Method for generating volume reflection holograms using a substrate-induced reconstruction beam and / or a substrate-induced diffraction beam in a single beam setup - Patents.com

Abstract

The present invention relates to a method for generating a volume reflection hologram using a substrate induced reconstruction beam and / or a substrate induced diffraction beam in a single beam setup, comprising the steps of: (i) providing at least one laser beam source (100) generating a recording beam (101, 102) having a first wave vector (201, 302b, 307); and (ii) providing a holographic recording medium (107, 200) on a transparent substrate (106), the substrate (106) being connected to the at least one laser beam source (102b, 307). (iii) providing a reflector arrangement (103, 103') arranged on the second flat surface of the substrate (106), wherein the recording beams (101, 102) irradiate the holographic recording medium (107), and the recording beams (101, 102) reflect the holographic recording medium (107) and the substrate (106). The reflected beam (104) passes through the plate (106) and is reflected by the reflector arrangement (103, 103') towards the holographic recording medium (107) and the substrate (106) as a reflected beam (104) having a second wave vector (202, 303b, 306), the reflected beam (104) generating an interference pattern with the recording beam in the holographic recording medium (107), the interference pattern having the form of a grating (110, 203) having a grating vector (305), the grating vector (305) being proportional to the wave vector of the reflected beam (104). the grating vector (305) is also identical to the difference vector of the wave vector (303a) of the diffracted beam and the wave vector (302a) of the reconstruction beam, a first plane in which the recording beam and the reflected beam extend is different from a second plane in which the reconstruction beam and the diffracted beam extend, and the grating vector (305) of the grating (110, 203) is parallel to a 30 intersection line of the first and second planes. The invention further relates to an apparatus for generating a volume reflection hologram using a substrate-induced reconstruction beam and / or a substrate-induced diffraction beam in a single beam setup.

Description

[Technical field]

[0001] The present invention relates to a method for generating a volume reflection hologram using a substrate-guided reconstruction beam and / or a substrate-guided diffraction beam in a single beam setup, and further to an apparatus for generating a volume reflection hologram using a substrate-guided reconstruction beam and / or a substrate-guided diffraction beam in a single beam setup. [Background technology]

[0002] Volume holograms, also called thick holograms in the literature, are used in many optical applications, such as display applications. By definition, a volume hologram has a thickness that is much larger than the wavelength of the light used to record the hologram. A volume hologram is an optically diffractive holographic optical element (HOE) based on an optically transparent medium, which has been written into the volume of the holographic recording layer as a phase grating or an absorption grating. Reflection volume holograms, in which the object and reference beams enter the holographic recording medium from opposite sides, are particularly relevant to the present application.

[0003] Typical recording media for volume holograms are metal halide emulsions, dichromated gelatin, photopolymers, and photochromic materials. Their functionality, chemical composition, and applications are described in the literature ["Optical holography", by P. Hariharan, Cambridge University Press (1996), ISBN 0 521 43348 7].

[0004] From the above mentioned documents it is known, for example, to split or direct a collimated laser beam into two separate object and reference beam paths using a beam splitter. Further optical elements such as lenses, spatial filters, etc. can be used, for example, to expand and / or homogenize the partial beams and / or to establish the desired wavefront. The object to be recorded in the recording medium as a hologram, also called the master element, is illuminated by the object beam and diffracts the light in the direction of the holographic recording medium. As an example, the holographic recording medium can be a photographic plate. Said plate is positioned in the device, in particular at the place where the two partial beams are interfered.

[0005] EP 0 821 293 discloses a holographic color display medium and a method for producing such a display medium. According to one embodiment shown in FIG. 22 of EP 0 821 293, a volume hologram photosensitive material is positioned close to the surface of a reflector with a number of micromirrors. A light beam can be impinged on the hologram photosensitive material, so that the light beam passes through the hologram photosensitive material and is reflected at the surface. The reflected and incident light interfere in the hologram photosensitive material, and interference fringes that vary from position to position are recorded. Due to the presence of an air gap between the volume hologram photosensitive material and the reflector with the number of micromirrors, the setup described in EP 0 821 293 describes an alternative approach that cannot be used for the generation of a volume reflection hologram using a substrate-induced reconstruction beam and / or a substrate-induced diffraction beam and therefore cannot be applied to solve the object of the present invention.

[0006] The substrate-induced reconstruction beam and / or the substrate-induced diffraction beam are each formed such that the reconstruction angle or the diffraction beam angle is less than the critical angle θ c, and therefore propagates via total internal reflection (TIR) ​​in the medium. The substrate-induced reconstruction beam and / or the substrate-induced diffracted beam are particularly characteristic for compact optical setups, where a light source is coupled through a thin waveguide or light guiding plate (light guide) and separated in "free space" by a volume hologram positioned on the light guide, or where the reconstruction beam is coupled into the light guide by a coupling hologram and propagates through the light guide as a substrate-induced diffracted beam.

[0007] For near-eye display applications, especially augmented reality smart glasses, eyebox size is a key component of mass adoption of this technology. Lightguide-based optical architectures for augmented reality near-eye display applications offer distinct advantages over eyebox expansion techniques.

[0008] For mass production of volume reflection holograms using a substrate-induced reconstruction beam and / or a substrate-induced diffraction beam, a single beam setup can be used, especially when the object or master element is a master hologram intended to generate copies. US 2007 / 024939 A1 discloses an apparatus in various embodiments, in which hologram copies can be generated in a semi-automatic to fully automatic manner by a transport device.

[0009] However, the fabrication of volume reflection holograms for coupling images into light guides is traditionally a challenging topic for mass manufacturing due to the phenomenon of total internal reflection (TIR), which is well suited for eyebox expansion. In a simple approach where a volume reflection hologram is recorded using a desired playback angle (with some compensation for the shrinkage of the photopolymer), an index matching medium is required to record the desired propagation angle above the critical angle in the medium and prevent unwanted extraneous gratings. WO 2017 / 108704 discloses a method and apparatus for fabricating volume reflection holograms for substrate-guided reconstruction beams that can be used for mass manufacturing of such holograms. Here, the use of index matching medium presents a significant challenge to process stability, even if a contact copy method utilizing a master hologram can be used.

[0010] Techniques that overcome the use of index-matching media are known to exist with some limited applicability to volume reflection holograms with substrate-induced reconstruction beams and / or substrate-induced diffraction beams. These techniques rely on optimizing the parameter space using the recording angle, recording wavelength, or both, to fabricate volume reflection holograms using recording angles that do not require the use of index-matching media. Physically, this amounts to preserving the lattice vector of the desired replay geometry using the more desirable recording geometry, a technique conventionally referred to as Bragg matching. This problem becomes even more complicated when multiple volume reflection holograms need to be recorded in a single or multiple photopolymer layers with the same replay geometry (collinearly aligned reconstruction beams) and different wavelength sensitivities. Typically, a two-beam recording setup is still required with all the difficulties related to process stability, especially vibrations. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] European Patent No. 0 821 293 [Patent Document 2] US Patent Application Publication No. 2007 / 024939 [Patent Document 3] International Publication No. 2017 / 108704 [Non-patent literature]

[0012] [Non-Patent Document 1] “Optical holography”, by P. Hariharan, Cambridge University Press (1996), ISBN 0 521 43348 7 Summary of the Invention [Problem to be solved by the invention]

[0013] To overcome the difficulties identified in the prior art, it is an object of the present invention to provide a method and apparatus for generating volume reflection holograms using a substrate-induced reconstruction beam and / or a substrate-induced diffraction beam in a single beam setup that does not rely on the use of index-matching media and is fully amenable to mass production of such holograms. [Means for solving the problem]

[0014] This and other objects are provided according to a first aspect of the invention, a method for generating a volume reflection hologram using a substrate induced reconstruction beam and / or a substrate induced diffraction beam in a single beam setup, comprising the following steps: providing at least one laser beam source generating a recording beam having a first wave vector; providing a holographic recording medium on a transparent substrate, the substrate having a first planar surface facing the at least one laser beam source and a second planar surface facing away from the at least one laser beam source, the holographic recording medium being disposed on the first planar surface or the second planar surface; providing a reflector arrangement disposed on the second planar surface of the substrate; Including, The recording beam illuminates a holographic recording medium; the recording beam passes through the holographic recording medium and the substrate and is reflected by the reflector arrangement towards the holographic recording medium and the substrate as a reflected beam having a second wave vector; the reflected beam generates an interference pattern with the recording beam in the holographic recording medium, the interference pattern having the form of a grating with a grating vector, the grating vector being the difference vector between the wave vector of the reflected beam and the wave vector of the recording beam, the grating vector also being identical to the difference vector between the wave vector of the diffracted beam and the wave vector of the reconstruction beam; a first plane in which the recording beam and the reflected beam extend is different from a second plane in which the reconstruction beam and the diffracted beam extend, and a grating vector of the grating is parallel to a line of intersection of the first and second planes; This is achieved by the method.

[0015] The method according to the invention uses a single beam setup in which the recording beam generates an interference pattern in the holographic recording medium with a reflected beam, which is a reflection of the recording beam after it has passed through the holographic recording medium. This single beam setup, which does not rely on a master hologram, is particularly useful when the wavefronts of the recording beam and the reflected beam each have a simple shape, e.g. a plane or a sphere.

[0016] The main advantage of the method according to the invention is that the user achieves maximum flexibility in the manufacturing process, following the principle found by the inventors that the first plane in which the recording and reflected beams spread is different from the second plane in which the reconstruction and diffracted beams spread, and the grating vector of the grating is collinear with the intersection line of the first and second planes. This allows, among other things, the user to ensure light propagation in the light guide by TIR, without relying on the use of index-matching media to achieve angles in the light guide that exceed the critical angle. The large flexibility gain in recording gratings in a holographic recording medium, preferably a photopolymer, is related both to the orientation of the recording and reflected beams relative to the holographic recording medium, and to the wavelength of the recording beam. Any desired propagation angle, for example parallel to the surface of the light guide, can be achieved by a simple rotation of the holographic recording medium after recording. The same is true for any diffraction angle relative to the diffracted beam in free space. The method according to the invention can be fully described by a formalism of energy and momentum conservation. Small corrections due to mechanical and optical changes in the holographic medium, for example shrinkage of photopolymer systems, as well as changes in the average refractive index due to recording and bleaching, can be easily incorporated.

[0017] According to a first embodiment of the invention, the reflector arrangement comprises at least two reflecting elements, which make it possible to align the recording beam and the reflected beam in a conformal orientation relative to each other.

[0018] According to a further embodiment of the invention, the beam path of the recording beam and the beam path of the reflected beam are arranged in a "free space" configuration in air or vacuum, which represents the simplest setup and ensures cost-efficient production of volume reflection holograms.

[0019] In a further alternative embodiment of the invention, the holographic recording medium is arranged on a second planar surface of the substrate facing away from the at least one laser beam source, and the reflector arrangement is formed by a beam guiding block, which has a first facet that is transparent to the recording beam and that contacts the holographic recording medium two-dimensionally and at least two further facets that reflect the recording beam towards the holographic recording medium as a reflected beam. In particular, the use of a transparent beam guiding block substantially improves the process stability by significantly reducing the vibrations of the optical components used to generate the reflected beam. The use of a transparent beam guiding block allows, among other things, the realization of a step-and-repeat recording process by laminating the recording film medium (transparent substrate and holographic recording medium) on the first facet as the beam entrance surface of the beam guiding block. Preferably, the beam guiding block is monolithic, i.e. has no internal boundary surfaces.

[0020] The beam guiding block can assume a wide variety of geometric shapes. According to a further embodiment of the invention, the beam guiding block is designed such that at least one of the at least two further facets reflects the recording beam as a reflected beam by total internal reflection (TIR). Preferably, all facets reflect the recording beam in the beam guiding block by TIR.

[0021] Alternatively, the beam guiding block is designed such that at least one of the at least two further facets has a reflective surface that does not require a TIR beam path, which can be achieved, for example, by applying a reflective coating to that facet.

[0022] According to a further embodiment of the invention, the wavelength of the recording beam is in the NIR range. This allows free-space recording of NIR reflector or edge-lit holograms for free-space or light guide eye tracking applications. This also allows the reconstruction wavelength to reach areas of interest where there is no NIR sensitized photopolymer.

[0023] According to a further embodiment of the present invention, a plurality of laser beam sources are provided, which generate collinear recording beams. In particular, the collinear recording beams generated by the plurality of laser beam sources have at least two different wavelengths, so as to generate at least two different volume reflection holograms in the holographic recording medium with a substrate-induced RGB reconstruction beam and / or a substrate-induced RGB diffraction beam. This therefore makes it possible to generate full-color volume reflection holograms as in-couplers (substrate-induced diffraction beams) or out-couplers (substrate-induced reconstruction beams) in the light guide.

[0024] As already mentioned, the method according to the invention has particular potential to be implemented in the mass production of volume reflection holograms. In particular, it is possible to realize a step-and-repeat process in which the transparent substrate and / or the holographic recording medium are provided as roll-fed material.

[0025] According to a second aspect of the present invention, the above and other objects are achieved by an apparatus for generating a volume reflection hologram using a substrate induced reconstruction beam and / or a substrate induced diffraction beam in a single beam setup, comprising: at least one laser beam source; Reflector arrangement and Equipped with the at least one laser beam source and the reflector arrangement are arranged relative to each other such that a transparent substrate carrying the holographic recording medium can be arranged between the at least one laser source and the reflector arrangement, the substrate having a first planar surface facing the at least one laser beam source and a second planar surface facing the reflector arrangement, the holographic recording medium being arranged on the first planar surface or the second planar surface; at least one laser source configured to generate a recording beam having a first wave vector; a beam path of the recording beam is configured such that the recording beam illuminates the holographic recording medium, passes through the holographic recording medium, and is reflected by the reflector arrangement towards the holographic recording medium as a reflected beam having a second wave vector; the beam path of the recording beam is further configured such that the reflected beam generates an interference pattern with the recording beam in the holographic recording medium, the interference pattern having a form of a grating having a grating vector, the grating vector being a difference vector between the wave vector of the reflected beam and the wave vector of the recording beam, the grating vector also being a difference vector between the wave vector of the diffracted beam and the wave vector of the reconstruction beam; a first plane in which the recording beam and the reflected beam extend is different from a second plane in which the reconstruction beam and the diffracted beam extend, and a grating vector of the grating is parallel to a line of intersection of the first and second planes; This is achieved by the device.

[0026] The device according to the invention can be realized in a cost-effective manner and ensures the generation of high-power volume reflection holograms. The particular advantages given in connection with the method according to claim 1 also apply to the device according to claim 13.

[0027] According to an advantageous embodiment of the device according to the invention, the reflector arrangement is formed by a beam guiding block, which has a first facet that is transparent to the recording beam and configured to contact the holographic recording medium two-dimensionally and at least two further facets that are configured to reflect the recording beam towards the holographic recording medium as a reflected beam. This design ensures in particular a very stable manufacturing process that is not susceptible to vibrations of the optical components involved. The beam guiding block can have a wide variety of geometric shapes, in particular a polyhedral geometry in which one facet of the polyhedron forms the first facet that contacts the holographic recording medium and two further facets that reflect the recording beam to form a reflected beam. Preferably, the beam guiding block is monolithic, i.e. consists of one piece of transparent material. According to a further embodiment, the material of the beam guiding block is index-matched to the holographic recording medium. According to the present disclosure, "index-matched" shall mean that the absolute value of the difference between the refractive index of the holographic recording medium and the refractive index of the material of the beam guiding block is <0.1, preferably <0.05, more preferably <0.02.

[0028] There are now several possibilities for improving and developing the method and device according to the invention, for which reference is firstly made to the dependent claims and secondly to the description of exemplary embodiments in conjunction with the drawings, in which: [Brief description of the drawings]

[0029] [Figure 1] FIG. 2 shows an apparatus for generating a volume reflection hologram using a substrate-induced reconstruction beam and / or a substrate-induced diffraction beam in a single beam setup in a first embodiment. [Diagram 2] FIG. 13 shows an apparatus for generating a volume reflection hologram using a substrate-induced reconstruction beam and / or a substrate-induced diffraction beam in a single beam setup in a second embodiment. [Diagram 3]FIG. 13 shows an apparatus for generating a volume reflection hologram using a substrate-induced reconstruction beam and / or a substrate-induced diffraction beam in a single beam setup in a third embodiment. [Figure 4a] FIG. 1 shows a first method of recording a volume reflection grating with varying grating spacing and grating tilt angle. [Figure 4b] FIG. 13 illustrates a second method for changing the grating tilt angle by tilting the holographic recording medium. [Figure 5a] FIG. 2 shows an example of the reconstruction or reconstruction of a volume reflection hologram produced in accordance with the present invention. [Figure 5b] FIG. 5b shows an example of recording a volume reflection hologram according to FIG. 5a in the same geometric configuration used for the reconstruction or playback of the volume reflection hologram. [Figure 5c] FIG. 5b shows an example of recording a volume reflection hologram according to FIG. 5a in a modified configuration. [Figure 6] FIG. 1 illustrates the interdependence of different recording wavelengths in a multi-spectral Bragg matched recording setup. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] FIG. 1 shows an apparatus for generating a volume reflection hologram with a substrate-induced reconstruction beam and / or a substrate-induced diffraction beam in a single beam setup in a first embodiment. 100 shows a single frequency laser present in air and emitting a recording beam 101 with a first wave vector. The recording beam 101 may be expanded by suitable optics (not shown) into a plane wave covering a certain cross-sectional area perpendicular to the propagation direction of the recording beam. The recording beam 101 passes through a holographic recording medium 107, which may be a photopolymer, as an example. The holographic recording medium 107 is placed on a transparent substrate 106, made of, for example, polycarbonate, cellulose triacetate, or cycloolefin copolymer. The transparent substrate 106 serves to mechanically support the holographic recording medium 107, here a photopolymer layer. The transparent substrate 106 has a first flat surface facing the laser beam source 100 and a second flat surface facing the opposite side to the laser beam source 100. Here, the holographic recording medium 107 is disposed on the second planar surface of the substrate 106 .

[0031] When the recording beam 101 illuminates the substrate 106 and the holographic recording medium 107, the beam is refracted by this optical coupling into a recording beam 102, whose propagation angle at the holographic recording medium 107 is measured towards its surface normal 111, denoted as γ. As mentioned above, the recording beam 101 passes through the holographic recording medium 107 and is reflected twice by the reflector arrangement 103 arranged on the second flat surface of the transparent substrate 106, finally forming a reflected beam 104 having a second wave vector. The reflected beam 104 passes through the holographic recording medium 107 as a reflected beam 104'. In this particular case, the propagation angle of the reflected beam 104' at the holographic recording medium 107 towards the surface normal 111 of the holographic recording medium 107 is also γ. The reflected beam 104' forms an interference pattern in the holographic recording medium 107 with the recording beam 102, which generates a volume Bragg grating structure 110 in the holographic recording medium 107 through a recording process.

[0032] FIG. 2 shows an apparatus for generating a volume reflection hologram using a substrate-induced reconstruction beam and / or a substrate-induced diffraction beam in a single beam setup in a second embodiment. In this second embodiment, the method for generating a volume reflection hologram using a substrate-induced reconstruction beam and / or a substrate-induced diffraction beam in a single beam setup is implemented as a step-and-repeat process. For this, a holographic recording medium 107 arranged on a transparent substrate 106 is provided as a roll-supplied material unwound from an unwinding roll 108a. To ensure a safe and reproducible recording process, the holographic recording medium 107 is laminated to a transparent carrier element 109, for example a flat surface of a flat glass sheet or another suitable transparent material, before recording. With the exception of the transparent carrier element 109, the beam paths implemented in the embodiment shown in FIG. 2 correspond to those of FIG. 1.

[0033] After the exposure and formation of the volume Bragg structure 110, the holographic recording medium 107 and the transparent substrate 106 are de-laminated from the surface of the transparent carrier element 109 by lifting the unwinding roll 108a and the take-up roll 108b in a controlled manner. Then, by rotating the unwinding roll 108a and the take-up roll 108b, the holographic recording medium 107 together with the transparent substrate 106 moves leftward for a defined distance 105. After this step propagation, the holographic recording medium 107 together with the transparent substrate 106 is re-laminated on the flat surface of the transparent carrier 109 by lowering the unwinding roll 108a and the take-up roll 108b in a controlled manner. The controlled delamination and controlled re-lamination may be supported by additional rollers and tools not shown in this specification. During the exposure period when the holographic recording medium 107 is in optical contact with the surface of the transparent carrier 109, the recording beam 101 illuminates the holographic recording medium 107, thus facilitating the exposure and formation of the volume Bragg grating structure 110 by interference of the recording beam 102 and the reflected beam 104'. Before the onset of delamination, while the holographic recording medium 107 propagates along the arrow 105 together with the transparent substrate 106, and until the completion of re-stacking, the recording beam 101 can be blocked, for example, by a shutter (not shown in Figs. 1-3). This allows realizing a roll-to-roll step-and-repeat process for generating volume reflection holograms using a substrate-induced reconstruction beam and / or a substrate-induced diffraction beam in a single beam setup.

[0034] FIG. 3 shows an apparatus for generating a volume reflection hologram with a substrate-induced reconstruction beam and / or a substrate-induced diffraction beam in a single beam setup in a third embodiment. In this embodiment, the flat transparent carrier element 109 is replaced by a transparent beam-guiding block 109', inside which the recording beam 102 is reflected and redirected as a reflected beam 104 to the holographic recording medium 107. The holographic recording medium 107 is thus laminated to, i.e. two-dimensionally in contact with, the first facet 109a' of the beam-guiding block 109'. Preferably, the beam-guiding block 109' is monolithic, i.e. made of one piece of material and does not include internal interfaces, in order to minimize losses in beam intensity. Furthermore, the beam-guiding block 109' is preferably index-matched to the holographic recording medium 107, i.e. the difference between the refractive index of the holographic recording medium and the beam-guiding block is <0.1, preferably <0.05, more preferably <0.02. Here, the recording beam 102 is reflected inside the beam guiding block 109' on facets 103' each having a reflective coating. Alternatively, the geometry of the beam guiding block 109' may be such that the recording beam hits the facets 103' at an angle larger than the critical angle, so that the reflection is performed by TIR. Using the beam guiding block 109' makes the recording setup very insensitive to vibrations.

[0035] FIG. 4a shows a first method of recording a volume reflection grating, where the grating period and the tilt angle of the grating can be varied. A recording beam with a first wave vector 201 located in the xz plane is incident on the holographic recording medium 200 at an angle θ (inside the holographic recording medium) measured towards a first surface 206 of the holographic recording medium 200 (xy plane). A reflected beam with a second wave vector 202 has the same length as the first wave vector 201 with an angle θ' measured towards a second surface 207 of the recording medium 200 (xy plane). Note that both angles θ and θ' are measured inside the holographic recording medium (not in air). The reflected beam (wave vector 202) generates an interference pattern with the recording beam (wave vector 201) in the holographic recording medium 200, the interference pattern having the form of a grating 203 with a grating vector K. The grating vector K (not shown in FIG. 4a) is the difference vector (e.g., vector 305 in FIGS. 5b and 5c) between the wave vector 202 of the reflected beam and the wave vector 201 of the recording beam. The length of the grating vector K is related to the grating period Λ via K=2·π / Λ.

[0036] When θ and θ' are of the same size (as shown in FIG. 4a), the grating planes are parallel to both sides 206, 207 of the holographic recording medium 200. The grating period Λ can be modified by θ and θ' and the same length k of the first and second wave vectors 201, 202, defined by k=2·π·n / λ, n indicating the refractive index of the holographic recording medium 200 and λ indicating the same vacuum wavelength of the recording beam (wave vector 201) and the reflected beam (wave vector 202). When θ and θ' are of different sizes, the grating planes are oblique (tilted) towards both sides 206, 207 of the holographic recording medium 200.

[0037] The grating vector, which is the difference vector between the reflected beam wave vector 202 and the recording beam wave vector 201, can be generated by multiple pairs of recording and reflected beams with the same length of the pairs and wave vectors of respective angles θ and θ' selected such that the same grating vector is generated by each pair of recording and reflected beams. All of the multiple pairs of recording and reflected beams lie in a common plane.

[0038] FIG. 4b shows a further method of changing the grating tilt angle by tilting the holographic recording medium 200 via rotation about an axis parallel to the y-axis (where the rotation axis is identical to the y-axis) and perpendicular to a common plane with multiple pairs of recording and reflected beams that can generate identical grating vectors at each pair's incidence angles θ and θ'.

[0039] However, using this scheme with all pairs of recording and reflected beams generating identical grating vectors located in a common plane limits the ability to select the first wave vector 201 and the second wave vector 202 such that both wave vectors 201, 202 can exist in free space such as air or vacuum (cosθ and cosθ'<1 / n, n being the refractive index of the holographic recording medium 200) and simultaneously have vacuum wavelengths that can address a part of the spectrum to which the holographic recording medium 200, preferably a photopolymer, exhibits recording sensitivity, or have vacuum wavelengths to which a single frequency laser source exists. The limitations become even more severe when not only one grating vector is involved, but two or more grating vectors, such as for example simultaneous co-aligned wavelength multiplexed recording of volume reflection gratings in the same volume of the holographic recording medium.

[0040] FIG. 5a shows an example of the reconstruction or reconstruction of a volume reflection hologram as part of an embodiment of the present invention. In this case, 300 denotes a reference plane, which may be an optical table used to expose a volume reflection hologram with a fixed right-handed coordinate system x, y, z. 301 denotes a plane of a holographic recording medium. Plane 301 is located in the xy plane of the fixed coordinate system, and reconstruction beam 302a is in air or vacuum. As shown in FIG. 5a, the wave vector 302a of the reconstruction beam is parallel to the z axis, and the reconstruction beam impinges perpendicularly on the holographic recording medium located in plane 301. The wave vector of the diffracted beam is denoted 303a and is located in the xz plane 304. The diffracted beam (wave vector 303a) is then generated by the interaction of the reconstruction beam (wave vector 302a) with a volume reflection hologram (not shown) recorded in a holographic recording medium, located in plane 301. In this example, the angle β measured inside the holographic recording medium is greater than the critical angle, so the diffracted beam is substrate guided in the holographic recording medium with an average refractive index n, i.e., propagates through the holographic recording medium via TIR. The wave vector 303a of the diffracted beam lies in the xz plane 304.

[0041] FIG. 5b shows an example of recording a volume reflection hologram according to FIG. 5a in the same geometric configuration used for the reconstruction or playback of the volume reflection hologram. This means that the wave vector 302a of the reconstruction beam in FIG. 5a is identical to the wave vector 302b of the recording beam, and the wave vector 303a of the diffracted beam in FIG. 5a is identical to the wave vector 303b of the reflected beam in FIG. 5b. A grating vector 305 characterizing the volume Bragg grating (not shown in FIG. 5a) is generated as the difference vector of the wave vector 303b of the reflected beam and the wave vector 302b of the recording beam. The same grating vector 305 is also given as the difference vector of the wave vector 303a of the diffracted beam and the wave vector 302a of the reconstruction beam (respectively in FIG. 5a). The grating vector 305, as well as the wave vector 303a of the diffracted beam, the wave vector 302a of the reconstruction beam, the wave vector 303b of the reflected beam, and the wave vector 302b of the recording beam are each located in the xz plane 304. The grating vector 305 is oblique by an angle α measured inside the holographic recording medium towards the z-axis of the fixed coordinate system. The reconstruction beam (wave vector 302a) and the recording beam (wave vector 302b) can be in air or vacuum (free space), but the diffracted beam (wave vector 303a) and the reflected beam (wave vector 303b) are guided to the substrate, which is undesirable for recording. The first plane in which the recording beam (wave vector 302b) and the reflected beam (wave vector 303b) extend is identical to the second plane in which the reconstruction beam (wave vector 302a) and the diffracted beam (wave vector 302a) extend, and coincides with the xz-plane 304.

[0042] FIG. 5c shows an example of the recording of the volume reflection hologram illustrated in FIG. 5a according to the present invention. To overcome the physical constraint that the diffracted beam (wave vector 303a) and the reflected beam (wave vector 303b) described in FIG. 5a and FIG. 5b are substrate-guided, the Bragg-matched recording of the grating vector 305 can be performed in a different way. First, the plane 301 of the holographic recording medium may be tilted by an angle α (which is also shown by the angle α in FIG. 1-FIG. 3) using a rotation around the y axis to become the plane 301′. Thereby, the desired gating vector 305 fixed in the coordinate system of the holographic recording medium belongs to the reference plane 300 and lies here in the yz-plane of a fixed coordinate system parallel to the z-axis. Thereby, the wave vector 307 of the new recording beam and the wave vector 306 of the new reflected beam can be selected, both wave vectors 306, 307 having the same length and both lying in the yz-plane of a fixed coordinate system belonging to the reference plane 300. The grating vector 305 is the difference vector of the wave vector 306 of the new reflected beam and the wave vector 307 of the new recording beam. Furthermore, by selecting a common length of the wave vectors 306 and 307 of the new reflected beam and the new recording beam (e.g., by appropriate selection of the vacuum wavelength), the new reflected beam and the new recording beam can be in air or vacuum (free space), thus avoiding substrate-induced recording and / or reflected beams. In this example of an embodiment of the present invention, the first plane in which the new recording beam and the new reflected beam (wave vectors 307, 306) extend is different from the second plane in which the reconstruction beam and the diffracted beam (wave vectors 302a, 303a) extend, and the grating vector of the grating (305) is parallel to the intersection line of the first and second planes.

[0043] As shown in Figure 6, a relationship exists between the readout wavelengths and the recording wavelengths for multispectral Bragg matched recording, for example red, green, and blue, to ensure the collinearity of the respective beam paths in the recording medium and the reflection arrangement, especially when beam guiding blocks are used (as shown in Figure 3). The x-axis of Figure 6 represents the range of recording wavelengths in the blue spectral range (440-480 nm), and the y-axis represents the range of recording wavelengths from blue to deep red (500 nm-700 nm) that depends on the choice of blue recording wavelength (values ​​on the x-axis). The dependence of the recording wavelengths in Figure 6 is calculated from readout wavelengths that are somewhere between 450 nm, 525 nm, and 650 nm, which are typical ranges for RGB scanning projectors, as shown qualitatively in Figures 5a-c. The calculation is based on the form of energy and momentum conservation applied in the field of light diffraction. The dashed lines 400 and 401 are the calculated recording wavelengths of green and red as a function of the chosen blue recording wavelength, in this case in the range of 440 nm-480 nm, for collinear RGB recording. To better illustrate the dependence of the recording wavelength as a function of the replay wavelength, lines 402 and 404 are the calculated recording wavelengths for replay wavelengths of 440 nm, 515 nm, and 640 nm. Lines 403 and 405 are the calculated recording wavelengths for replay wavelengths of 460 nm, 535 nm, and 660 nm. Line 406 represents a selection of blue recording wavelengths corresponding to a common fixed-frequency laser source at 457 nm. Points 407 and 408 represent common fixed-frequency laser sources at green and red, 532 nm and 660 nm, respectively. If 407 and 408 are on or very close to 400 and 401 along 406, then collinear recording can be successful with the respective fixed-frequency lasers. 408 meets this requirement for the replay wavelengths corresponding to 400 and 401. With the flexibility of the replay wavelengths and the selection of commonly available fixed-frequency lasers, one can potentially find combinations in which one or more fixed-frequency lasers can be used. With the development of high-power tunable laser sources, all recording wavelength requirements can be met, and thus collinear RGB recording in the described Bragg matching technique.

Claims

1. 1. A method for generating a volume reflection hologram using a substrate-induced reconstruction beam and / or a substrate-induced diffraction beam in a single beam setup, comprising: providing at least one laser beam source (100) generating a recording beam (101, 102) having a first wave vector (201, 302b, 307); Providing a holographic recording medium (107, 200) on a transparent substrate (106) the substrate (106) has a first flat surface facing the at least one laser beam source (100) and a second flat surface facing away from the at least one laser beam source (100), and the holographic recording medium (107) is disposed on the first flat surface or the second flat surface; A reflector arrangement (103, 103') disposed on the second flat surface of the substrate (106). ) and Including, The recording beams (101, 102) illuminate the holographic recording medium (107). Shoot, the recording beam (101, 102) passes through the holographic recording medium (107) and the substrate (106) and is reflected by the reflector arrangement (103, 103′) towards the holographic recording medium (107) and the substrate (106) as a reflected beam (104) having a second wave vector (202, 303 b, 306); the reflected beam (104) generates an interference pattern with the recording beam in the holographic recording medium (107), the interference pattern having the form of a grating (110, 203) with a grating vector (305), the grating vector (305) being the difference vector between the wave vector (303b, 306) of the reflected beam (104) and the wave vector (302b, 307) of the recording beam (101, 102), the grating vector (305) also being the difference vector between the wave vector (303a) of the diffracted beam and the wave vector (302a) of the reconstruction beam; a first plane in which the recording beam and the reflected beam extend is different from a second plane in which the reconstruction beam and the diffracted beam extend, and the grating vector (305) of the grating (110, 203) is parallel to an intersection line between the first plane and the second plane. method.

2. The method of claim 1, wherein the reflector arrangement comprises at least two reflecting elements (103, 103').

3. 3. The method according to claim 1, wherein the beam path of the recording beam (101) and the beam path of the reflected beam (104) are arranged in a "free space" configuration in air or vacuum.

4. 3. The method of claim 1, wherein the holographic recording medium (107) is arranged on the second planar surface of the substrate (106), and the reflector arrangement (103′) is formed by a beam guiding block (109′) that is transparent to the recording beam (102) and has a first facet that is in two-dimensional contact with the holographic recording medium (107) (109a′) and at least two further facets (103′) that reflect the recording beam (102) towards the holographic recording medium (107) and the substrate (106) as the reflected beam (104).

5. 5. The method of claim 4, wherein the beam guiding block (109') is designed such that at least one of the at least two further facets (103') reflects the recording beam (102) as the reflected beam (104) by total internal reflection (TIR).

6. 5. The method of claim 4, wherein the beam guiding block (109') is designed such that at least one of the at least two further facets (103') has a reflective surface that does not require a TIR beam path.

7. 5. The method of claim 4, wherein the material of the beam directing block (109') is index-matched to the holographic recording medium (107).

8. 3. The method according to claim 1, wherein the wavelength of the recording beam (101, 102) is in the NIR range.

9. 3. The method according to claim 1 or 2, wherein a plurality of laser beam (100) sources are provided, said plurality of laser beam sources generating collinear recording beams.

10. 10. The method of claim 9, wherein the collinear recording beams generated by the multiple laser beam sources have at least two different wavelengths so as to generate at least two different volume reflection holograms (110, 203) in the holographic recording medium (107, 200) using substrate-induced RGB reconstruction beams and / or substrate-induced RGB diffraction beams.

11. The method of claim 1 or 2, wherein the method is carried out in a step-and-repeat process.

12. The method of claim 11 , wherein at least the substrate (106) carrying the holographic recording medium (107) and / or the holographic recording medium (107) are roll-fed materials.

13. 1. An apparatus for generating a volume reflection hologram (110) using a substrate-induced reconstruction beam and / or a substrate-induced diffraction beam in a single beam setup, comprising: at least one laser beam source (100); A reflector arrangement (103) and Equipped with the at least one laser beam source (100) and the reflector arrangement (103) are arranged relative to each other such that a transparent substrate (106) carrying a holographic recording medium (107) can be arranged between the at least one laser beam source (100) and the reflector arrangement (103), the substrate (106) having a first planar surface facing the at least one laser beam source (100) and a second planar surface facing the reflector arrangement (103), the holographic recording medium (107) being arranged on the first planar surface or the second planar surface; the at least one laser beam source (100) is configured to generate a recording beam (101, 102) having a first wave vector (201, 302b, 307); a beam path of the recording beam (101, 102) is configured such that the recording beam (101, 102) illuminates the holographic recording medium, passes through the holographic recording medium, and is reflected by the reflector arrangement (103) towards the holographic recording medium (107, 200) as a reflected beam (104) having a second wave vector (202, 303b, 306); the beam path of the recording beam (101, 102) is further configured such that the reflected beam (104) generates an interference pattern with the recording beam in the holographic recording medium (107, 201), the interference pattern having the form of a grating (110, 203) having a grating vector (305), the grating vector (305) being the difference vector between the second wave vector (303b, 306) of the reflected beam and the first wave vector (201, 302b, 307) of the recording beam, the grating vector (305) also being the difference vector between the wave vector (303a) of a diffracted beam and the wave vector (302a) of a reconstruction beam; a first plane in which the recording beam and the reflected beam extend is different from a second plane in which the reconstruction beam and the diffracted beam extend, and the grating vector (305) of the grating is parallel to an intersection line between the first plane and the second plane. Device.

14. 14. The apparatus of claim 13, wherein the reflector arrangement (103, 103') is formed by a beam guiding block (109') that is transparent to the recording beam (102) and has a first facet (109a') configured to be in two-dimensional contact with the holographic recording medium (107) and at least two further facets configured to reflect the recording beam (102) towards the holographic recording medium as the reflected beam (104).

15. 15. The method of claim 14, wherein the material of the beam directing block (109') is index-matched to the holographic recording medium (107).

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