Diffractive optical waveguide and preparation method thereof

Abstract

The present application discloses a diffraction optical waveguide and a preparation method thereof, the diffraction optical waveguide comprising: a substrate, the substrate comprising a first surface and a second surface arranged opposite to each other, the first surface being a plane, the second surface being located on a plane parallel to the first surface, and the substrate thickness between a target area of ​​the second surface and the first surface being non-uniformly distributed; and a liquid crystal material layer, the liquid crystal material layer being formed on the second surface, the top of the liquid crystal material layer being flat, so that the thickness of the liquid crystal material layer in the target area is non-uniformly distributed, so that the diffraction efficiency of the liquid crystal polarizer grating formed in the target area is gradually increased along the direction of light propagation. The diffraction optical waveguide provided by the present application has extremely high diffraction efficiency and good uniformity.

Description

Technical Field

[0001] The present application relates to the field of augmented reality, and in particular to a diffractive optical waveguide and a method for preparing the same. Background Art

[0002] Waveguide display technology is one of the most challenging and complex problems in the field of augmented reality. Diffraction waveguide is a commonly used waveguide in waveguide display technology. Since the diffraction grating has multiple diffraction orders and the energy is dispersed, the improvement of energy utilization efficiency is the focus of grating design. Liquid crystal polarizer grating can efficiently diffract the incident light to a specific diffraction order, thereby achieving high diffraction efficiency (for example, up to 90%, or even 99%) for a specific wavelength range and light from a specific field of view.

[0003] However, the diffraction efficiency distribution of liquid crystal polarizer gratings is usually relatively uniform, which is very unfavorable to the uniformity requirements of diffraction light waveguides. Therefore, developing a diffraction light waveguide that has both the advantages of liquid crystal polarizer gratings and good uniformity has become a technical focus that technical personnel in this field need to solve urgently. Summary of the invention

[0004] In order to solve the above problems, the present application provides a diffraction optical waveguide with high diffraction efficiency and good uniformity and its preparation.

[0005] A diffractive optical waveguide, comprising:

[0006] A substrate, the substrate comprising a first surface and a second surface arranged opposite to each other, the first surface is a plane, the plane where the second surface is located is parallel to the first surface, and the thickness of the substrate between a target area of ​​the second surface and the first surface is non-uniformly distributed; and

[0007] A liquid crystal material layer is formed on the second surface, and the top of the liquid crystal material layer is flat, so that the thickness of the liquid crystal material layer in the target area is unevenly distributed, so that the diffraction efficiency of the liquid crystal polarizer grating formed in the target area gradually increases along the direction of light propagation.

[0008] In practice, the target area includes a first target area and a second target area, the liquid crystal polarizer grating formed in the first target area is used as a turning grating, the liquid crystal polarizer grating formed in the second target area is used as a coupling-out grating, and the substrate thickness distribution in the first target area is different from the substrate thickness distribution in the second target area.

[0009] In practice, the liquid crystal material layer is a multi-layer stacked structure, and the concentrations of chiral materials doped in different layers of liquid crystal materials are different.

[0010] It is feasible that the thickness of the liquid crystal polarizer grating formed in the target area gradually increases along the direction of light propagation, so that the diffraction efficiency of the liquid crystal polarizer grating formed in the target area gradually increases along the direction of light propagation, and the rate of change of the thickness of the liquid crystal polarizer grating along the direction of light propagation is less than or equal to 0.8um / mm.

[0011] In practice, the thickness of the liquid crystal polarizer grating formed in the target area satisfies the following relationship:

[0012]

[0013] in, is the distance along the direction of light propagation from the initial position of the liquid crystal polarizer grating The thickness of is the total length of the liquid crystal polarizer grating from the initial position along the direction of light propagation, is the maximum thickness of the liquid crystal polarizer grating along the direction of light propagation, It is the minimum thickness of the liquid crystal polarizer grating along the direction of light propagation.

[0014] A method for preparing a diffractive optical waveguide, used for preparing the aforementioned diffractive optical waveguide, comprising:

[0015] Obtaining a substrate, wherein the substrate includes two surfaces parallel to each other;

[0016] Processing one of the two surfaces so that the thickness of the substrate between the target area of ​​the surface and the other of the two surfaces is non-uniformly distributed;

[0017] forming a first material layer on the treated surface, and performing double-beam interference exposure on the first material layer to form an alignment layer having a periodic orientation;

[0018] A liquid crystal material layer is formed on the surface of the alignment layer, and the top of the liquid crystal material layer is flat.

[0019] In practice, the processing of one of the two surfaces so that the substrate thickness between the target area of ​​the surface and the other of the two surfaces is non-uniformly distributed comprises:

[0020] Based on the expected grating thickness distribution in the target area, the target area of ​​one of the two surfaces is continuously etched, and the etching beam spot size and the moving step are controlled so that the substrate thickness between the target area of ​​the surface and the other of the two surfaces is non-uniform and continuously distributed.

[0021] In practice, the processing of one of the two surfaces so that the substrate thickness between the target area of ​​the surface and the other of the two surfaces is non-uniformly distributed comprises:

[0022] Forming an embossed adhesive layer on one of the two surfaces, and performing structural transfer printing on the embossed adhesive layer using an embossing template, so that the thickness of the embossed adhesive layer is non-uniformly distributed in the target area;

[0023] The embossed adhesive layer is used as a mask for etching to transfer the surface profile change of the embossed adhesive layer to the substrate, so that the substrate thickness between the target area of ​​the surface and the other surface is unevenly distributed.

[0024] In practice, the processing of one of the two surfaces so that the substrate thickness between the target area of ​​the surface and the other of the two surfaces is non-uniformly distributed comprises:

[0025] Based on the expected grating thickness distribution in the target area, the target area of ​​one of the two surfaces is etched in sections and steps, so that the substrate thickness between the target area of ​​the surface and the other of the two surfaces is non-uniform and gradient distributed.

[0026] In practice, the number of the target area is more than one, and the liquid crystal material layer is formed on the surface of the alignment layer, and the top of the liquid crystal material layer is flat, including:

[0027] The liquid crystal material is dotted or sprayed on the alignment layer of each target area respectively, and the concentration of the chiral material doped in the liquid crystal material in different target areas is different.

[0028] The present application provides a diffraction optical waveguide and a preparation method thereof. The diffraction optical waveguide is designed to have two surfaces that are relatively arranged and parallel to each other, one of which is a plane and the other is a non-plane, so that the substrate thickness between the two surfaces in the target area is non-uniformly distributed, and the thickness modulation of the liquid crystal polarizer grating is cleverly converted into the contour modulation of the substrate surface, avoiding the problem that the thickness modulation is difficult to achieve when the liquid crystal material is solidified to form the liquid crystal polarizer grating. In this way, when a liquid crystal layer is formed in the target area of ​​the non-plane surface to form the liquid crystal polarizer grating, the thickness modulation of the liquid crystal polarizer grating can be achieved at the same time, and then the diffraction efficiency of the liquid crystal polarizer grating is modulated to gradually increase along the direction of light propagation, thereby achieving the purpose of optimizing the uniformity of the diffraction optical waveguide. Moreover, when there are multiple gratings in the diffraction optical waveguide that need to be distributed for diffraction efficiency modulation, each grating can be modulated separately, with high modulation flexibility and large degree of freedom. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0030] Figure 1 A schematic diagram of the structure of a diffractive optical waveguide provided in an embodiment of the present application;

[0031] Figure 2 A schematic diagram of the structure of another diffractive optical waveguide provided in an embodiment of the present application;

[0032] Figure 3 A schematic diagram of an intermediate structure of a diffractive optical waveguide prepared by the preparation method provided in an embodiment of the present application;

[0033] Figure 4 A schematic diagram of another intermediate structure of a diffractive optical waveguide prepared by the preparation method provided in an embodiment of the present application;

[0034] Figure 5 A schematic diagram of another intermediate structure of a diffractive optical waveguide prepared by the preparation method provided in an embodiment of the present application;

[0035] Figure 6 A schematic diagram of the structure of another diffractive optical waveguide provided in an embodiment of the present application;

[0036] Figure ID:

[0037] 110: substrate; 111: first surface; 112: second surface;

[0038] 100: target area;

[0039] 120: orientation layer;

[0040] 130: liquid crystal material layer; 131: first liquid crystal material layer; 132: second liquid crystal material layer; 133: third liquid crystal material layer;

[0041] 140: Liquid crystal polarizer grating;

[0042] 150: embossing adhesive layer;

[0043] 160: Protective layer. DETAILED DESCRIPTION

[0044] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0045] The technical solution of the present application is described in detail with specific embodiments below. The following specific embodiments can be combined with each other, and the same or similar concepts or processes may not be described in detail in some embodiments.

[0046] The present application provides a diffraction optical waveguide, comprising: a substrate, the substrate comprising a first surface and a second surface arranged opposite to each other, the first surface being a plane, the plane where the second surface is located being parallel to the first surface, and the thickness of the substrate between a target area of ​​the second surface and the first surface being non-uniformly distributed; and a liquid crystal material layer, the liquid crystal material layer being formed on the second surface, the top of the liquid crystal material layer being flat, so that the thickness of the liquid crystal material layer in the target area is non-uniformly distributed, so that the diffraction efficiency of the liquid crystal polarizer grating formed in the target area is gradually increased along the direction of light propagation.

[0047] Specifically, the substrate of the diffractive optical waveguide includes two surfaces arranged opposite to each other, one of which is a plane and the other surface has a variable surface profile, so that the substrate thickness between the two surfaces in a partial area (i.e., the target area) is non-uniformly distributed, and the substrate thickness between the two surfaces in a partial area is uniformly distributed. When the liquid crystal polarizer grating is formed by the liquid crystal material on the surface with the variable surface profile, the thickness of the liquid crystal polarizer grating can be modulated, that is, the diffraction efficiency of the liquid crystal polarizer grating can be modulated. Wherein, an orientation layer is included on the substrate, and when the liquid crystal material layer is formed on the orientation layer, the liquid crystal material molecules of the liquid crystal material can be oriented to form a spiral structure according to the surface orientation pattern of the orientation layer, thereby forming a liquid crystal polarizer grating.

[0048] For example, reference Figure 1The diffraction optical waveguide includes a substrate 110, an alignment layer 120 and a liquid crystal material layer 130. The substrate 110 includes a first surface 111 and a second surface 112 that are arranged opposite to each other. The first surface 111 is a plane, and the plane where the second surface 112 is located is parallel to the first surface 111. The thickness of the substrate between the first surface 111 and the second surface 112 in the target area 100 is non-uniformly distributed. In this way, the thickness of the liquid crystal polarizer grating 140 formed by the liquid crystal material in the target area 100 is non-uniformly distributed, that is, the diffraction efficiency of the liquid crystal polarizer grating 140 is non-uniformly distributed, so that the uniformity of the diffraction optical waveguide can be optimized by modulating the thickness distribution of the substrate between the first surface 111 and the second surface 112 in the target area 100.

[0049] It should be noted that when the orientation layer is formed, the exposure area is designed so that only the grating area has a periodic surface orientation pattern, while the non-grating area does not. In this way, after the liquid crystal material layer is formed on the surface of the orientation layer, the liquid crystal molecules in the grating area can be oriented to form a spiral structure according to the surface orientation pattern of the orientation layer, while the liquid crystal molecules in the non-grating area will not form a spiral structure. After the liquid crystal material layer is solidified, a liquid crystal polarizer grating is formed in the grating area, while there is no grating in the non-grating area. The target area of ​​the present application belongs to the grating area.

[0050] In practice, the target area includes a first target area and a second target area, the liquid crystal polarizer grating formed in the first target area is used as a turning grating, the liquid crystal polarizer grating formed in the second target area is used as a coupling-out grating, and the substrate thickness distribution in the first target area is different from the substrate thickness distribution in the second target area.

[0051] Specifically, the grating architecture of some diffraction waveguides includes coupling-in gratings and coupling-out gratings, the grating architecture of other diffraction waveguides includes coupling-in gratings, turning gratings and coupling-out gratings, and the grating structure of some diffraction waveguides also includes other functional gratings such as recycling gratings. Among them, for example, when replicating the pupil, the turning grating and the coupling-out grating will have uniformity performance requirements, that is, there is a need to modulate the diffraction efficiency distribution. The areas where these gratings with the need to modulate the diffraction efficiency distribution are located can all be target areas. Moreover, different gratings require different diffraction efficiency distributions, so the thickness distributions in different target areas are also different, and the thickness distributions in each target area can be modulated independently without affecting each other.

[0052] In practice, the liquid crystal material layer is a multi-layer stacked structure, and the concentrations of chiral materials doped in different layers of liquid crystal materials are different.

[0053] Specifically, after the chiral material is doped into the liquid crystal material, the self-spin speed of the liquid crystal molecules will change, and the tilt angle will also change accordingly. The higher the doping ratio of the chiral material, the faster the self-spin speed of the liquid crystal molecules can be, thus producing a more inclined grating angle.

[0054] For example, reference Figure 2 The liquid crystal material layer 130 includes a first liquid crystal material layer 131 , a second liquid crystal material layer 132 and a third liquid crystal material layer 133 , a total of three-layer stacked structure.

[0055] In practice, the multi-layered liquid crystal material layers may have the same rotation period or different rotation periods. For example, a second orientation layer is disposed on the first liquid crystal material layer, and the second orientation layer is processed to form a second surface orientation pattern, and the second surface orientation pattern provides a different orientation pattern from the first surface orientation pattern, so that the rotation period of the first liquid crystal material layer and the second liquid crystal material layer can be different. Moreover, the multi-layer liquid crystal polarizer grating can be configured to reduce the rainbow effect in the diffraction light waveguide. The thicker one in the multi-layer liquid crystal polarizer grating is used as the main grating, which mainly provides pupil expansion and outcoupling, and the thinner one is configured to suppress the undesired diffraction orders of the main grating, so that the rainbow effect can be reduced.

[0056] In practice, the thickness of the liquid crystal polarizer grating formed in the target area gradually increases along the direction of light propagation, so that the diffraction efficiency of the liquid crystal polarizer grating formed in the target area gradually increases along the direction of light propagation. It can be understood that the optimization of uniformity is to compensate for the attenuation of light energy by improving the diffraction efficiency. The thickness change of the liquid crystal polarizer grating should not be too fast or too slow. Too slow thickness change may lead to insufficient compensation, and too fast thickness change may lead to overcompensation, both of which may lead to failure to achieve a good uniformity optimization effect. Preferably, the rate of change of the thickness of the liquid crystal polarizer grating along the direction of light propagation is less than or equal to 0.8um / mm.

[0057] In practice, the thickness of the liquid crystal polarizer grating formed in the target area satisfies the following relationship:

[0058]

[0059] in, is the distance along the direction of light propagation from the initial position of the liquid crystal polarizer grating The thickness of is the total length of the liquid crystal polarizer grating from the initial position along the direction of light propagation, is the maximum thickness of the liquid crystal polarizer grating along the direction of light propagation, It is the minimum thickness of the liquid crystal polarizer grating along the direction of light propagation.

[0060] It can be understood that in some embodiments, the direction of light propagation in the liquid crystal polarizer grating is not unique. In this case, in each direction of light propagation, the thickness of the liquid crystal polarizer grating can satisfy the above relationship.

[0061] Specifically, the thickness of the liquid crystal polarizer grating ranges from 0.1 to 20 um, and the thickness of the liquid crystal polarizer grating in the coupling-in region is greater than the thickness of the liquid crystal polarizer grating in the turning region and the coupling-out region.

[0062] For example, in some embodiments, the maximum thickness of the liquid crystal polarizer grating in the turning region is defined as H, the maximum thickness of the liquid crystal polarizer grating in the coupling-in region is in the range of 1H-5H, and the maximum thickness of the liquid crystal polarizer grating in the coupling-out region is in the range of 0.7H-2H. Within the above thickness setting range of the liquid crystal polarizer grating, the diffraction optical waveguide can achieve high light energy utilization and good uniformity.

[0063] In the present application, the above diffraction optical waveguide is designed to have two surfaces that are relatively arranged and parallel to each other, one of which is a plane and the other is a non-plane, so that the substrate thickness between the two surfaces in the target area is unevenly distributed, and the thickness modulation of the liquid crystal polarizer grating is cleverly converted into the contour modulation of the substrate surface, avoiding the problem that the thickness modulation is difficult to achieve when the liquid crystal material is solidified to form the liquid crystal polarizer grating. In this way, when a liquid crystal layer is formed in the target area of ​​the non-plane surface to form a liquid crystal polarizer grating, the thickness modulation of the liquid crystal polarizer grating can be achieved at the same time, and then the diffraction efficiency of the liquid crystal polarizer grating is modulated to gradually increase along the direction of light propagation, thereby achieving the purpose of optimizing the uniformity of the diffraction optical waveguide. Moreover, when there are multiple gratings in the diffraction optical waveguide that need to be distributed for diffraction efficiency modulation, each grating can be modulated separately, with high modulation flexibility and large degree of freedom.

[0064] According to another aspect of the present application, the present application further provides a method for preparing a diffractive optical waveguide, which is used to prepare the diffractive optical waveguide in the aforementioned embodiment, and the preparation method specifically includes:

[0065] S10, obtaining a substrate, where the substrate includes two surfaces parallel to each other.

[0066] It is understood that the substrate of the diffractive optical waveguide should be transparent to visible light, may have a flat surface or a curved surface, and the substrate may include one or more types of dielectric materials, such as glass, quartz, plastic, polymer, poly (methyl methacrylate) (PMMA), crystal or ceramic, etc. The substrate of the diffractive optical waveguide may be rigid, semi-rigid, flexible or semi-flexible. Further, the substrate may also allow light from the external environment to pass through with little or no loss. The substrate of the diffractive optical waveguide obtained in the present application includes two surfaces parallel to each other.

[0067] S20, processing one of the two surfaces so that the thickness of the substrate between the target area of ​​the surface and the other of the two surfaces is non-uniformly distributed.

[0068] Practically, S20 includes: based on the expected grating thickness distribution in the target area, continuously etching the target area of ​​one of the two surfaces, controlling the etching beam spot size and the moving step length so that the substrate thickness between the target area of ​​the surface and the other of the two surfaces is non-uniform and continuously distributed.

[0069] Specifically, the present application can use an etching beam to perform mobile etching on one of the surfaces of the substrate, and control the beam spot size, beam energy, action time and moving step size of the etching beam, so that the substrate thickness between the target area of ​​the surface and the other of the two surfaces is non-uniform and continuously distributed. Among them, the etching beam can be a molecular gas group, an ion beam, etc., and the beam spot size can refer to the initial size of the action beam generated by the device, or it can be the size of the confinement window. For example, refer to Figure 3 , an etching beam is used to perform moving etching on one surface of the substrate, so as to form a groove on the surface with a depth that continuously increases along the etching moving direction. The moving etching may be the movement of the etching beam or the movement of the substrate.

[0070] Practically, S20 includes: forming an embossed adhesive layer on one of the two surfaces, and transferring the structure of the embossed adhesive layer using an embossing template so that the thickness of the embossed adhesive layer is non-uniformly distributed in the target area; etching using the embossed adhesive layer as a mask to transfer the surface profile change of the embossed adhesive layer to the substrate, so that the thickness of the substrate between the target area of ​​the surface and the other surface is non-uniformly distributed.

[0071] Specifically, the present application can process one of the two surfaces of the substrate by nanoimprinting combined with overall etching, so that the thickness of the substrate between the two surfaces in the target area is non-uniformly distributed. Figure 4 , an embossed adhesive layer 150 is formed on one of the two surfaces of the substrate, and the structure of the embossed adhesive layer 150 is transferred using an embossing template, so that the thickness of the embossed adhesive layer 150 is non-uniformly distributed in the target area 100; the embossed adhesive layer 150 is used as a mask for etching, and the surface profile change of the embossed adhesive layer 150 is transferred to the substrate, so that the thickness of the substrate between the two surfaces in the target area is non-uniformly distributed.

[0072] In practice, S20 includes: based on the expected grating thickness distribution in the target area, etching the target area of ​​one of the two surfaces in a zoned and step-by-step manner, so that the substrate thickness between the target area of ​​the surface and the other of the two surfaces is non-uniform and has a gradient distribution.

[0073] Specifically, the present application can process one of the two surfaces of the substrate by etching multiple times in different zones, so that the thickness of the substrate between the two surfaces in the target area is unevenly distributed. Figure 5 Through multiple photolithography etchings, different areas are etched at different depths each time to form steps of multiple depths to achieve thickness modulation.

[0074] S30, forming a first material layer on the treated surface, and performing double-beam interference exposure on the first material layer to form an alignment layer having a periodic orientation.

[0075] Specifically, a solution including the first material is spin-coated or spray-coated on the surface of the substrate and dried, and a double-beam exposure system is used to perform interference exposure on the grating area of ​​the first material layer by left-handed circularly polarized light and right-handed circularly polarized light, so as to form an orientation layer with a periodic orientation in the grating area. When the double-beam exposure is performed on the first material layer, only the area where the desired grating structure is located, that is, the grating area, is exposed. In the present application, the grating area includes the target area. The period range of the orientation layer in the target area is 250~500nm.

[0076] In one example, a solution including the first material is spin-coated on the surface of the substrate. After spin-coating, the solution is dried, such as by baking at an elevated temperature (eg, greater than about 100° C.), to remove the solvent.

[0077] S40, forming a liquid crystal material layer on the surface of the alignment layer, and the top of the liquid crystal material layer is flat.

[0078] Specifically, the alignment layer may have a periodic surface alignment pattern. Due to the periodic surface alignment pattern, the optical axis of the liquid crystal material at different positions on the alignment layer may rotate at different azimuths on the xz (horizontal) plane, and a periodic structure may be formed on the horizontal plane. The liquid crystal molecules in the liquid crystal material layer deposited on the alignment layer may be aligned with the alignment pattern in the alignment layer to form a spiral structure.

[0079] In practice, the number of target regions is more than one; S40 includes: spot coating or spray coating the liquid crystal material on the alignment layer of each target region respectively, and the concentration of the chiral material doped in the liquid crystal material in different target regions is different.

[0080] Specifically, in different areas, liquid crystal materials with different chiral material doping ratios are filled by dispensing or spraying, and then exposed and cured, so as to realize a liquid crystal polarizer grating whose thickness and tilt angle can be freely adjusted in different areas, making the grating control method more flexible and achieving better waveguide display effect.

[0081] In practice, the diffraction optical waveguide further comprises a base region, wherein the substrate thickness between the two surfaces in the base region is uniformly distributed, but the thickness is less than the original thickness of the substrate. The grating structure formed in the base region generally does not require diffraction efficiency distribution modulation, such as coupling gratings.

[0082] For example, reference Figure 6 , in different areas (target area and / or base area), liquid crystal materials with different chiral material doping ratios are filled by dispensing or spraying, the volume of the liquid crystal material is controlled, and it is exposed and cured to achieve a liquid crystal polarizer grating whose thickness and tilt angle in different areas can be freely adjusted.

[0083] It is feasible to spin-coat a layer of glue on the surface of the liquid crystal material after curing to serve as a protective layer. On the one hand, it is used to protect the internal liquid crystal polarizer grating. On the other hand, it can improve the unevenness of the waveguide surface caused by local coating of the liquid crystal material, thereby improving the total reflection effect of the waveguide and reducing the stray light of total reflection.

[0084] Continue to refer Figure 6 After the liquid crystal material is solidified, a layer of glue is spin-coated on the surface of the liquid crystal material to serve as a protective layer 160 .

[0085] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit it. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A diffractive optical waveguide, characterized in that: include: A substrate, the substrate comprising a first surface and a second surface arranged opposite to each other, the first surface is a plane, the plane where the second surface is located is parallel to the first surface, and the thickness of the substrate between a target area of ​​the second surface and the first surface is non-uniformly distributed; an alignment layer, wherein the alignment layer is formed on the second surface, the surface profile of the alignment layer is substantially consistent with the surface profile of the second surface, and when the alignment layer is formed, the exposure area is designed so as to have a periodic surface alignment pattern only in the target area; as well as, A liquid crystal material layer, wherein the liquid crystal material layer is formed on the alignment layer, and the liquid crystal material molecules of the liquid crystal material layer are oriented to form a spiral structure according to the surface alignment pattern of the alignment layer to form a liquid crystal polarizer grating; The top of the liquid crystal material layer is flat, so that the thickness of the liquid crystal material layer in the target area is non-uniformly distributed, and the diffraction efficiency of the liquid crystal polarizer grating formed in the target area gradually increases along the direction of light propagation.

2. The diffractive optical waveguide according to claim 1, characterized in that: The target area includes a first target area and a second target area, the liquid crystal polarizer grating formed in the first target area is used as a turning grating, the liquid crystal polarizer grating formed in the second target area is used as a coupling-out grating, and the substrate thickness distribution in the first target area is different from the substrate thickness distribution in the second target area.

3. The diffractive optical waveguide according to claim 1, characterized in that: The liquid crystal material layer is a multi-layer stacked structure, and the concentrations of chiral materials doped in different layers of liquid crystal materials are different.

4. The diffractive optical waveguide according to claim 1, characterized in that: The thickness of the liquid crystal polarizer grating formed in the target area gradually increases along the direction of light propagation, so that the diffraction efficiency of the liquid crystal polarizer grating formed in the target area gradually increases along the direction of light propagation, and the change rate of the thickness of the liquid crystal polarizer grating along the direction of light propagation is less than or equal to 0.8um / mm.

5. The diffractive optical waveguide according to claim 1, characterized in that: The thickness of the liquid crystal polarizer grating formed in the target area satisfies the following relationship: Where h is the thickness of the liquid crystal polarizer grating at a distance l from the initial position along the direction of light propagation, L is the total length of the liquid crystal polarizer grating from the initial position along the direction of light propagation, H max H is the maximum thickness of the liquid crystal polarizer grating along the direction of light propagation, min It is the minimum thickness of the liquid crystal polarizer grating along the direction of light propagation.

6. A method for preparing a diffractive optical waveguide, for preparing the diffractive optical waveguide as claimed in any one of claims 1 to 5, characterized in that: include: Obtaining a substrate, wherein the substrate includes two surfaces parallel to each other; Processing one of the two surfaces so that the thickness of the substrate between the target area of ​​the surface and the other of the two surfaces is non-uniformly distributed; forming a first material layer on the treated surface, and performing double-beam interference exposure on the first material layer to form an alignment layer having a periodic orientation; A liquid crystal material layer is formed on the surface of the alignment layer, and the top of the liquid crystal material layer is flat.

7. The preparation method according to claim 6, characterized in that: The step of processing one of the two surfaces so that the substrate thickness between the target area of ​​the surface and the other of the two surfaces is non-uniformly distributed comprises: Based on the expected grating thickness distribution in the target area, the target area of ​​one of the two surfaces is continuously etched, and the etching beam spot size and the moving step are controlled so that the substrate thickness between the target area of ​​the surface and the other of the two surfaces is non-uniform and continuously distributed.

8. The preparation method according to claim 6, characterized in that: The step of processing one of the two surfaces so that the substrate thickness between the target area of ​​the surface and the other of the two surfaces is non-uniformly distributed comprises: Forming an embossed adhesive layer on one of the two surfaces, and performing structural transfer printing on the embossed adhesive layer using an embossing template, so that the thickness of the embossed adhesive layer is non-uniformly distributed in the target area; The embossed adhesive layer is used as a mask for etching to transfer the surface profile change of the embossed adhesive layer to the substrate, so that the substrate thickness between the target area of ​​the surface and the other surface is unevenly distributed.

9. The preparation method according to claim 6, characterized in that: The step of processing one of the two surfaces so that the substrate thickness between the target area of ​​the surface and the other of the two surfaces is non-uniformly distributed comprises: Based on the expected grating thickness distribution in the target area, the target area of ​​one of the two surfaces is etched in sections and steps, so that the substrate thickness between the target area of ​​the surface and the other of the two surfaces is non-uniform and gradient distributed.

10. The preparation method according to claim 6, characterized in that: The number of the target areas is more than one, and a liquid crystal material layer is formed on the surface of the alignment layer, and the top of the liquid crystal material layer is flat, comprising: The liquid crystal material is dotted or sprayed on the alignment layer of each target area respectively, and the concentration of the chiral material doped in the liquid crystal material in different target areas is different.

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