Exit pupil expander

Abstract

According to an illustrative aspect of the present invention, an exit pupil expander, an EPE grating, is provided that is divided into at least two parts, the EPE grating comprising a plurality of grating bars in a first part and a plurality of grating bars in a second part, the plurality of grating bars in the first part being oriented in approximately the same direction as the plurality of grating bars in the second part and being offset in a direction perpendicular to the direction of the grating bars.

Description

[Technical Field]

[0001] FIELD OF THE INVENTION Embodiments of the present invention generally relate to an Exit Pupil Expander (EPE) for an optical waveguide device, such as a waveguide display. [Background technology]

[0002] In general, there is a need for improvements in exit pupil expanders, EPEs. Typically, light rays interfere with the EPE grating, causing non-uniformity in the outcoupled image. Therefore, there is a need to reduce the interference caused by light rays interfering with the EPE grating.

[0003] For example, U.S. Patent Application Publication No. 2018 / 0052501(A1) discusses EPE, which shows wave interference caused by a uniform grating in an orthogonal pupil expander (OPE). For example, by changing the parameters or material of the grating at different positions, the wave interference can be reduced and the brightness uniformity of the output image can be improved.

[0004] However, such solutions do not allow for independent adjustment of the phase of the diffracted beam without changing the amplitude response of the grating. In other words, in view of U.S. Patent Application Publication No. 2018 / 0052501(A1), more degrees of freedom are needed to modify the operation of the optical waveguide. Changing the width, height, and / or fill factor of the grating bars also does not achieve the desired effect. Therefore, there is a need to provide improved EPE gratings for optical waveguide devices, for example. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] U.S. Patent Application Publication No. 2018 / 0052501(A1) [Non-patent literature]

[0006] [Non-Patent Document 1] Joseph W. Goodman, "Introduction to Fourier Optics," 3rd Edition, 2004 (360 pages) Summary of the Invention [Means for solving the problem]

[0007] According to some aspects, the subject matter of the independent claims is provided. Some embodiments are defined in the dependent claims.

[0008] According to a first aspect of the present invention, there is provided an exit pupil expander (EPE) grating divided into at least two segments (hereinafter also referred to as "portions"), the EPE grating comprising a plurality of grating bars in the first portion and a plurality of grating bars in the second portion, the plurality of grating bars in the first portion being oriented in approximately the same direction as the plurality of grating bars in the second portion and being offset in a direction perpendicular to the direction of the grating bars.

[0009] Implementations of the first aspect may include at least one feature from the following bulleted list or any combination of the following features. The grating bars of the first portion and the grating bars of the second portion are offset to impart different phase shifts to light rays propagating along different paths in the EPE grating. Each of the plurality of grating bars of the second portion is offset by a distance relative to a corresponding grating bar of the first portion in a direction perpendicular to the direction of the grating bars. The first grating bars of the first part are corresponding grating bars of the first grating bars of the second part, and the second grating bars of the first part are corresponding grating bars of the second grating bars of the second part. The distance is smaller than the period of the EPE grating. Each of the plurality of bars in the second portion is laterally offset from a corresponding grating bar in the first portion by the distance. Each of the plurality of bars in the second portion is vertically offset from a corresponding grating bar in the first portion by the distance. The EPE grating is double periodic. The first portion of the EPE grating is configured to impart a first phase shift to a light beam deflected by the first portion, and the second portion of the EPE grating is configured to impart a second phase shift to a light beam deflected by the second portion. The first phase shift is different from the second phase shift. The amplitude of the light beam deflected in the first part is the same as the amplitude of the light beam deflected in the second part. The second part follows the first part with respect to the light beam guided to the EPE grating. The EPE grating further comprises a plurality of grating bars in the third portion, each of the plurality of bars in the third portion being offset from a corresponding grating bar in the first portion by a distance in a direction perpendicular to the direction of the grating bars. The distance between subsequent lattice bars in the first part is the same as the distance between subsequent lattice bars in the second part. The EPE grating is configured to diffusely couple light out of the EPE grating, and is preferably also configured to act as an incoupler. The EPE grating is configured to keep the amplitude of the light rays propagating through different paths in the EPE grating unchanged.

[0010] According to a second aspect of the present invention, there is provided an optical waveguide device for displaying an image, comprising an optical waveguide, an incoupling grating for diffractively coupling an image into the optical waveguide, an outcoupling grating for diffractively coupling the image out of the optical waveguide, and an EPE grating according to any of the preceding claims, wherein the EPE grating is located between the incoupling grating and the outcoupling grating to enlarge the exit pupil of the image on the outcoupling grating.

[0011] According to a third aspect of the present invention, there is provided a personal display device such as a head-mounted display (HMD) or a head-up display (HUD), comprising an optical waveguide device according to the second aspect. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 illustrates an example system in accordance with at least some embodiments of the present invention. [Figure 2a] 1A-1C illustrate illustrative examples of optical waveguide devices in accordance with at least some embodiments of the present invention. [Figure 2b] 1A-1C illustrate examples of an incoupling grating, an exit pupil expander, and an outcoupling grating in accordance with at least some embodiments of the present invention. [Figure 3] FIG. 1 shows an example of an exit pupil expander in accordance with at least some embodiments of this invention. [Figure 4] FIG. 2 shows a first example of staggered grating bars, in accordance with at least some embodiments of the present invention. [Figure 5a] FIG. 10 shows a second example of staggered grating bars, in accordance with at least some embodiments of the present invention. [Figure 5b] FIG. 10 shows a second example of staggered grating bars, in accordance with at least some embodiments of the present invention. [Figure 6] 10 illustrates an example of phase shifting in accordance with at least some embodiments of this invention. [Figure 7a] FIG. 10 shows an example of offset two-dimensional grating bars, in accordance with at least some embodiments of the present invention. [Figure 7b] FIG. 10 shows an example of offset two-dimensional grating bars, in accordance with at least some embodiments of the present invention. [Figure 8] FIG. 10 illustrates an example distribution of distance traveled in different segments, in accordance with at least some embodiments of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] Embodiments of the present invention relate to an exit pupil expander, e.g., an EPE grating, for an optical waveguide device. More specifically, embodiments of the present invention provide an EPE grating that reduces interference effects caused by light rays interfering with the EPE grating. According to embodiments of the present invention, the EPE grating is divided into at least two segments (also referred to as "portions") with different phase shifts. Each of the at least two portions may include a plurality of grating bars, and the grating bars of each portion may be configured to produce different phase shifts, e.g., different phase shifts controlled according to the Lohmann detour-phase principle, when light rays propagate through each portion along different paths. In this manner, the EPE grating may be configured to keep the amplitudes of light rays propagating along different paths the same, i.e., unchanged, to provide greater flexibility in modifying the operation of the optical waveguide. For example, interference caused by light rays interfering with the EPE grating may be reduced.

[0014] The EPE grating may include a plurality of grating bars in a first portion and a plurality of grating bars in a second portion, where the plurality of grating bars in the first portion are offset relative to the plurality of grating bars in the second portion. That is, the grating bars in the first and second portions may not be aligned, and each of the plurality of bars in the second portion may be offset from a corresponding grating bar in the first portion by a distance in a direction perpendicular to the direction of the grating bars. That is, offset may refer to the distance a grating bar is offset from a corresponding grating bar.

[0015] 1 illustrates an example system in accordance with at least some embodiments of the present invention. The system may include at least one light source 140. The at least one light source 140 may include, for example, a laser or light-emitting diode (LED) light source, with laser light sources having the advantage of being more strictly monochromatic than LEDs. Embodiments of the present invention are not limited to a particular light source and may be implemented using two or more types of light sources 140. The at least one light source 140, together with an optional mirror 130, may be configured to generate a light field with an angular interval that can be utilized by a waveguide display to generate its image.

[0016] Images may be encoded in a light field. The light field is shown schematically in FIG. 1 as field 100. In some embodiments, a physical primary display may display the image of light field 100, while in other embodiments, the system may not include a physical primary display and may simply encode images within light field 100 distributed within an angular interval. Light rays, i.e., light signals 104, from light field 100 may be transmitted directly or by a light guide 102, e.g., comprising mirrors and / or lenses, to a light guide 110 to form a waveguide-based display. Light guide 102 is optional, depending on the details of the particular embodiment, and may be omitted. In other words, light guide 102 is not present in all embodiments.

[0017] In waveguide 110, light ray 104 may travel by repeatedly reflecting within the waveguide and interacting with element 112a until it interacts with element 112 and is deflected from waveguide 110 into air as image-generating light ray 114 toward eye 120. Elements 112 and 112a may comprise, for example, semi-transmissive mirrors, surface relief gratings, or other diffractive structures. Element 112a may be configured, for example, to diffuse light ray 104 within waveguide 110 to accurately generate the image of the waveguide display. Light from different angular aspects of light field 100 interacts with element 112 such that light ray 114 generates the image encoded in light field 100 on the retina of eye 120.

[0018] Element 112 may then cause light ray 104 to exit waveguide 110 at the exit position. As a result, the user perceives an image encoded with light field 100 in front of their eye 120. Waveguide 110 may be at least partially transparent, so that if the waveguide display is head-mounted, for example, the user may also advantageously view their real surroundings through waveguide 110. As a result of the action of elements 112a and 112b, light is emitted from waveguide 110 at multiple angles in multiple elements 112. A waveguide display may have multiple waveguides 110 transmitting light to simulate different apparent depths in front of eye 120 and, optionally, the user's other eye, which is not shown in FIG. 1 for clarity.

[0019] In particular, embodiments of the present invention relate to an exit pupil expander (EPE) for a light guide device, such as a light guided diffractive display, including an incoupling grating, an EPE grating, and an outcoupling grating. Light guides can transmit optical frequencies. By visible frequencies, we mean light with wavelengths of approximately 400 to 700 nanometers. Light guides can be used in displays, where one or more waveguides can be used to transmit light from a light field to the appropriate location for emission into a user's eye.

[0020] Embodiments of the present invention can be used, for example, in head-mounted displays (HMDs) and head-up displays (HUDs) that utilize diffraction gratings. HMDs and HUDs can be implemented, for example, using light guide technology for augmented reality or virtual reality applications. In augmented reality, a user sees a view of the real world with a complementary display superimposed on it. In virtual reality, a user is deprived of a view of the real world and instead is given a view of a software-configured scene. In general, there is a need for improvements in EPE gratings, for example, for light guide devices.

[0021] 2a shows an example of a light guide device in accordance with at least some embodiments of the present invention. The light guide device 200 may include a light guide 110, an incoupling grating 202, an outcoupling grating 204, and an EPE grating 206. In the example of FIG. 2a, the exit pupil of the light guide device 200 may be expanded using the EPE grating 206 between the incoupling grating 202 and the outcoupling grating 204 of the light guide device 200.

[0022] As shown in FIG. 2a, light beams 104 from a light source 140 in FIG. 1, such as a projector, may be directed to an incoupling grating 202. The incoupling grating 202 may be configured to guide the light beams 104 into the light guide 110. That is, the incoupling region 202 may diffractively couple an image into the light guide 110. In some embodiments, as shown in FIG. 2a, the incoupling grating 202 may reside on a surface of the light guide 110, for example. However, in some embodiments, the incoupling grating 202 may be included within the light guide 110. Similarly, the light guide 110 may include an outcoupling grating 204 and / or an EPE grating 206, or the incoupling grating 202 and / or the EPE grating 206 may reside on a surface of the light guide 110.

[0023] Light ray 104 may undergo internal reflection within the waveguide and propagate via EPE grating 206 to outcoupling grating 204, laterally expanding the viewable area of ​​the display. In some illustrative embodiments, EPE grating 206 may thus reside along the path of light ray 104 between incoupling grating 202 and outcoupling grating 204 to expand the exit pupil of the image on outcoupling grating 204. Furthermore, outcoupling grating 204 may diffractively couple the image via light ray 114 out of light guide 110 and toward eye 120.

[0024] 2a shows an example with an incoupling grating 202, an outcoupling grating 204, and an EPE grating 206. However, in some embodiments, the EPE grating 206 may be configured to diffusely couple light out of the EPE grating 206. Alternatively, the EPE grating 206 may be configured to operate as an incoupler. For example, in a two-dimensional structure, the entire grating area may be made up of the same grating where light is coupled, diffused, and outcoupled.

[0025] 2b shows an example of an incoupling (IC) grating, EPE, and an outcoupling (OC) grating in accordance with at least some embodiments of the present invention. Typically, when an LED projector is used as the light source 140, for example, light rays interfere with the EPE grating 206, causing non-uniformities (e.g., fringes) in the outcoupled image. Thus, embodiments of the present invention provide an improved EPE that provides more flexibility to modify the behavior of the light guide, for example, to minimize image non-uniformities.

[0026] FIG. 3 illustrates an example of an EPE grating in accordance with at least some embodiments of the present invention. As illustrated in the example of FIG. 3, an EPE grating, such as EPE grating 206 of FIG. 2, may be divided into portions. Dashed lines in FIG. 3 indicate the boundaries of each portion. For example, the EPE grating may be divided into at least a first portion 310 and a second portion 320, as shown in FIG. 3. More specifically, the EPE grating may be divided into portions that produce different phase shifts, such as phase shifts controlled according to Roman's detour-phase principle. Appropriately selecting the phase shifts can reduce interference effects.

[0027] Each portion of the EPE grating 206 may include multiple grating bars, and the grating bars in different portions may be configured to produce diffracted incident light beams with different phase shifts in each portion as the light beam propagates through each portion along different paths. For example, the first portion 310 of the EPE grating 206 may include at least first and second grating bars, and the second portion 320 of the EPE grating 206 may include at least first and second grating bars, and the grating bars in the first and second portions may be configured to produce different phase shifts to reduce interference caused by light beams interfering with the EPE grating 206.

[0028] Some illustrative embodiments of the present invention may utilize Roman's detour-phase principle to reduce interference in EPE gratings, such that when light diffracts from the grating, it produces a set of reflected and transmitted diffraction orders with constant phase and amplitude determined by the properties of the grating. If the relative positions of the grating bars are misaligned within the period of the grating, for example, for the one-dimensional case, a phase shift occurs that can be determined by the following equation: t m (ds)=t m *exp (-i*2π*m*ds / d) (1) where d refers to the period of the grating, ds is the shift in the position of the grating bars, m is the diffraction order of the optical signal of interest (e.g., −2, −1, 0, 1, 2), and t m =t m(0) is the amplitude of the grating when the grating bars are not displaced. As a result, the phase of non-zero diffraction orders can be adjusted by displacing the grating bars, while the amplitude of all diffraction orders remains the same compared to a grating without bar displacement. Equation (1) may be generalized for two-dimensional gratings, for example, using periods dx and dy for m and n diffraction orders in the x and y directions, respectively. The period d of a grating may be referred to as the distance between successive grating bars in a given region. The phase shift may be controlled according to Roman's detour-phase principle, as described by Joseph W. Goodman in "Introduction to Fourier Optics," 3rd Edition, 2004 (page 360).

[0029] In this manner, different phase shifts may be achieved by, for example, offsetting the grating bars of one section, such as second section 320, relative to an adjacent section, such as first section 310, according to Roman's detour-phase principle, thereby reducing interference caused by rays interfering with EPE grating 206, while still maintaining the same amplitude of the diffracted rays.

[0030] That is, although the amplitude distribution may vary across the EPE grating 206, for example, the EPE grating 206 may be configured to control the phase shift according to Roman's detour-phase principle so that the amplitude remains unchanged for a single grating. Thus, the phase of the diffracted beam can be controlled without changing the amplitude. This provides greater freedom for modifying the waveguide's behavior. For example, in the initial design phase, the waveguide may be designed without phase shift, and then the amplitude may be kept the same while the phase shift is modified to reduce interference.

[0031] The magnitude of the shear may also vary from portion to portion and may be optimized to minimize the effects of interference. Alternatively, or additionally, the sizes of each portion may vary. That is, first portion 310 may have a first size and second portion 320 may have a second size, with the first size being different from the second size.

[0032] 3, incident light beam 104 may be guided toward the grating bars of the first portion 310 of the EPE grating. The grating bars of the first portion 310 may diffract incident light beam 104 into zero-order light beam 104a and first-order light beam 104b. The position of the grating bars in the first portion 310 does not affect the phase of zero-order light beam 104a, but does affect the phase of first-order light beam 104b. Zero-order light beam 104a may be referred to as an unpolarized light beam, and first-order light beam 104b may be referred to as a polarized light beam. Light beams of other diffraction orders other than zero (i.e., m=(−2, −1, 1, 2)) may be referred to as polarized light beams.

[0033] That is, the zero-order ray 104a may remain straight, i.e., bend, and the first-order ray 104b may not remain straight, i.e., bend.

[0034] The diffracted zeroth-order ray 104a may be further guided from the first portion 310 towards the grating bars in the second portion 320, which may diffract the incident zeroth-order ray 104a into zeroth-order ray 104c and first-order ray 104d. Furthermore, the position of the grating bars in the second portion 320 does not affect the phase of the zeroth-order ray 104c, but does affect the phase of the first-order ray 104d. Furthermore, the zeroth-order ray 104c may be referred to as an unpolarized ray, and the first-order ray 104d may be referred to as a polarized ray.

[0035] Furthermore, diffracted first-order ray 104b may be further guided from first portion 310 towards the grating bars of third portion 330, which may diffract ray 104b into unpolarized ray 104e and polarized ray 104f. The ray may be similarly guided through several portions of the EPE.

[0036] The light beams deflected by different portions may have different phases; in other words, the different portions may impart different phase shifts to the deflected light beams, but the amplitudes of the deflected (and undeflected) light beams may be the same. For example, the first portion 310 may be configured to impart a first phase shift to the light beam 104b deflected by the first portion 310, and the second portion 320 may be configured to impart a second phase shift to the light beam 104d deflected by the second portion 320. The amplitude of the light beam deflected by the first portion 310 may be the same as the amplitude of the light beam deflected by the second portion 320, although the first phase shift may be different from the second phase shift. Light beams propagating along different paths may strike the same location and interfere, and such interference can be controlled by adjusting the phase of the light beams.

[0037] The grating bars in each portion may be arranged in the same direction 305, but the grating bars in at least some portions may be offset relative to corresponding grating bars in adjacent portions in a direction 315 perpendicular to the grating bar direction 305, so that diffracted light rays propagating along different paths experience different phase shifts. For example, each of the plurality of bars in second portion 320 may be shifted, or offset, a distance in a direction 315 perpendicular to the grating bar direction 305 from the corresponding grating bar in first portion 310.

[0038] That is, by moving a grating bar within the distance between subsequent grating bars in a section, a phase shift can be realized in the deflected beams. The phases of the undeflected beams 104a, 104c, 104e may still not be displaced, and the amplitudes of all diffracted orders may remain the same regardless of the phase shift.

[0039] 4 shows a first example of offset grid bars according to at least some embodiments of the present invention. More specifically, FIG. 4 shows how first grid bar 321 and second grid bar 322 of second portion 320 may be offset by a distance 410 relative to corresponding first grid bar 311 and second grid bar 312 of first portion 320 in a direction 315 perpendicular to grid bar direction 305. That is, first grid bar 311 of first portion 310 may be the corresponding grid bar of first grid bar 321 of second portion 320, and second grid bar 312 of first portion 310 may be the corresponding grid bar of second grid bar 322 of second portion 320.

[0040] In this manner, the first grating bar 321 in the second portion 320 may be offset by a distance 410 from the corresponding grating bar, i.e., the first grating bar 311, in the first portion 310. Similarly, the second grating bar 322 in the second portion 320 may be offset by a distance 410 from the corresponding grating bar, i.e., the second grating bar 312, in the first portion 310. As a result, the first grating bar 311 and the second grating bar 312 in the first portion 310 are offset in a direction 315 perpendicular to the grating bar direction 305 relative to the corresponding first grating bar 321 and the second grating bar 322 in the second portion 320, thereby causing different phase shifts in the light beams propagating along different paths in the EPE grating, even if the amplitude of the diffracted light beam remains the same. That is, all grating bars in the second portion 320 may be offset by the same distance 410 relative to the corresponding grating bar in the first portion 310.

[0041] Distance 410 may be smaller than distance 420 between adjacent grating bars in a portion. That is, distance 410 may be smaller than the period of the EPE grating. For example, if the EPE grating includes two grating bars within the period of the EPE grating, both grating bars may be offset by the same amount.

[0042] 5a and 5b show a second example of offset grid bars according to at least some embodiments of the present invention. More specifically, FIGS. 5a and 5b show how each of the plurality of bars in second portion 320 may be offset relative to a corresponding grid bar in first portion 310 by a distance ds in a direction 315 perpendicular to grid bar direction 305, e.g., laterally. Distance ds (in Equation (1)) may correspond to distance 410 in FIG. 4. The distance between subsequent grid bars in a portion, e.g., first grid bar 311 and second grid bar 312 in first portion 310, is indicated by d (in Equation (1)), which may correspond to distance 420 in FIG. 4.

[0043] That is, the first grating bar 321 of the second portion 320 in FIG. 5b may be offset by a distance ds relative to the first grating bar 311 of the first portion 310 in FIG. 5a, and the second grating bar 322 of the second portion 320 in FIG. 5b may be offset by a distance ds relative to the second grating bar 312 of the first portion in FIG. 5a. In this manner, by shifting a grating bar in one portion relative to a grating bar in another portion within the distance between subsequent grating bars in the portion, i.e., within the period d, a phase shift can be realized in the diffraction orders, i.e., the deflected light beam. For example, as shown in FIG. 5b, the first grating bar 321 of the second portion 320 may be offset by a distance ds relative to the first grating bar 311 of the first portion 310, e.g., laterally, in the plane of the grating bar in the case of a one-dimensional grating. The laterally may refer to a direction extending from one side of the EPE grating to another side of the EPE grating.

[0044] 5a and 5b show cross-sectional views of grating bars 311, 312, 321, and 322. Grating bars generally may have a variety of shapes. However, embodiments of the present invention are not limited to any particular shape of the grating bars. For example, the cross-section of the grating bars may be rectangular or triangular. Furthermore, the width, height, fill factor, or any other characteristic of the grating bars may vary from portion to portion; for example, the grating bars in first portion 310 may have a different width than the grating bars in second portion 320.

[0045] FIG. 6 shows an example of phase shift in accordance with at least some embodiments of the present invention. More specifically, FIG. 6 shows an example of phase shift as a function of the ratio ds / d. In FIG. 6, the phase of the electric field component of the first diffraction order as a function of the Lohmann shift ds.

[0046] Figures 7a and 7b show examples of staggered two-dimensional grating bars in accordance with at least some embodiments of the present invention, i.e., Figures 7a and 7b show examples of double-period grating bars.

[0047] More specifically, Figures 7a and 7b illustrate how embodiments of the present invention can be applied to two-dimensional EPE grids. Figure 7a illustrates the position of the first grid bar 311 in the first portion 310, and Figure 7b illustrates the position of the first grid bar 321 in the second portion 320. As can be seen from Figure 7b, for two-dimensional grids, the first grid bar 321 in the second portion 320 may be offset in the plane of the grid bar, i.e., the lateral direction dx, and in a plane perpendicular to the plane of the grid bar, i.e., the vertical direction dy. The vertical direction may refer to the direction extending from the bottom of the EPE grid to the top of the EPE grid. As with the examples shown in Figures 5a and 5b, the grid bars shown in Figures 7a and 7b may have any shape or other characteristics, such as a rectangular or triangular shape.

[0048] 8 illustrates an example distribution of movement distances for different portions, in this case incoupling grating 202, outcoupling grating 204, and EPE grating 206, in accordance with at least some embodiments of the present invention. As shown in FIG. 8, portions of EPE grating 206, such as first portion 310 and second portion 320, may be configured such that the grating bars in different portions are moved by different distances to produce different phase shifts in the diffracted beams while keeping the amplitude of the diffracted beams the same, thereby reducing interference caused by beams interfering with EPE grating 206.

[0049] It should be understood that the disclosed embodiments of the invention are not limited to the particular structures, process steps, or materials disclosed herein, but are intended to cover equivalents thereof as recognized by those skilled in the relevant art. It should also be understood that the terminology used herein is used for the purpose of describing particular embodiments only, and is not intended to be limiting.

[0050] Throughout this specification, a reference to an embodiment or an embodiment means that a particular feature, structure, or characteristic described with respect to that embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrase "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. For example, when reference is made to a numerical value using terms such as about or substantially, the exact numerical value is also disclosed.

[0051] As used herein, a plurality of items, structures, components, and / or materials may be presented in a common list for convenience. However, these lists should be construed as individually identifying each member of the list as a separate and distinct element. Thus, no individual element of such a list should be construed as a de facto equivalent of other elements of the same list solely by virtue of presentation within a common group, unless otherwise indicated. In addition, various embodiments and examples of the present invention may be referred to herein along with alternatives for various of its components. It is understood that such embodiments, examples, and alternatives should not be construed as de facto equivalents of each other, but as independent and autonomous manifestations of the present invention.

[0052] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the above description, numerous specific details, such as illustrative lengths, widths, shapes, etc., are presented to provide a thorough understanding of embodiments of the present invention. However, one skilled in the relevant art will recognize that the present invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the present invention.

[0053] While the foregoing examples illustrate the principles of the present invention in one or more particular applications, it will be apparent to those skilled in the art that numerous changes in form, use, and details of implementation may be made without the exercise of inventive faculty and without departing from the principles and concepts of the present invention. Accordingly, it is not intended that the present invention be limited except as by the claims which follow.

[0054] In this document, the verbs "to comprise" and "to include" are used as open limitations which neither exclude nor require the presence of unrecited features. Features recited in dependent claims are mutually freely combinable, unless expressly stated otherwise. Furthermore, throughout this document, it is to be understood that the use of "a" or "an", i.e., the singular, does not exclude a plurality. [Industrial Applicability]

[0055] At least some embodiments of the present invention find industrial application in HMDs and HUDs.

[0056] Acronym List HMD Head-Mounted Display HUD Head-Up Display LCOS reflective LCD panel (Liquid Crystal on Silicon) LED Light-Emitting Diode MEMS Microelectromechanical System [Explanation of symbols]

[0057] 100 Right Field 102 Light Guide 104 Rays of light 110 Waveguide 112 elements 114 Directional light 120 eyes 130 Mirror 140 light source 202 Incoupling Lattice 204 Outcoupling lattice 206 Exit Pupil Expander 310,320 EPE segments 311,312,321,322 Lattice bars 305,315 directions 410 distance ds 420 distance d

Claims

1. 1. An Exit Pupil Expander (EPE) grating divided into at least two portions, the EPE grating comprising a plurality of grating bars in a first portion and a plurality of grating bars in a second portion, the plurality of grating bars in the first portion being oriented in generally the same direction as the plurality of grating bars in the second portion and being offset in a direction perpendicular to the direction of the grating bars; an EPE grating, wherein the plurality of grating bars in the first portion and the plurality of grating bars in the second portion are offset to impart different phase shifts to light rays propagating along different paths in the EPE grating.

2. 2. The EPE grating of claim 1, wherein each of the plurality of grating bars of the second portion is offset by a distance relative to a corresponding grating bar of the first portion in the direction perpendicular to the direction of the grating bars.

3. 3. The EPE grating of claim 2, wherein a first grating bar of the first portion is the corresponding grating bar of a first grating bar of the second portion, and a second grating bar of the first portion is the corresponding grating bar of a second grating bar of the second portion.

4. 4. The EPE grating according to claim 2 or 3, wherein the distance is less than the period of the EPE grating.

5. 5. An EPE grating according to claim 2, wherein each of the plurality of bars of the second portion is laterally offset from a corresponding grating bar of the first portion by the distance.

6. 6. An EPE grating according to claim 2, wherein each of the plurality of bars of the second portion is offset from a corresponding grating bar of the first portion by the distance in a direction perpendicular to the plane of the grating bar of the first portion.

7. 7. The EPE grating according to claim 1, wherein the EPE grating is double-periodic.

8. 8. An EPE grating according to claim 1, wherein the first portion of the EPE grating is configured to impart a first phase shift to light rays deflected by the first portion, and the second portion of the EPE grating is configured to impart a second phase shift to light rays deflected by the second portion.

9. The EPE grating of claim 8 , wherein the first phase shift is different from the second phase shift.

10. 10. An EPE grating according to claim 8 or 9, wherein the amplitude of the light beam deflected in the first portion is the same as the amplitude of the light beam deflected in the second portion.

11. 11. The EPE grating according to claim 1, wherein the second portion follows the first portion with respect to a light beam guided by the EPE grating.

12. 12. The EPE grating of claim 1, further comprising a plurality of grating bars in a third portion, each of the plurality of bars in the third portion being offset from a corresponding grating bar in the first portion by a distance in a direction perpendicular to the direction of the grating bars.

13. 13. An EPE grating according to any preceding claim, wherein the distance between subsequent grating bars in the first portion is the same as the distance between subsequent grating bars in the second portion.

14. 14. The EPE grating according to claim 1, wherein the EPE grating is configured to diffusely couple light out of the EPE grating and is also configured to act as an incoupling grating.

15. 15. The EPE grating of claim 1, wherein the EPE grating is configured to keep the amplitudes of light rays propagating through different paths in the EPE grating unchanged.

16. 1. An optical waveguide device for displaying an image, comprising: an optical waveguide; an incoupling grating for diffractively coupling the image into the optical waveguide; an outcoupling grating for diffractively coupling the image out of the optical waveguide; 16. An optical waveguide device comprising an EPE grating according to any one of claims 1 to 15, the EPE grating being between the incoupling grating and the outcoupling grating to expand the exit pupil of the image on the outcoupling grating.

17. 17. A personal display device such as a head-mounted display, HMD or head-up display, HUD, comprising the optical waveguide device according to claim 16.

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