Method of producing a holographic plate, apparatus and holographic plate

Abstract

A method of producing a holographic plate (2) comprising the steps of providing a photosensitive material (20) in a recording plane (19) and illuminating the recording plane (19) with a radiation of a light source (3) to produce a holographic pattern (15) is specified, wherein the radiation of the light source is split into a reference radiation (31) and an object radiation (32), wherein the reference radiation (31) runs through a reference arm (11), wherein the object radiation (32) runs through an object arm (12), wherein an object phase modulator (35) is arranged in the object arm (12), wherein the object radiation (32) and the reference radiation (31) interfere in the recording plane (19) and wherein a diffusing element (4) which is imaged onto the recording plane (19) is arranged in the reference arm (11) or in the object arm (12) between the object phase modulator (35) and the recording plane (19). Further a holographic plate (2) and an apparatus (1) are specified.

Description

[0001] METHOD OF PRODUCING A HOLOGRAPHIC PLATE, APPARATUS AND

[0002] HOLOGRAPHIC PLATE

[0003] The present application refers to a method of producing a holographic plate, to a holographic plate and to an apparatus for producing a holographic plate.

[0004] For augmented reality (AR) displays or virtual reality (VR) displays, off-axis retinal scanning displays (ORSD) may be used to project an image onto the retina of the user. A key factor for the user experience is the size of the eyebox. The term "eyebox" refers to a volume of space relative to the ORSD in which the user has to position their eye to be able to correctly see the full projected image.

[0005] Document WO 2023 / 161455 Al describes an optical system using pupil replication. Pupil replication means that the exit pupil of the projector is copied. As long as one of the replicated pupils overlaps with the pupil of the user's eye, at least one copy of the image will be projected onto the user's retina so that the user will see the projected image. This helps to increase the size of the eyebox. However, gaps remain between adjacent pupils where no projected light is present .

[0006] An object to be solved is to specify a way that helps to improve the user experience for such devices.

[0007] This object is obtained inter alia by a method, an apparatus and a holographic plate according to the independent claims. Further configurations and developments are subject of the dependent claims. A method of producing a holographic plate is speci fied .

[0008] According to at least one embodiment of the method, the method includes a step of providing a photosensitive material in a recording plane . For example , the photosensitive material comprises a photopolymer . For example , the photosensitive material is sensitive to radiation in the near ultraviolet , in the visible and / or in the near infrared spectral range . In this context , near ultraviolet radiation refers to the spectral range from 320 nm to 490 nm . Visible radiation refers to the spectral range from 420 nm to 780 nm . Near infrared radiation refers to the spectral range from 781 nm to 1 . 3 pm .

[0009] For example , the photosensitive material , such as the photopolymer, is configured to change its refractive index upon illumination .

[0010] According to at least one embodiment of the method, the method includes a step of illuminating the recording plane with a radiation of a light source to produce a holographic pattern .

[0011] For example , the light source is configured to emit coherent electromagnetic radiation . For example , the light source may comprise one or more lasers . For example , a laser includes a laser diode or a solid state laser .

[0012] For example , the laser source includes a laser emitting in the blue spectral range from 440 nm to 470 nm and / or in the green spectral range from 510 nm to 540 nm and / or in the red spectral range from 620 nm to 660 nm . Alternatively or in addition a laser may emit radiation in the near infrared spectral range.

[0013] According to at least one embodiment of the method, the radiation of the light source is split into a reference radiation and an object radiation, wherein the object radiation and the reference radiation interfere in the recording plane. The reference radiation runs through a reference arm and the object radiation runs through an object arm.

[0014] According to at least one embodiment of the method, an object phase modulator is arranged in the object arm. The object phase modulator is configured for imparting a phase profile on the radiation. In particular, during illumination of the recording plane, the object phase modulator imparts phase profiles on the object radiation. The reference radiation, in contrast, does not run through the object phase modulator. Thus, the interference of the reference radiation with the phase modulated object radiation causes an interference pattern which is specific to the phase profile formed by the object phase modulator.

[0015] According to at least one embodiment of the method, a diffusing element is imaged onto the recording plane. The diffusing element may be arranged in the reference arm or in the object arm between the object phase modulator and the recording plane. A further diffusing element may be arranged in the same arm as the diffusing element or in the other arms. In the latter case, both the reference arm and the object arm comprise at least one diffusing element that is imaged onto the recording plane. Features described in the following with respect to the diffusing element may also apply for the further diffusing element.

[0016] The diffusing element is in particular configured to diffuse the phase relation between the object radiation and the reference radiation during the recording of the holographic pattern in the recording plane. Thus, the interference pattern in the recording plane is affected by the phase profile formed by the object phase modulator and by the diffusing element.

[0017] In at least one embodiment of the method, the method of producing a holographic plate comprises the steps of providing a photosensitive material in a recording plane and illuminating the recording plane with a radiation of a light source to produce a holographic pattern wherein the radiation of the light source is split into a reference radiation and an object radiation. The reference radiation runs through a reference arm. The object radiation runs through an object arm. An object phase modulator is arranged in the object arm. The object radiation and the reference radiation interfere in the recording plane. A diffusing element which is imaged onto the recording plane is arranged in the reference arm or in the object arm between the object phase modulator and the recording plane.

[0018] It has been found that by using such a diffusing element during the recording of a holographic pattern, the optical properties of the holographic plate can be improved for or adapted to various applications. For example the holographic plate comprises a volume phase hologram, VPH, for example a transmission VPH or a reflection VPH.

[0019] For example, the holographic plate may be used in an AR or VR optical system, for example in an optical combiner to direct a projected image onto the user's eye. The diffused phase information of the holographic plate may result in improved properties when replayed by the projected light. This helps to obtain a large eyebox with a high homogeneity of the intensity of a projected image as perceived by the user.

[0020] According to at least one embodiment of the method, the object phase modulator comprises an object lens array with a plurality of object lenses. For example, the object radiation runs through only one of the object lenses at the same time.

[0021] For example, the lenses of the object lens array have a diameter in the order of millimeters. When recording a holographic plate for a pupil replicator, each of the object lenses may be used to produce one exit pupil, in particular exactly one exit pupil for the projected image.

[0022] For example, the object lens array comprises at least three or at least five and / or at most 100 or at most 50 lenses.

[0023] According to at least one embodiment of the method, the object radiation is sequentially guided through the object lenses. Thus, the object radiation shines through one object lens after the other. The illumination may occur during a plurality of substeps, wherein a time distance between two subsequent substeps may be smaller than the chemical reaction time interval of the photosensitive material. In this case the photosensitive material essentially behaves as if it were subjected to a continuous exposure. Consequently, diffractive optical elements associated to the respective object lenses can be produced multiplexed in a quasi-simultaneous manner. Thus, interference patterns produced during different substeps may be recorded such that the interference patterns interact with each other.

[0024] For example, the photosensitive material comprises monomers of a photopolymer, wherein the monomers diffuse towards zones of high intensity and polymerizing during the chemical reaction time interval when exposed to the interference pattern. In such a way the holographic pattern is recorded. In other words, the monomers of the photosensitive material agglomerate and polymerize due to the exposure with the interfering radiation generated by superimposing the phase modulated object radiation and the reference radiation during the illumination substeps.

[0025] According to at least one embodiment of the method, the diffusing element comprises or consists of a holographic diffuser. The holographic diffuser may be configured such that the diffusive effect is comparably low. For example, a diffraction angle of the holographic diffuser is in a range from 0.1° to 10°. For example, the holographic diffuser is a phase-only diffuser.

[0026] For example, the holographic plate is configured as a holographic pupil splitter in an optical combiner. As each of the potential sub-pupils may be diffused individually by the diffusing element, a sufficiently large eyebox size can be obtained without strong diffusers. Such strong diffusers typically create a lot of speckle. This might cause a hazy combiner .

[0027] According to at least one embodiment of the method, the diffusing element comprises a micro lens array. For example, the lenses of the micro lens array are achromatic. However, non-achromatic lenses may also be used. A diameter of the individual lenses of the micro lens array is comparably small. For example, the diameter of the individual lenses of the micro lens array acting as diffusing element is smaller than the diameter of the object lenses of the object lens array. For example, the diameter is smaller than a pixel size of the holographic plate to be produced.

[0028] According to at least one embodiment of the method, the object arm comprises an intermediate plane, wherein an area of the intermediate plane is imaged onto the recording plane. The intermediate plane is in particular arranged between the object phase modulator, for example embodied as object lens array, and the recording plane. For example, the object lenses are configured to individually illuminate the area of the intermediate plane. In other words, each of the object lenses illuminates the entire area of the intermediate plane to be imaged but, depending on the specific object lens, the angle of the object radiation impinging onto the intermediate plane varies from object lens to object lens.

[0029] According to at least one embodiment of the method, the diffusing element is arranged in the object arm in the intermediate plane. Consequently, each of the object lenses is configured to illuminate that area of the diffusing element which is to be imaged onto the recording plane. According to at least one embodiment of the method, the diffusing element is arranged in the reference arm. For example, the reference arm comprises a reference arm optics imaging the diffusing element onto the recording plane.

[0030] According to at least one embodiment of the method, a main surface of the diffusing element is arranged at an oblique angle with respect to the recording plane. In particular, the main surface of the diffusing element and of the recording plane are arranged such that an image plane of the diffusing element coincides with the recording plane. Thus, the diffusing element is imaged onto the recording plane even though the main surface of the diffusing element and of the recording plane are planes extending obliquely with respect to one another.

[0031] According to at least one embodiment of the method, the diffusing element is imaged onto the recording plane in a Scheimpflug configuration. For example, the reference arm comprises a reference optics imaging the diffusing element onto the recording plane, wherein a plane extending through the diffusing element, a lens plane of the reference optics and a plane running through the recording plane have a common intersection line.

[0032] Further, a holographic plate is specified.

[0033] In particular, the holographic plate may be produced according to the method described above. Thus, features described in connection with the method also apply for the holographic plate and vice versa. According to at least one embodiment of the holographic plate, the holographic plate is configured as a volume phase hologram of an optical combiner configured for an augmented and / or virtual reality device, for example a wearable device such as glasses or a headset.

[0034] Upon illumination by a projector, an optical combiner with the holographic plate is configured to provide a radiation of the projector through a plurality of pupils. In particular, the pupils are diffused so that the radiation at least partly fills gaps between the pupils.

[0035] By means of the diffused pupils, the holographic plate enlarges the eyebox of the AR or VR device and it also homogenizes the intensity across the projected image as perceived by the user.

[0036] Further, an apparatus for producing a holographic plate is specified.

[0037] According to at least one embodiment of the apparatus, the apparatus comprises a light source configured for emitting electromagnetic laser radiation and a beam splitter splitting the laser radiation into a reference radiation running through a reference arm towards a recording plane and into an object radiation running through an object arm towards the recording plane. An object phase modulator is arranged in the object arm. A diffusing element is configured to be imaged onto the recording plane wherein the diffusing element is arranged in the reference arm or in the object arm. By means of the diffusing element, the phase information provided by the object phase modulator is diffused during recording in the recording plane. Both the reference arm and the object arm may comprise a diffusing element that is imaged onto the recording plane. For example, the diffusing element is arranged in the object arm and a further diffusing element is arranged in the reference arm or vice versa.

[0038] According to at least one embodiment of the apparatus, the apparatus comprises a beam steering unit configured for directing the object radiation on different locations of the object phase modulator. For example, the beam steering unit is configured for directing the object radiation on individual object lenses of an object lens array. For example, the beam steering unit comprises or consists of two mirror galvanometers forming a two-dimensional scanner unit. For example, the two mirror galvanometers may be arranged within a focus plane of a telescope. Alternatively or in addition, the beam steering unit may comprise or consist of a two-dimensional optical phased array (OPA) forming a two- dimensional scanner unit or an acousto optical modulator (AOM) . For example, the optical phased array is arranged within a focus plane of a telescope.

[0039] Alternatively or in addition, the beam steering unit may comprise or consist of a digital light processing element (DLP) . For example a sequence of masks is cycled on the DLP to produce sequential beamlets. The object phase modulator, for example embodied as object lens array, is then illuminated by one of these beamlets at a time.

[0040] The apparatus may be used to perform the method described above. Thus, features described in connection with the method or the holographic plate may also apply for the apparatus and vice versa. After illumination of the photosensitive material , a finishing step may be performed . For example , a photopolymer may be hardened, for example thermally or by applying a radiation that does not have any signi ficant influence on the produced holographic pattern although minor shrinkage may occur . For example , a shrinkage rate during the finishing step is less than 5% or is less than 2 % .

[0041] A method, an apparatus and a holographic plate are explained in greater detail below by way of exemplary embodiments with reference to the drawings . In the exemplary embodiments and in the figures , similar or similarly acting parts are provided with the same reference signs . Generally, only the di f ferences with respect to the individual exemplary embodiments are described . Unless speci fied otherwise , the description of a part or feature in one exemplary embodiment applies to a corresponding part or feature in another exemplary embodiment as well .

[0042] In the figures :

[0043] Figure 1 shows a schematic block diagram of an exemplary embodiment of a method for producing a holographic plate ;

[0044] Figure 2A shows an exemplary embodiment of an apparatus for producing a holographic plate ;

[0045] Figure 2B shows a possible spatial arrangement of elements of the apparatus shown in Figure 2B ;

[0046] Figure 3A shows a schematic view of a part of an apparatus for producing a holographic plate according to an exemplary embodiment ; Figures 3B and 3C each show an exemplary embodiment of a beam steering unit ;

[0047] Figure 4A is a schematic perspective view of an exemplary embodiment of a wearable augmented reality display including a holographic plate described herein;

[0048] Figure 4B shows an exemplary embodiment of an augmented reality display including a holographic plate according to an exemplary embodiment in a sectional view; and

[0049] Figure 4C shows an exemplary embodiment of an arrangement of exit pupils of a wearable augmented reality display using a holographic plate described herein .

[0050] The figures are schematic representations . The elements illustrated in the figures and their relationships among one another are not necessarily true to scale . Rather, individual elements or layer thicknesses may be represented with an exaggerated si ze for the sake of better representability and / or for the sake of better understanding .

[0051] Figure 1 schematically illustrates an exemplary embodiment of a method of producing a holographic plate 2 . The reference signs refer associated structural features in the further figures .

[0052] In a method step S I , a photosensitive material 20 is provided in a recording plane 19 . For example , the photosensitive material is a photopolymer . The photosensitive material 20 may, for example , be placed and optionally adj usted in an apparatus embodied as described below . In a method step S2, the recording plane is illuminated with a radiation of a light source 3 to produce a holographic pattern. This method step will be described in more detail in connection with Figures 2A to 3C.

[0053] After the illumination step, the photosensitive material may be finished, for example by thermal and / or chemical treatment. This method step is not explicitly illustrated in Figure 1.

[0054] Figure 2A illustrates an apparatus 1 which may be used to perform the illumination of the recording plane 19 with a radiation 30 of a light source 3 (cf. Figure 3A) to produce a holographic pattern 15 in the photosensitive material 20 provided in the recording plane 19 (cf . Figure 2B) .

[0055] The radiation of the light source 3 is split into a reference radiation 31 and an object radiation 32. The reference radiation 31 runs through a reference arm 11. The object radiation 32 runs through an object arm 12. An object phase modulator 35 is arranged in the object arm 12. The object radiation 32 and the reference radiation 31 interfere in the recording plane 19, thereby producing the holographic pattern 15. A diffusing element 4 is imaged onto the recording plane 19.

[0056] In the exemplary embodiment shown in Figure 2A, one diffusing element 4 is arranged in the object arm 12 between the object phase modulator 35 and the recording plane 19. A further diffusing element 45 is arranged in the reference arm 11. Thus, in this Figure each arm comprises one diffusing element that is imaged onto the recording plane. However, the method does not necessarily require two diffusing elements in both the reference arm 11 and the object arm 12. Rather, one or more diffusing elements 4, 45, may also be arranged in only one of the reference arm and the object arm.

[0057] For example, the diffusing element 4 and / or the further diffusing element may comprise or consist of a holographic diffuser 41 or a micro lens 42 or a combination thereof.

[0058] In the exemplary embodiment of Figure 2A, the object phase modulator 35 comprises an object lens array 351 comprising a plurality of object lenses 352. An optical axis of the object arm 12 extends parallel or at least substantially parallel to a normal of the recording plane 19, for example with an angle of at most 10° to the normal of the recording plane. The object lens array 351 extends perpendicularly with respect to the optical axis of the object arm 12.

[0059] Figure 2A illustrates beam paths in the object arm 12 for two different substeps. During the illumination of the recording plane 19, the object radiation 32 runs through only one of the object lenses 352 at the same time. Figure 2A illustrates the object radiation in a first substep 321 and the object radiation in a second substep 322.

[0060] Exemplary embodiments of the apparatus 1 providing the individual illumination of the object lenses 352 are described in connection with Figures 3B and 3C.

[0061] The object lenses 352 are configured to individually illuminate an area of an intermediate plane 325 which is to be imaged onto the recording plane 19. Thus, in both illumination substeps the area of intermediate plane 325 is fully illuminated but from slightly different directions. The radiation coming from the individual object lenses 352 can be diffused by placing a diffusing element 4 in the object arm 12, in particular in the intermediate plane 325 so that the diffusing element 4 is imaged onto the recording plane 19. Due to the optical interference of the object radiation with the reference radiation 31 in the recording plane 19, the diffused phase information relating to the respective, individually diffused object lens 352 is recorded in the holographic pattern 15.

[0062] However, it is not necessary to provide the diffusing element 4 in the intermediate plane 325 of object arm 12. Instead, the diffusing element 4 may also be placed in the reference arm 11, so that there is no diffusing element in the object arm. This likewise results in a diffused phase information relating to the respective, individually diffused object lens 352 is recorded in the holographic pattern 15.

[0063] An optical axis of the reference arm 11 extends obliquely with respect to the recording plane 19. For example, an angle between the normal to the recording plane 19 and the optical axis of the reference arm 11 is in a range from 10° to 80°, for example in a range from 50° to 70°.

[0064] Using the apparatus as shown in Figure 2A a transmission volume phase hologram can be produced. A reflection volume phase hologram can be produced with substantially the same apparatus by directing the reference radiation 31 and the object radiation 32 from opposite sides onto the recording plane 19.

[0065] As illustrated in Figure 2B, the diffusing element 4 in the reference arm (corresponding to the further diffusing element 45 in Figure 2A) may be imaged onto the recording plane 19 in a Scheimpflug configuration 18 . As illustrated in Figure 2B, a main surface plane 40 of di f fusing element 4 , a lens plane 112 of a reference arm optics 111 and the recording plane 19 intersect in a common intersection line 181 extending perpendicular to the drawing plane . In this arrangement , the image plane of the di ffusing element 4 imaged by reference arm optics 111 coincides with the recording plane 19 .

[0066] Figure 3A schematically illustrates an exemplary embodiment of a part of the apparatus 1 that produces the reference radiation 81 and the obj ect radiation 82 of Figure 2A.

[0067] A light source 3 produces a radiation 30 . For example , the light source comprises three lasers emitting in the red, the blue and the green spectral range .

[0068] The radiation 30 impinges onto a beam splitter 33 , dividing the radiation 30 into the reference radiation 31 and the obj ect radiation 32 .

[0069] The obj ect radiation 32 is directed onto a beam steering unit 34 to illuminate the obj ect phase modulator 35 .

[0070] Figure 3A further shows an optional chopper 36 which may be used to block the radiation 30 during an of f time between two subsequent illumination substeps . The apparatus 1 may further comprise a synchroni zation unit 37 configured to synchroni ze the chopper and the beam steering unit 34 . However, a chopper is not necessarily required . For example , the light source 3 may be directly turned of f between subsequent illumination substeps . In this case , the of f time denotes the time between two subsequent on times of the light source 3 . Figures 3B and 3C illustrate two exemplary embodiments of a beam steering unit 34 that may be used to illuminate the object phase modulator 35 at different positions during different substeps of the illumination of the photosensitive material 20. The repositioning of the object radiation 32 between subsequent substeps may be performed during the off time so that the photosensitive material 20 is not exposed to the radiation 32 during the repositioning of the radiation by the beam steering unit. Thus, the holographic pattern 15 is not negatively affected by the spatially displaced beam paths of the object radiation 32.

[0071] The off time can be shorter than a chemical reaction time of the photosensitive material 20 so that for the photosensitive material 20 the recordation of the holographic pattern 15 appears to be simultaneous. If, for example, the photosensitive material 20 comprises monomers of a photopolymer, the monomers may diffuse towards zones of high intensity and when exposed to the interfering radiation. The off time may be so short that this process is not negatively affected. For example, the off time is at most 10 milliseconds .

[0072] Figure 3B shows a detail of the object arm 12 of the apparatus 1 according to an exemplary embodiment.

[0073] The object arm comprises two optical elements, for example two collimating lenses forming a telescope 122. The telescope 122 may have an arbitrary magnification. The telescope 122 is arranged between the beam splitter 33 and the object phase modulator 35, which is an object lens array 351, for instance . A scanner 341 , for example two mirror galvanometers are arranged in a focal plane of the telescope 122 . The two mirror galvanometers form a two-dimensional mirror galvanometer as a scanning unit of the beam steering unit 34 . The mirror galvanometers in particular deflect the obj ect radiation 32 by rotating a mirror through a galvanometer setup .

[0074] The mirror galvanometers within the focal plane 21 of the telescope 122 direct the impinging obj ect radiation 32 in a similar direction of propagation and displace it simultaneously . In such a way the obj ect radiation sequentially illuminates each obj ect lens 352 in the obj ect lens array 351 individually . Figure 3B illustrates illuminating one of the obj ect lenses 352 with the obj ect radiation in a first substep 321 and another one of the obj ect lenses 352 with the obj ect radiation in a second substep 322 .

[0075] Instead of the two mirror galvanometers 22 forming a two- dimensional mirror galvanometer as a scanner 341 of the beam steering unit 34 , a two-dimensional optical phased array 342 can be arranged in the focal plane of the telescope 122 . In that case , the two-dimensional optical phased array 342 forms the scanning unit of the beam steering unit 34 .

[0076] Figure 3C shows a detail of the obj ect arm 12 of the apparatus 1 according to another exemplary embodiment .

[0077] The apparatus 1 according to this exemplary embodiment comprises a digital light processing element 343 as scanning unit of the beam steering unit 34 . As in Figure 3B, an obj ect lens array 351 acts as an obj ect phase modulator 35 . The digital light processing element 343 comprises a plurality of microscopic small mirrors which can be switched electronically and independently from each other .

[0078] A sequence of binary images , for example masks , is cycled on the digital light processing element 343 to produce spatially separated obj ect radiation beamlets .

[0079] A telescope 122 may be arranged between the digital light processing element 343 and the obj ect lens array 351 . The telescope 122 may be formed by two collimating lenses , for instance . The digital light processing element 343 is magni fied and imaged by the telescope 122 on the obj ect lens array 351 .

[0080] Figure 3C illustrates illuminating one of the obj ect lenses 352 with the one of the beamlets of the obj ect radiation in a first substep 321 and another one of the obj ect lenses 352 with another one of the beamlets of the obj ect radiation in a second substep 322 .

[0081] The apparatus 1 is particularly suited to produce a holographic plate 2 , for example as a fan-out hologram 10 providing a plurality of individually di f fused pupils to expand the eyebox in a device for augmented reality or virtual reality applications , for example for head-up displays or for wearable AR or VR devices .

[0082] Figures 4A to 4C illustrates a wearable AR display 7 using a holographic plate 2 as described above . The display 7 comprises a support frame 71 with a central axis A7 and an optical system in the form of an of f-axis retinal scanning display mounted on the support frame 71 . The optical system comprises an image generator in the form a proj ector 72 , for example embodied as a scanning laser proj ector emitting an image light I and an eyepiece comprising an optical combiner 73 . The proj ector 72 is of fset from the central axis A7 .

[0083] In use , when the support frame 71 is mounted on a head of a user with the eyepiece including the optical combiner 73 positioned in a field of view of the user, the optical combiner 73 transmits ambient light from a scene located in front of the optical combiner 73 through the optical combiner 73 to an eye 73 of the user located behind the optical combiner 73 . The proj ector 72 proj ects the linearly-polari zed image light I defining an image towards the eye 75 of the user by way of the optical combiner 73 . The linearly- polari zed image light I may include one or more wavelengths such as one or more of red light , green light or blue light .

[0084] The optical combiner 73 replicates the image defined by the proj ected image light I a number of times at a plurality of positions in a plane 74 at the eye 75 of the user to expand an eyebox of the wearable AR display 7 . This pupil replication using a holographic plate is described in more detail in connection with Figure 4B . In this exemplary embodiment , the holographic plate forms a transmission volume phase hologram .

[0085] Figure 4B illustrates the optical system in use replicating an image defined by three di f ferent linearly-polari zed principal rays constituting the image light I at three di f ferent positions in the plane 74 at the eye 75 of the user to provide an expanded eyebox for each principal ray of the proj ected image light I . At these three di f ferent positions a pupil of the proj ected light provides the whole image light I as illustrated by the three principal rays present at each of the pupils in plane 74 .

[0086] For this purpose , the optical combiner 73 includes a fan-out hologram plate 10 with the holographic plate 2 which functions as an optical spreader for fanning-out the proj ected image light I to form spread image light . Thus , in Figure 4B each of the principal rays of image light I coming from proj ector 72 is split into three rays transmitted through the fan-out hologram plate 10 .

[0087] The optical combiner 73 further includes a reflector 76 in the form of a ' reflective pancake ' for collimating the spread image light and for reflecting the collimated light back through the holographic plate 2 to form collimated light which propagates to the plane 74 to provide the expanded eyebox in the plane 74 .

[0088] The reflector 76 is embodied as an optically-powered reflector and comprises a dichroic reflective coating . The dichroic reflective coating is configured to be highly reflecting in one or more narrow spectral bands , each narrow spectral band being arranged around a corresponding wavelength of the image light I , but to transmit light at other wavelengths of the ambient light .

[0089] The fan-out hologram plate 10 further includes a polari zation-dependent reflector 54 and a retarder 56 which comprises , or which is configured to act as , a quarter-wave plate .

[0090] The polari zation-dependent reflector 54 and the dichroic reflective coating of the optically-powered reflector 76 of the optical combiner 73 define an optical cavity, wherein the retarder 56 is located in the optical cavity . Thus , the retarder 56 changes the polari zation of the radiation while it passes through the retarder 56 .

[0091] By means of this configuration, the radiation transmitted through the holographic plate 2 passes the optical cavity four times before it is transmitted through the holographic plate 2 towards the eye 75 .

[0092] In ef fect , the reflective pancake optical combiner 73 provides a folded optical path for the image light I . As such, use of the reflective pancake optical combiner serves to reduce the physical thickness of the eyepiece including the optical combiner 73 resulting in a more compact eyepiece .

[0093] Figure 4C illustrates an exemplary embodiment of the arrangement of pupils 8 to expand the eyebox 74 . The main pupil of the image light I produced by proj ector 72 during operation is split into a subset of pupils 8 by means of holographic plate 2 as described above .

[0094] In the exemplary embodiment , 19 pupils are arranged in a hexagonal pattern . However, the number of pupils 8 may be varied in wide ranges . For example , the number of pupils 8 is in a range from 3 to 100 . A hexagonal pattern allows to obtain small distances between adj acent circular pupils , but other patterns , for example a rectangular pattern, may also be used . The number of pupils can be defined during recording the holographic plate , for example by a corresponding number of obj ect lenses as described above . The dashed lines of pupils 8 schematically illustrate a comparably sharp border that the pupils 8 would have, if the pupils 8 were not diffused. As described above, however, the pupils 8 are individually diffused. When replaying the holographic plate 2 with the light of the projector 72, the diffused phase information recorded in the holographic pattern 15 of the holographic plate 2 results in diffused pupils 8 of the image light I in the eyebox 74. Consequently, the image light I completely or at least party fills the gaps 81 between the individual pupils 8.

[0095] This allows to obtain an enlarged eyebox 74, wherein the homogeneity of the intensity of the image light I as perceived by the user is increased.

[0096] The invention described herein is not restricted by the description given with reference to the exemplary embodiments. Rather, the invention encompasses any novel feature and any combination of features, including in particular any combination of features in the claims, even if this feature or this combination is not itself explicitly indicated in the claims or exemplary embodiments.

[0097] References

[0098] 1 apparatus

[0099] 10 fan-out hologram

[0100] 11 reference arm

[0101] 111 reference arm optics

[0102] 112 lens plane of reference optics

[0103] 12 obj ect arm

[0104] 121 obj ect arm optics

[0105] 122 telescope

[0106] 15 holographic pattern

[0107] 18 Scheimpflug configuration

[0108] 181 intersection line

[0109] 19 recording plane

[0110] 2 holographic plate

[0111] 20 photosensitive material

[0112] 3 light source

[0113] 30 radiation

[0114] 31 reference radiation

[0115] 32 obj ect radiation

[0116] 321 obj ect radiation, first substep

[0117] 322 obj ect radiation, second substep

[0118] 325 area of intermediate plane

[0119] 33 beam splitter

[0120] 34 beam steering unit

[0121] 341 scanner

[0122] 342 optical phased array

[0123] 343 digital light processing element , DLP

[0124] 35 obj ect phase modulator

[0125] 351 obj ect lens array

[0126] 352 obj ect lens

[0127] 36 chopper

[0128] 37 synchroni zation unit 4 diffusing element

[0129] 40 main surface plane

[0130] 41 holographic diffuser

[0131] 42 micro lens array

[0132] 45 further diffusing element

[0133] 54 polarization-dependent reflector

[0134] 56 retarder

[0135] 7 augmented reality display device

[0136] 71 support frame

[0137] 72 projector

[0138] 73 optical combiner

[0139] 74 eyebox

[0140] 75 eye of a user

[0141] 76 reflector

[0142] 8 pupil

[0143] 81 gap

[0144] A7 central axis of the support frame

[0145] I image light

[0146] SI, S2 method step

Claims

Claims1. A method of producing a holographic plate (2) , comprises the steps of: a) providing a photosensitive material (20) in a recording plane (19) ; b) illuminating the recording plane (19) with a radiation of a light source (3) to produce a holographic pattern (15) ; wherein- the radiation of the light source (3) is split into a reference radiation (31) and an object radiation(32) ;- the reference radiation (31) runs through a reference arm (11) ;- the object radiation (32) runs through an object arm (12) ;- an object phase modulator (35) is arranged in the ob j ect arm ( 12 ) ;- the object radiation (32) and the reference radiation (31) interfere in the recording plane (19) ;- a diffusing element (4) which is imaged onto the recording plane (19) is arranged in the reference arm (11) or in the object arm (12) between the object phase modulator (35) and the recording plane (19) .

2. The method according to claim 1, wherein the object phase modulator (35) comprises an object lens array (351) with a plurality of object lenses (352) , wherein the object radiation (32) runs through only one of the object lenses (352) at the same time.

3. The method according to claim 2, wherein the object radiation (32) is sequentially guided through the object lenses (352) .

4. The method according to any of the preceding claims, wherein the diffusing element (4) comprises a holographic diffuser (41) .

5. The method according to any one of the preceding claims, wherein the diffusing element (4) comprises a micro lens array ( 42 ) .

6. The method according to claim 2 or any claim referring back to claim 2, wherein- in the object arm (12) an area of an intermediate plane(325) is imaged onto the recording plane (19) ; and- the object lenses (352) are configured to individually illuminate the area of the intermediate plane (325) .

7. The method according to claim 6, wherein the diffusing element (4) is arranged in the object arm (12) in the intermediate plane (325) .

8. The method according to any one of the preceding claims, wherein the diffusing element (4) is arranged in the reference arm (11) .

9. The method according to claim 8, wherein a main surface plane (40) of the diffusing element(4) is arranged at an oblique angle with respect to the recording plane (19) .

10. The method according to claim 8 or 9, wherein the diffusing element (4) is imaged onto the recording plane (19) in a Scheimpflug configuration (18) .

11. A holographic plate (1) produced according to one of the preceding claims.

12. The holographic plate according to claim 11, wherein the holographic plate (2) is configured as a volume phase hologram for an optical combiner (76) configured for an augmented and / or virtual reality display device (7) , wherein upon illumination by a projector (72) the optical combiner (76) with the holographic plate (2) is configured to provide a radiation of the projector (72) through a plurality of pupils (8) , wherein the pupils (8) are diffused, so that the radiation at least partly fills gaps (81) between the pupils (8) .

13. An apparatus (1) for producing a holographic plate (2) , the apparatus (1) comprising:- a light source (3) configured for emitting electromagnetic laser radiation (30) ,- a beam splitter (33) spitting the laser radiation into a reference radiation (31) running through a reference arm towards a recording plane (19) and into an object radiation (32) running through an object arm (12) towards the recording plane (19) ,- an object phase modulator (35) is arranged in the object arm ( 12 ) ; and- a diffusing element (4) configured to be imaged onto the recording plane (19) , wherein the diffusing element (4) is arranged in the reference arm (11) or in the object arm (12) .

14. The apparatus according to claim 13, wherein the apparatus (1) further comprises a beam steering unit (34) configured for directing the object radiation (32) on different locations of the object phase modulator (35) .

15. The apparatus according to claim 13 or 14, wherein the apparatus (1) comprises a further diffusing element (45) , wherein the diffusing element (4) is arranged in the object arm (12) and the further diffusing element (45) is arranged in the reference arm (11) or vice versa.

16. An apparatus according to any one of claims 13 to 14, wherein the apparatus (1) is configured to perform a method according to any one of claims 1 to 10.

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