Light structure

Abstract

A method for manufacturing a light-guiding optical element having multiple partially reflective surfaces is disclosed. The method includes: providing a plurality of transparent plates, each plate having two opposite surfaces polished such that the surfaces are parallel to each other; coating a first surface of a subset of the plates with a first coating; coating a second surface of the subset of the plates with a second coating; joining the plurality of transparent plates together to form a stack; and cutting the stack along a parallel plane at an angle to the faces of the transparent plates to form an optical element, wherein the first coating is a partially reflective coating having a first set of mechanical properties, and the second coating is selected from the group consisting of coatings similar to the first coating and non-reflective coatings having a second set of mechanical properties substantially similar to the first set of mechanical properties.

Description

Technical Field

[0001] The subject matter disclosed here relates to light-guiding optical elements, and more particularly to methods for manufacturing light-guiding optical elements. Prior Technology

[0002] Some head-mounted displays (HMDs) employ light-guide optical elements ("LOEs"), also known as waveguides. These LOEs operate by trapping light waves within a substrate through total internal reflection from the outer surface of the waveguide. The trapped light waves are then coupled out through an array of partially reflective surfaces. The inner surface of the waveguide is typically fabricated by bonding several transparent plates. Before bonding the plates, one surface of each plate is usually coated with a thin-film partially reflective coating. To maintain image quality, the plates should be as flat as possible. However, the thin-film and / or coating processes can stress the plates and cause them to warp, resulting in undesirable curvature and deterioration of the image quality transmitted by the LOE. Summary of the Invention

[0003] According to one aspect of the currently disclosed subject matter, a method for manufacturing a light-guiding optical element having a plurality of partially reflective surfaces is provided. The method includes: providing a plurality of transparent plates, each plate having two opposite surfaces polished such that the surfaces are parallel to each other; coating a first surface of a subset of the plates with a first coating; coating a second surface of the subset of the plates with a second coating; joining the plurality of transparent plates together to form a stack; and cutting the stack along a parallel plane at an angle to the faces of the transparent plates to form an optical element; wherein the first coating is a partially reflective coating having a first set of mechanical properties, and the second coating is selected from the group consisting of coatings similar to the first coating and non-reflective coatings having a second set of mechanical properties substantially similar to the first set of mechanical properties.

[0004] According to another aspect of the currently disclosed subject matter, a light-guiding optical element having at least a pair of parallel outer surfaces is provided, the light-guiding optical element comprising a plurality of partially reflective surfaces made of a plurality of transparent plates having surfaces that are not parallel to the paired parallel outer surfaces, the two opposite surfaces of each plate being polished such that the surfaces are parallel to each other; wherein each plate in the plate set is coated with a first coating on one surface and a second coating on a second surface, wherein the first coating is a partially reflective coating having a first set of mechanical properties, and the second coating is selected from the group consisting of coatings similar to the first coating and non-reflective coatings having a second set of mechanical properties substantially similar to the first set of mechanical properties.

[0005] According to some aspects of the currently disclosed subject matter, the subset of plates comprises alternating plates among a plurality of plates, and the second coating is a coating similar to the first coating. At least one of the plurality of plates may be coated with the first coating on a first surface and with a non-reflective coating having a second set of mechanical properties on a second surface.

[0006] Based on some aspects of the currently disclosed subject matter, the subset of plates includes all plates in a plurality of plates, and the second coating is a non-reflective coating having a second set of mechanical properties substantially similar to the first set of mechanical properties. Simple Explanation of the Diagram

[0007] To understand the invention and how it can be implemented in practice, embodiments will be described by way of non-limiting examples with reference to the drawings, in which: Figure 1A shows a side view of the LOE according to the prior art; Figure 1B shows an isometric view of the LOE according to the prior art; Figure 1C illustrates a method for manufacturing an LOE according to the prior art; Figure 2A illustrates a method for manufacturing an LOE according to a first embodiment of the currently disclosed subject matter; Figure 2B illustrates a particularly preferred method for manufacturing an LOE according to a first embodiment of the currently disclosed subject matter; Figure 3 illustrates a method for manufacturing an LOE according to a second embodiment of the currently disclosed subject matter; Figure 4A illustrates a stack cut along a parallel plane according to a first embodiment of the subject matter currently disclosed; Figure 4B illustrates a stack cut along a parallel plane according to a second embodiment of the currently disclosed subject matter; Figure 5A shows an alternative stack of Figure 4A cut along a parallel plane according to a second embodiment of the subject matter currently disclosed; Figure 5B shows an alternative stack of Figure 4B cut along a parallel plane according to a second embodiment of the currently disclosed subject matter; and Figures 6A and 6B illustrate an embodiment of a single-sided coating according to yet another embodiment of the subject matter currently disclosed. Implementation

[0008] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, those skilled in the art will understand that the subject matter currently disclosed can be practiced without these specific details. In other instances, well-known methods, processes, and components have not been described in detail so as not to obscure the subject matter currently disclosed.

[0009] With this in mind, now refer to Figures 1A through 1C, which show LOEs manufactured according to the prior art. Figure 1A shows a side view of an LOE 8 having multiple plates 4 and multiple partially reflective inner surfaces 2. Figure 1B shows an isometric view of the LOE 8. Figure 1C shows a prior art method for manufacturing the LOE 8, wherein each plate 4 is coated on one surface with a thin film partially reflective coating 12 before bonding. As further shown in Figures 4A through 4B, which illustrate alternative prior art methods, the plates are then bonded together to form a stack 6, and the stack is sliced ​​along a parallel plane 16 to form the LOE 8.

[0010] Typically, partial reflective coatings impose a certain amount of stress on the surface to which they are applied. When applied to one surface of each board but not to another, this stress can cause the board to warp, resulting in undesirable curvature within the board, which in turn can lead to poor LOE propagation quality images. The amount of warpage is typically determined by the mechanical properties of the specific coating (e.g., the stress characteristics of the thin-film coating) and / or by the coating process. For example, the coating material may have inherent properties related to the stress induced in the coated film. These properties include directional stress properties, which can be, for example, tensile or compressive. The stress may be caused by the different coefficients of thermal expansion between the coating and the board, or by the various coating layers and / or coating materials under temperature changes.

[0011] The inventors have discovered a method to mitigate the negative impact of stress caused by partially reflective coatings on a board, thereby reducing or completely eliminating warpage. Specifically, according to certain particularly preferred embodiments of the invention, some or all of the board is coated with a coating having similar stress characteristics on opposite surfaces, as will be described in more detail below.

[0012] Referring now to Figures 2A, 2B, and 3, two methods for manufacturing the LOE of the present invention according to certain embodiments of the currently disclosed subject matter are shown. Both methods initially provide a plurality of transparent plates 10, wherein two opposite surfaces of each plate are polished such that the opposite surfaces are parallel to each other. For clarity, in the foregoing description, the two opposite surfaces are sometimes referred to as the “left” surface and the “right” surface. It should be understood that, for the purposes of this invention, these descriptive names are intended only to distinguish the two opposite surfaces for the purposes of this specification and do not in any way indicate any particular order and / or optical properties of the plates during processing or in the final product.

[0013] As shown in Figures 2A, 2B, and 3, a first coating, serving as a partial reflective coating 12, is then applied to one of the surfaces of at least some of the plates in the plate 10 (referred to herein as a “subset” of plates). In Figures 2A, 2B, and 3, the plates in the subset are indicated as 10'. As an example, Figures 2A, 2B, and 3 show the partial reflective coating 12 applied to the left surface of plate 10'. Then, as will be described in detail below, a second coating is applied to a second surface of plate 10', which may be the same coating or a different coating depending on the specific embodiment.

[0014] In some implementations, different LOEs may require different amounts of reflectivity on different plates. Therefore, the reference to "first coating" or "partially reflective coating" should be understood as referring to one of a group of coatings that are all partially reflective but whose individual reflection parameters can vary. In some implementations, the reflection parameters can be gradually changed for each subsequent surface according to the specific LOE design requirements.

[0015] In the first embodiment, as shown in FIG2A, the subset of plates comprises alternating plates, and each plate 10' in this subset is coated with a partially reflective coating 12 on both its left and right surfaces. The remaining plates are uncoated. Because the coated plates are coated with a similar coating on both sides, any stress caused by the coating applied to such plates is applied equally to the two opposite surfaces, thereby reducing or eliminating the warping effect. It should be understood that a “similar” coating refers to a coating that applies stress in a similar manner, which may or may not be manufactured with the same composition, number of layers, and total thickness.

[0016] It should be noted that, as shown in Figures 4A and 4B, the plates are joined together to form an LOE, therefore it is not necessary to coat each plate with a partial reflective coating; that is, which of two adjacent plates has a coating has no effect on the optical properties. Therefore, the inventors have found it advantageous to apply a partial reflective coating to both surfaces of every other plate and not to the other plates. In this way, any deviation in the flatness of the plate caused by the coating is balanced by a similar deviation on the other side, thereby reducing or eliminating the overall curvature of the plate. Furthermore, the plates can be coated simultaneously on both sides.

[0017] In some particularly preferred embodiments, and as shown in FIG2B, in the presence of an odd number of optically active surfaces, plate 10'' (which may be the last plate shown in FIG2B, or it may alternatively be a plate located anywhere in the stack) is coated with a partially reflective coating 12 on one surface and a neutralizing coating 14 on the opposite surface, as will be described in further detail below with reference to the second embodiment shown in FIG3.

[0018] The partially reflective coating is preferably implemented as a multilayer dielectric coating, wherein the layers have alternating high and low refractive indices (and possibly intermediate values) and different layer thicknesses, wherein the thickness and layer properties are selected according to an algorithm to provide the desired optical properties, which are known in the art and can be obtained using standard software tools known in the art. Similarly, the second coating is preferably implemented using the same methods and techniques, but wherein the defined optical properties are those of the antireflective coating for all relevant angles. Antireflective properties can typically be achieved with fewer layers than the partially reflective coating, but it is preferred to use the same number of layers as the partially reflective coating to achieve similar mechanical properties, as discussed further below.

[0019] In the second embodiment, as shown in FIG3, the subset of plates includes all plates, and each plate 10' in this subset is coated with a partially reflective coating 12 on a first surface (e.g., shown as the left surface in FIG3) and with a neutralizing coating 14 on a second surface (e.g., shown as the right surface in FIG3). The neutralizing coating 14 has no discernible effect on light transmission (i.e., it is non-reflective), but has mechanical properties substantially similar to the partially reflective coating 12, namely stress characteristics. Therefore, when the neutralizing coating 14 is applied to the other surface (i.e., the surface without the partially reflective coating 12), substantially equal stress is applied to both surfaces of each coated plate, thereby reducing or eliminating the warping effect. In this embodiment, when each plate is coated, the plate is coated with a coating having equal stress characteristics on both sides. Therefore, similar to the first embodiment, any deviation in the flatness of the plate caused by the coating is balanced by the equal deviation on the other side, thereby reducing or eliminating the overall curvature of the plate.

[0020] In some embodiments, as mentioned, the neutralizing coating 14 is an anti-reflective coating. In some particularly preferred implementations, the anti-reflective coating is a coating with a similar layer composition and number of layers to a partially reflective coating, but the thickness to which it is applied is chosen to produce anti-reflective optical properties. In many cases, using the same number of layers with the same composition but different thicknesses provides sufficiently similar stress effects to reduce any residual warpage of the board to an acceptable level. However, using different layer compositions and / or number of layers for a “stress-neutralizing” coating also falls within the scope of this invention.

[0021] In some embodiments, a partial reflective coating and a neutralizing coating may be applied to the plate simultaneously. In some embodiments, the plate may warp after coating application and then be corrected. In some embodiments, stress may be induced by the coating process and / or during cooling after coating application.

[0022] It should be noted that "substantially similar" mechanical properties mean that the mechanical properties are similar enough that board warping is reduced, and preferably eliminated. In many cases, there will actually be a difference in layer thickness between the coatings on both sides of the board to achieve slightly different reflectivity / transmittance ratios and / or to achieve anti-reflective properties, but in many cases it has been found that using similar coating sequences (especially when the number and composition of the layers are the same) is "substantially similar" in the sense that the coatings achieve a sufficiently high degree of stress relief for high-quality products.

[0023] Referring now to Figures 4A and 4B, as in the prior art methods, in both of the above embodiments, the LOE 8 is then formed by joining the plates together to form a stack 6, and cutting the stack along a parallel plane 16 at an angle to the faces of the plates to form an optical element. In some embodiments, the plates may be joined by an adhesive-free bonding process (e.g., anodic bonding).

[0024] In some embodiments, as shown in FIG4A, the plates can be stacked without being angled, in which case the stack 6 is cut along a parallel plane angled relative to the surface of the plates. In other embodiments, as shown in FIG4B, the plates themselves are angled in the stack, in which case the stack 6 is cut straight. In some embodiments (not shown), the stack can be cut along another plane, for example to form a two-dimensional waveguide.

[0025] Referring to Figures 5A and 5B, in some embodiments, one or more end plates 18 having a different thickness than plate 10 may be attached to the ends of the stack 6. In some embodiments, the end plates 18 may also be coated with one or more coatings. These coatings may be the same as or different from the coating applied to plate 10. In some particularly preferred embodiments, no coating is applied to the end plates 18.

[0026] In some implementations, different LOEs may require different amounts of reflectivity on different plates. Therefore, the reference to "first coating" or "partially reflective coating" should be understood as referring to one of a group of coatings that are all partially reflective but whose individual reflection parameters can vary. In some implementations, the reflection parameters can be gradually changed for each subsequent surface according to the specific LOE design requirements.

[0027] Figures 6A and 6B schematically illustrate embodiments in which each of the plates 10 can be coated on only one side. Referring now to Figure 6A, in some embodiments, each of the plurality of plates 10 in the stack 6 is coated with a partial reflective coating 12'. Referring now to Figure 6B, the partial reflective coating 12' may comprise alternating layers of a high refractive index coating H having a refractive index Rh and a low refractive index coating L having a refractive index Rl, where Rh > Rl. For example, the partial reflective coating 12' may comprise alternating coatings H1, L1, H2, L2, etc., where H1 is applied to the plate surface, then L1 is applied on H1, then H2 is applied on L1, then L2 is applied on H2, and so on, as shown on the left-hand side of Figure 6B. It should be understood that coatings H and / or L themselves may comprise a variety of different coating materials that, when combined, achieve the desired refractive index, i.e., Rh for coating H and Rl for coating L.

[0028] Since each of the coatings H and L has its own inherent stress characteristics (tensile or compressive) when applied to the surface of the plate (which can vary depending on the specific temperature, thickness, coating process, etc.), the total stress of the partial reflective coating 12' can be calculated using known techniques such as the Stoney formula. As discussed above, the total stress of the partial reflective coating 12' may be undesirable and can cause warping of the plate 10 when applied to one side of each plate.

[0029] However, the inventors have discovered that some of the H or L layers can be replaced with alternative H' or L' (as the case may be) made of different materials or combinations of materials, as shown in the right-hand portion of FIG6B. In this case, the partial reflective coating 12' can be designed and constructed to have minimal tensile or compressive stress on the plate 10, or to have overall neutral stress in the sense of having no tensile or compressive stress. Therefore, the partial reflective coating 12' can now be applied to one side of each plate in the plate 10 without causing undesirable warping of the plate.

[0030] For example, consider a coating consisting of seventeen (17) alternating layers of SiO2 (R~1.42) and Ta2O5 (R~2.075) applied outward from the surface of the plate as follows: Layers 1, 3, 5, 7, 9, 11, 13, 15, 17 = TA2O5 Layers 2, 4, 6, 8, 10, 12, 14, 16 = SIO2

[0031] In this case, the partially reflective coating 12' will have a total tensile stress of over 216.4 MPa for approximately 6 minutes, which is undesirable for a single-sided coating. However, if some of the L layer (i.e., SiO2) is replaced with a layer of L' = MgF2 (R ~ 1.38), for example: Layers 1, 3, 5, 7, 9, 11, 13, 15, 17: TA2O5 Layers 2, 4, 6, 14, 16: SiO2 Layers 8, 10, and 12: MgF2 The MgF2 layer effectively neutralizes the total stress of the partial reflective coating 12', minimizing the total stress generated on the plate 10 by the partial reflective coating 12', and thus making it acceptable for a single-sided coating. It should be noted that the number of layers to be replaced is limited by the total stress in the design and can be varied depending on the amount of material, stress properties, design, and layer thickness.

[0032] It should also be noted that the deviation between the refractive index of the alternative layer H' or L' and the refractive index of the main layer H or L should preferably not exceed 6% (depending on the situation).

[0033] LOEs ​​manufactured according to the teachings of the currently disclosed subject matter can be used in a variety of applications, such as the various near-eye displays and related applications described in the patents and patent applications of Lums (Israel).

[0034] It should be understood that the invention is not limited in its application to the details set forth in the specification or shown in the drawings contained herein. The invention is capable of other embodiments and can be practiced and performed in various ways. Therefore, it should be understood that the wording and terminology used herein are for descriptive purposes and should not be considered limiting. Consequently, those skilled in the art will understand that the concepts upon which the content of this invention is based can be readily used as the basis for designing other structures, methods, and systems for performing some of the purposes of the currently disclosed subject matter.

[0035] Those skilled in the art will readily understand that various modifications and alterations can be applied to the embodiments of the invention as described above, without departing from the scope of the invention as defined in and by the appended claims.

[0036] 2: Partially reflective inner surface 4,10,10',10'': Board 6: Stacking 8:LOE 12,12': Partially reflective coating 14: Neutralizing coating 16: Parallel plane 18: End plate H,H1,H2,H',L,L1,L2,L': Coating Rh, Rl: Refractive index

Claims

1. An optical structure as an intermediate product in the manufacturing process of a light-guiding optical element, comprising: A stack of multiple transparent plates joined together, each transparent plate having two opposing surfaces polished to be parallel to each other, wherein the first of the two opposing surfaces of each transparent plate is coated with a partially reflective coating consisting of alternating high-refractive-index and low-refractive-index coatings, the refractive index of each high-refractive-index coating being greater than the refractive index of each low-refractive-index coating.

2. The optical structure as described in claim 1, wherein, All the high-refractive-index coatings of the partial reflective coating are formed from the same material.

3. The optical structure as described in claim 1, wherein, All layers of the first subset of the high refractive index coating of the partially reflective coating are formed of the same first material, and all layers of the second subset of the high refractive index coating of the partially reflective coating are formed of the same second material.

4. The optical structure as described in claim 3, wherein, Each of the first material and the second material has a refractive index, wherein the refractive index of the second material deviates from the refractive index of the first material by no more than 6%.

5. The optical structure as described in claim 1, wherein, All low-refractive-index coatings of the partially reflective coating are formed from the same material.

6. The optical structure as described in claim 1, wherein, All layers of a first subset of the low-refractive-index coating of the partial reflective coating are formed of the same first material, and all layers of a second subset of the low-refractive-index coating of the partial reflective coating are formed of the same second material.

7. The optical structure as described in claim 6, wherein, Each of the first material and the second material has a refractive index, wherein the refractive index of the second material deviates from the refractive index of the first material by no more than 6%.

8. The optical structure as described in claim 6, wherein, The first material is SiO2, and the second material is MgF2.

9. The optical structure as described in claim 1, wherein, At least one high-refractive-index layer of the partial reflective coating is formed of Ta2O5, and at least one low-refractive-index layer of the partial reflective coating is formed of SiO2.

10. The optical structure as described in claim 1, wherein, The partial reflective coating consists of at least one high-refractive-index layer formed from a variety of different coating materials.

11. The optical structure as described in claim 1, wherein, At least one low-refractive-index layer of the partially reflective coating is formed from a variety of different coating materials.

12. The optical structure as described in claim 1, wherein, Two of the transparent plates are the outermost plates of the stack, wherein an end plate is joined to at least one of the two outermost plates of the stack, and wherein the thickness of the end plate is greater than the thickness of each of the transparent plates.

13. The optical structure as described in claim 1, wherein, The transparent panels are arranged such that adjacent transparent panels in the stack are offset from each other.

14. The optical structure as described in claim 1, wherein, The stack includes a pair of parallel cut planes angled relative to the two opposing surfaces of the transparent plate.

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