A method for optimizing grating in a diffraction waveguide device and a diffraction waveguide device

Abstract

The present invention discloses a method for optimizing gratings in a diffraction waveguide device: based on the effective light path distribution of the image source edge field of view, the turning grating and the outcoupling grating are roughly partitioned into several sub-gratings; the layout parameters and topographical structural parameters of the initial turning grating and the outcoupling grating are obtained; sampling is performed, and ray tracing is performed for each field of view, recording the angle range of light rays passing through each sub-grating region; the diffraction efficiency within the angle range of light rays in each sub-grating region is calculated, and the fitness function of the grating is calculated; if the fitness function meets the requirements, the layout parameters and topographical structural parameters of the grating are determined; if the fitness function does not meet the requirements, the grating layout parameters and topographical structural parameters are modified, and multiple iterations are performed to meet the requirements. The present invention also provides a diffraction waveguide device that performs grating optimization using the above method. This method reduces the difficulty of existing grating optimization and provides a diffraction waveguide device based on this method, which can improve display uniformity.

Description

Technical Field

[0001] The present invention relates to the field of augmented reality display technology, and in particular to a method for optimizing a grating in a diffraction waveguide device and the diffraction waveguide device. Background Art

[0002] With the advent of the "metaverse" concept, augmented reality (AR) display technology has garnered significant attention. In AR displays implemented using geometric optics, the field of view (FOV) of the entire system is positively correlated with the volume of the optical structure. Increasing the FOV also increases the volume of the optical system. Waveguide AR displays, however, offer significant advantages in weight, size, and FOV, and are gradually becoming mainstream. Products like Hololens and Magic Leap, for example, all utilize waveguide structures.

[0003] Waveguide-based AR displays using surface-relief gratings offer advantages such as lightness, ease of mass production, a wide field of view, and excellent transmission. Turning and outcoupling gratings typically employ a gradual increase in diffraction efficiency using gradients in height, duty cycle, and tilt angle. However, optimizing the gratings requires considering a wide angular bandwidth, which increases the difficulty of grating optimization and results in poor display image uniformity. For example, Chinese patent publication number CN113625446A discloses a design method for an AR optical waveguide, which includes an input grating, an optical waveguide body, and an output grating. The design method includes: S101: optimizing the grating parameters of the input grating according to the angle at which the light beam is incident on the input grating and the wavelength of the light beam, wherein the grating parameters include the grating period, the grating depth, and the grating duty cycle; S102: determining the grating period and the grating depth of the output grating according to the grating period and the grating depth of the input grating; S103: partitioning the output grating; and S104: obtaining the grating duty cycle of each partition of the output grating with the total outcoupling optical power and non-uniformity of the output grating as optimization targets. And Chinese patent publication number CN113625447A discloses a design method for an outcoupling grating of an AR optical waveguide, the AR optical waveguide comprising an incoupling grating, an optical waveguide body, and the outcoupling grating. The design method comprises: S101: calculating the diffraction incoupling angle of the light beam diffracted into the optical waveguide body by the incoupling grating according to the angle at which the light beam is incident on the incoupling grating, the wavelength of the light beam, and the grating period of the incoupling grating; S102: calculating the distance between two adjacent outcoupling positions of the light beam on the outcoupling grating according to the diffraction incoupling angle and the thickness of the optical waveguide body; S103: calculating the brightness difference rate of the light beam after multiple outcouplings by the outcoupling grating according to the maximum diffraction efficiency of the outcoupling grating; S104: calculating the number of partitions of the outcoupling grating according to the length of the outcoupling grating, the sensitivity of the human eye to the light beam, the brightness difference rate, and the distance between the two adjacent outcoupling positions.

[0004] Therefore, how to reduce the difficulty of optimizing existing gratings and improve the display uniformity of diffraction waveguide structures is a research hotspot in this field. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for optimizing the grating in a diffraction waveguide device, which reduces the difficulty of existing grating optimization and provides a diffraction waveguide device based on this method, which can improve display uniformity.

[0006] The present invention is solved by the following technical solutions:

[0007] A method for optimizing a grating in a diffraction waveguide device, wherein the diffraction waveguide device includes an incoupling grating, a turning grating, and an outcoupling grating, the method comprising:

[0008] (1) According to the effective light path distribution of the edge field of view of the image source, the turning grating and the outcoupling grating are roughly divided into several sub-gratings;

[0009] (2) Obtaining the layout parameters and morphology structure parameters of the initialization turning grating and the outcoupling grating;

[0010] (3) Sampling the field of view on the image source, and performing ray tracing on each field of view, recording the angle range of the light passing through each sub-grating area;

[0011] (4) Calculate the diffraction efficiency of each sub-grating region within the light angle range and calculate the fitness function of the grating;

[0012] (5) If the fitness function meets the requirements, the initialization values ​​of step (2) can be determined as the layout parameters and morphology structure parameters of the grating; if the fitness function does not meet the requirements, the layout parameters and morphology structure parameters of step (2) are modified and iterated multiple times to meet the requirements.

[0013] In diffraction waveguide structures, current grating optimization mostly focuses on optimizing the grating's morphology within a certain range of incident angles. For gradient gratings, the incident angle range must cover the entire grating area. After light from a given field of view undergoes two-dimensional pupil expansion through the turning grating and the output grating, only a portion of the path corresponding to that path enters the exit pupil. This portion of the light path constitutes the effective light path of the diffraction waveguide structure. The technical concept of the present invention is to partition and optimize the grating based on the effective light paths corresponding to different fields of view. The turning grating and the output grating are optimized based on individual sub-grating regions. Each sub-grating region corresponds to the effective light path of light propagating in the waveguide, that is, each sub-grating region only covers a portion of the angular range of light propagating in the waveguide. Therefore, when optimizing a sub-grating region within the effective light path, only the angular range corresponding to that sub-grating region needs to be considered. The grating layout parameters are also included in the optimization range, increasing the selectable efficiency range of the grating, that is, increasing the selectable grating morphology parameter range, effectively reducing the difficulty of grating waveguide optimization.

[0014] Among them: in step (2), a genetic algorithm can be used; in step (4), RCWA can be called to calculate the diffraction efficiency within the light angle range of each sub-grating area; in step (5), the grating layout parameters and morphological structure parameters can be modified through selection, crossover, and mutation.

[0015] Furthermore, in step (1), the turning grating is divided into a plurality of sub-grating regions along the x-axis; or, the turning grating can be divided into a plurality of sub-grating regions along the y-axis; or, after the turning grating is divided into a plurality of sub-grating regions along the x-axis, it is further divided into a plurality of sub-grating regions along the y-axis; and the outcoupling grating is divided into a plurality of sub-grating regions along the y-axis;

[0016] Among them, the x-axis direction is the +1-order diffraction direction of the central field light after being coupled into the grating, and the y-axis direction is perpendicular to the x-axis direction and points to the direction of the out-coupling grating.

[0017] Furthermore, in step (2), the layout parameters include the shape and shape parameters of the grating area; and the topography structure parameters include the topography and topography parameters of the grating.

[0018] Furthermore, the shapes of the grating area can be rectangle, trapezoid, polygon, etc., and the shape parameters correspond to the length and width of the rectangle, the upper base, lower base, height, inclination, etc. of the trapezoid, the length of each side of the polygon, the angle between adjacent sides, etc.

[0019] Furthermore, the grating morphology can be selected from rectangular grating, trapezoidal grating, inclined grating, etc. The morphology parameters correspond to the thickness and duty cycle of the rectangular grating, the upper and lower bottom widths, thickness, inclination angle, etc. of the trapezoidal grating, and the inclination angle, thickness, duty cycle, etc. of the inclined grating.

[0020] Furthermore, in step (4), the fitness function is expressed as:

[0021] f=∑(yy i ) 2

[0022] When the turning grating and the outcoupling grating are optimized separately using the genetic algorithm, y is the target diffraction efficiency of the grating, and y i is the diffraction efficiency value of the grating under each field of view; when the turning grating and the outcoupling grating are optimized at the same time, y is the target efficiency value after the light passes through the turning grating and the outcoupling grating, y i is the efficiency value of light passing through the turning grating and the outcoupling grating in each field of view.

[0023] The present invention also provides a diffraction waveguide device, comprising an in-coupling grating, a turning grating and an out-coupling grating, wherein the turning grating and the out-coupling grating are obtained by optimizing the above method.

[0024] Preferably, according to the effective light path distribution of the edge field of view of the image source, the turning grating is first divided into a first turning grating, a second turning grating and a third turning grating along the x-axis, and the third turning grating is then divided into a third grating L and a third grating R along the y-axis.

[0025] The present invention considers the smaller areas of the first and second turning gratings and therefore does not divide them into separate zones. However, due to light diffusion, the third turning grating has a relatively larger area, so it is divided into left and right third turning gratings L and R. The left and right divisions of third gratings L and R ensure energy utilization at the edge of the field of view and improve the uniformity of diffraction waveguide transmission.

[0026] Furthermore, when the parameters of the first turning grating and the second turning grating are optimized, the light angle range of the first turning grating and the second turning grating includes all light angle values ​​of the entire image source, and the diffraction efficiency value of the second turning grating in the entire angular bandwidth is greater than the diffraction efficiency value of the first turning grating.

[0027] After being coupled into the grating, the light from the left side of the image source propagates through the waveguide at an angle of +α to the x-axis, i.e., it propagates within the region corresponding to the turning grating at an angle of +α to the x-axis. The light from the right side of the image source propagates through the waveguide at an angle of -α to the x-axis, i.e., it propagates within the region corresponding to the turning grating at an angle of -α to the x-axis. Therefore, the third turning grating is split along the y-axis into the third grating L and the third grating R.

[0028] Furthermore, when optimizing the parameters of the third turning grating L, the angular range of light passing through the third grating L is limited to the angle corresponding to the light incident on the turning grating from the lower right side of the image source. Its diffraction efficiency should be improved to ensure energy values ​​at the edge of the field of view, thereby improving overall display uniformity. Furthermore, when optimizing the parameters of the third turning grating R, the angular range of light passing through the third grating R includes the angle values ​​of light generated from the lower side of the image source; and the efficiency value of the third grating R is greater than that of the second turning grating.

[0029] Furthermore, the outcoupling grating is divided into multiple sub-grating areas along the y-axis, the light angle range of the outcoupling sub-grating area close to the turning grating includes the angle of the entire image source light transmitted here, and the light angle range of the outcoupling sub-grating area far from the turning grating includes the angle of the image source light transmitted here; wherein the light angle range is the angle range of the light passing through this grating area.

[0030] Once the grating period, azimuth angle, and wavelength of the incident light are determined, the diffraction angle and propagation path in the waveguide are also determined. Therefore, compared to existing methods, this invention utilizes grating partitioning and optimization based on effective light paths, which reduces the difficulty of grating optimization and makes it easier to achieve a highly uniform display effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is the ray path diagram corresponding to the upper left outgoing light of the image source and its effective ray path;

[0032] Figure 2 is the ray path diagram corresponding to the light emitted from the upper right of the image source and its effective ray path;

[0033] Figure 3 is the ray path diagram corresponding to the outgoing light from the lower left of the image source and its effective ray path;

[0034] Figure 4 is the ray path diagram corresponding to the outgoing light from the lower right of the image source and its effective ray path;

[0035] Figure 5 A flow chart of a method for reducing the difficulty of grating optimization provided by an embodiment;

[0036] Figure 6 A schematic diagram of a diffraction waveguide structure capable of improving uniformity provided in an embodiment;

[0037] Figure 7 A simulation result of the embodiment is shown;

[0038] Among them, 1. coupling-in grating; 2. turning grating; 3. coupling-out grating; 4. first turning grating 1; 5. second turning grating; 6. third turning grating L; 7. third turning grating R; 8. first coupling-out grating; 9. second coupling-out grating; 10. third coupling-out grating. DETAILED DESCRIPTION

[0039] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0040] Figures 1-4 is the light path distribution of the edge field of view and its effective light path distribution. Figure 1 For example, light is emitted from the upper left corner of the image source. After being diffracted by the coupling grating, it propagates at an angle of +α with the x-axis in the turning grating area. After being diffracted by the turning grating, it propagates at an angle of -β with the y-axis. The diffraction angle of the grating is calculated by the following formula:

[0041] n1sinθ1sinφ1=n0sinθ0sinφ0

[0042]

[0043] n0 and n1 are the refractive indices of the incident and diffraction regions, θ0 and φ0 are the incident and azimuth angles of the incident light, θ1 and φ1 are the incident and azimuth angles of the diffracted light, m ​​is the diffraction order, λ is the wavelength of the incident light, and Λ is the grating period. According to the grating period and orientation in the waveguide, the coupling grating and the coupling grating have the same period, d, and the period of the turning grating is d. f and the grating orientation ρ is set to satisfy the following conditions:

[0044]

[0045] At this time, the light will be emitted in parallel after passing through the diffraction waveguide. The light in the upper left field of view will be emitted in parallel after passing through the waveguide. Therefore, the light in this field of view that fills the exit pupil position will be located in the upper left area of ​​the outcoupling grating. The effective area of ​​the outcoupling grating under this field of view is Figure 1 In the middle shaded area, since the direction of the light is known, the effective position of the turning grating can be obtained according to the edge position of the effective area.

[0046] Figure 5 This is a flowchart of a method for reducing the difficulty of grating optimization provided in an embodiment. This method primarily uses a genetic algorithm and rigorous coupled-wave analysis (RCWA) to optimize grating layout parameters and topography parameters. The genetic algorithm is a numerical simulation optimization method based on a species evolution model. It has the advantages of simple programming and fast convergence. In this embodiment, this method is used to use the grating layout parameters and topography structure parameters as design variables, and RCWA is used to calculate the grating diffraction efficiency. After multiple iterations, the optimal result is obtained. Specifically:

[0047] S1. According to Figures 1-4 The effective light distribution in the edge field of view of the image source can be known, and based on this, the turning grating and the outcoupling grating can be roughly divided into several sub-gratings.

[0048] S2. Obtain the initial values ​​of the layout parameters and morphology structure parameters of the turning grating and the outcoupling grating.

[0049] S3. Sampling the input field of view on the image source, and performing ray tracing on each field of view, recording the angle range of the light passing through each sub-grating area.

[0050] S4. Call RCWA to calculate the diffraction efficiency of each sub-grating area within the light angle range and calculate the fitness function of the grating. The fitness function is expressed as:

[0051] f=∑(yy i ) 2

[0052] When the turning grating and the outcoupling grating are optimized separately using the genetic algorithm, y is the target diffraction efficiency of the grating, and y i is the diffraction efficiency value of the grating under each field of view; when the turning grating and the outcoupling grating are optimized at the same time, y is the target efficiency value after the light passes through the turning grating and the outcoupling grating, y i is the efficiency value of light passing through the turning grating and the outcoupling grating in each field of view.

[0053] S5. If the fitness function of the grating meets the requirements, the layout parameters and morphology structure parameters of the grating can be determined. If the fitness function of the grating does not meet the requirements, the grating layout parameters and morphology structure parameters can be modified through selection, crossover, and mutation, and multiple iterations can be performed to meet the design requirements.

[0054] The use of this method narrows the grating angle range that needs to be optimized in each sub-grating area, which can effectively reduce the difficulty of optimizing the surface relief grating. At the same time, the adjustable grating layout parameters can expand the grating efficiency range and provide more options for waveguide morphology structure parameters.

[0055] Figure 6 Schematic diagram of a diffraction waveguide structure with improved uniformity obtained according to the above grating optimization method, which includes: an in-coupling grating 1, a turning grating 2 and an out-coupling grating 3.

[0056] Taking into account that the upper and lower fields of view are distributed vertically in the grating area, and the left and right fields of view are distributed left and right in the grating, the initial distribution is determined based on this.

[0057] The turning grating 2 is first divided into three sub-regions along the x-axis, namely the first turning grating 4, the second turning grating 5 and the third turning grating. Considering that the areas of the first turning grating 4 and the second turning grating 5 are relatively small, no further sub-regions are formed.

[0058] The diffraction efficiencies of the first turning grating 4 , the second turning grating 5 and the third turning grating need to increase successively.

[0059] Due to light diffusion, the third turning grating has a relatively large area. Therefore, it is further divided along the y-axis into third turning grating L6 and third turning grating R7. Third turning grating L6 corresponds to light emitted from the lower right side of the image source. Therefore, the angular range of third turning grating L6 can be limited to the angle corresponding to the lower right side of the image source, and its diffraction efficiency should be improved to ensure energy values ​​at the edge of the field of view, thereby improving overall display uniformity. Third turning grating R7 corresponds to light emitted from the lower side of the image source. Therefore, the angular range must include angle values ​​of light rays from the lower side of the image source.

[0060] The outcoupling grating 3 is divided into three regions along the y-axis: the first outcoupling grating 8, the second outcoupling grating 9 and the third outcoupling grating 10. When optimizing the parameters of the outcoupling sub-region close to the turning grating, the incident angle needs to consider the angle at which the light from the entire image source is transmitted to this area. However, when optimizing the parameters of the outcoupling sub-region far from the turning grating, the incident angle only needs to consider the angle from the right side of the image source to this area. This reduces the difficulty of grating optimization and helps improve overall uniformity.

[0061] Figure 7The following figure shows a simulation result of an embodiment of the present invention, in which the grating morphology parameters are designed taking into account the limitations of actual processing. After the green light image is incident, the display image corresponding to the central field of view is as follows: Figure 7 As shown, it is calculated that the uniformity can reach 72%.

Claims

1. A method for optimizing a grating in a diffraction waveguide device, wherein the diffraction waveguide device comprises an incoupling grating, a turning grating, and an outcoupling grating, wherein: The method comprises: (1) According to the effective light path distribution of the edge field of view of the image source, the turning grating and the outcoupling grating are roughly divided into several sub-gratings; Specifically, according to the effective light path distribution of the edge field of view of the image source, the turning grating is first divided into a first turning grating, a second turning grating, and a third turning grating along the x-axis, and the third turning grating is further divided into a third grating L and a third grating R along the y-axis; the light angle ranges of the first turning grating and the second turning grating include all light angle values ​​of the entire image source, and the diffraction efficiency value of the second turning grating within the entire angular bandwidth is greater than the diffraction efficiency value of the first turning grating; the angle range of the light passing through the third grating L is limited to the angle of the light corresponding to the lower right side of the image source incident on the turning grating; the angle range of the light passing through the third grating R includes the angle value of the light generated on the lower side of the image source; and the efficiency value of the third grating R is greater than that of the second turning grating; The outcoupling grating is divided into three sub-grating regions along the y-axis; the ray angle range of the outcoupling sub-grating region close to the turning grating includes the angle of the entire image source light transmitted to this region, and the ray angle range of the outcoupling sub-grating region far from the turning grating includes the angle of the image source light transmitted to this region; wherein the ray angle range is the ray angle range of the light passing through this grating region; The x-axis is the +1-order diffraction direction of the central field light after being coupled into the grating, and the y-axis is perpendicular to the x-axis and points to the direction of the out-coupling grating. (2) Obtaining the layout parameters and morphology structure parameters of the initialization turning grating and the outcoupling grating; (3) Sampling the field of view on the image source, and performing ray tracing on each field of view, recording the angle range of the light passing through each sub-grating area; (4) Calculate the diffraction efficiency of each sub-grating region within the light angle range and calculate the fitness function of the grating; (5) If the fitness function meets the requirements, the initialization values ​​of step (2) can be determined as the layout parameters and morphology structure parameters of the grating; if the fitness function does not meet the requirements, the layout parameters and morphology structure parameters of step (2) are modified and iterated multiple times to meet the requirements.

2. The method for optimizing the grating in the diffraction waveguide device according to claim 1, wherein: In step (2), the layout parameters include the shape and shape parameters of the grating area; the topography structure parameters include the topography and topography parameters of the grating.

3. The method for optimizing a grating in a diffraction waveguide device according to claim 1, wherein: In step (4), the fitness function is expressed as: f=Σ(yy i ) 2 When the turning grating and the outcoupling grating are optimized separately using the genetic algorithm, y is the target diffraction efficiency of the grating, and y i is the diffraction efficiency value of the grating under each field of view; when the turning grating and the outcoupling grating are optimized at the same time, y is the target efficiency value after the light passes through the turning grating and the outcoupling grating, y i is the efficiency value of light passing through the turning grating and the outcoupling grating in each field of view.

4. A diffraction waveguide device comprising an incoupling grating, a turning grating and an outcoupling grating, characterized in that: The turning grating and the outcoupling grating are obtained by optimizing the method according to any one of claims 1 to 3.

5. The diffraction waveguide device according to claim 4, wherein: After being coupled into the grating, the light on the left side of the image source propagates in the waveguide at an angle of +α to the x-axis, that is, it propagates in the corresponding area of ​​the turning grating at an angle of +α to the x-axis; after being coupled into the grating, the light on the right side of the image source propagates in the waveguide at an angle of -α to the x-axis, that is, it propagates in the corresponding area of ​​the turning grating at an angle of -α to the x-axis.

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