Patent Publication Number: US-2022214487-A1

Title: Optical element, image waveguide method, head-mounted display apparatus and diffractive waveguide display

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of International application No. PCT/CN2020/130821, filed Nov. 23, 2020, which is incorporated herein by reference. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a means of guiding an image beam by using a waveguide medium. More particularly, the present disclosure relates to an optical element, an image waveguide method, a head-mounted display apparatus and a diffractive waveguide display by using the abovementioned means. 
     Description of Related Art 
     Virtual object manipulation technology mainly includes virtual reality (VR), augmented reality (AR), mixed reality (MR), etc. The VR is the technology of the interaction between a virtual object and a virtual environment. The AR is the technology of disposing a virtual object in a real environment. The MR is the technology of the interaction between a virtual object and a real environment. Current AR devices, such as head-mounted displays (HMDs) and head-up displays (HUDs), have been used in various applications, for example, entertainment, education, etc. However, the feature of the waveguide medium limits the field of view (FOV) of the current AR device to only about  40  degree angle, so that it can no longer meet the demand for user and cannot provide a user with good experience. In order to increase FOV, the AR device usually uses a glass lens with high refractive index as a waveguide medium to receive more light for providing larger images. However, the glass lens with the refractive index of 1.7 can only provide the FOV of about 40 degree angles. If a glass lens with a higher refractive index is required, its cost will increase correspondingly. Moreover, the refractive index of transparent glass has an upper limit and cannot increase without limit. 
     SUMMARY 
     An objective of the present invention is to provide a means of guiding an image beam by using a waveguide medium, which can increase the reflectance of the image beam in the waveguide medium to reduce the leakage of the image beam from the surface except light exiting surface, so the FOV can increase to 60 or more than 60 degree angle, thereby improving the experience of user. 
     According to the aforementioned object, an optical element is provided and suitable for guiding an image beam from an image source. The optical element includes a waveguide substrate, a first optical film structure, a second optical film structure and a sub-wavelength nanostructure. The waveguide substrate has a first side, a second side opposite to the first side, a light entering surface and a light exiting surface. The waveguide substrate is configured to allow the image beam to enter the interior thereof through the light entering surface, and configured to propagate the image beam between the first side and the second side of the waveguide substrate in a manner of total internal reflection, where the image beam exits the light exiting surface after one or more reflections. The first optical film structure and the second optical film structure are arranged on the first side and the second side of the waveguide substrate respectively. The sub-wavelength nanostructure is arranged on the first side of the waveguide substrate. The sub-wavelength nanostructure is configured to receive and diffract the image beam, so as to couple the image beam to the waveguide substrate. The first optical film structure and the second optical film structure are configured to reflect a part of the image beam at an incident angle inside the waveguide substrate smaller than a critical angle of the waveguide substrate. 
     According to an embodiment of the present invention, each of the first optical film structure and the second optical film structure includes at least one high refractive index layer and at least one low refractive index layer stacked alternately to each other. 
     According to another embodiment of the present invention, the material of the at least one high refractive index layer is tantalum oxide, titanium oxide or the combination thereof. 
     According to another embodiment of the present invention, the material of the at least one low refractive index layer is silicon oxide. 
     According to another embodiment of the present invention, the sub-wavelength nanostructure is arranged between the waveguide substrate and the first optical film structure. 
     According to another embodiment of the present invention, the sub-wavelength nanostructure is arranged on the first optical film structure. 
     According to another embodiment of the present invention, the sub-wavelength nanostructure and the first optical film structure are staggered in a direction along the thickness of the waveguide substrate. 
     According to another embodiment of the present invention, the first optical film structure covers the first side of the waveguide substrate completely, and the second optical film structure covers the second side of the waveguide substrate completely. 
     According to another embodiment of the present invention, the waveguide substrate includes a first waveguide layer and a second waveguide layer stacked alternately. The first waveguide layer and the second waveguide layer are adjacent to the first optical film structure and the second optical film structure respectively, where the refractive index of the first waveguide layer is higher than the refractive index of the second waveguide layer. 
     According to the aforementioned object, another optical element is provided and suitable for guiding an image beam from an image source. The optical element includes a plurality of light guide units. The light guide units are used for guiding a plurality of specific color beams in the image beam respectively, and the light guide units are stacked to each other. Each of the light guide units includes a waveguide substrate, a first optical film structure, a second optical film structure and a sub-wavelength nanostructure. The waveguide substrate has a first side, a second side opposite to the first side, a light entering surface and a light exiting surface. The waveguide substrate is configured to allow each of the specific color beams in the image beam to enter the interior thereof through the light entering surface, and configured to propagate each of the specific color beams between the first side and the second side of the corresponding waveguide substrate in a manner of total internal reflection, where each of the specific color beams exits the light exiting surface after one or more reflections. The first optical film structure and the second optical film structure are arranged on the first side and the second side of the waveguide substrate respectively. The sub-wavelength nanostructure is arranged on the first side of the waveguide substrate. In each of the light guide units, the sub-wavelength nanostructure is configured to receive and diffract the corresponding specific color beam, so as to couple the specific color beam to the waveguide substrate. The first optical film structure and the second optical film structure are configured to reflect a part of the specific color beam at an incident angle inside the waveguide substrate smaller than a critical angle of the waveguide substrate. 
     According to the aforementioned object, an image waveguide method is provided and includes the following steps. An image beam is collected. A plurality of specific color beams of the image beam is coupled to the optical element via diffraction. A sub-wavelength nanostructure of each of light guide units of the optical element receives and diffracts the corresponding specific color beam, so as to couple the specific color beam to the waveguide substrate. A first optical film structure and a second optical film structure of each light guide unit of the optical element reflect a part of the specific color beam at an incident angle inside the waveguide substrate smaller than a critical angle of the waveguide substrate. 
     According to the aforementioned object, a head-mounted display apparatus is provided and includes the abovementioned optical element. 
     According to the aforementioned object, a diffractive waveguide display is provided and includes the abovementioned optical element. The diffractive waveguide display further includes an image projection module, which is configured to project an image beam to the sub-wavelength nanostructure in the optical element. 
     According to another embodiment of the present invention, the image projection module is a laser projector. 
     According to another embodiment of the present invention, the laser projector includes a microelectromechanical mirror configured to couple each component beam of the image beam to the optical element at various incident angles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure can be more fully understood by reading the following detailed description of the embodiment, with reference made to the accompanying drawings as follows: 
         FIG. 1  is a schematic diagram of a diffractive waveguide display according to one embodiment of this disclosure. 
         FIG. 2  is a schematic diagram of optical paths in the optical element of  FIG. 1 . 
         FIG. 3A  is a schematic cross-sectional diagram of the partial structure of the optical element in  FIG. 1 . 
         FIG. 3B  is a schematic cross-sectional diagram of the partial structure of the optical element shown in  FIG. 3A  according to a various embodiment. 
         FIG. 4  is a schematic diagram of an example of the optical film structure in  FIG. 3A . 
         FIG. 5A  is a schematic cross-sectional diagram of an optical element according to another embodiment. 
         FIGS. 5B to 5D  are partial views of areas in the schematic cross-sectional diagram of the optical element shown in  FIG. 5A . 
         FIG. 6  is a schematic cross-sectional diagram of an optical element according to another embodiment of this disclosure. 
         FIG. 7  is a schematic diagram of various embodiment of the optical element as shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Specific embodiments of the present invention are further described in detail below with reference to the accompanying drawings, however, the embodiments described are not intended to limit the present invention and it is not intended for the description of operation to limit the order of implementation. 
     Terms used herein are only used to describe the specific embodiments, which are not used to limit the claims appended herewith. Unless limited otherwise, the term “a,” “an,” “one” or “the” of the single form may also represent the plural form. In addition, the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. 
     Reference numerals and/or letters may be repeated in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
       FIG. 1  is a schematic diagram of a diffractive waveguide display  100  according to one embodiment of this disclosure. The diffractive waveguide display  100  can have the AR function and include an image projection module  110  and an optical element  120 , in which the image projection module  110  is used for emitting an image beam, and the optical element  120  is suitable for guiding the image beam emitted by the image projection module  110 . The image projection module  110  may be a laser projector, a liquid crystal on silicon (LCoS) projector, or a digital light processing (DLP) projector, for example, but not limited thereto. If the image projection module  110  is the laser projector, the image projection module  110  can include a microelectromechanical mirror which is configured to couple each component beam of the image beam to the optical element  120  at various incident angles. The optical element  120  is configured to couple the image beam in a specific area and to output the image beam in another area. The diffractive waveguide display  100  can be applied in many kinds of products, such as a head-mounted display, a head-up display (HUD) or other appropriate electronic product. 
       FIG. 2  is a schematic diagram of optical paths in the optical element  120  of  FIG. 1 . As shown in  FIG. 2 , the optical element  120  can supply three areas to three diffraction optical elements (DOEs) DOE 1 -DOE 3 . In the area corresponding to the diffraction optical element DOE 1 , the image beam from the image source is coupled to the inside of the optical element  120 . The image beam keeps traveling inside the optical element  120  until it hits the area corresponding to the diffraction optical element DOE 2 . In the area corresponding to the diffraction optical element DOE 2 , the image beam coupled to the inside of the optical element  120  is separated into multiple beams laterally, and the traveling directions of the beams turn toward the area corresponding to the diffraction optical element DOE 3 . In the area corresponding to the diffraction optical element DOE 3 , the image beam coupled to the inside of the optical element  120  is separated laterally and coupled to the outside of the optical element  120 . Accordingly, via the design of the diffraction optical elements DOE 1 -DOE 3 , the image projected from the small image projection module  110  can extend into a larger size under the light and thin structure, so that the user can watch the image in the screen easily. 
       FIG. 3A  is a schematic cross-sectional diagram of the partial structure of the optical element  120  in  FIG. 1 . As shown in  FIG. 3A , the optical element  120  includes a waveguide substrate  121 , an optical film structure  122 ,  123  and a sub-wavelength nanostructure  124 . The waveguide substrate  121  has a first side  121 A and a second side  121 B opposite to the first side  121 A, and has a light entering surface  1211  and a light exiting surface (not shown in  FIG. 3A ). The optical film structure  122  and the sub-wavelength nanostructure  124  are arranged on the first side  121 A of the waveguide substrate  121 , whereas the optical film structure  123  is arranged on the second side  121 B of the waveguide substrate  121 . In the present embodiment, the optical film structure  122  on the first side  121 A also can be called “first optical film structure”, whereas optical film structure  123  on the second side  121 B also can be called “second optical film structure”. In addition, the sub-wavelength nanostructure  124  is arranged on the optical film structure  122 , that is, the waveguide substrate  121  and the sub-wavelength nanostructure  124  are located on two opposite sides of the optical film structure  122  respectively. In other embodiment, the sub-wavelength nanostructure  124  can be arranged between the waveguide substrate  121  and the optical film structure  122 , and further covered by the optical film structure  122 . Alternatively, the optical film structure  122  and the sub-wavelength nanostructure  124  can be arranged together on the first side  121 A of the waveguide substrate  121 , and the sub-wavelength nanostructure  124  are not covered by the optical film structure  122 . As  FIG. 3B  shows a schematic cross-sectional diagram of the partial structure of the optical element shown in  FIG. 3A  according to a various embodiment, the optical film structure  122  and the sub-wavelength nanostructure  124  are staggered in a direction along the thickness of the waveguide substrate  121  (i.e., do no overlap each other), thereby blocking the light rays which are not expected to be diffracted by the sub-wavelength nanostructure  124 , in which the light rays can be reflected by the optical film structure  122  to increase the overall coupling efficiency of the exit pupil expander (EPE) and to improve the uniformity of the output image. 
     Referring to  FIG. 3A  again, the waveguide substrate  121  is configured to allow the image beam projected by the image projection module  110  to enter the interior thereof through the light entering surface  1211  and configured to propagate the image beam between the first side  121 A and the second side  121 B of the waveguide substrate  121  in a manner of total internal reflection, where the image beam exits the light exiting surface (not shown in FIG. 3 A) after one or more reflections. The waveguide substrate  121  can be composed of transparent optical material, such as glass, epoxy resin, polymethyl methacrylate (PMMA), polystyrene (PS), polycarbonate (PC), cyclic olefin polymer (COP), cyclic olefin copolymer (COC) or other appropriate material. 
     The optical film structure  122 ,  123  is used for increasing the reflectance of the image beam in the waveguide substrate  121  after entering the inside of the waveguide substrate  121 . For example, as shown in  FIG. 3A , the image beam L 1  hits the second side  121 B of the waveguide substrate  121  at an incident angle θ 1  smaller than the critical angle θ C  of the waveguide substrate  121 . Since the optical film structure  123  can increase optical reflectance for the light rays (including the image beam L 1 ) at an angle smaller than the critical angle θ C , the reflection of the image beam L 1  on the second side  121 B of the waveguide substrate  121  can greatly increase. On the contrary, if the optical film structure  123  were non-existent, the image beam L 1  would exit the second side  121 B of the waveguide substrate  121  by refraction, thereby causing the leakage of the image beam L 1 . Accordingly, by arranging the optical film structure  123 , the total internal reflection of the image beam L 1  on the second side  121 B of the waveguide substrate  121  can be caused to prevent the leakage of the image beam L 1 , which is equivalent to increasing an angle range for accommodating light rays, i.e., magnifying the accommodated virtual image. In addition, the image beams L 2 , L 3  hit the second side  121 B of the waveguide substrate  121  at the incident angles θ 2 , θ 3  which are all greater than the critical angle θ C  of the waveguide substrate  121 . According to the principle of total internal reflection (TIR), the total internal reflection of the image beams L 2 , L 3  also can be caused on the second side  121 B of the waveguide substrate  121 . 
       FIG. 4  is a schematic diagram of an example of the optical film structure  122  in  FIG. 3A . As shown in  FIG. 4 , the optical film structure  122  includes at least one high refractive index layer  122 A and at least one low refractive index layer  122 B stacked alternately to each other. The material of the high refractive index layer  122 A may be titanium oxide (TiO 2 ), tantalum oxide (Ta 2 O 5 ) or other appropriate material with significantly high refractive index, whereas the material of the low refractive index layer  122 B may be silicon oxide (SiO 2 ) or other appropriate material with significantly low refractive index. The high refractive index layers  122 A and the low refractive index layers  122 B may have a thickness ranging about between 100 nm and 200 nm apiece. The numbers of the high refractive index layers  122 A and the low refractive index layers  122 B can be determined according to the design and the demand for application, and not limited to the depiction of FIG. 4 . For example, if there are a greater refractive index difference between the high refractive index layer  122 A and the low refractive index layer  122 B, each of the high refractive index layers  122 A and the low refractive index layer  122 B will have less number, or the thickness of each of the high refractive index layer  122 A and the low refractive index layer  122 B will be thinner. By the interference of the beams reflected by the high refractive index layers  122 A and the low refractive index layers  122 B, the reflectance of the waveguide substrate  121  can increase to 70% or more than 70% efficiently. In other embodiment, the high refractive index layer  122 A can be replaced by birefringent material, such as quartz crystal, calcite crystal, magnesium fluoride (MgF 2 ) crystal or other appropriate crystal with birefringence, or be replaced by a sub-wavelength grating structure, while the low refractive index layer  122 A can be replaced by other isotropic material. 
     Like the optical film structure  122 , the optical film structure  123  also can include at least one high refractive index layer and at least one low refractive index layers stacked alternately to each other. The stacked numbers, the arrangement and the materials of the high refractive index layer and the low refractive index layer in the optical film structure  123  can be the same as those of the optical film structure  122 , so the corresponding description refers to the previous description of the optical film structure  122  and is not repeated herein. 
     The sub-wavelength nanostructure  124  is configured to receive and diffract the image beam and to couple the image beam to the waveguide substrate  121 . Specifically, the sub-wavelength nanostructure  124  is the abovementioned optical element DOE 1 , which collects the image beams emitted by the image projection module  110  and couples a plurality of specific color beams in the image beam to the waveguide substrate  121  by diffraction. The sub-wavelength nanostructure  124  may be surface relief grating structure, holographic optical element structure, polarization volume grating structure or other structure having the function of optical diffraction. In some embodiments, as shown in  FIG. 3A , the sub-wavelength nanostructure  124  is a surface relief grating structure having the periodic arrangement in a grating. 
       FIG. 5A  is a schematic cross-sectional diagram of an optical element  200  according to another embodiment. As shown in  FIG. 5A , the optical element  200  includes a waveguide substrate  201 , optical film structures  202 ,  203  and sub-wavelength nanostructures  204 ,  205 , in which the optical film structure  202  is arranged on the first side  201 A of the waveguide substrate  201 , whereas the sub-wavelength nanostructures  204 ,  205  and the optical film structure  203  are all arranged on the second side  201 B of the waveguide substrate  201 . The waveguide substrate  201  and the optical film structures  202 ,  203  can be similar to the waveguide substrate  121  and the optical film structures  122 ,  123  in the optical element  120  of  FIG. 3A  respectively and thus not be repeated herein. According to the demand for application, the optical element  120  of the diffractive waveguide display  100  can be replaced by the optical element  200  shown in  FIG. 5A . 
       FIG. 5B  is a partial view of an area P 1  in the schematic cross-sectional diagram of the optical element  200  shown in  FIG. 5A . As shown in  FIGS. 5A and 5B , the image beams L 1 , L 2  which enter the waveguide substrate  201  through the light entering surface  201 A of the waveguide substrate  201  at various angles reflect at the reflective angles θ A2 , θ B2  that all increase significantly after the diffractive effect of the sub-wavelength nanostructure  204 . that is, the reflective angles θ A2 , θ B2  are greater than incident angles θ A1 , θ B1  respectively. Thus, the image beams L 1 , L 2  can reflect off the first side  201 A and the second side  201 A of the waveguide substrate  201  at the reflective angle greater than the critical angle to achieve multiple total internal reflections. Alternatively, the reflection can greatly increase by the reflective effect of the optical film structures  202 ,  203 . Then, by the diffractive effect of the sub-wavelength nanostructure  205 , the image beams L 1 , L 2  are separated into multiple component beams at various reflective angles. 
       FIG. 5C  is a partial view of an area P 2  in the schematic cross-sectional diagram of the optical element  200  shown in  FIG. 5A . As shown in  FIGS. 5A and 5C , the image beam L 1  is separated into a zero order beam L 1 A at a larger reflective angle θ A2  and a first order beam L 1 B at a smaller reflective angle θ′ A2  by the diffractive effect of the sub-wavelength nanostructure  205 . The zero order beam L 1 A can cause total internal reflection on the first side  201 A of the waveguide substrate  201  and thus remain in the waveguide substrate  201 , while the first order beam L 1 B hits an eyebox E through the light exiting surface  2010  and the optical film structure  202  of the waveguide substrate  201 . 
       FIG. 5D  is a partial view of an area P 3  in the schematic cross-sectional diagram of the optical element  200  shown in  FIG. 5A . Likewise, as shown in  FIGS. 5A and 5D , the image beam L 2  is separated into a zero order beam L 2 A at a larger reflective angle θ B2  and a first order beam L 2 B at a smaller reflective angle θ′ B2  by the diffractive effect of the sub-wavelength nanostructure  205 . The zero order beam L 2 A can cause total internal reflection on the first side  201 A of the waveguide substrate  201  and thus remain in the waveguide substrate  201 , while the first order beam L 2 B hits the eyebox E through the light exiting surface  2010  and the optical film structure  202  of the waveguide substrate  201 . The sub-wavelength nanostructures  204 ,  205  may be, for example, surface relief grating structure, holographic optical element structure, polarization volume grating structure or other structure having the function of optical diffraction. 
       FIG. 6  is a schematic cross-sectional diagram of an optical element  300  according to another embodiment of this disclosure. As shown in  FIG. 6 , the optical element  300  includes light guide units  300 A,  300 B,  300 C disposed in sequence from down to up and stacked to each other. Each of the light guide units  300 A,  300 B,  300 C has the same structure. The light guide unit  300 A includes a waveguide substrate  301 A, optical film structures  302 A,  303 A and sub-wavelength nanostructures  304 A,  305 A. The light guide unit  300 B includes a waveguide substrate  301 B, optical film structures  302 B,  303 B and sub-wavelength nanostructures  304 B,  305 B. The light guide unit  300 C includes a waveguide substrate  301 C, optical film structures  302 C,  303 C and sub-wavelength nanostructures  304 A,  305 C. In addition, the structure of each of the light guide units  300 A- 300 C is similar to the structure of the optical element  200  shown  FIG. 5 . For example, the waveguide substrate  301 A, the optical film structures  302 A,  303 A and the sub-wavelength nanostructures  304 A,  305 A of the light guide unit  300 A are similar to the waveguide substrate  201 , the optical film structures  202 ,  203  and the sub-wavelength nanostructures  204 ,  205  of the optical element  200  respectively. Likewise, according to the demand for application, the optical element  120  in the diffractive waveguide display  100  can be replaced by the optical element  300  shown FIG. 6 . 
     In the optical element  300  shown in  FIG. 6 , the light guide units  300 A,  300 B,  300 C are used for guiding the specific color beams R, G, B, and the sub-wavelength nanostructures  304 A- 304 C,  305 A- 305 C have the color selectivity apiece. Specifically, if the sub-wavelength nanostructures  304 A- 304 C,  305 A- 305 C are surface relief grating structures having the periodic arrangement in a grating apiece, the grating period of each surface relief grating structure can relate to the specific color beams R, G or B, so that the sub-wavelength nanostructures  304 A,  305 A have preferred diffractive efficiency on the specific color beam R, the sub-wavelength nanostructures  304 B,  305 B have preferred diffractive efficiency on the specific color beam G, and the sub-wavelength nanostructures  304 C,  305 C have preferred diffractive efficiency on specific color beam B. The reflective phenomena of the specific color light beams R, G, B in the waveguide substrates  301 A- 301 C respectively are similar to the reflective phenomena of the image light beams L 1  and L 2  in the waveguide substrate  201  shown in  FIG. 5 , and the specific color light beams R, G, B exit the light exiting surfaces of the waveguide substrates  301 A- 301 C after multiple total internal reflections. The specific color beams R, G, B can be red, green and blue beams respectively, or the combination of other color beams. For the specific color light beams R′, G′, B′ from the front environment, due to the low incident angles thereof, the light guide units  300 A,  300 B,  300 C have high transmittance to them. 
       FIG. 7  is a schematic diagram of various embodiment of the optical element  120  as shown in  FIG. 1 . As shown in  FIG. 7 , the waveguide substrate  121  in the optical element  120  is composed of two waveguide layers  121 _ 1 ,  121 _ 2  stacked to each other, in which the waveguide layers  121 _ 1 ,  121 _ 2  are adjacent to the optical film structures  122 ,  123  respectively, and the refractive index of the lower waveguide layer  121 _ 2  is greater than the refractive index of the upper waveguide layer  121 _ 1 , that is, the lower waveguide layer  121 _ 2  is an optically dense medium, whereas the upper waveguide layer  121 _ 1  is an optically rare medium. As a result, when a color beam travels in the waveguide substrate  121 , is can easily enter the optically dense lower waveguide layer  121 _ 2  from the optically rare upper waveguide layer  121 _ 1 . For the image beam L 1  at a larger incident angle, since the incident angle of the image beam L 1  is larger than the critical angle of the lower waveguide layer  121 _ 2 , according to the principle of total internal reflection (TIR), the total internal reflection of the image beam L 1  can be caused on the boundary between the waveguide layers  121 _ 1 ,  121 _ 2 , so that the image beam L 1  only travels in the waveguide layer  121 _ 2  and not enters the waveguide layer  121 _ 1  from the waveguide layer  121 _ 2 . The image beam L 2  at a smaller incident angle can be reflected by the optical film structure  123  of the lower optical film structure  121 _ 2  and travel toward the upper waveguide layer  121 _ 1 . Thus, the image beam L 2  can be refracted by the waveguide layer  121 _ 2  to the waveguide layer  121 _ 1 . In this time, since the image beam L 2  travels from the optically dense medium to the optically rare medium, based on Snell&#39;s law, the image beam L 2  can exit the upper waveguide layer  121 _ 1  at an increased refractive angle. If the refractive angle of the image beam L 2  increases to over the critical angle of the upper waveguide layer  121 _ 1 , by using the total internal reflection of the upper waveguide layer  121 _ 1 , the image beam L 2  can cause total internal reflection. If the refractive angle of the image beam L 2  does not exceed the critical angle of the upper waveguide layer  121 _ 1 , the optical film structure  122  disposed on the surface of the upper waveguide layer  121 _ 1  is used to make the image beam L 2  reflect, and then enter the lower waveguide layer  121 _ 2  of the optically dense medium again through the upper waveguide layer  121 _ 1  of the optically rare medium, so as to keep transmitting the image beam L 2  within the waveguide substrate  121  to the beyond. The double-layer substance design disclosed in the embodiment of  FIG. 7  can more effectively prevent the light leakage of the image beams L 1  and L 2 . With the design of more layers of substances, the mode shown in  FIG. 4  will be formed, thereby reducing the amount of the light leakage. 
     Consequently, by the design of the optical film structure according to the embodiment of the disclosure, the reflectance of the part of the image beam in the waveguide substrate at the angle less than the critical angle can increase. This effect is equivalent to reducing the critical angle, so as to form a similar function of an angular filter. Hence, only few light rays of the image beam at small angle is not reflected by the optical film structure and thus leaks. Other most of the light rays can travel in the waveguide substrate by total internal reflection or reflect by the optical film structure to reduce the leakage of the mage beam from the surface except the light exiting surface, so that the FOV can increase to 60 or more than 60 degree angle, thereby improving the experience of user. On the other hand, the embodiment of the present disclosure does not need to employ a high-cost, high-refractive index glass lens as the waveguide medium, which can effectively reduce the cost to manufacture the optical element. In addition, the material with high refractive index can be used for the optical film structure, which is not limited by the refractive index upper limit of the waveguide substrate and also can contribute to the flexibility of the material selection of the optical element. 
     Although the present disclosure has been described in considerable detail with reference to certain embodiments thereof, other embodiments are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present disclosure without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the present disclosure cover modifications and variations of this disclosure provided they fall within the scope of the following claims.