Patent Publication Number: US-11656465-B2

Title: Display device, optical element, and method of producing optical element

Description:
This is a Division of application Ser. No. 16/827,873 filed Mar. 24, 2020, which in turn claims the benefit of JP Application No. 2019-056555 filed Mar. 25, 2019. The disclosure of the prior applications is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a display device, an optical element, and a method of producing an optical element. 
     2. Related Art 
     As a display device including a diffraction element such as a holographic element, a display device has been proposed in which imaging light emitted from an imaging light generating device is deflected toward an eye of an observer by a diffraction element. Interference fringes are optimized in the diffraction element to obtain an optimum diffraction angle and optimum diffraction efficiency at a specific wavelength. However, the imaging light has a predetermined spectral width centered at a specific wavelength, and thus, light with a peripheral wavelength deviated from the specific wavelength may cause a decrease in resolution of an image. Thus, a display device has been proposed in which imaging light emitted from the imaging light generating device is directed by a first diffraction element of the reflective type toward a second diffraction element disposed in front of the first diffraction element and in which the second diffraction element deflects, toward the eye of the observer, the imaging light emitted from the first diffraction element. According to the configuration, the first diffraction element can compensate for light having a peripheral wavelength and cancel a color aberration, and a decrease in resolution of an image due to the light having the peripheral wavelength deviated from a specific wavelength can be suppressed (for example, see JP-A-2017-167181 described below). 
     It is conceivable to provide optical power required for wavelength compensation by using diffractive power of the first diffraction element and refractive power of a lens member having optical power and provided on one surface side of the first diffraction element. However, upon exposure of interference fringes of the first diffraction element, refraction at the curved surface of the lens member greatly affects the exposure. As a result, the exposure is performed with a wavefront that is different from an original wavefront. The interference fringes formed during the exposure affected by refraction by the lens member result in blur in the imaging light emitted to an exit pupil, and as a result, a problem of reduced resolution arises. 
     It is also conceivable to use plastics as a material of the lens member to reduce the weight of the lens member. However, when a plastic lens is used, the diffraction element is expanded or contracted during exposure, and as a result, desired diffraction performance cannot be achieved. 
     SUMMARY 
     In order to solve the above-described problem, a display device according to an aspect of the present disclosure includes a first optical unit having positive power, a second optical unit including a first diffraction element and having positive power, a third optical unit having positive power, and a fourth optical unit including a second diffraction element and having positive power, the first to fourth optical units are provided along an optical path of imaging light emitted from an imaging light generating device, the second optical unit further includes a first member provided at one surface side of the first diffraction element, and a second member provided on a side of the first member opposite to a side where the first diffraction element is located, the first member is transmissive and has an elastic modulus of 50 GPa or greater, and the second member is transmissive and has optical power. 
     In the display device according to the aspect, the first member may be formed from glass. 
     In the display device according to the aspect, a surface of the first member opposite to the first diffraction element may be a flat surface. 
     In the display device according to the aspect, a gap may be formed between the first member and the second member. 
     The display device according to the aspect may further include a spacer member forming a gap between the first member and the second member. 
     In the display device according to the aspect, the spacer member may be provided integrally with the second member. 
     In the display device according to the aspect, the second member may include a light shielding member provided at a surface of the second member facing the first member. 
     An optical element according to an aspect of the present disclosure includes a first diffraction element, a first member provided at one surface side of the first diffraction element, and a second member provided on a side of the first member opposite to a side where the first diffraction element is located, and the first member is transmissive and has an elastic modulus of 50 GPa or greater, and the second member is transmissive and has optical power. 
     In the optical element according to the aspect, the first member may be formed from glass. 
     In the optical element according to the aspect, a surface of the first member opposite to the first diffraction element may be a flat surface. 
     In the optical element according to the aspect, a gap may be formed between the first member and the second member. 
     The optical element according to the aspect may further include a spacer member forming a gap between the first member and the second member. 
     In the optical element according to the aspect, the spacer member may be provided integrally with the second member. 
     In the optical element according to the aspect, the second member may include a light shielding member provided at a surface of the second member facing the first member. 
     A method of producing an optical element according to an aspect of the present disclosure includes providing a hologram material layer for forming a hologram element on a first member that is transmissive and has an elastic modulus of 50 GPa or greater, performing interference exposure by irradiating, with object light and reference light, the hologram material layer on the first member, and providing a second member on a side, of the hologram element formed on the first member by the interference exposure, opposite to a side where the first member is located. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is an external view of a display device according to a first exemplary embodiment. 
         FIG.  2    is a schematic diagram of an optical system of the display device. 
         FIG.  3 A  is a schematic diagram of interference fringes of a diffraction element. 
         FIG.  3 B  is a schematic diagram of a different embodiment of interference fringes of a diffraction element. 
         FIG.  4    is a cross-sectional view illustrating a general configuration of a second optical unit. 
         FIG.  5 A  is a diagram illustrating a production step of the second optical unit. 
         FIG.  5 B  is a diagram illustrating a production step of the second optical unit. 
         FIG.  6    is a diagram illustrating an exposure step in a comparative example. 
         FIG.  7    is a schematic diagram of a diffraction characteristic in a volume hologram. 
         FIG.  8    is a schematic diagram of light emitted from a second diffraction element when diffraction angles are the same. 
         FIG.  9 A  is a schematic diagram when diffraction angles of a first diffraction element and the second diffraction element are set to a small angle. 
         FIG.  9 B  is a schematic diagram when the diffraction angles of the first and second diffraction elements are set to a large angle. 
         FIG.  10    is a diagram illustrating a relationship between the diffraction angles of the first diffraction element and the second diffraction element. 
         FIG.  11    is a schematic diagram of light emitted from the second diffraction element when the diffraction angles are different. 
         FIG.  12 A  is a diagram illustrating a first effect of a correction optical system. 
         FIG.  12 B  is a diagram illustrating a second effect of the correction optical system. 
         FIG.  12 C  is a diagram illustrating a third effect of the correction optical system. 
         FIG.  13    is an enlarged view of a prism. 
         FIG.  14    is a diagram schematically illustrating a light beam diagram of an optical system. 
         FIG.  15    is a cross-sectional view illustrating a general configuration of a second optical unit in a second exemplary embodiment. 
         FIG.  16    is a cross-sectional view illustrating a general configuration of a second optical unit in a third exemplary embodiment. 
         FIG.  17    is a cross-sectional view illustrating a configuration according to a modification example of the third exemplary embodiment. 
         FIG.  18    is a cross-sectional view illustrating a general configuration of a second optical unit in a fourth exemplary embodiment. 
         FIG.  19    is a diagram illustrating an exposure step of the first diffraction element. 
         FIG.  20    is a diagram illustrating an exposure step according to a modification example. 
         FIG.  21 A  is a cross-sectional view illustrating a general configuration of a second optical unit in a fifth exemplary embodiment. 
         FIG.  21 B  illustrates a shape of a light shielding film in a plan view. 
         FIG.  22    is a light beam diagram between a first diffraction element and a second diffraction element in an optical system in a sixth exemplary embodiment. 
         FIG.  23    is a schematic diagram of light emitted from the second diffraction element. 
         FIG.  24    is a schematic diagram illustrating a state in which the light illustrated in  FIG.  23    is incident on an eye. 
         FIG.  25    is a configuration diagram of a display device according to a modification example. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     First Exemplary Embodiment 
     Exemplary embodiments of the present disclosure will be described below with reference to the accompanying drawings. Note that, in each of the drawings below, to make each of layers and each of members a recognizable size, each of the layers and each of the members are illustrated to be different from an actual scale and an actual angle. 
       FIG.  1    is an external view illustrating an aspect of a visual appearance of a display device  100  according to the present exemplary embodiment.  FIG.  2    is a schematic diagram illustrating one aspect of an optical system  10  of the display device  100  illustrated in  FIG.  1   . Note that, as necessary in the drawings used in the following description, a front and rear direction of an observer wearing the display device is a direction along a Z axis, the front of the observer wearing the display device is a front side Z 1  as one side in the front and rear direction, and the rear of the observer wearing the display device is a rear side Z 2  as the other side in the front and rear direction. A left and right direction with respect to the observer wearing the display device is defined as a direction along an X axis, one side in the left and right direction corresponding to the right direction of the observer wearing the display device is defined as a front side X 1 , and the other side in the left and right direction corresponding to the left direction of the observer wearing the display device is defined as a left side X 2 . An up and down direction with respect to the observer wearing the display device is defined as a direction along a Y axis, one side in the up and down direction corresponding to the up direction of the observer wearing the display device is defined as an up side Y 1 , and the other side in the up and down direction corresponding to the down direction of the observer wearing the display device is defined as a down side Y 2 . 
     The display device  100  illustrated in  FIG.  1    is a head-mounted display device, and includes a right-eye optical system  10   a  that causes imaging light L 0   a  to be incident on a right eye Ea and a left-eye optical system  10   b  that causes imaging light L 0   b  to be incident on a left eye Eb. For example, the display device  100  is formed in a shape like glasses. Specifically, the display device  100  further includes a housing  90  that holds the right-eye optical system  10   a  and the left-eye optical system  10   b . The display device  100  is mounted to the head of the observer by the housing  90 . 
     In the display device  100 , the housing  90  includes a frame  91 , a temple  92   a  provided on the right side of the frame  91  and locked on the right ear of the observer, and a temple  92   b  provided on the left side of the frame  91  and locked on the left ear of the observer. The frame  91  includes storage spaces  91   s  on both sides of the frame  91 , and the storage spaces  91   s  house components such as an imaging light projecting device that constitute the optical system  10  described below. The temples  92   a  and  92   b  are foldably coupled to the frame  91  by hinges  95 . 
     The right-eye optical system  10   a  and the left-eye optical system  10   b  have the same basic configuration. Therefore, the right-eye optical system  10   a  and the left-eye optical system  10   b  will be described as the optical system  10  without distinction in the description below. 
     Next, a basic configuration of the optical system  10  of the display device  100  will be described with reference to  FIG.  2   . 
     As illustrated in  FIG.  2   , in the optical system  10  in the present exemplary embodiment, a first optical unit L 10  having positive power, a second optical unit L 20  having positive power, a third optical unit L 30  having positive power, and a fourth optical unit L 40  having positive power are disposed along an optical path of imaging light L 0  emitted from an imaging light generating device  31 . 
     In the present exemplary embodiment, the first optical unit L 10  having positive power is constituted of a mirror  40  and a projection optical system  32 . The second optical unit (optical element) L 20  having positive power includes a reflection-type first diffraction element  50  and a correction optical system  45 . The third optical unit L 30  having positive power is constituted of a light-guiding system  60 . The fourth optical unit L 40  having positive power is constituted of a reflection-type second diffraction element  70 . In the present exemplary embodiment, the first diffraction element  50  and the second diffraction element  70  are reflection-type diffraction elements. 
     In the optical system  10 , with focus on a traveling direction of the imaging light L 0 , the imaging light generating device  31  emits the imaging light L 0  toward the projection optical system  32 , and the projection optical system  32  emits the incident imaging light L 0  toward the mirror  40 . The mirror  40  includes a reflection surface  40   a  and reflects the imaging light L 0  toward the first diffraction element  50 . The imaging light L 0  reflected by the reflection surface  40   a  of the mirror  40  passes through the correction optical system  45  and is incident on the first diffraction element  50 . The imaging light L 0  diffracted by the first diffraction element  50  is emitted toward the light-guiding system  60 . The light-guiding system  60  emits the incident imaging light L 0  toward the second diffraction element  70 , and the second diffraction element  70  emits the incident imaging light L 0  toward the eye E of the observer. 
     In the present exemplary embodiment, the imaging light generating device  31  generates imaging light L 0 . 
     An aspect may be adopted where the imaging light generating device  31  includes a display panel  310  such as an organic electroluminescent display element. The aspect can provide a small-sized display device  100  capable of displaying a high-quality image. An aspect may be adopted where the imaging light generating device  31  includes an illumination light source (not illustrated) and a display panel  310  such as a liquid crystal display element that modulates illumination light emitted from the illumination light source. The aspect allows the illumination light source to be selected. Thus, the aspect has an advantage of increasing a degree of flexibility in a wavelength characteristic of the imaging light L 0 . Herein, an aspect may be adopted where the imaging light generating device  31  includes one display panel  310  that enables color display. Another aspect may be adopted where the imaging light generating device  31  includes a plurality of display panels  310  corresponding to respective colors and a synthesis optical system that synthesizes imaging light in respective colors emitted from the plurality of display panels  310 . Furthermore, an aspect may be adopted where the imaging light generating device  31  modulates laser light using a micro-mirror device. In this case, imaging light is generated by scanning the laser light by driving the micro-mirror device. 
     The projection optical system  32  is an optical system that projects the imaging light L 0  generated by the imaging light generating device  31 , and is constituted of a first lens  301 , a second lens  302 , a third lens  303 , and a fourth lens  304 . The first lens  301 , the second lens  302 , the third lens  303 , and the fourth lens  304  are constituted of a free-form lens or a rotationally symmetrical lens. The projection optical system  32  may be an eccentric optical system. In the example illustrated in  FIG.  2   , the number of lenses in the projection optical system  32  is four, but the number of lenses is not limited thereto. The projection optical system  32  may include five or more lenses. The lenses may be stuck together to form the projection optical system  32 . 
     The light-guiding system  60  includes a mirror  62  with a reflection surface  62   a  that is more recessed at the center than at peripheral portions. The light-guiding system  60  has positive power. The mirror  62  includes a reflection surface  62   a  inclined obliquely in the front and rear direction. The reflection surface  62   a  includes a spherical surface, an aspherical surface, a free-form surface, or the like. In the present exemplary embodiment, the mirror  62  is a total reflection mirror with the reflection surface  62   a  including a free-form surface. However, the mirror  62  may be a half mirror, and in this case, the range in which external light is visible can be widened. 
     Now, a configuration of the second optical unit L 20  including the first diffraction element  50  and a configuration of the fourth optical unit L 40  including the second diffraction element  70  will be described. 
     First, the configuration of the fourth optical unit L 40  will be described. In the following description, a configuration of the second diffraction element  70  constituting the fourth optical unit L 40  will be mainly described. 
       FIG.  3 A  is a schematic diagram of interference fringes  751  of the second diffraction element  70  illustrated in  FIG.  2   . As illustrated in  FIG.  3 A , the second diffraction element  70  is a partial reflection-type diffraction optical element constituted of a reflection-type volume hologram element. Thus, the second diffraction element  70  constitutes a partial transmissive reflective combiner. Therefore, external light is also incident on the eye E via the second diffraction element  70 , and thus the observer can recognize an image in which the imaging light L 0  formed by the imaging light generating device  31  and the external light (background) are superimposed on each other. 
     The second diffraction element  70  faces the eye E of the observer. An incident surface  71  of the second diffraction element  70  on which the imaging light L 0  is incident has a concave surface being recessed in a direction away from the eye E. In other words, the incident surface  71  has a shape having a central portion recessed and curved with respect to a peripheral portion in the incident direction of the imaging light L 0 . Thus, the imaging light L 0  can be efficiently condensed toward the eye E of the observer. 
     The second diffraction element  70  includes the interference fringes  751  with a pitch corresponding to a specific wavelength. The interference fringes  751  are recorded as a difference in refractive index and the like in a holographic photosensitive layer. The interference fringes  751  are inclined in one direction with respect to the incident surface  71  of the second diffraction element  70  so as to correspond to a specific incident angle. Therefore, the second diffraction element  70  diffracts and deflects the imaging light L 0  in a predetermined direction. The specific wavelength and the specific incident angle respectively correspond to a wavelength and an incident angle of the imaging light L 0 . The interference fringes  751  having the configuration can be formed by performing interference exposure on the holographic photosensitive layer by using reference light Lr and object light Ls. 
     In the present exemplary embodiment, the imaging light L 0  is used for color display, and thus includes red light LR, green light LG, and blue light LB, which will be described later. Thus, the second diffraction element  70  includes the interference fringes  751 R,  751 G, and  751 B having a pitch corresponding to the specific wavelength. For example, the interference fringes  751 R are formed, for example, at a pitch corresponding to the red light LR with a wavelength of 615 nm included in a wavelength range from 580 nm to 700 nm. The interference fringes  751 G are formed, for example, at a pitch corresponding to the green light LG with a wavelength of 535 nm included in a wavelength range from 500 nm to 580 nm. The interference fringes  751 B are formed, for example, at a pitch corresponding to the blue light LB with a wavelength of 460 nm, for example, in a wavelength range from 400 nm to 500 nm. The configuration can be formed by forming a holographic photosensitive layer having sensitivity corresponding to the respective wavelengths, and performing dual beam interference exposure on the holographic photosensitive layer by using reference light LrR, LrG, and LrB and object light LsR, LsG, and LsB having the respective wavelengths. 
     Note that, as illustrated in  FIG.  3 B , the interference fringes  751  in which the interference fringes  751 R,  751 G, and  751 B are superimposed on each other in one layer may be formed by dispersing a photosensitive material having sensitivity corresponding to the respective wavelengths in the holographic photosensitive layer, and then performing interference exposure on the holographic photosensitive layer by using the reference light LrR, LrG, and LrB and the object light LsR, LsG, and LsB having the respective wavelengths. Further, light having a spherical wave may be used as the reference light LrR, LrG, and LrB and the object light LsR, LsG, and LsB. 
     Next, the configuration of the second optical unit L 20  will be described. 
       FIG.  4    is a cross-sectional view illustrating a general configuration of a second optical unit L 20 . As illustrated in  FIG.  4   , the second optical unit L 20  includes the first diffraction element  50  and the correction optical system  45 . The first diffraction element  50  is constituted of a reflection-type volume hologram element having a basic configuration that is the same as the second diffraction element  70 . Thus the first diffraction element  50  includes interference fringes  50   a  having a pitch corresponding to a specific wavelength. 
     In the present exemplary embodiment, the first diffraction element  50  is provided integrally with the correction optical system  45 . The correction optical system  45  includes a first member  46  and a second member  47 , and as a whole, has the same function as a prism having a power to deflect the imaging light L 0 . The first member  46  is provided on an incident surface (one surface)  51  of the first diffraction element  50 . The second member  47  is provided on the opposite side of the first member  46  to the first diffraction element  50 . 
     The first member  46  is a member that is transmissive and has an elastic modulus of 50 GPa or greater and 100 GPa or less. In the present exemplary embodiment, the first member  46  is formed by using a glass plate having an elastic modulus of 80 GPa, for example. The first diffraction element  50  is affixed to a back surface  46   b  of the first member  46 . Note that a protective cover member formed from, for example, plastic, glass, or a hard coat, may be provided on a surface of the first diffraction element  50  facing the first member  46 . 
     The second member  47  is affixed to a front surface  46   a  of the first member  46 . An alignment mark for alignment with the second member  47  may be provided on the front surface  46   a  of the first member  46 . 
     The second member  47  is a member that is transmissive and has optical power. The second member  47  is formed from a material having a refractive index substantially equal to that of the first member  46 . For example, when the refractive index of the first member  46  is 1.5, the second member  47  is formed from a material having a refractive index of 1.3 to 1.9. 
     In the present exemplary embodiment, the second member  47  is formed from plastics such as acrylic resins, for example. The second member  47  has a back surface  47   b  that faces the first member  46  and a front surface  47   a  that faces away from the back surface  47   b . Since the back surface  47   b  is constituted of a flat surface, a gap is not formed between the back surface  47   b  and the front surface  46   a  of the first member  46  constituted of a flat surface. An alignment mark for alignment with the first member  46  may be provided on the back surface  47   b  of the second member  47 . 
     The front surface  47   a  is constituted of a surface having positive optical power. A surface having positive optical power refers to herein a lens shape such as a spherical surface, an aspheric surface, a cylindrical surface, or a free form surface. The front surface  47   a  of the second member  47  functions as a light incident/emission surface  45   a  of the correction optical system  45 . Note that the front surface  47   a  may be an inclined surface that is inclined with respect to the back surface  47   b . That is, the front surface  47   a  may be constituted of a flat surface as long as the front surface  47   a  has positive optical power. 
     As described above, in the present exemplary embodiment, the correction optical system  45  is formed by combining two pieces formed from glass and plastic, respectively. 
     Next, a method of producing the second optical unit L 20  will be described.  FIGS.  5 A and  5 B  are diagrams illustrating a production step of the second optical unit L 20 . 
     Similarly to the interference fringes  751  in the second diffraction element  70  illustrated in  FIGS.  3 A and  3 B , the interference fringes  50   a  in the first diffraction element  50  can be formed by performing dual beam interference exposure of the holographic photosensitive layer by using reference light and object light. 
     First, as illustrated in  FIG.  5 A , the first member  46  that is transmissive and has an elastic modulus of 50 GPa or greater and 100 GPa or less is prepared, and a holographic photosensitive layer  52  is provided on the back surface  46   b  of the first member  46  by using application treatment, for example. In other words, the first member  46  is used as a supporting member during exposure of the holographic photosensitive layer  52 . The holographic photosensitive layer  52  is formed from a hologram material in which a photosensitive monomer such as an acrylic polymer are dispersed in a binder resin such as an urethane resin, an epoxy resin, or a cellulose resin. 
     Then, dual beam interference exposure of the holographic photosensitive layer  52  is performed. In the dual beam interference exposure, to form the first diffraction element  50  as a hologram element, exposure is performed by causing the reference light Lr converging on a reference point RP to interfere, in the holographic photosensitive layer  52 , with the object light Ls emitted from an object point OP. 
     During the interference exposure step, expansion or contraction of the holographic photosensitive layer  52  occurs. At this time, deformation such as warping may occur in the supporting member supporting the holographic photosensitive layer  52 . If the supporting member deforms, the positions of the interference fringes are changed, and as a result, deterioration in diffraction performance, such as a change in diffraction angle, occurs. 
     According to the present exemplary embodiment, the first member  46  that supports the holographic photosensitive layer  52  has an elastic modulus of 50 GPa or greater and 100 GPa or less, and thus when expansion or contraction is about to occur during interference exposure, deformation of the holographic photosensitive layer  52  is suppressed by the first member  46 . Thus, it is possible to prevent deterioration in diffraction performance due to expansion or contraction during the interference exposure step, and thus to provide a highly reliable first diffraction element  50 . 
     In addition, in the producing method according to the present exemplary embodiment, it is possible to achieve an effect described below by performing dual beam interference exposure of the holographic photosensitive layer  52  supported on the first member  46  in which the light incident surface during the exposure is constituted of at least a flat surface. 
     Here, as a comparative example, a case in which a correction optical system formed by using a single member is used is considered.  FIG.  6    is a diagram illustrating an exposure step in the comparative example. As illustrated in  FIG.  6   , when the holographic photosensitive layer  52  provided on the correction optical system  45 A formed by a single component is subjected to the dual beam interference exposure, the object light Ls emitted from the object point OP is refracted by the light incident/emission surface  45   a   1  of the correction optical system  45 A, and thus the wavefront significantly changes. On the other hand, since the reference light Lr that converges on the reference point RP is directly incident on the holographic photosensitive layer  52 , wavefront change due to refraction at the surface of the correction optical system  45 A does not occur. 
     As described above, in the exposure step of the comparative example, the wavefront of the object light Ls is greatly affected by the refraction at the light incident/emission surface  45   a   1 . Thus, the holographic photosensitive layer  52  is exposed to a wavefront different from the original wavefront, as represented by a two-dot chain line in  FIG.  6   , and as a result, the desired diffraction performance as the first diffraction element  50  can not be achieved. 
     Note that, in  FIG.  6   , if the directions of rays of the exposure light are reversed, in other words, if light converging on the object point and light emitted from the reference point are used, the reference light is affected by the lens surface of the correction optical system  145 . Thus, also in this case, the holographic photosensitive layer  52  is exposed to a wavefront that is different from the original wavefront. 
     On the other hand, in the exposure step in the present exemplary embodiment, as illustrated in  FIG.  5 A , the object light Ls emitted from the object point OP is incident on the front surface  46   a  of the first member  46 . Since the front surface  46   a  located on the opposite side of the first member  46  to the first diffraction element  50  is constituted of a flat surface, the object light Ls is hardly affected by refraction at the front surface  46   a , and thus wavefront change is reduced. In addition, a change in the wavefront of the reference light Lr directly incident on the holographic photosensitive layer  52  is small. 
     As described above, when the producing method according to the present exemplary embodiment is adopted, the wavefronts of the object light Ls and the reference light Lr are hardly affected by refraction, and thus the desired position in the holographic photosensitive layer  52  can be exposed to form the interference fringes  50   a . Accordingly, the first diffraction element  50  having the desired diffraction performance can be produced. 
     Next, the second member  47  is provided on the opposite side, to the first member  46 , of the first diffraction element  50  formed on the first member  46 . Specifically, as illustrated in  FIG.  5 B , the back surface  47   b  of the second member  47  is affixed to the front surface  46   a  of the first member  46 . In the affixing, the first member  46  and the second member  47  can be accurately affixed together by using, as a reference, the alignment marks (not illustrated) provided on the front surface  46   a  of the first member  46  and the back surface  47   b  of the second member  47 . 
     As described above, the second optical unit L 20  including the first diffraction element  50  and the correction optical system  45  can be produced. With the second optical unit L 20  of the present exemplary embodiment, it is possible to reduce effects of refraction on the object light Ls and the reference light Lr during the interference exposure step, and to provide the first diffraction element  50  in which occurrence of expansion or contraction during the interference exposure step is suppressed. In other words, the first diffraction element  50  can achieve desired diffraction performance. Therefore, the second optical unit L 20  including the first diffraction element  50  is highly reliable. 
       FIG.  7    is a diagram illustrating a diffraction characteristic in a volume hologram constituting the first diffraction element  50  and the second diffraction element  70 .  FIG.  7    illustrates a difference in diffraction angle between a specific wavelength and a peripheral wavelength when a light beam is incident on one point on the volume hologram. In  FIG.  7   , when the specific wavelength is 531 nm, a deviation in the diffraction angle of light with a peripheral wavelength of 526 nm is indicated by a solid line L 526 , and a deviation in the diffraction angle of light with a peripheral wavelength of 536 nm is indicated by a dotted line L 536 . As illustrated in  FIG.  7   , even when a light beam is incident on the same interference fringes recorded in the hologram, a light beam having a longer wavelength diffracts more greatly, and a light beam having a shorter wavelength is less likely to diffract. Thus, when two diffraction elements, namely, the first diffraction element  50  and the second diffraction element  70  are used as in the present exemplary embodiment, proper wavelength compensation fails to be achieved unless considerations are given for the ray angle of incident light with a wavelength larger or smaller than the specific wavelength. In other words, color aberration occurring in the second diffraction element  70  fails to be canceled. 
     In the optical system  10  illustrated in  FIG.  2   , as described in JP-A-2017-167181, wavelength compensation, namely, a color aberration cancellation can be achieved because an incident direction and the like to the second diffraction element  70  is made appropriate in accordance with whether a sum of the number of times of formation of an intermediate image between the first diffraction element  50  and the second diffraction element  70  and the number of times of reflection by the mirror  62  is odd or even. 
     Here, a case in which diffraction angles of the first diffraction element  50  and the second diffraction element  70  are the same is considered. In other words, a case in which the diffraction angles of the first diffraction element  50  and the second diffraction element  70  are formed by the same diffraction element is considered.  FIG.  8    is a schematic diagram of light emitted from the second diffraction element  70  when the diffraction angles of the first diffraction element  50  and the second diffraction element  70  are the same.  FIG.  8    also illustrates, in addition to the light L 1  (solid line) having the specific wavelength of the imaging light L 0 , the light L 2  (dot-and-dash line) on the long wavelength side with respect to the specific wavelength and the light L 3  (dotted line) on the short wavelength side with respect to the specific wavelength. 
     As illustrated in  FIG.  8   , the imaging light L 0  incident on the first diffraction element  50  is diffracted and then deflected by the first diffraction element  50 . At this time, in the first diffraction element  50  formed of the volume hologram as illustrated in  FIG.  7   , the light L 2  on the long wavelength side with respect to the specific wavelength has a diffraction angle θ 2  greater than a diffraction angle θ 1  of the light L 1  having the specific wavelength. Further, the light L 3  on the short wavelength side with respect to the specific wavelength has a diffraction angle θ 3  smaller than the diffraction angle θ 1  of the light L 1  having the specific wavelength. Therefore, the imaging light L 0  emitted from the first diffraction element  50  is deflected and dispersed at each wavelength. 
     The imaging light L 0  emitted from the first diffraction element  50  is incident on the second diffraction element  70  via the light-guiding system  60 , and is then diffracted and deflected by the second diffraction element  70 . At this time, on the optical path from the first diffraction element  50  to the second diffraction element  70 , an intermediate image is formed once, and reflection by the mirror  62  is performed once. Therefore, when the incident angle is defined as an angle between the imaging light L 0  and a normal line of an incident surface of the second diffraction element  70 , the light L 2  on the long wavelength side with respect to the specific wavelength has an incident angle θ 12  larger than the incident angle θ 11  of the light L 1  with the specific wavelength, and the light L 3  on the short wavelength side with respect to the specific wavelength has an incident angle θ 13  smaller than the incident angle θ 11  of the light L 1  with the specific wavelength. As described above, the light L 2  on the long wavelength side with respect to the specific wavelength has a diffraction angle θ 2  larger than a diffraction angle θ 1  of the light L 1  with the specific wavelength. The light L 3  on the short wavelength side with respect to the specific wavelength has a diffraction angle θ 3  smaller than the diffraction angle θ 1  of the light L 1  with the specific wavelength. 
     Accordingly, the light L 2  on the long wavelength side with respect to the specific wavelength is incident on the first diffraction element  50  at a larger incident angle than the light L 1  with the specific wavelength. However, the light L 2  on the long wavelength side with respect to the specific wavelength has a larger diffraction angle than the light L 1  with the specific wavelength, and as a result, the light L 2  on the long wavelength side with respect to the specific wavelength and the light L 1  with the specific wavelength are substantially parallel when being emitted from the second diffraction element  70 . In contrast, the light L 3  on the short wavelength side with respect to the specific wavelength is incident on the first diffraction element  50  at a smaller incident angle than the light L 1  with the specific wavelength. However, the light L 3  on the short wavelength side with respect to the specific wavelength has a smaller diffraction angle than the light L 1  with the specific wavelength, and as a result, the light L 3  on the short wavelength side with respect to the specific wavelength and the light L 1  with the specific wavelength are substantially parallel when being emitted from the second diffraction element  70 . In this way, as illustrated in  FIG.  8   , the imaging light L 0  emitted from the second diffraction element  70  is incident as the substantially parallel light on the eye E of the observer. Thus, misalignment of image formation in the retina E 0  at each wavelength can be suppressed, and a color aberration generated by the second diffraction element  70  can be canceled. 
     When the color aberration is canceled by setting the diffraction angles of the first diffraction element  50  and the second diffraction element  70  to be the same in this way, a conjugated relationship is established between the first diffraction element  50  and the second diffraction element  70 . Here, the conjugated relationship refers to a relationship in which light emitted from a first position of the first diffraction element  50  is condensed by the light-guiding system  60  having positive power, and is incident on a second position corresponding to the first position of the second diffraction element  70 . 
     However, when the conjugated relationship is established by setting the diffraction angles of the first diffraction element  50  and the second diffraction element  70  to be the same as described above, the following problem arises. 
       FIG.  9 A  is a schematic diagram when the diffraction angles of the first diffraction element  50  and the second diffraction element  70  are set to a small angle.  FIG.  9 B  is a schematic diagram when the diffraction angles of the first diffraction element  50  and the second diffraction element  70  are set to a large angle. Note that, in  FIGS.  9 A and  9 B , each optical unit disposed along an optical axis is simplified and indicated by a thick arrow. 
     In  FIG.  9 A , the diffraction angles of the first diffraction element  50  and the second diffraction element  70  are set to a small angle α. In  FIG.  9 B , the diffraction angles of the first diffraction element  50  and the second diffraction element  70  are set to an angle β greater than the angle α. 
     As illustrated in  FIG.  9 A , when the diffraction angles of the first diffraction element  50  and the second diffraction element  70  are set to the small angle α, it is possible to reduce the size of the display device by disposing each optical member along a contour MC of a face of the observer. However, as illustrated in  FIG.  9 A , there is a problem in that the mirror  40  and the light-guiding system  60  interfere with each other and a part of the imaging light is missing. 
     On the other hand, as illustrated in  FIG.  9 B , when the diffraction angles of the first diffraction element  50  and the second diffraction element  70  are set to the large angle β, it is possible to avoid interference between the mirror  40  and the light-guiding system  60  by widening a gap therebetween. However, each optical member is disposed in a position away from the contour MC of the face of the observer, which results in a problem of increasing the size of the display device. 
     Thus, in the optical system  10  in the present exemplary embodiment, the first diffraction element  50  and the second diffraction element  70  have different diffraction angles.  FIG.  10    is a diagram illustrating a relationship between the diffraction angles of the first diffraction element  50  and the second diffraction element  70  in the optical system  10  in the present exemplary embodiment. 
     As illustrated in  FIG.  10   , in the optical system  10  in the present exemplary embodiment, a first diffraction angle α 1  of the imaging light L 0  in the first diffraction element  50  and a second diffraction angle β 1  of the imaging light L 0  in the second diffraction element  70  are different. Specifically, the second diffraction angle β 1  is greater than the first diffraction angle α 1 . According to the optical system  10  in the present exemplary embodiment, by setting the second diffraction angle β 1  to be greater than the first diffraction angle α 1 , the imaging light L 0  is incident on the eye E of the observer at a large angle of view, and each optical unit can also be disposed along the contour MC of the face of the observer. Therefore, the size reduction of the display device itself including the optical system  10  can be achieved. 
     Thus, as described above, the size reduction of the display device can be achieved by setting the diffraction angles α 1  and β 1  of the first diffraction element  50  and the second diffraction element  70  to be different from each other, but a new problem arises as described below. 
       FIG.  11    is a schematic diagram of light emitted from the second diffraction element  70  when the diffraction angles of the first diffraction element  50  and the second diffraction element  70  are different. Note that it is assumed that the correction optical system  45  is not disposed on the optical path illustrated in  FIG.  11   .  FIG.  11    illustrates light L 1  (solid line) of a specific wavelength of the imaging light L 0  as well as light L 2  (dot-and-dash line) on a long wavelength side and light L 3  (dotted line) on a short wavelength side relative to the specific wavelength. 
     As illustrated in  FIG.  11   , the imaging light L 0  incident on the first diffraction element  50  is diffracted and then deflected by the first diffraction element  50 . At this time, as illustrated in  FIG.  11   , the imaging light L 0  emitted from the first diffraction element  50  is deflected and dispersed at each wavelength. 
     The imaging light L 0  emitted from the first diffraction element  50  is diffracted and then deflected by the second diffraction element  70 . At this time, since the diffraction angle of the second diffraction element  70  is different from the diffraction angle of the first diffraction element  50 , the light L 2  on the long wavelength side and the light L 3  on the short wavelength side with respect to the light L 1  having the specific wavelength are emitted in a widened state, as illustrated in  FIG.  11   . In this way, as illustrated in  FIG.  11   , the imaging light L 0  emitted from the second diffraction element  70  is shifted in an image formation position in the retina E 0  at each wavelength, and thus there is a problem in that a color aberration cannot be canceled, and a resolution of the imaging light L 0  is reduced. 
     To resolve this problem, as illustrated in  FIG.  2   , the optical system  10  in the present exemplary embodiment includes, between the first optical unit L 10  and the fourth optical unit L 40  on the optical path of the imaging light L 0 , the correction optical system  45  that corrects an incident angle of the imaging light L 0  with respect to the second diffraction element  70 . More specifically, the correction optical system  45  is integrally provided on a light incident side and a light emitting side of the first diffraction element  50  constituting the second optical unit L 20 . The correction optical system  45  includes the light incident/emission surface  45   a  on which the imaging light L 0  is incident or from which the imaging light L 0  is emitted. 
     The correction optical system  45  has a shape in which a thickness on the side closer to the eye E of the observer is thick and a thickness on the side away from the eye E of the observer is thin. Further, it can also be said that the correction optical system  45  has a shape in which a thickness on the side closer to the second diffraction element  70  located on the left side X 2  with respect to the first diffraction element  50  is thick, and a thickness on the side closer to the imaging light generating device  31  located on the right side X 1  with respect to the first diffraction element  50  is thin. 
     The light incident/emission surface  45   a  is constituted of a surface being inclined so as to protrude toward the front side Z 1  as it is closer to the eye E of the observer. Further, it can also be said that the light incident/emission surface  45   a  is constituted of a surface being inclined so as to protrude toward the front side Z 1  as it is closer to the second diffraction element  70 . 
     Next, functions of the correction optical system  45  will be described with reference to drawings. 
       FIG.  12 A  is a diagram illustrating a first function of the correction optical system  45 ,  FIG.  12 B  is a diagram illustrating a second function of the correction optical system  45 , and  FIG.  12 C  is a diagram illustrating a third function of the correction optical system  45 . Note that it is assumed in  FIGS.  12 A,  12 B, and  12 C  that the second diffraction angle β 1  of the second diffraction element  70  is greater than the first diffraction angle α 1  of the first diffraction element  50 . 
       FIG.  13    is an enlarged view of the correction optical system  45 . The first member  46  and the second member  47  are omitted in  FIG.  13   .  FIG.  13    also illustrates, in addition to the light L 1  (solid line) having the specific wavelength of the imaging light L 0 , the light L 2  (dot-and-dash line) on the long wavelength side with respect to the specific wavelength and the light L 3  (dotted line) on the short wavelength side with respect to the specific wavelength. 
     As illustrated in  FIG.  12 A , the correction optical system  45  is provided on a light incident side of the first diffraction element  50  on the optical path of the imaging light L 0 . Thus, as illustrated in  FIG.  13   , the imaging light L 0  enters the correction optical system  45  through the light incident/emission surface  45   a . At this time, when the imaging light L 0  is incident on the correction optical system  45 , due to dispersion of the light, the light L 3  on the short wavelength side is refracted the most, the light L 2  on the long wavelength side is the smallest, and the light L 1  having the specific wavelength is refracted by the magnitude between the light L 3  on the short wavelength side and the light L 2  on the long wavelength side. Then, the light L 1  having the specific wavelength, the light L 2  on the long wavelength side, and the light L 3  on the short wavelength side are transmitted through the correction optical system  45  and are incident on the first diffraction element  50 . 
     The light L 1  having the specific wavelength, the light L 2  on the long wavelength side, and the light L 3  on the short wavelength side are dispersed by the correction optical system  45  and are thus incident on different places of the first diffraction element  50 . Further, incident angles of the light L 1  having the specific wavelength, the light L 2  on the long wavelength side, and the light L 3  on the short wavelength side with respect to the first diffraction element  50  are different from each other. 
     As described above, by dispersing the imaging light L 0 , the correction optical system  45  changes an incident position and incident angle with respect to the first diffraction element  50 , differently for each of the light L 1  having the specific wavelength, the light L 2  on the long wavelength side, and the light L 3  on the short wavelength side. 
     Here, a diffraction angle of the volume hologram constituting the first diffraction element  50  varies from place to place. The correction optical system  45  corrects, for example, an incident position of each of the light L 1  having the specific wavelength in the imaging light L 0 , the light L 2  on the long wavelength side, and the light L 3  on the short wavelength side with respect to the first diffraction element  50  to an appropriate position. In this way, the correction optical system  45  can correct an incident angle of the imaging light L 0  emitted from the first diffraction element  50  with respect to the second diffraction element  70  such that the light having the specific wavelength and the light having the peripheral wavelength are substantially parallel as illustrated in  FIG.  8    when being emitted from the second diffraction element  70 . In other words, the correction optical system  45  has a first function that is “performing correction so as to change an incident position of the imaging light L 0  incident on the first diffraction element  50  for each wavelength” as illustrated in  FIG.  12 A . 
     Furthermore, as illustrated in  FIG.  12 B , the correction optical system  45  corrects an incident angle of the imaging light L 0  with respect to the first diffraction element  50  at each wavelength, namely, for each of the light L 1  having the specific wavelength, the light L 2  on the long wavelength side, and the light L 3  on the short wavelength side. As a result, the incident angle of the imaging light L 0  with respect to the first diffraction element  50  is corrected by, for example, previously angling the light L 2  on the long wavelength side and the light L 3  on the short wavelength side in the imaging light L 0 , as illustrated in  FIG.  13   . In this way, the correction optical system  45  can cause the imaging light L 0  to be incident on the first diffraction element  50  such that the light having the specific wavelength and the light having the peripheral wavelength are substantially parallel as illustrated in  FIG.  8    when being emitted from the second diffraction element  70 . In other words, the correction optical system  45  has a second function, that is “correcting an incident angle of the imaging light L 0  with respect to the first diffraction element  50  for each wavelength”, as illustrated in  FIG.  12 B . 
     As illustrated in  FIG.  12 C , the correction optical system  45  is provided between the first diffraction element  50  and the second diffraction element  70  on the optical path of the imaging light L 0 . Thus, the imaging light L 0  emitted from the first diffraction element  50  is incident on the correction optical system  45  in a dispersed state at each wavelength. 
     As illustrated in  FIG.  13   , since a diffraction angle at the first diffraction element  50  varies depending on a place, the light L 1  having the specific wavelength, the light L 2  on the long wavelength side, and the light L 3  on the short wavelength side are diffracted at different angles. The light L 1  having the specific wavelength, the light L 2  on the long wavelength side, and the light L 3  on the short wavelength side that are diffracted by the first diffraction element  50  are transmitted through the correction optical system  45  again and emitted from the light incident/emission surface  45   a . The light L 1  having the specific wavelength, the light L 2  on the long wavelength side, and the light L 3  on the short wavelength side are emitted from the correction optical system  45  in different directions. In this way, the correction optical system  45  deflects the imaging light L 0  in a different direction for each wavelength, and thus incident angles of the light L 1  having the specific wavelength, the light L 2  on the long wavelength side, and the light L 3  on the short wavelength side with respect to the second diffraction element  70  can be each adjusted. 
     In other words, the correction optical system  45  compensates for a shortage of the diffraction angle of the imaging light L 0  in the first diffraction element  50 , and thereby an incident angle of the imaging light L 0  dispersed at each wavelength with respect to the second diffraction element  70  is corrected. In this way, the correction optical system  45  can correct an emission angle of the imaging light L 0  dispersed at each wavelength such that the light having the specific wavelength and the light having the peripheral wavelength are substantially parallel as illustrated in  FIG.  8    when being emitted from the second diffraction element  70 . In other words, the correction optical system  45  has a third function that is “performing correction so as to compensate for a shortage of a diffraction angle of the imaging light L 0  at the first diffraction element  50 ”, as illustrated in  FIG.  12 C . 
     As described above, the correction optical system  45  can achieve effects illustrated in  FIGS.  12 A,  12 B, and  12 C , and thus the imaging light L 0  emitted from the second diffraction element  70  can be incident on the eye E of the observer as substantially parallel light. Thus, misalignment of image formation in the retina E 0  at each wavelength can be suppressed, and a color aberration generated by the second diffraction element  70  can be canceled. Therefore, by including the second optical unit L 20  that includes the correction optical system  45 , high image quality can be acquired by canceling a color aberration generated by the second diffraction element  70  while adopting a structure in which the diffraction angles of the first diffraction element  50  and the second diffraction element  70  are different. In other words, the size reduction of the display device  100  can be achieved by setting diffraction angles to be different while appropriately performing wavelength compensation by the two diffraction elements. 
     As described above, the optical system  10  including the correction optical system  45  in the present exemplary embodiment can provide functions illustrated in  FIGS.  12 A,  12 B , and  12 C. Thus, the optical system  10  in the present exemplary embodiment can accurately correct an incident angle of the imaging light L 0  with respect to the second diffraction element  70  by using the correction optical system  45 . 
     Therefore, even when the first diffraction element  50  and the second diffraction element  70  having different diffraction angles are used, the optical system  10  in the present exemplary embodiment can cause the imaging light L 0  emitted from the second diffraction element  70  to be incident on the eye E of the observer as substantially parallel light by the correction optical system  45 . Thus, misalignment of image formation in the retina E 0  at each wavelength can be suppressed, and a color aberration generated by the second diffraction element  70  can be canceled. As a result, deterioration in resolution of imaging light can be prevented. 
     In other words, the optical system  10  in the present exemplary embodiment can acquire high image quality by canceling a color aberration generated by the second diffraction element  70  while adopting a structure in which the diffraction angles of the first diffraction element  50  and the second diffraction element  70  are different. In other words, the optical system  10  in the present exemplary embodiment can achieve the size reduction of the display device  100  by setting different diffraction angles while appropriately performing wavelength compensation by the two diffraction elements. 
       FIG.  14    is a diagram schematically illustrating a light beam diagram of the optical system  10  in the present exemplary embodiment. In  FIG.  14   , each optical unit disposed along the optical axis is indicated by a thick arrow. Further, in  FIG.  14   , a light beam of the imaging light emitted from the center of the imaging light generating device  31  is indicated by a solid line La, and a main light beam of the imaging light emitted from an end portion of the imaging light generating device  31  is indicated by a dot-and-dash line Lb. Further,  FIG.  14    illustrates travel of light emitted from the imaging light generating device  31 . Note that, in  FIG.  14   , all optical units are illustrated as a transmissive-type unit for simplification of the figure. In the following description, an “intermediate image” is a location where the light beam (solid line La) emitted from one pixel converges, and a “pupil” is a location where the main light beam (dot-and-dash line Lb) at each angle of view converges. 
     As illustrated in  FIG.  14   , the optical system  10  of the present exemplary embodiment includes the first optical unit L 10  having positive power, the second optical unit L 20  including the first diffraction element  50  and having positive power, the third optical unit L 30  having positive power, and the fourth optical unit L 40  including the second diffraction element  70  and having positive power, and the first to fourth optical units L 10  to L 40  are provided along an optical path of imaging light emitted from the imaging light generating device  31 . 
     In the optical system  10  in the present exemplary embodiment, a first intermediate image P 1  of the imaging light is formed between the first optical unit L 10  and the third optical unit L 30 , a pupil R 1  is formed between the second optical unit L 20  and the fourth optical unit L 40 , a second intermediate image P 2  of the imaging light is formed between the third optical unit L 30  and the fourth optical unit L 40 , and the fourth optical unit L 40  collimates the imaging light to form an exit pupil R 2 . At this time, the third optical unit L 30  causes the main light beam at the angle of view of the imaging light emitted from the second optical unit L 20  to be incident on the fourth optical unit L 40  as divergent light. 
     In the optical system  10  in the present exemplary embodiment, the pupil R 1  is formed between the second optical unit L 20  and the third optical unit L 30  between the second optical unit L 2  and the fourth optical unit L 40 . 
     Thus, according to the optical system  10  of the present exemplary embodiment, the first intermediate image P 1  of the imaging light is formed between the projection optical system  32  and the light-guiding system  60 , the pupil R 1  is formed in the vicinity of the light-guiding system  60 , the second intermediate image P 2  of the imaging light is formed between the light-guiding system  60  and the second diffraction element  70 , and the second diffraction element  70  collimates the imaging light to form the exit pupil R 2 . 
     In the optical system  10  in the present exemplary embodiment, the first intermediate image P 1  is formed between the first optical unit L 10  (projection optical system  32 ) and the second optical unit L 20  (first diffraction element  50 ). 
     According to the optical system  10  in the present exemplary embodiment, three conditions (Conditions 1, 2, and 3) described below are satisfied. 
     Condition 1: The light rays emitted from one point of the imaging light generating device  31  are formed into one point on the retina E 0 . 
     Condition 2: An incident pupil of the optical system and a pupil of an eye are conjugated. 
     Condition 3: A peripheral wavelength is compensated between the first diffraction element  50  and the second diffraction element  70 . 
     More specifically, as clearly seen from the dot-and-dash line Lb illustrated in  FIG.  14   , the light beam emitted from one point of the imaging light generating device  31  satisfies Condition 1 that an image is formed as one point in the retina E 0 , and thus the observer can visually recognize one pixel. Further, as clearly seen from the solid line La illustrated in  FIG.  14   , Condition 2 that the incident pupil of the optical system  10  and the pupil E 1  of the eye E are conjugated (conjugation of the pupil) is satisfied, and thus the entire region of the image generated by the imaging light generating device  31  can be visually recognized. Further, as described above, Condition 3 that the peripheral wavelength of the imaging light L 0  is compensated between the first diffraction element  50  and the second diffraction element  70  is satisfied by providing the correction optical system  45 , and thus a color aberration generated by the second diffraction element  70  can be canceled. 
     Second Exemplary Embodiment 
     Next, an optical system according to a second exemplary embodiment will be described. In the optical system in the above-described exemplary embodiment, the front surface of the first member in the correction optical system is constituted of a flat surface. However, the plate shape of the first member is not limited thereto. Note that components common to the first exemplary embodiment will be given an identical reference numeral and detail description will be omitted. 
       FIG.  15    is a cross-sectional view illustrating a general configuration of the second optical unit in the present exemplary embodiment. As illustrated in  FIG.  15   , a second optical unit L 21  in the present exemplary embodiment includes a first diffraction element  150  and a correction optical system  145 . The correction optical system  145  includes a first member  146  and the second member  47 . The first member  146  is provided on an incident surface (one surface)  151  of the first diffraction element  150 . The second member  47  is provided on the opposite side of the first member  146  to the first diffraction element  50 . 
     The first member  146  is a member that is transmissive and has an elastic modulus of 50 GPa or greater and 100 GPa or less. In the present exemplary embodiment, the first member  146  is formed from glass having an elastic modulus of 80 GPa, for example. The first diffraction element  150  is affixed to a back surface  146   b  of the first member  146 . The back surface  146   b  of the first member  146  is constituted of a curved surface, and the front surface  146   a  is constituted of a flat surface. The second member  47  is affixed to the front surface  146   a  constituted of a flat surface. 
     In the present exemplary embodiment, the first diffraction element  150  is provided on the back surface  146   b  of the first member  146  constituted of a curved surface. Thus, the incident surface  151  of the first diffraction element  150  on which the imaging light L 0  is incident is concaved to form a concave surface. In other words, the incident surface  151  of the first diffraction element  150  has a shape having a central portion recessed and curved with respect to a peripheral portion in the incident direction of the imaging light L 0 . Thus, the first diffraction element  150  can efficiently deflect the imaging light L 0  toward the light-guiding system  60 . 
     With the second optical unit L 21  in the present exemplary embodiment, similar effects to the above-described exemplary embodiment can be achieved. In other words, it is possible to reduce effects of refraction on the object light and the reference light during interference exposure, and to suppress occurrence of expansion and contraction, and thus, to provide the first diffraction element  150  having a desired diffraction performance. 
     Third Exemplary Embodiment 
     Next, an optical system according to a third exemplary embodiment will be described. In the optical systems in the above-described exemplary embodiments, the first member and the second member in the correction optical system are in close contact with each other. However, in a correction optical system in the present exemplary embodiment, a configuration in which, in the correction optical system of the second embodiment, the first member and the second member are spaced apart from each other is described. Note that components common to the second exemplary embodiment will be given an identical reference numeral and detail description will be omitted. 
       FIG.  16    is a cross-sectional view illustrating a general configuration of the second optical unit in the present exemplary embodiment. As illustrated in  FIG.  16   , a second optical unit L 22  includes the first diffraction element  150  and a correction optical system  245 . The correction optical system  245  includes the first member  146  and a second member  147  that are spaced apart from each other. 
     In the present exemplary embodiment, the first member  146  is provided on the incident surface  151  of the first diffraction element  150 . The second member  147  is provided on the opposite side of the first member  146  to the first diffraction element  150  so as to be spaced from the first member  146 . In other words, a gap G is provided between the front surface  146   a  of the first member  146  and the back surface  147   b  of the second member  147 . Note that the second member  147  is fixed to a holding member such as a lens barrel (not illustrated) so as to be spaced from the first member  146 . 
     With the correction optical system  245  in the present exemplary embodiment, effects similar to those in the above-mentioned exemplary embodiments can also be achieved. 
     Note that an anti-reflective coating such as a AR coat may be provided on at least one of the front surface  146   a  of the first member  146  and the back surface  147   b  of the second member  147 . 
       FIG.  17    is a cross-sectional view illustrating a configuration according to a modification example of the third exemplary embodiment. 
     As illustrated in  FIG.  17   , in a second optical unit L 23  of the present modification example, a spacer member  80  is disposed between the front surface  146   a  of the first member  146  and the back surface  147   b  of the second member  147 . The spacer member  80  holds the gap G between the first member  146  and the second member  147  so as to be a predetermined value. The spacer member  80  includes a first spacer portion  81  and a second spacer portion  82 . The first spacer portion  81  is provided at a part of a space between the front surface  146   a  of the first member  146  and the back surface  147   b  of the second member  147 , and the second spacer portion  82  is provided at a part of the remainder of the space between the front surface  146   a  of the first member  146  and the back surface  147   b  of the second member  147 . Each of the first spacer portion  81  and the second spacer portion  82  is formed from a different material. For example, one of the first spacer portion  81  and the second spacer portion  82  is formed from a plastic material, and the other of the first spacer portion  81  and the second spacer portion  82  is formed from an elastic member such as rubber. 
     Since the first member  146  formed from glass and the second member  147  formed from plastic are different in linear expansion coefficient, and thus also different in strain when heated. With the configuration of the present exemplary embodiment, the second spacer portion  82  formed from the elastic member can expand or contract, and thus the effect of thermal strain due to the difference in linear expansion coefficient can be mitigated. As a result, breakage of the correction optical system  245  due to thermal strain can be prevented. 
     With the configuration according to the modification example, effects similar to those in the above-mentioned exemplary embodiments can also be achieved. 
     Note that the spacer member may be provided integrally with the back surface  147   b  of the second member  147 . In this case, the number of parts can be reduced by integrally forming the second member  147  and the spacer member. 
     Note that the correction optical system  45  in the first exemplary embodiment may have a configuration in which the first member  46  and the second member  47  are spaced from each other. In other words, the gap G may be provided between the first member  46  and the second member  47 . 
     Fourth Exemplary Embodiment 
     Next, an optical system according to a fourth exemplary embodiment will be described. In the optical systems in the above-described exemplary embodiments, the front surface of the first member in the correction optical system is constituted of a flat surface. However, the front surface of the first member may be a curved surface. In the correction optical system in the present exemplary embodiment, a case in which the front surface of the first member is a curved surface is described. Note that components common to the first exemplary embodiment will be given an identical reference numeral and detail description will be omitted. 
       FIG.  18    is a cross-sectional view illustrating a general configuration of the second optical unit in the present exemplary embodiment. As illustrated in  FIG.  18   , a second optical unit L 24  includes a first diffraction element  350  and a correction optical system  345 . The correction optical system  345  includes a first member  346  and a second member  347 . The first member  346  is provided on an incident surface (one surface)  351  of the first diffraction element  350 . The second member  347  is provided on the opposite side of the first member  346  to the first diffraction element  350 . 
     The first member  346  is a member that is transmissive and has an elastic modulus of 50 GPa or greater and 100 GPa or less. In the present exemplary embodiment, the first member  346  is formed from glass having an elastic modulus of 80 GPa, for example. The first diffraction element  350  is affixed to a back surface  346   b  of the first member  346 . Both the back surface  346   b  and a front surface  346   a  of the first member  346  are constituted of a curved surface. The second member  347  is affixed to the front surface  346   a.    
     The second member  347  is a member that is transmissive and has optical power. In the present exemplary embodiment, the second member  347  is formed from plastics such as acrylic resins, for example. The second member  347  has a back surface  347   b  that faces the first member  346  and a front surface  347   a  that faces away from the back surface  347   b . The front surface  347   a  is constituted of a surface having positive optical power, and functions as the light incident/emission surface  345   a  of the correction optical system  345 . 
     A method of producing the second optical unit in the present exemplary embodiment will now be described.  FIG.  19    is a diagram illustrating the exposure step of the first diffraction element  350 . 
     As illustrated in  FIG.  19   , the first member  346  is prepared, and the holographic photosensitive layer  52  is provided on the back surface  346   b  of the first member  346  by using application treatment, for example. Next, dual beam interference exposure of the holographic photosensitive layer  52  is performed. In the dual beam interference exposure, to form the first diffraction element  350  as a hologram element, exposure is performed by causing the reference light Lr converging on a reference point RP to interfere, in the holographic photosensitive layer  52 , with the object light Ls emitted from an object point OP. 
     In the present exemplary embodiment, the front surface  346   a  of the first member  346  has, for example, a cylinder-like shape (cylindrical shape) that is defined based on the distance from the object point OP. Note that the front surface  346   a  of the first member  346  may have another shape that is similar to the cylindrical shape, for example, a free form surface. 
     For example, when the shortest distance from the object point OP to the holographic photosensitive layer  52  is defined as L and the thickness of the first member  346  is defined as Lg, the radius of curvature of the front surface  346   a  is defined as L−Lg. In this case, the object light Ls emitted from the object point OP is normally incident on the front surface  346   a  of the first member  346 , as illustrated in  FIG.  19   . 
     Here, when the front surface  46   a  of the first member  46  is a flat surface as in the first exemplary embodiment described above, it is difficult to completely prevent, from changing, the wavefront of the object light Ls that is emitted from the object point OP and has a spherical waveform depending on distance. On the other hand, in the configuration of the present exemplary embodiment, the object light Ls emitted from the object point OP is normally incident on the front surface  346   a  of the first member  346 , and thus the wavefront of the object light Ls incident on the front surface  346   a  is not affected by refraction. Therefore, with the exposure step of the present exemplary embodiment, the wavefront change of the reference light Lr incident on the holographic photosensitive layer  52  can be minimized. Thus, with the producing method of the present exemplary embodiment, it is possible to perform the exposure of the holographic photosensitive layer  52  with higher accuracy, and thus to produce the first diffraction element  350  having higher diffraction performance. 
     Note that the present exemplary embodiment describes reduction of effect of refraction on the wavefront of the object light Ls when the distance from the object point OP from which the object light Ls is emitted to the holographic photosensitive layer  52  is finite. 
     However, when the distance from the object point to the holographic photosensitive layer can be considered to be infinite, the wavefront of the object light emitted from the object point becomes parallel. In this case, as illustrated in  FIG.  20   , even when the first member  46  having the front surface  46   a  constituted of a flat surface is used as a supporting member, the effect of refraction on the wavefront of the object light Ls is reduced, and thus exposure of the holographic photosensitive layer  52  can be accurately performed. 
     Fifth Exemplary Embodiment 
     Next, an optical system according to a fifth exemplary embodiment will be described. In the optical system of the present exemplary embodiment, a case in which a light shielding film is formed in the second member of the correction optical system will be described. Note that components common to the first exemplary embodiment will be given an identical reference numeral and detail description will be omitted. 
       FIG.  21 A  is a cross-sectional view illustrating a general configuration of the second optical unit in the present exemplary embodiment. As illustrated in  FIG.  21 A , a second optical unit L 25  includes the first diffraction element  50  and a correction optical system  445 . The correction optical system  445  includes the first member  46 , the second member  47 , and a light shielding film (light shielding member)  48 . The light shielding film  48  is provided on the back surface  47   b  of the second member  47 . The light shielding film  48  is formed by a black coating film having, for example, light absorbing properties. Note that, as an alternative to a film, the light shielding film  48  may be formed from a black plastic or a black adhesive that can absorb light. 
       FIG.  21 B  illustrates the shape of the light shielding film  48  in a plan view. As illustrated in  FIG.  21 B , an opening  48   a  is provided in the light shielding film  48 . The opening  48   a  is sized to block components of the imaging light L 0  incident on the first diffraction element  50  or the imaging light L 0  diffracted by the first diffraction element  50  that may become stray light. As a result, the second optical unit L 25  can transmit, to the subsequent optical systems, imaging light in which stray light components are reduced. 
     Note that the light shielding film  48  may be provided on the front surface  46   a  of the first member  46 . When the light shielding film  48  is provided on the front surface  46   a  of the first member  46 , the light shielding film  48  may block exposure light during interference exposure of the first diffraction element  50 . Thus, after the interference exposure of the first diffraction element  50  is performed, the light shielding film  48  is formed on the front surface  46   a  of the first member  46 . 
     Sixth Exemplary Embodiment 
     Next, an optical system according to a sixth exemplary embodiment will be described. In the optical systems in the above-described exemplary embodiments, the case in which the correction optical system corrects the imaging light such that light having the specific wavelength, the light on the short wavelength side, and the light on the long wavelength side are incident on one point on the second diffraction element  70  is described. In the present exemplary embodiment, a case in which incident positions of light having a specific wavelength, light on a short wavelength side, and light on a long wavelength side are slightly different on a second diffraction element  70  is described. 
       FIG.  22    is a light beam diagram between the first diffraction element  50  and the second diffraction element  70  in an optical system  10 A in the present exemplary embodiment.  FIG.  23    is a schematic diagram of light emitted from the second diffraction element  70 .  FIG.  24    is a schematic diagram illustrating a state in which the light illustrated in  FIG.  23    is incident on an eye E. Note that, in  FIG.  22   , light having a specific wavelength is represented by a solid line Le, light having a wavelength of the specific wavelength −10 nm is represented by a dot-and-dash line Lf, and light having a wavelength of the specific wavelength +10 nm is represented by a two-dot chain line Lg. In  FIG.  24   , the leftmost picture of the figure illustrates a state in which the light having a wavelength of the specific wavelength −10 nm (the light represented by the dot-and-dash line Lf in  FIG.  23   ) enters the eye E. The rightmost picture of the figure illustrates a state in which the light having a wavelength of the specific wavelength +10 nm (the light represented by the two-dot chain line Lg in  FIG.  23   ) enters the eye E. Pictures between the leftmost and rightmost pictures illustrate states in which light having various wavelengths, from a wavelength of the specific wavelength −10 nm to a wavelength of the specific wavelength +10 nm, enters the eye E. Note that, while light of the specific wavelength incident on the eye E is not illustrated in  FIG.  24   , light of the specific wavelength incident on the eye E is an intermediate state between the state illustrated third from the left and the state illustrated fourth from the left. 
     As illustrated in  FIG.  23   , light in a peripheral wavelength shifted from a specific wavelength enters the second diffraction element  70  in different states. Here, in the second diffraction element  70 , as closer to an optical axis, the number of interference fringes is further reduced, and power of bending light is lower. Therefore, when light in a long wavelength side is caused to enter a side close to an optical axis and light in a short wavelength side is caused to enter a side close to an end, light in a specific wavelength and light in a peripheral wavelength are collimated. Consequently, an effect similar to wavelength compensation can be achieved. 
     In this case, positions of rays of light are different depending on a wavelength, as illustrated in  FIG.  23   . Therefore, a diameter of rays of light to enter a pupil is increased to a diameter φb from a diameter φa.  FIG.  24    illustrates the states of light beam intensity incident on the pupil at that time. As clearly seen from  FIG.  24   , the pupil cannot be filled near the specific wavelength, but the light having the peripheral wavelength can fill the pupil diameter since the light having the peripheral wavelength is incident on a position deviated from that of the light having the specific wavelength. As a result, it is possible to provide, to an observer, advantages, for example, improved visibility of an image. 
     Hereinbefore, the exemplary embodiment according to the display device of the present disclosure is described, but the present disclosure is not limited to the above exemplary embodiment, and is appropriately changeable without departing from the gist of the disclosure. 
     For example, in the exemplary embodiments described above, an example is given of the case in which the second diffraction angle of the imaging light L 0  at the second diffraction element  70  is greater than the first diffraction angle of the imaging light L 0  at the first diffraction element  50 . However, the present disclosure is not limited to this example. In other words, in the present disclosure, it is sufficient that the second diffraction angle of the second diffraction element  70  and the first diffraction angle of the first diffraction element  50  are different from each other, and the first diffraction angle may be greater than the second diffraction angle. In this way, even when the first diffraction angle is greater than the second diffraction angle, by providing the correction optical system, the size reduction of the display device can be achieved while appropriately performing wavelength compensation by the two diffraction elements. 
     Further, in the exemplary embodiments described above, the case in which the correction optical system  45  has all the functions illustrated in  FIGS.  12 A to  12 C  is described as an example. However, the correction optical system according to the present disclosure may include at least any of the functions. 
     Furthermore, in the optical systems in the exemplary embodiments described above, the correction optical system is used to solve problems that arises when the diffraction angles of the first diffraction element  50  and the second diffraction element  70  are different. However, the use of the correction optical system is not limited thereto. 
     Here, when the diffraction angles of the first diffraction element  50  and the second diffraction element  70  are the same, a conjugated relationship or a substantially conjugated relationship is established between the first diffraction element  50  and the second diffraction element  70 . 
     When a conjugated relationship or a substantially conjugated relationship is established between the first diffraction element  50  and the second diffraction element  70 , divergent light rays emitted from a first point of the first diffraction element  50  are collected by the light-guiding system  60  having positive power, and are incident at a second point of the second diffraction element  70  that corresponds to the first point. 
     Accordingly, when the first diffraction element  50  and the second diffraction element  70  satisfy a conjugated relationship or a substantially conjugated relationship, chromatic aberration caused by diffraction generated by the second diffraction element  70  can be compensated by the first diffraction element  50 . 
     Incidentally, the display device  100  has a structure in which the imaging light L 0  is incident from the oblique direction (obliquely incident) on the second diffraction element  70 . When the imaging light L 0  is obliquely incident on the second diffraction element  70  in this way, the ray shape of the imaging light L 0  on the second diffraction element  70  is distorted. 
     Thus, the ray shape of the imaging light incident on the eye E of the observer differs from the shape of the imaging light ray of the imaging light incident on the first diffraction element  50  and the second diffraction element  70 , and thus it is difficult to satisfy the above-described conjugated relationship or substantially conjugated relationship. 
     For example, when the light ray shape of the imaging light L 0  obliquely incident on the second diffraction element  70  can be corrected to a desired light ray shape such as a circular shape in advance, shapes of the imaging light incident on the first diffraction element  50  and the second diffraction element  70  become the same, and as a result, a conjugated relationship or a substantially conjugated relationship between the first diffraction element  50  and the second diffraction element  70  is established. The correction optical system  45  can be effectively used as a means for correcting the light ray shape of the imaging light L 0 . 
     Note that the oblique incidence of the imaging light L 0  with respect to the second diffraction element  70 , as described above, occurs even when the first diffraction element  50  and the second diffraction element  70  do not have a conjugated relationship. Regardless of whether there is a conjugated relationship, the correction optical system  45  can be used as a means for correcting the light ray shape of the imaging light L 0  obliquely incident on the second diffraction element  70 . 
     Modification Example 
       FIG.  25    is a configuration diagram of a display device  101  according to a modification example. As illustrated in  FIG.  25   , the display device  101  in the modification example includes the right-eye optical system  10   a  that causes the imaging light L 0   a  to be incident on the right eye Ea, the left-eye optical system  10   b  that causes the imaging light L 0   b  to be incident on the left eye Eb, and the frame  90  that holds the right-eye optical system  10   a  and the left-eye optical system  10   b.    
     The display device  101  in the present modification example has a configuration in which the imaging light L 0  travels from the up side Y 1  to the down side Y 2  in the right-eye optical system  10   a  and the left-eye optical system  10   b , and is thus emitted to an eye E of an observer. 
     The display device  101  in the present modification example also includes the above-described optical system  10 . Thus, the display device  101  in the present modification example can also achieve the size reduction of the device while appropriately performing wavelength compensation by two diffraction elements. 
     In the exemplary embodiments described above, the optical element according to the present disclosure is applied to the configuration of the second optical unit L 20 . However, the optical element according to the present disclosure may be applied to the configuration of the fourth optical unit L 40 . In this case, the second diffraction element constituting the fourth optical unit L 40  includes a first member that is transmissive and has an elastic modulus of 50 GPa or greater, and a second member that is transmissive and has optical power. 
     Application to Other Display Device 
     In the exemplary embodiments described above, the head-mounted display device  100  is exemplified, but the present disclosure may be applied to a head-up display, a handheld display, a projector optical system, and the like.