Light guide element and display device using the same

Abstract

The present invention provides a lightweight, wide-viewing-angle light guide element and a display device using the same. [Solution] The light guide element comprises a single-layer light guide substrate and a diffraction layer formed on the light guide substrate, wherein the diffraction layer comprises a first diffraction grating that incouples incident light incident on the light guide substrate into the light guide substrate and a second diffraction grating that outcouples total reflected light propagated through the light guide substrate to the outside of the light guide substrate, wherein the first diffraction grating incouples the incident light in an angular range of 60° or more including the normal direction of the light guide substrate at at least one wavelength of a first wavelength included in the 450nm ± 20nm band, a second wavelength included in the 530nm ± 20nm band, and a third wavelength included in the 630nm ± 20nm band, and the second diffraction grating outcouples the total reflected light in an angular range of 60° or more including the normal direction at at least one wavelength.

Description

[Technical Field]

[0001] The present invention relates to a light guide element and a display device using the same. [Background technology]

[0002] Augmented reality (AR) and mixed reality (MR) headsets are being developed for personal or professional use. AR / MR headsets require high resolution and a wide field of view, often employing heavy components for the AR / MR image display. This results in heavy headsets that are uncomfortable to wear due to their fixed position on the head. On the other hand, small, lightweight glasses-type displays that show simple information such as text and symbols are also being developed.

[0003] A configuration is known for displaying ARMR images in personal displays or augmented reality displays, which utilizes diffracted light guidance, in which an input coupling grating, an exit pupil magnification grating, and an output coupling grating are formed on a light guide plate, to display a projected image from a projector in front of the eye (see, for example, Patent Documents 1 and 2). As optical glass used for the light guide plate, lead-free, arsenic-free optical glass with a refractive index nd for the d line of 1.91 ≤ nd ≤ 2.05 is known (see, for example, Patent Document 3). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Special Publication No. 2020-521994 [Patent Document 2] Special Publication No. 2017-528739 [Patent Document 3] Patent No. 4970896 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] If a lightweight display device with functionality equivalent to an ARMR headset and a wide field of view could be realized, its application areas and user base would expand even further. The present invention aims to provide a lightweight, wide-field-of-view light guide element and a display device using the same. [Means for solving the problem]

[0006] In one aspect of the present invention, the light guide element comprises a single-layer light guide substrate and a diffraction layer formed on the light guide substrate. The diffraction layer comprises a first diffraction grating that incouples incident light incident on the light guide substrate into the light guide substrate, and a second diffraction grating that outcouples total reflected light propagating through the light guide substrate to the outside of the light guide substrate. The first diffraction grating incouples the incident light in an angular range of 60° or more, including the normal direction of the light guide substrate, at least one of the following wavelengths: a first wavelength included in the 450nm ± 20nm band, a second wavelength included in the 530nm ± 20nm band, and a third wavelength included in the 630nm ± 20nm band. The second diffraction grating outcouples the total reflected light in an angular range of 60° or more, including the normal direction, at at least one wavelength. [Effects of the Invention]

[0007] Lightweight and wide-viewing-angle light guide elements, and display devices using them, will be realized. [Brief explanation of the drawing]

[0008] [Figure 1] This is a schematic diagram showing an example of the configuration of the light guide element in the embodiment. [Figure 2A] This figure shows the coupling of incident light to the light guide substrate by an in-coupling grating. [Figure 2B] This diagram shows the total internal reflection light guide from the in-coupling grating to the out-coupling grating. [Figure 2C] This figure shows the emission of light from the out-coupling grating to the outside of the light guide substrate. [Figure 2D]It is a diagram showing an example of a composition in which the refractive index on the d line is greater than 2.05. [Figure 3] It is a diagram showing an example of a display device using the light guide element of the embodiment. [Figure 4] It is a diagram showing an operation example of the light guide element used in the display device. [Figure 5] It is a diagram of a configuration example of a light guide element using a line & space type one-dimensional diffraction grating. [Figure 6] It is a diagram of a configuration example of a light guide element using a two-dimensional diffraction grating in which a rectangular grating is a unit grating for the outcoupling grating. [Figure 7] It is a diagram of a configuration example of a light guide element using a two-dimensional diffraction grating in which a rectangular grating is a unit grating for both the incoupling grating and the outcoupling grating. [Figure 8] It is a diagram showing the grating shapes and optical characteristics of the examples and comparative examples. [Figure 9] It is a diagram showing an example of the material of the light guide substrate. [Figure 10] It is a diagram showing the refractive index difference between the light guide substrate and the diffraction layer of the examples and comparative examples. [Figure 11] It is a diagram showing the characteristics of the material used for the diffraction layer. [Figure 12] It is a diagram explaining the design of the NA (Numerical Aperture) diagram and FOV (Field of View). [Figure 13] It is a diagram explaining the design of the NA diagram and FOV. [Figure 14] It is a diagram explaining the design of the NA diagram and FOV. [Figure 15] It is a diagram explaining the diffraction guided wave directions of ±1st order and visibility. [Figure 16] It is a diagram explaining the FOV expansion effect by using ±1st order diffraction. [Figure 17] It is a diagram explaining the effect of using a two-dimensional lattice for the incoupling lattice. [Figure 18A] It is a diagram explaining the effect of using a rectangular lattice for the unit lattice. [Figure 18B]This figure shows the FOV light guide when a square grid is used as the unit cell. [Figure 18C] This figure shows the FOV light guide when a square grid is used as the unit cell. [Figure 19A] This figure shows the characteristics and diffraction pattern of the RGB light guide in Example 1. [Figure 19B] This figure shows the characteristics of the G light guide and the diffraction pattern of Example 1. [Figure 20A] This figure shows the characteristics and diffraction pattern of the RGB light guide in Example 2. [Figure 20B] This figure shows the characteristics of the G light guide and the diffraction pattern of Example 2. [Figure 21A] This figure shows the characteristics and diffraction pattern of the RGB light guide in Example 3. [Figure 21B] This figure shows the characteristics of the G light guide and the diffraction pattern of Example 3. [Figure 22A] This figure shows the characteristics and diffraction pattern of the RGB light guide in Example 4. [Figure 22B] This figure shows the characteristics of the G light guide and the diffraction pattern of Example 4. [Modes for carrying out the invention]

[0009] Figure 1 is a schematic diagram showing an example configuration of the light guide element 10 according to the embodiment. The light guide element 10 has a single-layer light guide substrate 11 and a diffraction layer 12 formed on the light guide substrate 11. The diffraction layer 12 has an in-coupling grating 121 that couples incident light incident on the light guide substrate 11 into the light guide substrate 11, and an out-coupling grating 123 that emits totally reflected light propagated through the light guide substrate 11 to the outside of the light guide substrate 11. As will be described later, an extended grating that guides the light coupled to the light guide substrate 11 to the out-coupling grating 123 may be formed on the light guide substrate 11. The direction of incidence and emission of light are not limited to the back surface of the light guide substrate 1. Light incident from the surface on which the diffraction layer 12 is formed can be coupled to the light guide substrate 11, and light can also be out-coupled from the surface on which the diffraction layer 12 is formed. The diffraction layer 12 may be formed on only one surface of the light guide substrate 11, or on both surfaces.

[0010] In this embodiment, a single-layer light guide substrate 11 is used to capture light of a first wavelength included in the blue wavelength band, light of a second wavelength included in the green wavelength band, and light of a third wavelength included in the red wavelength band with a high field of view (FOV), and emit light from the light guide substrate 11 with a high FOV. The blue wavelength band is, for example, 450 nm ± 20 nm. The green wavelength band is, for example, 530 nm ± 20 nm. The red wavelength band is, for example, 630 nm ± 20 nm.

[0011] Specifically, incident light is in-coupled at any of the first wavelength (λ1), second wavelength (λ2), and third wavelength (λ3) within an angular range of 60° or more, preferably 65° or more, and more preferably 70° or more, which includes the normal direction of the light guide substrate 11. Furthermore, light is out-coupled outside the light guide substrate 11 within an angular range of 60° or more, preferably 65° or more, and more preferably 70° or more, which includes the normal direction of the light guide substrate 11, at any of the wavelengths λ1, λ2, and λ3. The angle between the center of the angular range of the in-coupled or out-coupled light and the normal of the substrate is preferably ±15° or less, more preferably ±10° or less, even more preferably ±5° or less, and most preferably substantially coincides with the normal of the substrate.

[0012] The in-coupling grating 121 also in-couples incident light of any of the wavelengths λ1, λ2, and λ3 within a common angular range of 55° or more that includes the normal direction of the light guide substrate 11. The out-coupling grating 123 out-couples light of any of the wavelengths λ1, λ2, and λ3 within a common angular range of 55° or more that includes the normal direction of the light guide substrate 11. The angle between the center of the angular range of the in-coupling or out-coupling light and the normal of the substrate is preferably ±15° or less, more preferably ±10° or less, even more preferably ±5° or less, and most preferably substantially coincides with the normal of the substrate.

[0013] In other words, it is as follows: The angular range of the in-coupling or out-coupling light preferably includes at least the angular range of -42.5° to -12.5° or -12.5° to -42.5°, more preferably the angular range of -37.5° to -17.5° or -17.5° to -37.5°, even more preferably the angular range of at least -32.5° to -22.5° or -22.5° to -32.5°, and most preferably the angular range of -27.5° to 27.5°. That is, it is preferable to include an angular range of at least ±15°, more preferably ±20°, even more preferably ±25°, and most preferably ±27.5°.

[0014] In conventional configurations, three monochromatic light guide plates corresponding to each wavelength are used in a stacked configuration. While this makes it easier to achieve a high FOV because the FOV can be optimized for each wavelength, the entire optical element becomes thick and heavy. In this embodiment, a single-layer light guide substrate 11 is used to guide RGB light, realizing a small and lightweight light guide element 10. When a single-layer light guide substrate 11 is used, the overlapping FOV of each RGB wavelength becomes the overall FOV, but a high FOV can be achieved by devising the materials of the light guide substrate 11 and the diffraction layer 12, and the design of the in-coupling grating 121 and out-coupling grating 123. For example, both the light guide substrate 11 and the diffraction layer 12 are formed from inorganic materials and designed so that each has a predetermined refractive index. Furthermore, ±1st order diffraction is utilized. The rationale and analysis results of these configurations will be described later.

[0015] Figure 2A illustrates the coupling of incident light Lin to the light guide substrate 11 by the in-coupling grating 121. The plane on the light guide substrate 11 where the diffraction layer 12 is formed is the xy plane, and the thickness direction of the light guide substrate 11 is the z direction. The A-A' cross-sectional configuration is shown along with the configuration of the xy plane. For the sake of illustration, wavelengths are not distinguished, but light of each wavelength of RGB is incident on the light guide element 10.

[0016] The incident light Lin is coupled to the light guide substrate 11 by the in-coupling grating 121 as a diffracted wavefront in a predetermined direction, for example, the x-direction. The direction in which the incident light Lin is coupled to the light guide substrate 11 is not limited to the +x direction. As will be described later, a configuration in which the incident light propagates in the ±x directions may be adopted, or a configuration in which it propagates in two dimensions, the x-direction and the y-direction, may be adopted. The in-coupling grating 121 couples both light incident from the side of the diffraction layer 12 and light incident from the back surface of the light guide substrate 11 to the light guide substrate 11. Light perpendicularly incident from the direction normal to the light guide substrate 11 is also coupled into the light guide substrate 11. Furthermore, an FOV of 55° or more is achieved for all wavelengths of RGB.

[0017] Figure 2B illustrates total internal reflection light guidance from the in-coupling grating 121 to the out-coupling grating 123. The B-B' cross-sectional configuration is shown along with the xy-plane configuration. Light in-coupled to the light guide substrate 11 has its propagation direction transformed by the extended grating 122. The extended grating 122 has a line-and-space pattern extending diagonally with respect to the x-axis or y-axis. The extended grating 122 guides most of the light in the x-direction while diffracting some of the light in the -y-direction. The line-and-space pattern of the extended grating 122 replicates multiple diffracted rays along the x-direction, and each diffracted ray propagates in the -y-direction. Both the light propagating in the x-direction and the diffracted ray in the -y-direction propagate through total internal reflection within the light guide substrate 11.

[0018] Figure 2C illustrates the emission of light from the out-coupling grating 123 to the outside. It shows the C-C' cross-sectional configuration along with the xy-plane configuration. The out-coupling grating 123 guides most of the light by total internal reflection while diffracting some of the light for emission. The out-coupling grating 123 replicates multiple diffracted light rays along the light-guiding direction as they are emitted outside the light-guiding substrate 11. Since light can be emitted from both the diffraction layer 12 side and the back surface of the light-guiding substrate 11, the image formed by the emitted light can be viewed from either side.

[0019] The out-coupling grating 123 emits light that guides through the interior of the light guide substrate 11 via total internal reflection within a predetermined angular range that includes the normal direction of the light guide substrate 11. Specifically, out-coupling is achieved at an FOV of 60° or more including the normal direction for any of the RGB wavelengths. Furthermore, out-coupling is achieved at an FOV of 55° or more including the normal direction for all RGB wavelengths.

[0020] <Materials for light guide substrates> In Figures 2A to 2C, the light guide substrate 11 propagates the in-coupled light to the out-coupled position by total internal reflection. The higher the refractive index of the light guide substrate 11, the larger the angular range over which total internal reflection can guide the light. The refractive index nd of the light guide substrate 11 at the d-line (wavelength 587.56 nm) is greater than 2.05 (nd > 2.05). Examples of glass compositions with a refractive index nd at the d-line greater than 2.05 include the compositions shown in Figure 2D. If the refractive index nd is greater than 2.08, it becomes easier to design a higher FOV. In addition, in order to out-couple the incident light with minimal loss, it is preferable that the internal transmittance is high at all of λ1, λ2, and λ3. Since the transmittance loss due to absorption is greater for shorter wavelength light, the internal transmittance per 10 mm thickness of the light guide substrate 11 at a wavelength of 450 nm may be set to 90% or more, more preferably 95% or more. Table 1 lists the glass compositions that provide an internal transmittance of 95% or more for light with a wavelength of 450 nm.

[0021] [Table 1]

[0022] The light guide substrate 11 is, for example, a glass substrate. As the glass material, (1) Bi2O3-TeO2 glass or (2) La2O3-B2O3 glass may be used. Here, "composition" refers to the assembly of elements or components designed to total 100% in percentage units (mol%, weight%, etc.), excluding impurities that are inevitably mixed in and impurities and additives added intentionally in ppm (parts per million) units.

[0023] Examples of Bi2O3-TeO2-based glasses include those in which, when the total base composition is considered to be 100% based on the molar percentage of oxides, the Bi2O3 content is 20% to 50% and the TeO2 content is 10% to 35%.

[0024] Bi2O3 is a component that is preferably included to obtain high refractive index glass with high visible light transmittance. The lower limit is preferably 20% or more, more preferably 25% or more, and even more preferably 30% or more. The upper limit is preferably 45% or less, more preferably 40% or less, and even more preferably 35% or less.

[0025] TeO2 is a glass-forming component and may be included in glass because it can be used to obtain glass with high refractive index and high visible light transmittance. The TeO2 content is preferably 10% or more, more preferably more than 20%, and more preferably 25% or more. However, since too much TeO2 makes the glass unstable, it is preferably 35% or less, and more preferably 30% or less.

[0026] B2O3 is a glass-forming component and is preferably included to stabilize the glass, but if the content increases too much, it becomes difficult to achieve a high refractive index. The lower limit is preferably 10% or more, and more preferably 12% or more. The upper limit is preferably 40% or less, more preferably 35% or less, more preferably 30% or less, and more preferably 25% or less.

[0027] P2O5 is an optional component. P2O5 is a glass-forming component and is preferably included to stabilize the glass, but if the content increases too much, it becomes difficult to achieve a high refractive index. The lower limit is preferably 0% or more, the upper limit is preferably 20% or less, and more preferably 15% or less.

[0028] The sum of the B2O3 and P2O5 content is preferably 10% or more at the lower limit, and more preferably 20% or more. The upper limit is preferably 45% or less, more preferably 40% or less, and more preferably 35% or less.

[0029] Nb2O5, TiO2, Ta2O5, and WO3 are preferred components to increase the refractive index of the glass. Along with Bi2O3 and TeO2, these can also be used as the diffraction layer 12 described later. By increasing the ratio of these components, a configuration with a refractive index dispersion relationship similar to that of the diffraction layer 12 can be achieved. A higher ratio of Bi2O3-TeO2-Nb2O5-TiO2-Ta2O5-WO3 is preferable, preferably 55% or more, and more preferably more than 60%.

[0030] Examples of La2O3-B2O3-based glasses include those in which, when the total base composition is considered 100% based on the molar percentage of oxides, the La2O3 content is 10% to 40% and the B2O3 content is 10% to 35%.

[0031] La2O3 excels at increasing the refractive index and decreasing dispersion while maintaining glass stability, and is a component with high visible light transmittance, making it possible to obtain high refractive index glass with high visible light transmittance. For this reason, La2O3 may be included in the glass, but if the content increases too much, the resistance to devitrification decreases. The lower limit is preferably 10% or more, and more preferably 20% or more. The upper limit is preferably 40% or less, more preferably 30% or less, and even more preferably 25% or less.

[0032] B2O3 is a glass-forming component and is preferably included to stabilize the glass, but if the content increases too much, it becomes difficult to achieve a high refractive index. The lower limit is preferably 10% or more, and more preferably 12% or more. The upper limit is preferably 40% or less, more preferably 35% or less, more preferably 30% or less, and more preferably 25% or less.

[0033] SiO2 is an optional component. SiO2 is a glass-forming component and may be included to stabilize the glass, but if the content increases too much, it becomes difficult to achieve a high refractive index. The lower limit is preferably 0% or more, more preferably 5% or more, and more preferably 10% or more. The upper limit is preferably 30% or less, more preferably 20% or less, and more preferably 15% or less.

[0034] TiO2 is excellent at increasing the refractive index while maintaining glass stability, so it may be included in glass; however, if the content increases too much, the resistance to devitrification decreases. The lower limit is preferably 10% or more, more preferably 20% or more, and even more preferably 25% or more. The upper limit is preferably 40% or less, and even more preferably 35% or less.

[0035] ZrO2 is an optional component. ZrO2 is excellent at increasing the refractive index while maintaining glass stability, so it may be included in the glass, but if the content increases too much, the resistance to devitrification decreases. The lower limit is preferably 0% or more, and more preferably 5% or more. The upper limit is preferably 15% or less, and more preferably 10% or less.

[0036] Gd2O3, Nb2O5, Ta2O5, and WO3 are preferred components to increase the refractive index of the glass. Along with La2O3 and TiO2, these can also be used as the diffraction layer 12 described later. By increasing the ratio of these components, a structure with a refractive index dispersion relationship similar to that of the diffraction layer 12 can be achieved. A higher ratio of La2O3-TiO2-Gd2O3-Nb2O5-Ta2O5-WO3 is preferable, preferably 55% or more, and more preferably more than 60%. Furthermore, ZrO2 may also be included, and the ratio of La2O3-TiO2-Gd2O3-Nb2O5-Ta2O5-WO3-ZrO2 is preferably 55% or more, and more preferably 60% or more.

[0037] Using a glass material with the above composition, it is possible to realize a light guide substrate 11 with a refractive index of more than 2.05 for the d line (λ2), or a light guide substrate 11 with an internal transmittance of 95% or more per 10 mm thickness at 450 nm.

[0038] The light guide substrate 11 may be a single crystal substrate. A single crystal is a material in which the orientation of atoms or molecules is the same throughout the entire crystal. The light guide substrate 11 may be an isotropic single crystal substrate whose optical properties are independent of direction, or it may be a uniaxial single crystal substrate in which the crystal axis is oriented in a predetermined direction. In the case of a uniaxial substrate, it is desirable that the optical axis of the light guide substrate 11 be within ±4°, preferably within ±0.4°, of the normal. This is because the center of the line of sight when viewing actual scenery or text (not an ARMR image) through the light guide substrate roughly coincides with the normal of the substrate, and because if the direction of light propagation and the optical axis are misaligned, birefringence will cause the image to double, leading to a decrease in resolution.

[0039] Furthermore, lattice defects called dislocation defects can occur within the crystal. For example, dislocation defects with a diameter exceeding 1 μm, known as micropipes, often cause modulation of the refractive index near the defect. Even if the defect size does not affect the appearance of the light guide element, if such defects occur within the range of light guidance, the modulation of the refractive index can lead to a decrease in the resolution of the image displayed by the light guide element. Therefore, it is preferable that such defects are not present within the light guide element. The defect density of micropipes is approximately 10 defects / cm³. 2 Preferably, it is 1 piece / cm², and preferably 0.1 pieces / cm 2 It is preferable that it be so.

[0040] As the single-crystal light guide substrate 11, substrates such as TiO2, SrTiO3, KTaO3, LiNbO3, SiN, SiC, and diamond can be used.

[0041] Figure 3 shows an example of a display device 100 using the light guide element 10 of the embodiment. In this example, the display device 100 is an ARMR goggle. The light guide element 10 is used as an eyepiece for the right eye and an eyepiece for the left eye. The display device 100 has the light guide element 10 and a projector 110. A projector 110 is provided for each of the light guide elements 10 for the right eye and the left eye, and the light guide element 10 and the projector 110 are held on a wearable support 120. The light guide element 10 is formed from a single-layer light guide substrate 11, and the display device 100 as a whole is small and lightweight.

[0042] Figure 4 shows an example of the operation of the light guide element 10 used in the display device 100. The image projected from the projector 110 is diffracted into the light guide substrate 11 by the in-coupling grating 121. As described above, the in-coupling grating 121 diffracts RGB light, including perpendicularly incident light, into the interior of the light guide substrate 11 with an FOV of 55° or more. The in-coupled RGB light propagates through the interior of the light guide substrate 11 while undergoing total internal reflection, is diffracted by the out-coupling grating 123, and is emitted from the light guide substrate 11 with an FOV of 55° or more. This light image is incident on the human eye 20 and recognized as a color image.

[0043] The thickness of the light guide substrate 11 is, for example, 1 mm or less. The in-coupling grating 121 and out-coupling grating 123 are formed, for example, from thin films of inorganic material with a thickness of 100 to 1000 nm and have diffraction grating patterns formed at a predetermined pitch. When an extended grating 122 is used in conjunction with the in-coupling grating 121 and out-coupling grating 123, the extended grating 122 is also formed from the same thin film. The pitch of the diffraction grating is, for example, 300 to 500 nm. Instead of forming the diffraction layer 12 with a thin film of inorganic material, the diffraction grating may be formed directly on the surface of the light guide substrate 11. In this case, the surface region on which the diffraction grating is formed becomes the diffraction layer 12.

[0044] The in-coupling grid 121 achieves an FOV of 60° or more, preferably 65° or more, and more preferably 70° or more, for any wavelength of RGB. The in-coupling grid 121 achieves an FOV of 55° or more for any wavelength of RGB. The out-coupling grid 123 achieves an FOV of 60° or more, preferably 65° or more, and more preferably 70° or more, for any wavelength of RGB. The out-coupling grid 123 achieves an FOV of 55° or more for any wavelength of RGB.

[0045] The display device 100 using the light guide element 10 may be configured to display a color image in a range up to FOV 55°, and to display simple information or monochrome in the area beyond FOV 55°. For example, simple information such as characters or symbols, or icons or toolbars may be displayed at the edge of the field of view. A cover may be provided that covers at least one of the diffraction layer 12 and the back surface of the light guide substrate 11. In that case, the cover may become part of the light guide element 10 and be held by the support 120 in Figure 3. If a cover is used, it is desirable that it has high transmittance to visible light and does not affect the light image emitted from the out-coupling grating 123. The material of the cover may be glass or plastic.

[0046] <Configuration of diffraction gratings> Figures 5 to 7 show examples of lattice designs for the light guide element 10. Figure 5 shows an example configuration in which both the in-coupling lattice 121a and the out-coupling lattice 123a have a line-and-space pattern, which is a one-dimensional diffraction grating. Figure 5(A) shows an example where the in-coupling is diffraction in one axis direction, and in this example, diffraction in the +x direction is utilized. As explained with reference to Figures 2A to 2C, the RGB light diffracted in the +x direction by the in-coupling lattice 121a is redirected in the -y direction by the extended lattice 122 and emitted from the out-coupling lattice 123 with an FOV of 55° or more. Focusing on one wavelength of the RGB light, the light is emitted with an FOV of 60° or more, preferably 65° or more, and more preferably 70° or more.

[0047] The configuration in Figure 5(B) utilizes diffraction in the positive and negative directions along a single axis for in-coupling, in this example, diffraction in the ±x direction. The in-coupling grating 121a splits the incident light into two diffracted wavefronts, diffracting it in the +x and -x directions. Each of the diffracted light in the +x and -x directions is redirected by the extended grating 122 and out-coupled by the out-coupling grating 123. The field of view (FOV) of the emitted light is 55° or more for all RGB wavelengths, and if focusing on any one wavelength, the output FOV is 60° or more, preferably 65° or more, and more preferably 70° or more.

[0048] Figure 6 shows an example in which a line-and-space pattern, which is a one-dimensional diffraction grating, is used for the in-coupling grating 121a, and the out-coupling grating 123b is a two-dimensional diffraction grating with a rectangular unit cell. Figures 6(A) and (B) are the same as Figures 5(A) and (B), except that the unit cell of the out-coupling grating 123b is a rectangular unit cell. By using a rectangular unit cell, vignetting can be suppressed, as will be explained later. "Vignetting" refers to the phenomenon in which a part of the field of view to be displayed is missing from the input image due to a local decrease in light or brightness, and it occurs when light at a certain viewing angle cannot be guided by total internal reflection when light is diffracted and guided within the light guide substrate.

[0049] Figure 6(C) shows a design without the extended grating 122. The line and space pattern of the in-coupling grating 121a extends along the x-direction. This in-coupling grating 121a diffracts the incident light in the ±y directions, and in this example, diffraction in the -y direction is utilized. The light coupled to the light guide substrate 11 propagates in the -y direction while undergoing total internal reflection within the light guide substrate 11, and is emitted outside the light guide substrate 11 by the out-coupling grating 123b. In this case, since there is no extended grating 122, it is necessary to replicate the out-coupled light in two dimensions using the out-coupling grating 123b to improve visibility. Therefore, the out-coupling grating 123b is formed as a two-dimensional diffraction grating with a rectangular grating as the unit cell, and the propagating light is diffracted in two dimensions.

[0050] Figure 7 shows an example in which a rectangular grid is used as the unit cell for both the in-coupling grid 121b and the out-coupling grid 123b. Figures 7(A), (B), and (C) are the same as Figures 6(A), (B), and (C), except that the unit cell of the in-coupling grid 121b is a rectangular grid. By using a rectangular grid for both the in-coupling grid 121b and the out-coupling grid 123b, the FOV for RGB waveguides can be extended.

[0051] Figure 7(D) shows a design in which the in-coupling grid 121b and the out-coupling grid 123b partially overlap. A portion of the rectangular grid pattern of the out-coupling grid 123b also functions as the in-coupling grid 121b. Within the out-coupling grid 123b, the region that can be in-coupled with an FOV of 55° or more for each RGB color from the projector 110 (see Figure 3) becomes the in-coupling grid 121b.

[0052] The light guide element 10 may employ any of the lattice designs shown in Figures 5 to 7. In any case, it is desirable that the refractive index of the diffraction layer 12 be the same as or higher than that of the light guide substrate 11. As will be described later, it is desirable that the difference between the refractive index of the diffraction layer 12 and the refractive index of the light guide substrate 11 be 0.1 or less at each RGB wavelength.

[0053] The diffraction layer 12, which includes the in-coupling grating 121 and the out-coupling grating 123, is formed from, for example, ZrO2, HfO2, Ta2O5, Nb2O5, TeO2, MoO3, WO3, TiO2, SiN, SiON, SnO, ITO, Al2O3, Y2O3, AlN, MgO, or a mixture of two or more of these. Alternatively, the diffraction layer 12 may be formed from a glass material containing three or more inorganic elements. The diffraction layer 12 can be deposited on the surface of the light guide substrate 11 by vapor deposition, sputtering, etc. The in-coupling grating 121, the extended grating 122, and the out-coupling grating 123 are formed by etching a desired pattern, such as a line and space pattern or a rectangular lattice pattern, onto the diffraction layer 12.

[0054] When forming the diffraction layer 12 with a mixture of two or more materials, the dispersion of the refractive index, i.e., the wavelength dependence of the refractive index, may be matched to that of the light guide substrate 11 by cos-sputtering. The diffraction layer 12 may be formed by etching or by lift-off. The diffraction grating may be directly engraved onto a glass substrate, single crystal substrate, etc., as the diffraction layer 12. In this case, the light guide substrate 11 and the diffraction layer 12 can be formed integrally with a high refractive index material. When the light guide element 10 is applied to the display device 100 in Figure 3, the light guide substrate 11 is processed to a size, thickness, and shape suitable for an eyepiece. Since high FOV RGB light guidance is possible with a single-layer light guide substrate 11, it is thin and lightweight even when used as an eyepiece.

[0055] <Characteristics of the light guide element> Figure 8 shows the lattice shape and optical properties of the examples and comparative examples. The lattice shape includes the type of lattice pattern and the lattice pitch in the x and y directions. The optical properties show the specific wavelengths of λ1, λ2, and λ3, the refractive index of the light guide substrate 11 at λ3, the aspect ratio of the input image in the x and y directions, and the diagonal FOV. The relationship between the tangent of the diagonal FOV, the tangent of the horizontal FOV, and the tangent of the vertical FOV is expressed by the relationship between the diagonal and x and y directions of the aspect ratio of the projected image. For example, if we consider projecting an image with an aspect ratio of 4:3, the ratio of the tangents of the diagonal FOV, horizontal FOV, and vertical FOV is √(4^2+3^2), which is 5:4:3. Configurations that utilize diffraction in the ±1st order direction, as shown in Figure 7(B), are annotated as "±1st order utilization".

[0056] Throughout the examples and comparative examples, the common parameters are λ1 = 450 nm, λ2 = 532 nm, and λ3 = 633 nm. The aspect ratio of the input image is 16:9. The diffraction layer 12 is formed of an inorganic film with a refractive index higher than that of the light guide substrate 11 at λ1, λ2, and λ3, or a grating is formed directly on the light guide substrate 11. In these cases, the feasibility of total internal reflection light guidance is determined by the refractive index of the light guide substrate 11, so it is not necessary to consider the refractive index of the diffraction layer 12 when discussing the field of view. [Examples]

[0057] Example 1 uses a line and space (indicated as "L&S" in Figure 8) pattern for the in-coupling grating 121 and a two-dimensional diffraction grating with a rectangular unit cell for the out-coupling grating 123. The x-direction grating pitch of the in-coupling grating 121 is 310 nm. The x-direction pitch of the out-coupling grating 123 is 310 nm, and the y-direction pitch is 355 nm. A Bi2O3-B2O3-TeO2-P2O5-Nb2O5-ZnO glass substrate is used as the light guide substrate 11. The specific composition (mol%) is as follows: Bi2O3: 37.6, B2O3: 26.5, TeO2: 18.5, P2O5: 10.5, Nb2O5: 1.6, ZnO: 5.3

[0058] The refractive index of the light guide substrate 11 in Example 1 at λ3 is 2.08. As the wavelength decreases, the refractive index perceived by the wave increases, so the refractive index at the d line (wavelength 587.56 nm) is greater than 2.08. The diagonal FOV at λ2 in Example 1 is greater than 70°, and the maximum guided FOV at λ1, λ2, and λ3 is greater than 55°. [Examples]

[0059] In Example 2, a two-dimensional diffraction grating with a rectangular unit cell is used for both the in-coupling grating 121 and the out-coupling grating 123, utilizing diffraction in the ±1st order direction. For both the in-coupling grating 121 and the out-coupling grating 123, the grating pitch in the x-direction is 310 nm, and the pitch in the y-direction is 355 nm. As the light guide substrate 11, a Bi2O3-B2O3-TeO2-P2O5-Nb2O5-ZnO glass substrate with the same composition as in Example 1 is used. The refractive index of this light guide substrate 11 at λ3 is 2.08. In Example 2, the diagonal FOV at λ2 is greater than 70°, and the maximum waveguide FOV at λ1, λ2, and λ3 is greater than 55°. [Examples]

[0060] In Example 3, line and space patterns are used for both the in-coupling grating 121 and the out-coupling grating 123. The pitch in the x-direction of the in-coupling grating 121 is 270 nm, and the pitch in the y-direction of the out-coupling grating 123 is 300 nm. A single-crystal SiC substrate is used as the light guide substrate 11. The refractive index of this SiC substrate at λ3 is 2.63. In Example 3, the diagonal FOV at λ2 is greater than 100°, and the maximum waveguide FOV at λ1, λ2, and λ3 is greater than 65°. By using a light guide substrate 11 with a high refractive index, a high FOV can be achieved while using a line and space diffraction grating. [Examples]

[0061] In Example 4, a two-dimensional diffraction grating with a rectangular unit cell is used for both the in-coupling grating 121 and the out-coupling grating 123. The pitch of the in-coupling grating 121 in the x-direction is 270 nm, and the pitch in the y-direction is 310 nm. The pitch of the out-coupling grating 123 in the x-direction is 300 nm, and the pitch in the y-direction is 310 nm. As in Example 3, a single-crystal SiC substrate is used as the light guide substrate 11. The refractive index of the SiC substrate at λ3 is 2.63. In Example 4, the diagonal FOV at λ2 is greater than 110°, and the maximum waveguide FOV at λ1, λ2, and λ3 is greater than 85°. In Example 4, a high FOV can be achieved by using a high refractive index light guide substrate 11 and by using a two-dimensional diffraction grating with a rectangular unit cell for both the in-coupling grating 121 and the out-coupling grating 123.

[0062] <Comparative Example 1> In Comparative Example 1, the same light guide substrate 11 (refractive index 2.08 at λ3) as in Examples 1 and 2 is used, but a two-dimensional diffraction grating with a square unit cell is used for both the in-coupling grating 121 and the out-coupling grating 123. The grating pitch in the x and y directions for both the in-coupling grating 121 and the out-coupling grating 123 is 310 nm. In this case as well, diffraction in the ±1st order directions is utilized. In Comparative Example 1, the diagonal FOV at λ2 is less than 70°, and the maximum waveguide FOV at λ1, λ2, and λ3 is less than 55°. Although the same high refractive index light guide substrate 11 as in Examples 1 and 2 is used, the diagonal FOV is smaller compared to Examples 1-3 because a two-dimensional diffraction grating with a square unit cell is used for an input image with an aspect ratio of 16:9.

[0063] <Comparative Example 2> In Comparative Example 2, lead-free, arsenic-free optical glass described in Patent Document 3 is used as the light guide substrate. The composition (mol%) of this optical glass is as follows: GeO3:30.9 Bi2O3:25.0 B2O3:15.9 ZnO:10.0 SiO2:8.0 Li2O:5.0 BaO:5.0 Sb2O3:0.1

[0064] In Comparative Example 2, the refractive index at λ3 of the optical glass substrate is 1.99, and the refractive index nd for the d line is in the range of 1.91 ≤ nd ≤ 2.05. A two-dimensional diffraction grating with a rectangular unit cell is used for both the in-coupling grating 121 and the out-coupling grating 123, utilizing diffraction in the ±1st order direction. The pitch of the in-coupling grating 121 in the x-direction is 310 nm, and the pitch in the y-direction is 360 nm. The pitch of the out-coupling grating 123 in the x-direction is 310 nm, and the pitch in the y-direction is 370 nm. In Comparative Example 2, the diagonal FOV at λ2 exceeds 70°, but the maximum waveguide FOV at λ1, λ2, and λ3 is less than 55°. Although rectangular gratings are used in the in-coupling grating 121 and out-coupling grating 123, and diffraction in the ±1st order is utilized, a high FOV cannot be achieved for all RGB colors because the refractive index of the light guide substrate 11 at λ3 is 1.99.

[0065] <Comparative Example 3> In Comparative Example 3, the same optical glass substrate as in Comparative Example 2 is used. The refractive index of the optical glass substrate at λ3 is 1.99, and the refractive index nd for the d line is 1.91 to 2.05 (1.91 ≤ nd ≤ 2.05). In Comparative Example 3, line and space patterns are used for both the in-coupling grating 121 and the out-coupling grating 123. The pitch in the x-direction of the in-coupling grating 121 is 360 nm, and the pitch in the y-direction of the out-coupling grating 123 is 360 nm. In Comparative Example 3, the diagonal FOV at λ2 is less than 60°, and the maximum waveguide FOV for all of λ1, λ2, and λ3 is less than 35°. When a light guide substrate 11 with a refractive index nd of 2.05 or less for the d line is used, and line and space patterns are used for both the in-coupling grating 121 and the out-coupling grating 123, the FOV value required for the display device 100 cannot be achieved.

[0066] From the results in Figure 8, it is desirable that the refractive index nd of the light guide substrate in the d-line is greater than 2.05. By using a light guide substrate 11 with a refractive index nd greater than 2.05, a high FOV can be achieved even when line and space patterns are used for the in-coupling grating 121 and out-coupling grating 123.

[0067] Figure 9 shows an example of a crystalline material used in the light guide substrate 11. When a single crystal substrate is used for the light guide substrate 11, materials such as TiO2, SrTiO3, KTaO3, LiNbO3, SiC, and diamond can be used. Along with the crystal structure of these materials, the presence or absence of optical anisotropy, the ordinary refractive index no. on the d line, the extraordinary refractive index ne. on the d line, and the specific gravity (g / cm³) should be noted. 3 ), Mohs hardness, and absorption edge wavelength are shown.

[0068] SrTiO3, KTaO3, and diamond are optically isotropic. TiO2, LiNbO3, and SiC are uniaxial and exhibit birefringence, but the orientation of the optical axis relative to the normal is within ±4°, and the reduction in resolution due to the generation of double images caused by birefringence does not significantly affect the field of view when viewing transmitted real images. All of these crystals have absorption edges in the ultraviolet region and transmit visible light.

[0069] Figure 10 shows the refractive index difference between the light guide substrate and the diffraction layer in the examples and comparative examples. The refractive index of the light guide substrate 11, the refractive index of the diffraction layer 12, and the refractive index difference between the light guide substrate 11 and the diffraction layer 12 are shown at wavelengths of 450 nm (λ1), 532 nm (λ2), and 633 nm (λ3). The refractive index at each wavelength is calculated from the refractive index nd at the d-line and the Abbe number νd at the d-line, assuming normal dispersion. [Examples]

[0070] In Example 5, Ta2O5 is used for the diffraction layer 12. The light guide substrate 11 is the Bi2O3-B2O3-TeO2-P2O5-Nb2O5-ZnO glass substrate used in Examples 1 and 2. At each wavelength, the refractive index of the diffraction layer 12 is higher than that of the light guide substrate 11. The refractive index difference is 0.08 at 450 nm, 0.04 at 532 nm, and 0.02 at 633 nm, all of which are less than or equal to 0.1. [Examples]

[0071] In Example 6, ZrO2 is used for the diffraction layer 12. The light guide substrate 11 is the Bi2O3-B2O3-TeO2-P2O5-Nb2O5-ZnO glass substrate used in Examples 1 and 2. At each wavelength, the refractive index of the diffraction layer 12 is higher than that of the light guide substrate 11. The refractive index difference is 0.05 at 450 nm, 0.06 at 532 nm, and 0.08 at 633 nm, all of which are less than or equal to 0.1. [Examples]

[0072] In Example 7, Ta2O5 is used for the diffraction layer 12. The light guide substrate 11 is a Bi2O3-TiO2-Nb2O5-WO3-B2O3-P2O5-SiO2-BaO substrate. The specific composition (mol%) of the light guide substrate 11 is as follows: Bi2O3:21.0, TiO2:18.5, Nb2O5:16.5, P2O5:22.6, WO3:14.5, B2O3:2.8, BaO:2.8, SiO2:1.6. At each wavelength, the refractive index of the diffraction layer 12 is higher than that of the light guide substrate 11. The refractive index difference is 0.05 at 450 nm, 0.02 at 532 nm, and 0.01 at 633 nm, all of which are less than or equal to 0.1. [Examples]

[0073] In Example 8, ZrO2 is used for the diffraction layer 12. The light guide substrate 11 is the same Bi2O3-TiO2-Nb2O5-WO3-B2O3-P2O5-SiO2-BaO substrate used in Example 7, and its composition is also the same. At each wavelength, the refractive index of the diffraction layer 12 is higher than that of the light guide substrate 11. The refractive index difference is 0.02 at 450 nm, 0.04 at 532 nm, and 0.06 at 633 nm, all of which are less than 0.1. [Examples]

[0074] In Example 9, TiO5 is used for the diffraction layer 12, and a single-crystal substrate of LiNbO3 is used for the light guide substrate 11. At each wavelength, the refractive index of the diffraction layer 12 is higher than that of the light guide substrate 11. The refractive index difference is 0.07 at 450 nm, 0.03 at 532 nm, and 0.01 at 633 nm, all of which are less than or equal to 0.1. [Examples]

[0075] In Example 10, Nb2O5 is used for the diffraction layer 12. The light guide substrate 11 is the same LiNbO3 single crystal substrate as in Example 9. At each wavelength, the refractive index of the diffraction layer 12 is higher than that of the light guide substrate 11. The refractive index difference is 0.07 at 450 nm, 0.05 at 532 nm, and 0.03 at 633 nm, all of which are less than or equal to 0.1.

[0076] <Comparative Example 4> In Comparative Example 4, Ta2O5 is used for the diffraction layer 12. The light guide substrate 11 is the lead-free, arsenic-free optical glass substrate described in Patent Document 3, which was used in Comparative Examples 2 and 3. In this combination, the refractive index of the diffraction layer 12 is high, but the refractive index of the light guide substrate 11 at each wavelength is 2.05 or less, resulting in a large refractive index difference. The refractive index difference is 0.18 at a wavelength of 450 nm, 0.14 at a wavelength of 532 nm, and 0.11 at a wavelength of 633 nm, all exceeding 0.1.

[0077] <Comparative Example 5> In Comparative Example 5, ZrO2 is used for the diffraction layer 12. The light guide substrate 11 is the same as in Comparative Example 4, a lead-free, arsenic-free optical glass substrate as described in Patent Document 3. In this combination as well, the refractive index of the diffraction layer 12 is high, but the refractive index of the light guide substrate 11 at each wavelength is 2.05 or less, resulting in a large refractive index difference. The refractive index difference is 0.15 at a wavelength of 450 nm, 0.16 at a wavelength of 532 nm, and 0.16 at a wavelength of 633 nm, all exceeding 0.1.

[0078] If the refractive index of the diffraction layer 12 is lower than that of the light guide substrate 11, light with a numerical aperture (NA = n × sinθ) defined by the refractive index between the refractive index of the diffraction layer 12 and the refractive index of the light guide substrate 11 cannot exit from the light guide substrate 11 to the diffraction layer 12 and will not be diffracted. Therefore, at λ3, the refractive index of the diffraction layer 12 must be greater than that of the light guide substrate 11. On the other hand, if the difference Δn between the refractive index of the diffraction layer 12 and the refractive index of the light guide substrate 11 is large, reflection at the interface will increase, and the extraction efficiency at the out-coupling grating 123 will decrease. Therefore, at each wavelength of λ1, λ2, and λ3, it is desirable that the difference between the refractive index of the diffraction layer 12 and the refractive index of the light guide substrate 11 be 0.1 or less (Δn ≤ 0.1). In particular, at the wavelength of λ3, where total internal reflection guidance occurs within the light guide substrate 11 at a large reflection angle, the effect of interface reflection becomes large, so it is desirable that Δn be smaller than 0.1. At λ3, it is more desirable that Δn be smaller than 0.05, and even more desirable that it be smaller than 0.03. Furthermore, if the refractive index differences between λ1, λ2, and λ3 are Δnλ1, Δnλ2, and Δnλ3 respectively, the case where Δnλ1 ≥ Δnλ2 ≥ Δnλ3 makes it easier to reduce the effect of interfacial reflection across all wavelengths. Also, in λ1 and λ2, even when the refractive index of the diffraction layer 12 is lower than that of the light guide substrate 11, it is often possible to guide light to the desired FOV through total internal reflection. In such cases, a combination of materials may be selected that has lower dispersion than the light guide substrate 11 and matches the refractive index of the light guide substrate 11 and the diffraction layer 12 at wavelengths shorter than λ3.

[0079] Figure 11 shows the properties of the material used for the diffraction layer 12. As described above, in order to outcouple light to the outside of the light guide substrate 11, the refractive index of the diffraction layer 12 is set to be greater than or equal to the refractive index of the light guide substrate 11. As the material for the diffraction layer 12, for example, ZrO2, Ta2O5, Nb2O5, TeO2, MoO3, TiO2, WO3, etc. can be used. In addition to these, depending on the material of the light guide substrate 11, high refractive index materials such as HfO2, SiN, SiON, SnO, ITO, Al2O3, Y2O3, AlN, MgO, etc. can also be used.

[0080] The refractive index at each wavelength is calculated from the refractive index nd at the d-line and the Abbe number νd at the d-line, similar to FIG. 10. The larger the Abbe number, the lower the chromatic aberration. In the example of FIG. 11, the refractive index nd of all materials is greater than 2.10. When using the light guide substrate 11 with a refractive index nd exceeding 2.05, the material of the diffraction layer 12 can be selected according to the refractive index of the light guide substrate 11.

[0081] <Design of FOV> FIGS. 12 to 14 are diagrams for explaining the NA diagram and the design of the FOV. (A) of FIG. 12 is an NA diagram with the horizontal axis being the numerical aperture NAx in the x direction and the vertical axis being the numerical aperture NAy in the y direction. The donut-shaped region between the inner circle and the outer circle is the region where light can propagate by total reflection within the light guide substrate 11. The inner circle represents the NA at the critical angle, and the outer circle represents the NA at the maximum propagation angle.

[0082] (B) of FIG. 12 shows the propagation state within the light guide substrate 11. The numerical aperture NA is represented by n×sinθ. Let the incident angle of the light propagating by total reflection to the interface be θprop. Let the critical angle at which total reflection starts to occur at the interface be θc. The inner circle of the NA diagram is the NA when θprop = θc. When the numerical aperture NA at this time is set to 1 (NA = 1), the inner circle becomes a circle with a radius of 1.

[0083] At the outer circle, θprop is 90° (θprop = 90°). The radius of the outer circle is determined by the refractive index n of the light guide substrate 11. Therefore, the higher the refractive index of the light guide substrate 11 used, the wider the outer circle expands, and the wider the angular range that can be guided by total reflection waveguide becomes.

[0084] In (A) of FIG. 13, the black circle at the center of the NA diagram is the numerical aperture NA (or the incident angle) of the light incident on the incoupling grating 121. The white circle at the end of the arrow extending to the right from the black circle represents the NA of the light emitted from the outcoupling grating 123. The arrow between the black circle and the white circle indicates the NA that changes by diffraction. The diffraction order of the light that is guided by total reflection and emitted from the outcoupling grating 123 is the first order (m = 1).

[0085] Referring to Figure 13(B), the right side of the normal line drawn from the diffraction layer 12 is defined as the positive NA, and the left side as the negative NA. The angular region between the positive NA and the negative NA is the FOV. In the optical guide element 10 of this embodiment, the FOV is 55° or greater with respect to λ1, λ2, and λ3. If the pitch of the diffraction grating is Λ, the wavelength is λ, and the diffraction order is m, then the angle of incidence to the in-coupling grating 121 and the angle of emission from the out-coupling grating 123 are: nin×sinθin + mλ / Λ=nout×sinθout This is the relationship.

[0086] Figure 14 shows a design where the incident FOV and the exit FOV are aligned. In the NA diagram of Figure 14 (A), the rectangle in the center is the NA area corresponding to the incident FOV. The horizontal side of the rectangle corresponds to the FOV in the x-direction (FOVx), and the vertical side corresponds to the FOV in the y-direction (FOVy). Strictly speaking, the image of a rectangular field of view is a barrel shape inscribed within the rectangle on the NA diagram, but for convenience, a rectangular representation is used as a substitute. The diagonal FOV corresponds to the FOV at the diagonal of this rectangle. Light incident at this FOV is guided by diffracting it several times to fill the donut-shaped total internal reflection propagation region T, and then returned to its original position.

[0087] Referring to Figure 14(B), light incident in the central NA area of ​​Figure 14(A) is coupled into the light guide substrate 11 by the in-coupling grating 121, its direction is changed by the extension grating 212, and it propagates through the light guide substrate 11 while undergoing total internal reflection to the out-coupling grating 123. The total internal reflection propagated light is emitted by the out-coupling grating 123 with the same exit FOV as the incident FOV.

[0088] Figure 15 illustrates the ±1st order diffraction waveguide directions and visibility. As shown in Figure 14, when the incident FOV and exit FOV are aligned, in Figure 15 (A), the light with the incident angle corresponding to the positive FOV is in-coupled in the opposite direction to the propagation direction. When the light propagated by total internal reflection is emitted at the same angle as the incident angle by the out-coupling grating 23, the light is emitted in the direction of the user's eye 20. Similarly, the light with the incident angle corresponding to the negative FOV is in-coupled in the opposite direction to the propagation direction. Due to the out-coupling grating, the light emitted at the same angle as the incident angle is directed towards the user's eye 20, resulting in good image visibility.

[0089] In contrast, in Figure 15(B), if the positive incident FOV is in-coupled in the direction of propagation, the light emitted from the out-coupling grating 123 will be away from the user's eye 20. The same thing happens with the negative FOV, resulting in poor image visibility. Therefore, when designing to match the incident and out-coupling FOVs, the in-coupling grating 121 and out-coupling grating 123 are designed so that the light diffracts in the direction that provides the best image visibility.

[0090] Figure 16 illustrates the effect of extending the field of view (FOV) by utilizing diffraction in the ±1st order. This example uses diffraction in the +x and -x directions. In the NA diagram, the positive FOV is shown by a thick line, and the negative FOV is shown by a thin line. The solid line is the FOV of R light, the dashed line is the FOV of G light, and the dotted line is the FOV of B light.

[0091] The positive FOV is guided by all RGB elements in the total internal reflection propagation region T on the left side of the NA diagram. The negative FOV is guided by all RGB elements in the total internal reflection propagation region T on the right side of the NA diagram. Since there is a direction in which all RGB FOVs can be guided, vignetting is suppressed. An FOV extension effect can be obtained by utilizing diffraction in the ±1st order direction. For example, by using a light guide substrate with a refractive index of 2.08 at λ3, a diagonal FOV of 55° or more can be achieved for any of λ1, λ2, and λ3 included in RGB.

[0092] Figure 17 illustrates the effect of using a two-dimensional diffraction grating with a rectangular unit cell as the in-coupling grating. In Figure 17(A), when a line-and-space type one-dimensional pattern in-coupling grating 121a is used, light with a positive FOV that diffracts to the left in the figure is guided to the left side of the out-coupling grating 123b, and light with a negative FOV that diffracts to the right in the figure is guided to the right side of the out-coupling grating 123b. When the out-coupling grating 123b is a two-dimensional diffraction grating with a rectangular unit cell, diffraction occurs in the up, down, left, and right directions of the out-coupling grating 123b, and light is guided through the out-coupling grating 123b by two-dimensional diffraction. However, light heading towards the center of the out-coupling grating 123b may not be in-coupled by the in-coupling grating 121a, and as a result, light guidance may be insufficient in the central part of the out-coupling grating 123b.

[0093] In Figure 17(B), by using a two-dimensional diffraction grating in-coupling grating 121b, such as a rectangular grating, as the unit cell, light is diffracted and guided from the in-coupling grating 121b towards the out-coupling grating 123b, and the entire FOV, including the positive and negative FOVs, guides the central portion of the light-guiding substrate. The amount of light in the light image emitted from the out-coupling grating 123b is made uniform, improving visibility. By using a blazed diffraction grating as the two-dimensional diffraction grating, the diffraction efficiency that diffracts towards 123b can be selectively increased, thereby improving the utilization efficiency of incident light.

[0094] Figure 18A illustrates the effect of using a two-dimensional diffraction grating with a rectangular unit cell. The grating pitch in the x-direction is 310 nm, and the grating pitch in the y-direction is 355 nm. In the incident FOV in the center of the NA diagram, the right side is the positive FOV, and the left side is the negative FOV. The positive FOV is on the left side of the NA diagram, where all RGB wavelengths propagate within the total internal reflection propagation region T. The negative FOV is on the right side of the NA diagram, where all RGB wavelengths propagate within the total internal reflection propagation region T. In both the +y and -y directions, all RGB wavelengths are within the total internal reflection propagation region T, and no vignetting occurs. Using a single-layer light guide substrate 11 with a refractive index of 2.08 at λ3, a diagonal FOV of 55° or more can be targeted for all RGB wavelengths.

[0095] Figures 18B and 18C show the FOV light guidance when using a two-dimensional diffraction grating with a square unit cell. In Figure 18B, the grating pitch in the x and y directions is 310 nm, and in Figure 18C, the grating pitch in the x and y directions is 355 nm. In Figure 18B, R light is not sufficiently guided by total internal reflection above and below the y direction of the NA diagram, resulting in vignetting (V). In Figure 18C, B light is not sufficiently guided by total internal reflection in both the positive and negative FOV in the x direction of the NA diagram, resulting in vignetting (V). As shown in Figure 18A, by using a two-dimensional diffraction grating with a rectangular unit cell, the occurrence of vignetting can be suppressed and the quality of the color image can be maintained.

[0096] Considering these points, Diagonal FOV using FOVdiag The horizontal positive side FOV is FOVx+ (sign is positive). Horizontal direction - Side FOV is FOVx- (sign is positive) The vertical positive side FOV is FOVy+ (sign is positive). Vertical direction - Side FOV is FOVy - (sign is positive) The aspect ratio of the projected image is Ax:Ay When projecting an image, when the refractive index of the light guide substrate 11 at λ3 is n_λ3, at least the out-coupling grating is a two-dimensional diffraction grating with a rectangular grating as the unit grating, and when the pitches Λx and Λy in the x and y directions respectively satisfy the following equations, it is possible to design an optical element with an FOV of 55° or more in RGB and good visibility.

[0097] 1≦λ1 / Λx - sin(FOVx - ) (λ3 / Λx) 2 + sin(FOVy + ) 2 ≦(n_λ3) 2 (λ3 / Λx) 2 + sin(FOVy - ) 2 ≦(n_λ3) 2 (1)

[0098] 1≦λ1 / Λx - sin(FOVx + ) (λ3 / Λx) 2 + sin(FOVy + ) 2 ≦(n_λ3) 2 (λ3 / Λx) 2 + sin(FOVy - ) 2 ≦(n_λ3) 2 (2)

[0099] 1≦λ1 / Λy - sin(FOVy + ) λ3 / Λy + sin(FOVy - )≦(n_λ3) (λ3 / Λy + sin(FOVy - )) 2 + sin(FOVx + ) 2 ≦(n_λ3) 2 (λ3 / Λy + sin(FOVy - )) 2 + sin(FOVx - ) 2 ≦(n_λ3) 2 (3)

[0100] tan(((FOVx + ) + (FOVx - )) / 2) = Ax / (Ax 2 + Ay 2 ) 1 / 2 ·tan(FOVdiag / 2) (4) tan(((FOVy+)+(FOVy-)) / 2) =Ay / (Ax 2 +Ay 2 ) 1 / 2 · tan(FOVdiag / 2) (5) FOVdiag≧55° (6)

[0101] <FOV characteristics of Example 1> Figure 19A shows the characteristics and diffraction pattern of the RGB light guide in Example 1, and Figure 19B shows the characteristics and diffraction pattern of the G light guide in Example 1. The type and pitch of the diffraction grating in Example 1, and the refractive index of the light guide substrate 11 at each wavelength are as shown in Figure 8. In the RGB light guide in Figure 19A, the diagonal FOV of the incident light is 55°, the field of view is 0.5, and the aspect ratio of the incident image is 16:9. The half-angle in the x direction is 24.4°, and the half-angle in the y direction is 14.3°.

[0102] The NA diagram in Figure 19A shows the diffraction pattern when RGB light is perpendicularly incident on the in-coupling grating 121 (labeled "first grating" in the figure) and when RGB light is perpendicularly incident on the out-coupling grating 123 (labeled "second grating" in the figure). The first grating is a one-dimensional diffraction grating with a line and space pattern, and the second grating is a two-dimensional diffraction grating with a rectangular unit cell. In the diffraction pattern of the first grating, all positive FOVs are diffracted into the total internal reflection propagation region on the left, and all negative FOVs are diffracted into the total internal reflection propagation region on the right.

[0103] In the diffraction pattern of the second grating, all RGB signals are diffracted within the total internal reflection propagation region in both the ±x and ±y directions. The grating pitches of the in-coupling grating 121 and out-coupling grating 123 are set to the pitch that realizes the diffraction shown in this NA diagram. This grating design makes it possible to reproduce RGB images without vignetting with a diagonal FOV of 55° using a single-layer light guide substrate 11.

[0104] In the G light guide in Figure 19B, the diagonal FOV of the incident light is 70°, the field of view is 0.7, and the aspect ratio of the incident image is 16:9. The half-angle in the x-direction is 31.4°, and the half-angle in the y-direction is 18.9°.

[0105] The NA diagram in Figure 19B shows the diffraction pattern when G light is perpendicularly incident on the in-coupling grating 121 (labeled "first grating" in the figure) and when G light is perpendicularly incident on the out-coupling grating 123 (labeled "second grating" in the figure). In the diffraction pattern of the line-and-space type first grating, all positive FOV is diffracted into the total internal reflection propagation region on the left (thick dashed rectangle), and all negative FOV is diffracted into the total internal reflection propagation region on the right (thin dashed rectangle).

[0106] In the diffraction pattern of the second grating, which is a two-dimensional diffraction grating with a rectangular unit cell, G light diffracts into the total internal reflection propagation region in both the ±x and ±y directions. With this grating configuration, the field of view (FOV) can be extended with G light using a single-layer light guide substrate 11. However, even if R light or B light is incident perpendicularly on the second grating with a diagonal FOV of 70°, total internal reflection guidance is not guaranteed to be achieved in both the ±x and ±y directions.

[0107] <FOV characteristics of Example 2> Figure 20A shows the characteristics and diffraction pattern of the RGB light guide in Example 2, and Figure 20B shows the characteristics and diffraction pattern of the G light guide in Example 2. The type and pitch of the diffraction grating in Example 2, and the refractive index of the light guide substrate 11 at each wavelength are as shown in Figure 8. In the RGB light guide in Figure 20A, the diagonal FOV of the incident light is 55°, the field of view is 0.5, and the aspect ratio of the incident image is 16:9. The half-angle in the x direction is 24.4°, and the half-angle in the y direction is 14.3°.

[0108] The NA diagram in Figure 20A shows the diffraction pattern when RGB light is incident perpendicularly on the in-coupling grating 121 (labeled "first grating" in the figure) and when RGB light is incident perpendicularly on the out-coupling grating 123 (labeled "second grating" in the figure). Both the first and second gratings are two-dimensional diffraction gratings with a rectangular unit cell.

[0109] In the diffraction pattern of the first grating, in the x-axis direction, all positive FOVs diffract into the total internal reflection propagation region on the left, and all negative FOVs diffract into the total internal reflection propagation region on the right. In the y-axis direction, all RGB FOVs diffract into the total internal reflection propagation region. In the diffraction pattern of the second grating, all RGB diffract into the total internal reflection propagation region in both the ±x and ±y directions. The grating pitches of the in-coupling grating 121 and out-coupling grating 123 are set to achieve the diffraction shown in this NA diagram. This grating design makes it possible to reproduce RGB images with a diagonal FOV of 55° and no vignetting using a single-layer light guide substrate 11.

[0110] In the G light guide in Figure 20B, the diagonal FOV of the incident light is 70°, the field of view is 0.7, and the aspect ratio of the incident image is 16:9. The half-angle in the x-direction is 31.4°, and the half-angle in the y-direction is 18.9°.

[0111] The NA diagram in Figure 20B shows the diffraction pattern when G light is incident perpendicularly on the in-coupling grating 121 (labeled "first grating" in the figure) and when G light is incident perpendicularly on the out-coupling grating 123 (labeled "second grating" in the figure). In the diffraction pattern of the first grating, which has a rectangular unit cell, G is diffracted within the total internal reflection propagation region in both the ±x and ±y directions.

[0112] Even in the diffraction pattern of the second grating, which has a rectangular unit cell, G light diffracts into the total internal reflection propagation region in both the ±x and ±y directions. The field of view (FOV) can be extended with G light using a single-layer light guide substrate 11. However, even if R light or B light is incident perpendicularly on the second grating with a diagonal FOV of 70°, total internal reflection guidance is not guaranteed to be achieved in both the ±x and ±y directions.

[0113] <FOV characteristics of Example 3> Figure 21A shows the characteristics and diffraction pattern of the RGB light guide in Example 3, and Figure 21B shows the characteristics and diffraction pattern of the G light guide in Example 3. The type and pitch of the diffraction grating in Example 1, and the refractive index of the light guide substrate 11 at each wavelength are as shown in Figure 8. In the RGB light guide in Figure 21A, the diagonal FOV of the incident light is 65°, the field of view is 0.6, and the aspect ratio of the incident image is 16:9. The half-angle in the x direction is 29.0°, and the half-angle in the y direction is 17.3°.

[0114] The NA diagram in Figure 21A shows the diffraction pattern when RGB light is incident perpendicularly on the in-coupling grating 121 (labeled "first grating" in the figure) and when RGB light is incident perpendicularly on the out-coupling grating 123 (labeled "second grating" in the figure). Both the first and second gratings are one-dimensional diffraction gratings with a line and space pattern, but the direction in which the grating patterns extend is orthogonal between the first and second gratings.

[0115] In the diffraction pattern of the first grating, the incident FOV diffracts within the total internal reflection propagation region in both the +x and -x directions for all RGB channels. In the diffraction pattern of the second grating, the incident FOV diffracts within the total internal reflection propagation region in both the +y and -y directions for all RGB channels. The grating pitches of the in-coupling grating 121 and the out-coupling grating 123 are set to the pitch at which the diffraction in this NA diagram is realized. Using a single-layer light guide substrate 11 and a line-and-space one-dimensional diffraction grating, it is possible to reproduce RGB images without vignetting at a diagonal FOV of 65°.

[0116] In the G light guide in Figure 21B, the diagonal FOV of the incident light is 100°, the field of view is 1.2, and the aspect ratio of the incident image is 16:9. The half-angle in the x-direction is 46.1°, and the half-angle in the y-direction is 30.3°.

[0117] The NA diagram in Figure 21B shows the diffraction pattern when G light is incident perpendicularly on the in-coupling grating 121 (labeled "first grating" in the figure) and when G light is incident perpendicularly on the out-coupling grating 123 (labeled "second grating" in the figure). In the diffraction pattern of the line-and-space type first grating, both the positive and negative FOVs are diffracted within the total internal reflection propagation region in the ±x direction. In the diffraction pattern of the second grating, both the positive and negative FOVs are diffracted within the total internal reflection propagation region in the ±y direction. Using a single-layer light guide substrate 11, the diagonal FOV can be extended to 100° with G light. However, even when R light or B light is incident perpendicularly on the second grating with a diagonal FOV of 100°, total internal reflection guidance is not guaranteed to be achieved in both the ±x and ±y directions.

[0118] <FOV characteristics of Example 4> Figure 22A shows the characteristics and diffraction pattern of the RGB light guide in Example 4, and Figure 22B shows the characteristics and diffraction pattern of the G light guide in Example 4. The type and pitch of the diffraction grating in Example 4, and the refractive index of the light guide substrate 11 at each wavelength are as shown in Figure 8. In the RGB light guide in Figure 22A, the diagonal FOV of the incident light is 85°, the field of view is 0.9, and the aspect ratio of the incident image is 16:9. The half-angle in the x direction is 38.6°, and the half-angle in the y direction is 24.2°.

[0119] The NA diagram in Figure 22A shows the diffraction pattern when RGB light is perpendicularly incident on the in-coupling grating 121 (labeled "first grating" in the figure) and when RGB light is perpendicularly incident on the out-coupling grating 123 (labeled "second grating" in the figure). Both the first and second gratings are two-dimensional diffraction gratings with a rectangular unit cell. In the diffraction pattern of the first grating, in the x-axis direction, all positive FOVs are diffracted into the total internal reflection propagation region on the left, and all negative FOVs are diffracted into the total internal reflection propagation region on the right. In the y-axis direction, all RGB FOVs are diffracted into the total internal reflection propagation region.

[0120] In the diffraction pattern of the second grating, all RGB signals are diffracted within the total internal reflection propagation region in both the ±x and ±y directions. The grating pitches of the in-coupling grating 121 and out-coupling grating 123 are set to the pitch at which diffraction in this NA diagram is realized. With this grating design, a single-layer light guide substrate 11 can be used to reproduce RGB images without vignetting at a diagonal FOV of 85°.

[0121] In the G light guide in Figure 22B, the diagonal FOV of the incident light is 110°, the field of view is 1.4, and the aspect ratio of the incident image is 16:9. The half-angle in the x-direction is 51.2°, and the half-angle in the y-direction is 35.0°.

[0122] The NA diagram in Figure 21B shows the diffraction pattern when G light is incident perpendicularly on the in-coupling grating 121 (labeled "first grating" in the figure) and when G light is incident perpendicularly on the out-coupling grating 123 (labeled "second grating" in the figure). In the diffraction pattern of the first grating of the rectangular grating, G is diffracted within the total internal reflection propagation region in both the ±x and ±y directions.

[0123] Even in the diffraction pattern of the second grating, which uses a rectangular unit cell as a two-dimensional diffraction grating, the G light diffracts into the total internal reflection propagation region in both the ±x and ±y directions. Using a single-layer light guide substrate 11, the diagonal FOV can be extended to 110° for at least the G light. However, even if R light or B light is incident perpendicularly on the first or second grating with a diagonal FOV of 110°, total internal reflection guidance is not guaranteed to be achieved in both the ±x and ±y directions.

[0124] Although the present invention has been described above based on specific configuration examples, the present invention is not limited to the above-described configuration examples. The display device 100 using the light guide element 10 may be linked with a smartphone, a notebook personal computer (PC), etc. The display screen of the smartphone or notebook PC may be set to a working field of view with a diagonal FOV of 55°, and an indirect field of view with a diagonal FOV of 70° or more may be set around the working field of view. In this case, simple monochrome images or information may be displayed in the indirect field of view, within a range that does not interfere with daily activities.

[0125] This application claims priority based on Japanese Patent Application No. 2021-079180, filed on 7 May 2021, and includes the entire contents of that Japanese Patent Application. [Explanation of Symbols]

[0126] 10 Light guide elements 11 Light guide substrate 12 Diffraction layer 121, 121a, 121b In-coupling gratings (first diffraction gratings) 122 Extended Grid 123, 123a, 123b Out-coupling gratings (second diffraction gratings) 100 display device 110 Projector 120 Support

Claims

1. A single-layer light guide substrate, A diffraction layer formed on the light guide substrate, It has, The diffraction layer comprises a first diffraction grating that incouples incident light incident on the light guide substrate into the light guide substrate, and a second diffraction grating that outcouples total reflected light propagating through the light guide substrate to the outside of the light guide substrate. The first diffraction grating incouples the incident light in an angular range of 60° or more, including the normal direction of the light guide substrate, at least one of the following wavelengths: a first wavelength included in the 450 nm ± 20 nm band, a second wavelength included in the 530 nm ± 20 nm band, and a third wavelength included in the 630 nm ± 20 nm band. The second diffraction grating is a light guide element that outcouples the total reflected light in an angular range of 60° or more, including the normal direction, at at least one wavelength.

2. The first diffraction grating incouples the incident light in a common angular range of 55° or more, including the normal direction, at any of the first, second, and third wavelengths. The light guide element according to claim 1, wherein the second diffraction grating outcouples the total reflected light in a common angular range of 55° or more, including the normal direction, at any of the first wavelength, the second wavelength, and the third wavelength.

3. The light guide element according to claim 1 or 2, wherein the internal transmittance of the light guide substrate per 10 mm thickness for light with a wavelength of 450 nm is 95% or more.

4. The light guide element according to any one of claims 1 to 3, wherein the light guide substrate is an isotropic single crystal substrate, or a uniaxial crystal substrate whose optical axis is within ±4° of the normal direction of the light guide substrate.

5. The light guide substrate has a refractive index greater than 2.05 in the d-line. A light guide element according to any one of claims 1 to 4.

6. The composition of the light guide substrate is such that, when the total of the base composition is taken as 100% based on the oxide in mole percent, Bi ２ O ３ It contains 20% to 50% of [substance name] and 10% to 35% of TeO2. (1) Bi ２ O ３ -TeO ２ -based glass, or La ２ O ３ with a content of 10% to 40%, B ２ O ３ with a content of 10% to 35% (2) La ２ O ３ -B ２ O ３ -based glass The light guide element according to any one of claims 1 to 5.

7. The aforementioned light guide substrate is Bi ２ O ３ Contains 20% or more of Bi ２ O ３ -TeO ２ -Nb ２ O ５ -TiO ２ -Ta ２ O ５ - WO ３ A light guide element according to any one of claims 1 to 5, comprising 55 mol% or more of the above.

8. The aforementioned light guide substrate is TiO ２ SrTiO ３ , KTaO ３ LiNbo ３ A light guide element according to any one of claims 1 to 5, wherein the substrate is SiC or diamond.

9. The diffraction layer is ZrO ２ , HfO ２ Ta ２ O ５ , Nb ２ O ５ TeO ２ MoO ３ WO ３ , TiO ２ , SiN, SiON, SnO, ITO, Al ２ O ３ , Y ２ O ３ A light guide element according to any one of claims 1 to 8, which is formed of AlN, MgO, or a mixture of two or more of these.

10. The light guide element according to claim 9, wherein the refractive index of the diffraction layer at the third wavelength is greater than the refractive index of the light guide substrate at the third wavelength, and the difference in refractive index between the diffraction layer and the light guide substrate at the third wavelength is 0.1 or less.

11. The light guide element according to any one of claims 1 to 10, wherein the second diffraction grating is a two-dimensional diffraction grating with a rectangular unit cell, and when light of the first wavelength, the second wavelength, and the third wavelength is perpendicularly incident on the second diffraction grating from the light guide substrate, the grating pitch has such that (±1, 0)-order or (0, ±1)-order diffracted light is guided through the light guide substrate by total internal reflection at any wavelength.

12. The light guide element according to any one of claims 1 to 10, wherein the second diffraction grating is a two-dimensional diffraction grating with a rectangular unit cell, and has a grating pitch such that when light of any of the first, second, or third wavelengths is perpendicularly incident from the light guide substrate to the second diffraction grating, the (±1, ±1)th order diffracted light is guided through the light guide substrate by total internal reflection.

13. The first diffraction grating is a two-dimensional diffraction grating with a rectangular unit cell, and when light of the first, second, and third wavelengths is perpendicularly incident from the light guide substrate to the first diffraction grating, it has a grating pitch that guides (±1, 0)th order diffracted light or (0, ±1)th order diffracted light through the light guide substrate via total internal reflection at any wavelength, as described in any one of claims 1 to 12.

14. The first diffraction grating is a two-dimensional diffraction grating with a rectangular unit cell, and has a grating pitch such that when light of any of the first, second, or third wavelengths is perpendicularly incident on the first diffraction grating from the light guide substrate, the (±1, ±1)th order diffracted light is guided through the light guide substrate by total internal reflection, as described in any one of claims 1 to 12.

15. A light guide element according to any one of claims 1 to 14, Projector and A display device comprising the following: light projected from the projector enters the light guide element and is emitted from the second diffraction grating.

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