Apparatus for manufacturing hologram optical element, and method for manufacturing hologram optical element

Abstract

This device for manufacturing a hologram optical element is provided with: a laser light source (1); a splitting mirror (4) that splits laser light (L1) emitted from the laser light source (1) into laser light (L2) and laser light (L2); a mirror (6) that reflects the laser light (L2); a light guide plate (20) that includes a hologram optical element (11a); a coupler (10a) that causes laser light (L3) to be incident on the light guide plate (20); and a displacement unit that displaces the irradiation position of the laser light (L3).

Description

Hologram optical element manufacturing apparatus and method for manufacturing hologram optical element

[0001] The present disclosure relates to a manufacturing apparatus for a hologram optical element and a manufacturing method for a hologram optical element.

[0002] Conventionally, there have been known manufacturing apparatuses for manufacturing holographic optical elements. For example, in Patent Document 1, a light beam emitted from a laser light source is split into S-polarized light and P-polarized light by a polarizing beam splitter, and these lights are irradiated onto a recording medium to create a volume hologram (holographic optical element).

[0003] Japanese Patent Application Laid-Open No. 2021-12249

[0004] In conventional hologram manufacturing equipment, when a hologram light guide plate is exposed, the exposure light must be irradiated onto the hologram light guide plate at an angle that matches the angle of the light propagating within the hologram light guide plate. Typically, a coupler is used to adjust the angle of the exposure light relative to the hologram light guide plate. Typically, the coupler is positioned so that it covers the entire surface of the hologram light guide plate during exposure. Therefore, as the hologram light guide plate becomes larger, the coupler also becomes larger. Typically, the size and weight of a coupler are proportional to the cube of its length, so manufacturing a large hologram light guide plate requires a very heavy and large coupler. Furthermore, to accurately irradiate the hologram light guide plate with the exposure light, the space between the coupler and the hologram light guide plate must be filled with bubble-free matching oil. As the coupler becomes larger, pre-exposure setup becomes extremely difficult. Furthermore, manufacturing a large coupler requires a large glass block that is free of distortion and cracks, making manufacturing more difficult.

[0005] Therefore, an object of the present disclosure is to provide a method for manufacturing a holographic optical element that can reduce the size of the coupler.

[0006] In order to achieve the above object, an apparatus for manufacturing a holographic optical element according to one embodiment of the present disclosure includes a laser light source, a half mirror that splits light irradiated from the laser light source into a first light and a second light, a mirror that is created based on the shape of a product in which the holographic optical element is used and that reflects the first light, a light guide plate including the holographic optical element, a coupler that inputs the second light to the light guide plate so that the second light propagates while being totally reflected within the light guide plate, and a displacement unit that displaces the irradiation position of the second light relative to the coupler.

[0007] According to the present disclosure, the size of the coupler can be reduced.

[0008] A side view of an apparatus for manufacturing a hologram optical element according to an embodiment. A top view of an apparatus for manufacturing a hologram optical element according to an embodiment. A graph showing a light quantity distribution in an apparatus for manufacturing a hologram optical element according to an embodiment. A diagram for explaining light quantity distribution control in an apparatus for manufacturing a hologram optical element according to an embodiment. A cross-sectional view of a light guide plate using a volume hologram manufactured by an apparatus for manufacturing a hologram optical element according to an embodiment. A cross-sectional view showing a state of light propagation in a volume hologram according to an embodiment and a comparative example. A side view of an apparatus for manufacturing a hologram optical element according to a reference example.

[0009] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The following description of the preferred embodiments is merely exemplary in nature and is not intended to limit the present invention, its applications, or its uses. In the following description, the same parts will be designated by the same reference numerals, and detailed description will be omitted as appropriate.

[0010] The volume hologram used in this disclosure differs from a two-dimensional diffraction grating with fine periodic irregularities on its surface in that it records a three-dimensional sinusoidal refractive index distribution within a volume. By controlling the direction and period of this sinusoidal wave and the amplitude of the refractive index difference, it is possible to control the light distribution of the volume hologram.

[0011] (Embodiment) (Overall configuration of a manufacturing apparatus for a hologram optical element) Fig. 1 is a side view of a manufacturing apparatus for a hologram optical element according to this embodiment. Fig. 2 is a top view of the manufacturing apparatus for a hologram optical element according to this embodiment. In Fig. 1, the irradiation direction of the laser beam is the X direction, the thickness direction (vertical direction) of the volume hologram is the Y direction, and the direction perpendicular to the X and Y directions is the Z direction.

[0012] As shown in FIG. 1, the manufacturing apparatus for a holographic optical element according to this embodiment includes a laser light source 1, a condenser lens 2, a collimator lens 3, a branching mirror 4 (half mirror), a filter 5, mirrors 6 to 8, a slider 9 (displacement unit), couplers 10a and 10b, and a volume hologram 11 (light guide plate, holographic optical element).

[0013] The laser light source 1 is a light source that irradiates the condenser lens 2 with laser light L1. The laser light source 1 is a highly coherent laser light source. Therefore, after the laser light L1 is split into laser light L2 (object light) and laser light L3 (reference light) by a splitting mirror 4 (described later), even if the laser light L2 and L3 are separated by the same optical path length, they can still cause optical interference with each other. Furthermore, the laser light L1 has linear polarization characteristics, and if the polarization ratio is insufficient, the polarization direction may be controlled by inserting a wave plate or a polarizing plate.

[0014] The condenser lens 2 is a lens that condenses the laser light L1 emitted by the laser light source 1. The laser light L1 condensed by the condenser lens 2 becomes expanded light after being condensed (see Figure 1). The laser light L1 has a distribution generally known as a Gaussian distribution, in which the amount of light is high at the center and decreases toward the periphery. Since it is desirable for the light required for interference to have a nearly constant intensity distribution within the plane, the expanded central light is used, and other light is blocked and not used. Unnecessary light is omitted in Figure 1, and only the light beam that is used is shown.

[0015] The collimating lens 3 is a lens that converts the laser light L1 diffused by the condensing lens 2 into parallel light. Specifically, the collimating lens 3 is disposed so that its focal length f matches the distance to the focal point of the laser light L1 condensed by the condensing lens 2.

[0016] The splitting mirror 4 splits the laser light L1, which has been converted into parallel light by the collimator lens 3, into two light beams (laser light L2 (object light) and laser light L3 (reference light)).

[0017] The filter 5 is an element that controls the transmittance density of the transmitted laser light L2, and can be realized by means of changing the thickness of chrome plating, for example.

[0018] The mirror 6 reflects the laser light L2 that has passed through the filter 5. Specifically, the mirror 6 is made based on the shape of the product in which the volume hologram 11 is used. For example, if the volume hologram 11 is used as a light guide plate that projects an image onto the windshield of a vehicle, the mirror 6 is made in the shape of the vehicle's windshield. The light (laser light L2) reflected by the mirror 6 is irradiated onto the volume hologram 11. The transmittance density distribution of the filter 5 is set so that the intensity distribution of the laser light L2 irradiated onto the volume hologram 11 matches the light intensity emitted from the volume hologram 11 when light is introduced into the volume hologram 11 and reconstructed.

[0019] The mirror 7 is a mirror that reflects the laser light L3 incident from the branching mirror 4 to the mirror 8.

[0020] The mirror 8 is a mirror that reflects the laser light L3 incident from the mirror 7 onto the volume hologram 11. The mirror 8 is provided with a slider 9 that moves the mirror 8. In this embodiment, the slider 9 moves the mirror 8 in the X direction.

[0021] The coupler 10a is disposed on the surface of the volume hologram 11 that faces the mirror 8. The prism surface of the coupler 10a is shaped to match the angle of incidence of the laser light L3 (reference light).

[0022] The coupler 10b is disposed on the surface of the volume hologram 11 opposite the mirror 8. The prism surface of the coupler 10b is shaped to match the angle of incidence of the laser light L3 (reference light). The coupler 10b functions to extract the laser light L3 that propagates by total reflection within the volume hologram 11, thereby preventing the laser light L3 from being trapped within the volume hologram 11.

[0023] As explained above, the laser beams L2 and L3 incident on the volume hologram 11 overlap within the volume hologram 11, generating interference fringes in the volume hologram 11. The interference fringes are bright and dark sinusoidal fringes, and the volume hologram 11, which is made of a photosensitive material, is exposed to light in the bright areas and not in the dark areas. The exposed areas experience a refractive index change according to the amount of light energy. Therefore, a sinusoidal refractive index distribution is generated based on the sinusoidal interference fringes. The light irradiation is stopped when the required refractive index difference is generated, and then the volume hologram 11 is irradiated with light of a specific wavelength, such as ultraviolet light, to fix the refractive index distribution, thereby forming a holographic optical element.

[0024] The holographic optical element (volume hologram 11) records a refractive index difference based on the interference fringes of the two-beam interference of laser beams L2 and L3, resulting in a diffraction phenomenon based on these two beams. By irradiating light at the same direction and angle as laser beam L3 (reference beam), diffracted light is generated in the same direction and angle as laser beam L2 (object beam). When light is incident at the same direction and angle as laser beam L2 (object beam), laser beam L3 (reference beam) is generated. On the other hand, when laser beam L3 (reference beam) is incident from the opposite direction, laser beam L2 (object beam) generates light diffracted toward mirror 6. Therefore, when this light is reflected by mirror 6, the resulting light becomes parallel. This parallel beam can be used to construct a virtual image optical system, and parallel light can be easily obtained after reflection even when passing through a complex design shape by using a holographic light guide plate.

[0025] (Regarding the brightness distribution of the volume hologram) Figure 3 is a graph showing the light intensity distribution in the manufacturing apparatus for a hologram optical element according to the embodiment. Specifically, Figure 3(a) is a graph showing the light intensity distribution of the laser light L3 after passing through the branching mirror 4, Figure 3(b) is a graph showing the light intensity distribution of the laser light L2 after passing through the filter 5, and Figure 3(c) is a graph showing the light intensity distribution of the laser light irradiated onto the volume hologram 11. Note that in Figure 3(c), the laser lights L2 and L3 overlap in the central region.

[0026] 1, the laser beams L2 and L3 overlap on the volume hologram 11. As a result, interference fringes are recorded (formed) on the volume hologram 11.

[0027] As described above, the transmittance density distribution of the filter 5 is set so that the intensity distribution of the laser light L2 irradiated onto the volume hologram 11 matches the light intensity emitted from the volume hologram 11 when light is introduced into the volume hologram 11 and reconstructed. As a result, the interference fringes formed on the volume hologram 11 are configured so that the diffraction efficiency gradually increases from the left side to the right side of Fig. 1. Therefore, as shown in Fig. 3(c), a diffraction grating is recorded in the volume hologram 11 in the central region where the laser light L2 and L3 overlap.

[0028] Any method for setting the transmittance density distribution of the filter 5 may be used as long as it results in a light amount distribution (intensity distribution) of the laser light L2 transmitted through the filter 5. For example, the filter 5 may be composed of a quarter-wave plate 51, a phase modulation element 52, and a polarizing plate 53.

[0029] Specifically, the branching mirror 4 is configured with a polarizing prism splitter or the like, and transmits linearly polarized light (laser light L2) of the laser light L1 in a first polarization direction and reflects linearly polarized light (laser light L3) in a second polarization direction. The laser light L2 that has passed through the branching mirror 4 is then converted into circularly polarized light by a quarter-wave plate 51. The wavelength of the laser light L2 is then modulated by a phase modulation element 52. At this time, the laser light L2 is modulated to different wavelengths depending on the region where it enters the phase modulation element 52.

[0030] FIG. 4 is a diagram illustrating light intensity distribution control in the hologram optical element manufacturing apparatus according to this embodiment. In FIG. 4, the phase modulation element 52 has a gradually increasing phase modulation amount from region S1 at the top of the drawing to region S3 at the bottom. For example, the upper left region S1 of the phase modulation element 52 does not modulate the wavelength of the incident light, so the incident laser light L2 (L21) is reflected as circularly polarized light. Furthermore, the center region S2 of the phase modulation element 52 modulates the wavelength of the incident light by 5 / 8 wavelength, so the incident laser light L2 (L22) is converted to elliptically polarized light. Furthermore, the center region S3 of the phase modulation element 52 modulates the wavelength of the incident light by 1 / 2 wavelength, so the incident laser light L2 (L23) is converted to linearly polarized light.

[0031] The laser light L3 then passes through the polarizing plate 53. The polarizing plate 53 transmits light of the second polarization direction. Therefore, the laser light L2 (L23) reflected by a region of the phase modulation element 52 with a large amount of phase modulation (e.g., region S3) experiences less reduction in light intensity due to the polarizing plate 53, while the laser light L2 (L21) reflected by a region of the phase modulation element 52 with a small amount of phase modulation (e.g., region S1) experiences more reduction in light intensity due to the polarizing plate 53. For example, the amount of laser light L21 is about 50% of the amount of laser light L2, and the amount of laser light L23 is about 66% of the amount of laser light L2.

[0032] Fig. 5 is a cross-sectional view of a light guide plate using a volume hologram manufactured by the holographic optical element manufacturing apparatus according to this embodiment. Specifically, Fig. 5(a) is a cross-sectional view of a light guide plate using volume hologram 11 according to this embodiment, Fig. 5(b) is a graph showing the front luminance in Fig. 5(a), Fig. 5(c) is a cross-sectional view of a light guide plate 20' using volume hologram 11' in which the diffraction efficiency of interference fringes is constant, and Fig. 5(d) is a graph showing the front luminance in Fig. 5(c).

[0033] As shown in FIG. 5A , the light guide plate 20 according to this embodiment is composed of a volume hologram 11. The volume hologram 11 has a holographic optical element 11a disposed between transparent substrates 21 and 22. The transparent substrates 21 and 22 may be flat or curved, and their sizes may vary from the order of a few square millimeters to the order of several hundred square millimeters depending on the application. The light guide plate 20 is generally used in applications where light is extracted from a surface by arranging concave or convex prism patterns on the front and back surfaces of a transparent substrate or by incorporating a diffusing material. However, a holographic light guide plate using a holographic optical element is highly effective in extracting light in a specific direction and has desirable characteristics for applications requiring light distribution control.

[0034] 5(c), the light guide plate 20' is composed of a volume hologram 11'. The volume hologram 11' has a hologram optical element 11b disposed between transparent substrates 21 and 22. The volume hologram 11' (hologram optical element 11b) is formed so that the diffraction efficiency of the interference fringes is constant.

[0035] 5(a) and 5(c), a light source is disposed on the upper side of the light guide plate 20 (20') in the drawing (transparent substrate 21 side), and light emitted from the light source is totally reflected within the transparent base materials 21 and 22 and propagates to the right side of the drawing. The light is then emitted to the upper side of the drawing in accordance with the interference fringes recorded in the hologram optical element 11a (11b) disposed in the light guide plate 20 (20'). Note that even if the light source is disposed on the lower side of the light guide plate 20 (20') in the drawing (transparent substrate 22 side), the light is still emitted to the upper side of the drawing.

[0036] Here, the amount of light propagating through the light guide plate 20 (20') decreases (attenuates) as it moves away from the light source. Therefore, if a light guide plate 20' using a volume hologram 11' (hologram optical element 11b) with constant interference fringe diffraction efficiency is used, the luminance distribution will be such that the amount of light gradually decreases from the left side to the right side of the drawing, making it impossible to maintain a constant front luminance of the light guide plate (see FIG. 5(d)). In contrast, the volume hologram 11 (hologram optical element 11a) according to this embodiment is configured such that the diffraction efficiency gradually increases from the left side to the right side of the drawing (see FIG. 3(c)), making it possible to maintain a constant front luminance of the light guide plate 20 (see FIG. 5(b)).

[0037] 6A is a cross-sectional view showing the state of light propagation in a volume hologram according to a comparative example, and FIG. 6B is a cross-sectional view showing the state of light propagation in a volume hologram according to this embodiment. In FIG. 6, a volume hologram 11 has a holographic optical element 11a disposed between transparent substrates 21 and 22.

[0038] 6(a) and 6(b), during exposure, the laser light L3 propagates while being totally reflected within the volume hologram 11. In Figures 6(a) and 6(b), the reflection pitch during total reflection within the volume hologram 11 (the distance in the X direction from when a specific light is reflected by the bottom surface of the volume hologram 11 until when it is reflected again by the bottom surface of the volume hologram 11) is denoted by p.

[0039] In FIG. 6A, the width d of the laser light L3 (reference light) in the X direction is equal to or greater than p / 2 relative to the reflection pitch p. In this case, as shown in FIG. 6A, the laser light L3 propagating through the light guide plate 20 overlaps within the hologram optical element 11a. For example, at point A within the volume hologram 11, light incident from the upper left and light incident from the lower left overlap. This causes interference between these two beams. As a result, interference fringes parallel to the X direction are generated in the volume hologram 11 (hologram optical element 11a), recording a refractive index distribution. When a video signal is input to the volume hologram 11, this refractive index distribution reflects the light at a reflection angle different from the reflection at the air interface between the transparent substrates 21 and 22 in the volume hologram 11. This results in ghost images in the video signal, degrading the video quality.

[0040] 6B, in this embodiment, the width d of the laser light L3 (reference light) in the X direction is set to be less than p / 2 where p is the reflection pitch. This prevents optical interference between the lights propagating through the volume hologram 11 on the hologram optical element 11a, and makes it possible to prevent interference fringes parallel to the X direction from occurring within the hologram optical element 11a.

[0041] 7 is a side view of a hologram optical element manufacturing apparatus according to a reference example. Compared to the hologram optical element manufacturing apparatus according to the present embodiment, the hologram optical element manufacturing apparatus according to the reference example does not include slider 9, and couplers 10a' and 10b' are arranged to cover the entire surface of one side of volume hologram 11. In this way, in a typical hologram optical element manufacturing apparatus, couplers 10a' and 10b' are arranged to cover the entire surface of one side of volume hologram 11, thereby adjusting the angle of laser light L3 (reference light) with respect to the hologram light guide plate.

[0042] In contrast, this embodiment is provided with a slider 9 that moves the mirror 8 in the X direction. In the example of Fig. 6(b), only about half of the volume hologram 11 can be formed during exposure. Therefore, by moving the mirror 8 with the slider 9, the position of the laser light L3 (reference light) within the volume hologram 11 can be shifted, thereby exposing the entire volume hologram.

[0043] For example, the width d of the laser beam L3 in the X direction may be set to p / n, where n is an integer equal to or greater than 2. In this case, by moving the mirror 8 with the slider 9, the laser beam L3 in the hologram optical element 11a is exposed while being moved by p / n increments, thereby making it possible to cause two-beam interference between the laser beams L2 and L3 (object beam, reference beam).

[0044] Furthermore, the width of the laser light L3 (reference light) can be reduced by propagating the laser light L3 within the volume hologram 11, which allows the couplers 10a and 10b to be smaller. This eliminates the need to arrange the couplers so as to cover the entire surface of one side of the volume hologram, allowing the size of the couplers to be reduced.

[0045] The above-described configuration includes a laser light source 1, a branching mirror 4 (half mirror) that branches laser light L1 emitted from the laser light source 1 into laser light L2 (first light, object light) and laser light L3 (second light, reference light), a mirror 6 that is created based on the shape of a product in which the hologram optical element 11a is used and that reflects the laser light L2, a light guide plate 20 (volume hologram 11) that includes the hologram optical element 11a, a coupler 10a that causes the laser light L3 to enter the light guide plate 20 so that the laser light L3 propagates while being totally reflected within the light guide plate 20, and a slider 9 (displacement unit) that displaces the irradiation position of the laser light L3 relative to the coupler 10a. As a result, the laser light L3 propagates within the light guide plate 20 (volume hologram 11) while being totally reflected, and the hologram optical element 11a can be exposed by displacing the irradiation position of the laser light L3 relative to the coupler 10a with the slider 9. Therefore, it is no longer necessary to arrange the coupler so as to cover the entire surface of one side of the volume hologram as in the reference example, and it is no longer necessary to form the coupler 10a so as to cover the exposure range of the hologram optical element 11a, so the size of the coupler 10a can be reduced.

[0046] Other Embodiments The filter 5 may be made of a vapor-deposited film having a fixed concentration.

[0047] Furthermore, the mirror 6 may have the function of the filter 5. That is, a reflective film of a predetermined density may be formed directly on the reflective surface of the mirror 6.

[0048] Furthermore, in the above embodiment, the transparent substrates 21 and 22 of the volume hologram 11 are illustrated as flat plates, but they may also be curved. In this case, it is only necessary that the laser light L3 does not leak from the volume hologram 11 when it is reflected by the volume hologram 11. In other words, as long as the laser light L3 is totally reflected within the volume hologram 11, the transparent substrates 21 and 22 may have any shape.

[0049] In the above embodiment, the coupler 10b is disposed on the surface of the volume hologram 11 opposite to the surface on which the coupler 10a is provided, but it may be disposed on the same surface as the coupler 10a.

[0050] Furthermore, in the above embodiment, the movement of the irradiation position of the laser light L3 is realized by moving the mirror 8 with the slider 9, but the method of moving the irradiation position of the laser light L3 is not limited to this. For example, another mirror may be moved by a slider or the like. Furthermore, the volume hologram 11 and the couplers 10a and 10b may be moved by a moving means such as a slider. Furthermore, a shutter or the like may be provided to limit the irradiation range of the laser light L3, and the irradiation position of the laser light L3 may be moved by opening and closing the shutter. In this case, the laser light L3 is set to irradiate the entire coupler 10a.

[0051] In the above embodiment, the coupler 10a may be formed to have a width approximately equal to that of the laser light L3 (reference light). In this case, the coupler 10a may be moved relative to the volume hologram 11 by a slider or the like.

[0052] In the above embodiment, the couplers 10a and 10b may be of approximately the same size, or the coupler 10a may be larger than the coupler 10b, or the coupler 10b may be larger than the coupler 10a. Also, the coupler 10b may be arranged on the same surface as the coupler 10a.

[0053] Furthermore, in the above embodiment, the mirror 6 may be created based on the shape of a product in which the volume hologram 11 is used. That is, the mirror 6 may be created taking into account the shape of the product in which the volume hologram 11 is used. For example, the mirror 6 may have a shape similar to the shape of the product in which the volume hologram 11 is used. In this case, by adjusting the position of the volume hologram 11, the volume hologram 11 can be exposed according to the shape of the product in which the volume hologram 11 is used. Furthermore, for example, the mirror 6 may be created taking into account the manufacturing variation of the product in which the volume hologram 11 is used. In this case, the mirror 6 is designed based on the median of the manufacturing variation of the product in which the volume hologram 11 is used. Furthermore, for example, the mirror 6 may be created to match the shape of the projection surface onto which the light guide plate 20 projects light in a product in which the volume hologram 11 is used. Furthermore, for example, the mirror 6 may be created to match the shape of the projection surface onto which the light guide plate 20 projects light in a product in which the volume hologram 11 is used. Although the mirror 6 in FIG. 1 etc. is formed with a curved surface, the present invention is not limited to this and may be formed with only a flat surface.

[0054] The hologram optical element manufacturing apparatus of the present disclosure can be applied to hologram optical element systems such as projectors, bed-mounted displays, and head-up displays.

[0055] REFERENCE SIGNS LIST 1 laser light source 5 filter 8 mirror 9 slider (displacement portion) 10a, 10b coupler 11 volume hologram 11a hologram optical element 20 light guide plate 21, 22 transparent substrate

Claims

1. A hologram manufacturing device comprising: a laser light source; a half mirror that splits light irradiated from the laser light source into a first light and a second light; a mirror that is produced based on the shape of a product in which a hologram optical element is used and reflects the first light; a light guide plate including the hologram optical element; a coupler that makes the second light incident on the light guide plate so that the second light propagates while being totally reflected within the light guide plate; and a displacement unit that displaces the irradiation position of the second light relative to the coupler.

2. The hologram manufacturing apparatus according to claim 1, further comprising a filter for giving said first light an illuminance distribution to be recorded in said hologram optical element.

3. The hologram manufacturing apparatus according to claim 1, wherein the displacement section includes a slider that moves a mirror provided to reflect the second light and make the second light incident on the coupler.

4. The hologram manufacturing apparatus according to claim 1, wherein the displacement section includes a moving means for moving the volume hologram.

5. The hologram manufacturing apparatus according to claim 1, wherein, when a reflection pitch of the second light within the light guide plate is p, a width of the second light within the light guide plate is p / 2 or less.

6. A hologram manufacturing apparatus as described in claim 1, wherein, when the reflection pitch of the second light within the light guide plate is p and n is an integer greater than or equal to 2, the displacement amount of the irradiation position of the second light by the displacement section is p / n.

7. A hologram manufacturing method comprising: a step of a half mirror branching light irradiated from a laser light source; a step of a mirror created based on the shape of a product in which a hologram optical element is used reflecting a first light; a step of a coupler causing the second light to enter the light guide plate including the hologram optical element so that the second light propagates while being totally reflected within the light guide plate; and a step of a displacement unit displacing the irradiation position of the second light relative to the coupler.

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