Light guide member

Abstract

To provide a light guide member having a high refractive index and a high internal transmittance.SOLUTION: A light guide member is used in a waveguide structure for an image display device that guides image light entering from a display and emits the image light toward the eyes of a user. The light guide member is formed from a single crystal composed of lithium niobate.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a light-guiding member used in a waveguide structure of an image display device such as a head-mounted display or smart glasses. [Background technology]

[0002] In recent years, head-mounted displays and smart glasses have been put to practical use as devices that magnify two-dimensional images using an optical system and allow the user to observe the magnified virtual image with their eyes.

[0003] These head mounted displays and smart glasses are classified into see-through transparent and non-transparent types. Most smart glasses are transparent and are used to project new AR (Augmented Reality) and other technologies into the real world, so they must be small and portable. Most head mounted displays are non-transparent and are used to project VR (Virtual Reality) and other technologies, so they must have a high FOV (Field Of View) to provide a sense of immersion.

[0004] Such a head mounted display is described, for example, in Patent Document 1. The head mounted display described in Patent Document 1 includes a display device that displays an image, a light guide member to which the image displayed on the display element is incident, and a propagation means that causes the incident image to be totally reflected inside the light guide member and propagates it toward the pupil of the user. The light guide member is formed of a glass material with a refractive index of 2.0 to 2.1 so as to obtain a predetermined viewing angle. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2023-14127 Summary of the Invention [Problem to be solved by the invention]

[0006] According to the light guide member described in Patent Document 1, although the glass has a high refractive index of 2.0 or more, the internal transmittance at a wavelength of 400 nm when the plate thickness is 10 mm is about 0.61 to 0.75. In recent years, there has been a demand for higher internal transmittance in order to efficiently display clear images with good color reproducibility. In addition, there is a demand for a wider FOV in VR and AR applications, and materials with a higher refractive index are required.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a light-guiding member having a high refractive index and high internal transmittance. [Means for solving the problem]

[0008] As a result of intensive research, the present inventors have found that the above-mentioned problems can be solved by forming a light guide member from a single crystal of lithium niobate, and have completed the present invention as described below. That is, the present invention provides the following [1] to [7]. [1] A light-guiding member used in a waveguide structure of an image display device that guides image light incident from a display and outputs it toward a user's eye, the light-guiding member being formed from a single crystal of lithium niobate. [2] The light-guiding member according to the above item [1], characterized in that the light-guiding member has a first surface and a second surface opposing each other, and the plane orientation of the first surface and the plane orientation of the second surface are in the X-axis direction and the Z-axis direction, respectively, of a crystal axis. [3] The light-guiding member according to the above [2], wherein the difference between the maximum value and the minimum value of the distance from the first surface to the second surface is 2 μm or less. [4] The light-guiding member according to the above [2] or [3], wherein the parallelism between the first surface and the second surface is 2.5 arcsec or less. [5] The light-guiding member according to any one of the above [2] to [4], wherein the surface roughness Sq of the first surface and the surface roughness Sq of the second surface of the light-guiding member are each 1 nm or less. [6] The light-guiding member according to any one of the above [1] to [5], wherein the internal transmittance of the light-guiding member at a wavelength of 400 to 800 nm is 90% or more in terms of a value calculated for a thickness of 10 mm. [7] The light-guiding member according to any one of the above [1] to [6], wherein the light-guiding member has a plate thickness of 0.3 mm or more and 1.0 mm or less. Effect of the Invention

[0009] According to the present invention, it is possible to provide a light-guiding member having a high refractive index and high internal transmittance.

[0010] Furthermore, by using the light-guiding member of the present invention, it is possible to realize image display devices such as head-mounted displays and smart glasses that can efficiently display clear images and obtain a wider FOV. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram showing the crystal structure of lithium niobate. [Diagram 2] 1 is a graph showing the wavelength dependence of the refractive index of lithium niobate. [Diagram 3] FIG. 2 is a diagram showing an example of a waveguide structure of an image display device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The light guiding member of the present invention is used in a waveguide structure of an image display device that guides image light incident from a display and outputs it toward the user's eye. The light guiding member of the present invention is made of a single crystal of lithium niobate.

[0013] Examples of image display devices using the light-guiding member of the present invention include head-mounted displays and smart glasses. Fig. 3 shows an example of a waveguide structure of an image display device. Image light (RGB light) irradiated from a display 3 is diffracted by an input grating (diffraction grating) 4 and travels inside the light-guiding member 2 while repeating total reflection in the in-plane direction. The guided light is diffracted again by an output grating 5, exits the light-guiding member 2, and is guided to the user's eye.

[0014] The light guide member of the present invention is formed of a single crystal made of lithium niobate. The refractive index of lithium niobate for the wavelength of visible light is 2.2 or more, and a high FOV can be obtained by using the single crystal made of lithium niobate in the light guide member of an image display device such as a head mounted display or smart glasses.

[0015] The light guide member has a first surface and a second surface facing each other, and the surface orientation of the first surface and the surface orientation of the second surface are preferably the X-axis of the crystal axis. This can reduce the influence of pyroelectricity inherent to lithium niobate. Pyroelectricity is a property in which the polarization in a crystal changes due to a change in temperature, and the crystal becomes charged with static electricity. If pyroelectricity occurs during the manufacturing process, discharge may occur in the manufacturing process, causing undesirable problems. The angle deviation between the surface orientation of the first surface and the surface orientation of the second surface and the X-axis of the crystal axis is preferably -0.2 or more and +0.2 or less, respectively.

[0016] Furthermore, by setting the plane orientation of the first plane and the plane orientation of the second plane to the X-axis of the crystal axis, the first plane and the second plane have the same properties in the processing process, which makes the processing process easier.

[0017] The plane orientation of the first plane and the plane orientation of the second plane are preferably the Z-axis of the crystal axis, in addition to the X-axis of the crystal axis. A large bulk single crystal of lithium niobate can be grown by pulling a raw material melt melted in a crucible upward under a temperature gradient, known as the Czochralski method. By setting the Z-axis as the pulling axis, the crystallinity of the grown crystal is further improved. From the viewpoint of production efficiency, the plane orientation of the first plane and the plane orientation of the second plane are preferably the Z-axis of the pulling axis. The angular deviation between the plane orientation of the first plane and the plane orientation of the second plane and the Z-axis of the crystal axis is preferably -0.2 or more and +0.2 or less, respectively.

[0018] In the light guide member of the present invention, the difference between the maximum and minimum values ​​of the distance from the first surface to the second surface is preferably 2 μm or less. If the difference between the maximum and minimum values ​​of the distance from the first surface to the second surface is 2 μm or less, the blurring and bleeding of RGB light can be further reduced. From this viewpoint, it is more preferable that the difference between the maximum and minimum values ​​of the distance from the first surface to the second surface is 1 μm or less. Such a difference between the maximum and minimum values ​​of the distance from the first surface to the second surface is called TTV (Total Thickness Variation). For example, by polishing a single crystal of lithium niobate so that the difference between the maximum and minimum values ​​of the distance from the first surface to the second surface is 2 μm or less, the difference between the maximum and minimum values ​​of the distance from the first surface to the second surface can be made 2 μm or less.

[0019] In order to reliably guide the light of each wavelength of RGB through the grating, the parallelism between the first surface and the second surface in the light guide member of the present invention is preferably as high as possible, and is preferably 2.5 arcsec or less. If the parallelism between the first surface and the second surface is 2.5 arcsec or less, the light of RGB can be more reliably guided to the grating. From this viewpoint, the parallelism between the first surface and the second surface is more preferably 1.5 arcsec or less. The parallelism was evaluated using an index called a wedge angle (Wedge) according to JIS B0615-2:2017. In addition, for example, the parallelism between the first surface and the second surface in the light guide member can be made 2.5 arcsec or less by polishing a single crystal of lithium niobate so that the parallelism between the first surface and the second surface is 2.5 arcsec or less.

[0020] From the viewpoint that the light of each wavelength of RGB propagates in the light guide section and is totally reflected at the interface (surface) with air to guide the light, the surface roughness Sq of the first surface and the surface roughness Sq of the second surface of the light guide member of the present invention are preferably as small as possible, and are preferably 1 nm or less, and more preferably 0.5 nm or less. The surface roughness Sq of the first surface and the surface roughness Sq of the second surface are surface roughnesses defined in ISO25178. For example, the surface roughness Sq of the first surface and the surface roughness Sq of the second surface of the light guide member of the present invention can be set to 1 nm or less by polishing a single crystal of lithium niobate so that the surface roughness Sq of the first surface and the surface roughness Sq of the second surface are each 1 nm or less.

[0021] In order to guide the light of RGB accurately, it is preferable that the transmittance of each wavelength is high. The internal transmittance of the light of the light guide member of the present invention at wavelengths of 400 to 800 nm is preferably 90% or more in terms of thickness of 10 mm. By using a single crystal made of lithium niobate having an internal transmittance of 0.9 or more in terms of wavelengths of 400 to 800 nm when the plate thickness is 10 mm, the internal transmittance of the light of the light guide member of the present invention at wavelengths of 400 to 800 nm can be made 90% or more in terms of thickness of 10 mm. The higher the internal transmittance of the light of the light guide member of the present invention at wavelengths of 400 to 800 nm, the more preferable, and it is more preferable that the internal transmittance is 95% or more in terms of thickness of 10 mm. The upper limit of the range of the internal transmittance of the light of the light guide member of the present invention at wavelengths of 400 to 800 nm is not particularly limited, but is 100% or more in terms of thickness of 10 mm.

[0022] The thickness of the light guide member of the present invention is preferably 0.3 mm or more and 1.0 mm or less. When the thickness is 0.3 mm or more, the light guide member can be made more unlikely to break. Also, when the thickness is 1.0 mm or less, the light guide member can be made lighter, and the sense of fatigue when wearing the image display device can be reduced.

[0023] The air-equivalent optical path length of light traveling through the light guide member is shorter as the refractive index of the light guide member is higher, and the apparent viewing angle relative to the width of the image display element is larger as the refractive index of the light guide member is higher. Therefore, the higher the refractive index of the light guide member of the present invention, the better, and it is preferably 2.2 or more. By using a single crystal made of lithium niobate for the light guide member, the refractive index of the light guide member of the present invention can be made 2.2 or more.

[0024] Of course, there are individual differences, but it is believed that a viewing angle of approximately 45 degrees or more will allow for realistic images. EXAMPLES

[0025] Examples will be described below, but the present invention is not limited to these.

[0026] <Example 1> A 6-inch diameter lithium niobate (LN) single crystal ingot pulled by the CZ method with the X-axis direction as the crystal growth direction was sliced ​​to have an X-cut surface, and then chamfered and lapped.

[0027] Next, both sides were polished to a mirror finish using a double-sided polisher. The polishing conditions were 100 g / cm 2 The pressure was 1000 MPa, the rotation speed of the lower platen was 20 rpm, and colloidal silica was used as the polishing agent. By this polishing process, an LN substrate with a thickness of 0.5 mmt and a diameter of 6 inches was produced, the X-plane of which was the main surface.

[0028] The refractive index of this LN substrate was measured using a Prism Coupler Model 2010 / M manufactured by Metricon. Wavelengths of 473, 633, and 1064 nm were used. The measurement results are shown in Figure 2. The refractive index was high, at 2.2 or more, in the visible light region of 400 to 800 nm.

[0029] Other characteristics of the light guide member are as follows: Orientation of the first and second faces: X-axis of the crystal axis Internal transmittance: 91.1% TTV: 1.3um Parallelism: 1.8arcsec Surface roughness Sq: 1st side 0.5nm, 2nd side 0.5nm Thickness: 0.6mm

[0030] The crystal orientation was measured using an X-ray cut surface inspection machine manufactured by Rigaku Corporation. The internal transmittance was measured using the method described in JOGIS 17:2019, with 3mm and 10mm samples prepared and measured using a UV-3100 manufactured by Shimadzu Corporation. TTV and parallelism were measured using an UltraSort II manufactured by Tropel. Parallelism was evaluated using the index JIS B0615-2:2017 wedge angle. Roughness was measured in an area of ​​400 x 400um using a white light interferometer Zegage Plus manufactured by Zygo, and the Sq value was calculated. Sq refers to the Sq value defined in JISO25178. Thickness was measured with a micrometer.

[0031] Next, multi-layer anti-reflection films were formed on both sides of the LN substrate, and then the substrate was processed to a size of 60 x 50 mm to produce a light guide member.

[0032] Smart glasses were created using this light-guiding component, and the images projected on the smart glasses were visually inspected. It was confirmed that the viewing angle was sufficiently wide at 50 degrees, and that the images displayed were clear and free of blur or smearing.

[0033] <Example 2> A 6-inch diameter LN single crystal ingot pulled by the CZ method was sliced ​​into X-cut surfaces with the X-axis direction as the crystal growth direction, and then chamfered and lapped.

[0034] Next, both sides were polished to a mirror finish using a double-sided polisher. The polishing conditions were 150 g / cm 2 The pressure was 1000 MPa, the rotation speed of the lower platen was 15 rpm, and colloidal silica was used as the polishing agent. By this polishing process, an LN substrate with a thickness of 0.5 mmt and a diameter of 6 inches was produced, the X-plane of which was the main surface.

[0035] Other characteristics of the light guide member are as shown in Table 1.

[0036] Next, multi-layer anti-reflection films were formed on both sides of the LN substrate, and then the substrate was processed to a size of 60 x 50 mm to produce a light guide member.

[0037] Smart glasses were created using this light-guiding component, and the images projected on the smart glasses were visually inspected. It was confirmed that the viewing angle was sufficiently wide at 49 degrees, and that the images displayed were clear and free of blur or smearing.

[0038] <Example 3> A 6-inch diameter LN single crystal ingot pulled by the CZ method was sliced ​​into X-cut surfaces with the X-axis direction as the crystal growth direction, and then chamfered and lapped.

[0039] Next, both sides were polished to a mirror finish using a double-sided polisher. The polishing conditions were 100 g / cm 2 The pressure was set at 1000 psi, the rotation speed of the lower plate was set at 20 rpm, and colloidal silica was used as the polishing agent. By this polishing process, an LN substrate with a thickness of 0.3 mmt and a diameter of 6 inches was produced, the X-plane of which was the main surface.

[0040] Other characteristics of the light guide member are as shown in Table 1.

[0041] Next, multi-layer anti-reflection films were formed on both sides of the LN substrate, and then the substrate was processed to a size of 60 x 50 mm to produce a light guide member.

[0042] Smart glasses were created using this light-guiding component, and the images projected on the smart glasses were visually inspected. It was confirmed that the viewing angle was sufficiently wide at 52 degrees, and that the images displayed were clear and free of blur or smearing.

[0043] <Example 4> A 6-inch diameter LN single crystal ingot pulled by the CZ method was sliced ​​into X-cut surfaces with the X-axis direction as the crystal growth direction, and then chamfered and lapped.

[0044] Next, both sides were polished to a mirror finish using a double-sided polisher. The polishing conditions were 100 g / cm 2 The pressure was 1.0 mm, the rotation speed of the lower plate was 20 rpm, and colloidal silica was used as the polishing agent. By this polishing process, an LN substrate with a thickness of 1.0 mmt and a diameter of 6 inches was produced, the X-plane of which was the main surface.

[0045] Other characteristics of the light guide member are as shown in Table 1.

[0046] Next, multi-layer anti-reflection films were formed on both sides of the LN substrate, and then the substrate was processed to a size of 60 x 50 mm to produce a light guide member.

[0047] Smart glasses were created using this light-guiding component, and the images projected on the smart glasses were visually inspected. It was confirmed that the viewing angle was sufficiently wide at 51 degrees, and that the images displayed were clear and free of blur or smearing.

[0048] <Example 5> A 6-inch diameter LN single crystal ingot pulled by the CZ method with the Z-axis direction as the crystal growth direction was sliced ​​to obtain a Z-cut surface, which was then chamfered and lapped.

[0049] Next, both sides were polished to a mirror finish using a double-sided polisher. The polishing conditions were 100 g / cm 2 The pressure was 1.0 mm, the rotation speed of the lower plate was 20 rpm, and colloidal silica was used as the polishing agent. By this polishing process, an LN substrate with a thickness of 1.0 mmt and a 6-inch diameter Z surface as the main surface was produced.

[0050] Other characteristics of the light guide member are as shown in Table 1.

[0051] Next, multi-layer anti-reflection films were formed on both sides of the LN substrate, and then the substrate was processed to a size of 60 x 50 mm to produce a light guide member.

[0052] Smart glasses were created using this light-guiding component, and the images projected on the smart glasses were visually inspected. It was confirmed that the viewing angle was sufficiently wide at 50 degrees, and that the images displayed were clear and free of blur or smearing.

[0053] <Comparative Example 1> A 6-inch diameter LN single crystal ingot pulled by the CZ method was sliced ​​into Y-cut surfaces with the Y-axis direction as the crystal growth direction, and then chamfered and lapped.

[0054] Next, both sides were polished to a mirror finish using a double-sided polisher. The polishing conditions were 100 g / cm 2The pressure was 1000 MPa, the rotation speed of the lower platen was 20 rpm, and colloidal silica was used as the polishing agent. By this polishing process, an LN substrate with a thickness of 0.5 mmt and a diameter of 6 inches was produced, the Y surface of which was the main surface.

[0055] Other characteristics of the light guide member are as shown in Table 1.

[0056] Next, multi-layer anti-reflection films were formed on both sides of the LN substrate, and then the substrate was processed to a size of 60 x 50 mm to produce a light guide member.

[0057] When smart glasses were made using this light-guiding member and the image projected on the smart glasses was visually inspected, it was found that the color reproducibility was poor, with some colors differing from the original image on the display.It was also confirmed that the brightness was reduced, possibly due to the low transmittance.

[0058] <Comparative Example 2> A 6-inch diameter LN single crystal ingot pulled by the CZ method was sliced ​​into X-cut surfaces with the X-axis direction as the crystal growth direction, and then chamfered and lapped.

[0059] Next, both sides were polished to a mirror finish using a double-sided polisher. The polishing conditions were 100 g / cm 2 The pressure was 1000g, the rotation speed of the lower platen was 20 rpm, and colloidal silica was used as the polishing agent. We attempted to produce an LN substrate with a thickness of 0.2 mmt and a diameter of 6 inches, with the X-plane as the main surface, from this polishing process, but the substrate broke during processing, possibly due to its thin thickness, and we were unable to produce a substrate.

[0060] <Comparative Example 3> A 6-inch diameter LN single crystal ingot pulled by the CZ method was sliced ​​into X-cut surfaces with the X-axis direction as the crystal growth direction, and then chamfered and lapped.

[0061] Next, both sides were polished to a mirror finish using a double-sided polisher. The polishing conditions were 150 g / cm 2The pressure was 0.01 mm, the rotation speed of the lower platen was 5 rpm, and colloidal silica was used as the polishing agent. By this polishing process, an LN substrate with a thickness of 0.6 mmt and a diameter of 6 inches was produced, the X-plane of which was the main surface.

[0062] Other characteristics of the light guide member are as shown in Table 1.

[0063] Next, multi-layer anti-reflection films were formed on both sides of the LN substrate, and then the substrate was processed to a size of 60 x 50 mm to produce a light guide member.

[0064] When smart glasses were made using this light-guiding member and the image projected on the smart glasses was visually inspected, blurring and smearing were confirmed, possibly due to poor TTV and parallelism.

[0065] <Comparative Example 4> A 6-inch diameter LN single crystal ingot pulled by the CZ method was sliced ​​into X-cut surfaces with the X-axis direction as the crystal growth direction, and then chamfered and lapped.

[0066] Next, both sides were polished to a mirror finish using a double-sided polisher. The polishing conditions were 100 g / cm 2 The pressure was 1000 MPa, the rotation speed of the lower plate was 20 rpm, and cerium oxide was used as the polishing agent. By this polishing process, an LN substrate with a thickness of 0.6 mmt and a diameter of 6 inches was produced, the X-plane of which was the main surface.

[0067] Other characteristics of the light guide member are as shown in Table 1.

[0068] Next, multi-layer anti-reflection films were formed on both sides of the LN substrate, and then the substrate was processed to a size of 60 x 50 mm to produce a light guide member.

[0069] When smart glasses were produced using this light-guiding member and the image projected on the smart glasses was visually inspected, it was found that the brightness had decreased, possibly due to poor surface roughness.

[0070] Examples 1 to 5 and Comparative Examples 1 to 4 are shown in the following table. [Table 1]

[0071] The above-described embodiments are merely examples, and anything that has substantially the same configuration as the technical idea described in the claims of the present invention and exhibits similar effects is included within the technical scope of the present invention. [Explanation of symbols]

[0072] 1. Crystal structure of lithium niobate 2 Light guide member 3. Display 4 Input Grating 5 Output Grating

Claims

1. A light-guiding member used in a waveguide structure of an image display device that guides image light incident from a display and emits it toward a user's eye, The light-guiding member is formed of a single crystal of lithium niobate.

2. the light guide member has a first surface and a second surface opposed to each other, 2 . The light guide member according to claim 1 , wherein the plane orientation of the first surface and the plane orientation of the second surface are aligned in the X-axis direction or the Z-axis direction of a crystal axis, respectively.

3. 3. The light guide member according to claim 2, wherein a difference between a maximum value and a minimum value of a distance from the first surface to the second surface is 2 [mu]m or less.

4. 4. The light guide member according to claim 2, wherein the parallelism between the first surface and the second surface is 2.5 arcsec or less.

5. 4. The light guide member according to claim 2, wherein the first surface and the second surface of the light guide member each have a surface roughness Sq of 1 nm or less.

6. 3. The light guide member according to claim 1, wherein the internal transmittance of the light guide member at a wavelength of 400 to 800 nm is 90% or more in terms of a value calculated based on a thickness of 10 mm.

7. 3. The light guide member according to claim 1, wherein the light guide member has a plate thickness of 0.3 mm or more and 1.0 mm or less.

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