WO2025029761A1

WO2025029761A1 - Waveguide structures incorporating multiple grating types and methods of manufacture

Info

Publication number: WO2025029761A1
Application number: PCT/US2024/040093
Authority: WO
Inventors: Alastair John GRANT; Gerald BUXTON; Baeddan George HILL; Hyesog Lee; Yuyu LI; Joseph Stephen Thomas SMALLEY; Milan Momcilo Popovich
Original assignee: Digilens Inc.
Priority date: 2023-07-28
Filing date: 2024-07-29
Publication date: 2025-02-06

Abstract

Various embodiments include a waveguide structure. The waveguide structure may include a waveguide substrate including a top surface and a bottom surface; a first grating positioned on the bottom surface of the waveguide substrate; a second grating positioned on the top surface of the waveguide substrate; and a third grating positioned on the top surface of the waveguide substrate, wherein the second grating and the third grating at least partially overlap the first grating, wherein the first grating is a volume grating, wherein the second grating is a volume grating, and wherein the third grating is a polymer fringe grating. The waveguide structure may be a waveguide display such that the third grating is an input grating, the first grating is a fold grating, and the second grating is an output grating.

Description

WAVEGUIDE STRUCTURES INCORPORATING MULTIPLE GRATING TYPES AND METHODS OF MANUFACTURE

CROSS-REFERENCED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application 63/516,386 filed on Jul. 28, 2023, the disclosure of which is incorporated by reference in its entirety.

FIELD OF THE INVENTION

[0002] The present invention generally relates to waveguide structures which include multiple grating types.

BACKGROUND

[0003] Waveguides can be referred to as structures with the capability of confining and guiding waves (i.e., restricting the spatial region in which waves can propagate). One subclass includes optical waveguides, which are structures that can guide electromagnetic waves, typically those in the visible spectrum. Waveguide structures can be designed to control the propagation path of waves using a number of different mechanisms. For example, planar waveguides can be designed to utilize diffraction gratings to diffract and couple incident light into the waveguide structure such that the incoupled light can proceed to travel within the planar structure via total internal reflection (TIR).

[0004] Fabrication of waveguides can include the use of material systems that allow for the recording of holographic optical elements within or on the surface of the waveguides. One class of such material includes polymer dispersed liquid crystal (PDLC) mixtures, which are mixtures containing photopolymerizable monomers and liquid crystals. A further subclass of such mixtures includes holographic polymer dispersed liquid crystal (HPDLC) mixtures. Holographic optical elements, such as volume phase gratings, can be recorded in such a liquid mixture by illuminating the material with two or more mutually coherent laser beams. During the recording process, the monomers polymerize, and the mixture undergoes a photopolymerization-induced phase separation, creating regions densely populated by liquid crystal (LC) micro-droplets, interspersed with regions of clear polymer. The alternating liquid crystal-rich and liquid crystal-depleted regions form the fringe planes of the grating.

[0005] Waveguide optics, such as those described above, can be considered for a range of display systems and sensor applications. In many applications, waveguides containing one or more grating layers encoding multiple optical functions can be realized using various waveguide architectures and material systems, enabling new innovations in near-eye displays for Augmented Reality (AR) and Virtual Reality (VR), compact Heads Up Displays (HUDs) for aviation and road transport, and sensors for biometric and laser radar (LIDAR) applications. As many of these applications are directed at consumer products, there is a growing requirement for efficient low cost means for manufacturing holographic waveguides in large volumes.

SUMMARY OF THE INVENTION

[0006] In some aspects, the techniques described herein relate to a waveguide structure including: a waveguide substrate including a top surface and a bottom surface; a first grating positioned on the bottom surface of the waveguide substrate; a second grating positioned on the top surface of the waveguide substrate; and a third grating positioned on the top surface of the waveguide substrate, wherein the second grating and the third grating at least partially overlap the first grating, wherein the first grating is a volume grating, wherein the second grating is a volume grating, and wherein the third grating is a polymer fringe grating.

[0007] In some aspects, the techniques described herein relate to a waveguide structure, wherein the first grating is a volume Bragg grating (VBG).

[0008] In some aspects, the techniques described herein relate to a waveguide structure, wherein the first grating includes alternating polymer regions and inert material rich regions.

[0009] In some aspects, the techniques described herein relate to a waveguide structure, wherein inert material rich regions include liquid crystal and/or nanoparticles.

[0010] In some aspects, the techniques described herein relate to a waveguide structure, wherein the second grating is a volume Bragg grating (VBG).

[0011] In some aspects, the techniques described herein relate to a waveguide structure, wherein the second grating includes alternating polymer regions and inert material rich regions.

[0012] In some aspects, the techniques described herein relate to a waveguide structure, wherein inert material rich regions include liquid crystal and/or nanoparticles.

[0013] In some aspects, the techniques described herein relate to a waveguide structure, wherein the third grating includes alternating polymer regions and air gaps.

[0014] In some aspects, the techniques described herein relate to a waveguide structure, wherein a coating conformally coats the exposed surfaces of the waveguide substrate and the second grating and conformally coats polymer regions of the third grating.

[0015] In some aspects, the techniques described herein relate to a waveguide structure, wherein the coating completely fills the regions between adjacent polymer regions of the third grating to create alternating polymer regions and coating.

[0016] In some aspects, the techniques described herein relate to a waveguide structure, further including a third substrate, wherein the first grating is positioned between the third substrate and the waveguide substrate.

[0017] In some aspects, the techniques described herein relate to a waveguide structure, wherein the third grating is configured as an input grating, the first grating is configured as a fold grating, and a second grating is configured as an output grating.

[0018] In some aspects, the techniques described herein relate to a method of manufacturing a waveguide structure, the method including: providing a waveguide substrate including a top surface and a bottom surface; depositing a holographic mixture on the top surface of the waveguide substrate, wherein the holographic mixture includes monomers and an inert material; placing a cover substrate such that the holographic mixture is positioned between the waveguide substrate and the cover substrate; selectively exposing the holographic mixture in a first grating region and a second grating region to create volume gratings including alternating polymer regions and inert material rich regions to create a first grating and a second grating; removing the cover substrate; placing a barrier on the first grating region and the unexposed holographic mixture regions; evacuating the inert material in the inert material rich regions from the second grating to create alternating polymer regions and air gaps while the barrier protects the first grating; and removing the barrier from the first grating and the unexposed holographic mixture regions.

[0019] In some aspects, the techniques described herein relate to a method, wherein the barrier includes a tape and wherein removing the barrier from the first grating and the unexposed holographic mixture regions removes the unexposed holographic mixture.

[0020] In some aspects, the techniques described herein relate to a method, further including: depositing a holographic mixture on the bottom surface of the waveguide substrate, wherein the holographic mixture includes monomers and an inert material; placing a bottom substrate such that the holographic mixture is positioned between the waveguide substrate and the bottom substrate; and exposing the holographic mixture in a third grating region to create a volume grating including alternating polymer regions and inert material rich regions to create a third grating.

[0021] In some aspects, the techniques described herein relate to a method, further including removing the bottom substrate.

[0022] In some aspects, the techniques described herein relate to a method, further including conformally coating exposed portions of the waveguide substrate, the first grating, and polymer regions of the second grating.

[0023] In some aspects, the techniques described herein relate to a method, wherein the coating conformally coats the polymer regions of the second grating such that the air gaps are filled to produce alternating polymer regions and coating.

[0024] In some aspects, the techniques described herein relate to a method, wherein conformally coating includes an atomic layer deposition (ALD) process.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The description will be more fully understood with reference to the following figures and data graphs, which are presented as various embodiment of the disclosure and should not be construed as a complete recitation of the scope of the disclosure, wherein:

[0026] Fig. 1 is a first step for fabricating a waveguide structure.

[0027] Fig. 2 is a second step for fabricating a waveguide structure.

[0028] Fig. 3 is a third step for fabricating a waveguide structure. [0029] Fig. 4 is a fourth step for fabricating a waveguide structure.

[0030] Fig. 5 is a fifth step for fabricating a waveguide structure.

[0031] Fig. 6 is a sixth step for fabricating a waveguide structure.

[0032] Fig. 7 is a schematic of a three waveguide stack of a waveguide display in accordance with an embodiment of the invention.

[0033] Fig. 8A is an example of an evacuated polymer grating with a coating as described in the third grating of Fig. 4.

[0034] Fig. 8B is an example of an evacuated polymer grating with a coating as described in the third grating of Fig. 4.

DETAILED DESCRIPTION

[0035] Various embodiments of the invention include evacuated periodic structures (EPSs) which includes the use of a holographic mixture which may include, among other substances, a monomer and an inert substance. EPSs and various methods of manufacture including descriptions of holographic mixtures are described in U.S. Pat. Pub. No. 20220283376, entitled “Evacuated Periodic Structures and Methods of Manufacturing” and filed Mar. 7, 2022, and U.S. Pat. App. Pub. No. “Evacuating Bragg gratings and methods of manufacturing” and filed Aug. 28, 2020, which are hereby incorporated by reference in their entireties for all purposes. Examples of holographic mixtures are described in U.S. Pat. App. Pub. No. 20200271973, entitled “Holographic Polymer Dispersed Liquid Crystal Mixtures with High Diffraction Efficiency and Low Haze” and filed Feb. 24, 2020, which is hereby incorporated by reference in its entirety for all purposes. In an EPS process, a volume grating is formed after a holographic exposure process. This volume grating includes polymer rich regions (e.g. a polymer matrix) surrounding inert substance rich regions.

[0036] The inert fluid rich regions are removed to produce high modulation surface relief gratings (SRGs) after the inert fluid is removed. The SRGs may include a polymer matrix surrounding air gaps. The inert fluid may be a liquid crystal (LC). The LC may include cyanobiphenyl and/or terphenyl. It has been discovered that the modulation of the recorded holographic mixture structure is not significant to the overall performance of the resulting evacuated periodic structure. This differs from conventional holographic waveguides processes where high refractive index difference between regions create in the recorded hologram create high diffraction efficiency. In this process, the inert fluid is removed and thus the diffraction efficiency is created by the difference in refractive index between the polymer matrix and air gaps. In some embodiments, the inert fluid may be tuned to allow for optimal printability of the holographic mixture which results in high performance gratings. In some cases, after exposure but before evacuation, the resultant volume grating may have zero or close to zero diffraction efficiency. In some embodiments the air space of the SRG may extend down to the substate, while in other embodiments a bias layer formed from the polymer may exist.

[0037] It has been discovered that utilizing an inert substance which creates inert substance rich regions which are of low refractive index difference between the polymer matrix still creates a high diffraction efficiency grating after the subsequent removal of the inert substance. The removal of the inert substance creates alternating polymer matrix regions with air gap regions. The polymer matrix has a high refractive index difference with the air gap. Previous volume Bragg gratings (VBGs) utilize a holographic mixture including a monomer and liquid crystal. After holographic exposure, a volume grating of alternating polymer regions and liquid crystal regions were created. However, these polymer regions and liquid crystal regions have a high refractive index difference to create high diffraction efficiency. This is fundamentally different from the holographic mixture utilized in creating evacuated periodic structures as discussed above. The holographic mixture utilized to create evacuated periodic structures, may include an inert substance (e.g. inert fluid). The inert substance may be utilized to create optimal phase separation with the monomer during holographic exposure. The inert substance may include a liquid crystal. Optimum phase separation may be determined by choice of inert material and monomer and their initial concentrations diffusivity, exposure, and reaction temperature. In many embodiments, the inert substance may include nanoparticles. In many embodiments, the inert substance may not be a full liquid crystal but instead include various liquid crystal singles. In many embodiments, the inert substance may be tuned to allow for optimal printability of the holographic mixture which results in high performance gratings. It has been discovered that utilizing a holographic mixture which, after holographic exposure, creates a low refractive index difference between the polymer matrix and the inert substance allows for a high flexibility of substances which may be utilized for the inert fluid. The inert substance may be utilized to produce good phase separation or have high printability instead of optimized for producing a high refractive index difference after exposure.

[0038] EPS structures have been demonstrated in applicability for various gratings in waveguide based devices such as input gratings, output gratings, and/or fold gratings. However, other grating types may provide improved performance for at least some of the input gratings, output gratings, and/or fold gratings. The other grating types include VBGs, SRGs, or surface mounted VBGs. Surface mounted VBGs are VBGs which do not include a top substrate and thus are exposed. Examples of surface mounted VBGs are described in U.S. Pat. App. No. 18/620,834, entitled “Surface Mounted Volume Phase Structure and Methods of Manufacturing Thereof” and filed on March 28, 2024, which is hereby incorporated by reference in its entirety for all purposes. As is apparent, the surface mounted VBGs are an intermediate step in the fabrication of the EPSs and thus may be easily integrated on the same substrate as the EPSs. It has been discovered that surface mounted VBGs and EPSs may be fabricated on the same substrate with similar processes with just a single additional step of removing the inert fluid from the EPSs while the inert fluid remains for the surface mounted VBGs. Thus, on the same substrate may exist surface mounted VBGs and EPSs which may allow for the VBGs to be utilized in areas where the VBGs are advantageous and EPSs where the EPSs are advantageous. It has further been discovered that, on the other side of the substrate, an additional grating may exist which may overlap one or more of the surface mounted VBGs and EPSs. The EPS may be an evacuated polymer structure. VBG may be an intermediate step in the production of the EPS.

[0039] Figs. 1-6 are various steps for fabricating an example waveguide structure in accordance with an embodiment of the invention. Fig. 1 is a first step for fabricating a waveguide structure. In this step an initial waveguide structure is provided which includes a waveguide substrate 102. On a bottom surface of the waveguide substrate 102, a first grating 116 is positioned. The first grating 116 may be a volume grating which is fabricated utilizing a holographic exposure process. Examples of holographic exposure processes are described in Pub. No. 20220283376 and U.S. Pat. App. Pub. No. 20200271973 which are incorporated by reference above. The first grating 116 is positioned between a bottom substrate 104 and the waveguide substrate 102. The first grating 116 is surrounded on the same layer by a polymer 114. The first grating 116 may be a volume Bragg grating (VBG). The first grating 116 and the polymer 114 may be formed with a single layer of holographic material. The layer of holographic material may be selectively exposed to a holographic recording beam to form the first grating 116 while the remaining unexposed holographic material may form the polymer 114.

[0040] On a top surface of the waveguide substrate 102, a second grating 108 and a third grating 110 is positioned. The second grating 108 and the third grating 110 may be a volume grating which is fabricated utilizing the holographic exposure process discussed above. The second grating 108 and the third grating 110 are positioned between a top substrate 106 and the waveguide substrate 102. The second grating 108 and the third grating 110 are surrounded on the same layer by a polymer 112. The second grating 108 and the third grating 110 may be VBGs. The second grating 108 and the third grating 110 and the polymer 112 may be formed with a single layer of holographic material. The holographic material may be selectively exposed to a holographic recording beam to from the second grating 108 and the third grating 110 in different regions while the unexposed portion may form the polymer 112.

[0041] When the waveguide structure is utilized in a waveguide display, the first grating 116 may be fold grating, the second grating 108 may be the output grating, and the third grating 110 may be the input grating.

[0042] Fig. 2 is a second step for fabricating a waveguide structure. The description of Fig. 1 is applicable to Fig. 2 and will not be repeated in detail. The top substrate 106 may be removed to expose the second grating 108, the third grating 110, and the polymer 112. The second grating 108 and the polymer 112 may be masked while an inert material from inert material rich regions to form an evacuated periodic structure (EPS). The evacuated third grating 110a may be a periodic structure of alternating polymer regions and air gaps. The second grating 108 and the polymer 112 may be masked by a tape such as a Kapton tape.

[0043] Fig. 3 is a third step for fabricating a waveguide structure. The description of Fig. 2 is applicable to Fig. 3 and will not be repeated in detail. The polymer 112 is removed from areas surrounding the second grating 108 and the evacuated third grating 110a. It has been discovered that removing the tape used to mask the second grating 108 and the polymer 112 removes the polymer 112 while the second grating 108 remains. Without limitation to any particular theory, the tape adheres more to the polymer 112 than the second grating 108. Thus, removing the tape removes the polymer regions 112 however allows the second grating 108 to remain. After the tape is removed, the region where the second grating 108 is located does not have polymer material removed whereas the regions where the second grating 108 is not located have polymer material removed.

[0044] Fig. 4 is a fourth step for fabricating a waveguide structure. The description of Fig. 3 is applicable to Fig. 4 and will not be repeated in detail. A coating 402 may be deposited on the top surface of the waveguide substrate 102. The coating 402 coats the exposed surfaces of the waveguide substrate and the second grating 108. The coating 402 may be formed into the air gaps of the evacuated third grating 110a and onto the polymer regions of the evacuated third grating 110a to form a coated third grating 110b. The coating 402 may be formed through an atomic layer deposition (ALD) process. Various coating and backfilling processes of waveguide structures are discussed in Int. Pub. No. WO 2024/115967 which is hereby incorporated by reference in its entirety for all purposes.

[0045] Fig. 5 is a fifth step for fabricating a waveguide structure. The description of Fig. 4 is applicable to Fig. 5 and will not be repeated in detail. The bottom substrate 104 may be removed which exposes the polymer 114 and the first grating 116. In some embodiments, the bottom substrate 104 may not be removed and the fabricated structure described in connection with Fig. 4 may be the waveguide structure utilized in a waveguide display.

[0046] Fig. 6 is a sixth step for fabricating a waveguide structure. The description of Fig. 5 is applicable to Fig. 6 and will not be repeated in detail. The polymer 114 surrounding the first grating 116 may be removed. The polymer 114 may be removed by applying a tape and removing the tape.

[0047] As illustrated many different types of gratings are formed on a single waveguide substrate. In Fig. 6, the first grating 116 and the second grating 108 may be a surface mounted VBG whereas the third grating 110b is an EPS all formed on a single waveguide substrate 102. As illustrated, the third grating 110b may be a coated EPS. The second grating 108 and the third grating 110 may be formed on one side of the waveguide substrate 102 whereas the first grating 116 may be formed on an opposite side of the waveguide substrate 102. The second grating 108 may be a surface mounted VBG and may be an output grating. The third grating 110b may be an EPS and may be an input grating. The first grating 116 may be a surface mounted VBG and may be a fold grating. The first grating 116 may overlap with the second grating 108 and/or the third grating 110b. While this overlapping orientation is illustrated, in some embodiments, the first grating 116 may be spaced apart from one or more of the second grating 108 and/or the third grating 110b.

[0048] While a coating 402 is provided, in some embodiments, a coating 402 may not be incorporated in Fig. 6 such that the coating step described in connection with Fig. 4 may not be performed.

[0049] Fig. 7 is a schematic of a three waveguide stack of a waveguide display in accordance with an embodiment of the invention. A first waveguide 702 may input and output blue light, a second waveguide 704 may input and output green light, and a third waveguide 706 may input and output red light. The first waveguide 702 may be the waveguide structure described in connection with Fig. 4 in which the first grating 114 is left positioned between the bottom substrate 104 and the waveguide substrate 102. For this waveguide structure, it has been discovered that the substrates may be thin. For example, the bottom substrate 104 and the waveguide substrate 102 may both be 0.3 mm thickness. The second waveguide 704 and the third waveguide 706 may be the waveguide structure described in connection with Fig. 6 in which the first grating 114 is a surface mounted volume grating. For this configuration, the waveguide structure 102 may be thicker. The waveguide structure 102 may be 0.5 mm. The waveguide stack may be 1 .6 mm thickness after assembly.

[0050] It is understood that any combination of waveguides may be utilized. For example, all of the first waveguide 702, the second waveguide 704, and the third waveguide 706 may be the waveguide structure described in connection with Fig. 6. However, in other examples, any of the first waveguide 702, the second waveguide 704, and the third waveguide 706 may be the waveguide described in connection with Fig. 4 and the rest of the waveguides may be the waveguide structure described in connection with Fig. 6.

[0051] Fig. 8A is an example of an evacuated polymer grating with a coating as described in the third grating 110b of Fig. 4. The polymer grating includes polymer regions 804 positioned on a waveguide substrate 802. A conformal coating 806 coats the exposed portions of the polymer regions 804 and the waveguide substrate 802. Air gaps 804a are between adjacent portions of the coating 806.

[0052] Fig. 8B is an example of an evacuated polymer grating with a coating as described in the third grating 110b of Fig. 4. The polymer grating includes polymer regions positioned on a waveguide substrate. A conformal coating coats the exposed portions of the polymer regions and the waveguide substrate. The coating occupies the regions between adjacent polymer regions.

DOCTRINE OF EQUIVALENTS

[0053] While the above description contains many specific embodiments of the invention, these should not be construed as limitations on the scope of the invention, but rather as an example of one embodiment thereof. It is therefore to be understood that the present invention may be practiced in ways other than specifically described, without departing from the scope and spirit of the present invention. Thus, embodiments of the present invention should be considered in all respects as illustrative and not restrictive. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their equivalents.

Claims

WHAT IS CLAIMED IS:

1 . A waveguide structure comprising: a waveguide substrate including a top surface and a bottom surface; a first grating positioned on the bottom surface of the waveguide substrate; a second grating positioned on the top surface of the waveguide substrate; and a third grating positioned on the top surface of the waveguide substrate, wherein the second grating and the third grating at least partially overlap the first grating, wherein the first grating is a volume grating, wherein the second grating is a volume grating, and wherein the third grating is a polymer fringe grating.

2. The waveguide structure of claim 1 , wherein the first grating is a volume Bragg grating (VBG).

3. The waveguide structure of claim 2, wherein the first grating includes alternating polymer regions and inert material rich regions.

4. The waveguide structure of claim 3, wherein inert material rich regions comprise liquid crystal and/or nanoparticles.

5. The waveguide structure of claim 1 , wherein the second grating is a volume Bragg grating (VBG).

6. The waveguide structure of claim 5, wherein the second grating includes alternating polymer regions and inert material rich regions.

7. The waveguide structure of claim 6, wherein inert material rich regions comprise liquid crystal and/or nanoparticles.

8. The waveguide structure of claim 1 , wherein the third grating comprises alternating polymer regions and air gaps.

9. The waveguide structure of claim 1 , wherein a coating conformally coats the exposed surfaces of the waveguide substrate and the second grating and conformally coats polymer regions of the third grating.

10. The waveguide structure of claim 9, wherein the coating completely fills the regions between adjacent polymer regions of the third grating to create alternating polymer regions and coating.

11 . The waveguide structure of claim 1 , further comprising a third substrate, wherein the first grating is positioned between the third substrate and the waveguide substrate.

12. The waveguide structure of claim 1 , wherein the third grating is configured as an input grating, the first grating is configured as a fold grating, and a second grating is configured as an output grating.

13. A method of manufacturing a waveguide structure, the method comprising: providing a waveguide substrate comprising a top surface and a bottom surface; depositing a holographic mixture on the top surface of the waveguide substrate, wherein the holographic mixture includes monomers and an inert material; placing a cover substrate such that the holographic mixture is positioned between the waveguide substrate and the cover substrate; selectively exposing the holographic mixture in a first grating region and a second grating region to create volume gratings including alternating polymer regions and inert material rich regions to create a first grating and a second grating; removing the cover substrate; placing a barrier on the first grating region and the unexposed holographic mixture regions; evacuating the inert material in the inert material rich regions from the second grating to create alternating polymer regions and air gaps while the barrier protects the first grating; and removing the barrier from the first grating and the unexposed holographic mixture regions.

14. The method of claim 13, wherein the barrier comprises a tape and wherein removing the barrier from the first grating and the unexposed holographic mixture regions removes the unexposed holographic mixture.

15. The method of claim 13, further comprising: depositing a holographic mixture on the bottom surface of the waveguide substrate, wherein the holographic mixture includes monomers and an inert material; placing a bottom substrate such that the holographic mixture is positioned between the waveguide substrate and the bottom substrate; and exposing the holographic mixture in a third grating region to create a volume grating including alternating polymer regions and inert material rich regions to create a third grating.

16. The method of claim 15, further comprising removing the bottom substrate.

17. The method of claim 14, further comprising conformally coating exposed portions of the waveguide substrate, the first grating, and polymer regions of the second grating.

18. The method of claim 17, wherein the coating conformally coats the polymer regions of the second grating such that the air gaps are filled to produce alternating polymer regions and coating.

19. The method of claim 17, wherein conformally coating comprises an atomic layer deposition (ALD) process.