Optical system with cylindrical waveguide

Abstract

An optical system comprising: a cylindrical waveguide having concentric inner and outer surfaces defining a common cylindrical axis; and input optics arranged to receive light from an image source and pass the light into the cylindrical waveguide such that, at each point of incidence, all light rays originating from the same pixel of the image source are incident on the surface of the cylindrical waveguide at the same angle relative to the surface normal and at the same angle relative to a plane orthogonal to the cylindrical axis, so that the incoupled light maintains its direction angle as it propagates along the cylindrical waveguide. The optical system can be used to provide an optical display device that can form part of a head-mounted display.

Description

[0001] This application is a divisional application of the patent application with application number 2021800645086, application date September 17, 2021, and invention name “Optical system with cylindrical waveguide”. Technical Field

[0002] The present disclosure relates to an optical system that may form part of an optical display device, such as a head-mounted display. Background Art

[0003] In the field of augmented reality, virtual images are displayed to the user as superimposed on the real world using a transparent combiner that redirects the image from the projector to the user's eyes. Current solutions typically use flat or planar transparent waveguides made of glass or plastic substrates, where light is in-coupled into the waveguide via a diffraction grating (or similar) and passes through the interior of the waveguide (via total internal reflection) to a similar out-coupler, where the light is coupled out to the user's eyes. In this field, waveguides are typically several millimeters thick and are also called "light guides."

[0004] Reference Figure 1 , which shows a schematic diagram of such an optical system including a planar waveguide. The optical system includes: an object display (image source) 10; a collimating lens 20; a planar waveguide 30; an in-coupler 40 (e.g., a grating); and an out-coupler 50. The figure shows light rays 60 from a point on the object display 10. These lights are incident perpendicular to the surface of the waveguide. The figure also shows the path of a single light ray 70 through the waveguide 30 and via the out-coupler 50 to the user's eye 80. The thickness of the waveguide 30 is on the order of several millimeters, the spacing between the in-coupler 40 and the out-coupler 50 is on the order of centimeters, and the lateral period of the grating of the in-coupler 40 is on the order of a fraction of a micron.

[0005] In this typical planar waveguide design, the object display 10 is imaged by a collimating lens 20 at the focal length of the lens 20. This collimates the image-bearing light in both the horizontal and vertical (tangential and sagittal) planes.

[0006] The collimated light is coupled into the waveguide via an incoupler 40, which can be a linear diffraction grating or a holographic grating (other options include refractive optics such as a prism). This diffracts the light into the glass or plastic waveguide or substrate 30 at angles greater than the critical angle.

[0007] In the designs considered herein, diffraction gratings, holographic gratings, holograms, surface holograms, relief holograms, lithographically produced gratings, volume gratings, or tilted gratings may be used interchangeably and operate interchangeably. For ray tracing purposes, the lateral component of the grating period, the grating k-vector or momentum vector, is important. The specific material choice, production method, and cross-section may affect diffraction efficiency, diffraction orders, mechanical flexibility, and wavelength bandwidth, but not aberrations and resolution. When a linear grating receives collimated light, it diffracts but maintains that collimation, i.e., all rays from a given pixel change direction but remain parallel to each other.

[0008] When light strikes the outer surface of a waveguide, it reflects back inside due to total internal reflection (TIR). These angles are determined by the waveguide's refractive index, n. A typical value of n = 1.5 results in a critical angle of 42 degrees, and the guiding angle relative to the normal can be 60 to 70 degrees. Glass with a higher refractive index will allow for a larger field of view (FOV) due to the smaller critical angle.

[0009] One advantage of this approach is that all light rays from a given pixel propagate at the same angle within the flat waveguide, thus allowing light rays to overlap or swap within the waveguide without causing ghosting or other image artifacts.

[0010] This could be the key to achieving the main function of waveguides: pupil replication. In the field of augmented reality (AR), smaller optics are highly desirable. This means using small projectors. Small projectors produce small pupils (essentially bundles of light coming from the same pixel), which can be identified, for example, by looking at the small pupils of ordinary binocular microscopes (the small bright dots floating above the eyepieces).

[0011] The coupler 50 (waveguide output grating) can replicate the pupil by extracting light partially with a diffraction efficiency less than 100%, known as fractional diffraction efficiency. For example, a diffraction efficiency of 10% will extract 10% of the light and allow the remaining 90% to propagate further. This process will repeat the next time the propagating light encounters the output grating 50. More advanced variable diffraction efficiency gratings can help balance the light intensity between different pupil replicas.

[0012] This makes the task of extracting light through the output grating robust, as the output grating does not need to be located at a specific position. The exit pupil of the waveguide can be viewed as a single continuous pupil extending across the entire output grating 50. Likewise, the extended pupil does not move along the output grating as a function of the field (the location of the original pixel on the display).

[0013] Two-dimensional pupil expansion is achieved by utilizing partial diffraction to replicate the pupil first in one direction and then in the other direction (first by an intermediate linear grating and then by an output linear grating). In total, three or more gratings will be used, as implemented in the waveguide of the HoloLens (RTM) sold by Microsoft Corporation and the waveguide sold by WaveOptics, Ltd. A limitation of this approach is that the glass of the waveguide 30 must be flat, and consumers are accustomed to curved glass. It is also difficult to combine a flat waveguide with curved prescription glasses without making the device larger. There are also applications that use highly curved glass, such as motorcycle helmet visors, military masks, fighter pilot helmets, diving masks, fire protection masks, medical protection, welding masks, aircraft windshields, car or motorcycle windshields, etc. Trying to fit a flat waveguide in the limited space between the user and the visor is challenging.

[0014] There are also combined solutions based on free-space reflective optics, but these solutions typically have small eye-boxes (the area where the image is visible). They are not suitable for some applications, while waveguide solutions typically have larger eye-boxes, which makes them desirable. A large eye-box means that a single change in the AR glasses design can fit most people and the user can easily see the virtual image. Due to differences in interpupillary distance (IPD) among users, a small eye-box means that the AR glasses may need to be mechanically adjusted or adapted to the specific user, increasing cost and complexity.

[0015] The use of curved waveguides has been suggested in various documents, such as WO-2006 / 064301A1, US-2010 / 031571U9, DE-102017119440A1, US-8,842,368B2, US-8,810,913B2, US-9,733,475B1, US-2018 / 0292593A1, U S-2019 / 0317261A1, GB-2553382B, US-10,048,647B2, US-2016 / 0195720A1, US-2019 / 0072767A1, US-2018 / 0373115A1, US-2019 / 0369403A1, and US-2018 / 0348527A1. Similar proposals are found in Kalinina, Anastasiia, and Andrey Putilin, “Wide-field-of-view augmented reality eyeglasses using curved wedge waveguide (conference presentation)”, Digital Optics for Immersive Displays II. Vol. 11350, International Society for Optics

[0016] and Photonics, 2020, and DeHoog, Edward, Jason Holmstedt, and Tin

[0017] Aye’s “Field of view of limitations in see-through HMD using

[0018] geometric waveguides”, Applied optics 55.22(2016):5924-5930.

[0019] Some of these references simply mention curved waveguides. Others discuss curved waveguides in greater detail, but none explain how to replicate the pupil in a continuous manner without causing light from the same pixel to diverge when projected to infinity. For these approaches, typically only a single pupil is propagated through the waveguide, which does not allow for pupil expansion. Other designs rely on eye tracking. These designs often produce distorted outputs and require complex software to pre-distort the input image. Consequently, such solutions are expensive and complex to manufacture.

[0020] While there are many existing planar waveguide-based approaches on the market, with many proposed implementations, including those discussed above, there are no curved or cylindrical waveguide-based approaches on the market.

[0021] This appears to be due to the difficulty in propagating light through a waveguide without the image being highly distorted (resulting in poor quality) when it is output to the user. Alleviating this problem while achieving a product that can be manufactured for the mass market remains a challenge. Summary of the Invention

[0022] Against this background, there is provided an optical system according to claim 1, an optical display device as defined in claim 24 and a head mounted display according to claim 25. Further preferred and optional features are defined in the other claims and elsewhere in this disclosure.

[0023] The optical system considers the use of a cylindrical waveguide (also known as a cylindrical waveguide or cylindrical waveguide) with concentric (inner and outer) surfaces. A cylindrical waveguide using this structure can allow light to propagate between curved surfaces without aberration. This is advantageously achieved together with the following input optical device, which is arranged to receive light from an image source (particularly a pixelated image source or an image source with a light output that can be decomposed into pixels, at least in theory), and provide the light to the cylindrical waveguide. All light rays from the same pixel of the image source are incident on the cylindrical waveguide at the same angle relative to the normal of the incident surface at each incident point. Therefore, all light rays from the central pixel of the image source are incident orthogonally to the cylindrical surface at any point on the cylindrical waveguide. Light rays from non-central pixels are incident at the same angle relative to the surface normal at each incident point. In addition, all light rays from the same pixel of the image source are incident on the cylindrical waveguide at the same angle relative to a plane orthogonal to the cylindrical axis at each incident point. Therefore, the propagation direction of all light rays remains unchanged.

[0024] Given the input optics and waveguide structure, light received at the cylindrical waveguide (and typically coupled into it) maintains its direction angle as it propagates along the cylindrical waveguide. That is, all light rays from a given pixel, no matter how far they propagate, will approach the output grating at the same angle as measured between the ray and the surface at each ray's point of incidence. Typically, the input optics include coupling (or injection) optics.

[0025] The outcoupling optics may be arranged to receive light propagating along the cylindrical waveguide and present the light as an image to a subject (particularly an eye of a user, i.e., an observer). In some embodiments, an image source (e.g., an electronic display) and / or an image source mounting device (on which the image source may be positioned) may form part of the optical system.

[0026] The optical system can be used to implement an optical display device (e.g., a head-mounted display). Advantageously, the image source is configured to direct light into the coupling optics. A mounting structure configured to be wearable by a user (e.g., forming part of headphones, a mask, or optical glasses) can be used to position the optical system so that at least the coupling optics present light as an image to the user's eyes. In a preferred embodiment, the cylindrical waveguide is integrated with, embedded in, or fixed to the mask component of the glasses or the mounting structure.

[0027] A specific surface of the coupling-in optics for coupling the received light into the cylindrical waveguide is a diffraction grating. Advantageously, a linear diffraction grating with a constant period is used. The period is measured in the lateral direction along the grating surface. The coupling-in linear diffraction grating is advantageously applied to curved surfaces. A preferred implementation of such a coupling-in linear diffraction grating uses a flexible holographic material. The coupling-in linear diffraction grating can be attached to the surface of the cylindrical waveguide (and preferably conforms to the surface without an air gap). Alternative structures that do not use a coupling-in grating (or at least one grating of this form) can be considered. For example, the waveguide does not need to be completely cylindrical, such as only a portion of the waveguide is cylindrical with the above-mentioned characteristics. Light can enter the waveguide through another portion, so that the other portion can form part of the input optics.

[0028] Unlike conventional collimating optics for flat waveguides, the input optics proposed in this disclosure cannot be properly called a collimator in classical optical design terms, since the light rays from the pixels are not parallel. Rather, the preferred coupling-in (projection) optics comprise an optical device that is arranged to collimate or shape the light rays (only) in a plane that passes through (i.e., is neither parallel to nor completely includes) the cylinder axis (more preferably, perpendicular to the cylinder axis). For example, for glasses and helmets, the cylinder axis is vertical, and the plane is preferably horizontal. In this way, the light rays are incident on the input grating at the same angle in this plane. Wavefront shaping devices, such as cylindrical lenses and / or cylindrical mirrors, can be used for this task. In general, in optical design best practices, the center pixel light rays will advantageously be incident at an angle normal to the waveguide surface, making aberration management easier due to symmetry. Other pixels in this 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.

[0029] However, in a plane orthogonal to the cylinder axis (e.g., a horizontal plane), the light may not be collimated, but rather shaped so that rays from the same pixel have the same angle of incidence relative to the surface normal, where the corresponding normal is considered separately for each different point of incidence. All surface normals point toward the cylinder axis.

[0030] The simplest wavefront shape that satisfies this condition is a cylindrical (cylindrical) wavefront concentric with the cylindrical shape of the waveguide. Light from the central pixel will then propagate radially from the cylindrical axis of the waveguide and approach the surface at normal incidence. This wavefront is advantageously formed by positioning the image source (display) so that the center of the image source is on the waveguide axis. The cylindrical lens or cylindrical mirror is then arranged to have optical focusing power only in the vertical plane.

[0031] Optionally, additional or different input optics can be provided, for example, to optimize the performance of more or all pixels and / or minimize the size of the projector. This can include bringing the display closer using optics that focus in the horizontal plane. In the case of a concave mirror, the image source and / or coupling-in optics can be arranged so that light approaches the waveguide from the side opposite the mirror before being reflected and then diffracted.

[0032] Light can propagate through the cylindrical waveguide (e.g., between the coupling-in optics and the coupling-out optics) in a direction (defined by a vector) parallel to the cylindrical axis (e.g., vertical), or in a direction (defined by a vector) perpendicular to the cylindrical axis (which can be horizontal, particularly around the circumference of the waveguide), or in a direction defined by a vector between parallel to the cylindrical axis and perpendicular to the cylindrical axis (typically a diagonal).

[0033] The outcoupling optics typically comprise an outcoupling diffraction grating. In principle, a linear grating could be used. However, a practical application of a waveguide is one where the observer is inside the cylinder. Therefore, in this case, a simple linear grating may not be suitable for extracting the light, as it would focus the light in a horizontal plane at the axis of the cylinder (producing vertical rays), rather than further in front of the observer. Instead, the diverging lens properties can collimate the light in the horizontal direction (the light is already collimated in the vertical direction). This can be achieved by adding negative optical power to the output grating in the horizontal direction. By analogy, the grating is the sum of the function of a prism and the function of a cylindrical negative lens. There are many known examples of how to record such a grating. Such a grating would not be called "linear" compared to the input grating. By adding more focusing in two planes, the output grating can be selected to place the digital image at an arbitrary distance from the observer.

[0034] In the case where the cylindrical waveguide is embedded in a head-mounted structure that already corrects the user's vision (for example, prescription glasses), the input and output gratings can also be corrected to take this into account. The main factor is to maintain the above state of the light as it propagates within the waveguide, regardless of how the light approaches and leaves the entire stacked "sandwich".

[0035] In a preferred embodiment, the image source and / or image source mounting device can be positioned on the side of the inner surface of the cylindrical waveguide (closer to the cylinder axis), with the coupling-in optics positioned between the image source and the inner surface of the cylindrical waveguide. Alternatively, the image source and / or image source mounting device can be positioned on the side of the outer surface of the cylindrical waveguide (far away from the cylinder axis). A reflector (which, as described above, can also serve as an input wavefront shaping device) can then be arranged to receive light from the image source and reflect the received light toward the cylindrical waveguide. In such an embodiment, light from the image source can pass through the cylindrical waveguide before reaching the reflector.

[0036] An intermediate grating (or more than one intermediate grating) in the cylindrical waveguide can redirect, diffract, and / or split the light before the outcoupling optics. Advantageously, each of the one or more intermediate gratings can be a linear grating to preserve the angular properties of the propagating light (TIR state, i.e., all rays from the same pixel are incident on the cylindrical waveguide surface at the same angle relative to the surface normal and relative to a plane orthogonal to the cylinder axis), thereby allowing the two-dimensional pupil to be expanded without aberrations.

[0037] Multiple cylindrical waveguides can be used. The outcoupling optics can then create a different focus for the light from each waveguide.

[0038] Various combinations of the features described above and 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 discussed herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The present disclosure may be implemented in many ways and a preferred embodiment will now be described by way of example only with reference to the accompanying drawings, in which:

[0040] Figure 1 A schematic diagram of a conventional optical system including a planar waveguide is shown;

[0041] Figure 2 A cylindrical waveguide used in the present disclosure is schematically shown;

[0042] Figure 3 Schematically shows the Figure 2 Characteristics of cylindrical waveguides;

[0043] Figure 4 shows a schematic top view of an optical system according to a first embodiment of the present disclosure,

[0044] Figure 5a Shown according to Figure 4 A perspective view of an embodiment of the present invention;

[0045] Figure 5b Shown Figure 5a A top view of an embodiment of the present invention;

[0046] Figure 5c Shown Figure 5a A side view of an embodiment of the present invention;

[0047] Figure 6 shows a schematic top view of a cylindrical waveguide and simplified outcoupling optics for use in an embodiment of the present disclosure;

[0048] Figure 7An example flow chart illustrating an operational process according to the present disclosure; and

[0049] Figure 8 A schematic top view of an optical system according to a second embodiment of the present disclosure is shown.

[0050] Throughout the disclosure, the same reference numerals are used throughout the various drawings to denote the same features. DETAILED DESCRIPTION

[0051] The new method proposed in the present disclosure is advantageously used in augmented reality head-mounted devices. The method is based on a cylindrical waveguide and considers the system as a whole. In particular, the method can include matching the curvature of the waveguide to the wavefront of the input light. The manufacture and calibration of the system are also simple and inexpensive. In addition, the cylindrical shape of the waveguide is better adapted to the shape of prescription glasses because it is easier to assemble a cylindrical waveguide between spherical surfaces than to assemble a planar waveguide plate. This can reduce the total thickness from about 15 mm to about 5 mm. The waveguide can also be embedded in prescription glasses by using air gaps or refractive index changes (for example, see US-2018 / 0348527A1, the detailed description of which is incorporated herein by reference).

[0052] First refer to Figure 2 , schematically illustrates a cylindrical waveguide used in the present disclosure. The figure shows: an image source (object display) 110; and a cylindrical waveguide 120. Cylindrical waveguide 120 has two concentric surfaces with a constant thickness (t) between the two concentric surfaces. Image source 110 is located at the center of curvature 100 of cylindrical waveguide 120. The figure shows the distance (R1) between the center of curvature 100 and the inner surface of cylindrical waveguide 120; and the distance (R2) between the center of curvature 100 and the outer surface of cylindrical waveguide 120. Clearly, R2 = R1 + t, and this holds true regardless of where these two distances are measured on cylindrical waveguide 120. In other words, this cylindrical waveguide 120 has a common center of curvature, and the radii of the inner and outer surfaces of cylindrical waveguide 120 are separated by the thickness (t) of the waveguide. The simple concentric shape of this cylindrical waveguide 120 allows for lower manufacturing costs and easier tolerance control in large-scale production.

[0053] Next refer to Figure 3 , which schematically shows the Figure 2Characteristics of a cylindrical waveguide. Also shown are: a cylindrical lens 135; and a diffraction grating 140. Light from the central pixel of the image source 110 (located at the center of the radius of curvature of the concentric surface of the cylindrical waveguide 120) is collimated by the cylindrical lens 135 in one plane by focusing, which can be referred to as wavefront shaping. The light that undergoes this wavefront shaping is then coupled into the cylindrical waveguide 120 by the diffraction grating 140 at an angle β. The incident light is orthogonal to the surface of the input grating at every point along the grating surface. This angle β is set so that the angle between the normal and the internal light is less than the critical angle of the waveguide, β is typically less than 48 degrees, and more preferably at least 30 degrees and up to 40 degrees. Pixels other than the central pixel are coupled into the waveguide at slightly different angles (e.g., β+1), but this angle is constant at every point along the grating surface.

[0054] The above-cited paper by DeHoog et al. (“Field of view of limitations insee-through HMD using geometric waveguides”) has shown that if (and only if) the two surfaces of the cylindrical waveguide are concentric, such as Figure 2 and Figure 3 As shown in the cylindrical waveguide 120, after the light bounces once on the inner surface, the incident angle on the outer surface is the same. Therefore, the light will maintain its direction angle when propagating along the waveguide, which is different from the case of selecting any other surface radius. Figure 3 As shown, if a ray bounces off the outside surface at an angle α, it will form an angle β with the inside surface, but will then form an angle α on the next bounce off the outside surface. In other words, the angle of incidence inside the waveguide is the same for every other bounce off the waveguide surface. A ray from a pixel incident on the left, center, or right side of the grating, after bouncing off the outside surface, will approach the inside surface at the same angle throughout the waveguide. Pixels other than the center pixel will have different values for α and β, but the above relationship will still hold. This property allows pupil replication.

[0055] It has been recognized that a high performance and compact optical system can be achieved by making all light rays from the same point on the image source incident at the same angle on such a cylindrical waveguide 120. Incoupling optics, such as cylindrical lens 135 and diffraction grating 140, can be configured to meet this requirement.

[0056] While central ray angles are typically discussed, there are other angles of the field of view generated by the image source object 110 (e.g., a microdisplay) that generate an image for the user. These other ray angles are propagated through the cylindrical waveguide 120 in the same manner. Rays from the edges of the field of view (the edges of the object) will have some small aberrations (loss of image quality), but this is largely unnoticeable to the user and can be compensated for by optical devices (as is well known in the art). Ideally, aberrations should be kept below 1 arc minute, which is human visual acuity.

[0057] It should be understood here that these aberrations do not accumulate with the number of bounces. For example, if the light 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 replications, the angular resolution of the entire system will still maintain a resolution of + / - 0.1 degrees, regardless of how the light rays are swapped or shuffled. This is in contrast to waveguides, which accumulate aberrations as they propagate.

[0058] In a general sense and according to one aspect, an optical system can be considered, comprising: a cylindrical waveguide having an input optical device. The cylindrical waveguide has concentric (concave) inner and outer surfaces, thereby maintaining the angle of light propagating through the cylindrical waveguide. The concentric surfaces define a common cylinder axis (common centerline). The input optical device is arranged to receive light from an image source and pass the light into the cylindrical waveguide so that all light rays originating from the same point on the image source are incident on the surface of the cylindrical waveguide at the same angle relative to the surface normal at the point of incidence. The input optical device is also arranged so that all light rays originating from the same point on the image source are incident on the cylindrical waveguide at the same angle relative to a plane orthogonal to the cylinder axis at the point of incidence. The combination of these features means that light entering the cylindrical waveguide maintains its direction angle as it propagates along the cylindrical waveguide.

[0059] Preferably, the input optics include an incoupling optic configured to couple light into a surface (preferably, the interior surface) of the cylindrical waveguide. Advantageously, the outcoupling optics are arranged to receive light propagating along the cylindrical waveguide and present the light as an image to the subject. Optionally, the optical system may include an image source (a display, such as a microdisplay) and / or a mounting device for the image source (image source mounting device).

[0060] Other aspects of the present disclosure may be considered. For example, an optical display device may include: an optical system as described herein, the optical system including: an image source configured to direct light into a coupling-in optical device; and a mounting structure, in particular a mounting structure configured to be wearable by a user. The mounting structure may position the optical system so that the coupling-out optical device presents the light as an image to the user's eyes. For example, the mounting structure may include an eyeglass frame, a mask or helmet structure or other wearable mounting structure. Optionally, a head-mounted display including such an optical display device may be considered. In this case, the mounting structure may include an eyeglass or mask component. Advantageously, the cylindrical waveguide is integrated with, embedded in, or fixed to the eyeglass or mask component.

[0061] Another aspect that may be considered is a method of manufacturing and / or operating such an optical system. For example, a method of manufacturing the optical system may include providing a cylindrical waveguide having concentric inner and outer surfaces; and arranging input optics (e.g., including coupling-in optics) to receive light from an image source and direct the light into the cylindrical waveguide such that all light rays originating from a point on the image source are incident on the surface of the cylindrical waveguide at the same angle relative to the surface normal and at the same angle relative to a plane orthogonal to the cylinder axis at the point of incidence. The method may also include configuring coupling-out optics to receive light propagating along the cylindrical waveguide and present the light as an image to a subject. Similarly, a method of operating the optical system may include directing light from the image source to the input optics of the cylindrical waveguide such that all light rays originating from a point on the image source are incident on the cylindrical waveguide at the same angle relative to the surface normal and at the same angle relative to a plane orthogonal to the cylinder axis at the point of incidence. Preferably, the light propagating along the cylindrical waveguide is presented to the subject as an image (via the coupling-out optics).

[0062] Further preferred, optional and advantageous features will be discussed below, which are particularly applicable to all aspects disclosed herein.First, a specific embodiment will be discussed.

[0063] Now refer to Figure 4, which shows a schematic top view of an optical system according to a first embodiment of the present disclosure. In this figure, the plane of the page (and any plane parallel thereto) is considered to be horizontal, while the perpendicular plane emanating from the page (and any plane parallel thereto) is considered to be vertical. Thus, the cylindrical waveguide 120 can be visualized as, for example, similar to a cylindrical mask lying on a flat surface. The outcoupling diffraction grating 150 is also shown in this simplified diagram. The image source 110 is placed at a distance from the cylindrical waveguide 120 that is equal to the radius of curvature of the waveguide. As described above, the cylindrical lens 135 collimates (or shapes or adds power) the input light in one plane, and in particular adds power in the vertical plane. This is then matched as input to a linear diffraction grating 140, which, when applied to the curved surface of the cylindrical waveguide 120, propagates the image-bearing light along a rotationally symmetric waveguide (i.e., the image of the waveguide can be rotated about the cylindrical axis while the light from a given pixel will maintain the same angle relative to the waveguide surface). This makes the system constant with respect to the arrangement of the output diffraction grating 150, eliminating the need for precise alignment. The output diffraction grating 150 compensates by applying negative focusing power (i.e., divergence) to light in the opposite plane (the horizontal plane) to provide an extended eyebox for the user (not shown), whose output is set at infinity. This allows pupil replication to expand the output eyebox.

[0064] Therefore, this design generates a wavefront in the cylindrical waveguide 120 that can propagate rotationally symmetrically along the entire waveguide. The spherical wavefront emitted from the point light source 110 is in the horizontal plane (as described above, Figure 4 The curvature of the waveguide 120 itself is followed in the plane of the page) and is reflected by the cylindrical lens 135 in the vertical plane (as described above, in the plane from Figure 4 This means that after two bounces, the angle of the light is mapped back on itself and repeated infinitely. Therefore, the pupil can be replicated (extracted multiple times) to expand the eye movement range in one dimension (horizontally). The size of the vertical eye movement range is set by the vertical size of the cylindrical lens 135. It also relaxes the alignment tolerances (because the position of the coupler relative to the input pupil is not critical), the choice of waveguide thickness and the center ray steering angle.

[0065] Next refer to Figure 5a , showing that according to Figure 4 A perspective view of an embodiment of the invention. Figure 5b In the Figure 5a A top view of an embodiment of the present invention, Figure 5c In the Figure 5aLight 101 collimated in one plane is coupled into a cylindrical waveguide 120 , light 102 propagates through the waveguide 120 and is coupled out by a diffraction grating 150 to provide projected light 103 toward a user's eye 160 .

[0066] In this particular embodiment, the image source or object 110 is a microdisplay and is located at a distance from the cylindrical waveguide 120 that is the same as the waveguide's radius of curvature. This is achieved by physically positioning the image source 110 at this distance or virtually placing the image source 110 at this distance through an optical device (e.g., a lens). The image is collimated in one plane by a cylindrical plano-convex lens 135 onto an in-coupling diffraction grating 140 located on the concave inside surface of the cylindrical waveguide 120. The cylindrical lens 135 is oriented so that its focusing power is in the plane opposite the cylindrical waveguide 120. For example, if the curved waveguide is oriented horizontally (like a helmet visor or resting on a table), it has optical power in the horizontal plane and reflects light from a point source at the radius of curvature into a vertical line. The cylindrical lens 135 is then oriented in the opposite vertical plane and has optical power in the vertical plane and focuses light from the point source at the radius of curvature into a horizontal line.

[0067] Therefore, the image-bearing light is collimated in only one plane (the horizontal plane) before entering the waveguide, and the curvature of the cylindrical waveguide 120 aligns the light in the vertical plane. This allows the entire pupil of light entering the waveguide to propagate along the waveguide, allowing the pupil to expand at the output.

[0068] The focal length of cylindrical lens 135 determines the magnification of the object, and lens 135 is positioned one focal length away from object 110. If cylindrical lens 135 is placed immediately adjacent to cylindrical waveguide 120, it will have a focal length approximately equal to the waveguide's radius of curvature. For example, a typical radius of curvature for a mask-shaped waveguide is 200 mm, meaning that if the object is 200 mm away, the focal length of the cylindrical lens is 200 mm. For compactness, the 200 mm distance from the waveguide to the object can be reduced by folding the optical path with a mirror or virtually optically setting the object distance with a lens.

[0069] The choice of cylindrical lens 135 (diameter and / or focal length) determines the size of the vertical eyebox (determined by the diameter of the lens), and the focal length determines the magnification of the display 110 and, therefore, the field of view (FOV) of the image (along with the size of the display). Typically, a multi-element lens is used for the cylindrical lens 135 (e.g., as used in cameras), which provides good image quality across the entire FOV (small spot size RMS across the entire FOV). This is particularly desirable for pupil replication systems to accurately overlap the pupils and provide a high-resolution image. Although monochrome solutions are possible, for full-color microdisplays, the lens system is ideally achromatic. The FOV of the curved waveguide 120 can be largely determined by similar factors, but due to the nature of the curve around the user, the FOV will be extended compared to a planar waveguide. This is shown in the above-referenced paper by DeHoog et al. ("Field of view of limitations in see-through HMD using geometric waveguides").

[0070] The in-coupling diffraction grating 140 is a linear grating with equal surface spacing (pitch) between grating lines (or equivalently, equal fringe spacing in a volume holographic grating). The grating can be fabricated using photolithographic or interferometric methods. All light rays are collimated in one plane, incident orthogonally (90 degrees to the surface) across the entire width of the grating surface, and then diffracted at the same angle within the waveguide, which enables pupil replication.

[0071] Grating on a curved waveguide typically means that the collimated light is not incident orthogonally across the entire grating width due to the curvature of the waveguide. Typical solutions to this include varying the pitch of the grating to compensate, or recording the hologram directly on the curved surface, or performing photolithography on the curved surface, which is complex and expensive. In a preferred embodiment according to the present disclosure, the incoupling grating 140 is fabricated as a planar linear grating on a flat substrate (as is well known in the art and is relatively cheap and simple to manufacture compared to variable gratings). The grating 140 can be fabricated on any flexible holographic material, such as a photopolymer (e.g., Bayfol (RTM) sold by Covestro AG, or silver halide film), and then attached (laminated) to the cylindrical surface of the waveguide, conforming to the cylindrical surface. The grating 140 is index matched, preferably by lamination (or another index matching glue or liquid) so that the grating conforms to the shape of the cylindrical surface, and preferably so that there are no air gaps. Recording a hologram on a flat substrate, then removing the flexible holographic material and laminating it to a flat or cylindrical substrate (curved in only one dimension) is simple and inexpensive, whereas recording on a curved surface or laminating on a spherical surface (curved in two dimensions) is more difficult.

[0072] It is also possible to etch a slanted grating and use embossing or UV-curable resin techniques. The grating can then be transferred to a cylindrical waveguide.

[0073] The grating pitch is designed to diffract the central wavelength of the microdisplay 110 at a desired internal angle. Because the grating is nominally designed to diffract normal incident light at an angle, the grating has a tilt angle, and the pitch is usually specified as the spacing between the gratings measured along the planar surface of the grating. For input couplers, i.e., linear gratings, the pitch remains constant.

[0074] For planar waveguides, both couplers are typically linear and identical. The system acts like a periscope, presenting the viewer with a magnified image of the microdisplay superimposed on the real world. The overall design of the system means that the positional pixel information of the display is converted into angular information through collimation, and then back into positional information on the human retina.

[0075] Traditionally, for flat waveguides, the input and output gratings are linear, parallel, and have the same period to eliminate chromatic aberration. This is important for using broadband light sources, such as light-emitting diodes (LEDs). The output gratings according to the present disclosure have a variable period along the waveguide and do not eliminate chromatic aberration everywhere. However, the period of the output grating at the center can be chosen to be the same as the period of the input grating to minimize this chromatic aberration.

[0076] Alternatively, a narrowband light source such as a laser light source, a superluminescent light emitting diode (SLED), or a notch filtered narrow band LED can be used. Narrowband light sources can help minimize chromatic aberration. They can also limit the FOV of the output image, but a thin holographic volume grating can be used to mitigate this effect. For example, a typical thickness of a holographic volume grating can be at least 3 microns and a maximum of 6 microns. This can provide a typical spectral bandwidth of about 20 nm full width at half maximum (FWHM) and an angular bandwidth of about 6 degrees FWHM in air.

[0077] The cylindrical waveguide may form part of a head-mounted structure that already corrects the user's vision (e.g., prescription glasses). The incoupling grating 140 and the outcoupling diffraction grating 150 may then also take this into account without affecting the propagation of light inside the waveguide and its efficient outcoupling.

[0078] As an example, when the waveguide is embedded in a conventional prescription meniscus positive lens, the outer covering lens can have a convex spherical surface and a concave cylindrical surface. The inner liner lens can have a convex cylindrical surface and a concave spherical surface. Thus, the perspective optical properties will be defined by the outermost convex surface and the innermost concave surface, which will be selected based on the wearer's prescription. The focus of the digital image provided via the cylindrical waveguide will be independently defined by the innermost surface and the output grating optical properties (optical power). Those skilled in the art will also incorporate astigmatism in the perspective and digital image to suit the optician's prescription.

[0079] The output grating can be selected to place the digital image at an arbitrary distance from the observer by adding more focus in two planes. In the vertical plane, the light coupled into the waveguide is collimated, but the light output from the outcoupling grating does not need to be collimated. To allow multi-pupil extraction, only part of the light can be extracted in the first part of the outcoupling grating 150. To balance the uniformity of the light extracted across the coupling grating 150, the distal end of the outcoupling grating 150 (compared to the coupling grating 140) preferably has a higher efficiency than the near (receiving) end. The diffraction efficiency of the output grating is advantageously selected to be low enough at the receiving end of the output grating 150 to allow sufficient pupil replication (e.g., 5-25%), but high enough at the distal end to obtain satisfactory brightness (e.g., 20-100%).

[0080] Returning to the general meaning of the aspects discussed above, it will be appreciated that the incoupling optics may comprise an incoupling linear diffraction grating arranged to couple the received light into a cylindrical waveguide. This advantageously has a constant period. The incoupling linear diffraction grating is advantageously applied to curved surfaces, particularly those with index-matched curvature (without air gaps).

[0081] In another general sense, according to another aspect, an optical system may be considered, comprising: a cylindrical waveguide having concentric inner and outer surfaces; and an in-coupling linear diffraction grating applied to the curved surface and arranged to couple received light into the cylindrical waveguide. Optionally, the optical system may further include an out-coupling linear diffraction grating arranged to receive light propagating along the cylindrical waveguide and present the light as an image to an object. In a preferred embodiment, the wavefront shaping device may be configured to collimate or conform the received light in a single plane and direct the collimated light to the in-coupling linear diffraction grating. Methods of manufacturing and / or operating such an optical system may be further considered, for example as described below. The following further features may be applied to any aspect disclosed herein.

[0082] In yet another aspect, a method for fabricating a curved linear diffraction grating may be considered. The method includes: forming the linear diffraction grating on a planar surface; and attaching the linear diffraction grating to a curved surface of a (cylindrical) substrate such that the linear diffraction grating conforms to the curved surface. Similar to other aspects considered herein, the additional features discussed below (and elsewhere herein) may similarly apply to this aspect.

[0083]

[0014] Additional features applicable to all aspects will now be discussed. For example, the in-coupling linear diffraction grating can be made of a flexible holographic material. Optionally, the in-coupling linear diffraction grating is attached to (and / or conformed to) the inner surface of the cylindrical waveguide. In a preferred embodiment, the in-coupling linear diffraction grating has a straight grating. The grating thickness and angular bandwidth of the in-coupling linear diffraction grating can be configured to provide (substantially) uniform visible range color transmission across the width of the in-coupling linear diffraction grating.

[0084] Preferably, the input optics include a wavefront shaping device (which can be considered a form of collimator) configured to collimate or shape the received light into only 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 can be cylindrical in shape. In a preferred embodiment, the wavefront shaping device includes a plano-convex lens and / or a multi-element lens. Advantageously, the orientation of the wavefront shaping device is orthogonal to the orientation of the cylindrical waveguide.

[0085] Other general features will be discussed again below.Additional specific examples according to the present disclosure are now described.

[0086] Reference Figure 6 , which shows a schematic top view of the cylindrical waveguide 120 and a simplified diagram of the outcoupling optical device including the outcoupling diffraction grating 150. Figure 4 Likewise, the plane of the page in this figure (and any plane parallel to it) is considered to be horizontal, and the perpendicular plane emanating from the page (and any plane parallel to it) is considered to be vertical. Thus, the cylindrical waveguide 120 can be visualized, for example, as being similar to a cylindrical mask lying on a flat surface. Also shown in this simplified diagram is a cylindrical negative lens 155, which will be discussed further below. The light 151 output from the outcoupling diffraction grating 150 is collimated in the vertical plane and focused in the horizontal plane, as shown by line 152. The light 156 output from the cylindrical negative lens 155 is collimated in both the horizontal and vertical planes and has infinite focal length. This is a simplified diagram because the cylindrical negative lens 155 is actually optically integrated within the outcoupling diffraction grating 150. Therefore, optical power is contained in the hologram within the outcoupler to compensate and achieve collimation in both the horizontal and vertical planes at the output. Thus, the outcoupling diffraction grating 150 acts as a cylindrical lens to compensate for the cylindrical curvature introduced by the coupling-in optics and, as described below, in this manner, collimates the image substantially at infinity.

[0087] Due to the asymmetric collimation at the coupling-in optics, the coupling-out optics provide different focal positions for the horizontal (near) output image plane and the vertical (far or infinity) output image plane to provide images focused at infinity in both planes, thereby providing a high-quality image to the observer. This compensation is achieved by encoding the optical light power into the output grating. This method is known in the art, but not for this purpose. Figure 6 As shown, this is equivalent to placing a diverging cylindrical lens 155 (a plano-concave cylindrical lens; if the waveguide's radius of curvature is 200 mm, then the focal length of this lens would be -200 mm) with negative optical power equal to the waveguide's radius of curvature between the planar output grating and the user. Lens 155 is oriented perpendicular to the input cylindrical lens 135. If the input lens 135 focuses (or has optical power) in the vertical plane, the output compensation lens / grating will focus (or have optical power) in the horizontal plane to produce a spherically collimated output. As mentioned above, the centers of the outcoupling grating 150 have the same surface pitch (also called lateral pitch or in-plane pitch) as the input grating to allow for dispersion compensation.

[0088] The image is presented to the user 160 at infinity. This is often the desired use case, as it means that when the user focuses on a distant object in the real world, the virtual image will appear in focus, as is typical when, for example, a fighter pilot or motorcycle rider uses a visor. Consumer electronic devices using planar waveguides with pupil expansion also have images at infinity.

[0089] The output grating can have a varying diffraction grating efficiency, or a relatively low output efficiency (e.g., 10%). This can be achieved during the recording of the holographic outcoupler. While it is desirable that the input grating 140 have maximum diffraction efficiency (meaning that most of the light incident on it is coupled into the waveguide), the output grating 150 can have a low or variable efficiency, allowing pupil expansion. A small portion of the light is coupled out on the first interaction with the output grating 150, while most of the light continues to bounce downstream along the waveguide and a portion of the light is output on the second interaction, and so on. This allows for an expanded eye range in the horizontal plane.

[0090] As is known in the art, holographic waveguide gratings (linear outcouplers or dynamic outcouplers) can be fabricated by exposing a holographic material to two coherent light beams, where the waveguide beams are coupled into the holographic material via a prism. Three gratings can be multiplexed into a single holographic layer using lasers of three different wavelengths (e.g., red, green, and blue, RGB) to allow an observer to see a substantially white image from an RGB microdisplay. Alternatively, three separate layers can be stacked, one for each color.

[0091] By multiplexing multiple gratings into a single holographic layer, the field of view (FOV) provided to the user, over which a uniformly bright, uniformly colored image is viewed, can be increased. This can be achieved by varying the recording angle. Alternatively, multiple angle-multiplexed layers can be stacked.

[0092] The input and output gratings can be reflection holograms, transmission holograms, or any combination thereof. This will be understood from the above theory, as the desired effect is based solely on the lateral component of the grating pitch. The transverse (cross-sectional) pitch or period can be conveniently selected to suit the geometry of the reflection or transmission grating. It will also be understood that the linear grating described above is meant to be linear in the lateral direction, but can have variable transverse characteristics.

[0093] The collimated nature of the output light means that a large eye relief (i.e., the distance at which the eye can best see the image behind the output surface) can be achieved. This is often desirable, especially for applications with helmet visors rather than glasses. The larger the eye relief, the smaller the FOV.

[0094] The real-world view is largely unaltered by the curvature of the waveguide. A typical radius of curvature for glasses is 250mm, and for masks it's 150-200mm. Any curvature greater than 100mm (as is the case here) will result in a distortion of the real world that will be unnoticeable to the user. There will be only a very small astigmatism effect, unless compensated for by an additional overlapping lens (or lenses).

[0095] Returning to the general sense of the present disclosure, the outcoupling optics may be considered to comprise an outcoupling diffraction grating. In particular, the outcoupling diffraction grating may be configured to act as a cylindrical lens (e.g. focusing in one dimension only). Additionally or alternatively, the outcoupling linear diffraction grating may have a curved grating. In a preferred embodiment, the outcoupling linear diffraction grating may: have an internal grating angle arranged to collimate the received light in a plane or to focus the received light in a tangential plane and a sagittal plane at a predetermined distance, and / or an output or diffraction efficiency of no more than 25% (optionally 20%, 15% or 10%) at the end of the outcoupling diffraction grating closest to the light received from the input optics.

[0096] The outcoupling optics may include an output wavefront shaping device configured to collimate the received light in a single plane orthogonal to the single plane of the input wavefront shaping device. Additionally or alternatively, the outcoupling optics may include a cylindrical negative lens. Preferably, these aspects are integrated into the outcoupling diffraction grating.

[0097] The out-coupling linear diffraction grating may have the same surface pitch as the in-coupling linear diffraction grating. In some embodiments, the out-coupling linear diffraction may have an internal grating angle that is oriented oppositely to the internal grating angle of the in-coupling linear diffraction grating. This is particularly useful in situations where the light from the image source and the light reaching the observer (or the in-coupling optics and the out-coupling optics) are on the same side of each other. This may be referred to as a "U" grating. Alternatively, the angles of the in-coupling grating and the out-coupling grating are not oriented oppositely, and at least some of the light will be coupled out on the opposite side of the in-coupling light (in other words, the observer will be on the other side relative to the in-coupling light). This may be referred to as a "Z" grating.

[0098] From another perspective, the solutions according to the present disclosure can also be explained in terms of symmetry. These solutions use a cylindrical waveguide and a cylindrically symmetric wavefront, both of which are rotationally symmetric around their common axis.

[0099] Consider a pixel on a display or other image-generating device. The light wavefront from this pixel can be shaped into a cylindrical shape. A linear diffraction grating recorded on a flat substrate and laminated onto a cylindrical waveguide will have a constant period along the surface. Consequently, each ray of light will be deflected by the same angle, resulting in a light field that is symmetrical about the axis of rotation. As described above, a ray of light emitted between the cylindrical surfaces will maintain the two angles of incidence on the two surfaces for any number of reflections. This means that after every two reflections, the wave will completely reconstruct itself. Therefore, no ghost images are generated. Such a light field can propagate over any distance without any ray of light becoming different from the others.

[0100] Furthermore, pupil replication is achieved at the output grating. Partial decoupling of the light occurs during the initial interaction with the outcoupling optics, while the remaining light propagates and is coupled back in during the next interaction. In this case, the different interactions are perfectly matched, resulting in no ghost images. When all light reaches the outcoupling grating, the grating can diffract the light out of the waveguide, regardless of the grating's position or the position of any individual ray, since they will all arrive similarly.

[0101] A linear outcoupling grating with the same period as the incoupling grating diffracts the light again into a new cylindrical wavefront. It's well known that diffractive optical elements can cumulatively combine several functions. Like a cylindrical lens, the outcoupling grating also has one-dimensional focusing capabilities. This converts the diffracted light into collimated light. An observer receiving this light will see a star-like point at infinity.

[0102] The same explanation applies to the other pixels. As mentioned above, the wavefronts from these other pixels do not have to be exactly cylindrical. This is because the light rays from the non-central pixels hit the coupling optics at slightly different angles from the "perfect" normal (perpendicular). However, by all hitting the coupler at essentially the same angle relative to the surface normal at their respective intersection points, the resulting light rays will form a ray field that is rotationally symmetric about the cylinder axis and propagate in an indistinguishable manner.

[0103] Using modern optical design, it is possible to design a projector that forms such a light field with small errors, ideally 1 arc minute (human visual acuity).

[0104] Reference Figure 7, which shows an example flow chart of the operation process according to an embodiment of the present disclosure. In a display step 210, a dynamic 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 the vertical plane by a cylindrical lens 135 (or equivalently, a cylindrical reflector as will be discussed below) having optical power in the vertical plane. In a first incidence step 230, the image-bearing light (for a single pupil) is incident on the cylindrical waveguide 120 orthogonal to a surface (typically the inside surface) of the cylindrical waveguide 120. In a coupling-in step 240, the image-bearing light is coupled in at a constant angle (for all light rays from the same pixel) via the linear diffraction grating 140. Since the cylindrical waveguide 120 has concentric inside and outside surfaces, in a propagation step 250, the coupled-in light propagates rotationally symmetrically between the concentric surfaces. In an initial decoupling step 260, a first portion of the image-bearing light is extracted during an initial interaction with the decoupling diffraction grating 150. Then, in a further decoupling step 270, subsequent interactions (bounces) with the decoupling diffraction grating 150 result in further extraction of the propagated light, thereby providing pupil replication. In a second collimating step 280, the optical effect of the decoupling diffraction grating 150 adds negative cylindrical power in the horizontal plane. Finally, in an output step 290, the user's eye 160 is made to see a spherical collimated image at infinity superimposed on the real world.

[0105] Although specific embodiments have been described, those skilled in the art will appreciate that various modifications and substitutions may be made. In particular, the incoupling and outcoupling optics may vary significantly from the specific design described above.

[0106] As shown above, the light source can be placed closest to the inside surface of the cylindrical waveguide, and a cylindrical lens can be used to collimate the light. It is not necessary to use a cylindrical lens. For example, a cylindrical mirror can be used instead, potentially with all the same properties and benefits. It is also possible to place the light source after the outside surface and the cylindrical mirror used to collimate the light, for example, after the light passes through the waveguide.

[0107] Many designs can be used to optimize the performance of all pixels and minimize the projector's size. This includes utilizing optics that focus in the vertical plane to bring the display closer. For example, a concave mirror can be used to direct light approaching the waveguide from the opposite side of the mirror before being reflected and then diffracted. This is similar to the aberration management of afocal systems in classical optics. However, the handling of these aberrations is reformulated in this disclosure as a cylindrical coordinate system.

[0108] Reference Figure 8, shows a schematic top view of an optical system according to a second embodiment. In this embodiment, cylindrical lens 135 is replaced by a cylindrical mirror 136. Using this cylindrical mirror eliminates the need for image source 110 to be located closer to the inside surface of cylindrical waveguide 120. For simplicity, input diffraction grating 140 is not shown, but it is present in the same location as in the other described embodiments. In the illustrated embodiment, image source 110 is closer to the outside surface of cylindrical waveguide 120 than to the inside surface. The optical path length from image source 110 to the input diffraction grating remains the same as the radius of curvature of cylindrical waveguide 120. For example, a typical radius of curvature of a mask waveguide of 200 mm would mean the object is 200 mm away, and the cylindrical mirror has a radius of curvature of 400 mm. However, the distance between image source 110 and the input diffraction grating is different from this radius. Again, it can be seen that the incident light is normal to the surface of the input grating. The wavefront generated by the cylindrical mirror 136 is substantially the same as the wavefront generated by the cylindrical lens 135 and therefore has similar characteristics. As with the other embodiments disclosed herein, all other variations and options can be applied to implementations according to this embodiment.

[0109] The orientation of the cylindrical waveguide can be varied. The above embodiments orient the cylindrical waveguide with its cylindrical axis vertically (and therefore the cylindrical waveguide extending in the horizontal direction) as this is the normal way to align the mask. However, this is not necessary. Other orientations can be considered. Additionally or alternatively, light can enter and leave the cylindrical waveguide via different surfaces (e.g. on different sides of the waveguide). This can be achieved by positioning the in-coupling grating and out-coupling grating accordingly. In some embodiments, more than one in-coupling grating and / or out-coupling grating can be provided.

[0110] The incoupling grating and the outcoupling grating can each be a reflective grating or a transmissive grating and can be placed on the inside or outside surface of the waveguide (or on another surface of the waveguide). Those skilled in the art will appreciate variations on the embodiments shown herein.

[0111] The linear input grating can have any orientation angle. The light does not need to be redirected in the circumferential direction of the cylinder (perpendicular to the cylinder axis, which is horizontal in the above embodiment). The light can be directed along the cylinder axis (vertical). As an alternative, the light can be pointed at 45 degrees or any other diagonal direction. This allows design freedom, such as conveniently positioning the projection module at the temple of the glasses (glasses). It also makes sense to implement an intermediate grating for 2D pupil expansion. The intermediate linear grating can redirect and / or split the light while keeping the corresponding angle of each redirected light ray the same for rays from the same pixel.

[0112] The system allows for the use of either laser or LED light, which provides flexibility. Typically, LED light is used, such as LCOS (liquid crystal on silicon) coupled with LEDs or microLED microdisplays, but lasers can also be used if high efficiency and therefore high brightness are desired. Laser beam mirror scanning systems (MEMS, micro-electromechanical systems) can also be used. Lasers have some disadvantages in terms of cost, speckle (loss of resolution), and eye safety.

[0113] If a refractive element (e.g., a prism) is used as an in- or out-coupler together with a diffractive in- or out-coupler, uncompensated dispersion may limit the use of only narrow-band light sources (e.g., lasers). Furthermore, refractive couplers tend to be bulky and expensive.

[0114] By adding spherical optical power to the waveguide output, the virtual image can be set at different focal lengths. It is also possible to add an extra pair of lenses before and after the waveguide (a second lens to compensate for the effect of the first lens on the real world) to set the focal length even closer. Another possibility is to add an electrically addressable and switchable (liquid crystal based) holographic output grating that can be turned on or off to provide different focal planes for the image. Additionally or alternatively, the input grating can be switched in the same way to provide a larger FOV, which can be achieved by angle-multiplexing the grating. This switching can be synchronized with the time-multiplexed microdisplay.

[0115] Alternatively, multiple focal planes can be achieved by using multiple (stacked) cylindrical waveguides. The light will propagate as described above, but will exit with different focal points generated by the grating. This approach is described in detail in "Optical architectures for augmented, virtual and mixed reality headsets" (2020), BCKress, SPIE press.

[0116] Further adding axisymmetric optical power to the output of the cylindrical waveguide (similar to the optical power of a normal spherical lens) will result in the point being seen at a finite distance (say 1 m).

[0117] The cylindrical waveguide can form part of a larger (overall) waveguide structure, where only a portion may be cylindrical. Embodiments are contemplated where no coupling optics are required. For example, light may enter the waveguide from a non-cylindrical portion (e.g., due to an embedded image source) or may originate therefrom, and wavefront shaping may be performed in this portion. Thus, this portion of the waveguide may form part of the input optics.

[0118] The vertical eye range can also be extended by multiple input projectors that are shifted vertically. A typical approach for extending the vertical eye range in a planar waveguide uses a "turn" grating that propagates the pupil vertically to produce a 2D exit pupil expansion. There are a variety of methods for extending the eye range vertically, including input, turning, and output gratings such as those implemented in products by Vuzix Corporation or DigiLens Inc. One alternative is to use a "butterfly" turning grating that extends the eye range and also extends the FOV by splitting the FOV in two at the input and recombining it at the output (as used by the HoloLens (RTM) sold by Microsoft Corporation and described in the book by BCKress cited above). Another option is to use a reciprocal multiplexing grating that waveguides a portion of the light and couples a portion of the light out through the extended eye range (as used in products by WaveOptics, Ltd.).

[0119] All of these existing techniques benefit from using collimated light delivered by linear gratings and flat waveguides. According to the present disclosure using cylindrical waveguides, these techniques of light splitting and pupil replication via an intermediate linear grating can be implemented after the light is coupled into the cylindrical waveguide. The light can then be ultimately coupled out via a grating with negative cylindrical focusing.

[0120] The rotationally symmetric structure of embodiments according to the present disclosure allows the input and output gratings to be placed anywhere on the concentric cylindrical waveguide. For example, as with the typical flat horizontal configuration discussed above, the orientation can be vertical or at an angle to the waveguide (e.g., in a mask implementation). This allows for flexible placement of the projector and eye-box position in the final design. It also allows for the pupil replication and vertical eye-box expansion methods discussed in the previous paragraphs.

[0121] Eye tracking is an existing technology that can add additional functionality to AR devices. This is typically achieved with an infrared (IR) light source that illuminates the eye and a camera that detects the reflected light and determines the direction of the eye's gaze. In areas with limited space, such as a mask or glasses, this may not be feasible. Adding compact eye tracking functionality to a waveguide can be achieved with input and output gratings as described above, but the input and output gratings diffract infrared light (e.g., 850nm). The gratings can simply direct the IR light onto the eye, or the IR light reflected from the eye can be coupled via TIR from the grating at the eye's location to the output grating and thus to the camera.

[0122] Reference is further made to the general significance of the disclosure discussed above.For example, in a preferred embodiment, the radius of curvature of the cylindrical waveguide is at least 100 mm.

[0123] In an embodiment, the wavefront shaping device may include a concave (cylindrical) mirror. Optionally, the image source and / or the image source mounting device may be closer to the outer surface of the cylindrical waveguide than the inner surface of the cylindrical waveguide. A reflector (which is preferably the wavefront shaping device) may then be arranged to receive light from the image source and reflect the received light toward the cylindrical waveguide. In some embodiments, the reflector and the image source and / or the image source mounting device are configured so that the light from the image source passes through the cylindrical waveguide before reaching the reflector. In some embodiments, corresponding portions of the input optical device (e.g., an in-coupling grating) close to the cylindrical waveguide and the out-coupling optical device (e.g., an out-coupling grating) close to the cylindrical waveguide are located on opposite sides of the cylindrical waveguide.

[0124] In some embodiments, the input optical device further comprises one or more spherical lenses. Additionally or alternatively, the outcoupling optical device further comprises one or more spherical lenses. Spherical lenses can be used to change the optical path length of light and / or change the focus of light. In an embodiment, the input optical device may further comprise a waveguide portion integrated with a cylindrical waveguide. Advantageously, the waveguide portion forming at least a portion of the input optical device is non-cylindrical (columnar) and / or does not have concentric surfaces. In some embodiments, only a portion of the waveguide shape may be cylindrical.

[0125] In some embodiments, one or more intermediate gratings may be provided in the cylindrical waveguide. One, some or all of the one or more intermediate gratings may be linear. The one or more intermediate gratings may be arranged to redirect, diffract and / or split the light prior to the outcoupling optics. However, the relative angles of the light rays from the same pixel advantageously remain the same. The intermediate optical linear gratings advantageously maintain the angular properties of the propagating light (TIR condition, with all light rays from the same pixel incident on the cylindrical waveguide surface at the same angle relative to the surface normal and at the same angle relative to a plane orthogonal to the cylinder axis), thereby allowing a two-dimensional pupil expansion without aberrations.

[0126] The outcoupling optics may include an outcoupling diffraction grating having one or more of the following features: an internal grating angle arranged to refract the received light; a variable diffraction efficiency along the length of the outcoupling diffraction grating; and a switchable diffraction grating configuration (e.g., allowing modulation of the output light). Optionally, the incoupling diffraction grating may have a switchable diffraction grating configuration.

[0127] 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 more) cylindrical waveguides may be stacked. Some or all of the plurality of cylindrical waveguides may have a common cylindrical axis. In all of these cases, the input optical device may be arranged to cause some of the received light to enter each of the plurality of cylindrical waveguides, so that for each cylindrical waveguide, all light rays originating from the same pixel of the image source are incident on the surface of the corresponding cylindrical waveguide at the same angle relative to the surface normal and the same angle relative to the plane orthogonal to the corresponding cylindrical axis at each incident point, whereby the coupled-in light maintains its direction angle as it propagates along the corresponding cylindrical waveguide. Beneficially, the outcoupling optical device may be arranged to focus the light propagating along each cylindrical waveguide at different focal points. For example, the outcoupling optical device may be arranged to focus the light propagating along the first cylindrical waveguide at a first focal point, and to focus the light propagating along the second cylindrical waveguide at a second focal point different from the first focal point. Embodiments are conceivable with multiple image sources that are advantageously vertically displaced relative to each other.

[0128] All features disclosed herein may be combined in any combination (except combinations in which at least some 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-essential combinations may be used alone (not in combination).

Claims

1. An optical system comprising: a cylindrical waveguide having a first major surface and a second major surface, the first major surface and the second major surface being concentrically opposed and defining a common cylindrical axis; input optics comprising a collimating input mirror and an in-coupling diffraction grating; as well as Image source; wherein the input mirror is arranged proximate to a first surface of the waveguide and the image source is arranged proximate to a second surface of the waveguide such that light from the image source passes through the waveguide, is reflected by the mirror, is coupled into the waveguide by the incoupling diffraction grating, and propagates along the cylindrical waveguide about the common cylindrical axis, wherein the input optical device is arranged to receive light from the image source, reflect and diffract the light from the image source into the waveguide, so that all light rays originating from the same pixel of the image source are incident on the surface of the cylindrical waveguide at the same angle relative to the surface normal and the same angle relative to the plane orthogonal to the axis of the cylinder at each incident point, so that the coupled-in light maintains its direction angle while propagating along the cylindrical waveguide, and Wherein the input reflector is arranged to collimate the received light only in a single plane passing through the common cylindrical axis.

2. The optical system according to claim 1, wherein The incoupling diffraction grating is a linear diffraction grating having a constant period and arranged to diffractively couple light into the waveguide.

3. The optical system according to claim 1, wherein: The in-coupling diffraction grating is made of a flexible holographic material.

4. The optical system according to claim 3, wherein: The flexible holographic material is a photopolymer.

5. The optical system according to claim 1, wherein: The input mirror is a concave mirror.

6. The optical system according to claim 5, wherein: The input mirror is a cylindrical concave mirror.

7. The optical system according to claim 5, wherein: The input mirror has focusing capability only in the vertical plane.

8. The optical system according to claim 1, wherein: The input optical device is configured to arrange all light rays originating from the same pixel of the image source that are incident on the surface of the cylindrical waveguide at an angle relative to a plane orthogonal to the cylindrical axis so that the light rays propagate through the cylindrical waveguide in a direction parallel to the cylindrical axis or a direction perpendicular to the cylindrical axis or a direction defined by a vector parallel to the cylindrical axis and perpendicular to the cylindrical axis.

9. The optical system of claim 1 , further comprising the image source and / or image source mounting device, wherein the image source and / or the image source mounting device defines a position of a central pixel of the image source, and wherein an optical path length between the position of the central pixel of the image source and the cylindrical waveguide is substantially the same as a radius of curvature of the cylindrical waveguide.

10. An optical display device comprising the optical system according to any one of claims 1 to 9.

11. A head-mounted display comprising the optical display device according to claim 10.

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