Patent Description:
A glasses method and a non-glasses method have been widely commercialized and used for implementing a three-dimensional image. The glasses method may include a polarized glasses method and a shutter glasses method, and the non-glasses method may include a lenticular method and a parallax barrier method. These methods use the binocular parallax of the two eyes, but have a limitation in an increase of the number of viewpoints and may create a sense of fatigue to a viewer because a sense of depth recognized by the brain does not match the focal point of the eyes.

A holographic display method is gradually used as a three-dimensional image display method in which the sense of depth recognized by the brain and the focal point of eyes are matched with each other and full parallax may be provided. The holographic display method uses a principle to reproduce an image of the original object by irradiating and diffracting reference light to a holographic pattern that records interference fringes obtained by allowing object light reflected from the original object and the reference light to interfere with each other. The currently used holographic display method provides a spatial light modulator with a computer generated hologram (CGH), rather than a holographic pattern obtained by directly exposing the original object, as an electric signal. As the spatial light modulator forms a holographic pattern to diffract the reference light according to an input CGH signal, a three-dimensional image may be generated.

The holographic display apparatus may include a backlight unit for providing illumination light to the spatial light modulator. The backlight unit used in the holographic display apparatus has coherence and provides collimated illumination light to the spatial light modulator. The collimated coherent illumination light provided by the backlight unit may be diffracted by the spatial light modulator, thereby forming a holographic image.

<CIT> discloses an augmented reality display system configured to use fiducial markers to align 3D content with real objects. <CIT> discloses an optical display comprising a first waveguide comprising a first surface and a second surface, an input coupler, a fold grating, and an output grating. <CIT> discloses an apparatus for use in replicating an image associated with an input-pupil to an output-pupil including a planar optical waveguide including a bulk-substrate, and also including an input-coupler, an intermediatecomponent and an output-coupler.

One or more example embodiments of the present disclosure relate to a backlight unit and a holographic display apparatus including the backlight unit.

According to an aspect of the present disclosure, a backlight unit as defined in any of the claims <NUM>-<NUM> is provided. According to another aspect of the present disclosure, a holographic display apparatus as defined in claim <NUM> is provided.

The above and/or other aspects, features, and advantages of example embodiments will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:.

Hereinafter, a backlight unit and a holographic display apparatus including the backlight unit will be described in detail with reference to the accompanying drawings. Also, the size of each layer illustrated in the drawings may be exaggerated for convenience of explanation and clarity. Furthermore, the example embodiments are merely described below, by referring to the figures, to explain aspects of the present disclosure, and the example embodiments may have different forms. In the layer structure described below, when a constituent element is disposed "above" or "on" to another constituent element, the constituent element may include not only an element directly contacting on the upper/lower/left/right sides of the other constituent element, but also an element disposed above/under/left/right the other constituent element in a non-contact manner.

<FIG> is a schematic cross-sectional view of a holographic display apparatus <NUM> according to an embodiment. Referring to <FIG>, the holographic display apparatus <NUM> includes a backlight unit <NUM> may provide collimated coherent illumination light, a spatial light modulator <NUM> may reproduce a holographic image by modulating the illumination light, and a Fourier lens <NUM> may focus the holographic image in a space. Although <FIG> illustrates that the spatial light modulator <NUM> is disposed between the Fourier lens <NUM> and the backlight unit <NUM>, embodiments are not limited thereto. For example, the Fourier lens <NUM> may be disposed between the spatial light modulator <NUM> and the backlight unit <NUM>.

The spatial light modulator <NUM> may form a holographic pattern to diffract and modulate illumination light, according to a hologram data signal, for example, a computer generated hologram (CGH) data signal, provided by an image processor. To this end, the spatial light modulator <NUM> may include a plurality of display pixels that are two-dimensionally (2D) arranged. Furthermore, any of a phase modulator for performing phase modulation only, an amplitude modulator for performing amplitude modulation only, and a composite modulator for performing both of the phase modulation and the amplitude modulation may be used as the spatial light modulator <NUM>. Although <FIG> illustrates that the spatial light modulator <NUM> is a transmissive spatial light modulator, a reflective spatial light modulator may be used therefor. For a transmissive-type spatial light modulator, a semiconductor modulator based on a compound semiconductor, for example, gallium arsenide (GaAs), or a liquid crystal modulator may be used as the spatial light modulator <NUM>. For a reflective-type spatial light modulator, for example, a digital micromirror device (DMD), a liquid crystal on silicon (LCoS), or a semiconductor modulator may be used as the spatial light modulator <NUM>.

The backlight unit <NUM> may provide collimated coherent illumination light to the spatial light modulator <NUM>. The backlight unit <NUM> may include light sources <NUM> and <NUM> that may emit coherent light and a light guide structure <NUM> to expand and collimate a section of the light emitted from the light sources <NUM> and <NUM> to correspond to the size of the spatial light modulator <NUM>.

The light sources <NUM> and <NUM> may provide light traveling in opposite directions in the light guide structure <NUM>. To this end, the light sources <NUM> and <NUM> may include a first light source <NUM> disposed above one side edge of the upper surface of the light guide structure <NUM> and a second light source <NUM> disposed at the opposite side edge of the upper surface of the light guide structure <NUM>. The light emitted from the first light source <NUM> and the light emitted from the second light source <NUM> may travel in opposite directions in the light guide structure <NUM>. To provide light having a relatively high coherence, the first light source <NUM> and the second light source <NUM> may include, for example, a laser diode. In addition to the laser diode, any light source capable of emitting light having spatial coherence may be employed therefor. Although <FIG> illustrates, for convenience of explanation, that each of the first light source <NUM> and the second light source <NUM> is provided as a singular light source, each of the first light source <NUM> and the second light source <NUM> may include an array of a plurality of light sources.

Furthermore, the holographic display apparatus <NUM> may further include a 2D backlight unit <NUM> for providing illumination light for a 2D image. The illumination light for a 2D image provided by the 2D backlight unit <NUM> may not have coherence nor be necessarily collimated. The 2D backlight unit <NUM> may include, for example, a light-emitting diode (LED), as the light source, and may provide the spatial light modulator <NUM> with the light emitted from an LED light source by expanding the light with a light guide plate. While the holographic display apparatus <NUM> reproduces a holographic image, the backlight unit <NUM> may be turned on and the 2D backlight unit <NUM> may be turned off, and while the holographic display apparatus <NUM> reproduces a general 2D image, the backlight unit <NUM> may be turned off and the 2D backlight unit <NUM> may be turned on.

The holographic display apparatus <NUM> may further include the image processor that may generate a hologram data signal according to a holographic image to be provided to a viewer and provide the generated hologram data signal to the spatial light modulator <NUM>, and may control the operations of the backlight unit <NUM> and the 2D backlight unit <NUM>. Furthermore, the holographic display apparatus <NUM> may further include an eye tracker that may track the position of a viewer's pupil in real time and a beam deflector that may adjust the position of the holographic image focused by the Fourier lens <NUM> based on the position of the viewer's pupil provided by the eye tracker.

According to the embodiment, the light guide structure <NUM> may form illumination light by uniformly collimating the light emitted from the first light source <NUM> and the second light source <NUM>. To this end, the light guide structure <NUM> includes at least two light guide layers stacked in a thickness direction and at least two coupler layers respectively disposed to face each other in the different light guide layers. In <FIG>, the light guide structure <NUM> is illustrated to include a first light guide layer <NUM> where a first coupler layer CL1 is disposed and a second light guide layer <NUM> where a second coupler layer CL2 is disposed. In the above structure, the light emitted from the first light source <NUM> may be coupled by the first coupler layer CL1, and the light emitted from the second light source <NUM> may be coupled by the second coupler layer CL2. Each of the first coupler layer CL1 and the second coupler layer CL2 includes a plurality of couplers that guide incident light to the inside of the light guide structure <NUM>, expand the light traveling along the inside of the light guide structure <NUM> in two directions perpendicular to each other, and output the light to the outside of the light guide structure <NUM>.

For example, <FIG> is a schematic perspective view of a configuration of the first light guide layer <NUM> of the backlight unit <NUM> according to an embodiment. <FIG> is a schematic plan view of a configuration of the first light guide layer <NUM> of <FIG>. Referring to <FIG> and <FIG>, the first light guide layer <NUM> may include a first substrate S1 and a second substrate S2, which are stacked in the thickness direction, that is, a +z-axis direction, and the first coupler layer CL1 is disposed between the first substrate S1 and the second substrate S2. The first substrate S1 and the second substrate S2 may include a material such as glass or a polymer that is transmissive to light including an infrared light, a visible light, or an ultraviolet light. Furthermore, the first coupler layer CL1 may include a first output coupler OC1 and a first expansion coupler MC1, which are disposed adjacent to each other.

The first output coupler OC1 expands the light traveling along the inside of the light guide structure <NUM> in an x-axis direction. Furthermore, the first output coupler OC1 may output the light traveling along the inside of the light guide structure <NUM> to provide illumination light to the spatial light modulator <NUM>. To this end, the first output coupler OC1 may be disposed to face the spatial light modulator <NUM> of the holographic display apparatus <NUM>, and a y-axis directional width W and an x-axis directional length L1 of the first output coupler OC1 may be similar to the width and length of the spatial light modulator <NUM>.

The first expansion coupler MC1 is disposed at one side surface of the first output coupler OC1 in the x-axis direction. The first expansion coupler MC1 expands the light traveling along the inside of the light guide structure <NUM> in the +y-axis direction and provide the expanded light to the first output coupler OC1. To this end, as illustrated in <FIG>, the y-axis directional width W of the first expansion coupler MC1 may be the same as the y-axis directional width W of the first output coupler OC1. An x-axis directional length L2 of the first expansion coupler MC1 may be less than the x-axis directional length L1 of the first output coupler OC1.

Furthermore, <FIG> is a schematic perspective view of a configuration of the second light guide layer <NUM> of the backlight unit <NUM> according to an embodiment. <FIG> is a schematic plan view of a configuration of the second light guide layer <NUM> of <FIG>. Referring to <FIG> and <FIG>, the second light guide layer <NUM> may include a third substrate S3 and a fourth substrate S4, which are stacked in the thickness direction, that is, the z-axis direction, and the second coupler layer CL2 disposed between the third substrate S3 and the fourth substrate S4. The third substrate S3 and the fourth substrate S4 may include a material such as glass or a polymer that is transmissive to light including an infrared light, a visible light, or an ultraviolet light. Furthermore, the second coupler layer CL2 includes a second output coupler OC2, a second expansion coupler MC2 disposed adjacent to the second output coupler OC2 in the x-axis direction, a first input coupler IC1 and a second input coupler IC2 disposed at opposite sides of the second expansion coupler MC2 in the y-axis direction, a third input coupler IC3 disposed to face the first input coupler IC1 in the x-axis direction, and a fourth input coupler IC4 disposed to face the second input coupler IC2 in the x-axis direction.

The second output coupler OC2 expands the light traveling along the inside of the light guide structure <NUM> in the x-axis direction. Furthermore, the second output coupler OC2 outputs the light traveling along the inside of the light guide structure <NUM> to provide illumination light to the spatial light modulator <NUM>. The first output coupler OC1 and the second output coupler OC2 are disposed to face each other in the z-axis direction and may have the same size.

The second expansion coupler MC2 is disposed at one side surface of the second output coupler OC2 in the x-axis direction. The second expansion coupler MC2 expands the light traveling along the inside of the light guide structure <NUM> in the y-axis direction and provide the expanded light to the second output coupler OC2. To this end, the y-axis direction width of the second expansion coupler MC2 may be the same as the y-axis direction width of the second output coupler OC2, and the x-axis direction length of the second expansion coupler MC2 may be less than the x-axis direction length of the second output coupler OC2. The first expansion coupler MC1 and the second expansion coupler MC2 may be disposed to face each other in the z-axis direction and may have the same size.

The third input coupler IC3 is disposed to face the first light source <NUM> in the z-axis direction and may guide the light emitted from the first light source <NUM> to the inside of the light guide structure <NUM>. The light guided by the third input coupler IC3 to the inside of the light guide structure <NUM> may travel in the +x-axis direction and be incident on the first input coupler IC1. The first input coupler IC1 may slightly expand the incident light in the x-axis direction and change a traveling direction of the incident light in the +y-axis direction. The light having a direction changed by the first input coupler IC1 may travel in the light guide structure <NUM> in the +y-axis direction and be provided to the first expansion coupler MC1 and the second expansion coupler MC2.

Furthermore, the fourth input coupler IC4 is disposed to face the second light source <NUM> in the z-axis direction and may guide the light emitted from the second light source <NUM> to the inside of the light guide structure <NUM>. The light guided by the fourth input coupler IC4 to the inside of the light guide structure <NUM> may travel in the +x-axis direction and be incident on the second input coupler IC2. The second input coupler IC2 may slightly expand the incident light in the x-axis direction and changes a traveling direction of the incident light in the -y-axis direction. Then, the light having a direction changed by the second input coupler IC2 may travel in the light guide structure <NUM> in the -y-axis direction and be supplied to the first expansion coupler MC1 and the second expansion coupler MC2. Accordingly, the light emitted from the first light source <NUM> may travel in the +y-axis direction and be provided to the first expansion coupler MC1 and the second expansion coupler MC2, and the light emitted from the second light source <NUM> may travel in the -y-axis direction opposite to the traveling direction of the light emitted from the first light source <NUM> and be provided to the first expansion coupler MC1 and the second expansion coupler MC2.

The above-described first output coupler OC1, second output coupler OC2, first expansion coupler MC1, second expansion coupler MC2, and first to fourth input couplers IC1, IC2, IC3, and IC4 may be formed in a variety of types of surface gratings or volume gratings. A surface grating, which is a grating directly formed on a surface of a substrate, may include a diffractive optical element (DOE), for example, a binary phase grating or a blazed grating. A plurality of grating patterns of the DOE may serve as diffractive gratings and diffract the incident light. For example, the surface grating may diffract light incident in a specific angle range according to the size, height, cycle, duty ratio, or shape of grating patterns, causing destructive interference and constructive interference, thereby changing a traveling direction of the light. A volume grating may be formed separated from the substrate, and may include, for example, a holographic optical element (HOE), a geometric phase grating, a Bragg polarization grating, or a holographically formed polymer dispersed liquid crystal (H-PDLC). The volume grating may include cyclic fine patterns of materials having different refractive indexes. In particular, the third input coupler IC3 and the fourth input coupler IC4 may use gratings having a relatively high directivity and efficiency, for example, a blazed grating or a volume grating, such that the incident light is transmitted to the first input coupler IC1 and the second input coupler IC2 without loss.

The third input coupler IC3 and the fourth input coupler IC4 may be omitted. In this case, the first input coupler IC1 may guide the light emitted from the first light source <NUM> to the inside of the light guide structure <NUM> and the second input coupler IC2 may guide the light emitted from the second light source <NUM> to the inside of the light guide structure <NUM>. To this end, the first light source <NUM> may be disposed to face the first input coupler IC1, and the second light source <NUM> may be disposed to face the second input coupler IC2.

<FIG> is a schematic perspective view of a configuration of a light guide structure <NUM> according to an embodiment, in which the first light guide layer <NUM> of <FIG> and <FIG> and the second light guide layer <NUM> of <FIG> and <FIG> are bonded to each other. Referring to <FIG>, the light guide structure <NUM> has a structure in which the first light guide layer <NUM> is stacked on the second light guide layer <NUM>. Then, the first output coupler OC1 in the first light guide layer <NUM> is disposed to face the second output coupler OC2 in the second light guide layer <NUM>, and the first expansion coupler MC1 in the first light guide layer <NUM> is disposed to face the second expansion coupler MC2 in the second light guide layer <NUM>.

<FIG> is a schematic cross-sectional view taken along line A-A' in the light guide structure <NUM> of <FIG>, showing a light traveling and coupling operation. <FIG> is a schematic cross-sectional view showing a light traveling and coupling operation in the light guide structure <NUM> including the first expansion coupler MC1 and the second expansion coupler MC2. Referring to <FIG>, the light guide structure <NUM> may include the fourth substrate S4, the third substrate S3 stacked on the fourth substrate S4, the second substrate S2 stacked on the third substrate S3, and the first substrate S1 stacked on the second substrate S2. The first expansion coupler MC1 is disposed between the first substrate S1 and the second substrate S2. In contrast, the second expansion coupler MC2, the first input coupler IC1, and the second input coupler IC2 are disposed between the third substrate S3 and the fourth substrate S4.

As described above, the light incident on the third input coupler IC3 from the first light source <NUM> travels along the inside of the light guide structure <NUM> and is incident on the first input coupler IC1. The traveling direction of the light is changed by about <NUM>° by the first input coupler IC1 and travels in the +y-axis direction along the inside of the light guide structure <NUM>. As illustrated in <FIG>, the light may travel in the +y-axis direction in the light guide structure <NUM> by being totally reflected from an upper surface of the first substrate S1 and a lower surface of the fourth substrate S4 of the light guide structure <NUM>.

Furthermore, the light incident on the fourth input coupler IC4 from the second light source <NUM> travels along the inside of the light guide structure <NUM> and is incident on the second input coupler IC2. The traveling direction of the light is changed by about <NUM>° by the second input coupler IC2 and travels in the -y-axis direction along the inside of the light guide structure <NUM>. As illustrated in <FIG>, the light may travel in the -y-axis direction the inside of the light guide structure <NUM> by being totally reflected from the upper surface of the first substrate S1 and the lower surface of the fourth substrate S4 of the light guide structure <NUM>. Accordingly, the light emitted from the first light source <NUM> and the light emitted from the second light source <NUM> travels in the opposite directions in the light guide structure <NUM> including the first expansion coupler MC1 and the second expansion coupler MC2.

While traveling inside the light guide structure <NUM>, the light is repeatedly incident on the first expansion coupler MC1 and the second expansion coupler MC2. The first expansion coupler MC1 and the second expansion coupler MC2 may perform coupling only on the light incident in a specific direction. In other words, the first expansion coupler MC1 may couple part of the light incident at a first angle to be transmitted to the first output coupler OC1 and the second output coupler OC2 and may transmit light incident at angles other than the first angle. Furthermore, the second expansion coupler MC2 may couple part of the light incident at a second angle different from the first angle to be transmitted to the first output coupler OC1 and second output coupler OC2 and may transmit light incident at angles other than the second angle. In this state, the first angle and the second angle may have the same size but opposite signs with respect to a surface normal of the light guide structure <NUM>.

For example, referring to <FIG>, the first expansion coupler MC1 may couple part of the light traveling in the +y-axis direction obliquely downward from the first substrate S1 to the fourth substrate S4. Accordingly, the first expansion coupler MC1 may couple only the light emitted from the first light source <NUM>. Although <FIG> illustrates, for convenience of explanation, that the light coupled by the first expansion coupler MC1 exits the light guide structure <NUM> in a perpendicular direction, the light does not exit the light guide structure <NUM>, but travels in the light guide structure <NUM> in the -x-axis direction.

Furthermore, the second expansion coupler MC2 may couple part of the light input in the -y-axis direction obliquely downward from the first substrate S1 to the fourth substrate S4. Accordingly, the second expansion coupler MC2 may couple only the light emitted from the second light source <NUM>. Although <FIG> illustrates, for convenience of explanation, that the light coupled by the second expansion coupler MC2 exits the light guide structure <NUM> in a perpendicular direction, the light does not exit the light guide structure <NUM>, but travels in the light guide structure <NUM> in the -x-axis direction. As a result, the light may be expanded in the y-axis direction by the first expansion coupler MC1 and the second expansion coupler MC2 to be provided to the first output coupler OC1 and the second output coupler OC2.

The light coupled by the first expansion coupler MC1 and second expansion coupler MC2 in the above method travels in the light guide structure <NUM> in the -x-axis direction. While traveling in the -x-axis direction, as described in <FIG>, the light is totally reflected from the upper surface of the first substrate S1 and the lower surface of the fourth substrate S4 of the light guide structure <NUM> to be repeatedly incident on the first output coupler OC1 and the second output coupler OC2. Part of the light incident on the first output coupler OC1 and the second output coupler OC2 is coupled by the first output coupler OC1 and the second output coupler OC2 and output in the +z-axis direction through the upper surface of the first substrate S1 of the light guide structure <NUM>. In the process, the light may be expanded by the first output coupler OC1 and the second output coupler OC2 in the x-axis direction. Then, the light output from the light guide structure <NUM> may be incident on the spatial light modulator <NUM> as collimated illumination light.

<FIG> is a graph of individual output intensity and overall output intensity of the light traveling in opposite directions in the light guide structure <NUM> of <FIG>. In <FIG>, a graph A shows a relationship between the intensity of light emitted from the first light source <NUM> and coupled by the first expansion coupler MC1 and a coupling position of the first expansion coupler MC1, and a graph B shows a relationship between the intensity of the light emitted from the second light source <NUM> and coupled by the second expansion coupler MC2 and a coupling position of the second expansion coupler MC2. As illustrated in <FIG>, the intensity of the light coupled by the first expansion coupler MC1 gradually increases in the +y-axis direction, and the intensity of the light coupled by the second expansion coupler MC2 gradually increases in the -y-axis direction. As a result, the sum (A+B) of the intensity of the light coupled by the intensity of the light coupled by the first expansion coupler MC1 and the second expansion coupler MC2 is maintained at a relatively uniform level. Accordingly, an intensity distribution of the light finally output from the light guide structure <NUM> by the first output coupler OC1 and the second output coupler OC2 may be maintained to be uniform.

Referring back to <FIG>, to further maintain the uniformity of the sum of the intensity of the light coupled by the first expansion coupler MC1 and the second expansion coupler MC2, a thickness t1 of the first light guide layer <NUM> and a thickness t2 of the second light guide layer <NUM> of the light guide structure <NUM> may be selected to be different from each other. In other words, the thickness t1 that is the sum of the thickness of the first substrate S1 and the thickness of the second substrate S2 may be different from the thickness t2 that is the sum of the thickness of the third substrate S3 and the thickness of the fourth substrate S4. Then, compared to a case in which the thickness t1 of the first light guide layer <NUM> and the thickness t2 of the second light guide layer <NUM> are the same, regularity of positions where the light traveling in the +y-axis direction is incident on the first expansion coupler MC1 and regularity of positions where the light traveling in the -y-axis direction is incident on the second expansion coupler MC2 may be reduced, and thus the light may be more irregularly or uniformly distributed.

When the first light source <NUM> and the second light source <NUM> use laser diodes that emit light having a single wavelength, speckle noise may occur in illumination light due to interference of laser beams. Accordingly, to reduce the speckle noise, the first light source <NUM> and the second light source <NUM> may use laser diodes that emit light having a multi-peak wavelength distribution. For example, <FIG> is a graph of a wavelength distribution of light emitted from one light source. As illustrated in <FIG>, the speckle noise may be reduced when the light emitted from the first light source <NUM> and the second light source <NUM> has a multi-peak wavelength distribution in a narrow wavelength range of about <NUM> or less. Furthermore, a center wavelength of the first light source <NUM> and a center wavelength of the second light source <NUM> may be slightly different from each other. For example, a difference between the center wavelength of the light emitted from the first light source <NUM> and the center wavelength of the light emitted from the second light source <NUM> may be greater than <NUM> and equal to or less than <NUM>. Then, the occurrence of speckle noise in the illumination light provided by the spatial light modulator <NUM> may be further reduced.

As described above, the backlight unit <NUM> according to the embodiment provides uniform illumination light because one illumination light is generated by coupling the light traveling in the opposite directions. Accordingly, a strip pattern in the illumination light, which is formed as a bright pattern and a dark pattern are repeatedly distributed when only one light traveling in one direction in the light guide structure <NUM> is coupled, may be reduced or restricted. Furthermore, as the backlight unit <NUM> according to the example embodiment provides the illumination light in which speckle noise hardly exists, the quality of a holographic image produced by the holographic display apparatus <NUM> that includes the backlight unit <NUM> according to the example embodiment may be improved.

Furthermore, the backlight unit <NUM> according to the example embodiment may provide collimated coherent illumination light uniformly to a relatively large area, by using the light guide structure <NUM>, and may be manufactured to be relatively thin. Accordingly, the holographic display apparatus <NUM> including the backlight unit <NUM> according to the example embodiment may be manufactured to be relatively thin. The holographic display apparatus <NUM> may be applied to various fields such as three-dimensional (3D) mobile devices, 3D tablets, or 3D televisions (TVs).

The light guide structure <NUM> is described above to have two light guide layers, that is, the first light guide layer <NUM> and the second light guide layer <NUM>. However, embodiments are not limited thereto, and the light guide structure <NUM> may include two or more light guide layers. For example, <FIG> is a schematic cross-sectional view of a configuration of a light guide structure 110a of the backlight unit <NUM> according to another example embodiment. Referring to <FIG>, the light guide structure 110a may include an n-number of light guide layers <NUM>, <NUM>,. Here, n is a natural number greater than <NUM>. The first light guide layer <NUM> may include a first substrate S1, a first coupler layer CL1, and a second substrate S2, and the second light guide layer <NUM> may include a third substrate S3, a second coupler layer CL2, and a fourth substrate S4. An n-th light guide layer N may include a (2n-<NUM>)th substrate S(2n-<NUM>), an n-th coupler layer CLn, and a 2n-th substrate S2n. The layers from the n-th light guide layer N to the first light guide layer <NUM> may be sequentially stacked. The first to 2n-th substrates S1, S2,. , S2n-<NUM>, and S2n may be include a material such as glass or a polymer that is transmissive to light including an infrared light, a visible light, or an ultraviolet light.

Accordingly, the light guide structure 110a may include an n-number of coupler layers CL1, CL2,. One output coupler and one expansion coupler may be disposed in each of the first to n-th coupler layers CL1, CL2,. Accordingly, the light guide structure 110a may include an n-number of output couplers and an n-number of expansion couplers. The n-number of output couplers are disposed to face each other in the first to n-th coupler layers CL1, CL2,. , CLn different from each other, and the n-number of expansion couplers are disposed to face each other in the first to n-th coupler layers CL1, CL2,. , CLn different from each other. In the first to fourth input couplers IC1, IC2, IC3, and IC4, only one coupler layer among the first to n-th coupler layers CL1, CL2,. , CLn may be disposed.

The light traveling inside the light guide structure 110a may be totally reflected from the upper surface of the first substrate S1 and a lower surface of the 2n-th substrate S2n. While traveling inside the light guide structure <NUM>, the light may be coupled by the n-number of expansion couplers and then output-coupled by the n-number of output couplers to the outside of the light guide structure 110a. To maintain the intensity distribution of the output-coupled light uniform, the thicknesses of the first to n-th light guide layers <NUM>, <NUM>,. , N may be different from each other.

Furthermore, to bond the first to n-th light guide layers <NUM>, <NUM>,. , N to each other, a bonding layer <NUM> may be further disposed between two adjacent light guide layers among the first to n-th light guide layers <NUM>, <NUM>,. For example, the bonding layer <NUM> may be further disposed between the second substrate S2 of the first light guide layer <NUM> and the third substrate S3 of the second light guide layer <NUM>. To maintain the intensity distribution of the output-coupled light to be uniform, the bonding layer <NUM> may include a semi-transmissive layer that reflects part of the incident light and transmits the other part of the incident light. For example, the bonding layer <NUM> may reflect <NUM>% to <NUM>% of the incident light and transmit <NUM>% to <NUM>% thereof. Then, part of the light from the second substrate S2 that is incident on an interface between the second substrate S2 and the third substrate S3 may be reflected from the bonding layer <NUM> to travel back to the second substrate S2, and the other part of the light may be transmitted by the bonding layer <NUM> to continuously travel toward the third substrate S3. The bonding layer <NUM> may include, for example, a resin material having a refractive index that is different from a refractive index of the first to 2n-th substrates S1, S2,. , S2n-<NUM>, and S2n. Furthermore, the bonding layer <NUM> may include, instead of the resin material, dichroic coating that transmits part of the light incident at a preset specific angle and reflects the other part of light incident at an angle other than the preset specific angle. However, embodiment are not limited thereto. For example, the dichroic coating may transmit all light incident at an angle different from the preset specific angle.

Furthermore, a reflection plate <NUM> may be further disposed at the lowermost surface of the light guide structure 110a. For example, the reflection plate <NUM> may be disposed at a lower surface of the 2n substrate S2n. The reflection plate <NUM> may reflect light that is transmitted by the 2n substrate S2n to the outside, not being totally reflected from the lower surface of the 2n substrate S2n, to be obliquely reflected to the inside of the 2n substrate S2n, from among the light obliquely incident on the lower surface of the 2n substrate S2n. The light utilization efficiency of the backlight unit <NUM> may be improved by reducing loss of light by using the reflection plate <NUM>.

<FIG> is a schematic cross-sectional view of a configuration of the first light guide layer <NUM> of the backlight unit <NUM> according to an example not according to the invention. The first to n-th coupler layers CL1, CL2,. , CLn of <FIG> may include, for example, volume gratings, and may be manufactured separated from the first to 2n-th substrates S1, S2,. , S2n-<NUM>, and S2n. The light guide structure 110a may be manufactured by bonding two substrates corresponding thereto with the separately manufactured first to n-th coupler layers CL1, CL2,. , CLn inserted therebetween. However, instead of the volume gratings of <FIG>, as illustrated in <FIG>, the first to n-th coupler layers CL1, CL2,. , CLn may be formed directly on the surface of the substrate. For example, like a blazed grating or a binary phase grating, surface gratings having a plurality of cyclically fine grating patterns in which a plurality of recesses and a plurality of protrusions that are periodically arranged may be formed directly on the surface of the substrate by selectively using various processes such as imprinting or etching.

In <FIG>, for example, the first coupler layer CL1 is formed on the upper surface of the second substrate S2. The first substrate S1 may have a flat lower surface. A polymer layer <NUM>, as a planarization layer, may further fill a plurality of recesses in a periodic pattern of the first coupler layer CL1 formed on the upper surface of the second substrate S2. The polymer layer <NUM> may include the same material as the first substrate S1 or a material having the same refractive index as that of the first substrate S1. The polymer layer <NUM> may completely cover the first coupler layer CL1. However, embodiments are not limited thereto. For example, the second substrate S2 may have a flat upper surface, and the first coupler layer CL1 may be formed on the lower surface of the first substrate S1.

In the above-described example embodiments, one coupler layer is disposed between two substrates. The number of the first to 2n-th substrates S1, S2,. , S2n-<NUM>, and S2n is twice the number of the first to n-th coupler layers CL1, CL2,. However, embodiments are not limited thereto. For example, the coupler layers may be disposed on both surfaces of one substrate. For example, <FIG> is a schematic cross-sectional view of a configuration of a light guide structure 110b of the backlight unit <NUM> according to another example embodiment.

Referring to <FIG>, the first coupler layer CL1 is disposed between the first substrate S1 and the second substrate S2, and the second coupler layer CL2 is disposed between the second substrate S2 and the third substrate S3. Furthermore, an (n-<NUM>)th coupler layer CLn-<NUM> is disposed between an (n-<NUM>)th substrate Sn-<NUM> and the n-th substrate Sn. Accordingly, although the configuration of the first light guide layer <NUM> is the same as that described above, the second light guide layer <NUM>' may include the third substrate S3 and only the second coupler layer CL2 disposed on the upper surface of the third substrate S3, and an n-th light guide layer N' may include the n-th substrate Sn and only an (n-<NUM>)th coupler layer CLn-<NUM> disposed on the upper surface of the n-th substrate Sn. In the light guide structure 110b of <FIG>, the number of the first to n-th substrates S1, S2,. , Sn may be greater by one than the number of the first to (n-<NUM>)th coupler layers CL1, CL2,. , CLn-<NUM>.

Furthermore, to improve the uniformity of illumination light, the sum of the thickness t1 of the first substrate S1and the thickness t2 of the second substrate S2 may be different from the thickness of the third substrate S3. The thickness of the first light guide layer <NUM> may be different from the thickness of the second light guide layer <NUM>'. The thickness of the n-th light guide layer N' may be different from the thickness of the first light guide layer <NUM> or the thickness of the second light guide layer <NUM>'.

As illustrated in <FIG>, when the polymer layer <NUM> does not exist, a lower surface of a substrate disposed above may have a pattern shape complementary to the periodic patterns of the coupler layer disposed under the substrate disposed above the coupler layer. For example, the lower surface of the first substrate S1 may have a complementary pattern to the pattern of the first coupler layer CL1 disposed on the upper surface of the second substrate S2. Furthermore, the lower surface of the second substrate S2 may have a complementary pattern to the pattern of the second coupler layer CL2. In this connection, the coupler layers may be formed on both surfaces of the second to (n-<NUM>)th substrates S2, S3,. , Sn-<NUM>.

However, as illustrated in <FIG>, the polymer layer <NUM> may be further filled between the patterns of the first to (n-<NUM>)th coupler layers CL1, CL2,. , CLn-<NUM>. In this case, the first to n-th substrates S1, S2, S3,. , Sn may have flat lower surfaces, and the first to (n-<NUM>)th coupler layers CL1, CL2,. , CLn-<NUM> may be formed only on the upper surfaces of the second to n-th substrates S2, S3,.

<FIG> is a schematic cross-sectional view of a configuration of a light guide structure 110c of the backlight unit <NUM> according to an example not according to the invention. Referring to <FIG>, the first substrate S1 of the light guide structure 110c may have a thickness smaller than the thicknesses of the other substrates. For example, the first substrate S1 may have a thickness of about <NUM> or less, and substrates other than the first substrate S1 may each have a thickness of about <NUM> to about <NUM>. The first substrate S1 may include silicon oxide (SiO<NUM>) to serve as a protective layer.

<FIG> is a schematic cross-sectional view of a configuration of a light guide structure 110d of the backlight unit <NUM> according to an example not according to the invention. Referring to <FIG>, the light guide structure 110d may include a first light guide layer <NUM>' and the second light guide layer <NUM>'. The first light guide layer <NUM>' may include the first substrate S1 and the first coupler layer CL1 disposed on the upper surface of the first substrate S1. Furthermore, the second light guide layer <NUM>' may include the second substrate S2 and the second coupler layer CL2 disposed on the upper surface of the second substrate S2. The first substrate S1 may be disposed above the second substrate S2 and the lower surface of the first substrate S1 may have a pattern shape complementary to the cyclic pattern of the second coupler layer CL2. Furthermore, the polymer layer <NUM>, as a protective layer and a planarization layer, may be filled between the periodic patterns of the first coupler layer CL1. As illustrated in <FIG>, a coupler layer may be formed on the upper surface of the first substrate S1. In this case, the number of substrates and the number of coupler layers in the light guide structure 100d may be the same.

<FIG> is a schematic plan view of a configuration of the first light guide layer <NUM> of the backlight unit <NUM> according to an embodiment. <FIG> is a schematic plan view of a configuration of the second light guide layer <NUM> of the backlight unit <NUM> according to another example embodiment. In the example embodiments of <FIG>, all of the first to fourth input couplers IC1, IC2, IC3, and IC4 are disposed in the second light guide plate <NUM>. However, embodiments are not limited thereto. For example, as illustrated in <FIG>, the first input coupler IC1 and the third input coupler IC3 may be disposed in the first light guide plate <NUM>, and the second input coupler IC2 and the fourth input coupler IC4 may be disposed in the second light guide plate <NUM>. In other words, the first input coupler IC1 and the third input coupler IC3 may be disposed on the first coupler layer CL1 of the first light guide plate <NUM>, and the second input coupler IC2 and the fourth input coupler IC4 may be disposed on the second coupler layer CL2 of the second light guide plate <NUM>.

<FIG> is a schematic plan view of a configuration of the first light guide layer <NUM> of the backlight unit <NUM> according to an embodiment. <FIG> is a schematic plan view of a configuration of the second light guide layer <NUM> of the backlight unit <NUM> according to an embodiment. <FIG> is a schematic cross-sectional view of a configuration of a light guide structure according to an embodiment, in which the first light guide layer <NUM> of <FIG> and the second light guide layer <NUM> of <FIG> are bonded to each other. In particular, <FIG> is a cross-sectional view of the light guide structure taken along the first expansion coupler MC1 and the second expansion coupler MC2 to reveal the first expansion coupler MC1 and the second expansion coupler MC2.

Referring to <FIG>, an input coupler may not be disposed in the first light guide layer <NUM>, and only the first input coupler IC1 and the second input coupler IC2 may be disposed in the second light guide layer <NUM> at both sides of the second expansion coupler MC2 in the y-axis direction. The third input coupler IC3 and fourth input coupler IC4 may be disposed in a separate light guide layer adjacent to the lower surface of the second light guide layer <NUM>. For example, the third input coupler IC3 may be disposed in a first input light guide layer 1a that is disposed adjacent to the lower surface of the second light guide layer <NUM> to face the first input coupler IC1 in the z-axis direction, and the fourth input coupler IC4 may be disposed in a second input light guide layer 1b that is adjacent to the lower surface of the second light guide layer <NUM> to face the second input coupler IC2 in the z-axis direction as illustrated in <FIG>.

<FIG> is a schematic plan view of a configuration of the first light guide layer <NUM> of the backlight unit <NUM> according to an embodiment. <FIG> is a schematic plan view of a configuration of the second light guide layer <NUM> of the backlight unit <NUM> according to an embodiment. <FIG> is a schematic cross-sectional view of a configuration of a light guide structure according to an embodiment. in which the first light guide layer <NUM> of <FIG> and the second light guide layer <NUM> of <FIG> are bonded to each other. In particular, <FIG> is a cross-sectional view taken along the first expansion coupler MC1 and the second expansion coupler MC2 to reveal the first expansion coupler MC1 and second expansion coupler MC2.

Referring to <FIG>, in the first light guide layer <NUM>, the first input coupler IC1 may be disposed at the left side of the first expansion coupler MC1 in the -y-axis direction, and in the second light guide layer <NUM>, the second input coupler IC2 may be disposed at the right side of the second expansion coupler MC2 in the +y-axis direction. The third input coupler IC3 and fourth input coupler IC4 may be disposed in a separate light guide layer adjacent to the lower surface of the second light guide layer <NUM>. For example, the third input coupler IC3 may be disposed in the first input light guide layer 1a that is disposed adjacent to the lower surface of the second light guide layer <NUM> to face the first input coupler IC1 in the z-axis direction, and the fourth input coupler IC4 may be disposed in the second input light guide layer 1b that is disposed adjacent to the lower surface of the second light guide layer <NUM> to face the second input coupler IC2 in the z-axis direction.

In order for the holographic display apparatus <NUM> to reproduce a color holographic image, the backlight unit <NUM> may provide red illumination light, green illumination light, and blue illumination light to the spatial light modulator <NUM>. To this end, the backlight unit <NUM> may include a plurality of light guide structures for respectively providing red illumination light, green illumination light, and blue illumination light. For example, <FIG> schematically illustrates a configuration of the backlight unit <NUM> according to an example embodiment, in which the backlight unit provides red illumination light, green illumination light, and blue illumination light.

Referring to <FIG>, the backlight unit <NUM> may include a first light guide structure 110R that may provide red illumination light, a second light guide structure <NUM> that may provide green illumination light, and a third light guide structure 110B that may provide blue illumination light. Although <FIG> illustrates that, in the z-axis direction, the second light guide structure <NUM> is disposed above the first light guide structure 110R and the third light guide structure 110B is disposed above the second light guide structure <NUM>, embodiments are not limited thereto and the arrangement order of the first light guide structure 110R, the second light guide structure <NUM>, and the third light guide structure 110B may be selected differently as necessary. The first light source <NUM> and the second light source <NUM> may be disposed above the upper surface of the third light guide structure 110B, respectively at both edges thereof in the y-axis direction. Each of the first light guide structure 110R, the second light guide structure <NUM>, and the third light guide structure 110B may have the same configuration as that of the above-described light guide structure <NUM>.

Furthermore, each of the first light source <NUM> and the second light source <NUM> may include a red light source that may emit red light, a green light source that may emit green light, and a blue light source that may emit blue light. For example, <FIG> schematically illustrates an arrangement of a red light source, a green light source, and a blue light source in the backlight unit <NUM> of <FIG>. In particular, <FIG> illustrates the configuration of the backlight unit <NUM>, viewed from a different direction from that of <FIG>. For example, <FIG> illustrates the configuration of the backlight unit <NUM> viewed from the x-axis direction, and <FIG> illustrates the configuration of the backlight unit <NUM>, viewed from the y-axis direction.

Referring to <FIG>, the second light source <NUM> may include a red light source 122R, a green light source <NUM>, and a blue light source 122B. The red light source 122R, the green light source <NUM>, and the blue light source 122B may be linearly arranged in the x-axis direction above the upper surface of the third light guide structure 110B. At the opposite side of the first light guide structure 110R, the second light guide structure <NUM>, and the third light guide structure 110B viewed in <FIG>, the first light source <NUM> may also include a red light source, a green light source, and a blue light source linearly arranged in the x-axis direction.

The first light guide structure 110R may include a second input coupler IC2R to couple red light and a fourth input coupler IC4R to couple red light. The second light guide structure <NUM> may include a second input coupler IC2G to couple green light and a fourth input coupler IC4G to couple green light. The third light guide structure 110B may include a second input coupler IC2B to couple blue light and a fourth input coupler IC4B to couple blue light. At the opposite side of the first light guide structure 110R, the second light guide structure <NUM>, and the third light guide structure 110B viewed in <FIG>, the first light guide structure 110R may include a first input coupler to couple red light and a third input coupler to couple red light, the second light guide structure <NUM> may include a first input coupler to couple green light and a third input coupler to couple green light, and the third light guide structure 110B may include a first input coupler to couple blue light and a third input coupler to couple blue light.

Claim 1:
A backlight unit (<NUM>) comprising:
a light source (<NUM>, <NUM>) configured to emit light; and
a light guide structure (<NUM>) configured to guide the light emitted from the light source (<NUM>, <NUM>), the light guide structure (<NUM>) comprising:
a first coupler layer (CL1); and
a second coupler layer (CL2) facing the first coupler layer (CL1),
wherein the first coupler layer (CL1) comprises:
a first output coupler (OC1) configured to expand light traveling inside the light guide structure (<NUM>) in a first direction and output the expanded light in the first direction to the outside of the light guide structure (<NUM>); and
a first expansion coupler (MC1) configured to expand the light traveling inside the light guide structure (<NUM>) in a second direction perpendicular to the first direction and provide the expanded light in the second direction to the first output coupler (OC1), and
wherein the second coupler layer (CL2) comprises:
a second output coupler (OC2) disposed to face the first output coupler (OC1) and configured to expand light traveling inside the light guide structure (<NUM>) in the first direction and output the expanded light to the outside of the light guide structure (<NUM>); and
a second expansion coupler (MC2) disposed to face the first expansion coupler (MC1) and configured to expand light traveling inside the light guide structure (<NUM>) in the second direction and provide the expanded light to the second output coupler (OC2),
wherein the light guide structure (<NUM>) further comprises:
a first input coupler (IC1) disposed at a first side of the first and second expansion coupler (MC1, MC2) in the second direction and configured to provide light to the first and second expansion coupler (MC1, MC2); and
a second input coupler (IC2) disposed at a second side of the first and second expansion coupler (MC1, MC2) in the second direction, the second side being opposite the first side, wherein the second input coupler (IC2) is configured to provide light to the first and second expansion coupler (MC1, MC2).