Optical system with a cylindrical waveguide

Abstract

An optical system, comprising: a cylindrical waveguide having concentric inner and outer surfaces that define a common cylindrical axis; and an input optical device arranged to receive light from an image source and direct the light into the cylindrical waveguide such that, at each point of incidence, all the light rays from the same pixel of the image source are incident on the surface of the cylindrical waveguide at the same angle with respect to the surface normal and at the same angle with respect to a plane orthogonal to the cylindrical axis, whereby the coupled-in light maintains its angular direction 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 the application number 2021800645086, the application date of September 17, 2021, and the invention title of "Optical System with Cylindrical Waveguide". Technical Field

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

[0003] In the field of augmented reality, a transparent combiner is used to display a virtual image to a user in a manner superimposed on the real world, and the transparent combiner redirects the image from a projector to the user's eyes. Current solutions typically use a flat or planar transparent waveguide made of a glass or plastic substrate, where light is in-coupled into the waveguide via a diffraction grating (or the like), and travels through the interior of the waveguide (by total internal reflection) to a similar out-coupler, where the light is out-coupled to the user's eyes. In this field, the thickness of the waveguide is typically on the order of a few millimeters, and they are also referred to as "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 a ray of light 60 from a point on the object display 10. These lights are incident perpendicular to the surface of the waveguide. Also shown is the path of a single ray 70 passing through the waveguide 30 and reaching the user's eye 80 via the out-coupler 50. The thickness of the waveguide 30 is on the order of a few 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 micrometer.

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

[0006] The collimated light is coupled into the waveguide by the in-coupler 40, which can be a linear diffraction grating or a holographic grating (other options include refractive optical devices such as prisms). It diffracts the light into the glass or plastic waveguide or substrate 30 at an angle 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 operated interchangeably. For the purposes of ray tracing, it is the lateral component of the grating period of the grating k-vector or momentum vector that is important. Specific material choices, production methods, and cross-sections may affect diffraction efficiency, diffraction order, mechanical flexibility, and wavelength bandwidth, but do not affect aberration and resolution. When a linear grating receives collimated light, the linear grating diffracts but maintains that collimation, i.e., all rays from a given pixel change direction but remain parallel to each other.

[0008] When light impinges on the outer surface of the waveguide, due to total internal reflection (TIR), the light reflects back inside. These angles are determined by the refractive index n of the waveguide. Typical values are n = 1.5, the critical angle is 42 degrees, and the guiding angle with respect to the normal may be 60 to 70 degrees. Glass with a higher refractive index will allow a larger field of view (FOV) due to a smaller critical angle.

[0009] One advantage of this method is that all rays from a given pixel propagate within the flat waveguide at the same angle. Thus, it is allowed for the rays to overlap or swap inside the waveguide without causing the problem of ghosting or double images.

[0010] This may be the key to achieving the main function of the waveguide: pupil replication. In the field of augmented reality (AR), it is highly desirable to have smaller optical devices. This means using small projectors. Small projectors produce small pupils (essentially a beam of rays from the same pixel), which can be identified, for example, by looking at the small pupil of a common binocular microscope (the small bright spot floating above the eyepiece).

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

[0012] This makes the task of extracting light through the output grating robust because the output grating does not need to be located at a specific position. The exit pupil of the waveguide can be regarded as a continuous pupil extending over the entire output grating 50. Similarly, the extended pupil does not move along the output grating as a function of the field (the position 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 another (first through an intermediate linear grating and second through an output linear grating). In total, three or more gratings will be used, as implemented in the waveguides of the HoloLens (RTM) sold by Microsoft Corporation and the waveguides sold by WaveOptics, Ltd. The limitation of this method is that the glass of waveguide 30 must be flat, while consumers are accustomed to curved glass. It is also difficult to combine a flat waveguide with prescription glasses that are curved without making the device larger. There are also applications that use highly curved glass, such as the visors of motorcycle helmets, military visors, fighter pilot helmets, diving masks, fire insulation masks, medical protection, welding masks, aircraft windshields, automotive or motorcycle windshields, etc. Attempting to mount a flat waveguide in the limited space between the user and the mask is challenging.

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

[0015] The use of curved waveguides has been proposed in a variety of 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, US-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 discussed in Kalinina, Anastasiia and Andrey Putilin's "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 as well as 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 documents simply mention curved waveguides. Some of the other documents discuss curved waveguides in more detail, but none of them explain how to replicate the pupil in a continuous manner without the light rays from the same pixel diverging when projected to infinity. For these methods, usually only a single pupil propagates through the waveguide, which does not allow pupil expansion. Other designs rely on eye tracking. These designs typically produce distorted outputs and require complex software to pre-distort the input image. Therefore, such solutions are both expensive and complex to manufacture.

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

[0021] (cylindrical) waveguides. This seems to be due to the difficulty of avoiding highly distorted images (resulting in poor quality) when outputting to the user while allowing light to propagate through the waveguide. It remains a challenge to mitigate this problem while implementing a product that can be manufactured for the mass market. Summary of the Invention

[0022] In this context, 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 are provided. Other preferred and optional features are defined in other claims and elsewhere in this disclosure.

[0023] The optical system contemplates the use of a cylindrical waveguide (or cylindrical waveguide or cylindrical surface waveguide) having concentric (inner and outer) surfaces. Using a cylindrical waveguide of this structure can allow light to propagate between the curved surfaces without aberration. This is advantageously implemented with an input optical device arranged to receive light from an image source (in particular a pixelated image source or an image source having a light output that can at least theoretically be decomposed into pixels) and provide the light to the cylindrical waveguide. All the light rays from the same pixel of the image source enter the cylindrical waveguide at the same angle with respect to the incident surface normal at each point of incidence. Thus, all the light rays from the central pixel of the image source enter the cylindrical waveguide orthogonally to the cylindrical surface at any point on the cylindrical waveguide. The light rays from non-central pixels enter at the same angle with respect to the surface normal at each point of incidence. In addition, all the light rays from the same pixel of the image source enter the cylindrical waveguide at the same angle with respect to a plane orthogonal to the cylindrical axis at each point of incidence. Thus, the propagation direction of all the light rays remains unchanged.

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

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

[0026] 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 input optical device. A mounting structure configured to be worn by the user (e.g., forming part of headphones, a mask, optical glasses) may be used to position the optical system such that at least the output optical device presents light as an image to the user's eye. In a preferred embodiment, the cylindrical waveguide is integrated, embedded, or fixed with the glasses or the mask component of the mounting structure.

[0027] The specific surface of the input optical device 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 input linear diffraction grating is advantageously applied to a curved surface. A preferred implementation of such an input linear diffraction grating uses a flexible holographic material. The input linear diffraction grating may 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 an input grating (or at least one grating of this form) may be considered. For example, the waveguide does not need to be entirely cylindrical, such that only a portion of the waveguide is cylindrical with the above-described characteristics. Light may enter the waveguide through another portion, and thus this other portion may form part of the input optical device.

[0028] Unlike conventional collimating optical devices for flat waveguides, the input optical device proposed in this disclosure cannot be exactly called a collimator in terms of classical optical design terminology because the light rays from the pixels are not parallel. More precisely, the preferred coupling-in (projection) optical device includes such 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 nor completely inclusive of) 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. Typically in the best practices of optical design, the central pixel light rays will advantageously be incident at an angle orthogonal to the waveguide surface, which, due to symmetry, makes aberration management easier. The other pixels in this plane will produce light rays that are incident on the input grating at other angles but will be parallel to other light rays from the same pixel.

[0029] However, in a plane orthogonal to the cylinder axis (such as a horizontal plane), the light may not be collimated but shaped such that the light rays from the same pixel have the same angle of incidence with respect to the surface normal, where the corresponding normal is considered separately for each different point of incidence. All surface normals point towards 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. Then, the light rays from the central pixel will propagate radially from the cylinder axis of the waveguide and approach the surface at normal incidence. This wavefront is advantageously formed by positioning the image source (display) such that the center of the image source is on the waveguide axis. Then, the cylindrical lens or cylindrical mirror is arranged to have optical focusing ability only in the vertical plane.

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

[0032] Light can propagate through a cylindrical waveguide (e.g., between an optical input device and an optical output device) in a direction parallel to the axis of the cylinder (e.g., vertically), defined by vector, or in a direction perpendicular to the axis of the cylinder (which can be horizontal, especially around the circumference of the waveguide), defined by vector, or in a direction defined by a vector between parallel and perpendicular to the axis of the cylinder (typically diagonal).

[0033] The optical output device generally includes an output diffraction grating. In principle, a linear grating can be used. However, in the practical application of the waveguide, the observer is inside the cylinder. Therefore, in this case, a simple linear grating may not be suitable for extracting light because it focuses the light in the horizontal plane at the axis of the cylinder (producing vertical light rays), rather than farther in front of the observer. Instead, a diverging lens characteristic 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 a prism function and a cylindrical negative lens function. There are many known examples of how to record such a grating. Compared with the input grating, such a grating will not be called "linear". By adding more focusing in two planes, the output grating can be selected to place the digital image at any distance from the observer.

[0034] In the case where the cylindrical waveguide is embedded in a head-mounted structure (e.g., prescription glasses) that has corrected the user's eyesight, the input grating and the output grating can also be corrected to take this into account. The main factor is to maintain the above state of the light when the light propagates in the waveguide, regardless of how the light approaches and leaves the entire "sandwich" stack.

[0035] In a preferred embodiment, the image source and / or the image source mounting device can be located on one side (closer to the axis of the cylinder) of the inner surface of the cylindrical waveguide, where the optical input device is located between the image source and the inner surface of the cylindrical waveguide. Alternatively, as an alternative, the image source and / or the image source mounting device can be located on one side (the distal end away from the axis of the cylinder) of the outer surface of the cylindrical waveguide. Then, a mirror (as described above, which can also act as an input wavefront shaping device) can be arranged to receive light from the image source and reflect the received light towards the cylindrical waveguide. In such an embodiment, the light from the image source can pass through the cylindrical waveguide before reaching the mirror.

[0036] An intermediate grating (or more than one intermediate grating) in a cylindrical waveguide can redirect, diffract, and / or split light before the light is coupled out by an optical device. Advantageously, each of the one or more intermediate gratings can be a linear grating to maintain the angular characteristics of the propagating light (TIR condition, and all the light rays from the same pixel are incident on the cylindrical waveguide surface at the same angle with respect to the surface normal and at the same angle with respect to the plane orthogonal to the cylinder axis), thereby allowing the two-dimensional pupil to expand without aberration.

[0037] Multiple cylindrical waveguides can be used. Then, the light-coupling-out optical device can create different focal points for the light from each waveguide.

[0038] Various combinations of the features described above and elsewhere in this document can also be considered. Methods of manufacturing and / or operating an optical system or an optical display device can also be considered. These can have structural features corresponding to those discussed herein. Brief Description of the Drawings

[0039] The present disclosure can be implemented in various ways. Preferred embodiments will now be described only by way of example and with reference to the drawings, wherein:

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

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

[0042] Figure 3 A cylindrical waveguide for the present disclosure is schematically shown Figure 2 characteristics of the cylindrical waveguide;

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

[0044] Figure 5a An embodiment according to Figure 4 is shown in perspective view;

[0045] Figure 5b An embodiment of Figure 5a is shown in top view;

[0046] Figure 5c An embodiment of Figure 5a is shown in side view;

[0047] Figure 6 A schematic top view of a cylindrical waveguide and a simplified light-coupling-out optical device for an embodiment of the present disclosure is shown;

[0048] Figure 7Shows an example flowchart of an operating process according to the present disclosure; and

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

[0050] Throughout the disclosure, the same reference numerals are used to denote the same features in different drawings. Detailed Description

[0051] The novel 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 may include matching the curvature of the waveguide to the wavefront of the input light. The manufacturing and calibration of the system are also simple and inexpensive. In addition, the cylindrical shape of the waveguide better fits inside the shape of prescription glasses because it is easier to assemble a cylindrical waveguide between spherical surfaces than 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 an air gap or a refractive index change (see, for example, US-2018 / 0348527A1, which is hereby incorporated by reference in its entirety).

[0052] First, referring to Figure 2 , a cylindrical waveguide for the present disclosure is schematically shown. Illustrated are: 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. Illustrated is 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. Obviously, R2 = R1 + t, and this 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 radii of the inner and outer surfaces of the cylindrical waveguide 120 are separated by the thickness (t) of the waveguide. This simple concentric shape of the cylindrical waveguide 120 makes the manufacturing cost for mass production lower and the tolerances easier to control.

[0053] Next, referring to Figure 3 , which schematically shows the Figure 2Characteristics of the cylindrical waveguide are also shown: a cylindrical lens 135; and a diffraction grating 140. Light rays 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) are collimated by focusing in a plane by the cylindrical lens 135, which can be referred to as wavefront shaping. The light rays that have undergone this wavefront shaping are then coupled into the cylindrical waveguide 120 at an angle β by the diffraction grating 140. The incident light rays are orthogonal to the surface of the input grating at each point along the grating surface. The angle β is set such that the angle between the normal and the internal light ray 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 a slightly different angle (e.g., β + 1), but this angle is constant at each point along the grating surface.

[0054] The paper by DeHoog et al. cited above (“Field of view of limitations in see-through HMD using geometric waveguides”) has shown that if (and only if) the two surfaces of the cylindrical waveguide are concentric, as Figure 2 and Figure 3 shown by the cylindrical waveguide 120, then after the light rays bounce once on the inner surface, the angle of incidence on the outer surface is the same. Therefore, the light rays will maintain their direction angles as they propagate along the waveguide, which is different from the case of choosing any other surface radius. As Figure 3 shown, if the light rays bounce off the outer surface at an angle α, then it will form an angle β with the inner surface, but then form an angle α at the next bounce on the outer surface. In other words, every other bounce on the waveguide surface, the angle of incidence inside the waveguide is the same. Light rays incident from the left, middle, or right of the grating from a pixel will approach the inner surface at the same angle throughout the waveguide after bouncing off the outer surface. Pixels other than the central pixel will have different α and β values, but the above relationship still holds. This characteristic allows pupil replication.

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

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

[0057] It should be understood here that these aberrations do not accumulate with the number of bounces. For example, if rays from a given pixel diffract into the waveguide at 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 remain at a resolution of + / - 0.1 degrees, regardless of how these rays are exchanged or shuffled. This is in contrast to waveguides where aberrations accumulate with transmission.

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

[0059] Preferably, the input optics include an input coupling optic configured to couple light into the surface of the cylindrical waveguide (in a preferred embodiment, the inner surface). Advantageously, an output coupling optic is arranged to receive the light propagating along the cylindrical waveguide and present the light as an image to the object. Optionally, the optical system can include an image source (a display, e.g., a microdisplay) and / or mounting means for the image source (image source mounting means).

[0060] Other aspects in accordance with the present disclosure may be contemplated. 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 an input optical device; and a mounting structure, particularly a mounting structure configured to be worn by a user. The mounting structure may position the optical system such that the output optical device presents the light as an image to the user's eye. For example, the mounting structure may include a spectacle frame, a face mask, or a helmet structure or other wearable mounting structure. Optionally, a head-mounted display including such an optical display device may be contemplated. In such a case, the mounting structure may include spectacle or face mask components. Advantageously, the cylindrical waveguide is integrated, embedded, or fixed together with the spectacle or face mask components.

[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 an optical system may include: providing a cylindrical waveguide having concentric inner and outer surfaces; and arranging an input optical device (e.g., including an input optical device) to receive light from an image source and direct the light into the cylindrical waveguide such 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 with respect to the surface normal and at the same angle with respect to a plane orthogonal to the axis of the cylinder. The method may further include configuring an output optical device to receive the light propagating along the cylindrical waveguide and present the light as an image to an object. Similarly, a method of operating an optical system may include: directing light from an image source to the input optical device of the cylindrical waveguide such that all light rays originating from the same point on the image source are incident on the cylindrical waveguide at the same angle with respect to the surface normal and at the same angle with respect to a plane orthogonal to the axis of the cylinder. Preferably, the light propagating along the cylindrical waveguide is presented as an image to an object (by the output optical device).

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

[0063] Now referring 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 page plane (and any plane parallel thereto) is considered horizontal, while the vertical plane extending from the page (and any plane parallel thereto) is considered vertical. Thus, the cylindrical waveguide 120 can be visualized as, for example, a cylindrical face mask located on a flat surface. The output diffraction grating 150 is also shown in this simplified figure. The image source 110 is placed at a distance from the cylindrical waveguide 120 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, particularly adding power in the vertical plane. It is then input to match the linear diffraction grating 140, which, when applied to the curved surface of the cylindrical waveguide 120, propagates the image-bearing light along the rotationally symmetric waveguide (i.e., the image of the waveguide can be rotated around the cylindrical axis, while the light rays 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 a negative focusing power (i.e., divergence) to the light in the opposite plane (horizontal plane) to provide an extended eyebox to the user (not shown), with the output of this eyebox set at infinity. This allows pupil replication to expand the output eyebox.

[0064] Therefore, this design generates a wavefront within the cylindrical waveguide 120 that can propagate rotationally symmetrically along the entire waveguide. The spherical wavefront emitted from the point source 110 conforms to the curvature of the waveguide 120 itself in the horizontal plane (as described above, Figure 4 the plane of the page), and the power is increased in the vertical plane (as described above, in the plane in the Figure 4 direction extending from the page) by the cylindrical lens 135. This means that after two bounces, the angle of the light ray maps back to itself and repeats infinitely. Thus, the pupil can be replicated (extracted multiple times) to expand the eyebox one-dimensionally (horizontally). The size of the vertical eyebox is set by the vertical dimension of the cylindrical lens 135. It also relaxes the alignment tolerance (since the position of the coupler relative to the input pupil is not critical), the choice of waveguide thickness, and the central ray guiding angle.

[0065] Next, referring to Figure 5a , a perspective view of an embodiment according to Figure 4 is shown. In Figure 5b , a top view of the embodiment of Figure 5a is shown, and in Figure 5c , the embodiment of Figure 5aSide view of an embodiment. Light 101 collimated in a plane is coupled into a cylindrical waveguide 120. Light ray 102 propagates through waveguide 120 and is coupled out by diffraction grating 150 to provide projection light 103 to user's eye 160.

[0066] In this specific embodiment, image source or object 110 is a microdisplay, and the distance between it and cylindrical waveguide 120 is the same as the radius of curvature of the waveguide. This is achieved by physically positioning image source 110 at this distance or by optically (e.g., using a lens) virtually arranging image source 110 at this distance. The image is collimated in a plane by a cylindrical plano-convex lens 135 onto an input diffraction grating 140 located on the concave inner surface of cylindrical waveguide 120. Cylindrical lens 135 is oriented such that the focusing power of cylindrical lens 135 is in a plane opposite to cylindrical waveguide 120. For example, if the curved waveguide is horizontally oriented (oriented like a helmet visor or placed 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. Cylindrical lens 135 is then oriented in the opposite vertical plane and has optical power in the vertical plane and focuses light from a point source at the radius of curvature into a horizontal line.

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

[0068] The focal length of cylindrical lens 135 determines the magnification of the object, and lens 135 is set at a distance of one focal length from object 110. If cylindrical lens 135 is placed close to cylindrical waveguide 120, then cylindrical lens 135 will have a focal length approximately equal to the radius of curvature of the waveguide. For example, a typical radius of curvature of a visor-shaped waveguide is 200 mm, which means the object is 200 mm away and 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 by optically setting the object distance virtually with a lens.

[0069] The choice of the cylindrical lens 135 (diameter and / or focal length) determines the size of the vertical eye movement range (determined by the diameter of the lens), and the focal length determines the magnification of the display 110 and thus 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 (such as those used in cameras), and the multi-element lens provides good image quality (small spot size RMS over the entire FOV). This is particularly desirable for a pupil replication system to precisely overlap the pupils and provide a high-resolution image. Although a monochromatic solution is possible, for a full-color microdisplay, the lens system can ideally be achromatic. The FOV of the curved waveguide 120 can be determined to a large extent 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 paper by DeHoog et al. cited above ("Field of view of limitations in see-through HMD using geometric waveguides").

[0070] The coupled-in diffraction grating 140 is a linear grating that has equal surface spacing (pitch) between the grating lines (or equivalently, equal fringe spacing in a volume holographic grating). The grating can be fabricated by lithography or interference methods. All the light rays are collimated in one plane and orthogonally incident (at 90 degrees to the surface) over 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 bent waveguide generally means that due to the bending of the waveguide, collimated light does not impinge orthogonally across the entire width of the grating. Usual solutions to this include changing the pitch of the grating to compensate for this, or directly recording a hologram 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 input grating 140 is fabricated as a planar linear grating on a flat substrate (as is well known in the art and is relatively inexpensive and simple to fabricate compared to variable gratings). The grating 140 can be fabricated on any flexible holographic material, such as on a photosensitive polymer (e.g., Bayfol(RTM) sold by Covestro AG, or a silver halide film), and then attached (laminated) to the cylindrical surface of the waveguide, conforming to the cylindrical surface. The grating 140 is refractive-index matched, preferably by lamination (or another refractive-index matching glue or liquid), such that the grating conforms to the shape of the cylindrical surface and preferably such that there is no air gap. Recording a hologram on a flat substrate and then removing the flexible holographic material and laminating the flexible holographic material on a planar or cylindrical substrate (bent in only one dimension) is simple and inexpensive, while recording on a curved surface or laminating on a spherical surface (bent in two dimensions) is more difficult.

[0072] A slanted grating can also be etched and embossing or ultraviolet-curing resin techniques can be used. Then, the grating can be transferred to the cylindrical waveguide.

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

[0074] For a planar waveguide, both couplers are generally linear and identical. The system is like a periscope, superimposing a magnified image of the microdisplay over the real world for the observer. The overall design of the system means that the position pixel information of the display is converted from collimated to angular information through collimation and then back to position information on the human retina.

[0075] Traditionally, for a flat waveguide, the input and output gratings are linear, parallel, and have the same period to eliminate chromatic aberration. This is important for using a broadband light source, such as a light-emitting diode (LED). The output grating according to the present disclosure has a variable period along the waveguide and does 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. The narrowband light source can help minimize chromatic aberration. They can also limit the FOV of the output image, but a thin volume holographic grating can be used to mitigate this effect. For example, the typical thickness of the volume holographic grating can be at least 3 microns and at most 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 can form part of a head-mounted structure (e.g., prescription glasses) that has corrected the user's vision. Then, the input grating 140 and the output diffraction grating 150 can also take this into account without affecting the propagation of light inside the waveguide and its effective output coupling.

[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 according to 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 similarly incorporate astigmatism in both the perspective and the digital image to accommodate 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 focusing in two planes. In the vertical plane, the light coupled into the waveguide is collimated, but the light output from the output grating does not need to be collimated. To allow multi-pupil extraction, only a portion of the light can be extracted from the first part of the output grating 150. To balance the uniformity of the light extracted across the coupling grating 150, the distal end (vis-à-vis the input grating 140) of the output grating 150 preferably has a higher efficiency than the proximal (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 significance of the aspects discussed above, it can be understood that the input optical device may include an input linear diffraction grating, which is arranged to couple the received light into a cylindrical waveguide. This advantageously has a constant period. The input linear diffraction grating is advantageously applied to a curved surface, particularly a curved surface with a refractive index matching thereto (without an air gap).

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

[0082] In yet another aspect, a method of manufacturing a curved linear diffraction grating can be considered. The method includes: forming a linear diffraction grating on a planar surface; and attaching the linear diffraction grating to the 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) can be similarly applied to this aspect.

[0083] Now, additional features applicable to all aspects are discussed. For example, the input linear diffraction grating can be made of a flexible holographic material. Optionally, the input linear diffraction grating is attached to (and / or conforms to) the inner surface of the cylindrical waveguide. In a preferred embodiment, the input linear diffraction grating has a straight grating. The grating thickness and angular bandwidth of the input linear diffraction grating can be configured to have (substantially) uniform visible range color transmission across the width of the input linear diffraction grating.

[0084] Preferably, the input optical device includes a wavefront shaping device (which can be considered a form of collimator), which is configured to collimate the received light in only a single plane or conform the received light. More preferably, the (input) wavefront shaping device is configured such that a single plane passes through the cylindrical axis of the cylindrical waveguide. For example, the wavefront shaping device can be of a cylindrical 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. Now, additional specific examples according to the present disclosure are described.

[0086] Referring to Figure 6 , which shows a schematic top view of a cylindrical waveguide 120 and a simplified view of an output optical device including an output diffraction grating 150. Similar to Figure 4 , the page plane (and any plane parallel thereto) in this figure is considered horizontal, while the vertical plane (and any plane parallel thereto) extending from the page is considered vertical. Thus, the cylindrical waveguide 120 can be visualized as, for example, a cylindrical face mask located on a flat surface. A cylindrical negative lens 155 is also shown in this simplified view, which will be discussed further below. The light rays 151 output from the output diffraction grating 150 are collimated in the vertical plane and focused in the horizontal plane, as shown by line 152. The light rays 156 output from the cylindrical negative lens 155 are collimated in both the horizontal and vertical planes and have an infinite focal length. This is a simplified view because the cylindrical negative lens 155 is actually optically integrated within the output diffraction grating 150. Thus, the optical power is contained in the hologram within the output coupler to compensate for and achieve collimation in both the horizontal and vertical planes at the output. Thus, the output diffraction grating 150 acts as a cylindrical lens to compensate for the cylindrical curvature introduced by the input optical device and, as described below, in this way, substantially collimates the image at infinity.

[0087] Due to the asymmetric collimation at the input optical device, the output optical device has different focal positions for the horizontal (near) output image plane and the vertical (far or infinite) output image plane to provide an image focused at infinity in both planes, thereby providing a high-quality image to the observer. This compensation is achieved by encoding the optical power into the output grating. This method is known in the art but not for this purpose. As Figure 6 shown, this is equivalent to placing a diverging cylindrical lens 155 (a plano-concave cylindrical lens; if the radius of curvature of the waveguide is 200 mm, then the focal length of this lens will be -200 mm) with a negative optical power equal to the radius of curvature of the waveguide between the planar output grating and the user. The 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, then the output compensation lens / grating will focus (or have optical power) in the horizontal plane to produce a spherically collimated output. As described above, the center of the output grating 150 has the same surface pitch (also referred to as the 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 typically the desired usage scenario as it means that when the user focuses on a distant object in the real world, the virtual image will appear in focus, which is typical, for example, when a fighter pilot or motorcycle rider uses a visor. Consumer electronic devices using a planar waveguide with pupil expansion also have an image 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 coupler. While it is desirable for the input grating 140 to have a maximum diffraction efficiency (meaning that most of the light incident thereon is coupled into the waveguide), the output grating 150 can have a low or variable efficiency, allowing for pupil expansion. A small portion of the light is coupled out during 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 during the second interaction, and so on. This allows for an expanded eye movement range in the horizontal plane.

[0090] As is known in the art, a holographic waveguide grating (linear coupler or dynamic coupler) can be fabricated by exposing a holographic material to two coherent light beams, where the waveguide beam is coupled into the holographic material through 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. As an alternative, three separate layers can be stacked, one for each color.

[0091] By multiplexing multiple gratings into a single holographic layer, the FOV provided to the user can be increased, on which a uniformly bright and color-uniform image can be seen. This can be achieved by changing the recording angle. As an alternative, 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 only based on the lateral component of the grating pitch. The lateral (cross-sectional) pitch or period can be conveniently selected to accommodate the geometry of the reflection or transmission grating. It can also be understood that the above linear grating means being linear in the lateral direction, but can have variable transverse characteristics.

[0093] The collimation characteristics of the output light mean that a large eye relief (i.e., the distance at which the eye can optimally see the image behind the output surface) can be achieved. This is typically desirable, especially for applications with a helmet visor rather than glasses. The larger the eye relief, the smaller the FOV typically is.

[0094] The real-world scene is largely unchanged by the bending of the waveguide. The normal radius of curvature of the glasses is 250 mm, and the normal radius of curvature of the face mask is 150 - 200 mm. Any curvature greater than 100 mm (which is the case here) will not be noticed by the user as distorting the real world. 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 optical device can be considered to include an outcoupling diffraction grating. In particular, the outcoupling diffraction grating can be configured to act as a cylindrical lens (e.g., focusing in only one dimension). Additionally or alternatively, the outcoupling linear diffraction grating can have a curved grating. In a preferred embodiment, the outcoupling linear diffraction grating can: have an internal grating angle that is arranged to collimate the received light in a plane or focus the received light in the tangential plane and the sagittal plane at a predetermined distance, and / or the output or diffraction efficiency at the end of the outcoupling diffraction grating closest to the light received from the input optical device does not exceed 25% (optionally, 20%, 15% or 10%).

[0096] The outcoupling optical device can 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 optical device can include a cylindrical negative lens. Preferably, these aspects are integrated in the outcoupling diffraction grating.

[0097] The outcoupling linear diffraction grating can have the same surface pitch as the incoupling linear diffraction grating. In some embodiments, the outcoupling linear diffraction can have an internal grating angle with an orientation opposite to that of the incoupling linear diffraction grating. This is especially used in the case where the light from the image source and the light reaching the observer (or the incoupling optical device and the outcoupling optical device) are on the same side of each other. This can be referred to as a "U" grating. Alternatively, the angles of the incoupling grating and the outcoupling grating are not opposite in orientation, and at least some of the light will be outcoupled on the opposite side of the incoupled light (in other words, the observer will be on the other side relative to the incoupled light). This can 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 about 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. Then, each ray of light will be deflected by the same angle, causing the light field to be symmetric about the axis of rotation. As described above, the rays of light emitted between the cylindrical surfaces will maintain two angles of incidence on the two surfaces under any number of reflections. This means that after every two reflections, the wave will coincide exactly with itself. Thus, no ghosting will occur. Such a light field can propagate any distance without any ray of light becoming distinguishable from the others.

[0100] In addition, pupil replication at the output grating is achieved. Partial coupling - out of the light occurs in the initial interaction with the coupling - out optical device, and the remaining light propagates and is coupled - out in the next interaction. In this case, the different interactions match perfectly and no ghosting is caused. When all the rays of light reach the coupling - out grating, regardless of the position of the grating or the position of any one ray, the grating can diffract the rays out of the waveguide because they will all arrive similarly.

[0101] A linear coupling - out grating with the same period as the coupling - in grating diffracts the rays of light again into a new cylindrical wavefront. It is well - known that diffractive optical elements can combine several functions in a cumulative manner. Like a cylindrical lens, the coupling - out grating also has one - dimensional focusing ability. This will convert the diffracted light into collimated light. An observer receiving such rays of light will see a star - like point at infinity.

[0102] The above explanation can also apply to other pixels. As described above, the wavefronts from these other pixels do not have to be exactly cylindrical. This is because the rays of light from non - central pixels are incident on the coupling - in optical device at angles slightly different from the "perfect" normal (perpendicular) angle. However, by hitting the coupler at angles that are essentially the same with respect to the surface normal at their respective intersection points, the resulting rays of light will form a ray field that is rotationally symmetric about the cylinder axis and propagate in an indistinguishable manner.

[0103] Using modern optical design, a projector can be designed to form such a light field with small errors, ideally an error of 1 arc minute (human visual acuity).

[0104] Refer to Figure 7, which shows an example flowchart of an operation process according to an embodiment of the present disclosure. In the display step 210, a dynamic image is displayed on an image source 110 (e.g., a microdisplay). In the first collimation step 220, an image-carrying light is collimated in a vertical plane by a cylindrical lens 135 having optical power in the vertical plane (or equivalently, a cylindrical mirror as will be discussed below). In the first incidence step 230, the image-carrying light (for a single pupil) is incident on the cylindrical waveguide 120 orthogonally to the surface (usually the inner surface) of the cylindrical waveguide 120. In the coupling-in step 240, the image-carrying light is coupled in via a linear diffraction grating 140 at a constant angle (for all rays from the same pixel). Since the cylindrical waveguide 120 has concentric inner and outer surfaces, in the propagation step 250, the coupled-in light propagates rotationally symmetrically between the concentric surfaces. In the initial coupling-out step 260, a first portion of the image-carrying light is extracted in an initial interaction with the coupling-out diffraction grating 150. Then, in a further coupling-out step 270, subsequent interactions (bounces) with the coupling-out diffraction grating 150 result in further extraction of the propagating light, thus providing pupil duplication. In the second collimation step 280, the optical effect of the coupling-out diffraction grating 150 adds negative cylindrical power in the horizontal plane. Finally, in the output step 290, the user's eye 160 sees a spherical collimated image at infinity superimposed on the real world.

[0105] Although specific embodiments have been described, those skilled in the art will recognize that various variations and substitutions can be made. Specifically, the coupling-in and coupling-out optical devices can be significantly different from the specific designs described above.

[0106] 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. It is not necessary to use a cylindrical lens. For example, a cylindrical mirror can be used as an alternative, and the cylindrical mirror will potentially have all the same characteristics and benefits. The light source can also be placed after the outer surface and the cylindrical mirror for collimating the light, e.g., after the light passes through the waveguide.

[0107] Many designs can be used to optimize the performance of all pixels and minimize the volume of the projector. This includes using optical devices focused in the vertical plane to bring the display closer. For example, a concave mirror can be used to approach the light to the waveguide from the side opposite to the concave mirror before being reflected and then diffracted. This is similar to the aberration management of an afocal system in classical optics. However, the handling of these aberrations is reformulated in a cylindrical coordinate system in the present disclosure.

[0108] Referring to Figure 8, which shows a schematic top view of an optical system according to a second embodiment. In this embodiment, the cylindrical lens 135 is replaced by a cylindrical mirror 136. With such a cylindrical mirror, the image source 110 does not have to be located closer to the inner surface of the cylindrical waveguide 120. For simplicity, the input diffraction grating 140 is not shown, but it is present at the same location as in the other embodiments described. In the embodiment shown, 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 input diffraction grating remains the same as the radius of curvature of the cylindrical waveguide 120. For example, a typical radius of curvature of a 200 mm faceplate-shaped waveguide would mean that the object is 200 mm away, and the cylindrical mirror has a radius of curvature of 400 mm. However, the distance between the image source 110 and the input diffraction grating is different from this radius. Again, it can be seen that the incident light rays are orthogonal 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 thus has similar characteristics. For all other embodiments disclosed herein, all other variations and options can be applied to the embodiments according to this embodiment.

[0109] The orientation of the cylindrical waveguide can be changed. In the above embodiment, the cylindrical axis of the cylindrical waveguide is vertically oriented (so the cylindrical waveguide extends in the horizontal direction) because this is the normal way to align the faceplate. However, this is not necessary. Other directions can be considered. Additionally or alternatively, light can enter and exit the cylindrical waveguide through different surfaces (e.g., on different sides of the waveguide). The input grating and the output grating can be positioned accordingly to achieve this. In some embodiments, more than one input grating and / or output grating can be provided.

[0110] Each of the input grating and the output grating can be a reflective grating or a transmissive grating and can be placed on the inner surface or the outer surface of the waveguide (or on another surface of the waveguide). Those skilled in the art will understand these variations of the embodiments shown here.

[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 oriented along the cylinder axis (vertically). Alternatively, the light can be directed at 45 degrees or any other diagonal direction. This allows for design freedom, such as conveniently positioning the projection module at the temple (glasses). This is also meaningful for implementing an intermediate grating for 2D pupil expansion. The intermediate linear grating can redirect and / or split the light while maintaining the same corresponding angle for each redirected ray for light rays from the same pixel.

[0112] The system allows the use of lasers or LED lights, which allows for flexibility. Typically, LED light is used, such as LCOS (Liquid Crystal on Silicon) plus LED or microLED microdisplays, but if high efficiency and thus high brightness are desired, lasers can also be used. A laser beam mirror scanning system (MEMS, Micro-Opto-Electro-Mechanical System) can also be used. Lasers have some disadvantages in terms of cost, speckle (resolution loss), and eye safety.

[0113] If a refractive element (e.g., a prism) is used as an input coupler or output coupler together with a diffractive input coupler or output coupler, uncompensated dispersion may make it necessary to use a narrowband light source (e.g., a laser). Additionally, refractive couplers tend to be large and expensive.

[0114] By adding spherical optical power to the waveguide output, a virtual image can be set at different focal lengths. A pair of additional lenses (a second lens to compensate for the effect of the first lens on the real world) can also be added before and after the waveguide to set the focal length closer. Another possibility is to add an electrically addressable switchable (liquid crystal-based) holographic output grating, which 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 an angular multiplexing grating. This switching can be synchronized with a time-division multiplexed microdisplay.

[0115] Optionally, multiple focal planes can be achieved by using multiple (stacked) cylindrical waveguides. Light will propagate as described above, but exit in cases where the grating produces different foci. This method is described in detail in "Optical architectures for augmented, virtual and mixed reality headsets" (2020), B.C. Kress, SPIE press.

[0116] Further adding axially symmetric optical power (similar to the optical power of an ordinary spherical lens) to the output of the cylindrical waveguide will result in seeing the point at a finite distance (e.g., 1m).

[0117] The cylindrical waveguide can form part of a larger (overall) waveguide structure, where only a part may be cylindrical. Embodiments that do not require input optics can be considered. For example, light can enter or originate from the non-cylindrical part of the waveguide (e.g., due to an embedded image source), and wavefront shaping can be performed in this part. Thus, this part of the waveguide can form part of the input optics.

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

[0119] All of these existing techniques benefit from using collimated light transmitted by a linear grating and a flat waveguide. According to the present disclosure using a cylindrical waveguide, these techniques of splitting light and replicating the pupil by an intermediate linear grating can be implemented after the light is coupled into the cylindrical waveguide. Then, the light can finally be coupled out through a grating with a negative cylindrical focusing function.

[0120] The rotationally symmetric structure according to an embodiment of the present disclosure allows the input grating and the output grating to be placed anywhere on the concentric cylindrical waveguides. For example, like the typical planar horizontal configuration discussed above, the orientation can be vertical or at an angle to the waveguide (e.g., in a head-mounted display implementation). This allows for flexible placement of the projectors and the eye movement range positions in the final design. It also allows for the pupil replication and vertical eye movement range extension methods discussed in the previous paragraphs.

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

[0122] Further referring to the general meaning 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 to the inner surface of the cylindrical waveguide. Then, a mirror (which is preferably the wavefront shaping device) may be arranged 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 mounting device are configured such that light from the image source passes through the cylindrical waveguide before reaching the mirror. In some embodiments, corresponding portions of the input optical device (e.g., the coupling-in grating) adjacent to the cylindrical waveguide and the output optical device (e.g., the coupling-out grating) adjacent to the cylindrical waveguide are located on opposite sides of the cylindrical waveguide.

[0124] In some embodiments, the input optical device further includes one or more spherical lenses. Additionally or alternatively, the output optical device further includes one or more spherical lenses. The spherical lenses may be used to change the optical path length of the light and / or change the focusing of the light. In an embodiment, the input optical device may further include a waveguide portion integrated with the cylindrical waveguide. Advantageously, the waveguide portion forming at least a part of the input optical device is non-cylindrical (non-cylindrical) and / or does not have concentric surfaces. In some embodiments, only a part of the waveguide shape is 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 before the output optical device. However, the relative angles of the light rays from the same pixel advantageously remain the same. The intermediate optical linear grating advantageously preserves the angular characteristics of the propagating light (TIR condition, and all light rays from the same pixel are incident on the surface of the cylindrical waveguide at the same angle with respect to the surface normal and at the same angle with respect to the plane orthogonal to the cylindrical axis), thereby allowing aberration-free two-dimensional pupil expansion.

[0126] The output optical device may include an output 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 output diffraction grating; and a switchable diffraction grating configuration (e.g., allowing modulation of the output light). Optionally, the input diffraction grating may have a switchable diffraction grating configuration.

[0127] Multiple 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 multiple cylindrical waveguides may have a common cylindrical axis. In all of these cases, the input optical device may be arranged such that some of the received light enters each of the multiple cylindrical waveguides, such that for each cylindrical waveguide, all light rays 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 at the same angle relative to a plane orthogonal to the corresponding cylindrical axis at each point of incidence, and the coupled-in light thus maintains its direction angle as it propagates along the corresponding cylindrical waveguide. Advantageously, the output optical device may be arranged to focus the light propagating along each cylindrical waveguide at different focal points. For example, the output 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 having multiple image sources may be contemplated, and these image sources are advantageously vertically shifted relative to each other.

[0128] All features disclosed herein 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 apply to all aspects of the invention and may be used in any combination. Similarly, features described in non-essential combinations may be used alone (without combination).

Claims

1. An optical system, comprising: A cylindrical waveguide having concentric inner and outer surfaces defining a common cylindrical axis; An input optical device arranged to receive light from an image source and to direct the light into the cylindrical waveguide such that all rays from the same pixel of the image source impinge on the surface of the cylindrical waveguide at each point of incidence at the same angle with respect to the surface normal and at the same angle with respect to a plane orthogonal to the cylindrical axis, whereby the coupled-in light maintains its angular direction as it propagates along the cylindrical waveguide in a direction around the common cylindrical axis.

2. The optical system according to claim 1, wherein, The input optical device includes a coupling-in optical device configured to couple the light into the surface of the cylindrical waveguide.

3. The optical system according to claim 2, wherein, The coupling-in optical device includes a coupling-in linear diffraction grating having a constant period, applied to a curved surface, and arranged to couple the received light into the cylindrical waveguide.

4. The optical system according to claim 3, wherein, The coupling-in linear diffraction grating is made of a flexible holographic material, and / or wherein the coupling-in linear diffraction grating is attached to the inner or outer surface of the cylindrical waveguide, and / or wherein the coupling-in linear diffraction grating is switchable.

5. The optical system according to claim 3, wherein the optical system meets one or more of the following: The grating thickness and angular bandwidth of the coupling-in linear diffraction grating are configured such that there is uniform visible range color transmission across the width of the coupling-in linear diffraction grating; The coupling-in linear diffraction grating has straight gratings; and The coupling-in linear diffraction grating is configured to redirect and / or split the received light into multiple groups, maintaining the same relative ray angles within each group.

6. The optical system according to claim 1, wherein, The input optical device includes a wavefront shaping device configured to collimate the received light in only a single plane.

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

8. The optical system according to claim 6, wherein, The wavefront shaping device has a cylindrical shape.

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

10. The optical system according to claim 9, wherein, The wavefront shaping device includes a multi-element lens or mirror.

11. The optical system according to claim 10, wherein, The input optical device further includes a waveguide portion integrated with the cylindrical waveguide.

12. The optical system according to claim 11, wherein, The input optical device is configured to set all rays from the same pixel of the image source impinging on the surface of the cylindrical waveguide at an angle with respect to a plane orthogonal to the cylindrical axis such that the rays propagate through the cylindrical waveguide in a direction parallel to the cylindrical axis or perpendicular to the cylindrical axis or in a direction defined by a vector between parallel and perpendicular to the cylindrical axis.

13. The optical system according to claim 12, further comprising the image source and / or an image source mounting device that defines the position of the central pixel of the image source, and wherein the optical path length between the position of the central pixel of the image source and the cylindrical waveguide is substantially the same as the radius of curvature of the cylindrical waveguide.

14. The optical system according to claim 13, further comprising: the image source and / or the image source mounting device, the image source and / or the image source mounting device being located at a position closer to the outer surface of the cylindrical waveguide relative to the inner surface of the cylindrical waveguide; and a mirror, the mirror being arranged to receive light from the image source and reflect the received light towards the cylindrical waveguide.

15. The optical system according to claim 14, wherein, The mirror and the image source and / or the image source mounting device are configured such that light from the image source passes through the cylindrical waveguide before reaching the mirror.

16. The optical system according to claim 14, wherein, The mirror is a wavefront shaping device.

17. The optical system according to claim 15, further comprising: an output optical device, the output optical device being arranged to receive light propagating along the cylindrical waveguide and present the light as an image to an object.

18. The optical system according to claim 17, wherein, The output optical device includes an output diffraction grating.

19. The optical system according to claim 18, wherein, The output diffraction grating has one or more of the following: a configuration acting as a cylindrical lens; a curved grating; an internal grating angle arranged to collimate the received light in a plane or to focus the received light in the tangential plane and the sagittal plane at a predetermined distance; an internal grating angle arranged to refract the received light; a diffraction efficiency of not more than 25% at the end of the output diffraction grating closest to the light received from the input optical device; a variable diffraction efficiency along the length of the output diffraction grating; and a switchable diffraction grating configuration.

20. The optical system according to any one of claims 17 to 19, wherein Each of the input optical devices and / or the output optical device further includes a corresponding spherical lens; and / or wherein the output optical device further includes a cylindrical negative lens.

21. The optical system according to any one of claims 17 to 19, wherein, The portions of the input optical device and the output optical device each adjacent to the cylindrical waveguide are located on opposite sides of the cylindrical waveguide.

22. The optical system according to any one of claims 17 to 19, further comprising: an intermediate linear grating in the cylindrical waveguide, the intermediate linear grating being arranged to perform one or more of the following operations: redirecting light; diffracting light; and splitting light before the output optical device while maintaining the property that all light rays from the same pixel of the image source are incident on the surface of the cylindrical waveguide at the same angle with respect to the surface normal and at the same angle with respect to a plane orthogonal to the cylindrical axis at each incident point.

23. The optical system according to any one of claims 17 to 19, wherein, The cylindrical waveguide is a first cylindrical waveguide, and the optical system further includes: a second cylindrical waveguide having concentric inner and outer surfaces, wherein the input optical device is arranged to cause some of the received light to enter the first cylindrical waveguide and some of the received light to enter the second cylindrical waveguide, such that for each cylindrical waveguide, all light rays from the same pixel of the image source are incident on the surface of the corresponding cylindrical waveguide at the same angle with respect to the surface normal and at the same angle with respect to a plane orthogonal to the corresponding cylindrical axis at each incident point, and the coupled-in light thus maintains its direction angle as it propagates along the corresponding cylindrical waveguide; and An output optical device, which is arranged to focus the light propagating along the first cylindrical waveguide at a first focal point and focus the light propagating along the second cylindrical waveguide at a second focal point different from the first focal point.

24. An optical display device, comprising: The optical system according to any one of the preceding claims, the optical system including an image source configured to direct light towards an input optical device; And A mounting structure configured to be worn by a user and configured to position the optical system such that the output optical device presents light as an image to the user's eyes.

25. A head-mounted display, comprising the optical display device according to claim 24, wherein, The mounting structure includes a glasses or mask component, and the cylindrical waveguide is integrated, embedded or fixed on the glasses or mask component.

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