Light guide member

Abstract

The present invention is a light guide member for use 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, and the present invention is characterized in that light guide member is formed from a single crystal composed of lithium niobate. According to the present invention, it is possible to provide a light guide member having a high refractive index and a high internal transmittance.

Description

Light guide member

[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.

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

[0003] These head-mounted displays and smart glasses are classified into see-through and non-transparent types. Most smart glasses are see-through and project new AR (Augmented Reality) and other technologies into the real world, so they must be small and portable. Meanwhile, most head-mounted displays are non-transparent and project VR (Virtual Reality) and other technologies, so they must have a wide 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-guiding member onto which the image displayed on the display element is incident, and a propagation means that totally reflects the incident image inside the light-guiding member and propagates it toward the user's pupil. The light-guiding member is formed from a glass material with a refractive index of 2.0 to 2.1 so as to obtain a predetermined viewing angle.

[0005] JP 2023-14127 A

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

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

[0008] As a result of extensive research, the present inventors have found that the above-mentioned problems can be solved by forming a light-guiding member from a single crystal of lithium niobate, and have thus completed the present invention. 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 eyes, the light-guiding member being formed from a single crystal of lithium niobate. [2] The light-guiding member according to [1] above, characterized in that the light-guiding member has a first surface and a second surface facing each other, and the plane orientations of the first surface and the second surface are aligned with the X-axis and Z-axis of the crystal axes, respectively. [3] The light-guiding member according to [2] above, characterized in 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. [4] The light-guiding member according to [2] or [3] above, characterized in that 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 [2] to [4] above, 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 [1] to [5] above, wherein the internal transmittance of the light-guiding member at wavelengths of 400 to 800 nm is 90% or more in terms of a 10 mm thickness. [7] The light-guiding member according to any one of [1] to [6] above, wherein the plate thickness of the light-guiding member is 0.3 mm or more and 1.0 mm or less.

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

[0011] 1A and 1B are diagrams showing the crystal structure of lithium niobate, a graph showing the wavelength dependence of the refractive index of lithium niobate, and a diagram showing an example of a waveguide structure of an image display device.

[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 a user's eyes. The light-guiding member of the present invention is formed from a single crystal of lithium niobate.

[0013] Examples of image display devices that use 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) emitted from a display 3 is diffracted by an input grating (diffraction grating) 4 and travels through the light-guiding member 2 while repeatedly undergoing 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 eyes.

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

[0015] The light-guiding member has a first surface and a second surface facing each other, and the plane orientation of the first surface and the plane orientation of the second surface are preferably aligned with the X-axis of the crystal axis. This reduces the influence of pyroelectricity inherent in lithium niobate. Pyroelectricity is a property in which polarization within a crystal changes with temperature, resulting in static electricity. If pyroelectricity occurs during the manufacturing process, discharge may occur during the manufacturing process, causing undesirable problems. The angular deviation between the plane orientation of the first surface and the plane 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. Large bulk single crystals of lithium niobate can be grown by pulling a raw material melt melted in a crucible upward under a temperature gradient, a method known as the Czochralski method. Setting the Z-axis as the pulling axis further improves the crystallinity of the grown crystal. 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, which is the pulling axis. The angular deviations between the plane orientations of the first plane and the second plane and the Z-axis of the crystal axis are preferably −0.2 or more and +0.2 or less, respectively.

[0018] In the light-guiding 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. When the difference between the maximum and minimum values ​​of the distance from the first surface to the second surface is 2 μm or less, blurring and bleeding of RGB light can be further reduced. From this perspective, it is even 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 lithium niobate single crystal 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 light of each wavelength of RGB through the grating, in the light-guiding member of the present invention, the parallelism between the first surface and the second surface is preferably as high as possible, and is preferably 2.5 arcsec or less. When the parallelism between the first surface and the second surface is 2.5 arcsec or less, RGB light can be more reliably guided to the grating. From this perspective, 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 the wedge angle (Wedge) in accordance with JIS B0615-2:2017. Furthermore, for example, by polishing a lithium niobate single crystal so that the parallelism between the first surface and the second surface is 2.5 arcsec or less, the parallelism between the first surface and the second surface in the light-guiding member can be 2.5 arcsec or less.

[0020] From the viewpoint that light of each wavelength of RGB propagates within the light-guiding section and is guided by total reflection at the interface (surface) with air, the surface roughness Sq of the first surface and the surface roughness Sq of the second surface of the light-guiding 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 ISO 25178. For example, by polishing a lithium niobate single crystal 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, the surface roughness Sq of the first surface and the surface roughness Sq of the second surface of the light-guiding member of the present invention can each be 1 nm or less.

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

[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. A thickness of 0.3 mm or more can make the light guide member even less likely to break. Furthermore, a thickness of 1.0 mm or less can make the light guide member lighter, thereby reducing fatigue when wearing the image display device.

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

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

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

[0026] Example 1 A lithium niobate (LN) single crystal ingot with a diameter of 6 inches, which was 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 using a double-sided polishing machine to achieve a mirror finish. ２ The polishing was performed at a pressure of 1000 kJ / cm, a rotation speed of the lower surface plate of 20 rpm, and colloidal silica was used as the polishing agent. By this polishing process, an LN substrate having a thickness of 0.5 mmt and a diameter of 6 inches and an X-plane as the main surface was produced.

[0028] The refractive index of this LN substrate was measured using a Prism Coupler Model 2010 / M manufactured by Metricon. Wavelengths used were 473, 633, and 1064 nm. 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 properties of the light guide member were as follows: Plane orientation of the first and second surfaces: X-axis of the crystal axis Internal transmittance: 91.1% TTV: 1.3 μm Parallelism: 1.8 arcsec Surface roughness Sq: 0.5 nm on the first surface, 0.5 nm on the second surface Thickness: 0.6 mm

[0030] Crystal orientation was measured using an X-ray cut surface inspection machine manufactured by Rigaku Corporation. Internal transmittance was measured using the method described in JOGIS 17:2019, with 3 mm and 10 mm samples prepared and measured using a UV-3100 model manufactured by Shimadzu Corporation. Total thickness variation (TTV) and parallelism were measured using an UltraSort II manufactured by Tropel. Parallelism was evaluated using the wedge angle index (Wedge) specified in JIS B0615-2:2017. Roughness was measured over a 400 x 400 μm area using a Zygo white light interferometer, Zegage Plus, and the Sq value was calculated. Sq refers to the Sq value defined in JIS 25178. Thickness was measured using a micrometer.

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

[0032] Smart glasses were made using this light-guiding member, and the images displayed on the smart glasses were visually inspected. It was confirmed that the viewing angle was a sufficiently wide 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 with the X-axis direction as the crystal growth direction was sliced ​​to have an X-cut surface, and then chamfered and lapped.

[0034] Next, both sides were polished using a double-sided polishing machine to achieve a mirror finish. ２ The polishing was performed at a pressure of 1000 kJ / cm, a rotation speed of the lower surface plate of 15 rpm, and colloidal silica was used as the polishing agent. By this polishing process, an LN substrate having a thickness of 0.5 mmt and a diameter of 6 inches and an X-plane as the main surface was produced.

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

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

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

[0038] Example 3 An LN single crystal ingot with a diameter of 6 inches, which was 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.

[0039] Next, both sides were polished using a double-sided polishing machine to achieve a mirror finish. ２ The polishing pressure was 1000 kJ / min, the rotation speed of the lower surface plate was 20 rpm, and colloidal silica was used as the polishing agent. By this polishing process, an LN substrate having a thickness of 0.3 mmt and a diameter of 6 inches and an X-plane as the main surface was produced.

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

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

[0042] Smart glasses were made using this light-guiding member, and the images displayed on the smart glasses were visually inspected. It was confirmed that the viewing angle was a sufficiently wide 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 with the X-axis direction as the crystal growth direction was sliced ​​to have an X-cut surface, and then chamfered and lapped.

[0044] Next, both sides were polished using a double-sided polishing machine to achieve a mirror finish. ２ The polishing was performed at a pressure of 1.0 mmt and a rotation speed of the lower surface plate of 20 rpm, using colloidal silica as an abrasive. By this polishing process, an LN substrate having a thickness of 1.0 mmt and a diameter of 6 inches and an X-plane as a main surface was produced.

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

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

[0047] Smart glasses were made using this light-guiding member, and the images displayed on the smart glasses were visually inspected. It was confirmed that the viewing angle was a sufficiently wide 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 give Z-cut surfaces, which were then chamfered and lapped.

[0049] Next, both sides were polished using a double-sided polishing machine to achieve a mirror finish. ２ The polishing was performed at a pressure of 1.0 mmt and a rotation speed of the lower platen of 20 rpm, using colloidal silica as an abrasive. An LN substrate having a thickness of 1.0 mmt and a diameter of 6 inches and a Z-plane as the main surface was produced by this polishing process.

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

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

[0052] Smart glasses were made using this light-guiding member, and the images displayed on the smart glasses were visually inspected. It was confirmed that the viewing angle was a sufficiently wide 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 with the Y-axis direction as the crystal growth direction was sliced ​​to have a Y-cut surface, and then chamfered and lapped.

[0054] Next, both sides were polished using a double-sided polishing machine to achieve a mirror finish. ２ The polishing was performed at a pressure of 1000 kJ / cm, a rotation speed of the lower surface plate of 20 rpm, and colloidal silica was used as the polishing agent. By this polishing process, an LN substrate having a thickness of 0.5 mmt and a diameter of 6 inches and a Y-plane as the main surface was produced.

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

[0056] Next, multilayer anti-reflection films were formed on both sides of the LN substrate, which was then processed to a size of 60 x 50 mm to prepare 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 with the X-axis direction as the crystal growth direction was sliced ​​to have an X-cut surface, and then chamfered and lapped.

[0059] Next, both sides were polished using a double-sided polishing machine to achieve a mirror finish.２ The pressure was 1000 kJ / cm, the rotation speed of the lower surface plate was 20 rpm, and colloidal silica was used as the abrasive. An attempt was made 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, through this polishing process, but the substrate broke during processing, possibly due to its thin thickness, and it was not possible to produce a substrate.

[0060] Comparative Example 3 A 6-inch diameter 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.

[0061] Next, both sides were polished using a double-sided polishing machine to achieve a mirror finish. ２ The polishing pressure was 1000 kJ / min, the rotation speed of the lower surface plate was 5 rpm, and colloidal silica was used as the polishing agent. By this polishing process, an LN substrate having a thickness of 0.6 mmt and a diameter of 6 inches and an X-plane as the main surface was produced.

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

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

[0064] Smart glasses were produced using this light-guiding member, and the image projected on the smart glasses was visually inspected. Blurred and smeared images were observed, possibly due to poor TTV and parallelism.

[0065] Comparative Example 4 A 6-inch diameter 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.

[0066] Next, both sides were polished using a double-sided polishing machine to achieve a mirror finish. ２ The polishing pressure was 1000 kJ / min, the rotation speed of the lower platen was 20 rpm, and cerium oxide was used as an abrasive. By this polishing process, an LN substrate having a thickness of 0.6 mmt and a diameter of 6 inches and an X-plane as a main surface was produced.

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

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

[0069] Smart glasses were produced using this light-guiding member, and the image projected on the smart glasses was visually inspected. It was confirmed 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 table below.

[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 that exhibits similar effects is included within the technical scope of the present invention.

[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 the waveguide structure of an image display device that guides image light incident from a display and emits it toward the user's eyes, said light-guiding member being formed from a single crystal of lithium niobate.

2. The light guide member according to claim 1, characterized in that the light guide 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 respectively in the X-axis or Z-axis direction of a crystal axis.

3. The light guide member according to claim 2, wherein the difference between the maximum and minimum values ​​of the distance from the first surface to the second surface is 2 μm or less.

4. The light-guiding member according to claim 2 or 3, wherein the parallelism between the first surface and the second surface is 2.5 arcsec or less.

5. A light-guiding member according to claim 2 or 3, characterized in that 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 guide member according to claim 1 or 2, characterized in that the internal transmittance of said light guide member in the wavelength range of 400 to 800 nm is 90% or more in terms of a value calculated based on a thickness of 10 mm.

7. The light-guiding member according to claim 1 or 2, characterized in that the plate thickness of the light-guiding member is 0.3 mm or more and 1.0 mm or less.

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