阅读说明：本技术 背光单元以及包括该背光单元的全息显示装置 (Backlight unit and holographic display device including the same ) 是由 崔七星 宋薰 李泓锡 于 2020-04-03 设计创作，主要内容包括：提供一种背光单元和包括该背光单元的全息显示装置,该背光单元包括光源和配置为引导从光源发射的光的导光结构,该导光结构包括第一耦合器层和第二耦合器层,该第一耦合器层包括第一输出耦合器和第一扩展耦合器,该第一输出耦合器配置为在第一方向上扩展光并将已在第一方向上被扩展的光输出到导光结构的外部,该第一扩展耦合器配置为在垂直于第一方向的第二方向上扩展光并将已在第二方向上被扩展的光提供给第一输出耦合器,第二耦合器层包括第二输出耦合器和第二扩展耦合器,该第二输出耦合器配置为在第一方向上扩展光并将已被扩展的光输出到导光结构的外部,该第二扩展耦合器配置为在第二方向上扩展光并将已被扩展的光提供给第二输出耦合器。(There is provided a backlight unit and a holographic display device including the backlight unit, the backlight unit including a light source and a light guide structure configured to guide light emitted from the light source, the light guide structure including a first coupler layer and a second coupler layer, the first coupler layer including a first output coupler configured to expand the light in a first direction and output the light expanded in the first direction to the outside of the light guide structure and a first expansion coupler configured to expand the light in a second direction perpendicular to the first direction and provide the light expanded in the second direction to the first output coupler, the second coupler layer including a second output coupler configured to expand the light in the first direction and output the light expanded in the second direction to the outside of the light guide structure, the second expanding coupler is configured to expand the light in a second direction and provide the expanded light to the second output coupler.)

1. A backlight unit, comprising:

a light source configured to emit light; and

a light guide structure configured to guide light emitted from the light source, the light guide structure including:

a first coupler layer; and

a second coupler layer facing the first coupler layer, wherein the first coupler layer comprises:

a first output coupler configured to expand light propagating inside the light guide structure in a first direction and output the light that has been expanded in the first direction to the outside of the light guide structure; and

a first expanding coupler configured to expand light propagating inside the light guiding structure in a second direction perpendicular to the first direction and to provide the light that has been expanded in the second direction to the first output coupler, an

Wherein the second coupler layer comprises:

a second output coupler configured to expand light propagating inside the light guide structure in the first direction and output the expanded light to the outside of the light guide structure; and

a second expanding coupler configured to expand light propagating inside the light guiding structure in the second direction and provide the expanded light to the second output coupler.

2. The backlight unit of claim 1, wherein the first output coupler faces the second output coupler and the first extension coupler faces the second extension coupler.

3. The backlight unit of claim 2, wherein the first extension coupler is configured to couple a portion of light incident at a first angle and provide the coupled light to the first output coupler, and is configured to transmit light incident at an angle different from the first angle, an

The second expansion coupler is configured to couple a portion of light incident at a second angle different from the first angle and provide the coupled light to a second output coupler, and is configured to transmit light incident at an angle different from the second angle.

4. The backlight unit of claim 3, wherein the first angle and the second angle have the same magnitude and opposite sign relative to a surface normal of the light guiding structure.

5. The backlight unit of claim 3, wherein the first extension coupler is disposed adjacent to a side surface of the first output coupler in the first direction, and

wherein the second extension coupler is disposed adjacent to a side surface of the second output coupler in the first direction.

6. The backlight unit of claim 1, wherein the light guide structure further comprises:

a first input coupler disposed adjacent to a first side surface of the second extension coupler in the second direction and configured to provide light to the first side surface of the second extension coupler; and

a second input coupler disposed adjacent to a second side surface of the second extension coupler in the second direction and configured to provide light to the second side surface of the second extension coupler.

7. The backlight unit of claim 6, wherein the first and second input couplers are disposed in the first coupler layer.

8. The backlight unit of claim 6, wherein the first input coupler is disposed in the first coupler layer and the second input coupler is disposed in the second coupler layer.

9. The backlight unit of claim 6, wherein the light guide structure further comprises:

a third input coupler disposed to face a side surface of the first input coupler in the first direction and configured to provide light to the side surface of the first input coupler; and

a fourth input coupler disposed to face a side surface of the second input coupler in the first direction and configured to provide light to the side surface of the second input coupler.

10. The backlight unit of claim 9, wherein the third and fourth input couplers are disposed in the first coupler layer.

11. The backlight unit of claim 9, wherein the third input coupler is disposed in the first coupler layer and the fourth input coupler is disposed in the second coupler layer.

12. The backlight unit of claim 9, wherein the light source comprises a first light source configured to emit light to the first input coupler or the third input coupler and a second light source configured to emit light to the second input coupler or the fourth input coupler.

13. The backlight unit according to claim 12, wherein a difference between a center wavelength of light emitted from the first light source and a center wavelength of light emitted from the second light source is greater than 0nm and less than or equal to 10 nm.

14. The backlight unit of claim 1, wherein the light guide structure further comprises:

a first substrate disposed over the first coupler layer;

a second substrate disposed below the first coupler layer;

a third substrate disposed above the second coupler layer and below the second substrate; and

a fourth substrate disposed below the second coupler layer.

15. The backlight unit of claim 14, wherein the light guide structure further comprises a semi-transmissive layer disposed between the second substrate and the third substrate and configured to reflect a portion of incident light and transmit the remaining portion of the incident light.

16. The backlight unit according to claim 14, wherein the light guide structure further comprises a reflective plate disposed at a lower surface of the fourth substrate.

17. The backlight unit according to claim 14, wherein a sum of thicknesses of the first substrate and the second substrate is different from a sum of thicknesses of the third substrate and the fourth substrate.

18. The backlight unit according to claim 14, wherein the first substrate has a thickness of 15nm or less and comprises SiO₂。

19. The backlight unit as claimed in claim 14, wherein the first output coupler and the first extension coupler have grating structures in which a plurality of recesses and a plurality of protrusions are periodically disposed, respectively, and

wherein the light guide structure further comprises a polymer filling the plurality of recesses of the grating structure.

20. The backlight unit of claim 1, wherein the light guide structure comprises:

a first substrate disposed over the first coupler layer;

a second substrate disposed below the first coupler layer and above the second coupler layer; and

a third substrate disposed below the second coupler layer.

21. The backlight unit as claimed in claim 20, wherein a sum of a thickness of the first substrate and a thickness of the second substrate is different from a thickness of the third substrate.

22. The backlight unit according to claim 1, wherein the light source comprises:

a first wavelength light source configured to emit light of a first wavelength;

a second wavelength light source configured to emit light of a second wavelength different from the first wavelength; and

a third wavelength light source configured to emit light of a third wavelength different from the first wavelength and the second wavelength, respectively.

23. The backlight unit of claim 22, wherein the light guide structure comprises:

a first light guiding structure configured to guide light of the first wavelength emitted from the first wavelength light source;

a second light guiding structure configured to guide the light of the second wavelength emitted from the second wavelength light source; and

a third light guiding structure configured to guide the light of the third wavelength emitted from the third wavelength light source.

24. A holographic display device, comprising:

a backlight unit configured to provide collimated illumination light; and

a spatial light modulator configured to generate a holographic image by modulating the collimated illumination light received from the backlight unit,

wherein the backlight unit includes:

a light source configured to emit light; and

a light guide structure configured to guide light emitted from the light source,

wherein the light guide structure includes:

a first coupler layer; and

a second coupler layer facing the first coupler layer,

wherein the first coupler layer comprises:

a first output coupler configured to expand light propagating inside the light guide structure in a first direction and output the light that has been expanded in the first direction to the outside of the light guide structure; and

a first expanding coupler configured to expand light propagating inside the light guiding structure in a second direction perpendicular to the first direction and to provide the light that has been expanded in the second direction to the first output coupler, an

Wherein the second coupler layer comprises:

a second output coupler configured to expand light propagating inside the light guide structure in the first direction and output the light that has been expanded in the first direction to the outside of the light guide structure; and

a second expanding coupler configured to expand light propagating inside the light guiding structure in the second direction and provide the light that has been expanded in the second direction to the second output coupler.

25. A backlight unit, comprising:

a light source configured to emit light; and

a light guide structure configured to guide light emitted from the light source,

wherein the light source comprises:

a first light source configured to emit first light and provided on a first edge of an upper surface of the light guide structure; and

a second light source configured to emit second light and provided on a second edge of the upper surface of the light guide structure opposite the first edge, wherein the light guide structure comprises:

a first grating layer comprising:

a first output grating configured to expand the first light propagating inside the light guide structure in a first direction and output the first light that has been expanded in the first direction to the outside of the light guide structure; and

a first expansion grating configured to expand the first light propagating inside the light guiding structure in a second direction perpendicular to the first direction and to provide the first light that has been expanded in the second direction to the first output grating, an

A second grating layer comprising:

a second output grating configured to expand the second light propagating inside the light guide structure in the first direction and output the second light expanded in the first direction to the outside of the light guide structure; and

a second expansion grating configured to expand the second light propagating inside the light guide structure in the second direction and provide the second light that has been expanded in the second direction to the second output grating, the second grating layer being provided on a lower surface of the first grating layer.

26. The backlight unit of claim 25, wherein the light guide structure further comprises:

the first substrate is arranged on the first grating layer;

the second substrate is arranged below the first grating layer;

a third substrate disposed above the second grating layer and below the second substrate; and

and the fourth substrate is arranged below the second grating layer.

27. The backlight unit as claimed in claim 26, wherein a sum of thicknesses of the first and second substrates is different from a sum of thicknesses of the third and fourth substrates.

Technical Field

Example embodiments of the present disclosure relate to a backlight unit and a holographic display device including the same, and more particularly, to a backlight unit capable of providing uniform illumination light and a holographic display device including the same.

Background

The glasses method and the non-glasses method have been widely commercialized and used to implement 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 lens method and a parallax barrier method. These methods use binocular parallax of both eyes, but have a limitation in increasing the number of viewpoints, and may cause a feeling of fatigue to a viewer due to a sense of depth recognized by the brain not matching the focuses of the eyes.

A hologram display method is increasingly used as a three-dimensional image display method in which a sense of depth recognized by the brain and the focus of the eye are matched with each other and full parallax can be provided. The hologram display method uses a principle of reproducing an image of an original object by irradiating and diffracting reference light to a hologram pattern that records interference fringes obtained by allowing object light and reference light reflected from the original object to interfere with each other. The currently used holographic display method provides a Computer Generated Hologram (CGH) to a spatial light modulator, not a pattern obtained by directly exposing an original object, as an electrical signal. When the spatial light modulator forms a hologram pattern to diffract the reference light according to the input CGH signal, a three-dimensional image may be generated.

The holographic display may comprise a backlight unit for providing illumination light to the spatial light modulator. A backlight unit used in a holographic display device provides collimated illumination light having coherence and provides the collimated illumination light to a spatial light modulator. Collimated coherent illumination light provided by the backlight unit may be diffracted by the spatial light modulator to form a holographic image.

Disclosure of Invention

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

According to an aspect of an exemplary embodiment, there is provided a backlight unit including: a light source configured to emit light and a light guide structure configured to guide the light emitted from the light source, the light guide structure including a first coupler layer and a second coupler layer facing the first coupler layer, wherein the first coupler layer includes a first output coupler configured to expand the light propagating inside the light guide structure in a first direction and output the light expanded in the first direction to the outside of the light guide structure, and a first expansion coupler configured to expand the light propagating inside the light guide structure in a second direction perpendicular to the first direction and supply the light expanded in the second direction to the first output coupler, and wherein the second coupler layer includes a second output coupler configured to expand the light propagating inside the light guide structure in the first direction and output the expanded light to the outside of the light guide structure, the second expanding coupler is configured to expand light propagating inside the light guiding structure in a second direction and provide the expanded light to the second output coupler.

The first output coupler may face the second output coupler, and the first extension coupler faces the second extension coupler.

The first extension coupler may be configured to couple light incident at a first angle and provide the coupled light to the first output coupler and transmit light incident at an angle different from the first angle, and the second extension coupler may be configured to couple light incident at a second angle different from the first angle and provide the coupled light to the second output coupler and transmit light incident at an angle different from the second angle.

The first angle and the second angle may have the same magnitude and opposite signs with respect to a surface normal of the light guiding structure.

The first extension coupler may be disposed adjacent to a side surface of the first output coupler in the first direction, and the second extension coupler may be disposed adjacent to a side surface of the second output coupler in the first direction.

The light guide structure may further include: a first input coupler disposed adjacent to a first side surface of the second extension coupler in the second direction and configured to provide light to the first side surface of the second extension coupler; and a second input coupler disposed adjacent to a second side surface of the second extension coupler in the second direction and configured to provide light to the second side surface of the second extension coupler.

The first input coupler and the second input coupler may be disposed in a first coupler layer.

The first input coupler may be disposed in a first coupler layer and the second input coupler may be disposed in a second coupler layer.

The light guide structure may further include: a third input coupler disposed to face a side surface of the first input coupler in the first direction and configured to provide light to the side surface of the first input coupler; and a fourth input coupler disposed to face a side surface of the second input coupler in the first direction and configured to provide light to the side surface of the second input coupler.

The third input coupler and the fourth input coupler may be disposed in the first coupler layer.

The third input coupler may be disposed in the first coupler layer and the fourth input coupler may be disposed in the second coupler layer.

The light source may include a first light source configured to emit light to the first input coupler or the third input coupler and a second light source configured to emit light to the second input coupler or the fourth input coupler.

The difference between the center wavelength of light emitted from the first light source and the center wavelength of light emitted from the second light source may be greater than 0nm and less than or equal to 10 nm.

The light guide structure may further include: a first substrate disposed over the first coupler layer; a second substrate disposed below the first coupler layer; a third substrate disposed above the second coupler layer and below the second substrate; and a fourth substrate disposed below the second coupler layer.

The light guide structure may further include a semi-transmissive layer disposed between the second substrate and the third substrate and configured to reflect a portion of incident light and transmit the remaining portion of the incident light.

The light guide structure may further include a reflective plate disposed at a lower surface of the fourth substrate.

The sum of the thicknesses of the first substrate and the second substrate may be different from the sum of the thicknesses of the third substrate and the fourth substrate.

The first substrate may have a thickness of about 15nm or less and include SiO₂。

The first output coupler and the first extension coupler may respectively have a grating structure in which a plurality of grooves and a plurality of protrusions are periodically disposed, and the light guide structure may further include a polymer filling the plurality of grooves of the grating structure.

The light guide structure may include: a first substrate disposed over the first coupler layer; a second substrate disposed below the first coupler layer and above the second coupler layer; and a third substrate disposed below the second coupler layer.

The sum of the thicknesses of the first substrate and the second substrate may be different from the thickness of the third substrate.

The light source may include a first wavelength light source configured to emit light of a first wavelength, a second wavelength light source configured to emit light of a second wavelength different from the first wavelength, and a third wavelength light source configured to emit light of a third wavelength different from the first and second wavelengths, respectively.

The light guide structure may include: a first light guide structure configured to guide light of a first wavelength emitted from a first wavelength light source; a second light guiding structure configured to guide light of a second wavelength emitted from the second wavelength light source; and a third light guiding structure configured to guide light of a third wavelength emitted from the third wavelength light source.

According to another aspect of an example embodiment, there is provided a holographic display device including: a backlight unit configured to provide collimated illumination light; and a spatial light modulator configured to generate a holographic image by modulating collimated illumination light received from the backlight unit, wherein the backlight unit includes a light source configured to emit light and a light guide structure configured to guide the light emitted from the light source, wherein the light guide structure includes a first coupler layer and a second coupler layer facing the first coupler layer, wherein the first coupler layer includes a first output coupler configured to expand light propagating inside the light guide structure in a first direction and output the light expanded in the first direction to an outside of the light guide structure and a first expansion coupler configured to expand light propagating inside the light guide structure in a second direction perpendicular to the first direction and supply the light expanded in the second direction to the first output coupler, and wherein the second coupler layer includes a second output coupler and a second expansion coupler, the second output coupler is configured to expand light propagating inside the light guide structure in a first direction and output the expanded light to the outside of the light guide structure, and the second expansion coupler is configured to expand light propagating inside the light guide structure in a second direction and provide the expanded light to the second output coupler.

According to another aspect of an example embodiment, there is provided a backlight unit including: a light source configured to emit light; and a light guide structure configured to guide light emitted from the light source, wherein the light source includes: a first light source configured to emit first light and provided on a first edge of an upper surface of the light guide structure; and a second light source configured to emit second light and provided on a second edge opposite to the first edge of the upper surface of the light guide structure, wherein the light guide structure includes a first grating layer and a second grating layer, the first grating layer including a first output grating configured to expand the first light propagating inside the light guide structure in a first direction and output the first light having been expanded in the first direction to the outside of the light guide structure, and a first expansion grating configured to expand the first light propagating inside the light guide structure in a second direction perpendicular to the first direction and provide the first light having been expanded in the second direction to the first output grating, the second grating layer including a second output grating configured to expand the second light propagating inside the light guide structure in the first direction and output the second light having been expanded in the first direction to the outside of the light guide structure, the second expansion grating is configured to expand second light propagating inside the light guide structure in a second direction and to provide the second light that has been expanded in the second direction to a second output grating, the second grating layer being provided on a lower surface of the first grating layer.

The light guide structure may further include: the first substrate is arranged on the first grating layer; the second substrate is arranged below the first grating layer; a third substrate disposed above the second grating layer and below the second substrate; and a fourth substrate disposed below the second grating layer.

The sum of the thicknesses of the first substrate and the second substrate may be different from the sum of the thicknesses of the third substrate and the fourth substrate.

Drawings

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

FIG. 1 is a schematic cross-sectional view of a holographic display according to an example embodiment;

fig. 2 is a schematic perspective view of a configuration of a first light guide layer of a backlight unit according to an example embodiment;

FIG. 3 is a schematic plan view of the configuration of the first light guiding layer of FIG. 2;

fig. 4 is a schematic perspective view of a configuration of a second light guide layer of a backlight unit according to an example embodiment;

FIG. 5 is a schematic plan view of the configuration of the second light guiding layer of FIG. 4;

FIG. 6 is a schematic perspective view of a configuration of an entire light guiding structure by assembling the first light guiding layer of FIG. 2 and the second light guiding layer of FIG. 4;

FIG. 7 is a schematic cross-sectional view taken along line A-A' in the light guiding structure shown in FIG. 6, illustrating light propagation and coupling operations;

FIG. 8 is a graph of individual output intensity and overall output intensity of light propagating in opposite directions in the light guiding structure shown in FIG. 7;

FIG. 9 is a graph of a wavelength distribution of light emitted from a light source;

fig. 10 is a schematic cross-sectional view of a configuration of a light guide structure of a backlight unit according to another exemplary embodiment;

fig. 11 is a schematic cross-sectional view of a configuration of a first light guide layer of a backlight unit according to another example embodiment;

fig. 12 is a schematic cross-sectional view of a configuration of a light guide structure of a backlight unit according to another exemplary embodiment;

fig. 13 is a schematic cross-sectional view of a configuration of a light guide structure of a backlight unit according to another exemplary embodiment;

fig. 14 is a schematic cross-sectional view of a configuration of a light guide structure of a backlight unit according to another exemplary embodiment;

fig. 15 is a schematic plan view of a configuration of a first light guide layer of a backlight unit according to another example embodiment;

fig. 16 is a schematic plan view of a configuration of a second light guide layer of a backlight unit according to another example embodiment;

fig. 17 is a schematic plan view of a configuration of a first light guide layer of a backlight unit according to another example embodiment;

fig. 18 is a schematic plan view of a configuration of a second light guide layer of a backlight unit according to another example embodiment;

FIG. 19 is a schematic cross-sectional view of a configuration of a light guiding structure according to an example embodiment, wherein the first light guiding layer of FIG. 17 and the second light guiding layer of FIG. 18 are bonded to each other;

fig. 20 is a schematic plan view of a configuration of a first light guide layer of a backlight unit according to another example embodiment;

fig. 21 is a schematic plan view of a configuration of a second light guide layer of a backlight unit according to another example embodiment;

fig. 22 is a schematic cross-sectional view of a configuration of a light guiding structure according to an example embodiment, wherein the first light guiding layer of fig. 20 and the second light guiding layer of fig. 21 are bonded to each other;

fig. 23 schematically illustrates a configuration of a backlight unit according to an example embodiment, in which the backlight unit supplies red illumination light, green illumination light, and blue illumination light; and

fig. 24 schematically illustrates an arrangement of red, green, and blue light sources in the backlight unit of fig. 23.

Detailed Description

Reference will now be made in detail to example embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. In this regard, the example embodiments may have different forms and should not be construed as limited to the descriptions set forth herein. Accordingly, only example embodiments are described below to illustrate aspects by referring to the drawings. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items. Expressions such as "at least one of … …" when followed by a list of elements modify the entire list of elements rather than individual elements within 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.

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

Fig. 1 is a schematic cross-sectional view of a holographic display 200 according to an example embodiment. Referring to fig. 1, the holographic display device 200 may include: a backlight unit 100 that can provide collimated coherent illumination light; a spatial light modulator 210 that can reproduce a hologram image by modulating the illumination light; and a fourier lens 220 that can focus the holographic image in a space. Although fig. 1 illustrates that the spatial light modulator 210 is disposed between the fourier lens 220 and the backlight unit 100, the embodiment is not limited thereto. For example, the fourier lens 220 may be disposed between the spatial light modulator 210 and the backlight unit 100.

The spatial light modulator 210 may form a holographic pattern from a hologram data signal (e.g., a Computer Generated Hologram (CGH) data signal) provided by an image processor to diffract and modulate the illumination light. To this end, the spatial light modulator 210 may include a plurality of display pixels arranged in two dimensions (2D). Further, any one of a phase modulator for performing only phase modulation, an amplitude modulator for performing only amplitude modulation, and a complex modulator for performing both phase modulation and amplitude modulation may be used as the spatial light modulator 210. Although fig. 1 shows that the spatial light modulator 210 is a transmissive spatial light modulator, a reflective spatial light modulator may be used therefor. For the transmissive type spatial light modulator, a semiconductor modulator based on a compound semiconductor such as gallium arsenide (GaAs) or a liquid crystal modulator may be used as the spatial light modulator 210. 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 210.

The backlight unit 100 may provide collimated coherent illumination light to the spatial light modulator 210. The backlight unit 100 may include light sources 121 and 122 that may emit coherent light, and a light guide structure 110 for expanding and collimating a portion of the light emitted from the light sources 121 and 122 to correspond to the size of the spatial light modulator 210.

The light sources 121 and 122 may provide light traveling in opposite directions in the light guide structure 110. To this end, the light sources 121 and 122 may include a first light source 121 disposed over one side edge of the upper surface of the light guide structure 110 and a second light source 122 disposed at an opposite side edge of the upper surface of the light guide structure 110. The light emitted from the first light source 121 and the light emitted from the second light source 122 may travel in opposite directions in the light guide structure 110. In order to provide light with relatively high coherence, the first light source 121 and the second light source 122 may comprise, for example, laser diodes. In addition to the laser diode, any light source capable of emitting light with spatial coherence may be used therefor. Although fig. 1 shows that each of the first and second light sources 121 and 122 is provided as a single light source for convenience of explanation, each of the first and second light sources 121 and 122 may include an array of a plurality of light sources.

In addition, the holographic display device 200 may further include a 2D backlight unit 101 for providing illumination light for the 2D image. The illumination light for the 2D image provided by the 2D backlight unit 101 may not have coherence nor need to be collimated. The 2D backlight unit 101 may include, for example, a Light Emitting Diode (LED) as a light source, and may provide light emitted from the LED light source to the spatial light modulator 210 after expanding the light with a light guide plate. When the holographic display device 200 reproduces a holographic image, the backlight unit 100 may be turned on and the 2D backlight unit 101 may be turned off, and when the holographic display device 200 reproduces a general 2D image, the backlight unit 100 may be turned off and the 2D backlight unit 101 may be turned on.

The holographic display device 200 may further include an image processor that may generate a hologram data signal according to a hologram image to be provided to a viewer and provide the generated hologram data signal to the spatial light modulator 210, and may control operations of the backlight unit 100 and the 2D backlight unit 101. Further, the holographic display device 200 may further include: an eye tracker that can track the position of the pupil of the viewer in real time; and a beam deflector that can adjust the position of the holographic image focused by the fourier lens 220 based on the position of the pupil of the viewer provided by the eye tracker.

According to example embodiments, the light guide structure 110 may form the illumination light by uniformly collimating the light emitted from the first and second light sources 121 and 122. To this end, the light guide structure 110 may include at least two light guide layers stacked in a thickness direction, and at least two coupler layers in different light guide layers are respectively disposed to face each other. For example, in fig. 1, light guiding structure 110 is shown to include a first light guiding layer 10 having a first coupler layer CL1 disposed therein and a second light guiding layer 20 having a second coupler layer CL2 disposed therein. In the above structure, light emitted from the first light source 121 may be coupled through the first coupler layer CL1, and light emitted from the second light source 122 may be coupled through the second coupler layer CL 2. Each of the first and second coupler layers CL1 and CL2 may include a plurality of couplers that may guide incident light to the inside of the light guide structure 110, expand light propagating along the inside of the light guide structure 110 in two directions perpendicular to each other, and output the light to the outside of the light guide structure 110.

For example, fig. 2 is a schematic perspective view of a configuration of a first light guide layer 10 of a backlight unit 100 according to an example embodiment. Fig. 3 is a schematic plan view of the configuration of the first light guide layer 10 of fig. 2. Referring to fig. 2 and 3, the first light guide layer 10 may include a first substrate S1 and a second substrate S2 stacked in a thickness direction (i.e., a + z-axis direction), with a first coupler layer CL1 disposed between the first substrate S1 and the second substrate S2. The first and second substrates S1 and S2 may include a material, such as glass or polymer, that transmits light including infrared light, visible light, or ultraviolet light. Further, the first coupler layer CL1 may include a first output coupler OC1 and a first extension coupler MC1 disposed adjacent to each other.

The first output coupler OC1 spreads the light propagating along the inside of the light guiding structure 110 in the x-axis direction. Further, the first output coupler OC1 may output light propagating along the inside of the light guide structure 110 to provide illumination light to the spatial light modulator 210. For this, the first output coupler OC1 may be disposed to face the spatial light modulator 210 of the holographic display device 200, and the width W in the y-axis direction and the length L1 in the x-axis direction of the first output coupler OC1 may be similar to the width and length of the spatial light modulator 210.

The first expansion coupler MC1 is disposed at one side surface of the first output coupler OC1 in the x-axis direction. The first expanding coupler MC1 may expand light propagating along the inside of the light guiding structure 110 in the + y-axis direction and provide the expanded light to the first output coupler OC 1. For this, as shown in fig. 3, the width W of the first extension coupler MC1 in the y-axis direction may be the same as the width W of the first output coupler OC1 in the y-axis direction. The length L2 of the first extension coupler MC1 in the x-axis direction may be less than the length L1 of the first output coupler OC1 in the x-axis direction.

Further, fig. 4 is a schematic perspective view of the configuration of the second light guide layer 20 of the backlight unit 100 according to an example embodiment. Fig. 5 is a schematic plan view of the configuration of the second light guide layer 20 of fig. 4. Referring to fig. 4 and 5, the second light guide layer 20 may include third and fourth substrates S3 and S4 stacked in a thickness direction (i.e., a z-axis direction) and a second coupler layer CL2 disposed between the third and fourth substrates S3 and S4. The third and fourth substrates S3 and S4 may include a material, such as glass or polymer, that transmits light including infrared light, visible light, or ultraviolet light. Further, the second coupler layer CL2 may include 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 on 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 can expand the light propagating along the inside of the light guiding structure 110 in the x-axis direction. Further, the second output coupler OC2 may output light propagating along the inside of the light guide structure 110 to provide illumination light to the spatial light modulator 210. 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 expanding coupler MC2 may expand the light propagating along the inside of the light guiding structure 110 in the y-axis direction and provide the expanded light to the second output coupler OC 2. For this reason, the y-axis direction width of the second extension 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 extension coupler MC2 may be smaller than the x-axis direction length of the second output coupler OC 2. The first extension coupler MC1 and the second extension 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 121 in the z-axis direction and may guide light emitted from the first light source 121 to the inside of the light guide structure 110. Light guided by the third input-coupler IC3 to the interior of the light guiding structure 110 may propagate in the + x-axis direction and be incident on the first input-coupler IC 1. The first input coupler IC1 may slightly spread incident light in the x-axis direction and change the propagation direction of the incident light to the + y-axis direction. Light having a direction changed by the first input coupler IC1 may propagate in the light guide structure 110 in the + y-axis direction and be supplied to the first and second extension couplers MC1 and MC 2.

Further, the fourth input coupler IC4 is disposed to face the second light source 122 in the z-axis direction, and may guide light emitted from the second light source 122 to the inside of the light guide structure 110. Light guided by the fourth input coupler IC4 to the interior of the light guiding structure 110 may propagate in the + x-axis direction and be incident on the second input coupler IC 2. The second input coupler IC2 may slightly spread incident light in the x-axis direction and change the propagation direction of the incident light to the-y-axis direction. Then, light having a direction changed by the second input coupler IC2 may propagate in the light guide structure 110 in the-y-axis direction and be supplied to the first and second extension couplers MC1 and MC 2. Accordingly, light emitted from the first light source 121 may propagate in the + y-axis direction and be provided to the first and second extension couplers MC1 and MC2, and light emitted from the second light source 122 may propagate in the-y-axis direction opposite to the propagation direction of the light emitted from the first light source 121 and be provided to the first and second extension couplers MC1 and MC 2.

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 as various types of surface gratings or volume gratings (volume gratings). The surface grating, which is a grating formed directly on the surface of the substrate, may include a Diffractive Optical Element (DOE), such as a binary phase grating or a blazed grating. The multiple grating patterns of the DOE may function as diffraction gratings and diffract incident light. For example, the surface grating may diffract light incident within a specific angle range according to the size, height, period, duty cycle, or shape of the grating pattern, causing destructive interference and constructive interference, thereby changing the propagation direction of the light. The volume grating may be formed separately from the substrate and may comprise, 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 comprise a periodic fine pattern of materials having different refractive indices. Specifically, the third and fourth input coupler ICs 3 and 4 may use gratings having relatively high directivity and efficiency, such as blazed gratings or bulk gratings, so that incident light is transmitted to the first and second input coupler ICs 1 and 2 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 light emitted from the first light source 121 to the inside of the light guide structure 110, and the second input coupler IC2 may guide light emitted from the second light source 122 to the inside of the light guide structure 110. To this end, the first light source 121 may be disposed to face the first input coupler IC1, and the second light source 122 may be disposed to face the second input coupler IC 2.

Fig. 6 is a schematic perspective view of a configuration of a light guide structure 110 according to an example embodiment, in which the first light guide layer 10 of fig. 2 and 3 and the second light guide layer 20 of fig. 4 and 5 are bonded to each other. Referring to fig. 6, the light guide structure 110 has a structure in which the first light guide layer 10 is stacked on the second light guide layer 20. Thus, the first output coupler OC1 in the first light guiding layer 10 is disposed facing the second output coupler OC2 in the second light guiding layer 20, and the first expanding coupler MC1 in the first light guiding layer 10 is disposed facing the second expanding coupler MC2 in the second light guiding layer 20.

Fig. 7 is a schematic cross-sectional view taken along line a-a' in the light guiding structure 110 of fig. 6, illustrating light propagation and coupling operations. Fig. 7 is a schematic cross-sectional view showing light propagation and coupling operations in the light guiding structure 110 including the first and second extension couplers MC1 and MC 2. Referring to fig. 7, the light guide structure 110 may include a fourth substrate S4, a third substrate S3 stacked on the fourth substrate S4, a second substrate S2 stacked on the third substrate S3, and a first substrate S1 stacked on the second substrate S2. The first extension coupler MC1 is disposed between the first substrate S1 and the second substrate S2. In contrast, the second extension 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, light incident on the third input-coupler IC3 from the first light source 121 propagates along the inside of the light guide structure 110 and is incident on the first input-coupler IC 1. The propagation direction of the light is changed by about 90 ° by the first input coupler IC1 and propagates in the + y-axis direction along the inside of the light guide structure 110. As shown in fig. 7, light may propagate in the + y-axis direction in the light guide structure 110 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 110.

Further, light incident on the fourth input coupler IC4 from the second light source 122 propagates along the inside of the light guiding structure 110 and is incident on the second input coupler IC 2. The propagation direction of the light is changed by about 90 ° by the second input coupler IC2 and propagates in the-y-axis direction along the inside of the light guide structure 110. As shown in fig. 7, light may propagate inside the light guide structure 110 in the-y-axis direction 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 110. Accordingly, light emitted from the first light source 121 and light emitted from the second light source 122 propagate in opposite directions in the light guide structure 110 including the first and second extension couplers MC1 and MC 2.

While propagating within the light guiding structure 110, light is repeatedly incident on the first and second extension couplers MC1 and MC 2. The first and second extension couplers MC1 and MC2 may perform coupling only on light incident in a specific direction. In other words, the first expanding coupler MC1 may couple a portion of light incident at a first angle such that it is transmitted to the first output coupler OC1, and may transmit light incident at an angle different from the first angle. Further, the second expanding coupler MC2 may couple a portion of light incident at a second angle different from the first angle to be transmitted to the second output coupler OC2, and may transmit light incident at an angle different from the second angle. In this state, the first angle and the second angle may have the same magnitude but opposite signs with respect to the surface normal of the light guiding structure 110.

For example, referring to fig. 7, the first extension coupler MC1 may couple a portion of light propagating in the + y-axis direction obliquely downward from the first substrate S1 to the fourth substrate S4. Accordingly, the first extension coupler MC1 may couple only light emitted from the first light source 121. Although fig. 7 shows that the light coupled by the first extension coupler MC1 exits the light guide structure 110 in the vertical direction for convenience of explanation, in reality, the light does not exit the light guide structure 110 but propagates in the light guide structure 110 in the-x-axis direction.

Further, the second extension coupler MC2 may couple part of light input in the-y-axis direction obliquely downward from the first substrate S1 to the fourth substrate S4. Accordingly, the second extension coupler MC2 may couple only light emitted from the second light source 122. Although fig. 7 shows that the light coupled by the second extension coupler MC2 exits the light guide structure 110 in the vertical direction for convenience of explanation, in reality, the light does not exit the light guide structure 110 but propagates in the light guide structure 110 in the-x-axis direction. As a result, light expanded in the y-axis direction by the first expansion coupler MC1 may be provided to the first output coupler OC1, and light expanded in the y-axis direction by the second expansion coupler MC2 may be provided to the second output coupler OC 2.

The light coupled by the first and second extension couplers MC1 and MC2 in the above method propagates in the light guiding structure 110 in the-x-axis direction. When propagating in the-x-axis direction, as illustrated in fig. 7, 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 110 to be repeatedly incident on the first and second output couplers OC1 and OC 2. A part of the light incident on the first and second output couplers OC1 and OC2 is coupled by the first and second output couplers OC1 and OC2 and output in the + z-axis direction through the upper surface of the first substrate S1 of the light guide structure 110. In this process, the light may be spread in the x-axis direction by the first output coupler OC1 and the second output coupler OC 2. The light output from the light guiding structure 110 may then be incident on the spatial light modulator 210 as collimated illumination light.

Fig. 8 is a graph of individual output intensity and overall output intensity of light traveling in opposite directions in the light guide structure 110 of fig. 7, in which the vertical axis represents light intensity and the horizontal axis represents distance from the left side of the light guide structure 110 in the y-axis direction. In fig. 8, a curve a shows a relationship between the intensity of light emitted from the first light source 121 and coupled by the first extension coupler MC1 and the coupling position of the first extension coupler MC1, and a curve B shows a relationship between the intensity of light emitted from the second light source 122 and output by the second extension coupler MC2 and the output position of the second extension coupler MC 2. As shown in fig. 8, the intensity of light output by the first extension coupler MC1 gradually decreases in the + y-axis direction, and the intensity of light output by the second extension coupler MC2 gradually decreases in the-y-axis direction. As a result, the sum (a + B) of the intensities of the light output by the first and second extension couplers MC1 and MC2 is maintained at a relatively uniform level. Therefore, the intensity distribution of the light finally output from the light guiding structure 110 through the first and second output couplers OC1 and OC2 may be maintained uniform.

Referring back to fig. 7, in order to further maintain uniformity of the sum of the intensities of the light output by the first and second extension couplers MCl and MC2, the thickness t1 of the first light guide layer 10 and the thickness t2 of the second light guide layer 20 of the light guide structure 110 may be selected to be different from each other. In other words, the thickness t1, which is the sum of the thicknesses of the first and second substrates S1 and S2, may be different from the thickness t2, which is the sum of the thicknesses of the third and fourth substrates S3 and S4. Accordingly, compared to the case where the thickness t1 of the first light guide layer 10 and the thickness t2 of the second light guide layer 20 are the same, the regularity of the position where light propagating in the + y-axis direction is incident on the first extended coupler MC1 and the regularity of the position where light propagating in the-y-axis direction is incident on the second extended coupler MC2 may be reduced, and thus the light may be distributed more irregularly or more uniformly.

When the first and second light sources 121 and 122 use laser diodes that emit light having a single wavelength, speckle noise may occur in the illumination light due to interference of laser beams. Therefore, in order to reduce speckle noise, the first and second light sources 121 and 122 may use laser diodes that emit light having multimodal wavelength distributions. For example, fig. 9 is a graph of a wavelength distribution of light emitted from one light source. As shown in fig. 9, when the light emitted from the first light source 121 and the second light source 122 has a multimodal wavelength distribution in a narrow wavelength range of about 20nm or less, speckle noise can be reduced. Further, the center wavelength of the first light source 121 and the center wavelength of the second light source 122 may be slightly different from each other. For example, the difference between the center wavelength of light emitted from the first light source 121 and the center wavelength of light emitted from the second light source 122 may be greater than 0nm and equal to or less than 10 nm. Thus, the occurrence of speckle noise in the illumination light provided by the spatial light modulator 210 can be further reduced.

As described above, since one illumination light is generated by coupling lights propagating in opposite directions, the backlight unit 100 according to example embodiments may provide uniform illumination light. Accordingly, it is possible to reduce or limit a stripe pattern in the illumination light, which is formed as a bright pattern and a dark pattern that are repeatedly distributed when only one light propagating in one direction is coupled in the light guide structure 110. Further, since the backlight unit 100 according to the example embodiment provides illumination light with little speckle noise, the quality of a hologram image generated by the hologram display device 200 including the backlight unit 100 according to the example embodiment may be improved.

In addition, the backlight unit 100 according to example embodiments may uniformly provide collimated coherent illumination light to a relatively large area by using the light guide structure 110, and may be manufactured to be relatively thin. Accordingly, the holographic display device 200 including the backlight unit 100 according to example embodiments may be manufactured to be relatively thin. The holographic display device 200 may be applied to various fields, such as a three-dimensional (3D) mobile device, a 3D tablet, or a 3D Television (TV).

The light guide structure 110 is described above as having two light guide layers, i.e., a first light guide layer 10 and a second light guide layer 20. However, embodiments are not limited thereto, and the light guide structure 110 may include one or more light guide layers. For example, fig. 10 is a schematic cross-sectional view of a configuration of a light guide structure 110a of a backlight unit 100 according to another example embodiment. Referring to fig. 10, the light guide structure 110a may include N light guide layers 10, 20, … …, N. Here, n is a natural number greater than 2. The first light guide layer 10 may include a first substrate S1, a first coupler layer CL1, and a second substrate S2, and the second light guide layer 20 may include a third substrate S3, a second coupler layer CL2, and a fourth substrate S4. The nth light guiding layer N may include a (2N-1) th substrate S (2N-1), an nth coupler layer CLn, and a 2 nth substrate S2N. Layers from the nth light guide layer N to the first light guide layer 10 may be sequentially stacked. The first to 2 n-th substrates S1, S2, … …, S (2n-1) and S2n may include a material transmitting light including infrared light, visible light or ultraviolet light, such as glass or polymer.

Thus, the light guiding structure 110a may comprise n coupler layers CL1, CL2, … …, CLn. One output coupler and one extension coupler may be provided in each of the first to nth coupler layers CL1, CL2, … …, CLn. Thus, the light guiding structure 110a may comprise n output couplers and n extension couplers. The n 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 expansion couplers are disposed to face each other in the first to n-th coupler layers CL1, CL2, … …, CLn different from each other. The first to fourth input coupler ICs 1, IC2, IC3, and IC4 may be disposed on only one coupler layer among the first to nth coupler layers CL1, CL2, … …, CLn.

The light propagating within the light guide structure 110a may be totally reflected from the upper surface of the first substrate Sl and the lower surface of the 2 n-th substrate S2 n. While propagating within the light guiding structure 110, light may be coupled through the n extended couplers and then out-coupled to the outside of the light guiding structure 110a through the n output couplers. In order to keep the intensity distribution of the outcoupled light uniform, the thicknesses of the first to nth light guiding layers 10, 20, … …, N may be different from each other.

In addition, in order to bond the first to nth light guide layers 10, 20, … …, N to each other, the bonding layer 15 may be further provided between two adjacent light guide layers among the first to nth light guide layers 10, 20, … …, N. For example, the bonding layer 15 may be further disposed between the second substrate S2 of the first light guide layer 10 and the third substrate S3 of the second light guide layer 20. In order to keep the intensity distribution of the outcoupled light uniform, the bonding layer 15 may include a semi-transmissive layer that reflects a part of incident light and transmits the other part of the incident light. For example, the bonding layer 15 may reflect 10% to 90% of incident light and transmit 90% to 10% thereof. Then, part of the light incident on the interface between the second substrate S2 and the third substrate S3 from the second substrate S2 may be reflected from the bonding layer 15 to propagate back to the second substrate S2, while the other part of the light may be transmitted by the bonding layer 15 to continue to propagate toward the third substrate S3. The bonding layer 15 may include, for example, a resin material having a refractive index different from those of the first to 2 n-th substrates S1, S2, … …, S (2n-1), and S2 n. Further, the bonding layer 15 may include a dichroic coating layer that transmits a portion of light incident at a preset specific angle and reflects other portions of light incident at an angle different from the preset specific angle, instead of the resin material. However, the embodiment is not limited thereto. For example, the dichroic coating may transmit all light incident at an angle different from a preset specific angle.

In addition, the reflective plate 11 may be further disposed at the lowermost surface of the light guide structure 110 a. For example, the reflection plate 11 may be disposed at the lower surface of the 2 n-th substrate S2 n. The reflection plate 11 may reflect light transmitted to the outside by the 2 n-th substrate without being totally reflected from the lower surface of the 2 n-th substrate S2n, from among light obliquely incident on the lower surface of the 2 n-th substrate S2n, so as to be obliquely reflected to the inside of the 2 n-th substrate S2 n. By reducing the loss of light by using the reflection plate 11, the light utilization efficiency of the backlight unit 100 can be improved.

Fig. 11 is a schematic cross-sectional view of a configuration of a first light guide layer 10 of a backlight unit 100 according to another example embodiment. The first to nth coupler layers CL1, CL2, … …, CLn of fig. 10 may include, for example, a bulk grating, and may be manufactured separately from the first to 2 nth substrates S1, S2, … …, S2n-1, and S2 n. Each of the first to nth light guide layers 10, 20, … …, N in the light guide structure 110a may be fabricated by bonding together two substrates corresponding thereto and interposing the first to nth coupler layers CL1, CL2, … …, CLn, which are separately fabricated, therebetween. However, instead of the bulk grating of fig. 10, as shown in fig. 11, the first to nth coupler layers CL1, CL2, … …, CLn may be directly formed on the surface of the substrate. For example, like a blazed grating or a binary phase grating, a surface grating having a plurality of cyclic (circulant) fine grating patterns in which a plurality of depressions and a plurality of protrusions are periodically arranged may be directly formed on the surface of a substrate by selectively using various processes such as imprinting or etching.

In fig. 11, for example, a first coupler layer CL1 is formed on an upper surface of the second substrate S2. The first substrate S1 may have a flat lower surface. The polymer layer 16 as a planarization layer may further fill a plurality of recesses in the periodic pattern of the first coupler layer CL1 formed on the upper surface of the second substrate S2. The polymer layer 16 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 16 may completely cover the first coupler layer CL 1. However, the embodiment is 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 a lower surface of the first substrate S1.

In the above-described exemplary embodiments, one coupler layer is disposed between two substrates. The number of the first to 2 n-th substrates S1, S2, … …, S (2n-1), and S2n is twice the number of the first to n-th coupler layers CL1, CL2, … …, CLn. However, the embodiment is not limited thereto. For example, the coupler layers may be disposed on both surfaces of one substrate. For example, fig. 12 is a schematic cross-sectional view of a configuration of a light guide structure 110b of a backlight unit 100 according to another example embodiment.

Referring to fig. 12, a first coupler layer CL1 is disposed between the first substrate S1 and the second substrate S2, and a second coupler layer CL2 is disposed between the second substrate S2 and the third substrate S3. Further, the (n-1) th coupler layer CL (n-1) is disposed between the (n-1) th substrate S (n-1) and the nth substrate Sn. Accordingly, although the configuration of the first light guide layer 10 is the same as that described above, the second light guide layer 20 'may include the third substrate S3 and only the second coupler layer CL2 disposed on the upper surface of the third substrate S3, and the nth light guide layer N' may include the nth substrate Sn and only the (N-1) th coupler layer CL (N-1) disposed on the upper surface of the nth substrate Sn. In the light guide structure 110b of fig. 12, the number of the first to nth substrates S1, S2, … …, Sn may be one greater than the number of the first to (n-1) th coupler layers CL1, CL2, … …, CL (n-1).

Further, in order to improve uniformity of the illumination light, the sum of the thickness t1 of the first substrate S1 and 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 10 may be different from the thickness of the second light guide layer 20'. The thickness of the nth light guiding layer N 'may be different from the thickness of the first light guiding layer 10 or the thickness of the second light guiding layer 20'.

As shown in fig. 12, when the polymer layer 16 is not present, the lower surface of the substrate disposed above may have a pattern shape complementary to the periodic pattern of the coupler layer disposed below the substrate. For example, the lower surface of the first substrate S1 may have a pattern complementary to the pattern of the first coupler layer CL1 disposed on the upper surface of the second substrate S2. In addition, the lower surface of the second substrate S2 may have a pattern complementary to that of the second coupler layer CL 2. In this regard, coupler layers may be formed on both surfaces of the second to (n-1) th substrates S2, S3, … …, S (n-1).

However, as shown in FIG. 11, the polymer layer 16 may be further filled between the patterns of the first to (n-1) th coupler layers CL1, CL2, … …, CL (n-1). In this case, the first to nth substrates S1, S2, S3, … …, Sn may have flat lower surfaces, and the first to (n-1) th coupler layers CL1, CL2, … …, CL (n-1) may be formed only on the upper surfaces of the second to nth substrates S2, S3, … …, Sn.

Fig. 13 is a schematic sectional view of a configuration of a light guide structure 110c of a backlight unit 100 according to another example embodiment. Referring to fig. 13, the first substrate S1 of the light guide structure 110c may have a thickness smaller than that of the other substrates. For example, the first substrate S1 may have a thickness of about 15nm or less, and the substrates other than the first substrate S1 may each have a thickness of about 0.1mm to about 2 mm. The first substrate S1 may include silicon oxide (SiO)₂) To serve as a protective layer.

Fig. 14 is a schematic sectional view of a configuration of a light guide structure 110d of a backlight unit 100 according to another example embodiment. Referring to fig. 14, the light guide structure 110d may include a first light guide layer 10 'and a second light guide layer 20'. The first light guide layer 10' may include a first substrate S1 and a first coupler layer CL1 disposed on an upper surface of the first substrate S1. In addition, the second light guide layer 20' may include a second substrate S2 and a second coupler layer CL2 disposed on an upper surface of the second substrate S2. The first substrate S1 may be disposed over 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 CL 2. In addition, the polymer layer 16 as a protective layer and a planarization layer may be filled between the periodic patterns of the first coupler layer CL 1. As shown in fig. 14, 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 110d may be the same.

Fig. 15 is a schematic plan view of a configuration of a first light guide layer 10 of a backlight unit 100 according to another example embodiment. Fig. 16 is a schematic plan view of a configuration of the second light guide layer 20 of the backlight unit 100 according to another example embodiment. In the example embodiments of fig. 2 to 6, all of the first to fourth input couplers IC1, IC2, IC3, and IC4 are disposed in the second light guide layer 20. However, the embodiment is not limited thereto. For example, as shown in fig. 15, a first input coupler IC1 and a third input coupler IC3 may be disposed in the first light guide layer 10, and a second input coupler IC2 and a fourth input coupler IC4 may be disposed in the second light guide layer 20. 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 guiding layer 10, 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 guiding layer 20.

Fig. 17 is a schematic plan view of a configuration of a first light guide layer 10 of a backlight unit 100 according to another example embodiment. Fig. 18 is a schematic plan view of a configuration of the second light guide layer 20 of the backlight unit 100 according to another example embodiment. Fig. 19 is a schematic cross-sectional view of a configuration of a light guide structure according to an example embodiment, in which the first light guide layer 10 of fig. 17 and the second light guide layer 20 of fig. 18 are bonded to each other. Specifically, fig. 19 is a cross-sectional view of the light guiding structure taken along the first and second extension couplers MC1 and MC2 to reveal the first and second extension couplers MC1 and MC 2.

Referring to fig. 17 to 19, the input coupler may not be disposed in the first light guide layer 10, and only the first and second input couplers ICl and IC2 may be disposed in the second light guide layer 20 at both sides of the second extension coupler MC2 in the y-axis direction. The third input coupler IC3 and the fourth input coupler IC4 may be disposed in separate light guiding layers adjacent to the lower surface of the second light guiding layer 20. For example, a third input coupler IC3 may be disposed in the first input light guide layer 1a disposed adjacent to the lower surface of the second light guide layer 20 to face the first input coupler IC1 in the z-axis direction, and a fourth input coupler IC4 may be disposed in the second input light guide layer 1b disposed adjacent to the lower surface of the second light guide layer 20 to face the second input coupler IC2 in the z-axis direction, as shown in fig. 19.

Fig. 20 is a schematic plan view of a configuration of a first light guide layer 10 of a backlight unit 100 according to another example embodiment. Fig. 21 is a schematic plan view of a configuration of the second light guide layer 20 of the backlight unit 100 according to another example embodiment. Fig. 22 is a schematic cross-sectional view of a configuration of a light guide structure according to an example embodiment, in which the first light guide layer 10 of fig. 20 and the second light guide layer 20 of fig. 21 are bonded to each other. Specifically, fig. 22 is a sectional view taken along the first extension coupler MC1 and the second extension coupler MC2 to reveal the first extension coupler MC1 and the second extension coupler MC 2.

Referring to fig. 20 to 22, in the first light guide layer 10, the first input coupler IC1 may be disposed at the left side of the first extension coupler MC1 in the-y-axis direction, and in the second light guide layer 20, the second input coupler IC2 may be disposed at the right side of the second extension coupler MC2 in the + y-axis direction. The third input coupler IC3 and the fourth input coupler IC4 may be disposed in separate light guiding layers adjacent to the lower surface of the second light guiding layer 20. For example, the third input coupler IC3 may be disposed in the first input light guide layer 1a disposed adjacent to the lower surface of the second light guide layer 20 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 disposed adjacent to the lower surface of the second light guide layer 20 to face the second input coupler IC2 in the z-axis direction.

In order for the holographic display device 200 to reproduce a color hologram image, the backlight unit 100 may supply red illumination light, green illumination light, and blue illumination light to the spatial light modulator 210. To this end, the backlight unit 100 may include a plurality of light guide structures for respectively providing red illumination light, green illumination light, and blue illumination light. For example, fig. 23 schematically shows a configuration of a backlight unit 100 according to an example embodiment, in which the backlight unit supplies red illumination light, green illumination light, and blue illumination light.

Referring to fig. 23, the backlight unit 100 may include a first light guide structure 110R that may provide red illumination light, a second light guide structure 110G that may provide green illumination light, and a third light guide structure 110B that may provide blue illumination light. Although fig. 23 illustrates that the second light guide structure 110G is disposed over the first light guide structure 110R and the third light guide structure 110B is disposed over the second light guide structure 110G in the z-axis direction, embodiments are not limited thereto, and the arrangement order of the first, second, and third light guide structures 110R, 110G, and 110B may be variously selected as necessary. The first and second light sources 121 and 122 may be disposed over the upper surface of the third light guide structure 110B at both edges of the third light guide structure 110B in the y-axis direction, respectively. Each of the first, second, and third light guide structures 110R, 110G, and 110B may have the same configuration as that of the light guide structure 110 described above.

In addition, each of the first and second light sources 121 and 122 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. 24 schematically illustrates an arrangement of red, green, and blue light sources in the backlight unit 100 of fig. 23. Specifically, fig. 24 shows a configuration of the backlight unit 100 viewed from a direction different from that of fig. 23. For example, fig. 23 shows the configuration of the backlight unit 100 viewed from the x-axis direction, and fig. 24 shows the configuration of the backlight unit 100 viewed from the y-axis direction.

Referring to fig. 24, the second light source 122 may include a red light source 122R, a green light source 122G, and a blue light source 122B. The red, green, and blue light sources 122R, 122G, and 122B may be linearly arranged in the x-axis direction over the upper surface of the third light guide structure 110B. On the opposite sides of the first, second, and third light guide structures 110R, 110G, and 110B as viewed in fig. 24, the first light sources 121 may also include red, green, and blue light sources linearly arranged in the x-axis direction.

The first light guiding structure 110R may include a second input coupler IC2R for coupling red light and a fourth input coupler IC4R for coupling red light. The second light guiding structure 110G may include a second input coupler IC2G for coupling green light and a fourth input coupler IC4G for coupling green light. The third light guiding structure 110B may include a second input coupler IC2B for coupling blue light and a fourth input coupler IC4B for coupling blue light. On the opposite side of the first, second and third light guiding structures 110R, 110G, 110B as seen in fig. 24, the first light guiding structure 110R may comprise a first input coupler for coupling red light and a third input coupler for coupling red light, the second light guiding structure 110G may comprise a first input coupler for coupling green light and a third input coupler for coupling green light, and the third light guiding structure 110B may comprise a first input coupler for coupling blue light and a third input coupler for coupling blue light.

A fourth input coupler IC4R for coupling red light is disposed to face the red light source 122R in the z-axis direction, and the fourth input coupler IC4R guides the red light emitted from the red light source 122R to the inside of the first light guide structure 110R. A fourth input coupler IC4G for coupling green light is disposed to face the green light source 122G in the z-axis direction, and the fourth input coupler IC4G guides green light emitted from the green light source 122G to the inside of the second light guide structure 110G. Further, a fourth input coupler IC4B for coupling blue light is disposed facing the blue light source 122B in the z-axis direction, the fourth input coupler IC4B guiding the blue light emitted from the blue light source 122B to the inside of the third light guiding structure 110B. Therefore, the fourth input coupler IC4R for coupling red light, the fourth input coupler IC4G for coupling green light, and the fourth input coupler IC4B for coupling blue light are disposed at different positions in the x-axis direction. Thus, the first light guiding structure 110R may provide collimated red illumination light to the spatial light modulator 210 by guiding and expanding red light emitted from the red light source 122R. The second light guiding structure 110G may provide collimated green illumination light to the spatial light modulator 210 by guiding and expanding green light emitted from the green light source 122G, and the third light guiding structure 110B may provide collimated blue illumination light to the spatial light modulator 210 by guiding and expanding blue light emitted from the blue light source 122B.

While the above-described backlight unit and holographic display device including the same have been particularly shown and described with reference to example embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present disclosure as defined by the following claims. The exemplary embodiments should be considered in a descriptive sense only and not for purposes of limitation. Therefore, the scope of the present disclosure is defined not by the detailed description of the present disclosure but by the appended claims, and all changes within the scope will be construed as being included in the present disclosure.