Light guide element and display device using the same

Abstract

The purpose of the present invention is to provide a light-guide element which is lightweight and which has a wide viewing angle, and a display device which uses the same.　A light-guide element (10) according to the present invention comprises a monolayer light-guide substrate (11) and a diffraction layer (12) formed on the light-guide substrate (11). The diffraction layer (12) has a first diffraction grating (121) which is for in-coupling, into the light-guide substrate (11), of incident light that is incident on the light-guide substrate (11), and a second diffraction grating (123) which for out-coupling, to the outside of the light-guide substrate (11), of total reflection light that has propagated through the light-guide substrate (11). At at least one wavelength among a first wavelength (λ1) included in a 450 nm ± 20 nm band, a second wavelength (λ2) included in a 530 nm ± 20 nm band, and a third wavelength (λ3) included in a 630 nm ± 20 nm band, the first diffraction grating (121) achieves in-coupling of the incident light in an angle range of not less than 60° including the normal direction to the light-guide substrate (11). At said at least one wavelength, the second diffraction grating (123) achieves out-coupling of said total reflection light in an angle range of not less than 60° including said normal direction.

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) / mixed reality (MR) headsets are being developed for both personal and professional use. ARMR headsets require high resolution and a wide viewing angle, so they often use heavy components to display the ARMR image, making the entire headset heavy and uncomfortable to wear because it is fixed to the head. On the other hand, small, lightweight glasses-type displays that show simple information such as letters and symbols are also being developed.

[0003] A configuration is known in which an optical element for displaying an ARMR image in a personal display or an augmented reality display uses a diffractive light guide in which an input coupling grating, an exit pupil expansion 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 Document 1 and Patent Document 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 Summary of the Invention [Problem to be solved by the invention]

[0005] If a lightweight display device with the same functionality and wide viewing angle as the ARMR headset could be realized, the application field and range of users would be further expanded.The present invention aims to provide a lightweight light-guiding element with a wide viewing angle and a display device using the same. [Means for solving the problem]

[0006] In one aspect of the present invention, a light-guiding element includes a single-layer light-guiding substrate and a diffraction layer formed on the light-guiding substrate, the diffraction layer includes a first diffraction grating that incouples incident light that has entered the light-guiding substrate into the light-guiding substrate, and a second diffraction grating that outcouples totally reflected light that has propagated through the light-guiding substrate to the outside of the light-guiding substrate; the first diffraction grating incouples the incident light in an angular range of 60° or more including a normal direction of the light-guiding substrate at at least one wavelength selected from a first wavelength included in a 450 nm±20 nm band, a second wavelength included in a 530 nm±20 nm band, and a third wavelength included in a 630 nm±20 nm band; The second diffraction grating outcouples the totally reflected light at the at least one wavelength in an angular range of 60° or more including the normal direction. [Effects of the Invention]

[0007] A lightweight light guide element with a wide viewing angle and a display device using the same are realized. [Brief explanation of the drawings]

[0008] [Figure 1] 1A and 1B are schematic diagrams illustrating a configuration example of a light guide element according to an embodiment. [Figure 2A] 1A and 1B illustrate coupling of incident light into a light-guiding substrate by an incoupling grating. [Figure 2B] FIG. 10 illustrates total internal reflection guiding of light from an in-coupling grating to an out-coupling grating. [Figure 2C] 10A and 10B are diagrams illustrating light exiting from an outcoupling grating to the outside of a light-guiding substrate. [Figure 2D]FIG. 10 is a diagram showing an example of a composition having a refractive index greater than 2.05 at the d line. [Figure 3] 1 is a diagram illustrating an example of a display device using a light guide element according to an embodiment. [Figure 4] 10A and 10B are diagrams illustrating an example of the operation of a light guide element used in a display device. [Figure 5] FIG. 10 is a diagram illustrating an example of the configuration of a light-guiding element using a line-and-space one-dimensional diffraction grating. [Figure 6] FIG. 10 is a diagram illustrating an example of the configuration of a light-guiding element that uses a two-dimensional diffraction grating in which a rectangular lattice serves as a unit lattice for an out-coupling grating. [Figure 7] 10A and 10B are diagrams illustrating an example of the configuration of a light-guiding element that uses two-dimensional diffraction gratings in which rectangular lattices are used as unit lattices for both the in-coupling grating and the out-coupling grating. [Figure 8] FIG. 10 is a diagram showing grating shapes and optical characteristics of an example and a comparative example. [Figure 9] 10A and 10B are diagrams illustrating an example of a material for the light guide substrate. [Figure 10] FIG. 10 is a diagram showing the difference in refractive index between the light guide substrate and the diffractive layer in the example and the comparative example. [Figure 11] FIG. 10 is a diagram showing the properties of materials used in the diffractive layer. [Figure 12] FIG. 1 is a diagram illustrating the design of an NA (Numerical Aperture) diagram and an FOV (Field of View). [Figure 13] 1 is a diagram illustrating the NA diagram and FOV design. [Figure 14] 1 is a diagram illustrating the NA diagram and FOV design. [Figure 15] 1A and 1B are diagrams illustrating ±1st-order diffraction waveguiding directions and visibility. [Figure 16] 10A and 10B are diagrams illustrating the FOV expansion effect achieved by utilizing ±1st-order diffraction. [Figure 17] 10A and 10B are diagrams illustrating the effect of using a two-dimensional intercoupling lattice. [Figure 18A] FIG. 10 is a diagram illustrating the effect of using a rectangular lattice as a unit lattice. [Figure 18B]FIG. 10 is a diagram showing FOV light guiding when a square lattice is used as the unit lattice. [Figure 18C] FIG. 10 is a diagram showing FOV light guiding when a square lattice is used as the unit lattice. [Figure 19A] 4A and 4B are diagrams illustrating the characteristics of RGB light guide and a diffraction image of the first embodiment. [Figure 19B] 10A and 10B are diagrams showing the characteristics of G light guide and a diffraction image in Example 1. [Figure 20A] 10A and 10B are diagrams illustrating the characteristics of RGB light guide and a diffraction image of Example 2. [Figure 20B] 10A and 10B are diagrams showing the characteristics and diffraction images of G light guided in Example 2. [Figure 21A] 10A and 10B are diagrams illustrating the characteristics of RGB light guide and a diffraction image of Example 3. [Figure 21B] 10A and 10B are diagrams showing the characteristics of G light guide and a diffraction image in Example 3. [Figure 22A] 10A and 10B are diagrams illustrating the characteristics of RGB light guide and a diffraction image of Example 4. [Figure 22B] 10 is a diagram showing the characteristics and diffraction image of G light guided in Example 4. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0009] FIG. 1 is a schematic diagram illustrating an exemplary configuration of a light-guiding element 10 according to an embodiment. The light-guiding element 10 includes a single-layer light-guiding substrate 11 and a diffraction layer 12 formed on the light-guiding substrate 11. The diffraction layer 12 includes an in-coupling grating 121 that couples incident light into the light-guiding substrate 11 into the light-guiding substrate 11, and an out-coupling grating 123 that outputs the totally reflected light propagating through the light-guiding substrate 11 to the outside of the light-guiding substrate 11. As described below, the light-guiding substrate 11 may include an extended grating that guides the light incoupled into the light-guiding substrate 11 to the out-coupling grating 123. The incident and out-coupling directions of light are not limited to the rear surface of the light-guiding substrate 11. Light incident from the surface on which the diffraction layer 12 is formed can be coupled into the light-guiding substrate 11, or light can 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-guiding substrate 11, or on both surfaces.

[0010] In the embodiment, a single-layer light guide substrate 11 is used, and 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 are introduced into the light guide substrate 11 with a wide FOV and emitted from the light guide substrate 11 with a wide 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, at any one of the first wavelength (λ1), second wavelength (λ2), and third wavelength (λ3), incident light is incoupled over an angular range of 60° or more, preferably 65° or more, and more preferably 70° or more, including the normal direction of light guide substrate 11. Furthermore, at any one of the wavelengths λ1, λ2, and λ3, light is outcoupled to the outside of light guide substrate 11 over an angular range of 60° or more, preferably 65° or more, and more preferably 70° or more, including the normal direction of light guide substrate 11. The angle between the center of the angular range of incoupled or outcoupled light and the normal to the substrate is preferably ±15° or less, more preferably ±10° or less, even more preferably ±5° or less, and most preferably approximately coincides with the normal to the substrate.

[0012] The incoupling grating 121 also incouples incident light for any of the wavelengths λ1, λ2, and λ3 in a common angular range of 55° or more including the normal direction to the light-guiding substrate 11. The outcoupling grating 123 outcouples light for any of the wavelengths λ1, λ2, and λ3 in a common angular range of 55° or more including the normal direction to the light-guiding substrate 11. The angle between the center of the angular range of the incoupled or outcoupled light and the normal to the substrate is preferably ±15° or less, more preferably ±10° or less, even more preferably ±5° or less, and most preferably approximately coincident with the normal to the substrate.

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

[0014] In conventional configurations, three monochromatic light guide plates corresponding to each wavelength are stacked. Because the FOV can be optimized for each wavelength, a high FOV is easily achieved, but the overall optical element becomes thick and heavy. In this embodiment, a single-layer light guide substrate 11 guides RGB light, resulting in a compact and lightweight light guide element 10. When a single-layer light guide substrate 11 is used, the overlapping FOV of the FOVs for each RGB wavelength becomes the overall FOV. However, a high FOV can be achieved by optimizing the materials of the light guide substrate 11 and the diffraction layer 12 and the design of the incoupling grating 121 and the outcoupling grating 123. For example, both the light guide substrate 11 and the diffraction layer 12 can be formed from inorganic materials and designed to have a predetermined refractive index. Additionally, ±1st-order diffraction is utilized. The rationale for these configurations and analytical results will be described later.

[0015] FIG. 2A is a diagram illustrating the coupling of incident light Lin to the light guide substrate 11 by the incoupling grating 121. The plane on the light guide substrate 11 on which the diffraction layer 12 is formed is defined as the xy plane, and the thickness direction of the light guide substrate 11 is defined as the z direction. The cross-sectional structure of A-A' is shown together with the structure of the xy plane. For convenience of illustration, For convenience, the 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 into the light-guiding substrate 11 by the incoupling 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 into the light-guiding 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 the incident light propagates in two dimensions, the x direction and the y direction, may be adopted. The incoupling grating 121 couples into the light-guiding substrate 11 both light incident from the side of the diffraction layer 12 and light incident from the back surface of the light-guiding substrate 11. Normal incident light from the normal direction to the light-guiding substrate 11 is also coupled into the light-guiding substrate 11. In addition, an FOV of 55° or more is realized for all RGB wavelengths.

[0017] FIG. 2B is a diagram illustrating total internal reflection light guide from the in-coupling grating 121 to the out-coupling grating 123. The diagram shows the cross-sectional structure along the line B-B' along with the structure of the xy plane. The propagation direction of the light incoupled into the extended grating 122 is changed by the extended grating 122. The extended grating 122 has a line and space pattern that extends obliquely with respect to the x-axis or y-axis. The extended grating 122 guides most of the light in the X-direction, while diffracting a portion of the light in the -y-direction. The line and space pattern of the extended grating 122 replicates many diffracted beams along the x-direction, and each diffracted beam propagates in the -y-direction. Both the light propagating in the x-direction and the light diffracted in the -y-direction propagate within the light-guiding substrate 11 while being totally reflected.

[0018] FIG. 2C is a diagram for explaining the emission of light from the outcoupling grating 123 to the outside. It shows the CC' cross-sectional structure along with the xy plane structure. The outcoupling grating 123 is mostly The outcoupling grating 123 guides a portion of the light by total reflection, while diffracting and emitting a portion of the light. The outcoupling grating 123 replicates multiple diffracted beams along the light guiding direction and emits them outside the light-guiding substrate 11. Since light can be emitted from both the diffractive layer 12 side and the back surface of the light-guiding substrate 11, the image formed by the emitted light can be seen from either side.

[0019] The outcoupling grating 123 outputs light that is guided by total reflection inside the light-guiding substrate 11 in a predetermined angle range including the normal direction of the light-guiding substrate 11. Specifically, outcoupling is achieved at any wavelength of RGB with an FOV of 60° or more including the normal direction. Also, outcoupling is achieved at all wavelengths of RGB with an FOV of 55° or more including the normal direction.

[0020] <Light guide substrate materials> 2A to 2C, the light guide substrate 11 transmits incoupled light to the outcoupling position by total internal reflection. The higher the refractive index of the light guide substrate 11, the wider the angle range in which light can be guided by total internal reflection. 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 having a refractive index nd at the d-line greater than 2.05 include the composition shown in FIG. 2D. A refractive index nd greater than 2.08 facilitates the design of a wider FOV. Furthermore, to outcouple incident light with minimal loss, it is preferable that the internal transmittance be high at all wavelengths λ1, λ2, and λ3. Because transmittance loss due to absorption is greater for shorter wavelength light, the internal transmittance of the light guide substrate 11 per 10 mm of thickness for light with a wavelength of 450 nm may be set to 90% or higher, more preferably 95% or higher. Examples of glass compositions that provide light guide substrate 11 with an internal transmittance of 95% or more for light with a wavelength of 450 nm include the compositions shown in Table 1.

[0021] [Table 1]

[0022] The light guide substrate 11 is, for example, a glass substrate. As the glass material, (1) Bi2O3-TeO2-based glass or (2) La2O3-B2O3-based glass may be used. Here, when we say "composition," we mean the percentage (mol%, weight percent), excluding unavoidable impurities and intentionally added impurities and additives in ppm (Parts per Million) units. It refers to an assembly of elements or components that are designed to total 100% (e.g., 100% of the total).

[0023] Examples of Bi2O3-TeO2-based glass include glasses containing 20% ​​to 50% Bi2O3 and 10% to 35% TeO2, when the total of the matrix composition in mole percent based on oxides is 100%.

[0024] Bi2O3 is a component that is preferably contained to obtain a high refractive index glass with a high visible light transmittance, and the lower limit is preferably 20% or more, more preferably 25% or more, and even more preferably 30% or more, and 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 to obtain a glass with a high refractive index and high visible light transmittance. The TeO2 content is preferably 10% or more, more preferably more than 20%, and even more preferably 25% or more. However, since too much TeO2 makes the glass unstable, it is preferably 35% or less, and even more preferably 30% or less.

[0026] B2O3 is a glass-forming component and is preferably contained to stabilize the glass, but if the content is too high, 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 contained to stabilize the glass, but if the content is too high, it becomes difficult to achieve a high refractive index. The lower limit is preferably 0% or more, and the upper limit is preferably 20% or less, and more preferably 15% or less.

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

[0029] Nb2O5, TiO2, Ta2O5, and WO3 are components that are preferably contained to increase the refractive index of the glass. These, along with Bi2O3 and TeO2, can also be used as the diffraction layer 12, which will be 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. The higher the ratio of Bi2O3-TeO2-Nb2O5-TiO2-Ta2O5-WO3, the better, preferably 55% or more, and more preferably more than 60%.

[0030] Examples of La2O3-B2O3-based glass include glasses containing 10% to 40% La2O3 and 10% to 35% B2O3, when the total of the matrix composition in mole percent based on oxides is 100%.

[0031] La2O3 is an excellent component that increases the refractive index and reduces dispersion while maintaining glass stability, and has high visible light transmittance, making it possible to obtain a glass with a high refractive index and high visible light transmittance. For this reason, La2O3 may be contained in the glass, but if the content is too high, devitrification resistance 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 contained to stabilize the glass, but if the content is too high, 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 contained to stabilize the glass, but if the content is too high, 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 may be contained in the glass because it has an excellent effect of increasing the refractive index while maintaining glass stability, but if the content is too high, devitrification resistance will decrease. 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 more preferably 35% or less.

[0035] ZrO2 is an optional component. ZrO2 may be contained in the glass because it is excellent at increasing the refractive index while maintaining glass stability, but if the content is too high, devitrification resistance 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 components that are preferably included to increase the refractive index of the glass. These components, including La2O3 and TiO2, can also be used in the diffraction layer 12 described below. Increasing the ratio of these components can achieve a configuration with a refractive index dispersion relationship similar to that of the diffraction layer 12. The higher the ratio of La2O3-TiO2-Gd2O3-Nb2O5-Ta2O5-WO3, the better, preferably 55% or more, and more preferably greater than 60%. ZrO2 may also be included, with the ratio of La2O3-TiO2-Gd2O3-Nb2O5-Ta2O5-WO3-ZrO2 being 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 having a refractive index of more than 2.05 for the d-line (λ2), or a light guide substrate 11 having an internal transmittance of 95% or more per 10 mm thickness at 450 nm.

[0038] The light-guiding substrate 11 may be a single-crystal substrate. A single crystal refers to a crystal in which the orientation of the atomic or molecular arrangement is the same throughout the crystal. The light-guiding substrate 11 may be an isotropic single-crystal substrate whose optical properties are independent of direction, or a uniaxial single-crystal substrate whose crystal axis faces a predetermined direction. In the case of a uniaxial substrate, it is desirable that the optical axis of the light-guiding substrate 11 be within ±4° of the normal, preferably within ±0.4°. This is because the center of the line of sight when viewing an actual scene or text, not an ARMR image, through the light-guiding substrate roughly coincides with the normal to the substrate, and because misalignment of the light propagation direction and the optical axis results in a double image due to birefringence, leading to reduced resolution.

[0039] Furthermore, lattice defects called dislocation defects may occur within the crystal. For example, dislocation defects with a diameter of more than 1 μm, known as micropipes, often cause refractive index modulation 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 guiding, the refractive index modulation will lead to a decrease in the resolution of the image displayed by the light guide element, so it is preferable that such defects do not exist within the light guide element. The defect density of micropipes is 10 / cm 2 It is preferable that the density is 1 particle / cm², and 0.1 particles / cm² is preferable. 2 It is more preferable that:

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

[0041] FIG. 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 a eyepiece for the left eye. The display device 100 includes the light guide element 10 and a projector 110. The light guide element 10 for the right eye and the light guide element 10 for the left eye are each provided with a projector 110, and the light guide element 10 and the projector 110 are held by a wearable support 120. The light guide element 10 is formed of a single-layer light guide substrate 11, and the display device 100 is small and lightweight overall.

[0042] 4 shows an example of the operation of the light-guiding element 10 used in the display device 100. An image projected from the projector 110 is diffracted by the incoupling grating 121 into the light-guiding substrate 11. As described above, the incoupling grating 121 diffracts RGB light into the light-guiding substrate 11 with an FOV of 55° or more, including perpendicular incident light. The incoupled RGB light propagates inside the light-guiding substrate 11 while being totally reflected, is diffracted by the outcoupling grating 123, and is emitted from the light-guiding substrate 11 with an FOV of 55° or more. This optical image is incident on the human eye 20 and is recognized as a color image.

[0043] The thickness of the light-guiding substrate 11 is, for example, 1 mm or less. The incoupling grating 121 and the outcoupling grating 123 are, for example, formed of a thin film of an inorganic material having a thickness of 100 to 1000 nm, and have a diffraction grating pattern formed at a predetermined pitch. When an extended grating 122 is used in combination with the incoupling grating 121 and the outcoupling grating 123, the extended grating 122 is also formed of 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 an inorganic material, a diffraction grating may be formed directly on the surface of the light-guiding substrate 11. In this case, the surface region on which the diffraction grating is formed becomes the diffraction layer 12.

[0044] The incoupling grating 121 achieves an FOV of 60° or more, preferably 65° or more, and more preferably 70° or more for any wavelength of RGB. The incoupling grating 121 achieves an FOV of 55° or more for any wavelength of RGB. The outcoupling grating 123 achieves an FOV of 60° or more, preferably 65° or more, and more preferably 70° or more for any wavelength of RGB. The outcoupling grating 123 achieves an FOV of 55° or more for any wavelength of RGB.

[0045] A display device 100 using a light-guiding element 10 may be configured to display color images within a field of view (FOV) of up to 55°, and to present simple information or display monochrome images in areas beyond 55°. For example, simple information such as characters or symbols, icons, toolbars, etc., may be displayed at the edge of the field of view. A cover may be provided to cover at least one of the diffraction layer 12 and the rear surface of the light-guiding substrate 11. In this case, the cover may be part of the light-guiding element 10 and may be held by the support 120 shown in FIG. 3. If a cover is used, it is desirable that the cover be highly transparent to visible light and not affect the optical image emitted from the outcoupling grating 123. The cover may be made of glass or plastic.

[0046] <Diffraction grating configuration> 5 to 7 show examples of grating designs for the light-guiding element 10. FIG. 5 shows a configuration example in which both the incoupling grating 121a and the outcoupling grating 123a have line-and-space patterns that are one-dimensional diffraction gratings. FIG. 5A shows an example in which incoupling is uniaxial diffraction, and in this example, diffraction in the +x direction is utilized. As described with reference to FIGS. 2A to 2C, the RGB light diffracted in the +x direction by the incoupling grating 121a is redirected to the -y direction by the extension grating 122 and emitted from the outcoupling grating 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 of Figure 5(B) utilizes diffraction in the positive and negative directions of a single axis for incoupling, in this example, diffraction in the ±x directions. Incoupling grating 121a splits the incident light into two diffracted wavefronts, diffracting them in the +x and -x directions. The diffracted light in the +x and -x directions is redirected by extension grating 122 and then outcoupled by outcoupling grating 123. The FOV of the output light is 55° or greater for all RGB wavelengths, and when focusing on any one wavelength, the output FOV is 60° or greater, preferably 65° or greater, and more preferably 70° or greater.

[0048] Figure 6 shows an example in which the incoupling grating 121a is a one-dimensional diffraction grating with a line-and-space pattern, and the outcoupling grating 123b is a two-dimensional diffraction grating with a rectangular lattice unit. (A) and (B) of Figure 6 are the same as (A) and (B) of Figure 5, except that the outcoupling grating 123b uses a rectangular lattice unit. Using a rectangular lattice unit can suppress vignetting, as described below. "Vignetting" refers to the loss of part of the field of view desired to be displayed in an input image due to a localized decrease in light or brightness. This phenomenon occurs when light at a certain viewing angle cannot be guided by total reflection when diffracting and guiding light within a light-guiding substrate.

[0049] FIG. 6C shows a design that does not use the extended grating 122. The line-and-space pattern of the incoupling grating 121a extends along the x direction. This incoupling grating 121a diffracts incident light in the ±y directions, and in this example, diffraction in the -y direction is utilized. Light incoupled into the light-guiding substrate 11 propagates in the -y direction while being totally reflected within the light-guiding substrate 11 and is emitted outside the light-guiding substrate 11 by the outcoupling grating 123b. In this case, since the extended grating 122 is not present, it is necessary to replicate the outcoupling light in two dimensions using the outcoupling grating 123b to improve visibility. Therefore, the outcoupling grating 123b is formed as a two-dimensional diffraction grating with a rectangular lattice as a unit cell, and the propagating light is diffracted two-dimensionally.

[0050] 7 shows an example in which rectangular lattices are used as unit lattices for both the incoupling grating 121b and the outcoupling grating 123b. (A), (B), and (C) of FIG. 7 are the same as (A), (B), and (C) of FIG. 6, except that the unit lattice of the incoupling grating 121b is a rectangular lattice. By using rectangular lattices for both the incoupling grating 121b and the outcoupling grating 123b, the FOV for the RGB waveguide can be expanded.

[0051] 7D shows a design in which the incoupling grating 121b and the outcoupling grating 123b partially overlap. Part of the rectangular grating pattern of the outcoupling grating 123b also functions as the incoupling grating 121b. The area of ​​the outcoupling grating 123b that can incouple each of the RGB colors from the projector 110 (see FIG. 3) with an FOV of 55° or more becomes the incoupling grating 121b.

[0052] The light-guiding element 10 may employ any of the grating designs shown in Figures 5 to 7. Whichever pattern is used, it is desirable that the refractive index of the diffractive layer 12 be the same as or higher than the refractive index of the light-guiding substrate 11. As will be described later, it is desirable that the difference between the refractive index of the diffractive layer 12 and the refractive index of the light-guiding substrate 11 be 0.1 or less for each RGB wavelength.

[0053] The diffraction layer 12, including the incoupling grating 121 and the outcoupling grating 123, is formed of, for example, ZrO2, HfO2, Ta2O5, Nb2O5, TeO2, MoO3, WO3, TiO2, SiN, SiON, SnO, ITO, Al2O3, YO3, AlN, MgO, or a mixture of two or more of these. Alternatively, the diffraction layer 12 may be formed of a glass material containing three or more inorganic elements. The diffraction layer 12 can be formed on the surface of the light-guiding substrate 11 by vapor deposition, sputtering, or the like. The incoupling grating 121, the extended grating 122, and the outcoupling grating 123 are formed by etching the diffraction layer 12 to form a desired pattern, such as a line-and-space pattern or a rectangular grating pattern.

[0054] When the diffraction layer 12 is formed from a mixture of two or more materials, a film with a refractive index dispersion, i.e., a wavelength dependency of the refractive index, matching that of the light-guiding substrate 11 may be formed by co-sputtering. The diffraction layer 12 may be formed by etching or lift-off. The diffraction layer 12 may be formed by directly engraving a diffraction grating into a glass substrate, single-crystal substrate, or the like. In this case, the light-guiding substrate 11 and the diffraction layer 12 can be integrally formed using a high-refractive-index material. When the light-guiding element 10 is applied to the display device 100 shown in Figure 3, the light-guiding substrate 11 is processed to a size, thickness, and shape suitable for an eyepiece. Because a single-layer light-guiding substrate 11 enables RGB light guidance with a wide FOV, it is thin and lightweight even when used as an eyepiece.

[0055] <Light guide element characteristics> FIG. 8 shows the grating shapes and optical characteristics of the example and comparative example. The grating shapes include the type of grating pattern and the grating pitch in the x and y directions. The optical characteristics include the specific wavelengths λ1, λ2, and λ3, the refractive index of the light-guiding 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 aspect ratio of the projected image and the x and y directions. For example, when projecting an image with an aspect ratio of 4:3, the ratio of the tangents of the diagonal FOV, the horizontal FOV, and the vertical FOV is √(4^2 + 3^2), which is 5:4:3. The configuration utilizing diffraction in the ±1st-order directions, as shown in FIG. 7B, is annotated as "±1st-order use."

[0056] Common parameters throughout the examples and comparative examples are λ1 of 450 nm, λ2 of 532 nm, and λ3 of 633 nm. The aspect ratio of the input image is 16:9. The diffraction layer 12 is formed of an inorganic film with a higher refractive index than the light-guiding substrate 11 at λ1, λ2, and λ3, or a grating is formed directly on the light-guiding substrate 11. In these cases, the refractive index of the light-guiding substrate 11 determines whether total internal reflection light guidance is possible, so the refractive index of the diffraction layer 12 does not need to be taken into account when discussing the viewing angle. [Example]

[0057] In Example 1, a line and space (denoted as "L&S" in FIG. 8) pattern is used for the incoupling grating 121, and a two-dimensional diffraction grating in which the unit lattice is a rectangular lattice is used for the outcoupling grating 123. The grating pitch in the x direction of the incoupling grating 121 is 310 nm. The pitch in the x direction of the outcoupling grating 123 is 310 nm, and the pitch in the y direction 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: Bi2O3:37.6B2O3:26.5TeO2:18.5P2O5:10.5Nb2O 5:1.6ZnO:5.3

[0058] The refractive index of the light-guiding substrate 11 at λ3 in Example 1 is 2.08. The shorter the wavelength, the greater the refractive index experienced by the wave, 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 waveguiding FOV at all of λ1, λ2, and λ3 is greater than 55°. [Example]

[0059] In Example 2, two-dimensional diffraction gratings with rectangular unit cells are used for both the incoupling grating 121 and the outcoupling grating 123, and diffraction in the ±1st order directions is utilized. The grating pitch in the x direction of each of the incoupling grating 121 and the outcoupling grating 123 is 310 nm, and the pitch in the y direction is 355 nm. The light-guiding substrate 11 is a Bi2O3-B2O3-TeO2-P2O5-Nb2O5-ZnO glass substrate with the same composition as in Example 1. The refractive index of this light-guiding substrate 11 at λ3 is 2.08. In Example 2, the diagonal FOV at λ2 is greater than 70°, and the maximum waveguiding FOV at all of λ1, λ2, and λ3 is greater than 55°. [Example]

[0060] In Example 3, a line-and-space pattern is used for both the incoupling grating 121 and the outcoupling grating 123. The pitch of the incoupling grating 121 in the x direction is 270 nm, and the pitch of the outcoupling grating 123 in the y direction is 300 nm. A single-crystal SiC substrate is used as the light-guiding 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 waveguiding FOV at all of λ1, λ2, and λ3 is greater than 65°. By using a light-guiding substrate 11 with a large refractive index, a wide FOV can be achieved while using a line-and-space diffraction grating. [Example]

[0061] In Example 4, two-dimensional diffraction gratings with rectangular unit lattices are used for both the incoupling grating 121 and the outcoupling grating 123. The pitch of the incoupling grating 121 in the x direction is 270 nm and the pitch in the y direction is 310 nm. The pitch of the outcoupling grating 123 in the x direction is 300 nm and the pitch in the y direction is 310 nm. As in Example 3, a SiC single crystal substrate is used as the light-guiding 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 waveguiding FOV at all of λ1, λ2, and λ3 is greater than 85°. In Example 4, a high refractive index light-guiding substrate 11 is used, and two-dimensional diffraction gratings with rectangular unit lattices are used for the incoupling grating 121 and the outcoupling grating 123, thereby achieving a high FOV.

[0062] <Comparative Example 1> Comparative Example 1 uses the same light-guiding substrate 11 (refractive index at λ3: 2.08) as in Examples 1 and 2, but uses two-dimensional diffraction gratings with square unit lattices for both the in-coupling grating 121 and the out-coupling grating 123. The grating pitches in the x and y directions are 310 nm for each of the in-coupling grating 121 and the out-coupling grating 123. In this case, diffraction in the ±1st order directions is also utilized. In Comparative Example 1, the diagonal FOV at λ2 is smaller than 70°, and the maximum waveguiding FOV at all of λ1, λ2, and λ3 is smaller than 55°. While using the same high-refractive index light-guiding substrate 11 as in Examples 1 and 2, the diagonal FOV is smaller than in Examples 1 to 3 by using two-dimensional diffraction gratings with square unit lattices for an input image with an aspect ratio of 16:9.

[0063] <Comparative Example 2> In Comparative Example 2, the 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 :30.9Bi2O3:25.0B2O3:15.9ZnO :10.0SiO2:8.0Li2O:5.0BaO :5.0Sb2O3:0.1

[0064] The optical glass substrate of Comparative Example 2 has a refractive index of 1.99 at λ3, and a refractive index nd for the d-line in the range of 1.91≦nd≦2.05. Two-dimensional diffraction gratings with rectangular unit lattices are used for both the incoupling grating 121 and the outcoupling grating 123, utilizing diffraction in the ±1st order directions. The pitch of the incoupling grating 121 in the x direction is 310 nm, and the pitch in the y direction is 360 nm. The pitch of the outcoupling 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 all of λ1, λ2, and λ3 is less than 55°. Rectangular gratings are used for incoupling grating 121 and outcoupling grating 123, and diffraction in the ±1st order directions is utilized. However, since the refractive index at λ3 of light guide substrate 11 is 1.99, a high FOV cannot be achieved for all RGB colors.

[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, a line-and-space pattern is used for both the incoupling grating 121 and the outcoupling grating 123. The pitch of the incoupling grating 121 in the x-direction is 360 nm, and the pitch of the outcoupling grating 123 in the y-direction is 360 nm. In Comparative Example 3, the diagonal FOV at λ2 is smaller than 60°, and the maximum waveguide FOV at all of λ1, λ2, and λ3 is smaller than 35°. When a light-guiding substrate 11 having a refractive index nd of 2.05 or less at the d-line is used and a line-and-space pattern is used for both the incoupling grating 121 and the outcoupling grating 123, the FOV value required for the display device 100 cannot be realized.

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

[0067] FIG. 9 shows an example of a crystalline material used for the light guide substrate 11. When a single crystal substrate is used for the light guide substrate 11, TiO2, SrTiO3, KTaO3, LiNbO3, SiC, diamond, etc. can be used. In addition to the crystalline structure of these materials, the presence or absence of optical anisotropy, the ordinary refractive index n0 at the d line, the extraordinary refractive index n0 at the d line, the specific gravity (g / cm 3 ), Mohs hardness, and absorption edge wavelength.

[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 double image caused by birefringence when viewing a transmitted image of a real image does not significantly affect the FOV. All of these crystals have an absorption edge in the ultraviolet region, allowing visible light to pass through.

[0069] 10 shows the refractive index difference between the light guide substrate and the diffractive layer of the example and the comparative example. The graph shows the refractive index of the light guide substrate 11, the refractive index of the diffractive layer 12, and the refractive index difference between the light guide substrate 11 and the diffractive layer 12 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 vd at the d-line, assuming normal dispersion. [Example]

[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. The refractive index of the diffraction layer 12 is higher than that of the light guide substrate 11 at each wavelength. The refractive index differences are 0.08 at a wavelength of 450 nm, 0.04 at a wavelength of 532 nm, and 0.02 at a wavelength of 633 nm, all of which are 0.1 or less. [Example]

[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. The refractive index of the diffraction layer 12 is higher than that of the light guide substrate 11 at each wavelength. The refractive index difference is 0.05 at a wavelength of 450 nm, 0.06 at a wavelength of 532 nm, and 0.08 at a wavelength of 633 nm, all of which are 0.1 or less. [Example]

[0072] In Example 7, Ta2O5 is used for the diffraction layer 12. The light guide substrate 11 is made of Bi2O3-Ti The light-guiding substrate 11 is an O2-Nb2O5-WO3-B2O3-P2O5-SiO2-BaO substrate. The specific composition (mol %) of the light-guiding 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-guiding substrate 11. The refractive index differences are 0.05 at a wavelength of 450 nm, 0.02 at a wavelength of 532 nm, and 0.01 at a wavelength of 633 nm, all of which are less than 0.1. [Example]

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

[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 a wavelength of 450 nm, 0.03 at a wavelength of 532 nm, and 0.01 at a wavelength of 633 nm, all of which are 0.1 or less. [Example]

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

[0076] <Comparative Example 4> In Comparative Example 4, the diffraction layer 12 is made of Ta2O5. 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. With 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 differences are 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, like Comparative Example 4, is the lead-free, arsenic-free optical glass substrate described in Patent Document 3. Even with 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 differences are 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-guiding substrate 11, light with a numerical aperture (NA = n × sinθ) defined by the difference between the refractive indexes of the diffraction layer 12 and the light-guiding substrate 11 cannot exit the light-guiding substrate 11 to the diffraction layer 12 and is not diffracted. Therefore, the refractive index of the diffraction layer 12 must be greater than that of the light-guiding substrate 11 at λ3. On the other hand, if the difference Δn between the refractive indexes of the diffraction layer 12 and the light-guiding substrate 11 is large, reflection at the interface increases, reducing the extraction efficiency of the outcoupling grating 123. Therefore, it is desirable that the difference between the refractive index of the diffraction layer 12 and the refractive index of the light-guiding substrate 11 be 0.1 or less (Δn≦0.1) at each of the wavelengths λ1, λ2, and λ3. In particular, at the wavelength λ3, which is totally reflected and guided within the light-guiding substrate 11 at a large reflection angle, the influence of interface reflection is significant, so it is desirable that Δn be less than 0.1. At λ3, Δn is more desirably less than 0.05, and even more desirably less than 0.03. Furthermore, when the refractive index differences at λ1, λ2, and λ3 are Δnλ1, Δnλ2, and Δnλ3, respectively, the influence of interfacial reflection is likely to be reduced over all wavelengths when Δnλ1 ≥ Δnλ2 ≥ Δnλ3 holds. Even when the refractive index of the diffractive layer 12 is lower than the refractive index of the light-guiding substrate 11 at λ1 and λ2, it is often possible to guide light to a desired FOV through total reflection, and a combination of materials may be selected that has lower dispersion than the light-guiding substrate 11 and matches the refractive index of the light-guiding substrate 11 and the diffractive layer 12 at wavelengths shorter than λ3.

[0079] 11 shows the characteristics of materials used for the diffraction layer 12. As described above, in order to outcouple light to the outside of the light-guiding substrate 11, the refractive index of the diffraction layer 12 is set to be equal to or higher than the refractive index of the light-guiding substrate 11. Examples of materials that can be used for the diffraction layer 12 include ZrO2, Ta2O5, Nb2O5, TeO2, MoO3, TiO2, and WO3. In addition, high-refractive-index materials such as HfO2, SiN, SiON, SnO, ITO, Al2O3, YO3, AlN, and MgO can also be used depending on the material of the light-guiding substrate 11.

[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 a 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] <FOV Design> FIGS. 12 to 14 are diagrams for explaining the NA diagram and the FOV design. (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, and 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. If 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] For 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 in which total reflection waveguide can occur.

[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 changed by diffraction. The diffraction order of the light that propagates by total reflection and is emitted from the outcoupling grating 123 is the first order (m = 1).

[0085] 13B, when viewed from the normal line dropped down from the diffractive layer 12, the right side is defined as a positive NA and the left side as a negative NA. The angular region between the positive NA and the negative NA is the FOV. In the light guide element 10 of this embodiment, the FOV is 55° or more for all of λ1, λ2, and λ3. If the pitch of the diffraction grating is Λ, the wavelength is λ, and the diffraction order is m, the angle of incidence on the incoupling grating 121 and the angle of emergence from the outcoupling grating 123 are expressed as follows: nin×sinθin + mλ / Λ=nout×sinθout The relationship is as follows.

[0086] Figure 14 shows a design that aligns the input and output FOVs. The rectangle at the center of the NA diagram in Figure 14(A) is the NA area corresponding to the input FOV. The horizontal sides of the rectangle correspond to the FOV in the x direction (FOVx), and the vertical sides correspond to the FOV in the y direction (FOVy). Strictly speaking, the image of a rectangular field of view on the NA diagram will be a barrel-shaped image inscribed in the rectangle, but for convenience we will use a rectangular representation instead. The diagonal FOV corresponds to the FOV at the diagonal of this rectangle. Light incident on this FOV is guided while being diffracted several times to fill the donut-shaped total internal reflection propagation region T, and then returned to its original position.

[0087] 14(B), light incident on the central NA area in FIG. 14(A) is coupled into the light-guiding substrate 11 by the in-coupling grating 121, its direction is changed by the extension grating 212, and it propagates to the out-coupling grating 123 while being totally reflected within the light-guiding substrate 11. The totally reflected propagating light is output by the out-coupling grating 123 with an output FOV that is the same as the input FOV.

[0088] FIG. 15 is a diagram illustrating the ±1st-order diffraction waveguide direction and visibility. When the input FOV and output FOV are configured to be aligned as in FIG. 14, light with an incident angle corresponding to the plus (positive) FOV is incoupled in the direction opposite to the traveling direction in (A) of FIG. 15. When light propagated by total internal reflection is output by the outcoupling grating 23 at the same angle as the incident angle, the light is output toward the user's eye 20. Similarly, light with an incident angle corresponding to the minus (negative) FOV is incoupled in the direction opposite to the traveling direction. The light output by the outcoupling grating at the same angle as the incident angle is directed toward the user's eye 20, resulting in good image visibility.

[0089] 15B, when the positive side incident FOV is incoupled in the direction of travel, the light emitted by the outcoupling grating 123 misses the user's eye 20. The same thing happens with the negative side FOV, resulting in poor image visibility. Therefore, when designing to align the incident and output FOVs, the incoupling grating 121 and the outcoupling grating 123 are designed so that light is diffracted in the direction that maximizes image visibility.

[0090] Figure 16 explains the effect of expanding the FOV by utilizing diffraction in the ±1st order directions. In this example, we take diffraction in the +x and -x directions as an example. In the NA diagram, the FOV on the positive side is shown with a thick line, and the FOV on the negative side with a thin line. The solid line is the FOV for R light, the dashed-dotted line is the FOV for G light, and the dotted line is the FOV for B light.

[0091] In the positive FOV, all of the RGB are guided in the total internal reflection propagation region T on the left side of the NA diagram. In the negative FOV, all of the RGB are guided in the total internal reflection propagation region T on the right side of the NA diagram. Because there is a direction in which the entire RGB FOV can be guided, vignetting is suppressed. The FOV is expanded by utilizing diffraction in the ±1st order directions. For example, by using a light-guiding substrate with a refractive index of 2.08 at λ3, a diagonal FOV of 55° or more can be achieved for all of λ1, λ2, and λ3 included in RGB.

[0092] 17A and 17B are diagrams illustrating the effect of using a two-dimensional diffraction grating with a rectangular unit grating as the incoupling grating. In FIG. 17A, when an incoupling grating 121a with a one-dimensional line-and-space pattern is used, light in the positive FOV diffracted to the left side of the figure is guided to the left side of the outcoupling grating 123b, and light in the negative FOV diffracted to the right side of the figure is guided to the right side of the outcoupling grating 123b. When the outcoupling grating 123b is a two-dimensional diffraction grating with a rectangular unit grating, diffraction occurs in the up, down, left, and right directions of the outcoupling grating 123b, and light is guided by two-dimensional diffraction through the outcoupling grating 123b. However, light directed toward the center of the outcoupling grating 123b may not be incoupled by the incoupling grating 121a. As a result, light may not be guided sufficiently in the center of the outcoupling grating 123b.

[0093] In Figure 17 (B), by using a two-dimensional diffraction grating, incoupling grating 121b, as the unit lattice, such as a rectangular lattice, light is diffracted and guided from incoupling grating 121b toward outcoupling grating 123b, and the entire FOV, including the plus-side and minus-side FOV, is guided through the central portion of the light-guiding substrate. The amount of light emitted from outcoupling grating 123b is uniform, improving visibility. By using a blazed diffraction grating as the two-dimensional diffraction grating, the diffraction efficiency of diffraction toward 123b can be selectively increased, thereby improving the utilization efficiency of incident light.

[0094] FIG. 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. The right side of the incident FOV at the center of the NA diagram is the positive FOV, and the left side is the negative FOV. The positive FOV is on the left side of the NA diagram, and all of the R, G, and B colors propagate within the total internal reflection propagation region T. The negative FOV is on the right side of the NA diagram, and all of the R, G, and B colors propagate within the total internal reflection propagation region T. Even in the +y and -y directions, all R, G, and B wavelengths are within the total internal reflection propagation region T, and no vignetting occurs. Using a single-layer light-guiding substrate 11 with a refractive index of 2.08 at λ3, a diagonal FOV of 55° or more can be achieved for all R, G, and B colors.

[0095] Figures 18B and 18C show the FOV light guided using a two-dimensional diffraction grating with a square unit lattice. 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, the R light is not guided sufficiently by total internal reflection at the top and bottom of the y direction of the NA diagram, resulting in vignetting (V). In Figure 18C, the B light is not guided sufficiently by total internal reflection at both the positive and negative FOVs in the x direction of the NA diagram, resulting in vignetting (V). Using a two-dimensional diffraction grating with a rectangular unit lattice, as in Figure 18A, suppresses vignetting and maintains color image quality.

[0096] In light of these contents, Diagonal FOV is FOVdiag Horizontal + side FOV is FOVx+ (sign is positive) Horizontal -side FOV is FOVx- (positive sign) Vertical + side FOV is FOVy+ (positive sign) Vertical -side FOV is FOVy- (positive sign) The aspect ratio of the projected image is Ax:Ay When projecting an image, if the refractive index of the light guide substrate 11 at λ3 is n_λ3, then at least In both cases, when the outcoupling grating is a two-dimensional diffraction grating with a rectangular lattice as the unit lattice, and the pitches Λx and Λy in the x and y directions are rectangular lattices that satisfy the following formulas, 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> FIG. 19A shows the characteristics and diffraction image of RGB light guide of Example 1, and FIG. 19B shows the characteristics and diffraction image of G light guide of Example 1. The type and pitch of the diffraction grating of Example 1 and the refractive index of light guide substrate 11 at each wavelength are as shown in FIG. 8. In the RGB light guide of FIG. 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 image when RGB light is perpendicularly incident on incoupling grating 121 (labeled "first grating" in the figure) and the diffraction image when RGB light is perpendicularly incident on outcoupling 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 image of the first grating, all of the plus-side FOV is diffracted into the total internal reflection propagation region on the left, and all of the minus-side FOV is diffracted into the total internal reflection propagation region on the right.

[0103] In the diffraction image of the second grating, all of the R, G, and B colors are diffracted within the total internal reflection propagation region in the ±x and ±y directions. The grating pitches of the incoupling grating 121 and the outcoupling grating 123 are set to a pitch that achieves the diffraction shown in this NA diagram. This grating design enables the use of a single-layer light-guiding substrate 11 to reproduce RGB images with a diagonal FOV of 55° and no vignetting.

[0104] 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 image when G light is perpendicularly incident on incoupling grating 121 (labeled "first grating" in the diagram) and the diffraction image when G light is perpendicularly incident on outcoupling grating 123 (labeled "second grating" in the diagram). In the diffraction image of the line-and-space first grating, all of the positive-side FOV is diffracted into the total internal reflection propagation region on the left (thick dash-dotted rectangle), and all of the negative-side FOV is diffracted into the total internal reflection propagation region on the right (thin dash-dotted rectangle).

[0106] The diffraction image of the second grating, which is a two-dimensional diffraction grating with a rectangular unit lattice, is in the ±x direction. In the ±y directions, G light is diffracted into the total internal reflection propagation region. With this grating configuration, the FOV can be expanded for G light using a single-layer light-guiding substrate 11. However, even if R light or B light is perpendicularly incident on the second grating with a diagonal FOV of 70°, total internal reflection light guiding is not necessarily achieved in both the ±x and ±y directions.

[0107] <FOV characteristics of Example 2> FIG. 20A shows the RGB light guide characteristics and diffraction image of Example 2, and FIG. 20B shows the G light guide characteristics and diffraction image of Example 2. The type and pitch of the diffraction grating and the refractive index of light guide substrate 11 at each wavelength of Example 2 are as shown in FIG. 8. In the RGB light guide of FIG. 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 Fig. 20A shows the diffraction image when RGB light is perpendicularly incident on incoupling grating 121 (denoted as "first grating" in the diagram) and the diffraction image when RGB light is perpendicularly incident on outcoupling grating 123 (denoted as "second grating" in the diagram). Both the first grating and the second grating are two-dimensional diffraction gratings with rectangular unit lattices.

[0109] In the diffraction image of the first grating, in the x-axis direction, all of the positive-side FOV is diffracted into the left total internal reflection propagation region, and all of the negative-side FOV is diffracted into the right total internal reflection propagation region. In the y-axis direction, all of the RGB FOV is diffracted into the total internal reflection propagation region. In the diffraction image of the second grating, all of the RGB is diffracted into the total internal reflection propagation region in the ±x and ±y directions. The grating pitches of the incoupling grating 121 and outcoupling grating 123 are set to achieve diffraction 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-guiding substrate 11.

[0110] 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 image when G light is perpendicularly incident on incoupling grating 121 (labeled "first grating" in the diagram) and the diffraction image when G light is perpendicularly incident on outcoupling grating 123 (labeled "second grating" in the diagram). In the diffraction image of the first grating, whose unit lattice is a rectangular lattice, G is diffracted within the total internal reflection propagation region in both the ±x and ±y directions.

[0112] Even in the diffraction image of the second grating, whose unit cell is a rectangular lattice, G light is diffracted into the total internal reflection propagation region in the ±x and ±y directions. The FOV can be expanded for G light by using a single-layer light-guiding substrate 11. However, even if R light or B light is perpendicularly incident on the second grating with a diagonal FOV of 70°, total internal reflection light guiding is not necessarily achieved in both the ±x and ±y directions.

[0113] <FOV characteristics of Example 3> FIG. 21A shows the RGB light guide characteristics and diffraction image of Example 3, and FIG. 21B shows the G light guide characteristics and diffraction image of Example 3. The type and pitch of the diffraction grating and the refractive index of light guide substrate 11 at each wavelength of Example 1 are as shown in FIG. 8. In the RGB light guide of FIG. 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] 21A shows the diffraction image when RGB light is perpendicularly incident on incoupling grating 121 (referred to as "first grating" in the diagram) and the diffraction image when RGB light is perpendicularly incident on outcoupling grating 123 (referred to as "second grating" in the diagram). Both the first grating and the second grating are one-dimensional diffraction gratings with line and space patterns, but the extending directions of the grating patterns of the first grating and the second grating are orthogonal to each other.

[0115] In the diffraction image of the first grating, the incident FOV is diffracted into the total internal reflection propagation region in the +x and -x directions for all RGB. In the diffraction image of the second grating, the incident FOV is diffracted into the total internal reflection propagation region in the +y and -y directions for all RGB. The grating pitches of the incoupling grating 121 and outcoupling grating 123 are set to a pitch that achieves the diffraction of this NA diagram. Using a single-layer light-guiding substrate 11 and a line-and-space one-dimensional diffraction grating, RGB images can be reproduced with a diagonal FOV of 65° without vignetting.

[0116] 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 image when G light is perpendicularly incident on the incoupling grating 121 (labeled "first grating" in the figure) and the diffraction image when G light is perpendicularly incident on the outcoupling grating 123 (labeled "second grating" in the figure). In the diffraction image of the line-and-space type first grating, both the positive and negative FOVs are diffracted into the total internal reflection propagation region in the ±x direction. In the diffraction image of the second grating, both the positive and negative FOVs are diffracted into the total internal reflection propagation region in the ±y direction. Using a single-layer light-guiding substrate 11, the diagonal FOV can be expanded to 100° for G light. However, even if R or B light is perpendicularly incident on the second grating with a diagonal FOV of 100°, total internal reflection light-guiding is not necessarily achieved in both the ±x and ±y directions.

[0118] <FOV characteristics of Example 4> FIG. 22A shows the RGB light guide characteristics and diffraction image of Example 4, and FIG. 22B shows the G light guide characteristics and diffraction image of Example 4. The type and pitch of the diffraction grating and the refractive index of light guide substrate 11 at each wavelength of Example 4 are as shown in FIG. 8. In the RGB light guide of FIG. 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 image when RGB light is perpendicularly incident on the incoupling grating 121 (labeled "first grating" in the figure) and the diffraction image when RGB light is perpendicularly incident on the outcoupling grating 123 (labeled "second grating" in the figure). Both the first grating and the second grating are two-dimensional diffraction gratings with rectangular unit lattices. In the diffraction image of the first grating, in the x-axis direction, all of the positive-side FOV is diffracted into the total internal reflection propagation region on the left, and all of the negative-side FOV is diffracted into the total internal reflection propagation region on the right. In the y-axis direction, all of the RGB FOV is diffracted into the total internal reflection propagation region.

[0120] In the diffraction image of the second grating, all of the RGB light is diffracted within the total internal reflection propagation region in the ±x and ±y directions. The grating pitches of the incoupling grating 121 and outcoupling grating 123 are set to a pitch that achieves the diffraction of this NA diagram. With this grating design, a single-layer light-guiding substrate 11 can be used to reproduce RGB images with a diagonal FOV of 85° without vignetting.

[0121] 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 image when G light is perpendicularly incident on incoupling grating 121 (labeled "first grating" in the diagram) and the diffraction image when G light is perpendicularly incident on outcoupling grating 123 (labeled "second grating" in the diagram). In the diffraction image of the first rectangular grating, G is diffracted within the total internal reflection propagation region in both the ±x and ±y directions.

[0123] Even in the diffraction image of the second grating, which is a two-dimensional diffraction grating with a rectangular unit lattice, G light is diffracted into the total internal reflection propagation region in the ±x and ±y directions. Using a single-layer light-guiding substrate 11, the diagonal FOV can be expanded to 110° for at least G light. However, even if R light or B light is perpendicularly incident on the first or second grating with a diagonal FOV of 110°, total internal reflection light guiding is not necessarily 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 configuration examples. The display device 100 using the light guide element 10 may be linked to a smartphone, a notebook personal computer (PC), or the like. 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 monochromatic images or information may be displayed in the indirect field of view to the extent that they do not interfere with daily activities.

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

[0126] 10 Light guide element 11 Light guide substrate 12 Diffraction layer 121, 121a, 121b Incoupling grating (first diffraction grating) 122 Extended Lattice 123, 123a, 123b Outcoupling grating (second diffraction grating) 100 display device 110 Projector 120 Support

Claims

1. a single-layer light-guiding substrate; a diffraction layer formed on the light guide substrate; and the diffraction layer includes a first diffraction grating that incouples incident light that has entered the light-guiding substrate into the light-guiding substrate, and a second diffraction grating that outcouples totally reflected light that has propagated through the light-guiding substrate to the outside of the light-guiding substrate; the first diffraction grating incouples the incident light at at least one wavelength selected from a first wavelength included in a 450 nm±20 nm band, a second wavelength included in a 530 nm±20 nm band, and a third wavelength included in a 630 nm±20 nm band, within an angular range of 60° or more including a normal direction of the light-guiding substrate; the second diffraction grating outcouples the totally reflected light at the at least one wavelength in an angular range of 60° or more including the normal direction; the diffraction layer is formed of 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; a refractive index of the diffractive layer at the third wavelength is greater than a refractive index of the light-guiding substrate at the third wavelength, and a difference in refractive index between the diffractive layer and the light-guiding substrate at the third wavelength is 0.1 or less.

2. A single-layer light-guiding substrate; a diffraction layer formed on the light guide substrate; and the diffraction layer includes a first diffraction grating that incouples incident light that has entered the light-guiding substrate into the light-guiding substrate, and a second diffraction grating that outcouples totally reflected light that has propagated through the light-guiding substrate to the outside of the light-guiding substrate; the first diffraction grating incouples the incident light at at least one wavelength selected from a first wavelength included in a 450 nm±20 nm band, a second wavelength included in a 530 nm±20 nm band, and a third wavelength included in a 630 nm±20 nm band, within an angular range of 60° or more including a normal direction of the light-guiding substrate; the second diffraction grating outcouples the totally reflected light at the at least one wavelength in an angular range of 60° or more including the normal direction; the first diffraction grating is a two-dimensional diffraction grating having a rectangular unit lattice, and has a grating pitch such that when light of the first wavelength, the second wavelength, and the third wavelength is perpendicularly incident on the first diffraction grating from the light-guiding substrate, (±1, 0)-order diffracted light or (0, ±1)-order diffracted light is guided by total internal reflection within the light-guiding substrate at any wavelength.

3. A single-layer light-guiding substrate; a diffraction layer formed on the light guide substrate; and the diffraction layer includes a first diffraction grating that incouples incident light that has entered the light-guiding substrate into the light-guiding substrate, and a second diffraction grating that outcouples totally reflected light that has propagated through the light-guiding substrate to the outside of the light-guiding substrate; the first diffraction grating incouples the incident light at at least one wavelength selected from a first wavelength included in a 450 nm±20 nm band, a second wavelength included in a 530 nm±20 nm band, and a third wavelength included in a 630 nm±20 nm band, within an angular range of 60° or more including a normal direction of the light-guiding substrate; the second diffraction grating outcouples the totally reflected light at the at least one wavelength in an angular range of 60° or more including the normal direction; the first diffraction grating is a two-dimensional diffraction grating having a rectangular unit lattice, and has a grating pitch such that when light of any one of the first wavelength, the second wavelength, and the third wavelength is perpendicularly incident on the first diffraction grating from the light-guiding substrate, (±1, ±1)-order diffracted light is guided by total reflection within the light-guiding substrate.

4. the first diffraction grating incouples the incident light at any of the first wavelength, the second wavelength, and the third wavelength in a common angular range of 55° or more including the normal direction; 4. The light guide element according to claim 1, wherein the second diffraction grating outcouples the totally reflected light in a common angular range of 55° or more including the normal direction for any of the first wavelength, the second wavelength, and the third wavelength.

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

6. 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.

7. the light-guiding substrate has a refractive index at the d-line of greater than 2.05; The light guide element according to any one of claims 1 to 3.

8. The composition of the light guide substrate is, when the total of the base composition is 100% in mole percent based on oxides, Bi ２ O ３ The content of Bi is 20% to 50%, and the content of TeO 2 is 10% to 35%. ２ O ３ -TeO ２ La-based glass or ２ O ３ The content of is 10% to 40%, B ２ O ３ (2) La ２ O ３ -B ２ O ３ Glass The light guide element according to any one of claims 1 to 3, wherein

9. The 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 -W.O. ３ The light guide element according to any one of claims 1 to 3, comprising 55 mol% or more of

10. The light guide substrate is made of TiO ２ , SrTiO ３ , KTaO ３ , LiNbO ３ 4. The light guide element according to claim 1, wherein the substrate is made of silicon carbide, silicon carbide, or diamond.

11. The diffraction layer is made of ZrO ２ , HfO ２ , Ta ２ O ５ , Nb ２ O ５ , TeO ２ , MoO ３ , W.O. ３ , TiO ２ , SiN, SiON, SnO, ITO, Al ２ O ３ , Y ２ O ３ 4. The light guide element according to claim 2, wherein the light guide element is made of AlN, MgO, or a mixture of two or more of these.

12. 12. The light-guiding element according to claim 11, wherein a refractive index of the diffractive layer at the third wavelength is greater than a refractive index of the light-guiding substrate at the third wavelength, and a refractive index difference between the diffractive layer and the light-guiding substrate at the third wavelength is 0.1 or less.

13. The light-guiding element according to any one of claims 1 to 3, wherein the second diffraction grating is a two-dimensional diffraction grating having a rectangular lattice as a unit lattice, and has a grating pitch such that when light of the first wavelength, the second wavelength, and the third wavelength is perpendicularly incident on the second diffraction grating from the light-guiding substrate, (±1, 0)-order or (0, ±1)-order diffracted light is guided by total reflection within the light-guiding substrate at any wavelength.

14. 4. The light-guiding element according to claim 1, wherein the second diffraction grating is a two-dimensional diffraction grating having a rectangular unit lattice, and has a grating pitch such that when light of any one of the first wavelength, the second wavelength, and the third wavelength is perpendicularly incident on the second diffraction grating from the light-guiding substrate, (±1, ±1)-order diffracted light is guided by total reflection within the light-guiding substrate.

15. 4. The light-guiding element according to claim 1, wherein the first diffraction grating is a two-dimensional diffraction grating having a rectangular unit lattice, and has a grating pitch such that, when light of the first wavelength, the second wavelength, and the third wavelength is perpendicularly incident on the first diffraction grating from the light-guiding substrate, (±1, 0)-order diffracted light or (0, ±1)-order diffracted light is guided by total reflection within the light-guiding substrate at any wavelength.

16. 3. The light-guiding element according to claim 1, wherein the first diffraction grating is a two-dimensional diffraction grating having a rectangular unit lattice, and has a grating pitch such that, when light of any one of the first wavelength, the second wavelength, and the third wavelength is perpendicularly incident on the first diffraction grating from the light-guiding substrate, diffracted light of (±1, ±1) orders is guided by total reflection within the light-guiding substrate.

17. The light guide element according to any one of claims 1 to 3, A projector and wherein light projected from the projector enters the light-guiding element and is emitted from the second diffraction grating.

Images