Optical arrangement, display assembly, and display device

Abstract

An optical arrangement (101) comprises a transparent waveguide (110) configured to guide light of a design wavelength therein, and an out-coupling arrangement (120) with a diffractive grating (130) configured to couple the light out of the waveguide. The diffractive grating (130) comprises a two—dimensional grating having a slanted configuration and comprising at least two grating layers (135, 136) with a refractive index difference between adjacent grating layers.

Description

[0001] OPTICAL ARRANGEMENT , DISPLAY ASSEMBLY , AND DISPLAY DEVICE

[0002] BACKGROUND

[0003] In various transparent display devices such as augmented or extended reality glasses or head-up displays HUDs , an image from an image source is transmitted into a transparent waveguide and coupled out of it towards the viewer or user of the display device .

[0004] Said out-coupling may be effected by an out-coupling arrangement comprising a diffractive grating . Slanted grating profile may provide certain advantages in the out-coupling efficiency and spectral performance .

[0005] The usability of a display device may be adversely affected by leakage light undesirably coupled to the direction opposite to the viewer or user of the display device . For example , in the case of AR glasses , such light may cause a glow adversely obscuring the eyes of the user, or leaking information towards the outside world . Typically, adj usting the grating design to increase the out-coupling efficiency to the useful direction also increases the undesired out-coupling to the opposite direction .

[0006] Another factor affecting the feasibility of a display device is the uniformity of the brightness of the image displayed to the user or viewer of the display device .

[0007] SUMMARY

[0008] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description . This summary is not intended to identify key features or essential features of the claimed subj ect matter, nor is it intended to be used to limit the scope of the claimed subj ect matter .

[0009] According to a f irst aspect , an optical arrangement i s disclosed, which comprises a transparent waveguide configured to guide light of a design wavelength therein, and an out-coupling arrangement with a diffractive grating, configured to couple the light out of the waveguide . The diffractive grating comprises a two-dimensional grating having a slanted configuration and comprises at least two grating layers with a refractive index difference between two adj acent grating layers .

[0010] In an embodiment , the design wavelength lies in the range of 350 to 800 nm, for example , in the range of 380 to 780 nm .

[0011] In an embodiment which may be in accordance with the previous embodiments , the two-dimensional grating has , in a first extending orientation Dl za first grating period Pi and, in a second extending orientation D2 , a second grating period P2 different from the first grating period Pi . Then, the first grating period Pi may be equal to or higher than half of the design wavelength, and the second grating period P2 may be less than half of the design wavelength .

[0012] In an embodiment in accordance with any of the two previous embodiments , the two-dimensional grating comprises two non-parallel overlayed one-dimensional grating structures with the first and the second grating periods Pi , P2 , and the one of the grating structures with the first grating period Pi has a slanted grating profile .

[0013] In an embodiment which may be in accordance with any of the two previous embodiments , the two-dimensional grating has , in the second extending orientation D2 , a non-constant fill factor configured to vary out-coupling efficiency of the diffractive grating .

[0014] In an embodiment which may be in accordance with any of the three previous embodiments , the first extending orientation Di and the second extending orientation D2 lie at an orientation angle in the range of 30 to 90 degrees , for example , in the range of 50 to 90 degrees to each other .

[0015] In another embodiment which may be in accordance with any of the previous embodiments , the diffractive grating comprises a birefringent material .

[0016] In an embodiment which may be in accordance with any of the embodiments above , the diffractive grating comprises at least two grating layers , for example , three , four , five , or six layers . Each of the at least two grating layers may then have a refractive index different from those of the other grating layers .

[0017] In an embodiment which may be in accordance with any of the embodiments above , the refractive index difference lies in the range of 0.2 to 2.0, for example, in the range of 0.5 to 1.5.

[0018] In an embodiment which may be in accordance with any of the embodiments above, the diffractive grating has a slant angle in the range of 5 to 70 degrees, for example, in the range of 30 to 70 degrees, for example, in the range of 40 to 60 degrees.

[0019] In an embodiment which may be in accordance with any of the embodiments above, the diffractive grating comprises grating features formed as surface-relief structures. Those grating features may be two-dimensional grating features formed by material islands or voids each having a peripheral edge surface delimiting the lateral extension of the grating feature, the peripheral edge surface being defined by lateral edges of the grating layers .

[0020] In an embodiment which may be in accordance with the previous embodiment, the grating features are covered by a solid material. The solid material may have a refractive index less than or equal to 1.5.

[0021] A grating feature being covered by a solid material refers to said solid material not forming a part of the grating feature. Such solid material thereby does not form a grating layer.

[0022] According to a second aspect, a display assembly is disclosed, which comprises an optical arrangement in accordance with the first aspect. According to a third aspect , an augmented or extended reality display device is disclosed, which comprises a display assembly in accordance with the second aspect . The augmented reality display device may be a near-to- eye display NED or a head-up display HUD .

[0023] Further embodiments of the above aspects may be implemented within the scope of the claims .

[0024] BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present disclosure will be better understood from the following detailed description read in view of the accompanying drawings , wherein :

[0026] FIG . 1 shows cross-sectional and top views of a two- dimensional diffractive grating forming part of an optical arrangement of a display assembly;

[0027] FIG . 2 shows a cross-sectional view of another two- dimensional grating;

[0028] Fig . 3 shows optical coupling arrangement simulation results ; and

[0029] FIGs . 4 and 5 show display devices incorporating an optical arrangement .

[0030] Unless specif ical ly stated to the contrary, any of the aforementioned drawings may be schematic and drawn not to scale such that any element in said drawing may be drawn with inaccurate proportions with respect to other elements in said drawing in order to emphasi ze certain structural aspects of the embodiment of said drawing .

[0031] Moreover, corresponding elements in the embodiments of any two drawings of the aforementioned drawings may be di sproportionate to each other in said two drawings in order to emphasi ze certain structural aspects of the embodiments of said two drawings .

[0032] DETAILED DESCRIPTION

[0033] It is apparent to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways . The invention and its embodiments are thus not limited to the examples described above , instead they may vary within the scope of the claims .

[0034] The optical arrangement 101 of FIG . 1 forms a part of , or serves as , a display assembly 100 . Such display assembly may form at least a part of a display device , such as a near-to-eye display, a head-mounted display, a see-through display, or a head-up display . In other embodiments , optical arrangements may be implemented for non-imaging applications such as back light or front light illumination of displays , or general lighting purposes .

[0035] The optical arrangement 101 comprises a transparent waveguide 110 which is configured to guide light to propagate therein, for example , by total internal reflections at opposite surfaces thereof . The waveguide 110 may be a plate-like or sheet-like configuration . Such configuration or shape may refer to the waveguide extending substantially along or parallel to a fictitious planar or curved two-dimensional waveguide base plane 111 . The waveguide may have lateral dimensions parallel to such waveguide base plane 111 substantially larger than a thickness in a direction perpendicular to the waveguide base plane 111 . A waveguide base plane may alternatively be considered as a waveguide base surface .

[0036] "Transparent" refers to the waveguide 110 being at least partially transmissive for the design wavelength ( s ) in a direction perpendicular to the waveguide base plane . For example , in the case of the design wavelength ( s ) lying in the visible spectrum range , the transparency may enable the optical arrangement 101 to form a part of a transparent display assembly through which the viewer of the display assembly can see .

[0037] In the example of FIG . 1 , the upper surface 112 of the waveguide follows the waveguide base plane 111 . In the case of a waveguide with a uniform thickness having parallel bottom and upper surfaces , both of those surfaces are also parallel to the waveguide base plane . In the case of non-parallel upper and bottom surfaces , the base plane may be defined as a geometrical centre plane of the waveguide .

[0038] The waveguide may be made of and comprise any appropriate glass or plastic material ( s ) suitable for transparent waveguides . For example , it may comprise or be formed of Dense Tantalum Flint TafD or any other appropriate transparent material .

[0039] The optical arrangement 101 comprises an out-coupling arrangement 120 configured to couple at least part of the light propagating in the waveguide out of it .

[0040] The out-coupling arrangement comprises a diffractive grating 130 . A "diffractive grating" refers to a grating with one or more periodic grating structures , at least one of which is dimensioned to operate dif f ractively at the design wavelength ( s ) . Thereby, the out-coupling may be effected by diffraction of light .

[0041] The waveguide and the diffractive grating may be designed for operation at one or more design wavelengths . The design wavelength ( s ) may be selected to lie in the range of visible wavelengths . For example , one or more design wavelengths may l ie in the range of about 350 to 800 nm, for example , in the range of about 380 to 780 nm .

[0042] Although not illustrated in the drawings , the out- coupling arrangement may additionally comprise any appropriate other layer ( s ) , structure ( s ) , or element ( s ) .

[0043] The diffractive grating 130 of FIG . 1 is a two- dimensional grating . A two-dimensional grating generally refers to a grating structure having periodicity in at least two non-parallel directions . A two-dimensional grating such as the grating of FIG . 1 thus has periodically repeating grating features in each of the at least two directions . In a simple case such as that of FIG . 1 , a two-dimensional grating may comprise two one-dimensionally periodic grating structures 140 , 150 overlayed non-parallel to each other . In other embodiments , more complex grating structures providing periodicities in at least two nonparallel directions may be used . For example , a two- dimensional grating may comprise repeating unit cells , each unit cell itself comprising a two-dimensional arrangement of a plurality of grating features .

[0044] In other embodiments , a one-dimensional diffractive grating may be used .

[0045] Two grating structures being overlayed refers to the resulting structure of the two-dimensional grating being a combination or sum of the two one-dimensional grating structures .

[0046] A one-dimensional grating structure may be considered having its periodic structure extending and being periodic in a longitudinal direction of that grating structure . That longitudinal direction thereby determines an extending orientation of that grating structure . Orientations of two grating periods relative to each other may be defined by means of the extending orientations or the longitudinal directions of the grating structures .

[0047] Non-parallel refers to the two grating structures 140 , 150 having their grating periodicities extending in different directions , i . e . having their extending orientations Dl zD2 lying at a non- zero orientation angle to each other . In the example of FIG . 1 , the non- zero orientation angle [3 is substantially 90 degrees . The two one-dimensional grating periods thus extend substantially perpendicular to each other . In other embodiments , the orientation angle between the two grating structures may be different . In various embodiments , the orientation angle between the two onedimensional grating structures may l ie in the range of ZZZ to YYY degrees .

[0048] The diffractive grating 130 lies on the upper surface 112 of the waveguide 110 . There may be any appropriate additional intermediate layer ( s ) between the waveguide and the diffractive grating . For example , one or more material layers may exist between the diffractive grating 130 and the body of the waveguide 110 , for example , to serve as an anti-reflection structure .

[0049] In the vertical or thickness direction, the diffractive grating 130 comprises a base part 131 having a non- zero thickness , and grating features defined by the first and second grating structures 140 , 150 , the grating features extending from the base part . In other embodiments , diffractive gratings may be implemented without any base part . Such grating may alternatively be considered having a base part with a zero thickness .

[0050] The base part extends along or parallel to a substantially parallel grating base plane 132 , or grating base surface . In other embodiments , a grating base plane may be planar or curved . In the example of FIGs . la and lb, the grating base plane is paral lel to the waveguide base plane 111 . In other embodiments , the grating base plane and the waveguide base plane bay be non-parallel . The one-dimensional grating structure 140 , the longitudinal cross-sectional profile of which is shown in drawing b) of FIG . 1 , has a slanted grating profile . That grating structure 140 is hereinafter referred also to as a first grating structure . Correspondingly, the other grating structure 150 , the longitudinal cross- sectional profile of which is shown in drawing b) of FIG . 1 , may be considered as a second grating structure . In embodiments with one-dimensional diffractive grating, the single grating structure may be in accordance with the slanted first grating structure 140 .

[0051] "Longitudinal cross-sectional" profile refers to the cross section of the grating structure along a section plane which is parallel to the extending orientation of the grating structure and perpendicular to the grating base plane 132 .

[0052] A slanted grating profile of a one-dimensional or a two- dimensional grating generally refers to a cross- sectional profile of a grating comprising periodically repeating grating features which are directed inclined or slanted, i . e . non-perpendicularly, relative to the grating base plane .

[0053] In a slanted grating profi le , there i s thus a non- zero slant angle between the normal N of the grating base plane and a centre line of a grating feature of the grating structure . The slant angle is defined in a cross section of the grating in a direction parallel to the proj ection of the centre line along the grating base plane . In the example of FIG. 1, said direction coincides with the extending orientation Dx of the first grating structure 140. In the extending orientation D2 of the second grating structure 150, such angle is zero, i.e. the second grating structure has a perpendicular, thus not slanted, grating profile. The centre line 154 of the grating features in the form of projections 151 thus extends perpendicularly to the grating base plane 132, thus parallel to the normal N of the grating base plane 132.

[0054] In other embodiments, in a two-dimensional diffractive grating with a slanted configuration, a slanted grating feature may be directed so as to produce the projection of the centre line of the grating feature along the grating base plane be directed at any angle relative to the extending orientations of the grating periods.

[0055] In the example of FIG. la, the grating features in the form of projections 141 of the first grating structure 140 have a constant width, i.e. the side surfaces 142, 143 of the cross-section of the projection are parallel. Then, the centre line 144 defining the slant angle a is parallel to the side surfaces.

[0056] In other embodiments, a projection of the grating structure may have a non-constant width with the opposite side surfaces thereof lying at a non-zero angle to each other. A centre line of such projection may be defined as the geometrical centre line of the crosssection of the projection. The slant angle may lie, for example, in the range of 5 to 70 degrees. It may be, for example, about 5 10, 15, 30, 40, 60 or 70 degrees.

[0057] As mentioned above, the second grating structure 150, the cross-sectional longitudinal profile of which is illustrated in drawing b) of FIG. 1, has a perpendicular, i.e. non-slanted grating profile. In other embodiments, also the second grating structure may have a slanted grating profile with a slant angle being the same as or different from that of the first grating structure .

[0058] The overlayed non-parallel first and second grating structures 140, 150 result in the diffractive grating 130 having a two-dimensional array of pillars 133 as grating features of the diffractive grating. In the example of FIG 1, the cross-section of the pillars is substantially rectangular. In other embodiments, pillars or other types of grating features may have any other appropriate cross-section, such as round or oval.

[0059] The pillars 133 are an example of two-dimensional grating features formed by discrete material islands. Each material island comprises a plurality of stacked grating layers 135, 136, 137. -In other embodiments, an opposite configuration may be implemented where two- dimensional grating features are formed by discrete voids, i.e. cavities or holes, formed in a multi-layered stack of grating layers.

[0060] Both the pillars and voids are examples of surfacerelief grating features, i.e. grating features of a surface-relief grating structure. In the example of FIGs. la, lb, and 1c, the pillars 133 have substantially planar, substantially horizontal top surfaces 134. In other embodiments, a top surface may be slanted, i.e. tilted relative to the grating base plane .

[0061] The first grating structure 140 has a first grating period Pi which is selected to enable diffraction of light at the design wavelength. The first grating period is at least equal to half of the design wavelength. It may be, for example, substantially equal to the design wavelength .

[0062] In the example of FIGs. la, lb, and 1c, the second grating structure 150 has a second grating period P2 which is shorter than the first grating period Pi. In an embodiment, the second grating period P2 is less than half of the design wavelength. Thus, at the design wavelength, the second grating structure may be, and operate as, as a sub-wavelength, thus a non-dif f ractive grating .

[0063] In other embodiments, also a second grating period may lie in a range making the second grating structure to operate dif f ractively . Such grating periods may be the same or different.

[0064] The diffractive grating 130 is formed as a surface relief structure on the upper surface 112 of the waveguide 110. In other embodiments, buried grating structures may be used.

[0065] In the example of FIG. 1, the designed out-coupling direction is upwards in the drawings a) and b) of FIG: 1. Thus, the diffractive grating 130 lying on the upper surface 112 of the waveguide 110 operates in a transmissive mode, i.e. is a transmissive grating. In other embodiments, a diffractive grating of an out- coupling arrangement may lie on or close to the surface of the waveguide opposite to the designed out-coupling direction. Such diffractive grating may then operate in a reflective mode, i.e. be a reflective grating.

[0066] The diffractive grating 130 has a bottom grating layer 135, a middle grating layer 136, and a top grating layer 137.

[0067] In the example of FIG. 1, the bottom grating layer 135 covers the base part 131 and the lower part of the pillars 133 of the diffractive grating 130. The middle grating layer 136 lies, in the direction of the normal N of the grating base plane, in the middle part of the pillars, on the bottom grating layer 135 and below the top grating layer 137. In other embodiments, the grating layer thicknesses may be selected differently.

[0068] The total thickness of the at least two grating layers of the diffractive grating above the base part may lie, for example, in the range of 50 to 500 nm. This total thickness defines the height of the pillars or other types of grating features, i.e. the grating height H in the direction of the normal N of the grating base plane 132.

[0069] The grating 130 of FIG. 1 thereby exemplifies a "grating layer" referring to a layer forming a part of a thickness of a grating feature. In the direction of the normal of a grating base plane, the material of the grating feature thus changes between the grating layers . Thereby, at each level in the direction of the normal of the grating base plane , the grating feature is formed by one grating layer only . A material coating a grating and a grating feature thereof by lying not only on top of the grating feature but also on a side surface thereof is not part of the grating feature , and does not form a grating layer .

[0070] Each of the two-dimensional grating features 141 formed by material islands has a peripheral edge surface 160 delimiting the lateral extension of the grating feature . The peripheral edge surface thus refers to the side surfaces connecting the upper surface of the base part 131 ( the upper surface of the waveguide 110 in embodiments with a base part with a zero thickness ) and the top surface 134 of the pil lar-formed grating feature .

[0071] A peripheral edge surface , such as that of FIG . 1 covers the entire perimeter of the grating feature . It thus encircles the grating feature cross section entirely . In the example of FIG . 1 , the peripheral edge surface 160 of the grating feature 141 comprises thus the opposite grating feature side surfaces 142 , 143 shown in drawing a) of FIG . 1 as well as the corresponding opposite side surfaces shown in drawing b) of FIG . 1 .

[0072] In the case of a grating feature with a circular or other curved cross-section, the peripheral edge surface is formed by one single curved grating feature side surface encircling the entire perimeter of the grating feature . The peripheral edge surface 160 is defined by lateral edge surfaces 135' , 136' , 137' of the gating layers. Each lateral edge surface encircles the entire perimeter of the grating feature. A lateral edge surface of a grating layer thereby delimits the lateral extension of the grating layer itself, and also a part of the height of the peripheral edge surface of the grating feature.

[0073] The lateral edge surface 135' of the bottom grating layer 135 covers the part of the thickness of the bottom grating layer 135 above the base part only. The lateral edges 136' , 137' of the middle and top grating layers 136, 137 cover the entire thicknesses of those layers.

[0074] In the case of a grating feature formed as a void in a multi-layer stack of grating layers, the peripheral edge surface of the grating feature is correspondingly defined by grating layer lateral edge surface (s) encircling the void.

[0075] The lateral extension, i.e. the extension in the direction parallel to or along the grating base plane 132 being delimited by lateral edge surface (s) of a plurality of grating layers means that, for example, a coating layer in the form of a layer of a material covering both the top surface 134 and a side surface of the grating feature is not part of the grating feature. Such layer it thus not a grating layer, even though it may affect the optical performance of the grating. The materials of the three grating layers of the example of FIG. 1 have been selected such that there is a refractive index change over the interface between each two adjacent grating layers. This may be implemented by each of the three grating layers having a refractive index n4, n2, ns different from those of the two other grating layers. Alternatively, the top and bottom grating layers

[0076] 135, 137 may have the same refractive index differing from the refractive index of the middle grating layer

[0077] 136.

[0078] In other embodiments, a multi-layered diffractive grating may comprise any appropriate number of at least two grating layers, the materials of the layers being selected to provide a refractive index difference between each two adjacent grating layers. Thus, there can be, for example, two, three, four, five, six, or even more layers.

[0079] A refractive index difference between adjacent grating layers of a diffractive grating may be, for example, in the range of 0.2 to 2.0, for example, in the range of 0.5 to 1.5.

[0080] The grating layers of the grating structure may comprise or be formed of materials with a high refractive index, such as 2.0 or higher. Examples of high refractive index materials comprise, for example, titanium oxide TiCy, silicon nitride Si3N4, and hafnium oxide HfCy.

[0081] The out-coupling arrangements discussed above may provide significant advantages.

[0082] For example, the slanted grating profile of a diffractive grating profile may provide a high out- coupling efficiency. The layered grating structure may further decrease the undesired out-coupling to the direction opposite to the useful direction. Such out- coupling is known in the art also as a "world-side leakage" or "leakage to the world side" . The table below summari zes observations from selected simulations carried out for different slant angles and numbers of grating layers for visible wavelengths .

[0083] The above table shows that the undesired out-coupling to the direction opposite to the useful direction may be strongly decreased by the claimed optical arrangement .

[0084] Further examples of the effect of the grating layers of the diffractive grating are discussed below with reference to FIG . 3 .

[0085] The two-dimensionality may further provide , for example , the advantage of the grating being capable of not only coupling out light of the waveguide but also of expanding the expanding the light field and spread the light uniformly over an out-coupling area . Further, a two-dimensional grating may couple out light propagating within the waveguide in different directions .

[0086] The multi-layered configuration with variable refractive index profile with additional interfaces between materials of different refractive indices provides an increased degree of freedom of designing the grating for various applications . For example , a multi- layered structure may enable designing the slanted grating structure so as to have , for a given coupl ing ef ficiency, a slant angle smaller than the slant angle for a slanted grating structure formed of one single material .

[0087] In embodiments with the second grating period P2 being sub-wavelength, special advantages may be achievable in terms of providing additional grating modulating characteristics . For example , a sub-wavelength grating may result in the material of the grating serving in accordance with an effective refractive index lying between 1 . 0 ( in the case of no material or air f illing between the pi llars 133 ) and the refractive indices of the grating layers of the diffractive grating . Such ef fective index may be dependent on the fi ll factor of the grating structure , as well as solid material ( s ) possibly present between the pillars .

[0088] Fill factor refers to the portion of the grating period being covered the grating feature .

[0089] An effective refractive index may affect the out- coupling efficiency of the first grating structure . On the other hand, the effective refractive index may be made variable , i . e . having different values , in different parts of the area of the out-coupling arrangement . This can be effected, for example , by varying the fill factor of the second grating structure . Thereby, it is possible to modulate the out-coupling efficiency of the diffractive grating . For example , the out-coupling efficiency may be adj usted to compensate the decrease of the intens ity of the light propagating in the waveguide caused by part of the light having already been coupled out. This may enable, for example, providing a substantially uniform out-coupling intensity throughout the area of the out-coupling arrangement or a specific part thereof.

[0090] An example of the variable fill factor is illustrated in drawing c) of FIG. 1 where the distance between pillars in the direction of the second extending orientation D2 is changed between different parts of the coupling arrangement. In the example of FIG. 1, also the second grating period P2 is varied. In other embodiments, the second grating period may be constant although the fill factor of the second grating structure varies .

[0091] The modulating capability may be even further enhanced by the diffractive grating comprising, at least in one of its grating layers, a birefringent material. Then, the grating may be designed to operate differently for different states of polarization of the light.

[0092] In the example of FIG. 1, the diffractive grating 130 has no solid material filling in between the pillars 133 thereof. FIG. 2 illustrates another embodiment where the grating structures 240, 250 are covered by a solid filling material 238 filling the space between the pillars 233. The solid material may be selected so as to have a low refractive index, such as less than or equal to 1.5, for example, less than or equal to 1.2. The solid filling material may be or comprise, for example, silicon oxide SiO2, magnesium fluoride MgF2, or some plastic with a refractive index in said range. Fig . 3 shows graphs of simulated performance of slanted one-dimensional grating structures with a slant angle of 40 degrees . The graphs show the out-coupling efficiencies for different wavelengths as a function of the grating height for to the useful or intended direction T1 ( 1storder transmi ssive diffraction) and the ratio of the out-coupling efficiency to the opposite undesired direction R1 ( 1storder reflective diffraction) to Tl . The simulated grating structures were as follows : A) a single layer HO2 grating, B) a single layer SiCy grating, C) a four-layer grating with a TiCy layer of 20 nm, a SiCy layer of 50 nm, a TiCy layer with a varying thickness , and a SiCy layer of 80 nm . The simulations were calculated by Fourier Modal Method ( FMM) for single grating interaction .

[0093] The simulations show the multi-layer grating being superior to the single layer gratings suffering from high leakage to the world side . The grating C) has the additional advantage to have a low Rl / Tl ratio for a large range of grating thicknesses ("grating height" in the graphs ) , which facilitates adj usting of the outcoupling efficiencies for different colors .

[0094] Fig . 4 illustrates a display assembly in the form of augmented reality AR glasses 400 , also known as extended reality glasses or mixed reality glasses , with two lenses 410 . The AR glasses have an image source 402 configured to emit light L for producing an image and transmit it to an in-coupling arrangement 460 which may comprise , for example , a diffractive grating and which is configured to couple the light into the lens 402 to propagate therein . The lens 410 thereby serves as a waveguide . The lens further comprises an outcoupling arrangement 420 with a diffractive grating 430 which may be in accordance with any of those discussed above with reference to FIGs . 1 to 3 . The lens thereby forms an implementation of an optical arrangement 401 as discussed above with reference to FIGs . 1 to 3 . The out- coupling arrangement is configured to couple light L propagating in the lens out of it towards the eye of the user of the glasses . Thereby, the image is displayed to the user . The AR glasses are an example of a near-to- eye display NED .

[0095] FIG . 5 shows a display assembly 500 in the form of a windscreen 510 . The windscreen serves as a waveguide into which light from an image source 502 can be coupled via or by an in-coupling arrangement 560 . Light propagating within the windscreen is expanded to a larger area by an intermediate pupil expansion arrangement 570 which may comprise , for example , diffractive grating ( s ) . There is further an out-coupling arrangement 520 incorporated in the windscreen 510 , comprising a diffractive grating 530 which may be in accordance with any of those discussed above with reference to FIGs . 1 to 3 . The windscreen 510 thereby forms an implementation of an optical arrangement 501 as di scussed above with reference to FIGs . 1 to 3 . The out-coupling arrangement 520 is configured to couple light propagating in the windscreen out of it towards the driver of the vehicle . Thereby, the image is displayed to the driver . The optical arrangement 501 in the form of the windscreen is an example of a head up display HUD . The optical arrangements discussed above with reference to the FIGs. may be manufactured using principles and processes as such known in the art. For example, the diffractive gratings may be manufactured using various thin film deposition and etching techniques.

[0096] It will be understood that any benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages.

[0097] The term "comprising" is used in this specification to mean including the feature (s) or act(s) followed thereafter, without excluding the presence of one or more additional features or acts. It will further be understood that reference to 'an' item refers to one or more of those items.

Claims

CLAIMS1. An optical arrangement (101) comprising a transparent waveguide (110) configured to guide light of a design wavelength therein, and an out-coupling arrangement (120) with a diffractive grating (130) configured to couple the light out of the waveguide, wherein the diffractive grating (130) comprises a two-dimensional grating having a slanted configuration and comprising at least two grating layers (135, 136) with a refractive index difference between adjacent grating layers.

2. An optical arrangement (101) as defined in claim 1, wherein the design wavelength lies in the range of 350 to 800 nm, for example, in the range of 380 to 780 nm.

3. An optical arrangement (101) as defined in claim 1 or 2, wherein the two-dimensional grating has, in a first extending orientation Dlza first grating period Pi and, in a second extending orientation D2, a second grating period P2 different from the first grating period Pi.

4. An optical arrangement (101) as defined in claim 3, wherein the first grating period Pi is equal to or higher than half of the design wavelength, and the second grating period P2 is less than half of the design wavelength .

5. An optical arrangement (101) as defined in claims 3 or 4, wherein the two-dimensional grating comprises two non-parallel overlayed one-dimensional grating structures (140, 150) with the first and the secondgrating periods Pi, P2, the one of the grating structures (140) with the first grating period Pi having a slanted grating profile.

6. An optical arrangement (101) as defined in any of claims 4 to 5, wherein the two-dimensional grating (130) has, in the second extending orientation Di,a nonconstant fill factor configured to vary out-coupling efficiency of the diffractive grating (130) .

7. An optical arrangement (101) as defined in any of claims 3 to 6, wherein the first extending orientation Di and the second extending orientation D2 lie at an orientation angle in the range of 30 to 90 degrees, for example, in the range of 50 to 90 degrees to each other.

8. An optical arrangement (101) as defined in any of the preceding claims, wherein the diffractive grating (130) comprises a birefringent material.

9. An optical arrangement (101) as defined in any of the precedent claims, wherein the diffractive grating (130) comprises at least three grating layers (135, 136, 137) .

10. An optical arrangement (101) as defined in any of the preceding claims, wherein each of the at least two grating layers (135, 136, 137) has a refractive index different from those of the other layers.

11. An optical arrangement (101) as defined in any of the preceding claims, wherein the refractive indexdifference lies in the range of 0.2 to 2.0, for example, in the range of 0.5 to 1.5.

12. An optical arrangement (101) as defined in any of the preceding claims, wherein the diffractive grating has a slant angle in the range of 5 to 70 degrees, for example, in the range of 30 to 70 degrees, for example, in the range of 40 to 60 degrees.

13. An optical arrangement (101) as defined in any of the preceding claims, wherein the diffractive grating (130) comprises grating features (141) formed as surface-relief structures.

14. An optical arrangement (101) as defined in claim 13, wherein the grating features (141) are two-dimensional grating features formed by material islands or voids each having a peripheral edge surface (160) delimiting the lateral extension of the grating feature, the peripheral edge surface being defined by lateral edge surfaces (135' , 136' , 137' ) of the grating layers (135, 136, 137) .

15. An optical arrangement (201) as defined claim 13 or 14, wherein the grating features (241) are covered by a solid material (238) .

16. An optical arrangement (201) as defined in claim 15, wherein the solid material (238) has a refractive index less than or equal to 1.5.

17. A display assembly (400, 500) comprising an optical arrangement (401, 501) as defined in any of the preceding claims.

18. An augmented reality display device comprising a display assembly (400, 500) as defined in claim 17, such as a near-to-eye display NED or a head-up display HUD.

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