Diffractive optical element and display device

Abstract

The present invention relates to a diffractive optical element, which comprises a first substrate (10) and a second substrate (11), wherein a liquid crystal layer (14) is arranged between the first substrate (10) and the second substrate (11). The diffractive optical element further comprises strip electrodes (12) located on the first substrate (10) and strip electrodes (13) located on the second substrate (11). The electrodes on the first substrate are arranged at an angle greater than 50° relative to the electrodes on the second substrate. In addition, the electrodes on the first substrate and the electrodes on the second substrate are controlled in such a manner that a defined out-of-plane field is generated in the corresponding overlapping region between the electrodes on the first substrate and the electrodes on the second substrate. The diffractive optical element can be used in a display device for displaying a preferred three-dimensional scene.

Description

[0001] The present invention relates to a diffractive optical element for changing the direction of light or for guiding light to a predetermined direction or for adapting or adjusting a field of view.

[0002] The present invention further relates to a display device having such a diffractive optical element, which is used for displaying two-dimensional and / or three-dimensional objects or scenes.

[0003] In many applications, such as in optical data transmission or displays that represent information in two-dimensional and / or three-dimensional form, optical elements can be used to redirect light or establish a field of view. In such displays (particularly in holographic displays), diffractive optical elements are often used to control the deflection of light. For example, diffractive optical elements can be used for observer tracking in displays to track light used to create a preferred three-dimensional scene or to direct light to the observer's new position as the observer's position relative to the display changes to the observer's new position.

[0004] In the case of a holographic display device having at least one virtual observer area (also referred to as a virtual observer window), a scene or object generated in holographic form can only be observed when the eyes of an observer of the scene are located at the position of the virtual observer area, thereby enabling the observer to observe the represented scene through this virtual observer area. To do this, the observer must assume a fixed position relative to the display device. However, if the observer moves to another position relative to the display device, in order for the observer to continue to observe the presented scene or information, the virtual observer area must be moved to the new position of the observer's eyes.

[0005] In display devices, one known observer tracking option is to track or define the direction of light by appropriately encoding a phase response and a hologram into a spatial light modulation device arranged to modulate the incident light and generate a reconstructed scene.

[0006] Another option for observer tracking is to use a diffractive optical element or device, such as a diffractive device. Such a diffractive optical element or device is designed to be controllable.

[0007] For example, patent document US 8,860,896 B2 discloses a phase modulator comprising a first substrate and a second substrate, an electrode arrangement, and a liquid crystal layer with liquid crystal molecules. The first substrate is arranged opposite the second substrate, and the liquid crystal layer is arranged between the two substrates. The electrode arrangement is only on one substrate and has strip electrodes, while the other substrate has planar electrodes or no electrodes. This phase modulator can also realize a variable diffraction grating by controlling the in-plane electrodes on one substrate. An in-plane field is generated between every two strip electrodes on the same substrate. This makes it possible to form a variable orientation of the liquid crystal molecules in the liquid crystal layer, thereby generating a variable grating period. By writing a diffraction grating with a specific grating period into the phase modulator, light can be deflected.

[0008] Patent document US 2012 / 0206667 A1 describes a phase deflector that uses an out-of-plane field between stripe electrodes on a first substrate and electrodes on a second substrate. The electrodes on the second substrate are either planar or stripe-shaped and extend parallel to the electrodes on the first substrate. By incorporating a diffraction grating with a defined grating period into the phase deflector, light can be deflected perpendicular to the electrode line arrangement. By varying the period of the incorporation, the angle of light deflection can be varied.

[0009] Patent document US 2014 / 0055692A1 describes the use of a diffraction device for observer tracking in a holographic display device. The diffraction device also includes strip electrodes located on at least one substrate. The deflection of light in a direction perpendicular to the electrode lines is also affected by writing a diffraction grating with a defined grating period into the diffraction device. The grating period can be changed here to change the deflection angle of the light. Different configurations of the diffraction device are described. In some configurations, in each case, an out-of-plane electric field between the strip electrodes on the first substrate and the electrodes on the second substrate is used, and the electrodes on the second substrate are planar or also strip-shaped and extend parallel to the electrodes of the first substrate.

[0010] In all prior art elements or devices described in detail herein, an in-plane field or an out-of-plane field is combined with a respective liquid crystal (LC) mode to adjust the diffraction grating by means of the respective field.

[0011] The diffractive device with the ability to generate an in-plane field can be based on, for example, the HAN (Hybrid Alignment Nematic) mode or the CIPR (Continuous in Plane Rotation) mode, as described in patent document US Pat. No. 8,860,896 B2.

[0012] Diffractive devices with the ability to generate out-of-plane fields can be based on ECB (electrically controlled birefringence) mode, and they can also be based on liquid crystal mode using smectic liquid crystals, in which case the liquid crystal molecules can rotate in-plane in the presence of an out-of-plane field.

[0013] In holographic display devices or displays in general, observer tracking requires deflection of light in both horizontal and vertical directions.

[0014] Prior art (such as that described in patent document US 2014 / 0055692A1) discloses a device consisting of at least two diffraction devices for this purpose. The electrodes of the first diffraction device and the electrodes of the second diffraction device are rotated substantially 90 degrees relative to each other in the device, or the two diffraction devices are arranged to rotate relative to each other. For example, in this case, the first diffraction device deflects the incident light in the horizontal direction, while the second diffraction device deflects the light in the vertical direction. In order to perform observer tracking in the depth direction (z direction) as well, at least two diffraction devices are used. A cylindrical lens function is written into each of the at least two diffraction devices, wherein at least two cross-cylindrical lens functions approximate a spherical lens. For example, the diffraction device can also be rotated or tilted 45 degrees relative to the horizontal direction or horizontal line, so that the first diffraction device deflects the light 45 degrees and the second diffraction device deflects the light 135 degrees.

[0015] Typically, a holographic display device comprises at least one light modulation device and at least two diffraction devices for light deflection and thereby for observer tracking.

[0016] Typically, light modulating devices and diffraction devices have ITO (Indium Tin Oxide) electrodes located on at least one substrate. The refractive index of the electrodes differs significantly from that of the surrounding substrate (e.g., glass) and the liquid crystal layer. For example, light reflected from the boundary surface between the ITO electrode and the substrate can impair the contrast of the displayed scene. Absorption of light in the ITO electrodes can further reduce the brightness of the displayed scene. Furthermore, these effects can be enhanced by adding an ITO electrode layer to the display device.

[0017] The prior art includes further documents, such as patent document US 2013 / 0222384A1, which discloses optional possible uses of diffraction devices. This includes the use of diffraction devices to achieve a large viewing angle or a large field of view, for example in head-mounted displays (HMDs). For such applications of diffraction devices, a pair of one diffraction device for horizontal light deflection and one diffraction device for vertical light deflection or a pair of diffraction devices for horizontal and vertical focusing of light is usually used. For at least one diffraction device used in a head-mounted display, it is also particularly advantageous to use at least one reflective diffraction device upstream or downstream of an electrode (for example a metal layer or a dielectric mirror layer) in any direction of light passing through the diffraction device, the at least one reflective diffraction device having, for example, a reflective metal electrode located on a substrate or a transparent electrode combined with a reflective layer arranged in some other way.

[0018] However, a pair of diffraction devices for deflecting light horizontally and vertically is typically required at approximately the same location or position in a holographic display device. However, the use of reflective diffraction devices requires a minimum distance between the diffraction device for vertically deflecting and / or focusing the light and the diffraction device for horizontally deflecting and / or focusing the light so that the modulated light extends continuously and properly through both diffraction devices in the optical path.

[0019] It is therefore an object of the present invention to avoid the disadvantages of the prior art and to provide a feasible solution which allows reducing the number of diffractive devices required in a display device.

[0020] Another object of the present invention is to provide a feasible solution for a reflective diffraction device which allows light to be deflected in a first defined direction and in a second defined direction at approximately the same position in a beam path of a display device.

[0021] This object is achieved according to the invention by a diffractive optical element having the features of claim 1 .

[0022] The present invention provides a diffractive optical element that avoids the aforementioned shortcomings of the prior art. The diffractive optical element comprises a first substrate and a second substrate, with a liquid crystal layer disposed therebetween. Furthermore, the diffractive optical element comprises strip electrodes on the first substrate and strip electrodes on the second substrate, wherein the electrodes on the first substrate are arranged at an angle greater than 50° relative to the electrodes on the second substrate. The electrodes on the first substrate and the second substrate are controllable such that a defined out-of-plane field is generated in the respective overlapping regions of the electrodes on the first substrate and the electrodes on the second substrate.

[0023] To this end, two substrates are arranged parallel to each other, each having strip-shaped electrodes. The number of electrodes, as well as their width and spacing, can vary, allowing the diffractive optical element to have different numbers of electrodes depending on the application. However, in a preferred embodiment of the present invention, the electrode width and spacing are identical within the same substrate and on the first and second substrates. However, it is also possible, for example, for the electrodes on a substrate to have a defined distance from each other or the same width in one region, and a different distance from each other or different width in another region on the same substrate. This applies to both the first and second substrates. However, the electrodes on the first substrate are arranged at an angle relative to the electrodes on the second substrate. This angle is advantageously greater than 50°. Preferably, the electrodes on the two substrates are arranged perpendicular to each other or at an angle of approximately 90° to each other. The liquid crystal layer embedded between the first and second substrates is controlled by an out-of-plane field generated in the overlap or intersection region between one electrode on the first substrate and one electrode on the second substrate, respectively. Depending on the liquid crystal mode employed, the phase configuration curve in the diffractive optical element is generated by the out-of-plane or in-plane alignment of the liquid crystal molecules in the liquid crystal layer. The voltages applied to the electrodes of the first and second substrate generate in each case an out-of-plane field between the respective electrodes of the two substrates, causing the liquid crystal molecules in the liquid crystal layer to rotate and align accordingly according to the liquid crystal mode used.

[0024] A diffractive optical element of this design can not only guide light into a single, defined direction, but also allow light to be deflected into two different directions simultaneously. This means that the diffractive optical element can deflect incident light into a single, defined direction based on control by the electrodes, but can also deflect or guide light into two different directions simultaneously if desired. Therefore, it is advantageous to design the diffractive optical element so that incident light can be deflected into at least one direction.

[0025] Thus, in this manner, a diffractive optical element is provided that can deflect incident light into a first direction and a second direction arranged at an angle relative to the first direction. Thus, the diffractive optical element can function as a diffractive device. For example, the diffractive optical element can be used as a diffractive device for deflecting light in a display device that displays two-dimensional and / or three-dimensional information or scenes.

[0026] According to the present invention, a single diffractive optical element in the form of a diffractive device can be used to simultaneously deflect light in a display device into both a substantially horizontal and a substantially vertical direction. Consequently, two separate diffractive devices, as envisioned in the prior art, are no longer necessary. A display device including such a diffractive optical element according to the present invention, which can be used as a diffractive device, can thus be configured in a more compact and cost-effective manner.

[0027] Diffractive optical elements can also be used to increase the size of the field of view in a display device, such as in a head-mounted display.

[0028] Advantageous further configurations and developments of the invention are apparent from the dependent claims.

[0029] In an advantageous embodiment of the invention, the diffractive optical element can be provided to comprise at least one diffraction grating having a defined grating period.

[0030] By applying a defined voltage to control the electrodes, a diffraction grating with a predetermined and defined grating period can be written into or formed within a diffractive optical element. The grating period is variable. By varying the voltage applied to the electrodes, the grating period can be changed, thereby varying the light deflection angle. This allows the formation of a predetermined phase configuration curve, thereby achieving a predetermined light deflection using the diffractive optical element.

[0031] Advantageously, the diffractive optical element can be designed in such a manner that a diffraction grating having a defined grating period that deflects light incident on the diffractive optical element in a predetermined direction is generated by individually controlling electrodes on only one substrate.

[0032] Therefore, by individually and differently controlling the electrodes on the first substrate or the second substrate, a diffraction grating of a defined grating period can be written into the diffraction optical element so that light can be deflected in one direction. Instead of applying different voltage values ​​to the electrodes on the other substrate, only one voltage value that is equal to all electrodes is applied. The direction of light deflection depends on the arrangement of the electrodes on the corresponding substrate. In other words, if the electrodes on the substrate (whether the first substrate or the second substrate) are arranged horizontally, the light will be deflected in the vertical direction by the diffraction grating formed in the diffraction optical element. However, if the electrodes are arranged vertically on the substrate, the light is deflected in the horizontal direction by the formed diffraction grating. In order to deflect light in the horizontal direction using a diffraction optical element, it is necessary to consider a substrate on which electrodes are installed that are controlled by a control device and on which the electrodes are arranged vertically. This can be related to the first substrate or the second substrate of the diffraction optical element. A deflection of light in only one direction may be sufficient, for example, when a diffractive optical element is used as a diffractive device in a display device and tracking of light in only one direction is required, for example when an observer of the information or the presented scene is moved only horizontally or only vertically to another position, so that the observer can still observe the presented scene.

[0033] However, if it is necessary to deflect or guide light into two different directions, for example, when the observer moves to a new position in the horizontal and vertical directions, in one configuration of the present invention, it can be arranged that two diffraction gratings, each with a defined grating period, can be simultaneously formed by separately controlling the electrodes on the first substrate and the electrodes on the second substrate, and the two diffraction gratings are used to deflect light incident on the diffractive optical element into two predetermined directions at angles to each other.

[0034] Furthermore, a diffraction grating with a specific or defined grating period can be written into the same diffractive optical element or formed into a diffraction grating with a specific or defined grating period by separately controlling electrodes on another substrate, thereby deflecting light in a second direction (preferably perpendicular) at an angle to the first direction. This simultaneously forms two diffraction gratings in the diffractive optical element, each having a defined grating period. Therefore, the electrodes of the first substrate and the electrodes of the second substrate can be simultaneously controlled by a control device, thereby simultaneously forming two diffraction gratings with two phase responses for deflecting light in two different directions. For example, the first deflection direction of light can be horizontal, and the second deflection direction of light can be vertical, or vice versa. By writing different grating periods into the diffractive optical element on one substrate and the other substrate, different deflection angles can be achieved, for example, in the horizontal and vertical directions. The grating period written into one substrate or into both substrates (e.g., from the edge to the middle of the substrate) can also be varied to write a lens function. For example, horizontal deflection can be combined with vertical focusing, or vice versa, or focusing can be performed with different strengths in the horizontal and vertical directions.

[0035] Therefore, it can be advantageously arranged that the electrodes on the first substrate are in a substantially horizontal arrangement and the electrodes on the second substrate are in a substantially vertical arrangement, or that the electrodes on the first substrate are in a substantially vertical arrangement and the electrodes on the second substrate are in a substantially horizontal arrangement. In this way, the electrodes of the first substrate are arranged at an angle of substantially 90° relative to the electrodes on the second substrate. Therefore, the two deflection directions of the light are substantially perpendicular to each other. This basically corresponds to the vertical deflection of the light and the horizontal deflection of the light. However, as mentioned above, it is also possible that the electrodes of the two substrates are arranged at an angle of less than 90° relative to each other. However, it is advantageous that the angle should not be less than 50°, otherwise there will no longer be any special difference in the deflection direction of the light.

[0036] As an alternative to this arrangement of the electrodes on the first substrate and the electrodes on the second substrate, it can also be arranged that the electrodes on the first substrate and the electrodes on the second substrate are both arranged at an angle to the horizontal line, wherein the electrodes of the first substrate are arranged at an angle to the electrodes of the second substrate.

[0037] The strip electrodes can also be arranged on the first and second substrates in the same manner, rotated or tilted at a defined angle relative to the horizontal or a generally mathematically horizontal line. The angle at which the electrodes are arranged on each substrate can range from 0° to 90°, preferably from 30° to 60°. However, a preferred angle is approximately 45°. The electrodes on the first substrate are also arranged at an angle relative to the electrodes on the second substrate, such as an angle between 50° and 90°. For example, if the angle of the electrodes on the first substrate to the horizontal is approximately 45° and the angle of the electrodes on the second substrate to the horizontal is approximately 135°, then light will be deflected in a first direction of approximately 45° and in a second direction of approximately 135°. Of course, the arrangement of the electrodes on the two substrates can also be reversed, so that the first deflection of light will be at approximately 135° and the second deflection of light will be at 45°. Of course, the electrodes can also be arranged at other angles relative to the horizontal on the substrates.

[0038] It can also be advantageously provided that periodically repeating different out-of-plane electric fields are provided in adjacent overlapping regions of the electrodes of the first substrate and the electrodes of the second substrate.

[0039] This is the case when two diffraction gratings, each with a defined grating period, are formed simultaneously in a diffractive optical element. In this way, a defined out-of-plane field is achieved.

[0040] When a first diffraction grating for deflecting light in a first defined direction and a second diffraction grating for deflecting light in a second defined direction are formed simultaneously, the distribution of electric field flux lines for forming the first diffraction grating and the distribution of electric field flux lines for forming the second diffraction grating generated in the liquid crystal layer between the electrodes of the first substrate and the electrodes of the second substrate can be different. This means that the change in electric field from one electrode to the next can be different for the electrodes on the first substrate than for the electrodes on the second substrate.

[0041] More specifically, electrodes on a substrate (e.g., a first substrate) can each be controlled with different voltage values, using voltages that periodically repeat with a defined grating period associated with the first substrate having a first grating period. Electrodes on another substrate (e.g., a second substrate) can also be controlled with different voltage values, in which case they are also controlled with voltages that repeat with a grating period associated with the second substrate having a second grating period. The (first and second) grating periods can be the same or different, meaning that the grating periods on the first and second substrates can be set independently of each other.

[0042] Alternatively, the electrodes on the substrate can be controlled with different voltage values ​​so that the voltage is periodically repeated locally within a small range (e.g., on a few dozen electrodes), but the grating period can vary over a larger range (e.g., on a few thousand electrodes). For example, four periods of 10 entities can be written on 40 electrodes, and then five periods of 10 entities can be written on the next 50 electrodes.

[0043] For example, this can be used to write diffraction structures (such as lens functions), in which case the invention is not limited to simple lens functions, but includes any structure. Similarly, the grating period on one substrate can be independently changed in a desired defined direction and position. Another grating period on another substrate can be changed in another desired defined direction (for example, vertical direction) and position. For example, on a first substrate, equal grating periods can be written in the horizontal direction across the horizontal extent of the substrate in order to deflect light to a given angle in the horizontal direction. However, on a second substrate, a grating period that varies as the vertical extent of the substrate in the vertical direction changes can be written.

[0044] In addition to the lens function, different phase functions can be written to correct the aberrations on one substrate and the other separately.

[0045] In another configuration of the present invention, a single diffraction grating can be formed in a diffractive optical element by controlling the voltage values ​​of electrodes on one substrate to be equal and controlling the electrodes on the other substrate to have different, periodically repeating voltage values ​​using a control device. For example, the electrodes on a substrate (e.g., the first substrate) can all be controlled to have a voltage value of 0 volts, while the electrodes on the other substrate (e.g., the second substrate) are each controlled to have different voltage values ​​(e.g., 0 volts, 2.66 volts, and 5.33 volts). These applied voltages are periodically repeated according to the grating period required for the diffraction grating to be formed in the diffractive optical element. In this way, deflection of light in only one limited direction can be achieved using the diffractive optical element according to the present invention.

[0046] In a diffraction optical element, whether only one diffraction grating is formed or two different diffraction gratings are formed, the grating period is naturally not limited to the simple example of only 3 electrodes used here. In particular, it can also have a longer grating period, such as 20 or 100 electrodes and a more complex voltage curve.

[0047] According to the invention, a liquid crystal pattern is provided, with which at least one diffraction grating having a defined grating period can be formed by an out-of-plane field.

[0048] The present invention uses a liquid crystal mode for which a periodic diffraction structure for forming a diffraction grating can be written into a diffractive optical element by an out-of-plane electric field.

[0049] For example, a suitable liquid crystal mode may be ECB mode (electrically controlled birefringence mode), ULH mode (uniform horizontal helical mode) or another VA mode (vertically aligned mode). In other words, a liquid crystal mode that is advantageously set may be ECB mode, ULH mode or VA mode.

[0050] In one embodiment of the present invention, an ECB mode may be used, wherein the optical axes of the liquid crystal molecules of the liquid crystal layer are also rotated out of plane in an out-of-plane field. In an embodiment of the present invention, linearly polarized light may preferably be used.

[0051] In another embodiment of the present invention, liquid crystal molecules in the liquid crystal layer can be used whose optical axes have a substantially in-plane rotation in the out-of-plane field. These can be smectic liquid crystals or other cholesteric liquid crystals. For example, cholesteric liquid crystals are used in ULH mode. When using smectic or cholesteric liquid crystals and the corresponding liquid crystal modes, circularly polarized light can be preferably used.

[0052] In another configuration of the present invention, it can be advantageously provided that a phase modulation of ≥2π (preferably ≥4π) is achieved on the light in the liquid crystal layer.

[0053] In the liquid crystal layer, a phase modulation range of the light of greater than 2π should advantageously be provided. This means that, when an electric field is applied, by selecting the field strength, a phase between a minimum and a maximum can be set, with the difference between the maximum and the minimum being greater than 2π.

[0054] In a particularly preferred embodiment, the liquid crystal layer is used to adjust the phase modulation range of light by at least 4π (i.e., ≥4π). This is applicable to transmissive diffractive optical elements. To achieve a phase modulation range of ≥4π within the liquid crystal layer, the thickness of the liquid crystal layer can be appropriately adjusted, which means that the liquid crystal layer has a greater thickness than, for example, a liquid crystal layer of a reflective diffractive optical element.

[0055] In the case of a reflective diffractive optical element, this value for the maximum phase modulation of light ≥ 4π applies to twice the path length of the light through the liquid crystal layer, i.e., the distance the light travels through and back. For many liquid crystal modes, a double passage of light through the liquid crystal layer also results in a doubling of the phase modulation. For example, for the ECB mode, a phase modulation of ≥ 2π for a single passage through the liquid crystal layer will correspond to a phase modulation of ≥ 4π for a round trip.

[0056] Furthermore, it can advantageously be provided that the range of the phase modulation is selected to be a range in which the phase of the light has a linear characteristic with respect to the voltage applied to the electrodes of the at least one substrate by the at least one control device.

[0057] According to the invention, use is made of the range in which the phase modulation in the liquid crystal layer has an approximately linear dependence of the phase on the voltage applied to the substrate electrodes and thus on the applied out-of-plane field.

[0058] For example, for the ECB mode used in a liquid crystal layer, the dependence of the phase modulation on the voltage applied to the electrode arrangement typically results in an approximately S-shaped curve. At very small and also very large phase values, the dependence of the phase on the applied voltage deviates significantly from the desired linear behavior according to the S-shaped curve. However, for intermediate phase values, the variation of the phase value with the voltage is approximately linear. According to the present invention, only the linear portion of the curve for the dependence of the phase modulation on the voltage applied to the substrate electrodes is used. Preferably, the maximum phase modulation of the light in the liquid crystal layer is selected to be sufficiently large, for example ≥5π, so that the portion of the phase modulation corresponding to the region with linear behavior of the phase of the light with respect to the voltage applied to the substrate electrodes also includes a phase modulation range of approximately 4π.

[0059] In one configuration of the present invention, the diffractive optical element is designed to be reflective. In a preferred configuration of the present invention, the diffractive optical element can be reflective and have a liquid crystal mode in which the liquid crystal molecules of the liquid crystal layer are subjected to an out-of-plane field with their optical axes rotated in-plane. To this end, for example, the liquid crystal layer can include smectic liquid crystals, or the liquid crystal mode used can be a ULH mode.

[0060] According to the invention, the diffractive optical element can be designed to be transmissive or reflective.

[0061] The object of the invention is also achieved by a display device having the features of claim 17 .

[0062] The present invention provides a display device for displaying two-dimensional and / or three-dimensional objects or scenes, comprising an illumination device, at least one spatial light modulation device, and at least one diffractive optical element according to the present invention.

[0063] The display device may, in particular, be a holographic display device that preferably reconstructs a three-dimensional scene or object in a holographic manner and presents it to at least one observer. An illumination device, including at least one laser or light-emitting diode (LED) as at least one light source, emits at least approximately coherent light, which is then directed to at least one spatial light modulator. The spatial light modulator modulates this light using correspondingly represented information and, with the aid of an optical system, reconstructs the scene or object to be presented. A diffractive optical element according to the present invention can be used and arranged in a display device so as to function as a diffraction device and deflect light incident thereon into at least one defined direction or to produce an enlarged field of view. Therefore, in one embodiment, the diffractive optical element can be configured as a tracking element in the display device. When the observer's position changes, the diffractive optical element can be controlled accordingly, as described in the display device, so that the diffractive optical element can be used to guide light to the observer's new position relative to the display device. To determine or detect the observer's position, the display device may include a position detection system, such as a camera. This allows the observer to observe the presented scene from their new position.

[0064] In another embodiment, a diffractive optical element may be used in a light modulating device, for example, to generate segmented multiple images of a magnified field of view.

[0065] In another embodiment, a diffractive optical element may be used in, for example, a display device that generates a light modulating device image (eg, in a heads-up display or a head-mounted display) to shift the depth plane of the light modulating device image.

[0066] For example, the display device may also include two diffractive optical elements according to the present invention for tracking an observer in the lateral and depth directions (z direction) and another diffractive optical element for segmenting multiple images of the light modulation device and moving the multiple image segments in the depth plane.

[0067] In an advantageous configuration of the invention, an illumination device for emitting polarized light can be provided.

[0068] Linearly polarized light or else circularly polarized light can be used here. The use of linearly polarized light in combination with the ECB mode as liquid crystal mode is particularly advantageous, in which the optical axes of the liquid crystal molecules rotate out of plane also in an out-of-plane field.

[0069] When the liquid crystal mode is ULH mode (i.e., when cholesteric liquid crystals or smectic liquid crystals are used in the liquid crystal layer), circularly polarized light can be preferably used. These liquid crystals have their optical axes substantially rotated in-plane when an out-of-plane field is generated.

[0070] According to the present invention, the electrodes on the first substrate and / or the electrodes on the second substrate may be controlled by providing at least one control device.

[0071] The at least one control device may be designed such that it can apply a desired voltage to the electrodes of the first substrate and the electrodes of the second substrate so as to propagate or generate an out-of-plane (electric) field in the liquid crystal layer between the two substrates.

[0072] In another configuration, it is also possible to use a control device to control the electrodes of the first substrate, and use another control device to control the electrodes of the second substrate.

[0073] There are various possibilities for advantageously configuring the teaching of the present invention and / or for combining the above-described exemplary embodiments or configurations with one another. For this purpose, reference is made, on the one hand, to the patent claims that are dependent on the independent claims and, on the other hand, to the following description of preferred exemplary embodiments of the present invention with the aid of the drawings, which also generally illustrate preferred configurations of the teaching. The invention is explained in principle with the aid of the exemplary embodiments described, without intending to restrict the invention thereto.

[0074] In the figure:

[0075] Figure 1 A schematic diagram showing a liquid crystal mode (specifically, an ECB mode) in a liquid crystal layer according to the prior art is shown;

[0076] Figure 2 A schematic diagram of a diffraction device using an ECB mode according to the prior art is shown;

[0077] Figure 3 shows a schematic diagram of another diffraction device according to the prior art;

[0078] Figure 4 The schematic diagram shows the principle of two diffraction devices for deflecting light using ECB mode according to the prior art in a top view;

[0079] Figure 5 A schematic diagram of a diffractive optical element according to the present invention is shown in perspective view without applying an electric field;

[0080] Figure 6 The perspective diagram shows the Figure 5 A schematic diagram of a diffractive optical element according to the present invention in the presence of an electric field;

[0081] Figure 7 Shown according to Figure 6 A top view of a schematic diagram of a diffractive optical element for explaining a generated electric field;

[0082] Figure 8 Schematic diagram showing in top view the principle of a diffraction device according to the prior art using a liquid crystal pattern, the phase modulation of which depends on the sign of the applied voltage;

[0083] Figure 9 Schematic diagram showing the principle of a diffractive optical element using a liquid crystal pattern depending on a voltage sign according to the present invention;

[0084] Figure 10 A graph showing an S-shaped curve of the dependence of the phase modulation on the voltage applied to the electrode arrangement, wherein the ECB mode is used in the liquid crystal layer; and

[0085] Figure 11 The schematic diagram of FIG. 1 shows the principle of the display device according to the present invention in a top view.

[0086] It should be briefly mentioned that identical elements / parts / components are provided with identical reference numerals in the accompanying drawings as well.

[0087] exist Figure 1 In the present invention, the liquid crystal mode ECB known from the prior art is briefly described. By explaining the ECB mode and showing the optical elements, Figure 1 In the left image a), no voltage is applied to the optical element, and in the right image b), a voltage is applied to the optical element. Figure 1 The optical element comprises two substrates S1 and S2, which are opposite and parallel to each other. The two substrates S1 and S2 each comprise a planar electrode E1, E2. A liquid crystal layer LL with liquid crystal molecules LM is arranged between the two substrates S1 and S2. In each case, an alignment layer AL1, AL2 is arranged between the electrodes E1 and E2 and the liquid crystal layer LL, the alignment layers AL1, AL2 being arranged for pre-alignment of the liquid crystal molecules LM. Alignment layers are known in the art and will therefore not be discussed further here, in particular since a detailed description of the alignment layer does not contribute any essential features to the present invention. In the case of the ECB mode, in the absence of an electric field, the liquid crystal molecules are aligned substantially parallel to the substrate surfaces due to the alignment layers AL1, AL2, as Figure 1 In the case of the ECB mode, the alignments on the two substrates S1 and S2 are set to be antiparallel relative to each other.

[0088] according to Figure 1 As shown in Figure b), if different voltages Va and Vb are applied to electrodes E1 and E2 on substrates S1 and S2, an out-of-plane field is formed between electrodes E1 and E2. This out-of-plane field is represented by the dashed arrows. This out-of-plane field causes the liquid crystal molecules LM to align vertically. In other words, the out-of-plane field forces the liquid crystal molecules LM from their parallel alignment relative to the surfaces of substrates S1 and S2 to an alignment perpendicular to the substrate surfaces. However, the degree of vertical alignment of the liquid crystal molecules LM obtained varies depending on the strength of the applied field or the voltage applied to electrodes E1 and E2.

[0089] Depending on the polarization state of light incident on the optical element, the alignment of the liquid crystal molecules LM in an electric field causes a rotation of the light's polarization, which can be used to modulate the light's amplitude or phase. Phase modulation of the light is particularly possible when using linearly polarized light, where the polarization direction of the light is parallel to the orientation of the liquid crystal molecules generated by the alignment layer.

[0090] Figure 2 The principle of using the ECB mode in a diffraction device according to the prior art is shown in a side view. The diffraction device comprises two substrates S1 and S2, between which a liquid crystal layer LL is arranged. The substrate S1 also comprises a planar electrode E1, and a plurality of individual linear electrodes E2 to E3 are arranged. N The electrodes E2 to En are arranged on the substrate S2 in parallel with each other. The two substrates S1 and S2 here include an alignment layer (not shown) for pre-aligning the liquid crystal molecules LM in the liquid crystal layer LL. The alignment layer is used to align the liquid crystal molecules LM parallel to the substrates S1 and S2 and perpendicular to the electrodes E2, ..., E2 on the substrate S2. N .

[0091] Different voltages V0, V1, V2, and V3 are applied to the linear electrodes E2, E3, E4, E5, E6, ..., E3 on the substrate S2. N , and applying a voltage V0 to a planar electrode E1 on a substrate S1, generating individual electrodes E2, E3, E4, ..., E N The different electric fields between the planar electrode E1 and the planar electrode E2 cause the liquid crystal molecules LM to align to different degrees along the arrows shown (i.e., the vertical direction here). Figure 2 As shown, the applied voltages V0, V1, V2 and V3 are repeated in a periodic manner. Thus, for each electrode E2, ..., E N Set different phase modulations.

[0092] Figure 3 Shown according to Figure 2 FIG. 1 is a side view of a diffraction device of the prior art, except that, due to the alignment layer (also not shown here), the liquid crystal molecules LM, which can also be aligned parallel to the substrates S1 and S2, are now also parallel to the individual linear electrodes E2, E3, E4, E5, E6, ..., E7 on the substrate S2. N .therefore, Figure 3 shows the projection on the short axis of the liquid crystal molecule LM. Figure 2 The diffraction device described in the embodiment of the present invention has a planar electrode E1 and each of the electrodes E2, ..., E N Different electric fields are generated between the two surfaces, which leads to corresponding alignment of the liquid crystal molecules LM.

[0093] By comparing with the prior art, the following provides a preliminary description of an exemplary embodiment according to the present invention. Here, the ECB mode will also be used for comparison.

[0094] Figure 4 The use of two diffraction devices according to the prior art is shown in a top view during use, for example in patent document US 2014 / 0055692 A1, where a display device is used as a light tracking device. To simplify the description, a linear dependence of the phase on the voltage applied to the electrode arrangement of the diffraction device is assumed.

[0095] according to Figure 4 The first diffraction device of the diagram a) comprises electrodes E2, E3, ..., E3 in strip or linear form and arranged vertically on the first substrate. N and a planar electrode E1 on a second substrate, which is identical to the first substrate here and is not shown for the sake of clarity. The two substrates S1 and S2 are arranged in parallel and have the greatest similarity relative to each other. Since the diffraction device is shown in a top view and for the sake of clarity, the substrates S1 and S2 and any layers present (such as alignment layers or liquid crystal layers) are not shown, the electrodes E2, E3, ..., E N The planar electrodes E1 are connected to each other, so only the electrode arrangement is considered here. A diffraction grating with a grating period of four electrodes is written into the diffraction device and a diffraction grating is formed. To this end, the strip electrodes E2, E3, ..., E N Apply appropriate voltage and each strip electrode E2, E3, ..., E N Appropriate voltages are applied between the planar electrode E1 and the planar electrode E1 to generate an electric field, and periodically repeated phase values ​​of 0, 0.5π, π, and 1.5π are written into the diffraction device. In this exemplary embodiment, a voltage of 0 volt (0V) is applied to the planar electrode E1, and periodically repeated voltages of 0 volt (0V), 2 volts (2V), 4 volts (4V), and 6 volts (6V) are applied to the strip electrodes E2, E3, ..., E N to achieve this.

[0096] When using the ECB mode, the phase of the liquid crystal molecules adjusted in the liquid crystal layer does not depend on the sign, but rather on the magnitude of the voltage applied to the electrodes. Therefore, in this exemplary embodiment, voltages of 0 volts, -2 volts, -4 volts, and -6 volts can also be applied to adjust or generate the same diffraction grating having the grating period of the four electrodes.

[0097] Figure 4 Schematic diagram b) shows a second diffraction device, with Figure 4In contrast to the first diffraction device shown in FIG. a), the second diffraction device includes electrodes E2, E3, ..., E3 in a strip or linear form and arranged horizontally on a first substrate. N And a planar electrode E1 on the second substrate. Here, for the sake of clarity, only the electrode arrangement is considered. In the diagram b), a diffraction grating with a grating period of three electrodes is written into the diffraction device and a diffraction device is formed. To this end, the diffraction device is formed by forming a diffraction grating on the strip electrodes E2, E3, ..., E N By applying a suitable voltage between the planar electrode E1 and the strip electrodes E2, E3, E4, ..., E3 and E4 with voltages of 0 volt (0V), 2.66 volts (2.66V) and 5.33 volts (5.33V), the periodically repeated phase values ​​0, 0.66π and 1.33π can be written here. N This is achieved by applying appropriate voltages between them in order to adjust these phase values.

[0098] The light is deflected in the horizontal direction by the first diffraction device according to diagram a), and in the vertical direction by the second diffraction device according to diagram b).

[0099] Figure 5 A diffractive optical element according to the present invention is shown in a perspective view. The diffractive optical element comprises a first substrate 10 and a second substrate 11. The first substrate 10 includes horizontal electrodes 12 in the form of strips or lines, while the second substrate 11 includes vertical electrodes 13 in the form of strips or lines. Alternatively, the first substrate 10 may have strip electrodes aligned and arranged vertically, while the second substrate may have strip electrodes aligned and arranged horizontally, or the electrodes 12 and 13 may be arranged at an angle relative to the horizontal. The strip electrodes 12 on the first substrate and the strip electrodes 13 on the second substrate 11 are arranged parallel to each other. In this manner, a diffractive optical element is formed having strip electrodes 12 and 13 on the two substrates 10 and 11 that intersect with each other. That is, the electrodes 12 on the first substrate 10 are arranged at an angle (approximately 90° in this example) relative to the electrodes on the second substrate 11, forming a mutually intersecting electrode arrangement. A liquid crystal layer 14 containing liquid crystal molecules 15 is disposed between the two parallel substrates 10 and 11. Alignment layers 16 and 17 for pre-aligning the liquid crystal molecules 15 in the liquid crystal layer 14 have been applied to the two substrates 10 and 11. The alignment layers 16 and 17 are applied to the substrates 10 and 11, for example, by rubbing, so that the liquid crystal molecules 15 are pre-aligned parallel to the second substrate 11, parallel to the electrodes 13 provided on the substrate 11, and parallel to the first substrate 10, but perpendicular to the electrodes 12 provided on the substrate 10.

[0100] according to Figure 5 The diffractive optical element is shown in the absence of a voltage between the electrodes 12 and 13 of the first substrate 10 and the second substrate 11 , so that there is no electric field that can propagate within the liquid crystal layer 14 .

[0101] By applying a periodic voltage to the electrode 12 of the first substrate 10 and the electrode 13 of the second substrate 11, two diffraction gratings each having a defined grating period can be written into the diffraction optical element and two diffraction gratings can be formed. For the two diffraction gratings, the grating period can be adjusted independently. Figure 6 The formation of two diffraction gratings in a diffractive optical element is shown in perspective view, using the same Figure 5 The diffractive optical element shown is the same as the diffractive optical element. Figure 5 and Figure 6 In the embodiment of the present invention, since the angle between the electrode 12 of the first substrate 10 and the electrode 13 of the second substrate 11 is approximately 90°, the electrode 12 is aligned in the horizontal direction and the electrode 13 is aligned in the vertical direction, a diffraction grating with a defined grating period in a horizontal form and a diffraction grating with a defined grating period in a vertical form are written into the diffractive optical element and form the diffractive optical element. Two diffraction gratings are formed simultaneously. In this way, light can be simultaneously deflected into two different angled directions using only a single optical element. However, the electrodes 12 and 13 can also be arranged at an angle relative to the horizontal line on the substrates 10 and 11. For example, the electrode 12 can be arranged at an angle of approximately 45° relative to the horizontal line, and the electrode 13 can be arranged at an angle of approximately 135° relative to the horizontal line on the substrates 10 and 11. It is also feasible that the electrodes 12 and 13 are arranged so as not to be exactly perpendicular to each other (i.e., at an angle of 90°). Therefore, the electrodes 12 and 13 can also be arranged at an angle of approximately 80° relative to each other, where the angle is preferably greater than 50°.

[0102] For clarity, Figure 6 Only the application of electric fields to the two electrodes 12 of the first substrate 10 and the two electrodes 13 of the second substrate 11 is shown. To this end, voltages Vu1 and Vu2 are applied to the electrodes 13 of the second substrate 11 by a control device (not shown), and voltages Vo1 and Vo2 are applied to the electrodes 12 of the first substrate. Here, the value of voltage Vu1 is different from the value of voltage Vu2, and the value of voltage Vo1 is different from the value of voltage Vo2. In the overlapping area or intersection range of the upper electrode and the lower electrode in each case, a defined electric field can be found here, such as Vo1-Vu1 or Vo1-Vu2 or Vo2-Vu1 or Vo2-Vu2. The higher the voltage value applied to the electrodes 12 and 13, the greater the mobility of the liquid crystal molecules in the electric field, that is, the greater the out-of-plane rotation of the optical axis of the liquid crystal molecules. As shown Figure 6As shown, the voltage Vu2 applied to the electrode 13 of the second substrate 11 has a higher voltage value than the voltage value Vu1 applied to the other electrode 13 of the second substrate 11. Therefore, an out-of-plane field is formed between the electrodes 12 and 13, and the out-of-plane rotation of the optical axis of the liquid crystal molecule 15 is greater when the high voltage Vu2 is applied than when the low voltage Vu1 is applied. This means that when the high voltage Vu2 is applied to the electrode 13, the optical axis of the liquid crystal molecule 15 in this area is rotated from the direction according to the direction of rotation. Figure 5 The liquid crystal molecules 15 in the region of the electrode 13 to which the low voltage or lower voltage Vu1 is applied also perform out-of-plane rotation, but with a lower rotation, so that the optical axis of the liquid crystal molecules 15 is not perpendicular to the second substrate 11, but forms a defined angle with the second substrate 11, which is determined by the applied voltage value.

[0103] Therefore, the out-of-plane electric field generated between the respective electrodes 12 and 13 will in each case lead to a different degree of out-of-plane alignment, Figure 6 In the embodiment, the liquid crystal molecules 15 are aligned to a greater extent parallel or perpendicular to the substrates 10 and 11. Thus, the liquid crystal molecules 15 are aligned to a greater extent parallel or perpendicular to the substrates 10 and 11, and thus the phase modulation of light incident on the diffractive optical element differs in the vertical direction or the horizontal direction depending on the electrode.

[0104] Figure 7 Shows the Figure 5 and Figure 6 FIG1 is an example of a diffractive optical element in which appropriate voltages are applied to the electrodes 12 and 13 of the two substrates 10 and 11. The diffractive optical element is shown in a top view of the first substrate 10, and only the electrode arrangement is considered.

[0105] Here, the following voltages are applied to the electrodes 12 of the first substrate 10 in a periodically repeated manner: 0 volts (0V), -2.66 volts (-2.66V) and -5.33 volts (-5.33V), while voltages of 0 volts (0V), 2 volts (2V), 4 volts (4V) and 6 volts (6V) with opposite signs are applied to the electrodes 13 of the second substrate 11. For example, in the overlapping region or intersection region 20 between the electrode 13 on the second substrate 11 to which a voltage of 6 volts is applied and the electrode 12 on the first substrate 10 to which a voltage of -5.33 volts is applied, a total voltage of 6 volts + 5.33 volts = 11.33 volts is therefore applied. Therefore, assuming that there is a linear relationship between the voltage applied to the electrodes 12 and 13 and the phase of the light, and if the liquid crystal layer 14 is also capable of modulating a phase exceeding 2π, a phase modulation of the light of 2.83π is performed in the overlapping region 20 of these electrodes 12 and 13. In accordance with Figure 7 In an exemplary embodiment, each electrode 12 and 13 forms a phase difference of 0.5π in the horizontal direction from one electrode 12 to the next or adjacent electrode 12, and a phase difference of 0.67π in the vertical direction from one electrode 13 to the next or adjacent electrode 13 in the corresponding overlapping region 20. Preferably, a phase modulation greater than 4π is achievable. The grating period of the diffraction grating can be varied and defined by using voltage values ​​applied to each adjacent electrode in a different manner to determine the defined period.

[0106] When two diffraction gratings are simultaneously formed in a diffractive optical element, the out-of-plane electric field generated between electrodes 12 of first substrate 10 and electrodes 13 of second substrate 11 produces different magnetic flux distributions in liquid crystal layer 14 for forming the first diffraction grating and the second diffraction grating. Furthermore, different out-of-plane electric fields exist in adjacent overlapping regions 20 formed by interdigitated electrodes 12 and 13, but these fields repeat periodically depending on the value of the periodically applied voltage.

[0107] Thus, it is now possible to deflect incident light into a first direction (e.g., vertical direction) and a second direction (e.g., horizontal direction) using only a single optical element, in a manner similar to the combination of two diffraction devices according to the prior art. When using the ECB mode or the VA mode, the light incident on the diffractive optical element should have linear polarization in order to be able to perform corresponding phase modulation on the light.

[0108] It is also possible to change the signs of the voltages applied to the electrodes of the two substrates of the diffractive optical element so that a positive voltage is applied to the first substrate and a negative voltage is applied to the second substrate.

[0109] If incident light passing through a diffractive optical element is required to be deflected in only one direction, a single diffraction grating with a defined grating period can be generated within the diffractive optical element. This diffractive optical element can then be used to deflect light in the desired direction. To this end, the electrodes of the substrates are all subjected to the same voltage, while voltages with different, periodically repeating values ​​are applied to the electrodes of the other substrate of the diffractive optical element. This creates an out-of-plane field between the electrodes of the two substrates, causing the liquid crystal molecules in the liquid crystal layer to undergo corresponding out-of-plane rotation. Thus, a diffraction grating with a defined grating period can be written into or formed into a diffractive optical element, and can deflect light in a defined direction.

[0110] Figure 7 The exemplary embodiment of the diffractive optical element shown and described in is suitable for use in a liquid crystal mode, in which the alignment of the liquid crystal molecules depends entirely on the magnitude of the voltage applied to the electrodes of the two substrates.

[0111] Furthermore, the diffractive optical element according to the present invention can also be applied to a liquid crystal pattern in a liquid crystal layer, in which the phase modulation of light depends on the sign of the voltage applied to the two substrate electrodes.

[0112] For a liquid crystal mode with in-plane rotation of the optical axis of the liquid crystal molecules in an out-of-plane electric field, the phase modulation of light (in this case, circularly polarized light) during a single pass through the liquid crystal layer is proportional to twice the rotation angle of the liquid crystal molecules. In the case of light passing twice through the liquid crystal layer of a reflective diffractive optical element, the phase modulation is proportional to four times the rotation angle of the optical axis of the liquid crystal molecules with a suitable configuration incorporating an additional retardation layer (i.e., the optical thickness of the liquid crystal layer is set to correspond to a half-wave layer), which is disposed between the liquid crystal layer and the reflector and corresponds to a quarter-wave layer, and passes through the retardation layer between the first and second passes of the liquid crystal layer. In the case of a rotation of the optical axis of the liquid crystal molecules of up to ±90 degrees, the result of a reflective diffractive optical element can be a phase modulation range of 4π (between -2π for a rotation angle of -90 degrees and +2π for a rotation angle of 90 degrees).

[0113] Here, a comparison will also be made between the prior art and the diffractive optical element according to the invention, again taking only the electrode arrangement of the elements into account.

[0114] Figure 8 In each of illustrations a) and b) a diffraction device according to the prior art is shown which generates a phase modulation of light by a sign-dependent in-plane rotation of the optical axes of the liquid crystal molecules. Figure 8 FIG. a) shows a diffraction device in a top view, which comprises two substrates (not shown here) with a liquid crystal layer (also not shown) embedded between the two substrates. One of the two substrates comprises strip electrodes E2, E3, ..., E3 arranged vertically. N , and the other substrate includes a planar electrode E1. Figure 4 Similarly, the white area represents the planar electrode E1. A voltage of 0V is applied to the planar electrode E1. In order to generate a phase modulation less than 0π, the strip electrodes E2, E3, ..., E N Apply negative voltage and N Applying a positive voltage generates a phase modulation greater than 0π. As can be seen from the diagram a), a voltage of 0 volt is applied to the planar electrode E1, and a voltage of 0 volt is applied to the strip electrodes E2, E3, ..., E NThe following voltages are applied in a periodic, repetitive pattern: -3 volts (-3V), -1 volt (1V), 1 volt (1V), 3 volts (3V). Here, an out-of-plane field is also generated in the liquid crystal layer, but the optical axes of the liquid crystal molecules are correspondingly aligned in the plane. For example, in the ULH mode, the liquid crystal molecules have a helical arrangement in the cholesteric phase. The out-of-plane field deforms the helix. This deformation of the helix corresponds to a rotation of the optical axis of the liquid crystal molecules. In this case, the optical axis of the liquid crystal molecules does not correspond to the orientation of a single liquid crystal molecule, but is formed by the average orientation of many liquid crystal molecules.

[0115] For example, smectic liquid crystal molecules have a spontaneous polarization, which results in alignment in an electric field. Due to the relative orientation of the polarization and the molecular axis, the effect of the alignment of the polarization parallel to the electric field is that the optical axis of the liquid crystal molecules rotates perpendicular to the electric field. Therefore, when an out-of-plane field is applied, the optical axis also rotates in a plane perpendicular to the field direction, resulting in in-plane rotation.

[0116] In this way, a diffraction grating with a grating period of 4 is obtained in the diffraction device, and its phase levels are -0.75π, -0.25π, 0.25π and 0.75π.

[0117] The diffraction device according to diagram a) then deflects the incident light into the horizontal direction.

[0118] Figure 8 FIG (b) shows a diffraction device having a structure corresponding to that of FIG (a), but comprising strip electrodes E2, E3, ..., E3 arranged vertically on one of the two substrates. N Similarly, a voltage value of 0 volt (0V) is applied to the planar electrode E1, and then voltage values ​​of -2.66 volts (-2.66V), 0 volts (0V) and +2.66 volts (+2.66V) are applied to the strip electrodes E2, E3, ..., E N At electrodes E1 and E2, E3, ..., E N An out-of-plane field is generated between the two liquid crystals, thereby achieving phase levels of -0.66π, 0π, and +0.66π through the in-plane rotation of the optical axes of the liquid crystal molecules. In this way, a diffraction grating with a grating period of 3 is realized in the diffraction device.

[0119] The diffraction device according to diagram b) then deflects the incident light into the vertical direction.

[0120] Figure 9 A diffractive optical element according to the present invention is shown, which uses a liquid crystal mode, such as a ULH mode, in which the phase modulation of light depends on the sign of the voltage applied to the electrodes, wherein the liquid crystal molecules rotate or align in the plane in the out-of-plane field generated by the liquid crystal layer. Figure 7 As mentioned above, for the sake of clarity, only the electrode arrangement is shown here.

[0121] Here, the basis is also shown in a top view Figure 9 The diffractive optical element also includes a first substrate and a second substrate, and a liquid crystal layer having liquid crystal molecules is provided between the first substrate and the second substrate. Figure 9 Only the first substrate 100 is shown, and the first substrate 100 has electrodes 120 in the form of strips or lines, here in a vertical direction. The second substrate, which is not visible, also includes electrodes 130 in the form of strips or lines. Like electrodes 130, electrodes 120 are each arranged parallel to each other on their respective substrates. In addition, the angle of electrodes 130 of the second substrate relative to electrodes 120 of the first substrate 100 is set to 90° here, which means that electrodes 120 and 130 are arranged crosswise relative to each other on their respective substrates. However, in the case of such a diffractive optical element, it is not required that electrodes 120 and 130 are arranged at an angle of exactly 90° relative to each other; instead, electrodes 120 and 130 can also be arranged at an angle within a range of approximately 50° to approximately 90° relative to each other.

[0122] In the case of forming two diffraction gratings, each having a defined grating period that is variable by means of a control device (not shown), a voltage may be applied to each of the electrodes 120 and 130. Figure 9 As shown, voltages of -2.66 volts (-2.66V), 0 volts (0V), and +2.66 volts (+2.66V) are applied to the electrode 120 of the first substrate 100, and voltages of +3 volts (3V), 1 volt (1V), -1 volt (-1V), and -3 volts (-3V) are applied to the electrode 130 of the second substrate. The voltage values ​​applied to the electrodes 120 and 130 are each repeated periodically. For example, for the upper left corner overlapping area or intersection 200 of the two electrodes 120 and 130 on the first substrate 100 shown, as shown Figure 9 As shown, the applied voltage is -2.66 volts, while the voltage applied to the second substrate (not shown) is +3 volts. Then, the potential difference ΔU between the two electrodes 120 and 130 in the overlapping area 200 is determined according to the following formula: the voltage value U2 of the first substrate electrode - the (negative) voltage value U1 of the second substrate electrode (-2.66V-3V) = -5.66V. Therefore, there is a potential difference ΔU = -5.66V in the overlapping area 200. Therefore, for this negative voltage applied to the electrodes 120 and 130 of the diffractive optical element in the overlapping area 200 in the liquid crystal layer, there is a phase modulation of light of 1.41π. In Figure 9These voltages applied to the electrodes 120 and 130 of the two substrates generate two diffraction gratings with a defined grating period by generating an out-of-plane field in the liquid layer and a corresponding alignment or orientation by the in-plane rotation of the optical axes of the liquid crystal molecules. This produces the phase values ​​shown, for example -1.41π, -0.91π or alternatively -0.75π, -0.25π, for phase modulation of the incident light in the corresponding overlapping region 200 of the electrodes 120 and 130. In addition, Figure 9 As shown, between any two adjacent electrodes 120 or 130, the phase difference is in each case 0.5π as seen in the horizontal direction, and 0.67π (modulo 2π) as seen in the vertical direction.

[0123] Therefore, the diffraction optical element can deflect the light (preferably circularly polarized light) incident in this way into one direction (here the horizontal direction) or another direction (here the vertical direction). To this end, two diffraction gratings with a defined grating period can be formed simultaneously in the diffraction optical element. However, it is also feasible here that, if required, only one diffraction grating with a defined grating period can be formed in the diffraction optical element. Then, the diffraction grating is formed by the electrodes on one of the two substrates, so that different periodically repeated voltages are applied to these electrodes, and the voltages of all electrodes and electrodes of the other substrate are equal. Therefore, an out-of-plane field is also generated, but only one diffraction grating is written into the diffraction optical element. Phase modulation of light ≥2π (preferably ≥4π) is also provided here.

[0124] For example, if the maximum phase modulation of the liquid crystal layer is less than 4π, the diffraction efficiency of the diffractive optical element may be slightly limited in some cases, since generally speaking, when the optical axis of the liquid crystal molecules rotates within the plane, the maximum rotation angle of the optical axis is less than 90 degrees. However, the configuration of the diffractive optical element with a smaller optical phase modulation range allows the desired deflection in two different directions.

[0125] In general, it is preferred to use a series of phase modulations in the liquid crystal layer of the diffractive optical element so that the phase is approximately linearly dependent on the voltage applied to the substrate electrodes and thus with the applied out-of-plane field. This will refer to Figure 10 illustrate.

[0126] For the ECB mode used in the liquid crystal layer, the result of the dependence of the phase modulation on the voltage applied to the electrode arrangement of the diffractive optical element is usually an approximately S-shaped curve, such as Figure 10 In the case of very small and very large phase values, according to Figure 10 The S-shaped curve of the phase dependence on the applied voltage is significantly different from the expected linear behavior. However, for the average phase value, such as Figure 10 As shown by the dotted line in the figure, the phase value varies approximately linearly with the applied voltage. Preferably, only the linear portion of the curve of the dependence of the phase modulation on the voltage applied to the substrate electrodes is utilized. Preferably, the maximum phase modulation of the light in the liquid crystal layer is selected to be sufficiently large, for example ≥5π, so that the phase modulation corresponds to the range of linear behavior of the phase of the light with respect to the voltage applied to the substrate electrodes (see Figure 10 The portion (the area indicated by the dotted line) also includes a phase modulation range of approximately 4π.

[0127] Diffractive optical elements can be designed to be either transmissive or reflective. To achieve phase modulation of 4π or greater with a transmissive diffractive optical element, the thickness of the liquid crystal layer must be adjusted or appropriately designed, i.e., greater than that of a reflective diffractive optical element. This means that, because light passes through the liquid crystal layer twice, a reflective diffractive optical element must be thinner for phase modulation of 4π or greater than a transmissive diffractive optical element.

[0128] In addition, since in the out-of-plane field generated in the liquid crystal layer, there may also be unwanted in-plane fields between the electrodes on the two substrates, the area of ​​the region where unwanted in-plane fields can be generated should be as small as possible. This effect can be guaranteed when the strip electrodes on the two substrates are configured so that the area between the electrodes on each substrate is ideally kept as small as possible. This means that there should only be very small gaps between the electrodes arranged parallel to each other on each substrate. Therefore, in the usual line / space representation (line width and spacing) for fine resolution structures, the line (line width) should be at the maximum value and the space (spacing) should be at the minimum value. This is applicable while taking into account customary manufacturing tolerances. For example, a space that is too small increases the risk of short circuits between adjacent electrodes in the production of the electrode structure. In practice, the space will be selected to be as small as possible, but also large enough for the electrode structure to have a good yield.

[0129] To achieve the maximum deflection angle in the diffractive structure, a small spacing between the electrodes is advantageous. However, if the spacing is very small, a specific distance between the electrodes must be maintained to avoid short circuits, and the line / space ratio becomes unfavorable again. An example is an electrode spacing of 2 microns, an electrode width of 1.5 microns, and a spacing of 0.5 microns from the next electrode. It will be understood that the present invention is not intended to be limited to these values.

[0130] Figure 11A display device, in particular a holographic display device, for representing two-dimensional and / or three-dimensional information, for example an object or a scene is shown. The display device comprises an illumination device 30 for emitting light. The illumination device here may comprise at least one light source, which preferably emits polarized light, in particular linearly polarized light or circularly polarized light. Furthermore, the display device comprises at least one spatial light modulator 31 having pixels, which is illuminated by the light from the illumination device in order to modulate the amplitude and / or phase of the light in accordance with the scene or object to be represented. An optical system 32 is used together with the at least one spatial light modulator 31 to reconstruct and represent a scene, which is preferably generated holographically. Downstream of the at least one spatial light modulator 31 in the direction of the light, a diffractive optical element 33 is arranged, which may be as shown in FIG. Figure 5 、 6 , 7 and 9. At least one spatial light modulation device 31 and a diffractive optical element 33 are connected to a control device 34, by which these elements 31 and 33 can be controlled accordingly. However, the diffractive optical element 33 can also be controlled by a dedicated control device. Therefore, a corresponding voltage can be applied to the electrodes of the diffractive optical element 33 by the control device 34 so as to generate at least one diffraction grating with a defined grating period. Depending on the control of the electrodes of the diffractive optical element 33, this has a variable diffraction grating or a variable diffraction structure. By means of the diffraction grating present in the diffraction optical element 33, the light modulated by the at least one light modulation device 31 can be variably diffracted in a definable manner and deflected into at least one desired lateral or axial direction.

[0131] This diffractive optical element is capable of laterally tracking at least one virtual observer window 36 generated in the observer plane 35, through which the observer's eye 37 must view the represented scene. If the observer moves to a different position, the virtual observer window 36 can be tracked by the diffractive optical element 33 to the new eye position, now designated by reference numeral 37'. The virtual observer window at the observer's new observer position is designated by reference numeral 36'. To this end, a corresponding diffraction grating with the required grating period is written into the diffractive optical element 33 by applying a corresponding predefined voltage to the substrate electrodes of the diffractive optical element 33. The diffractive optical element can thus track the virtual observer window 36 only in one defined lateral direction or simultaneously in two defined lateral directions at an angle to one another. Tracking of the virtual observer window 36 in the axial direction (z-direction) can also be achieved by the diffractive optical element by writing a lens function into the diffractive optical element. This depends on the new position of the observer relative to the spatial light modulation device 31.

[0132] The diffractive optical element provided in the display device according to the present invention allows it to have a more compact configuration in terms of structure than a case where two diffraction devices are provided for deflecting light.

[0133] In addition to using diffractive optical elements in display devices that preferably display three-dimensional scenes or objects, diffractive optical elements can also be used in other devices because diffractive optical elements can, in principle, deflect light by diffraction. Therefore, diffractive optical elements can generally be used in devices that need or require light deflection.

[0134] The present invention is not limited to the exemplary embodiments shown here. Further combinations of embodiments or exemplary embodiments are also possible. In summary, it should be particularly noted that the exemplary embodiments described above are only used to illustrate the claimed teachings, but this teaching is not intended to be limited to the exemplary embodiments.

Claims

1. A diffractive optical element, comprising: - a first substrate and a second substrate, wherein a liquid crystal layer is provided between the first substrate and the second substrate, - strip electrodes on the first substrate and strip electrodes on the second substrate, wherein the electrodes on the first substrate are arranged at an angle greater than 50° relative to the electrodes on the second substrate, - the electrodes on the first substrate and the electrodes on the second substrate are controlled in such a way that a defined out-of-plane field is generated in respective overlapping regions of the electrodes on the first substrate and the electrodes on the second substrate; as well as -In the case where a first diffraction grating for deflecting light into a first defined direction and a second diffraction grating for deflecting light into a second defined direction are formed simultaneously, the electric field variation from one electrode to the next electrode is different for the arrangement of the electrodes on the first substrate and the arrangement of the electrodes on the second substrate. 2 . The diffractive optical element according to claim 1 , wherein the diffractive optical element is designed to deflect incident light into at least one direction. 3 . The diffractive optical element according to claim 2 , further comprising at least one diffraction grating having a defined grating period.

4. The diffractive optical element according to claim 1, wherein a diffraction grating having a defined grating period can be formed by individually controlling electrodes on only one of the substrates to deflect light incident on the diffractive optical element in a predetermined direction.

5. The diffractive optical element according to claim 1 , wherein two diffraction gratings each having a defined grating period can be simultaneously formed by individually controlling electrodes on the first substrate and electrodes on the second substrate to deflect light incident on the diffractive optical element in two predetermined directions set at an angle to each other.

6. The diffractive optical element according to claim 1 , wherein the electrodes on the first substrate are in a substantially horizontal arrangement, and the electrodes on the second substrate are in a substantially vertical arrangement, or the electrodes on the first substrate are in a substantially vertical arrangement, and the electrodes on the second substrate are in a substantially horizontal arrangement. 7 . The diffractive optical element according to claim 1 , wherein the electrodes on the first substrate and the electrodes on the second substrate are arranged at an angle to a horizontal line, wherein the electrodes on the first substrate are arranged at an angle to the electrodes on the second substrate. 8 . The diffractive optical element according to claim 1 , wherein different periodically repeating out-of-plane electric fields are provided in adjacent overlapping regions of the electrodes of the first substrate and the electrodes of the second substrate.

9. The diffraction optical element according to claim 1 , wherein, when a first diffraction grating for deflecting light in a first defined direction and a second diffraction grating for deflecting light in a second defined direction are simultaneously formed, distributions of electric field magnetic flux lines for forming the first diffraction grating and distributions of electric field magnetic flux lines for forming the second diffraction grating generated in the liquid crystal layer between the electrodes of the first substrate and the electrodes of the second substrate are different.

10. The diffractive optical element according to claim 1, wherein a single diffraction grating is formed by controlling electrodes on one substrate to have equal voltage values ​​and controlling electrodes on the other substrate to have respective different periodically repeated voltage values ​​using a control device. 11 . The diffractive optical element according to claim 1 , wherein the diffractive optical element is configured as a liquid crystal mode, and at least one diffraction grating having a defined grating period is formed by an out-of-plane field using the liquid crystal mode. 12 . The diffractive optical element according to claim 11 , wherein the liquid crystal mode is an ECB mode, a ULH mode, or a VA mode. 13 . The diffractive optical element according to claim 1 , wherein the diffractive optical element can achieve a phase modulation of light in the liquid crystal layer of ≥2π, or ≥4π. 14 . The diffractive optical element according to claim 13 , wherein the range of the phase modulation is selected to be a range in which the phase of the light has a linear characteristic with respect to the voltage applied to at least one electrode of the substrate. 15 . The diffractive optical element according to claim 1 , wherein the diffractive optical element is designed to be transmissive or reflective. 16 . The diffractive optical element according to claim 15 , wherein the diffractive optical element is designed to be reflective and has a liquid crystal pattern that causes liquid crystal molecules of the liquid crystal layer to rotate in a plane in an out-of-plane field.

17. A display device for displaying two-dimensional and / or three-dimensional objects or scenes, comprising an illumination device, at least one spatial light modulation device and at least one diffractive optical element according to any one of claims 1 to 16. The display device according to claim 17 , wherein the illumination device is configured to emit polarized light.

19. The display device according to claim 17, wherein at least one control device is configured to control the electrodes on the first substrate and / or the electrodes on the second substrate of at least one of the diffractive optical elements.

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