Method of producing a holographic plate, holographic plate and apparatus

Abstract

A method of producing a holographic plate (2) is specified, the method comprising the steps of: a) providing a photosensitive material (20) in a recording plane (19); and 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); - 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); and - the recording plane (19) is sequentially illuminated in a plurality of substeps (321, 322), wherein the illumination of the recording plane is modified between the substeps (321, 322). Further, a holographic plate (2) and an apparatus (1) are specified.

Description

[0001] Description

[0002] METHOD OF PRODUCING A HOLOGRAPHIC PLATE, HOLOGRAPHIC PLATE

[0003] AND APPARATUS

[0004] The present application relates to a method of producing a holographic plate, to a holographic plate and to an apparatus .

[0005] In many optical applications diffusers are used. However, conventional diffusers are not suitable for applications where a transparent, haze-free vision through the diffusor is required .

[0006] It is an object to provide a way to obtain specific diffusive properties .

[0007] This object is achieved 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.

[0008] A method of producing a holographic plate is specified.

[0009] 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 .

[0010] For example , the photosensitive material , such as the photopolymer, is configured to change its refractive index upon illumination .

[0011] 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 . In the context of the present application, the term " light" is not limited to electromagnetic radiation in the visible spectral range .

[0012] 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 .

[0013] 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 . The light source may emit radiation of a single peak emission wavelength or radiation of two or three di f ferent peak emission wavelengths simultaneously .

[0014] According to at least one embodiment of the method, the radiation of the light source is split into a reference radiation and an obj ect radiation, wherein the obj ect 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.

[0015] For example, an object phase modulator may be arranged in the object arm. The object phase modulator may be configured for imparting a phase profile on the radiation. In particular, during illumination of the recording plane, the object phase modulator may impart 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.

[0016] 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. 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.

[0017] 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 may be affected by the phase profile formed by the object phase modulator and by the diffusing element. According to at least one embodiment of the method, the recording plane is sequentially illuminated in a plurality of substeps. In particular, the illumination of the recording plane is modified between the substeps. As the object radiation or the reference radiation passes through the diffusing element, each substep provides information on the diffusing element. Thus, the produced holographic plate may include multiplexed information on the diffusing element.

[0018] In at least one embodiment of a method of producing a holographic plate, the method comprising 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. The radiation of the light source is split into a reference radiation and an object radiation, wherein the reference radiation runs through a reference arm, the object radiation runs through an object arm, and 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. The recording plane is sequentially illuminated in a plurality of substeps, wherein the illumination of the recording plane is modified between the substeps.

[0019] The holographic plate produced in this way may include multiplexed information on the diffusing element. Thus, the holographic plate may act as a diffuser only for radiation incident at specific angular and wavelength ranges of the radiation associated to the respective substeps. Outside these ranges, the holographic plate may be transparent and free of haze. In other words, the method takes advantage of the effect that the Bragg selectivity of the holographic plate, i.e. the ability to diffract specific angular and wavelength ranges based on properties of the photosensitive material as recording medium and on the recording configuration, in order to produce a diffusing element in the holographic plate that enables a clear vision through it. The specific wavelength and angular ranges defined during the recording of the holographic pattern, in contrast, are diffused when replaying the holographic plate.

[0020] For example, the holographic plate comprises a volume phase hologram, VPH, such as a transmission VPH or a reflection VPH.

[0021] For example, the holographic plate may be used in an AR optical system such as an optical combiner configured 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. For example, this helps to obtain a large eyebox with a high homogeneity of the intensity of a projected image as perceived by the user. At the same time, the holographic plate may allow for a clear vision through the holographic plate to see the environment.

[0022] This helps to obtain a large eyebox with a high homogeneity of the intensity of a projected image as perceived by the user .

[0023] However, the holographic plate may also be used for other applications such displays like head-up displays or any other application requiring transparency and diffusive properties at the same time. For example, the holographic plate may be used for consumer devices or medical applications.

[0024] According to at least one embodiment of the method, a time distance between two subsequent substeps is smaller than a chemical reaction time interval of the photosensitive material. Thus, the photosensitive material essentially behaves as if it were subjected to a continuous exposure. Consequently, diffractive optical elements, in particular diffusing optical elements associated to the respective substep 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. At the same time, the holographic plate may allow for a clear vision through it.

[0025] 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 object radiation and the reference radiation during the illumination substeps .

[0026] According to at least one embodiment of the method, an angle of incidence of the radiation illuminating the recording plane is changed between the substeps. Thus, the holographic plate may act as a diffuser for different angles of incidence. In other words, the multiplexed illumination on the diffusing element may result from different angles of incidence of the radiation illuminating the recording plane during the substeps. For example, an angle of incidence of the radiation illuminating the diffusing element is changed between the substeps. For example, the same area of the diffusing element may be imaged onto the recording plane for all substeps.

[0027] According to at least one embodiment of the method, the object phase modulator comprises an object lens array with a plurality of object lenses, wherein the object radiation is sequentially guided through exactly one of the object lenses in each substep. For example, each object lens of the object lens array illuminates the same area on the recording plane, but from a slightly different angle. In particular, the diffusing element may be arranged in the object arm between the object phase modulator and the recording plane. However, the diffusing element may also be arranged in the reference arm.

[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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] According to at least one embodiment of the method, a peak emission wavelength of the radiation illuminating the recording plane is tuned between the substeps. For example, the peak emission wavelengths used for two different substeps may differ from one another by at least 0.1 nm or at least 0.5 nm or at least 1 nm and / or at most 500 nm or at most 100 nm or at most 20 nm. In particular, the wavelength changes may occur in discrete steps between the illumination substeps. During the substeps of the illumination, the wavelength may be kept constant. Thus, the multiplexed information on the diffusing element in the finished holographic plate results from different peak emission wavelengths of the radiation illuminating the recording plane during the substeps. When replaying the holographic plate, the holographic plate may act as a diffuser for radiation impinging from the same direction, but having different wavelengths .

[0034] According to at least one embodiment of the method, a position of the diffusing element with respect to the radiation illuminating the diffusing element is changed between the substeps. Thus, the multiplexed illumination on the diffusing element in the finished holographic plate may result from a movement of the diffusing element with respect to the radiation illuminating the diffuser during the substeps. For example, the movement of the diffusing element includes a displacement, in particular along its main extension plane, and / or a rotation. For example, the movement occurs in discrete steps, wherein the position of the diffusing element is stationary during the substeps of the illumination .

[0035] According to at least one embodiment of the method, the diffusing element comprises at least one of: a holographic diffuser, a ground glass diffuser, a photolithographic diffuser, a prismatic diffuser, 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.

[0036] The object radiation and / or the reference radiation may be collimated, divergent, convergent, or have a freeform wavefront .

[0037] 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 significant 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%.

[0038] According to at least one embodiment of the method, the holographic plate produced as described above is used as a master to produce a copy of the holographic plate. In this context, the term "copy" in particular means that the master and the produced copy substantially have the same or at least substantially the same optical properties and / or fulfil the same or at least substantially the same optical function. If, for example, the master is a reflection volume phase hologram configured to reflect radiation impinging at a specific angle of incidence, this likewise applies to the copy. Further, diffusive properties of the master may be transferred to the copy . Thus , the copy is a holographic plate that substantially has the same or at least substantially the same properties as the master . Features and embodiments disclosed in connection with the holographic plate or the master may also apply to the copy or vice versa .

[0039] According to at least one embodiment of the method, a photosensitive material is provided on the master . At this stage , the finishing step has already been performed, so that photosensitive material has already been developed and the holographic plate acting as master has been finished . For example , the photosensitive material directly adj oins the master .

[0040] Materials suited for the production of the master may also be used for the production of the copy . For example , the same material is used for the production of the master and for the production of the copy . However, the materials do not necessarily have to be the same .

[0041] According to at least one embodiment of the method, the master is illuminated with either the reference radiation or the obj ect radiation to produce the copy of the master in the photosensitive material .

[0042] In particular, the same apparatus may be used for the production of the master and for the production of the copy . At this step, the master is located in the recording plane . However, the apparatus may be slightly modi fied, for example with respect to the di f fusing element ( s ) and / or the light source . I f the master is illuminated with the reference radiation, the master may reproduce the obj ect radiation . Likewise , the master may reproduce the reference radiation i f the master is illuminated with the obj ect radiation . In both cases , the radiation illuminating the master and the radiation reproduced by the master may interfere resulting in a holographic pattern within the photosensitive material . Thus , the holographic pattern of the master may be copied .

[0043] In at least one embodiment of the method of producing a holographic plate , the method comprises a step of providing a photosensitive material on a master comprising a holographic pattern; and a step of illuminating the holographic pattern of the master with radiation from a light source to produce a copy of the master in the photosensitive material , wherein a di f fusing element is arranged in a beam path between the light source and the master when illuminating the holographic pattern of the master . Thus , a di f fusing element is used during the production of the copy .

[0044] The master can be produced as described above . In departure therefrom, it is not necessarily required for the master to be produced using a di f fusing element . However, one or more di f fusing elements may be used both during the production of the master and during the replication of the master to produce the copy .

[0045] According to at least one embodiment of the method, the master is illuminated with incoherent light to produce the copy . For example , a light emitting diode ( LED) is used . As one of the beams that interfere within the photosensitive material is produced by the master, a comparable short coherence length may be suf ficient to replicate the master . The use of the light source emitting the reference beam of electromagnetic radiation with low coherence helps to avoid unintended ef fects such as speckle patterns that may occur i f coherent light sources such as lasers are used . In this way, manufacturing of the holographic plate can be simpli fied . Also , parasitic inference may be avoided during the production of the holographic plate . However, a coherent light source may be used for the replication of the master as well .

[0046] According to at least one embodiment of the method, the photosensitive material is provided in a beam path between the light source and the master . Thus , the radiation passes the photosensitive material before it impinges onto the master . For example , the master is a reflection volume phase hologram .

[0047] According to at least one embodiment of the method, the master is provided in a beam path between the light source and the photosensitive material . Thus , the radiation passes through the master before it impinges onto the photosensitive material . For example , the master is a transmission volume phase hologram .

[0048] Further, a holographic plate is speci fied .

[0049] In particular, the holographic plate may be produced according to the methods described above . Thus , features described in connection with the method also apply for the holographic plate and vice versa .

[0050] 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 .

[0051] Upon illumination by a proj ector, an optical combiner with the holographic plate is configured to provide a radiation of the proj ector through a plurality of pupils . In particular, the pupils are di f fused so that the radiation at least partly fills gaps between the pupils .

[0052] By means of the di f fused pupils , the holographic plate enlarges the eyebox of the AR or VR device and it also homogeni zes the intensity across the proj ected image as perceived by the user .

[0053] Further, an apparatus for producing a holographic plate is speci fied .

[0054] 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 obj ect radiation running through an obj ect arm towards the recording plane . A di f fusing element is configured to be imaged onto the recording plane wherein the di f fusing element is arranged in the reference arm or in the obj ect arm . The apparatus is configured for sequentially illuminating the recording plane in a plurality of substeps , wherein the illumination of the recording plane is modi fied between the substeps . 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.

[0055] 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.

[0056] 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.

[0057] According to at least one embodiment of the apparatus, wherein the apparatus is configured to tune a peak emission wavelength of the radiation illuminating the diffusing element between the substeps. For example, the same light source is used to provide the radiation having di f ferent peak emission wavelengths . Alternatively, di f ferent light sources may be used .

[0058] According to at least one embodiment of the apparatus , the apparatus is configured to change a position of the di f fusing element with respect to the radiation illuminating the di f fuser between the substeps .

[0059] According to at least one embodiment of the apparatus , the apparatus is configured to change an angle of incidence of the radiation illuminating the recording plane between the substeps .

[0060] The apparatus may be used to perform the method described above . Thus , features described in connection with the methods or the holographic plate may also apply for the apparatus and vice versa .

[0061] Further, features described in connection with at least one embodiment of the method, the apparatus , or the holographic plate may be combined with features described in connection with other embodiments of the method, the apparatus , or the holographic plate , unless they are contradictory .

[0062] The methods , the apparatus and the 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 .

[0063] In the figures :

[0064] Figure 1 shows a schematic block diagram of an exemplary embodiment of a method for producing a holographic plate ;

[0065] Figure 2A shows an exemplary embodiment of an apparatus for producing a holographic plate ;

[0066] Figure 2B shows a possible spatial arrangement of elements of the apparatus shown in Figure 2B ;

[0067] Figure 2C shows an exemplary embodiment of an apparatus for producing a holographic plate ;

[0068] Figure 3A shows a schematic view of a part of an apparatus for producing a holographic plate according to an exemplary embodiment ;

[0069] Figures 3B and 3C each show an exemplary embodiment of a beam steering unit ;

[0070] Figure 4A is a schematic perspective view of an exemplary embodiment of a wearable augmented reality display including a holographic plate described herein;

[0071] Figure 4B shows an exemplary embodiment of an augmented reality display including a holographic plate according to an exemplary embodiment in a sectional view; Figure 4C shows an exemplary embodiment of an arrangement of exit pupils of a wearable augmented reality display using a holographic plate described herein;

[0072] Figures 5A and 5B shows an exemplary embodiment of a method for producing a holographic plate by way of intermediate steps ; and

[0073] Figure 6 shows an exemplary embodiment of a method for producing a holographic plate .

[0074] 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 .

[0075] Figure 1 schematically illustrates an exemplary embodiment of a method of producing a holographic plate 2 . The reference signs refer to associated structural features in the further figures for the sake of better understanding .

[0076] 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 .

[0077] In a method step S2 , the recording plane is illuminated with a radiation of a light source 3 to produce a holographic pattern 15 . In particular, the illumination may be performed in a plurality of substeps , wherein the illumination of the recording plane is modified between the substeps. This method step will be described in more detail in connection with Figures 2A to 3C.

[0078] 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.

[0079] 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) .

[0080] 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. 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.

[0081] In the exemplary embodiment shown in Figure 2A, one diffusing element 4 is arranged in the object arm 12. 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. 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 array 42 or a combination thereof. Alternatively or in addition, a ground glass diffuser, a photolithographic diffuser, or a prismatic diffuser, or a combination thereof may also be used.

[0082] The recording plane 19 is sequentially illuminated in a plurality of substeps, wherein the illumination of the recording plane is modified between the substeps.

[0083] In the exemplary embodiment of Figure 2A, an angle of incidence of the radiation illuminating the diffusing element 4 is changed between the substeps.

[0084] As illustrated in Figure 2A, this can be achieved via an object phase modulator 35 arranged in the object arm 12, wherein the diffusing element 4 is arranged in the object arm between the object phase modulator 35 and the recording plane 19. 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.

[0085] 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. Thus, the object radiation 32 is sequentially guided through exactly one of the object lenses 352 in each substep 321, 322. Figure 2A illustrates an angle of incidence 401 with respect to a normal to the diffusing element 4 for the radiation of the second substep 322 and an angle of incidence 191 of the radiation impinging onto the recording plane 19 with respect to a normal to the recording plane 19.

[0086] Exemplary embodiments of the apparatus 1 providing the individual illumination of the object lenses 352 are described in connection with Figures 3B and 3C.

[0087] 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.

[0088] 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 32 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. Thus, the produced holographic plate may represent a transparent optical element that acts as a diffuser for radiation impinging from specific angles only . 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.

[0089] 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°.

[0090] 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. This is illustrated in Figure 2C.

[0091] 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 diffusing 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 diffusing element 4 imaged by reference arm optics 111 coincides with the recording plane 19. The Scheimpflug configuration 18 may also be used for the embodiment of Figure 2C to produce a reflection volume phase hologram .

[0092] 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.

[0093] 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 .

[0094] The radiation 30 impinges onto a beam splitter 33 , dividing the radiation 30 into the reference radiation 31 and the obj ect radiation 32 .

[0095] The obj ect radiation 32 is directed onto a beam steering unit 34 to illuminate the obj ect phase modulator 35 .

[0096] 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 .

[0097] Figures 3B and 3C illustrate two exemplary embodiments of a beam steering unit 34 that may be used to illuminate the obj ect phase modulator 35 at di f ferent positions during di f ferent 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.

[0098] 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 .

[0099] Figure 3B shows a detail of the object arm 12 of the apparatus 1 according to an exemplary embodiment.

[0100] 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 .

[0101] 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 .

[0102] 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 .

[0103] 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 .

[0104] Figure 3C shows a detail of the obj ect arm 12 of the apparatus 1 according to another exemplary embodiment .

[0105] 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 . A sequence of binary images, for example masks, is cycled on the digital light processing element 343 to produce spatially separated object radiation beamlets.

[0106] A telescope 122 may be arranged between the digital light processing element 343 and the object lens array 351. The telescope 122 may be formed by two collimating lenses, for instance. The digital light processing element 343 is magnified and imaged by the telescope 122 on the object lens array 351.

[0107] Figure 3C illustrates illuminating one of the object lenses 352 with the one of the beamlets of the object radiation in a first substep 321 and another one of the object lenses 352 with another one of the beamlets of the object radiation in a second substep 322.

[0108] In departure from Figures 2A and 2C, the illumination of the recording plane may also be varied between the substeps in different ways. This may be done instead of or in addition to the illumination from different angles of incidence.

[0109] For example, a peak emission wavelength of the radiation emitted by the light source 3 illuminating the recording plane 19 is tuned between the substeps. For example, the peak emission wavelengths used for two different substeps may differ from one another by at least 0.1 nm or at least 0.5 nm or at least 1 nm and / or at most 500 nm or at most 100 nm or at most 20 nm.

[0110] Thus, the multiplexed information on the diffusing element 4 and / or the further diffusing element 45 in the finished holographic plate 2 results from different peak emission wavelengths of the radiation illuminating the recording plane during the substeps. When replaying the holographic plate, the holographic plate may act as a diffuser for radiation impinging from the same direction, but having different wavelengths .

[0111] Alternatively or in addition, a position of the diffusing element 4 and / or the further diffusing element 45 with respect to the radiation illuminating the diffusing element and / or the further diffusing element 45 is changed between the substeps. Thus, the multiplexed illumination on the diffusing element 4 and / or the further diffusing element 45 in the finished holographic plate 2 may result from a movement of the diffusing element with respect to the radiation illuminating the diffusing element 4 and / or the further diffusing element 45 during the substeps. For example, the movement of the diffusing element includes a displacement, in particular along its main extension plane, and / or a rotation.

[0112] Thus, the finished holographic plate 2 may include superimposed information on different diffusing properties.

[0113] The apparatus 1 and the method are particularly suited to produce a holographic plate 2 acting as a diffuser for specific wavelength ranges and / or specific ranges of the angle of incidence of the impinging radiation when replaying the holographic plate. For other radiation components, the holographic plate 2 may be transparent and free of haze.

[0114] For example, the holographic plate 2 may be used in a display, such as a head-up display or a display of a portable or wearable electronic device. For example, the holographic plate 2 may be used for an augmented and / or virtual reality device such as glasses , or a headset , or a helmet .

[0115] For example , 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 .

[0116] 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 .

[0117] 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 . 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 .

[0118] 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 .

[0119] 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 .

[0120] 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 . 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 .

[0121] 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 .

[0122] 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 .

[0123] 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 .

[0124] 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 .

[0125] 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 projector 72 during operation is split into a subset of pupils 8 by means of holographic plate 2 as described above.

[0126] 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 adjacent 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 object lenses as described above.

[0127] 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.

[0128] 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.

[0129] At the same time, the holographic plate does not impede the user' s vision of the surroundings as the holographic plate is transparent to the radiation from the surroundings even though it acts as a di f fuser for the radiation emitted by the pro j ector 72 .

[0130] Figures 5A and 5B illustrate an exemplary embodiment of a method of producing a holographic plate by replicating a master 25 . The master 25 is a holographic plate that may be produced as described above .

[0131] In the exemplary embodiment of Figure 5A a photosensitive material 20 is provided on the master 25 . The master 25 with the photosensitive material 20 is illuminated with the reference radiation 31 . In particular, the photosensitive material may be in direct contact to the holographic pattern 15 of the master 25 .

[0132] In particular, the apparatus 1 used to produce the master 25 may also be used to replicate the master 25 wherein the master 25 is located in the recording plane 19 .

[0133] The master 25 is a reflection volume phase hologram . The photosensitive material 20 is arranged in a beam path of the reference radiation 31 on its way to the master 25 .

[0134] Figure 5A illustrates a wavefront 318 and a coherence volume 319 of the reference radiation 31 . A holographic pattern 15 of the master 25 reproduces the obj ect radiation 32 . A wavefront 328 of the obj ect radiation and a coherence volume 329 are illustrated in the Figure .

[0135] The reference radiation 31 and the reproduced obj ect radiation 32 interfere within the photosensitive material 20 thereby exposing the photosensitive material 20 to replicate the holographic pattern 15 of the master in the photosensitive material 20 .

[0136] By means of a finishing step including developing the photosensitive material 20 , a holographic plate may be produced that represents a copy of the master and consequently has substantially the same optical properties as the master 25 . Figure 5B illustrates the finished copy 26 detached from the master 25 . The master 25 can be used in subsequent steps to produce further copies 26 .

[0137] As the obj ect radiation 32 is reproduced by the holographic pattern 15 , the coherence volume 319 of the reference radiation 31 and the coherence volume 329 of the reproduced obj ect radiation 32 may overlap in the photosensitive material 20 even i f an incoherent light source such as an LED is used . Alternatively, coherent radiation may be used to illuminate the master 25 .

[0138] In departure from the representation in Figure 5A, the master 25 may also be illuminated with the obj ect radiation 32 . In this case , the master 25 would reproduce the reference radiation 31 resulting in interference ef fects as described above .

[0139] As described above , a di f fusing element 4 and / or a further di f fusing element 45 may be used to produce the master 25 . In this case , the same or substantially the same setup of the apparatus 1 may be used to produce the copy of the master 25 . In particular, the di f fusing element ( s ) may be present during the production of the copy of the master 25 as well .

[0140] Alternatively, at least one of the di f fusing elements or all di f fusing elements may be removed for the production of the copy .

[0141] As a further alternative , a di f fusing element may be used for the reproduction of the master 25 only . Thus , di f fusing elements can be dispensed with during the production of the master 25 .

[0142] The exemplary embodiment of Figure 6 substantially corresponds to the exemplary embodiment of Figures 5A and 5B .

[0143] However, the master 25 to be copied is a transmission volume phase hologram . The photosensitive material 20 is arranged on the master 25 such that the reference radiation 31 that illuminates the master 25 has to pass through the master 25 before it reaches the photosensitive material 20 . As described in connection with Figure 5A, the master 25 may also be illuminated with the obj ect radiation 32 to reproduce the reference radiation 31 by means of the holographic pattern 15 .

[0144] Using the methods and / or the apparatus described above , holographic plates may be produced that in particular take advantage of the Bragg selectivity of volume phase holograms to produce a see-through transparent optical element free of haze capable of di f fusing only that light that is incident at a speci fic angle and within a speci fic wavelength range . Conventional di f fusers , in contrast , are typically broadband in terms of wavelength and angle of incidence , so that they are not suited for applications where haze- free transparency is required such as for applications where the optical element is to be placed close to the user' s eye . The di f fused light may be of a single color or comprise several spectral components such as in the red, green, and blue spectral range to obtain a full color RGB representation .

[0145] This patent application claims the priority of German patent application 10 2024 116 523 . 5 , the disclosure content of which is hereby incorporated by reference .

[0146] 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 i f this feature or this combination is not itsel f explicitly indicated in the claims or exemplary embodiments .

[0147] References

[0148] 1 apparatus

[0149] 10 fan-out hologram

[0150] 11 reference arm

[0151] 111 reference arm optics

[0152] 112 lens plane of reference optics

[0153] 12 obj ect arm

[0154] 121 obj ect arm optics

[0155] 122 telescope

[0156] 15 holographic pattern

[0157] 18 Scheimpflug configuration

[0158] 181 intersection line

[0159] 19 recording plane

[0160] 191 angle of incidence

[0161] 2 holographic plate

[0162] 20 photosensitive material

[0163] 25 master

[0164] 26 copy

[0165] 3 light source

[0166] 30 radiation

[0167] 31 reference radiation

[0168] 318 wavefront of reference radiation

[0169] 319 coherence volume of reference radiation

[0170] 32 obj ect radiation

[0171] 321 obj ect radiation, first substep

[0172] 322 obj ect radiation, second substep

[0173] 325 area of intermediate plane

[0174] 328 wavefront of obj ect radiation

[0175] 329 coherence volume of obj ect radiation

[0176] 33 beam splitter

[0177] 34 beam steering unit

[0178] 341 scanner 342 optical phased array

[0179] 343 digital light processing element, DLP

[0180] 35 object phase modulator

[0181] 351 object lens array

[0182] 352 object lens

[0183] 36 chopper

[0184] 37 synchronization unit

[0185] 4 diffusing element

[0186] 40 main surface plane

[0187] 401 angle of incidence

[0188] 41 holographic diffuser

[0189] 42 micro lens array

[0190] 45 further diffusing element

[0191] 54 polarization-dependent reflector

[0192] 56 retarder

[0193] 7 augmented reality display device

[0194] 71 support frame

[0195] 72 projector

[0196] 73 optical combiner

[0197] 74 eyebox

[0198] 75 eye of a user

[0199] 76 reflector

[0200] 8 pupil

[0201] 81 gap

[0202] A7 central axis of the support frame

[0203] I image light

[0204] SI, S2 method step

Claims

Claims1. A method of producing a holographic plate (2) , comprising the steps of: a) providing a photosensitive material (20) in a recording plane (19) ; and 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) ;- 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) ; and- the recording plane (19) is sequentially illuminated in a plurality of substeps (321, 322) , wherein the illumination of the recording plane is modified between the substeps (321, 322) .

2. The method according to claim 1, wherein a time distance between two subsequent substeps (321, 322) is smaller than a chemical reaction time interval of the photosensitive material (20) .

3. The method according to claim 1 or 2, wherein an angle of incidence of the radiation illuminating the recording plane (19) is changed between the substeps (321, 322) .

4. The method according to claim 3, wherein- an object phase modulator (35) is arranged in the object arm ( 12 ) ;- the diffusing element (4) is arranged in the object arm(12) between the object phase modulator (35) and the recording plane (19) ;- the object phase modulator (35) comprises an object lens array (351) with a plurality of object lenses (352) , wherein the object radiation (32) is sequentially guided through exactly one of the object lenses (352) in each substep.

5. The method according to any one of the preceding claims, wherein a peak emission wavelength of the radiation illuminating the recording plane (19) is tuned between the substeps .

6. The method according to any one of the preceding claims, wherein a position of the diffusing element (4) with respect to the radiation illuminating the diffusing element is changed between the substeps.

7. The method according to any one of the preceding claims, wherein the diffusing element (4) comprises at least one of: a holographic diffuser, a ground glass diffuser, a photolithographic diffuser, a prismatic diffuser, a micro lens array.

8. The method according to any one of the preceding claims, wherein the holographic plate (2) produced according to any one of the preceding claims is used as a master (25) to produce a copy (26) of the holographic plate (2) .

9. The method according to claim 8, wherein the method further comprises the steps of: a) providing a photosensitive material (20) on the master(25) ; b) illuminating the master (25) with either the reference radiation (31) or the object radiation (32) to produce a copy(26) of the master (25) in the photosensitive material (20) .

10. A method of producing a holographic plate (2) , comprising the steps of: a) providing a photosensitive material (20) on a master (25) comprising a holographic pattern (15) ; and b) illuminating the holographic pattern (15) of the master(25) with radiation from a light source (3) to produce a copy(26) of the master (25) in the photosensitive material (2) ; wherein a diffusing element is arranged in a beam path between the light source (3) and the master (25) in step b) .

11. The method according to claim 10, wherein the master (25) is illuminated with incoherent light to produce the copy (26) .

12. The method according to any one of claims 8 to 11, wherein the photosensitive material (20) is provided in a beam path between the light source (3) and the master (25) .

13. The method according to any one of claims 8 to 11, wherein the master (25) is provided in a beam path between the light source (3) and the photosensitive material (20) .

14. The method according to any one of claims 10 to 13, wherein the master (25) is produced using a method according to any one of claims 1 to 7.

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

16. 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) , 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) ; wherein the apparatus is configured for sequentially illuminating the recording plane (19) in a plurality of substeps, wherein the illumination of the recording plane (19) is modified between the substeps.

17. The apparatus according to claim 16, wherein the apparatus (1) is configured to tune a peak emission wavelength of the radiation illuminating the diffusing element (4) between the substeps (321, 322) .

18. The apparatus according to claim 16 or 17, wherein the apparatus (1) is configured to change a position of the diffusing element (4) with respect to the radiation illuminating the diffusing element (4) between the substeps.

19. The apparatus according to any one of claims 16 to 18,wherein the apparatus is configured to change an angle of incidence (191) of the radiation illuminating the recording plane between the substeps.

20. An apparatus according to any one of claims 16 to 19, wherein the apparatus (1) is configured to perform a method according to any one of claims 1 to 8.

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