Optical element

Abstract

An optical element (1) for an augmented reality display comprises a first side (2) facing a scene and a second side (3) being opposite to the first side (2) and facing a user's eye (4). The optical element (1) further comprises an optically-powered reflector (5) arranged at the first side (2) of the optical element (1), a polarization volume grating (6) arranged at the second side (3) of the optical element (1), and an optical cavity (7) formed by the optically- powered reflector (5) and the polarization volume grating (6). The optical element (1) is configured to reflect, and control the divergence of, image light (8) that is incident on the second side (3) of the optical element (1), such that the image light (8) traverses the optical cavity (7) four times and exits the optical element (1) as projected image light (8') from the second side (3) towards the user's eye (4). Further, an optical system, a wearable augmented reality display (13) and a vehicle (17) are provided.

Description

[0001] Description

[0002] OPTICAL ELEMENT

[0003] This disclosure relates to an optical element for an augmented reality display, an optical system for an augmented reality display, a wearable augmented reality display and a vehicle comprising the optical system .

[0004] An optical element for an augmented reality display may comprise a single optical component that is optically-powered to deflect image light . Such a configuration may deliver sharp on-axis performance but large field curvature . This means that the focus of the user needs to be accommodated to bring of f-axis field points into focus . While accommodation is often allowable in visual instruments such as telescopes , it may present problems in a binocular near-eye display applications . In addition, such optical elements may have poor optical performance .

[0005] At least one obj ect of particular embodiments is to provide an optical element for an augmented reality display with improved optical performance .

[0006] In at least one embodiment , an optical element is provided for an augmented reality display .

[0007] The augmented reality, AR, display can be an of f-axis retinal scanning display, ORDSD . ORDSDs are a type of display that are used in virtual and augmented reality applications such as in wearable heads-up display . ORSDs are designed to allow a user to see proj ected content in their field of view while viewing an external environment . ORSDs work by using a proj ector secured to a user' s head to proj ect an image onto the retina of the user which causes the user to see displayed content floating in space in front of them .

[0008] The optical element may be called optical combiner or optical lens , in particular reflective pancake lens , in the following . The optical element may be configured to combine the proj ected content , i . e . image light , with light from the external environment . That is , the optical combiner may be configured to combine image light and ambient light . In the following, it is distinguished between said image light and ambient light . The optical combiner may have the form of an optical lens and / or may act as an optical lens . As explained in the following, image light hits the optical element at a side of the optical element and leaves the optical element at the same side . Thus , the optical element is reflective for image light . The term pancake lens means that the optical element can be formed as flat and thin lens assembly .

[0009] In at least one embodiment , the optical element comprises a first side facing a scene and a second side being opposite to the first side and facing a user' s eye .

[0010] This can mean that the optical element is arranged between a user, in particular the user' s eye , and an external environment , in particular the scene . The optical element has a main plane of extension that runs in lateral directions .

[0011] The first side and the second side are essentially parallel to the main plane of extension of the optical element . However, the first and / or the second side can also be curved / spherical . A transversal direction, that is perpendicular to the lateral directions , relates to the thickness of the optical element . Further, the transversal direction may in the following refer to a main direction of the field of view of the user.

[0012] In at least one embodiment, the optical element comprises an optically-powered reflector arranged at the first side of the optical element.

[0013] That a component is "optically-powered" can mean that it can manipulate light in a specific way. For example, light is redirected / def lected, for example by refracting, diffracting, reflecting and / or transmission. As a further example, the polarization of light interacting with the optically-powered component can be changed. As a second further example, wavelengths of light interacting with the optical-powered component are filtered, or only light having specific wavelengths interact with the optically-powered component.

[0014] The optically-powered reflector faces the scene. The optically-powered reflector is configured to reflect image light, i.e. light that is projected onto the optical element. However, the optically-powered reflector may be substantially transparent for ambient light. Thus, the optically-powered reflector may leave ambient light unaffected or substantially unaffected. Image light, however, can be deflected and / or at least partially focused and / or at least partially collimated.

[0015] In at least one embodiment, the optical element comprises a polarization volume grating arranged at the second side of the optical element.

[0016] That is, the polarization volume grating, PVG, may face the user's eye. As the optically-powered reflector, the PVG may affect image light but may leave ambient light unaffected or substantially unaffected. The polarization volume grating is a structured optical component designed to manipulate the polarization of light through the creation of periodic variations in refractive index within a volume of a transparent material. These periodic variations can be in one, two, or three dimensions, and are responsible for the diffraction properties of the grating. In an example, the PVG is formed by patterned (cholesteric) liquid crystals with differing orientations. For example, a helical twist is induced along the transversal direction while the liquid crystals are homogeneous in lateral directions. Further, the PVG may be formed by holographic recording. The PVG may be highly polarization-dependent, meaning its diffraction efficiency and / or transmission / ref lection behavior can vary significantly with the polarization state of incident light. Like other diffraction gratings, polarization volume gratings operate based on the Bragg condition. This condition requires that the incident angle of light has to match the grating period to achieve constructive interference, resulting in efficient diffraction. The grating period in polarization volume gratings can be precisely engineered, allowing for tunable wavelength selectivity. This means that they can be designed to diffract specific wavelengths of light while transmitting others. Further, PVGs can have high angular sensitivity, meaning that small changes in the incident angle can result in significant changes in the diffracted angle. In other words, the PVG is also optically-powered.

[0017] In at least one embodiment, the optical element comprises an optical cavity formed by the optically-powered reflector and the polarization volume grating. This can mean that the optical cavity is arranged between the optically-powered reflector and the polari zation volume grating . The optically-powered reflector and the polari zation volume grating form boundaries of the optical cavity . The optical cavity defines a folded optical path along which the image light propagates . The optical cavity comprises air or is air- filled . However, it is also possible that the optical cavity comprises a solid transparent material or that a solid transparent material is located in the optical cavity . For example , the solid transparent material comprises a polymer or glass .

[0018] In at least one embodiment , the optical element is configured to reflect , and control the divergence of , image light that is incident on the second side of the optical element , such that the image light traverses the optical cavity four times and exits the optical element as proj ected image light from the second side towards the user' s eye .

[0019] As mentioned above , the optical element is reflective for image light . The image light enters the optical element at the second side . Then, the image light is transmitted by the PVG, traverses the optical cavity and is reflected by the optically-powered reflector . The image light traverses the optical cavity a second time and is reflected by the PVG . The image light traverses the optical cavity a third time and is again reflected by the optically-powered reflector . The image light traverses the optical cavity a fourth time , is transmitted by the PVG and exits the optical element at the second side . Then, the image light propagates towards the user' s eye . The image light is deflected by both the PVG and the optically-powered reflector depending on its wavelength and its incident angle onto both the PVG and the optically- powered reflector . Thus , the divergence of the image light can be controlled . It is possible to focus and / or collimate the image light at least partially, such that the image light exits the optical element as proj ected image light . Thus , the proj ected image can be focused by the user' s eye onto a retina of the user' s eye .

[0020] In at least one embodiment , an optical element for an augmented reality display comprises a first side facing a scene and a second side being opposite to the first side and facing a user' s eye . The optical element further comprises an optically-powered reflector arranged at the first side of the optical element , and a polari zation volume grating arranged at the second side of the optical element , and an optical cavity formed by the optically-powered reflector and the polari zation volume grating . The optical element is configured to reflect , and control the divergence of , image light that is incident on the second side of the optical element , such that the image light traverses the optical cavity four times and exits the optical element as proj ected image light from the second side towards the user' s eye .

[0021] The optical element described here is based on the following considerations , among others .

[0022] The proposed optical element forms an improved design for a reflective pancake lens usable for augmented reality applications . Image light that enters the optical element at the second side leaves the optical element at the second side as proj ected image light . Further, ambient light from the scene can pass the optical element from the first side to the second side substantially unhindered . Thus , image light and ambient light are combined to form an augmented reality effect .

[0023] While conventional optical element may have poor optical performance, the optical performance of the proposed design is improved by an improved resolution over field without the need for accommodation. The design makes significant improvement to the optical quality by adding a polarization volume grating, PVG, with diffractive optical power. Thus, there is no need for simple polarization reflectors or chiral mirrors without diffractive optical power. Degrees of freedom are added to the optical design that allow to improve the optical performance, in particular the sharpness, resolution, modulation transfer function (MTF) , distortion, field curvature .

[0024] Polarization volume gratings are diffractive elements that exhibit both Bragg selectivity and polarization selectivity. The polarization volume grating diffracts light of one circular polarization and transmits light of the other circular polarization undiffracted. By using the polarization volume grating, diffractive power is added to the optical element and aberration can be corrected due to the reflection from the PVG.

[0025] According to at least one embodiment, the optical element is configured to transmit ambient light that is incident on the first side of the optical element, such that the ambient light traverses the optical cavity once and exits the optical element substantially without refraction.

[0026] In other words, the refractive power of the optical element for ambient light is substantially zero. Further, the optical element provides low aberrations (blurring, distortion) for ambient light . Thus , the optical element can be designed to be optically transparent for ambient light . Ambient light that enters the optical element at the first side of the optical element passes the optically-powered reflector, the optical cavity and the PVG substantially unhindered . Thus , ambient light traverses the optical cavity only once . Advantageously, ambient light is not reflected and / or refracted by the optical element or parts of the optical element , so that a complete and undistorted image of the scene reaches the user ' s eye . In particular, no ghost images are generated by reflections inside the optical element . The optical element may comprise further components to suppress at least partially unwanted internal reflections of ambient light . For example , a polari zer is arranged on the optically- powered reflector outside the optical cavity . In this way it is possible to manipulate the polari zation of ambient light before it enters the optical element . The polari zed ambient light may be unaf fected by the PVG, such that it passes the PVG without reflection .

[0027] According to at least one embodiment , the optical element is configured to act as optical combiner for combining the ambient light and the proj ected image light that exits the optical element towards the user' s eye .

[0028] Thus , in use , the optical element ef fectively combines the ambient light which is incident on the first side of the optical element with the proj ected image light which exits the second side of the optical element . Thus , an augmented reality ef fect can be generated . According to at least one embodiment , the optical element further comprises an optical spreader configured to spread the image light so as to form spread image light , wherein the optical spreader is arranged at the second side of the optical element outside the optical cavity .

[0029] In the following, the terms image light , spread image light and proj ected image light may be used interchangeably depending on the context . In particular, image light entering the optical element upon incidence on the second side may be trans formed / modi f led by the optical element to proj ected image light or spread proj ected image light , respectively . Before image light enters the optical cavity, it might be spread by the optical spreader . This can mean that the optical spreader is configured to spread or fan-out or separate the incident image light . The optical spreader may be further configured to selectively spread or fan-out light incident on the optical spreader according to an angle of incidence of the light incident on the optical spreader . Speci fically, the optical spreader is configured to spread or fan-out the image light incident on the optical spreader at higher angles of incidence but to transmit the ambient light from the scene without spreading or fanning-out the ambient light . Spreading the image light relates to pupil replication and is used to expand the "eyebox" of an augmented reality device , e . g . an ORSD . The term "eyebox" refers to a volume of space relative to the ORSD in which the user' s eye has to be positioned to be able to correctly see the full , proj ected image . I f an ORSD has a small eyebox, the range of eye positions at which the user can correctly see the full image is small . I f an ORSD has a large eyebox, the range of eye positions at which the user can correctly see the full image is greater which thus provides a better user experience . I f the eye position is moved outside of the eyebox, the user will see only part of the projected image or not see it at all. This is because it is only in the eyebox that the user's pupil and thus retina is correctly aligned with the optical path of the light projected by the ORSD. This means that the focus of the user needs to be accommodated to bring off-axis field points into focus. While accommodation is often allowable in visual instruments, e.g. telescopes, it may present problems with vergence accommodation conflict in a binocular near eye display. An approach for overcome this issue is to use pupil replication. The term pupil replication refers to copying the exit pupil of an image generator / projector that generates the image light, that is, a copy of the full image being projected. Pupil replication works by replicating the full image a number of times and projecting each copy to a different position in front of the user's eye. As long as one of the replicated pupils overlaps with the pupil of the user's eye, at least one copy of the image will be projected onto the user's retina and the user will be able to see the projected image correctly. This has the effect of increasing the size of the eyebox as the user's eye can be in any position where there is overlap with at least one replicated pupil. Therefore, the optical spreader spreads, for example fans-out or separates, rays of image light into multiple directions to form different rays of spread image light which are incident on the second side of the optical element defined by the PVG.

[0030] According to at least one embodiment, the optical spreader is or comprises at least one of an optical beam-expander, an optical diffuser, a hologram and a volume phase hologram,

[0031] VPH. In particular, the optical spreader is or comprise a volume phase hologram, VPH. For example, the VPH comprises one or more VPH gratings. These gratings are free of, or have no, surface grooves. For VPH gratings diffraction may occur as incoming light crosses through a film with periodic index modulations sealed between consecutive substrates. For example, the volume phase hologram comprises an active optical structure. Particularly, the impact of the active optical structure of the volume phase hologram on impinging / passing light is only Bragg diffraction. Thus, impinging image light can be diffracted in multiple directions based on its wavelength and on its incident angle. Ambient light can pass the VPH effectively unaffected.

[0032] According to at least one embodiment, the polarization volume grating is configured to selectively reflect or transmit light based on the polarization of the light. In particular, the polarization volume grating is configured to transmit image light of a first circular polarization and to reflect image light of an opposite second circular polarization.

[0033] For example an image generator projects image light of a first circular polarization onto the optical element. Since the first circular polarization is aligned with the transmission requirement of the PVG, the PVG transmits each of the circular polarized rays of the (spread) image light towards the optically-powered reflector. The transmitted image light of the first circular polarization is reflected by the optically-powered reflector, whereby the handedness of the polarization is reversed or flipped. Thus, corresponding rays of first reflected image light having a second circular polarization which is opposite the first circular polarization is formed. Each ray of first reflected image light propagates back through the optical cavity and hits the PVG . Since the second circular polari zation is not aligned with the transmission requirement of the PVG, said rays are reflected by the PVG to form corresponding rays of second reflected image light having the second circular polari zation . Said rays propagate through the optical cavity to hit the optically-powered reflector, where upon reflection the handedness is reversed again, such that corresponding rays of third reflected image light having the first circular polari zation are formed . After passing the optical cavity a fourth time said rays hit the PVG . Since the polari zation state is aligned with transmission requirement of the PVG, the PVG transmits said rays . Therefore , by means of the polari zation dependent PVG image light is forced to pass the optical cavity four times before exiting the optical element at the second side .

[0034] According to at least one embodiment , the polari zation volume grating is configured to di f fract image light based on a wavelength and / or an incident angle of the image light , such that the divergence of the image light is controlled .

[0035] In other words , the PVG is optically-powered . The PVG exhibits Bragg selectivity . The PVG is configured to deflect the image light in order to control the divergence of it . The optically-powered reflector and the PVG act together to control the divergence of the image light . In particular, the divergence of the image light is controlled upon interaction of the image light with the PVG and the optically-powered reflector . Thus , image light can leave the optical element as proj ected image light , e . g . as at least partially collimated and / or at least partially focused image light . Since the PVG is optically-powered, a total optical power of the optical element is enhanced, which improves the optical performance ( sharpness , resolution, modulation trans fer function MTF, distortion, field curvature ) of the optical element and adds degrees of freedom to the design of the optical element .

[0036] According to at least one embodiment , the polari zation volume grating is flat or planar .

[0037] This can mean that the PVG is parallel to the main plane of the extension of the optical element . A main direction of the user' s field of view can be perpendicular to the PVG .

[0038] Further, the second side of the optical element can form a flat / planar surface . Having a flat or planar surface can have the benefit that the optical element is more compact and easy to manufacture . Further, aberration can be reduced . However, it is also possible to implement a curved or non- planar PVG . This can further improve the optical power of the PVG and enhance the optical performance .

[0039] According to at least one embodiment , the optically-powered reflector is configured to reflect light in one or more narrow spectral bands , in particular to reflect image light and reverse its handedness of polari zation .

[0040] In particular, the optically-powered reflector is configured to reflect light in one or more narrow spectral bands , wherein each spectral band being arranged around a wavelength of the image light . Further, the optically-powered reflector may be configured to have a reflectance in each spectral band of 90% or greater, 95% or greater, or 99% or greater . In other words , the optically-powered reflector may be configured to transmit light at other wavelengths , i . e . outside the narrow spectral bands . This can mean that the optically-powered reflector is configured to reflect ambient light at wavelengths inside the one or more narrow spectral bands but to transmit ambient light at wavelengths outside the one or more narrow spectral bands . Thus , a main portion of ambient light can pass the optically-powered reflector unhindered, while image light having predetermined wavelengths is essentially completely reflected by the optically-powered reflector . This prevents the generation of ghost images as well as reduces power consumption of an image generator due to minimal light leakage .

[0041] According to at least one embodiment , the optically-powered reflector is configured to reflect image light based on a wavelength and / or an incident angle of the image light , such that the divergence of the image light is controlled .

[0042] The optically-powered reflector is configured to deflect the image light in order to control the divergence of it . As mentioned above , the optically-powered reflector and the PVG act together to control the divergence of the image light . Thus , image light can leave the optical element proj ected image light , e . g . collimated and / or focused image light . Since the reflector is optically-powered, a total optical power of the optical element is enhanced, which improves the optical performance ( sharpness , resolution, modulation trans fer function MTF, distortion, field curvature ) of the optical element and adds degrees of freedom to the design of the optical element .

[0043] According to at least one embodiment , the optically-powered reflector is or comprises at least one of a curved mirror, a Fresnel reflector, a di f fractive mirror, a volume phase hologram, a polari zation volume grating, and a transparent substrate with a reflective coating .

[0044] For example , the optically-powered reflector is an optically- powered dichroic reflector or comprises a transparent substrate with a dichroic reflective coating . The dichroic reflective coating may be disposed on a surface of the transparent substrate facing the scene . It should be noted the optically-powered reflector may also be implemented as a di f fracting element which is operated in a reflection mode , such that light rays are scattered backwards .

[0045] According to at least one embodiment , the optical element is free of a further polari zer and / or a retarder, in particular quarter-wave plate .

[0046] By means of the PVG a further polari zer and / or retarder is not required . In particular, the optical cavity can be free of a polari zer or retarder . Thus , manufacturing of the optical element is facilitated and degrees of freedom to the design of the optical element are added . Further, circular polari zed light can be used as image light incident on the second side of the optical element .

[0047] Further, an optical system for an augmented reality display is provided . The optical system comprises the optical element according to one of the above-mentioned embodiments . This means that all features disclosed for the optical element are also disclosed for the optical system and vice versa .

[0048] According to at least one embodiment , the optical system comprises the optical element as defined above and an image generator configured to generate the image light that is incident on the second side of the optical element .

[0049] By means of the generated image light content can be proj ected onto the user' s retina . The image generator may be arranged at the second side of the optical element , such that image light from the image generator is incident on the second side . For example , the image generator is configured to focus the image light to a focal plane and the second side is located at or near the focal plane . In particular, the optical spreader can be located at the focal plane . For example , the image generator is or comprises an image proj ector, a scanning laser proj ector such as an of f-axis scanning laser proj ector, an LED-based proj ector, a digital micro-mirror array proj ector or a digital light processing, DLP, proj ector . For example , the image generator is mounted at a support structure that is connected to the optical element , such that the image generator is permanently aligned with the optical element . It is also possible that the image generator is implemented as OLED-based proj ector comprising a transparent display that is arranged on the second side of the optical element with or without optical spreader in between . The image generator may emit image light of one or more wavelengths such as one or more of red light , green light or blue light . The image generator may emit light of speci fic wavelengths matching the Bragg requirement of the PVG and the reflection requirement of the optically-powered reflector . The image generator may emit circular polari zed light , for example , right-handed circular polari zed light , RHCP, or left-handed circular polari zed light , LHCP .

[0050] Further, a wearable augmented reality display and a vehicle are provided . The wearable augmented reality display and the vehicle comprise the optical system as defined above . This means that all features disclosed for the optical element and the optical system are also disclosed for the wearable augmented reality display and the vehicle , and vice versa .

[0051] According to at least one embodiment , the wearable augmented reality display comprises the optical system as defined above , and a support frame for mounting the optical system on a user so that the optical element is positioned in a field of view of the user .

[0052] For example , the wearable augmented reality display is implemented in AR / VR smart glasses . For example , the wearable augmented reality display is a near-eye display .

[0053] According to at least one embodiment , the vehicle comprises the optical system as defined above , and further comprises a windscreen, wherein the optical element is incorporated into the windscreen .

[0054] Further embodiments of the wearable augmented reality display or the vehicle become apparent to the skilled reader from the aforementioned embodiments of the optical element and the optical system for an augmented reality display and vice- versa .

[0055] The following description of Figures may further illustrate and explain aspects of the optical element , the optical system and device comprising the optical system . Components and parts of the optical element that are functionally identical or have an identical ef fect are denoted by identical reference symbols . Identical or ef fectively identical components and parts might be described only with respect to the Figures where they occur first . Their description is not necessarily repeated in successive Figures .

[0056] Figure la shows an optical element according to an embodiment .

[0057] Figure lb shows an exemplary phase profile for a polari zation volume grating .

[0058] Figure 2 shows an optical element according to a further embodiment .

[0059] Figures 3a and 3b show exemplary optical simulations of optical elements .

[0060] Figure 4 shows an optical system according to an embodiment .

[0061] Figure 5 shows an optical element according to an embodiment .

[0062] Figure 6 shows a wearable augmented reality display according to an embodiment .

[0063] Figure 7 shows a vehicle according to an embodiment .

[0064] An optical element is speci fied . In particular, a reflective pancake lens with polari zation volume grating for improved image quality is speci fied .

[0065] The optical element can be used, e . g . , in AR / VR applications , smart glasses , near eye displays . The optical element allows for an improved image quality of the reflective di f fractive pancake lens inter alia by using a polari zation volume grating .

[0066] The technical problem solved is the improvement of the optical performance (e.g. sharpness / resolution / MTF, distortion, field curvature) over field for the reflective pancake lens. A reflective pancake lens can be composed of a single spherical reflector plus a planar hologram / polarization reflector. The configuration delivers sharp on axis performance but large field curvature. This means that the user needs to accommodate their focus to bring off-axis field points into focus.

[0067] While accommodation is often allowable in visual instruments (e.g. telescopes) , it may present problems with vergence accommodation conflict in a binocular near eye display. For this reason it is desirable to find optical designs for the optical element that have improved resolution over field without the need for accommodation .

[0068] For example, documents US11448803B1, US20180120579A1 and DE 102022126674.5 describe optical elements and are herewith enclosed by reference.

[0069] Many optical elements are "transmissive" pancake lenses, where the image generator and eye are on opposite sides of the lens and light traverses the thickness three times. The optical element described herein e.g. relates to a pancake optic where the "reflector"- image generator and eye are on the same side of the lens and light traverses the element four times . By the optical element described herein significant improvement to the optical quality is reached by adding a diffractive optical element to the pancake lens. Specifically the new designs use a polarization volume grating, PVG, with optical power to replace the simple polarization reflector or chiral mirror. Note that a chiral mirror has similar polarization behavior to the PVG, but has no diffractive optical power. It is simple a reflector / mirror.

[0070] The technical feature used to solve the problem is inter alia to replace the polarizer and quarter wave plate with a polarization volume grating, PVG. In doing so, degrees of freedom are added to the optical design that allow us to improve the optical performance of the display path (e.g. MTF over field) . Since the reflective pancake lens must be see-through, there are strong constraints on the optical design - namely, the refractive power should be zero on see-through, with low aberrations (blurring, distortion) .

[0071] To add degrees of freedom, the polarizer element, which is usually just a reflector, can include diffractive power. Polarization volume gratings are diffractive elements that exhibit both Bragg selectivity and polarization selectivity. This element will diffract light of one circular polarization and simply transmit light undiffracted of the other circular polarization. By using this element, we add diffractive power and aberration correction to the reflection from the PVG.

[0072] An example design is shown in the figures. It shows that an improved performance is achieved by using a PVG element in the reflective pancake lens. It is just one embodiment of how the optical functions of the curved reflector and PVG can be combined for improved optical performance .

[0073] The addition of a powered diffractive element (the polarization volume grating) allows for additional degrees of freedom in the pancake lens design, reduced aberrations and improved overall image quality. The figures show examples .

[0074] Figure 1 shows an example design of a reflective pancake lens using a PVG element. Figure lb shows the phase function of the PVG element.

[0075] Figure 2 shows the polarization behavior of a single ray. The two circular polarizations are arbitrary (could also be reversed) . Note that the handedness of the polarization flips upon reflection from the outer reflector (normal reflection) but is maintained when reflecting from the PVG. This behavior means that no quarter wave plate is required to achieve the four passes through the pancake lens .

[0076] Figure 3A and 3B show the improvement achieved by adding a PVG element to the optical simulation and optimizing the phase function and outer reflector curvature. Significant reduction in aberrations is achieved.

[0077] The present application discloses an optical element comprising a diffractive optical element, e.g. a polarization volume grating, and an outer reflector downstream of the diffractive optical element. Referring back to Figure 1 , Figure la shows an exemplary embodiment of the optical element 1 . The optical element 1 can be provided for an augmented reality display . The optical element 1 comprises a first side 2 facing a scene and a second side 3 being opposite to the first side 2 and facing a user' s eye 4 . An optically-powered reflector 5 is arranged at the first side 2 of the optical element 1 . A polari zation volume grating 6 is arranged at the second side of the optical element . An optical cavity 7 is formed by the optically-powered reflector 5 and the polari zation volume grating, PVG 6 . The optical element 1 is configured to reflect , and control the divergence of , image light 8 that is incident on the second side 3 of the optical element 1 , such that the image light 8 traverses the optical cavity 7 four times and exits the optical element 1 as proj ected image light 8 ' from the second side 3 towards the user' s eye 4 .

[0078] The optical element 1 has a main plane of extension which runs in lateral direction x, y . A transversal direction z , that is perpendicular to the lateral directions x, y, is in the following defined as viewing axis of the user . The optical element 1 has a thickness along the transversal direction z . The thickness can be small , so that a reflective pancake lens is formed .

[0079] How the image light 8 is incident on the second side 3 of the optical element 1 is not shown in Figure la . For that it is referred to Figure 2 . Figure la shows multiple light rays 8a- c of image light 8 . Subsets 8a, 8b, 8c of light rays reach the user' s eye 4 as at least partially collimated light rays 8a' , 8b' , 8c' that are parallel to each other . Each subset 8a, 8b, 8c is generated by a principal image light beam that is incident on the second side 3 of the optical element 1 . An optical spreader 9 that is arranged on the second side 3 spreads or fans out said image light beam to produce the respective subsets 8a, 8b, 8c .

[0080] Further subsets of light rays 8a-c reach the user' s eye 4 as focused light , as illustrated in Figure 1 . The pupil of the user' s eye 4 is placed at or near the focal plane of the optical element 1 . Since the optical element 1 may have multiple focal points within the focal plane , an eyebox for the optical element 1 is expanded . In other words , the optical element 1 may enable pupil replication by replicating the full image a number of times and proj ecting each copy to a di f ferent position in front of the user' s eye 4 . As long as one of the replicated pupils overlaps with the pupil of the user' s eye 4 , at least one copy of the image will be proj ected onto the user' s retina and the user will be able to see the proj ected image correctly .

[0081] As mentioned above , the optical element 1 according to Figure la comprises an optical spreader 9 that is configured to spread the image light 8 so as to form spread image light 8 , wherein the optical spreader 9 is arranged at the second side 3 of the optical element 1 outside the optical cavity 7 . This can mean that the optical spreader 9 is closer to the user' s eye 4 than the PVG 6 . The PVG 6 and / or the optical spreader 9 can be flat or planar, as shown in Figure la . For example , the optical spreader 9 is or comprises at least one of an optical beam-expander, an optical di f fuser, a hologram and a volume phase hologram, VPH . The optically-powered reflector 5 is or comprises at least one of a curved mirror, a Fresnel reflector, a di f fractive mirror, a volume phase hologram, a polari zation volume grating, and a transparent substrate with a reflective coating, in particular dichroic reflective coating . As shown in Figure la, the optically-powered reflector 5 can be curved .

[0082] In the following, the PVG 6 and the optically-powered reflector 5 are described in more detail .

[0083] In Figure lb an exemplary phase profile for the PVG 6 is shown . On the x-axis the radial position r in millimeters is depicted, wherein the radial position r refers to the distance of a peripheral point in the plane of the PVG 6 to the center of the PVG 6 . On the y-axis , the phase cp on periods of 2n radians is depicted .

[0084] In Figure 2 a folded optical path along which the image light 8 propagates is shown . Image light 8 is incident on the second side 3 of the optical element 1 . Spreading of the image light 8 by means of the optical spreader 9 is not shown in Figure 2 for sake of clarity . The PVG 6 is configured to selectively reflect or transmit light based on the polari zation of the light , in particular to transmit image light 8 of a first circular polari zation and to reflect image light 8 of an opposite second circular polari zation .

[0085] The image light 8 incident on the second side 3 of the optical element 1 may have a circular polari zation which aligns with the transmission requirement of the PVG 6 . Thus , the image light 8 is transmitted by the PVG 6 . Further, the PVG 6 is configured to di f fract image light 8 based on a wavelength and / or an incident angle of the image light 8 , such that the divergence of the image light 8 is controlled . In particular, the Bragg selectivity of the PVG 6 is designed to match with the wavelength and the incidence angle of the image light 8 . Thus , the image light 8 undergoes di f fraction at the PVG 6 , such that the image light 8 is deflected, but maintains its polari zation state , as shown in Figure 2 .

[0086] The image light 8 traverses the optical cavity 7 and is first reflected by the optically-powered reflector 5 . The optically-powered reflector 5 is configured to reflect light in one or more narrow spectral bands , in particular to reflect image light 8 and reverse its handedness of polari zation . For example , i f the image light 8 had a right- handed circular polari zation, RHCP, the first reflected image light has a left-handed circular polari zation, LHCP, or vice versa . The wavelengths of the image light 8 are matched to the reflection requirement of the optically-powered reflector 5 . Further, the optically-powered reflector 5 is configured to reflect image light 8 based on a wavelength and / or an incident angle of the image light , such that the divergence of the image light 8 is controlled . Thus , the first reflected image light 8 is deflected again .

[0087] The first reflected image light 8 traverses the optical cavity 7 for the second time and is second reflected by the PVG 6 , since its polari zation state is not aligned with the transmission requirement of the PVG 6 . It is deflected by di f fraction and traverses the optical cavity 7 a third time as second reflected image light 8 . Its polari zation state does not change upon interaction with the PVG 6 .

[0088] After traversing the optical cavity 7 , the second reflected image light 8 hits the optically-powered reflector 5 , where it is third reflected and where the handedness of its polari zation is reversed again . As the optically-powered reflector 5 is optically-powered, the third reflected image light 8 is deflected . The third reflected image light 8 traverses the optical cavity 7 a fourth time and is transmitted by the PVG 6 , since it has a polari zation state that matches the transmission requirement of the PVG 6 . Thus , the third reflected image light 8 exits the optical element 1 towards the user' s eye as proj ected image light 8 ' .

[0089] Figures 3A and 3B show optical simulation results of exemplary optical elements 1 . Figure 3A shows the simulation results of an optimi zed optical element 1 without PVG 6 , as shown on the top right in Figure 3A. On the top left of Figure 3A various spot diagrams are shown . The surface of the optical spreader 9 , that is implemented as volume phase hologram (VPH) in this example , is chosen as imaging surface . In the spot diagrams , IMA describes the location of the centroid on the imaging surface . OBJ describes the location of the field points in terms of field angles . On the bottom of Figure 3A various transverse ray fan plots are shown . Corresponding simulation results and plots for an optimi zed optical element 1 with PVG 6 ( as depicted on the top right in Figure 3B ) are shown in Figure 3B . It can be seen that the aberration can be signi ficantly reduced by using the PVG 6 . The PVG 6 adds optical power to the optical element 1 which leads to improved image quality .

[0090] In Figure 4 an optical system 10 is shown . The optical system 10 comprises the optical element 1 as shown in Figure 1 , for example . It further comprises an image generator 12 configured to generate the image light 8 that is incident on the second side 3 of the optical element 1 . The image generator 12 is configured to focus the image light 8 to a focal plane and the optical spreader 9 may be located at this focal plane. Different principal beams of the image light 8a to 8c are projected onto the optical spreader 9. As explained in context of Figure 1, the image spreader 9 spreads, for example fans-out or separates, the respective beams 8a to 8c to form subsets of rays of image light 8. For example, the optical spreader 9 spreads, for example fans-out or separates, the principal circular polarized (e.g. RHCP) beam 8a of image light 8 into three different directions to form three different circular polarized (e.g. RHCP) rays of spread image light 8 which are incident on the second side 3 of the optical element 1 defined by the PVG 6. These rays 8a leave the optical element 1 at the second side 3 towards the user' s eye as projected image light 8' . The projected image light 8' includes at least partially focused light. The projected image light 8' is focused onto a focal plane 22 at the eye 4 of the user including the eyebox 24. The projected image light 8' also includes at least partially collimated light that is formed by the fanned-out rays generated by the optical spreader 9 from each of the principal beams 8a to 8c.

[0091] In Figure 5 a further function of the optical element 1 is shown. The optical element 1 is configured to transmit ambient light 11 that is incident on the first side 2 of the optical element 1, such that the ambient light 11 traverses the optical cavity 7 once and exits the optical element 1 substantially without refraction. This can mean that ambient light 11 reaches the eyebox 24 substantially unaffected by the optical element 1. For example, ambient light 11 has other wavelengths outside the spectral band of the optically- powered reflector 5 and not matching the Bragg condition of the PVG and / or the VPH. The ambient light 11 is also not affected by the optical spreader 9. It is therefore possible for the optical element 1 to act as optical combiner for combining the ambient light 11 and the proj ected image light 8 ' that exits the optical element 1 towards the user' s eye 4 .

[0092] In Figure 6 a wearable augmented reality display 13 , comprising the optical system 10 of Figure 4 and a support frame 14 for mounting the optical system 10 on a user so that the optical element 1 is positioned in a field of view of the user .

[0093] The wearable augmented reality, AR, display 13 has a central axis 15 , wherein the optical system 10 is implemented as an of f-axis retinal scanning display mounted on the support frame 14 . The optical system 10 comprises the image generator 12 in the form of a scanning laser proj ector 12 and an eyepiece 16 . The optical element 1 is integrated in the eyepiece 16 . The proj ector 12 is of fset from the central axis 15 . In use , when the support frame 14 is mounted on the head of a user with the eyepiece 16 positioned in a field of view of the user, the eyepiece 16 transmits ambient light 11 from a scene located in front of the eyepiece 16 through the eyepiece 16 to the eye 4 of the user located behind the eyepiece 16 and the proj ector 12 proj ects circular-polari zed image light 8 defining an image towards the eye 4 of the user by way of the eyepiece 16 . The circular-polari zed image light 8 may include one or more wavelengths such as one or more of red light , green light or blue light . As explained in more detail above , the eyepiece 16 including the optical element 1 replicates the image defined by the image light 8 , proj ected by the image generator 12 onto the second side 3 of the optical element 1 , a number of times at a plurality of positions in a plane 22 at the eye 4 of the user to expand an eyebox 24 of the wearable AR display 13 . In Figure 7 a vehicle 17 comprising the optical system 10 according to Figure 4 is shown . The vehicle 17 comprises a windscreen 18 , wherein the optical element 1 is incorporated into the windscreen 18 . The image generator 12 can be arranged in the vicinity of the windscreen 18 and can proj ect image light 8 onto the windscreen 18 . The principle of operation corresponds to the principle of operation of the wearable AR display 13 according to Figure 6 , and is not reproduced here .

[0094] The embodiments of the optical element and the optical system disclosed herein have been discussed for the purpose of familiari zing the reader with novel aspects of the idea . Although preferred embodiments have been shown and described, many changes , modi fications , equivalents and substitutions of the disclosed concepts may be made by one having skill in the art without unnecessarily departing from the scope of the claims .

[0095] It will be appreciated that the disclosure is not limited to the disclosed embodiments and to what has been particularly shown and described hereinabove . Rather, features recited in separate dependent claims or in the description may advantageously be combined . Furthermore , the scope of the disclosure includes those variations and modi fications , which will be apparent to those skilled in the art and fall within the scope of the appended claims .

[0096] The term " comprising" , insofar it was used in the claims or in the description, does not exclude other elements or steps of a corresponding feature or procedure . In case that the terms " a" or " an" were used in conj unction with features , they do not exclude a plurality of such features . Moreover, any reference signs in the claims should not be construed as limiting the scope.

[0097] This patent application claims the priority of German patent application 102023119320.1 and the priority of German patent application 102023128569.6, the disclosure content of which are hereby incorporated by reference.

[0098] References

[0099] 1 optical element

[0100] 2 first side

[0101] 3 second side

[0102] 4 eye

[0103] 5 optically-powered reflector

[0104] 6 polari zation volume grating

[0105] 7 optical cavity

[0106] 8 image light

[0107] 9 optical spreader

[0108] 10 optical system

[0109] 11 ambient light

[0110] 12 image generator

[0111] 13 wearable augmented reality display

[0112] 14 support frame

[0113] 15 central axis

[0114] 16 eyepiece

[0115] 17 vehicle

[0116] 18 windscreen

[0117] 22 focal plane

[0118] 24 eyebox x, y lateral directions z transversal direction

Claims

Claims1. Optical element (1) for an augmented reality display, the optical element (1) comprising:- a first side (2) facing a scene,- a second side (3) being opposite to the first side (2) and facing a user's eye (4) ,- an optically-powered reflector (5) arranged at the first side (2) of the optical element (1) , and- a polarization volume grating (6) arranged at the second side (3) of the optical element (1) ,- an optical cavity (7) formed by the optically-powered reflector (5) and the polarization volume grating (6) , wherein- the optical element (1) is configured to reflect, and control the divergence of, image light (8) that is incident on the second side (3) of the optical element(I) , such that the image light (8) traverses the optical cavity (7) four times and exits the optical element (1) as projected image light (8' ) from the second side (3) towards the user's eye (4) .

2. Optical element (1) according to claim 1, wherein the optical element (1) is further configured to transmit ambient light (11) that is incident on the first side (2) of the optical element (1) , such that the ambient light(II) traverses the optical cavity (7) once and exits the optical element (1) substantially without refraction.

3. Optical element (1) according to claim 2, wherein the optical element (1) is configured to act as optical combiner for combining the ambient light (11) and theprojected image light (8' ) that exits the optical element(1) towards the user's eye (4) .

4. Optical element (1) according to one of claims 1 to 3, further comprising an optical spreader (9) configured to spread the image light (8) so as to form spread image light (8) , wherein the optical spreader (9) is arranged at the second side (3) of the optical element (1) outside the optical cavity (7) .

5. Optical element (1) according to claim 4, wherein the optical spreader (9) is or comprises at least one of an optical beam-expander, an optical diffuser, a hologram and a volume phase hologram, VPH.

6. Optical element (1) according to one of claims 1 to 5, wherein the polarization volume grating (6) is configured to selectively reflect or transmit light based on the polarization of the light, in particular to transmit image light (8) of a first circular polarization and to reflect image light (8) of an opposite second circular polarization .

7. Optical element (1) according to one of claims 1 to 6, wherein the polarization volume grating (6) is configured to diffract image light (8) based on a wavelength and / or an incident angle of the image light (8) , such that the divergence of the image light (8) is controlled.

8. Optical element (1) according to one of claims 1 to 7, wherein the polarization volume grating (6) is flat or planar .

9. Optical element (1) according to one of claims 1 to 8, wherein the optically-powered reflector (5) is configured to reflect light in one or more narrow spectral bands, in particular to reflect image light (8) and reverse its handedness of polarization.

10. Optical element (1) according to one of claims 1 to 9, wherein the optically-powered reflector (5) is configured to reflect image light (8) based on a wavelength and / or an incident angle of the image light (8) , such that the divergence of the image light (8) is controlled.

11. Optical element (1) according to one of claims 1 to 10, wherein the optically-powered reflector (5) is or comprises at least one of a curved mirror, a Fresnel reflector, a diffractive mirror, a volume phase hologram, a polarization volume grating, and a transparent substrate with a reflective coating.

12. Optical element (1) according to one of claims 1 to 11, wherein the optical element (1) is free of a further polarizer and / or a retarder, in particular a quarter-wave plate .

13. Optical system (10) for an augmented reality display, the optical system (10) comprising:- the optical element (1) according to one of claims 1 to 12,- an image generator (12) configured to generate the image light (8) that is incident on the second side14. A wearable augmented reality display (13) , comprising the optical system (10) of the preceding claim and a support frame (14) for mounting the optical system (10) on a user so that the optical element (1) is positioned in a field of view of the user.

15. A vehicle (17) , comprising the optical system (10) of claim 13, wherein the vehicle (17) comprises a windscreen (18) , and wherein the optical element (1) is incorporated into the windscreen (18) .

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