阅读说明：本技术 光学设备 (Optical device ) 是由 瓦尔特·德拉齐克 奥克萨那·什拉姆科娃 于 2019-12-20 设计创作，主要内容包括：公开了一种光学设备和包括该光学设备的眼镜装置。所述光学设备包括衍射光栅,该衍射光栅被配置为在所述光学设备上衍射给定波长的入射光,所述衍射光栅具有高于所述给定波长的光栅间距并且被配置为以具有等于或大于2的绝对值的衍射级衍射所述入射光,其中所述光学设备包括光波导,该光波导被配置为引导以具有等于或大于2的绝对值的衍射级衍射的所述光。所述衍射光栅包括具有折射率n-(3)的第一介电材料的衬底和沉积在所述衬底上的具有折射率n-(2)的至少一个第二介电材料,其中n-(3)＜n-(2)或者n-(3)＝n-(2)。(An optical apparatus and an eyeglass device including the same are disclosed. The optical device comprises a diffraction grating configured to diffract incident light of a given wavelength on the optical device, the diffraction grating having a grating pitch higher than the given wavelength and being configured to diffract the incident light in diffraction orders having an absolute value equal to or greater than 2, wherein the optical device comprises an optical waveguide configured to guide the light diffracted in diffraction orders having an absolute value equal to or greater than 2. The diffraction grating comprises a refractive index n 3 Of a first dielectric materialA substrate and a refractive index n deposited on said substrate 2 At least one second dielectric material of (2), wherein n 3 ＜n 2 Or n 3 ＝n 2 。)

1. An optical device comprising a diffraction grating configured to diffract incident light of a given wavelength on the optical device, the diffraction grating having a grating pitch higher than the given wavelength and being configured to diffract the incident light in diffraction orders having an absolute value equal to or greater than 2, wherein the optical device comprises an optical waveguide configured to guide the light diffracted in diffraction orders having an absolute value equal to or greater than 2, wherein the diffraction grating comprises a grating having a refractive index n₃And a substrate of a first dielectric material deposited on said substrate and having a refractive index n₂At least one second dielectric material of (2), wherein n₃＜n₂Or n₃＝n₂。

2. The optical device of claim 1, wherein the diffraction grating comprises a base pattern comprising the second dielectric material, the base pattern configured to form a nano-jet beam associated with an edge of the base pattern from the light.

3. The optical device of claim 2, wherein the base pattern of the diffraction grating is configured according to at least one of the following arrangements:

-said basic pattern comprises a refractive index n on top of said substrate₃Of a block of said first dielectric material having a refractive index n₃Is inserted into the block of the first dielectric material having a refractive index n₂In a block of said second dielectric material, or,

-said basic pattern comprises a refractive index n on top of said substrate₃Of a block of said first dielectric material having a refractive index n₃Is placed in the block of the first dielectric material with a refractive index n₂Between two blocks of said second dielectric material, having a refractive index n₃And the bulk and refractive index of the first dielectric material of (a) is n₂Has the same height, or

Said baseThe base pattern comprises a refractive index n of the same width and height on top of the substrate₂Is separated by a distance, or,

-said basic pattern comprising on top of said substrate a layer having a refractive index n₂The block of the second dielectric material, the block having a U-shape.

4. The optical apparatus of claim 3, wherein when the base pattern comprises a refractive index n having the same width and height on top of the substrate₂Of said second dielectric material, said two blocks being separated by a distance W₁Separated, the two blocks having a height H₂Wherein And isWherein W₂Is the width of each of the two blocks, and θ'_B1And θ ″)_B1Is a respective angle of nano-jet beam radiation associated with an edge of the base pattern from the light, whereinWherein theta is_iIs the angle of the incident light with respect to the normal to the top surface of the diffraction grating andn₁is the refractive index of the bulk dielectric in which the diffraction grating is placed.

5. The optical apparatus of claim 3, wherein when the base pattern is included in the linerOn top of the base having a refractive index n₂When said block of second dielectric material has a U-shape, said U-shape comprises a block of second dielectric material having a height H₁Has a height H₂And width W₂Two lobes of (a), wherein H₁Is lower than H₂And is andwhereinθ_iIs the angle of the incident light with respect to the normal to the top surface of the diffraction grating, and n₁is the refractive index of the bulk dielectric in which the diffraction grating is placed.

6. The optical device of any one of claims 4 or 5, W₁Is the distance separating the two blocks or the width of the central block of the block having a U-shape, and W₂Is the width of each of the two blocks or the width of each of the two lobes, where W is₁And W₂Dependent on the grating pitch d of the diffraction grating, whereinAnd is

7. The optical device according to any of claims 1-6, wherein the base pattern has a symmetrical geometry.

8. The optical device according to any of claims 1-6, wherein the base pattern has an asymmetric geometry.

9. The optical device of any one of claims 1-8, wherein the diffraction grating is configured to diffract light of a set of wavelengths including more than one wavelength, and wherein the grating pitch is above a highest wavelength in the set of wavelengths.

10. The optical device according to any one of claims 1-9, comprising one diffraction grating per red, green and blue.

11. The optical device of any one of claims 1-9, wherein the diffraction grating is configured for in-coupling light into the optical waveguide or for extracting light out of the optical waveguide.

12. The optical device according to any one of claims 1-9, wherein the diffraction grating is configured for in-coupling light entering the optical waveguide, and wherein the optical waveguide comprises a further diffraction grating configured for extracting light from the optical waveguide, the further diffraction grating having a grating pitch higher than a wavelength of the light, and the further diffraction grating being configured for diffracting the light in diffraction orders having an absolute value equal to or larger than 2.

13. The optical apparatus of claim 2, wherein the base pattern of the diffraction grating comprises a layer having an index of refraction n on top of the substrate₂Has a U-shape, a base angle between a top surface of the U-shape and a side surface of the U-shape being different from 90 °, and wherein n is₃＝n₂。

14. An eyewear apparatus comprising at least one optical device according to any one of claims 1 to 13.

15. The eyewear apparatus of claim 14, comprising:

a light display engine configured for emitting an image to be displayed,

optics configured to couple incident light from the light display engine to the light guide,

-the optical waveguide configured for directing incident light towards an eye of a user such that the image is visible to the user.

1. Field of the invention

The present disclosure relates to the field of optics and photonics, and more particularly to optical devices comprising at least one diffraction grating. It may be applied in the field of comfortable and wearable optics, i.e. AR/VR glasses (augmented reality/virtual reality), and various other electronic consumer products including displays and/or lightweight imaging systems.

2. Background of the invention

This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.

AR/VR glasses are considered to be the next generation of human-machine interfaces, thus raising the significant interest of major industrial players in the fields of consumer electronics and mobile devices.

The development of AR/VR glasses (and more generally, glasses electronics) is associated with a number of challenges, including reducing the size and weight of such devices and improving image quality (in terms of contrast, field of view, color depth, etc.), which should be sufficiently realistic to enable a truly immersive user experience.

The trade-off between image quality and the physical size of optical components has prompted the search for ultra-compact optical components that can be used as building blocks for more complex optical systems, such as AR/VR glasses. Such an optical component should also be easy to manufacture and replicate.

In such AR/VR glasses, various types of refractive and diffractive lenses and beam forming components are used to direct light from a microdisplay or projector to the eye of the eye, allowing the formation of a virtual image superimposed with an image of the physical world seen with the naked eye (in the case of AR glasses) or captured by a camera (in the case of VR glasses).

Some types of AR/VR glasses use optical waveguides where light propagates into the optical waveguide through TIR (total internal reflection) only over a limited range of internal angles. The FoV (field of view) of a waveguide depends on the material of the waveguide.

The FoV of a waveguide is defined as the maximum span it can propagate into the waveguide by TIRGenerally, as shown in FIG. 1A, the maximum angular span that can be coupled into the waveguide is defined by two rays: having an angle of incidenceCritical ray of (in fig. 1A)) And has an angle of incidenceGrazing ray of (FIG. 1A)). The critical ray is at a critical angleLight diffracted into the waveguide, critical angleByIs defined in which n₂Is the refractive index of the waveguide material and λ is the wavelength of the incident light. The grazing rays having grazing incidenceDiffracting the rays at the input angle into the waveguide. The theoretical FoV of the waveguide proposed above is for a single mode system, where one single diffraction mode is used to carry the image: or +1 or-1 diffraction mode.

In WO2017180403, a waveguide with an extended field of view is proposed, where bimodal image propagation is used. In this approach, the diffraction mode +1 is used to carry the right-hand image in one direction (negative angle of incidence on the non-coupler), while the-1 mode is used to propagate normal angles of incidence into the waveguide in the opposite direction. Such a system is shown in FIG. 1B, which shows the critical and glancing angles for the two diffraction modes. In WO2017180403, combining the two half-images is done so that the user sees one single image due to the pupil expander and the out-coupler at the exit of the waveguide. The benefit of this system is to double the field of view, since each half of the image can use the entire angular bandwidth of the waveguide in each propagation direction.

However, such optical waveguides and most optical waveguides include diffraction gratings. The period d (also called the grating pitch) of such a diffraction grating depends on the wavelength λ of the incident light and the refractive index n of the waveguide material₂And can be defined by the following formula

If we consider the ratio between grating pitch and wavelength: d/λ, in the case of equation 1 given above, then 3/2 < n₂< 2 and 2/3 < d/λ < 4/5, and in any case d/λ < 1 is a value that can be defined as a subwavelength. Equation 1 means in any case that the diffraction grating has a subwavelength structure.

In US20160231568, a waveguide for spectacles is disclosed in which the grating pitch of the structure is between 250 and 500 nm. This geometry makes the grating very difficult to manufacture. Since the structure is sub-wavelength and the required precision is even challenging for e-beam lithography, lithography is not possible.

Accordingly, there is a need for an improved optical waveguide that includes a diffraction grating.

3. Summary of the invention

An optical device comprising a diffraction grating configured to diffract incident light, the diffraction grating having a grating pitch higher than a wavelength of the incident light and being configured to diffract the incident light in diffraction orders having an absolute value equal to or greater than 2, wherein the optical device comprises an optical waveguide configured to guide the incident light diffracted in diffraction orders having an absolute value equal to or greater than 2.

Thus, according to the present disclosure, a diffraction grating is dedicated to a wavelength or a set of wavelengths. The diffraction grating is configured such that a major portion of incident light is diffracted in diffraction orders having an absolute value equal to or greater than 2. Using higher diffraction orders than in prior art systems means | M | > 1, where M is a diffraction order, has the effect of multiplying the wavelength by the order, which is used in the diffraction equation. Since the grating pitch is a function of the product M λ, this means that the grating pitch is multiplied by M, and the structure for the inner coupler is much larger. This opens up new possibilities in manufacturing technology, since nanoimprinting can be used. We also get fewer lines per mm for the grating density and can optimize the manufacturing process since the structure will no longer be sub-wavelength but super-wavelength.

In accordance with the present disclosure, there is provided,where d is the grating pitch and λ is the wavelength, the diffraction grating thus has a super-wavelength structure which imposes less constraints on fabrication than a sub-wavelength structure.

According to an embodiment of the present disclosure, the diffraction grating includes a grating having a refractive index n₃And a substrate of a first dielectric material deposited on said substrate and having a refractive index n₂At least one second dielectric material of (2), wherein n₃＜n₂Or n₃＝n₂。

According to an embodiment of the present disclosure, the diffraction grating comprises a base pattern comprising the second dielectric material, the base pattern being configured to form a nano-jet beam associated with an edge of the base pattern from the incident light on the diffraction grating. According to this embodiment, the base pattern of the diffraction grating comprises edges forming steps such that when light is incident on the optical device, a nano-jet beam is formed associated with the edges of the base pattern.

Advantageously, according to this embodiment of the present disclosure, the nano-jet formation phenomenon is utilized to convert energy from a major portion of incident light into a second order diffracted beam. Using a base pattern configured to form a nano-jet beam from the edges of the base pattern allows for providing high diffraction efficiency and high diffraction uniformity.

The prior art system cannot operate at higher orders of diffraction because the configuration of the prior art system provides diffraction efficiencies close to 0. Using a base pattern configured to form a nano-jet beam allows for achieving high diffraction efficiency while diffraction uniformity is on average and at least equal to that produced by prior art systems at lower diffraction orders.

According to another embodiment of the present disclosure, the base pattern is configured according to any one of the following arrangements:

the basic pattern comprises a refractive index n on top of the substrate₃Of a first dielectric material of (a), the refractive index being n₃Is inserted into the block of the first same dielectric material having a refractive index n₂In a block of said second dielectric material, or

The base pattern comprises a refractive index n on top of the substrate₃Of a block of said first dielectric material having a refractive index n₃Is placed in a refractive index n₂Between two blocks of said second dielectric material, having a refractive index n₃And the bulk and refractive index of the first dielectric material of (a) is n₂Have the same height, or

The base pattern comprises a refractive index n of the same width and height on top of the substrate₂Two blocks of said second dielectric material, said two blocks being separated by a distance, or

Said basic pattern comprising a refractive index n on top of said substrate₂The block of the second dielectric material, the block having a U-shape.

According to another embodiment of the disclosure, when the base pattern is comprised on top of the substrateHaving a refractive index n of the same width and height₂Of said second dielectric material, said two blocks being separated by a distance W₁Separated, the two blocks having a height H₂WhereinEyes of a userWherein W₂Is the width of each of the two blocks, and θ'_B1And θ ″)_B1Is a respective angle of nano-jet beam radiation associated with an edge of a base pattern from the light incident on the at least one diffraction grating, whereinWherein theta is_iIs the angle of the incident light with respect to the normal to the top surface of the diffraction grating andn₁is the refractive index of the bulk dielectric in which the diffraction grating is placed.

According to another embodiment of the disclosure, when the base pattern comprises on top of the substrate a layer having a refractive index n₂When said second dielectric material has a block with a U-shape, said U-shape comprises a height H₁Has a height H of separation of the central block₂And a width W₂Two blocks of (2), wherein H₁Is lower than H₂And is andwhereinWherein theta is_iIs the angle of the incident light with respect to the normal to the top surface of the diffraction grating, andn₁to it isThe refractive index of the bulk dielectric in which the diffraction grating is placed.

According to another embodiment of the present disclosure, W₁Is the distance separating two blocks or the width of the central block with the block in the shape of a U, W₂Is the width of each of the two blocks or the width of each of the two lobes, W₁And W₂Dependent on the grating pitch d of the diffraction grating, whereinAnd is

According to another embodiment of the present disclosure, the base pattern has a symmetrical geometry.

According to this embodiment, the optical device can be used in dual mode. For example, it may be implemented in a waveguide that separates the right-hand side and the left-hand side of the input image to double the field of view of the waveguide.

According to another embodiment of the present disclosure, the base pattern has an asymmetric geometry.

According to this embodiment, the optical device is designed for single mode

Diffraction, e.g. for shifting the image to a single side of the waveguide. This embodiment allows for an even larger grating pitch.

According to another embodiment of the present disclosure, the diffraction grating is configured to diffract light of a set of wavelengths including more than one wavelength, and the grating pitch is greater than a highest wavelength of the set of wavelengths.

According to another embodiment of the invention, an optical device according to any of the above disclosed embodiments comprises one diffraction grating per red, green and blue.

According to another embodiment of the present invention, the optical device is an optical waveguide. The optical device may advantageously be used as a waveguide, for example for AR/VR glasses.

According to another embodiment of the disclosure, the diffraction grating is configured for in-coupling light into the optical waveguide or for extracting light out of the optical waveguide.

According to another embodiment of the disclosure, the diffraction grating is configured for in-coupling light entering the optical waveguide, and the optical waveguide comprises a further diffraction grating configured for extracting light out of the optical waveguide, the further diffraction grating having a grating pitch larger than a wavelength of the extracted light, and the further diffraction grating is configured for diffracting the extracted light in diffraction orders having an absolute value equal to or larger than 2.

According to another aspect of the present disclosure, an eyeglass apparatus is disclosed. Such an eyeglass arrangement comprises at least one optical device according to any of the above embodiments.

According to an embodiment of the present invention, an eyeglass device includes:

a light display engine configured for emitting an image to be displayed,

-a set of optics configured to couple incident light from the light display engine to the light guide,

-the optical waveguide is configured for guiding the incident light towards an eye of a user such that the image is visible to the user.

4. Description of the drawings

The disclosure may be better understood with reference to the following description and accompanying drawings, which are given by way of example and do not limit the scope of protection, and in which:

figure 1A shows the definition of theoretical critical and grazing rays in a single mode,

FIG. 1B shows the definition of theoretical critical and grazing rays in the dual mode,

figure 2 shows the diffraction pattern of an incident plane wave,

FIG. 3 shows the definition of significant rays of the negative angle input space,

fig. 4A and 4B show an exemplary 60 ° field of view light engine 60. .

Fig. 5 shows a schematic view of the light engine of fig. 4A and 4B, wherein the waveguide is provided with an in-coupling grating.

Fig. 6 shows exemplary geometries and pitch dimensions of the basic pattern of the diffraction grating.

Fig. 7 shows the diffraction performance of the grating structure shown in fig. 6.

Fig. 8 shows the diffraction performance of the grating structure shown in fig. 6 without high refractive index material.

Fig. 9 shows (a) a cross-sectional view of a two-material super element (mataement) with the insert shown in fig. 6; (b) power density distribution in the xy plane of a superelement with the following parameters: n is₁＝1.0，n₂＝2.105，W₁＝260nm，W₂＝100nm，H＝700nm，H₂＝300nm，λ＝625nm，θ_i＝0°。

Fig. 10 shows (a) a cross-sectional view of a bi-material super-element with an insert as shown in fig. 6; (b) power density distribution in the xy plane of a superelement with the following parameters: n is₁＝1.0，n₂＝2.105，W₁＝260nm，W₂＝100nm，H＝700nm，H₂＝300nm，λ＝625nm，θ_i＝30°。

Fig. 11 shows (a) Hy component distribution and (b) power density distribution of the super element shown in fig. 6 in the xy plane, where the parameters are: n is₁＝1.0，n₂＝2.105，n₃＝1.52，W₁＝260nm，W₂＝100nm，H＝700nm，H₂＝300nm，λ＝625nm。

Fig. 12 shows exemplary geometries and pitch dimensions of an alternative embodiment of the basic pattern of the diffraction grating.

Figure 13 shows the performance of a grating based on the geometry shown in figure 12.

Fig. 14A shows a cross-sectional view (left part) of a bi-material super-element with an equal height insert and the power density distribution (right part) of said super-element in the xy-plane, with the parameters: n is₁＝1.0，n₂＝2.105，n₃＝1.52，W₁＝260nm，W₂＝130nm，H＝305nm，H₂＝300nm，λ＝625nm and theta_i＝0°。

Fig. 14B shows a cross-sectional view (left part) of a bi-material super-element with an equal height insert and the power density distribution (right part) of said super-element in the xy-plane, with the parameters: n is₁＝1.0，n₂＝2.105，n₃＝1.52，W₁＝260nm，W₂＝130nm，H＝305nm，H₂300nm, 625nm and theta_i＝30°。

Fig. 15 shows (a) Hy component distribution and (b) power density distribution of the super element shown in fig. 12 in the xy plane, where the parameters are: n is₁＝1.0，n₂＝2.105，n₃＝1.52，W₁＝260nm，W₂＝130nm，H＝305nm，H₂＝300nm，λ＝625nm。

Fig. 16 shows another exemplary geometry of the basic pattern of the diffraction grating according to an embodiment of the present disclosure.

Figure 17 shows the performance of the structure shown in figure 16.

Fig. 18 shows an exemplary regular structure of a diffraction grating according to.

Fig. 19 shows the diffraction efficiency of the structure shown in fig. 18.

Fig. 20 shows a cross-section of the single-material super-element shown in fig. 16 (left part) and 18 (right part) with the following parameters: n is₁＝1.0，n₂＝2.105，W₁＝260nm，W₂＝130nm，H＝200nm，θ_i＝0°。

Fig. 21 shows a cross-sectional view of the single-material super-element shown in fig. 16 (left part) and 18 (right part) with the following parameters: n is₁＝1.0，n₂＝2.105，W₁＝260nm，W₂＝130nm，H＝200nm，θ_i＝30°。

Fig. 22A shows (a) Hy component distribution and (b) power density distribution in the xy plane for a single NJ element shown in fig. 18, where the parameters are: n is₁＝1.0，n₂＝2.105，n₃＝1.52，W₁＝260nm，W₂＝130nm，H₂＝200nm，λ＝625nm。

Fig. 22B shows (c) Hy component distribution and (d) power density distribution of the twin structure (twin structure) super element shown in fig. 16 in the xy plane, where the parameters are: n is₁＝1.0，n₂＝2.105，n₃＝1.52，W₁＝260nm，W₂＝130nm，H₂＝200nm，λ＝625nm。

Figure 23 shows the power density distribution in the xy-plane of the twin structural super element shown in figure 16, with the parameters: n is₁＝1.0，n₂＝2.105，W₁＝260nm，W₂＝130nm，H₂＝200nm，λ＝625nm；n₃(in the left column) ═ 2.105, n₃(in the right column) ═ 1.52.

Fig. 24 shows another exemplary geometry of the basic pattern of the diffraction grating according to an embodiment of the present disclosure.

Figure 25 shows the performance of the U-shaped structure shown in figure 24.

Fig. 26 shows (a) Hy component distribution and (b) power density distribution of the U-shaped super element shown in fig. 24 in the xy plane, where the parameters are: n is₁＝1.0，n₂＝2.105，n₃＝1.52，W₁＝260nm，W₂＝130nm，H₂＝200nm，H₁＝50nm，λ＝625nm。

Fig. 27 shows the performance for different pitch sizes with d 823 ± 5nm for the U-shaped structure shown in fig. 24: the top shows the performance at 818nm and the bottom shows the performance at 832 nm.

Fig. 28 shows exemplary geometries and pitch dimensions of another embodiment of the basic pattern of the diffraction grating.

Fig. 29 shows the performance of the geometry shown in fig. 28, showing the +2 diffraction order efficiency as a function of the angle of incidence.

Figure 30 shows a schematic perspective view of an eyeglass apparatus according to an embodiment of the present disclosure,

figure 31 shows a schematic front view of the eyewear device of figure 30,

figure 32 shows an exemplary diffraction grating according to an embodiment of the present disclosure,

FIG. 33 shows a single-material super-element (n) with non-vertical edges₂＝n₃) And a single-material super-element (n) having a top surface that is not parallel to the xz-plane (fig. 33a, 33b)₂＝n₃) Is shown in cross-sectional view (figure 33c),

figure 34(a) shows a cross-sectional view of a diffraction grating cell; fig. 34(b) shows the diffraction performance of the grating shown in fig. 34 (a).

5. Detailed description of the preferred embodiments

In accordance with the present principles, an optical device will be described that includes at least one diffraction grating. Such an optical device may be used as a waveguide for AR/VR glasses, for example, in accordance with embodiments of the present disclosure.

According to the present disclosure, the optical device presents a specific diffraction grating, which may be used for in-coupling light into the optical device and/or out-coupling light from the optical device.

In accordance with the present principles, a diffraction grating is configured to diffract incident light in diffraction orders having an absolute value equal to or greater than 2.

5.1 diffraction Pattern

Fig. 2 schematically shows what happens to a Plane Wave (PW) incident on a linear grating (DG). The plane wave is split into diffraction modes (-2, -1, 0, +1, +2), which are angularly spaced beams of local maximum intensity. Although only diffraction pattern numbers-2, -1, 0, 1, and 2 are shown, higher orders are present. The zeroth order is generally of greater power and is the natural mode in which normal refraction or reflection will occur. A diffraction pattern of reflection may also be present.

Typically, a grating produces many diffraction orders. In many applications, it is desirable to use only the first order and avoid all other modes. An example of such a grating is a thick hologram made of the interference of two plane waves.

The purpose of the structure of the diffraction grating is then to prioritize the first diffraction order and to suppress the remaining diffraction orders as much as possible.

All diffraction orders are linked to the incident beam by the following mathematical formula: n is₂(λ)sinθ₂-n₁(λ)sinθ₁＝MλT，

Wherein n is₁(lambda) and n₂(λ) is the refractive index of the dielectric into which the light propagates separately, where, in most cases, n₁(λ)＝1，θ₁Is the angle of incidence, θ₂Is the diffraction angle, M is the diffraction order, λ is the wavelength, T is the grating frequency, which is expressed in lines per μ M if the wavelength is also expressed in μ M.

For a diffraction order M of 0, the formula reduces to the well-known snell-cartesian law of refraction. However, this equation does not take into account different phenomena. Depending on the polarization of the input beam, the geometry of the basic structures used to construct the array, and the materials used, there may be lost modes. Furthermore, this equation does not take into account the energy redistribution of the basic structure.

5.2 specifying wavelength

Typically, the light engine includes a light source and a display. The light sources are power LEDs that are driven in a time sequence. The design of the planar optics should be adapted to the wavelength of the LED. Examples of common LEDs for near-eye projection are: 459nm blue, 530nm true green and 625nm red. The diffraction process is very dispersive. The diffraction angle is different for different wavelengths because it varies linearly with wavelength (M λ T), which is a considerable variation. Therefore, it is necessary to find a method of minimizing color difference. One way to deal with this is to have one waveguide per color band, because for each color band (e.g., red, green, blue), the diffraction grating of each waveguide is configured differently depending on the color band. Therefore, if RGB true-color images are considered, three waveguides are required, which may complicate their design.

5.3 ultra-wavelength in-coupling grating design

Fig. 3 shows the definition of some important rays for characterizing a waveguide. Capital lettersAndrespectively representing grazing rays and critical rays. If the subscript is 1, then,the ray is outside the waveguide and if the index is 2, the ray is inside the waveguide. In principle, the maximum input angular bandwidth of the grating according to the figure isAnd the maximum waveguide angular bandwidth isThat is to say, raysIs not a desirable option because it is not possible to extract the ray. Thus, in practice, the angular bandwidth inside the waveguide will be limited to(the angular sector indicated by ABDW on FIG. 3), which corresponds to the angular range of the input

With respect to the angle sign convention, a positive angle measure is oriented in a trigonometric direction, which means on the graphAnd all othersAnother convention is: the diffracted rays in the graph are all positive values, the diffraction pattern is positive, M > 0, and if we apply the diffraction equation to this set of rays, we get the following 4:

in order to select a grating period d capable of diffracting radiation as shown in fig. 3, some selection may be required, in particular with respect to the radiation inside the waveguideThe selection may be based on the distance the image must travel in the waveguide before being extracted, the amount of TIR bounce, and the thickness of the waveguide.

For example, if an image of about 4cm needs to be extracted from the injection in the waveguide, this is the distance between the exit pupil of the light engine in the glasses branch and the eye. Then, when it is assumed that the light engine and the eye are in the same half-space with respect to the waveguide, this means that the extraction ports are even. X_i ^C，GThe distance between the point of input of the critical or grazing ray inside the waveguide and the point at which the ray bounces the ith time on the face of the waveguide. The index i represents the number of times a ray inside the waveguide is bounced before being extracted, i may also be referred to as an extraction port. If i is even, the extraction port is on the same side of the waveguide as the light engine, and if i is odd, the extraction port is on the opposite side of the waveguide from the light engine. The way this definition of the actual usage abstraction port is at point X₂ ^C，X₂ ^G]With a diffraction grating disposed therebetween, the result will be an image output by the collection waveguide by diffraction between these two points. In other words, the image is collected at the second extraction point, i.e., the image is considered to be extracted at port No. 2.

Some values useful for the design of the system can be defined in table 1, according to an example of a 24 degree field of view required for a glass substrate with a refractive index of 1.5. The values in the "input" column are the appropriate values that have been selected for designing the system, while the values in the "calculate" column are the parameters of the system obtained by using the "input" values.

Table 1: parameters of the grating design

In this exemplary system, a reasonable value for the grazing angle in the waveguide may be selected to be between 60 ° and 90 °, e.g., the grating angle is selected to be approximatelyThis value of the grazing angle makes it easier to extract from the waveguide, however,can also be selected, e.g.In practice, it is desirable to design a diffraction grating that diffracts all θ 1 angles of a particular sign into one direction and diffracts angles having the opposite sign into the opposite direction.

In this way, the waveguide will operate in dual modes, according to which the field of view is divided in half, half of the field of view (i.e. half of the image) being directed in one direction and the other half being directed in the other direction, with the positive result of doubling the field of view. This means thatNeeds to be selected andthe same angular sign and in the vicinity of the normal.

Another condition is that there should be no cross-talk between the positive and negative stages, which means that for a given orientation of the impinging optocoupler there should always be one and only one diffraction direction, and no energy goes into the other direction. In fig. 3, the condition is forAnd in order not to reduce the input field of view, preferably,from equation 2 discussed above, the grating pitch can be obtained as:

by using equation 3 discussed above,obtained according to the following formula:

the maximum angular span of the negative-angle input beam isCoupled into a waveguide to span an angular bandwidth

For n₂∈[3/2，2]And considering the second diffraction order, | M | > 2, the relationship between the pitch size and the wavelength is in any case d/λ ≧ 1, which means that the structure of the grating will be super-wavelength.

If instead of coupling the first diffraction order into the waveguide, the pitch of the grating is chosen in such a way that higher orders are coupled, the pitch of the grating is larger and the limitations of the microfabrication process are avoided.

Table 2 shows the grating designed to couple two orders and the use thereofThe difference between the gratings at one level. The difference in pitch size is almost doubled. For the second diffraction order, we get the following values of spacing for the RGB inner coupler: d₆₂₅＝822.4nm，d₅₃₀＝697.4nm，d₄₆₀A value of 605.3nm, rather than the very small pitch size obtained for the first diffraction order: d₆₂₅＝411.2nm，d₅₃₀＝348.7nm，d₄₆0＝302.7nm。

Gratings that use the second diffraction order are called super-wavelength gratings because their pitch is always larger than the wavelength of the color band for which they are designed. Gratings that use the first diffraction order are referred to as sub-wavelength gratings because their pitch is smaller than the wavelength of the color band for which they are designed.

The field of view FoV in table 2 is given for a system using two ± 2 diffraction orders. It is twice the field of view of previous systems that operate with only one direction into the waveguide.

The advantage of using the order ± 1 is that for very low diffraction orders a symmetric response curve is provided in both directions, while emphasizing the order +1 or-1 with very high diffraction efficiency and with high diffraction uniformity.

As described below, the nanojet-based diffraction grating disclosed herein allows a symmetric response of ± 2 diffraction orders with very high diffraction uniformity, which is advantageous.

Table 2: pitch and angular bandwidth of grating

Fig. 4A and 4B illustrate exemplary illustrative light engines that provide a 60 ° field of view.

The light engine includes a display that emits incident light to a set of optics that couple the incident light to an exit pupil disposed at a final surface after a final lens of the set of optics, where the rays intersect into a smallest circular cross-section.

As shown in fig. 5, the diffraction grating will be placed at the exit pupil. In fig. 4A, only rays of negative incidence angle enter the in-coupling grating (exit pupil). These rays come from the upper part of the display. In fig. 4B, only light rays at normal incidence angle enter the in-coupling grating (exit pupil). These rays come from the lower part of the display.

Fig. 5 shows the light engine from fig. 4A and 4B, with a Waveguide (WG) provided with an in-coupling grating. The order M is 2 on the left hand side and the order M is-2 on the right hand side. If the display is turned on, both ray paths exist simultaneously. There is an angular range of ± 2.97 ° along the central horizontal portion of the display from which light is not coupled into the waveguide. This part of the display will always be black.

Fig. 5 is a schematic diagram of a light engine obtained from a simulation. Only the incoupling of incident light into the waveguide is shown. On the simulated view, the light is extracted at the output without using an extraction port.

5.4 Nanofluidic based geometric elements for Dual-mode and 2-order diffraction gratings

In all of the following subsections, different exemplary geometries will be presented that achieve high performance for the new principles presented in this disclosure.

Furthermore, a set of equations is provided to demonstrate the contribution of the edge diffraction phenomenon disclosed in "Near field focusing by edge diffraction", a. boriking, v. drazic, r. keying, m.damghann, o.shramkova, l.blond é, optical bulletin, volume 43, phase 16, page 4053-.

The proposed data was obtained using COMSOL multi-physics field software. The proposed analysis of the field and power distribution within the so-called superelement of the grating helps to explain the physical properties of the edge diffraction phenomenon and to obtain an optimal topology. Assume that the system is illuminated by a linearly polarized plane wave E ═ 0, 0, 1 (TE). The effect of the parameters of the individual superelements on the system function is taken into account. As demonstrated in the above cited documents, the nano-jet (NJ) beamforming phenomenon is only associated with the edges of the system. Based on the analysis of the wedge diffraction phenomenon disclosed in "Near field focusing by edge diffraction", a. borisikin, v. drazic, r. keying, m.damghaian, o.shramkova, l.blond é, optical bulletin, volume 43, phase 16, page 4053 + 4056 (2018) ", the deflection angle of the nano-jet (NJ) beam in denser dielectrics can be obtained with normal incidence of electromagnetic waves (θ i 90 °).

Wherein n is according to an embodiment of the present disclosure_LIs the refractive index of the bulk dielectric, n_HIs the refractive index of the higher index material.

In the case of a single wedge of a single material element, constructive interference phenomena between the jet wave generated by the wedge and the plane wave refracted by the edge of the wedge result in the generation of an NJ beam.

It should also be noted that, in the following, n unless otherwise specified₁Is the refractive index of the bulk dielectric, n₂Denotes the refractive index of the high refractive index material, and n₃Indicating a lower index of refraction, such as that of a glass substrate.

5.2.1 two-Material solution with insert

An exemplary geometry of a base pattern of a diffraction grating configured to diffract light having a diffraction order equal to or greater than 2 in absolute value is disclosed in fig. 6.

In fig. 6, the base pattern includes two dielectric materials: refractive index n₂Of dielectric material ME2 placed at a refractive index n₃On top of the dielectric material layer ME 3. Layer ME3 presents block B3 with the same dielectric material, block B3 being inserted in block ME 2.

The structure of fig. 6 has been optimized to provide optimum performance for the red wavelength λ 625 nm. Other slightly varying parameters may be used for the variable aspect of the diffraction profile. This structure is symmetrical because it requires the positive and negative diffraction orders to be fed in a symmetrical manner.

To fabricate this structure, a glass etch is first required to create a first structure in the base material of the waveguide (layer ME 3). An e-beam lithographic resist was then spin-coated on top of the structure and exposed and etched again to add a second component (ME2 block).

Both ME2 and ME3 components are dielectric transparent materials. As can be seen from the dimensions shown in fig. 6, the structure has a much larger pitch than the prior art: 822.4 nm instead of 496 nm. The aspect ratio is 700/460 ≈ 1.5, which is a low aspect ratio, while the depth is not much larger than the width.

FIG. 7 shows the diffraction performance of the grating of FIG. 6, which shows η for a field of view of 54 ° for 2 × (30 ° -3 °)_max≈65％，Each design has absolutely no crosstalk between the +2 and-2 orders. Some angles with diffraction in the +2 and-2 orders should be avoided at this design stage. The 0 order, +1 order, and-1 order are not coupled into the waveguide. They are transmitted through it and therefore do not reduce the contrast of the virtual image projected by the light engine display.

In fig. 7, when second order diffraction is used instead of first order diffraction, the diffraction uniformity is of the same order as in the prior art system. Very high diffraction efficiencies were obtained, as highlighted in fig. 8, indicating the diffraction efficiency of the same system without nanojet enhancement (i.e., the high index material ME2 was removed to leave only the etched waveguide). In this case, the diffraction efficiencies of the +2 and-2 orders are not important, as shown in fig. 8.

Fig. 9 shows (a) a cross-sectional view of a bi-material super-element with an insert such as that shown in fig. 6, and (b) the power density distribution in the xy-plane of such a super-element with the following parameters: n is₁＝1.0，n₂＝2.105，W₁＝260nm，W₂＝100nm，H＝700nm，H₂＝300nm。λ＝625nm，θi＝0°。

The symmetric superelement will have a refractive index n₃Width W₁And height H₂Having a refractive index n₂Width 2W₂+W₁And total heightH (as shown in fig. 9). Suppose n₁Is the refractive index of the main dielectric, and n₁＜n₂＜n₃. As a result, in the proposed system, for θ_i＝O°(θ_iIs the angle of incidence), we will observe the radiation of 4 nanojets with the following offset angles:

for the proposed symmetric system, two opposite edges of the block (ME2 in fig. 9 (a)) are generated with refractive index n₂Has a radiation angle theta of propagation inside the block ME2_B1Of the 2 nano-jets (NJ1, see dashed line starting at the top edge of block ME2 in fig. 9 (a)), the hot spot of the power distribution inside the superelement in fig. 9(b) corresponds to the intersection of these two NJs. It should be noted that in this cross-sectional view of fig. 9, no refraction phenomena at the boundary between the insert B3 and the main block ME2 are considered.

At a refractive index of n₂Has a radiation angle theta inside the block ME2_B2Of a second pair NJ of n refractive indices₃The edge of the center block B3 of (NJ2, see dashed line from the top edge of block B3 in fig. 9 (a)). The propagation direction of these NJs changes due to total internal reflection of the wave at the vertical edge of block ME 2. As a result, at the bottom surface of the superelement, we can observe two less dense hot spots at the intersection of NJ1 and NJ2 (the intersection of dashed lines NJ1 and NJ2 on each side of block B3, refer to CR1 and CR 2).

Fig. 10 shows (a) a cross-sectional view of a bi-material super-element with an interposer, such as that shown in fig. 6, and (b) the power density distribution in the xy-plane of the super-element with the following parameters: n is₁＝1.0，n₂＝2.105，W₁＝260nm，W₂＝100nm，H＝700nm，H₂＝300nm。λ＝625nm，θ_i＝30°。

The radiation angles theta' and theta "of the opposite edges of the system are not equal (see fig. 10 (a)). As a result, for block ME 2:

in a similar manner, the radiation angle of the nano-jet of the insert (block B3) may be determined as:

the reflection of the generated wave results in the generation of a new NJ hot spot (cross-over point) at the edge of the constituent parts of the super-element and an asymmetric redistribution of the total power inside the super-element, as shown in fig. 10 (b).

As shown in FIG. 6, the blocks ME2 and B3 are placed at a refractive index n₃On a substrate ME 3. Fig. 11(a) shows the Hy component field distribution (i.e. the distribution corresponding to the projection of the magnetic field on the y-axis) of the superelement shown in fig. 6 in the xy-plane, and fig. 11(b) shows the power density distribution of the superelement shown in fig. 6 in the xy-plane with the parameters: n is₁＝1.0，n₂＝2.105，n₃＝1.52，W₁＝260nm，W₂＝100nm，H＝700nm，H₂300 nm. For three different angles of incidence theta_i，λ＝625nm。

Under the super-element, NJ is obtained inside the substrate (ME3) after a corresponding wave refraction at the surface of the substrate. The power distribution shown in fig. 11(b) shows dense lobes within the substrate resulting from wave interference. Constructive interference between the refracted waves obtained from wedge diffraction of the periodic array of superelements results in a redistribution of power between the respective diffraction orders. The central insert (B3) with the lower refractive index helps to suppress almost completely the intensity of the 0 diffraction order in case of normal incidence, while significantly reducing this intensity in case of oblique incidence. Furthermore, the presented topology helps to change the propagation direction of the waves diffracted by the wedges of the elements of the diffraction grating and to increase the intensity of the ± 2 nd diffraction orders (see fig. 7).

5.2.2 Bi-Material and contour insert

The structure of fig. 6 is not the only structure that can be used to achieve the correct performance, even though it may be the preferred structure for a microfabrication process. Another geometry of the basic pattern according to another embodiment, which can achieve comparable performance, is shown in fig. 12.

In the embodiment shown in fig. 12, the base pattern comprises a material having the same refractive index n as the material of the substrate ME3₃Block B3 of the same dielectric material. Block B3 is placed on top of substrate ME3 and is located at refractive index n₂Between two blocks of dielectric material ME21 and ME 22. Blocks B3, ME21 and ME22 have the same height.

The values shown in fig. 12 have been optimized for a wavelength of 625 nm. However, a slightly different value may be used for the wavelength of 625 nm. Also, other values may be used for other wavelengths as long as the grating pitch d is greater than the wavelength of light dedicated for diffraction.

Figure 13 shows the performance of a diffraction grating based on the geometry of figure 12.

As can be seen from fig. 13, the maximum diffraction efficiency is 55%, which can be considered low. By using a catalyst having a higher n₂With the material of value (2.3 to 2.5), the efficiency will increase significantly. n is₂The value used is 2.105 based on available silicon nitride e-beam lithography compatible materials.

When considering the combination placement at having a refractive index n₂Width W₂And 2 similar blocks of the total height H have a refractive index n between them₃Width W_iAnd a block of height H, as shown in fig. 12, a nano-fluidic analysis of energy transfer to a higher level can be performed.

Suppose n₁Is the refractive index of the main dielectric, and n₁＜n₂＜n₃。

FIGS. 14A and 14B illustrate cross-sectional views of a two-material metamaterial having an index of refraction n₂Two blocks ME22 and ME21 and having a refractive index n₃With respect to the super element with the following parameters, the three blocks ME22, ME21 and B3 have equal height (left part of fig. 14A) and xyPower density distribution in the plane (right part of fig. 14A): n is₁＝1.0，n₂＝2.105，n₃＝1.52，W₁＝260nm，W₂＝130nm，H＝305nm，H₂300 nm. λ 625nm, for fig. 14A, θ_i0 ° (normal incidence), for fig. 14B, θ_i＝30°。

As a result, in the case of normal incidence (FIG. 14A), the refractive index n is respectively increased by₂And a dielectric ME21 having a refractive index n₁And on the other hand has a refractive index n₂And a dielectric ME22 having a refractive index n₁The outer edge between the main dielectrics of (a) generates two NJs. The two NJ beams being at an angle theta_B1And (5) spreading. In the presence of a refractive index n₂Block ME22 (or corresponding block ME21) and having a refractive index n₃Respectively, to generate two NJs at the edges between the blocks B3. Two NJ are each at an angle θ_B2Propagation (see dashed line in fig. 14A).

Having a refractive index n in FIG. 14A₂Corresponds to the intersection of two NJs with different radiation angles. The propagation of the jet beam diffracted by the outer wedge into the central part of the superelement (insert B3) results in a new hot spot outside the structure along the symmetry axis of the superelement.

It should be noted that the illustrated schematic distribution of NJ does not take into account refraction phenomena that cause the intersection point of NJ to shift along the axis of symmetry. The presence of such NJ hot spots explains the high intensity of the 0 diffraction order at normal incidence for each superelement (see fig. 13).

Fig. 15(a) shows the Hy component field distribution of the super element shown in fig. 12 in the xy plane, and fig. 15(b) shows the power density distribution of the super element shown in fig. 12 in the xy plane, where the parameters are: n is₁＝1.0，n₂＝2.105，n₃＝1.52，W₁＝260nm，W₂＝130nm，H＝305nm，H₂300 nm. When the geometry and refractive index shown in FIG. 12 is n₃When placed together, λ 625nm for three different angles of incidence.

By changing the incident angle of the electromagnetic wave from 0 degrees to +30 degrees, the wave diffracted by the inner wedge (wedge of the interposer) is transmitted into the substrate (see fig. 15(a)), and is input to the 2 nd diffraction order. The power distribution shown in fig. 15(b) shows dense lobes within the substrate resulting from interference of diffracted waves. For negative angles of incidence, there is a strengthening of the ± 2 nd diffraction order. As in the case of a superelement with an insert (fig. 6), the phenomenon of total internal reflection plays a key role by changing the propagation direction of the diffracted waves and causing a redistribution of power between the respective diffraction orders.

5.2.3 twinning Structure

Fig. 16 shows a basic pattern of another embodiment in accordance with the present principles. In this embodiment, the base pattern of the structure is based on deposition and electron beam irradiation at a refractive index n₃Has a refractive index n on a glass substrate ME3₂A high refractive index single material of (2). According to this embodiment, this results in having a refractive index n₂And two blocks of a single material ME21 and ME22 of the same size are placed on top of substrate ME3 and separated by a determined distance W₁. The space separating the two squares ME21 and ME22 is naturally filled with a host dielectric (having a refractive index n)₁)。

For such geometries, no glass etching is required, and no multiple electron beam lithography is required, which is advantageous for micro-fabrication. In the example given here, the structure is also very shallow, having a height H of 200nm₂Much smaller than in the embodiment with a height of 700nm shown in fig. 6.

FIG. 17 shows the performance of the twin structure of FIG. 16, and on FIG. 17 we can see from 3 to 30 and-30 to-3 degrees η_max＝75％、Γ＝64％。

The performance is very good since a maximum diffraction efficiency of 75% and a diffraction uniformity of 64% are achieved. These values are excellent and represent a real improvement when compared to the structures of fig. 6 and 12.

For comparison with the above results, FIG. 18 shows the refractive index deposited on a glass substrate having a refractive index of n3Is n₂Of a single material. Fig. 19 shows the very poor second order diffraction efficiency in this case.

FIG. 20(a) shows a cross-sectional view of a single material super-element of the embodiment shown in FIG. 16, the structure of FIG. 20(a) having a refractive index n₂Two blocks of a single material ME21 and ME22 filled with a material having a refractive index n₁Distance W of the main dielectric₁And (4) separating. Fig. 20(B)) shows a cross-sectional view of a single-material super-element made up of a single block (B1). In fig. 20(a) and 20(b), the parameters are as follows: n is₁＝1.0，n₂＝2.105，W₁＝260nm，W₂＝130nm，H＝200nm，θ_i＝0°。

FIGS. 21(a) and (b) show the same views as FIGS. 20(a) and (b), but the incident angle θ of the electromagnetic wave_i＝30°。

Fig. 22A shows the Hy component field distribution (a) and the power density distribution (B) in the xy plane for a single NJ element B1 in the right part of fig. 20 for three different angles of incidence, and fig. 22B shows the Hy component field distribution (c) and the power density distribution (d) in the xy plane for the dual-structure super elements ME21 and ME22 in the left part of fig. 20, with the parameters in both cases as follows: n is₁＝1.0，n₂＝2.105，n₃＝1.52，W₁＝260nm，W₂＝130nm，H₂＝200nm，λ＝625nm。

The phenomenon of total internal reflection helps to change the response of the system in the case of a single material element. In FIGS. 20(a) and 21(a), W₁Is the distance between the elements, n₁Is the refractive index of the main dielectric, n₁＜n₂＜n₃。

In the single NJ element system shown in fig. 20(B) and 21(B), there are two NJs associated with the 2 outer edges (left edge and right edge) of the block B1 (see fig. 20(B), the dashed line corresponding to having an offset angle θ_B1NJ beam).

There are two additional edges to θ in the dual block system or dual structure shown in FIG. 20(a)_i0 degrees, resulting in an offset angle θ_B1Four NJ (two NJ in each block for left and right edges) ofThe radiation (see the dotted line in FIG. 20(a), the dotted line indicated by IJW corresponds to the NJ beam (FIG. 20(a)) generated by the inner edges in the twin system by varying the distance between the inner edges, the conditions for constructive and destructive interference for respective waves diffracted by the right or left edges of the block and propagating in the same direction are satisfied.

For normal incidence, the radiation angle of all NJ's is the same θ_B1. It appears that in the case of a twin structure, the presence of two additional inner edges results in a reduction of the strength of the central NJ (oriented along the symmetry axis of the superelement according to the intersection of NJs produced by the outer edges of one or more blocks) and in the beginning of 2 additional NJs with a higher strength according to the intersection of NJs produced by the outer and inner edges of the blocks, than the power distribution of a single-material system and a two-material system.

For oblique incidence, obtaining a radiation angle of θ'_B1And has a radiation angle theta ″)_B1And a second pair of NJs. To theta'_B1And θ ″)_B1The equations for (a) are already discussed above in the two-material solution with insert (fig. 10) and are not repeated here.

For certain angles of incidence, the combination of the constructive and destructive phenomena of each pair and the total internal reflection of the waves again results in a redistribution of intensity between the respective diffraction orders of the periodic array of superelements. As a result, as shown by comparison of fig. 17 and 19, the ± 1 st order diffraction is suppressed, and the intensity of the ± 2 nd order diffraction increases.

By considering some parameter constraints, the ± 2 order diffraction of the twin element topology can be further enhanced. To obtain the maximum intensity of the 2 nd diffraction order, in the case of negative incidence angles, the following parameters are considered:

1. selectingOf NJ produced by changing the left vertical edge of a blockDirection;

2. selectingTo avoid reflection of NJ by the right vertical edge of the block;

3. the width of the respective patches and the distance between the patches depends on the period of the diffraction grating:and isIn the ideal case, in order to provide constructive interference between the NJ generated by the left or right edges of the block, it is preferable to haveOr However, optimizing these parameters, it is desirable that good diffraction uniformity of the system must be considered over a wide range of angles of incidence. Therefore, it is not possible to provide constructive interference for all angles of incidence within the market.

4. Note that within each block, NJ generated by the opposite edges intersect at point a (see fig. 21). The distance between the top of each block and the intersection point can be determined asThe angle of departure of focal point A from the vertical axis in order to obtain the maximum NJ input to the corresponding diffraction orderShould approximate the angle of the corresponding diffraction order distribution. Also, focal point A should be selected to be close to the boundary (H) between the bulk and the substrate₂→H_A)。

The use of twin elements with the above parameters provides enhanced +2 diffraction orders at normal incidence.

Considering a diffraction grating as shown in fig. 16, which presents a periodic array of twin superelements disposed on a substrate having a lower refractive index, the refraction of a wave diffracted by the edges of the superelements at the boundary between the substrate and the elements affects the angular NJ deviation. Figure 23 shows the power density distribution in the xy-plane of a twin structured super element with the following parameters: n is₁＝1.0，n₂＝2.105，W₁＝260nm，W₂＝130nm，H₂200nm, 625 nm; and for the left column n₃2.105 for the right column n₃＝1.52。

5.2.4U-shaped structures

Figure 24 shows another embodiment of a base pattern of a diffraction grating of an optical device according to the present principles. According to this embodiment, the basic pattern comprises a refractive index n₂Of a single material, in the form of a U-shaped block ME2, located at a refractive index n₃On top of the substrate ME 3. The values presented in fig. 24 are examples only. More precisely, the U-shaped form is generally referred to as the form forming the letter U in fig. 24. As can be seen in FIG. 24, the block ME2 has two heights H₂Width W₂Has a refractive index of n₂A flap or block of a single material, and the two flaps or blocks are defined by a height H₁Width W₁Is a refractive index n₂Separate strips of the same single material.

From a geometrical point of view, a high refractive index n₂A single material is deposited and electron beam irradiated on a glass substrate ME 3. Glass etching is not required and multiple electron beam lithography is not required, and both of these facts are advantageous for micro-fabrication.

The structure is also very shallow, 200nm in height, compared to the 700nm height embodiment disclosed in fig. 6.

As shown in fig. 25, since the maximum diffraction efficiency of 65% is obtained, it is slightly lower than that obtained by the twin structure, and thus the performance is very good. However, the diffraction uniformity had a value of Γ 87%, which was excellent. These properties are excellent and represent a real improvement when compared to the structures of fig. 6 and 12.

FIG. 26 shows the Hy component distribution (top) and the power density distribution (bottom) in the xy-plane for the U-shaped super element shown in FIG. 24, with the parameters: n is₁＝1.0，n₂＝2.105，n₃＝1.52，W₁＝260nm，W₂＝130nm，H₂＝200nm，H₁＝50nm，λ＝625nm。

Having a higher refractive index n₂The U-shaped super-element of (a) helps to reduce the intensity of the central NJ at normal incidence and increase the intensity of the lobe. For the U-shaped topology, the intensity of the ± 2 diffraction orders at small angles of incidence can be increased and diffraction uniformity can be improved, as shown by the performance shown in fig. 25.

The height of the central block (H1) can be obtained by considering that for some specific angle of incidence, NJ generated by the left edge of the left block (in case of negative angle of incidence) or by the right edge of the right block (in case of normal angle of incidence) is not reflected by the opposite edge and does not change the direction of propagation. For the remaining angles of incidence, a possible choice is

To theta'_B1Has been discussed in the previous two-material solution with insert (figure.

Width W of central band separating two lobes of U-shape₁Should also satisfy W in the twin structure₁And W₂The defined relationship.

5.2.5 Pitch tolerance

The values provided for the U-shaped structure should be selected to be tolerance robust and in order to check the precision required for manufacturing, the performance of different pitch dimensions is disclosed with figure 27. The original spacing was 823 nm.

Fig. 27 shows the tolerance for the pitch size of d 818nm and d 832 nm. The top of fig. 27 shows the performance with d 818 nm. It has η_max＝65％Γ is 81.8%. The bottom of fig. 27 shows the performance for a spacing of d 832 nm. It has η_max＝78.5％、Γ＝76.2％。

Fig. 27 shows that for d 823 ± 5nm, the diffraction order ± 2 still has high diffraction efficiency and uniformity.

5.4 Nano-jet enhanced Single mode higher order diffraction

The above principles (with super-wavelength grating pitch and second order diffraction) can also be extended to an incoupler that shifts the image to only a single side of the waveguide, rather than shifting the positive angle to one direction and the negative angle to the other.

For this reason, the geometry needs to break the symmetry in order to enhance one diffraction order. Figure 28 shows the geometry and pitch dimensions of an alternative embodiment of the base pattern of the diffraction grating. As can be seen in fig. 28, the pitch of the grating is even larger than that of the grating disclosed above (in this case 988nm), which is almost a micron-sized space with an aspect ratio close to 1.

According to this embodiment, the base pattern is similar to the geometry shown in fig. 6, but the height on the right side is lower than the height on the left side to break the symmetry.

The performance of this geometry is shown in fig. 29. The horizontal axis spans 12 degrees. The curve represents the +2 diffraction order efficiency as a function of incident angle. This configuration achieves 84% maximum efficiency and 54% uniformity for input angles spanning ± 12 degrees.

5.5 diffraction Grating Using structures with modified base Angle

FIG. 33 shows a single-material super element (n) with non-vertical edges₂＝n₃) And a single-material super-element (n) having a top surface that is not parallel to the xz-plane (fig. 33a, 33b)₂＝n₃) Cross-sectional view (fig. 33 c).

Structures having vertical edges and/or top surfaces that are not parallel to the xz plane are contemplated herein. To demonstrate the effect of the base angles of the components of the diffraction grating, consider the U-shaped element shown on figure 24. The bottom angle of the top surface of the U-shaped block is shown as alpha₁And alpha₂. The bottom angles correspond to the angles between the top surface of the U and the sides of one of the U-shaped blocks, respectively.

Considering here the refractive index n of a single material super-element, U-shaped structure₂And refractive index n of the substrate₃Are equal.

Fig. 33 shows the general topology of a single material element. As can be seen from these configurations, the base angle α_jNot equal to 90 °, where j is 1 or 2. The NJ beam radiation angle may be determined using the following approximate consensus:wherein j is 1 or 2, n₁Is the refractive index of the bulk dielectric, and n₂Is the refractive index of the microlens material.

It should also be mentioned that the angle of the NJ distribution is modified due to internal reflection at the non-vertical edges of the element.

The structure presented in fig. 33(c) is optimized to provide the best performance for the blue wavelength at in-460 nm. The system consists of a linear polarization plane wave H ═ 0; 0; 1} (TM) irradiation. For n₂The pitch dimension d of the grating was 487.4nm, 1.9 and the modified U-shaped structure used the same reference as used in figure 24, with W₁＝60nm；H₁＝220nm；W₂＝180nm；H₂360nm and n₃＝1.9。

To change the angle of the scattered jet wave, the base angle of the top of the U-shaped cell was changed by adding a symmetrical pyramid with a height Δ H of 360nm (see fig. 34 (a)). Fig. 34(b) shows the diffraction performance of the grating having the above parameters. There is absolutely no crosstalk between the +2 and-2 stages of each design. 0. The +1 and-1 orders are not coupled into the waveguide, they transmit through and do not reduce the virtual image contrast.

This modification of the U-shaped topology is a material (n) with a lower refractive index equal to that of the substrate₃＝n₂) Providing a very high second order diffraction efficiency. Unfortunately, the diffraction uniformity of this system is not very high and the system is very sensitive to the angle of incidence.

5.6 diffraction Grating for AR/VR glasses

Figure 32 illustrates an exemplary diffraction grating according to an embodiment of the present disclosure. According to this embodiment, the basic pattern of the diffraction grating has a U shape as shown in fig. 24.

According to embodiments of the present disclosure, a diffraction grating having a base pattern according to any of the embodiments disclosed herein may be dedicated to diffracting only a given wavelength. For example, when used in an optical waveguide, one diffraction grating per RGB color may be used. This embodiment allows minimizing chromatic aberrations and a grating dedicated to narrow bands has a much better performance in terms of FoV.

According to another embodiment of the present disclosure, a diffraction grating is configured to diffract light of a set of wavelengths including more than one wavelength. In this case, the NJ structure base pattern of the diffraction grating is configured such that the grating pitch is higher than the highest wavelength in the set of wavelengths. For example,whereinAnd M ═ 2, where n is the refractive index of the substrate.

According to embodiments of the present disclosure, a diffraction grating having a base pattern according to any of the embodiments disclosed herein may be used in an optical waveguide, for example in a waveguide in AR/VR glasses.

According to this embodiment, the diffraction grating may be configured to in-couple light entering the optical waveguide or extract light from the optical waveguide according to a formation position of the diffraction grating on the waveguide.

According to another embodiment of the present disclosure, the optical waveguide may comprise two diffraction gratings according to any of the embodiments disclosed herein: one diffraction grating is configured for in-coupling light into the optical waveguide and the other diffraction grating is configured for extracting light from the optical waveguide.

Each diffraction grating has a grating pitch higher than the wavelength of light that it is configured to in-couple or out-couple, and both diffraction gratings are configured to diffract the light in diffraction orders having an absolute value equal to or greater than 2.

According to embodiments of the present disclosure, an eyewear apparatus is disclosed, comprising an optical device according to any of the above disclosed embodiments.

Fig. 30 illustrates a schematic perspective view of an eyeglass apparatus according to an embodiment of the present disclosure, and fig. 31 illustrates a schematic front view of the eyeglass apparatus illustrated in fig. 30.

According to an embodiment of the present disclosure, such an eyeglass device includes:

a light display engine (not shown) configured for emitting an image to be displayed, the light engine display may for example be placed on a branch of the eyewear apparatus,

-an OPTICS group (OPTICS) configured for coupling incident light from the light display engine to the light guide (WG).

According to an embodiment, an optical Waveguide (WG) is configured to guide incident light towards an eye of a user such that an image is visible to the user.

According to the embodiment shown in fig. 30, an optical Waveguide (WG), also denoted by "1", includes an input grating ("2" in fig. 30) that functions as an incoupling device. The input grating may be a diffraction grating according to any of the above embodiments.

The optical waveguide also includes a vertical pupil expander ("3" on fig. 30), a horizontal pupil expander ("4" on fig. 30), and an output grating ("5" on fig. 30) or output coupler.

According to an embodiment of the present disclosure, the output grating may be a diffraction grating according to any of the above embodiments.

As shown in fig. 31, in order to have a stereoscopic view, images are emitted from two light engines (not shown) respectively placed on two branches of the glass apparatus. The eyewear apparatus also includes two Waveguides (WG) on each side of the apparatus, and two sets of OPTICS (OPTICS) for directing light from respective light engines to respective waveguides.