Low-leakage meta-grating structure

Abstract

This invention discloses a low-leakage meta-grating structure, which is composed of a first free-form surface structure and a second free-form surface structure forming an inverted goblet structure. The first free-form surface structure is symmetrically distributed along the longitudinal centerline of the meta-grating unit and opens downwards. The free-form surface structures are also symmetrically distributed along the longitudinal centerline of the meta-grating unit and open downwards. The spacing between the two free-form surface structures can be adjusted. This meta-grating structure is applied to the coupling region of a two-dimensional grating waveguide. As a two-dimensional coupling grating structure, the light beam is coupled into the input grating, propagates through total internal reflection within the two-dimensional grating waveguide, and after reaching the coupling region, it is expanded by the meta-grating structure and coupled out to the human eye. Furthermore, because the proposed meta-grating structure is an inverted goblet structure, it can reduce the efficiency of the light beam coupling out to the environment.

Description

Technical Field

[0001] This invention relates to a low-leakage meta-grating structure, belonging to the field of diffraction optics technology, and particularly to a two-dimensional extended meta-grating structure suitable for near-eye display systems with light leakage. Background Technology

[0002] Augmented reality (AR) technology has matured significantly in recent years and, as a next-generation computing platform, has a strong potential to replace mobile phones as the next generation of portable smart devices. Augmented reality near-eye display devices are an important component of AR technology.

[0003] Currently, mature augmented reality near-eye display solutions mainly fall into four categories: prism solutions, birdbath solutions, freeform surface solutions, off-axis holographic lens solutions, and waveguide solutions. However, most of these require complex mirror combinations, resulting in a large overall module size and limiting their application in smart wearables, specifically augmented reality glasses. Waveguides, due to their compact and lightweight advantages, have the potential to be applied to various miniaturization needs, thus becoming the best augmented reality glasses solution currently available.

[0004] Waveguide solutions are specifically divided into two types: geometric waveguides and diffractive waveguides. The most common geometric waveguide solution is the array waveguide, where the beam emitted by the micro-projection optical engine is coupled into the waveguide by a mirror, and then passes through numerous semi-transparent and semi-reflective mirrors in the output region before entering the human eye. This solution has limited field of view and orbital range, and due to the large number of coatings, it is difficult to guarantee product yield. Imaging is prone to issues such as black background stripes, uneven brightness, and ghosting, so geometric waveguide solutions cannot provide the best display effect for the human eye. The grating structure design process for diffractive waveguide solutions is more complex, but it offers greater design freedom. By calculating and optimizing the grating structure parameters, diffractive waveguide augmented reality near-eye devices can achieve excellent imaging effects and a larger field of view. Diffractive waveguide solutions are further divided into surface relief grating waveguides and volume holographic grating waveguides. Among these, the application range of volume holographic grating waveguides is limited by the difficulty in large-scale mass production. Surface relief grating waveguides have become the most researched technology solution due to their ease of production using nanoimprinting and semiconductor etching technologies.

[0005] Surface relief gratings refer to periodically varying structures formed on a surface, specifically various periodic grooves. Based on differences in the groove's contour, shape, and tilt angle, surface relief gratings can be classified into one-dimensional and two-dimensional gratings. Two-dimensional grating waveguides are divided into coupling-in and coupling-out regions. The coupling-out region utilizes a two-dimensional grating structure to combine expansion and coupling functions, fully utilizing the effective area of ​​the waveguide. Currently, commercially available two-dimensional surface relief grating waveguides suffer from low coupling efficiency and severe light leakage problems.

[0006] CN118465901A discloses a two-dimensional coupled meta-grating structure, a two-dimensional grating waveguide, and a wearable device for near-eye display. The two-dimensional coupled meta-grating structure includes multiple periodically arranged grating periodic units. Each grating periodic unit includes a double-waisted freeform surface structure, a teardrop-shaped structure, a freeform surface shape structure, and a single-waisted freeform surface shape structure. The double-waisted freeform surface structure is arranged at the center of the meta-grating unit, with its double waists symmetrically distributed along the transverse centerline. The longitudinal two sides of the double-waisted freeform surface structure are symmetrically arranged as follows: the freeform surface shape structure and the teardrop-shaped structures on both sides are arranged on the front side of the two sides, and the single-waisted freeform surface shape structure is arranged on the rear side of the two sides and extends longitudinally. The provided two-dimensional coupled meta-grating structure is more effective in adjusting the coupling efficiency for multiple viewing angles, effectively increasing the coupling efficiency and improving the display brightness of the entire waveguide.

[0007] CN116299816A discloses a forked metasurface grating, optical waveguide, and near-eye display device for suppressing high-order diffraction. It includes at least one first grating structure and at least one second grating structure. The size of the first grating structure is larger than that of the second grating structure. The coupling efficiency can be adjusted by changing the size ratio of the two grating structures. The first and second grating structures are forked structures with the same intersection angle. This forked structure can better reduce light leakage on the outer side. The edges of the first and second grating structures are serrated. In any grating structure, the distance between two adjacent serrations is the same, and each serration is randomly and normally distributed along the vector direction of the one-dimensional grating. By serrifying the edges of the grating structures, it is beneficial to suppress higher-order diffraction components of light, thereby improving imaging quality.

[0008] Currently, conventional solutions use rhomboid or other shaped structures for the coupling region, such as... Figure 4 As shown, in a typical scheme, due to the characteristics of a conventional grating, there are diffraction-level energy outputs in both the direction of the human eye and the direction of the external environment. The output energy in the two directions is approximately the same. The light leakage into the external environment will, on the one hand, leak information and become stray light, and on the other hand, reduce the actual output efficiency of the waveguide to the human eye. Summary of the Invention

[0009] The purpose of this invention is to innovatively propose a novel meta-grating structure based on the design of two-dimensional extended diffractive waveguides. This structure is easy to fabricate, has more adjustable parameters, suppresses light leakage efficiency, and improves coupling efficiency. Traditional two-dimensional grating structures, such as rhomboid and cylindrical grating structures, when used in waveguides, exhibit comparable beam coupling efficiency towards the eye and light leakage efficiency towards the environment. This light leakage creates stray light output into the environment, exposing displayed content, and simultaneously affects the effective coupling efficiency towards the eye. This invention innovates on the grating structure by proposing a novel two-dimensional grating structure. In grating waveguide applications, this structure significantly suppresses beam coupling towards the environment, thereby greatly avoiding the problems encountered by conventional two-dimensional grating structures.

[0010] The technical solution adopted in this invention is a low-leakage meta-grating structure, which is composed of a first freeform surface structure 1 and a second freeform surface structure 2, forming an inverted goblet structure. The first freeform surface structure 1 is symmetrically distributed along the longitudinal centerline of the meta-grating unit and opens downwards. The freeform surface structure 2 is symmetrically distributed along the longitudinal centerline of the meta-grating unit and opens downwards; the spacing d between the two freeform surface structures can be adjusted; the lateral period ranges from 50 to 200 nm, and the height of the inverted goblet structure ranges from 20 to 1000 nm.

[0011] This meta-grating structure is applied to the coupling region of a two-dimensional grating waveguide. As a two-dimensional coupling grating structure, the light beam is coupled into the coupling grating and propagates through total internal reflection within the two-dimensional grating waveguide. After reaching the coupling region, it is expanded by the meta-grating structure and coupled out to the human eye. At the same time, since the proposed meta-grating structure is an inverted cup structure, it can reduce the efficiency of the light beam coupling out to the environment.

[0012] Furthermore, a coating is applied to this metagrating structure, either through a common coating or a deposition coating method; the coating material is such as TiO2. 2， Metal oxides such as Si3N4 and AL2O3 are used for coatings with thicknesses ranging from 10nm to 200nm.

[0013] Furthermore, the period of the grating ranges from 200nm to 1000nm, and the ratio of the longitudinal period to the transverse period of the grating structure is [missing value]. For a two-dimensional grating two-dimensional pupil expansion scheme, the period of the coupled grating needs to satisfy the K-domain principle, and the consistency of the periods of the coupled and coupled gratings is necessary to ensure the normal coupling out of the beam.

[0014] The following grating equation condition must be satisfied for the incident beam:

[0015]

[0016] Where sinθin >1 / n, where n is the refractive index of the waveguide substrate, λ is the wavelength, d is the grating period, and θ air Let φ be the incident elevation angle of the light beam. air Let θ be the incident azimuth angle of the light beam. in sinθ is the coupling angle of the beam within the waveguide. in >1 / n ensures that the beam propagates through total internal reflection within the waveguide, satisfying the total internal reflection condition.

[0017] Furthermore, for the beam to propagate throughout the waveguide, the K-vector closure condition must be satisfied. The K-vector closure condition is described by the following equation: K in K is the grating vector obtained when the beam is coupled in. EPE K is the grating vector obtained when the beam expands its pupil. out The grating vector obtained when the beam is coupled out is such that the angle at which the beam is coupled out to the human eye is the same as the angle at which the beam exits from the optomechanism, i.e., K is satisfied. in +K EPE +K out =0.

[0018] Compared with existing technologies, the performance of the grating waveguide designed in this invention depends on the grating structure used. The diffraction order and corresponding efficiency in each direction of the grating structure affect the waveguide's pupil expansion and coupling efficiency. In conventional two-dimensional grating structures, the efficiency of the beam coupled to the human eye and the beam coupled to the environment is roughly the same. This can cause light pollution or leakage of display information in the direction of the environment, and also affect the overall efficiency of the waveguide. Therefore, the shape of the grating structure causes the aforementioned light leakage drawback. This invention addresses the above problems from the perspective of grating structure by proposing a novel two-dimensional meta-grating structure, namely an inverted goblet-like structure. Due to its innovative grating shape, this structure effectively suppresses the beam coupling from the waveguide to the environment, i.e., light leakage. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of a micro-projection optical engine displaying near the eye via a coupled grating.

[0020] Figure 2 This is a schematic diagram of a two-dimensional pupil diffraction waveguide based on a one-dimensional grating.

[0021] Figure 3 This is a schematic diagram of a two-dimensional pupil diffraction waveguide based on a two-dimensional grating.

[0022] Figure 4 This is a schematic diagram of the structure proposed in this invention.

[0023] Figure 5 This is a schematic diagram of a two-free-form surface structure.

[0024] Figure 6This is a diagram of a periodically arranged super-configuration grating structure.

[0025] Figure 7 This is a K-vector image using this raster structure.

[0026] Figure 8 This is a schematic diagram of an embodiment. Detailed Implementation

[0027] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0028] like Figure 1 As shown, the micro-projection optical engine couples light into the waveguide substrate through a coupling grating, so that the light satisfies the total internal reflection condition and propagates within the substrate through total internal reflection. In the area of ​​the coupling grating, some light rays propagate further and some light rays are coupled out to the human eye, thereby achieving the purpose of near-eye display.

[0029] like Figure 2 The diagram shows a two-dimensional pupil-expanding diffractive waveguide based on a one-dimensional grating. The beam coupled into the coupling region is expanded in two directions, the turning region and the coupling region, and finally the beam is coupled out to the human eye in the coupling region.

[0030] like Figure 3 The diagram shows a two-dimensional pupil-expanding diffractive waveguide based on a two-dimensional grating. The coupling region couples the light rays into the waveguide base, ensuring total internal reflection. The coupling region couples a portion of the light rays out to the eye, while the remaining light rays propagate further within the coupling region. The ratio of the longitudinal period to the transverse period of the grating structure in the coupling region is [missing information]. Currently, the two-dimensional classical grating structures used in the coupling region are cylindrical and rhombic structures.

[0031] This invention proposes a two-dimensional coupled metagrating structure for two-dimensional extended diffractive waveguides, such as... Figure 4 The diagram shown is a top view of the structure proposed in this invention, representing four periods. The shaded area represents the grating structure. It is composed of a first free-form surface structure 1 and a second free-form surface structure 2. The free-form surface structure 1 is symmetrically distributed along the longitudinal centerline of the meta-grating unit and opens downwards. The free-form surface structure 2 is also symmetrically distributed along the longitudinal centerline of the meta-grating unit and opens downwards, allowing adjustment of the spacing d between the two structures. The lateral period size ranges from 50 to 200 nm, and the proposed structure height ranges from 20 to 1000 nm. Figure 5 a and b belong to different morphological structures as described above. This grating structure is applied to, for example... Figure 3 The coupling region shown is used to suppress the energy of light leakage, thereby preventing information leakage into the environment and stray light. At the same time, the suppression of light leakage improves the effective coupling efficiency of the waveguide to the human eye.

[0032] Figure 6 It is a periodically arranged super-configuration grating structure. Figure 7 The diagram shows the K-vector of this grating structure, illustrating the vector directions of some diffraction orders. Coatings, such as TiO2 and Al2O3, can be deposited on this structure, with thicknesses ranging from 10 nm to 200 nm.

[0033] The period of the grating ranges from 200nm to 1000nm, and the ratio of the longitudinal period to the transverse period of the grating structure is as follows: For a two-dimensional grating two-dimensional pupil expansion scheme, the coupled grating period needs to meet the following requirements: Figure 7 The K-domain schematic diagram shown demonstrates the consistency of the input and output grating periods, ensuring proper beam output.

[0034] The following grating equation condition must be satisfied for the incident beam:

[0035]

[0036] Where sinθ in >1 / n, where n is the refractive index of the waveguide substrate, λ is the wavelength, d is the grating period, and θ air Let φ be the incident elevation angle of the light beam. air Let θ be the incident azimuth angle of the light beam. in sinθ is the coupling angle of the beam within the waveguide. in A value greater than 1 / n ensures that the beam propagates through total internal reflection within the waveguide, satisfying the total internal reflection condition.

[0037] For the propagation of the light beam throughout the waveguide, such as Figure 1 and Figure 2 As shown, the condition for the closure of the K vector must be satisfied. The condition for the closure of the K vector can also be described as follows: K in K is the grating vector obtained when the beam is coupled in. EPE K is the grating vector obtained when the beam expands its pupil. out This is the grating vector obtained when the beam is coupled out, which ensures that the angle at which the beam is coupled out to the human eye is consistent with the angle at which the beam is emitted from the optomechanical system.

[0038] K in +K EPE +K out =0

[0039] In the embodiments, the structure designed according to the above description is as follows: Figure 8 As shown in figure a, this is the actual structure used, which ensures that... Figure 7As shown, under various efficiencies of forward propagation and pupil dilation, the efficiency coupled to the human eye is 1.89%, and the efficiency coupled to the environment can be suppressed to 0.23%. The grating period is 760nm x 440nm, the grating height is 150nm, the refractive index of the grating layer and the substrate layer is 1.9136, and the incident wavelength is green light at 530nm. Figure 8 Figure b shows a comparative structure calculated using a common cylindrical structure with a height of 150 nm and a diameter of 200 nm. The grating period is also 760 x 440 nm, with green light incident at a wavelength of 530 nm. The refractive indices of the grating layer and the substrate layer are both 1.9136. The simulated efficiency of the light leakage to the human eye is 2.39%, and the efficiency of the light leakage to the environment is 1.64%. The two efficiencies are not significantly different, and the efficiency of the light leakage to the environment is not suppressed. The efficiency comparison between the two structures demonstrates the superior performance of the current structure in suppressing light leakage.

[0040] Without increasing the processing difficulty, improving the degree of freedom of adjustment of the two-dimensional grating in the diffractive waveguide can better control the coupling efficiency, further suppress light leakage efficiency, achieve higher coupling efficiency, and at the same time avoid information leakage and environmental stray light.

Claims

1. A low-leakage meta-grating structure, characterized in that: The inverted goblet structure is formed by a first free-form surface structure and a second free-form surface structure. The first free-form surface structure is symmetrically distributed along the longitudinal centerline of the meta-grating unit and opens downwards. The second free-form surface structure is symmetrically distributed along the longitudinal centerline of the meta-grating unit and opens downwards. The spacing d between the two free-form surface structures can be adjusted. The lateral period ranges from 50 to 200 nm, and the height of the inverted goblet structure ranges from 20 to 1000 nm. This meta-grating structure is applied to the coupling region of a two-dimensional grating waveguide. As a two-dimensional coupling grating structure, the light beam is coupled into the coupling grating and propagates through total internal reflection within the two-dimensional grating waveguide. After reaching the coupling region, it is expanded by the meta-grating structure and coupled out to the human eye. At the same time, since the proposed meta-grating structure is an inverted cup structure, it can reduce the efficiency of the light beam coupling out to the environment.

2. The low-leakage meta-grating structure according to claim 1, characterized in that: A coating is applied to this metagrating structure; the coating can be a common coating or a deposited coating; the coating material is TiO2. 2， Si3N4 and AL2O3 metal oxides, with a coating thickness ranging from 10nm to 200nm.

3. The low-leakage meta-grating structure according to claim 1, characterized in that: The grating period ranges from 200nm to 1000nm, and the ratio of the longitudinal period to the transverse period of the grating structure is [missing value]. For a two-dimensional grating two-dimensional pupil expansion scheme, the period of the coupled grating needs to satisfy the K-domain principle, and the consistency of the periods of the coupled and coupled gratings is necessary to ensure the normal coupling out of the beam. The following grating equation condition must be satisfied for the incident beam: in >1 / n, where n is the refractive index of the waveguide substrate. λ is the wavelength, and d is the grating period. The incident elevation angle of the light beam. The angle of incidence of the light beam. The coupling angle of the light beam within the waveguide. >1 / n ensures that the beam propagates through total internal reflection within the waveguide, satisfying the total internal reflection condition.

4. The low-leakage meta-grating structure according to claim 1, characterized in that: For the beam to propagate throughout the waveguide, the condition for K-vector closure must be satisfied. The condition for K-vector closure is described by the following equation: ， The grating vector when the beam is coupled in. The grating vector during beam pupil expansion. This is the grating vector when the beam is coupled out, ensuring that the angle at which the beam is coupled out to the human eye is consistent with the angle at which the beam exits from the optomechanical system.

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