CURVED LIGHT GUIDE, IMAGE OPTICS AND HMD

Description

[0001] The present invention relates to a curved optical fiber for a head-mounted display (HMD). The invention also relates to imaging optics for an HMD and the HMD itself.

[0002] A head-mounted display (HMD) is a display system in which an image is projected into the wearer's field of vision using imaging optics. Often, it is desirable to allow an unobstructed view of the surroundings alongside the displayed image. This precludes solutions where an opaque image source and its associated imaging optics are positioned directly in front of the eye. Therefore, in one class of HMDs, sometimes also called smart glasses, the source images are typically generated outside the eye's field of vision, and the resulting image is projected in front of the eye in the desired viewing direction using imaging optics.For this purpose, the HMD includes, for example, an optical fiber through which the imaging beam path for generating the virtual image is guided by internal reflection from an input structure located outside the field of view to an at least partially transparent output structure located within the eye's field of view. Input and output can be achieved using refractive, reflective, or diffractive structures. Diffractive input and output structures are described, for example, in DE 10 2015 122 055 A1, US 2006 / 012181 A1, and US 2010 / 0220295 A1. Examples of reflective or refractive input or output coupling structures are described, for example, in US 2012 / 00002294 A1, in DE 10 2011 007 812, in WO 2015 / 075206 A1 and WO 2015 / 075207A1 A1.

[0003] To achieve a large output pupil, US 8,320,032 B2 proposes multiple coupling. While US 8,320,032 B2 describes a planar optical fiber, US 8,830,584 B2 applies the principle of multiple coupling to spherical optical fibers. For ideal multiple coupling, in the case of a spherically concentric optical fiber, the coupling structures must lie parallel to the parallels of latitude on the sphere. This allows light to be transferred from the input structure to the output structure and coupled out multiple times. In this way, an ideal point-to-point imaging effect can be achieved at the center of the sphere. Multiple coupling is also described in DE 10 2015 122 055 A1.

[0004] In US 2013 / 0088780 A1, the principle of point-to-point imaging is used to achieve different focal planes using multiple optical fibers. However, the optical fibers must be curved away from the eye, so that they have a convex curvature as seen from the eye. For aesthetic reasons, however, the optical fibers should have a concave curvature as seen from the eye, so that the head-mounted display can take the shape of ordinary eyeglasses.

[0005] Furthermore, US patent 8,842,368 B2 describes a head-mounted display with diffractive input and output coupling, in which curved diffractive structures are used to generate a pupil image. However, this pupil image would not be compatible with multiple output coupling.

[0006] US patent 2012 / 0300311 A1 describes a head-mounted display with a cylindrically curved optical fiber and diffractive input and output coupling. To ensure that a collimated image coupled into the optical fiber can be output as a collimated image with an enlarged exit pupil, the input and output coupling gratings have variable grating spacings in the direction of curvature of the optical fiber.

[0007] DE 10 2012 213 685 A1 describes a display device in which beams coupled into a curved optical fiber are converted into collimated beams by means of an additional optic arranged between the image sensor and the optical fiber before coupling, and are coupled out of the optical fiber as collimated beams. The collimation can be performed by the coupling grating used for coupling instead of by the additional optic 9.

[0008] DE 10 2015 122 055 A1 describes a display device that includes a correction arrangement for correcting imaging errors.

[0009] German patent DE 10 2008 009 095 A1 describes a transmitted light display system in which an image is visible through a disc into which an intermediate image generated by an image sensor on a diffusing disc is coupled by means of an imaging lens. A correction lens is arranged between the diffusing disc and the imaging lens, with the aid of which the image quality is to be optimized.

[0010] German patent DE 602 05 722 T2 describes a relay lens system that projects an image generated by an image source onto a curved visor worn in front of the eye, from where the light coming from the relay lens system is reflected towards the eye. The relay lens system includes an optical element that performs chromatic correction.

[0011] WO 2008 / 023367 A1 describes a coupling module for coupling a beam path into an optical fiber. The coupling module comprises a polarization beam splitter and a delay plate.

[0012] US patent 2012 / 0300311 A1 describes a head-mounted display with a cylindrically curved light guide. The optical effect caused by the curvature of the light guide is compensated for by the input and output coupling structures. This light guide ensures that a collimated image introduced into the light guide is also coupled out as a collimated image with a magnified output pupil.

[0013] DE 10 2015 116 297 A1 describes an imaging optic comprising a spectacle lens and an optical element, also referred to as an imaging light guide, through which a light beam is coupled into the spectacle lens. The spectacle lens may have a spherical curvature.

[0014] Compared to the aforementioned prior art, a first object of the present invention is to provide an advantageous optical fiber for a head-mounted display (HMD) which, when using an optical fiber with a concave curvature as seen from the eye, enables the generation of a virtual image at infinity or at a fixed distance in front of the eye. A further object of the present invention is to provide an advantageous imaging optic for an HMD with which the generation of a virtual image at infinity or in a fixed image plane in front of the eye is possible when using an optical fiber with a spherically concave curvature as seen from the eye.

[0015] A third object of the present invention is to provide an HMD which has a spherically concave curved light guide as seen from the eye and which is able to generate a virtual image in infinity or a fixed image plane in front of the eye.

[0016] The first problem is solved by a curved optical fiber according to claim 1, the second problem by an imaging optic according to claim 10 and the third problem by an HMD according to claim 15.

[0017] The dependent claims contain advantageous embodiments of the invention.

[0018] A curved optical fiber according to the invention for a head-mounted display (HMD) is, when used as intended, positioned in front of at least one eye and has a spherically concave curvature as viewed from the eye. It comprises a rear surface facing the eye and a front surface facing away from the eye. It also comprises an input structure for coupling light into the optical fiber such that light is guided through the optical fiber by reflection at the front surface and / or the rear surface, and an output structure for coupling the light guided through the optical fiber out. Such an optical fiber exerts a focusing effect on the coupled light. According to the invention, the optical fiber therefore has a diverging structure whose focal length is less than or equal to the radius of curvature of the optical fiber.The diverging structure can be superimposed on either the output coupling structure or the input coupling structure, thus eliminating the need to integrate an additional optical element into the optical fiber. Alternatively, the focal length of the diverging structure can be divided into two substructures, one of which can be superimposed on the input coupling structure and the other on the output coupling structure.

[0019] By integrating a diverging structure into the optical fiber, it becomes possible to use the fiber to generate a virtual image that, from the eye's perspective, lies beyond the fiber. If the focal length of the diverging structure is equal in magnitude to the radius of curvature of the optical fiber, the image is located at infinity. If the focal length of the diverging structure is smaller in magnitude than the radius of curvature of the optical fiber, meaning the diverging effect of the structure is greater than the converging effect of the optical fiber, the virtual image is located at a distance from the eye that is determined by the strength of the diverging effect, i.e., by the focal length of the diverging structure.

[0020] The diverging structure can be, in particular, a diffraction lens, for example, in the form of a volume hologram or a blaze grating. The diverging effect of the diffraction lens is determined by its focal length. To generate a virtual image at infinity, a diffraction lens is used whose focal length is equal in magnitude to the radius of curvature of the optical fiber. This applies particularly when the centers of curvature of the front and back surfaces of the optical fiber coincide. If the centers of curvature of the front and back surfaces do not coincide, or if the optical fiber has a curvature not covered by the claimed invention that deviates from spherical curvature, for example, because it is to be used simultaneously to correct a visual impairment within the context of smart glasses, this relationship applies only approximately.The exact focal length of the diffraction lens used to generate a virtual image depends on the specific curvature of the optical fiber and can be determined approximately or experimentally. For example, the focal length of the diffraction lens can be less than or equal to the smaller of the two radii of curvature of the front and rear surfaces of the optical fiber if the centers of curvature of the front and rear surfaces do not coincide. In this case, the virtual image may not be located at infinity, but at a large distance from the optical fiber, so it can be considered to be approximately at infinity. In the case of an experimental determination, for example,The point is determined at which a divergent beam of rays emanating from a point light source is imaged to infinity by the optical fiber without the diverging structure, and the distance of this point from the back surface of the optical fiber is considered its radius of curvature.

[0021] A diffraction lens focuses light to a focal point by means of interference. Constructive interference occurs at the focal point, while destructive interference occurs outside the focal point. Typically, the focal point is formed by the first diffraction maximum that occurs during diffraction. To achieve the desired diffraction pattern, the diffraction lens can either have alternating transparent and opaque zones or alternating zones with shorter and longer optical path lengths for the light passing through the lens. The spacing and width of the zones of the diffraction lens depend on the wavelength of the light and the desired focal length of the lens.In the case of alternating transparent and opaque zones, diffraction is generated because only certain parts of the wavefront can pass through the diffraction lens. These passing parts interfere constructively in some directions and destructively in others. In the case of zones with different optical path lengths, a phase shift occurs between the wavefront parts passing through the different zones of the diffraction lens. This phase shift causes the different wavefront parts to interfere constructively in some directions and destructively in others. Designing the diffraction lens with transparent and opaque zones allows for its fabrication using relatively simple means.In contrast, the production of a diffraction lens with zones of different optical path lengths offers the advantage that, in an augmented reality application, it does not affect the ambient light passing through the light guide from the outside and thus does not weaken the perception of the environment.

[0022] The diverging structure, however, leads to longitudinal and lateral chromatic aberration. Longitudinal chromatic aberration means that the focal points of light of different colors (wavelengths) are located at different distances from the diverging structure, so that at a given focal point only light of one color is focused, while light of other colors at that focal point is slightly defocused. Lateral chromatic aberration, on the other hand, results from different imaging scales of the diverging structure for light of different colors (wavelengths) guided through the optical fiber, leading to colored fringes in the image. While longitudinal chromatic aberration is usually small enough to be dispensed with, lateral chromatic aberration can become so large at the edges of the image field that it is perceived as disturbing.To minimize the lateral chromatic aberration induced by the diverging structure, it is therefore advantageous if the diverging effect of the diverging structure is centered on the center of the image field.

[0023] Only a specific angular spectrum of the light path can be transmitted through the optical fiber at any given time. The smaller the angular spectrum, the smaller the image field that can be transmitted through the optical fiber. To transmit a large angular spectrum for a large image field, the input and output coupling structures can each comprise several substructures that couple different parts of the light's angular spectrum into or out of the optical fiber. These different parts of the angular spectrum can then be designed to each correspond to the maximum of the angular spectrum that the optical fiber can transmit. The total angular spectrum of the transmitted light then results from the combination of these two parts, allowing for the transmission of a large angular spectrum and thus generating a large image field.

[0024] Furthermore, diffractive input and output coupling structures are each optimized for a specific wavelength. With increasing deviation from this optimized wavelength, no usable interference for output coupling occurs. The wavelength spectrum of light that can be coupled in or out via the input and output coupling structures is therefore limited. If an image with a broad wavelength spectrum, ideally a color image, is to be generated, it is therefore advantageous if the output coupling structure comprises several substructures that couple different parts of the spectral range of the light being coupled into or out of the optical fiber.To generate a color image, a particular embodiment of the optical fiber according to the invention includes an input structure and an output structure, each comprising three substructures, one optimized for blue light, one for green light, and another for red light. If the angular spectrum is then divided into multiple parts, these substructures must each be present multiple times, namely once for each part of the angular spectrum. Thus, in the case of an angular spectrum divided into two parts and three colors, there are a total of six substructures. If the angular spectrum is to be divided into n parts, there are accordingly 3n substructures for the input structure and 3n substructures for the output structure.It is possible to transmit the different spectral ranges and / or different parts of the angular spectrum through a single optical fiber, or to divide the optical fiber into multiple sub-fibers, each carrying different spectral wavelength ranges and / or different parts of the angular spectrum. These sub-fibers can then be considered separate transmission channels. The presence of separate transmission channels reliably prevents crosstalk between the transmitted signals. Conversely, transmitting multiple spectral ranges and / or parts of the angular spectrum through a single optical fiber reduces the complexity of the fiber and thus simplifies its manufacturing.

[0025] The coupling of light rays into the optical fiber occurs such that the coupled light rays travel along great circles that pass through points on the spherical front surface and / or the spherical back surface of the optical fiber, defined in spherical coordinates by the polar angles 0° and 180°. These points are generally not themselves part of the spherical front surface and / or the spherical back surface of the optical fiber, as these surfaces only represent a portion of an imaginary spherical surface containing these points. On the Earth's surface, such great circles correspond to the lines of longitude. Therefore, these great circles are referred to as the lines of longitude of the optical fiber in the following discussion.In this way, for all meridians along which the coupled light rays travel, there exists a great circle that intersects all these meridians at right angles—that is, a great circle that the light rays pass through at right angles, or would pass through if they were coupled out before reaching this great circle. The great circle intersecting the meridians at right angles thus runs essentially parallel to the structural elements of the coupling and coupling structures, i.e., parallel to the grating structures in the case of diffractive coupling, or parallel to the boundary lines between Fresnel structures in the case of reflective or refractive coupling. On the Earth's surface, this intersecting great circle would correspond to the equator. Therefore, the great circle intersecting the meridians at right angles will henceforth be called the equator of the optical fiber.It is particularly advantageous if the diverging structure is integrated into the output coupling structure, since the path of the light rays along meridians within the optical fiber is then not affected by the diverging effect of the diverging structure. The path of the light rays along meridians within the optical fiber allows the output coupling structure to be positioned closer to the equator of the optical fiber than the input coupling structure, according to the invention. It can be advantageous if the output coupling structure is positioned no more than 5 degrees, preferably no more than 3 degrees, from the equator of the optical fiber. In particular, the output coupling structure can be positioned such that the equator of the optical fiber also intersects the output coupling structure, and especially such that the equator intersects the output coupling structure symmetrically.Due to the proximity of the output coupling structure to the equator of the optical fiber, the meridians along which the light rays propagate intersect the structural elements of the output coupling structure at nearly right angles, so that the light rays strike the structural elements of the output coupling structure at nearly right angles. This minimizes differences in pupil shape resulting from varying numbers of reflections within the optical fiber. Different numbers of reflections within the optical fiber can cause the diffraction lens to be used at different locations, leading to pupil distortion that may require correction. By arranging the output coupling structure near the equator, pupil distortion can be reduced and potentially minimized.

[0026] According to the invention, an imaging optic is also provided, comprising an imaging beam path, an optical fiber, and at least one image-forming optical element. The imaging optic is characterized by the fact that it includes an optical fiber according to the invention, wherein the optical fiber serves as the at least one image-forming optical element. The imaging optic can, in particular, consist exclusively of the optical fiber according to the invention. Because the optical fiber simultaneously functions as the image-forming optical element, it becomes possible to produce a compact imaging optic. The optical fiber according to the invention makes it possible to provide a virtual image, which, as seen from the eye, is located at a finite or infinite distance beyond the optical fiber, even with a concave curvature of the optical fiber as seen from the eye, particularly a spherical-concave curvature.

[0027] In a further development of the imaging optics according to the invention, a correction element for pre-correcting a trapezoidal pupil distortion induced by the optical fiber is positioned upstream of the coupling structure of the light guide in the imaging beam path. This correction element generates a pupil distortion inverse to the pupil distortion induced by the optical fiber. The two pupil distortions then compensate each other. Additionally or alternatively, a compensation optic for compensating a static lateral chromatic aberration can be positioned upstream of the coupling structure of the optical fiber in the imaging beam path. The static lateral chromatic aberration is induced by the optical fiber containing the diverging structure and is independent of eye movement. It can therefore be readily compensated with an optical compensation optic.This compensation optic induces a lateral chromatic aberration inverse to the one induced by the optical fiber, thus canceling out the two aberrations. Furthermore, if a manipulator comprising a diffractive optical element and a refractive optical element, arranged perpendicular to the optical axis of the imaging beam path and thus displaceable relative to each other, is positioned upstream of the optical fiber's coupling structure in the imaging beam path, and if an eye tracker is also present to detect the eye's gaze direction, it is possible to pre-correct a dynamic, field-constant lateral chromatic aberration introduced by the optical fiber. A dynamic lateral chromatic aberration arises from eye movements, at least when the chromatic bandwidth of the transmitted light exceeds a certain threshold.

[0028] Furthermore, a polarization beam splitter and a delay plate can be positioned upstream of the optical fiber coupling structure in the imaging beam path. The polarization beam splitter, in conjunction with the delay plate, enables the coupling of the imaging beam path through the coupling structure while maintaining compact dimensions for the imaging optics.

[0029] According to a further aspect of the invention, a head-mounted display (HMD) is provided with a display for showing an initial image and an imaging optic according to the invention for generating a virtual image from the initial image. Since the imaging optic according to the invention can be compact due to the use of the light guide as an image-forming optical element, it is possible to provide a compact HMD by using the imaging optic according to the invention. This allows a virtual image to be generated when the light guide is concavely curved from the eye's perspective, and this virtual image is located beyond the light guide at infinity or at a specific distance from the light guide.

[0030] Further features, properties and advantages of the present invention will become apparent from the following description of exemplary embodiments with reference to the accompanying figures. Figure 1 shows an ideal point-to-point imaging pattern, such as can be achieved with a spherical optical fiber, which is useful for understanding the claimed invention. Figure 2 shows a curved optical fiber with an imaging pattern to infinity. Figure 3 shows an example of a diffractive output coupling structure for coupling out a beam of light transmitted through the optical fiber. Figure 4 shows a diffraction lens. Figure 5 shows the diffraction pattern resulting from a superposition of the output coupling structure. Figure 3 and the diffraction lens Figure 4 Resulting output structure. Figure 6 is a representation to illustrate the field angle of a considered display. Figure 7 shows how the field angle is derived from Figure 6 can be split between two different optical fibers. Figure 8 shows the extraction of the signal from the optical fibers. Figure 7transmitted beams. Figure 9 shows an optical fiber with three substructures, each suitable for coupling out a different spectral range. Figure 10 shows an embodiment of an imaging optic with a wavefront manipulator for correcting lateral chromatic aberration induced by the optical fiber. Figure 11 shows an embodiment of an imaging optic with a correction element upstream of the optical fiber's coupling structure for correcting pupil distortion induced by the optical fiber. Figure 12 shows an example of a compact imaging optic.

[0031] Before proceeding with reference to Figure 2 The principle underlying the invention will be discussed with reference to Figure 1 explains the effect of a spherical optical fiber.

[0032] In Figure 1A spherical optical fiber 1 is shown, comprising a diffractive input coupling structure 3 located at the edge of the optical fiber and a diffractive output coupling structure 5 located at the center of the optical fiber. The connecting line between the center of the input coupling structure 3 and the center of the output coupling structure 5 extends essentially along a meridian 7 of the sphere of the optical fiber 1, i.e., along a great circle that passes through those points of a sphere of which the optical fiber is a part, which are given in spherical coordinates by the polar angles 0° and 180°. In the present embodiment, the output coupling structure 5 is located on the equator 9 of this sphere, such that it is symmetrically intersected by the equator 9.

[0033] The diffractive input coupling structure 3 and the diffractive output coupling structure 5 are each formed by diffraction gratings with a specific grating vector, whose grating lines run parallel to lines of latitude, i.e., parallel to the equator 9. By means of the diffractive input coupling structure 3, a divergent beam 2 originating from the center of the sphere of the optical fiber 1 is deflected such that the individual rays of the beam in the optical fiber 1 strike the front surface 4 of the optical fiber 1, which, in the intended use of the optical fiber in an HMD, points away from the eye, such that they are reflected by total internal reflection towards the rear surface 6 of the optical fiber 1, which, in the intended use of the optical fiber in an HMD, points towards the eye, where they are again reflected by total internal reflection towards the front surface 4 of the optical fiber 1.In this way, the rays of the beam are guided to the output coupling structure 5 by multiple total internal reflections at the front surface 4 and the rear surface 6 of the optical fiber 1, where they are deflected by the diffraction grating such that they emerge from the rear surface 6 of the optical fiber 1. The beam emerging from the rear surface 6 of the optical fiber 1 is a convergent beam 8 such that its rays are refocused onto the center of the sphere of the optical fiber 1. The in . Figure 1 The optical fiber 1 shown, with the diffractive coupling structure 3 and the diffractive coupling structure 5, thus ideally maps a point located in the center of the sphere of the optical fiber 1 onto itself.

[0034] For use as part of the imaging optics of an HMD, the light guide 1, as shown in Figure 1However, this is only of limited use. In a head-mounted display (HMD), it is usually desirable to generate an image at infinity that the HMD user can view with relaxed eyes, or an image located at a defined distance from the eye beyond the light guide, so that the user has the impression that the image is floating at a certain distance in front of their eye. In other words, an imaging optic encompassing the light guide should be able to generate a virtual image beyond the light guide. While it would theoretically be possible to generate such a virtual image by means of a lens positioned between the output structure and the eye, using beams coupled out by the output structure (5), this is not always feasible.However, placing a lens between the light guide and the eye is undesirable because, firstly, there is very little space for such a lens between the light guide and the eye, and secondly, the risk of injury to the eye from an optical element located directly in front of the eye in the event of a blow to the HMD should be avoided. Furthermore, the lens would also affect the image of the surroundings.

[0035] According to the invention, it is therefore proposed to integrate a diverging structure into the optical fiber 1 itself, the focal length of which is less than or equal to the radius of curvature of the optical fiber 1. This diverging structure diverges the beam of rays coupled out of the optical fiber 1 by means of the coupling structure 5, as described in Figure 2The light beam is depicted as a parallel beam 10 if the focal length of the diverging structure is equal in magnitude to the radius of curvature of the optical fiber 1, or as a divergent beam if the focal length of the diverging structure is less than the radius of curvature of the optical fiber 1 (not shown in the figure). If the focal length of the diverging structure is equal in magnitude to the radius of curvature of the optical fiber 1, the optical fiber 1 thus constitutes an image-forming optical element of an imaging optic that generates a virtual image at infinity.If, on the other hand, the focal length of the diverging structure is smaller than the radius of curvature of the optical fiber 1, the beam of light coupled out of the optical fiber 1 is divergent, so that the optical fiber 1 represents an image-forming optical element of an imaging optic that generates a virtual image at a defined distance from the eye beyond the optical fiber 1. The imaging optic can consist solely of the optical fiber or include other optical elements, such as elements for correcting image aberrations, as described below.

[0036] The diverging structure can be a refractive, reflective, or diffractive structure integrated into the optical fiber 1. In the present embodiment, a diffractive structure in the form of a diffraction lens is used. The use of a diffractive structure is advantageous because it can be manufactured very thinly, so that the thickness of the optical fiber 1 does not need to increase, or only increases minimally. Furthermore, the diffractive diverging structure can be integrated into the diffractive output coupling structure 5 and / or the diffractive input coupling structure 3. In the case of multiple output coupling, it is particularly advantageous to integrate the diffractive diverging structure into the diffractive output coupling structure 5, as this does not affect the light path within the glass. This will be illustrated below with reference to the Figures 3 to 5 This is explained using the example of integration into the decoupling structure 5.

[0037] Figure 3Figure 1 shows an exemplary diffractive output coupling structure 5 in the form of a diffraction grating, where the gray levels indicate the phase shift to be generated in the range of 0–1 λ. The output coupling structure has parallel grating structures which, in the present embodiment, extend along latitude circles of the sphere of the optical fiber 1. A diffractive output coupling structure 5, as shown in Figure 1, is described in Figure 2. Figure 3 As shown, this leads to the in Figure 1 shown convergent beam 8 and thus to the point-draw-point mapping from Figure 1 .

[0038] Figure 4Figure 1 shows a diffraction lens 11. In the present embodiment, the focal length of the diffraction lens 11 is equal to the radius of curvature of the sphere of the optical fiber 1. The diffraction lens 11 is a concentric diffraction grating with a ring-shaped grating structure. The ring-shaped grating structure can be formed either by alternating ring-shaped transparent and opaque zones or by alternating ring-shaped zones with different optical path lengths, so that rays passing through adjacent zones experience different phase shifts. In both cases, interference occurs after passing through the diffraction lens, which strengthens the rays in certain regions of space through constructive interference and weakens them in other regions through destructive interference.The deflection angles at which constructive and destructive interference occur depend on the distance between two adjacent ring-shaped structures. The closer the distance, the larger the angles at which the first-order diffraction maximum (i.e., the first constructive interference) occurs. By decreasing the distances towards the edge of the diffraction lens, the deflection angles can be adjusted so that the deflected rays converge with constructive interference at a specific distance from the lens. This distance is then the focal distance of the diffraction lens 11. For example, the +1 diffraction order can be used to focus light incident on the diffraction lens 11, and the -1 diffraction order can be used to scatter light incident on the diffraction lens 11.

[0039] If the diffraction order +1 is used, a parallel beam of light would become a convergent beam, which is focused at the focus of the diffraction lens 11. Conversely, if the diffraction order -1 is used, a parallel beam of light would become a divergent beam upon passing through the diffraction lens 11. In the present embodiment, the diffraction lens 11 is designed such that the diffraction order -1 is used.

[0040] The in Figure 4 The diffraction lens 11 shown is connected to the output coupling grating of the output coupling structure 5, as shown in Figure 3 The superimposed images are shown. The result of the superimposition is in Figure 5 shown. Such a coupling structure 13 is found in the in Figure 2 The lens light guide 1 shown replaces the coupling structure 5. Figure 5 Use.

[0041] As already mentioned, in the present embodiment, the diverging effect in the form of the diffraction lens 11 is fully integrated into the output coupling structure 13. However, it is also possible, in principle, to integrate the diverging structure, for example in the form of the described diffraction lens, into the input coupling structure 3. Furthermore, it is also possible to integrate a diverging effect into both the input coupling structure 3 and the output coupling structure 13, the sum of which is equivalent to the diverging effect of the diffraction lens. Figure 4 This corresponds to the following. A specialist can easily construct corresponding divided diffraction lenses. However, a diverging structure integrated exclusively into the output coupling structure 13 offers the advantage over the other variants that the diverging effect does not affect the light transmission through the optical fiber.

[0042] The ones relating to the Figures 3 to 5The described grid structures with different optical path lengths can be produced, for example, by holographic exposure on the curved surface of the light guide. While it is also possible, in principle, to create grid structures using transparent and opaque areas of the light guide, with the opaque areas being printed on, this results in a loss of ambient light passing through the light guide. However, particularly in the context of an HMD designed as an augmented reality display, where the displayed image is intended to be perceived as superimposed onto the surroundings, grid structures with different optical path lengths offer the advantage that the ambient light is not attenuated.

[0043] Furthermore, it should be mentioned that the position of the input coupling structure 3 and the position of the output coupling structure 13 on the optical fiber can, in principle, be freely chosen. However, it must be noted that the rays belonging to a field point travel along non-parallel great circles within the optical fiber, which, depending on the position of the input and output coupling structures, leads to more or less trapezoidal pupil distortion and thus to astigmatism in the field. Ideally, the output coupling structure is therefore located near the equator 7 of the sphere of the optical fiber 1 in order to minimize pupil distortion and thus astigmatism. Typically, with a pupil diameter of 3 mm, a geometric point size of 1 to 2 arcminutes at the field edge can be achieved, which is generally not perceived as particularly bothersome.

[0044] In relation to Figure 2In the described embodiment of the invention, an optical fiber 1 with a single light-guiding structure serves to guide the light from the input coupling structure 3 to the output coupling structure 13. With such an optical fiber, the field angle ranges to be transmitted are relatively limited and are insufficient to transmit an image in 16:9 format with an image diagonal of 70°. In order to nevertheless enable the transmission of such an image, a field angle range extending over a field angle α is divided into several sub-field angle ranges, and these sub-field angle ranges are then transmitted separately from one another through the optical fiber. This is described below using the example of dividing the field angle range extending over the field angle α into two field angle ranges extending over field angles α1 and α2. The division of the field angle range into two field angle ranges is shown in Figure 6schematically represented. The figure shows a display 15 and the field angle α at which the display 15 is viewed. This field angle is divided into two field angles α1 and α2, which do not overlap in this example. However, it is also possible in principle to allow an overlap of the partial field angles α1 and α2. The partial field angles α1 and α2 can be chosen such that they each represent the largest angle of an optical fiber, as used in Figure 2 The transmissible field angle range is shown. To transmit the entire field angle range given by the field angle α, two optical fibers are then used as described in the diagram. Figure 2 The light guides 11 and 12 of light guide 1 have been described and are arranged one behind the other. This arrangement is shown schematically in the Figure 7 and 8 depicted. The Figure 7The figure shows a cross-section along a meridian, depicting the two sublight guides 11 and 12 arranged one behind the other, with the coupling structures 31 and 32. Each coupling structure 31, 32 is assigned its own display 151, 152, which each displays the image area perceptible under the corresponding field angle range α1, α2. As already mentioned, the field angles α1, α2 are chosen such that the field angle ranges covered by them can be completely transmitted by the respective light guide 11, 12. For example, if the field angle α is 40° and the corresponding field angle range is from -20° to +20°, the partial field angles α 1 , α 2 can be, for example, 20°, where the partial field angle α1 covers a field angle range from -20° to 0° and the partial field angle α 2 covers a field angle range from 0° to +20°.Since the sublight guides 1 1 , 1 2 arranged one behind the other transmit the complete image together, they together form the light guide 1.

[0045] The coupling from the light guide 1, which is constructed from sublight guides 1 1 , 1 2, is in Figure 8 schematically represented. Each of the downlight guides 11, 12 has an output coupling structure 131, 132, which is essentially the output coupling structure 13 consisting of Figure 2This corresponds to the following. However, the structures of the basic gratings 5, onto which the diffraction lens is superimposed, are chosen such that the light guided through the sublight guides 11, 12 is coupled out at the correct angles. The coupling structures 31, 32 are correspondingly adapted to the basic gratings of the coupling structures 131, 132. As a result, the field angle ranges defined by the partial field angles α1, α2 are recombined to form the field angle range with the opening angle α. This procedure is also called field stitching and is described in detail in WO 2017 / 102795 A1. Reference is made to this publication for further details on field stitching.

[0046] By using a suitable number of sub-light guides in light guide 1, a sufficiently large field of view can be achieved. Since diffractive coupling and coupling structures, in particular, can be manufactured very flat, a larger number of sub-light guides can also be arranged in series.

[0047] Furthermore, it is generally not possible to transmit the entire wavelength spectrum of visible light through a single optical fiber using only a single input and output coupling structure. However, it is possible to equip the optical fiber with sub-light guides, each designed to transmit a different wavelength range of visible light. For example, one sub-light guide, with its input and output coupling structures, could be designed to transmit red light, another to transmit green light, and a third to transmit blue light. In this way, a complete color image can be transmitted using three sub-light guides. This is shown schematically in Figure 9Figure 1 shows a light guide 1 with three sub-light guides 11<, 12<, 13< and their associated output coupling structures 131<, 132<, 133<. Each sub-light guide 11<, 12<, 13< with its associated input coupling structure (not shown) and output coupling structure 131<, 132<, 133< is designed to transmit a color, for example, the first sub-light guide 11< for transmitting red light, the second sub-light guide 12< for transmitting green light, and the third sub-light guide 13< for transmitting blue light. Therefore, a color image can be transmitted using the three sub-light guides 11<, 12<, 13<.

[0048] If, as with regard to the Figures 6 to 8Since multiple field angle ranges are to be transmitted, and each field angle range is to represent a color image, three substructures 1<1, 1<2, 1<3, for red, green, and blue light are required for each field angle range defined by a partial field angle α1, α2. In the example above, this results in a total of six substructures: three each for the field angle range defined by partial field angle α1 and three each for the field angle range defined by partial field angle α2. As already mentioned, the substructures can be manufactured quite thinly using diffractive coupling and diffractive coupling, so that a light guide 1 with six substructures can be produced without problems with an acceptable overall thickness.

[0049] The in Figure 4The diffraction lens 11 shown is not color-corrected. This is because the angles at which constructive interference occurs depend on the wavelength of the light. The lack of color correction leads to longitudinal chromatic aberration and lateral chromatic aberration.

[0050] The longitudinal chromatic aberration results from the fact that the angles for the first diffraction maximum differ, meaning the lens has different focal lengths for different wavelengths. For example, if the diffraction lens is designed so that its focal length for green light corresponds to the radius of curvature of the optical fiber, the focal length for red light is slightly shorter and the focal length for blue light is slightly longer than the radius of curvature of the optical fiber 1. This results in a certain amount of blurring in the red and blue parts of the image when the green part is sharp. With a bandwidth of 50 nm at a wavelength of 500 nm (green light), the loss of sharpness due to the blurring in the red and blue image components is approximately 0.6 diopters, assuming a radius of curvature of 150 mm for the optical fiber 1 and a focal length of 150 mm for the diffraction lens 11. A loss of sharpness of 0.6 diopters can still be considered acceptable.

[0051] Lateral chromatic aberration results from the different magnifications and thus the different image scales of the individual colors in the respective focal planes, which arise from the different focal lengths. While the lateral chromatic aberration is not apparent on the optical axis, it increases with distance from the optical axis. If the exit pupil of the diffraction lens is located 20 mm behind the output coupling structure, lateral chromatic aberrations appear at the field edge of an image field of ±12°, measuring on the order of approximately 10 arcminutes at the field edge. Such a lateral chromatic aberration is unacceptable and must be corrected.

[0052] One way to correct the lateral chromatic aberration is to place a correction element upstream of the coupling structure 3 of the optical fiber 1 in an imaging device encompassing the optical fiber 1. This correction element generates a lateral chromatic aberration inverse to the lateral chromatic aberration induced by the diffraction lens 11, so that the lateral chromatic aberration of the diffraction lens 11 and the lateral chromatic aberration of the correction element exactly cancel each other out. An example of such a correction element is shown in Figure 10The figure schematically depicts an HMD with an image sensor 15, on which an initial image is displayed, which is to be transformed into a virtual image by an imaging optic. This imaging optic comprises, in addition to a light guide 1 according to the invention, a correction element 17 for correcting the lateral chromatic aberration. In the present embodiment, this correction element 17 comprises a conventional refractive optical element 19 and a diffractive optical element 21. These two are matched to each other such that together they induce a lateral chromatic aberration which compensates for the lateral chromatic aberration of the diffraction lens 11 of the output coupling structure 13. Furthermore, the refractive optical element 19 and the diffractive optical element 21 can be shifted relative to each other, thereby also compensating for dynamic lateral chromatic aberration.A dynamic lateral chromatic aberration arises from the user's eye movement, which causes lateral chromatic aberrations occurring along edges in the field to change their extent. This change in the extent of the lateral chromatic aberration can be compensated for by appropriately shifting the two optical elements 19 and 21 relative to each other. In the present embodiment, an eye tracker 23 is provided to determine the gaze direction. Based on the determined gaze direction, a control unit 25 controls the relative shift between the refractive optical element 19 and the diffractive element 21 of the correction element 17. If the dynamic lateral chromatic aberration is not to be compensated, the refractive optical element 19 and the diffractive optical element 21 can be fixed relative to each other, and the eye tracker 23 and the control unit 25 can be omitted.

[0053] The correction element 17 also makes it possible to compensate for the previously mentioned pupil distortion, which occurs due to the guidance of the light rays along the meridians, by appropriate design of the refractive optical element 19 and / or the diffractive optical element 21.

[0054] However, if the goal is simply to correct pupil distortion, this can also be achieved with simpler means. Figure 11 Figure 1 shows an imaging optic for a head-mounted display (HMD) with a light guide 1 according to the invention and a correction element that serves solely to compensate for pupil distortion. A parabolic mirror 27 with a radius of curvature of 480 mm is used as the correction element. The radius of curvature of the light guide 1 and the focal length of the diffraction lens of the output coupling structure 13 are assumed to be 150 mm. The starting point of the optical system in Figure 10The beam of light striking the parabolic mirror 27 from the left is located outside the image at the focal point of the parabolic mirror. Linear astigmatism can also be reduced by an analogously generated quadratic pupil distortion in the longitudinal direction.

[0055] Pupil distortion can be corrected not only with the correction elements described above, but also with other off-axis elements. Correcting pupil distortion also makes longer transmission distances in the optical fiber possible.

[0056] In the imaging device according to the invention, it is possible to arrange the display 15 closer to the coupling structure if a polarization beam splitter 29 and a delay plate 32 in the form of a λ / 4 plate are positioned upstream of the coupling structure, as is the case in Figure 12This is shown schematically. Coupling using a polarization beam splitter and a delay plate is described, for example, in WO 2008 / 023367 A1, there in particular with reference to Figure 3 , described. Reference is therefore made to the disclosure in WO 2008 / 023367 A1 with regard to coupling using a polarization beam splitter and a delay plate.

[0057] The present invention has been described in detail with reference to exemplary embodiments for illustrative purposes. However, it is apparent to a person skilled in the art that deviations from these exemplary embodiments are possible within the scope of the invention. For example, the number of partial field angles can be greater than two, and the number of color channels to be transmitted can be less than or greater than three. Furthermore, it is generally possible to use a refractive lens, such as a Fresnel lens, instead of a diffraction lens to produce the diverging effect. As already mentioned, the diverging effect can be superimposed on the input coupling structure instead of the output coupling structure. It is also possible to divide the diverging effect and superimpose part of it on the input coupling structure and the other part on the output coupling structure. Other input and output coupling structures can also be used, e.g.,...Reflective structures, such as partially reflective mirrors instead of diffractive structures, or the described diffractive structures can be replaced by blaze gratings. In the latter case, it is possible to optimize the diffraction pattern to a specific diffraction order by appropriately selecting the blaze angle. The diffraction lens can also be implemented as a blaze grating. Dichroic partially reflective mirrors, for example, can then be used to couple out a specific wavelength range. Although the coupling structures are located on the back surface of the optical fiber in the described embodiments, they could also be located on the front surface, so that the light, for example, first passes approximately perpendicularly through the back surface and only then reaches the deflecting / diffracting structure on the front surface.Accordingly, after passing through the optical fiber, the light can be deflected by a deflecting / diffracting structure on the front surface towards the rear surface, through which it then exits the optical fiber, for example, at an almost perpendicular angle. The invention is therefore not intended to be limited by the described embodiments, but only by the appended claims. Reference symbol list

[0058] 1. Spherical optical fiber 2. Divergent beam 3. Coupling structure 4. Front surface 5. Coupling structure 6. Rear surface 7. Longitudinal circle 8. Convergent beam 9. Equator 10. Parallel beam 11. Diffraction lens 13. Superimposed Coupling structure 15. Display 17. Correction element 19. Refractive optical element 21. Diffractive optical element 23. Eye tracker 25. Control unit 27. Correction element 29. Polarizing beam splitter 31. Delay plate

Claims

1. Spherically curved light guide (1) for an HMD, wherein, when used as intended, the light guide (1) is arranged in front of at least one eye and has a concave curvature with a radius of curvature as seen from the eye, wherein the spherically curved light guide comprises: - a back surface (6) facing the eye and a front surface (4) facing away from the eye, - an input coupling structure (3) for coupling light into the light guide (1) in such a way that light is guided through the light guide (1) with reflection at the front surface (4) and / or the back surface (6), and - an output coupling structure (13) for coupling out the light guided through the light guide (1), wherein the light guide (1) has a diverging structure (11) for light guided through the light guide, the absolute value of the focal length of which structure is less than or equal to the radius of curvature of the light guide (1), characterized in that the input coupling structure (3) is configured in such a way that it couples rays of light into the light guide (1) such that the rays of light coupled in pass in the light guide (1) along circles (7) of longitude of the light guide (1), and the output coupling structure (5, 13) lies closer than the input coupling structure (3) to the great circle (9) of the light guide (1) that intersects the circles (7) of longitude at right angles.

2. Light guide (1) according to Claim 1, characterized in that the diverging structure (11) is superimposed on the output coupling structure (5, 13) and / or the input coupling structure (3).

3. Light guide (1) according to Claim 1 or Claim 2, characterized in that the diverging structure is a diffraction lens (11).

4. Light guide (1) according to Claim 3, characterized in that the diffraction lens (11) comprises zones which differ from one another in terms of their optical path length for the light guided through the light guide (1).

5. Light guide (1) according to Claim 3, characterized in that the diffraction lens (11) comprises transparent and non-transparent zones.

6. Light guide (1) according to any of Claims 1 to 5, characterized in that the diverging effect of the diverging structure (11) is centred on the centre of the image field.

7. Light guide (1) according to any of Claims 1 to 6, characterized in that the light to be coupled into the light guide (1) has an angular spectrum and / or a spectral range and the input coupling structure (3) and the output coupling structure (13) each comprise a plurality of substructures (31, 32 ,131, 132, 131, 132 ,133), which respectively couple different parts of the spectral range and / or different parts of the angular spectrum into the light guide (1) and out of the light guide (1).

8. Light guide (1) according to Claim 7, characterized in that comprises a plurality of sub-light guides (11, 12, 11, 12, 13), each of which respectively guides a different one of the different parts of the spectral range and / or of the different parts of the angular spectrum from the input coupling structure (3) to the output coupling structure (13).

9. Light guide (1) according to any of Claims 1 to 8, characterized in that the input coupling structure (3) is arranged in the light guide (1) in such a way that the input coupling can take place through the back surface (6) of the light guide (1).

10. Imaging optical unit comprising an imaging beam path, a light guide (1) and at least one image-forming optical element, characterized in that it comprises a light guide (1) according to any of Claims 1 to 9, wherein the light guide (1) serves as the at least one image-forming optical element.

11. Imaging optical unit according to Claim 10, characterized in that a correction element (17, 27) for correcting in advance a pupil distortion induced by the light guide is disposed upstream of the input coupling structure (3) of the light guide (1) in the imaging beam path.

12. Imaging optical unit according to Claim 10 or Claim 11, characterized in that a compensation optical unit (27), which compensates for a lateral chromatic aberration induced by the light guide (1), is disposed upstream of the input coupling structure (3) of the light guide (1) in the imaging beam path.

13. Imaging optical unit according to any of Claims 10 to 12, characterized in that the compensation optical unit (27) comprises a diffractive optical element (21) and a refractive optical element (19), which are arranged such that they are displaceable relative to one another perpendicular to the optical axis of the imaging beam path, and in that an eye tracker (23) is present.

14. Imaging optical unit according to any of Claims 10 to 13, characterized in that a polarization beam splitter (29) and a retardation plate (31) are disposed upstream of the input coupling structure (3) of the light guide (1) in the imaging beam path.

15. HMD comprising a display (15) for displaying an output image and an imaging optical unit for generating a virtual image from the output image, characterized in that the imaging optical unit is an imaging optical unit according to any of Claims 10 to 14.