JP2015105990A - Optical device and electronic device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electronic device configured to increase diffraction efficiency in a wide range of a field of view, to observe a large-size virtual image.SOLUTION: On a light incident surface of a light guide plate 200 formed of glass or light-transmissive resin material or the like, a first diffraction optical element 102, which is a surface-relief diffraction grating formed of SiN or the like, is arranged. A refraction index of the first diffraction optical element 102 is higher than that of the light guide plate 200. An image forming apparatus 500 makes TE-polarized image light 10 incident on the first diffraction optical element 102.

Description

  The present invention relates to an optical device including a light guide plate and a diffractive optical element, and an electronic apparatus including the optical device.

  In an optical system for reducing the thickness of a head-mounted display, for example, a light guide plate and a diffractive optical element that inputs light to the light guide plate and outputs light from the light guide plate are used (for example, Patent Document 1). And Patent Document 2).

  In Patent Document 1 and Patent Document 2, in order to observe a virtual image over a wide field of view, studies are made on the refractive index of the light guide plate, the pitch of the diffraction grating, and the wavelength range used for virtual image display.

  However, in Patent Document 1 and Patent Document 2, a configuration for maintaining high diffraction efficiency over a wide field of view is not studied and is not described.

  On the other hand, although not an image display device, Patent Document 3 discloses a configuration in which the refractive index of the diffraction grating is made higher than the refractive index of the substrate.

Special table 2008-535032 gazette US Pat. No. 6,757,105 JP 2009-237351 A

  However, in the configuration of Patent Document 3, a light guide unit having the same refractive index as that of the diffraction grating is arranged at the bottom of the diffraction grating structure. In this configuration, the diffraction grating having the same refractive index as that of the light guide plate is used. This is equivalent to the case where is arranged, and diffracted light cannot be efficiently input to the light guide plate.

  In view of the above-described circumstances, it is an object of the present invention to provide an optical device and an electronic apparatus that can increase diffraction efficiency over a wide visual field range and can observe a large virtual image. .

  In order to solve the above-described problem, an aspect of the optical device according to the present invention includes a light guide and a plurality of protrusions extending in a first direction along one surface of the light guide. A first diffractive optical element arranged apart from each other along a second direction intersecting with one direction, and the refractive index of each of the plurality of convex portions of the first diffractive optical element is the light guide It is characterized by being higher than the refractive index.

  According to the aspect of the optical device according to the present invention described above, when the first diffractive optical element is on the incident side, the light incident on the first diffractive optical element is diffracted by the first diffractive optical element, Introduced into the light guide. At this time, since the refractive index of each of the plurality of convex portions of the first diffractive optical element is higher than the refractive index of the light guide, the diffracted light is efficiently introduced into the light guide plate. Further, when the first diffractive optical element is on the exit side, the diffracted light efficiently introduced into the light guide can be efficiently extracted by the first diffractive optical element.

  In one aspect of the optical device according to the present invention described above, the first diffractive optical element may be provided corresponding to a light incident portion of the light guide. In this case, the light incident on the first diffractive optical element is diffracted by the first diffractive optical element and introduced into the light guide. At this time, since the refractive index of each of the plurality of convex portions of the first diffractive optical element is higher than the refractive index of the light guide, the diffracted light is efficiently introduced into the light guide plate.

  In one aspect of the optical device according to the present invention described above, the first diffractive optical element may be provided corresponding to a light emitting portion of the light guide. In this case, the diffracted light efficiently introduced into the light guide can be efficiently extracted by the first diffractive optical element.

  In one aspect of the optical device according to the present invention described above, a plurality of convex portions extending in the third direction along one surface of the light guide body are along a fourth direction intersecting the third direction. The first diffractive optical element is provided corresponding to a light incident portion of the light guide, and the second diffractive optical element is provided on the light guide. It may be provided corresponding to the light emitting part of the body. In this case, the light incident on the first diffractive optical element is diffracted by the first diffractive optical element and introduced into the light guide. At this time, since the refractive index of each of the plurality of convex portions of the first diffractive optical element is higher than the refractive index of the light guide, the diffracted light is efficiently introduced into the light guide plate. And the diffracted light efficiently introduced into the light guide can be efficiently taken out by the second diffractive optical element.

  In one aspect of the optical device according to the present invention described above, a base layer is provided between the light guide and the first diffractive optical element, and the refractive index of each of the plurality of convex portions of the first diffractive optical element is It is preferable that the refractive index of the light guide and the base layer is higher. In this case, even when the plurality of convex portions of the first diffractive optical element are formed by etching, since the base layer is present, the phenomenon of etching to the light guide is prevented and the depth of the convex portion is set to a desired depth. The depth can be controlled.

  In one aspect of the optical device according to the present invention described above, the refractive index of the base layer may be the same as that of the light guide. In this case, the light guide and the base layer can be considered as an integral member, and the diffracted light is efficiently introduced into the light guide plate via the base layer.

  In one aspect of the optical device according to the present invention described above, the first diffractive optical element is preferably a surface relief type diffraction grating. In this case, a diffractive optical element having a convex portion having a refractive index higher than that of the light guide plate can be easily produced.

  Next, an electronic apparatus according to the present invention includes the above-described optical device according to the present invention and an image forming unit that emits image light. Such an electronic apparatus may be provided with an image forming unit such as a liquid crystal display and a collimator optical system, and can be adapted to be mounted on the observer's head such as a head-mounted display.

  In the electronic apparatus described above, it is preferable that the image light is TE polarized light. In this case, TE-polarized image light is introduced into the light guide more efficiently than the diffractive optical element.

  In the electronic apparatus according to the present invention, the “image forming unit” refers to, for example, a liquid crystal display that displays an image, a laser scanning display that causes an observer to recognize the image by scanning laser light, and an image. It includes an optical system that condenses and converts image light emitted from the display.

It is a perspective view which shows an example of the whole image of the head mounted display which concerns on 1st Embodiment. It is principal part sectional drawing which shows an example of the internal structure of the optical system for left eyes of the head mount display which concerns on 1st Embodiment, and a waveguide. It is a top view which shows the optical system for left eyes of the head mounted display which concerns on 1st Embodiment. It is an expanded sectional view of the 1st diffractive optical element and a light guide plate. It is a wave vector diagram in case the refractive index of a diffraction grating and the refractive index of a light-guide plate are 1.62, and incident angle (theta) a is -10 degrees. Wave vector diagram when the refractive index of the diffraction grating and the refractive index of the light guide plate are 1.62 and the incident angle θa = 4.1 °. It is a wave vector diagram when the refractive index of the diffraction grating is 2.05, the refractive index of the light guide plate is 1.62, and the incident angle θa is −10 °. Wave vector diagram when the refractive index of the diffraction grating is 2.05, the refractive index of the light guide plate is 1.62, and the incident angle θa = 10.1 °. It is a wave vector diagram in case the refractive index of a diffraction grating and the refractive index of a light-guide plate are 2.05, and incident angle (theta) a is -10 degrees. It is a wave vector diagram in case the refractive index of a diffraction grating and the refractive index of a light-guide plate are 2.05, and incident angle (theta) a = 10.1 degrees. It is a graph which shows the relationship between the incident angle and the 1st-order diffraction efficiency in various combinations of the refractive index of a diffraction grating, the refractive index of a light-guide plate, and the depth of a diffraction grating. It is principal part sectional drawing of the optical device in which the diffractive optical element and the foundation | substrate were formed with the material of refractive index higher than a light-guide plate. It is principal part sectional drawing of the optical device in which the diffractive optical element was formed with the material of refractive index higher than a light-guide plate. It is an expanded sectional view of the 1st diffractive optical element and light guide plate in a 2nd embodiment. It is principal part sectional drawing which shows the state which etched after forming the film | membrane for diffractive optical elements on a light-guide plate. It is principal part sectional drawing which shows the state which provided the base layer on the light-guide plate and formed the film after forming the film | membrane for diffractive optical elements on a base layer.

  Hereinafter, various embodiments according to the present invention will be described with reference to the accompanying drawings. In the drawings, the ratio of dimensions of each part is appropriately changed from the actual one. In the embodiment described below, the optical device of the present invention will be described as an example in which the optical device is applied to a head-mounted display that is an example of an image display device that is mounted on the head of an observer. This embodiment shows one aspect of the present invention and does not limit the present invention, and can be arbitrarily changed within the scope of the technical idea of the present invention.

<First Embodiment>
(Overall configuration of head-mounted display)
FIG. 1 is a perspective view illustrating an example of an overall image of a head-mounted display 100 as an example of an electronic apparatus according to the first embodiment. As shown in FIG. 1, the head mounted display 100 according to the present embodiment is a head mounted display having an appearance like glasses, and for an observer wearing the head mounted display 100. It is possible to recognize image light due to a virtual image and to allow an observer to observe an external image in a see-through manner.

  Specifically, the head mounted display 100 includes a light guide plate 200, a pair of left and right temples 131 and 132 that support the light guide plate 200, and a pair of image forming apparatuses 111 and 112 attached to the temples 131 and 132. . Here, in the drawing, the first display device 100A in which the left side of the light guide plate 200 and the image forming device 111 are combined is a portion that forms a virtual image for the right eye, and functions alone as an image display device. In the drawing, the second display device 100B in which the right side of the light guide plate 200 and the image forming device 112 are combined is a portion that forms a virtual image for the left eye, and functions alone as an image display device.

  The internal structure and the light guide plate of the head mounted display 100 will be described. FIG. 2A is a cross-sectional view of an essential part schematically showing an example of the internal structure of the head-mounted display and the light guide plate according to the present embodiment. FIG. 2B is a plan view corresponding to FIG. 2A. In addition, although illustration is abbreviate | omitted, about the internal structure of the optical system for right eyes, and the light-guide plate, it has the structure which reversed FIG. 2A and swapped right and left. As illustrated in FIG. 2A, the second display device 100B includes an image forming unit 500 and a light guide plate 200.

  The image forming unit 500 includes an image display device 501 and a projection optical system 502. Among these, the image display device 501 is a liquid crystal panel in the present embodiment, and the vibration direction of the electric vector of the light emitted from the liquid crystal panel is the direction along the pattern of the first diffractive optical element 102 (y direction). Thus, the output side polarizing plate of the liquid crystal panel is arranged. That is, the light incident on the light guide plate 200 is changed to a TE wave with respect to the first diffractive optical element 102. The image light 10 has red (R), green (G), and blue (B) wavelength components. However, in the present embodiment, description will be made assuming that only green light is used. As the image display device 501, an organic EL panel can be used.

  The projection optical system 502 is a collimator lens that converts image light emitted from each point on the image display device 501 into a light beam in a parallel state and enters the light guide plate 200. The collimator lens is made of a material that preserves the plane of polarization of incident polarized light, such as glass or a resin having low birefringence.

  The light guide plate 200 is formed of glass or a light transmissive resin material, and has a flat plate shape extending in parallel with the YX plane in the drawing. The light incident surface of the light guide plate 200 is provided with a first diffractive optical element 102 that diffracts incident light in the direction of the end surface on the light exit surface side. The light exit surface diffracts the light propagating through the light guide plate 200. A second diffractive optical element 103 that is taken out into the air is provided. These diffractive optical elements are all surface relief type diffraction gratings.

  The image light 10 emitted from the pixel of the image display device 501 is converted into parallel light by the projection optical system 502 and is incident on the first diffractive optical element 102 as light having an angle corresponding to the position of the pixel. The first diffractive optical element 102 diffracts the image light 10 and causes the image light 10 to enter the light guide plate 200. The image light 10 incident on the light guide plate 200 propagates while repeating total reflection in the light guide plate 200 and reaches the second diffractive optical element 103. The image light reaching the second diffractive optical element 103 is taken out little by little every time it enters the diffraction grating surface of the second diffractive optical element 103 and reaches the eye 50 of the observer.

  In order to increase the size of the virtual image that can be observed by the observer, it is necessary to increase the viewing angle of the image light 10 incident on the light guide plate 200 from the image display device 501. Since the light propagated through the light guide plate 200 is emitted from the second diffractive optical element 103 at the same angle as the incident light on the first diffractive optical element 102, the first diffractive optical element 102 and the second diffractive optical element 103 are It is necessary to have a configuration capable of maintaining high diffraction efficiency over a wide viewing angle, that is, a wide incident angle range.

(Refractive index of diffractive optical element)
In the present invention, in order to achieve high diffraction efficiency over a wide incident angle range, the first diffractive optical element 102 and the second diffractive optical element 103 are made of a material having a refractive index higher than that of the light guide plate 200. . This will be described in detail below.

  FIG. 3 shows an enlarged cross section of the first diffractive optical element 102 and the light guide plate 200. The refractive index of the light guide plate 200 is ns, and the refractive index of the first diffractive optical element 102 is ng. Let na be the refractive index of the medium on which light enters the first diffractive optical element 102. In this embodiment, since this medium is air, na = 1.

  The region where the first diffractive optical element 102 is formed is called a grating layer, and the effective refractive index of the grating layer is nge. Although the effective refractive index of the grating layer varies depending on the polarization direction of incident light, here, the effective refractive index for TE-polarized light with high diffraction efficiency is considered. TE polarized light is polarized light whose electric field direction is parallel to the direction of the grating pattern of the first diffractive optical element 102. The effective refractive index for TE-polarized light is as follows:

(Equation 1)
nge = [a · ng ² + (1−a) · na ² ] ^1/2
It is represented by

  Here, the magnitude of the first-order diffraction efficiency is considered qualitatively. The wavelength of light incident on the first diffractive optical element 102 is λ, the wave number in air is ka, the wave number in the grating layer (in the medium having an effective refractive index of nge) is kge, and the wave number of light in the light guide plate is ks. Where each wave number is represented by the following equation.

(Equation 2)
ka = 2π · na / λ
kge = 2π · nge / λ
ks = 2π · ns / λ

  The grating vector K of the first diffractive optical element 102 is expressed by the following expression using the grating pitch P.

(Equation 3)
K = 2π / P

  The incident angle to the first diffractive optical element 102 is θa, the refraction angle to the grating layer is θgi, the diffraction angle when diffracted in the grating layer is θgo, and the refraction angle to the light guide plate 200 is θs. Is used to consider the coupling efficiency of incident light and diffracted light in the grating layer.

  FIG. 4 shows a wave vector diagram when the incident angle θa is −10 °. In FIG. 4, the angle in the clockwise direction is represented by a positive angle. Further, the wavelength λ = 0.53 μm, the light guide plate refractive index ns = 1.62, and the grating refractive index ng = 1.62. When the grating packing ratio a is 0.5, the effective refractive index nge of the grating layer for TE polarized light is as follows.

(Equation 4)
nge = 1.35

  Incident light having a wave number ka enters the grating layer at an incident angle θa = −10 °. This incident light is refracted by the effective refractive index nge of the grating layer and enters the grating layer. At this time, the refraction angle in the lattice layer is θgi = −7.4 °, and the wave number is kge.

  The light incident on the grating layer is diffracted and incident on the light guide plate 200 after being diffracted in the grating layer. In order to obtain 39 ° which is slightly larger than 38.1 ° in the case of 1.62, the diffraction angle θgo in the grating layer needs to be 49.2 °.

  A grating vector K for diffracting light having a refraction angle θgi incident in the grating layer in the direction of the diffraction angle θgo is expressed by the following expression.

(Equation 5)
K = kge · [sin (θgo) −sin (θgi)]

At this time, the grating pitch P of the first diffractive optical element 102 is 0.444 μm. The difference Δk in the z direction between the grating vector K and the wave vector of the diffracted light represents the amount of phase mismatch under the Bragg condition. When Δk = 0, that is, when the wave vector and the diffraction vector of the refracted light and diffracted light in the grating layer are closed, the diffraction efficiency is maximized as the Bragg condition, but the deviation from the Bragg condition increases as Δk increases. Increases and the diffraction efficiency decreases. In the case of FIG. 4, Δk = 5.4 μm ⁻¹ .

  FIG. 5 shows a wave vector diagram when light having an incident angle θa = 4.1 °, for example, is incident on the first diffractive optical element 102 (grating pitch P = 0.444 μm) determined in this way.

The refraction angle θgi to the grating layer is 3.1 °, the diffraction angle θgo in the grating layer is 70 °, and the refraction angle θs in the light guide plate 200 is 51.3 °. In this case, the phase mismatch amount Δk increases to 10.5 μm ⁻¹ and the diffraction efficiency is expected to be lowered. When the incident angle θa is larger than 4.1 °, the diffraction angle θgo in the grating layer becomes larger than 70 °, and the phase mismatch amount Δk further increases.

  On the other hand, the diffraction efficiency when the refractive index ng of the diffraction grating is 2.05, which is larger than the refractive index 1.62 of the light guide plate, will be considered. FIG. 6 shows a wave vector diagram when the incident angle θa is −10 °. When the grating packing ratio a is 0.5, the effective refractive index nge of the grating layer for TE polarized light is nge = 1.61, and the nge when the grating refractive index is 1.62 is larger than 1.35. The circle of kge representing the wave number in the lattice layer becomes large and approaches the circle of wave number ks in the light guide plate.

As in the case of FIG. 4, when the grating pitch P of the first diffractive optical element 102 is determined so that the light having the incident angle θa = −10 ° has a refraction angle θs = 39 ° in the light guide plate 200, FIG. As in the case, P = 0.444 μm, and the size of the lattice vector K is the same as in FIG. At this time, the diffraction angle θgo the next 39.2 ° in the lattice layer, the phase mismatching amount Δk is expected that small 4.2 .mu.m ^-1 next than 5.4 [mu] m ^-1, the diffraction efficiency becomes higher.

  FIG. 7 shows a wave vector diagram when light having an incident angle θa = 10.1 °, for example, is incident on the first diffractive optical element 102 (grating pitch P = 0.444 μm) determined in this way.

The refraction angle θgi to the grating layer is 6.2 °, the diffraction angle θgo in the grating layer is 58 °, and the refraction angle θs in the light guide plate 200 is 57.8 °. In this case, the phase mismatch amount Δk is 8.9 μm ⁻¹ .

In the case of FIG. 4 (light guide plate refractive index ns = 1.62, grating refractive index ng = 1.62), the phase mismatch amount Δk is already 10.5 μm ⁻¹ when the incident angle θa is 4.1 °. However, in the case of FIG. 7 (light guide plate refractive index ns = 1.62, grating refractive index 2.05), the phase mismatch amount Δk is 8 even when the incident angle θa is 10.1 °. .9μm is only about ^-1, it is expected that the incident angle θa is maintained high 10.1 ° even diffraction efficiency.

  Light guide plate refractive index na = 1.62, grating refractive index ng = 1.62, diffraction grating depth d = 0.200 μm, light guide plate refractive index ns = 1.62, grating refractive index 2.05, FIG. 10 shows an example of a result obtained by calculating the first-order diffraction efficiency in the case where the depth d of the diffraction grating is 0.175 μm.

  As can be seen from FIG. 10, when the grating refractive index ng is 1.62, which is the same as the light guide plate refractive index ns = 1.62, the first-order diffraction efficiency greatly decreases as the incident angle approaches 10 °. On the other hand, when the grating refractive index ng is 2.05, which is larger than the light guide plate refractive index ns = 1.62, the diffraction efficiency is maintained even when the incident angle is 10 °. That is, it is understood that the diffraction efficiency is maintained over a wide range of incident angles, and display with a small difference in brightness between the left and right sides of the screen can be performed even when a large-size virtual image is displayed.

  In the above description, the diffraction efficiency when light is introduced into the light guide plate 200 by the first diffractive optical element 102 is shown. However, the second diffractive optical element 103 that extracts light from the light guide plate 200 performs the reverse process. Therefore, by using a diffraction grating having a high refractive index, light in a wide angle range can be extracted from the light guide plate 200 and delivered to the observer's eyes. That is, by using a diffraction grating having a high refractive index as the first diffractive optical element and the second diffractive optical element, high diffraction efficiency is maintained over a wide incident angle range, and even if a large-size virtual image is displayed, both sides are displayed. A display with little difference in brightness between right and left can be performed.

  For example, as shown in FIGS. 4 and 5, when the light guide plate refractive index ns = 1.62 and the grating refractive index ng = 1.62, the virtual image screen size corresponding to the range of −10 ° to + 4 ° as the incident angle. If the aspect ratio of the screen is 16: 9, it will be 28 inches diagonally 2.5 meters away. As can be seen from FIG. 10, since the diffraction efficiency changes greatly in this angular range, it is expected that a large distribution of brightness in the screen will occur.

  However, as shown in FIGS. 6 and 7, when the refractive index of the light guide plate is ns = 1.62 and the refractive index of the grating is 2.05, by increasing the refractive index of the diffractive optical element, −10 ° to + 10 ° High diffraction efficiency can be maintained in the incident angle range. If the aspect ratio of the screen is 16: 9, the virtual image screen size corresponding to this incident angle range is 40 inches diagonally 2.5 meters away, and the virtual image screen size can be increased. In particular, when the refractive index of the light guide plate is ns = 1.62 and the refractive index of the grating is 2.05, the change in diffraction efficiency in this incident angle range is small, so that the brightness uniformity in the screen is high.

  The diffractive optical element can be formed by using a glass substrate having a refractive index of about 1.62 as a light guide plate, forming SiN (silicon nitride) on the glass substrate, and etching the SiN film. . It can also be created using the lift-off method.

In the present embodiment, SiN is cited as a high refractive index material used for the diffractive optical grating, but other optical thin films having a high refractive index include hafnium oxide HfO ₂ (refractive index 1.95) and zirconium dioxide ZrO _2. (Refractive index 2.05), indium tin oxide ITO (refractive index 2.05), tantalum pentoxide Ta ₂ O ₅ (refractive index 2.1), titanium dioxide TiO ₂ (refractive index 2.4), or the like is used. Can do.

(Comparative example)
Next, as shown in FIG. 11, a diffractive optical element 301 and its base layer 302 are formed of a material having a higher refractive index than that of the light guide plate 400, as shown in FIG. Consider an optical device having a structure in which the element 301 and the base layer 302 are provided on the light guide plate 400.

  Patent Document 3 describes that the diffractive optical element and its underlayer are made of a material having a refractive index of 1.9, but here, for comparison with the optical device of the present embodiment. In the following description, it is assumed that the diffractive optical element 301 and the base layer 302 are made of a material having a refractive index of 2.05. It is assumed that the refractive index ng of the diffractive optical element 301 and the refractive index ns of the base layer 302 serving as the light guide layer are both 2.05. FIG. 8 shows a wave vector diagram when the incident angle θa is −10 °.

  When the grating packing ratio a is 0.5, the effective refractive index nge of the grating layer with respect to TE polarization is 1.61, and the circle representing the magnitude kge of the wave vector in the grating layer is shown in FIGS. It becomes the same as the circle of kge in the optical device of the embodiment. On the other hand, the refractive index ns of the underlayer 302 is 2.05, which is larger than the effective refractive index nge of the lattice layer, so that the circle representing the magnitude ks of the wave vector in the underlayer 302 is larger than the effective refractive index circle. .

As in the case of the optical device of this embodiment shown in FIG. 4 or FIG. 6, the grating of the diffractive optical element 301 so that the light having the incident angle θa = −10 ° has the refraction angle θs = 30 ° in the base layer 302. The pitch P is obtained. Since the refractive index ns of the underlayer 302 is 2.05, the critical angle is 29.2 °, and 30 ° slightly larger than the critical angle is defined as the refractive angle θs in this case. The grating pitch P of the diffractive optical element 301 for diffracting the light having the incident angle θa = −10 ° to the refraction angle θs = 30 ° in the base layer 302 is 0.442 μm, and the magnitude of the grating vector K is determined. At this time, the diffraction angle θgo in the grating layer is 39.5 °, and the phase mismatch amount Δk is 4.2 μm ⁻¹ . This value is the same as the phase mismatch amount in the case of the optical device of the present embodiment shown in FIG. 6 (when ng = 2.05), and it is predicted that the diffraction efficiency will be high.

For example, light having an incident angle θa = 10.1 ° is incident on the diffractive optical element 301 (grating pitch P = 0.442 μm) determined in this way. A wave vector diagram in this case is shown in FIG. As shown in FIG. 9, the refraction angle θgi to the grating layer is 6.3 °, the diffraction angle θgo in the grating layer is 58.4 °, and the refraction angle θs in the base layer 302 is 42.1 °. . In this case, the phase mismatch amount Δk is 9.0 μm ⁻¹ , and in the case of the optical device of the present embodiment shown in FIG. 6 (light guide plate refractive index ns = 1.62, grating refractive index ng = 2.05). It is expected that the diffraction efficiency is maintained high even when the incident angle θa is 10.1 °. The result of calculating the first-order diffraction efficiency in this configuration is shown in FIG.

  Next, the refractive index of the light guide plate will be considered. As shown in FIG. 2A, the light guide plate 200 has a function of transmitting the image light 10 from the image display device 501 arranged beside the face to the eyes 50 of the observer. Accordingly, the light guide plate 200 has a length of about 100 mm, a width of about 30 mm, and a thickness of about 1.5 mm. When glass is used as the material of the light guide plate 200, the upper limit of the refractive index of the glass is about 2, and the density of the glass having a high refractive index is that of a glass having a refractive index of about 1.5, which is generally used as optical glass. About 1.7 times larger.

  When the optical system of the present invention is used as an electronic apparatus such as a head-mounted display, it is desired to reduce the weight of the optical system. For this purpose, it is preferable to use glass having a low density, that is, a low refractive index.

  Also in the comparative example shown in FIG. 11, the light guide plate 400 is made of glass having a low refractive index (for example, 1.52). In the comparative example, the diffractive optical element 301 and its base 302 are formed on the surface of the light guide plate 400 with a material having a refractive index ng higher than the refractive index (for example, 2.05).

  In the structure of this comparative example, since the diffractive optical element 301 is formed of a high refractive index material, light incident on the diffractive optical element 301 from the air side has high diffraction efficiency as described with reference to FIGS. Light is introduced into the base 302 which is a high refractive index layer.

  However, in the structure of the comparative example, since there is a difference in refractive index at the interface between the base 302 that is a high refractive index layer and the light guide plate 400, interface reflected light is generated. The interface reflected light is emitted to the air side by the diffractive optical element 301 while being repeatedly reflected between the diffractive optical element 301 and the interface. That is, the amount of light diffracted by the diffractive optical element 301 enters the light guide plate 400 and propagates in the direction of the diffractive optical element on the exit side is reduced.

  Therefore, unlike the optical device of the present embodiment shown in FIG. 12, the diffractive optical element 102 having a high refractive index has a refractive index lower than the refractive index of the diffractive optical element 102 without using a base that is a high refractive index layer. It is preferably formed on the surface of the light guide plate 200.

Second Embodiment
Next, a second embodiment of the present invention will be described with reference to FIGS. When the diffractive optical element 102 is formed by etching, a diffractive optical element film 102A is formed on the light guide plate 200 as shown in FIG. 14, and a resist 300 is formed on the film 102A. Etch as shown. At this time, an etched region 303 is formed not only in the film 102A but also in the light guide plate 200, and the depth of the diffractive optical element 102 cannot be controlled, and a desired diffraction efficiency may not be obtained. is there.

  Therefore, in the present embodiment, as shown in FIG. 15, the base film 201 is formed on the light guide plate 200, and then the same etching is performed. The base film 201 is made of a material that is difficult to be etched as compared with the material of the film 102A of the diffractive optical element 102. Even when etching similar to the above is performed, the depth of the etched region 303 is as described above. It becomes shallower than the case. Therefore, the depth of the diffractive optical element 102 can be controlled, and a desired diffraction efficiency can be obtained.

FIG. 13 shows a cross-section of the main part of the optical device after etching in this embodiment. The refractive index nt of the base film 201 needs to be approximately the same as the refractive index ns of the light guide plate 200 and smaller than the refractive index ng of the diffractive optical element 102. In addition, the base film 201 needs to be harder to be etched than the material of the diffractive optical element 102. As such a base film, Al ₂ O ₃ (aluminum oxide) having a refractive index of about 1.63 can be used (light guide plate refractive index 1.62, lattice refractive index 2.05).

<Modification>
The present invention is not limited to the above-described embodiments, and for example, various modifications described below are possible. Of course, each embodiment and each modification may be combined as appropriate.

(1) The refractive index of the light guide plate, the refractive index of the grating, or the refractive index of the base film can be changed as appropriate. In addition, it is possible to use plastic instead of glass as the light guide plate. However, since the present invention is established when the polarization with respect to the diffraction grating is TE polarized light, it is necessary that depolarization does not occur during propagation of the light guide plate.
(2) In the first embodiment and the second embodiment described above, surface relief type diffraction gratings are used as the first diffractive optical element 102 and the second diffractive optical element 103 which are diffractive optical elements on the incident side. The present invention is not limited to this, and an inclined surface relief diffraction grating or a blazed diffraction grating can also be applied.

DESCRIPTION OF SYMBOLS 10 ... Image light, 50 ... Eye of observer, 100 ... Head mounted display, 100A ... Image display apparatus, 100B ... Image display apparatus, 102 ... 1st diffractive optical element, 103 ... 2nd diffractive optical element, 111, DESCRIPTION OF SYMBOLS 112 ... Image forming apparatus 131, 132 ... Temple, 200 ... Light guide plate, 201 ... Base film, 300 ... Resist, 500 ... Image forming apparatus, 501 ... Image display apparatus, 502 ... Projection optical system.

Claims (9)

A light guide;
A first diffractive optical element in which a plurality of convex portions extending in a first direction along one surface of the light guide are arranged apart from each other along a second direction intersecting the first direction; Have
An optical device, wherein a refractive index of each of the plurality of convex portions of the first diffractive optical element is higher than a refractive index of the light guide. The optical device according to claim 1.
The optical device, wherein the first diffractive optical element is provided corresponding to a light incident portion of the light guide. The optical device according to claim 1.
The optical device, wherein the first diffractive optical element is provided corresponding to a light emitting portion of the light guide. The optical device according to claim 1.
A second diffractive optical element in which a plurality of convex portions extending in a third direction along one surface of the light guide are arranged apart from each other along a fourth direction intersecting the third direction. Have
The first diffractive optical element is provided corresponding to a light incident part of the light guide,
The optical device, wherein the second diffractive optical element is provided corresponding to a light emitting portion of the light guide. The optical device according to any one of claims 1 to 4,
An underlayer is provided between the light guide and the first diffractive optical element;
The refractive index of each of the plurality of convex portions of the first diffractive optical element is higher than the refractive indexes of the light guide and the base layer. The optical device according to claim 5.
The optical device, wherein the refractive index of the underlayer is the same as that of the light guide. The optical device according to any one of claims 1 to 6,
The first diffractive optical element is a surface relief type diffraction grating;
An optical device characterized by that.   An electronic apparatus comprising the optical device according to claim 1 and an image forming apparatus that emits image light. The electronic device according to claim 8,
The electronic apparatus is characterized in that the image light is TE polarized light.

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