Focal length modulation optical system and holographic display device having the same

Abstract

Disclosed are a focal length modulation optical system and a holographic display device having the focal length modulation optical system. The holographic display device comprises: a light source configured to emit light of multiple colors; a focal length modulation optical system comprising at least one variable focal length lens, wherein the at least one variable focal length lens is configured to change the focal position of the incident light by electrically controlling the at least one variable focal length lens based on the color of the light incident on the variable focal length lens; and a spatial light modulator configured to form a holographic image by diffracting the light output from the focal length modulation optical system.

Description

Technical Field

[0001] Example embodiments of the present disclosure relate to a focus modulation optical system and a holographic display device including the same. Background Art

[0002] The holographic display device converts a general image signal into a holographic image signal, and transmits the converted image to the left eye and the right eye by using an optical system, thereby displaying a three-dimensional (3D) image. The holographic display device is considered to be an ideal 3D display that does not cause eye fatigue while displaying a 3D image with clearer depth. However, in order to achieve the above characteristics, a high-resolution spatial light modulator (SLM) with wavelength-sized pixels, a surface light source with coherence, and a relatively large amount of calculation for computer-generated holograms (CGH) are required.

[0003] Recently, in order to reduce the data processing amount and the conditions of resolution, a binocular hologram method has been proposed, which provides a holographic image only to the field of view corresponding to each eye of the viewer. For example, a holographic image having only a viewpoint corresponding to the left eye field of view of the viewer and a holographic image having a viewpoint corresponding to the right eye field of view are generated and provided to the left and right eyes of the viewer. In this case, since there is no need to generate holographic images for other viewpoints, the data processing amount can be significantly reduced, and thus the current commercial display device can meet the resolution conditions of the spatial light modulator.

[0004] In a holographic display device, chromatic aberration occurs when colored light is used, and the chromatic aberration may deteriorate image quality. Summary of the invention

[0005] One or more example embodiments provide methods and apparatus for a focus modulation optical system that can reduce chromatic aberration.

[0006] Another one or more example embodiments provide a method and apparatus for a holographic display device including a focus modulation optical system that can reduce chromatic aberration.

[0007] Additional aspects will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of example embodiments.

[0008] According to one aspect of an example embodiment, a focal length modulation optical system is provided, comprising: at least one variable focal length lens configured to change a focal position of incident light according to color by electrical control, wherein the at least one variable focal length lens is variable so that each color of light forms a focus at the same position.

[0009] The at least one variable focal length lens may include a first substrate, a first electrode layer disposed on the first substrate, a second substrate disposed opposite to the first substrate, a second electrode layer disposed on the second substrate, and a liquid crystal layer disposed between the first substrate and the second substrate, the liquid crystal layer including a plurality of active lenses, and the plurality of active lenses are configured to change based on a voltage applied between the first electrode layer and the second electrode layer.

[0010] At least one of the first electrode layer and the second electrode layer may include strip-shaped electrodes disposed apart from each other.

[0011] At least one of an array pitch, a shape, and a curvature of the plurality of active lenses may be adjusted based on a voltage applied to the strip electrodes.

[0012] The focus modulation optical system may further include a fixed focal length lens configured to focus light incident on the variable focal length lens.

[0013] The color light may include a first color light, a second color light and a third color light, wherein the at least one variable focal length lens includes: a first variable focal length lens configured to focus the first color light at a first position; a second variable focal length lens configured to focus the second color light at the first position; and a third variable focal length lens configured to focus the third color light at the first position, wherein the first variable focal length lens, the second variable focal length lens and the third variable focal length lens may be arranged sequentially.

[0014] The focal length modulation optical system may also include: a first wavelength selective delay device provided in front of the first variable focal length lens; a second wavelength selective delay device provided between the first variable focal length lens and the second variable focal length lens; and a third wavelength selective delay device provided between the second variable focal length lens and the third variable focal length lens.

[0015] The first wavelength selective delay device can be configured to convert the first color light from the second polarization to the first polarization, and transmit the second color light of the second polarization and the third color light of the second polarization. The second wavelength selective delay device can be configured to convert the first color light from the first polarization to the second polarization, convert the second color light from the second polarization to the first polarization, and transmit the third color light of the second polarization. The third wavelength selective delay device can be configured to convert the third color light from the second polarization to the first polarization, convert the second color light from the first polarization to the second polarization, and transmit the first color light of the second polarization.

[0016] The focal length modulation optical system may further include a wavelength selective retarder provided in front of the at least one variable focal length lens, the wavelength selective retarder being configured to selectively delay a phase based on a color of the incident light.

[0017] According to one aspect of another example embodiment, a holographic display device is provided, comprising: a light source configured to emit light of multiple colors; a focal length modulation optical system comprising at least one variable focal length lens, wherein the at least one variable focal length lens is configured to change a focusing position of incident light based on a color of light incident on the at least one variable focal length lens by electrically controlling the at least one variable focal length lens; and a spatial light modulator configured to form a holographic image by diffracting light output from the focal length modulation optical system.

[0018] The at least one variable focal length lens may include a first substrate, a first electrode layer disposed on the first substrate, a second substrate disposed opposite to the first substrate, a second electrode layer disposed on the second substrate, and a liquid crystal layer disposed between the first substrate and the second substrate, the liquid crystal layer including a plurality of active lenses, and the plurality of active lenses are configured to change based on a voltage applied between the first electrode layer and the second electrode layer.

[0019] At least one of the first electrode layer and the second electrode layer may include strip-shaped electrodes disposed apart from each other.

[0020] At least one of an array pitch, a shape, and a curvature of the plurality of active lenses may be configured to change based on a voltage applied to the strip electrodes.

[0021] The holographic display device may further include a wavelength selective retarder, which is provided in front of the at least one variable focal length lens and is configured to selectively delay the phase based on the color of the incident light.

[0022] The holographic display device may further include a fixed focal length lens configured to focus light to be emitted onto the wavelength selective retarder.

[0023] The holographic display device may further include a fixed focal length lens configured to focus light to be emitted onto the at least one variable focal length lens.

[0024] The at least one variable focal length lens may include: a first variable focal length lens configured to focus a first color light at a first position; a second variable focal length lens configured to focus a second color light at the first position; and a third variable focal length lens configured to focus a third color light at the first position, and the first variable focal length lens, the second variable focal length lens and the third variable focal length lens may be arranged sequentially.

[0025] The holographic display device may also include: a first wavelength selective delay device provided in front of the first variable focal length lens; a second wavelength selective delay device provided between the first variable focal length lens and the second variable focal length lens; and a third wavelength selective delay device provided between the second variable focal length lens and the third variable focal length lens.

[0026] The holographic display device may also include a controller configured to control the light source to simultaneously emit the first color light, the second color light and the third color light, and control the first variable focal length lens, the second variable focal length lens and the third variable focal length lens to adjust the focusing position corresponding to each of the first color light, the second color light and the third color light, respectively.

[0027] The holographic display device may further include a beam deflector configured to deflect the plurality of color lights emitted from the light source.

[0028] The holographic display device may further include a position detection sensor configured to detect a position of a viewer viewing the holographic image.

[0029] The holographic display device may further include a controller configured to control the light source to emit a plurality of color lights in a time sequence, wherein the variable focal length lens may be configured to adjust a focusing position corresponding to the plurality of color lights in a time sequence.

[0030] According to one aspect of another example embodiment, a holographic display device is provided, which includes: a light source configured to emit a plurality of color lights; a beam deflector configured to deflect the plurality of color lights emitted from the light source; a light guide plate configured to guide the plurality of color lights deflected from the beam deflector; a focal length modulation optical system including a fixed focal length lens and a plurality of variable focal length lenses, the plurality of variable focal length lenses being configured to change the focusing position of the plurality of color lights output from the light guide plate based on the colors of the incident plurality of color lights by electrically controlling the plurality of variable focal length lenses; and a spatial light modulator configured to form a holographic image by diffracting the plurality of color lights output from the focal length modulation optical system.

[0031] The plurality of variable focal length lenses may include a first variable focal length lens configured to focus a first color light at a first position; a second variable focal length lens configured to focus a second color light at the first position; and a third variable focal length lens configured to focus a third color light at the first position. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The above and / or additional aspects will become apparent and more readily understood from the following description of example embodiments in conjunction with the accompanying drawings, in which:

[0033] Figure 1schematically illustrates a holographic display device according to an example embodiment;

[0034] Figure 2 shows the chromatic aberration that occurs when multiple wavelengths of light are incident on a fixed focal length diffractive lens;

[0035] Figure 3 shows a focal length modulation optical system according to an example embodiment;

[0036] Figure 4 An example of a variable focal length lens of a focal length modulation optical system according to an example embodiment is shown;

[0037] Figure 5 Show Figure 4 Operation of variable focal length lenses;

[0038] Figure 6 illustrates the operation of a focus modulation optical system according to an example embodiment;

[0039] Figure 7 shows a process of removing chromatic aberration in a focus modulation optical system according to an example embodiment;

[0040] Figure 8 Show Figure 3 The simultaneous operating mechanism of red light, green light and blue light in the focus modulation optical system shown;

[0041] Fig. 9 An example of further providing a fixed focal length lens in a focal length modulation optical system according to an example embodiment is shown;

[0042] Fig.10 shows a focal length modulation optical system according to an example embodiment; and

[0043] Fig.11 Show Fig.10 The time sequential operating mechanism of red light, green light and blue light in the focus modulation optical system is shown. DETAILED DESCRIPTION

[0044] Reference will now be made in detail to a focal length modulation optical system and a holographic display device having the focal length modulation optical system according to example embodiments, examples of which are shown in the accompanying drawings, wherein the same reference numerals refer to the same elements throughout. For clarity and ease of description, the size of each element shown in the accompanying drawings may be exaggerated. Although terms such as "first", "second", etc. may be used to describe various components, these components should not be limited to the above terms. The above terms are only used to distinguish one component from another component.

[0045] It will also be understood that the terms "including" and / or "comprising" used herein indicate the presence of the stated features or components, but do not exclude the presence or addition of one or more other features or components. In addition, when a material layer is described as being present on another layer, the material layer may be present directly on the other layer, or a third layer may be interposed therebetween. Because the materials forming each layer in the following embodiments are exemplary, other materials may be used. The use of the terms "a" and "the" and similar indicators in the context of describing the present disclosure should be interpreted as covering both the singular and the plural.

[0046] Expressions such as "at least one of," when following a list of elements, modify the entire list of elements and do not modify the individual elements in the list. For example, the expression "at least one of a, b, and c" should be understood to include: only a, only b, only c, both a and b, both a and c, both b and c, or all of a, b, and c.

[0047] Figure 1 A holographic display device 100 according to an example embodiment is schematically shown.

[0048] The holographic display device 100 may include a light source 110 that provides a light beam, a beam deflector 120 that deflects the light beam from the light source 110, a focal length modulation optical system 150 that adjusts the focal length according to the wavelength of the incident light beam, and a spatial light modulator 160 that forms a holographic image by diffracting the incident light beam. In addition, the holographic display device 100 may further include a light guide plate 130 that guides the light beam deflected by the beam deflector 120 toward the spatial light modulator 160, and a controller 170 that controls the focal length to change according to the wavelength of the incident light beam in the focal length modulation optical system 150. In addition, the controller 170 may control the deflection direction of the beam deflector 120. In addition, the holographic display device 100 may further include a position detection sensor 180 that tracks the position of the viewer.

[0049] The light source 110 may be a coherent light source that emits coherent light including a plurality of wavelength bands. In order to provide light having high coherence, for example, a laser diode (LD) may be used as the light source 110. However, when the light has a certain degree of spatial coherence, the light may be diffracted and modulated by the spatial light modulator 160, and thus, for example, a light emitting diode (LED) may be used as the light source 110. In addition to the LED, any light source that can emit light having spatial coherence may be used as the light source 110. In addition, in Figure 1 In FIG. 1 , the light source 110 is shown as only one block, but the light source 110 may include a plurality of point light source arrays. For example, the light source 110 may include a plurality of red emitting lasers, a plurality of green emitting lasers, and a plurality of blue emitting lasers.

[0050] The beam deflector 120 can deflect the light beam from the light source 110. For example, the light beams L1 and L2 can travel toward the left eye and the right eye, respectively. The incident light beam can be deflected in two directions in a time sequence by the controller 170, or deflected in two directions at the same time. The beam deflector 120 may include a beam deflection panel that changes the traveling direction of light by using an electrical signal. Examples of the beam deflector may include a micro-electromechanical system (MEMS) scanner, a galvano mirror, a linear spatial light modulator (SLM), and a liquid crystal beam deflector.

[0051] The light guide plate 130 may be formed of transparent glass or transparent plastic. For example, polymethyl methacrylate (PMMA) may be used as the material of the light guide plate 130. The light guide plate 130 may have, for example, a rectangular parallelepiped shape to serve as an optical waveguide for guiding a light beam. A light beam incident on an end portion of one side of the light guide plate 130 may be transmitted to an end portion of the opposite side thereof by total internal reflection therein.

[0052] The focus modulation optical system 150 may correct chromatic aberration by modulating the focal length of the incident light beam according to the wavelength of the incident light beam. The focus modulation optical system 150 may include, for example, a variable focal length lens.

[0053] Figure 2 1 shows that chromatic aberration occurs when light beams with different wavelengths are incident on a fixed focal length lens 10. The focal length is inversely proportional to the wavelength λ of the light. For example, when red light L R , Green Light L G and Blu-ray L B When incident on the fixed focal length lens 10, the red light L R Focus at the shortest distance, green light L G Focusing at the second shortest distance, blue light L B Focus is at the longest distance, so chromatic aberration occurs.

[0054] In an example embodiment, the focal length modulation optical system 150 may reduce or eliminate chromatic aberration by modulating the focal length according to the wavelength of the incident light beam.

[0055] The spatial light modulator 160 can form a holographic pattern to diffract and modulate an incident light beam according to a hologram signal provided by the image signal processor 190. As the spatial light modulator 160, any one of a phase modulator capable of performing only phase modulation, an amplitude modulator capable of performing only amplitude modulation, and a combined modulator capable of performing both phase modulation and amplitude modulation can be used. Figure 1 The spatial light modulator 160 is shown to be a transmissive spatial light modulator, but a reflective spatial light modulator may be used. For the transmissive type, the spatial light modulator 160 may use, for example, a semiconductor modulator based on a compound semiconductor such as gallium arsenide (GaAs), or a liquid crystal device (LCD). For the reflective type, the spatial light modulator 160 may use, for example, a digital micromirror device (DMD) or liquid crystal on silicon (LCoS).

[0056] The light beam emitted from the light source 110 may be deflected by the beam deflector 120, guided by the light guide plate 130, magnified by the focal length modulation optical system 150 to a size corresponding to the size of the spatial light modulator 160, and then incident on the spatial light modulator 160. The light guide plate 130 may include: an input coupler 131 that receives the light beam deflected by the beam deflector 120 and incident on the light guide plate 130; and an output coupler 132 that outputs the light beam traveling by being guided by the light guide plate 130. The input coupler 131 may include, for example, a collimating lens. The collimating lens may make the incident light beam a parallel light beam to be incident on the light guide plate 130. Therefore, the light coupling efficiency with respect to the light guide plate 130 may be increased. The output coupler 132 may include a grating. The grating may improve the light output efficiency by utilizing the diffraction efficiency of the output light.

[0057] The directional light beam output from the light guide plate 130 may be incident on the spatial light modulator 160 through the focal length modulation optical system 150. The spatial light modulator 160 may form a holographic pattern having an interference pattern to modulate the incident light beam. As the incident light beam is diffracted and modulated by the holographic pattern formed by the spatial light modulator 160, a holographic image may be reproduced at a position in a specific space. For example, a left-eye holographic image may be reproduced at a left-eye position, and a right-eye holographic image may be reproduced at a right-eye position.

[0058] The position detection sensor 180 can track the position of the viewer and send the position of the viewer to the controller 170. The controller 170 can control the beam deflector 120 according to the position of the viewer so that the beam can be directed to the eyes of the viewer. The beam deflector 120 can deflect the beam by electrical control to adjust the traveling direction of the beam.

[0059] Figure 3 A focal length modulation optical system 200 according to an embodiment is shown. The focal length modulation optical system 200 may include a plurality of wavelength selection delay devices and a variable focal length lens corresponding to each of the plurality of wavelength selection delay devices. The wavelength selection delay device may selectively delay the phase of the incident light according to the wavelength and color of the incident light. For example, the focal length modulation optical system 200 may include a first wavelength selection delay device 205, a first variable focal length lens 210, a second wavelength selection delay device 215, a second variable focal length lens 220, a third wavelength selection delay device 225, and a third variable focal length lens 230.

[0060] Figure 4 An example of a variable focal length lens 300 is shown. The variable focal length lens 300 may be a liquid crystal lens that controls the focal length by electrically changing the arrangement of liquid crystals. The variable focal length lens 300 may include a first substrate 310, a first electrode layer 320 including a plurality of first electrodes 321 spaced apart from each other and arranged on the first substrate 310, a second substrate 350 disposed opposite to the first substrate 310, a second electrode layer 340 provided on the second substrate 350 and having a second electrode 341, and a liquid crystal layer 330 provided between the first substrate 310 and the second substrate 350.

[0061] The first substrate 310 and the second substrate 350 may include insulating substrates, and may be formed of, for example, glass or transparent plastic.

[0062] Each of the plurality of first electrodes 321 may be independently driven. Figure 5 , the liquid crystal layer 330 may include a plurality of active lenses 331 formed as an array of liquid crystal molecules changes according to an electric field formed between the first electrode layer 320 and the second electrode layer 340. At least one of an array pitch P, a shape, and a curvature of the active lens 331 that changes according to a voltage applied between the first electrode 321 and the second electrode 341 may be changed. The focal length of the incident light beam may be controlled according to the array pitch P, the shape, and the curvature of the active lens 331.

[0063] The first electrode 321 may have, for example, a strip shape and may be arranged in parallel with each other. The second electrode 341 as a common electrode may have a single panel shape. However, example embodiments are not limited thereto, and the second electrode 341 may be a plurality of electrodes having a strip shape and spaced apart from each other. The first electrode 321 and the second electrode 341 may include a transparent conductive material, such as indium tin oxide (ITO), indium zinc oxide (IZO), and indium tin zinc oxide (ITZO).

[0064] The liquid crystal layer 330 may include liquid crystal (LC) molecules. The LC molecules may be initially aligned such that their long axis directions are parallel to one direction, for example, direction x. An alignment layer for alignment of the LC molecules may be further provided on at least one of the first substrate 310 and the second substrate 350.

[0065] Reference Figure 5 , the active lens 331 may be formed as the array of LC molecules is changed by the voltage applied between the first electrode 321 and the second electrode 341. The shape of the active lens 331 may be changed according to the voltage applied between the first electrode 321 and the second electrode 341. The active lens 331 is formed because the refractive index of the LC molecules in the long axis direction and the refractive index thereof in the short axis direction are different from each other. In other words, with respect to the light incident on the LC molecules, the LC molecules have different refractive indices for light polarized parallel to the long axis direction and light polarized parallel to the direction perpendicular to the long axis direction. The liquid crystal layer 330 refracts only polarized light in one direction, and transmits polarized light in other directions without refraction.

[0066] like Figure 4 As shown, in the initial alignment state where no voltage is applied between the first electrode 321 and the second electrode 341, all LC molecules are aligned in directions parallel to each other, so that light of a certain polarization can have the same refractive index at any position. Figure 5 As shown, when a certain electric distribution is formed in the liquid crystal layer 330 according to the voltage applied between the first electrode 321 and the second electrode 341, the LC molecules are arranged depending on the direction of the electric field. For example, the long axis direction of the LC molecules can be arranged along the electric field direction when the dielectric anisotropy is positive, and arranged in a direction perpendicular to the electric field direction when the dielectric anisotropy is negative. Different voltages can be applied to the first electrode 321, and therefore, the arrangement form of each LC molecule is different at each position. Because the refractive index of each LC molecule is different in the long axis direction and the short axis direction, a refractive index gradient can be obtained according to the distribution of the LC molecules in the long axis direction. The active lens can be formed according to the refractive index gradient in the liquid crystal layer 330. By controlling the distribution of the LC molecules in the long axis direction according to the electric field distribution of the liquid crystal layer 330, the array pitch, shape and curvature of the active lens can be adjusted.

[0067] Since the long axis direction of the LC molecules is different at each position in the liquid crystal layer 330, the active lens 331 is formed by the arrangement of the LC molecules in the long axis direction. Therefore, the active lens 331 can only work on the light polarized in the direction parallel to the long axis direction of the LC molecules in the initial alignment state, for example, in the direction parallel to the direction x. In the following description, the polarization parallel to the direction x is referred to as the first polarization and is represented by the symbol express.

[0068] When LC molecules such as Figure 5 When arranged as shown, the long axis direction of the LC molecules at each position is different, but the short axis direction thereof is the same in the direction y. Therefore, when the light incident on the liquid crystal layer 330 is polarized in the direction y, no refraction occurs. In the following description, the polarization parallel to the direction y is referred to as the second polarization and is represented by the symbol "⊙".

[0069] In other words, when the light incident on the liquid crystal layer 330 has a first polarization parallel to the direction x polarization When the focal length of the light is changed by the active lens 331, when the light of the second polarization (⊙) polarized in the direction parallel to the direction y is incident, the light passes through the liquid crystal layer 330 without being changed.

[0070] For example, when a sequentially increasing voltage is applied to n first electrodes 321, where n is a natural number, and when a sequentially increasing voltage is applied to more than n first electrodes 321, the active lens 331 is changed, and thus the focal length of light can be changed. The controller 370 can control the voltage applied to the first electrode 321. According to an example embodiment, the shape of the active lens can be formed by applying a first voltage to two adjacent first electrodes 321 and then not applying a voltage to the next two first electrodes 321 or applying the same voltage as the second electrode 341 thereto.

[0071] Figure 6 The operation of the wavelength selective retarder 304 and the variable focal length lens 300 corresponding thereto is shown according to an example embodiment.

[0072] For example, when the first wavelength light L of the second polarization (⊙) R , the second wavelength light L of the second polarization (⊙) G and the third wavelength light L of the second polarization (⊙) B When incident on the focal length modulation optical system, the wavelength selective delay device 304 converts the first wavelength light L R Switch to first polarization And output the second wavelength light L in the second polarization state (⊙) G and the third wavelength light L B The output light is incident on the variable focal length lens 300 .

[0073] The variable focal length lens 300 may, for example, focus only on the first polarization Therefore, the first polarization The first wavelength of light L R is refracted and outputted at the first focal length, and the second wavelength light L of the second polarization (⊙) G and the third wavelength light L B is output without change in focal length.

[0074] Figure 7 Show detailed implementation Figure 3 An example of a focal modulation optical system 200.

[0075] Reference Figure 7 , the focal length modulation optical system 400 may include the following structure, in which the first wavelength selection delay 305, the first variable focal length lens 301, the second wavelength selection delay 306, the second variable focal length lens 302, the third wavelength selection delay 307 and the third variable focal length lens 303 are stacked in sequence. The first variable focal length lens 301, the second variable focal length lens 302 and the third variable focal length lens 303 may each have a wavelength selected from the first variable focal length lens 307 and the second variable focal length lens 303. Figure 6 The structure of the variable focus lens 300 described is basically the same structure.

[0076] For example, when the first wavelength light L of the second polarization (⊙) R , the second wavelength light L of the second polarization (⊙) G and the third wavelength light L of the second polarization (⊙) B When incident on the focal length modulation optical system 400, the first wavelength selective delay device 305 can transfer the first wavelength light L R The polarization of And can output the second wavelength light L in the second polarization state (⊙) G and the third wavelength light L B The first variable focal length lens 301 can change the first wavelength light L output from the first wavelength selective delay device 305. R The focal length of the second polarization (⊙) is outputted as the second wavelength light L G and the third wavelength light L B There is no change in focal length.

[0077] The second wavelength selective retarder 306 disposed on the optical path of the light passing through the first variable focal length lens 301 can G The polarization of The first wavelength light L R The light source is converted to the second polarization state (⊙), and outputs the third wavelength light L in the second polarization state (⊙). B Without changing, the second variable focal length lens 302 can change the second wavelength light L output from the second wavelength selective delay device 306. G The focal length can output the first wavelength light L of the second polarization (⊙) R and the third wavelength light L B There is no change in focal length.

[0078] The third wavelength selective retarder 307 disposed on the optical path of the light passing through the second variable focal length lens 302 can B The polarization of The second wavelength light L G Convert to the second polarization (⊙), and output the first wavelength light L in the second polarization (⊙) state R Without changing, the third variable focal length lens 303 can change the third wavelength light L output from the third wavelength selective delay device 307. B The focal length can output the first wavelength light L of the second polarization (⊙) R and the second wavelength light L G The controller 370 may control the first to third variable focal length lenses 301 , 302 , and 303 .

[0079] The controller 370 may control voltages applied to the first to third variable focal length lenses 301 , 302 , and 303 so that focal points of corresponding color lights may be formed at the same position regardless of the wavelength of incident light.

[0080] The operation of the focal length modulation optical system 400 to control the focal length is described below.

[0081] White light or light of various wavelength bands may be incident on the focus modulation optical system 400. The incident light may be separated into a first polarization The first color light L R , the second color light L of the second polarization (⊙) G and the third color light L of the second polarization (⊙) B .

[0082] First polarization The first color light L R , the second color light L of the second polarization (⊙) G and the third color light L of the second polarization (⊙) B The first variable focal length lens 301 may include a first active lens 331R which is electrically controlled and may allow a first polarization The first color light L R The focus position of the first variable focal length lens 301 is the first position. The light is refracted, so only the first polarization can be adjusted The first color light L R The focal length of the second polarization (⊙) of the second color light L G and the third color light L of the second polarization (⊙) BIt can be output by the first variable focal length lens 301 without change.

[0083] Next, the first polarization The first color light L R , the second color light L of the second polarization (⊙) G and the third color light L of the second polarization (⊙) B The second color light L G The polarization of the In addition, the second wavelength selective retarder 306 converts the first polarization The first color light L R The third color light L changes to the second polarization (⊙), but does not affect the second polarization (⊙) B The first color light L of the second polarization (⊙) R , first polarization The second color light L G and the third color light L of the second polarization (⊙) B The light is incident on the second variable focal length lens 302 through the second wavelength selective retarder 306 .

[0084] The second variable focal length lens 302 may include a second active lens 331G which is electrically controlled and may allow the second color light L G The focus is formed at the first position. Because the second variable focal length lens 302 only focuses on the first polarization The light of the first color L acts as the second polarization (⊙) of the incident light. R and the third color light L of the second polarization (⊙) B Not affected.

[0085] Next, the first color light L of the second polarization (⊙) R , first polarization The second color light L G and the third color light L of the second polarization (⊙) B The third color light L B The second polarization (⊙) is changed to the first polarization In addition, the third wavelength selective retarder 307 converts the first polarization The second color light L G The first color light L is changed to the second polarization (⊙) without affecting the second polarization (⊙). R . It is possible to use the second color light L G and the third color light L BThe optical anisotropic material with wavelength selectivity is used as the material of the third wavelength selective retarder 307. The third wavelength selective retarder 307 can change the second color light L G and the third color light L B The first color light L of the second polarization (⊙) R , the second color light L of the second polarization (⊙) G and the first polarization The third color light L B The light is incident on the third variable focal length lens 303 through the third wavelength selective retarder 307 .

[0086] The third variable focal length lens 303 may include a third active lens 331B which is electrically controlled and may allow the third color light L B The focus is formed at the first position. In addition, because the third variable focal length lens 303 only focuses on the first polarization The light of the first color L in the second polarization (⊙) R , the second color light L of the second polarization (⊙) G and the first polarization The third color light L B Among them, only the first polarization The third color light L B can be focused at the first position by the third variable focal length lens 303. Through the above process, the first color light L R , the second color light L G and the third color light L B The focus may be on the same first position.

[0087] Figure 8 Show Figure 3 The simultaneous operation mechanism of red light, green light and blue light in the focus modulation optical system 200 is shown.

[0088] In a holographic display device of an exemplary embodiment, when multiple frames of images are displayed, at each frame, the red light L R , Green Light L G and Blu-ray L B can be incident on the focal length modulation optical system at the same time. For example, in the first frame, the red light L from the light source R , Green Light L G and Blu-ray L B Can be incident on the variable focal length lens at the same time. Used to adjust the red light L R The focal length voltage V(R) is used to adjust the green light L G The voltage V(G) of the focal length and the voltage used to adjust the blue light L BA voltage V(B) for adjusting the focal length of the variable focal length lens can be applied to the variable focal length lens. The focal length can be adjusted in the same manner at the second frame. In the simultaneous operation mechanism, since the focus modulation optical system operates simultaneously with respect to multiple color lights, the frame rate can be relatively high.

[0089] Fig. 9 A focal length modulation optical system 200A is shown according to an example embodiment. Figure 3 In addition to the elements of the focal length modulation optical system 200 shown, the focal length modulation optical system 200A may further include a fixed focus lens 240. The fixed focus lens 240 may be disposed in front of the first wavelength selective retarder 205. The fixed focus lens 240 may be a lens configured to focus any one of the wavelengths of light. For example, the fixed focus lens 240 may be a lens configured to focus green light. However, example embodiments are not limited thereto, and the fixed focus lens 240 may focus on appropriate color light other than green light. For example, the fixed focus lens 240 may be designed to primarily reduce chromatic aberration with respect to each color of light. The fixed focus lens 240 may be difficult to manufacture and may incur considerable manufacturing costs in order to more accurately correct chromatic aberration. However, the fixed focus lens 240 may be manufactured at low cost to preliminarily reduce chromatic aberration.

[0090] Next, through the first wavelength selective delay 205, the second wavelength selective delay 215 and the third wavelength selective delay 225 and the first variable focal length lens 210, the second variable focal length lens 220 and the third variable focal length lens 230, the focal length relative to each color light can be adjusted, so that the chromatic aberration can be further corrected more accurately.

[0091] In other words, after preliminarily correcting the chromatic aberration by using the fixed focal length lens 240, the chromatic aberration may be more accurately corrected by the wavelength selective retarder and the variable focal length lens.

[0092] Fig.10 A focus modulating optical system 400 ′ is shown according to an example embodiment.

[0093] The focus modulation optical system 400 ′ may include a fixed focus lens 410 and a variable focus lens 420 .

[0094] The variable focal length lens 420 may have a reference Figure 4 and Figure 5 Describes the structure. Figure 4 and Figure 5 By adjusting the voltage applied between the first electrode 321 and the second electrode 341 according to the wavelength of the incident light, the variable focal length lens 420 can control the active lens 331. Therefore, the chromatic aberration can be reduced or eliminated by adjusting the focal length of each color light.

[0095] exist Fig.10 In the focal length modulation optical system 400' shown, each color light can be input in time sequence. Fig.11 Shows red light L R , Green Light L G and Blu-ray L B The chronological operation mechanism.

[0096] In a holographic display device of an exemplary embodiment, when an image having a plurality of frames is displayed, at each frame, the red light L R , Green Light L G and Blu-ray L B is sequentially input to the focal length modulation optical system 400'. For example, in the first frame, the red light L from the light source R can be incident on the focus modulation optical system 400' and used to adjust the red light L R A voltage V(R) of the focal length of the red light L may be applied to the variable focal length lens 420. R can be focused at the first position. Then, the green light L G can be incident on the variable focal length lens 420 and is used to adjust the green light L G A voltage V(G) of the focal length may be applied to the focal length variable focal length lens 420. The green light L G can be focused at the first position. Then, the blue light L B can be incident on the variable focal length lens 420 and is used to adjust the blue light L B A voltage V(B) of the focal length of the blue light L may be applied to the variable focal length lens 420. B can be focused at a first position. The focal length can be adjusted at a second frame in the same manner. In a time-sequential operation mechanism, since the focus modulation optical system operates time-sequentially with respect to a plurality of color lights, the frame rate can be relatively lower than the frame rate of a simultaneous operation mechanism. On the contrary, the focus modulation optical system can be made thinner than a simultaneously operating focus modulation optical system. When the operation speed of the variable focal length lens is fast enough, the necessary frame rate can be ensured and the focus modulation optical system can be reduced or miniaturized. In addition, although in Fig.10 The fixed focal length lens 410 is provided in the embodiment, but by adjusting the focal length by using only the variable focal length lens 420 without using the fixed focal length lens 410, chromatic aberration can be reduced.

[0097] It should be understood that the example embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each example embodiment should generally be considered as available for other similar features or aspects in other example embodiments.

[0098] Although example embodiments have been described with reference to the drawings, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope as defined by the following claims.

[0099] This application claims the benefit of Korean Patent Application No. 10-2018-0093992 filed on August 10, 2018, in the Korean Intellectual Property Office, and Korean Patent Application No. 10-2018-0118499 filed on October 4, 2018, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference in their entirety.

Claims

1. A focal length modulation optical system, comprising: at least one variable focal length lens configured to change the focus position of incident light according to color by electrical control, wherein the at least one variable focal length lens is variable so that each color of light forms a focus at the same position, The color light includes a first color light, a second color light and a third color light, wherein the at least one variable focal length lens comprises: a first variable focal length lens configured to focus the first color light at a first position; a second variable focal length lens configured to focus the second color light at the first position; and a third variable focal length lens configured to focus the third color light at the first position, and wherein the first variable focal length lens, the second variable focal length lens and the third variable focal length lens are arranged sequentially, and The focal length modulation optical system further comprises: a first wavelength selective retarder provided in front of the first variable focal length lens; a second wavelength selective retarder provided between the first variable focal length lens and the second variable focal length lens; and A third wavelength selective retarder is provided between the second variable focal length lens and the third variable focal length lens.

2. The focal length modulation optical system according to claim 1, wherein the at least one variable focal length lens comprises: a first substrate; providing a first electrode layer on the first substrate; a second substrate disposed opposite to the first substrate; providing a second electrode layer on the second substrate; as well as A liquid crystal layer is provided between the first substrate and the second substrate, the liquid crystal layer including a plurality of active lenses configured to change based on a voltage applied between the first electrode layer and the second electrode layer. 3 . The focal length modulation optical system according to claim 2 , wherein at least one of the first electrode layer and the second electrode layer comprises strip-shaped electrodes disposed apart from each other. 4 . The focal length modulation optical system of claim 3 , wherein at least one of an array pitch, a shape, and a curvature of the plurality of active lenses is adjusted based on a voltage applied to the strip electrodes. 5 . The focal length modulation optical system of claim 1 , further comprising a fixed focal length lens configured to focus light incident on the variable focal length lens.

6. The focal length modulation optical system according to claim 1, wherein the first wavelength selective retarder is configured to convert the first color light from the second polarization to the first polarization, and transmit the second color light of the second polarization and the third color light of the second polarization, wherein the second wavelength selective retarder is configured to convert the first color light from the first polarization to the second polarization, convert the second color light from the second polarization to the first polarization, and transmit the third color light of the second polarization, and The third wavelength selective retarder is configured to convert the third color light from the second polarization to the first polarization, convert the second color light from the first polarization to the second polarization, and transmit the first color light of the second polarization.

7. The focal length modulation optical system according to claim 1, further comprising a wavelength selective retarder provided in front of the at least one variable focal length lens, the wavelength selective retarder being configured to selectively delay a phase based on a color of incident light.

8. A holographic display device, comprising: a light source configured to emit a plurality of colors of light; A focal length modulation optical system comprising at least one variable focal length lens configured to change a focus position of incident light by electrically controlling the at least one variable focal length lens based on a color of light incident on the at least one variable focal length lens; as well as a spatial light modulator configured to form a holographic image by diffracting light output from the focus modulation optical system, wherein the at least one variable focal length lens comprises: a first variable focal length lens configured to focus a first color of light at a first position; a second variable focal length lens configured to focus the second color light at the first position; and a third variable focal length lens configured to focus the third color light at the first position, and The first variable focal length lens, the second variable focal length lens and the third variable focal length lens are sequentially arranged, and The holographic display device further comprises: a first wavelength selective retarder provided in front of the first variable focal length lens; a second wavelength selective retarder provided between the first variable focal length lens and the second variable focal length lens; and A third wavelength selective retarder is provided between the second variable focal length lens and the third variable focal length lens.

9. The holographic display device according to claim 8, wherein the at least one variable focal length lens comprises: a first substrate; a first electrode layer disposed on the first substrate; a second substrate disposed opposite to the first substrate; a second electrode layer disposed on the second substrate; as well as A liquid crystal layer is disposed between the first substrate and the second substrate, the liquid crystal layer including a plurality of active lenses configured to change based on a voltage applied between the first electrode layer and the second electrode layer.

10. The holographic display device according to claim 9, wherein at least one of the first electrode layer and the second electrode layer comprises strip electrodes that are disposed separately from each other.

11. The holographic display device according to claim 10, wherein at least one of an array pitch, a shape, and a curvature of the plurality of active lenses is configured to change based on a voltage applied to the strip electrodes.

12. The holographic display device according to claim 8 further includes a wavelength selective delay device, which is provided in front of the at least one variable focal length lens and is configured to selectively delay the phase based on the color of the incident light.

13. The holographic display device according to claim 12 further includes a fixed focal length lens, which is configured to focus the light to be emitted on the wavelength selective delay device.

14. The holographic display device according to claim 10 further includes a fixed focal length lens, which is configured to focus light to be emitted on the at least one variable focal length lens.

15. The holographic display device according to claim 8 further includes a controller, which is configured to control the light source to simultaneously emit a first color light, a second color light, and a third color light, and to control the first variable focal length lens, the second variable focal length lens, and the third variable focal length lens to adjust a focusing position corresponding to each of the first color light, the second color light, and the third color light, respectively.

16. The holographic display device according to claim 8 further includes a beam deflector, which is configured to deflect the multiple color lights emitted from the light source.

17. The holographic display device according to claim 8, further comprising a position detection sensor configured to detect a position of a viewer viewing the holographic image.

18. The holographic display device according to claim 8, further comprising a controller configured to control the light source to emit a plurality of color lights in a time sequence. The variable focal length lens is configured to adjust focus positions corresponding to the plurality of color lights in a time sequence.

19. A holographic display device, comprising: a light source configured to emit a plurality of colors of light; a beam deflector configured to deflect the plurality of colors of light emitted from the light source; a light guide plate configured to guide the plurality of color lights deflected from the beam deflector; a focal length modulation optical system including a fixed focal length lens and a plurality of variable focal length lenses, the plurality of variable focal length lenses being configured to change a focus position of the plurality of color lights output from the light guide plate by electrically controlling the plurality of variable focal length lenses based on the colors of the incident plurality of color lights; as well as a spatial light modulator configured to form a holographic image by diffracting the plurality of color lights output from the focus modulation optical system, The plurality of variable focal length lenses include: a first variable focal length lens configured to focus a first color of light at a first position; a second variable focal length lens configured to focus the second color light at the first position; and a third variable focal length lens configured to focus the third color light at the first position, and The holographic display device further comprises: a first wavelength selective retarder provided in front of the first variable focal length lens; a second wavelength selective retarder provided between the first variable focal length lens and the second variable focal length lens; and A third wavelength selective retarder is provided between the second variable focal length lens and the third variable focal length lens.

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