensation members 42 to interface smoothly with the offset portions of the plates 12, 12T, 12B, filling the voids created by the lateral offset(s), in particular when the press apparatus 20 is actuated.

[0079] The stepped configuration of the compensation members 42 can be achieved by staggering compensation members at different heights and optionally different thickness adjacent to one another. In principle, the dimensions (e.g., height and width) of the compensation members 42 may be designed according to the dimensions of the steps 14. For example, for steps of uniform height (measured in a direction perpendicular to the faces of the plates 12) and width, compensation members having uniform thickness (width) and having a constant height differential may be used. It is noted, however, that a group of multiple compensation sub-members of smaller width (where the widths of the sub-members may be uniform or non-uniform) may be placed adjacent to one another to form a compensation member, so that the total combined widths of the sub-members corresponds to the width of a corresponding step. Similarly, a group of multiple compensation sub-members of smaller height (where the heights of the sub-members may be uniform or non-uniform) may be placed one on top of another so that the total combined heights of the sub-members corresponds to the height of a corresponding step.

[0080] FIG. 6 shows an enlarged representation of a compensation arrangement 40 spaced apart from the stack 10 in order to more clearly illustrate the stepped configuration 44 of the compensation members 42, and the correspondence between the stepped configuration 44 and the plate steps 14. Large block arrows are used in FIG. 6 to represent how the compensation arrangement 40 and the stack 10 can be interfaced (interlocked) together at the steps 14 and stepped configuration 44. As shown, the height differential (vertical direction in the figure) between adjacent compensation members corresponds to the heights of the steps 14, and the widths (horizontal direction in the figure) of the compensation members correspond to the widths of the steps 14. This stepped configuration 44 allows the contact surfaces 43 of the compensation members 42, which are the exposed vertical and horizontal surfaces shown in FIG. 6, to seamlessly fill the voids created by the lateral offset(s) by contacting the exposed parts of the parallel faces (and preferably the edges) of the plates 12, 12T, 12B that form the steps 14, in particular when the press apparatus 20 is actuated. As shown in FIG. 6, a portion of one of the contact surfaces 43 of the shortest compensation members 42 is also configured to contact side surfaces 37 and 39 of the upper and lower block members 34 and 36, respectively. This is due to the alignment of the thick plates 12T and 12B with the block members 34 and 36, respectively, which creates parts of additional steps.

[0081] The compensation provided by the compensation members 42 allows the press apparatus 20 to apply uniform pressure across all of the plates 12, 12T, 12B in the stack 10. This pressure can be applied by actuating the press apparatus 20 to decrease the distance between the press members 22 and 24 such that the press members 22 and 24 provide an inward force on the plates 12, toward the center of the stack 10. Specifically, when the press apparatus 20 is actuated, the pressing members 22 and 24 apply pressure to the upper and lower block members 34 and 36, respectively, which in turn apply pressure to the thick transparent plates 12T and 12B, respectively, which in turn apply pressure to the internal plates 12 of the stack 10, all while the offset (stepped) regions of the stack 10 are supported by the compensation arrangement 40 (from below and above). The applied pressure is approximately uniform on each adhesive layer, which consequently squeezes out excess adhesive from the interfaces and redistributes the excess adhesive to exposed regions (e.g., external faces, sides, etc.) of the stack 10. The redistributed excess adhesive can then be removed from the exposed regions of the stack 10.

[0082] In order to achieve preferred results, the press arrangement is preferably configured to apply pressure so that the pressure on the adhesive between the plates is in the range between approximately 0.2 MPa and 1 MPa. This preferred pressure range results in squeezing out a substantial amount of adhesive (least 95%) from the interfaces between the plates so that suitably thin adhesive layers are achieved, while at the same not damaging (e.g., cracking or breaking) the plates. Typically, pressing forces (load force) in the range between approximately 140 kilogramforce and 700 kilogram-force (for plates having interface dimension of approximately 70 mm by 100 mm) will achieve the requisite pressure needed to sufficiently thin the adhesive layers without damaging the plates.

[0083] The compensation members 42 are preferably rectangular cuboid shaped members, and are constructed from a material, typically metallic material, that is solid and dense enough to apply suitable pressure to the plates. Examples of suitable metallic materials from which the compensation members 42 can be constructed include, but are not limited to, titanium, cast iron, stainless steel (e.g., grade 410), and the like. In certain embodiments, the compensation members 42 and the block members 34 and 36 are constructed from the same material. In certain embodiments, the contact surfaces 43 of the compensation members 42 are formed from polished glass plates that are bonded or otherwise attached to the compensation members 42. The compensation members 42 and polished glass plates that form the contact surfaces 43 preferably have similar coefficient of thermal expansion.

[0084] In the illustrated embodiment, there are two sets of compensation members 42, one at each end of the stack 10. This is due to the staggering of the stack creating a set of steps at both end regions of the stack (i.e., each step at one end of the stack has a corresponding stack at the other end of the stack). The first set of the compensation members (on the left side of the relevant figures) is deployed in association with the lower pressing member 22, and the second set of the compensation members (on the right side of the relevant figures) is deployed in association with the upper pressing member 24. The first set of compensation members 42 is located at (associated with) a first (left) end region of the stack 10 and compensates for the offset between one or more pairs of adjacent plates 12, 12T and 12B at the left end region of the stack 10. The second set of compensation members 42 is located at (associated with) a second (right) end region of the stack 10 and compensating for the offset between one or more pairs of adjacent plates 12, 12T and 12B at the right end region of the stack 10.

[0085] In certain embodiments, some or all of the compensation members 42 may have adjustable height, measured along a dimension perpendicular to the parallel faces of the plates 12 of the stack. Adjustment of the height of a compensation member may provide further control of the pressure applied by the compensation member. Control of the height adjustment may be provided by an actuator 48, which may be a pneumatic actuator, hydraulic actuator, or any other suitable actuation mechanism. In the embodiment illustrated in FIGS. 5 A and 5B, each compensation member 42 is height-adjustable and has an associated actuator, implemented as a pneumatic drive piston. This enables pressure applied by each compensation member to be controlled independently. The actuators 48, as well as the actuator 28, may be electrically connected with a computerized control system (not shown) to enable computerized control of the actuation mechanisms.

[0086] FIG. 7 illustrates another embodiment in which a pair of insulating members 50 and 52 is deployed. The insulating members 50 and 52 increase the uniformity of the pressure applied on each of the block members 34 and 36. The upper insulating member 50, which may be implemented as a cushion, for example an air cushion, is deployed between the upper block member 34 and the upper pressing member 22. The deployment of the upper insulating member 50 may be achieved by attaching, for example via adhesive bonding, the upper insulating member 50 to the pressure-applying surface 23 of the upper pressing member 22. The upper insulating member 50 may be positioned so that that a portion of the insulating member 50 contacts the majority and preferably the entirety of the upper surface area 35 of the upper block member 34 during actuation of the press apparatus 20. In certain embodiments, for example embodiments in which the compensation members are not provided with height-control adjustment, it is preferable that a portion of the upper insulating member 50 also contacts the majority and more preferably the entirety of the upper surface area 45 of the adjacent (right-side) compensation members during actuation of the press apparatus 20.

[0087] The lower insulating member 52, which may also be implemented as a cushion (e.g., air cushion), is deployed between the lower block member 36 and the lower pressing member 24. The insulating member 52 may be attached, for example via adhesive bonding, to the pressure-applying surface 25 of the lower pressing member 24. The lower insulating member 52 may be positioned relative to the lower block member 36 and the adjacent (left-side) compensation members in a same or similar fashion as the positioning of the upper insulating member 50 relative to the upper block member 34 and the right-side compensation members.

[0088] Insulating members may also be placed at the contact surfaces of the compensation members 42 such that the insulating members provide insulation between the compensation members 42 and the plates 12 so as to increase the uniformity of the pressure on the plates 12 in the areas of the steps 14.

[0089] It is noted that although the embodiment illustrated in FIG. 7 is shown as being without height-adjustment-control (e.g., actuator 48) of the compensation members 42, it should be readily apparent that the compensation members 42 in the embodiment of FIG. 7 can also be provided with adjustable height and height-adjustment-control.

[0090] According to certain embodiments, after the staggered stack 10 of plates 12 is successfully pressed using the techniques described above, at least some (preferably most, if not all) of the excess adhesive redistributed (i.e., squeezed out from the interfaces) to the exposed regions (e.g., external faces, sides, etc.) of the stack 10 can be removed from said exposed regions. The remaining optical adhesive at the interfaces 17 can then be solidified (cured), for example using UV curing or heat curing, so that the stack 10 becomes a bonded stack.

[0091] In certain embodiments, the bonded stack can then be further processed, for example processed to fabricate one or more LOEs by slicing the stack along a series of parallel cutting planes that are obliquely inclined relative to the faces of the plates of the stack (for example as illustrated in FIG. 2). The LOEs may then be polished at the major external surfaces generated at the cutting planes. Optionally, one or more of the LOEs extracted from the bonded stack may be further sliced along cutting planes perpendicular to the major external surfaces in order to cut the LOEs into smaller / shorter LOEs. FIG. 8 shows one such exemplary LOE, designated 60. The LOE 60 has a pair of mutually parallel major external surfaces 62, generated by cutting the bonded stack at a pair of the parallel cutting planes, and a plurality of mutually-parallel partially-reflecting internal surfaces (facets) 64 formed from the interfaces 17 (provided with the partially-reflecting coating) that are obliquely inclined relative to the major external surfaces 62.

[0092] As another non-limiting example of further processing of the bonded stack, the bonded stack can be cut as part of a two-dimensional (2D) expansion LOE fabrication processes. Such 2D expansion LOEs include two aperture expansion regions, each having a set of mutually-parallel facets that are non-parallel to each other. Details of an exemplary 2D expansion LOE can be found, for example, in commonly owned US Patent No. 10,739,512. By way of one example use of the bonded stack in production a 2D expansion LOE, the bonded stack can be cut along two planes to form an optical block having a plurality of partially-reflecting facets (formed from the interfaces 17) that can be used to form a first aperture expansion region of a 2D LOE (represented as region 16 in the aforementioned patent).

[0093] In embodiments in which the coating is a fully-reflecting coating, slicing the bonded stack along the parallel cutting planes produces one or more substrates, each having a pair of mutually parallel major external surfaces and a plurality of mutually-parallel fully-reflecting internal surfaces that are obliquely inclined relative to the major external surfaces. In such embodiments, one or more of the substrates may be further sliced along a series of parallel cutting planes that are perpendicular to the major external surfaces and in which the cutting planes are spaced apart so that each fully-reflecting internal surface is bounded by a pair of the cutting planes. This cutting along the series of parallel cutting planes cuts the substrates into smaller substrates, where each of the smaller substrates has a single fully-reflecting internal surface embedded therein.

[0094] It is noted that although the illustrated embodiments show a plurality of compensation members sub-divided into two sets of compensation members (with each set itself having a plurality of compensation members), this arrangement of compensation members is based on the parameters (e.g., the number of steps, and the dimensions of the steps) of the non-limiting example staggered stack 10 shown in FIG. 3. In principle, for any staggered stack configuration, a corresponding suitable arrangement of compensation members can be deployed to provide compensation for the offsets (steps) of the staggered stack.

[0095] Some extreme examples of staggered stack configurations are illustrated in FIGS. 9 - 11. Looking first at FIG. 9, there is shown a staggered stack configuration composed of only a pair of thin plates 12 that are offset one from the other, with thick plates 12T and 12B provided without offset, such that a single step 14 is formed at each end of the stack. In such a configuration, a single compensation member may be deployed at each end of the stack to compensate for the single step. The compensation member at the left end of the stack may compensate by providing pressure from below, and the compensation member at the right end of the stack may compensate by providing pressure from above.

[0096] Another, more extreme example, is shown in FIG. 10. Here, a staggered stack configuration may be composed of only a pair of thin plates of unequal length, where one of the thin plates overhangs the other thin plate at both ends of the stack. Here too the thick plates 12T and 12B are provided without offset, such that a single step 14 is formed at each end of the stack, but in contrast to the steps in the configuration of FIG. 9, the steps in the configuration of FIG. 10 have the same orientation. In such a configuration, a single compensation member may be deployed at each end of the stack to compensate for the single step. The compensation member at the left end of the stack may compensate by providing pressure from below, and the compensation member at the right end of the stack may also compensate by providing pressure from below. An even more extreme example of a staggered stack configuration is shown in FIG. 11. Here, the stack is composed of only a pair of thin plates 12 of unequal length where, one of the thin plates overhangs the other thin plate only at a first (left) end of the stack. At the other (right) end of the stack, the thin plates 12 are aligned. This results in the formation of only one step 14, located at the left end of the stack. In such a configuration, a single compensation member may be deployed at the left end of the stack to compensate for the single step, by providing pressure from below. No compensation member is needed at the right end of the stack.

[0097] Turning now to FIG. 12, there is shown a flow diagram of a process (method) 1200 having multiple stages for bonding transparent plates and optionally fabricating one or more LOEs from the bonded plates, according to embodiments of the first aspect of the present disclosure described above. Reference is also made to FIGS. 3 - 8.

[0098] At stage 1202, a plurality of parallel-faced transparent plates 12 are arranged in a staggered stack 10, with a lateral offset between one or more pairs of adjacent transparent plates 12 to define one or more plate steps 14. As part of stage 1202, or as part of a separate stage, optical adhesive 13 is provided at interfaces 17 between adjacent transparent plates 12, 12T, and 12B of the stack 10. As part of stage 1202, or as part of a separate stage, the staggered stack 10 is placed between a pair of pressing members (22 and 24) of press apparatus 20, as described above. Optical coating (e.g., partially-reflecting coating) is provided at one face (or in some cases both faces) at each of the interfaces 17 between the thin plates 12. It is noted that providing the optical coating at the requisite faces of the plates 12 is typically performed prior to the execution of stage 1202, but can also be performed during building-up of the stack, and can be performed using any suitable plate coating technique known in the art.

[0099] At stage 1204, a plurality of compensation members 42 is provided between the pressing members 22 and 24. The compensation members 42 are in a stepped configuration 44, and the plate steps 14 and the stepped configuration 44 are correspondingly configured such that the compensation members 42 compensate for the lateral offset between the one or more pairs of adjacent transparent plates 12.

[0100] It will be appreciated that stages 1202 and 1204 may be performed simultaneously or contemporaneously, or in a different order than as illustrated in FIG. 12. For example, the compensation members 42 may be provided before the plates 12 are arranged in the stack 10. In certain embodiments, the compensation members 42 can be part of the press apparatus 20.

[0101] At stage 1206, pressure is applied to the plates 12, 12T, and 12B via the pressing members 22 and 24, for example via actuation of the press apparatus 20, as described above.

[0102] At stage 1208, the optical adhesive is solidified, for example via UV or heat curing so that the pressed stack becomes a bonded stack. Solidification may be performed prior to the pressed stack 10 being removed from between the press members 22 and 24, or after removal of the pressed stack from between the press members 22 and 24.

[0103] At stage 1210, the bonded stack 10 may be further processed, for example in LOE fabrication processing, for example by cutting the bonded stack 10 along parallel cutting planes obliquely inclined to the faces of the plates 12 to produce one or more LOE 60, or by trimming the bonded stack 10 to form an optical block with internal facets for use in 2D LOE fabrication. Additional further processing steps may include, for example, polishing steps, including polishing surfaces of the bonded stack or surfaces of the LOE(s) extracted from the bonded stack or surfaces of the optical block extracted from the bonded stack.

[0104] As discussed in the background section, in order for optical products constructed from stacks of bonded transparent product to have optimal mechanical and optical properties, the layer of adhesive between the plates should be as thin as possible. One way to achieve thinning of the adhesive layers is by applying pressure to the stack, as discussed above. However, the pressure must be applied without warping or deforming the plates, particularly at the edges of the plates. The following sections of this document present embodiments according to a second aspect of the present disclosure, which provide another method for bonding transparent plates, which can be used for light-guide optical element (LOE) fabrication. The method includes, among other things, stages for applying pressure without warping or deforming the plates. At least some of stages of the method are illustrated schematically in FIGS. 13 - 19, and shown in the flow diagram of FIG. 20. An arrangement for executing at least some of the method stages is also illustrated schematically in FIGS. 13 - 19.

[0105] Looking first at FIG. 13, a plurality of parallel-faced transparent (thin) plates 12 is arranged in a stack 70. Although the stack 70 is shown as being a staggered stack with each single thin plate being offset relative to its neighboring plate, the methods according to the present set of embodiments can be used for more complex staggered stack configurations, including configurations in which the staggering is on groups of multiple plates (for example as illustrated in FIG. 3) and thick plates that top off (at the top and bottom) the stack. Furthermore, the methods according to the present set of embodiments can be used for less complex stack configurations, such as the configurations illustrated in FIGS. 9 - 11. Moreover, the methods according to the present set of embodiments may also be used for non-staggered stacks of plates.

[0106] Similar to as described in the embodiments according to the first aspect of the present disclosure, an optical adhesive, which may be a liquid optical curable adhesive, is provided at each of the interfaces 17 between adjacent thin plates 12 of the stack 70.

[0107] The adhesive layers are exaggerated for clarity in FIG. 13, and are represented in the figure by elongated oblong dotted-pattern-filled shapes, designated 13. In embodiments in which the stacked plates are used to fabricate LOEs, an at least partially-reflecting coating, that provides partially-reflecting optical properties, is provided at one face at each of the interfaces 17. It is noted, however, that the bonding methods described herein may be used to produce other types of optical devices aside from LOEs, including optical elements having fully-reflecting internal surfaces. Therefore, other types of coating, such as fully-reflecting coating, can be provided at the interfaces 17.

[0108] As shown in FIG. 14, the stack 70 may be placed inside of a flexible container 80 having a sidewall 84 and an opening 82 (typically a small opening, the size of which is exaggerated in the figure for clarity of illustration) in the sidewall 84. The interior surface of the sidewall 84 defines an interior volume 86 of the container 80, within which the stack 70 may be placed. In certain embodiments, the interior surface of the sidewall 84 may be non-smooth or grooved (i.e., may have non-smooth regions / areas or may have grooves in one or more regions / areas of the interior surface).

[0109] To apply pressure to the stack, gas (typically air) inside of the interior volume 86 is removed (sucked out) from the container 80 through the opening 82. This gas removal may be achieved, for example, by using a gas removal mechanism 89, which together with the container 80 forms a pressure arrangement. The gas removal mechanism 89 may be implemented, for example, as a vacuum or suction pump mechanism, fluidically coupled to the opening 82. The gas removal mechanism 89 may create a temporary seal of the opening 82 during the gas removal process. The removal of the gas (the flow of the gas being represented in FIG. 14 as thick black arrows) causes the size of the interior volume 86 to decrease and causes the container 80 (i.e., the sidewall 84) to deform around the stack 70, as illustrated in FIG. 15. This pressurized deformation of the container 80 (sidewall 84) around the stack 70 applies an applied pressure on sides of the stack 70, and creates pressure differentials at regions 88 around the stack 70 within the container 80 (within the interior volume 86). This applied pressure and the pressure differentials cause excess adhesive 13’ from the interfaces 17 between the adjacent plates 12 to be redistributed to the regions 88. In certain embodiments, the excess adhesive may also accumulate at the non-smooth or grooved regions / areas of the interior surface of the sidewall 84.

[0110] In further detail, and with reference to FIG. 15, most of the external surface area of the container 80 is pushed inward by the external air pressure (and optionally enhanced by external pressure applied externally) during the gas removal process. The flow of the gas out of the opening 82 is represented in the figure by thick black arrows, and the inward pushing of the external surface area is represented in the drawing by large block arrows. The inward pushing of the external surface area applies the applied pressure on all sides of the stack 70. The properties of the container sidewall 84 cause the regions 88 to be formed in the container 80 when the gas is removed (i.e., when the external surface area is pushed inward) from the interior volume 86. These regions 88 are in the form of pockets (or wrinkles or creases) of small interior volume, bound by deformed portions of the sidewall 84. These pocket regions 88, which are formed at small areas around the stack 70, for example areas close to the some of the steps (defined by the offsets between plates) and / or close to the top and / or bottom faces of the stack 70, have lower pressure at the small areas around the stack 70, which consequently causes excess adhesive 13’ to be squeezed out from in between the plates 12 (i.e., from the interfaces) to fill the interior volumes of the regions 88.

[0111] During this process, approximately uniform pressure is applied on all sides of the stack 70, such that distortion at edges of the plates 12 is mitigated. In most practical cases, no distortion, or a negligible amount of distortion, is expected at the edges of the plates 12.

[0112] In order to achieve preferred results, the approximately uniform pressure applied to the stack 70 is preferably such that the pressure on the adhesive between the plates is in the range between approximately 0.2 MPa and 1 MPa, which can be achieved by providing a load force to the plates in the range between approximately 140 kilogram-force and 700 kilogram-force (for plates having interface dimension of approximately 70 mm by 100 mm).

[0113] In certain embodiments, the container 80 is heated while being deformed under pressure around the stack 70, which may advantageously thin-out the adhesive at the interfaces between the plates thereby increasing the flow rate of the excess adhesive from the interfaces during the pressure-application process. The temperature to which the container 80 is heated can be selected so that sufficient thinning-out of the adhesive is achieved. This temperature is typically in the range between approximately 60° C and 80° C (depending on the type of adhesive). The container 80 may be heated using any suitable mechanism or heating arrangement. In one non-limiting example, an autoclave is used, which applies both heat and external pressure to the container 80. In another non-limiting example, the container 80 may be placed in a water bath and an immersion circulator may be used to heat the temperature of the water and hence the container 80. Autoclaves and immersion circulation may be of particular advantage when precise temperature control is desired. As another non-limiting example, thermal radiation sources such as heat lamps may be employed to heat the container 80.

[0114] In certain embodiments, external pressure may be applied in combination with the pressure applied via the gas removal process, preferably while the container 80 undergoes heating.

[0115] The container 80 may be implemented, for example, as a tube or bladder with flexible, but preferably semi-rigid, sidewall 84. The sidewall 84 should have properties, including material properties and suitable thickness, that enable formation of the regions 88 when the sidewall 84 is deformed around the stack 70. In particular, the sidewall 84 should be suitably flexible enough to deform around the stack while at the same time suitably rigid enough to induce formation of the regions 88. The properties of the sidewall 84 preferably also provide the sidewall 84 with nonsmooth or grooved regions of the internal surface. Certain types of thermoplastics provide suitable properties (including thermoforming with good transparency). Examples of suitable thermoplastic materials from which the sidewall 84 of the container 80 can be constructed include, but are not limited to, polyethylene (PET), polypropylene (PP), and the like. Preferably the sidewall 84 has a thickness in a range between approximately 0.1 mm and 0.6 mm.

[0116] Referring now to FIG. 16, in certain embodiments, after the pressure is applied and the excess adhesive 13’ is successfully redistributed to the regions 88 (and optionally also at the nonsmooth or grooved regions / areas of the interior surface of the sidewall 84), the container 80 may be inflated, for example by introducing gas (e.g., air) into the container 80 via the opening 82. In certain embodiments, the gas removal mechanism 89 may also be configured to introduce gas into the container 82. In other embodiments, a separate inflation mechanism can be used, such as, for example, a pressurized gas tank with a control-release valve connected to an output nozzle. The flow of gas into the container 80 is represented in the figure as thick black arrows. The introduction of the gas causes the sidewall 84 to expand, thereby increasing the size of the interior volume 86. At least some (preferably most, if not all) of the excess adhesive 13’, which is now attached to the interior surface of the sidewall 84 and / or to the external / exposed faces of the plates 12, can then be removed.

[0117] The stages of removal of gas from the container, inflation of the container, and removal of excess adhesive may be repeated, as needed (until a stopping condition is satisfied) in order to achieve effective plate stacking with minimal excess adhesive between the plates. FIG. 17 shows the stack 70 after removal of gas from the container, inflation of the container, and removal of excess adhesive, resulting in minimal adhesive at the interfaces 17 between the plates 12.

[0118] After pressing the stack, the remaining optical adhesive at the interfaces 17 can then be solidified (cured), for example using UV curing or heat curing, so that the stack 70 becomes a bonded stack. In certain embodiments, the adhesive is solidified prior to inflation of the container 80 (i.e., while the container is still in the pressurized state). In certain embodiments, the bonded stack can then be further processed, for example processed to fabricate one or more LOEs (by slicing the stack 70 along parallel cutting planes 92 oblique to the plate faces, as shown in FIG. 18) or processed as part of 2D expansion LOE fabrication processes, similar to as described in embodiments according to the first aspect. It is noted that the techniques described above with reference to FIGS. 15 and 16 can be used with LOEs instead of plates in order to form a bonded stack of LOEs. Such a bonded stack of LOEs can be used in 2D expansion LOE fabrication processes, for example as described in commonly owned US Patent No. 11,886,008. In such embodiments, each of the plates, as an LOE, has a plurality of mutually-parallel partially-reflecting internal surfaces that are obliquely inclined relative to the parallel faces of the plate.

[0119] According to certain embodiments, the density of the adhesive layers provided at the interfaces 17 can be reduced (thinned-out) prior to the pressure-application stage, to reduce the viscosity of the adhesive at the interfaces. This may be achieved by diluting the adhesive with an additive, which may be a solvent (diluent), preferably prior to providing the adhesive at the faces of the plates. The reduced adhesive viscosity increases the flow rate of the excess adhesive from the interfaces during the pressure-application process, which may reduce the overall pressure needed to sufficiently redistribute the adhesive to the regions 88.

[0120] The solvent in the adhesive may then be evaporated as part of further processing of the stack. Evaporation of the solvent may be performed at the stack-level or the slice-level, but there is advantage to performing evaporation at the slice-level. Specifically, since the surface are of the slices is substantially larger than the stack surface area, the solvent evaporation at the slice-level is significantly faster. In order to further expedite the solvent evaporation, the slices (or the stack) can be placed inside of a vacuum chamber. FIG. 19 shows a plurality of slices 94, for example extracted from the stack 70, placed inside of a vacuum chamber 100. The evaporation of the solvent is represented in FIG. 19 by the curved arrows.

[0121] After evaporation of the solvent, further solidification of the adhesive may be considered, since the adhesive is more susceptive to chemical bonding in the absence of the solvent.

[0122] It should be appreciated that although the optional steps of adhesive dilution via additive (e.g., solvent) and evaporation of said additive have been described in the context of embodiments according to the second aspect of the present disclosure, these optional steps are also applicable for use with embodiments according to the first aspect, as well as conventional LOE fabrication processes.

[0123] It is noted that in the configuration illustrated in FIG. 14, the pressure applied at the corners of the staggered stack 70 may bend the thin plates 12 at the top and bottom of the stack and deform the stack, due to lack of symmetry of the stack (i.e., due to the staggered plate structure). To mitigate deformation of the stack under pressure, the stack may be topped off at the top and bottom with high-stiffness plates (i.e., plates having stiffness sufficient enough to oppose bending of the thin plates 12). FIG. 20 schematically illustrates such an embodiment, in which the stack 70 is topped off at the top and bottom with additional plates 12S having high stiffness. These plates 12S have stiffness that is sufficient to oppose the bending of the thin plates 12, thereby improving the flatness of the overall stack structure under pressure.

[0124] Turning now to FIG. 21, there is shown a flow diagram of a process (method) 2100 having multiple stages for bonding transparent plates and optionally fabricating an LOE from the bonded plates, according to the second embodiments of the present disclosure described above. Reference is also made to FIGS. 13 - 20.

[0125] At optional stage 2102, the density of an optical adhesive is thinned-out by diluting the adhesive with an additive (e.g., solvent) to reduce the viscosity of the adhesive.

[0126] At stage 2104, a plurality of parallel-faced transparent plates 12 are arranged in a stack 70, which can be a staggered stack with a lateral offset between one or more pairs of adjacent transparent plates 12 to define one or more plate steps. As part of stage 2104, or as part of a separate stage, optical adhesive 13 is provided at interfaces 17 between adjacent transparent plates 12 of the stack 70. Optical coating (e.g., partially-reflecting coating) is also provided at one face (or in some cases both faces) at each of the interfaces 17. It is noted that providing the optical coating at the requisite faces of the plates 12 is typically performed prior to the execution of stage 2104, and can be performed using any suitable plate coating technique known in the art.

[0127] At stage 2106, the staggered stack 70 is placed in flexible container 80 having opening 82.

[0128] At stage 2108, gas is removed from the container 80 via the opening 82 so as to cause the container 80 to deform around the stack thereby applying an applied pressure on sides of the stack and creating pressure differentials at regions 88 around the stack 70 within the container 80. The applied pressure and pressure differentials cause excess adhesive from the interfaces 17 to be redistributed to the regions 88.

[0129] At stage 2110, the container 80 is inflated, and the excess adhesive is removed from the stack and / or the interior surface of the container sidewall.

[0130] Optionally, stages 2108 and 2110 can be repeated as needed, as represented by the feedback arrow 2111.

[0131] At stage 2112, the optical adhesive is solidified, for example via UV or heat curing, so that the stack becomes a bonded stack. As discussed above, this stage is preferably performed while the container is deformed around the container 80 (i.e., immediately after execution of stage 2108)

[0132] At stage 2114, the bonded stack may be further processed. This is represented in FIG. 20 as a slicing stage to extract slices (for example as shown in FIG. 18), for example as part of LOE fabrication processing. It is noted, however, that other processing stages can be executed instead of, or in addition to slicing, including polishing (and optionally further cutting), and 2D LOE fabrication processing stages. If stage 2102 is executed, polishing (and further cutting) is typically performed as a separate subsequent stage, as shown in FIG. 20 as stage 2118.

[0133] If stage 2102 is executed, then stage 2116 may also be executed, where the additive in the adhesive is evaporated, for example by placement of the slices in vacuum chamber 100. As mentioned above, the evaporation stage may be performed at the stack-level instead of the slice- level. After stage 2116, polishing (and further cutting) of the slices may be performed at stage 2118. Between stages 2116 and 2118, stage 2112 may be re-executed (i.e., further solidification).

[0134] It should be noted that the two aspects of the present disclosure presented herein are of independent utility, such that the embodiments according to the first aspect and the embodiments according to the second aspect can each be practiced in their own right. Notwithstanding the above, embodiments are contemplated which employ both of the two aspects together. Embodiments which employ both of the two aspects together have certain advantages, including reduced processing (bonding) time since employing the two aspects in combination will decrease the amount of time needed to achieve the desired adhesive layer thinness. Such embodiments entail performing at least the following stages: 1) placing a staggered stack of plates (having plate steps) in a flexible container, 2) placing the flexible container (with the stack therein) between a pair of oppositely disposed pressing members with a compensation arrangement having compensation members arranged in a stepped configuration in correspondence with the steps of the stack, 3) removing gas from the container to apply pressure to the stack, and 4) applying pressure to the plates via the pressing members.

[0135] It will be appreciated that the stage of removing gas from the container can be performed prior to placement of the container between the pressing members. Many variations of such combined embodiments are contemplated herein, including variations in which the gas removal stage is performed contemporaneously or simultaneously with, and in certain cases subsequent to, the stage of applying pressure via the pressing members.

[0136] Despite the advantage of such combined embodiments, certain modifications of the first or second aspects may be needed in order to practice embodiments that employ both of the two aspects together. For example, the sidewall of the container at the plate steps may change the geometry of the steps as “seen” by the compensation members. Specifically, the thickness of sidewall at the plate steps may lead to non-uniformity of the plate steps and non-planarity of the horizontal and / or vertical portions of the steps. Accordingly, the geometry of the compensation members may require adjustment to account for the sidewall of the container.

[0137] It will be appreciated that the positioning of the plates relative to one another (to achieve offset and / or alignment) can be performed using any suitable optical alignment apparatus / device(s) / tool(s) that perform suitable optical alignment techniques / methods. Such suitable optical alignment apparatus / device(s) / tool(s) can include, for example, one or more computerized control device, one or more computerized processing device, one or more optical subsystem having, for example, one or more light source, one or more light detector / sensor (including optical sensors), one or more optical component (e.g., one or more lens, one or more folding optic, one or more prism, etc.), autocollimators, and the like. Details of non-limiting examples of suitable optical alignment apparatus / device(s) / tool(s) / method(s) that can be used for aligning the various optical structures described herein can be found in various publications by Lumus Ltd. (Israel), including, for example, International Patent Application No. PCT / IL2021 / 051377 and International Patent Application No. PCT / IL2021 / 051378.

[0138] The present disclosure has described various cutting and slicing stages in which optical structures are cut along cutting lines and / or planes in order to produce various other optical structures or optical products. In certain embodiments some or all of the surfaces of these optical structures, including and in particular those surfaces that result from these cutting stages, can be polished to, for example, increase optical quality. In certain embodiments, polishing can be performed as part of, or subsequent to, these cutting stages, and prior to subsequent optical coupling stages. In the above-described fabrication methods, the cutting or slicing of the various optical structures described herein can be performed by any suitable cutting apparatus / device / tool, as should be understood by those of ordinary skill in the art. The polishing of the faces and surfaces of the various optical structures described herein can be performed by any suitable polishing apparatus / device / tool, as should be understood by those of ordinary skill in the art.

[0139] The descriptions of the various embodiments of the present disclosure have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.

[0140] As used herein, the singular form, “a”, “an” and “the” include plural references unless the context clearly dictates otherwise.

[0141] The word “exemplary” is used herein to mean “serving as an example, instance or illustration”. Any embodiment described as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments and / or to exclude the incorporation of features from other embodiments.

[0142] It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination or as suitable in any other described embodiment of the invention. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements. Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A method comprising: arranging a plurality of parallel-faced transparent plates in a stack with a lateral offset between one or more pairs of adjacent transparent plates to define one or more plate steps, wherein optical adhesive is provided at interfaces between adjacent transparent plates of the stack, and wherein a coating is provided at one face at each of the interfaces, the stack being placed between first and second pressing members; providing a plurality of compensation members between the first and second pressing members in a stepped configuration, wherein the one or more plate steps and the stepped configuration are correspondingly configured such that the compensation members compensate for the offset between the one or more pairs of adjacent transparent plates; and applying pressure to the plurality of transparent plates via the first and second pressing members.

2. The method of claim 1, wherein the stepped configuration of the compensation members is provided at least in part by dimensions of the compensation members.

3. The method of claim 1, wherein at least some of the compensation members have adjustable height, wherein the height is measured along a dimension perpendicular to the parallel faces of the transparent plates.

4. The method of claim 1 , wherein the plurality of compensation members includes a first set of compensation members associated with the first pressing member and a second set of compensation members associated with the second pressing member.

5. The method of claim 1 , wherein the plurality of compensation members includes a first set of compensation members and a second set of compensation members, the first set of compensation members associated with a first end region of the stack and compensating for the offset between one or more pairs of adjacent transparent plates at the first end region of the stack, the second set of compensation members associated with a second end region of the stack and compensating for the offset between one or more pairs of adjacent transparent plates at the second end region of the stack.

6. The method of claim 1, wherein the stack is placed between first and second pressing member such that a first block member is positioned between the first pressing member and a first transparent plate at a top end of the stack, and such that a second block member is positioned between the second pressing member and a second transparent plate at a bottom end of the stack.

7. The method of claim 6, further comprising: deploying a first insulating member between the first block member and the first pressing member; and deploying a second insulating member between the second block member and the second pressing member.

8. The method of claim 1, wherein the coating provides partially-reflecting optical properties.

9. The method of claim 8, further comprising: solidifying the adhesive so that the stack forms a bonded stack; and cutting the bonded stack along at least two parallel cutting planes obliquely inclined relative to the faces of the transparent plates to extract one or more parallel-faced substrates having a plurality of mutually-parallel partially-reflective internal surfaces formed from the interfaces.

10. The method of claim 1, further comprising: placing the stack in a flexible container having an opening; and removing gas from the container via the opening so as to cause the container to deform around the stack thereby applying an applied pressure on sides of the stack and creating pressure differentials at regions around the stack within the container, wherein the applied pressure and pressure differentials cause excess adhesive from the interfaces to be redistributed to the regions.

11. A press arrangement comprising: a first pressing member; a second pressing member opposite the first pressing member, the first and second pressing members configured to be spaced apart to receive a stack of parallel-faced transparent plates, the transparent plates arranged in the stack with a lateral offset between one or more pairs of adjacent transparent plates to define one or more plate steps, wherein optical adhesive is provided at interfaces between adjacent transparent plates of the stack, and wherein a coating is provided at one face at each of the interfaces; a plurality of compensation members provided between the first and second pressing members in a stepped configuration, wherein the one or more plate steps and the stepped configuration are correspondingly configured such that the compensation members compensate for the offset between the one or more pairs of adjacent transparent plates; and an actuator associated with at least one of the first or second pressing members and configured to move at least one of the first or second pressing members to apply pressure to the plurality of transparent plates.

12. A method comprising: arranging a plurality of parallel-faced transparent plates in a staggered stack, wherein optical adhesive is provided at interfaces between adjacent transparent plates of the stack, and wherein a coating is provided at one face at each of the interfaces; placing the stack in a flexible container having an opening; and removing gas from the container via the opening so as to cause the container to deform around the stack thereby applying an applied pressure on sides of the stack and creating pressure differentials at regions around the stack within the container, wherein the applied pressure and pressure differentials cause excess adhesive from the interfaces to be redistributed to the regions.

13. The method of claim 12, further comprising: inflating the container; and removing at least some of the redistributed excess adhesive.

14. The method of claim 13, further comprising: repeatedly removing gas from the container, inflating the container, and removing at least some redistributed excess adhesive, until a stopping condition is satisfied.

15. The method of claim 12, further comprising: thinning the adhesive provided at the interfaces by diluting the adhesive with an additive so as to reduce the viscosity of the adhesive.

16. The method of claim 15, wherein the additive includes a solvent.

17. The method of claim 15, further comprising: cutting the stack along at least two parallel cutting planes obliquely inclined relative to the faces of the transparent plates to extract at least one parallel-faced substrate having a plurality of mutually-parallel internal surfaces formed from the interfaces; and placing one or more of the at least one substrate in a chamber to expedite evaporation of the solvent.

18. The method of claim 12, further comprising: solidifying the adhesive so that the stack forms a bonded stack.

19. The method of claim 18, wherein the solidifying the adhesive is performed while the container is deformed around the stack.

20. The method of claim 12, wherein the coating provides partially-reflecting optical properties.

21. The method of claim 20, further comprising: cutting the stack along at least two parallel cutting planes obliquely inclined relative to the faces of the transparent plates to extract at least one parallel-faced substrate havinga plurality of mutually-parallel partially-reflective internal surfaces formed from the interfaces.

22. The method of claim 12, wherein the applied pressure is substantially uniform on all sides of the stack thereby mitigating distortion at edges of the transparent plates.

23. The method of claim 12, wherein the stack is topped off at the top and bottom with high-stiffness plates having stiffness that is sufficient to oppose the bending of the remaining plates of the stack under the applied pressure.

24. The method of claim 12, further comprising: heating the container while the container is deformed around the stack.

25. The method of claim 24, further comprising: applying external pressure to the container while the container is undergoing the heating.

26. The method of claim 12, wherein the staggered stack is such that there is a lateral offset between one or more pairs of adjacent transparent plates defining one or more plate steps, the method further comprising: placing the stack between first and second pressing members; providing a plurality of compensation members between the first and second pressing members in a stepped configuration, wherein the one or more plate steps and the stepped configuration are correspondingly configured such that the compensation members compensate for the offset between the one or more pairs of adjacent transparent plates; and applying pressure to the plurality of transparent plates via the first and second pressing members.

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