Waveguide grating for optical pupil expansion, design method, and display device

Abstract

The application discloses a waveguide grating and a design method thereof. The method comprises the following steps: obtaining basic parameters; initializing a grating structure; changing an optical unit structure through iterative operation; calculating performance parameters of the grating; and determining the optical unit structure corresponding to the performance parameters meeting an optimization target as part of an output of an optimization scheme of the waveguide grating, wherein the performance parameters comprise intermediate coupling-out efficiency, first side coupling-out efficiency and second side coupling-out efficiency, and the optimization target comprises maximizing the side coupling-out efficiency under the condition that the difference between the intermediate coupling-out efficiency and the two side coupling-out efficiencies is less than or equal to a threshold value, and the difference between the two side coupling-out efficiencies is equal to another threshold value. The application further discloses a display device comprising the waveguide grating. The design method has great flexibility, significantly reduces the calculation amount of the optimization processing, and improves the design efficiency.

Description

Technical Field

[0001] This invention relates to diffraction-based display technology, particularly waveguide gratings for optical pupil expansion, design methods thereof, and display devices having such waveguide gratings. Background Technology

[0002] Diffraction-based display technology has developed rapidly in recent years and can be applied to display devices such as near-eye displays, head-mounted displays, and head-up displays. Diffractive waveguides are an important optical device used in diffractive display technology. A diffractive waveguide for display applications has a coupling grating and a coupling grating disposed on a waveguide substrate. The coupling grating couples incident light carrying image information into the waveguide substrate; the coupling grating propagates and expands the light carrying image information while simultaneously coupling the light out of the waveguide substrate, forming a coupled light field. The eye receives the light from the coupled light field, thereby allowing, for example, to observe the image carried by the incident light.

[0003] The coupling grating of a diffractive waveguide can be a two-dimensional grating. The optical unit structure in a two-dimensional grating is usually a circular, rectangular, or rhomboid structure with a cross-section. When light is coupled into such a coupling grating, a bright line appears in the middle, which will also reduce the splitting energy on both sides, which is not conducive to the light energy spreading to both sides and affects the uniformity of the waveguide light.

[0004] To improve brightness and uniformity, researchers have attempted to modify the shape of the optical unit structure of two-dimensional gratings. For example, Chinese patent application CN 110914724A utilizes three sets of one-dimensional gratings with an included angle of 60° to construct a new grating pattern (see Figures 14 and 18 in CN 110914724A). The optical unit structure in the intersecting grating pattern has multiple straight sides, which are substantially parallel to the sides of the three sets of one-dimensional rectangular gratings mentioned above. Another example is Chinese patent CN 212460098U, where the optical unit structure of the two-dimensional grating is formed by the intersection of two sets of one-dimensional rectangular gratings with an included angle of approximately 60°. Further deformation operations are performed on the intersecting unit structure, resulting in a modified parallelogram. This modified parallelogram has acute vertices at both ends and two vertices at the top and bottom in the middle, with a small gap between these two vertices. Furthermore, the four long sides on both sides are parallel to each other, and the four short sides in the middle are also parallel to each other. The coupling grating with an improved parallelogram optical unit structure can reduce the central bright stripe of the coupled light field and significantly improve the uniformity between different fields of view of the coupled light field.

[0005] However, the machining of the multiple vertices and notches in the improved parallelogram is difficult, and the machining accuracy cannot be guaranteed, making mass production challenging. Furthermore, from the perspective of waveguide grating design methods, there is an urgent need to propose a flexible design method that can effectively obtain waveguide gratings with good coupling efficiency and uniformity. Summary of the Invention

[0006] The purpose of this invention is to provide a diffractive waveguide for diffractive displays and a display device including the diffractive waveguide, so as to at least partially overcome the shortcomings of the prior art.

[0007] According to one aspect of the present invention, a method for designing a waveguide grating for optical pupil expansion is provided, the waveguide grating comprising a waveguide substrate and a grating structure formed on the waveguide substrate, the design method comprising the following processes:

[0008] (1) Obtain the basic parameters of the waveguide grating, the basic parameters including the refractive index of the waveguide substrate, the refractive index of the grating structure layer and the operating wavelength of the waveguide substrate;

[0009] (2) Initialize the grating structure such that the grating structure includes a plurality of optical unit structures arranged in an array in a plane and has a first grating vector, a second grating vector and a third grating vector, wherein the direction of the third grating vector is between the directions of the first grating vector and the second grating vector, and the third grating vector is the vector sum of the first grating vector and the second grating vector, and the optical unit structure has structural parameters;

[0010] (3) Through multiple iterative calculations, the structural parameters of the optical unit structure are changed. Based on the basic parameters and the structural parameters of the optical unit structure, the performance parameters of the grating structure are calculated. The structural parameters of the optical unit structure corresponding to the performance parameters that satisfy the optimization objective are determined as optimization parameters. The performance parameters include intermediate coupling efficiency CO, first lateral coupling efficiency SO1, and second lateral coupling efficiency SO2. The optimization objective includes:

[0011] In satisfying Less than or equal to the first threshold V1 and Under the condition that it is less than or equal to the second threshold V2, the sum of the first lateral coupling efficiency SO1 and the second lateral coupling efficiency SO2 is the maximum value, where:

[0012] The intermediate coupling efficiency CO = η e / η a ,η aη represents the light energy of an incident light beam that propagates in the waveguide substrate via total internal reflection and is incident on the grating structure in the plane approximately along the direction of the third grating vector. e The light energy of the incident beam after diffraction by the grating structure and coupling out from the intermediate coupled beam of the waveguide substrate.

[0013] First lateral coupling efficiency η b η is the light energy of the first lateral propagating beam, which propagates in the waveguide substrate via total internal reflection in the plane, substantially along the direction of the first grating vector, after the incident beam has been diffracted by the grating structure. c The light energy of the first laterally propagated beam coupled out from the waveguide substrate after diffraction is the light energy of the first laterally coupled beam.

[0014] Second lateral coupling efficiency η b′ η is the light energy of the second lateral propagating beam, which propagates in the waveguide substrate via total internal reflection in the plane, substantially along the direction of the second grating vector, after the incident beam has been diffracted by the grating structure. c′ The light energy of the second laterally propagated beam coupled out from the waveguide substrate after diffraction; and

[0015] (4) Output the optimized scheme of the waveguide grating, the optimized scheme including the optimized parameters of the optical unit structure.

[0016] Advantageously, the performance parameters may further include intermediate total internal reflection efficiency CR, first lateral total internal reflection efficiency SR1, and second lateral total internal reflection efficiency SR2, and the optimization objectives may further include: intermediate total internal reflection efficiency CR being greater than or equal to a third threshold V3, first lateral total internal reflection efficiency SR1 being greater than or equal to a fourth threshold V4, and second lateral total internal reflection efficiency SR2 being greater than or equal to a fifth threshold V5, wherein:

[0017] The intermediate total internal reflection efficiency CR = η f / η a η f The light energy of the zero-order diffracted beam after the incident beam has passed through the grating structure is given.

[0018] The first lateral total internal reflection efficiency SR1 = η d / η b η d The light energy of the zero-order diffracted beam after the first lateral propagating beam has passed through the grating structure is given.

[0019] The second lateral total reflection efficiency SR2 = η d′ / ηb′ , η d′ is the light energy of the zero-order diffracted beam after the second laterally propagating beam is diffracted by the grating structure.

[0020] In some embodiments, as the number of iterations increases, the first threshold V1 gradually decreases from an initial value until it decreases to between 5% and 20%, preferably between 10% and 15%.

[0021] Advantageously, the initial value may be between 70% and 90%.

[0022] Advantageously, the first threshold V1 is gradually decreased in stages.

[0023] Advantageously, when the performance parameter satisfying the optimization objective meets: where 70%≤α≤95%, the first threshold V1 is decreased to βV1, where α≤β<1.

[0024] Advantageously, the second threshold V2 is between 0 and 20%.

[0025] Advantageously, the third threshold V3 is between 80% and 95%.

[0026] Advantageously, the fourth threshold V4 and the fifth threshold V5 are between 80% and 95%.

[0027] Advantageously, the initialization includes: selecting an initial structure of the optical unit structure; and determining an arrangement of the optical unit structures.

[0028] Advantageously, the initial structure of the optical unit structure has a diamond-shaped, circular, elliptical or rectangular cross-section.

[0029] Advantageously, the determining the arrangement of the optical unit structures comprises: arranging the optical unit structures into a plurality of rows, the plurality of rows extending along a first direction and arranged at predetermined intervals in a second direction perpendicular to the first direction, the plurality of optical unit structures in each row are arranged along the first direction with a period P, and the optical unit structures in two adjacent rows are staggered by a predetermined distance s along the first direction, s=P / n, where 1<n≤10, preferably n=2.

[0030] In some embodiments, n≠2, and the fourth threshold V4 and the fifth threshold V5 satisfy: V4≠V5.

[0031] Advantageously, the modification of the structural parameters of the optical unit structure includes: using a simulated annealing (SA) algorithm, a genetic algorithm (GA) algorithm, or a simulated annealing-particle swarm optimization (SA-PSO) algorithm to change the cross-section of the optical unit structure.

[0032] Advantageously, determining the structural parameters of the optical unit structure corresponding to the performance parameters that satisfy the optimization objective as optimization parameters includes:

[0033] Record the performance parameters as the optimal performance parameters;

[0034] After changing the structural parameters of the optical unit structure and calculating new performance parameters based on the changed structural parameters, the new performance parameters are compared with the optimal performance parameters. If the new performance parameters better meet the optimization objective compared to the optimal performance parameters, the new performance parameters are recorded to replace the optimal performance parameters.

[0035] After ceasing further changes to the structural parameters of the optical unit structure, the structural parameters of the optical unit structure corresponding to the optimal performance parameters are extracted and used as the optimization parameters of the optical unit structure.

[0036] Advantageously, the design method may also include the following processes:

[0037] (5) Based on the optimization scheme of the waveguide grating, optical waveguide tracing simulation is performed on the waveguide grating to calculate the uniformity and coupling efficiency of the coupled light field formed after the light passes through the entire waveguide grating multiple times.

[0038] According to another aspect of the present invention, a waveguide grating for optical pupil expansion is provided, the waveguide grating comprising a waveguide substrate and a grating structure formed on the waveguide substrate, the grating structure comprising a plurality of optical unit structures arranged in an array in a plane and having a first grating vector, a second grating vector, and a third grating vector, the direction of the third grating vector being between the directions of the first grating vector and the second grating vector, and the third grating vector being the vector sum of the first grating vector and the second grating vector, the optical unit structure having structural parameters configured such that:

[0039]

[0040] and

[0041] SO1 + SO2 ≥ A

[0042] in:

[0043] V1 is the first threshold;

[0044] V2 is the second threshold;

[0045] A is the predetermined target value; CO is the intermediate coupling efficiency, CO = η e / η a ,η a η represents the light energy of an incident light beam that propagates in the waveguide substrate via total internal reflection and is incident on the grating structure in the plane approximately along the direction of the third grating vector. e The light energy of the incident light beam after it has been diffracted by the grating structure and then coupled out from the middle of the waveguide substrate;

[0046] SO1 represents the first lateral coupling efficiency. η b η is the light energy of the first lateral propagating beam, which propagates in the waveguide substrate via total internal reflection in the plane, substantially along the direction of the first grating vector, after the incident beam has been diffracted by the grating structure. c The light energy of the first laterally propagated beam coupled out from the waveguide substrate after diffraction is the light energy of the first laterally coupled beam.

[0047] SO2 represents the second lateral coupling efficiency. η b′ η is the light energy of the second lateral propagating beam, which propagates in the waveguide substrate via total internal reflection in the plane, substantially along the direction of the second grating vector, after the incident beam has been diffracted by the grating structure. c′ The light energy of the second laterally propagated beam coupled out from the waveguide substrate after diffraction is the light energy of the second laterally coupled beam.

[0048] Advantageously, the structural parameters are further configured such that: CR≥V3; SR1≥V4; and SR2≥V5, where: V3 is a third threshold; V4 is a fourth threshold; and V5 is a fifth threshold;

[0049] CR is the intermediate total internal reflection efficiency, CR = η f / η a η f The light energy of the zeroth-order diffracted beam after the incident beam has passed through the grating structure;

[0050] SR1 is the first lateral total internal reflection efficiency, SR1 = η d / η b η dis the light energy of the zero-order diffracted beam after the first laterally propagating beam is diffracted by the grating structure;

[0051] SR2 is the second lateral total reflection efficiency, SR2=η d′ / η b′ , η d′ is the light energy of the zero-order diffracted beam after the second laterally propagating beam is diffracted by the grating structure.

[0052] Advantageously, the first threshold value V1 is between 5% and 20%, preferably between 10% and 15%.

[0053] Advantageously, the second threshold value V2 is between 0 and 20%.

[0054] Advantageously, the third threshold value V3 is between 80% and 95%.

[0055] Advantageously, the fourth threshold value V4 and the fifth threshold value V5 are between 80% and 95%.

[0056] Advantageously, the optical unit structures are arranged in a plurality of rows, the plurality of rows extend along a first direction and are arranged at predetermined intervals in a second direction perpendicular to the first direction, a plurality of the optical unit structures in each row are arranged at a period P along the first direction, and the optical unit structures in two adjacent rows are staggered by a predetermined distance s along the first direction, s=P / n, wherein 1<n≤10, preferably n=2.

[0057] In some embodiments, n≠2, and the fourth threshold value V4 and the fifth threshold value V5 satisfy: V4≠V5.

[0058] According to another aspect of the present invention, there is provided a display device comprising the waveguide grating as described above.

[0059] Advantageously, the display device is a near-eye display device and comprises a lens and a frame for holding the lens close to an eye, the lens comprising the waveguide grating.

[0060] Advantageously, the display device is an augmented reality display device or a virtual reality display device.

[0061] The waveguide grating design method according to embodiments of the present invention has great flexibility, provides a practical approach for optimizing the grating structure under different conditions, and the waveguide grating design method according to embodiments of the present invention greatly reduces the calculation amount of optimization and improves the efficiency of optimal design of waveguide gratings. The waveguide grating according to embodiments of the present invention has excellent uniformity and out-coupling efficiency due to meeting the corresponding design requirements. Description of the Drawings

[0062] Other features, objects, and advantages of the invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0063] Figure 1 This is a general flowchart of the waveguide grating design method according to an embodiment of the present invention;

[0064] Figure 2 This is a schematic diagram of a waveguide grating, which schematically shows... Figure 1 An example of the grating structure obtained through initialization in the design method shown;

[0065] Figure 3 schematically shown Figure 2 The grating vector of the grating structure in the waveguide grating shown;

[0066] Figure 4 The diagram schematically illustrates how light is coupled out of the waveguide substrate and propagates within the waveguide substrate through diffraction and total internal reflection in a two-dimensional grating structure.

[0067] Figure 5 A flowchart illustrating an example of a waveguide grating design method according to an embodiment of the present invention;

[0068] Figure 6 A flowchart illustrating another example of a waveguide grating design method according to an embodiment of the present invention;

[0069] Figure 7 The curves showing the intermediate and lateral coupling efficiencies as a function of the number of iterations in the performance parameters that satisfy the optimization objective, obtained in an exemplary design example, are shown.

[0070] Figure 8 The curves showing the intermediate total internal reflection efficiency and lateral total internal reflection efficiency as a function of the number of iterations are presented in the performance parameters that meet the optimization objectives obtained in the above exemplary design example.

[0071] Figure 9 The process of changing the cross-sectional shape of the optical unit of the grating structure in the above exemplary design example is illustrated schematically;

[0072] Figure 10 The optimized waveguide grating obtained in the above exemplary design example is illustrated schematically;

[0073] Figure 11 The schematic diagram illustrates the viewport of the waveguide grating;

[0074] Figure 12 A schematic diagram illustrating the partitioning of the waveguide grating window; and

[0075] Figure 13For the waveguide grating with the initialized grating structure and the waveguide grating with the optimized grating structure in the above exemplary design examples, Figure 12 The diagram shows the distribution of light energy within the field of view in the central partition. Detailed Implementation

[0076] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. It is to be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. For ease of description, only the parts relevant to the invention are shown in the drawings. It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other.

[0077] Figure 1 This is a general flowchart of a waveguide grating design method 100 according to an embodiment of the present invention. The method 100 can design waveguide gratings for optical pupil expansion. Figure 2 A waveguide grating 10 for optical pupil expansion is schematically shown.

[0078] like Figure 2 As shown, the waveguide grating 10 includes a waveguide substrate 10a and a grating structure 1 formed on the waveguide substrate 10a. According to an embodiment of the present invention, the grating structure 1 serves as a coupling grating 11 in the waveguide grating 10, for simultaneously coupling light coupled therein (… Figure 2 The dashed arrows schematically illustrate the direction of light coupling (IN) as it propagates and expands, while simultaneously coupling the light out of the waveguide substrate 10a, forming an output light field. Figure 2 In the image, the grating structure 1 is shown in a magnified form within the dashed box next to the waveguide grating 10.

[0079] like Figure 1 As shown, design method 100 includes the following processes:

[0080] S110: Obtain the basic parameters of the waveguide grating;

[0081] S120: Initializes the grating structure of the waveguide grating;

[0082] S130: Through multiple iterative calculations, the structural parameters of the optical unit structure of the grating are changed. Based on the basic parameters and the structural parameters of the optical unit structure, the performance parameters of the grating are calculated, and the structural parameters of the optical unit structure corresponding to the performance parameters that satisfy the optimization objective are determined as the optimization parameters; and

[0083] S140: Optimized scheme for output waveguide grating.

[0084] The basic parameters obtained in process S110 include the refractive index of the waveguide substrate, the refractive index of the grating structure layer, and the operating wavelength of the waveguide substrate. Optionally, in some embodiments, other parameters, such as the thickness of the waveguide substrate and the desired field of view range of the waveguide grating, may be further obtained in process S110.

[0085] In process S120, the grating structure is initialized, which involves constructing the grating structure. For example... Figure 2 As shown, the initialized grating structure 1 includes a plane ( Figure 2 Multiple optical unit structures 1a are arranged in an array within the plane (xy) shown, and have a first grating vector G1, a second grating vector G2 and a third grating vector G3. Figure 3 The diagram schematically illustrates the grating vectors G1, G2, and G3 of the grating structure 1, wherein the direction of the third grating vector G3 lies between the directions of the first grating vector G1 and the second grating vector G2, and the third grating vector G3 is the vector sum of the first grating vector G1 and the second grating vector G2. After initialization, the optical unit structure 1a has multiple structural parameters, including but not limited to parameters relating to the cross-sectional shape and size of the optical unit structure 1a, and / or parameters relating to the depth, height, or morphology of the optical unit structure 1a in the direction perpendicular to the xy plane.

[0086] In process S130, the performance parameters of the grating structure are calculated, and the optimization parameters of the optical unit structure are determined based on the calculated performance parameters and the optimization objective. To facilitate understanding of the optimization objective and the performance parameters used in the design method according to embodiments of the present invention, the following first refers to... Figure 4 This paper describes how light is coupled out of the waveguide substrate through diffraction and total internal reflection in grating structure 1, and how it propagates within the waveguide substrate.

[0087] Figure 4 The middle circle is used to schematically represent the illumination spot LS of light in the xy plane. The illumination spot LS located within the region of grating structure 1 represents the position where light diffracts on grating structure 1. It should be understood that... Figure 4 The distance between the two diffraction points of the light was magnified to more clearly show the diffraction effect of the grating structure.

[0088] See Figure 4 The straight arrow 'a' indicates propagation in the waveguide substrate via total internal reflection and approximately along the third grating vector G3 in the xy plane (see...). Figure 3 An incident beam a is incident on grating structure 1 in the direction of ); after diffraction by grating structure 1, the incident beam a is coupled outward from the waveguide substrate to form an intermediate coupled beam e. Figure 4 The image in the middle is marked with "×".

[0089] After the incident beam a is diffracted by the grating structure, a zero-order diffracted beam f is obtained. This zero-order diffracted beam f can continue to propagate approximately along the direction of the third grating vector G3 within the waveguide substrate.

[0090] Meanwhile, after being diffracted by grating structure 1, the incident beam a still lies approximately along the first grating vector G1 and the second grating vector G2 in the xy plane (see... Figure 3 The direction of the beam splits into two lateral propagation beams, namely the first lateral propagation beam b and the second lateral propagation beam b'.

[0091] The first laterally propagating beam b, after being diffracted by the grating structure, is coupled outward from the waveguide substrate as a beam c; similarly, the second laterally propagating beam b', after being diffracted by the grating structure, is coupled outward from the waveguide substrate as a beam c'. The first laterally coupled beam c and the second laterally coupled beam c'... Figure 4 The image in the middle is marked with "×".

[0092] Meanwhile, the first lateral propagating beam b, after being diffracted by the grating structure, becomes a zero-order diffracted beam d, which can continue to propagate approximately along the direction of the first grating vector G1 within the waveguide substrate through total internal reflection; similarly, the second lateral propagating beam b', after being diffracted by the grating structure, becomes a zero-order diffracted beam d', which can continue to propagate approximately along the direction of the second grating vector G2 within the waveguide substrate.

[0093] Return to reference Figure 1 Next, we will introduce the processing of S130.

[0094] According to an embodiment of the present invention, the performance parameters of the grating structure calculated in processing S130 include the intermediate coupling efficiency CO, the first lateral coupling efficiency SO1, and the second lateral coupling efficiency SO2.

[0095] Specifically, the intermediate coupling efficiency CO = η e / η a , where η a Let η be the light energy of the incident beam a. e The light energy of the beam e coupled out from the middle;

[0096] First lateral coupling efficiency Where η b η is the light energy of the first lateral propagating beam b. c The light energy of the first laterally coupled beam c;

[0097] Second lateral coupling efficiency Where η b′ For the light energy of the second lateral propagating beam b', η c′The light energy of the second laterally coupled beam c' is given.

[0098] Accordingly, the optimization objectives adopted in processing 130 include: satisfying Less than or equal to the first threshold V1 and Under the condition that it is less than or equal to the second threshold V2, the sum of the first lateral coupling efficiency SO1 and the second lateral coupling efficiency SO2 is the maximum value.

[0099] As can be seen, the first lateral coupling efficiency is essentially... Second lateral coupling efficiency The intermediate coupling efficiency CO = η e / η a Taking all factors into consideration, we can see the relationship between these three coupling efficiencies (CO, SO1, SO2) and their magnitudes, reflecting the relationship between the light energy coupled at the middle position and at the two lateral positions. Therefore, by limiting... If the light energy difference is less than or equal to the first threshold V1, the difference between the light energy of the intermediate coupled beam e and the two lateral coupled beams b and b' can be limited, thereby ensuring the uniformity of the coupled light field of the grating structure.

[0100] The first threshold V1 can be set according to the desired uniformity of the coupled optical field. In some embodiments, to achieve good uniformity, the first threshold V1 is preferably between 5% and 20%. Taking into account both the desired uniformity and the effectiveness and efficiency of the design method 100, the first threshold V1 is preferably controlled between 10% and 15%. According to different embodiments of the present invention, in processing S130, the first threshold can be predetermined and fixed, or it can be gradually reduced as the optimization process proceeds. This will be discussed in conjunction with... Figure 6 This paper introduces a specific example of gradually reducing the first threshold based on the optimization effect at each stage to improve uniformity and achieve effective and efficient waveguide grating optimization design, which will not be elaborated here.

[0101] Grating structure 1 can be Figure 4 When a structure has a symmetrical structure in the y-direction, it also possesses symmetrical optical properties. Therefore, the first lateral coupling efficiency SO1 and the second lateral coupling efficiency SO2 are equal, i.e., SO1 = SO2 = SO. Thus, in this case, the above conditions... Less than or equal to the first threshold V1, this can be specifically implemented in processing S130 as follows: Less than or equal to the first threshold V1.

[0102] According to an embodiment of the present invention, the optimization objective requires Less than or equal to the second threshold V2. When the diffraction properties of the grating structure 1 in the directions of the first grating vector G1 and the second grating vector G2 are not symmetrical, by limiting the difference between the first lateral coupling efficiency SO1 and the second lateral coupling efficiency SO2, a uniform coupled light field can still be obtained on both sides relative to the opposite side. This difference The smaller the value, the more uniform the two sides are relative to each other. Preferably, the second threshold V2 is between 0% and 20%.

[0103] The optimization objective requires the sum of the first lateral coupling efficiency SO1 and the second lateral coupling efficiency SO2 to be maximized. This is because in existing waveguide gratings, the coupled optical field typically has a central bright line corresponding to the central coupled beam, while the portions on either side of the coupled optical field corresponding to the first and second lateral coupled beams are relatively dark. According to an embodiment of the present invention, by selecting an optical unit structure that maximizes the sum of the first lateral coupling efficiency SO1 and the second lateral coupling efficiency SO2, a grating structure with brighter lateral portions of the coupled optical field can be obtained, overcoming the shortcomings of existing waveguide gratings.

[0104] Grating structure 1 in Figure 4 When the structure and optical properties are symmetrical in the y-direction, SO1 = SO2 = SO. In this case, the above optimization objective, namely the sum of the first lateral coupling efficiency SO1 and the second lateral coupling efficiency SO2, is the maximum value. In processing S130, it can be simplified to the first lateral coupling efficiency SO1 or the second lateral coupling efficiency SO2 being the maximum value.

[0105] In some embodiments, the performance parameters of the grating structure calculated in processing S130 may further include the intermediate total internal reflection efficiency CR, the first lateral total internal reflection efficiency SR1, and the second lateral total internal reflection efficiency SR2.

[0106] Specifically, the intermediate total internal reflection efficiency CR = η f / η a , where η a Let η be the light energy of the incident beam a. f Let f be the light energy of the zeroth-order diffracted beam f after the incident beam a has passed through the grating structure;

[0107] First lateral total internal reflection efficiency SR1 = η d / η b , where η b η is the light energy of the first lateral propagating beam b. d The light energy of the zero-order diffracted beam d after the first lateral propagating beam b passes through the grating structure;

[0108] Second lateral total internal reflection efficiency SR2=η d′ / η b，, where η b， For the light energy of the second lateral propagating beam b', η d′ The light energy of the zero-order diffracted beam d' after the second lateral propagating beam b' passes through the grating structure.

[0109] Accordingly, in such an embodiment, the optimization objective may further include: the intermediate total reflection efficiency CR is greater than or equal to the third threshold V3, the first lateral total reflection efficiency SR1 is greater than or equal to the fourth threshold V4, and the second lateral total reflection efficiency SR2 is greater than or equal to the fifth threshold V5.

[0110] The intermediate total internal reflection efficiency CR, the first lateral total internal reflection efficiency SR1, and the second lateral total internal reflection efficiency SR2 characterize the efficiency of light energy transferred in the original direction after diffraction, thereby affecting the propagation distance of light in the directions of the first grating vector G1, the second grating vector G2, and the third grating vector G3, respectively: the larger CR, SR1, and SR2 are, the farther the light propagates, and the larger the pupil expansion range that the grating structure can achieve. According to an embodiment of the present invention, the third threshold V3 is preferably between 80% and 95%. According to an embodiment of the present invention, the fourth threshold V4 and the fifth threshold V5 are preferably between 80% and 95%, respectively.

[0111] In process S130, the structural parameters of the optical unit structure of the grating structure are changed multiple times through iterative calculations. After each change of structural parameters, the aforementioned performance parameters are calculated, and the performance parameters that satisfy the optimization objective and their corresponding structural parameters of the optical unit structure are selected. The selected structural parameters of the optical unit structure are determined as optimization parameters and output as part of the optimization scheme of the waveguide grating in process S140. The optimization scheme output in process S140 may also include the basic parameters obtained in process S110 and the first to third grating vectors determined by initializing the grating structure in process S120; however, this is not limiting. If the aforementioned basic parameters and / or grating vectors are preset conditions for the waveguide grating design, the optimization scheme output in process S140 may no longer include the corresponding content.

[0112] Compared to existing waveguide grating design methods disclosed in, for example, Chinese patent applications CN 110914724A and CN 212460098 U, which determine the shape of the optical unit structure by imitating the superimposed pattern of two or three one-dimensional grating structures, the design method 100 according to the embodiments of the present invention has greatly improved flexibility. It breaks through the design concept of designing two-dimensional gratings based on the superposition of one-dimensional grating structures and provides a practical way to optimize grating structures under different conditions.

[0113] Furthermore, in the waveguide grating design method 100 according to an embodiment of the present invention, a specific optimization target is set for the optical unit structure of the grating structure. This optimization target is set based on the performance index (performance parameter) of a single diffraction or a very small number of diffractions of light in the grating structure. Therefore, during the optimization process of the design method 100 (corresponding to process S130), key evaluation indicators of the optical coupling efficiency and uniformity of the grating structure can be obtained with only a small amount of optical simulation computation. Thus, compared to grating optimization design methods that set optimization targets based on the overall coupling efficiency and / or uniformity of the grating structure, the waveguide grating design method 100 according to an embodiment of the present invention significantly reduces the computational load of optimization and improves the efficiency of waveguide grating optimization design.

[0114] Continue to refer to Figure 1 Optionally, the waveguide grating design method 100 according to an embodiment of the present invention further includes:

[0115] S150: An optimized scheme based on waveguide gratings is used to verify the optical efficiency of waveguide gratings.

[0116] Specifically, in processing S150, based on the optimized waveguide grating scheme, optical waveguide tracing simulation can be performed on the waveguide grating to calculate the uniformity and coupling efficiency of the coupled light field formed after multiple diffractions through the entire waveguide grating. The calculated uniformity and coupling efficiency can be used to verify whether the optimized waveguide grating output in processing S140 meets the design requirements of the waveguide grating.

[0117] Reference above Figures 1 to 4 The overall flow of the design method 100 according to an embodiment of the present invention has been described. For ease of understanding, the following will refer to... Figure 5 and Figure 6 Two examples of design methods according to embodiments of the present invention are presented.

[0118] Figure 5 This is a flowchart of an example of a waveguide grating design method according to an embodiment of the present invention, namely design method 100A. The processes S110 and S140 in design method 100A are related to the reference... Figure 1 The processes S110 and S140 described are the same, and will not be repeated here.

[0119] like Figure 5 As shown, in design method 100A, the grating structure is initialized through processes S121 and S122. Specifically, in process S121, the initial structure of the optical unit structure is selected; in process S122, the arrangement of the optical unit structure is determined. It should be understood that the order of processes S121 and S122 can be reversed, and the present invention is not limited in this respect.

[0120] In the design method according to embodiments of the present invention, the initial structure of the optical unit structure can be selected as having a cross-section of a rhombus, circle, ellipse, rectangle, or any other suitable shape. Selecting the initial structure of the optical unit structure may also include setting other structural parameters of the optical unit structure, including but not limited to depth or height; the present invention is not limited in this respect.

[0121] Return to reference Figure 2 The optical structure 1 shown in the figure, in process S122, determines the arrangement of the optical unit structure 1a, which may include: arranging the optical unit structure 1a into multiple rows, in Figure 2 In the diagram, the multiple rows extend along the y-direction and are arranged at predetermined intervals D in the x-direction perpendicular to the y-direction. Multiple optical unit structures 1a within each row are arranged periodically P along the y-direction, and the optical unit structures in adjacent rows are staggered by a predetermined distance s along the y-direction, where s = P / n. <n≤10。

[0122] exist Figure 2 In the example shown, n = 2, and the resulting grating structure 1 has a symmetrical structure in the y direction.

[0123] In some other cases, n can also be set to ≠ 2. In this case, during the optimization process, the fourth threshold V4 and the fifth threshold V5 can be made to satisfy: V4 ≠ V5.

[0124] exist Figure 5 In the example shown, the optimization process in the waveguide grating design method according to an embodiment of the present invention (corresponding to) Figure 1 The process S130 in the design method 100 shown is implemented by including multiple steps. Specifically, in process S131, the performance parameters of the grating structure are calculated; in process S132, it is determined whether the calculated performance parameters meet the optimization objective. The setting of the performance parameters and optimization objective involved is the same as that in the design method 100 described above, and will not be repeated here.

[0125] In process S132, if the judgment result is negative, the process proceeds to process S133. In process S133, the structural parameters of the optical unit structure are changed, and then the flow returns to process S131. Preferably, in process S133, a simulated annealing algorithm, a genetic algorithm, or a combination of particle swarm optimization and simulated annealing is used to change the cross-section of the optical unit structure, thereby changing the structural parameters of the optical unit structure. While a simple simulated annealing algorithm or genetic algorithm has stronger global search capabilities, its convergence speed is slower. Using a simulated annealing algorithm combined with particle swarm optimization can effectively improve the convergence speed.

[0126] In process S132, if the judgment result is positive, then proceed to process S134; in process S134, the current performance parameter is recorded as the optimal performance parameter.

[0127] After processing S134, proceed to processing S135 to determine if the condition for ending the iteration is met. If the result of processing S135 is negative, proceed to processing S133 to change the structural parameters of the optical unit structure and perform the next iteration calculation.

[0128] The conditions for ending the iteration can be set according to different design needs, and the present invention is not limited in this respect. The conditions for ending the iteration may be, for example, that the number of iterations reaches a certain threshold, or that one or more related indicators of the optimization objective exceed a certain threshold. For example, if other requirements in the optimization objective are met, the sum of the first lateral coupling efficiency SO1 and the second lateral coupling efficiency SO2 exceeds a threshold. The conditions for ending the iteration may also be, for example, a combination of the above-mentioned conditions.

[0129] In design method 100A, during processing S132, it is determined whether the optimization objective is met. The newly calculated performance parameter can be compared with the previously recorded optimal performance parameter. If the new performance parameter is more in line with the optimization objective than the previously recorded optimal performance parameter, the new performance parameter is recorded to replace the previous optimal performance parameter.

[0130] If the judgment result in process S135 is positive, the iteration ends and proceeds to process S136. In process S136, the structural parameters of the optical unit structure corresponding to the optimal performance parameters are extracted as optimization parameters.

[0131] Figure 6 This is a flowchart illustrating another example of a waveguide grating design method according to an embodiment of the present invention. Figure 6 The design method 100B shown is... Figure 5 The design method 100A shown is basically the same, except that: design method 100B also includes determining whether the condition for lowering the first threshold is met in process S134a. If the determination result is positive, then process S134b is entered; in process S134b, the first threshold is lowered.

[0132] Processes S134a and S134b are configured such that as the number of iterations increases, the first threshold V1 is gradually reduced from its initial value until it is reduced to between 5% and 20%, preferably to between 10% and 15%. The initial value of the first threshold is preferably between 70% and 90%.

[0133] The difference between the intermediate coupling efficiency CO and the first lateral coupling efficiency SO1 and the second lateral coupling efficiency SO2 at different stages of the optimization process. Significant changes are possible. By setting a high initial value for the first threshold, the optimization process can avoid failing to find performance parameters and corresponding optical unit structures that meet the optimization target due to an initially low first threshold, thus ensuring the effectiveness of the design method. At the same time, by gradually decreasing the first threshold as the number of iterations increases, the optimization process can be directed towards improving the uniformity of the coupled light field of the grating structure.

[0134] Preferably, the first threshold V1 is gradually reduced in stages. In a preferred implementation, when the performance parameter (V1) satisfies the optimization objective... Figure 6 The example shown represents the optimal performance parameters recorded in processing S134, which satisfy the following: When 70% ≤ α ≤ 95% (determined in processing S134a), the first threshold V1 is reduced to βV1, where α ≤ β < 1 (the first threshold is reduced in processing S134b).

[0135] It should be understood that, in Figure 6 In the example shown, processes S134a and S134b are set after process S134, i.e., after the current performance parameter is recorded as the optimal performance parameter; however, this is merely exemplary and not limiting. For example, the process of adjusting the first threshold (e.g., processes S134a and S134b) can be set after calculating the performance parameters of the grating structure based on the structural parameters of the changed optical unit structure (process S131). Alternatively, such a process can be set, for example, after determining in process S135 that the condition for ending the iteration is not met, and before proceeding to process S133 to change the optical unit structure. In summary, in other examples of the design method according to embodiments of the present invention, the process of adjusting the first threshold can be set in different stages of the optimization process.

[0136] To facilitate understanding, the following will be combined with... Figures 7 to 12 A design example is provided, illustrating the design method according to an embodiment of the present invention. In this design example:

[0137] 1) The basic parameters of the waveguide grating include: refractive index of the waveguide substrate 1.816, refractive index of the grating structure layer 1.816, and operating wavelength 532nm;

[0138] 2) Initialized grating structure as follows Figure 2As shown (see grating structure 1), the optical unit structure has a rhomboid cross-sectional shape with a side length of 260 nm and an acute apex angle of 50°. The optical unit structure is a concave hole structure formed in the waveguide substrate with a depth of 50 nm. The predetermined interval D = 450 nm, the period P = 420 nm, and the predetermined distance s = 210 nm. The grating structure formed by this arrangement has a symmetrical first grating vector G1 and second grating vector G2, and a third grating vector G3 located between the first grating vector G1 and the second grating vector G2.

[0139] 3a) Set the initial value of the first threshold V1 = 90%, the second threshold V2 = 0, the third threshold V3 = 85%, the fourth threshold V4 = 85%, and the fifth threshold V5 = 85%;

[0140] 3b) Assuming the incident angle of the light incident grating structure is 40.6°, the calculated performance parameters of the initialized grating structure are: CO = 3.75e-3, SO1 = SO2 = 4.45e-4, CR = 0.903, SR1 = SR2 = 0.895;

[0141] 3c) The parameters α = 0.8 and β = 0.8 are used to control the reduction of the first threshold. When the first threshold V1 is reduced to below 10%, the first threshold no longer changes.

[0142] 3d) The optical unit structure was optimized using the SA algorithm (by changing the structural parameters of the optical unit structure). The process ended after 3500 iterations. The calculated values ​​of the performance parameters of the grating structure after optimization were: CO = 9.05e-4, SO1 = SO2 = 8.25e-4, CR = 0.867, SR1 = SR2 = 0.889.

[0143] Figure 7 The curves showing the changes in intermediate coupling efficiency and lateral coupling efficiency as a function of the number of iterations in the performance parameters that meet the optimization objectives obtained in the above design example are shown. Figure 8 The curves showing the intermediate total internal reflection efficiency and lateral total internal reflection efficiency as a function of the number of iterations are presented in the performance parameters that meet the optimization objective obtained in the above design example. From... Figure 7 and Figure 8 As can be seen, the design method according to the embodiments of the present invention can efficiently converge and optimize in the direction of improving the lateral coupling efficiency on both sides and balancing the intermediate coupling efficiency and the lateral coupling efficiency, while ensuring that the intermediate total reflection efficiency and the lateral total reflection efficiency on both sides are above a predetermined level.

[0144] Figure 9The diagram illustrates the change process of the cross-sectional shape of the optical unit of the grating structure in the above design example, where figure (a) shows the initial cross-sectional shape, figure (d) shows the final optimized cross-sectional shape, and figures (b) and (c) show the cross-sectional shapes obtained in the intermediate stages.

[0145] Figure 10 The optimized waveguide grating obtained in the above design example is illustrated schematically. For example... Figure 10 As shown, the optimized waveguide grating 10' includes a grating structure 1' formed on the waveguide substrate 10a. This grating structure 1' has the optical unit structure array arrangement described in item 2) of the above design example, wherein the optical unit structure 1a' has the following characteristics: Figure 9 The cross-sectional shape is shown in figure (d). This grating structure 1' is used as a coupling grating 11' in the waveguide grating 10'.

[0146] To illustrate the design effect of the waveguide grating design method according to the embodiments of the present invention, simulation calculations were performed on the optical efficiency of the waveguide grating 10 with the initialized grating structure 1 and the waveguide grating 10' with the optimized grating structure 1'.

[0147] To better understand the simulation results, let's first look at... Figure 11 and Figure 12 , Figure 11 and Figure 12 The diagram schematically illustrates the eyebox of a waveguide grating. Specifically, as shown... Figure 11 As shown and as previously described, waveguide gratings 10 and 10' include grating structures 11 and 11' formed on the waveguide substrate. When the eye E is within a certain range of the coupled light field of the grating structures 11 and 11', the eye E can observe the complete image displayed through the waveguide grating. This range is the viewing window EB of the waveguide grating. Figure 11 As shown in the diagram, the viewing window EB has a certain range within the plane of the waveguide substrate parallel to the waveguide gratings 10 and 10'. Figure 12 The area of ​​the viewport EB is schematically shown as a rectangle, divided into 9 partitions. In the above simulation of the waveguide grating's optical effect, the uniformity and coupling efficiency in each of the 9 partitions were simulated and calculated.

[0148] also, Figure 13 In the above exemplary design example, waveguide grating 10 and waveguide grating 10' are... Figure 12 The light energy distribution map (the distribution map of light energy as the field of view changes) within the field of view of the central partition (i.e., partition number 5) of the window shown. Figure 13 The left-hand chart shows the light energy distribution of waveguide grating 10 in the central partition, and the right-hand chart shows the light energy distribution of waveguide grating 10' in the central partition. Figure 13 In the chart shown, the horizontal axis represents the field of view angle that varies around the x-axis, and the vertical axis represents the field of view angle that varies around the y-axis. Different gray levels represent different brightness levels. The numbers in the rectangular grayscale image in the middle of the chart indicate the maximum and minimum light intensities in the corresponding distribution map.

[0149] The waveguide grating 10 obtained from the above optical effect simulation calculation (e.g.) Figure 2 The uniformity index UNI and coupling efficiency index EFF of each window partition (as shown in the figure) are shown in Table 1 below:

[0150] [Table 1]

[0151]

[0152] The above optical effect simulation calculations yielded waveguide grating 10' (e.g. Figure 10 The uniformity index UNI and coupling efficiency index EFF of each window partition (as shown in the figure) are shown in Table 2 below:

[0153] [Table 2]

[0154]

[0155] Here, the uniformity index UNI is the ratio of the maximum light intensity to the minimum light intensity in the coupled light field of each window partition. The smaller the ratio, the better the uniformity. The coupling efficiency EFF is the ratio of the average light intensity at each field of view angle of the coupled light field in each window partition to the light intensity of the coupled light from the grating structure. The larger the EFF value, the higher the coupling efficiency.

[0156] Comparing Tables 1 and 2, it can be seen that the waveguide grating has been effectively optimized by the design method according to the embodiment of the present invention, which significantly improves the uniformity of the coupled optical field; the overall coupling efficiency of each partition changes little.

[0157] At the same time, from Figure 13 As can be seen, before optimization, the light intensity of waveguide grating 10 in the central partition (partition number 5) of window EB varies greatly across different field of view angles, resulting in an uneven light energy distribution with a bright center and dark sides. In contrast, after optimization, the uniformity of light intensity distribution across different field of view angles is significantly improved in the same partition, particularly in the two side fields of view (FOVX ranges of -15° to -6° and 6° to 15°). With the overall coupling efficiency of waveguide grating 10 and waveguide grating 10' in the central partition being roughly the same, simulation calculations show that the coupling efficiency of waveguide grating 10 in the two side fields of view is 1.8e-4, while the coupling efficiency of waveguide grating 10' in the two side fields of view after optimization is 2.7e-4. This indicates that optimization greatly improves the coupling efficiency of the two side fields of view, resulting in a more balanced coupling efficiency between the central and side fields of view.

[0158] According to an embodiment of the present invention, there is also provided a waveguide grating obtained by the above design method, for example Figure 10 the waveguide grating 10' shown. The waveguide grating comprises a waveguide substrate and a grating structure formed on the waveguide substrate, the grating structure comprises a plurality of optical unit structures arranged in an array in a plane and has a first grating vector, a second grating vector and a third grating vector, and the direction of the third grating vector is between the directions of the first grating vector and the second grating vector (see, for example, Figure 3 ). In the waveguide grating according to an embodiment of the present invention, the optical unit structures have structural parameters configured to satisfy:

[0159] and SO1+SO2≥A.

[0160] As exemplarily shown in the above design example, the structural parameters of the optical unit structure 1a' can be further configured to satisfy: CR≥V3; SR1≥V4; and SR2≥V5.

[0161] The first threshold V1 can be between 5% and 20%, preferably between 10% and 15%.

[0162] The second threshold V2 can be between 0 and 20%.

[0163] The third threshold V3 can be between 80% and 95%.

[0164] The fourth threshold V4 and the fifth threshold V5 can each be between 80% and 95%.

[0165] In the waveguide grating according to an embodiment of the present invention, the optical unit structures in the optical structure may be arranged in a plurality of rows, the plurality of rows extend along a first direction and are arranged at predetermined intervals in a second direction perpendicular to the first direction, the plurality of optical unit structures in each row are arranged at a period P along the first direction, and the optical unit structures in two adjacent rows are staggered by a predetermined distance s along the first direction, s=P / n, where 1<n≤10, preferably n=2.

[0166] When n≠2, the fourth threshold V4 and the fifth threshold V5 may not be equal, that is, V4≠V5.

[0167] The waveguide grating according to an embodiment of the present invention can be applied to a display device. Such a display device is, for example, a near-eye display device, which comprises an eyeglass lens and a frame for holding the eyeglass lens close to an eye, wherein the eyeglass lens may comprise the waveguide grating according to an embodiment of the present invention as introduced above. Preferably, the display device can be an augmented reality display device or a virtual reality display device.

[0168] The above description is merely a preferred embodiment of this application and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of the invention involved in this application is not limited to technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the inventive concept. For example, technical solutions formed by substituting the above features with (but not limited to) technical features with similar functions disclosed in this application.

Claims

1. A design method for a waveguide grating for optical pupil expansion, the waveguide grating comprising a waveguide substrate and a grating structure formed on the waveguide substrate, the design method comprising the following processes: (1) Obtain the basic parameters of the waveguide grating, the basic parameters including the refractive index of the waveguide substrate, the refractive index of the grating structure layer and the operating wavelength of the waveguide substrate; (2) Initialize the grating structure such that the grating structure includes a plurality of optical unit structures arranged in an array in a plane and has a first grating vector, a second grating vector and a third grating vector, wherein the direction of the third grating vector is between the directions of the first grating vector and the second grating vector, and the third grating vector is the vector sum of the first grating vector and the second grating vector, and the optical unit structure has structural parameters; (3) Through multiple iterative calculations, the structural parameters of the optical unit structure are changed. Based on the basic parameters and the structural parameters of the optical unit structure, the performance parameters of the grating structure are calculated, and the structural parameters of the optical unit structure corresponding to the performance parameters that satisfy the optimization objective are determined as optimization parameters. The performance parameters include the intermediate coupling efficiency CO, the first lateral coupling efficiency SO1, and the second lateral coupling efficiency SO2. The optimization objective includes: In satisfying Less than or equal to the first threshold V1 and Under the condition that it is less than or equal to the second threshold V2, the sum of the first lateral coupling efficiency SO1 and the second lateral coupling efficiency SO2 is the maximum value, where: The intermediate coupling efficiency , The light energy of the incident light beam that propagates in the waveguide substrate by total internal reflection and is incident on the grating structure in the plane approximately along the direction of the third grating vector. The light energy of the incident beam after diffraction by the grating structure and coupling out from the intermediate coupled beam of the waveguide substrate. First lateral coupling efficiency , The light energy of the first lateral propagating beam, which propagates within the waveguide substrate in a total internal reflection manner in the plane generally along the direction of the first grating vector after the incident beam has been diffracted by the grating structure. The light energy of the first laterally propagated beam coupled out from the waveguide substrate after diffraction is the light energy of the first laterally coupled beam. Second lateral coupling efficiency , The light energy of the second lateral propagating beam, which is the light energy of the incident beam after diffraction by the grating structure, propagating in the plane generally along the direction of the second grating vector within the waveguide substrate in a manner of total internal reflection. The light energy of the second laterally propagated beam coupled out from the waveguide substrate after diffraction; and (4) Output the optimized scheme of the waveguide grating, the optimized scheme including the optimized parameters of the optical unit structure.

2. The design method as described in claim 1, wherein, The performance parameters also include the intermediate total internal reflection efficiency CR, the first lateral total internal reflection efficiency SR1, and the second lateral total internal reflection efficiency SR2. Furthermore, the optimization objectives include: the intermediate total internal reflection efficiency CR being greater than or equal to a third threshold V3, the first lateral total internal reflection efficiency SR1 being greater than or equal to a fourth threshold V4, and the second lateral total internal reflection efficiency SR2 being greater than or equal to a fifth threshold V5, wherein: The intermediate total internal reflection efficiency , The light energy of the zero-order diffracted beam after the incident beam has passed through the grating structure is given. First lateral total reflection efficiency , The light energy of the zero-order diffracted beam after the first lateral propagating beam has passed through the grating structure is given. Second lateral total reflection efficiency , The light energy is the zero-order diffracted beam after the second lateral propagating beam has passed through the grating structure.

3. The design method as described in claim 1 or 2, wherein, As the number of iterations increases, the first threshold V1 gradually decreases from its initial value until it decreases to between 5% and 20%.

4. The design method of claim 3, wherein, As the number of iterations increases, the first threshold V1 gradually decreases from its initial value until it drops to between 10% and 15%.

5. The design method of claim 3, wherein, The initial value is between 70% and 90%.

6. The design method of claim 3, wherein, The first threshold V1 is gradually reduced in stages.

7. The design method of claim 5, wherein, The first threshold V1 is gradually reduced in stages.

8. The design method as described in claim 6, wherein, When the performance parameter satisfying the optimization target satisfies: wherein the first threshold V1 is reduced to wherein .

9. The design method as described in claim 7, wherein, When the performance parameter satisfying the optimization target satisfies: wherein the first threshold V1 is reduced to wherein .

10. The design method of claim 1 or 2, wherein, The second threshold V2 is between 0% and 20%.

11. The design method as described in claim 2, wherein, The third threshold V3 is between 80% and 95%.

12. The design method of claim 2, wherein, The fourth threshold V4 and the fifth threshold V5 are between 80% and 95%.

13. The design method of claim 12, wherein, As the number of iterations increases, the first threshold V1 gradually decreases from its initial value until it decreases to between 5% and 20%. The first threshold V1 decreases gradually in stages, and When the performance parameter satisfying the optimization target satisfies: wherein the first threshold V1 is reduced to wherein .

14. The design method of claim 1 or 2, wherein, The initialization includes: The initial structure of the optical unit structure is selected; and Determine the arrangement of the optical unit structure.

15. The waveguide grating design method as described in claim 14, wherein, The initial structure of the optical unit has a rhomboid, circular, elliptical, or rectangular cross-section.

16. The design method of claim 15, wherein, The method of determining the arrangement of the optical unit structure includes: The optical unit structures are arranged in multiple rows, which extend along a first direction and are arranged at predetermined intervals in a second direction perpendicular to the first direction. Within each row, multiple optical unit structures are arranged with a period P along the first direction, and the optical unit structures in adjacent rows are staggered by a predetermined distance s along the first direction, where s = P / n, where 1 <n 10.

17. The design method of claim 16, wherein, n=2。 18. The design method of claim 2, wherein, The initialization includes: (1) selecting the initial structure of the optical unit structure, and (2) determining the arrangement of the optical unit structure; The initial structure of the optical unit has a rhomboid, circular, elliptical, or rectangular cross-section. The method of determining the arrangement of the optical unit structures includes: arranging the optical unit structures into multiple rows, the multiple rows extending along a first direction and arranged at predetermined intervals in a second direction perpendicular to the first direction, the multiple optical unit structures in each row arranged along the first direction with a period P, and the optical unit structures in adjacent rows being staggered along the first direction by a predetermined distance s, s = P / n, where 1 <n 10 and n≠2, and the fourth threshold V4 and the fifth threshold V5 satisfy: V4 ≠V5.

19. The design method as described in claim 1 or 2, wherein, The modification of the structural parameters of the optical unit structure includes: using a simulated annealing algorithm, a genetic algorithm, or a combination of particle swarm optimization and simulated annealing algorithm to change the cross-section of the optical unit structure.

20. The design method of claim 1 or 2, wherein, The step of determining the structural parameters of the optical unit structure corresponding to the performance parameters that satisfy the optimization objective as optimization parameters includes: Record the performance parameters as the optimal performance parameters; After changing the structural parameters of the optical unit structure and calculating new performance parameters based on the changed structural parameters, the new performance parameters are compared with the optimal performance parameters. If the new performance parameters better meet the optimization objective compared to the optimal performance parameters, the new performance parameters are recorded to replace the optimal performance parameters. After ceasing further changes to the structural parameters of the optical unit structure, the structural parameters of the optical unit structure corresponding to the optimal performance parameters are extracted and used as the optimization parameters of the optical unit structure.

21. The design method as described in claim 1 or 2, further comprising the following steps: (5) Based on the optimization scheme of the waveguide grating, perform optical waveguide tracing simulation on the waveguide grating, and calculate the uniformity and coupling efficiency of the coupled light field formed after the light passes through the entire waveguide grating multiple times.

22. A waveguide grating for optical pupil expansion, the waveguide grating comprising a waveguide substrate and a grating structure formed on the waveguide substrate, the grating structure comprising a plurality of optical unit structures arranged in an array in a plane and having a first grating vector, a second grating vector, and a third grating vector, the direction of the third grating vector being between the directions of the first grating vector and the second grating vector, and the third grating vector being the vector sum of the first grating vector and the second grating vector, the optical unit structure having structural parameters configured such that: V1； V2; and SO1 + SO2 A, in: V1 is the first threshold; V2 is the second threshold; A is a predetermined target value; CO is the intermediate out-coupling efficiency, , is the optical energy of an incident light beam that propagates in the waveguide substrate in a total reflection manner and is incident to the grating structure in a direction substantially along the third grating vector in the plane, is the optical energy of an intermediate out-coupled light beam that is out-coupled from the waveguide substrate after diffraction of the incident light beam by the grating structure. SO1 is a first side-out coupling efficiency, , is a first side-propagating light beam of optical energy propagating in the waveguide substrate in a total internal reflection manner in a direction generally along the first grating vector as a result of diffraction of the incident light beam by the grating structure, is a first side-out coupled light beam of optical energy coupled out of the waveguide substrate as a result of diffraction of the first side-propagating light beam, SO2 is a second lateral out-coupling efficiency , SO2 is a second lateral out-coupling efficiency SO2 is a second lateral out-coupling efficiency 23. The waveguide grating of claim 22, wherein, The structural parameters are further configured such that: CR V3; SR1 V4; and SR2 V5, in: V3 is the third threshold; V4 is the fourth threshold; V5 is the fifth threshold; CR stands for intermediate total internal reflection efficiency. , The light energy of the zeroth-order diffracted beam after the incident beam has passed through the grating structure; SR1 represents the first lateral total internal reflection efficiency. , The light energy of the zero-order diffracted beam after the first lateral propagation beam has passed through the grating structure; SR2 represents the second highest lateral total internal reflection efficiency. , The light energy of the zero-order diffracted beam after the second lateral propagating beam has passed through the grating structure.

24. The waveguide grating as claimed in claim 22 or 23, wherein, The first threshold V1 is between 5% and 20%.

25. The waveguide grating as claimed in claim 24, wherein, The first threshold V1 is between 10% and 15%.

26. The waveguide grating as claimed in claim 22 or 23, wherein, The second threshold V2 is between 0% and 20%.

27. The waveguide grating as claimed in claim 23, wherein, The third threshold V3 is between 80% and 95%.

28. The waveguide grating as claimed in claim 23, wherein, The fourth threshold V4 and the fifth threshold V5 are between 80% and 95%.

29. The waveguide grating as described in claim 22 or 23, wherein, The optical unit structures are arranged in multiple rows, which extend along a first direction and are arranged at predetermined intervals in a second direction perpendicular to the first direction. Within each row, multiple optical unit structures are arranged with a period P along the first direction, and the optical unit structures in adjacent rows are staggered by a predetermined distance s along the first direction, where s = P / n, where 1 <n 10.

30. The waveguide grating as claimed in claim 29, wherein, n=2。 31. The waveguide grating as claimed in claim 23, wherein, The optical unit structures are arranged in multiple rows, which extend along a first direction and are arranged at predetermined intervals in a second direction perpendicular to the first direction. Within each row, multiple optical unit structures are arranged with a period P along the first direction, and the optical unit structures in adjacent rows are staggered by a predetermined distance s along the first direction, where s = P / n, where 1 <n 10 and n≠2, and the fourth threshold V4 and the fifth threshold V5 satisfy: V4 ≠V5.

32. A display device comprising a waveguide grating as claimed in any one of claims 22-31.

33. The display device as claimed in claim 32, wherein, The display device is a near-eye display device and includes a lens and a frame for holding the lens close to the eye, the lens including the waveguide grating.

34. The display device as claimed in claim 32 or 33, wherein, The display device is an augmented reality display device or a virtual reality display device.

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