WO2007129783A1

WO2007129783A1 - Optical waveguide

Info

Publication number: WO2007129783A1
Application number: PCT/KR2006/002306
Authority: WO
Inventors: Chinsoo Hong; Chang Kyo Kim
Original assignee: Chinsoo Hong; Chang Kyo Kim
Priority date: 2006-05-10
Filing date: 2006-06-15
Publication date: 2007-11-15
Also published as: KR100827712B1; KR20070109215A

Abstract

The present invention relates, in general, to optical waveguides and, more particularly, to an optical waveguide, which can realize the efficient transmission of light and the attainment of uniform brightness without requiring a diffusion sheet for correcting luminance to make it uniform. Further, the present invention provides an optical waveguide, the optical waveguide dispersing light incident from a light source, placed on one side of the optical waveguide, to cause the light to be transmitted through a top surface of the optical waveguide. The top surface has repeating patterns formed thereon, the patterns having an arbitrary width. The patterns protrude from the top surface to a predetermined height, which varies as the patterns become distant from the light source.

Description

Description OPTICAL WAVEGUIDE

Technical Field

[1] The present invention relates, in general, to optical waveguides and, more particularly, to an optical waveguide, which can realize the efficient transmission of light and the attainment of uniform brightness without requiring a diffusion sheet for correcting luminance to make it uniform. Background Art

[2] Generally, a display device, such as a Liquid Crystal Display (LCD), uses a two- dimensional light source (hereinafter referred to as a 'surface light source') for causing light, emitted from a one-dimensional light source, to be incident on an optical waveguide, and to be transmitted through one surface of the optical waveguide. In this case, the efficiency of the surface light source has become an important factor for determining the overall efficiency of a display device. Generally, the efficiency of light on an LCD is 3 to 10%, and loss of light may occur at an optical waveguide, polarizers, a color filter, etc. Technology for display devices has progressed to reduce manufacturing cost while minimizing the loss of light. In particular, an optical waveguide for efficiently transmitting light through the entire surface thereof has been actively developed.

[3] Meanwhile, in the case of light transmitted through one surface of the optical waveguide, luminance is not uniform with respect to respective portions of the corresponding surface, through which the light is transmitted. In order to correct the non- uniformity of light, diffusion sheets are installed on the surface of the optical waveguide through which light is transmitted. That is, the diffusion sheets function to cause the luminance of a display device to be uniform by diffusing and dispersing light that exists inside the optical waveguide. However, there is a problem in that, when the above-described diffusion sheet is inevitably used to realize uniform luminance, the diffusion sheets act as another factor reducing the efficiency of light. Disclosure of Invention Technical Problem

[4] Accordingly, the present invention has been made to solve the above problems occurring in the prior art, and an object of the present invention is to provide an optical waveguide, in which blade patterns or modified blade patterns, which have a diffraction grating having spatial period comparable to the wavelength of visible rays, are formed on the surface through which light is transmitted, in the shape of a concentric circles, and then the width or height of the patterns are varied according to the a distance from a light source to a position on the surface, thus causing light, transmitted through the optical waveguide, to have uniform brightness over the entire surface thereof, without using a diffusion sheet. Technical Solution

[5] In order to accomplish the object mentioned above , the present invention provides an optical waveguide, the optical waveguide dispersing light incident from a light source, placed on one side of the optical waveguide, to cause the light to be transmitted through a top surface of the optical waveguide, wherein the top surface has repeating patterns formed thereon, the patterns having an arbitrary width, and the patterns protrude from the top surface to a predetermined height, which varies as the patterns become distant from the light source. Advantageous Effects

[6] Accordingly, the optical waveguide of the present invention is advantageous in that blade patterns having a period comparable to the wavelength of visible light, or modified blade patterns are formed on the surface, through which light is transmitted, in the shape of concentric circles, and the width or height of the patterns vary according to the distance to a light source, so that the transmitted light has uniform brightness over an entire transmission surface, and there is no need to use diffusion sheets. Further, the present invention is advantageous in that, when the diffusion sheets is not used, a display device having higher efficiency can be produced, and the costs of manufacturing the display device can also be reduced. Brief Description of the Drawings

[7] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

[8] FIG. 1 illustrates the traveling direction of transmitted light at an interface consisting of materials with different refractive indices;

[9] FIGS. 2 and 3 illustrate the transmission angle and the reflection angle of light, incident on an interface consisting of materials with different refractive indices, according to an incidence angle;

[10] FIG. 4 is a schematic view showing the section and the top of an optical waveguide, where no diffraction grating is formed;

[11] FIG. 5 illustrates a total reflection area on the optical waveguide of FIG. 4;

[12] FIGS. 6 to 8 illustrate various patterns of diffraction gratings;

[13] FIGS. 9 to 11 illustrate transmittances according to the height of the diffraction gratings of FIGS. 6 to 8;

[14] FIG. 12 illustrates the average transmittance of an optical waveguide, with the blade pattern of FIG. 8, according to the color and polarization state of light;

[15] FIG. 13 is a top view of the optical waveguide of FIG. 4;

[16] FIG. 14 illustrates the relative transmittance (RT) of an optical waveguide applicable to the present invention according to a location on the optical waveguide;

[17] FIG. 15 illustrates the relative transmittance of FIG. 14 in three dimensions;

[18] FIG. 16 illustrates transmittance according to the height of a blade pattern when the incidence angle of light is 70 degrees;

[19] FIG. 17 illustrates the relationship between the height of a blade pattern and transmittance;

[20] FIG. 18 illustrates the height of a blade pattern according to a location on an optical waveguide applicable to the present invention;

[21] FIG. 19 illustrates the height of the blade pattern of FIG. 18 in three dimensions;

[22] FIG. 20 illustrates blade patterns formed on the top of an optical waveguide according to an embodiment of the present invention;

[23] FIG. 21 illustrates a perspective view showing the overall external shape of an optical waveguide according to an embodiment of the present invention, and a cross sectional view thereof taken in a diagonal direction;

[24] FIG. 22 is a sectional view of an optical waveguide according to an embodiment of the present invention; and

[25] FIG. 23 is a top view of an optical waveguide having bar-shaped patterns.

Best Mode for Carrying Out the Invention

[26] Reference should now be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components.

[27] Hereinafter, an optical waveguide according to embodiments of the present invention will be described in detail with reference to the attached drawings.

[28] If a patterned surface is formed using a metal mesh having a spatial period of several mm, electromagnetic waves having a wavelength of several cm can be blocked by the a patterned surface. The metal mesh is composed of horizontal metallic lines or vertical metallic lines, and the electric field of electromagnetic waves is composed of the sum of components varying in the horizontal direction of the metal mesh, and components varying in the vertical direction of the metal mesh. Electrons existing in metal are moved in a direction opposite to the electric field, collide with atoms forming the metal, and completely consume the energy obtained by an externally applied electric field. Since electromagnetic waves, having electric field components formed in the same direction as the metallic lines, are completely absorbed by the metal, they cannot be transmitted through the metallic lines. Further, electromagnetic waves formed in a direction perpendicular to the metallic lines are transmitted through the metallic lines. Therefore, if a metal mesh is formed, electromagnetic waves having a wavelength shorter than the interval of the metal mesh cannot be transmitted through the metal mesh.

[29] Meanwhile, if electromagnetic waves are incident on an arbitrary device composed of metallic lines arranged at regular intervals, only electromagnetic waves having perpendicular components are transmitted through the device. Therefore, the transmitted electromagnetic waves are polarized in one direction, and such a device is called a polarizer.

[30] In order to manufacture the same polarizer while reducing the scale of the wavelength of the electromagnetic waves to that of visible light, the interval of metallic lines ( sinθ,,- sm ■ θ „, = mλ d hereinafter referred to as a 'period') must be comparable to the wavelength of visible light (about 400 to 700 nm), so that the width of each metallic line must be narrower than the period. Such a device is called a lamellar grating, and is basically different from a typical diffraction grating in that the typical diffraction grating has a period greater than a wavelength, and satisfies the following Math Figure 1, a diffraction grating Math Figure. [31]

sinθ_o- sinθ,^ — — a

[32] In Math Figure 1,

is the incidence angle of light, incident on the diffraction grating,

is the reflection angle or transmission angle (a reflective diffraction grating in the case of a reflection angle, and a transmission diffraction grating in the case of a transmission angle),

is a diffraction order that is an arbitrary integer, and d is the period of the diffraction grating. The typical diffraction grating satisfies , so that

may have several values other than 0, each value of m indicating the traveling direction of light. [33] However, the lamella grating satisfies

or

, so that only light reflected or transmitted to realize m =0 in Math Figure 1 is always obtained. In an actual experiment, since light traveling in various directions can be obtained, unlike the above case, Math Figure 1 cannot be applied to the lamella grating.

[34] The lamella grating is used in the region of infrared rays or visible rays, but cannot be used in the region of ultraviolet rays. Light in the region of ultraviolet rays is absorbed by the collective movement of electrons within metal (plasma frequencies falls within the region of ultraviolet rays), so that the lamella grating cannot function as a polarizer in the region of ultraviolet rays. In order to use the lamella grating in the region of visible rays, the period of the grating must be approximately several hundreds of nm, that is, the wavelength of visible rays.

[35] If metallic lines are used for such a lamella grating, the components of electromagnetic waves, transmitted through the lamella grating and having the same direction as the metallic lines, are absorbed in the metallic lines, and are converted into the motion of electrons. The motion of electrons results in the re-emission of electromagnetic waves. The components of electromagnetic waves, having a direction perpendicular to that of the metallic lines, are transmitted through the lamella grating. Accordingly, the period of the lamella grating causes the electromagnetic waves to be reflected or transmitted in a specific direction. A specific property, obtained using metal, is the fact that the more intense electromagnetic waves are transmitted through the geometrically open area among the lamellar grating lines.

[36] Meanwhile, when a high molecular substance besides metal, is used for the lamella grating, the above-described phenomenon occurs, that is, electromagnetic waves are reflected or transmitted in a specific direction. Further, since the high molecular substance has a very low imaginary refractive index in the region of visible range, the absorption of light caused by the high molecular substance itself becomes negligibly small.

[37] FIG. 1 illustrates the traveling direction of transmitted light at an interface consisting of media with different refractive indices. The following Math Figure 2 is an Math Figure for obtaining a horizontal wave number k parallel to the interface consisting of media with refractive indices n and n , as shown in FIG. 1,

[38]

[39] where

and

Λ:_σ( = 2π/λ_o) denote a wavelength and a wave number in a vacuum, respectively, and

Λ denotes the period of the lamella grating formed on the interface. Further, a wave number on an arbitrary medium is proportional to a refractive index, so that a wave number

in a direction perpendicular to the interface consisting of media with refractive indices n and n is given by the following Math Figure 3. [40]

2 Λ

K_n ^~y (k_ofijf-k xn

[41] In Math Figure 3,

denotes the refractive index n or n . o

[42] The

, indicating the transmission angle or the reflection angle on the interface, is given by the following Math Figure 4, based on the relationship between the incidence angle θ _* and a diffraction order m

[43] λ nsιn§ +m

[44] FIGS. 2 and 3 illustrate the transmission angle θ / and the reflection angle θ, corresponding to the incidence angle

, incident on the interface consisting of media with different refractive indices In the drawings, a period

A on the interface is set to 400nm, and is based on Math Figures 3 and 4 As shown in FIGS. 1 to 3, it can be seen that the transmission angle

is related to the period

Λ

, regardless of the shape of a lamella grating

[45] FIG 4 is a schematic diagram to depict the section and the top of an optical waveguide, where no diffraction grating is formed, and FIG. 5 is a diagram showing a total reflection at some region on the optical waveguide of FIG 4.

[46] As shown in FIG 4, the optical waveguide is a thin sheet with a round corner, and a point-like source, in particular, a luminescent diode or light emitting diode (LED), is installed on the round corner of the optical waveguide Further, the shortest distance between a white LED and the optical waveguide is d, the depth of the optical waveguide is t, the length of one side of the optical waveguide is L, and the incidence angle of the light, which is emitted from the LED and incident on the top surface of the optical waveguide, is θ ,„ [47] FIGS. 6 to 8 illustrate various patterns of diffraction gratings. In detail lustrates a diffraction grating, the top surface of which has a sinusoidal pattern, FIG. 7 illustrates a diffraction grating, the section of which is a triangular pattern, and FIG. 8 illustrates a diffraction grating, the section of which has a blade pattern, that is, a right triangular shape.

[48] FIGS. 9 to 11 illustrate transmittances according to the height of the diffraction gratings of FIGS. 6 to 8. In particular, when a lamella grating having a period

Λ of 400nm, that is, the above pattern having a width of 400nm, is formed on the top surface of the optical waveguide of FIG. 4, that is, the interface between the optical waveguide and an external medium, the transmittances of the optical waveguide obtained when the incidence angle

Θ _OT of light rays emitted from the LED is 70 degrees are shown. Accordingly, as shown in FIG. 9, the transmittance of the optical waveguide having a sinusoidal pattern is a maximum of about 8%. In FIG. 10, the maximum transmittance of the optical waveguide with a triangular pattern is shown to be 7%. As shown in FIG. 11, the maximum transmittance of the optical waveguide with a blade pattern is to be about 14%. It can be seen that, among various types of optical waveguides, the optical waveguide having the blade pattern exhibits the highest transmittance.

[49] FIG. 12 illustrates the average transmittance of the optical waveguide having the blade pattern of FIG. 8 according to the color and polarization state of light. Further, although not shown in the drawing, transmittance similar to the average transmittance of FIG. 12 can be obtained even when the top apex of the blade of FIG. 12 is moved, and then an asymmetrical triangle is formed. Accordingly, hereinafter, an optical waveguide having a blade pattern is described.

[50] FIG. 13 is a top view of the optical waveguide of FIG. 4, and Math Figure 5 is an

Math Figure for obtaining the incidence angle θ

[51]

n₀swQ-nco$_irι

[52] In Math Figure 5, n is the refractive index of the optical waveguide of FIG. 13, and

is the refractive index of an external medium of the optical waveguide, and θ is the incidence angle of the light incident on the curved surface of FIG. 13 with respect to a side surface indicated by a dotted line. Generally, LED emits the light intensity corresponding to

∞sθ _/π in a direction made at an angle of

from the central axis. Accordingly, an angle w relative to the central axis can be obtained by using an analytic methodthe with the following Math Figure 6. [53]

cosw^cosθcosθ^

[54] If Math Figure 5 is applied to Math Figure 6,

can be expressed using

® in and θ

[55] Meanwhile, if the optical waveguide is considered to be a nonabsorbing medium, a transmission coefficient corresponding to a polarization direction can be obtained by the following Math Figure 7.

[56]

2cosθ_/Hcosθ cos(θ _/n-θ)

2cosθ _/rtcosθ

¹¹ cos(θ_OT-θ)sin(θ+θ_JJ [57] In Math Figure 7, is the transmission coefficient of light rays obtained when the electric field of the incident light is perpendicular to an incident plane, and

*l l is the transmission coefficient obtained when the electric field is horizontal to the incident plane. [58] Meanwhile, transmittance

T satisfies the following Math Figure 8 together with Math Figure 7. [59] nsinft m ,-,

w₀cosθ

[60] That is, if Math Figure 7 is applied to Math Figure 8, transmittance can be calculated. [61] The intensity of light emitted from the LED at a solid angle of dΩ is determined such that, as the distance r increases, the light must irradiate wider area with respect to a single point on the optical waveguide spaced apart from the LED by the distance r (proportional to r ). The intensity of the light emitted from the LED is proportional to the cosine of the angle w relative to the central axis of the LED. As shown in FIG. 4, the size of area through which the light is transmitted may vary depending on the incidence angle of the light, incident on the top surface of the optical waveguide. At this area the light intensity must satisfy a cosine law, and is given as

COSθ _/ra . Further, the transmittance

T of light, emitted from the LED and transmitted through the optical waveguide, is given by Math Figure 8. As described above, on the basis of Math Figures 5 to 8, Relative Transmittance (RT) is given by the following Math Figure 9 depending on a location on the top surface of the optical waveguide. [62]

[63] In Math Figure 9,

Λ is a proportional factor to be determined so as to cause a maximum amount of light to be transmitted through the top surface of the optical waveguide. Accordingly, the results obtained by configuring relative transmittance (RT) of the optical waveguide having a size of 1"(1 inch) when the proportional factor

A is set to, for example, 1, are shown in FIGS. 14 and 15. FIG. 14 illustrates relative transmittance (RT) according to a location on an optical waveguide applicable to the present invention, and FIG. 15 illustrates the relative transmittance (RT) of FIG. 14 in three dimensions. If the relative transmittance (RT) must be obtained, as shown in FIGS. 14 and 15, a method of obtaining relative transmittance (RT) using the height of a blade pattern is described.

[64] FIG. 16 illustrates transmittance according to the height of a blade pattern when the incidence angle ^θ in of light is 70 degrees. It can be seen through FIG. 16 that, if the height of the blade pattern is increased until the height becomes 250nm, the transmittance

T is also increased, regardless of colors. Therefore, as shown in FIGS. 14 and 15, it can be seen that light can be uniformly transmitted through the entire top surface of the optical waveguide only when the height of the blade pattern must be varied as a location on the optical waveguide becomes distant from the LED, in consideration of the fact that, as the location on the optical waveguide becomes distant from the LED, the relative transmittance (RT) is lowered.

[65] FIG. 17 illustrates the relationship between the height of the blade pattern and transmittance. As shown in FIG. 17, the relationship between transmittance and the height of the blade is indicated by a solid line at an arbitrary incidence angle of ^θ in

(for example, 70 degrees). The height of the blade pattern according to the location on the optical waveguide can be obtained from the relationship between the height of the blade pattern and transmittance in FIG. 17, using the relative transmittanc FIGS. 14 and 15. This is shown in FIGS. 18 and 19. FIG. 18 illustrates the height of a blade pattern according to a location on an optical waveguide applicable to the present invention, and FIG. 19 illustrates the height of the blade pattern in three dimensions. [66] FIG. 20 illustrates the patterns formed on the top surface of an optical waveguide according to an embodiment of the present invention, in particular, blade patterns. As shown in FIG. 20, when such a blade pattern does not exist, the incidence angle of light

is greater than a critical angle, so that Total Internal Reflection (TIR) occurs, and thus the transmittance is 0. In contrast, on the optical waveguide, on which the blade patterns are formed, light is refracted in a traveling direction in the sequence of red (R), Green (G) and Blue (B), as shown in FIG. 20, even when the light are incident at an angle greater than the critical angle. For example, when the period,

, of patterns is set to 400nm, the angle at which light corresponding to three colors (RGB) progress is within 20 degrees, so that light is uniformly transmitted through the front surface of the optical waveguide. Therefore, there is no need to use a diffusion sheets.

[67] FIG. 21 illustrates a perspective view showing the overall external shape of an optical waveguide according to an embodiment of the present invention, and a cross sectional view thereof.

[68] As shown in FIG. 21, blade patterns having the shape of a concentric circle around an LED are formed on the top surface of the optical waveguide. The light proressing through the top surface has uniform brightness only in case the RT values of Math Figure 9 are the same at any location on the optical waveguide. In order to satisfy the condition, Math Figure 9 is satisfied only when the height of the blade patterns is small at a location on the optical waveguide close to the LED, and is large at a location on the optical waveguide far away from the LED, as described above.

[69] FIG. 22 is a sectional view of an optical waveguide according to an embodiment of the present invention.

[70] As shown in FIG. 22, light emitted from an LED 1 placed on one side of an optical waveguide 2 is incident on the optical waveguide 2. Light transmittance varies according to an incidence angle and a polarization state. In this case, reflected light become useless if they are left as they are, so that a reflector 7 is installed to cause the light rays to be transmitted through the optical waveguide 2.

[71] In this case, even when the optical waveguide 2 has a size of 1 inch, the incidence angle of light rays, which are incident on the top surface of the optical wa having blade patterns 3 and 4 formed thereon after being transmitted through the optical waveguide 2 from the LED 1, is greater than a critical angle. Accordingly, total reflection is conducted. As a location becomes distant from the LED 1, the area irradiated by light increases. Further, as the location deviates from the central axis of the LED 1, the intensity of light emitted from the LED 1 is weakened. Accordingly, the transmittance of Math Figure 9 must be equally obtained at every location in order to emit light in the forward direction of the optical waveguide 2 at uniform intensity, in consideration of all transmittances of light primarily transmitted through the optical waveguide 2 from the LED 1. For this reason, the height of the blade pattern 4, arranged relatively far away from the LED 1, is greater than that of the blade pattern 3 arranged close to the LED 1.

[72] In FIG. 22, reference numerals 5 and 6, not described, denote reflecting plates for reflecting light. In this case, the reflector 7 and the reflecting plates 5 and 6 are preferably made of materials such as aluminum or silver in order to obtain maximum reflection efficiency.

[73] The optical waveguide according to the present invention is not limited to the above embodiments, but can be variously modified within the allowable range of the technical spirit of the present invention. For example, FIG. 23 is a top view of an optical waveguide having bar-shaped patterns. In FIG. 23, the overall external shapes of the patterns are linear bars, and the patterns having such a bar shape are arranged in a checkerboard shape on the optical waveguide and together form concentric circles. Further, the optical waveguide is implemented so that patterns having the same distance to the LED have the same height, thus causing the overall pattern shape to be a concentric circle, and allowing the height of patterns to gradually vary as a location becomes distant from the LED.

Industrial Applicability

[74] As described above, the optical waveguide of the present invention is advantageous in that blade patterns having a period comparable to the wavelength of visible light, or modified blade patterns are formed on a surface, through which light is transmitted, in the shape of a concentric circle, and the width or height of the patterns varies according to the distance to a light source, so that the transmitted light has uniform brightness over entire transmission surface, and there is no need to use diffusion sheets. Further, the present invention is advantageous in that, when the diffusion sheets are not used, a display device having higher efficiency can be produced, and the costs of manufacturing the display device can also be reduced.

[75] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that variot fications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

[1] An optical waveguide, the optical waveguide dispersing light incident from a light source, placed on one side of the optical waveguide, to cause the light to be transmitted through a top surface of the optical waveguide, wherein: the top surface has repeating patterns formed thereon, the patterns having an a rbitrary width, and the patterns protrude from the top surface to a predetermined height, which varies as the patterns become distant from the light source.

[2] The optical waveguide according to claim 1, wherein the light is visible, and the width of patterns is comparable to about a wavelength of the visible light.

[3] The optical waveguide according to claim 2, wherein the height of the patterns is determined by relative transmittance given by the following Math Figure:

R T=A

CO5 θ( tOS θ ,_;,) T where

is a proportional factor, r- is a distance between the light source and an arbitrary location on the top surface,

is an incidence angle of a visible ray incident on the location, θ is an angle between a central axis, arbitrarily designated between the light source and the optical waveguide, and the location, and

is transmittance of the visible ray on the location. [4] The optical waveguide according to claim 3, wherein the transmittance is adjusted by varying the width of the patterns. [5] The optical waveguide according to any of claims 1 to 4, wherein: the patterns have a shape of a concentric circle around the light source, and the patterns have a section having an asymmetric triangular shape. [6] The optical waveguide according to any of claims 1 to 4, wherein: the patterns have a shape of a concentric circle around the light source, and the patterns have a section having a protrusion. [7] The optical waveguide according to any of claims 1 to 4, wherein: the patterns have external shapes that are linear bars, and the linear bar-shaped patterns are arranged in a checkerboard shape and together form a c circle, and linear bar-shaped patterns having the same distance to the light source have the same height within a predetermined range.