Optical device

Abstract

Optical device (1) for manipulating light beams (2a), (2b), comprising arranged in the beam paths of the light beams (2a), (2b): - a beam splitter (3) for reflecting or transmitting light beams and - a first notch beam splitter (4a) configured for splitting light beams (2a) having a first wavelength into transmitted light beams and reflected light beams and for substantially transmitting light beams (2b) having a second wavelength.

Description

[0001] Optical device

[0002] The invention concerns an optical device for manipulating light beams.

[0003] A 3D display is a display device capable of conveying depth to the viewer. Many 3D displays are stereoscopic displays, which produce a basic 3D effect by means of stereopsis, but can cause eye strain and visual fatigue due to the vergence-accommodation conflict. I. e., conventional stereoscopic three-dimensional displays suffer from the vergence-accommodation conflict because the stimulus to accommodation is fixed by the display panel and viewing optics, but that to vergence changes with image contents. This is in particular a serious problem in AR and VR applications. To produce a more realistic 3D effect, it would be necessary to also use different focal lengths for the displayed content. Thus, using multifocal or varifocal displays can overcome the vergence-accommodation conflict. Therein, it can be sufficient to use two or three different focal planes (e. g. for each main color: red, blue, green) to sufficiently alleviate the vergence-accommodation conflict.

[0004] Most current solutions use mechanically or electrically actuated lenses. US 20210208407 A1 shows a head mounted display system to display an image, the head mounted display system comprising: a display engine to generate light for a display;

[0005] the system configured to apply color specific settings to a subset of colors of the light, such that the subset of colors has different settings than another portion of the light; and an optical combiner to output the one or more colors of light to generate the image. The color specific settings may comprise different focal distances to provide a multi-focal display. For example, the green light may be at a near distance, while the red and blue lights are at an infinite focal distance. For this purpose, multiple waveguides are stacked upon each other. However, this requires one waveguide per focal plane and does not allow different focal planes for all main colors. Further, stacking more combiners results in complexity issues. Also, US 20180136474 A1 shows a stacked waveguide assembly, which again results in a very complex device. Further, Zhan, T., Xiong, J., Zou, J. et al. Multifocal displays: review and prospect. PhotoniX 1, 10 (2020) (https: / / doi. org / 10.1186 / s43074-020-00010-0) describes different multifocal display technologies. There are numerous potential optical architectures for providing the multiplanar functionality, which are based on different ways for multiplexing the information and / or generating the focal planes. The multifocal enabling methods can be categorized into power-based and distancebased ones, depending on the focal plane generating mechanism. The other classification standard is how the 2D information is multiplexed into 3D scenes. The current multiplexing channels include space, time, polarization, wavelength and their hybrids. Wavelength-multiplexing is based on the fact that the human visual system's color response renders the fine visible spectrum considerably redundant for display applications.

[0006] Tao Zhan, Junyu Zou, Matthew Lu, Enguo Chen, and Shin-Tson Wu, " Wavelength-multiplexed multi-focal-plane seethrough near-eye displays, " Opt. Express 27, 27507-27513 (2019) demonstrates a wavelength-multiplexed multifocal display system. A spectral notch filter was used as the wavelength-sensitive component to generate distinct depths. Two laser projectors with close but different wavelengths (532 nm and 517 nm) provide two images. The notch filter has a sharp spectral stopband, such that the 517 nm light can pass through it, while the 532 nm light is totally reflected. Thus, the two images are separated into two depths by the spectral notch filter and an eyepiece lens system creates different focal planes for the different optical path lengths. However, this system requires a large space, which is disadvantageous e. g. for head-mounted displays. Further, light may be directly scattered from the beam splitter to the viewer, without having reached the spectral notch filter, thereby limiting the image quality.

[0007] It is an obj ective of the present invention to overcome or alleviate at least one of the problems of the prior art. In particular it is an obj ective of the present invention to propose an optical device for manipulating light beams, in particular for imposing different optical path lengths on light beams, which is more compact and / or has a higher image quality.

[0008] This is achieved by an optical device for manipulating light beams, comprising arranged in the beam paths of the light beams:

[0009] - a beam splitter for reflecting or transmitting light beams and

[0010] - a first notch beam splitter configured for splitting light beams having a first wavelength into transmitted light beams and reflected light beams and for substantially transmitting light beams having a second wavelength.

[0011] The beam splitter is configured for transmitting or reflecting light beams. The first notch beam splitter splits light beams of the first wavelength into transmitted light beams and reflected light beams, while light beams of the second wavelength are substantially transmitted. Since the beam splitter and the first notch beam splitter are arranged in the beam paths of the light beams, at least a fraction of the light beams of the first wavelength are reflected back and forth between the beam splitter and the first notch beam splitter at least once, thereby changing (in particular increasing) the optical path length of light beams of the first wavelength compared to light beams of the second wavelength. Used in a display device, this allows the different wavelengths to be translated into different focal planes. Since the first notch beam splitter is configured for splitting light beams having the first wavelength into transmitted light beams and into reflected light beams (i. e. also for partially transmitting light beams), the optical device may be almost flat in a direction of the beam paths and light beams can be coupled out of the optical device in a forwards direction substantially parallel to the direction of light beams coming into the optical device. Further, the light beams of all beam paths can be guided by the same elements, reducing the complexity.

[0012] The second wavelength is in particular different from the first wavelength. The beam splitter and the first notch beam splitter can be arranged in any order in the beam paths; i. e. the light beams can first reach the first notch beam splitter or can first reach the beam splitter. In particular, the beam splitter and the first notch beam splitter are arranged in the beam paths of the light beams such that at least a fraction of the light beams reaches each of them at least twice.

[0013] The beam splitter is in particular configured for reflecting or transmitting light beams of at least the first wavelength and the second wavelength. Optionally, the beam splitter reflects or transmits light beams substantially wave-length independent (in particular in the visible spectrum, which can be defined as between 300 and 850 nm). Optionally, the beam splitter is configured for splitting light beams (in particular substantially wave-length independent, e. g. in the visible spectrum) into transmitted light beams and into reflected light beams, wherein e. g. fixed fractions of the incoming light beams could be transmitted and reflected, respectively, or the light beams are split into reflected light beams and transmitted light beams dependent on / based upon the polarization of the light beams.

[0014] The first notch beam splitter transmits optionally at least 25%, further optionally at least 40%, of the light beams of the first wavelength and / or reflects optionally at least 25%, further optionally at least 40%, of the light beams of the first wavelength (in terms of intensity). Optionally, the first notch beam splitter splits the light beams of the first intensity substantially equally into transmitted and reflected light beams (i. e. the transmitted and the reflected light beams have substantially the same intensity). Optionally, the first notch beam splitter is configured for splitting light beams having the first wavelength into transmitted light beams and into reflected light beams dependent on / based upon the polarization of the light beams. Optionally, the first notch beam splitter is configured for substantially transmitting light beams of the first wavelength having a first polarization state and for substantially reflecting light beams of the first wavelength having a second polarization state different from the first polarization state (optionally orthogonal to the first polarization state). Optionally, the first notch beam splitter is configured for substantially transmitting light beams having the second wavelength po-larization-independently. The first notch beam splitter could also be configured for splitting light beams having the first wavelength into transmitted light beams and into reflected light beams based upon fixed fractions of the incoming light beams. Optionally, the first notch beam splitter directs the reflected light beams at the beam splitter.

[0015] Optionally, the optical device is configured for emitting light beams that were transmitted by the beam splitter and by the first notch beam splitter. Optionally, light beams of the first wavelength are reflected once by each of the beam splitter and the first notch beam splitter and are transmitted once by each of the beam splitter and the first notch beam splitter. In particular, the optical device is configured for emitting light beams of the first wavelength that were reflected by the beam splitter and by the first notch beam splitter (at least) once and that were transmitted by the beam splitter and by the first notch beam splitter (at least) once.

[0016] Transmitting / reflecting / absorbing throughout this disclosure refers to substantially transmitting / reflecting / absorbing, i. e. in particular disregarding imperfections. Substantially transmitting / ref lecting / absorbing throughout this disclosure means in particular that at least 80%, optionally at least 90%, further optionally at least 95%, even further optionally substantially all, of the intensity of the respective light is respectively transmitted / reflected / absorbed.

[0017] In particular, the beam splitter and the first notch beam splitter are configured such that at least a fraction of the light beams having the first wavelength are reflected back and forth (in one direction or the other) at least once between the beam splitter and the first notch beam splitter.

[0018] Optionally, the optical device comprises a second notch beam splitter arranged in the beam paths and configured for splitting light beams having the second wavelength into transmitted light beams and into reflected light beams and for substantially transmitting light beams having the first wavelength, in particular such that at least a fraction of the light beams having the second wavelength is reflected back and forth (in one direction or the other) at least once between the beam splitter and the second notch beam splitter, preferably via the first notch beam splitter. Thus, also the optical beam path of the light beams of the second wavelength is changed (in comparison to light beams of the first wavelength).

[0019] Optionally, the second notch beam splitter is configured for splitting light beams having the second wavelength into transmitted light beams and into reflected light beams dependent on / based upon the polarization of the light beams. Optionally, the second notch beam splitter is configured for substantially transmitting light beams of the second wavelength having a first polarization state (in particular the same first polarization state mentioned in the context of the first notch beam splitter) and for substantially reflecting light beams of the second wavelength having a second polarization state (in particular the same second polarization state mentioned in the context of the first notch beam splitter) different from the first polarization state (optionally orthogonal to the first polarization state). Optionally, the second notch beam splitter is configured for substantially transmitting light beams having the first wavelength polarization-independently. The second notch beam splitter could also be configured for splitting light beams having the second wavelength into transmitted light beams and into reflected light beams based upon fixed fractions of the incoming light beams. In particular, the second notch beam splitter directs the reflected light beams at the first notch beam splitter and / or at the beam splitter. In particular, the first notch beam splitter is arranged in the beam paths of the light beams between the beam splitter and the second notch beam splitter.

[0020] Optionally, the optical device is configured for emitting light beams that were transmitted by the beam splitter, by the first notch beam splitter and by the second notch beam splitter. Optionally, light beams of the first wavelength are reflected once by each of the beam splitter and the first notch beam splitter and are transmitted (at least) once by each of the beam splitter, the first notch beam splitter and the second notch beam splitter and light beams of the second wavelength are reflected once by each of the beam splitter and the second notch beam splitter and are transmitted (at least) once by each of the beam splitter, the first notch beam splitter and the second notch beam splitter.

[0021] Optionally, the beam splitter comprises an optical polarizing element for reflecting or transmitting light beams (in particular of at least the first and the second wavelengths) depending on their polarizations. The optical polarizing element acts in particular substantially wavelength independent, i. e. acts preferably for at least the first wavelength and the second wavelength wave-length in the same way. The optical polarizing element is in particular a polarizing beam splitter or a linear polarizer. This allows to either polarize light coupled into the optical device or only couple in light of a certain polarization (in particular if the beam splitter is arranged in the beam paths prior to the first (and in particular second) notch filter), or to polarize light coupled out of the optical device or only couple out light of a certain polarization.

[0022] Optionally, the optical device comprises a polarization changing element configured for imparting a relative phase shift of π between two orthogonal polarization components of light beams after a pass from the first notch beam splitter to the beam splitter and a pass from the beam splitter to the first notch beam splitter (in any order). This allows to distinguish light beams that have already been reflected once (i. e. have been passed forwards and backwards once and therefore have been imprinted an additional path length based on their wavelength) from initial light beams.

[0023] Optionally, the optical device comprises a ( first) quarter-wave plate arranged in the beam paths between the beam splitter and the first notch beam splitter (in particular between the beam splitter on the one hand and the first notch beam splitter and the second notch beam splitter on the other hand). In particular, the polarization changing element comprises or is the ( first) quarter-wave plate.

[0024] Optionally, the optical device comprises an outcoupling polarizing optical element for transmitting or absorbing light beams depending on their polarizations arranged in the beam paths of light beams transmitted by the first notch beam splitter and by the beam splitter, and optionally by the second notch beam splitter. This allows to only couple out light that was reflected back and forth between the beam splitter and the respective notch beam splitter and that has thus been imprinted the respective additional path length based upon the wavelength. The outcoupling polarizing element is optionally a linear polarizer, in particular an absorptive linear polarizer. This is particularly advantageous if the beam splitter is arranged prior to the first / second notch beam splitter in the beam paths of the light beams. The outcoupling polarizing optical element is optionally arranged in the beam paths of the light beams after having been transmitted by the beam splitter and the first notch beam splitter (and optionally the second notch beam splitter).

[0025] Optionally, the optical device comprises a (second) quarter wave plate in the beam paths of the light beams transmitted by the first notch beam splitter and by the beam splitter, and optionally also transmitted by the second notch beam splitter, ahead of the outcoupling polarizing optical element. The (second) quarter wave plate allows to adapt the polarization of light beams transmitted by the notch beam splitters and the beam splitter such that the light beams are transmitted by the outcoupling polarizing optical element.

[0026] Optionally, the optical device comprises a lens (or at least one lens) arranged in the beam paths of light beams transmitted by the first notch beam splitter and by the beam splitter, and optionally by the second notch beam splitter, and configured for creating different focal planes for the (emitted) light beams depending on the optical path lengths of the beam paths travelled by the light beams from being received by the optical device up to the lens. Thus, the different optical path lengths of the light beams, which depend on the wavelength, are translated into different focal planes for different wavelengths. Optionally, the optical device is arranged in the beam paths behind the outcoupling polarizing optical element. The lens can comprise a diffractive lens and / or a refractive lens. Optionally, the focal length of the lens is wavelength dependent, in particular different for the first wavelength and the second wavelength. In particular, additionally, the material of the lens (es) and / or a coating of the lens (es) may create different focal planes depending on the wavelength.

[0027] Optionally, the first wavelength differs from the second wavelength by between 1 nm and 30 nm, further optionally by between 2 nm and 20 nm. I. e., optionally, light beams of the first wavelength and of the second wavelength are precepted by the human visual system as substantially the same color, while the slightly differing wavelengths allow the notch beam splitters to select different path lengths based upon the slightly different sub-wavelengths. E. g., the first wavelength could be 520 nm and the second wavelength could be 530 nm, which would both be precepted as green, while the slightly different wavelengths allow the distinction by the notch beam splitters and thus the different optical path lengths.

[0028] Optionally, the optical device comprises a third notch beam splitter arranged in the beam paths of the light beams and configured for splitting light beams having a third wavelength into transmitted light beams and reflected light beams (wherein the reflected light beams are in particular reflected via the first and optionally the second notch beam splitter to the beam splitter) and for substantially transmitting light beams having the first wavelength and light beams having the second wavelength, wherein the first notch beam splitter and the second notch beam splitter are further configured for substantially transmitting light beams having the third wavelength,

[0029] wherein optionally the third wavelength differs from the first wavelength and from the second wavelength by more than 40 nm. Thus, the third notch beam splitter can be used for imprinting another focal length on light beams of a third wavelength. In particular, the third wavelength could represent a different color than the first and the second wavelength, while the first and the second wavelength could be close enough to be perceived as substantially the same color.

[0030] In particular, the third notch beam splitter is configured such that at least a fraction of the light beams having the third wavelength are reflected back and forth (in one direction or the other) at least once between the beam splitter and the third notch beam splitter, in particular via (i. e. by being transmitted by and passing through) the first and optionally the second notch beam splitter. Optionally, the third notch beam splitter is configured for splitting light beams having the third wavelength into trans-mitted light beams and into reflected light beams dependent on / based upon the polarization of the light beams. Optionally, the third notch beam splitter is configured for substantially transmitting light beams of the third wavelength having a first polarization state (in particular the same first polarization state mentioned in the context of the first notch beam splitter) and for substantially reflecting light beams of the third wavelength having a second polarization state (in particular the same second polarization state mentioned in the context of the first notch beam splitter) different from the first polarization state (optionally orthogonal to the first polarization state). Optionally, the third notch beam splitter is configured for substantially transmitting light beams having the first wavelength and the second wavelength polarization-inde-pendently. The third notch beam splitter could also be configured for splitting light beams having the third wavelength into transmitted light beams and into reflected light beams based upon fixed fractions of the incoming light beams. In particular, the third notch beam splitter directs the reflected light beams at the first notch beam splitter and / or the second notch beam splitter and / or at the beam splitter. In particular, the first notch beam splitter and the second notch beam splitter are arranged in the beam paths of the light beams between the beam splitter and the third notch beam splitter.

[0031] In an optional embodiment, the first notch beam splitter, optionally the second notch beam splitter and optionally the third notch beam splitter are each associated with a respective first wavelength and a respective second wavelength, wherein the respective first wavelength and the respective second wavelength associated with each of the notch beam splitters are different from each other and are different from the respective first wavelength and the respective second wavelength of each of the other ones of notch beam splitters, and the first notch beam splitter, optionally the second notch beam splitter and optionally the third notch beam splitter are each configured for substantially transmitting light beams having the respective first wavelength and light beams having the respective second wavelength and are each configured for substantially transmitting light beams having the respective first wavelength and the respective second wavelength associated with any of the other notch beam splitters. E. g. the first notch beam splitter can be configured for splitting light beams with a wavelength of 530 nm and light beams with a wavelength of 630 nm in transmitted and reflected light beams and for substantially transmitting light beams with a wavelength of 520 nm and light beams with a wavelength of 640nm; and the first notch beam splitter can be configured for splitting light beams with a wavelength of 520 nm and light beams with a wavelength of 640 nm in transmitted and reflected light beams and for substantially transmitting light beams with a wavelength of 530 nm and light beams with a wavelength of 630nm.

[0032] Optionally, the optical device comprises a fourth notch beam splitter arranged in the beam paths of the light beams and configured for splitting light beams having a fourth wavelength into transmitted light beams and reflected light beams and for substantially transmitting light beams having the first wavelength and light beams having the second wavelength and light beams having the third wavelength,

[0033] wherein the first notch beam splitter and the second notch beam splitter and the third notch beam splitter are further configured for substantially transmitting light beams having the fourth wavelength,

[0034] wherein optionally the fourth wavelength differs from the first wavelength and from the second wavelength by more than 40 nm and optionally the third wavelength differs from the fourth wavelength by between 1 nm and 30 nm.

[0035] E. g., the first wavelength and the second wavelength are chosen within the same one of the following wavelength intervals, and optionally the third wavelength (and further optionally the fourth wavelength) is chosen from a different one of the following wavelength intervals: 590 nm to 760 nm; 495 nm to 590 nm; 380 nm to 495 nm. Optionally, the first wavelength and the second wavelength are sub-wavelengths of a first color (i. e. wavelength within the spectrum of a certain color), and the third wavelength is a subwavelength of a second color. Optionally, the optical device is configured for manipulating the optical path lengths of light beams of one or more sub-wavelengths of the first color, of one or more sub-wavelengths of the second color, and of one or more sub-wavelengths of one or more further colors. In particular, the optical device comprises arranged in the beam paths of the light beams for each sub-wavelength of each color a notch beam splitter, configured for splitting light beams having the respective sub-wavelength into transmitted light beams and reflected light beams and for substantially transmitting light beams having any of the other sub-wavelengths. Optionally, the notch beam splitters associated with a first sub-wavelength of the first color and of the second color and optionally of the one or more further colors are arranged close to each other; the notch beam splitters associated with a second sub-wavelength of the first color and of the second color and optionally of the one or more further colors are arranged close to each other; and the notch beam splitters associated with a first sub-wavelength of the first color and of the second color and optionally of the one or more further colors are distanced from the notch beam splitters associated with a second sub-wavelength of the first color and of the second color and optionally of the one or more further colors. In particular, optionally, a distance (in particular normal / minimal distance) between:

[0036] - each of the notch beam splitters associated with a first sub-wavelength of the first color and of the second color and optionally of the one or more further colors

[0037] and

[0038] - each of the notch beam splitters associated with a first sub-wavelength of the first color and of the second color and optionally of the one or more further colors

[0039] is at least two times, preferably at least five times, more preferably at least ten times, larger than the distance between - each respective one of the notch beam splitters associated with a first sub-wavelength of the first color and of the second color and optionally of the one or more further colors and

[0040] - each of the respective other ones of the notch beam splitters associated with a first sub-wavelength of the first color and of the second color and optionally of the one or more further colors. Thus, there is substantially the same focal depth for a sub-wavelength of each color (e. g. redl, bluel, greenl have substantially the same first focal depth and red2, blue2, green2 have substantially the same second focal depth different from the first focal depth).

[0041] More concretely, the optical device is configured for manipulating light beams of a number of sub-wavelengths of a number of colors and comprises arranged in the beam paths of the light beams:

[0042] - a beam splitter for reflecting or transmitting light beams and

[0043] - for each of the number of sub-wavelengths of each of the number of colors a respective notch beam splitter configured for splitting light beams having the respective sub-wavelength into transmitted light beams and reflected light beams and for substantially transmitting light beams having any of the other of the number of sub-wavelengths of any of the number of colors. Optionally, the number of colors is at least two or at least three. The number of sub-wavelengths of at least one of the number of colors is at least one, optionally at least two, further optionally at least three. The number of sub-wavelengths of each of the number of colors is at least one, optionally at least two, further optionally at least three. The colors can for example be red, green and blue. Optionally, the sub-wavelengths of a color are within a spectral range of the respective color. Optionally, the sub-wavelengths of a first color (e. g. red or orange) lie between 590 nm and 760 nm and / or the sub-wavelengths of a second color (e. g. green or yellow) lie between 495 nm and 590 nm and / or the sub-wavelengths of a third color (e. g. blue or violet) lie between 380 nm and 495 nm. Optionally, each subwavelength of each color differs from the other sub-wavelengths of the same color by at least 1 nm, preferably at least 2 nm, more preferably at least 5 nm. Optionally, each sub-wavelength of each color differs from the next higher sub-wavelength of the respective color (if such a next higher sub-wavelength exists) and the next lower sub-wavelength of the respective color (if such a next lower sub-wavelength exists) by less than 30 nm, further optionally by less than 25 nm, further optionally by less than 20 nm, further optionally by less than 15 nm.

[0044] The invention further concerns a display device, comprising:

[0045] - the optical device according to any of the embodiments as mentioned in this disclosure,

[0046] - a first display element, in particular an LCD-panel, comprising a plurality of cells each for selecting incident light beams (in particular dependent on the focus plane to be displayed in an area of the image), wherein

[0047] either selected light beams are transmitted and non-selected light beams are reflected or absorbed (in particular in case the display element is transmissive)

[0048] or selected light beams are reflected and non-selected light beams are transmitted or absorbed (in particular in the case the display element is reflective), and

[0049] a backlight element for illuminating the plurality of cells of the first display element, wherein the backlight element is configured for illuminating a first subset of the plurality of cells with light beams having the first wavelength and a second subset of the plurality of cells with light beams having the second wavelength;

[0050] wherein the optical device is arranged to receive the light beams selected by the first display element.

[0051] The cells of the first display element can thus either let certain light beams pass or not. By selecting light beams of the first or the different wavelengths, also different optical path lengths (and in consequence, focal depths) can be selected.

[0052] Thus, the first display element allows to select the focal plane for different areas of an image to be displayed. In particular, the first display element selects incident light beams dependent on a focus plane to be presented in an area of the image to be displayed. In particular, the first display element is a local dimming display element. Preferably, the display element is a transmissive display element. Optionally, the first subset of the plurality of cells is distinct from the second subset of the plurality of cells. In this disclosure, image refers to any visual content or visual information to be presented / displayed by the display device.

[0053] Optionally, the backlight element is further configured for illuminating a third subset of the plurality of cells of the first display element with light beams having the third wavelength. Optionally, the third subset of the plurality of cells is distinct from the first subset of the plurality of cells and from the second subset of the plurality of cells. Optionally, the backlight element is further configured for illuminating a fourth subset of the plurality of cells of the first display element with light beams having the fourth wavelength. Optionally, the fourth subset of the plurality of cells is distinct from the first subset of the plurality of cells and from the second subset of the plurality of cells and from the third subset of the plurality of cells.

[0054] Optionally, the backlight element is configured for illuminating with light beams of each of the number of sub-wavelengths of each of the number of colors a respective subset of the plurality of cells for each of the number of sub-wavelengths of each of the number of colors. In particular, the subset of the plurality of cells of each of the number of sub-wavelengths of each of the number of colors is distinct from the subsets of all other sub-wavelengths of the same and the other colors. In Particular, the number of subsets is the same as the total number of sub-wavelengths (i. e. the sum of the number of sub-wave-lengths of all colors).

[0055] Optionally, each cell of the first display element corresponds to one sub-pixel of the first display element. Optionally, a first sub-wavelength of each of the number of colors illuminates one sub-pixel of each one of a first group of pixels of the display element, a second sub-wavelength of each of the number of colors illuminates one sub-pixel of each of a second group of pixels, and optionally a third sub-wavelength of each of the number of colors illuminates one sub-pixel of each of a third group of pixels. E. g. each pixel of the first group has three subpixels, wherein a first one of the subpixels is illuminated with a red-1 sub-wavelength, a second one of the subpixels is illuminated with a green-1 sub-wavelengths and a third one of the subpixels is illuminated with a blue-1 sub-wavelength; and each pixel of the second group has three subpixels, wherein a first one of the subpixels is illuminated with a red-2 sub-wave-length, a second one of the subpixels is illuminated with a green-2 sub-wavelengths and a third one of the subpixels is illuminated with a blue-2 sub-wavelength. E. g. red-1, green-1 and blue-1 represent a lowest sub-wavelength of the respective color, red-2, green-2 and blue-2 represent a next higher subwavelength of the respective color.

[0056] Optionally, the display device comprises:

[0057] - a second display element, in particular an LCD-panel, arranged for receiving light beams selected by the first display element. In particular, the second display element is arranged in the beam paths of the light beams between the first display element and the optical device. Preferably, the second display element is transmissive. In particular, the second display element is configured for forming the actual image to be displayed (in terms of the pixel content (i. e. color content of each pixel) of the image to be displayed). Optionally, the second display element comprises a plurality of cells (e. g. pixels (in the one color case) or color-subpixels) each for selecting incident light beams, wherein either selected light beams are transmitted and non-selected light beams are reflected or absorbed (in particular in the case of a transmissive display element) or selected light beams are reflected and non-selected light beams are transmitted or absorbed (in particular in the case of a reflective display element) (in particular dependent on the image to be displayed). Optionally, selected light beams coming from the first display element may each illuminate more than one cell of the second display element. I. e., there may or may not be a 1: 1 correspondence between the light beams from the cells of the first display element and the light beams arriving at the cells of the second display element. Optionally, light beams coming from different cells of the first display element may be at least partially overlapping at the second display element.

[0058] Optionally, the backlight element comprises:

[0059] - a first light source for emitting light having the first wavelength, - a second light source for emitting light having the second wavelength,

[0060] - a first bus waveguide for receiving light from the first light source,

[0061] - a second bus waveguide for receiving light from the second light source,

[0062] - at least two fan-out waveguides associated with the first bus waveguide, wherein each of the at least two fan-out waveguides associated with the first bus waveguide is arranged for illuminating a corresponding cell of the first subset of the first display element (and in particular different ones of the at least two fan-out waveguides associated with the first bus waveguide are arranged for illuminating different ones of the cells of the first subset of the first display element), wherein for each of the at least two fan-out waveguides associated with the first bus waveguide there is provided for a fan-out optical coupler for coupling light from the first bus waveguide to the respective one of the at least two fan-out waveguides associated with the first bus waveguide, and

[0063] - at least two fan-out waveguides associated with the second bus waveguide, wherein each of the at least two fan-out waveguides associated with the second bus waveguide is arranged for illuminating a corresponding cell of the second subset of the first display element (and in particular different ones of the at least two fan-out waveguides associated with the second bus waveguide are arranged for illuminating different ones of the cells of the second subset of the first display element), wherein for each of the at least two fan-out waveguides associated with the second bus waveguide there is provided for a fanout optical coupler for coupling light from the second bus waveguide to the respective one of the at least two fan-out waveguides associated with the second bus waveguide. This allows to distribute light of different wavelengths and precisely illuminate particular cells of the first display element with light beams having the right wavelength. The backlight element comprises optionally at least 10, further optionally at least 100 fan-out waveguides associated with the first bus waveguide and / or optionally at least 10, further optionally at least 100 fan-out waveguides associated with the second bus waveguide. Optionally, the backlight element comprises

[0064] - a primary waveguide for receiving light from the first light source,

[0065] - a third bus waveguide for receiving light from the first light source,

[0066] - at least two fan-out waveguides associated with the third bus waveguide, wherein each of the at least two fan-out waveguides associated with the third bus waveguide is arranged for illuminating a corresponding cell of the first subset of the first display element, wherein for each of the at least two fanout waveguides associated with the third bus waveguide there is provided for a fan-out optical coupler for coupling light from the third bus waveguide to the respective one of the at least two fan-out waveguides associated with the third bus waveguide, wherein the fan-out waveguides associated with the first bus waveguide and the fan-out waveguides associated with the third bus waveguide are arranged for illuminating pair-wise different cells of the first subset of the first display element,

[0067] - a bus optical coupler associated with the first bus waveguide for coupling light from the primary waveguide to the first bus waveguide,

[0068] - a bus optical coupler associated with the third bus waveguide for coupling light from the primary waveguide or the first bus waveguide to the third bus waveguide. This allows to easily distribute light of the first wavelength to a plurality of fanout waveguides for illuminating the first subset of cells of the first display element.

[0069] Optionally, the backlight element comprises for each of the number of sub-wavelengths of each of the number of colors

[0070] - a respective light source for emitting light having the respective sub-wavelength,

[0071] - at least one bus waveguide associated with the respective light source for receiving light from the respective light source, wherein for each of the at least one bus waveguides associated with the respective light source at least two fan-out waveguides associated with the respective bus waveguide are provided, wherein each of the at least two fan-out waveguides associated with the respective bus waveguide is arranged for illuminating a corresponding cell of a subset of cells of the first display element, which subset corresponds to the respective sub-wavelength of the respective color, wherein for each of the at least two fan-out waveguides associated with the respective bus waveguide there is provided for a fan-out optical coupler for coupling light from the respective bus waveguide to the respective one of the at least two fan-out waveguides associated with the respective bus wave-guide. Optionally, the subsets of cells are distinct from each other. Optionally, different ones of the fan-out waveguides illuminate different cells of the fanout waveguides. There can also be provided for more than one light source for emitting light of each sub-wavelength, and associated bus waveguide(s), fan-out waveguides and fan-out optical couplers.

[0072] Optionally, the backlight element comprises for each of the number of sub-wavelengths of each of the number of colors

[0073] - a primary waveguide for receiving light from the respective light source for emitting light having the respective subwavelength, wherein for each of the at least one bus waveguides associated with the respective light source there is provided for a bus optical coupler associated with the respective bus waveguide for coupling light from the respective primary waveguide to the respective bus waveguide.

[0074] Optionally, the backlight element further comprises a transparent substrate, wherein the first bus waveguide, the second bus waveguide, the at least two fan-out waveguides associated with the first bus waveguide and the at least two fan-out waveguides associated with the second bus waveguide (and optionally the primary waveguide, the third bus waveguide and the at least two fan-out waveguides associated with the third bus waveguide) are provided in the transparent substrate,

[0075] wherein optionally the first bus waveguide, the second bus waveguide, the at least two fan-out waveguides associated with the first bus waveguide and the at least two fan-out waveguides associated with the second bus waveguide (and optionally the primary waveguide, the third bus waveguide and the at least two fan-out waveguides associated with the third bus waveguide) are provided in the transparent substrate by femtosecond direct laser writing.

[0076] Optionally, the display device comprises a diffuser arranged in the beam paths of light beams between the backlight element and the optical device, optionally in the beam paths ahead of the second display element and / or behind the first display element. This allows to hide the light beam array from a viewer and to ensure homogenous illumination of the second display element.

[0077] Optionally, the first light source is a laser light source and the second light source is a second laser light source and the display device comprises a despeckling element arranged in the beam paths of light beams between the backlight element and the optical device, optionally in the beam paths ahead of the second display element and after the light beams were selected by the first display element. In particular, the despeckling device is arranged after a first pass of the light beams of the first display element. The despeckling device allows to reduce laser speckle.

[0078] Optionally, the second display element is a stereoscopic display element.

[0079] Arranged in the beam paths of the light beams in this disclosure refers in particular to the light beams that are (in the end) transmitted to the lens, i. e. to the (in the end) desired light beams. It will be understood that references to wavelengths in this disclosure may refer to spectral bands of wavelengths.

[0080] Preferably, references to wavelengths in this disclosure, in particular concerning transmission / reflection / absorption (bands), refer to the center wavelengths (power-weighted mean wavelength) (of these bands).

[0081] The present invention may have one or more of these advantages: - not requiring eye tracking, in particular resulting in no motion to photon latency, no wrong focus positions and / or reduced costs;

[0082] - not requiring mechanical parts, in particular resulting in reduced costs and / or increased reliability and rigidity;

[0083] - no reduction in frame rate, resulting in all standard frame rates being possible, supporting any backlight duty cycle and / or every frame having the correct focus;

[0084] - not compromising image quality, in particular supporting 4k or higher resolutions and / or not affecting local dimming.

[0085] Optionally, the beam splitter is configured for reflecting or transmitting light beams, wherein light beams reflected at the beam splitter are directed towards the first notch beam splitter.

[0086] The invention further concerns a device comprising:

[0087] - the optical device according to any one of the embodiments as mentioned in this disclosure;

[0088] a backlight element for providing the light beams having the first wavelength and the light beams having the second wavelength and optionally the light beams having the third wavelength and optionally the light beams having the fourth wavelength. ( I. e., the invention also concerns a device similar to the display device mentioned above, wherein the first display element is merely optional. References in this disclosure to the backlight element refer to preferred embodiments of the device and / or of the display device. Optionally, the display device comprises the device and the remaining features of the display device mentioned in any of the embodiments of this disclosure. ) Optionally, the backlight element of the device comprises:

[0089] - a first light source for emitting light having the first wavelength,

[0090] - a second light source for emitting light having the second wavelength,

[0091] - optionally a third light source for emitting light having the third wavelength,

[0092] - optionally a fourth light source for emitting light having the fourth wavelength.

[0093] By way of example, the disclosure is further explained with respect to some selected embodiments shown in the drawings. However, these embodiments shall not be considered limiting for the disclosure.

[0094] Fig. 1 schematically shows a first preferred embodiment of an optical device according to the present invention.

[0095] Fig. 2 schematically shows a second preferred embodiment of the optical device.

[0096] Fig. 3 schematically shows a third preferred embodiment of the optical device.

[0097] Fig. 4 schematically shows a first preferred embodiment of a display device 30.

[0098] Fig. 5 schematically shows a second preferred embodiment of a display device 30.

[0099] Fig. 6 shows a simulation of light beams passing a first display element.

[0100] Fig. 7 shows a preferred embodiment of a backlight element for the display device.

[0101] Fig. 1 schematically shows a first preferred embodiment of an optical device 1 for manipulating light beams 2a, 2b. The optical device 1 comprises:

[0102] - a beam splitter 3 for reflecting or transmitting light beams,

[0103] - a first notch beam splitter 4a configured for splitting light beams 2a having a first wavelength into transmitted light beams and reflected light beams and for substantially transmitting light beams 2b having a second wavelength,

[0104] - a second notch beam splitter 4b arranged in the beam paths and configured for splitting light beams 2b having the second wavelength into transmitted light beams and into reflected light beams and for substantially transmitting light beams 2a having the first wavelength, and

[0105] - a polarization changing element 6 configured for imparting a relative phase shift of π between two orthogonal polarization components of light beams after a forwards and a backwards pass, in particular after a pass from the first notch beam splitter 4a to the beam splitter 3 and a pass from the beam splitter 3 to the first notch beam splitter 4a. In this embodiment, the polarization changing element 6 is a quarter-wave plate 7 arranged in the beam paths of the light beams between the beam splitter 3 on the one hand and the first notch beam splitter 4a and the second notch beam splitter 4b on the other hand.

[0106] The light beams 2a, 2b having the first wavelength (e. g. 520 nm) and second wavelength (e. g. 530 nm) are shown as being provided from the right. A light beam 2a having the first wavelength is exemplarily shown in the top and a light beam 2b having the second wavelength is exemplarily shown in the bottom. The second notch beam splitter 4b substantially transmits the light beams 2a having the first wavelength and partially transmits and partially reflects the light beams 2b having the second wavelength. Then, the first notch beam splitter 4a substantially transmits the light beams 2b having the second wavelength (those that were transmitted by the second notch beam splitter 4b) and partially transmits and partially reflects the light beams 2a having the first wavelength. The light beams 2a, 2b that were reflected by either the first or the second notch beam splitter 4a, 4b leave the optical device 1 or are absorbed (and are "wasted" / "lost" light).

[0107] The light beams 2a, 2b are polarized. The light beams 2a, 2b transmitted by the first notch beam splitter 4a and by the second notch beam splitter 4b are e. g. left-handed (L) circular polarized. The light beams 2a, 2b of the first wavelength and second wavelength transmitted by both notch beam splitters 4a, 4b pass the quarter-wave plate 7 and become linearly vertically (V) polarized (i. e. p-polarized). The beam splitter 3 is an optical polarizing element 5 for reflecting or transmitting light beams depending on their polarizations, in particular a polarizing beam splitter or a linear polarizer. In particular, the optical polarizing element 5 is configured to substantially transmit linearly horizontally (H) polarized (i. e. s-polarized) light and to substantially reflect vertically (V) polarized light. I. e., the light beams 2a, 2b of the first wavelength and the second wavelength are substantially reflected by the optical polarizing element 5 upon reaching it for the first time. The light beams 2a, 2b of the first wavelength and the second wavelength subsequently again pass the quarter-wave plate, becoming left-handed (L) circularly polarized.

[0108] Then, the first notch beam splitter 4a substantially transmits the light beams 2b having the second wavelength and partially transmits and partially reflects the light beams 2a having the first wavelength. The light beams 2a having the first wavelength that were reflected at the first notch beam splitter 4a then are right-handed (R) polarized due to the reflection. The second notch beam splitter 4b substantially transmits the light beams 2a having the first wavelength (which were transmitted by the first notch beam splitter 4a and are wasted light) and partially transmits and partially reflects the light beams 2b having the second wavelength (which were transmitted by the first notch beam splitter 4a). The light beams 2b having the second wavelength that were reflected at the second notch beam splitter 4b then are right-handed (R) polarized due to the reflection and are subsequently substantially transmitted by the first notch beam splitter 4a. The light beams 2b having the second wavelength that were transmitted at the second notch beam splitter 4b at this point (i. e. at the second pass) are also wasted light.

[0109] The light beams 2a, 2b having the first wavelength and the second wavelength (which were reflected at the beam splitter 3 and then reflected at the respective notch filter 4a, 4b) pass through the quarter-wave plate 7, changing their polarization horizontal (H) polarization. Thus, they are subsequently substantially transmitted by the optical polarizing element 5. The optical device further comprises a lens 10, arranged in the beam paths of these light beams 2a, 2b of the first wavelength and the second wavelength (that were ultimately transmitted once by the beam splitter 3, the first notch beam splitter 4a and the second notch beam splitter 4b). In consequence, the light beams 2a, 2b of the first and second wavelength travelled different optical path lengths from entering the optical device to the lens 10. The lens is configured for creating different focal planes for the light beams 2a, 2b depending on the optical path lengths of the beam paths travelled by the light beams 2a, 2b from being received by the optical device 1 up to the lens 10 and redirects the light beams 2a, 2b to a viewer' s eye. In this embodiment, about 75% of the original light is wasted (by the reflection at the first and second notch beam splitters 4a, 4b upon reaching them the first time and by the transmission at the first and second notch beam splitters 4a, 4b upon reaching them the second time, in each case 50% of the remaining light is lost).

[0110] In conclusion, the optical device 1 creates different focal planes / depths for the light beams 2a of the first wavelength than for the light beams 2b of the second wavelength. The first wavelength can e. g. differ from the second wavelength by between 1 nm and 30 nm, such that they are perceived by the human visual system as substantially the same color, while their slight wavelength difference allows the notch beam splitters 4a, 4b to distinguish between the two.

[0111] Fig. 2 schematically shows a second preferred embodiment of the optical device 1. The second embodiment differs from the first embodiment mainly in that the order of the beam splitter 3 on the one hand and the first notch beam splitter 4a and the second notch beam splitter 4b on the other hand are reversed and in that there is provided for:

[0112] - an outcoupling polarizing optical element 8 for transmitting light beams depending on their polarizations arranged in the beam paths of light beams transmitted by the first notch beam splitter 4a, by the second notch beam splitter 4b and by the beam splitter 3, and

[0113] - a second quarter wave plate 9 in the beam paths of the light beams transmitted by the first notch beam splitter 4a, by the second notch beam splitter 4b and by the beam splitter 3 ahead of the outcoupling polarizing optical element 8.

[0114] Light beams 2a, 2b of the first and second wavelength enter the optical device 1 ( from the right). The light beams 2a, 2b may be polarized (shown as H-polarized here). The light beams 2a, 2b cross the beam splitter 3, which is an optical polarizing element 5 (in particular a linear polarizer or a polarization beam splitter), and which is configured to substantially transmit H-polarized light and substantially reflect V-polarized light beam. I. e., the light beams 2a, 2b are substantially transmitted at this point. The light beams 2a, 2b of the first and the second wavelength become circularly R-polarized going through the first quarter-wave plate 7.

[0115] The first notch beam splitter 4a splits the light beams 2a of the first wavelength into transmitted light beams and reflected light beams. Substantially 50% of the light beams 2a of the first wavelength (in terms of the intensity) is transmitted and substantially 50% reflected. The transmitted light of the first wavelength is lost / wasted (and blocked / absorbed by the outcou-pling optical element 8 after passing the second quarter-wave plate 9). The reflected light beams 2a of the first wavelength then is L-polarized (due to the reflection) and moves again across the first quarter-wave plate 7, changing polarization to V. This allows the light beams 2a of the first wavelength to be further substantially reflected at the optical polarizing element 5, and then going through the first quarter-wave plate 7 again (changing polarization to L). Then, at the first notch beam splitter 4a, the light beams 2a of the first wavelength are again split into transmitted light beams and reflected light beams. In this case, the reflected light beams 2a of the first wavelength are wasted (i. e. another about 25% of the original intensity going into the optical device 1 ). The transmitted light beams 2a of the first wavelength pass through the second notch beam splitter 4b substantially unaltered and subsequently their polarization is changed to H upon passing the second quarter-wave plate 9. The outcoupling optical element 8 substantially transmits H-polarization (corresponding to the wanted light) and substantially blocks V-polarization (corresponding to the unwanted light).

[0116] The first notch beam splitter 4a substantially transmits the light beams 2b of the second wavelength, which are then split at the second notch beam splitter 4b, wherein the transmitted light corresponds to the unwanted light and is blocked at the optical outcoupling element 8. The wanted reflected light travels via the first notch beam splitter 4a and the first quarter-wave plate 7 to the optical polarizing element 5, where the light beams are reflected again, via the first quarter-wave plate 7 and the first notch beam splitter 4a to the second notch beam splitter 4b. This time, the transmitted fraction of the light beams 2a is the wanted light and passes the second quarter-wave plate 9 and the optical outcoupling element 8. The light beams 2a, 2b of the first and second wavelength transmitted by the optical outcoupling element 8 are then emitted from the lens 10.

[0117] Fig. 3 schematically shows a third preferred embodiment of the optical device 1. This embodiment is similar to the first embodiment, such that only the differences will be pointed out. In contrast to the first embodiment, light beams 2c of a light wavelength are provided and the optical device 1 comprises a third notch beam splitter 4c arranged in the beam paths and configured for splitting light beams having a third wavelength into transmitted light beams and reflected light beams and for substantially transmitting light beams 2a having the first wavelength and light beams 2b having the second wavelength. The beam paths of the light beams 2a, 2b of the first and second wavelength are substantially the same as in the first embodiment, since they pass the third notch beam splitter 4c substantially unaltered.

[0118] The first notch beam splitter 4a and the second notch beam splitter 4b are further configured for substantially transmitting light beams having the third wavelength. Thus, the beam path of the light beams 2c of the third wavelength is similar to the light beams 2a, 2b of the first and second wavelength, only that the light beams 2c of the third wavelength pass the first and the second notch beam splitters 4a, 4b substantially unaltered and are split into transmitted and reflected light beams at the third notch beam splitter 4c. In this way, the optical path length for the light beams 2c of the third wavelength is different from the one for the light beams 2a, 2b of the first and second wavelength, and a third focal plane can be generated.

[0119] For example, the first wavelength, the second wavelength and the third wavelength can pairwise differ from each other by between 1 nm and 30 nm, thus being perceived as substantially the same color by the human visual system and at the same time having different focal planes. Alternatively, the third wavelength could differ e. g. by more than 40 nm from the first and the second wavelength, to represent a different color.

[0120] By adding light beams of further wavelengths, and further notch beam splitters (substantially transmitting light beams of all the other wavelengths while splitting light beams of the respective wavelength into transmitted light beams and reflected light beams), it is possible to generate further focal planes.

[0121] Fig. 4 schematically shows a first preferred embodiment of a display device 30. The display device 30 comprises a backlight element 33 for providing light beams 34a having the first wavelength and light beams 34b having the second wavelength, a first display element 31, a despeckling element 47, a diffuser 46, a second display element 35 and an optical device 1 in any of the embodiments as described in this disclosure (e. g. the first or the second embodiment).

[0122] The first display element 31 is used for selecting the focal depth for the areas of the image to be displayed by respectively selecting light beams 34a, 34b of the first or the second wavelength for the respective areas of the image to be displayed. In this embodiment, the first display element 31 is an LCD-panel and comprises a plurality of cells 32 each for selecting incident light beams, wherein selected light beams are transmitted and non-selected light beams are reflected or absorbed. ( In an alternative embodiment, selected light beams could be reflected and non-selected light beams transmitted or absorbed. )

[0123] The backlight element 33 is configured for illuminating a first subset 32a of the plurality of cells 32 of the first display element 31 with light beams 34a having the first wavelength and a second subset 32b of the plurality of cells 32 with light beams 34b having the second wavelength. The backlight element 33 comprises:

[0124] - a first light source 36a (in particular a first laser light source) for emitting light having the first wavelength, - a second light source 36b (in particular a second laser light source) for emitting light having the second wavelength, - a first bus waveguide 37a for receiving light from the first light source 36a, and - a second bus waveguide 37b for receiving light from the second light source 36b.

[0125] In this embodiment, the bus waveguides 37a, 37b extend from both sides of the light sources 36a, 36b, and light is directly coupled from the light sources 36a, 36b to the bus waveguides 37a, 37b. Associated with the first bus waveguide 37a a number of fan-out waveguides 38a is provided. For each of the fan-out waveguides 38a a fan-out optical coupler 39a is provided for coupling light from the first bus waveguide 37a to the respective fan-out waveguide 38a. Each fan-out waveguide 38a is arranged for emitting the light at a corresponding cell 32 of the first subset 32a of the first display element 31. Analogously, associated with the second bus waveguide 37b a number of fan-out waveguides 38b is provided. For each of the fan-out waveguides 38b a fan-out optical coupler 39b is provided for coupling light from the second bus waveguide 37b to the respective fan-out waveguide 38b. Each fan-out waveguide 38b is arranged for emitting the light at a corresponding cell 32 of the second subset 32b of the first display element 31. Each fan-out waveguide 38a illuminates a different cell 32 of the first subset 32a than all the other fan-out waveguides 38a and each fan-out waveguide 38b illuminates a different cell 32 of the second subset 32b than all the other fan-out waveguides 38b. The first and the second subset 32a, 32b are distinct from each other. Thus, the first display element 31 allows to decide for each region whether light beams 34a of the first wavelength and / or light beams 34b of the second wavelength are passed. In the depicted state, on the left-hand side, three light beams 34a of the first wavelength are passed while the light beams 34b of the second wavelength are blocked and on the right-hand side three light beams 34b of the second wavelength are passed while the light beams 34a of the first wavelength are blocked.

[0126] In this embodiment, the backlight element 33 further comprises a transparent substrate 45, wherein the first bus waveguide 37a, the second bus waveguide 37b, the at least two fan-out waveguides 38a associated with the first bus waveguide 37a and the at least two fan-out waveguides 38b associated with the second bus waveguide 37b are provided in the transparent substrate 45, in particular by femtosecond direct laser writing.

[0127] The selected light beams 34a, 34b are directed at the despeckling device 47 for reducing laser speckles. The despeckling device 47 also comprises a mirror, reflecting the light beams 34a, 34b such that they pass through the first display element 31 and the backlight element 33 again, substantially unaltered.

[0128] Therein, each of the light beams 34a, 34b passes through the same cell 32 of the first display element 31 as previously. ( Instead of the despeckling device 47, only a mirror could be provided. )

[0129] The light beams 34a, 34b then pass the diffuser 46, which serves to hide the light beam array from a viewer and to ensure homogenous illumination of the second display element 35. In this embodiment, there is provided for an (air) spacer 48 between the diffuser 46 and the second display element 35, which allows the light beams 34a, 34b to expand further. The (selected) light beams 34a, 34b reach the second display element 35, which in this embodiment is another LCD panel. The light beams 34a, 34b are partially overlapping at the second display element 35 to ensure a homogenous illumination. The second display element 35 serves to display the image (in terms of the color / pixel content) to be shown. The light beams 34a, 34b of the first and second wavelength passing the second display element 35 reach the optical device 1, which manipulates them, as laid out above, such that the focal lengths are different for the light beams 34a of the first wavelength than for the light beams 34b of the second wavelength. Thus, an image can be displayed with different focal lengths allowing proper accommodation of the viewer' s eye.

[0130] Fig. 5 schematically shows a second preferred embodiment of a display device 30. This embodiment is generally the same as the first embodiment and only differs in the backlight element 33. In Fig. 5, only the light beams 34a of the first wavelength and the respective elements are shown and described for simplicity; however, the optical device 30 analogously comprises the respective elements for the light beams 34b of the second wavelength. Again, the first bus waveguide 37a and the associated fan-out waveguides 38a are provided in the transparent substrate 45. The first laser light source 36a emits light of the first wavelength, which is coupled into first bus waveguides 37a. By evanescent coupling at the optical fan-out couplers 39a, the light is then coupled into the respective fan-out waveguides 38a, which are curved partially in the direction of an emission surface 49 of the transparent substrate 45. The fan-out waveguides 38a terminate within the transparent substrate at a distance from the emission surface 49 and emit the light beams 34a at optical redirecting features 50 formed at the emission surface 49, where the light beams 34a are reflected once such that they are emitted substantially vertically (slightly deviating from completely vertically) to the main extension plane of the emission surface 49. Similar to the first embodiment of the display device 30, the light beams 34a are directed at the first display element 31 and are reflected (e. g. at the despeckling device 47 ) to pass the first display element 31 a second time.

[0131] Fig. 6 shows a closer simulation (in particular a Zemax simulation) of light beams 34a. As can be seen, the light beams 34a pass through the same cell 32 of the first display element 31 in the forwards as in the backwards pass.

[0132] Returning to Fig. 5, after passing the first display element 31, the light beams 34a reach the backlight element 33 again and impinge on the emission surface 49 in regions which are parallel to the main extension plane of the emission surface 49, such that they pass through the transparent substrate 45 substantially unaltered and continue as described in the context of the first embodiment of the display device 30.

[0133] Fig. 7 shows a preferred embodiment of a backlight element 33 for the display device 30. In this embodiment, the backlight element 33 is configured for providing light beams of six different wavelengths. E. g., the first and the second wavelength could be in the red spectrum, the third and the fourth wavelength in the green spectrum and the fifth and the sixth wavelength in the blue spectrum. Therein, the first and the second wavelength, the third and the fourth wavelength, and the fifth and the sixth wavelength slightly differ from each other. In this way, the display device 30 can provide a full color image with two different focal planes, which are dependent on whether the light beams of the first / third / fifth or the light beams of the second / fourth / sixth wavelength are selected by the first display element 31.

[0134] The backlight element 33 comprises six laser light sources 36a, 36b, 36a', 36b', 36a' ', 36b' ' for respectively emitting the first (e. g. 630 nm), second (e. g. 640 nm), third (e. g. 520 nm), fourth (e. g. 530 nm), fifth (e. g. 450 nm) and sixth (e. g. 460 nm) wavelength. For the sake of simplicity, only the waveguides associated with the light sources 36a, 36a', 36a' ', 36b are shown.

[0135] For spreading the light from the first light sources 36a over the emission surface 49, the first light sources 36a couples light into the primary waveguide 40. From the respective primary waveguide 40, light is coupled to a number of bus waveguides 37a, 41 (in this case two bus waveguides associated with the first light source 36a) at the respective bus optical couplers 43, 44 (e. g. by evanescent coupling). A number of fan-out waveguides 42 is associated with the bus waveguide 41 and light of the first wavelength is coupled from the bus waveguide 41 to each of the fan-out waveguides 42 at respective fan-out optical couplers (e. g. by evanescent coupling, not shown explicitly in Fig. 7 ). Similarly, a number of fan-out waveguides 38a is associated with the bus waveguide 37a and light of the first wavelength is coupled from the bus waveguide 37a to each of the fanout waveguides 38a at respective fan-out optical couplers 39a. Further, a number of fan-out waveguides 51 is associated directly with the primary waveguide 40 and light of the first wavelength is coupled directly from the primary waveguide 40 to each of the fan-out waveguides 51 at respective fan-out optical couplers. Each of the fan-out waveguides 38a, 42, 51 receiving light from the first light source 36a is arranged for emitting the light of the first wavelength at a corresponding cell 32 of the first subset 32a of the first display element 31, wherein there is a one-to-one correspondence between fan-out waveguides 38a, 42, 51 and cells 32 of the first subset 32a. Analogous to the first light source 36a and the first wavelength, a primary waveguide, bus waveguides and fan-out waveguides are connected to each one of the light sources 36a', 36b', 36a' ', 36b' ' for spreading light of the second, third, fourth, fifth and sixth wavelength over the emission surface, wherein each of the fan-out waveguides receiving light from each of the light sources 36b, 36a', 36b', 36a' ', 36b' ' illuminates a corresponding cell of a corresponding subset of the first display element 31.

[0136] It will be understood that for example a third focal depth can be added to the display device 30 by adding three further light sources with three further wavelengths. Fig. 8 shows the cells 32 of a preferred embodiment of the first display element 31 having nine subsets 32a, 32b, 32c; 32a', 32b', 32c'; 32a' ', 32b' ', 32c' ' of cells 32 (indicated by different hatching).

[0137] Therein, 32a, 32b, 32c provide three different wavelengths in the red spectrum for three different focal depths appearing red; 32a', 32b', 32c' provide three different wavelengths in the green spectrum for three different focal depths appearing green; and 32a' ', 32b' ', 32c' ' provide different wavelengths in the blue spectrum for three different focal depths appearing blue. Each pixel 52 comprises one cell 32 (corresponding to a subpixel) of each subset 32a, 32b, 32c; 32a', 32b', 32c'; 32a' ', 32b' ', 32c' '.

Claims

Claims:

1. Optical device ( 1 ) for manipulating light beams (2a), (2b), comprising arranged in the beam paths of the light beams (2a), (2b):- a beam splitter (3) for reflecting or transmitting light beams and- a first notch beam splitter (4a) configured for splitting light beams (2a) having a first wavelength into transmitted light beams and reflected light beams and for substantially transmitting light beams (2b) having a second wavelength.

2. Optical device ( 1 ) according to claim 1, comprising a second notch beam splitter (4b) arranged in the beam paths and configured for splitting light beams (2b) having the second wavelength into transmitted light beams and into reflected light beams and for substantially transmitting light beams (2a) having the first wavelength.

3. Optical device ( 1 ) according to claim 1 or 2, wherein the beam splitter (3) comprises an optical polarizing element (5) for reflecting or transmitting light beams depending on their polarizations, in particular a polarizing beam splitter or a linear polarizer.

4. Optical device ( 1 ) according to any one of the previous claims, comprising a polarization changing element ( 6) configured for imparting a relative phase shift of π between two orthogonal polarization components of light beams after a pass from the first notch beam splitter (4a) to the beam splitter (3) and a pass from the beam splitter (3) to the first notch beam splitter ( 4a).

5. Optical device ( 1 ) according to any one of the previous claims, comprising a quarter-wave plate (7 ) arranged in the beam paths between the beam splitter (3) and the first notch beam splitter ( 4a).

6. Optical device ( 1 ) according to any one of the previous claims, comprising an outcoupling polarizing optical element ( 8 )for transmitting or absorbing light beams depending on their polarizations arranged in the beam paths of light beams transmitted by the first notch beam splitter (4a) and by the beam splitter (3), and optionally by the second notch beam splitter (4b).

7. Optical device ( 1 ) according to claim 6, comprising a quarter wave plate ( 9) in the beam paths of the light beams transmitted by the first notch beam splitter (4a) and by the beam splitter (3), and optionally by the second notch beam splitter (4b), ahead of the outcoupling polarizing optical element ( 8 ).

8. Optical device ( 1 ) according to any one of the previous claims, comprising a lens ( 10) arranged in the beam paths of light beams transmitted by the first notch beam splitter (4a) and by the beam splitter (3), and optionally by the second notch beam splitter (4b), and configured for creating different focal planes for the light beams depending on the optical path lengths of the beam paths travelled by the light beams from being received by the optical device ( 1 ) up to the lens ( 10).

9. Optical device ( 1 ) according to any one of the previous claims, wherein the first wavelength differs from the second wavelength by between 1 nm and 30 nm.

10. Optical device ( 1 ) according to any one of the previous claims, comprising a third notch beam splitter arranged in the beam paths and configured for splitting light beams having a third wavelength into transmitted light beams and reflected light beams and for substantially transmitting light beams (2a) having the first wavelength and light beams (2b) having the second wavelength,wherein the first notch beam splitter (4a) and the second notch beam splitter (4b) are further configured for substantially transmitting light beams having the third wavelength, wherein the third wavelength differs from the first wavelength and from the second wavelength by more than 40 nm.

11. Display device (30), comprising:- the optical device ( 1 ) according to any one of the previous claims,- a first display element (31 ), in particular an LCD-panel, comprising a plurality of cells (32 ) each for selecting incident light beams, whereineither selected light beams are transmitted and non-selected light beams are reflected or absorbedor selected light beams are reflected and non-selected light beams are transmitted or absorbed, anda backlight element (33) for illuminating the plurality of cells (32 ) of the first display element (31 ), wherein the backlight element (33) is configured for illuminating a first subset (32a) of the plurality of cells (32 ) with light beams (34a) having the first wavelength and a second subset (32b) of the plurality of cells (32 ) with light beams (34b) having the second wavelength;wherein the optical device ( 1 ) is arranged to receive the light beams selected by the first display element (31 ).

12. Display device (30) according to claim 11, comprising:- a second display element (35), in particular an LCD-panel, arranged for receiving light beams selected by the first display element ( 31 ).

13. Display device (30) according to any one of claims 11 or 12, wherein the backlight element (33) comprises:- a first light source (36a) for emitting light having the first wavelength,- a second light source (36b) for emitting light having the second wavelength,- a first bus waveguide (37a) for receiving light from the first light source (36a),- a second bus waveguide (37b) for receiving light from the second light source (36b),- at least two fan-out waveguides (38a) associated with the first bus waveguide (37a), wherein each of the at least two fanout waveguides (38a) associated with the first bus waveguide (37a) is arranged for illuminating a corresponding cell (32 ) of the first subset (32a) of the first display element (31 ), wherein for each of the at least two fan-out waveguides (38a) associated with the first bus waveguide (37a) there is provided for a fan-out optical coupler (39a) for coupling light from thefirst bus waveguide (37a) to the respective one of the at least two fan-out waveguides (38a) associated with the first bus waveguide (37a), and- at least two fan-out waveguides (38b) associated with the second bus waveguide (37b), wherein each of the at least two fan-out waveguides (38b) associated with the second bus waveguide (37b) is arranged for illuminating a corresponding cell (32 ) of the second subset (32b) of the first display element (31 ), wherein for each of the at least two fan-out waveguides (38b) associated with the second bus waveguide (37b) there is provided for a fan-out optical coupler (39b) for coupling light from the second bus waveguide (37b) to the respective one of the at least two fan-out waveguides (38b) associated with the second bus waveguide (37b).

14. Display device (30) according to claim 13, wherein the backlight element (33) comprises- a primary waveguide (40) for receiving light from the first light source (36a),- a third bus waveguide (41 ) for receiving light from the first light source (36a),- at least two fan-out waveguides (42 ) associated with the third bus waveguide (41 ), wherein each of the at least two fanout waveguides (42 ) associated with the third bus waveguide (41 ) is arranged for illuminating a corresponding cell (32 ) of the first subset (32a) of the first display element (31 ), wherein for each of the at least two fan-out waveguides (42 ) associated with the third bus waveguide (41 ) there is provided for a fanout optical coupler for coupling light from the third bus waveguide (41 ) to the respective one of the at least two fan-out waveguides (42 ) associated with the third bus waveguide (41 ), wherein the fan-out waveguides (38a) associated with the first bus waveguide (37a) and the fan-out waveguides (42 ) associated with the third bus waveguide (41 ) are arranged for illuminating pair-wise different cells (32 ) of the first subset (32a) of the first display element (31 ),- a bus optical coupler (43) associated with the first bus waveguide (37a) for coupling light from the primary waveguide (40) to the first bus waveguide (37a),- a bus optical coupler (44 ) associated with the third buswaveguide (41 ) for coupling light from the primary waveguide (40) or the first bus waveguide (37a) to the third bus waveguide (41 ).

15. Optical device (30) according to any one of claims 13 to 14, wherein the backlight element (33) further comprises a transparent substrate (45), wherein the first bus waveguide (37a), the second bus waveguide (37b), the at least two fan-out waveguides (38a) associated with the first bus waveguide (37a) and the at least two fan-out waveguides (38b) associated with the second bus waveguide (37b) are provided in the transparent substrate (45),wherein optionally the first bus waveguide (37a), the second bus waveguide (37b), the at least two fan-out waveguides (38a) associated with the first bus waveguide (37a) and the at least two fan-out waveguides (38b) associated with the second bus waveguide (37b) are provided in the transparent substrate (45) by femtosecond direct laser writing.

16. Display device (30) according to claim 10 and anyone of claims 11 to 15, wherein the backlight element (33) is further configured for illuminating a third subset of the plurality of cells with light beams having the third wavelength.

17. Display device (30) according to any one of claims 10 to 16, comprising a diffuser (46) arranged in the beam paths of light beams between the backlight element (33) and the optical device ( 1 ), optionally in the beam paths ahead of the second display element (35) and / or behind the first display element (31 ).

18. Display device (30) according to any one of claims 10 to 17, wherein the first light source (36a) is a laser light source and the second light source (36b) is a second laser light source and the display device (30) comprises a despeckling element (47 ) arranged in the beam paths of light beams between the backlight element (33) and the optical device ( 1 ), optionally in the beam paths ahead of the second display element (35) and after the light beams were selected by the first display element (31 ).

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