Optical fiber for a display device

Description

[0001] The present invention relates to an optical waveguide for a display device and to methods for manufacturing such an optical waveguide. The invention also relates to a device for generating a virtual image that uses such an optical waveguide.

[0002] A head-up display, also known as a HUD, is a display system that allows the viewer to maintain their line of sight by projecting the content into their field of vision. While such systems were originally used primarily in aviation due to their complexity and cost, they are now also being installed in large-scale production in the automotive sector.

[0003] Head-up displays generally consist of an image generator, an optical unit, and a mirror unit. The image generator generates the image. The optical unit directs the image to the mirror unit. The image generator is often also referred to as an imaging unit or PGU (Picture Generating Unit). The mirror unit is a partially reflective, translucent screen. The viewer therefore sees the content displayed by the image generator as a virtual image and, at the same time, the real world behind the screen. In the automotive sector, the windshield is often used as the mirror unit, and its curved shape must be taken into account in the display. Due to the interaction of the optical unit and the mirror unit, the virtual image is an enlarged representation of the image generated by the image generator.

[0004] The viewer can only view the virtual image from the position of the so-called eyebox. An eyebox is an area whose height and width correspond to a theoretical viewing window. As long as one eye of the viewer is within the eyebox, all elements of the virtual image are visible to the viewer. If, however, the eye is outside the eyebox, the virtual image is only partially visible to the viewer or not visible at all. The larger the eyebox, the less restricted the viewer is in choosing their seating position.

[0005] The size of the virtual image in conventional head-up displays is limited by the size of the optical unit. One approach to enlarging the virtual image is to couple the light coming from the imaging unit into an optical fiber. The light coupled into the optical fiber, which carries the image information, is totally reflected at its interfaces and is thus guided within the optical fiber. In addition, a portion of the light is coupled out at numerous positions along the propagation direction, so that the image information is distributed across the surface of the optical fiber. In this way, the optical fiber dilates the exit pupil. The effective exit pupil is composed of images of the aperture of the imaging system.

[0006] Against this background, US 2016 / 0124223 A1 describes a display device for virtual images. The display device comprises an optical waveguide that causes light coming from an imaging unit, which is incident through a first light incidence surface, to undergo repeated internal reflection to move in a first direction away from the first light incidence surface. The optical waveguide also causes a portion of the light guided in the optical waveguide to exit to the outside through regions of a first light exit surface extending in the first direction. The display device further comprises a first light-incident-side diffraction grating that diffracts incident light to cause the diffracted light to enter the optical waveguide, and a first light-outgoing diffraction grating that diffracts light incident from the optical waveguide.

[0007] A conventional full-color head-up display based on fiber optics typically consists of three stacked, monochrome fiber optics, one each for red, green, and blue. These fiber optics each consist of a glass substrate, a thin hologram layer, and another glass substrate as a cover layer. The glass is typically thicker than 1 mm. They are also rigid and resistant to bending.

[0008] The substrates used to manufacture such an optical waveguide must possess excellent surface properties, such as flatness. Such properties are very difficult to obtain on the market without complex surface treatment of the substrates, and they are expensive. Furthermore, the substrate thickness increases the thickness of the optical waveguide, which consists of three monochrome optical waveguides. This, in turn, increases the thickness of the entire product containing the optical waveguide, such as a head-up display. Furthermore, superposition errors occur between the colors transported by each of the monochrome optical waveguides when the optical waveguides are viewed at a steep angle. This is often the case, particularly when used as a head-up display in motor vehicles.

[0009] WO 2016 / 113288 A1 describes a method for producing an optical casting comprising at least one volume holographic optical element by means of a casting process. In the method, a casting mold is provided, which comprises a first mold part and a second mold part, which can be connected to form the casting mold. A holographic optical element is positioned and aligned on the first mold part and / or the second mold part. In one or more casting steps, the holographic optical element is embedded in casting material.

[0010] It is an object of the present invention to provide an improved optical waveguide and methods for producing such an optical waveguide.

[0011] This object is achieved by a method having the features of claim 1 or 2 and by an optical waveguide having the features of claim 8. Preferred embodiments of the invention are the subject of the dependent claims. According to a first aspect of the invention, a method for producing an optical waveguide comprises the steps: Applying a layer of a translucent material to a first impression plate; curing the applied translucent material to form a cover layer; applying a hologram layer to the cover layer; applying a substrate to the hologram layer; and exposing and curing the hologram layer.

[0012] For an embodiment of the optical waveguide in which the substrate is made of glass, the material layer is produced by applying the material, for example a varnish or an optically clear adhesive, to a mold plate, i.e. to a reference surface that has the desired properties, particularly with regard to flatness. The material is then cured. A thin holographic layer is then applied, the layer thickness of which can be defined by spacers. Finally, a glass substrate is applied to the structure consisting of the material layer and thin hologram layer, and the hologram layer is exposed and cured. The exposure and curing of the thin hologram layer is possible before and after the application of the glass substrate. The glass substrate can serve as a mechanically stable carrier.

[0013] One advantage of the described process is the elimination of glass for the construction of an optical waveguide. This allows for a reduced-thickness optical waveguide. Furthermore, the properties of the mold's mold plate, such as its flatness, are transferred to the cover layer formed on it. This achieves surface properties that even exceed those of glass materials. Furthermore, the production of the cover layer is more cost-effective than using the glass substrates currently available on the market with corresponding surface properties.

[0014] An optical waveguide produced using this method according to the invention comprises a substrate, a cover layer, and an optically active layer located between the two. The cover layer consists of a translucent material that has undergone a curing process. The cover layer is applied in a mold with a precisely defined geometry and cured. It thus has a precise surface shape without the need for complex post-processing. Fluctuations in surface quality, such as those previously encountered with glass substrates used as cover layers, are avoided. The high quality of the formed surfaces also results in a significant improvement in total internal reflection, reducing angular errors.

[0015] According to a further aspect of the invention, a method for producing an optical waveguide comprises the steps: Applying a layer of a translucent material to a first mold plate; curing the applied translucent material to form a cover layer; applying a hologram layer to the cover layer; exposing and curing the hologram layer; applying a layer of a translucent material to the hologram layer; shaping the applied translucent material using a second mold plate; and curing the applied translucent material to form a substrate.

[0016] For an embodiment of the optical waveguide in which neither the substrate nor the cover layer is made of glass, the material to produce the material layer, for example a varnish or an optically clear adhesive, is applied to a molding plate, i.e. to a reference surface which has the desired properties, in particular with regard to flatness. The material is then cured. A thin holographic layer is then applied, the layer thickness of which can be defined by spacers. The hologram layer is then exposed and cured. To create the second material layer, which can also consist of a varnish or an optically clear adhesive, for example, the material is applied to the hologram layer. The surface of the material layer is molded using a counter-plate which is brought into contact with the material layer under the application of force.The same translucent material is preferably used for the substrate and the cover layer, but different materials can also be used.

[0017] One advantage of the described process is the elimination of two glasses for the construction of an optical waveguide. Furthermore, the properties of the mold's impression plate, such as its flatness, are transferred to the cover layer formed on it.

[0018] In an optical waveguide manufactured using this method according to the invention, the substrate also consists of a translucent material that has undergone a curing process. This has the advantage that the substrate can also be manufactured cost-effectively while still maintaining the desired surface properties. Furthermore, a somewhat flexible optical waveguide can be realized in this way.

[0019] The two material layers can also be used as a carrier material for transferring the thin holographic layer to a substrate or to another waveguide, since the material layers can be detached from the thin hologram layer without damaging it.

[0020] According to one aspect of the invention, a separating layer is arranged between the first impression plate and the layer of translucent material, or between the second impression plate and the layer of translucent material. Preferably, one or more additional layers are inserted between the impression plates and the respective adjacent material layers, which serve to improve the detachability of the material layers from the respective impression plate. This is particularly useful when the material used is an adhesive, which can only be removed from the impression plate with a certain amount of effort without an additional layer between it and the impression plate.

[0021] According to one aspect of the invention, the translucent material is a lacquer or an optically clear adhesive (OCA), i.e., a curing adhesive. This typically has a refractive index that corresponds to the transparent materials it is used to bond. Such adhesives are known to those skilled in the art and can be easily adjusted to a desired refractive index—here, that of the glass used or to be replaced. These materials have the advantage of being cost-effective and easy to process.

[0022] According to one aspect of the invention, the translucent material subjected to a curing process has a refractive index greater than or equal to 1.4. It thus has optical properties equivalent to those of glass and can be used as a replacement for otherwise used glass without the need for complex recalculation of the optical properties. A refractive index of n=1.5±0.02 has proven particularly suitable for use in optical fibers.

[0023] According to one aspect of the invention, the first impression plate or the second impression plate has a structure. As an alternative to a flat material layer, structuring the impression plates can also achieve a targeted structuring of the shape of the material layer. In this way, different thicknesses of the lacquer layer or the optical fiber can be realized in different areas. These are, for example, depressions or elevations in the low millimeter range.

[0024] According to one aspect of the invention, an optical waveguide for a display device is produced by means of a method according to the invention.

[0025] According to a further aspect of the invention, an apparatus for generating a virtual image comprises: an imaging unit for generating an image; an optical unit for projecting the image toward a mirror unit for generating the virtual image; and an optical waveguide according to the invention for expanding an exit pupil.

[0026] The optical fiber according to the invention enables head-up displays to be implemented with a reduced space requirement. Its use is particularly advantageous for full-color head-up displays based on optical fibers, which require three stacked, monochrome optical fibers.

[0027] Preferably, a device according to the invention for generating a virtual image is used in a means of transport to create a virtual image for an operator of the means of transport. The means of transport can be, for example, a motor vehicle or an aircraft. Of course, the solution according to the invention can also be used in other environments or for other applications, e.g., in trucks, in rail technology and public transport, in cranes and construction machinery, etc.

[0028] Further features of the present invention will become apparent from the following description and the appended claims taken in conjunction with the figures. Figure overview

[0029] Fig. 1 schematically shows a head-up display according to the prior art for a motor vehicle; Fig. 2 shows an optical waveguide with two-dimensional magnification; Fig. 3 schematically shows a head-up display with an optical waveguide; Fig. 4 schematically shows a head-up display with an optical waveguide in a motor vehicle; Fig. 5 shows three examples of an optical waveguide in longitudinal section; Fig. 6 schematically shows a first embodiment of an optical waveguide according to the invention; Fig. 7 schematically shows a second embodiment of an optical waveguide according to the invention; Fig. 8 shows a manufacturing detail for the optical waveguide from Fig. 7 ; Fig. 9 schematically shows a first manufacturing method for an optical waveguide according to the invention; Fig. 10 shows a modification of the manufacturing method from Fig. 9 ; and Fig. 11 schematically shows a second manufacturing method for an optical waveguide according to the invention. Character description

[0030] To better understand the principles of the present invention, embodiments of the invention are explained in more detail below with reference to the figures. Like reference numerals are used in the figures for like or equivalent elements and are not necessarily described again for each figure. It is understood that the invention is not limited to the illustrated embodiments and that the described features can also be combined or modified without departing from the scope of the invention as defined in the appended claims.

[0031] First, based on the Figures 1 to 4 the basic idea of a head-up display with fiber optic cable is explained.

[0032] Fig. 1shows a schematic diagram of a head-up display according to the prior art for a motor vehicle. The head-up display has an image generator 1, an optical unit 2, and a mirror unit 3. A beam SB1 emanates from a display element 11, which is reflected by a folding mirror 21 onto a curved mirror 22, which reflects it toward the mirror unit 3. The mirror unit 3 is depicted here as the windshield 31 of a motor vehicle. From there, the beam SB2 travels toward an eye 61 of a viewer.

[0033] The viewer sees a virtual image VB, which is located outside the motor vehicle above the hood or even in front of the motor vehicle. Due to the interaction of optical unit 2 and mirror unit 3, the virtual image VB is an enlarged representation of the image displayed by display element 11. A speed limit, the current vehicle speed, and navigation instructions are symbolically displayed here. As long as the eye 61 is within the eyebox 62 indicated by a rectangle, all elements of the virtual image are visible to the eye 61. If the eye 61 is outside the eyebox 62, the virtual image VB is only partially visible to the viewer or not visible at all. The larger the eyebox 62, the less restricted the viewer is in choosing their seating position.

[0034] The curvature of the curved mirror 22 is adapted to the curvature of the windshield 31 and ensures that the image distortion is stable across the entire eyebox 62. The curved mirror 22 is rotatably mounted by means of a bearing 221. The resulting rotation of the curved mirror 22 enables the eyebox 62 to be moved and thus the position of the eyebox 62 to be adjusted to the position of the eye 61. The folding mirror 21 ensures that the path traveled by the beam SB1 between the display element 11 and the curved mirror 22 is long, while at the same time the optical unit 2 remains compact. The optical unit 2 is separated from the environment by a transparent cover 23. The optical elements of the optical unit 2 are thus protected, for example, from dust in the interior of the vehicle. An optical film or polarizer 24 is also located on the cover 23.The display element 11 is typically polarized, and the mirror unit 3 acts as an analyzer. The purpose of the polarizer 24 is therefore to influence the polarization to achieve uniform visibility of the useful light. A glare shield 25 serves to reliably absorb the light reflected across the interface of the cover 23, preventing glare to the viewer. In addition to sunlight SL, light from another interfering light source 64 can also reach the display element 11. In combination with a polarization filter, the polarizer 24 can also be used to block out incoming sunlight SL.

[0035] Fig. 2shows a schematic spatial representation of an optical waveguide 5 with two-dimensional magnification. In the lower left area, a coupling hologram 53 can be seen, by means of which light L1 coming from an imaging unit (not shown) is coupled into the optical waveguide 5. In this area, it propagates to the top right in the drawing, according to arrow L2. In this area of the optical waveguide 5 is a folded hologram 51, which acts similarly to many partially transparent mirrors arranged one behind the other, and generates a light beam that is broadened in the Y direction and propagates in the X direction. This is indicated by three arrows L3.In the part of the optical waveguide 5 extending to the right in the figure, there is an output hologram 52, which also functions similarly to many partially transparent mirrors arranged one behind the other and, indicated by arrows L4, outputs light upwards in the Z direction from the optical waveguide 5. This results in a broadening in the X direction, so that the original incident light beam L1 leaves the optical waveguide 5 as a light beam L4 enlarged in two dimensions.

[0036] Fig. 3shows a spatial representation of a head-up display with three optical waveguides 5R, 5G, 5B, which are arranged one above the other and each represent an elementary color red, green, and blue. Together, they form the optical waveguide 5. The holograms 51, 52, 53 present in the optical waveguide 5 are wavelength-dependent, so that one optical waveguide 5R, 5G, 5B is used for each of the elementary colors. An image generator 1 and an optical unit 2 are shown above the optical waveguide 5. The optical unit 2 has a mirror 20, by means of which the light generated by the image generator 1 and shaped by the optical unit 2 is deflected in the direction of the respective input hologram 53. The image generator 1 has three light sources 14R, 14G, 14B for the three elementary colors. It can be seen that the entire unit shown has a low overall height compared to its light-emitting surface.

[0037] Fig. 4shows a head-up display in a motor vehicle similar to Fig. 1 , but here in a spatial representation and with an optical fiber 5. The schematically indicated image generator 1 can be seen, which generates a parallel beam SB1, which is coupled into the optical fiber 5 by means of the mirror plane 523. The optical unit is not shown for the sake of simplicity. Several mirror planes 522 each reflect a portion of the light incident on them towards the windshield 31, the mirror unit 3. From this, the light is reflected towards the eye 61. The observer sees a virtual image VB above the hood or at an even greater distance in front of the vehicle. With this technology, too, the entire optical system is installed in a housing that is separated from the surroundings by a transparent cover. As with the head-up display from Fig. 1 A retarder can be arranged on this cover.

[0038] Fig. 5shows three examples of an optical waveguide 5 in longitudinal section. The optical waveguide 5 in partial image (a) has an ideally flat upper boundary surface 501 and an ideally flat lower boundary surface 502, both of which are arranged parallel to one another. It can be seen that a parallel light beam L1, which propagates from left to right in the optical waveguide 5, remains unchanged and parallel in cross-section due to the parallelism and flatness of the upper and lower boundary surfaces 501, 502. The optical waveguide 5 in partial image (b) has upper and lower boundary surfaces 501, 502 that are not completely flat and are also not parallel to one another. The optical waveguide 5 therefore has a thickness that varies in the direction of light propagation. It can be seen that the light beam L1 is no longer parallel after just a few reflections and also does not have a homogeneous cross-section.The optical waveguide 5 in sub-image (c) has upper and lower boundary surfaces 501, 502 that deviate even more from the ideal shape than those in sub-image (b). Therefore, the light beam L1 also deviates even more from the ideal shape. The flatness of the boundary surfaces 501, 502 is therefore of great importance for the quality of light propagation in the optical waveguide.

[0039] Fig. 6shows a first embodiment of an optical waveguide 5 according to the invention. In this embodiment, a substrate 54 made of glass is used. A thin hologram layer 56 is arranged on the substrate 54. A cover layer 55 made of a translucent material that has undergone a curing process is arranged on the hologram layer 56. The translucent material can be, for example, a lacquer or an optically clear adhesive. The refractive index of the material is preferably greater than or equal to 1.4. If necessary, the cover layer 55 can be structured.

[0040] Fig. 7shows a second embodiment of an optical waveguide 5 according to the invention. In this embodiment, a substrate 54 is used, which also consists of a light-transmitting material that has been subjected to a curing process. Arranged on the substrate 54 is a thin hologram layer 56, onto which a cover layer 55 is applied. As before, the cover layer 55 consists of a light-transmitting material that has been subjected to a curing process. The same light-transmitting material is preferably used for the substrate 54 and the cover layer 55, but different materials can also be used. In this embodiment, too, the light-transmitting material can be a lacquer or an optically clear adhesive. Here, too, the refractive index of the light-transmitting material is preferably greater than or equal to 1.4. If necessary, the substrate 54 or the cover layer 55 can have a structure.

[0041] Fig. 8 shows a manufacturing detail for the optical fiber 5 from Fig. 7 To produce this structure, two impression plates 70, 71 are used, each with the desired surface properties. To remove the optical fiber 5 from the production structure, the impression plates 70, 71 are moved apart, whereby the optical fiber 5 detaches from the impression plates 70, 71.

[0042] Fig. 9 shows, in schematic form, a first manufacturing method for an optical waveguide according to the invention. First, a layer of a curable, light-transmitting material is applied S1 to a first mold plate. This layer is cured S2 to form the cover layer. Subsequently, a hologram layer is applied S3 to the cover layer. A substrate is then applied S4 to this hologram layer, and the hologram layer 56 is exposed S5 and cured S6.

[0043] Fig. 10 shows in schematic form a Fig. 9 A modified manufacturing process for an optical waveguide according to the invention. According to this advantageous variant, exposure S5 and curing S6 take place before the substrate is applied S4. The remaining steps correspond to those of Fig. 9

[0044] Fig. 11shows, in schematic form, another manufacturing method for an optical waveguide according to the invention. First, a layer of a curable, translucent material is applied S1 to a lower mold plate. This layer is cured S2 to form the cover layer. This is followed by the application S3 of a hologram layer to the cover layer, and the exposure S5 and curing S6 of the hologram layer. A further layer of a curable, translucent material is then applied S7 to the cured hologram layer. Using a second mold plate, the material layer is shaped S8, followed by the curing S9 of the material layer to form the substrate. Finally, the mold plates are moved apart S10, whereby the optical waveguide detaches from the mold plates.

[0045] In all embodiments of the method, one or more additional layers can be inserted between the impression plates and the respective adjacent material layers, which serve as separating layers to improve the detachability of the material layers from the respective impression plate. List of reference symbols

[0046] 1Image generator / imaging unit 11Display element 14, 14R, 14G, 14BLight source 2Optical unit 20Mirror 21Folding mirror 22Curved mirror 221Bearing 23Transparent cover 24Optical film / polarizer 25Anti-glare screen 3Mirror unit 31Windshield 5 Optical fiber 501 Upper boundary surface 502 Lower boundary surface 51 Folding hologram 52 Output hologram 521 Output region 522 Mirror plane 523 Mirror plane 53 Input hologram 531 Input region 54 Substrate 55 Cover layer 56 Hologram layer 61Eye / Viewer 62Eyebox 64Light source 70First impression plate 71Second impression plate L1...L4Light S1Applying a material layer to a first impression plate S2Curing the material layer to form a cover layer S3Applying a hologram layer to the cover layer S4Applying a substrate to the hologram layer S5Exposing the hologram layer S6Curing the hologram layer S7Applying a material layer to the hologram layer S8Shaping the material layer S9Curing the material layer to form a substrate S10Separating the impression plates SB1, SB2Bench of rays SLSunlight VBVirtual image

Claims

1. Method for producing an optical waveguide (5), including the steps of: - applying (S1) a layer of a light-transmissive material onto a first mould plate (70); - curing (S2) the applied light-transmissive material to form a cover layer (55); - applying (S3) a hologram layer (56) onto the cover layer (55); - applying (S4) a substrate (54) onto the hologram layer (56); and - exposing (S5) and curing (S6) the hologram layer (56).

2. Method for producing an optical waveguide (5), including the steps of: - applying (S1) a layer of a light-transmissive material onto a first mould plate (70); - curing (S2) the applied light-transmissive material to form a cover layer (55); - applying (S3) a hologram layer (56) onto the cover layer (55); - exposing (S5) and curing (S6) the hologram layer (56); - applying (S7) a layer of a light-transmissive material onto the hologram layer (56); - shaping (S8) the applied light-transmissive material by means of a second mould plate (71); and - curing (S9) the applied light-transmissive material to form a substrate (54).

3. Method according to Claim 2, wherein the second mould plate (71) is brought into contact with the layer of the light-transmissive material under the influence of force.

4. Method according to any of the preceding claims, wherein a separating layer is arranged between the first mould plate (70) and the layer of the light-transmissive material or between the second mould plate (71) and the layer of the light-transmissive material.

5. Method according to any of the preceding claims, wherein a layer thickness of the hologram layer (56) is defined by spacers during the application (S3) of the hologram layer (56) onto the cover layer (55).

6. Method according to any of the preceding claims, wherein the light-transmissive material is a lacquer or an optically clear adhesive.

7. Method according to any of the preceding claims, wherein the light-transmissive material that has been subjected to a curing process has a refractive index of greater than or equal to 1.4.

8. Optical waveguide (5) for a display device, wherein the optical waveguide (5) comprises a substrate (54), a cover layer (55) and an intervening hologram layer (56) and is fabricated by means of a method according to any of the preceding claims.

9. Device for generating a virtual image (VB), having: - an image-generating unit (1) for producing an image; and - an optics unit (2) for projecting the image in the direction of a mirror unit (3) for generating the virtual image (VB); characterized in that the device comprises at least one optical waveguide (5) according to Claim 8 for expanding an exit pupil.

10. Means of transport having a device according to Claim 9 for generating a virtual image (VB) for an operator of the means of transport.