Patent Description:
Certain head-mounted displays (HMD) employ a two-dimensional waveguide that operates by trapping light waves inside a substrate by total internal reflections from the external surfaces of the waveguide. The light waves which are trapped inside the waveguide are coupled out by an array of partially reflecting surfaces. Typically, the coupled out light waves pass through an additional waveguide before being transmitted to the eye of a user. In order to maintain the quality of the propagating image, there must be a very high degree of perpendicularity between two or more surfaces of the waveguide. Typically these waveguides have two pairs of parallel external surfaces opposite each other (i.e. top and bottom, front and back) in which the two pairs have to be mutually perpendicular. <CIT> discloses a method of making a waveguide using gaps for turning light comprising: forming a pseudo-randomized pattern of pits in one surface of each of a number N of optically transparent flat sections; tilting the pitted N optically transparent flat sections at a same angle with each of the pitted N surfaces facing in the same direction; stacking the tilted pitted N optically transparent flat sections into a stack with each of the pitted N surfaces facing the in the same direction so each pitted surface abuts an all flat surface of a respective adjacent optically transparent flat section; bonding the stack; slicing across the stack to form a waveguide including the N bonded sections tilted at the same angle, and polishing external planar surfaces of the sliced waveguide.

In order to understand the invention and to see how it can be carried out in practice, embodiments will be described, by way of non-limiting examples, with reference to the accompanying drawings, in which:.

The object of the invention is a method and an apparatus for determining an angle of a target surface of a waveguide, thereby achieving accurate perpendicular polishing of the target surface relative to a reference surface, in accordance with the appended claims.

However, it will be understood by those skilled in the art that the presently disclosed subject matter may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the presently disclosed subject matter.

Bearing this in mind, the presently disclosed subject matter is particularly applicable to a waveguide such as that disclosed in <CIT>, which comprises a plurality of internal partially reflecting surfaces which are parallel to each other but angled relative to waveguide's side external surfaces, although the invention may also be applied to advantage in any case where optical components must be polished to generate high-quality mutually-perpendicular polished surfaces, even without internal partially-reflective surfaces. Referring now to <FIG>, there is illustrated a top view of a waveguide <NUM> having side external surfaces <NUM>, front and back surfaces <NUM>, and angled internal surfaces 102a-102x, <FIG> illustrates a perspective view of waveguide <NUM> in which is shown a side surface <NUM>, top surface <NUM>, and front surface <NUM>. In waveguides such as the one disclosed in the aforementioned PCT, there must be a very high degree of perpendicularity between the top and bottom surfaces relative to the side external surfaces, and in certain preferred implementations also between the top and bottom surfaces relative to the internal surfaces.

As such, there is herein provided a method of polishing an external surface of a waveguide in order to achieve accurate perpendicularity between the surface to be polished ("target surface") and at least one other, typically abutting, surface ("reference surface") of the waveguide. In some embodiments it may be desirable for the target surface to be polished accurately perpendicular to two different non-parallel reference surfaces simultaneously, such as an external surface and an internal surface.

Referring now to <FIG>, the method includes providing a polishing apparatus <NUM> for use in polishing a target surface of the waveguide. The polishing apparatus <NUM> includes an annular polishing plate <NUM> having a flat exterior (top as shown) surface <NUM>. Polishing plate <NUM> is configured to hold an object to be polished and facilitate slidable contact between the object to be polished and a polisher (not shown), such that point of contact between the polisher and the object defines a polishing plane (which generally moves during polishing as the object's surface is ground down). During polishing, polishing apparatus <NUM> contacts the polisher such that flat surface <NUM> is parallel to the polishing plane. Therefore it can be said that flat surface <NUM> defines a plane (hereinafter "reference plane") which is always parallel to the polishing plane. Although <FIG> shows polishing apparatus <NUM> with flat surface <NUM> facing upwards, this is for clarity only, as in most cases during polishing the polishing apparatus <NUM> is turned upside down onto the polisher.

Polishing apparatus <NUM> further includes at least one adjustable mounting apparatus <NUM> configured to hold the waveguide <NUM> during polishing of the target surface. Adjustable mounting apparatus <NUM> is further configured to hold the waveguide at any one of a plurality of angular orientations relative to the flat surface <NUM> of the polishing apparatus <NUM>. As will be further detailed with reference to <FIG>, the adjustable mounting apparatus <NUM> facilitates rotation of the waveguide <NUM> about a plurality of axes, thereby allowing a user to set the desired plane upon which the target surface will be polished to (i.e. made parallel to after polishing is complete).

In some embodiments, as shown in <FIG>, polishing apparatus <NUM> can include a plurality of adjustable mounting apparatuses <NUM>, where each mounting apparatus <NUM> holds a different waveguide <NUM> and each mounting apparatus <NUM> is independently adjustable, thereby allowing simultaneous polishing of a plurality of waveguides <NUM>. In some embodiments, polishing apparatus <NUM> further includes a rotatable base <NUM> which allows for bringing each mounting apparatus sequentially into alignment with the optical alignment sensor, as will be detailed below. Free rotation may also be allowed during polishing.

In some embodiments, the method further includes mounting a plurality of sacrificial blocks <NUM> (e.g. using an adhesive bonding material) on flat surface <NUM> of polishing plate <NUM> at various points. Sacrificial blocks <NUM> may be desired in some cases for balancing and/or load distribution during polishing. In addition or in the alternative, sacrificial blocks <NUM> may also be desired to relieve some of the pressure placed on the waveguide(s) <NUM> during polishing. This is particularly valuable where the polishing process would initially reach a corner or edge of the waveguide, which would otherwise result in a localized application of excessive loading of the polisher. By use of sacrificial blocks <NUM>, the load of the polishing process is always distributed over a relatively large area, maintaining parallelism of the polisher to the reference plane and avoiding damage to corners or edges of the waveguide. In this case, prior to polishing, target surface of waveguide <NUM> should be adjacent to, but below, the top surface of sacrificial blocks <NUM>. By "adjacent, but below", it is meant that to the naked eye the two surfaces appear to lie on the same plane but in fact there is a miniscule difference in their relative elevation such that the target surface is somewhat lower. Additionally or alternatively, top surface of sacrificial blocks <NUM> can be used as an alternative reference plane parallel to the polishing plane, as will be detailed below. In some cases, by employing sacrificial blocks pre-polished to provide two parallel faces and uniform thickness, a sufficiently accurate reference surface can be achieved by adhering the blocks to flat surface <NUM> with pressure. Additionally, or alternatively, the plurality of sacrificial blocks <NUM> may first be simultaneously polished after mounting in order to ensure that the top surfaces thereof lie on identical planes, i.e. are coplanar, and secondly that the top surfaces of the sacrificial blocks are accurately parallel to the polishing plane. In some embodiments, the sacrificial blocks <NUM> can be made out of glass, or out of the same material as waveguide <NUM>, or any other suitable material.

<FIG> illustrates an enlarged view of adjustable mounting apparatus <NUM> according to some embodiments of the presently disclosed subject matter. Mounting apparatus <NUM> includes a tilting stage <NUM> on which is fixed a mounting plate <NUM> configured to receive the waveguide and to hold the waveguide during polishing, for example via a temporary adhesive bond, or alternatively by clamping. Mounting plate <NUM> is secured to tilting stage <NUM> by clamps which are tightened by fasteners <NUM> (e.g. screws). Mounting apparatus <NUM> further comprises rotation screws 406a-406b configured to facilitate rotation (tilting) of tilting stage <NUM> about at least two perpendicular axes (e.g. tilt and roll). In certain embodiments, mounting apparatus can include a third "elevation" screw 406c to facilitate adjusting the height (i.e. elevation) of tilting stage <NUM> relative to polishing plate <NUM>. In the preferred but non-limiting implementation illustrated here, all three adjustment screws 406a-406c are essentially similar, each raising or lowering one region of a three-point support structure. However, the presence of three adjustment points allows for overall raising or lowering of the tilting stage <NUM>. In certain embodiments it may be desirable to set the height of the waveguide such that at least part of the target surface is located below, but adjacent to, an initial polishing plane (e.g. top surface of sacrificial blocks <NUM>). In certain embodiments, the height of the waveguide can be adjusted, via operation of screws 406a-406c, to a predetermined difference relative to the initial polishing plane such that sacrificial blocks <NUM> take all, or most of the load during the initial stage of polishing.

During polishing, substantial stress can be placed on tilting stage <NUM>, which could lead to unintended slippage of adjustment screws 406a-406c, and consequent undesirable deviation in the orientation of the tilting stage <NUM>. To prevent such deviation, in some embodiments, mounting apparatus <NUM> can further include a locking mechanism configured to lock the orientation of tilting stage <NUM> at a given angular orientation (and height). In this case, the method preferably further includes locking the angular orientation and/or height of the tilting stage by use of the locking mechanism prior to polishing.

<FIG> illustrates a schematic non-limiting example of a locking mechanism according to certain embodiments of the presently disclosed subject matter, serving also as a more detailed exemplary structure of each of the aforementioned adjustment screws 406a-406c. Adjustment of the height of the region of tilting stage <NUM> supported by each adjustment screw is achieved by turning wheel <NUM> which turns a hollow bolt <NUM>, which in turn raises or lowers a rider <NUM> engaged with a region of the tilting stage <NUM>. When the adjustment screw is correctly adjusted, wheel <NUM> is fixed to prevent further turning by tightening a screw <NUM> which locks wheel <NUM>. An additional clamping screw <NUM> is connected via a cable <NUM> to a cable end <NUM>, located on the opposite side of a support member of polishing plate <NUM>. Once all adjustments to orientation have been finalized, screw <NUM> is rotated to tighten cable <NUM>, thereby securing the tilting stage <NUM> to the polishing plate <NUM> at the given orientation. In some embodiments, the locking mechanism can further include metallic ends <NUM> to prevent tilting stage <NUM> from sinking relative to polishing plate <NUM> during tensioning of cable <NUM>. In some embodiments, the locking mechanism can further include a spring <NUM> located between cable end <NUM> and polishing plate <NUM> to maintain residual tension of the cable <NUM> when screw <NUM> is loosened, e.g. to make adjustments to the orientation of tilting stage <NUM>.

Referring now to <FIG>, in some preferred embodiments, the method further includes positioning a height sensing apparatus <NUM> configured to detect the height difference between the target surface of waveguide <NUM> and the top surface <NUM> of sacrificial blocks <NUM>, and to set the desired height difference prior to polishing. In some embodiments, a height sensing apparatus <NUM> can also be used, after adjusting the first waveguide to the desired height, to adjust the height of the second and subsequent waveguides in polishing apparatus <NUM> to the same height as the first waveguide.

In some embodiments, as illustrated in <FIG>, the method further includes positioning one or more optical alignment sensors <NUM> (e.g. an autocollimator, etc.), each configured to emit one or more collimated light beams and to receive reflections thereof, and positioning one or more light reflecting apparatuses <NUM> (e.g. mirror, pentaprism, etc.) configured to reflect a collimated light beam exactly <NUM> degrees. For each reference surface <NUM> a corresponding optical alignment sensor <NUM> and corresponding light reflecting apparatus <NUM> is positioned such that a first collimated light beam is reflected off of a surface <NUM> parallel to the reference plane, and a second collimated light beam <NUM>, perpendicular to the first collimated light beam at the reflection point, is reflected off of the given reference surface <NUM>. Suitable optical alignment sensors include the Nikon Autocollimator 6B-LED/6D-LED made by Nikon Corporation. It should be appreciated that a single, broad collimated light beam can also be used, in which case the reference to a first and second collimated light beams should be understood to refer to two different parts of a single collimated light beam.

In some embodiments the method further includes, by use of the adjustable mounting apparatus <NUM>, aligning waveguide <NUM> within polishing apparatus <NUM> such that the polishing plane is perpendicular to each reference surface. This is accomplished by adjusting the angular orientation of the waveguide such that for each given reference surface, the reflections received by the corresponding optical alignment sensor <NUM> align therein, thereby being indicative of perpendicularity between the reference plane (and by extension the polishing plane) and the given reference surface, as will be further detailed below with reference to <FIG>.

The method further includes polishing the target surface of the waveguide by bringing it into slidable contact with the polisher, thereby achieving accurate perpendicular polishing of the target surface relative to each reference surface. In some embodiments, accurate perpendicularity includes perpendicularity to within <NUM> arcminute. In some embodiments, accurate perpendicularity includes perpendicularity to within <NUM> arcseconds.

In some embodiments, as detailed above, the method can include locking the orientation of the waveguide via the locking mechanism of the mounting apparatus prior to polishing.

For greater clarity reference is made to <FIG>, illustrating a conceptual diagram of the alignment method detailed above. Cube <NUM> has two abutting non-parallel surfaces <NUM> and <NUM>. Surface <NUM> is desired to be polished accurately perpendicular to surface <NUM>. Surface <NUM> is known in advance to be parallel with the polishing plane, therefore the desired outcome is to orient cube <NUM> on the polishing apparatus such that surface <NUM> is accurately perpendicular to surface <NUM>. An autocollimator <NUM> and forty-five degree tilted mirror <NUM> are together positioned such that autocollimator <NUM> emits a first collimated beam <NUM> onto surface <NUM> and a second collimated light beam <NUM> (which may be a different region of a single broad collimated beam) onto surface <NUM>, and receives reflections therefrom. If the reflections from the two collimated light beams are precisely parallel so that their images appear aligned within the autocollimator, surface <NUM> is perpendicular to the polishing plane, and otherwise not. In such a case, cube <NUM> should be rotated left or right until the collimated light beams align.

As illustrated in <FIG>, the reflections of two collimated beams received within an autocollimator can be considered aligned when, as viewed in a viewfinder <NUM> of the autocollimator, alignment symbols in the received reflections <NUM>, <NUM> coincide. Imperfectly overlapping reflections are considered misaligned and indicative of non-perpendicularity between the surfaces off of which the collimated light beams were reflected.

<FIG> illustrates an alternative method of aligning the waveguide. In this method, the waveguide is bonded to a large, preferably glass, block <NUM> having accurately perpendicular surfaces, with the target surface of the waveguide facing the polisher. The surface of block <NUM> to which the waveguide is bonded to is perpendicular to the polishing plane and therefore the target surface will be made perpendicular to it after polishing. If the target surface is simultaneously to be made perpendicular with an internal surface of the waveguide, an optical alignment sensor and light reflecting apparatus can be used to reflect a first collimated light beam off of the internal surface of the waveguide and a second collimated light beam off of a top surface of block <NUM>. The waveguide's orientation relative to block <NUM> is then adjusted until the reflected beams overlap.

<FIG> illustrates an alternative, or additional, alignment method whereby a ray <NUM> is reflected from internal facets <NUM>. The rays <NUM> (preferably originated and reflected onto an autocollimator) have a different angle within the waveguide <NUM> from outside the component because of refraction by the component <NUM>. Nevertheless, the alignment procedure is still effective as previously described.

In certain cases, the light from the autocollimator is not monochromatic. Consequently, the reflected light will be dispersed by the aforementioned refraction, thereby degrading alignment accuracy. According to this invention, this limitation can be eliminated by using proper orientation of the autocollimator projected image. <FIG> shows a typical image projected by the autocollimator. If the dispersion orientation of ray <NUM> on the image plan is expected to be at orientation illustrated by arrow <NUM>, the orientation of the collimator projected image should be adjusted (rotated) until it is aligned in parallel and perpendicular to the dispersion orientation, as shown in <FIG>. The reflected image (<FIG>) illustrates the resultant dispersion of the vertical line <NUM> so that, in most cases, it is rendered invisible. The line aligned along the direction of dispersion remains sharp. Shift of this line in perpendicular direction <NUM> relative to the projected reflected image from surface <NUM> indicates non-perpendicularity, as previously described.

Referring now to <FIG>, in some embodiments, it may be desirable to re-check the perpendicularity between the polishing plane and the reference surface(s) at various times throughout the polishing process. In order to facilitate such checking without separating the polishing apparatus from the polisher, in some embodiments one or more openings <NUM> can be made, e.g. by drilling, through the polishing plate, each opening allowing a collimated light beam to reach its intended surface. Additionally, since the sacrificial blocks are in contact with the polisher and not accessible by the optical alignment sensor, a substitute surface parallel to the polishing plane may be used. In this case, one or more blocks <NUM>, each having a flat surface parallel to the polishing plane can be attached to the polishing plate and used as the reference plane in place of the sacrificial blocks.

In certain particularly preferred implementations of the device and method of the present invention, alignment of the polishing plane perpendicular to a surface of the waveguide is performed simultaneously for both an external surface and an internal partially reflective surface of the waveguide. The adjustment may be performed using two autocollimators simultaneously and performing the adjustment of each alternately and iteratively. In some cases, the adjustment process may be automated.

For greater clarity, in waveguide applications where a top or and/or bottom surface is required to be accurately perpendicular to the side external surfaces and the angled internal surfaces, two optical alignment sensors and two light reflecting apparatuses can be used to achieve accurate perpendicularity between the target surface and two non-parallel reference surfaces (i.e. a side external surface and an internal surface). The first optical alignment sensor and light reflecting apparatus emit and receive reflections from a surface parallel to the polishing plane and the first reference surface, respectively. The second optical alignment sensor and light reflecting apparatus emit and received reflections from the surface parallel to the polishing plane and the second reference surface, respectively. The angular orientation of the waveguide is then adjusted using the adjustable mounting apparatus until the reflections received by the first optical alignment sensor align therein and the reflections received by the second optical alignment sensor also align therein, thereby indicating perpendicularity between the polishing plane, first reference surface, and second reference surface simultaneously.

It is to be understood that the invention is not limited in its application to the details set forth in the description contained herein or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Hence, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for designing other structures, methods, and systems for carrying out the several purposes of the presently disclosed subject matter.

Claim 1:
A method for determining an angle of a target surface of a waveguide (<NUM>) to be polished relative to at least one reference surface of the waveguide (<NUM>), thereby achieving accurate perpendicular polishing of the target surface relative to a reference surface, wherein the waveguide (<NUM>) includes a plurality of mutually parallel partially reflective internal surfaces (102a-102x), and wherein the at least one reference surface includes one or more surfaces parallel to the partially reflective internal surfaces (102a-102x),
the method characterized by comprising:
illuminating the target surface with one or more first collimated light beams that impinge substantially perpendicular to the target surface;
illuminating the reference surface with one or more second collimated light beams that impinge substantially perpendicular to the at least one reference surface;
collecting, by at least one optical sensor (<NUM>), one or more first light beams reflected from the target surface in response to illumination by the one or more first collimated light beams and one or more second light beams reflected from the at least one reference surface in response to illumination by the one or more second collimated light beams, wherein at least one of the first light beams or the second light beams are directed to the at least one optical sensor (<NUM>) via a reflecting element (<NUM>); and
determining the angle of the target surface relative to the at least one reference surface based on an angle of the one or more first light beams and an angle of the one or more second light beams as measured by the optical sensor (<NUM>).