Optical system having a cylindrical waveguide

Abstract

The optical system includes a cylindrical waveguide and an input optical element. The cylindrical waveguide has concentric inner and outer surfaces defining a common cylindrical axis. The input optical element is arranged to receive light from an image source and to direct the received light into the cylindrical waveguide such that all light rays from the same pixel of the image source are incident on one face of the cylindrical waveguide at each point of incidence at the same angle relative to the surface normal and to a plane perpendicular to the cylindrical axis. This causes the captured light to maintain its direction angle as it propagates along the cylindrical waveguide. An optical display device, which may form part of a head-mounted display, may be provided using the optical system.

Description

[Technical field]

[0001] The present disclosure relates to optical systems that may form part of optical display devices such as head mounted displays. [Background technology]

[0002] In the field of augmented reality, a virtual image is displayed to the user superimposed on the real world using a transparent combiner, which redirects the image from the projector towards the user's eye. Current solutions use a flat or planar transparent waveguide, typically made from a glass or plastic substrate. In this case, light is coupled into the waveguide via a diffraction grating (or similar), passes inside the waveguide (by total internal reflection) and reaches a similar out-coupler, where it is extracted and reaches the user's eye. In the field, the waveguide is typically only a few millimeters thick and is also called a "light guide".

[0003] Referring to FIG. 1, a schematic diagram of such an optical system is shown, which includes a planar waveguide. The optical system includes an object display (image source) 10, a collimator lens 20, a planar waveguide 30, an incoupler 40 (such as a diffraction grating), and an outcoupler 50. A number of light rays 60 from a point on the object display 10 are shown. These light rays are incident perpendicularly on the surface of the waveguide. The path of a single light ray 70 is also shown, which passes through the waveguide 30 and through the outcoupler 50 to the user's eye 80. The thickness of the waveguide 30 is on the order of a few millimeters, the distance between the incoupler 40 and the outcoupler 50 is on the order of a few centimeters, and the lateral period of the diffraction grating of the incoupler 40 is on the order of a fraction of a micron.

[0004] In such a typical planar waveguide design, the object display 10 is imaged by a collimator lens 20 at the focal length of the collimator lens 20. The collimator lens 20 collimates the image-bearing light in both the horizontal and vertical (tangential and sagittal) planes.

[0005] The collimated light is coupled into the waveguide by incoupler 40, which may be a linear or holographic grating (other options include refractive optical elements such as prisms), that diffracts the light at angles greater than the critical angle so that it enters the waveguide or substrate 30, which may be made of glass or plastic.

[0006] Designs contemplated herein may be used interchangeably and interoperably, including diffraction gratings, holographic gratings, holograms, surface holograms, embossed holograms, lithographically fabricated gratings, volume gratings, or slant gratings. For ray tracing, the transverse component of the grating period of the k-vector or momentum vector of the grating is important. The choice of specific material, fabrication means, and cross-section may affect the diffraction efficiency, diffraction order, mechanical flexibility, and wavelength bandwidth, but do not affect aberrations and resolution. When a linear grating receives collimated light, it diffracts the collimated light but maintains collimation; that is, all the rays from a given pixel change direction but remain parallel to each other.

[0007] When light hits the outer surface of the waveguide, it bounces back inside due to total internal reflection (TIR). This angle is determined by the refractive index n of the waveguide. A typical value is n=1.5, which gives a critical angle of 42 degrees and a guide angle of approximately 60-70 degrees to the normal. Glasses with a higher refractive index have a lower critical angle, allowing for a larger field of view (FOV).

[0008] One advantage of this approach is that all rays from a given pixel travel at the same angle inside the flat waveguide, so all rays can be superimposed or swapped inside the waveguide without causing double or ghost image problems.

[0009] This may be the key to realizing the main function of the waveguide, i.e. pupil duplication. In the field of Augmented Reality (AR), relatively small optical elements are highly desirable. This means using miniature projectors, which produce small pupils (essentially bundles of light rays originating from the same pixel), which can be seen, for example, by looking at the small pupil of a typical pair of binoculars (a small bright spot floating above the eyepiece).

[0010] The outcoupler 50 (waveguide output grating) can replicate the pupil by partially extracting the light with less than 100% diffraction efficiency (also called partial diffraction efficiency). For example, a diffraction efficiency of 10% will extract 10% of the light and propagate the remaining 90% further. This process is repeated the next time the propagated light encounters the output grating 50. More advanced gratings with variable diffraction efficiency help balance the light intensity between the different pupil replicas.

[0011] This ensures that the output grating fulfills the light extraction task in that it does not need to be located at a specific location: the exit pupil of the waveguide can be treated as one continuous pupil extending over the entirety of the output grating 50. Similarly, the expanding pupil does not move along the output grating depending on the field of view (location of the generating pixel on the display).

[0012] Two-dimensional pupil expansion can be achieved by first replicating the pupil in one direction by partial diffraction with a linear intermediate grating, and then replicating it in another direction by partial diffraction with a linear output grating. A total of three or more gratings may be used. This is achieved in the HoloLens® waveguide sold by Microsoft Corporation and in waveguides sold by Waveoptics, Ltd.

[0013] A limitation of this approach is that the glass of the waveguide 30 must be flat, while consumers are accustomed to curved glasses. Combining a flat waveguide with curved prescription glasses is difficult without making the unit thicker. There are also applications where high curved glass is used, such as motorcycle helmet visors, military visors, fighter pilot helmets, diving goggles, firefighting thermo visors, medical safety glasses, welding masks, airplane windshields, and car or scooter windshields. Trying to fit a flat waveguide into the limited space available between the user and the visor is challenging.

[0014] There are also combiner solutions based on free-space reflective optics, but these solutions generally have a small eyebox (area where the image is visible). These are unsuitable for some applications. In contrast, waveguide solutions have a large eyebox, which makes the waveguide solution desirable. A large eyebox means that a single variation in the design of the AR glasses can fit most people, allowing the user to easily see the virtual image. A small eyebox means that the AR glasses need to be mechanically adjusted or fitted to a specific user due to different inter-pupillary distances (IPDs) for different users, increasing cost and complexity.

[0015] The use of curved waveguides has been suggested in various documents such as Patent Documents 1 to 15. Similar suggestions are also discussed in Non-Patent Documents 1 and 2. [Prior art documents] [Patent documents]

[0016] [Patent Document 1] International Publication No. 2006 / 064301 [Patent Document 2] US Patent Application Publication No. 2010 / 031571 [Patent Document 3] German Patent Application Publication No. 102017119440 [Patent Document 4] U.S. Patent No. 8,842,368 [Patent Document 5] U.S. Patent No. 8,810,913 [Patent Document 6] U.S. Patent No. 9,733,475 [Patent Document 7] US Patent Application Publication No. 2018 / 0292593 [Patent Document 8] US Patent Application Publication No. 2019 / 0317261 [Patent Document 9] British Patent No. 2553382 [Patent Document 10] U.S. Patent No. 10,048,647 [Patent Document 11] US Patent Application Publication No. 2016 / 0195720 [Patent Document 12] US Patent Application Publication No. 2019 / 0072767 [Patent Document 13] US Patent Application Publication No. 2018 / 0373115 [Patent Document 14] US Patent Application Publication No. 2019 / 0369403 [Patent Document 15] US Patent Application Publication No. 2018 / 0348527 [Non-patent literature]

[0017] [Non-Patent Document 1] Kalinina, Anastasiia and Andrey Putilin, "Wide-field-of-view augmented reality eyeglasses using curved wedge waveguide (Conference presentation)," Digital Optical Elements for Immersive Displays II. Vol. 11350, International Society of Optical Engineering, 2020 [Non-Patent Document 2] DeHoog, Edward, Jason Holmstedt, and Tin Aye, "Field of view of limitations in see-through HMD using geometric waveguides," Applied Optics 55.22 (2016): 5924-5930.

[0018] Some of these documents only briefly mention curved waveguides. Others discuss curved waveguides in more detail, but none explain how to continuously replicate the pupil without diverging rays from the same pixel when projected to infinity. In these approaches, there is usually only one pupil propagating through the waveguide, so pupil expansion is not possible. Other designs rely on eye tracking. These designs generally produce an output with aberrations and require complex software to pre-distort the input image. Thus, such solutions would be expensive and complex to manufacture.

[0019] There are many existing planar waveguide solutions on the market, but despite many suggested examples including the one mentioned above, no curved or cylindrical waveguide products are commercially available. This is likely due to the difficulty of propagating light in a waveguide without introducing significant aberrations (degrading the quality) into the image when output to the user.

[0020] Mitigating this problem while still achieving a product that can be manufactured for mass market remains a challenge. Summary of the Invention [Means for solving the problem]

[0021] Against this background there is provided an optical system as claimed in claim 1, an optical display device as claimed in claim 24 and a head mounted display as claimed in claim 25. Further preferred and optional features are defined in the other claims and in the rest of the disclosure.

[0022] An optical system using a cylindrical waveguide with concentric (inner and outer) surfaces is conceivable. Such a cylindrical waveguide structure allows light to be transferred between curved surfaces without aberrations. This is advantageously realized with a number of input optical elements arranged to receive light from an image source (particularly a pixelated image source or an image source with an optical output that can be divided into a number of pixels, at least in theory) and to feed this light into the cylindrical waveguide. All light rays from the same pixel of the image source enter the cylindrical waveguide at the same angle relative to the normal to the surface of incidence at each point of incidence. Thus, all light rays from the central pixel of the image source enter perpendicular to the cylindrical surface at any point on the cylindrical waveguide. Light rays from non-central pixels enter at the same angle relative to the surface normal at each point of incidence. Furthermore, all light rays from the same pixel of the image source enter the cylindrical waveguide at the same angle relative to the plane perpendicular to the cylinder axis at each point of incidence. That is, the propagation direction of all light rays remains the same.

[0023] Given the input optics and waveguide structure, light received at the cylindrical waveguide (generally coupled into the cylindrical waveguide) maintains its orientation angle as it propagates along the cylindrical waveguide. That is, all light rays from a given pixel approach the output grating at the same angle measured between the light rays and the surface at the point of incidence of each light ray, no matter how far they propagate. Generally, the input optics comprises an incoupling (or exit) optic.

[0024] The outcoupling optics may be positioned to receive light propagated along the cylindrical waveguide and cause this light to appear as an image on an object (particularly, to a user's eye, i.e., viewer). In some embodiments, an image source (such as an electronic display) or an image source mount (on which the image source can be positioned), or both, may form part of the optical system.

[0025] The optical system may be used to implement an optical display device (e.g., a head mounted display). Advantageously, the image source is configured to direct light to the in-coupling optics. A mounting device configured to be worn by a user (e.g., part of a headset, visor, optical glasses) may be used to position the optical system such that the out-coupling optics causes the light to appear as an image to the user's eye. In a preferred embodiment, the cylindrical waveguide is integrated into or embedded in or attached to the glasses or visor component of the mounting device.

[0026] A particular surface of the incoupling optical element for coupling the received light into the cylindrical waveguide is a diffraction grating. Advantageously, an incoupling linear grating with a certain period is used. This period is measured laterally along the surface of the incoupling grating. Advantageously, the incoupling linear grating is attached to a curved surface. A preferred embodiment of this incoupling linear grating uses a flexible holographic material. The incoupling linear grating may be attached to the surface of the cylindrical waveguide (and preferably conforms to said surface without gaps). There are several alternative configurations that do not use an incoupling grating (or at least one of its forms). For example, the entire waveguide does not have to be cylindrical, only a portion of the waveguide is cylindrical with the above mentioned properties. The light may enter the waveguide through another portion, which may thus form part of the input optical element.

[0027] Unlike traditional collimating optics used for flat waveguides, the input optics proposed in this disclosure would not be called a collimator exactly in classical optical design terminology, since not all the rays from one pixel are parallel. Rather, a suitable incoupling (projector) optic comprises an optical device arranged to collimate or adapt light (only) in a plane (i.e., a plane that is neither parallel nor entirely) passing through the cylinder axis, more preferably in a plane perpendicular to the cylinder axis. For example, in the case of glasses and helmets, the cylinder axis is vertical and the plane is preferably horizontal. In this way, all the rays are incident on the input grating at the same angle in this plane. For this purpose, wavefront shaping devices such as cylindrical lenses or mirrors can be used. As is customary in best practice optical design, all the rays of the central pixel are advantageously incident at an angle perpendicular to the plane of the waveguide, which will make aberration management easier due to symmetry. Other pixels in the plane will produce rays that are incident on the input grating at other angles, but will be parallel to other rays from the same pixel.

[0028] However, in one plane perpendicular to the cylinder axis (e.g. the horizontal plane), the light does not have to be collimated, but should be shaped so that all light rays from the same pixel have the same angle of incidence with respect to the surface normal, which is considered separately for the different points of incidence. All surface normals point towards the cylinder axis.

[0029] The simplest wavefront shape that satisfies this condition is a cylindrical wavefront concentric with the cylindrical shape of the waveguide. In this case, each ray from the central pixel propagates radially from the cylindrical axis of the waveguide and enters the face at normal incidence. This wavefront is advantageously created by placing the image source (display) with its center aligned with the waveguide axis. In this case, a cylindrical lens or mirror is placed so that it has focusing power only in the vertical plane.

[0030] In some cases, additional or different input optics may be provided, for example to optimize performance for more or all pixels, or to minimize the volume of the projector, or both. This may include utilizing optics that focus in the horizontal plane for closer displays. If a concave mirror is used, the image source and / or incoupling optics may be positioned such that light enters the waveguide from the side of the waveguide opposite the mirror and is reflected from it and subsequently diffracted.

[0031] Light may propagate in a cylindrical waveguide (e.g., between an incoupling optical element and an outcoupling optical element) in the following directions: parallel to the cylinder axis (defined by a vector) (e.g., vertical), perpendicular to the cylinder axis (defined by a vector) (which may be horizontal, in particular horizontal around the circumference of the waveguide), or in a direction defined by a vector between parallel and perpendicular to the cylinder axis (typically, oblique).

[0032] The outcoupling optics usually comprises an outcoupling grating. In principle, a linear grating could also be used. However, in practical use of the waveguide, the viewer is inside the cylinder. A simple linear grating is therefore not suitable for extracting light in this situation, since it would focus the light on the cylinder axis in a horizontal plane (vertical light) and not far in front of the viewer. In contrast, a diverging lens property can collimate the light horizontally (it is already collimated vertically). This can be achieved by adding a negative refractive power to the outcoupling grating in the horizontal direction. By analogy, the grating is the sum of a prism function and a cylindrical negative lens function. There are many known examples of how such gratings can be recorded. In contrast to the input grating, such a grating would not be called "linear". It is possible to choose an output grating in order to place the digital image at any distance from the viewer by further focusing in both planes.

[0033] If the cylindrical waveguide is embedded inside a head mounted structure that already corrects the user's vision (e.g. prescription glasses), the input and output gratings may also be corrected to take this into account. It is essential to maintain the above-mentioned state of the light as it propagates inside the waveguide, regardless of how it enters and exits the "sandwich" of all the overlapping layers.

[0034] In a preferred embodiment, the image source or the image source mount or both may be positioned on the side of the inner surface of the cylindrical waveguide (closer to the cylindrical axis). In this case, the incoupling optical element is disposed between the image source and the inner surface of the cylindrical waveguide. Alternatively, the image source or the image source mount or both may be positioned on the side of the outer surface of the cylindrical waveguide (away from the cylindrical axis). In this case, a mirror (which may also function as an input wavefront shaping device, as described above) may be positioned to receive light from the image source and reflect the received light toward the cylindrical waveguide. In such an embodiment, the light from the image source may pass through the cylindrical waveguide before reaching the mirror.

[0035] One or more intermediate optical gratings in the cylindrical waveguide may redirect, diffract and / or split the light before the outcoupling optics. Advantageously, the one or more intermediate optical gratings may each be a linear grating, and the angular properties of the propagating light (TIR condition and all light rays from the same pixel are incident on the cylindrical waveguide surface at the same angle with respect to the surface normal and with respect to the plane perpendicular to the cylinder axis) are maintained, allowing two-dimensional pupil expansion without aberrations.

[0036] Multiple cylindrical waveguides may be used, in which case the outcoupling optics may produce different focal points for the light from each waveguide.

[0037] Various combinations of the above features and features described elsewhere herein are also contemplated. Methods of manufacturing and / or operating optical systems or optical display devices are also contemplated. These may have structural features corresponding to those described herein.

[0038] The present disclosure can be put into practice in a number of ways and some preferred embodiments will now be described, by way of example only, with reference to the accompanying drawings in which: [Brief description of the drawings]

[0039] [Figure 1] 1 shows a schematic diagram of an existing optical system with a planar waveguide. [Diagram 2] 1 illustrates a schematic of a cylindrical waveguide as used in the present disclosure. [Diagram 3] 3 shows a schematic diagram of the cylindrical waveguide of FIG. 2 used in the present disclosure; [Figure 4] 1 shows a schematic top view of an optical system according to a first embodiment of the present disclosure; [Figure 5a] 5 shows a perspective view of the embodiment in FIG. 4. [Figure 5b] FIG. 5b shows a schematic top view of the embodiment of FIG. 5a. [Figure 5c] FIG. 5b shows a side view of the embodiment of FIG. 5a. [Figure 6] FIG. 1 shows a schematic top view of a cylindrical waveguide and simplified outcoupling optics used in certain embodiments of the present disclosure. [Figure 7] 1 illustrates an example flowchart of an operational process according to the present disclosure. [Figure 8] 2 shows a schematic top view of an optical system according to a second embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] Throughout this disclosure, the same reference numbers are used when the same features are shown in different drawings.

[0041] The new approach proposed in this disclosure is advantageously targeted to augmented reality head-mounted devices. The approach is based on cylindrical waveguides and considers the whole system. In particular, this may include matching the curvature of the waveguide to the wavefront of the input light. The system is also simple and cheap to manufacture and align. The cylindrical shape of the waveguide also fits better inside the shape of prescription glasses, since it is easier to fit a cylinder between spheres than a flat waveguide plate. This allows the total thickness to be reduced from about 15 mm to about 5 mm. It is also possible to embed the waveguide in prescription glasses by using air gaps or refractive index changes (see, for example, US Pat. No. 5,333,933, the details of which are incorporated herein by reference).

[0042] Referring first to FIG. 2, a cylindrical waveguide for use in the present disclosure is shown in schematic form, with an image source (object display) 110 and a cylindrical waveguide 120. The cylindrical waveguide 120 has two concentric surfaces with a constant thickness (t) between them. The image source 110 is located at the center of curvature 100 of the cylindrical waveguide 120. The distance (R1) between the center of curvature 100 and the inner surface of the cylindrical waveguide 120 and the distance (R2) between the center of curvature 100 and the outer surface of the cylindrical waveguide 120 are shown. It is clear that R2=R1+t, which is true regardless of where these two distances are measured on the cylindrical waveguide 120. In other words, such a cylindrical waveguide 120 has a common center of curvature, and the radius of the inner surface and the radius of the outer surface of the cylindrical waveguide 120 are separated by the thickness (t) of the waveguide. The simple concentric shape of such cylindrical waveguide 120 makes it cheaper to manufacture and the tolerances easier for mass production.

[0043] Referring now to FIG. 3, the cylindrical waveguide of FIG. 2 is shown in schematic form for use in the present disclosure. Also shown is a cylindrical lens 135 and a diffraction grating 140. The light rays from the central pixel of the image source 110 (located at the center of the radius of curvature of each concentric surface of the cylindrical waveguide 120) are collimated into one plane by focusing with the cylindrical lens 135. This may also be called wavefront shaping. Such wavefront shaped light rays are then coupled into the cylindrical waveguide 120 by the incoupling diffraction grating 140 at an angle β. These incident light rays are normal to the face of the input diffraction grating at every point along the face of the diffraction grating. The angle β is such that the angle between the normal and the internal ray is less than the critical angle of the waveguide, and β is typically less than 48 degrees, more preferably at least 30 degrees and at most 40 degrees. Pixels other than the central pixel are coupled into the waveguide at a slightly different angle, say β+1, but this angle is constant at every point along the face of the grating.

[0044] The above-mentioned non-patent document 2 proves that if (and only if) the two faces of a cylindrical waveguide are concentric, as shown by the cylindrical waveguide 120 in Figs. 2 and 3, the angle at which a ray of light is incident on the outer face is the same after one bounce on the inner face. Thus, the ray of light maintains its direction angle as it propagates along the waveguide. This would not be the case if the radii of the faces were chosen differently. As shown in Fig. 3, if the ray of light bounces on the outer face at an angle α, it will make an angle β with the inner face, and then on the next bounce on the outer face it will make an angle α. In other words, the angle of incidence on the inside of the waveguide will be the same for every other bounce on the faces of the waveguide. The ray of light incident on the left, center, and right sides of the grating from the pixels, respectively, will approach the inner face at the same angle throughout the entire waveguide after bouncing on the outer face. The pixels other than the center pixel will have different α and β values, but the above relationship still holds. This property makes pupil duplication possible.

[0045] It has been recognized that a high performance and compact optical system can be achieved by having all light rays from the same point on the image source enter such a cylindrical waveguide 120 at the same angle. For example, in the incoupling optics, the cylindrical lens 135 and the diffraction grating 140 can be configured to meet this requirement.

[0046] Although generally discussed in terms of the central ray angle, other angles occur at the image source object 110 (e.g., a microdisplay), which results in the field of view of the image to the user. These other ray angles propagate in the same way through the cylindrical waveguide 120. There will be some small aberrations (image quality defects) in the rays from the edge of the field of view (edge ​​of the object), but these are hardly noticeable to the user (as is well known in the art) and can be corrected by optical means. Ideally, the aberrations should be kept below 1 minute of visual angle, which is the human visual acuity.

[0047] It should be understood that these aberrations do not accumulate with the number of bounces. For example, if a ray from a given pixel diffracts into the waveguide over an angular range of 60.0±0.1 degrees, then after any number of bounces and pupil duplications, the angular resolution of the entire system will still maintain a resolution of ±0.1 degrees, regardless of how the rays are permuted or shuffled. This is in contrast to a waveguide, where aberrations accumulate with transmission.

[0048] Thus, in a general sense and according to one aspect, an optical system may be considered that includes a cylindrical waveguide with an input optical element. The cylindrical waveguide has concentric (concave) inner and outer surfaces, which maintain the angle of light propagating within the cylindrical waveguide. The concentric surfaces define a common cylindrical axis (common centerline). The input optical element is positioned to receive light from an image source and to direct this light into the cylindrical waveguide. The input optical element is then positioned such that all light rays from the same point on the image source enter a surface of the cylindrical waveguide at the same angle relative to the surface normal at the point of incidence. The input optical element is also positioned such that all light rays from the same point on the image source enter the cylindrical waveguide at the same angle relative to a plane perpendicular to the cylindrical axis at the point of incidence. This combination of features means that light entering the cylindrical waveguide maintains its directional angle as it propagates along the cylindrical waveguide.

[0049] The input optical element preferably comprises an incoupling optical element arranged to couple light to one side of the cylindrical waveguide (in preferred embodiments, the inner side). Advantageously, an outcoupling optical element is arranged to receive the light propagated along the cylindrical waveguide and make it appear as an image on the object. Optionally, the optical system may comprise an image source (a display such as a microdisplay) or a mount for the image source (image source mount) or both.

[0050] Further aspects according to the present disclosure can be envisaged. For example, an optical display device may comprise an optical system as described herein, with an image source arranged to direct light to an incoupling optic, and a mounting device, in particular arranged to be worn by a user. The mounting device may position the optical system so that the outcoupling optic causes the light to appear as an image to the user's eye. For example, this may comprise a spectacle frame, a visor structure or a helmet structure, or another wearable mount. If necessary, a head-mounted display with such an optical display device may be considered. In this case, the mounting device may comprise a spectacle or visor component. Advantageously, the cylindrical waveguide is integrated or embedded or attached to the spectacle or visor component.

[0051] Other aspects of the method of manufacturing and / or operating such an optical system may be considered. For example, the method of manufacturing the optical system may include providing a cylindrical waveguide having concentric inner and outer surfaces, and arranging an input optical element (e.g., with an incoupling optical element) to receive light from an image source and to direct the light into the cylindrical waveguide such that all light rays from the same point of the image source are incident on one side of the cylindrical waveguide at the same angle with respect to the surface normal and with respect to a plane perpendicular to the cylinder axis at each point of incidence. The method may further include configuring an outcoupling optical element to receive the light propagated along the cylindrical waveguide and direct the light to appear as an image on the object. Similarly, a method of operating an optical system may include directing light from an image source to an input optic of a cylindrical waveguide such that all light rays from the same point of the image source enter the cylindrical waveguide at the same angle relative to the surface normal and to a plane perpendicular to the cylinder axis at each point of incidence. The light propagated along the cylindrical waveguide is preferably made to appear as an image on the object (by an outcoupling optic).

[0052] In the following, further preferred, optional and advantageous features, which are particularly applicable to all aspects disclosed herein, are described. First, one particular embodiment is described.

[0053] Now referring to FIG. 4, a schematic diagram of an optical system according to a first embodiment of the present disclosure is shown from above. In this drawing, the paper plane (and any parallel planes) are considered to be horizontal, and the vertical plane (and any parallel planes) coming out of the paper plane are considered to be vertical. Thus, the cylindrical waveguide 120 can be visualized in one plane, for example, as a cylindrical visor. Also shown in this drawing is an outcoupling grating 150. The image source 110 is placed at a distance from the cylindrical waveguide 120 equal to the radius of curvature of the cylindrical waveguide 120. As explained above, the cylindrical lens 135 collimates (or matches or magnifies) the input light in only one plane, in particular the magnification in the vertical plane. This is then matched as input to the linear grating 140. When applied to the curved surface of the cylindrical waveguide 120, this causes the image-bearing light to propagate along the waveguide with rotational symmetry (i.e., the image of the waveguide can be rotated around the cylindrical axis so that the light rays from a given pixel maintain the same angle with respect to the waveguide surface). This makes the system invariant to the placement of the output grating 150, eliminating the need for precise alignment. The output grating 150 compensates by applying a negative focusing power to the light (i.e., dispersing the light) in the opposite plane (the horizontal plane), providing the user (not shown) with an expanded eyebox with the output set to infinity. This allows the output eyebox to be expanded by pupil duplication.

[0054] This design therefore produces a wavefront in the cylindrical waveguide 120 that can propagate rotationally symmetrically along the entire waveguide. The spherical wavefront emitted from the point image source 110 naturally conforms to the curvature of the waveguide 120 in the horizontal plane (in the plane of the paper in FIG. 4, as discussed above) and is magnified in power by the cylindrical lens 135 in the vertical plane (out of the plane of the paper in FIG. 4, as discussed above). This means that the angle of the ray is mapped to the same angle after two bounces, and this is repeated infinitely. Thus, the eyebox can be expanded in one dimension (horizontal) since pupil duplication (multiple extractions) is possible. The size of the vertical eyebox is set by the vertical size of the cylindrical lens 135. This relaxes the alignment tolerances (since the location of the outcoupler relative to the entrance pupil is not critical), and also the selection of the waveguide thickness and the guiding angle of the central ray.

[0055] Referring now to Figure 5a, a perspective view of the embodiment shown in Figure 4 is shown. Figure 5b shows a top view of the embodiment of Figure 5a. Figure 5c shows a side view of the embodiment of Figure 5a. Light 101 collimated in one plane is coupled into a cylindrical waveguide 120, where multiple light rays 102 propagate through the waveguide 120 and are extracted by a diffraction grating 150 to project light 103 to the user's eye 160.

[0056] In this particular embodiment, the image source or object 110 is a microdisplay and is disposed at a distance from the cylindrical waveguide 120 equal to the radius of curvature of the waveguide. This is achieved by physically placing the image source 110 at this distance or by virtually placing the image source 110 at this distance by optical means (such as a lens). The image is collimated in one plane by a cylindrical plano-convex lens 135 and captured by an incoupling grating 140 located on the inner concave surface of the cylindrical waveguide 120. The cylindrical lens 135 is oriented such that its focusing power is on the opposite surface of the cylindrical waveguide 120. For example, if a curved waveguide is oriented horizontally (as when oriented like a helmet visor or resting on a table), it will have focusing power in the horizontal plane and will reflect light from a point source into a vertical line with the radius of curvature. In this case, cylindrical lenses 135 are oriented in opposite vertical planes and have focusing power in the vertical plane, causing light from a point source to be focused into a horizontal line with a radius of curvature.

[0057] Therefore, the image-bearing light is collimated in only a single plane (the horizontal plane) before it enters the waveguide. The curvature of the cylindrical waveguide 120 focuses this light in the vertical plane. This allows the entire pupil of light entering the waveguide to propagate along the waveguide, thus enabling pupil expansion at the output side.

[0058] The focal length of the cylindrical lens 135 determines the magnification of the object. The lens 135 is set at a position one focal length away from the object 110. If the cylindrical lens 135 is placed next to the cylindrical waveguide 120, the focal length of the cylindrical lens 135 is approximately equal to the radius of curvature of the waveguide. For example, a typical radius of curvature of 200 mm for a visor-type waveguide means that the object is 200 mm away and the focal length of the cylindrical lens was 200 mm. To achieve compactness, the distance of 200 mm from the waveguide to the object can be reduced by folding the optical path with multiple mirrors or by virtually setting the distance of the object optically using multiple lenses.

[0059] The choice of cylindrical lens 135 (diameter and / or focal length) determines the size of the vertical eyebox (defined by the lens diameter). The focal length also determines the magnification of the display 110 and therefore the image field of view (FOV) (the size of the display). Typically, the cylindrical lens 135 is a multi-element lens (as used in cameras). This provides good image quality (small spot size RMS across the field of view) over the entire field of view. This is especially desirable for pupil replication systems to accurately overlay multiple pupils to provide high resolution images. Ideally, the lens system is achromatic for full-color microdisplays, but it can also be monochromatic. The field of view of the curved waveguide 120 may be determined primarily by similar factors, but its curved nature around the user results in an expanded field of view compared to a planar waveguide. This is shown in Non-Patent Document 2.

[0060] The incoupling grating 140 is a linear grating with equal spacing (pitch) between the grating lines (or the equivalent of equal fringe spacing in a volume holographic grating). This grating can be fabricated by lithography or interferometry. All the rays collimated in one plane are incident normal (90 degrees to the surface) over the entire width of the grating surface and then diffracted at the same angle inside the waveguide. This allows for pupil duplication.

[0061] Having an incoupling grating on a curved waveguide generally means that the collimated light is not perpendicularly incident over the entire width of the incoupling grating due to the curvature of the waveguide. Common solutions to this include varying the pitch of the incoupling grating to correct this, or recording a hologram directly on the curved surface or lithographically etching it onto the curved surface (complex and expensive). In a preferred embodiment according to the present disclosure, the incoupling grating 140 is fabricated on a flat substrate as a planar linear grating (which is well known in the art and relatively cheap and simple compared to tunable gratings). The grating 140 can be fabricated on any flexible holographic material such as photopolymer (e.g., Bayfol® from Covestro, or silver halide film) and can be fitted to and attached (laminated) to the cylindrical surface of the waveguide. The incoupling grating 140 is index matched, preferably by lamination (or another index matching adhesive or liquid) to match the shape of the cylindrical surface and desirably without voids. It is easy and cheap to record a hologram on a flat substrate, remove it, and laminate flexible holographic material to a planar or cylindrical substrate (curved in only one dimension), but it is more difficult to record on a curved surface or laminate to a spherical surface (curved in two dimensions).

[0062] It may also be possible to etch a slant grating and use embossing or UV curable resin techniques. This grating is transferred onto a cylindrical waveguide.

[0063] The grating pitch is designed to diffract the central wavelength of the microdisplay 110 at the desired internal angle. Because the grating is nominally designed to diffract normally incident light at an angle, it has a tilt angle, and the pitch is usually defined as the intergrating spacing measured along the plane of the grating, which is kept constant for the input coupler, i.e., a linear grating.

[0064] In the case of planar waveguides, both couplers are typically linear and identical. In this case, the system acts like a periscope, and a magnified image of the microdisplay appears to the viewer superimposed on the real world. The overall design of the system means that pixel position information of the display is converted by collimation into angular information and then back to position information at the human retina.

[0065] Traditionally, in a planar waveguide, the input and output gratings are linear, parallel, and have the same period to cancel chromatic aberration, which is important for use with broadband light sources such as light emitting diodes (LEDs). The output grating according to the present disclosure has a variable period along the waveguide so that chromatic aberration does not cancel everywhere. However, to minimize this aberration, the period of the outcoupling grating at the center can be chosen to be the same as the period of the incoupling grating.

[0066] Alternatively, narrowband sources such as laser sources, super-bright light-emitting diodes (SLEDs), or notch-filtered narrowband LEDs can be used. Narrowband sources can help minimize chromatic aberrations. They can also limit the field of view of the output image, but to mitigate this, thin volume holographic gratings can be used. For example, a typical thickness for a volume holographic grating can be as small as 3 microns and as large as 6 microns. This can result in a typical spectral bandwidth of approximately 20 nm full width half maximum (FWHM) and an angular bandwidth in air of approximately 6 degrees full width half maximum.

[0067] The cylindrical waveguide may form part of a head-mounted structure that already corrects the user's vision (e.g. prescription glasses), in which case the incoupling grating 140 and outcoupling grating 150 may take this into account, but without compromising the propagation of light inside the waveguide and its efficient extraction.

[0068] As an example, if a waveguide is embedded in an otherwise normal prescription positive meniscus lens, the outer cover lens may have a convex spherical surface and a concave cylindrical surface. The inner liner lens may have a convex cylindrical surface and a concave spherical surface. The optical transparency will then be determined by the outermost convex surface and the innermost concave surface selected to match the wearer's prescription. The focus of the digital image provided through the cylindrical waveguide will be determined independently by the innermost surface together with the optical properties (optical power) of the output grating. The skilled person will also accommodate the transparency and the digital image for astigmatism according to the prescription from the optician.

[0069] The outcoupling grating can be selected to place the digital image at any distance from the viewer by adding further focusing in both planes. In the vertical plane, the light coupled into the waveguide is collimated, but the light exiting the outcoupling grating does not need to be collimated. To allow for multiple pupil extraction, only a portion of the light may be extracted at a first portion of the outcoupling grating 150. To improve uniformity of the extracted light across the outcoupling grating 150, it is desirable for the far end of the outcoupling grating 150 (relative to the incoupling grating 140) to have a higher efficiency than the near (receiving) end. The diffraction efficiency of the output grating is advantageously selected to be low enough (e.g., 5-25%) at the receiving end of the output grating 150 to allow sufficient pupil duplication, but high enough (e.g., 20-100%) at the far end to provide sufficient brightness.

[0070] Returning to the general gist of one embodiment above, it can be seen that the incoupling optical element may comprise an incoupling linear grating arranged to couple the received light into a cylindrical waveguide, advantageously having a constant period. The incoupling linear grating is advantageously mounted on a curved surface, in particular with index matching (without an air gap).

[0071] In another general spirit, according to a further aspect, an optical system may be conceived that includes a cylindrical waveguide having concentric inner and outer surfaces, and an incoupling linear grating attached to the curved surface and arranged to couple received light into the cylindrical waveguide. Optionally, the optical system may further include a linear outcoupling grating arranged to receive light propagated along the cylindrical waveguide and to make the light appear as an image on an object. In one preferred embodiment, the wavefront shaping device may be configured to collimate or conform the received light to a single plane and direct the collimated light to the incoupling linear grating. Methods of manufacturing and / or operating such an optical system may further be conceived, as exemplified below. The following further features are applicable to any of the aspects disclosed herein.

[0072] In yet another aspect, a method for manufacturing a curved linear grating can be envisaged. The method includes forming a linear grating on a plane and attaching the linear grating to a curved surface of a (cylindrical) substrate such that the linear grating conforms to the curved surface. As with other aspects envisaged herein, the further features below (and described elsewhere herein) are also applicable to this aspect in a similar manner.

[0073] Further features applicable to all aspects are described below. For example, the incoupling linear grating may be made of flexible holographic material. Optionally, the incoupling linear grating is attached to (and / or conforms to) the inner surface of the cylindrical waveguide. In some preferred embodiments, the incoupling linear grating comprises a plurality of line gratings. The grating thickness and angular bandwidth of the incoupling linear grating may be configured for (nearly) uniform visible color transmission across the width of the incoupling linear grating.

[0074] The input optical element preferably comprises a wavefront shaping device (which may be in the form of a collimator) configured to collimate or adapt the received light to a single plane. More preferably, the (input) wavefront shaping device is configured such that the single plane passes through the cylindrical axis of the cylindrical waveguide. For example, the wavefront shaping device may be cylindrical. In some preferred embodiments, the wavefront shaping device comprises a plano-convex lens or a multi-element lens or both. Advantageously, the orientation of the wavefront shaping device is orthogonal compared to the orientation of the cylindrical waveguide.

[0075] Further general features are restated below: Next, further specific examples according to the present disclosure are described.

[0076] Referring to FIG. 6, a schematic diagram of a cylindrical waveguide and simplified outcoupling optics is shown from above. As with FIG. 4, the plane of the drawing (and any parallel planes) is considered to be horizontal, and the vertical plane (and any parallel planes) emerging from the plane of the drawing is considered to be vertical. Thus, the cylindrical waveguide 120 can be visualized on a plane, for example, like a cylindrical visor. Also shown in this simplified diagram is a cylindrical negative lens 155, which will be further described below. The multiple light rays 151 emerging from the outcoupling grating 150 are collimated in one vertical plane and focus in a horizontal plane, as shown by lines 152. The multiple light rays 156 emerging from the cylindrical negative lens 155 are collimated in both the horizontal and vertical planes, and have an infinity focus. This is a simplification, in practice a cylindrical negative lens 155 is optically integrated into the outcoupling grating 150 such that the powers for correcting and collimating to both planes at exit are included in the hologram in the outcoupler, so that the outcoupling grating 150 acts as a cylindrical lens to correct the cylindrical curvature introduced by the incoupling optics, thereby collimating the image essentially to infinity, as explained below.

[0077] Due to the asymmetric collimation in the incoupling optics, the outcoupling optics directs the image focused at infinity to both planes by correcting the different focus positions of the horizontal (near) and vertical (far or infinity) output image planes, providing a high quality image to the viewer. This correction is achieved by coding the optical power on the output grating. This technique is known in the art, but not for this purpose. As shown in FIG. 6, this corresponds to placing a diverging cylindrical lens 155 with a negative optical power equal to the radius of curvature of the waveguide between the planar output grating and the user (for a plano-convex cylindrical lens, if the radius of curvature of the waveguide is 200 mm, the focal length of the lens is -200 mm). The orientation of the lens 155 is perpendicular to the input cylindrical lens 135. If the input lens 135 is focused (or has a power) in the vertical plane, the output correction lens or grating is focused (or has a power) in the horizontal plane to produce a spherically collimated output. As mentioned above, the center of the outcoupling grating 150 has the same surface pitch (also known as lateral pitch or in-plane pitch) as the input grating to allow for chromatic dispersion correction.

[0078] To the user 160, the image appears to be at infinity, which is typically the desired use case since it means that a user such as a fighter pilot or motorbike rider sees a focused virtual image while focusing on a real-world distant object, as is typical when using a visor. Consumer devices that use planar waveguides with pupil expansion also have images at infinity.

[0079] The output grating can have a variable grating efficiency or a relatively low output efficiency (e.g., 10%). This is achievable during recording of the holographic outcoupler. While it is desirable for the input grating 140 to have maximum diffraction efficiency (i.e., most of the incident light is coupled into the waveguide), the output grating 150 can have a low or variable efficiency to allow for pupil expansion. A small portion of the light is coupled out on the first interaction with the output grating 150, while most bounces back down the waveguide, and some of the light is output on the second interaction, and so on. This allows for an expanded eyebox in the horizontal plane.

[0080] Holographic waveguide gratings (either linear or prescription outcouplers) can be fabricated by exposing a holographic material to two coherent light beams and coupling the guided beams into the material through a prism, as is known in the art. Three different wavelengths (e.g., red, green, and blue, RGB) lasers can be used to multiplex three gratings into a single holographic layer, allowing the viewer to see a nearly white image from an RGB microdisplay. Alternatively, three separate layers can be stacked for each color.

[0081] The user sees an image with uniform brightness and saturation across the field of view, which can be increased by multiplexing multiple diffraction gratings onto a single holographic layer. This can be achieved by varying the recording angle, or multiple angle multiplexed layers can be stacked.

[0082] The input and output gratings can be reflection holograms, transmission holograms, or a combination of both. This will be understood from the above theory, since the desired effect is based only on the transverse component of the grating pitch. The transverse (cross-sectional) pitch or period can be suitably selected to match the geometry of the reflection or transmission grating. It can be understood that the above linear gratings are meant to be transversely linear, but can also have variable transverse characteristics.

[0083] The collimation of the output light means that a long eye relief (i.e. the distance from the output face to the eye at which the image can be optimally viewed) can be achieved. This is desirable in many cases, especially for applications with helmet visors rather than glasses. Longer eye relief generally results in a smaller field of view.

[0084] The real world field of view is barely altered by the curvature of the waveguide. A radius of curvature of 250mm is typical for glasses, and 150-200mm for visors. Any curvature greater than 100mm (as in this case) is not noticeable to the user as contributing to real world distortion. Only very small astigmatism effects are present, unless corrected by one or more additional superimposed lenses.

[0085] Returning to the general gist of the present disclosure, the outcoupling optical element may be considered to comprise an outcoupling grating. In particular, the outcoupling grating may be configured to function as a cylindrical lens (e.g., focusing in only one dimension). Additionally or alternatively, the linear outcoupling grating may have a plurality of curved gratings. In some preferred embodiments, the linear outcoupling grating may have an internal grating angle, or a power efficiency or diffraction efficiency, or both. The internal grating angle is arranged to collimate the received light in one plane or to focus the received light in tangential and sagittal planes at a predetermined distance. The power efficiency or diffraction efficiency is 25% or less (optionally 20%, 15%, or 10% or less) at one end of the outcoupling grating closest to the light received from the input optical element.

[0086] The outcoupling optics may comprise an output wavefront shaping device configured to collimate the received light into a single plane orthogonal to the single plane of the input wavefront shaping device. Additionally or alternatively, the outcoupling optics may comprise a cylindrical negative lens. Such an aspect is preferably incorporated into the outcoupling grating.

[0087] The outcoupling linear grating may have the same surface pitch as the incoupling linear grating. In some embodiments, the outcoupling linear grating may have an internal grating angle that is oriented in the opposite direction to the internal grating angle of the incoupling linear grating. This is particularly used when the light from the image source and the light to the viewer (or the incoupling and outcoupling optics) are on the same side of each other. This may also be called a "U-shaped" grating. Alternatively, the angles of the incoupling and outcoupling gratings are not oriented in opposite directions, and at least some light is extracted on the opposite side to the captured light (in other words, the viewer is on the opposite side to the captured light). This may also be called a "Z-shaped" grating.

[0088] From another perspective, the approaches of the present disclosure can be described in terms of symmetry: they use cylindrical waveguides and cylindrically symmetric wavefronts, which are rotationally symmetric about their common axis.

[0089] Consider a pixel on a display or other image generating means. The light wavefront from this pixel can be shaped cylindrically. A linear diffraction grating recorded on a flat substrate and laminated to a cylindrical waveguide has a constant period along the surface. In this case, the ray field is symmetric about the axis of rotation, since every ray is deflected by the same angle. As explained above, a ray launched between cylindrical surfaces maintains two angles of incidence on the two surfaces, regardless of the number of bounces. This means that after every two reflections, the wave will match exactly with itself. This does not produce a double image. Such a ray field can propagate any distance, without any ray being different from the others.

[0090] Also, pupil duplication at the output grating is realized. At the initial interaction with the outcoupling optics, part of the light is extracted, and the remaining light continues to propagate and is extracted at the next interaction. In this case, the different interactions are perfectly coincident, so no ghost images are caused. When all of the rays reach the outcoupling grating, the grating can diffract these rays out of the waveguide, regardless of the position of the grating or the position of any of the rays. This is because all of these rays arrive in the same way.

[0091] If a linear outcoupling grating with the same period as the incoupling grating is used, each ray is diffracted again into a new cylindrical wavefront. It is well known that diffractive optical elements can combine several functions additively. The outcoupling grating, like a cylindrical lens, also has one-dimensional light-gathering power. This converts the diffracted light into a collimated beam. An observer receiving such light will experience a star-like point at infinity.

[0092] The above description can be repeated for other pixels. As mentioned above, the wavefronts from these other pixels do not need to be precisely cylindrical. This is because the rays from the non-central pixels enter the incoupling optics at angles slightly different from the "perfectly" normal (perpendicular) angles. However, by having all rays strike the incoupler at approximately the same angle to the surface normal at their respective intersection points, these rays form a ray space with rotational symmetry about the cylindrical axis and propagate indistinguishably.

[0093] Using modern optical design, it is possible to design a projector that creates such a light field with small tolerances, ideally within 1 minute of visual angle (human visual acuity).

[0094] Referring to FIG. 7, an example flow chart of an operational process for several embodiments according to the present disclosure is shown. In a display step 210, a moving image is displayed on an image source 110 (e.g., a microdisplay). In a first collimation step 220, the image-bearing light is collimated in a vertical plane by a cylindrical lens 135 (or equivalently a cylindrical mirror, described below) having a power in the vertical plane. In a first incidence step 230, the image-bearing light (for a single pupil) strikes the cylindrical waveguide 120 in a manner perpendicular to its face (typically the inner face). In a capture step 240, the image-bearing light is captured at a constant angle (for all rays from the same pixel) through an incoupling linear diffraction grating 140. Since the cylindrical waveguide 120 has concentric inner and outer faces, the captured light propagates between these concentric faces with rotational symmetry in a propagation step 250. In an initial extraction step 260, an initial portion of the image-bearing light is extracted upon a first interaction with the outcoupling grating 150. Subsequent interactions (bounces) with the outcoupling grating 150 further extract the propagating light for pupil replication in a further extraction step 270. The optical effect of the outcoupling grating 150 in a second collimation step 280 adds negative cylindrical power in the horizontal plane. Finally, in an output step 290, a spherically collimated image at infinity is superimposed on the real world and viewed by the user's eye 160.

[0095] Having described specific embodiments, it will be appreciated by those skilled in the art that various modifications and variations are possible, in particular that the incoupling and outcoupling optics may vary significantly from the specific designs set forth above.

[0096] As shown above, the light source can be placed closest to the inner surface of the cylindrical waveguide and a cylindrical lens can be used to collimate the light. A cylindrical lens does not have to be used. For example, a cylindrical mirror can be used instead. A cylindrical mirror can have all of the same properties and advantages. The light source can also be placed beyond the outer surface and a cylindrical mirror can be used to collimate the light, for example after it has passed through the waveguide.

[0097] Many designs can be used to optimize performance across all pixels and minimize the projector volume. This includes using optics that focus in a vertical plane to bring the display closer. For example, a concave mirror may be used to make the light enter the waveguide from the opposite side of the mirror before reflecting and diffracting. This is similar to the aberration management of an afocal system in classical optics. However, in this disclosure, the treatment of these aberrations is reformulated in a cylindrical coordinate system.

[0098] Referring to FIG. 8, a schematic top view of an optical system according to a second embodiment is shown. In this embodiment, a cylindrical mirror 136 is used instead of the cylindrical lens 135. The use of such a mirror eliminates the need to position the image source 110 closer to the inner surface of the cylindrical waveguide 120. For simplicity, the incoupling grating 140 is not shown, but is present in the same position as in the other described embodiments. In the illustrated embodiment, the image source 110 is closer to the outer surface of the cylindrical waveguide 120 than to the inner surface of the cylindrical waveguide 120. The optical path length from the image source 110 to the incoupling grating is kept the same as the radius of curvature of the cylindrical waveguide 120. For example, a typical radius of curvature of a visor-shaped waveguide is 200 mm, which means that the object is located 200 mm away and the radius of curvature of the cylindrical mirror is 400 mm. However, the distance between the image source 110 and the input grating is not the same as this radius of curvature. It can be seen that the incident light is again normal to the surface of the input grating. The effect on the wavefront by the cylindrical mirror 136 is essentially the same as that by the cylindrical lens 135, and therefore the cylindrical mirror 136 has similar properties. All other variations and options may be applied to implementations of this embodiment as well as other embodiments disclosed herein.

[0099] The orientation of the cylindrical waveguide can be varied. In the above embodiment, the cylindrical axis of the cylindrical waveguide is oriented vertically (so the cylindrical waveguide extends horizontally). This is the usual way to align the visor. However, this is not required. Other orientations are also possible. Additionally or alternatively, light may enter and exit the cylindrical waveguide through several different faces, for example through several different sides of the waveguide. Incoupling and outcoupling gratings can therefore be positioned to achieve this. In some embodiments, there may be several incoupling gratings or outcoupling gratings or both.

[0100] Each of the incoupling grating and the outcoupling grating can be a reflective grating or a transmissive grating, and can be located on the inner or outer surface of the waveguide (or on another surface of the waveguide). Those skilled in the art will appreciate such variations on the embodiments described herein.

[0101] The linear input grating can have any orientation angle. The light does not need to be redirected around the cylinder (perpendicular to the cylinder axis, which is horizontal in the above embodiment). The light can be directed along the cylinder axis (vertical). Alternatively, the light can be directed at 45 degrees or in any other oblique direction. This allows design freedom, for example to position the projection module nicely in the temple of glasses. This is also important for realizing multiple intermediate gratings for two-dimensional pupil expansion. The linear intermediate grating may redirect and / or split the light while keeping the angle of each redirected light beam the same as the multiple light beams from the same pixel.

[0102] The system is versatile as it can use laser or LED light. Typically LED light is used, such as LCoS (Liquid Crystal on Silicon) plus LED or micro LED micro displays, but laser light can also be used if high efficiency and therefore brightness is desired. Mirror scanning systems for laser beams (MEMS: microoptoelectromechanical system) can also be used. Laser light has some drawbacks in terms of cost, speckle (loss of resolution) and eye safety concerns.

[0103] When refractive elements (e.g., prisms) are used as in- or out-couplers together with diffractive in- or out-couplers, uncorrected chromatic dispersion may allow only narrowband sources (e.g., lasers) to be used. Furthermore, refractive couplers tend to be bulky and expensive.

[0104] By adding a spherical refractive power to the waveguide output, the virtual image can be set to different focal lengths. A further pair of lenses (a second lens to correct the effect of the first lens on the real world) in front of and behind the waveguide can set the focal length closer. Multiple electrically addressable switchable (liquid crystal based) holographic output gratings can be added. These output gratings can be switched on or off to provide different focal planes for the image. Additionally or alternatively, the input grating may be switched as well to provide a larger field of view, which can be achieved by angle multiplexed multiple gratings. This switching can be synchronized with a time multiplexed microdisplay.

[0105] In some cases, multiple focal planes may be achieved through the use of multiple (stacked) cylindrical waveguides. Light propagates as before, but exits multiple diffraction gratings to produce multiple different focal points. This approach is detailed in "Optical architectures for augmented, virtual and mixed reality headsets" (2020), BCKress, published by the International Society for Optical Engineering (SPIE).

[0106] Furthermore, if we add an axially symmetric power to the output of a cylindrical waveguide, like a typical spherical lens, we will experience a point at a finite distance, say 1m.

[0107] The cylindrical waveguide may be part of a larger (integral) waveguide structure, in which case only a portion of it may be cylindrical. There are some possible embodiments in which no incoupling optics are required. For example, the light may enter the waveguide through a non-cylindrical portion of the waveguide, or may be generated within the waveguide in a non-cylindrical portion of the waveguide (e.g. due to an embedded image source). Wavefront shaping may then take place in this portion. This portion of the waveguide may therefore form part of the input optics.

[0108] The vertical eyebox can also be enlarged by vertically offsetting multiple input projectors. A common technique used to extend the vertical eyebox into a planar waveguide is to use a "rotating" grating that propagates the pupil vertically, resulting in a two-dimensional enlargement of the exit pupil. There are various ways to enlarge the eyebox vertically, including in-coupling gratings, rotating gratings, and output gratings, as implemented in products by Vuzix Corporation and DigiLens Inc. One option is to use a "butterfly" rotating grating, which enlarges the eyebox and further expands the FOV by splitting the FOV in two at the input and recombining it at the output (as used in the HoloLens® sold by Microsoft and described in the book by BCKress mentioned above). A further option is to use a mutually multiplexed grating. This diffraction grating (as used in products by Waveoptics, Ltd.) guides some of the light and extracts some of the light across an expanded eyebox.

[0109] Both of these existing approaches benefit from the use of linear gratings and collimated light routed through planar waveguides. In accordance with the present disclosure using cylindrical waveguides, these approaches of splitting the light and replicating the pupil with linear intermediate gratings may be implemented after coupling the light into a cylindrical waveguide, where the light is finally extracted by a grating with negative cylindrical focusing.

[0110] In the rotationally symmetrical structure of some of the embodiments according to the present disclosure, the input and output gratings can be mounted anywhere on one concentric cylindrical waveguide. For example, they could be oriented vertically or diagonally across the waveguide (e.g. in a visor implementation) similar to the general planar horizontal configuration above. This allows flexibility in projector mounting and eyebox location in the final design. Pupil duplication and eyebox vertical expansion methods described in the paragraphs above are also possible.

[0111] Eye tracking is an existing technique that may add additional functionality to AR devices. This is typically achieved by an infrared (IR) light source that shines light on the eye and a camera to detect the reflected light and determine the eye's gaze direction. In areas where space is limited (such as visors or glasses), this may not be possible. With input and output gratings as described above, but diffracting infrared light (e.g., 850 nm), adding compact eye tracking functionality to the waveguide may be achieved. The gratings may simply direct IR light to the eye, or they may take the IR light reflected from the eye from the grating at the eyebox to the output grating and then to the camera via TIR.

[0112] Further referring to the general gist of the present disclosure above, for example, in some of the preferred embodiments, the radius of curvature of the cylindrical waveguide is at least 100 mm.

[0113] In some embodiments, the wavefront shaping device may comprise a concave mirror (cylindrical mirror). If necessary, the image source and / or the image source mount may be positioned closer to the outer surface of the cylindrical waveguide than the inner surface of the cylindrical waveguide. A mirror (preferably a wavefront shaping device) may then be positioned to receive light from the image source and reflect the received light towards the cylindrical waveguide. In some embodiments, the mirror and the image source and / or the image source mount are configured such that the light from the image source passes through the cylindrical waveguide before reaching the mirror. In some embodiments, portions of the input optical element (e.g., an incoupling grating) closer to the cylindrical waveguide and the outcoupling optical element (e.g., an outcoupling grating) closer to the cylindrical waveguide are on opposite sides of the cylindrical waveguide.

[0114] In certain embodiments, the input optical element further comprises one or more ball lenses. Additionally or alternatively, the outcoupling optical element further comprises one or more ball lenses. The ball lenses may be used to change the optical path length of the light or to change the focusing of the light, or both.

[0115] In some embodiments, the input optical element may further comprise a waveguide portion integrated into the cylindrical waveguide. Advantageously, the waveguide portion forming at least part of the input optical element is non-cylindrical and / or has no concentric faces. In some embodiments, only a part of the waveguide shape may be cylindrical.

[0116] In some embodiments, one or more intermediate optical gratings may be provided in the cylindrical waveguide. One or some or all of the one or more intermediate optical gratings may be linear. The one or more intermediate optical gratings may be arranged to redirect the light, diffract the light, and / or split the light before the outcoupling optics, but advantageously the relative angles of the light rays from the same pixel are kept the same. The linear intermediate optical grating advantageously maintains the angular properties of the propagating light (TIR condition and all light rays from the same pixel are incident on the cylindrical waveguide surface at the same angle with respect to the surface normal and with respect to the plane perpendicular to the cylinder axis). This allows for two-dimensional pupil expansion without aberrations.

[0117] The outcoupling optical element may comprise an outcoupling grating having one or more of an internal grating angle arranged to refract the received light, a diffraction efficiency that is variable according to the length of the outcoupling grating, and a switchable grating configuration (e.g., allowing modulation of the output light). Optionally, the incoupling grating may have a switchable grating configuration.

[0118] A plurality of cylindrical waveguides may be provided. For example, a second cylindrical waveguide having concentric inner and outer surfaces may be provided. The first and second (or plurality) cylindrical waveguides may be stacked. Some or all of the plurality of cylindrical waveguides may have a common cylindrical axis. In all such cases, the input optical element may be arranged to direct a portion of the received light into each of the plurality of cylindrical waveguides, such that in each cylindrical waveguide, all light rays from the same pixel of the image source are incident on one face of the respective cylindrical waveguide at each point of incidence at the same angle relative to the surface normal and to a plane perpendicular to the respective cylindrical axis. This causes the captured light to maintain its directional angle as it propagates along the respective cylindrical waveguide. Advantageously, the outcoupling optical element may be arranged to focus the light propagated along each cylindrical waveguide to a different focal point. For example, the outcoupling optics may be positioned to focus light propagated along a first cylindrical waveguide to a first focal point and to focus light propagated along a second cylindrical waveguide to a second, different focal point.

[0119] One can envisage an embodiment with multiple image sources, which are advantageously vertically offset from one another.

[0120] All of the features disclosed in this specification may be combined in any combination, except combinations in which at least some of such features and / or steps are mutually exclusive. In particular, the preferred features of the invention are applicable to all aspects of the invention and may be used in any combination. Likewise, features described in non-basic combinations may be used separately (not in combination).

Claims

1. 1. An optical system comprising: a cylindrical waveguide having concentric inner and outer surfaces defining a common cylindrical axis; An input side optical element, the input optical element is positioned to receive light from an image source and to couple the light into the cylindrical waveguide; the arrangement being such that all light rays from the same pixel of the image source are incident on one face of the cylindrical waveguide at each point of incidence at the same angle relative to the surface normal and to a plane perpendicular to the cylinder axis; whereby the captured light maintains its directional angle as it propagates along the cylindrical waveguide in a direction about the common cylindrical axis. An input side optical element; An optical system comprising:

2. 2. The optical system of claim 1, The input optical element includes an incoupling optical element configured to couple the light to one side of the cylindrical waveguide. Optical system.

3. 3. The optical system of claim 2, the incoupling optical element comprises an incoupling linear type diffraction grating; The incoupling linear diffraction grating is It has a certain period, is attached to a curved surface, and arranged to couple the received light into the cylindrical waveguide; Optical system.

4. 4. The optical system of claim 3, the incoupling linear grating is made of a flexible holographic material, or The incoupling linear diffraction grating is attached to the inner surface or the outer surface of the cylindrical waveguide, or The incoupling linear grating is switchable, or A combination of these, Optical system.

5. 5. An optical system according to claim 3 or claim 4, the grating thickness and angular bandwidth of the incoupling linear grating are configured for uniform visible color transmission across the width of the incoupling linear grating; and The incoupling linear type diffraction grating has a plurality of linear diffraction gratings; and The incoupling linear diffraction grating is redirecting the received light; or Dividing the received light into a plurality of groups, or both; configured to maintain the same relative angles of the beams within each group; One or more of these apply: Optical system.

6. An optical system according to any one of claims 1 to 5, the input optical element comprises a wavefront shaping device configured to collimate received light into only a single plane; Optical system.

7. 7. An optical system according to claim 6, the wavefront shaping device is configured such that the single plane passes through the cylindrical axis of the cylindrical waveguide; Optical system.

8. 8. An optical system according to claim 6 or claim 7, The wavefront shaping device has a cylindrical shape. Optical system.

9. 9. The optical system of claim 8, the orientation of the cylindrical shape of the wavefront shaping device is orthogonal compared to the orientation of the cylindrical waveguide; Optical system.

10. An optical system according to any one of claims 5 to 9, The wavefront shaping device comprises a multi-element lens or a mirror. Optical system.

11. An optical system according to any one of claims 1 to 10, The input optical element further comprises a waveguide portion integral with the cylindrical waveguide.

12. An optical system according to any one of claims 1 to 11, The input side optical element is configured to set the angle of all light rays incident on one face of the cylindrical waveguide from the same pixel of the image source relative to a plane perpendicular to the cylindrical axis; This setting is such that the ray: In a direction parallel to the cylinder axis, or In a direction perpendicular to the cylinder axis, or In a direction defined by a vector between parallel and perpendicular to the cylinder axis, adapted to propagate within the cylindrical waveguide; Optical system.

13. An optical system according to any one of claims 1 to 12, The optical system comprises: the image source, or an image source mount defining a location for a central pixel of the image source; or Both Further equipped with an optical path length between the position of the central pixel of the image source and the cylindrical waveguide is approximately the same as a radius of curvature of the cylindrical waveguide; Optical system.

14. An optical system according to any one of claims 1 to 13, The optical system comprises: the image source or image source mount, or both, disposed closer to the outer surface of the cylindrical waveguide than to the inner surface of the cylindrical waveguide; a mirror positioned to receive light from the image source and reflect the received light towards the cylindrical waveguide; Further equipped Optical system.

15. 15. The optical system of claim 14, The mirror and the image source or the image source mount or both are light from the image source passes through the cylindrical waveguide before reaching the mirror; Optical system.

16. An optical system according to claim 14 or claim 15, when dependent on claim 6, The mirror is the wavefront shaping device. Optical system.

17. An optical system according to any one of claims 1 to 16, The optical system further comprises an outcoupling optical element. The outcoupling optic is positioned to receive light that has propagated along the cylindrical waveguide and to cause the light to appear as an image on an object.

18. 18. The optical system of claim 17, the outcoupling optical element comprises an outcoupling grating; Optical system.

19. 20. The optical system of claim 18, The outcoupling grating is A configuration that functions as a cylindrical lens; A curved diffraction grating; The received light is collimated into a plane or tangent-shifted. an internal grating angle arranged to focus the axial and sagittal planes at a given distance; an internal grating angle arranged to refract the received light; a diffraction efficiency of 25% or less at an end of the outcoupling grating closest to the light received from the input optical element; a diffraction efficiency that is variable according to the length of the outcoupling grating; Switchable grating configuration and having one or more of: Optical system.

20. An optical system according to any one of claims 17 to 19, the input optical element or the outcoupling optical element or both comprise corresponding ball lenses, or The outcoupling optical element further comprises a cylindrical negative lens, or It's both. Optical system.

21. An optical system according to any one of claims 17 to 20, the input optical element and the outcoupling optical element have respective portions closer to the cylindrical waveguide on either side of the cylindrical waveguide; Optical system.

22. An optical system according to any one of claims 17 to 21, the optical system further comprising a linear intermediate optical grating in the cylindrical waveguide; The linear intermediate optical diffraction grating is While maintaining the property that all of the light rays from the same pixel of the image source are incident on one face of the cylindrical waveguide at each point of incidence at the same angle relative to the surface normal and at the same angle relative to a plane perpendicular to the cylindrical axis, Changing the direction of light, Diffracting light, Splitting the light before the outcoupling optics. and arranged to perform one or more of the following: Optical system.

23. An optical system according to any one of claims 1 to 22, the cylindrical waveguide is a first cylindrical waveguide; The optical system comprises: a second cylindrical waveguide having concentric inner and outer surfaces, the input optical element is arranged to direct a portion of the received light into the first cylindrical waveguide and a portion of the received light into the second cylindrical waveguide; the arrangement being such that, in each of the first cylindrical waveguide and the second cylindrical waveguide, all of the light rays from the same pixel of the image source are incident on a surface of each of the cylindrical waveguides at each point of incidence at the same angle with respect to the surface normal and with respect to a plane perpendicular to the respective cylindrical axis; This allows the captured light to maintain its directional angle as it propagates along each of the cylindrical waveguides. a second cylindrical waveguide; an outcoupling optic positioned to focus light propagated along the first cylindrical waveguide at a first focal point and to focus light propagated along the second cylindrical waveguide at a second, different focal point; The optical system further comprises:

24. 1. An optical display device, comprising: An optical system according to any one of claims 17 to 23, which is derived from claim 2, comprising an image source configured to direct light to the in-coupling optics; A mounting device comprising: configured to be wearable by a user; and a mounting device configured to position the optical system such that the outcoupling optics causes the light to appear as an image to an eye of the user; and An optical display device comprising:

25. A head mounted display comprising the optical display device according to claim 24, the mounting device comprises an eyeglass component or a visor component; The cylindrical waveguide is integrated into, embedded in, or attached to the eyeglass component or the visor component. Head-mounted display.

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