WO2021073725A1

WO2021073725A1 - Rotated subwavelength grating for high efficiency thin waveguide

Info

Publication number: WO2021073725A1
Application number: PCT/EP2019/077893
Authority: WO
Inventors: Gaurav BOSE; Antonie VERHOEVEN
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2019-10-15
Filing date: 2019-10-15
Publication date: 2021-04-22

Abstract

An apparatus includes a waveguide, a first diffraction grating on a front side of the waveguide and a second diffraction grating on a back side of the waveguide. The small period back side gratings, rotated around the propagation direction, will rotate the state of polarization of an incoming light beam after a first Total Internal reflection (TIR). The back side diffraction grating is designed such that it does not change the propagation direction of the guiding light and will reduce backcoupling from the incoupler, hence increasing the net guided energy towards the user' s eye. The back side diffraction grating of the disclosed embodiments will help to increase the guiding efficiency and thereby result in a brighter augmented reality image.

Description

ROTATED SUBWAVELENGTH GRATING FOR HIGH EFFICIENCY THIN

WAVEGUIDE

TECHNICAL FIELD [0001] The aspects of the present disclosure relate generally to imaging optics and more particularly to a diffraction grating for augmented reality glasses.

BACKGROUND

[0002] Augmented Reality (AR) uses a digital platform to overlay the virtual contents onto the real world. The virtual content is in the form of videos, text or image format. It aims to enhance the user experience by bringing digital closer to the human senses. The users may use mobile devices, phones, tablets or headsets to view the world using AR.

[0003] Head Mounted Displays (HMD) are one example of an AR device that projects virtual images onto the user’s retina without impairing sight of the real world. The Head Mounted Display unit of this type will typically make use of micro-nano structures or gratings.

[0004] The micro-nano structures or gratings of such display devices are used to couple and guide the light through from the projector into the waveguide. Due to reciprocity of diffraction, any light that is coupled into the waveguide will couple out (backcoupling) with the same amount if it interacts with or hits the grating again. As a result the net light guiding towards the eye of the user is reduced.

[0005] Accordingly, it would be desirable to provide a diffraction grating for a waveguide device that addresses at least some of the problems identified above. SUMMARY

[0006] The aspects of the disclosed embodiments are directed to providing a diffraction grating for a waveguide. This object is solved by the subject matter of the independent claims. Further advantageous modifications can be found in the dependent claims.

[0007] According to a first aspect the above and further objects and advantages are obtained by an apparatus. In one embodiment, the apparatus includes a waveguide, a first diffraction grating on a front side of the waveguide and a second diffraction grating on a back side of the waveguide. The aspects of the disclosed embodiments are directed to using small period back side gratings (rotated around the propagation direction) that will rotate the state of polarization after a first Total Internal reflection (TIR). The backside grating is designed such that it does not change the propagation direction of the guiding light and will reduce backcoupling from the incoupler, hence increasing the net guided energy towards the eye of the user. The backside diffraction grating of the disclosed embodiments will help to increase the guiding efficiency and thereby result in a brighter augmented reality image.

[0008] In a possible implementation form of the apparatus grating lines of the second diffraction grating are rotated relative to a normal plane of grating lines of the first diffraction grating. Small period back side gratings are rotated around the propagation direction in a manner that will rotate the state of polarization of an incoming light signal after a first Total Internal reflection (TIR). The grating does not change the propagation direction of the guiding light and will reduce backcoupling from the incoupler, which increases the net guided energy towards the user’s eye.

[0009] In a possible implementation form of the apparatus a rotation of the grating lines of the second diffraction grating is in the range of 0 degree to 180 degree relative to the normal plane of the grating lines of the first diffraction grating. Small period back side gratings are rotated around the propagation direction in a manner that will rotate the state of polarization of an incoming light signal after a first Total Internal reflection (TIR). The grating does not change the propagation direction of the guiding light and will reduce backcoupling from the incoupler, which increases the net guided energy towards the user’s eye.

[0010] In a possible implementation form of the apparatus the rotation of the grating lines of the second diffraction grating relative to the grating lines of the first diffraction grating is configured to rotate a polarization of incoming light after a first total internal reflection. Small period back side gratings are rotated around the propagation direction in a manner that will rotate the state of polarization of an incoming light signal after a first Total Internal reflection (TIR). The grating does not change the propagation direction of the guiding light and will reduce backcoupling from the incoupler, which increases the net guided energy towards the user’s eye.

[0011] In a possible implementation form of the apparatus a diameter of an aperture of the second diffraction grating is less than a diameter of an aperture of the first diffraction grating. The size of the bottom grating can be chosen such that it mostly impacts the smaller diffraction angles with little negative impact for the higher diffraction angles. The design can enable thinner and lighter weight glasses.

[0012] In a possible implementation form of the apparatus a grating line density of the second diffraction grating is greater than a grating line density of the first diffraction grating. The size of the bottom grating can be chosen such that it mostly impacts the smaller diffraction angles with little negative impact for the higher diffraction angles. The design can enable thinner and lighter weight glasses. [0013] In a possible implementation form of the apparatus the first diffraction grating is an incoupler and the second diffraction grating is a subwavelength diffraction grating. The aspects of the disclosed embodiments incouple light with higher efficiency for all Field of View (FOV) angles and provide a brighter augmented reality image. [0014] In a possible implementation form of the apparatus, the apparatus comprises a near eye display. The aspects of the disclosed embodiments increase the guiding efficiency and result in a brighter AR image with improved image quality in a near eye display device.

[0015] In a possible implementation form of the apparatus, the apparatus comprises a head mounted display. The aspects of the disclosed embodiments increase the guiding efficiency and result in a brighter AR image with improved image quality in a head mounted display device.

[0016] In a possible implementation form of the apparatus, the apparatus comprises wearable augmented reality glasses. The aspects of the disclosed embodiments increase the guiding efficiency and result in a brighter AR image with improved image quality in augmented reality glasses. The glasses can be thinner and lighter weight.

[0017] These and other aspects, implementation forms, and advantages of the exemplary embodiments will become apparent from the embodiments described herein considered in conjunction with the accompanying drawings. It is to be understood, however, that the description and drawings are designed solely for purposes of illustration and not as a definition of the limits of the disclosed invention, for which reference should be made to the appended claims. Additional aspects and advantages of the invention will be set forth in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. Moreover, the aspects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS [0018] In the following detailed portion of the present disclosure, the invention will be explained in more detail with reference to the example embodiments shown in the drawings, in which:

[0019] Figure 1 illustrates a schematic block diagram of an exemplary apparatus incorporating aspects of the disclosed embodiments. [0020] Figure 2 illustrates a schematic block diagram of an exemplary apparatus incorporating aspects of the disclosed embodiments.

[0021] Figure 3 is a graph illustrating polarization rotation in an exemplary apparatus incorporating aspects of the disclosed embodiments.

[0022] Figure 4 is a graph illustrating exemplary sizing of a backside diffraction grating in an apparatus incorporating aspects of the disclosed embodiments.

[0023] Figure 5 is a graph illustrating coupling efficiency in an exemplary apparatus incorporating aspects of the disclosed embodiments.

DETAIFED DESCRIPTION OF THE DISCFOSED EMBODIMENTS [0024] Referring to Figure 1, a schematic block diagram of an exemplary waveguide apparatus 100 incorporating aspects of the disclosed embodiments is illustrated. The aspects of the disclosed embodiments are directed to using small period back side gratings (rotated around propagation direction) that will rotate the state of polarization of an incoming light signal after a first Total Internal reflection (TIR). The diffraction grating of the disclosed embodiments is designed such that it does not change the propagation direction of the guiding light and will reduce backcoupling from the incoupler, hence increasing the net guided energy towards the user’s eye. The diffraction grating of the disclosed embodiments will help to increase the guiding efficiency, resulting in a brighter AR image with improved image quality, while enabling the display glass to be thinner and lighter weight.

[0025] As shown in Figure 1, in one embodiment, the apparatus 100 includes a waveguide 102. The waveguide 102, also referred to as a substrate, generally comprises a glass substrate. In alternate embodiments, the waveguide 102 can comprise any suitable waveguide structure.

[0026] In the example of Figure 1 , the waveguide 102 includes a first diffraction grating

104. The first diffraction grating 104 is generally disposed on what is described as top or front side 106 of the waveguide 102. For the purposes of the description herein, the first diffraction grating will be referred to as a front side diffraction grating.

[0027] The waveguide 102 also includes a second diffraction grating 108. In the example of Figure 1, the second diffraction grating 108 is disposed on a bottom or back side 110 of the waveguide 102. For the purposes of the description herein, the second diffraction grating 108 will be referred to as a back side diffraction grating. [0028] The top or front side 106 of the waveguide 102 is generally the side from which an incoming light signal LI is received. The front side diffraction grating 104 can also be referred to as an incoupler. The back side diffraction grating 108 is generally configured not to change a propagation direction of the incoming or guiding light signal LI and reduces back coupling from the front side diffraction grating 104. The propagation direction of the incoming light signal LI within the waveguide 102 is illustrated by arrow 1 in Figure 1.

[0029] Referring also to Figures 2A-2C, in one embodiment, the back side diffraction grating 108 is configured to rotate a state of the polarization of the incoming light signal LI after a first total internal reflection (TIR) in the waveguide 102, illustrated by arrow or ray 2. As shown in Figure 2B, the back side diffraction grating 108 includes a plurality of subwavelength finite aperture gratings 120. The plurality of subwavelength finite aperture gratings 120 generally comprise a series of microstructures 122, as illustrated in Figure 2C. The microstructures 122 are disposed next to one another in a periodic manner. In diffraction optics a series of microstructures of the same shape that are placed next to one another in a periodic manner is referred to as a grating.

[0030] The microstructures 122 can be used to manipulate the phase, amplitude and propagation direction of the light. The repetition period of the microstructures, referred to as dl in Figure 2B, will determine the direction(s) in which the light will propagate. If the period dl is small enough, meaning to say: dl_sub<wl_0/n_glass with dl being the periodicity of the grating 108, wl_0 the wavelength at vacuum and n glass the refractive index of the medium, there will only be a single diffraction order propagating after interaction with the micro structure 122. This back side diffraction grating 108 is also referred to as a sub-wavelength grating. This diffraction order has the same propagation direction as a simple reflection from the interface would have. If the period dl is larger than this value, (dl>wl_0/n_glass), the light could split up in multiple unwanted directions and the diffraction grating is no longer a sub wavelength grating.

[0031] The back side grating 108 of the disclosed embodiments is a subwavelength grating and is used to control the polarization of the light signal LI. In one embodiment, the back side diffraction grating 108 is configured to be rotated relative to the front side diffraction grating 104. The amount of rotation of the back side diffraction grating 108 can be dependent upon the average number of expected interactions of the light signal LI with the back side diffraction grating 104. For example, if the light signal LI will reflect and hit the front side diffraction grating only once, a polarization rotation angle of approximately 90 degrees is recommended. If the light signal LI will reflect and interact with the front side diffraction grating 104 multiple times, a recommended polarization rotation can be taken from the graph in Figure 3.

[0032] The graph in Figure 3 illustrates exemplary polarization rotation angles of the back side diffraction grating based on a number of expected successive interactions of LI with the front side diffraction grating. The successive interactions of LI with the front side diffraction grating 104 may also be referred to herein as “hits”.

[0033] The polarization rotation angle of the back side diffraction grating 108 can be realized by rotating the orientation of the back side diffraction grating 108 with respect to an orientation of the front side diffraction grating 104, generally along the azimuthal direction Z, as is illustrated in Figures 2A and 2B. In the example of Figure 2B, the grating lines 120 of the backside diffraction grating 108 are rotated in the XY plane relative to the grating lines 124 of the front side diffraction grating 104. The amount of azimuthal rotation needed to achieve the desired polarization rotation depends on the subwavelength grating design. The term “azimuthal rotation” as used herein is defined as the rotation in the X-Y plane (e.g. keeping the Z axis as the axis you rotate ‘around’).

[0034] As an example, referring to Figures 2A-2C, for the backside diffraction grating

108, the visible wavelength of 535 nm has been chosen and a plurality of subwavelength finite aperture gratings 120 with a 2.8 (Titanium) refractive index on a substrate 102 of refractive index of n glass = 2. The microstructure or grating element 122 in Figure 2C has a height of h = 250 nm, a period of dl = 150 nm and a fill factor of 0.5. The light is incident inside the waveguide 102 at a high angle (above Total internal Reflection angle (usually 30 degrees) for substrate refractive index of n_glass=2). The plurality of subwavelength finite aperture gratings 120 of the backside diffractiongrating 108 are rotated approximately 20 degrees with respect to the grating plane normal. In the examples of Figures 1 and 2, the grating plane normal is the Z-direction (as the normal component of a surface points directly perpendicular to that surface). While specific rotation angles are referred to above, the aspects of the disclosed embodiments are not so limited. In alternate embodiments, the range of rotation angles can include 0 to 180 degrees.

[0035] The design of the front side diffraction grating 104 and back side diffraction grating 108 are typically done separately. The polarization of the input light LI to the front side diffraction grating 104 is assumed to be parallel to the grating lines 124 of the front side diffraction grating 104. Only certain propagation angles benefit from the back side diffraction grating 108. As such, the grating equation referred to above can be used to determine these propagation angles and use this information to design the back side diffraction gratings 108. It should be noted that many types sub-wavelength grating designs are possible.

[0036] The back side diffraction grating 108 can also be referred to as a finite aperture.

A finite aperture refers to the size or diameter of the back side diffraction grating 108, such as shown in Figure 1 and Figure 2B. The finite size is used to target those diffraction angles that will interact with, or hit, the back side 112 of the front side diffraction grating 104. Those rays that propagate with sufficiently large angle after their first interaction with the front side diffraction grating 104, (see ray 1 and ray 2 in Figure 1), will not hit the front side diffraction grating 104 for a second time. As such the size of the back side diffraction grating 108 can be chosen such that it impacts the smaller diffraction angles with little negative impact for the higher diffraction angles.

[0037] Figure 4 illustrates optimal aperture sizing for the back side diffraction grating

108. As Figure 4 demonstrates, if multiple interactions are considered, the size of the back side diffraction grating 108 (Lbot) will increase when the ratio between the Length (L_top) of the front side diffraction grating 104 over the thickness (h) of the waveguide or glass substrate 102 (Ltop/h) is large. In other words, the waveguide 102 is thin compared to the length L_top of the front side diffraction grating 104. The left most starting point of the backside diffractiongrating 108 is where the minimum (positive diffraction) angle that is Totally Internally Reflecting (TIR) hits or interacts with the bottom side 116 of the waveguide 102 for the first time.

[0038] In one embodiment, the back side diffraction grating 108, which can also be referred to as a subwavelength or metasurface grating, is not limited to a binary grating. In alternate embodiments, the back side diffraction grating 108 can be used with any type of subwavelength grating. [0039] Figure 5 illustrates the improvement in the coupling efficiency with a backside diffraction grating 108 of the disclosed embodiments. In this example, line 202 illustrates the coupling efficiency with the back side diffraction grating 108 of the disclosed embodiments. Line 204 illustrates the coupling efficiency without the back side diffraction grating 108 of the disclosed embodiments. From this figure it is clear that the aspects of the waveguide design with the back side diffraction grating 108 outperforms a waveguide design without, as more light is coupled into the waveguide for the majority of incidence angle (Theta).

[0040] The apparatus 100 shown in Figure 1 generally comprises an augmented reality display unit. This can include for example, but is not limited to, display glasses, head mounted display devices or near, in one embodiment. [0041] The aspects of the disclosed embodiments provide a waveguide assembly that makes it possible to in couple light with higher field of view with higher efficiency for all Field of View (FOV) angles. The use of rotated binary gratings with sub wavelength features on the back side enables a waveguide design that provides for thinner and lighter weight display glasses, such as those use in augmented reality systems and head mounted displays. The aspects of the disclosed embodiments allow higher efficiency from the in-coupling region to be coupled in the waveguide. This results in a brighter augmented reality image. The use of rotated binary gratings also provides for uniform light distribution inside the pupil, which also improves image quality. [0042] Thus, while there have been shown, described and pointed out, fundamental novel features of the invention as applied to the exemplary embodiments thereof, it will be understood that various omissions, substitutions and changes in the form and details of devices and methods illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit and scope of the presently disclosed invention. Further, it is expressly intended that all combinations of those elements, which perform substantially the same function in substantially the same way to achieve the same results, are within the scope of the invention. Moreover, it should be recognized that structures and/or elements shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.

Claims

1. An apparatus (100) comprising : a waveguide (102); a first diffraction grating (104) on a front side (106) of the waveguide (102); and a second diffraction grating (108) on a back side (110) of the waveguide (102).

2. The apparatus (100) according to claim 1 wherein grating lines of the second diffraction grating (108) are rotated relative to a normal plane of grating lines of the first diffraction grating (104).

3. The apparatus (100) according to any one of the preceding claims wherein a rotation of the grating lines of the second diffraction grating (108) is in the range of 0 degree to 180 degree relative to the normal plane of the grating lines of the first diffraction grating (104).

4. The apparatus (100) according to any one of the preceding claims where the rotation of the grating lines of the second diffraction grating (108) relative to the grating lines of the first diffraction grating (104) is configured to rotate a polarization of incoming light after a first total internal reflection.

5. The apparatus (100) according to any one of the preceding claims wherein a diameter of an aperture the second diffraction grating (108) is less than a diameter of an aperture of the first diffraction grating (104).

6. The apparatus (100) according to any one of the preceding claims wherein a grating line density of the second diffraction grating (108) is greater than a grating line density of the first diffraction grating (104).

7. The apparatus (100) according to any one of the preceding claims wherein the first diffraction grating (104) is an incoupler and the second diffraction grating (108) is a subwavelength diffraction grating.

8. The apparatus (100) according to any one o f the preceding claims wherein the apparatus (100) comprises a near eye display.

9. The apparatus (100) according to any one o f the preceding claims wherein the apparatus (100) comprises a head mounted display.

10. The apparatus (100) according to any one o f the preceding claims wherein the apparatus (100) comprises wearable augmented reality glasses.