Light guide

Abstract

The invention relates to a backlighting means which is suitable in particular for a holographic display apparatus and comprises at least one light guide (LG) having at least two incoupling devices (Gin1, Gin2), at least one of which is switchable, for coupling in light from at least two mutually incoherent light sources (Ls1, Ls2) in such a way that, in chronological succession, by alteration of the switching state of the at least one switchable incoupling device (Gin1; Gin2), light from at least one light source (Ls1; Ls2) is collimated by means of an optical collimation unit and is coupled in at an incoupling device (Gin1; Gin2), and light from at least one other light source (Ls2; Ls1) is collimated by means of an optical collimation unit and is coupled in at another incoupling device (Gin2; Gin1), and incoupled light from the at least two light sources (Ls1; Ls2) propagates in a zigzag in the light guide (LG), preferably by way of total internal reflection, and can be coupled out by means of an outcoupling device (Gout), so that a spatial overlap of light from the at least two light sources (Ls1; Ls2) can be generated during the outcoupling process.

Description

[0001] light guide

[0002] Holographic display devices with at least one light source and at least one spatial light modulator (SLM) often require collimated illumination of this light modulator. The light emitted by at least one light source, usually small compared to the SLM, such as a laser, must be collimated and expanded to the size of the SLM.

[0003] The complexity of expanding and collimating the illumination depends, among other things, on the size of the SLM. For holographic projection devices in which the SLM is designed as a microdisplay and typically has a diagonal of less than 1 inch, for example, 0.7 inches or 0.5 inches, conventional collimation optics based on lenses (a "beam expander") can be used to provide collimated illumination of the SLM. The lenses used in the collimation optics can be only slightly larger than the SLM to be illuminated, allowing for a compact illumination setup.

[0004] For holographic direct-view displays, where the SLM typically has a diagonal of more than 10 inches, and in some cases even more than 20 inches, special solutions are required, as conventional lens optics would be heavy and expensive and not feasible. A wedge backlight, for example, is known from the prior art, as described in US 2013 / 0170004 A1 and US 2014 / 0376207 A1. Here, the light is expanded by incident light at very large angles relative to the normal and deflected by multiple gratings. Such a device can be made compact even for a large SLM, but typically requires a somewhat greater thickness than a backlight unit (BLU) in a conventional 2D display.

[0005] For certain applications, such as mobile displays, where size and weight are particularly important, a BLU based on a thin light guide has significant advantages.

[0006] According to the prior art, such as from DE 10 2012 100 209 A1, coupling into a light guide by means of a coupling grating and zigzag propagation of the light in the light guide, in which reflection at the surfaces of the light guide occurs by total internal reflection, and coupling out a portion of the light after each zigzag using an output grating. One difficulty here is that the size of the coupling surface in the light guide is limited. Typically, a coupling grating should be highly efficient in order to couple in as much light from the light source as possible. If coupled-in light rays were to strike the coupling grating again after a zigzag in the light guide, the coupling grating would undesirably couple these light rays out again with equally high efficiency.The size of the coupling area is therefore limited to the distance that light rays that are coupled, for example, at the left edge of the coupling grating have traveled after a zigzag to the right.

[0007] Since collimation of the light usually occurs before coupling into the optical fiber and the coupled light propagates in a zigzag pattern in the optical fiber, it is not easily possible to achieve an overlap during coupling out with the coupling grating, which has a high efficiency, as would be useful for a uniform intensity distribution of the coupled out light.

[0008] US 2021 / 0318481 A1 describes an optical fiber with an inner boundary layer at which part of the light is reflected and part is transmitted. As the light propagates through the optical fiber and encounters the inner boundary layer, the light is split into different paths that the light can take through the optical fiber. This allows, among other things, an overlap of light that has traveled different paths during output, which contributes to a more uniform output intensity over a large area. However, this can also lead to disruptive interference between light rays that propagate through the optical fiber along similarly long but different paths.

[0009] The object of the invention is to find a solution to couple out overlapping segments from a light guide in which collimated light propagates in a zigzag pattern, without causing disturbing interference between these segments.

[0010] According to the invention, a backlight unit (BLU) for a holographic display device is proposed, comprising at least one light guide having at least two coupling devices, at least one of which is switchable, for coupling in light from at least two mutually incoherent light sources, such that, in succession in time, upon or by changing the switching state of the at least one switchable coupling device, light from at least one light source is collimated by means of collimating optics and coupled in at one coupling device, and light from at least one other light source is collimated by means of collimating optics and coupled in at another coupling device, and coupled-in light from the at least two light sources is propagated in the light guide in a zigzag manner - preferably under total internal reflection - and coupled out again by means of an outcoupling device such thatthat during the coupling out a spatial overlap or spatial adjoining of light from at least two light sources is created.,

[0011] The at least one switchable coupling device can, for example, comprise either a switchable grating or a passive grating or a reflective element and an additional switchable component.

[0012] In the latter case, the passive grating can be, for example, a polarization-selective grating or the reflective element can be a reflective polarizer and the switchable component a polarization switch.

[0013] The switchable component can also be positioned downstream of the passive grating or reflective element in the beam path. For example, a switchable component in the form of a polarization switch can also be positioned downstream of the light coupling from the optical fiber.

[0014] The invention is described in more detail below and illustrated with the aid of figures.

[0015] Fig. 1 shows a schematic of a prior art light guide. Light rays emanating from a light source Ls are collimated using a lens. The light rays then strike a coupling grating Gin, in this case a transmissive one, and are coupled into a light guide LG, so that they propagate in a zigzag pattern within the light guide. The thick dashed lines Be1, Be2, and Be3 on the underside of the light guide schematically represent the area that the coupled-in light hits after a certain number of reflections. The black arrows Lue1 and Lue2 show that there are gaps in between where no light rays reach. If an output coupling grating were attached to the underside of the light guide, light could be output in the area of ​​the thick dashed lines. However, gaps would arise in the area of ​​the black arrows during output coupling because there are no light rays in the light guide that can be output.However, for a BLU to illuminate an SLM over a wide area, a coupling without gaps is required.

[0016] To avoid these gaps, one would, for example, as Fig. 2 shows, try to widen the coupling area by enlarging the coupling grating Gin compared to Fig. 1 and further widening the light from the light source Ls using the lens Le in order to illuminate the wider coupling grating Gin. As Fig. 2 shows, however, the problem arises at the widened coupling area that light rays, after being reflected on the underside of the light guide LG, hit the coupling grating Gin again and can be undesirably coupled out of the light guide LG at the coupling grating Gin, marked in Fig. 2 by the arrows Lback.

[0017] Typically, a coupling grating is designed to have the highest possible coupling efficiency so that as much light as possible is coupled from the light source Ls into the light guide LG. In this case, the coupling grating Gin would also function efficiently as an output coupling grating. Of the light rays that hit the coupling grating Gin again after being reflected on the underside, a very large proportion would be output, leaving only a few or no light rays propagating further in the light guide. Despite the wider coupling grating Gin, the area effectively used for propagation in the light guide is limited. Although the coupling grating Gin was made wider, the area Be, which the light hits after a certain number of reflections in the light guide, has not widened.

[0018] The maximum width of the coupling grating at which such unwanted coupling does not occur corresponds to the distance the light coupled in at the left edge propagates in a zigzag pattern to the right. However, this does not allow for overlap or contiguity of the coupled-out light areas during coupling.

[0019] Fig. 3a shows an embodiment according to the invention.

[0020] Light from two light sources Ls1 and Ls2 is coupled one after the other into a light guide LG, in this example from two different or opposite sides of the light guide LG. The coupling devices, which are designed as transmissive coupling gratings Gin1 and Gin2, are each designed and dimensioned so small that the light does not hit the same coupling grating again after a zigzag pattern or after a reflection at the boundary surface in the light guide LG opposite the coupling grating. In this document, the term coupling device is used synonymously with the term coupling grating. A light beam that propagates in a zigzag pattern in the light guide usually propagates - depending on the design of the coupling device - in a plane that is essentially perpendicular to the surface orinterface of the light guide LG and propagates in this plane in a zigzag shape in a direction of travel that can correspond to the longitudinal extent of the light guide LG. If light is coupled into the light guide LG by a coupling grating Gin2 in such a way that it propagates in a zigzag pattern at an angle a to the normal in the light guide LG of thickness d, then after a distance d tan(a) (depicted as "a" in Fig. 3a) it strikes the opposite surface or interface of the light guide LG and after 2 d tan(a) it strikes the surface or interface of the light guide LG at which it was coupled.

[0021] To prevent a light beam that is coupled in at the left edge of the coupling grating Gin2 from then hitting the coupling grating Gin2 again after a zigzag path in the light guide LG, the width b2 of this grating must be less than 2 d tan(a). The same applies to the width b1 of the grating Gin1. On the other hand, to achieve an overlap during coupling out with the combination of the two coupling gratings Gin1 and Gin2, the sum of the widths of both gratings must be b1 + b2 > 2 d tan(a). In a preferred embodiment of the invention, both gratings are of equal width b1 = b2 and d tan a < b1 < 2 d tan(a). The grating Gin1 is designed to be switchable, while the grating Gin2 is a passive grating in the preferred embodiment. Alternatively, it is also possible to design both gratings Gin1 and Gin2 to be switchable, although this is not necessary in the case of Gin2.Both gratings have the same grating period, so they produce the same diffraction angle of 90° - a for light of a specific wavelength. The grating Gin1 is switched off while light from the light source Ls2 is coupled into the grating Gin2, so that the light rays coupled in with the grating Gin2 that hit the grating Gin1 continue to propagate in the light guide and are not undesirably coupled out again at the grating Gin1. In this respect, a switched off grating Gin1 means that the light rays coupled into the light guide by the other coupling grating Gin2 are reflected at the grating Gin1 or in the region of the grating Gin1 and continue to propagate in the light guide, since no light is deflected by diffraction when the grating Gin1 is switched off.

[0022] For example, grating Gin1 is initially switched on, and light source Ls1 is also switched on, but light source Ls2 is switched off. As a result, light from light source Ls1 is coupled into the light guide via grating Gin1, with this coupled light propagating to the right and avoiding the other input grating Gin2. The light from light source Ls1 can then be coupled out of the light guide LG through an output grating Gout, although this coupling still has gaps.

[0023] Then grating Gin1 and light source Ls1 are switched off and light source Ls2 is switched on instead. Grating Gin2 is preferably designed to be passive. However, it can alternatively be designed to be switchable and in this case would be switched on together with light source Ls2. Light from light source Ls2 is coupled into light guide LG by means of grating Gin2. This light then strikes grating Gin1. Since grating Gin1 is switched off and does not deflect any light when switched off, there is no unwanted coupling of light from light guide LG. Instead, the light is reflected and propagates further in light guide LG and can also be coupled out through grating Gout. The arrangement is such that the coupling out of light from light source Ls2 closes the gaps in the coupling out of light source Ls1.

[0024] The light sources Ls1 and Ls2 and the gratings Gin1 and Gin2 are arranged next to each other or relative to each other such that, after a certain number of zigzags, the light beams coupled into the two gratings Gin1 and Gin2 arrive offset from each other at the underside of the light guide LG for coupling out of the light guide LG. When coupled from opposite sides, the light coupled into the coupling grating Gin2 preferably exhibits half a zigzag more in the light guide than the light coupled into the coupling grating Gin1.

[0025] Preferably, the light beams from the light sources Ls1 and Ls2 arrive at the underside of the light guide LG offset by half the width of the respective coupling area for coupling out of the light guide. To achieve this, when the light is coupled in from opposite sides, the arrangement of the gratings Gin1 and Gin2 is designed such that a light beam coupled in at the center of the coupling grating Gin2 approximately hits the edge of the (switched off) coupling grating Gin1. Conversely, a light beam coupled in at the edge of the coupling grating Gin2 approximately hits the center of the (switched off) coupling grating Gin1. For this purpose, the gratings Gin2 and Gin1 are arranged on the light guide LG such that the left edge of Gin2 is shifted by 3 d tan(a) relative to the center of Gin1.

[0026] The two coupling gratings Gin1, Gin2 could also be arranged at least partially or (almost) completely overlapping on opposite sides of the light guide LG with respect to their vertical projection. In the illustration in Fig. 3a, the light source Ls1, the collimation lens Lei, and the coupling grating Gin1 would be arranged further to the left. In this case, it may be advantageous if the two coupling gratings Gin1, Gin2 are both switchable.

[0027] Fig. 3b shows an arrangement according to the invention in which the light from both light sources Ls1 and Ls2 is coupled into the light guide on the same side. This arrangement is advantageous for keeping the overall thickness of the lighting device low. The lateral distance between the two coupling gratings Gin1 and Gin2 is somewhat greater here than in Fig. 3a. Light that is coupled in at grating Gin1 travels a full additional zigzag to reach an overlapping coupling-out position compared to light that is coupled in at grating Gin2. The region Be11, in which the light that was coupled in at Gin1 strikes the underside of the light guide for the first time after half a zigzag, does not yet show any overlap with light that was coupled in at grating Gin2.Only when the light from Gin1 hits the underside of the light guide after an additional zigzag in the area Be12 and after another zigzag in the area Be3, there is an overlap with light that was coupled at the coupling grating Gin2 and hits the underside of the light guide in the area Be21.

[0028] Preferably, the two coupling gratings Gin1 and Gin2 are offset from each other such that a light beam coupled at the left edge of Gin1 arrives at Gin2 after two zigzags, and a light beam coupled at the right edge of Gin1 arrives at Gin2 after one zigzag symmetrical to the center. This is indicated by the two arrows in Figure 3b.

[0029] In Fig. 3b, at least the grating Gin2 is switchable and is switched off while light from the light source Ls1 is coupled into the light guide LG at the grating Gin1. In Figures 3a and 3b, the light beams from both light sources Ls1, Ls2, after one or more zigzags, strike an output grating Gout, at which light from both light sources Ls1, Ls2 is output. Preferably, the output grating Gout has the same grating period as the two input gratings Gin1, Gin2, so that the light beams are output perpendicular to the surface of the light guide LG. However, it is also possible to use different grating periods.

[0030] Figures 3a and 3b each illustrate the invention by way of example using transmissive input gratings Gin1, Gin2 and transmissive output gratings Gout. However, the invention is not limited to the use of transmissive gratings. Reflective gratings or combinations of transmissive and reflective gratings can also be used.

[0031] Fig. 3c shows an arrangement largely similar to that of Fig. 3b, with the difference that the coupling grating Gin1 is reflective. Furthermore, the light source Ls1 is arranged on the opposite side of the light guide.

[0032] Instead of reflective gratings, the use of other reflective coupling elements or reflective coupling devices is also possible. Fig. 4 shows another embodiment of the invention in which such reflective coupling elements are used. In this case, the left passive reflective coupling element is a mirrored, inclined surface R of the optical fiber. The right coupling element is a reflective wire grid polarizer (WGP), which reflects light of a specific linear polarization and transmits light of a linear polarization perpendicular to it.

[0033] For example, light sources Ls1 and Ls2 are laser light sources that emit linearly polarized light, with the emitted light from each of the two laser light sources being oriented such that it has mutually perpendicular polarization. Alternatively or additionally, polarizers P1 and P2 can be present in the light path between light sources Ls1, Ls2 and the light guide LG. These polarizers are oriented such that light from light sources Ls1 and Ls2 hits the light guide with different polarizations.

[0034] The polarization for light rays from light source Ls1, which are coupled into the light guide via the left reflective coupling element, is selected such that these light rays are transmitted by the Wire Grid Polarizer WGP. The polarization for light rays from light source L2, which strike the Wire Grid Polarizer WGP, is selected such that these rays are reflected by the Wire Grid Polarizer WGP.

[0035] Light sources Ls1 and Ls2 are switched on sequentially. Light from both light sources propagates in the light guide and is output by means of an output coupling device Gout. The distance between the reflective input coupling surface R and the WGP is selected such that the light rays from light source Ls1 impinge on the underside of the light guide in the areas Be11, Be2, and so on, which are offset from the areas Be21, Be22, Be23, and so on, where light impinges, which is coupled from light source Ls2 via the WGP into the light guide LG.

[0036] A polarization switch (PS) is located in the light path after the light is coupled out of the fiber optic cable. This is part of the switchable coupling device. The polarization switch is synchronized with the light sources Ls1 and Ls2 so that the polarization is rotated by 90° for one of the two light sources, Ls1 or Ls2, while the polarization remains unchanged for the other light source.

[0037] This ensures that the light from both light sources has the same polarization after being coupled out of the light guide, as is necessary, for example, to illuminate a spatial light modulator (SLM) that modulates polarized light, such as a liquid-crystal-based SLM. The two reflective coupling elements are spaced apart such that a light beam coupled into the center of the coupling surface of the first reflective coupling element approximately strikes the edge of the second coupling element, and vice versa, if necessary, to achieve an offset and thus an overlap of the coupled-out light beams from both light sources Ls1, Ls2.

[0038] Fig. 5 shows a schematic of an optical simulation with an arrangement similar to Fig. 3b with two transmissive coupling gratings Gin1 and Gin2 on the same side of the light guide LG. Light from the two light sources Ls1 and Ls2 is collimated by the two lenses Lei and Le2 and hits the two coupling gratings Gin1 and Gin2.

[0039] The light is coupled in using two gratings Gin1 and Gin2, of which at least the Gin2 grating is switchable. The light is coupled in sequentially by switching the light sources on and off. This is synchronized by switching on the Gin2 grating when Ls2 is switched on and off when Gin2 is switched off and Ls1 is switched on. The light is then coupled out of the light guide LG using an output grating Gout on the side opposite the light coupling side.

[0040] Advantageously, several optical fibers can be combined as BLUs for an SLM, where light propagates in a zigzag pattern in two mutually perpendicular directions. For example, light can initially propagate in a zigzag pattern in a first, perpendicular, or vertical direction in a first optical fiber LG1. This light can then be coupled out of the optical fiber and coupled into a second optical fiber LG2, where it then propagates in a second, or horizontal, direction in a zigzag pattern. The direction of propagation of the light is determined by the deflection direction of the coupling elements.

[0041] Such an arrangement with two light guides is shown in Fig. 6. The upper light guide LG1 shown there, into which light from the light sources Ls1 and Ls2 is coupled, corresponds to the arrangement shown in Fig. 5.

[0042] Light that is coupled out of a first light guide LG1, which is designed as in Fig. 5, is coupled into a second light guide LG2 by means of a coupling grating Gin3, as can be seen in Fig. 6. The coupling grating Gin3 has an orientation, in particular with regard to its deflection direction, which is rotated by 90 degrees compared to the gratings Gin1 and Gin2, so that the deflection direction of the light during coupling is also rotated by 90°. In Fig. 6 the light propagates in the first light guide LG1 (in the perspective view shown) from front to back or in the direction of the longitudinal axis of the elongated light guide LG1, and in the second light guide LG2 from right to left, i.e. also in the direction of the longitudinal axis of the more elongated light guide LG2. The light guide LG1 is designed in the form of a narrow strip. The coupling grating Gin3 in the second light guide LG2 has approximately the same size as the light guide LG1.The light guide LG2 is rectangular and has approximately the same size as the SLM (not shown) that is to be illuminated with the lighting device.

[0043] However, the arrangement shown in Fig. 6 then has strip-shaped gaps in the propagation direction of the light in the second light guide LG2.

[0044] To achieve an overlap in the coupling out of the light in both dimensions in this second optical fiber LG2, two coupling gratings Gin3 and Gin4 can also be used there, at least one of which is switchable. This light, which is coupled in at the second coupling grating Gin4, can come from an additional optical fiber LG3, as shown in Fig. 7.

[0045] Figs 7a and 7b show different perspective views of the same arrangement. As shown in Fig. 7a, light from two light sources Ls1 and Ls2 is coupled into a first light guide LG1 and propagates in a first dimension or in the direction of the longitudinal axis of the elongated light guide LG1 in a zigzag pattern, in this Fig. 7a now from back to front. Light from two further light sources Ls3 and Ls4 is coupled into another light guide LG3 arranged next to it and propagates there, preferably parallel to the first light guide LG1, also in a zigzag pattern or in the direction of the longitudinal axis of the elongated light guide LG3, in this Fig. 7a from front to back. The light guides LG1 and LG3 are essentially identical and aligned parallel to one another. The light is coupled out again from both light guides LG1, LG3 by means of an outcoupling device. The outcoupling device is shown in Fig.7a, since it is located on the underside of the respective optical fibers LG1 and LG3. The position corresponds to the output coupling device Gout shown in Fig. 5. Both optical fibers LG1 and LG3 are arranged such that the output light then strikes one of the input gratings Gin3 and Gin4 of the further optical fiber LG2, where it can be coupled in and propagated in this optical fiber LG2 in a direction perpendicular to the first two optical fibers LG1 and LG3.

[0046] All light guides each have at least one switchable coupling grating. For example, light from light source Ls1 and then from light source Ls2 is coupled successively into light guide LG1, then outcoupled from there and coupled into light guide LG2 via coupling grating Gin3. While the other coupling grating Gin4 of LG2 is switched off, light from light source Ls3 and then from light source Ls4 is coupled into the further light guide LG3, then outcoupled from there and coupled into light guide LG2 by switching on its coupling grating Gin4. This further light guide LG2 also has an outcoupling grating Gout3 on its underside, which couples the light towards the SLM.

[0047] To better illustrate the propagation of the light rays in the light guides LG1, LG2 and LG3, Fig. 7b shows the same arrangement as Fig. 7a, although for better clarity only selected light rays are shown in Fig. 7b. Light from the light sources LS1 and LS2, which is coupled (at different times) into the light guide LG1 at the coupling gratings Gin1 and Gin2, propagates there in a zigzag from left to right. After each zigzag, a portion of the light is coupled out of the light guide LG1. For better clarity, however, Fig. 7b only shows those light rays from each of the two light sources Ls1 and Ls2 that are each coupled out of the light guide LG1 after a specific number of zigzags. These light rays are then coupled into the light guide LG2 using the coupling grating Gin3 and also propagate there in a zigzag direction in a direction rotated by 90° compared to the light guide LG1.

[0048] Likewise, light from light sources Ls3 and Ls4 is coupled (time-shifted from one another) into the light guide LG3 at the coupling gratings Gin5 and Gin6, where it propagates in a zigzag pattern from right to left. After each zigzag, a portion of the light is coupled out of the light guide LG3. For clarity, Fig. 7b again shows only those light rays from each of the two light sources Ls3 and Ls4 that are coupled out of the light guide LG3 after a fixed number of zigzags and are then coupled into the light guide LG2, where they propagate in a zigzag pattern rotated by 90° relative to the light guide LG3.

[0049] In this way, propagation in vertical and horizontal directions is achieved in the light guides, allowing the surface of an SLM to be illuminated. When the light is coupled out of the light guide LG2, both a horizontal and a vertical overlap of the light rays from the four light sources Ls1, Ls2, Ls3, and Ls4 is created, or at least a border of the areas of the light guide LG2 is created where light rays from the four light sources Ls1, Ls2, Ls3, and Ls4 arrive. The first two light guides are advantageously arranged so that the light propagation in them is antiparallel, i.e., in the view shown in Fig. 7b, from left to right in light guide LG1 and from right to left in light guide LG3. This contributes to an improved intensity distribution of the BLU.

[0050] With the backlight according to the invention, only a relatively small rectangular or square area - for example 10 mm x 10 mm - is required for the input coupling device Gin1, Gin2, for example 1 / 5, 1 / 10, 1 / 20, 1 / 30 or even less of the surface of the light guide on which the at least one input coupling device Gin1, Gin2 and / or the output coupling device Gout is arranged. For example, two elongated light guides LG1, LG3 with a length of approximately 1 m or longer, in conjunction with another rectangular or square light guide LG2 of appropriate dimensions, can illuminate an SLM or an LCD panel with a screen diagonal of, for example, 50" or 60" or even larger with light from the at least two light sources Ls1, Ls2, Ls3 and Ls4. The at least one light guide LG1, LG2, LG3 can, for example, comprise or be made from borosilicate glass. The light absorption orThe light loss in the light guide is usually less than 10% and can be reduced to 1% or even less.

[0051] The light from the light sources, coupled into the light guide LG via the coupling device Gin1, Gin2, could exhibit a Gaussian or flat-top distribution perpendicular to the wave vector. This can be used to reduce or compensate for intensity fluctuations in the light from the different light sources, which propagate in a zigzag pattern in the light guide. It can also compensate for the sum of the light intensities in the overlapping area during coupling out.

[0052] The at least two coupling devices Gin1, Gin2 could also be arranged at oppositely spaced ends or locations of the at least one light guide LG, wherein the two coupling devices Gin1, Gin2 can be designed such that the coupled-in light from the two light sources Ls1, Ls2 propagates in a zigzag shape in opposite directions in the light guide LG. For example, the light that is coupled into the light guide LG by light source Ls1 propagates from right to left, and the light that is coupled into the light guide LG by light source Ls2 propagates from left to right. The coupling-out device Gout could be designed (conjugated) such that the light from the two light sources Ls1, Ls2 leaves the light guide LG at substantially the same angle relative to the surface of the light guide LG.As described in the exemplary embodiments, by appropriately positioning the coupling devices, an offset or a boundary for the light can be created when coupling out of a light guide. Light from the other light source thus fills the gaps when coupling out of the light guide that would arise with a single light source and a coupling device. Because the light sources are incoherent with each other and are used one after the other, no disruptive interference occurs in the coupled-out light.

[0053] The invention is not limited to the embodiments described in detail here.

Claims

Patent claims 1. Background lighting, in particular for a holographic display device, with at least one light guide (LG) which has at least two coupling devices (Gin1, Gin2), at least one of which is switchable, for coupling in light from at least two mutually incoherent light sources (Ls1, Ls2), in such a way that, one after the other, by changing the switching state of the at least one switchable coupling device (Gin1; Gin2), light from at least one light source (Ls1; Ls2) is collimated by means of collimation optics (Lei; Le2) and coupled in at one coupling device (Gin1; Gin2), and light from at least one other light source (Ls2; Ls1) is collimated by means of collimation optics (Le2; Lei) and coupled in at another coupling device (Gin2;Gin1) is coupled in, and coupled-in light from the at least two light sources (Ls1, Ls2) in the light guide (LG) is propagated in a zigzag manner - preferably under total reflection - and can be coupled out by means of a coupling-out device (Gout) in such a way that a spatial overlap or a spatial adjoining of light from the at least two light sources (Ls1, Ls2) can be generated during the coupling-out.; 2. Backlight according to claim 1, wherein the at least one switchable coupling device (Gin1, Gin2) has a switchable grating, in particular a switchable polarization grating.

3. Backlight according to claim 1 or 2, wherein the at least one switchable coupling device (Gin1, Gin2) comprises a passive grating or a reflective element (R, WGP) and a switchable component.

4. The backlight of claim 3, wherein the passive grating comprises a polarization-selective grating or the reflective element comprises a reflective polarizer and the switchable component comprises a polarization switch.

5. Backlight according to claim 4, wherein the switchable component is arranged in the beam path after the passive grating or after the reflective element.

6. Backlight according to one of the preceding claims, wherein the switchable component is designed in the form of a polarization switch, which can also be arranged after coupling out of the light from the light guide (LG).

7. Background lighting according to one of the preceding claims, wherein at least one of the two coupling devices (Gin1, Gin2) is dimensioned such that the light coupled in via one coupling device (Gin1, Gin2) does not strike the same one coupling device (Gin1, Gin2) again after reflection at the boundary surface in the light guide (LG) opposite the one coupling device (Gin1, Gin2).

8. Backlight according to one of the preceding claims, in particular according to claim 7, wherein the width (b1, b2) of the at least one coupling device (Gin1, Gin2) is less than 2 d tan(a), where a is the angle to the normal of the boundary surface of the light guide (LG) and d is the thickness of the light guide (LG).

9. Backlight according to one of the preceding claims, wherein the widths (b1, b2) of the at least two coupling devices (Gin1, Gin2) are dimensioned such that b1 + b2 > 2 d tan(a), where a is the angle to the normal of the boundary surface of the light guide (LG) and d is the thickness of the light guide (LG), in order to achieve a spatial overlap or a spatial adjoining during the coupling-out.

10. Backlight according to one of the preceding claims, in particular according to claim 8, wherein the widths (b1, b2) of the at least two coupling devices (Gin1, Gin2) are of equal width, i.e. b1=b2, and wherein d tan(a) < b1 < 2 d tan(a).

11. Backlight according to one of the preceding claims, wherein the at least two coupling devices (Gin1, Gin2) are designed in the form of coupling gratings which have the same grating period and therefore produce an identical diffraction angle 90° - α for light of a specific wavelength when coupling in light.

12. Background lighting according to one of the preceding claims, wherein the light sources (Ls1, Ls2) and the coupling devices (Gin1, Gin2) are arranged relative to one another such that after a predetermined number of zigzags the light beams coupled in at the two coupling devices (Gin1, Gin2) arrive offset from one another at an output side of the light guide (LG) for coupling out of the light guide (LG).

13. Backlight according to one of the preceding claims, wherein light from the light sources (Ls1, Ls2) can be coupled into the light guide (LG) from opposite sides.

14. Backlight according to one of the preceding claims, in particular according to claim 13, wherein the coupling devices (Gin1, Gin2) are arranged relative to one another in such a way that a light beam which is coupled in the middle by one coupling device (Gin2) approximately hits an edge of the other coupling device (Gin1).

15. Backlight according to one of the preceding claims, in particular according to claim 13 or 14, wherein the coupling devices (Gin1, Gin2) are arranged on the light guide (LG) in such a way that a left edge of one coupling device (Gin2) is laterally offset by 3 d tan(a) relative to the center of the other coupling device (Gin1), where a is the angle to the normal of the boundary surface of the light guide (LG) and d is the thickness of the light guide (LG).

16. Backlight according to one of the preceding claims, wherein light from the light sources (Ls1, Ls2) can be coupled into the light guide (LG) from the same side.

17. Backlight according to one of the preceding claims, in particular according to claim 16, wherein the at least two coupling devices (Gin1, Gin2) are arranged offset from one another such that a light beam which is coupled in at a left edge of one coupling device (Gin1) arrives at the other coupling device (Gin2) after two zigzags and a light beam which is coupled in at a right edge of one coupling device (Gin1) arrives at the other coupling device (Gin2) after one zigzag symmetrical to the center of the other coupling device (Gin2).

18. Backlight according to one of the preceding claims, wherein the at least one coupling device (Gin1, Gin2) and / or the coupling device (Gout) are designed to be reflective or transmissive or have a combination of transmissive and reflective designs.

19. Backlight according to one of the preceding claims, wherein the at least one coupling device (Gin1, Gin2) has a passive reflective coupling element (R), for example with a mirrored inclined surface of the light guide (LG), or a polarization-specific component, for example a reflective wire grid polarizer (WGP), wherein the polarization-specific component or the wire grid polarizer (WGP) reflects light of a specific linear or circular polarization and transmits light of a linear or other circular polarization perpendicular thereto.

20. Backlight according to one of the preceding claims with at least two light guides (LG1, LG2), wherein the light of the at least one light source (Ls1, Ls2) is coupled into a first light guide (LG1) in such a way that it propagates in a zigzag shape in a first direction in the first light guide (LG1), and wherein the light is coupled out of the first light guide (LG1) and coupled into a second light guide (LG2) in such a way that it propagates in a zigzag shape in a second direction in the second light guide (LG2), wherein the first direction is substantially perpendicular to the second direction.

21. Backlight according to claim 20 with a further light guide (LG3), which is designed in a similar way to the first light guide (LG1) and is aligned substantially parallel to the first light guide (LG1) and in which light from at least one further light source (Ls3, Ls4) is coupled into the further light guide (LG3) in such a way that it propagates in a zigzag shape substantially parallel or antiparallel to the first direction of the propagation direction in the first light guide (LG1), and wherein the first light guide (LG1) and the further light guide (LG3) are arranged relative to one another in such a way that a spatial overlap or a spatial adjoining of light from the at least four light sources (Ls1, Ls2, Ls3, Ls4) can be generated during the coupling-out.

22. Background lighting according to one of the preceding claims, wherein light from the light source (Ls1, Ls2) can be coupled out of the light guide (LG) by means of the coupling-out device (Gout) designed in the form of a grating, wherein the coupling-out device (Gout) can, for example, comprise a volume grating.

23. Backlight according to one of the preceding claims, wherein the at least two coupling devices (Gin1, Gin2) are arranged at opposite ends of the at least one light guide (LG), wherein the light from two light sources (Ls1, Ls2) coupled in via the two coupling devices (Gin1, Gin2) propagates in a zigzag shape in opposite directions in the light guide (LG) and wherein the coupling-out device (Gout) is (conjugated) designed such that the light from the two light sources (Ls1, Ls2) leaves the light guide (LG) at substantially the same angle relative to the surface of the coupling-out device (Gout).

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