WO2005080861A1 - Dispositif de couplage optique pour ensemble de retro-eclairage d'ecran - Google Patents

Dispositif de couplage optique pour ensemble de retro-eclairage d'ecran Download PDF

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Publication number
WO2005080861A1
WO2005080861A1 PCT/GB2005/000542 GB2005000542W WO2005080861A1 WO 2005080861 A1 WO2005080861 A1 WO 2005080861A1 GB 2005000542 W GB2005000542 W GB 2005000542W WO 2005080861 A1 WO2005080861 A1 WO 2005080861A1
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WIPO (PCT)
Prior art keywords
light
waveguides
array
waveguide
lightguide
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PCT/GB2005/000542
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English (en)
Inventor
Anna Taylor O'neil
Iman Ibrahim Khandaker
Yongmin Lee
Samir Mezouari
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Microsharp Corporation Limited
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Publication of WO2005080861A1 publication Critical patent/WO2005080861A1/fr

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/0001—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems
    • G02B6/0011—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems the light guides being planar or of plate-like form
    • G02B6/0013—Means for improving the coupling-in of light from the light source into the light guide
    • G02B6/0023—Means for improving the coupling-in of light from the light source into the light guide provided by one optical element, or plurality thereof, placed between the light guide and the light source, or around the light source
    • G02B6/0028—Light guide, e.g. taper
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/2804—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/0001—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems
    • G02B6/0011—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems the light guides being planar or of plate-like form
    • G02B6/0066—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems the light guides being planar or of plate-like form characterised by the light source being coupled to the light guide
    • G02B6/0068—Arrangements of plural sources, e.g. multi-colour light sources
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/2804—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers
    • G02B6/2808—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using a mixing element which evenly distributes an input signal over a number of outputs
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/2804—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers
    • G02B6/2808—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using a mixing element which evenly distributes an input signal over a number of outputs
    • G02B6/2813—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using a mixing element which evenly distributes an input signal over a number of outputs based on multimode interference effect, i.e. self-imaging

Definitions

  • the present invention relates to a light -coupling device for a display backlight assembly.
  • Backlights are used in many types of display. Smaller displays include mobile phones and personal digital assistants (PDAs) . Larger displays include laptop computers, desktop monitors and, in particular, liquid crystal display (LCD) TVs that are increasingly attaining larger and larger screen diagonals.
  • LCD liquid crystal display
  • Liquid crystal display devices and other backlit display devices use a lightguide to couple light from a substantially linear source, such as a cold cathode fluorescent lamp (CCFL) , to produce a substantially planar output.
  • CCFL cold cathode fluorescent lamp
  • the ' performance of a backlit display is mainly governed by its uniformity and brightness. A combination of good uniformity and high brightness is highly desirable.
  • the backlight module of a flat panel display uses a complex layered structure comprising several light management films, such as diffusers and brightness enhancement films, as well as a lightguide plate.
  • Lightguides in the form of hollow structures are described in EP-A-0 , 377 , 309, US-A-5 , 986 , 728 and GB-A-
  • a prismatic optical film is used in combination with diffuser films with the aim of achieving an even distribution of light at high brightness over the surface that is being illuminated.
  • Light-deflection elements may be provided on the surface of a lightguide by coating it with a material that scatters or reflects light, such as a resin containing light-diffusing particles or a resin having a refractive index that is different from that of the material for the lightguide plate substrate. This is generally achieved through a variety of methods: inkjet printing, screen- printing, chemical etching, or by injection moulding.
  • US-A- 5,377,084 describes the printing of patterns of a large number of fine spots in screen print onto a lightguide in the form of a transparent plate through the use of a frosted white paint mixed with fine glass beads so that light incident on the transparent plate experiences substantially irregular reflection.
  • a dot pattern of a reflective material such as alumina, can be formed on the transparent plate, as described in US-A-5 , 600 , 462.
  • US-A- 5,363,294, US-A-5 , 667, 289, US 3,241,256 also describe arrangements of this type.
  • a lightguide may have dispersed through it light-diffusing particles having a refractive index that is different from that of the substrate material.
  • US-A-6,027,221, US-A-5 , 967 , 637 and JP 10-052839 disclose lightguides manufactured by injection moulding, having finely spaced, well-defined features produced by a relatively low cost process.
  • US-A-5 , 776, 636 discloses a method of manufacturing lightguides in which microlenses are produced using a photo-curable polymer.
  • the surface of a lightguide plate may be provided with a relief pattern to scatter or reflect light.
  • US-A-5, 618, 096 and US-A-6, 123 , 431 disclose light-emitting panels which may be provided with light-extracting deformities on one or both sides to control the distribution of light emitted from the panel.
  • Figure 1 illustrates a prior art lightguide 1, provided with discrete LED light sources 2a-c situated adjacent to one side surface of the lightguide, forming an edge-lit backlight assembly. Backlight assemblies are typically restricted to a small number of discrete light sources 2a-c, and this creates hot-spot regions 3 in the illumination of the lightguide 1.
  • US-B-6, 473, 554 discloses a low-profile backlight apparatus, which includes a waveguide having one or more point light sources within the waveguide and one or more refractive index interfaces within the waveguide which are arranged to redirect light therein to an emitting surface of the waveguide. As a result of the light source being embedded in the waveguide, the waveguide tends to be significantly thicker than the light source.
  • US-A-5, 036,435 discloses an illumination panel formed by the arrangement of many optical fibres with a light-leaking portion for backlight illumination.
  • US-A-4, 845, 596 describes a surface illumination device comprising parallel, spaced optical conductors which are placed above an optically reflecting surface.
  • US-A-5 , 097, 396 discloses the use of optical fibres for a backlighting panel in which the fibres terminate at different locations throughout the lightguide.
  • US-A- 5,307,245 discloses a similar arrangement, but which uses laser etching of the fibres to terminate the fibres in a zigzag pattern.
  • Figure 2 shows a prior art backlight assembly in which an optical fibre bundle 5 is used to couple light from a LED light source 6 into a lightguide 7, to illuminate a LCD panel 8.
  • the above optical fibre-based systems all suffer from a number of problems. For example, the manufacture of the lightguide panels is difficult, especially the assembly of the many tens of fibres required in parallel and spaced arrangements. Separating and aligning the individual fibres within an optical fibre bundle is time consuming. In addition, the generally circular cross section of the fibres results in a circular pattern of light emission, which in turn leads to poor geometric overlap with the rectangular cross section of the lightguide.
  • the transition device may comprise a solid transparent member or a plurality of optical fibres.
  • This arrangement suffers from similar problems to those above.
  • the transition device With the solid transparent member transition device, the transition device must be individually tailored to fit the shape and size of any particular panel member edge. Such bespoke components do not readily find general application.
  • US-A-5, 005, 108 itself suggests eliminating the transition devices in certain applications and focusing the light directly on the panel input surfaces instead, to cut down on system losses which are otherwise caused by such transition devices.
  • the present invention aims to address the above and other objectives by providing an improved backlight illumination apparatus.
  • a light-coupling device for a display backlight assembly
  • the display backlight assembly comprising a light source and a lightguide
  • the light-coupling device comprising an array of waveguides of efficient geometric cross section such as polygonal, circular, elliptical, or any other shape, the waveguides being adapted to be interposed between the light source and a side surface of the lightguide to provide a light path from the light source to the lightguide, the waveguides being optically connected such that at least one waveguide in the array is optically connected through a coupling element to at least one other waveguide in the array.
  • the cross section is geometrically efficient in both the sense that each individual waveguide has a geometrically efficient cross section and in the sense that the set of waveguides have a geometrically efficient cross- section where they are coupled to the lightguide plate.
  • the array of waveguides of geometric cross section provides an improved light-coupling efficiency between the light source and the lightguide.
  • the cross- sectional shape of the waveguides in the array matches that of the side surface of the lightguide. For example, with an array of rectangular waveguides arranged to be interposed between a rectangular side surface of a lightguide, light passing through the rectangular waveguide array is coupled to the lightguide efficiently.
  • the optical efficiency in backlights can be significantly increased - benefiting from improved mode overlap and/or improved packing.
  • Efficiency gains of 25% and more may be obtained using the light- coupling device of the present invention. This can be increased considerably as the thicknesses of the waveguides and the lightguide approach the theoretical minimum.
  • the provision of an array of waveguides of geometric cross section permits the use of powerful, efficient and/or more bulky light sources, since these may be located remotely from the lightguide. In this way, the thickness of the lightguide can be substantially independent of the dimensions of the light source.
  • the waveguide array of geometric cross section allows the separation of the light source from the proximity of the lightguide, making the display thickness independent of the light source thickness.
  • the waveguide array of geometric cross section can be significantly slimmer than a conventional lightguide. This is especially so when the array of waveguides comprises a high refractive index guiding region, which can have cross-sectional dimensions as little as half the wavelength of the guided light. However, the waveguides may alternatively have arbitrarily large dimensions .
  • the light-coupling device of the present invention is advantageously able to provide a compact profile, while maintaining comparable, if not superior, performance to that of the above prior art, in terms of brightness and uniformity.
  • the light-coupling device also offers an energy efficient means of implementing reduced area backlight illumination, for purposes including but not limited to energy saving and switchable parallax barriers for 3D displays .
  • the light-coupling device of the invention is capable of changing the relative intensity of light emitted by individual waveguides within the array.
  • the light-coupling device may be arranged to lower the intensity of light emitted from the central waveguides of the array while simultaneously increasing the intensity of light emitted from the outer waveguides of the array, by means of optical coupling within the array between the waveguides.
  • a waveguide array of geometric cross section for distributing light within a display backlight comprising: light source, lightguide, and said waveguide array of geometric cross section, wherein said waveguide array has end portions on at least one side of said lightguide.
  • an array of waveguides for illumination within a backlight comprising a light source, and said waveguide array being of efficient geometric cross section for distributing light.
  • the waveguide array of geometric cross section comprises optical media exploiting a complex refractive index mismatch for optical confinement.
  • the waveguide array of geometric cross section enforces optical confinement in two dimensions by exploiting a complex refractive index mismatch at a material boundary.
  • the waveguide array of geometric cross section has at least one facet coupling to at least one side of said lightguide.
  • the waveguide array of geometric cross section consists of symmetric waveguides, wherein the waveguiding layers are bounded by a material with a lower refractive index.
  • the waveguide array of geometric cross section consists of asymmetric waveguides, wherein the waveguiding layers are bounded by materials, both with lower refractive indices.
  • the waveguide array of geometric cross section is a channel waveguide.
  • the channel waveguide consists of a waveguide region surrounded on all sides by confining media of lower refractive indices.
  • the waveguide array of geometric cross section allows the separation of source, or sources, from the proximity of the lightguide making the display thickness independent of the source thickness.
  • the light source includes incandescent, fluorescent, and phosphorescent sources.
  • the light source is selected from the group consisting of Light Emitting Diodes (LEDs) , laser diodes, Cold Cathode Fluorescent Lamps (CCFLs) , UHP lamps, metal halide lamps, halogen lamps, xenon arc lamps, solid state lasers, gas lasers, and fibre lasers.
  • the light source may be any known source of light.
  • the waveguide array of geometric cross section is coupled to at least one source.
  • arrays of waveguides of geometric cross section are coupled to a number of sources less than or equal to the number of arrays of waveguides .
  • the arrays of waveguides of geometric cross section are coupled to a number of sources greater than the number of arrays of waveguides.
  • the one or more waveguide arrays of geometric cross section are directly butt-coupled to the one or more light sources.
  • the one or more waveguide arrays are directly butt-coupled to a lightguide.
  • the one or more waveguide arrays of geometric cross section are coupled to one or more light source, using one or more coupling element.
  • the waveguide array is coupled to said lightguide, using a coupling element.
  • the coupling element comprises mode conversion elements.
  • the mode conversion elements comprise S- bends, or Multi-Mode Interference (MMI) couplers, or Y couplers, or radiative star couplers, or evanescent couplers, or confluent couplers, or prism couplers, or grating couplers, or any other known coupler.
  • the couplers incorporate a tapered feature or tapered features.
  • the mode conversion elements incorporate a tapered feature or tapered features.
  • the tapered feature or tapered features comprise linear tapers, or adiabatic tapers, or exponential tapers .
  • the mode conversion elements comprise any combination of couplers, or tapered features.
  • the mode conversion elements comprise an optical multiplexer/demultiplexer.
  • the at least one waveguide array of geometric cross section is coupled to the lightguide in at least one sector.
  • the at least one waveguide array of geometric cross section is coupled to a number of sectors equal to the number of arrays of waveguides.
  • the at least one waveguide array of geometric cross section is coupled to a number of sectors greater than the number of arrays of waveguides .
  • the at least one waveguide array of geometric cross section is coupled to a number of sectors less than the number of arrays of waveguides .
  • the at least one waveguide array of geometric cross section has perturbations on at least one material boundary, which cause light by said waveguide array of geometric cross section to be emitted therefrom.
  • the waveguide array of geometric cross section can be significantly slimmer than a conventional lightguide, since the higher refractive index guiding region can have a thickness as little as half the wavelength of the guided light within the waveguide medium; however these waveguides can have larger thickness.
  • the at least one waveguide array of geometric cross section is fabricated using planar and/or roller based processing techniques so waveguide layout and waveguide perturbations can be implemented more easily than for optical fibre based arrays.
  • the waveguide array of geometric cross section is used in reverse as an optical collection device.
  • the light source is embedded in the said waveguide array of geometric cross section.
  • the said waveguide array of geometric cross section may have individual waveguides with a hexagonal, quadrilateral, rectangular, or square cross section.
  • the at least one waveguide array of geometric cross section has a flexible structure.
  • one embodiment of the present invention is implemented using refractive index perturbations to couple light out of plane and into a substantially orthogonal waveguide or lightguide.
  • the at least one waveguide array of geometric cross section is used to illuminate the keypad in a device, whereby the substantially light-transmitting keypad may constitute the lightguide plate or a plurality of lightguide plates.
  • the at least one waveguide array of geometric cross section has coupling elements which are passive couplers or/and active couplers.
  • Passive couplers achieve their effect through geometry; active couplers work through geometry and additional switching elements.
  • the said coupling elements are active couplers.
  • Active couplers can be implemented by electro- optic, acousto-optic, or microfluidic array techniques or by any other known means .
  • the said active couplers illuminate the lightguide, or the waveguide itself, in rows.
  • the said active couplers illuminate the lightguide, or the waveguide itself, in columns.
  • the active couplers can be used for a 3D display.
  • the said radiative star couplers can be used in an MxN arrangement, to recombine M narrow-bandwidth light sources into N channels with a substantially broader spectral distribution: effecting superior spectral uniformity.
  • Another embodiment of the light-coupling device provides the ability to distance the source from the lightguide, allowing the display to be significantly slimmer. Since waveguides can be designed to be significantly slimmer than traditional lightguides, in another embodiment the waveguide is substituted for the lightguide allowing a slimmer display. According to another embodiment of the waveguide array, there is provided a more efficient backlight with high uniformity. This is especially advantageous in the case of discrete sources, such as an LED array.
  • the lightguide when illuminated with LEDs the lightguide can have a number of hot-spots, i.e. very bright regions, near each source - see Figure 1.
  • This undesirable characteristic can be substantially ameliorated by provision of a waveguide array of efficient geometric cross section with adiabatically tapered waveguides, or tapered waveguides of other suitable profiles, such as but not limited to linear or exponential tapers.
  • These units can be manufactured by methods such as, but not limited to, injection moulding, embossing, or UV curing. This allows a mass production approach, rather than the small scale, bespoke solutions of other technologies.
  • the display backlight assembly achieves higher optical efficiency allowing reduced power consumption.
  • a light-coupling device in combination with a lightguide, the light-coupling device comprising an array of waveguides of geometric cross section adapted to be interposed between a light source and a side surface of the lightguide to provide a light path from the light source to the lightguide, the waveguides being optically connected such that at least one waveguide in the array is optically connected through a coupling element to at least one other waveguide in the array.
  • a display backlight assembly comprising: a light source; a lightguide having at least one side surface; and a light-coupling device comprising an array of waveguides of geometric cross section, the waveguides being interposed between the light source and one of the side surfaces of the lightguide to provide a light path from the light source to the lightguide, the waveguides being optically connected such that at least one waveguide in the array is optically connected through a coupling element to at least one other waveguide in the array.
  • a light-coupling device for a backlit display assembly, the backlit display assembly comprising a light source and a display region, and the light-coupling device comprising an array of waveguides adapted to be interposed between the light source and the display region, the waveguides extending in a first direction and comprising a plurality of light-deflecting elements such that light received from the light source and conveyed by the waveguides is redirected by the deflecting elements toward said display region in a direction transverse to said first direction, the waveguides being optically connected such that at least one waveguide in the array is optically connected through a coupling element to at least one other waveguide in the array.
  • a light-coupling device for a display backlight assembly, the display backlight assembly comprising at least two light sources and a lightguide with a plurality of side surfaces, and the light-coupling device comprising one or more arrays of waveguides of geometric cross section, the waveguides being adapted to be interposed between the light sources and one or more side surfaces of the lightguide to provide a light path from the light sources to the lightguide, the waveguides being optically connected such that at least one waveguide in one array is optically connected through a coupling element to at least one other waveguide in the same array.
  • a light-coupling device for a display backlight assembly, the display backlight assembly comprising at least one light source and at least one lightguide with a plurality of side surfaces, and the light-coupling device comprising one or more arrays of waveguides of geometric cross section, the waveguides being adapted to be interposed between the at least one light source and one or more side surfaces of each of the at least one lightguide to provide a light path from the at least one light source to the at least one lightguide, the waveguides being optically connected such that at least one waveguide in one array is optically connected through a coupling element to at least one other waveguide in the same array.
  • a preferred embodiment of the invention provides a light-coupling device, for a display backlight assembly comprising at least one light source and at least one lightguide, the device comprising at least one array of waveguides of geometric cross section, the waveguides adapted to be interposed between the light source (s) and at least one side surface of the lightguide (s) to provide a light path therebetween, the waveguides being optically connected such that at least one waveguide in a given array is optically connected through a coupling element to at least one other waveguide in the same array.
  • the waveguide array transmits light to a plurality of different locations throughout the assembly.
  • a waveguide array network may comprise symmetric and/or asymmetric couplers, to provide a desired even or non-uniform distribution of light to the lightguide.
  • a waveguide array may be formed integrally with a lightguide.
  • a light source may be contained within a waveguide array.
  • a waveguide array may be formed of flexible material.
  • a coupling element may be an active coupler.
  • Figure 1 shows a prior art edge-lit lightguide having multiple light sources
  • Figure 2 shows schematically a prior art backlight arrangement employing optical fibre coupling from light source to lightguide
  • Figure 3 shows a waveguide array backlight assembly in accordance with a first embodiment of the present invention
  • Figure 4 shows a waveguide array backlight assembly in accordance with a second embodiment of the present invention
  • Figures 5a, 5b and 5c show cross-sectional views of a rectangular waveguide in accordance with further, respective embodiments of the present invention
  • Figure 6 shows a backlight assembly in which light is coupled to a plurality of edges of a lightguide, in accordance with a further embodiment of the present invention
  • Figure 7 shows schematically a multimode interference (MMI) coupler suitable for use as a coupling device in the embodiment of Figure 6
  • Figures 8a and 8b show schematically a Y coupler and an MMI coupler
  • FIG. 13a shows an example of a waveguide array comprising a network of optical pathways between the waveguides in the same general manner as shown in Figure 6, with the junctions between the waveguides being in accordance with Figure 8a; and
  • Figure 15 shows an embodiment similar to Figure 14 but arranged to provide an output intensity which varies along the output nodes of the network formed by the waveguide array.
  • FIG. 3 there is shown a display backlight assembly in accordance with a first embodiment of the invention.
  • the display backlight assembly shows a lightguide 20, in the form of a rectangular plate. Adjacent the lightguide 20 is an array of waveguides 10.
  • the waveguides 10 are arrayed on a substrate 11, which provides support to the waveguides.
  • the waveguides 10 illustrated have a cross section in the shape of a rectangle.
  • the rectangular waveguide modes in this embodiment therefore match the rectangular geometry of the lightguide modes, thus enabling efficient optical coupling between the waveguide array 10 and the lightguide 20.
  • the light enters the lightguide 20 through one of the side surfaces of the lightguide. Because of the similar shapes of the waveguide cross sections and the side surface of the lightguide 20, the light is able to fill the lightguide efficiently and evenly, so that a substantially uniform illumination of the lightguide may be achieved.
  • the geometrical shape of each of the waveguide cross sections is similar and, preferably still, congruent. This helps to provide even illumination from the waveguide array 10. However, in certain applications, it may be desirable to change the cross-sectional shape and/or size of each waveguide 10 depending on the position of the waveguide in the array.
  • the cross section of the waveguides 10 matches the side surface of the lightguide 20 in terms both of shape and of aspect ratios, so that there is a substantially constant geometric mode between the waveguide array and the lightguide.
  • the waveguide array 10 of Figure 3 extends laterally substantially fully along a lateral dimension of the side surface of the lightguide 20, which helps to ensure that full and uniform illumination is provided. However, in some applications it may not be necessary for the array to extend fully,- for example, if the upper surface of the lightguide includes a non-illuminating border or the like.
  • the waveguides 10 may be arranged in a regular array, for the purposes of even light distribution into the lightguide 20.
  • the waveguides 10 of the array may, for example, be spaced from each other by an equal distance.
  • the waveguides 10 may be arrayed in a non-planar array, with, for example, alternating waveguides being located equally between adjacent waveguides, but positioned vertically above or below those adjacent waveguides.
  • Such an array may still be termed a regular array.
  • the waveguides may be irregularly arrayed.
  • the irregular array may still, however, be symmetric.
  • the spacing between adjacent waveguides 10 may increase or decrease towards the middle of the array.
  • the array may be irregular, but ordered.
  • the light-transmitting lightguide 20 used in the display backlight assembly of the present embodiment may be formed by either a hollow structure defining an optical cavity or a solid light-guiding plate. Both forms have a face or faces through which light is input from a light source, and two opposed major surfaces, through one of which the light output emerges, to provide illumination to a liquid crystal display device, for example.
  • a solid lightguide typically made of an optically clear polymer, has first and second opposed major faces, wherein the first major face comprises a window through which light can be emitted from the light-transmitting plate and the second major face has a specularly reflecting surface.
  • the edge-coupled light may be arranged to propagate from the input surface toward the opposite end surface, confined by total internal reflection (TIR) .
  • TIR total internal reflection
  • the light may be extracted from the lightguide by frustration of the TIR by various methods.
  • the distribution of scattered light from the light-transmitting plate may be optimised via feature design, size, spacing and distribution over the surface of the light-transmitting plate.
  • the term 'light-deflection element' in this context indicates a structure capable of reflecting light at such an angle that it is emitted from the optical cavity of the light-transmitting plate through the window.
  • the light-deflection elements are structures of circular shape in the plan view and partly circular in the sectional view.
  • This configuration facilitates the construction of slim displays with high optical efficiency and correspondingly good battery life.
  • This embodiment at least matches the slimness and uniformity of the optical fibre-based prior art systems, whilst being significantly easier to manufacture.
  • the waveguide array 10 of geometric cross section may incorporate integrated optical devices such as, but not limited to, couplers, gratings and mode converters.
  • Mode conversion elements include couplers and multiplexers/demultiplexers.
  • FIG. 4 shows a second embodiment of the invention.
  • the display backlight assembly shown in Figure 4 includes a waveguide array 10 interposed between a light source 60 and a lightguide 20.
  • the array of waveguides 10 is again supported on a substrate 11.
  • a coupling element 50 is positioned between the array of waveguides and the light source 60.
  • the coupling element 50 serves to couple light from a point source, for example, to the full geometrical distribution of light propagation supported by the waveguide array 10.
  • the rectangular waveguide array 10 is only partially shown in Figure 4. This is because the array need not be a flat waveguide array, but may alternatively be curved or bent.
  • the waveguides 10 have a rigid structure, but, alternatively, the waveguide array may be formed of a flexible material, such that the lightguide 20 and light source 60 may be moved relative to one another, while still being in optical communication with one another by means of the array of flexible rectangular waveguides 10.
  • the waveguide array 10 of geometric cross section of this embodiment may incorporate integrated optical devices such as, but not limited to, couplers, gratings and mode converters.
  • the light source 60 may be coupled to the waveguide array 10 by a combination of coupling elements 50.
  • similar coupling mechanisms may be used between the waveguide array 10 and the lightguide 20.
  • the waveguide array 10 of geometric cross section offers the advantages of flexibility, slimness and ability to distance the light source 60 from the lightguide 20.
  • the display backlight assembly includes a light source 60 which may be selected from a number of devices: Light Emitting Diodes (LEDs) , laser diodes, Cold Cathode
  • CCFLs Fluorescent Lamps
  • UHP lamps metal halide lamps, halogen lamps, xenon arc lamps, solid state lasers, gas lasers, and fibre lasers, or any other known source.
  • the air gap is eliminated by allowing the waveguide array 10 to be directly joined to the light source 60.
  • the array of waveguides 10 of geometric cross section may be interposed between a light source 60 and a lightguide 20.
  • the light-coupling efficiency between the waveguide array 10 and the lightguide 20 may be increased by eliminating air gaps - and indeed any significant refractive index change - at the interface between the array and the lightguide.
  • the array of waveguides 10 may be simply positioned adjacent a side surface of the lightguide 20 to confront that surface, it is preferable for the array to be joined to the lightguide surface.
  • the lightguide 20 and the array of waveguides 10 are formed integrally.
  • the function of the lightguide 20 may be performed by the waveguide array 10 itself.
  • the waveguide array 10 therefore includes light-deflecting elements or perturbations as described above in relation to the lightguide 20, so that the waveguide array may be arranged to backlight a display panel, in the same way that the lightguide 20 otherwise would.
  • Figures 5a-c illustrate different embodiments of the invention, in which waveguides 10 of the array are provided with a layer of material 12 around the waveguides in a longitudinal direction.
  • the material 12 may be formed fully or partially around the waveguides 10 and may in fact be two or more different materials. In any case, the material 12 has a refractive index which is lower than that of a central portion 13 of the waveguides, which preferably has a relatively high refractive index. In this way, light passing along the waveguide array 10 is substantially confined within the waveguides as a result of the refractive index interfaces within the individual waveguides 10.
  • the material may be part of the supporting substrate 11, or may be formed by coating or cladding the central portion 13 of the waveguides 10.
  • the building blocks of optical integrated circuits typically use planar and rectangular waveguide geometries.
  • the light is confined in a central high refractive index portion of the waveguide, surrounded by regions of lower refractive index. If all of the surrounding regions are of the same lower refractive index, the waveguide is termed a "symmetric" waveguide. There may be more than one optical material surrounding the waveguides. If these regions are of different, lower refractive indices, the waveguide is termed an "asymmetric" waveguide.
  • the higher refractive index region is immersed in lower refractive index material (s) .
  • the higher refractive index regions are coated with lower refractive index materials.
  • Figures 5a and 5b show cross sections of rectangular waveguides 10, comprising a central portion 13 (of refractive index n 2 ) surrounded by regions 11, 12 of differing refractive indices (n x and n 3 ) .
  • the index of refraction n 2 is greater than ni or n 3 .
  • the cladding material 12 is arranged to form a covering for all of the waveguides 10 in the array, whereas in Figure 5b, each waveguide is separately clad. In both cases, one of the surfaces of the waveguides 10 is bounded by the supporting substrate 11.
  • the material 12 may alternatively form a layer between the central portion 13 and the substrate 11.
  • Figure 5c shows a special case of a channelled waveguide 10 where a material of i surrounds a material of n 2 . Additionally, it is also possible in another embodiment to provide cladding which has a plurality of refractive indices up to the value of n 2 , such as a graded refractive index material. This may be formed of one or more layers of material .
  • Figure 6 shows another embodiment of the invention where the light source 60 is coupled to the lightguide 20 through a plurality of its edges. The light may be provided by one or more light sources 60. For example, a single light source 60 may be coupled to the waveguide array 10, respective sections of which provide paths to two or more of the side surfaces of the lightguide 20 (not shown) .
  • a light source 60 may be provided in respect of each side surface, as shown.
  • the waveguide array 10 forms a network in that there is at least one optical pathway between at least one waveguide and another waveguide: the waveguides have one or more optical pathways between each other, the pathways joining at junctions which comprise coupling elements 30.
  • one waveguide branches into at least two sub-branches defining respective further waveguides .
  • Figure 6 illustrates two sets of coupling element: a first set of coupling elements 30 are arranged within the waveguide array 10, to redirect and distribute light passing through the array; and a second set of coupling elements 40 are arranged between the waveguides 10 and the lightguide 20.
  • the coupling elements 40 between the waveguide array 10 and the lightguide 20, and the coupling elements 30 creating the divisions within the waveguide array may take a multiplicity of forms.
  • Coupling elements 30, 40 are used to facilitate the efficient transfer of light from the one or more light sources 60 to each sector of the waveguide array 10 of geometric cross section.
  • the methods used for coupling an optical waveguide between two waveguides are different from those used between free space and an optical waveguide.
  • Some couplers are selective in the mode coupled into the waveguide, while others are multimode; in this case it is possible to use both single and multimode waveguides.
  • Single mode waveguides can be used for transmission of substantially monochromatic light and can approach the minimum thickness achievable using classical waveguides.
  • Passive coupler fabrication can be carried out using the same techniques used to fabricate the waveguide array 10, and so this fabrication can occur simultaneously.
  • the types of coupling devices 30, 40 which may be used are multimode interference (MMI) couplers 31, as shown in Figure 7; Y couplers 32, as shown in Figure 8a; and evanescent couplers 33, as shown in Figure 8b.
  • MMI multimode interference
  • grating couplers, confluent couplers, prism couplers and other known forms may be used.
  • Tapers 31 at the input and output of couplers can be added for enhanced performance .
  • Figures 9a and 9b show linear tapers and adiabatic tapers respectively, but exponential tapers and other equivalent forms may alternatively be employed.
  • the improved coupling efficiency allows power consumption reductions for improved battery life, in particular for handheld devices.
  • the optical efficiency in backlights can be significantly increased - benefiting from improved mode overlap and/or improved packing. This efficiency increase may be at least 25%, and can be considerably higher as the thicknesses (dimensions) of the waveguides 10 and lightguide 20 approach a theoretical minimum.
  • Figures 10a and 10b show views of a portion of a display backlight assembly according to a further embodiment of the present invention.
  • the assembly shown is an integrated optical component (IOC) , or "integrated optical circuit", which may be readily inserted into a larger arrangement of components, as a waveguide/lightguide unit or module, arranged to extract light from a lower plane to a higher plane in the arrangement .
  • IOC integrated optical component
  • Light may pass into the IOC at a lower plane, through a waveguide 10a. From the waveguide 10a, the light may pass into the waveguide array
  • the waveguide 10 of the array are therefore angled to achieve the light transfer between the planes, although they may be curved and preferably are shallow-curved.
  • the waveguide 10a has perturbations formed therein, at the interfaces with the waveguide array 10, so that light is more efficiently extracted into the array.
  • a cone-shaped insert may be used for coupling the waveguide 10a to the waveguide array 10.
  • Figures 10a and 10b illustrate an embodiment where a lightguide 20 and a substantially rigid waveguide/waveguide array 10a, 10 are used to facilitate the assembly process of an "integrated optical circuit" with waveguiding structures on a plurality of planes.
  • a waveguide array 10 has been described above with reference to the above embodiments, there may be more than one array. These may be arranged to provide light paths to respective different side surfaces of the lightguide 20, or to the same side surface. In the latter case, the second array may be generally parallel to and spaced vertically and/or horizontally from the first array, in a stacked arrangement.' This may be particularly advantageous with lightguides 20 of large dimensions, for which more than one layer of waveguide arrays 10 can be used.
  • the integrated optical system incorporates either active or passive optical couplers, or any combination of the two.
  • Passive couplers achieve their effect through geometry, whereas active couplers work both through geometry and additional switching elements.
  • the resulting system allows energy saving by only illuminating that part of a display which is being used.
  • the system also provides for the implementation of high efficiency parallax barriers for 3D displays.
  • One such embodiment is illustrated in Figure 11.
  • the use of active couplers allows control of light transmission through the waveguides 10 to an individual sector or sectors of the lightguide 20 and this allows the display to be illuminated in any region or regions of the display.
  • Figure 11 shows how single (SI) or multiple sources (SI and S2) can be used to illuminate all or part of a display.
  • Active couplers can be switched on and off depending upon the region that needs illuminating, as will be known to those skilled in the art. It will be understood that the active couplers may be provided by electro-optical, acousto-optical, or microfluidic array coupling techniques, although others which are known are included.
  • the lightguide 20 is not marked. This is because the illumination region, comprising the dark region and the illuminated region in the figure, may be provided in one of two ways. Firstly, the illumination region may be provided by the lightguide 20, in a similar manner to that described above, i.e., by providing light-deflecting elements within the lightguide 20, which act to redirect the light out of the lightguide and towards a display panel, for example.
  • the illumination region may be provided by the waveguide array 10 itself. That is, the waveguide array 10 may comprise light-deflecting elements (not shown) within the waveguides, so that light is redirected towards a display region, e.g. a display panel, keypad etc., preferably located above the waveguide array, by the light-deflecting elements.
  • the waveguide array 10 may be arranged to perform the function of both a waveguide and a lightguide.
  • the illuminated substantially transparent object, such as the keypad to constitute the lightguide.
  • the possibility of not employing an independent lightguide 20 with no other function within in the device has the advantage of reducing the size, manufacturing cost and complexity of such a backlit display assembly.
  • One additional benefit of this embodiment is that it is not necessary to distribute light-deflecting elements in set patterns within a lightguide, especially towards the corners and centre of the lightguide, so that a desired distribution of illuminating light is produced.
  • Individual waveguides may have an identical distribution of light -deflecting elements therein.
  • the spatial arrangement of the individual waveguides themselves as opposed to the arrangement of the light -deflecting elements themselves within individual waveguides, may be varied to achieve an even, or desired, distribution of light.
  • the distribution of the light-deflecting elements within the waveguides may be additionally or alternatively varied to achieve the desired illumination performance.
  • FIGS 12a-c show cross-sectional views of two back- to-back displays 70, 80, in accordance with further embodiments of the invention.
  • the displays 70, 80 share backlight assembly 1, which is interposed therebetween and which therefore is illuminated when the first display 70 or the relevant portion of the second display 80 are in use.
  • the other portion of the second display 80 is illuminated by backlight assembly 2.
  • backlight assembly 2 extends fully behind the second display 80 to be capable of illuminating the whole of the display.
  • Backlight assembly 1 still provides illumination to the first display. However, this may cause some interference should both displays be in use at the same time, so in Figure 12c, there is a barrier positioned between backlight assemblies 1 and 2.
  • Figures 13a and 13b show an application of the embodiments of Figures 12a-c, in which the display 70 is illuminated by one light source, and display 80 is illuminated by the same or another light source. Display 70 is illuminated when the phone is being used. Display 80, which can be split into sections, is illuminated when the full display (PDA mode) is being used. Active couplers can be used instead of multiple light sources in order to illuminate the desired display, depending upon use.
  • the embodiments of the waveguide array 10 can be used to illuminate the substantially light-transmitting keypad using active/passive couplers 30 and 40.
  • a light source/sources can be used to illuminate the keypad, 90 and 100.
  • Waveguides which form a network through interconnection of the light-carrying media via branch structure geometries are examples of embodiments of the invention, as illustrated in Figure 6.
  • the light from a relatively compact symmetric source such as a 5 mm diameter approximately circular cross section light beam emitted by a LED
  • a relatively compact symmetric source such as a 5 mm diameter approximately circular cross section light beam emitted by a LED
  • the designs disclosed demonstrate the wide degree of control in the redistribution of the light's geometrical distribution that is facilitated by these embodiments of the invention. Variations of these examples which take light of a given input geometrical distribution emitted by a light source and output light of another geometrical distribution suitable for introduction into a lightguide plate will be obvious to those skilled in the art. It will be obvious to those skilled in the art that the examples below may be generalized to coupling a plurality of light sources into a LCD lightguide, for example as is shown in Figure 6.
  • Example 1 Figure 14 shows a waveguide array network which accepts a relatively symmetric input light geometrical distribution which is subsequently redistributed by the waveguide network to produce an output light geometrical distribution which is elongated and relatively asymmetric with respect to the input light geometrical distribution.
  • the input light is accepted by the waveguide network and redirected through four layers of Y couplers 32, as shown in Figure 8a, to produce a relatively asymmetric output light distribution.
  • This arrangement is derived from a network architecture that is similar to that disclosed in Figure 6, where the junction structure is as disclosed in Figure 8a.
  • the coupling elements 30 are employed sequentially leading finally to a set of coupling elements, one example of said elements being 40, which is directed into the lightguide, 20.
  • each coupling element is a symmetric Y coupler 32 as shown in Figure 8a. This symmetric coupling element splits the input light intensity equally into each output arm of the coupler statistically on average. In an ideal lossless system, after n stages of couplers are traversed in series, there are 2 ⁇ output nodes, and the intensity output by each node is (l/2) n of the intensity accepted by the single input node.
  • the fourth layer comprises 16 output nodes each outputting 1/16 of the light intensity accepted by the waveguide network single input node.
  • each Y coupler 32 ( Figure 8a) is used as a coupling element 30 and the light is spilt 50:50 (percent) into each of the arms of each Y coupler 32.
  • the light splits 50:50 again.
  • one embodiment may involve taking a conical distribution with circular cross section of incident light at the input end of the light-coupling device, and emitting light with a highly elongated rectangular distribution from the output end, i.e. into the lightguide (s) .
  • a circle has infinite rotational symmetry.
  • a rectangle has 180 degree rotational symmetry. The geometric properties of a rectangle along the orthogonal pair of directions aligned with the non-equal, long and short, sides of the rectangle are different or "asymmetric" .
  • Example 2 Figure 15 shows a similar waveguide network to that disclosed in Figure 14, the principal difference being that the Y couplers employed are of various degrees of asymmetry in Figure 15 in contrast to the symmetric Y couplers in Figure 14.
  • the net effect of the asymmetric Y couplers in Figure 15 is to produce a lower intensity of light emerging from the output nodes at the centre of the set of output nodes than that which emerges from the output nodes towards the edges of the set of output nodes .
  • Such an output distribution of light intensity may be desirable in certain LCD systems to compensate for non-uniform light intensity propagation through the rest of the LCD display, for example, or may be desirable for aesthetic reasons.
  • the system is assumed to be lossless, and uses a similar light source to that considered in Example 1.
  • the first layer Y coupler is a symmetric 50:50 Y coupler.
  • the second layer of Y couplers each produce an 80:20 intensity split: the two central arms/waveguides of this layer each receive 20 percent of the light intensity from its respective upstream waveguide branching from the first coupler stage. This produces a non-uniform intensity distribution in the arms of this layer of 40:10:10:40 from left to right across the figure.
  • the two central couplers in the waveguide network are symmetric 50:50 Y couplers.
  • the outer two couplers of this stage are 60:40 asymmetric couplers on the edge of the waveguide network, for which the larger portion of the light intensity is directed into the arm/waveguide on the edge of the network.
  • the arrangement of the second example is able to produce a non-uniform distribution of the light intensity which is emitted by the waveguide network, the intensity distribution having a geometrical distribution which may be suitable for efficient coupling into a LCD lightguide.

Abstract

Cette invention concerne un dispositif de couplage optique conçu pour un ensemble de rétro-éclairage d'écran qui comprend une source de lumière (60, S1, S2) et un guide optique (20), lequel dispositif comprend un réseau (10) de guides d'ondes présentant une section transversale géométrique, lesquels guides d'ondes sont conçus pour être intercalés entre la source de lumière (60, S1, S2) et une surface latérale du guide optique (20) afin qu'une raie spectrale soit générée entre ladite source de lumière et ladite surface latérale du guide optique, les guides d'ondes étant optiquement connectés de façon qu'au moins un guide d'ondes du réseau (10) soit optiquement connecté au moyen d'un élément de couplage (30) à au moins un autre guide d'ondes du réseau (10). Un réseau de guides-d'ondes peut comprendre des coupleurs actifs ou passifs, symétriques et/ou asymétriques (30-35) permettant de fournir au guide optique (20) la distribution de lumière uniforme ou non uniforme voulue. Le réseau de guides d'ondes (10) peut être intégré au guide optique (20) et peut être constitué de matériau flexible. Une source de lumière (60, S1, S2) peut être logée dans le réseau de guides d'ondes (10). Cette invention peut être appliquée aux écrans à cristaux liquides (70, 80), aux claviers en caoutchouc (90, 100), aux commutateurs tactiles, aux panneaux rigides et analogue.
PCT/GB2005/000542 2004-02-16 2005-02-16 Dispositif de couplage optique pour ensemble de retro-eclairage d'ecran WO2005080861A1 (fr)

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