Patent Description:
Various types of display, such as near-eye displays, may employ a light-guide optical element (LOE) to expand an input image in one or more dimensions. Where two-dimensional expansion is required, two LOEs may be used, including a first LOE configured to expand an image in one dimension, and a second LOE configured to expand the image in the other dimension. Of particular relevance to the present invention are reflective LOE's, where at least one of the first and second LOEs is implemented as a transparent block bounded by two parallel major external surfaces configured to support propagation of light rays therebetween via total internal reflection (TIR) and having a set of mutually-parallel partially-reflecting internal surfaces (or "facets") located between, and non-parallel to, the major external surfaces. A collimated image propagating within the LOE is progressively partially deflected by facets of the first set of facets towards the second set of facets, and by the second set of facets outwards towards an eye of an observer, thereby presenting an image to the observer.

<CIT> discloses an optical aperture multiplier includes a first optical waveguide having a rectangular cross-section and including partially reflecting surfaces at an oblique angle to a direction of elongation of the waveguide.

<CIT> discloses a method for fabricating an optical device that includes a light waves-transmitting substrate having at least two major surfaces and edges and a plurality of partially reflecting surfaces carried by the substrate, wherein the partially reflecting surfaces are parallel to each other and not parallel to any of the edges of the substrate.

<CIT> discloses an optical structure is presented for producing therefrom two or more light guide optical elements (LOEs). The optical structure is configured such that each LOE comprises an optically transparent body for guiding input light in a general propagation direction through the body by total internal reflections of light from major surfaces of the body and comprises light directing surfaces for coupling the input light out of the body in one or more output directions.

The present invention is a method for producing light-guide optical elements according to the appended claims.

According to the teachings of an embodiment of the present invention there is provided, a method for producing light-guide optical elements (LOEs) each having a pair of mutually-parallel major external surfaces for guiding image illumination propagating within the LOE by internal reflection at the major external surfaces, each LOE further having an active region comprising a set of mutually-parallel partially-reflecting surfaces located between, and oriented non-parallel to, the major external surfaces, and at least one secondary region, at least one of the partially-reflecting surfaces terminating at a boundary between the active region and the secondary region, the method comprising the steps of: (a) bonding together a plurality of parallel-faced plates at a plurality of interfaces so as to form a stack of plates, one face at each of the interfaces having a coating to provide partially-reflecting optical properties; (b) cutting and polishing the stack of plates to form a boundary plane intersecting at least one of the interfaces; (c) bonding a block of transparent material to the stack at the boundary plane to form a precursor structure; and (d) slicing the precursor structure along a plurality of parallel planes so as to form a plurality of slices, each slice containing a part of the stack for providing the active region of the LOE and a part of the block to provide the secondary region of the LOE.

According to a further feature of an embodiment of the present invention, the boundary plane is cut along a plane obliquely oriented relative to a plane of the interfaces.

According to a further feature of an embodiment of the present invention, the block of transparent material is index-matched to the plurality of plates.

According to a further feature of an embodiment of the present invention, the block of transparent material is a block of optically continuous material.

According to a further feature of an embodiment of the present invention, the block of transparent material is a continuous uniform block.

According to a further feature of an embodiment of the present invention, prior to the slicing, the precursor structure is cut along at least one edge plane, a part of the edge plane defining, after the slicing, an edge of each LOE.

According to a further feature of an embodiment of the present invention, the plurality of parallel planes are perpendicular to the interfaces.

According to a further feature of an embodiment of the present invention, the plurality of parallel planes are obliquely angled relative to the interfaces.

According to a further feature of an embodiment of the present invention, an edge is formed to the active region of the LOE, wherein the boundary is non-parallel to the edge so that a length of the partially-reflecting surfaces in a direction parallel to the major external surfaces progressively decreases from partially-reflecting surface to partially-reflecting surface along at least a quarter of the set of partially-reflecting surfaces.

According to a further feature of an embodiment of the present invention, the coatings are configured to provide sequentially varying reflectivity for successive of the interfaces.

According to a further feature of an embodiment of the present invention, the plates have thicknesses differing from each other such that the interfaces are non-uniformly spaced.

According to a further feature of an embodiment of the present invention, the method further comprises the steps of: (a) cutting and polishing the stack of plates to form an additional boundary plane intersecting at least one of the interfaces, the additional boundary plane being non-coplanar with the boundary plane; and (b) bonding an additional block of transparent material to the stack at the boundary plane to form the precursor structure, and wherein the slicing is performed so that each slice additionally contains a part of the additional block.

There is also provided according to the teachings of an embodiment of the present invention, an intermediate work product sliceable along a plurality of parallel planes to form a plurality of light-guide optical elements (LOEs) each having a pair of mutually-parallel major external surfaces for guiding image illumination propagating within the LOE by internal reflection at the major external surfaces, each LOE further having an active region comprising a set of mutually-parallel partially-reflecting surfaces located between, and oriented non-parallel to, the major external surfaces, and at least one secondary region, at least one of the partially-reflecting surfaces terminating at a boundary between the active region and the secondary region, the intermediate work product comprising: (a) a stack formed from a plurality of parallel-faced plates bonded together at a plurality of interfaces, one face at each of the interfaces having a coating to provide partially-reflecting optical properties, the stack being cut and polished at a boundary plane intersecting at least one of the interfaces; and (b) a block of transparent material bonded to the stack at the boundary plane.

According to a further feature of an embodiment of the present invention, the boundary plane is obliquely oriented relative to a plane of the interfaces.

Certain embodiments of the present invention provide a method for manufacturing a light-guide optical element (LOE) for achieving optical aperture expansion for the purpose of a head-up display, and most preferably a near-eye display, which may be a virtual reality display, or more preferably an augmented reality display. <FIG> illustrate certain particularly preferred examples of optical arrangements and corresponding devices for which the production methods of the present invention are particularly relevant, although the production methods are not limited to such applications.

An exemplary implementation of a device in the form of a near-eye display, generally designated <NUM>, employing an LOE <NUM> according to the teachings of an embodiment of the present invention, is illustrated schematically in <FIG>. The near-eye display <NUM> employs a compact image projector (often referred to in this field as a "POD") <NUM> optically coupled so as to inject an image into LOE (interchangeably referred to as a "waveguide," a "substrate" or a "slab") <NUM> within which the image light is trapped in one dimension by internal reflection at a set of mutually-parallel planar external surfaces. The light impinges of a set of partially-reflecting surfaces (interchangeably referred to as "facets") that are parallel to each other, and inclined obliquely to the direction of propagation of the image light, with each successive facet deflecting a proportion of the image light into a deflected direction, also trapped/guided by internal reflection within the substrate. This first set of facets are not illustrated individually in <FIG>, but are located in a first region of the LOE designated <NUM>. This partial reflection at successive facets achieves a first dimension of optical aperture expansion.

In a first set of preferred but non-limiting examples of the present invention, the aforementioned set of facets are orthogonal to the major external surfaces of the substrate. In this case, both the injected image and its conjugate undergoing internal reflection as it propagates within region <NUM> are deflected and become conjugate images propagating in a deflected direction. In an alternative set of preferred but non-limiting examples, the first set of partially-reflecting surfaces are obliquely angled relative to the major external surfaces of the LOE. In the latter case, either the injected image or its conjugate forms the desired deflected image propagating within the LOE, while the other reflection may be minimized, for example, by employing angularly-selective coatings on the facets which render them relatively transparent to the range of incident angles presented by the image whose reflection is not needed.

The first set of partially-reflecting surfaces deflect the image illumination from a first direction of propagation trapped by total internal reflection (TIR) within the substrate to a second direction of propagation, also trapped by TIR within the substrate.

The deflected image illumination then passes into a second substrate region <NUM>, which may be implemented as an adjacent distinct substrate or as a continuation of a single substrate, in which a coupling-out arrangement (either a further set of partially reflective facets or a diffractive optical element) progressively couples out a proportion of the image illumination towards the eye of an observer located within a region defined as the eye-motion box (EMB), thereby achieving a second dimension of optical aperture expansion. The overall device may be implemented separately for each eye, and is preferably supported relative to the head of a user with the each LOE <NUM> facing a corresponding eye of the user. In one particularly preferred option as illustrated here, a support arrangement is implemented as an eye glasses frame with sides <NUM> for supporting the device relative to ears of the user. Other forms of support arrangement may also be used, including but not limited to, head bands, visors or devices suspended from helmets.

Reference is made herein in the drawings and claims to an X axis which extends horizontally (<FIG>) or vertically (<FIG>), in the general extensional direction of the first region of the LOE, and a Y axis which extends perpendicular thereto, i.e., vertically in <FIG> and horizontally in <FIG>.

In very approximate terms, the first LOE, or first region <NUM> of LOE <NUM>, may be considered to achieve aperture expansion in the X direction while the second LOE, or second region <NUM> of LOE <NUM>, achieves aperture expansion in the Y direction. The details of the spread of angular directions in which different parts of the field of view propagate will be addressed more precisely below. It should be noted that the orientation as illustrated in <FIG> may be regarded as a "top-down" implementation, where the image illumination entering the main (second region) of the LOE enters from the top edge, whereas the orientation illustrated in <FIG> may be regarded as a "side-injection" implementation, where the axis referred to here as the Y axis is deployed horizontally. In the remaining drawings, the various features of certain embodiments of the present invention will be illustrated in the context of a "top-down" orientation, similar to <FIG>. However, it should be appreciated that all of those features are equally applicable to side-injection implementations, which also fall within the scope of the invention. In certain cases, other intermediate orientations are also applicable, and are included within the scope of the present invention except where explicitly excluded.

The POD employed with the devices of the present invention is preferably configured to generate a collimated image, i.e., in which the light of each image pixel is a parallel beam, collimated to infinity, with an angular direction corresponding to the pixel position. The image illumination thus spans a range of angles corresponding to an angular field of view in two dimensions.

Image projector <NUM> includes at least one light source, typically deployed to illuminate a spatial light modulator, such as an LCOS chip. The spatial light modulator modulates the projected intensity of each pixel of the image, thereby generating an image. Alternatively, the image projector may include a scanning arrangement, typically implemented using a fast-scanning mirror, which scans illumination from a laser light source across an image plane of the projector while the intensity of the beam is varied synchronously with the motion on a pixel-by-pixel basis, thereby projecting a desired intensity for each pixel. In both cases, collimating optics are provided to generate an output projected image which is collimated to infinity. Some or all of the above components are typically arranged on surfaces of one or more polarizing beam-splitter (PBS) cube or other prism arrangement, as is well known in the art.

Optical coupling of image projector <NUM> to LOE <NUM> may be achieved by any suitable optical coupling, such as for example via a coupling prism with an obliquely angled input surface, or via a reflective coupling arrangement, via a side edge and/or one of the major external surfaces of the LOE. Details of the coupling-in configuration are not critical to the invention, and are shown here schematically as a non-limiting example of a wedge prism <NUM> applied to one of the major external surfaces of the LOE.

It will be appreciated that the near-eye display <NUM> includes various additional components, typically including a controller <NUM> for actuating the image projector <NUM>, typically employing electrical power from a small onboard battery (not shown) or some other suitable power source. It will be appreciated that controller <NUM> includes all necessary electronic components such as at least one processor or processing circuitry to drive the image projector, all as is known in the art.

Turning now to FIGS. 2A-2F, the optical properties of an implementation of the near-eye display are illustrated in more detail. Specifically, there is shown a more detailed view of a light-guide optical element (LOE) <NUM> formed from transparent material, including a first region <NUM> containing a first set of planar, mutually-parallel, partially-reflecting surfaces <NUM> having a first orientation, and a second region <NUM> containing a second set of planar, mutually-parallel, partially-reflecting surfaces <NUM> having a second orientation non-parallel to the first orientation. A set of mutually-parallel major external surfaces <NUM> extend across the first and second regions <NUM> and <NUM> such that both the first set of partially-reflecting surfaces <NUM> and the second set of partially-reflecting surfaces <NUM> are located between the major external surfaces <NUM>. Most preferably, the set of major external surfaces <NUM> are a pair of surfaces which are each continuous across the entirety of first and second regions <NUM> and <NUM>, although the option of having a set down or a step up in thickness between the regions <NUM> and <NUM> also falls within the scope of the present invention. Regions <NUM> and <NUM> may be immediately juxtaposed so that they meet at a boundary, which may be a straight boundary or some other form of boundary, or there may be one or more additional LOE region interposed between those regions, to provide various additional optical or mechanical function, depending upon the particular application. In certain particularly preferred implementations, particularly high quality major external surfaces are achieved by employing continuous external plates between which the separately formed regions <NUM> and <NUM> are sandwiched to form the compound LOE structure.

The optical properties of the LOE may be understood by tracing the image illumination paths backwards. The second set of partially-reflecting surfaces <NUM> are at an oblique angle to the major external surfaces <NUM> so that a part of image illumination propagating within the LOE <NUM> by internal reflection at the major external surfaces from the first region <NUM> into the second region <NUM> is coupled out of the LOE towards an eye-motion box <NUM>. The first set of partially-reflecting surfaces <NUM> are oriented so that a part of image illumination propagating within the LOE <NUM> by internal reflection at the major external surfaces from the coupling-in region (coupling prism <NUM>) is deflected towards the second region <NUM>.

One dimension of the angular spread of the projected image from image projector <NUM> is represented in <FIG> by the cone of illumination spreading from the POD aperture on the right side of the LOE towards the left side of the LOE. In the non-limiting example illustrated here, the central optical axis of the POD defines a direction of propagation within the LOE aligned with the X axis, and the angular spread (within the LOE) is roughly ±<NUM>°. (It should be noted that the angular FOV becomes larger in air due to the change in refractive index. ) The first set of partially-reflecting surfaces <NUM> are illustrated in first region <NUM>, and the second set of partially-reflecting surfaces <NUM> are illustrated in second region <NUM>.

The near-eye display is designed to provide a full field-of-view of the projected image to an eye of the user that is located at some position within the permitted range of positions designated by an "eye-motion box" (EMB) <NUM> (that is, a shape, typically represented as a rectangle, spaced away from the plane of the LOE from which the pupil of the eye will view the projected image). In order to reach the eye-motion box, light must be coupled-out from the second region <NUM> by the second set of partially-reflecting surfaces <NUM> towards the EMB <NUM>. In order to provide the full image field-of-view, each point in the EMB must receive the entire angular range of the image from the LOE. Tracing back the field-of-view from the EMB indicates a larger rectangle <NUM> from which relevant illumination is coupled-out of the LOE towards the EMB.

<FIG> illustrates a first extremity of the field of view, corresponding to the bottom-left pixel of the projected image. A beam of a width corresponding to the optical aperture of the projector as coupled into the LOE is shown propagating leftwards and upwards from the POD and being partially reflected from a series of partially-reflecting surfaces <NUM>. As illustrated here, only a subset of the facets generate reflections that are useful for providing the corresponding pixel in the image viewed by the user, and only a sub-region of those facets contributes to the observed image of this pixel. The relevant regions are illustrated by heavy black lines, and the rays corresponding to this pixel in the redirected image reflected from facets <NUM> and then coupled-out by facets <NUM> reaching the four corners of the EMB <NUM> are shown. Here and throughout the description, it will be noted that only the in-plane propagation directions of the rays are illustrated here during propagation within the LOE, but the rays actually follow a zigzag path of repeated internal reflection from the two major external surfaces, and one entire dimension of the image field of view is encoded by the angle of inclination of the rays relative to the major external surfaces, corresponding to the pixel position in the Y dimension. By way of one additional example, deflected and coupled-out rays corresponding to the top-left extremity of the image as viewed at the top-left corner of the EMB are shown in dash-dot lines.

<FIG> illustrates the same configuration as <FIG>, but here shows the rays corresponding to the bottom-right pixel of the field-of-view reaching the four corners of the EMB, again with the relevant regions of the relevant partially-reflecting surfaces <NUM> denoted by a heavy line.

It will be apparent that, by additionally tracing correspond ray paths for all fields (directions or pixels) of the image reaching all regions of the EMB, it is possible to map out an envelope of all ray paths from the coupling-in region propagating within the LOE, deflected by one of the first set of partially-reflecting surfaces and coupled out by one of the second set of partially-reflecting surfaces in a direction reaching the eye-motion box, and this envelope defines an "imaging area" of each facet <NUM> which is needed for deflecting part of the image illumination which contributes to the image reaching the EMB, while the remainder of the facet <NUM> lying outside the envelope is a "non-imaging area" which does not contribute to the required image. A simplified outline of this envelope corresponding to the "imaging areas" of all of the facets <NUM> is shown in heavy lines in <FIG>.

It has been found that the parts of the facets in the "non-imaging area" may in certain cases have an adverse effect on image quality, for example, supporting unintended multiple-reflection light paths with give rise to ghost images of the input image illumination and/or external radiation from ambient light sources. In order to minimize such effects, according to certain particularly preferred implementations of the present invention, it is preferable to implement facets <NUM> as "partial facets" such that the partially-reflecting properties are only present within a subregion of the cross-sectional area of region <NUM> which includes the "imaging area" of each facet plane, and preferably excludes at least the majority of the "non-imaging area" for some or all of the facets. Such an implementation is illustrated schematically in <FIG>. The active (partially-reflecting) area of the facets preferably extends slightly beyond the minimum required to complete the geometrical requirements for the EMB image projection. According to certain particularly preferred implementations, the distance of the furthest partially-reflecting facet encountered along a line from the coupling-in location progressively increases with increasing angle clockwise as shown, away from the boundary with the second region <NUM> over a majority of the angular range of the image projected from projector <NUM>. This leaves one or more regions, labeled here as <NUM>a, <NUM>b and <NUM>c, which are within region <NUM> and are preferably implemented without partially reflecting facets.

In <FIG>, the optical axis of the projector <NUM> is illustrated as being parallel to the X-axis. It should be appreciated that the optical axis is not actually parallel to the X axis but rather lies in the X-Z plane, with a Z-component into the page chosen such that the entire range of angles in the depth dimension of the FOV undergo total internal reflection at the major substrate surfaces. For simplicity of presentation, the graphic representations herein, and the description thereof, will relate only to the in-plane (X-Y) component of the light ray propagation directions, referred to herein as the "in-plane component" or the "component parallel to the major external surfaces of the LOE.

<FIG> illustrates a similar implementation in which the optical axis of the projector is rotated so as to align one side of the field with the upper edge of the region <NUM>. In this case, there are two regions, labeled <NUM>a and <NUM>b which are preferably implemented without partially-reflective facets.

<FIG> illustrates a typical production method for manufacturing an LOE region <NUM> or <NUM> such as is illustrated in <FIG>. The method involves first stacking and bonding a plurality of transparent plates <NUM> optically coated with an at least partially reflective coating, thereby forming a stack <NUM>. The interfaces between plates correspond to the facets of the LOE. The stack is typically topped off (at the top and/or bottom) with a transparent plate having a thickness of several times that of the other plates. The stack is cut into slices <NUM> at the desired angle relative to the facet surfaces. Each slice is then shaped (e.g., by cutting and/or grinding, followed by polishing) to form parallel external surfaces with the facets oriented at a specific predetermined angle relative to the external surfaces based on the required LOE configuration <NUM>. In other words, the LOE is shaped out of a slice from a stack of parallel coated glass plates, where the angle and orientation of the partially reflecting surfaces is determined by the slicing angle and the orientation of the subsequent cutting.

The above manufacturing process is efficient, in that a single stack of plates can be used to manufacture a plurality of similar LOEs through the slicing, cutting and polishing steps mentioned above. The use of thick end plates for the stack allows production of a region of clear glass before the first facet and/or after the last facet, but only at a boundary parallel to the facets. This approach cannot, however, directly form inactive regions such as regions <NUM>a, <NUM>b and <NUM>c of <FIG> and <FIG>, since these regions intersect multiple stacked and bonded transparent plates that form faceted region <NUM>.

Thus, in order to produce the waveguide described in <FIG> or <FIG>, or other similar waveguides with partial facets, additional steps are needed beyond the manufacturing method described above with reference to <FIG>.

According to one particularly preferred aspect of the present invention, there is provided a method for producing light-guide optical elements (LOEs), where each LOE has a pair of mutually-parallel major external surfaces for guiding image illumination propagating within the LOE by internal reflection at the major external surfaces, an active region having a set of mutually-parallel partially-reflecting surfaces located between, and oriented non-parallel to, the major external surfaces, and at least one secondary region, where at least one of the partially-reflecting surfaces terminates at a boundary between the active region and the secondary region. The method includes at least the following steps, as illustrated in the block diagram of <FIG>, and schematically in <FIG>:.

In the example illustrated in <FIG>, the final LOE <NUM> is cut out from each slice <NUM> as shown, thereby forming an LOE <NUM> with at least one region <NUM>a from which this set of facets is excluded.

Preferably, block <NUM> is formed from a transparent material index-matched to the plurality of plates, so that the boundary between the region with facets and the region without facets does not generate significant optical aberration. For the same reason, it may be preferable for attachment of block <NUM> to stack <NUM> to be performed with index-matched optical adhesive. Block <NUM> itself is preferably a block of optically continuous material, meaning that it does not have internal features which cause noticeable optical aberrations, scattering or deflection of light. Most preferably, block <NUM> is implemented as a continuous uniform block of transparent material, typically glass.

The orientation and position of boundary plane <NUM> is chosen according to the location of the desired boundary in the final LOE structure. In most cases, the orientation will be a plane obliquely oriented relative to a plane of the interfaces. This is illustrated schematically as angle α (greater than <NUM>°) in <FIG>.

The method of <FIG> and <FIG> sets out the steps for manufacturing LOEs with a single inactive region <NUM>a, as illustrated in <FIG>, but is readily adapted by repetition of steps <NUM> and <NUM> to form additional boundary planes and to add additional transparent blocks to the precursor structure prior to slicing for forming two or more inactive regions from which the set of facets are excluded. <FIG> illustrates a further example of an LOE <NUM> having two inactive regions <NUM>a and <NUM>b, and <FIG> illustrates a number of such LOEs <NUM> being sliced from a corresponding precursor structure <NUM> so that each slide includes a region of the stack and a region of both blocks.

Although illustrated herein primarily in the example of a first dimension of expansion of an optical aperture, where the facets are responsible for a deflection from a first guided direction to a second guided direction of image light propagation with the LOE, the same principles are applicable to an LOE employed for a second (or any other) stage of optical aperture expansion. By way of example, <FIG> illustrates an optical arrangement including a first LOE <NUM> as illustrated in <FIG> for performing a first dimension of optical aperture expansion and a second LOE <NUM> for implementing a second dimension of optical aperture expansion and coupling-out the image illumination towards the eye of the observer. In this case, coupling-out facets <NUM> (shown schematically excessively spaced apart in order to facilitate understanding of the illustration) are limited to an active region of LOE <NUM>, and are excluded from an inactive region <NUM> in which the facets are not needed for directing any part of the image to any part of the EMB. The facets stop at a boundary plane <NUM>. Here too, the inactive region <NUM> is preferably generated together with the rest of the LOE by slicing a precursor assembly (not shown) including a transparent block bonded to a stack of coated plates, all as disclosed above with reference to <FIG> and <FIG>.

<FIG> illustrate a further example of the method of the present invention generally similar to the previous examples. In this case, the precursor structure <NUM> (<FIG>) is formed by attachment to stack <NUM> of a first block <NUM>a at a first boundary plane <NUM>a, and a second block 50b at a second boundary plane 48b. In this case, the cutting of second boundary plane 48b also includes cutting of part of the first block <NUM>a. <FIG> is a side view illustrating cutting lines <NUM> along which the precursor structure is preferably cut prior to slicing. This cutting preferably defines one or more edge planes, parts of which define, after slicing, an edge of each LOE. The resulting pre-shaped precursor structure <NUM>' is shown in <FIG> and <FIG>. Subsequent slicing of precursor structure <NUM>' along the slicing planes (dashed lines <NUM> of <FIG>) results in near-final LOE structures <NUM>, typically requiring only final polishing and any other steps required for assembling the LOE as part of the overall optical design.

It will be noted that the production methods of the present invention are applicable to a wide range of LOE structures for different applications, and can be adapted to provide different parameters of the LOE. For example, in certain implementations, the parallel slicing planes are oriented perpendicular to the interfaces of the stack, resulting in partially-reflecting surfaces that are orthogonal to the major external surfaces of the LOE. For other applications, the parallel slicing planes are obliquely angled relative to the interfaces, thereby generating an LOE with partially-reflecting surfaces that are oblique to the major external surfaces of the LOE.

The method of the present invention may also be implemented with sequences of partially-reflecting surfaces that implement various additional features, all according to the requirements of a particular optical arrangement. Examples include, but are not limited to, variable facet spacing, where the plates have thicknesses differing from each other such that the interfaces are non-uniformly spaced, and varying reflectivity, where the coatings are configured to provide sequentially varying reflectivity for successive of the interfaces.

Clearly, depending upon the desired geometry of the final optical device, the inactive region of the LOE may extend along a larger or smaller proportion of the region of facets. According to certain particularly preferred implementations, the boundary plane is non-parallel to an edge of the LOE so that a length of the partially-reflecting surfaces in a direction parallel to the major external surfaces progressively decreases from partially-reflecting surface to partially-reflecting surface along at least a quarter of the set of partially-reflecting surfaces.

As mentioned above in the context of <FIG>, the two-dimensional optical expansion examples illustrated herein in a "top-down" context can all equally be applied to "sideway" configurations, where an image is injected from a POD located laterally outside the viewing area and is spread by a first set of facets vertically and then by a second set of facets horizontally for coupling into the eye of the user. All of the above-described configurations and variants should be understood to be applicable also in a side-injection configuration.

Throughout the above description, reference has been made to the X axis and the Y axis as shown, where the X axis is either horizontal or vertical, and corresponds to the first dimension of the optical aperture expansion, and the Y axis is the other major axis corresponding to the second dimension of expansion. In this context, X and Y can be defined relative to the orientation of the device when mounted on the head of a user, in an orientation which is typically defined by a support arrangement, such as the aforementioned glasses frame of <FIG>. Other terms which typically coincide with that definition of the X axis include: (a) at least one straight line delimiting the eye-motion box, that can be used to define a direction parallel to the X axis; (b) the edges of a rectangular projected image are typically parallel to the X axis and the Y axis; and (c) a boundary between the first region <NUM> and the second region <NUM> typically extends parallel to the X axis.

Claim 1:
A method for producing light-guide optical elements (LOEs, <NUM>) each having a pair of mutually-parallel major external surfaces (<NUM>) for guiding image illumination propagating within the LOE (<NUM>) by internal reflection at the major external surfaces (<NUM>), each LOE (<NUM>) further having an active region comprising a set of mutually-parallel partially-reflecting surfaces (<NUM>) located between, and oriented non-parallel to, the major external surfaces (<NUM>), and at least one secondary region, all of said partially-reflecting surfaces (<NUM>) terminating at a boundary between the active region and the secondary region, the method comprising the steps of:
a) bonding together a plurality of parallel-faced plates (<NUM>) at a plurality of interfaces so as to form a stack (<NUM>) of plates (<NUM>), one face at each of said interfaces having a coating to provide partially-reflecting optical properties;
b) cutting and polishing said stack (<NUM>) of plates (<NUM>) to form a boundary plane (<NUM>) intersecting at least one of said interfaces;
c) bonding a block (<NUM>) of transparent material to said stack at said boundary plane (<NUM>) to form a precursor structure (<NUM>); and
the method characterized in that it further comprises:
d) slicing said precursor structure (<NUM>) along a plurality of parallel planes so as to form a plurality of slices (<NUM>), each slice containing a part of said stack (<NUM>) for providing the active region of the LOE and a part of said block (<NUM>) to provide the secondary region of the LOE.