Optical grating, optical waveguide, and optical system

Abstract

An optical grating for optically coupling a light ray propagating within an optical waveguide by total internal reflection to a light ray propagating outside, and toward or away from, the waveguide is provided. The optical grating includes an optically transparent adhesive layer including a plurality of substantially parallel linear adhesive prisms and an overcoat disposed on the plurality of substantially parallel linear adhesive prisms. For at least a first visible wavelength in a visible wavelength range extending from about 420 nm to about 680 nm, the optically transparent adhesive layer and the overcoat have respective indices n1 and n2, where n1 ≥ 1.2, n2 / n1 > 1. For a substantially normally incident collimated light beam having the at least the first visible wavelength, and for at least a first polarization state, a first order diffraction efficiency of the multilayer grating is greater than about 40%.

Description

[0001] OPTICAL GRATING, OPTICAL WAVEGUIDE, AND OPTICAL SYSTEM

[0002] Technical Field

[0003] The present disclosure relates generally to an optical grating. The present disclosure further relates to an optical waveguide and an optical system including the optical waveguide.

[0004] Cross-Reference

[0005] This application claims the benefit of U.S. Patent Application Serial No. 63 / 373,586, filed on August 26, 2022, which is incorporated herein by reference in its entirety.

[0006] Background

[0007] Optical systems are used in realizing augmented reality glasses. Generally, a waveguide combiner is used to transport light from a projector to an eye of a viewer. In some cases, the waveguide combiner has surface relief gratings to transport the light. However, such surface relief gratings may result in significant loss of incident light and / or a process of manufacturing such surface relief gratings may be complex and time-consuming.

[0008] Summary

[0009] In a first aspect, the present disclosure provides an optical waveguide including an optical core configured to propagate an image light therealong. The optical waveguide further includes first and second spaced apart multilayer gratings disposed on the optical core. The first multilayer grating is configured to receive an image light emitted by an image projector and inject at least a portion of the received image light into the optical core. The injected image light propagates along the optical core primarily by total internal reflection. The second multilayer grating is configured to receive at least a portion of the injected image light and extract at least a portion of the received injected image light from the optical core for viewing by a viewer. Each of the first and second multilayer gratings includes an optically transparent adhesive layer disposed on the optical core. The optically transparent adhesive layer includes an adhesive structured first major surface facing away from the optical core and an opposing adhesive second major surface facing, and bonded to, the optical core. The structured first major surface defines a plurality of substantially parallel linear adhesive prisms extending along substantially a same in-plane first direction and arranged along an in-plane orthogonal second direction. Each linear adhesive prism includes a wider base facing the optical core, a narrower top facing away from the optical core, and opposing first and second sides converging toward each other as they extend from opposite ends of the base to opposite corresponding ends of the top. The first and second sides make respective angles al and a2 with the second direction. The angle al is greater than about 60 degrees and greater than the angle a2 by at least 5 degrees. Each of the first and second multilayer gratings further includes an overcoat disposed on the structured first major surface of the optically transparent adhesive layer opposite the optical core such that for each of the linear adhesive prisms, a first portion of the overcoat includes opposing substantially parallel first and second major surfaces and substantially conformably covers at least 80% of the second side of the linear adhesive prism. In addition, for at least a first visible wavelength in a visible wavelength range extending from about 420 nanometers (nm) to about 680 nm, the optically transparent adhesive layer and the overcoat have respective indices nl and n2, where nl > 1.2 and n2 > 1.6.

[0010] In a second aspect, the present disclosure provides an optical multilayer grating for optically coupling a light ray propagating within an optical waveguide by total internal reflection to a light ray propagating outside, and toward or away from, the waveguide. The multilayer grating includes an optically transparent unitary adhesive layer having a plurality of spaced apart adhesive prisms and a continuous adhesive land portion extending continuously across substantially an entire length and an entire width of the unitary adhesive layer and unitarily joining bases of the adhesive prisms. Each of the adhesive prisms includes a top opposite to the base of the prism, and at least opposing first and second sides extending from, and connecting, the base to the top. The optical multilayer grating further includes an overcoat disposed on the optically transparent unitary adhesive layer such that: a first portion of the overcoat substantially conformably covers at least 80% of the second side of each of the adhesive prisms, and includes substantially parallel opposing first and second major surfaces, and has an average thickness h2c; and any second portion of the overcoat that substantially conformably covers at least 50% of the first side of each of the adhesive prisms, includes substantially parallel opposing first and second major surfaces, and has an average thickness h2a, where h2c / h2a > 1.5. In addition, for at least a first visible wavelength in a visible wavelength range extending from about 420 nm to about 680 nm, the optically transparent adhesive layer and the overcoat have respective indices nl and n2, nl > 1.2, n2 / nl >1. Further, for a substantially normally incident collimated light beam having the at least the first visible wavelength, and for at least a first polarization state, a first order diffraction efficiency of the multilayer grating is greater than about 40% .

[0011] In a third aspect, the present disclosure provides an optical grating for optically coupling a light ray propagating within an optical waveguide by total internal reflection to a light ray propagating outside, and toward or away from, the waveguide. The optical grating includes a plurality of spaced apart substantially parallel and substantially planar facets extending along an in-plane same first direction and arranged along an in-plane orthogonal second direction. Each substantially planar facet includes an average thickness T along a normal to the substantially planar facet and making an angle A with a third direction orthogonal to the in-plane first and second directions, where T / A > 1 nm / deg. Each substantially planar facet is defined by a support material. For at least a first visible wavelength in a visible wavelength range extending from about 420 nm to about 680 nm, the support materials and the substantially planar facets have respective indices nl and n2, where nl > 1.2 and n2 / nl > 1. In addition, for a substantially normally incident collimated light beam having the at least the first visible wavelength, and for at least a first polarization state, a first order diffraction efficiency of the optical grating is greater than about 50%. The details of one or more examples of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims.

[0012] Brief Description of the Drawings

[0013] Exemplary embodiments disclosed herein may be more completely understood in consideration of the following detailed description in connection with the following figures. The figures are not necessarily drawn to scale. Like numbers used in the figures refer to like components. However, it will be understood that the use of a number to refer to a component in a given figure is not intended to limit the component in another figure labeled with the same number.

[0014] FIG. 1 shows a schematic sectional view of an optical system, according to an embodiment of the present disclosure;

[0015] FIGS. 2A-2C show schematic sectional magnified views of a portion of an optical multilayer grating of the optical system, according to an embodiment of the present disclosure;

[0016] FIG. 3 shows a schematic sectional magnified view of a portion of an optical multilayer grating of the optical system, according to another embodiment of the present disclosure;

[0017] FIG. 4 shows a schematic top view of an optical system, according to another embodiment of the present disclosure;

[0018] FIG. 5 shows a schematic top view of an optical system, according to yet another embodiment of the present disclosure;

[0019] FIGS. 6A-6D show exemplary plots depicting diffraction efficiencies for optical waveguides having different parameters of optical multilayer gratings; and

[0020] FIGS. 7A-7D show exemplary heatmap graphs depicting diffraction efficiencies for optical waveguides having different parameters of the optical multilayer gratings.

[0021] Detailed Description

[0022] In the following description, reference is made to the accompanying figures that form a part thereof and in which various embodiments are shown by way of illustration. It is to be understood that other embodiments are contemplated and may be made without departing from the scope or spirit of the present disclosure. The following detailed description, therefore, is not to be taken in a limiting sense.

[0023] In the following disclosure, the following definitions are adopted.

[0024] As used herein, all numbers should be considered modified by the term “about”. As used herein, “a,” “an,” “the,” “at least one,” and “one or more” are used interchangeably.

[0025] As used herein as a modifier to a property or attribute, the term “generally”, unless otherwise specifically defined, means that the property or attribute would be readily recognizable by a person of ordinary skill but without requiring absolute precision or a perfect match (e.g., within + / - 20 % for quantifiable properties). The term “substantially”, unless otherwise specifically defined, means to a high degree of approximation (e.g., within + / - 10% for quantifiable properties) but again without requiring absolute precision or a perfect match.

[0026] The term “about”, unless otherwise specifically defined, means to a high degree of approximation (e.g., within + / - 5% for quantifiable properties) but again without requiring absolute precision or a perfect match.

[0027] As used herein, the terms “first” and “second” are used as identifiers. Therefore, such terms should not be construed as limiting this disclosure. The terms “first” and “second” when used in conjunction with a feature or an element can be interchanged throughout the embodiments of this disclosure.

[0028] As used herein, “at least one of A and B” should be understood to mean “only A, only B, or both A and B”.

[0029] As used herein “a first order diffraction efficiency” refers to a ratio of powers between a diffracted light beam and an incident light beam.

[0030] As used herein, “a collimated beam” refers to an electromagnetic radiation that has parallel rays and spreads minimally as it propagates.

[0031] Optical systems are used in realizing augmented reality glasses. Generally, a waveguide combiner is used to transport light from a projector to an eye of a viewer. In some cases, the waveguide combiner has surface relief gratings to transport the light. The surface relief gratings are used for incoupling, via an in-coupler, expanding, and out-coupling, via an out-coupler, light towards the eyes of the viewer. However, the light incident from the projector into the in-coupler may be diffracted to directions both towards and away from the out-coupler. The light diffracted away from the out-coupler may be lost. In some cases, the light diffracted away from the out-coupler may be significant. Therefore, such surface relief gratings may result in significant loss of incident light and / or a process of manufacturing such surface relief gratings may be complex and time-consuming.

[0032] The present disclosure relates to an optical waveguide including an optical core configured to propagate an image light therealong. The optical waveguide further includes first and second spaced apart multilayer gratings disposed on the optical core. The first multilayer grating is configured to receive an image light emitted by an image projector and inject at least a portion of the received image light into the optical core. The injected image light propagates along the optical core primarily by total internal reflection. The second multilayer grating is configured to receive at least a portion of the injected image light and extract at least a portion of the received injected image light from the optical core for viewing by a viewer. Each of the first and second multilayer gratings includes an optically transparent adhesive layer disposed on the optical core. The optically transparent adhesive layer includes an adhesive structured first major surface facing away from the optical core and an opposing adhesive second major surface facing, and bonded to, the optical core. The structured first major surface defines a plurality of substantially parallel linear adhesive prisms extending along substantially a same in-plane first direction and arranged along an in-plane orthogonal second direction. Each linear adhesive prism includes a wider base facing the optical core, a narrower top facing away from the optical core, and opposing first and second sides converging toward each other as they extend from opposite ends of the base to opposite corresponding ends of the top. The first and second sides make respective angles al and a2 with the second direction. The angle al is greater than about 60 degrees and greater than the angle a2 by at least 5 degrees. Each of the first and second multilayer gratings further includes an overcoat disposed on the structured first major surface of the optically transparent adhesive layer opposite the optical core such that for each of the linear adhesive prisms, a first portion of the overcoat includes opposing substantially parallel first and second major surfaces and substantially conformably covers at least 80% of the second side of the linear adhesive prism. In addition, for at least a first visible wavelength in a visible wavelength range extending from about 420 nanometers (nm) to about 680 nm, the optically transparent adhesive layer and the overcoat have respective indices nl and n2, where nl > 1.2 and n2 > 1.6.

[0033] Further, in some cases, a first order diffraction efficiency of the multilayer grating is greater than about 40%. In other words, the first multilayer grating may diffract more light towards the second multilayer grating as compared to the surface relief gratings which do not include the optically transparent adhesive layer and / or the overcoat. Therefore, the optical system of the present disclose may reduce the loss of incident light.

[0034] Further, some surface relief gratings having a similar optical performance may include slanted gratings having undercuts. However, manufacturing such slanted grating may not be trivial, especially in process steps requiring replication. Hence, the process of manufacturing the undercuts of the slanted gratings may be complex and time-consuming. Since the structured first major surface defines the plurality of substantially parallel linear adhesive prisms and the overcoat may be selectively disposed on the structured first major surface of the optically transparent adhesive layer, the optical waveguide of the present disclosure may not require an undercut. Hence, manufacturing of the optical waveguide may be less time-consuming and less complex than the process of manufacturing the undercuts of the slanted gratings of the surface relief gratings having the undercuts.

[0035] Referring now to figures, FIG. 1 illustrates a schematic sectional view of an optical system 400, according to an embodiment of the present disclosure.

[0036] The optical system 400 includes an optical waveguide 300. The optical waveguide 300 defines mutually orthogonal x, y, and z-axes. The x and y-axes are in-plane axes of the optical waveguide 300, while the z-axis is a transverse axis disposed along a thickness of the optical waveguide 300. In other words, the x and y-axes are disposed along a plane of the optical waveguide 300, while the z-axis is perpendicular to the plane of the optical waveguide 300.

[0037] The optical system 400 further includes an image projector 40. The image projector 40 is configured to emit an image light 20. In some embodiments, the image light 20 includes one or more of a letter, a number, a symbol, a logo, a text, an alphanumeric, and a picture. The waveguide 300 includes an optical core 10. The optical core 10 is configured propagate an image light 22 therealong. In FIG. 1, the optical core 10 propagates the image light 22 along the x- axis.

[0038] In some embodiments, the optical core 10 has an average thickness of between about 100 microns and about 2000 microns. In some embodiments, the optical core 10 may have the average thickness of between about 150 microns and about 1500 microns, about 200 microns and about 1250 microns, about 250 microns and about 1250 microns, or about 300 microns and about 1000 microns.

[0039] In some embodiments, the optical core 10 includes one or more of a glass, a polymer, a crystal, a silica, a sol-gel, and silicon dioxide. In some embodiments, the optical core 10 has an index of refraction n of greater than about 1.4 at the at least a first visible wavelength in a visible wavelength range extending from about 420 nanometers (nm) to about 680 nm. In some embodiments, the optical core 10 has the index of refraction n of greater than about 1.5, greater than about 1.6, greater than about 1.8, or greater than about 2 at the at least the first visible wavelength.

[0040] In some embodiments, the at least the first visible wavelength in the visible wavelength range includes at least one blue wavelength in the range from about 420 nm to about 480 nm. In some embodiments, the at least the first visible wavelength in the visible wavelength range includes at least one green wavelength in a range from about 490 nm to about 560 nm. In some embodiments, the at least the first visible wavelength in the visible wavelength range includes at least one red wavelength in a range from about 590 nm to about 670 nm. In some embodiments, the at least the first visible wavelength in the visible wavelength range includes at least one blue wavelength, at least one green wavelength, and at least one red wavelength.

[0041] The optical waveguide 300 further includes an optical grating 30a, 30b disposed on the optical core 10. In some embodiments, the optical grating 30a, 30b may be interchangeably referred to as “the optical multilayer grating 30a, 30b”. The optical grating 30a, 30b may be collectively or individually referred to as “the optical grating 30”.

[0042] The optical grating 30a, 30b is configured to at least do one of: receive the image light 20 emitted by the image projector 40 and inject at least a portion 21 of the received image light 20 into the optical core 10; or extract at least a portion 24 of the image light 22 propagating along the optical core 10 primarily by total internal reflection from the optical core 10 for viewing by a viewer 90. Specifically, the optical grating 30a, 30b may be configured to receive the image light 20 emitted by the image projector 40 and inject at least the portion 21 of the received image light 20 into the optical core 10 and / or extract at least the portion 24 of the image light 22 propagating along the optical core 10 primarily by total internal reflection from the optical core 10 for viewing by a viewer 90.

[0043] The received image light 20 may be interchangeably referred to as “the light ray 20”. The portion 21 of the received image light 20 may be interchangeably referred to as “the light ray 21”. Similarly, the portion 24 of the image light 22 may be interchangeably referred to as “the light ray 24”. Therefore, the optical grating 30a, 30b optically couples the light ray 21 propagating within the optical waveguide 300 by total internal reflection to the light ray 20, 24 propagating outside, and toward or away from, the waveguide 300.

[0044] In some embodiments, the optical waveguide 300 includes two optical multilayer gratings. For example, in the illustrated embodiment of FIG. 1, the optical waveguide 300 includes the optical multilayer grating 30a and the optical multilayer grating 30b. The optical multilayer grating 30a may be interchangeably referred to as “the first multilayer grating 30a”. Similarly, the optical multilayer grating 30b may be interchangeably referred to as “the second multilayer grating 30b”.

[0045] The first and second spaced apart multilayer gratings 30a, 30b are disposed on the optical core 10. The first multilayer grating 30a is configured to receive the image light 20 from the image projector 40 and is configured to inject at least the portion 21 into the optical core 10. In other words, the first multilayer grating 30a is configured to receive the emitted image light 20 and inject at least the portion 21 of the received light 20 into the optical core 10. The injected image light 21 propagates along the optical core 10 primarily by total internal reflection.

[0046] In some embodiments, the first multilayer grating 30a is configured to inject at least 10% of the received image light 20 into the optical core 10. In some embodiments, the first multilayer grating 30a may be configured to inject at least 15%, at least 20%, at least 25%, at least 30%, at least 35%, at least 40%, at least 50%, at least 55%, or at least 60% of the received image light 20 into the optical core 10.

[0047] The second multilayer grating 30b is configured to receive at least a portion 23 of the injected image light 21 and extract at least the portion 24 of the received incident light 23 from the optical core 10 for viewing by the viewer 90. In some embodiments, the second multilayer grating 30b is configured to extract at least 1% of the received injected image light 23 from the optical core 10 for viewing by the viewer 90. In some embodiments, the second multilayer grating 30b is configured to extract at least 2%, at least 5%, at least 10%, at least 15%, at least 20%, at least 25%, at least 30%, at least 35%, at least 40%, at least 50%, at least 55%, or at least 60% of the received injected image light 23 from the optical core 10 for viewing by the viewer 90.

[0048] In some embodiments, the optical core 10 is substantially optically transmissive at the at least the first visible wavelength, such that the optical core 10 absorbs less than about 30% of injected image light 21 at the at least the first visible wavelength as the injected image light 21 propagates along the optical core 10 from the first multilayer grating 30a to the second multilayer grating 30b. In some embodiments, the optical core 10 absorbs less than about 25%, less than about 20%, less than about 15%, less than about 10%, less than about 5%, or less than about 1% of the injected image light 21 at the at least the first visible wavelength as the injected image light 21 propagates along the optical core 10 from the first multilayer grating 30a to the second multilayer grating 30b.

[0049] Each of the first and second multilayer gratings 30a, 30b includes an optically transparent adhesive layer 50a, 50b disposed on the optical core 10. Specifically, the first multilayer grating 30a includes the optically transparent adhesive layer 50a and the second multilayer grating 30b includes the optically transparent adhesive layer 50b. The optically transparent adhesive layer 50a, 50b may be collectively or individually referred to as “the optically transparent adhesive layer 50”.

[0050] The optically transparent adhesive layer 50a, 50b includes an adhesive structured first major surface 51a, 51b facing away from the optical core 10 and an opposing adhesive second major surface 52a, 52b facing, and bonded to, the optical core 10. Specifically, the optically transparent adhesive layer 50a includes the adhesive structured first major surface 51a facing away from the optical core 10 and the adhesive second major surface 52a facing, and bonded to, the optical core 10. Similarly, the optically transparent adhesive layer 50b includes the adhesive structured first major surface 51b facing away from the optical core 10 and the adhesive second major surface 52b facing, and bonded to, the optical core 10. The adhesive structured first major surface 51 a, 5 lb may be collectively or individually referred to as “the adhesive structured first major surface 51”. The opposing adhesive second major surface 52a, 52b may be collectively or individually referred to as “the adhesive second major surface 52”.

[0051] The structured first major surface 51a, 51b defines a plurality of substantially parallel linear adhesive prisms 53a, 53b extending along substantially a same in-plane first direction and arranged along an in-plane orthogonal second direction. In FIG. 1, the first direction extends substantially along the y-axis and the second direction extends substantially along the x-axis. In the illustrated embodiment of FIG. 1, the structured first major surface 51a defines the plurality of substantially parallel linear adhesive prisms 53a and the structured first major surface 51b defines the plurality of substantially parallel linear adhesive prisms 53b. The plurality of substantially parallel linear adhesive prisms 53a, 53b may be collectively or individually referred to as “the linear adhesive prisms 53”.

[0052] In some embodiments, the substantially parallel linear adhesive prisms 53 a, 53b of each of the first and second multilayer gratings 30a, 30b form a periodic pattern along the in-plane second direction (i.e., substantially along the x-axis) along which the prisms 53a, 53b are arranged. In some embodiments, the periodic pattern has a period in a range from about 50 nm to about 2000 nm. In some embodiments, the periodic pattern has the period in the range from about 100 nm to about 1500 nm or from about 200 nm to about 1000 nm.

[0053] In some embodiments, the optically transparent adhesive layer 50a, 50b of each of the first and second multilayer gratings 30a, 30b has a maximum thickness hmax of less than about 10 microns. In some embodiments, the maximum thickness hmax is less than about 5 microns, less than about 4 microns, less than about 3 microns, less than about 2 microns, or less than about 1 micron. In some embodiments, the optically transparent adhesive layer 50a, 50b of each of the first and second multilayer gratings 30a, 30b has the maximum thickness hmax of less than about 2000 nm. In some embodiments, the maximum thickness hmax is less than about 1500 nm, less than about 1000 nm, less than about 900 nm, less than about 800 nm, less than about 700 nm, less than about 600 nm, less than about 500 nm, less than about 400 nm, or less than about 300 nm.

[0054] In some embodiments, the optically transparent adhesive layer 50a, 50b of each of the first and second multilayer gratings 30a, 30b has a minimum thickness hmin of greater than about 5 nm. In some embodiments, the minimum thickness hmin is greater than about 10 nm, greater than about 15 nm, or greater than about 20 nm.

[0055] In some embodiments, the optically transparent adhesive layer 50a, 50b of each of the first and second multilayer gratings 30a, 30b is disposed directly on the optical core 10 such that the adhesive second major surface 52a, 52b of the optically transparent adhesive layer 50a, 50b is directly bonded to, and makes physical contact with, an uppermost major surface 11 of the optical core 10 across substantially an entire length (i.e., substantially along the y-axis) and an entire width (i.e., substantially along the x-axis) of the optically transparent adhesive layer 50a, 50b.

[0056] In some embodiments, the optically transparent adhesive layer 50a, 50b of each of the first and second multilayer gratings 30a, 30b includes one or more of a natural or synthetic rubber-based pressure sensitive adhesive, an acrylic pressure sensitive adhesive, a vinyl alkyl ether pressure sensitive adhesive, a silicone pressure sensitive adhesive, a polyester pressure sensitive adhesive, a polyamide pressure sensitive adhesive, a poly-alpha-olefin pressure sensitive adhesive, a polyurethane pressure sensitive adhesive, and a styrenic block copolymer based pressure sensitive adhesive.

[0057] In some embodiments, the optically transparent adhesive layer 50a, 50b of each of the first and second multilayer gratings 30a, 30b has a unitary construction. Specifically, the optically transparent adhesive layer 50a of the first multilayer grating 30a has the unitary construction and the optically transparent adhesive layer 50b of the second multilayer grating 30b has the unitary construction. In such embodiments, the optically transparent adhesive layer 50a, 50b may be interchangeably referred to as “the optically transparent unitary adhesive layer 50a, 50b”.

[0058] Further, in some embodiments, the optically transparent adhesive layers 50a, 50b of the first and second multilayer gratings 30a, 30b are connected and, in combination, form a unitary construction.

[0059] As shown in FIG. 1, the multilayer grating 30a, 30b further includes an overcoat 60 disposed on the optically transparent unitary adhesive layer 50a, 50b. Specifically, each of the first and second multilayer gratings 30a, 30b includes the overcoat 60 disposed on the structured major surface 51a, 51b of the optically transparent adhesive layer 50a, 50b opposite the optical core 10. In some embodiments, the overcoat 60 includes titanium. In some embodiments, the overcoat 60 includes titanium dioxide (TiO2). In some embodiments, the overcoat 60 includes one or more of TiO2, zirconium oxide (ZrO), Nb2O5, Ta2O5, HfO2, SiAlOxNy, Si3N4, Nb-doped TiO2, and ZrO2.

[0060] Referring to FIGS. 1 and 2A-2C, for a substantially normally incident collimated light beam 25 (shown in FIG. 1) having the at least the first visible wavelength, and for at least a first polarization state, a first order diffraction efficiency of the multilayer grating 30a, 30b is greater than about 40%. In some embodiments, for the substantially normally incident collimated light beam 25 having the at least the first visible wavelength, and for at least the first polarization state, the first order diffraction efficiency of the multilayer grating 30a, 30b is greater than about 45%, greater than about 50%, greater than about 55%, greater than about 60%, greater than about 65%, greater than about 70%, greater than about 75%, greater than about 80%, or greater than about 85%. In some embodiments, the first polarization state is along the x-axis. In some other embodiments, the first polarization state is along the y-axis.

[0061] In some embodiments, for the substantially normally incident collimated light beam 25 having the at least the first visible wavelength, and for each of the first polarization state and an orthogonal second polarization state, the first order diffraction efficiency of the multilayer grating 30a, 30b is greater than about 50%. In some embodiments, for the substantially normally incident collimated light beam 25 having the at least the first visible wavelength, and for each of the first polarization state and the second polarization state, the diffraction efficiency of the multilayer grating 30a, 30b is greater than about 55%, greater than about 60%, greater than about 65%, greater than about 70%, greater than about 75%, greater than about 80%, or greater than about 85%. In some embodiments, the first polarization state is along the x-axis and the second polarization state is along the y-axis.

[0062] Therefore, the first multilayer grating 30a may diffract more light (e.g., the image light 20) towards the second multilayer grating 30b as compared to surface relief gratings without the optically transparent adhesive layer 50 and / or the overcoat 60. Therefore, the optical system 400 may reduce the loss of the image light.

[0063] FIGS. 2A-2C illustrate schematic sectional magnified views of a portion of the optical multilayer grating 30a, 30b shown in FIG. 1, according to an embodiment of the present disclosure.

[0064] Referring to FIGS. 1 and 2A-2C, each of the adhesive prisms 53 includes a top 55 opposite to a base 54 of the prism 53, and at least opposing first and second sides 56, 57, extending from, and connecting, the base 54 to the top 55. The base 54 is wider than the top 55. The base 54 is interchangeably referred to as “the wider base 54”. The top 55 is interchangeably referred to as “the narrower top 55”. Specifically, each linear adhesive prism 53 includes the wider base 54 facing the optical core 10, the narrower top 55 facing away from the optical core 10, and the opposing first and second sides 56, 57. The wider base 54 includes opposing ends 54-1 and 54-2 and the narrower top 55 includes opposite ends 55-1 and 55-2. The opposing first and second sides 56, 57 converge toward each other as they extend from the opposite ends 54-1, 54-2 of the base 54 to the opposite corresponding ends 55-1, 55-2 of the top 55.

[0065] In some embodiments, for each of the linear adhesive prisms 53 of the optically transparent adhesive layer 50a, 50b of each of the first and second multilayer gratings 30a, 30b, the narrower top 55 has a width of less than about 50 nm. In some embodiments, the narrower top 55 has the width of less than about 45 nm, less than about 40 nm, less than about 35 nm, less than about 30 nm, less than about 25 nm, less than about 20 nm, less than about 15 nm, less than about 10 nm, less than about 5 nm, less than about 4 nm, less than about 3 nm, less than about 2 nm, or less than about 1 nm.

[0066] In some embodiments, for each of the linear adhesive prisms 53 of the optically transparent adhesive layer 50a, 50b of each of the first and second multilayer gratings 30a, 30b, the wider base 54 of the prism 53 is wider than the narrower top 55 of the prism 53 by at least 5 nm. In some embodiments, the wider base 54 is wider than the narrower top 55 by at least 10 nm, at least 15 nm, at least 20 nm, at least 30 nm, at least 40 nm, or at least 50 nm. As shown in FIGS. 2 A and 2B, in some embodiments, for each of the linear adhesive prisms 53 of the optically transparent adhesive layer 50a, 50b of each of the first and second multilayer gratings 30a, 30b, the narrower top 55 is substantially planar.

[0067] Further, as shown in FIG. 2A, the first and second sides 56, 57 make respective angles al and a2 with the second direction (i.e., substantially along the x-axis). The angle al is greater than about 60 degrees. In some embodiments, the angle al may be greater than about 65 degrees, greater than about 70 degrees, greater than about 75 degrees, greater than about 80 degrees, or greater than about 85 degrees.

[0068] Further, the angle al is greater than the angle a2 by at least 5 degrees. In some embodiments, the angle al may be greater than the angle a2 by at least 10 degrees, at least 15 degrees, at least 20 degrees, at least 25 degrees, at least 30 degrees, at least 35 degrees, or at least 40 degrees.

[0069] Referring to FIGS. 1 and 2A-2C, in some embodiments, the optically transparent adhesive layer 50a, 50b of each of the first and second multilayer gratings 30a, 30b includes a continuous adhesive land portion 58 extending continuously across substantially the entire length (i.e., substantially along the y-axis) and the entire width (i.e., substantially along the x-axis) of the adhesive layer 50a, 50b and unitarily joining the wider bases 54 of the adhesive prisms 53. In some embodiments, the continuous adhesive land portion 58 of the optically transparent adhesive layer 50a, 50b of each of the first and second multilayer gratings 30a, 30b has an average thickness of greater than about 5 nm. In some embodiments, the average thickness of the continuous adhesive land portion 58 is greater than about 10 nm, greater than about 15nm, or greater than about 20 nm.

[0070] In some embodiments, as shown in FIG. 1, the optical waveguide 300 further includes an optically transparent connecting adhesive layer 59 that is disposed on the optical core 10 between the first and second multilayer gratings 30a, 30b and extends continuously across substantially an entire separation 70 between, and joins, the first and second multilayer gratings 30a, 30b.

[0071] In some embodiments, the optically transparent adhesive layers 50a, 50b of the first and second multilayer gratings 30a, 30b include a common continuous adhesive land portion 58, 59 that extends continuously across substantially the entire separation 70 between the first and second multilayer gratings 30a, 30b and substantially across the entire length and the entire width of each of the first and second multilayer gratings 30a, 30b and unitarily joins the wider bases 54 of all the adhesive prisms 53 in the first and second multilayer gratings 30a, 30b.

[0072] In some embodiments, the optically transparent unitary adhesive layer 50a, 50b includes the plurality of spaced apart adhesive prisms 53a, 53b and the continuous adhesive land portion 58 extending continuously across substantially the entire length and the entire width of the unitary adhesive layer 50a, 50b and unitarily joining the bases 54 of the adhesive prisms 53.

[0073] For at least the first visible wavelength in the visible wavelength range, the optically transparent adhesive layer 50a, 50b and the overcoat 60 have respective indices nl and n2. The value of the index nl is greater than or equal to 1.2, i.e., nl > 1.2. In some embodiments, nl > 1.25, nl > 1.30, nl > 1.35, nl > 1.4, nl > 1.45, or nl > 1.5. The value of the index n2 is greater than or equal to 1.6, i.e., n2 > 1.6. In some embodiments, n2 > 1.65, n2 > 1.7, n2 > 1.75, n2 > 1.8, n2 > 1.85, n2 > 1.9, n2 > 2, n2 > 2.05, n2 > 2.1, n2 > 2.15, n2 > 2.2, n2 > 2.25, n2 > 2.3, or n2 > 2.35. In some embodiments, n2 / nl is greater than 1, i.e., n2 / nl > 1. In some embodiments, n2 / nl > 1.1, n2 / nl > 1.2, n2 / nl > 1.3, n2 / nl > 1.4, n2 / nl > 1.5, n2 / nl > 1.6, n2 / nl > 1.7, or n2 / nl > 1.8.

[0074] In some embodiments, each of the linear adhesive prisms 53 has a maximum thickness hl . The maximum thickness may be measured from the base 54 to the top 55 of each of the linear adhesive prisms 53. In some embodiments, the maximum thickness hl of each of the linear adhesive prisms 53 is equal to a difference between the maximum thickness hmax (shown in FIG. 1) of the optically transparent adhesive layer 50a, 50b and the minimum thickness hmin (shown in FIG. 1) of the optically transparent adhesive layer 50a, 50b.

[0075] A first portion 61 of the overcoat 60 substantially conformably covers at least 80% of the second side 57 of each of the adhesive prisms 53 and includes substantially parallel opposing first and second major surfaces 61-1, 61-2. In other words, for each of the linear adhesive prisms 53, the first portion 61 of the overcoat 60 includes the opposing substantially parallel first and second major surfaces 61-1, 61-2 and substantially conformably covers at least 80% of the second side 57 of the linear adhesive prism 53. In some embodiments, the first portion 61 substantially conformably covers at least 85%, at least 90%, at least 95%, at least 98%, or at least 99% of the second side 57 of the linear adhesive prism 53.

[0076] The first portion 61 has an average thickness h2c. In some embodiments, the first portion 61 of the overcoat 60 has the average thickness h2c of between about 5 nm and about 1000 nm. In some embodiments, the first portion 61 has the average thickness h2c between about 10 nm and about 900 nm, about 50 nm and about 800 nm, or about 100 nm and about 700 nm.

[0077] In some embodiments, the first portion 61 may be interchangeably referred to as “the planar facet 61”. The optical grating 30a, 30b includes the plurality of spaced apart substantially parallel and substantially planar facets 61 extending along the in-plane same first direction (i.e., substantially along the y-axis) and arranged along the in-plane orthogonal second direction (i.e., substantially along the x- axis). Each substantially planar facet 61 is defined by a support material. In some embodiments, the support material may be the optically transparent adhesive layer 50. The support material may therefore be interchangeably referred to as “the optically transparent adhesive layer 50”. The support materials 50 and the substantially planar facets 61 have the respective indices nl and n2, where nl > 1.2 and n2 / nl > 1.

[0078] Each substantially planar facet 61 includes an average thickness T (depicted by h2c in FIG. 2A) along a normal 63 to the substantially planar facet 61. In some embodiments, the substantially planar facets 61 make an angle A (depicted by b2 in FIG. 2 A) with a third direction orthogonal to the in-plane first and second directions. The third direction substantially extends along the z-axis. A ratio of the average thickness T and the angle A is greater than 1 nm / deg, i.e., T / A > 1 nm / deg. In some embodiments, T / A > 2 nm / deg, T / A > 3 nm / deg, T / A > 4 nm / deg, T / A > 5 nm / deg, T / A > 6 nm / deg, T / A > 7 nm / deg, T / A > 8 nm / deg, T / A > 9 nm / deg, or T / A > 10 nm / deg. Further, any second portion 62 of the overcoat 60 that substantially conformably covers at least 50% of the first side 56 of each of the linear adhesive prisms 53 of the optically transparent adhesive layer 50a, 50b of each of the first and second multilayer gratings 30a, 30b has an average thickness h2a. In some embodiments, any second portion 62 of the overcoat 60 that substantially conformably covers at least 50% of the first side 56 of each of the linear adhesive prisms 53, includes substantially parallel opposing first and second major surfaces 62-1, 62-2, and has the average thickness h2a of less than about 500 nm.

[0079] In some embodiments, the average thickness h2a is less than about 350 nm, less than about 300 nm, less than about 250 nm, less than about 200 nm, less than about 150 nm, less than about 100 nm, or less than about 50 nm. A ratio of the average thickness h2c and the average thickness h2a is greater than or equal to 1.5, i.e., h2c / h2a > 1.5. In some embodiments, h2c / h2a > 2, h2c / h2a > 3, h2c / h2a > 5, h2c / h2a > 10, h2c / h2a > 15, or h2c / h2a > 20.

[0080] In some embodiments, the second portion 62 may substantially conformably cover at least 60%, at least 70%, at least 80%, or at least 90% of the first side 56 of any of the linear adhesive prisms 53 of the optically transparent adhesive layer 50a, 50b of each of the first and second gratings 30a, 30b. In some embodiments, the second portion 62 makes an angle bl with the third direction orthogonal to the in-plane first and second directions.

[0081] Referring to FIG. 2B, in some embodiments, a third portion 65 of the overcoat 60 includes opposing substantially parallel first and second major surfaces 65-1, 65-2. The third portion 65 substantially conformably covers at least 80% of the narrower top 55 of each of the linear adhesive prisms 53 of the optically transparent adhesive layer 50a, 50b of each of the first and second multilayer gratings 30a, 30b. In some embodiments, the third portion 65 may substantially conformably cover at least 85%, at least 90%, at least 95%, at least 98%, or at least 99% of the narrower top 55 of each of the linear adhesive prisms 53 of the optically transparent adhesive layer 50a, 50b of each of the first and second multilayer gratings 30a, 30b. In some embodiments, the third portion 65 has an average thickness h2b (shown in FIG. 2B). The third portion 65 of the overcoat 60 has the average thickness h2b of between about 5 nm and about 1000 nm. In some embodiments, the third portion 65 has the average thickness h2b between about 10 nm and about 900 nm, about 50 nm and about 800 nm, or about 100 nm and about 700 nm.

[0082] In some embodiments, for each pair of the neighboring linear adhesive prisms 53 in the plurality of substantially parallel linear adhesive prisms 53 of the optically transparent adhesive layer 50a, 50b of each of the first and second multilayer gratings 30a, 30b, a fourth portion 64 of the overcoat 60 substantially conformably covers at least 40% of a portion of the continuous adhesive land portion 58 between the adhesive prisms 53 in the pair of neighboring linear adhesive prisms 53.

[0083] In some embodiments, for each pair of neighboring linear adhesive prisms 53 in the plurality of substantially parallel linear adhesive prisms 53 of the optically transparent adhesive layer 50a, 50b of each of the first and second multilayer gratings 30a, 30b, the fourth portion 64 of the overcoat 60 includes opposing substantially parallel first and second major surfaces 64-1, 64-2 and substantially conformably covers at least 40% of the portion of the continuous adhesive land portion 58 that joins the adhesive prisms 53 in the pair of adhesive prisms 53. In some embodiments, the fourth portion 64 substantially conformably covers at least 45%, at least 50%, at least 55%, at least 60%, at least 65%, or at least 70% of the portion of the continuous adhesive land portion 58 that joins the adhesive prisms 53 in the pair of adhesive prisms 53.

[0084] In some embodiments, the fourth portion 64 has an average thickness h2d of less than about 1000 nm. In some embodiments, the fourth portion 64 has the average thickness h2d of less than about 900 nm, less than about 800 nm, less than about 700 nm, less than about 600 nm, less than about 500 nm, less than about 400 nm, less than about 300 nm, or less than about 200 nm.

[0085] In some embodiments, no portion of the overcoat 60 having an average coating thickness of greater than about 20 nm covers any portion, other than the second side 57, of any linear adhesive prism 53 in the optically transparent adhesive layer 50a, 50b in each of the first and second gratings 30a, 30b. In some embodiments, no portion of the overcoat 60 having the average coating thickness of greater than about 30 nm, greater than about 40 nm, greater than about 50 nm, greater than about 60 nm, greater than about 70 nm, greater than about 80 nm, greater than about 90 nm, or greater than about 100 nm covers any portion, other than the second side 57.

[0086] Further, as shown in FIG. 2C, in some embodiments, for each of the linear adhesive prisms 53 of the optically transparent adhesive layer 50a, 50b of each of the first and second multilayer gratings 30a, 30b, the opposing first and second sides 56, 57 of the prisms 53 converge toward each other and form an acute angle el greater than about 30 degrees therebetween. In some embodiments, the acute angle el is greater than about 40 degrees, greater than about 50 degrees, greater than about 60 degrees, greater than about 70 degrees, greater than about 80 degrees, or greater than about 85 degrees.

[0087] In some other embodiments, for each of the linear adhesive prisms 53 of the optically transparent adhesive layer 50a, 50b of each of the first and second multilayer gratings 30a, 30b, the opposing first and second sides 56, 57 of the prisms 53 converge toward each other and form the acute angle el of less than about 35 degrees therebetween. In some embodiments, the acute angle el is less than about 30 degrees, less than about 25 degrees, or less than about 20 degrees.

[0088] Since the overcoat 60 may be selectively disposed on the structured first major surface 51 of the optically transparent adhesive layer 50, the optical waveguide 300 may not require an undercut. Hence, manufacturing the optical waveguide 300 may be less time-consuming and less complex than a process of manufacturing undercuts of slanted gratings.

[0089] FIG. 3 illustrates a schematic sectional magnified view of a portion of the optical multilayer grating 30 shown in FIG. 1, according to another embodiment of the present disclosure.

[0090] Referring to FIGS. 1-3, the structured first major surface 51 of the optically transparent adhesive layer 50a, 50b defines a plurality of substantially parallel linear adhesive prisms 53’. The linear adhesive prisms 53’ are substantially similar to the linear adhesive prisms 53 as illustrated in FIGS. 2A-2C. However, in the illustrated embodiment of FIG. 3, for each of the linear adhesive prisms 53’ of the optically transparent adhesive layer 50a, 50b of each of the first and second multilayer gratings 30a, 30b, a narrower top 55’ is a curved surface. In some embodiments, for at least one of the linear adhesive prisms 53’ of the optically transparent adhesive layer 50a, 50b of at least one of the first and second multilayer gratings 30a, 30b, at least one of the opposing first and second sides 56’, 57’ of the prism 53’ is a curved surface.

[0091] FIG. 4 illustrates a schematic top view of the optical system 400, according to another embodiment of the present disclosure. The optical system 400 of FIG. 4 is substantially similar to the optical system 400 shown in FIG. 1. However, the first multilayer grating 30a is arranged at an angle with the second multilayer grating 30b. Specifically, the plurality of substantially parallel linear adhesive prisms 53b (shown in FIG. 1) extend along substantially a same in-plane first direction substantially along a y’-axis and are arranged along an in-plane orthogonal second direction substantially along a x’-axis.

[0092] Referring to FIGS. 1-4, in some embodiments, the in-plane first directions of the first and second multilayer gratings 30a, 30b make an angle cl of less than about 20 degrees with each other. In some embodiments, the angle cl may be less than about 15 degrees, less than about 10 degrees, or less than about 5 degrees with each other.

[0093] In some embodiments, the same first direction (i.e., substantially along the y-axis) along which the substantially parallel linear adhesive prisms 53 of the first multilayer grating 30a extend makes the angle cl of less than about 15 degrees with the same first direction (i.e., substantially along the y’-axis) along which the substantially parallel linear adhesive prisms 53 of the second multilayer grating 30b extend.

[0094] In some embodiments, the angle cl between the first direction along which the substantially parallel linear adhesive prisms 53 of the first multilayer grating 30a extend and the first direction along which the substantially parallel linear adhesive prisms 53 of the second multilayer grating 30b extend is less than about 10 degrees or less than about 5 degrees. In such cases, the first multilayer grating 30a may be positioned substantially parallel to the second multilayer grating 30b. Specifically, the substantially parallel linear adhesive prisms 53 of the first multilayer grating 30a may extend substantially parallel to the substantially parallel linear adhesive prisms 53 of the second multilayer grating 30b. In other words, the in-plane first directions of the first and second multilayer gratings 30a, 30b make the angle cl of less than about 15 degrees with each other.

[0095] Similarly, in some embodiments, the in-plane second directions of the first and second multilayer gratings 30a, 30b make an angle c2 of less than about 20 degrees with each other. In some embodiments, the angle c2 between the second directions is less than about 15 degrees, less than about 10 degrees, or less than about 5 degrees with each other.

[0096] In some embodiments, the same second direction (i.e., substantially along the x-axis) along which the substantially parallel linear adhesive prisms 53 of the first multilayer grating 30a are arranged makes the angle c2 of less than about 15 degrees with the same second direction (i.e., substantially along the x’-axis) along which the substantially parallel linear adhesive prisms 53 of the second multilayer grating 30b are arranged. In other words, the in-plane second directions of the first and second multilayer gratings 30a, 30b make the angle c2 of less than about 15 degrees with each other.

[0097] In some other embodiments, the same first direction along which the substantially parallel linear adhesive prisms 53 of the first multilayer grating 30a extend makes the angle cl of greater than about 15 degrees with the same first direction along which the substantially parallel linear adhesive prisms 53 of the second multilayer grating 30b extend. In some embodiments, the angle cl is greater than about 20 degrees, greater than about 25 degrees, greater than about 30 degrees, greater than about 35 degrees, greater than about 40 degrees, greater than about 45 degrees, greater than about 50 degrees, greater than about 55 degrees, or greater than about 60 degrees. In other words, the in-plane first directions of the first and second multilayer gratings 30a, 30b make the angle cl of greater than about 15 degrees with each other.

[0098] Similarly, in some other embodiments, the in-plane second directions of the first and second multilayer gratings 30a, 30b make the angle c2 of greater than about 10 degrees with each other. In some embodiments, the angle c2 between the second directions of the first and second multilayer gratings 30a, 30b is greater than about 15 degrees, greater than about 20 degrees, greater than about 25 degrees, greater than about 30 degrees, greater than about 35 degrees, greater than about 40 degrees, or greater than about 45 degrees with each other.

[0099] In some embodiments, the same second direction along which the substantially parallel linear adhesive prisms 53 of the first multilayer grating 30a are arranged makes the angle c2 of greater than about 15 degrees with the same second direction along which the substantially parallel linear adhesive prisms 53 of the second multilayer grating 30b are arranged. In some embodiments, the in-plane second directions of the first and second multilayer gratings 30a, 30b make the angle c2 of greater than about 10 degrees with each other. In some embodiments, the in-plane second directions of the first and second multilayer gratings 30a, 30b make the angle c2 of greater than about 15 degrees, greater than about 20 degrees, greater than about 25 degrees, greater than about 30 degrees, greater than about 35 degrees, greater than about 40 degrees, or greater than about 45 degrees with each other.

[0100] In some embodiments, the first and second multilayer gratings 30a, 30b are spaced apart by at least a distance DI of 1 millimeter (mm). In some embodiments, the first and second multilayer gratings 30a, 30b are spaced apart by the distance DI of at least 2 mm, at least 5 mm, at least 10 mm, at least 15 mm, or at least 20 mm. In some other embodiments, the first and second gratings 30a, 30b are spaced apart by the distance DI of less than about 50 mm. In some embodiments, the first and second gratings 30a, 30b may be spaced apart by the distance DI of less than about 45 mm, less than about 40 mm, less than about 35 mm, less than about 30 mm, less than about 25 mm, less than about 20 mm, less than about 15 mm, or less than about 10 mm.

[0101] FIG. 5 illustrates a schematic top view of an optical system 410, according to yet another embodiment of the present disclosure. The optical system 410 of FIG. 5 is substantially similar to the optical system 400 shown in FIG. 1. However, the optical system 410 includes an optical waveguide 310 instead of the optical waveguide 300 shown in FIG. 1. The optical waveguide 310 is substantially similar to the optical waveguide 300. However, the optical waveguide 310 further includes a third multilayer grating 30c spaced apart from the first and second multilayer gratings 30a, 30b. The third multilayer grating 30c is substantially similar to the first and second multilayer gratings 30a, 30b in construction. The third multilayer grating 30c also includes the optically transparent adhesive layer 50 (shown in FIG. 1) and the overcoat 60 (shown in FIG. 1) disposed on the optically transparent adhesive layer 50. The optically transparent adhesive layer 50 of the third multilayer grating 30c is disposed on the optical core 10 and includes the plurality of substantially parallel linear adhesive prisms 53 (shown in FIG. 1).

[0102] The optical waveguide 310 is configured to receive at least a portion 26 of the injected image light 21 along a first propagating direction 26a and redirect the injected image light 21 as a redirected image light 27 propagating along a different second propagating direction 27a along the optical core 10 primarily by total internal reflection. The second multilayer grating 30b is configured to receive at least a portion 28 of the redirected image light 27 and extract at least the portion 24 of the received redirected image light 28 from the optical core 10 for viewing by the viewer 90.

[0103] In some embodiments, the optically transparent adhesive layers 50 of the first, second, and third multilayer gratings 30a, 30b, 30c include a common continuous adhesive land portion 58” that extends continuously across substantially entire separations 71, 72 between the first, second, and third multilayer gratings 30a, 30b, 30c and substantially across an entire length and an entire width of each of the first, second, and third multilayer gratings 30a, 30b, 30c. The common continuous adhesive land portion 58” unitarily joins the wider bases 54 (shown in FIG. 2A) of all the adhesive prisms 53 in the first, second, and third multilayer gratings 30a, 30b, 30c. The entire length and the entire width of the first multilayer grating 30a extend along the x-axis and the y-axis, respectively. The entire length and the entire width of the second multilayer grating 30b extend along the x’-axis and the y’-axis, respectively. The entire length and the entire width of the third multilayer grating 30c extend along x”- axis and y”-axis, respectively.

[0104] FIGS. 6A-6D illustrate exemplary plots depicting diffraction efficiencies of exemplary optical waveguides (e.g., the optical waveguides 300, 310) having different parameters of multilayer gratings (e.g., the first, second, and third multilayer gratings 30a, 30b, 30c).

[0105] Referring to FIGS. 1 to 6A-6B, a plot 602 shown in FIGS. 6 A and 6B depicts diffraction efficiencies of the multilayer gratings of the optical waveguides having different ratios n2 / nl of the respective indices n2, nl of the overcoat 60 and the optically transparent adhesive layer 50. The ratio n2 / nl is expressed as n2 / nl in the abscissa. The diffraction efficiency is expressed in percentage (%) in the ordinate.

[0106] The index n2 of the overcoat 60 is kept constant and the diffraction efficiencies for various indices nl of the optically transparent adhesive layer 50 is varied to measure the diffraction efficiencies of the multilayer gratings. As is apparent from the plot 602 of FIG. 6A, a diffraction efficiency of greater than about 40% is achieved when the ratio n2 / nl is greater than about 1. Further, as is apparent from the plot 602 of FIG. 6B, a diffraction efficiency of greater than about 50% is achieved when the ratio n2 / nl is greater than about 1.2.

[0107] Referring to FIGS. 1 to 6C, a plot 604 shown in FIG. 6C depicts diffraction efficiencies of the multilayer gratings of the optical waveguides having different ratios h2c / h2a of the height h2c of the first portion 61 and the height h2a of the second portion 62 of the overcoat 60. The ratio h2c / h2a is expressed as h2c / h2a in the abscissa. The diffraction efficiency is expressed in percentage (%) in the ordinate.

[0108] The height h2c is kept constant and the height h2a is varied to measure diffraction efficiencies of the multilayer gratings. As is apparent from the plot 604 of FIG. 6C, a diffraction efficiency is higher for a higher ratio of h2c / h2a. Specifically, a diffraction efficiency of greater than about 40% is achieved when the ratio h2c / h2a is greater than about 2.

[0109] Referring to FIGS. 1 to 6D, a plot 606 shown in FIG. 6D depicts diffraction efficiencies of the multilayer gratings of the optical waveguides having different ratios h2c / b2 of the height h2c of the first portion 61 of the overcoat 60 and the angle b2. The ratio h2c / b2 is expressed as h2c / b2 in the abscissa. The diffraction efficiency is expressed in percentage (%) in the ordinate.

[0110] The height h2c is kept constant and the angle b2 is varied to measure diffraction efficiencies of the multilayer gratings. As is apparent from the plot 606 of FIG. 6D, a diffraction efficiency is higher for a higher ratio of h2c / b2. Specifically, a diffraction efficiency of greater than about 50% is achieved when the ratio h2c / b2 is greater than about 1.

[0111] FIGS. 7A-7D illustrate exemplary heatmap graphs 702, 704, 706, 708, respectively. The heatmap graphs 702, 704, 706, 708 depict diffraction efficiencies for respective exemplary optical waveguides (i), (ii), (iii), (iv) having different parameters of multilayer gratings.

[0112] In FIGS. 7A-7D, the diffraction efficiency is expressed in percentage (%) in the right ordinate. Further, the period of the periodic pattern of the substantially parallel linear adhesive prisms 53 is about 400 nm, the minimum thickness hmin of the optically transparent adhesive transparent layer 50 is about 20 nm, and the index of refraction n of the optical core 10 is about 2.

[0113] FIG. 7A illustrates a heat map graph 702 showing an impact of the index nl of the optically transparent adhesive layer 50 and the index n2 of the coating 60 on the diffraction efficiency of the optical waveguide (i). The index nl is shown in the left ordinate and the index n2 is shown in the abscissa. The third portion 65 (shown in FIG. 2B) of the coating 60 has the average thickness h2b of about 170 nm and the angle b2 is about 15 degrees. As is apparent from the heat map graph 702, the optical waveguide (i) has a greater diffraction efficiency for a lower index nl and a greater index n2.

[0114] FIG. 7B illustrates a heat map graph 704 showing an impact of the average thickness h2a of the second portion 62 (shown in FIG. 2A) with respect to the average thickness h2b of the third portion 65 (shown in FIG. 2B) of the overcoat 60 on the diffraction efficiency of the optical waveguide (ii). The average thickness h2a of the second portion 62 is expressed in nm and shown in the left ordinate. The average thickness h2b of the third portion 65 is expressed in nm and shown in the abscissa. The index nl of the optically transparent adhesive layer 50 is about 1.457 and the index n2 of the coating 60 is about 2.4. Further, the maximum thickness hl is about 280 nm, and the angle b2 is about 15 degrees. As is apparent from the heat map graph 704, the optical waveguide (ii) has a greater diffraction efficiency for a lower average thickness h2a and a greater average thickness h2b.

[0115] FIG. 7C illustrates a heat map graph 706 showing an impact of the angle b2 with respect to the average thickness h2b on the diffraction efficiency of the optical waveguide (iii). The angle b2 is expressed in degrees and shown in the left ordinate and the average thickness h2b is expressed in nm and shown in the abscissa. The index nl of the optically transparent adhesive layer 50 is about 1.457 and the index n2 of the coating 60 is about 2.4. Further, the maximum thickness hl is about 280 nm. As is apparent from the heat map graph 706, the optical waveguide (iii) has a greater diffraction efficiency for a lower angle b2 and a greater average thickness h2b.

[0116] FIG. 7D illustrates a heat map graph 708 showing an impact of the maximum thickness hl and the average thickness h2b on the diffraction efficiency of the optical waveguide (iv). The index nl of the optically transparent adhesive layer 50 is about 1.457 and the index n2 of the coating 60 is about 2.4. Further, the angle b2 is about 15 degrees. The maximum thickness hl is shown in the left ordinate and the average thickness h2b is shown in the abscissa. As is apparent from the heat map graph 708, the optical waveguide (iv) has a higher diffraction efficiency for a greater maximum thickness hl and a greater average thickness h2b.

[0117] Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims are to be understood as being modified by the term “about”. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein.

[0118] Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and / or equivalent implementations can be substituted for the specific embodiments shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this disclosure be limited only by the claims and the equivalents thereof.

Claims

CLAIMS1. An optical waveguide comprising: an optical core configured to propagate an image light therealong; and first and second spaced apart multilayer gratings disposed on the optical core, the first multilayer grating configured to receive an image light emitted by an image projector and inject at least a portion of the received image light into the optical core, the injected image light propagating along the optical core primarily by total internal reflection, the second multilayer grating configured to receive at least a portion of the injected image light and extract at least a portion of the received injected image light from the optical core for viewing by a viewer, each of the first and second multilayer gratings comprising: an optically transparent adhesive layer disposed on the optical core and comprising an adhesive structured first major surface facing away from the optical core and an opposing adhesive second major surface facing, and bonded to, the optical core, the structured first major surface defining a plurality of substantially parallel linear adhesive prisms extending along substantially a same in-plane first direction and arranged along an in-plane orthogonal second direction, each linear adhesive prism comprising a wider base facing the optical core, a narrower top facing away from the optical core, and opposing first and second sides converging toward each other as they extend from opposite ends of the base to opposite corresponding ends of the top, the first and second sides making respective angles al and a2 with the second direction, the angle al greater than about 60 degrees and greater than the angle a2 by at least 5 degrees; and an overcoat disposed on the structured first major surface of the optically transparent adhesive layer opposite the optical core such that for each of the linear adhesive prisms, a first portion of the overcoat comprises opposing substantially parallel first and second major surfaces and substantially conformably covers at least 80% of the second side of the linear adhesive prism, wherein, for at least a first visible wavelength in a visible wavelength range extending from about 420 nm to about 680 nm, the optically transparent adhesive layer and the overcoat have respective indices nl and n2, nl > 1.2, n2 > 1.6.

2. The optical waveguide of claim 1, further comprising an optically transparent connecting adhesive layer that is disposed on the optical core between the first and second multilayer gratings and extends continuously across substantially an entire separation between, and joins, the first and second multilayer gratings.

3. The optical waveguide of claim 1, wherein the optically transparent adhesive layer of each of the first and second multilayer gratings is disposed directly on the optical core such that the adhesivesecond major surface of the optically transparent adhesive layer is directly bonded to, and makes physical contact with, an uppermost major surface of the optical core across substantially an entire length and an entire width of the optically transparent adhesive layer.

4. The optical waveguide of claim 1, wherein for each of the linear adhesive prisms of the optically transparent adhesive layer of each of the first and second multilayer gratings, the opposing first and second sides of the prism converge toward each other and form an acute angle of less than about 35 degrees therebetween.

5. The optical waveguide of claim 1, wherein the overcoat comprises titanium.

6. The optical waveguide of claim 1, wherein no portion of the overcoat having an average coating thickness of greater than about 20 nm covers any portion, other than the second side, of any linear adhesive prism in the optically transparent adhesive layer in each of the first and second multilayer gratings.

7. The optical waveguide of claim 1, further comprising a third multilayer grating spaced apart from the first and second multilayer gratings and configured to receive at least a portion of the injected image light along a first propagating direction and redirect the injected image light as a redirected image light propagating along a different second propagating direction along the optical core primarily by total internal reflection, the second multilayer grating configured to receive at least a portion of the redirected image light and extract at least a portion of the received redirected image light from the optical core for viewing by the viewer, wherein the third multilayer grating comprises: an optically transparent adhesive layer disposed on the optical core and comprising a plurality of substantially parallel linear adhesive prisms; and an overcoat disposed on the optically transparent adhesive layer.

8. An optical multilayer grating for optically coupling a light ray propagating within an optical core by total internal reflection to a light ray propagating outside, and toward or away from, the waveguide, the multilayer grating comprising: an optically transparent unitary adhesive layer comprising a plurality of spaced apart adhesive prisms and a continuous adhesive land portion extending continuously across substantially an entire length and an entire width of the unitary adhesive layer and unitarily joining bases of the adhesive prisms, each of the adhesive prisms comprising a top opposite to the base of the prism, and at least opposing first and second sides extending from, and connecting, the base to the top; and an overcoat disposed on the optically transparent unitary adhesive layer such that:a first portion of the overcoat substantially conformably covers at least 80% of the second side of each of the adhesive prisms, comprises substantially parallel opposing first and second major surfaces, and has an average thickness h2c; and any second portion of the overcoat that substantially conformably covers at least 50% of the first side of each of the adhesive prisms, comprises substantially parallel opposing first and second major surfaces, and has an average thickness h2a, h2c / h2a > 1.5; wherein, for at least a first visible wavelength in a visible wavelength range extending from about 420 nm to about 680 nm, the optically transparent adhesive layer and the overcoat have respective indices nl and n2, nl > 1.2, n2 / nl > 1; and wherein, for a substantially normally incident collimated light beam having the at least the first visible wavelength, and for at least a first polarization state, a first order diffraction efficiency of the multilayer grating is greater than about 40%.

9. An optical grating for optically coupling a light ray propagating within an optical waveguide by total internal reflection to a light ray propagating outside, and toward or away from, the waveguide, the optical grating comprising: a plurality of spaced apart substantially parallel and substantially planar facets extending along an in-plane same first direction and arranged along an in-plane orthogonal second direction, each substantially planar facet comprising an average thickness T along a normal to the substantially planar facet and making an angle A with a third direction orthogonal to the in-plane first and second directions, T / A > 1 nm / deg, each substantially planar facet defined by a support material, wherein, for at least a first visible wavelength in a visible wavelength range extending from about 420 nm to about 680 nm, the support materials and the substantially planar facets have respective indices nl and n2, nl > 1.2, n2 / nl > 1; and wherein, for a substantially normally incident collimated light beam having the at least the first visible wavelength, and for at least a first polarization state, a first order diffraction efficiency of the optical grating is greater than about 50%.

10. An optical waveguide comprising: an optical core configured to propagate an image light therealong; and the optical grating of claim 9 disposed on the optical core, the optical grating configured to at least do one of: receive an image light emitted by an image projector and inject at least a portion of the received image light into the optical core; or extract at least a portion of an image light propagating along the optical core primarily by total internal reflection from the optical core for viewing by a viewer.

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