WO2011119104A1

WO2011119104A1 - A method and structure for coupling light from a light source into a planar waveguide

Info

Publication number: WO2011119104A1
Application number: PCT/SG2010/000244
Authority: WO
Inventors: Kazuya Takayama; Ieng Kin Lao
Original assignee: Nitto Denko Corporation
Priority date: 2010-03-24
Filing date: 2010-07-01
Publication date: 2011-09-29
Also published as: SG183141A1; WO2011119105A1; SG182817A1; SG181950A1; WO2011119107A1; WO2011119106A1

Abstract

A method and structure for coupling light from a light source into a planar waveguide. The method comprises the steps of providing the light source above the planar waveguide; providing one or more inclined light guiding channels in a thin-film layer between the planar waveguide and the light source; coupling light from the light source into the guiding channels such that the light coupled into each guiding channel undergoes at least one internal reflection; coupling the light exiting each guiding channel into the planar waveguide for propagation in the waveguide; and providing a capping layer between the light source and the light guiding channels in the thin-film layer; wherein the light source is non-directional, and the capping layer is configured for reducing refraction and reflection of incident light within a window or direction of maximum coupling efficiency of the coupling from the guiding channels into the waveguide.

Description

A METHOD AND STRUCTURE FOR COUPLING LIGHT FROM A LIGHT SOURCE INTO A PLANAR WAVEGUIDE

FIELD OF INVENTION

A method and structure for coupling light from a light source into a planar waveguide.

BACKGROUND

Waveguides can be used for various applications. Possible examples include biosensors for point of care applications or field measurements, or for the waveguides to be used as optical interconnects. Current light coupling technologies, in the case of the light source and/or the photo detector being mounted on the waveguide surface, can be categorized into two main groups: Reflection and/or Diffraction methods.

In an example reflection method, a mirror structure created by microfabrication techniques, such as a dicing saw with 45 degree or 90 degree edge, reactive ion etching, direct exposure with a shadow mask, or laser ablation, can reflect the light from the light source into the waveguide and also can reflect the light from the waveguide to the outside. In an example diffraction method, a grating structure composed of a physical periodic structure on the waveguide surface, a so called "surface relief grating", and/ or composed of an optical periodic structure in the waveguide medium, a so called "index modulated grating", can diffract the light into the waveguide and can diffract the light from the waveguide to the outside.

Another light coupling technology uses a mode coupling method in which an energy transition is introduced when both the electro-magnetic field intensity profile excited by the light source and the propagation mode profile in the waveguide core are closely overlapped. An example of a mode coupling technology is a prism-coupling method for measuring refractive index and thickness of polymer films.

In the above reflection methods, in terms of the size of the coupling area, a mirror structure will have approximately the same size as the core of the waveguide because the mirror plane will extend substantially across the waveguide core. This coupling area is small as a result, and light is typically coupled into the waveguide with a focused laser light source. In the above diffraction methods, in the case of a grating structure, the waveguide is usually "single-mode" because the diffraction direction is determined by the "phase" difference among diffracted lights from each grating element. Since the refractive index difference between the core material and the cladding material, and the waveguide dimensions are small for single-mode operation, the diverging angle of the propagating light from the waveguide core channel to the waveguide core "pad", in which the grating structure is formed, is also small. This again results in a small coupling area for such coupling techniques. Also, diffraction methods are wavelength-selective.

For the above mode coupling method, it is possible to have a large coupling area as long as both the wave vectors of the incident light and of the propagating mode along the thickness of the film are equal. This means that the mode coupling method is "thickness sensitive". In the case of the above prism coupling method, the prism angle and waveguide film thickness must be determined precisely, which increases fabrication complexity, and severely limits the suitability of such methods for mass-production of coupling structures.

An evanescent wave coupling technology has been described in EP 1939955A2. However, this patent describes the thicknesses of the waveguide ranging from 10 nm to 10 μπι, and that the waveguide is designed for low-order mode, i.e. less than 20 modes. As there thus may be limitations for the thickness of the waveguide, this method may still share some of the problems described above for mode coupling methods.

Other current solutions offer to couple light from the end-face of the waveguide core based on alignment tolerances of automatic assembly systems using e.g. image recognition usually designed for optical fiber pigtailing. However, such solutions do not offer the advantage of having the coupling area on the top, which enables pick and place for mass production manufacturing at a low cost. A need therefore exists to provide a method and system for light coupling that seeks to address at least some of the above-mentioned problems.

SUMMARY In accordance with a first aspect of the present invention, there is provided a method of coupling light from a light source into a planar waveguide, the method comprising the steps of providing the light source above the planar waveguide; providing one or more inclined light guiding channels in a thin-film layer between the planar waveguide and the light source; coupling light from the light source into the guiding channels such that the light coupled into each guiding channel undergoes at least one internal reflection; coupling the light exiting each guiding channel into the planar waveguide for propagation in the waveguide; and providing a capping layer between the light source and the light guiding channels in the thin-film layer; wherein the light source is non-directional, and the capping layer is configured for reducing refraction and reflection of incident light within a window or direction of maximum coupling efficiency of the coupling from the guiding channels into the waveguide.

Each guiding channel may comprise a first end surface facing the light source and substantially parallel to the planar waveguide, and a second end surface facing, and substantially parallel to, the planar waveguide.

The capping layer may be configured for directing light incident in an area between adjacent first surfaces of the guiding channels towards one of the adjacent first surfaces.

The capping layer may be configured such that light incident in the area between the adjacent first surfaces of the guiding channels is directed towards the one of the adjacent first surfaces via at least one total internal reflection at a top surface of the capping layer.

The capping layer may be configured for manipulating an angle of incidence of the light incident in the area between adjacent first surfaces of the guiding channels into the one of the adjacent first surfaces The capping layer may comprise an enhancer structure comprising first enhancer elements substantially perpendicular to the window or direction, and second enhancer elements substantially parallel to the window or direction.

The method may further comprise providing a second set of one or more inclined light guiding channels in a thin-film layer on the planar waveguide; coupling light from the waveguide into the guiding channels such that the light coupled into each guiding channel undergoes at least one total internal reflection; and coupling the light out of each guiding channel.

The method may further comprise providing a detector on the second set of guiding channels.

Each guiding channel of the second set may comprise a first end surface facing the detector and substantially parallel to the planar waveguide, and a second end surface facing, and substantially parallel to, the planar waveguide.

In accordance with a second aspect of the present invention, there is provided a multi-layer structure for coupling light from a light source into a planar waveguide, the structure comprising the planar waveguide; one or more inclined light guiding channels in a thin-film layer on the planar waveguide disposed such that the light coupled into each guiding channel undergoes at least one total internal reflection and such that the light exiting each guiding channel is coupled into the planar waveguide for propagation in the waveguide; and a capping layer on the light guiding channels in the thin-film layer; wherein the light source is non-directional, and the capping layer is configured for reducing refraction and reflection of incident light within a window or direction of maximum coupling efficiency of the coupling from the guiding channels into the waveguide.

Each guiding channel may comprises a first end surface facing the light source and substantially parallel to the planar waveguide, and a second end surface facing, and substantially parallel to, the planar waveguide.

The capping layer may be configured such that light incident in the area between the adjacent first surfaces of the guiding channels is directed towards the one of the adjacent first surfaces via at least one total internal reflection at a top surface of the capping layer. The capping layer may be configured for manipulating an angle of incidence of the light incident in the area between adjacent first surfaces of the guiding channels into the one of the adjacent first surfaces

The capping layer may comprise an enhancer structure comprising first enhancer elements substantially perpendicular to the window or direction, and second enhancer elements substantially parallel to the window or direction.

The structure may further comprise a second set of one or more inclined light guiding channels in a thin-film layer on the planar waveguide, disposed for coupling light from the waveguide into the guiding channels such that the light coupled into each guiding channel undergoes at least one total internal reflection, and for coupling the light out of each guiding channel.

The structure may further comprise a detector on the second set of guiding channels.

The structure may further comprise the light source.

The structure may further comprise one or more transparent layers between any one of the layers of the structure.

The structure may further comprise one or more adhesion promoting layers between any one of the layers of the structure.

In accordance with a third aspect of the present invention, there is provided a method for fabricating a multi-layer structure as defined in the first aspect.

The method may comprise one or more of a group consisting of photolithography processing, UV exposure processing, laser excitation processing, unexposed layer processing, imprinting processing, hot embossing/thermal compression processing, and molding processing.

The method may comprise layer-by-layer processing, bonding processing, or a combination of both. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which:

Figure 1 a) shows a schematic diagram illustrating a structure for coupling light into a thin-film waveguide according to an example embodiment. Figure 1b) shows a schematic diagram illustrating the light propagation in the structure of Figure 1a).

Figure 1c) shows a schematic diagram of a detail of the structure of Figure

1a).

Figure 2 shows a schematic diagram illustrating one example approach to avoid light coupled into the waveguide escaping from the adjacent channels in the structure of Figure 1 a), according to an example embodiment. Figure 3 shows a schematic diagram illustrating another approach to avoid light coupled into the waveguide escaping from the adjacent channels in the structure of Figure a), according to another example embodiment.

Figure 4 shows a schematic drawing embodiment of a light coupling structure for coupling light into a thin-film planar waveguide.

Figure 5 shows a schematic diagram illustrating light propagation in one light coupling structure according to an example embodiment. Figure 6a) shows a schematic diagram illustrating light propagation in a light coupling structure according to another example embodiment.

Figure 6b) shows a schematic diagram illustrating light propagation in a light coupling structure according to another example embodiment. Figure 6c) shows a schematic diagram illustrating light propagation in a light coupling structure according to another example embodiment. Figure 7a) shows a schematic diagram of a structure for coupling light out of a thin-film planar waveguide according to an example embodiment.

Figure 7b) is a schematic diagram illustrating coupling light out of a thin-film * planar waveguide in the structure of Figure 7a).

Figure 7c) shows a detail of the structure of Figure 7a).

Figure 8a) shows a schematic drawing illustrating an example architecture for a multi-layer structure according to an example embodiment.

Figure 8b) shows a schematic drawing illustrating an example architecture for a multi-layer structure according to an example embodiment.

Figure 8c) shows a schematic drawing illustrating an example architecture for a multi-layer structure according to an example embodiment.

Figure 8d) shows a schematic drawing illustrating an example architecture for a multi-layer structure according to an example embodiment. Figure 8e) shows a schematic drawing illustrating an example architecture for a multi-layer structure according to an example embodiment.

Figure 8f) shows a schematic drawing illustrating an example architecture for a multi-layer structure according to an example embodiment.

Figure 8g) shows a schematic drawing illustrating an example architecture for a multi-layer structure according to an example embodiment. Figure 9 shows a flowchart illustrating the formation of single or multiple planar waveguide components for formation of a waveguide layer in an example embodiment. Figure 10 shows a flowchart illustrating the formation of single or multiple planar waveguide components for formation of the inclined guiding channel layer in an example embodiment.

Figure 11 a) is a schematic drawing illustrating the formation of inclined light guiding channels using a laser head, according to an example embodiment.

Figure 1 1 b) is a schematic drawing illustrating the formation of inclined light guiding channels using a laser head, according to an example embodiment. Figure 12a) is a schematic drawing illustrating the formation of inclined guiding channels according to an example embodiment.

Figure 12b) is a schematic drawing illustrating the formation of inclined guiding channels according to an example embodiment.

Figure 12c) is a schematic drawing illustrating the formation of inclined guiding channels according to an example embodiment.

Figure 13 shows a flowchart illustrating the formation of a capping layer in an example embodiment.

Figure 14 shows a flowchart illustrating the formation of a capping layer in another example embodiment. Figure 15a) shows a flowchart illustrating the formation of a capping layer in other example embodiment. Figure 15b) shows a flowchart illustrating a method of forming either a waveguide, or a waveguide with inclined light coupling channel or capping layer, according to an example embodiment. Figure 16 shows a schematic diagram illustrating coupling of light from a light source into a thin-film planar waveguide via inclined coupling channels according to an example embodiment.

Figure 17 shows simulation results of the coupling efficiency as a function of the incident light angle, for the structure of Figure 16.

Figure 18 shows an intensity profile of a non-directional light source.

Figure 19 shows a coupling efficiency profile corresponding to the profile of Figure 17 using the same coordinate system compared to the light source intensity profile of Figure 18, and send it around an incident angle of 78.8°.

Figure 20a) is a schematic drawing illustrating minimising reflection in a preferred window or direction of light coupling, according to an example embodiment.

Figure 20b) shows a schematic diagram illustrating minimising refraction in the preferred window or direction of maximum coupling efficiency, according to an example embodiment.

Figure 21 shows a schematic drawing illustrating a multi-layer structure comprising an enhancer structure formed in a capping layer, according to an example embodiment. Figure 22 shows a schematic drawing illustrating a multi-layer structure comprising an enhancer structure formed in a capping layer, according to an example embodiment. Figure 23 shows a flowchart illustrating a method of coupling light from a light source into a planar waveguide according to an example embodiment.

DETAILED DESCRIPTION

The example embodiments described seek to provide methods and systems for coupling light into and out of a planar waveguide that can provide both a large coupling area as well as offering solutions over a large thickness range. Providing a large coupling area and providing over a large thickness range are both advantageous with regard to improving coupling efficiency. Also, with a large coupling area, optical elements can advantageously be used without the need for precise alignment. The term "thin-film" used throughout the description and the claims is intended to distinguish the respective components of the structures in the example embodiments from macroscopic structures typified by light pipes, light wedges, optical fibres, etc. As will be understood by a person skilled in the art, the term "thin-film" signifies that material making up the thin-film is formed on a substrate, noting that the substrate may be removed after fabrication of the thin film using techniques understood in the art. The term "thin-film" is not intended to limit to a particular thickness range of the film. In the described example embodiments, by way of example but not limitation, film thicknesses are typically in the range from about 1 μίη to 1000 μηη, and preferably from about 5 μητι to 250 μΐτι.

Figure 1a) shows a schematic diagram illustrating a structure 100 for coupling light into a thin-film waveguide 102. The structure 100 comprises one or more inclined light guiding channels 103 to 106 in a thin-film layer 108 formed on the thin-film planar waveguide 102. It is noted that the inclined light guiding channels are not exploited as a grating coupler in the example embodiments. Rather, light is coupled into and out of the light guiding channels, with at least one internal reflection occurring within the light guiding channels. Also, in the case of the grating structure, incident light, diffracted light, and grating period or pitch must satisfy a diffraction relationship, as is understood in the art. For example, for 90 degrees with 1^st order of diffraction, the grating pitch must be shorter than the wavelength. Considering an applicable wavelength range between about 400 nm and about 1700 nm for the example embodiments, the range of grating pitch would be between 212 nm and 1202 nm. In example embodiments, the pitch of the inclined light guiding channels is longer than about 10 Mm, more preferably, longer than about 40 μιπ, i.e the inclined light guiding channels are not functioning as a grating coupler.

In the example embodiment, the effectiveness of light coupling into the waveguide 102 is strongly related to the channel 103-106 angle, which can be determined based on an incidence angle of the inputted light, and the refractive indices of the channel 103-106 materials and of the waveguide 102 materials.

The channels may be substantially non-wavelength selective. The material and the multimode size of the channels 103-106 preferably have a low transmission loss within a large wavelength range, and the refractive index may have weak dependence on the wavelength. The channels 103-106 may be substantially non-wavelength selective in a wavelength range from about 400 nm to about 1700 nm, for example. in order to obtain a high effectiveness of light coupling into the waveguide 102, it is preferred to maximize the effective light coupling region,∑W_ln , to the overall light coupling region, W,. Several approaches may be adopted in order to obtain a high effectiveness of light coupling into the waveguide 102. One example is to increase the number of inclined channels by reducing the width G_in of the gaps. Preferably, the pitch is maintained larger than about 10μιη. The depth of the inclined channels 103-106, d„ is preferably determinend by the equation,

d zae_inc +W, : < i//tan0,

, where 6_inc is the incident angle of the light from a light source, so that the said incident light reflect at least one time by inclined walls of the channels 103-106. (Figure 1c). It may be preferred for the reflected light rays to move in a forward direction relative to the inclined channels 103-106 in the waveguide 102, towards light out-coupling channels (not shown).

Figure 1b) shows a schematic diagram similar to the diagram of Figure 1a), but illustrating the light propagation in the structure 100 and the waveguide 102.

To avoid light coupled into the waveguide 102 escaping from the adjacent channels, several approaches can be adopted and are explained in detail below. With reference to Figure 2, one example approach considers all the light coupling in channels as one single region 200 to determine the width of the overall light coupling structure (W,). The reflected light which is coupled in by the first inclined channel 202 will hit the location after the last inclined channel 204, therefore the coupled-in light will not be lost. The width of the overall coupling structure (W) can be determined based on the incidence angle of the light which is coupled into the waveguide 206 through the light coupling channels, the thickness of the waveguide 206 and the refractive indices of the waveguide and channel materials.

The pitch and distribution of the individual inclined channels e.g. 202, 204 are not critical in this approach. This approach may be more suitable for thicker or waveguides, which allow for a larger lateral displacement of light coupled into the waveguide 206 between respective internal reflections at the bottom and top of the waveguide 206.

With reference to Figure 3, another example approach considers the pitch and location of the individual inclined channels e.g. 300, 302. By calculating the light paths e.g. 304, 306 and a design such that the reflected light will only hit the downstream gap locations e.g. 308, 310, the pitch and location of the individual inclined channels can be determined. The pitch and the distribution can be determined based on the incidence angle of the light that is coupled into the waveguide 312 through the light coupling channels, the thickness of the waveguide 312 and the refractive indices of the waveguide and channel materials. The pitch and the distribution may be uniform, periodical, or non-uniform. This approach may be more suitable for thinner waveguides, where the lateral displacement of the light coupled into the waveguide 312 between respective internal reflections is smaller.

Figure 4 shows a schematic diagram of another example embodiment of a light coupling structure 400 for coupling light into a thin-film planar waveguide 402. The structure 400 in this example embodiment comprises, in addition to one or more inclined light coupling channels 403 to 406 formed in a thin-film layer 410 above the waveguide 402, a capping layer 411 formed on, and extending across the inclined channels 403 to 406.

As shown in Figure 5, the capping layer 500 can be fiat. The capping layer 500 can improve the coupling efficiency compared to an embodiment in which no capping layer is used, as a result of light e.g. 502 entering the capping layer 500 in the gap region 504 between adjacent inclined channels 506, 508, being directed into the inclined channel 508 after multiple internal reflections at top and bottom surfaces of the capping layer 500 respectively. Alternatively, the capping layer can comprise some structures that may enhance the light coupling effect. For example, with reference to Figures 6a) to 6c), the capping layer 600a, b, c, d can comprise a curved shape at the gap regions e.g. 604 at a bottom and/or top surface of the capping layer 600a, b, c, d. The curved shape may be preferred to be formed on the side of active optical elements (not shown) as a result of downstream processing for the optical elements during a monolithic integration process. The curved shapes are for example designed such that light entering the capping layer 600a, b, c substantially in the gap regions e.g. 604 is internally reflected such that the light enters the adjacent inclined channel 608 after at least one internal reflection at the top surface of the capping layer 600a, b, c. Additionally or alternatively, the angle of light entering the adjacent inclined channel 608 can be manipulated as a result of the reflection at the curved shapes to optimize or control the coupling into the waveguide 610. In yet another embodiment, the capping layer 600d may comprise a micro lens structure 612 to assist focusing of light. Materials for the lens can include, but are not limited to, for example glass, plastic, quartz. Potential advantages provided by including a capping layer in example embodiments can include:

(1) Flattening the top surface for the overall light coupling structure, which can facilitate downstream processes, such as deposition of organic light transmissive devices during a monolithicaliy integrated process.

(2) Sealing of non-solid cladding materials (gas, liquid; e.g.: air, water) around the light coupling channels to prevent leakage.

(3) Enhanced light coupling efficiency by:

- Guiding the light that is incident at the gap regions to the light coupling channel regions (see Figure 5).

- Using curved shapes such as a concave or convex surface at the gap region, not only can the light that is incident at the gap regions be guided to the light coupling channel regions, but the incident angle will also change which may provide a more favourable incident angle at the light coupling channels, (see Figures 6a) to 6c)). The curved shape, e.g. a convex / concave surface, is preferred on the side of active optical elements as a result of downstream processing for the optical elements during a monolithic integration process.

- Concentrating more light to the region of the respective light coupling channels where there exists an optimum or better coupling efficiency compared to other regions.

The efficiency of light coupling into the channel may not be uniform. For example, for each channel, there may exist a particular position at its top surface where coupling efficiency is at its highest. This may be the case, for example, when the capping layer comprises a micro lens structure, (see Figure 6d))

- Changing the angle of the inputted light. For example, a grating at the capping layer or the capping layer providing a function similar to a polarizer. (4) Modulate the optical characteristic of the input light. For example, in the case of a broadband input light source and waveguide application such as bio-sensing, that require specific wavelength range, the capping layer can have an optical filter function, noting that the light coupling structure itself is preferably not wavelength selective.

Figure 7a) shows a schematic diagram of a structure 700 for coupling light out of a thin-film planar waveguide 702 according to an example embodiment. One or more inclined channels 703 to 706 are provided in a thin-film layer above the planar waveguide 702 to maximize the efficiency of light coupled out, and to make the direction of the coupled out light closer to the direction perpendicular to waveguide 702. Figure 7b) shows a schematic diagram similar to Figure 7a), illustrating the mode propagation in the waveguide 702, and coupling-out of different modes via respective inclined channels 703 to 706. The following design criteria are suggested. However, the present invention is not limited to the following design criteria.

- d₀: The depth of the light coupling structure 700 is preferably determined by the equation,

, where 6g_Uide is the largest angle that derived from the propagation constant of the guided light, so that the said guided light reflect at least one time by the inclined walls of the channels, as illustrated in Figure 7c.

- W₀: can be determined by the waveguide thickness d_WG> the highest propagation mode and lowest propagation mode inside the waveguide (i.e.

and T respectively).

- The widths W_on of the individual channels: can be determined by the highest and lowest modes for the light ray emitted from the individual channel (for example: For channel 1 , they are T_i+n+m and T_j+n respectively). Moreover it may be preferred that W₀₁≤ W_o2 < W_o3≤W_o4.

- The inclination angle of individual channels: can be determined by the highest and lowest modes for the light ray emitted from the individual channel (for example: For channel 1 , they are T_i+n+m and T_i+n respectively). Moreover it may be preferred that θ₀ι≥

Width of individual gap G_0n between channels: can be determined by the lowest mode at the previous channel and the highest mode at the following channel (for example: For G₀i, they are T^ and T_i+j+k respectively). Moreover, it may be preferred that

Figures 8a) to 8g) show respective schematic drawings illustrating different example architectures for multi-layer structures embodying the present invention. It is noted that each architecture may comprise one or more light coupling structures for coupling light into the planar waveguide and/or one or more coupling structures for coupling light out of the planar waveguide, as described above with reference to Figures 1 to 7.

Figure 8a) shows a stand-alone multilayer structure 800 comprising a waveguide layer 802, an inclined channel layer 804, a capping layer 806, and an organic light transmissive device layer 808. In this embodiment, the light source can be integrated with the multi layer structure 800, for example by providing the organic light transmissive device large 808 in the form of an organic light emitting device (OLED) incorporating an electroluminescence layer, as is understood in the art.

Figure 8b) shows another multilayer structure 810 comprising a waveguide layer 812, an inclined light coupling channel layer 814, and an organic light transmissive device layer 816. In this embodiment, no capping layer is provided between a light transmissive device, such as an OLED, formed in the organic light transmissive device layer 816 and the inclined light coupling channel layer 814.

Figure 8c) shows another multilayer structure 820 which is similar to the structure 800 shown in Figure 8a), except that no organic light transmissive device layer (compare 808 in Figure 8a)) is provided. The same layers have been given the same numerals as in Figure 8a). The multilayer structure 820 can be used in conjunction with separate/stand-alone light sources. Figure 8d) shows another multilayer structure 830 which is similar to the structure 810 shown in Figure 8b), with the exception that no organic light transmissive device layer (compare 816 in Figure 8b)) is provided. The same layers have been given the same numeral as in Figure 8b). The multilayer structure 830 can be used in conjunction with separate/stand-alone light sources.

It is possible to have a transparent (for example, transparent at wavelength between about 400nm to 1700nm) substrate in between each layer of the multilayer structures. One or more than one transparent substrates may be incorporated, in between any one of the layers.

For example, Figure 8e) shows a multilayer structure 840, which is similar to the multilayer structure 800 of Figure 8a), with a transparent substrate layer 842 incorporated between the organic light transmissive device layer 808, and the capping layer 806. The same layers have been given the same reference numerals as in Figure 8a).

In between each layer of the multilayer structures, it is possible to have a transparent adhesive layer. One or more than one adhesive layers may be incorporated in between any one of the layers.

For example, Figure 8f) shows a multilayer structure 850, which is similar to the structure 800 shown in Figure 8a), but incorporating an adhesive layer 852 between the organic light transmissive device layer 808 and the capping layer 806. The same layers have been given the same numerals as in Figure 8a). For completeness, it is noted that the multilayer structure in example embodiments can have both one or more transparent substrates and one or more adhesive layers between any one of the layers of the multilayer structures. The multilayer structures can be attached on a substrate or it can be a standalone multilayer structure, i.e. with the substrate removed. For example, Figure 8g) shows an example multilayer structure 860 similar to the multilayer structure 840 of Figure 8e), but including a substrate 862. The same layers have been given the same reference numerals as in Figure 8e). The multilayered structures in example embodiments, including the multilayer structures described above with reference to Figures 8a) to 8g), may additionally be mounted on a separate supporting substrate. A number of different processes may be used for the attachment of the multilayer structures to the supporting substrate, including, but not limited to, a photon and/or heat activated bonding mechanism directly between the multilayer structure and the supporting substrate, or a lamination-type process using a bonding layer between the multilayer structure and the supporting substrate, with pressure and/or photons and/or heat being provided as part of the lamination process.

An anti-reflection coating (ARC) layer may also be formed on the substrate 862 by depositing ARC materials on the substrate 862. Figure 9 shows a flowchart 900 illustrating the formation of single or multiple planar waveguide components for formation of the waveguide layer in an example embodiment. Here, the flowchart starts at step 902 with the provision of a substrate or a previously formed layer functioning as the substrate. At step 904, optional cleaning and conditioning are allowed for, as well as in step 906 allowing for deposition of an optional adhesion promoting layer. At step 908, the material(s) for formation of the waveguide layer are coated onto the substrate or cladding layer functioning as a substrate, or optionally on the deposited adhesion promoting layer. The formation and dimensioning of the waveguide can then be performed for example by a photolithography processing, step 910, or imprinting processing, step 912, or through hot embossing/thermal compression processing, step 914. The processes 910, 912, and 914 are each understood in the art, and will not be described in detail herein. Steps 904 to 910/912/914/916 may be repeated for fabricating multiple waveguide components making up the overall waveguide layer of multilayer structures embodying the present invention.

Figure 10 shows a flowchart 1000 illustrating the formation of single or multiple planar waveguide components for formation of the inclined guiding channel layer in an example embodiment. Here, the flowchart starts at step 1002 with the provision of a substrate or a previously formed layer functioning as the substrate. At step 1004, optional cleaning and conditioning are allowed for, as well as in step 006 allowing for deposition of an optional adhesion promoting layer. At step 1008, the material(s) for formation of the waveguide layer are coated onto the substrate or cladding layer functioning as a substrate, or optionally on the deposited adhesion promoting layer. The formation and dimensioning of the inclined channels can then be performed for example by an inclined UV exposure processing, step 1010, or laser excitation processing or laser writing, step 1012, or through imprinting, step 1014, or through hot embossing thermal compression processing, step 1016. The processes 1014, and 1016are each understood in the art, and will not be described in detail herein.

An inclined UV exposure process 1010 may be carried out. The inclined UV exposure may mean that the UV exposure may be carried out at an inclined angle relative to the device substrate. Similarly, the laser excitation or laser writing 1012 may be carried out by either inclining the substrate or by inclining the laser ray.

Figures 11a) and 11 b) are schematic drawings illustrating formation of the inclined light guiding channels using a laser head 1100 generating a coliimated beam for laser excitation processing. In Figure 11a), the substrate coated with the waveguide material(s) is being inclined relative to the laser head 1100, using a suitable substrate holder/manipulator understood in the art (not shown). In Figure 11 b) the laser emission from the laser head 100 is changed either by directly manipulating the laser head using a manipulation stage (not shown) understood in the art, or through suitable optics such as scanners and/or lenses, as are understood in the art. In the example configuration shown in Figure 11a) and 11b), the energy is directly illuminated to the light coupling channel material layer 1102 from the front surface, and therefore does not pass through the (substrate) layers below or affect the layers via the laser emission . Thus, preferably it does not matter whether the layer(s) below are opaque or transparent.

Figures 12a) to 12c) are schematic drawings illustrating formation of the inclined guiding channels according to one example embodiment. In Figure 12a), the substrate or a previously formed layer functioning as a substrate, indicated at 1202, coated with the material(s) layer 1204 for formation of the inclined light guiding channels is provided. In this example embodiment, the material(s) layer 1204 has undergone a baking step after coating. Next, inclined UV exposure or inclined laser excitation are performed (compare for example 11a) and 11b)), resulting in exposed areas e.g. 1206 to 1208 in the material(s) layer 1204, as shown in 12b). After an optional post exposure bake, a development step is performed, in the example embodiment such that only the exposed regions 1206 to 1208 remain, as indicated in Figure 12c). The structure as shown in Figure 12c) may be subjected to another baking step if required. The gaps between the channels 1206 to 1208 can be filled or otherwise, depending for example on the need to improve coupling efficiency by the material of channel versus gap (air/fluid/etc).

Figure 13 shows a flowchart 1300 illustrating the formation of a capping layer in an example embodiment, more particular using an in-situ partial UV exposure of the same material(s) layer as for the inclined coupling channel formation (compare Figure 10). Here, the flowchart starts at step 1302 after an initial UV exposure, and optional post exposure bake, as part of the photolithography processing for formation of the inclined coupling channel layer (compare step 1010 in Figure 10), or after the laser excitation, and optional post exposure bake, as part of the laser excitation processing (compare step 1012 in Figure 10). Without a developing step, at step 1302 UV exposure of the same material(s) layer with reduced energy dosage by, for example, partial exposure or using a grey mask, is performed for formation, dimensioning, and shaping of the capping layer. An optional post exposure bake is allowed for at step 1304, followed by a development step 1306. At step 1308, an optional baking step is allowed for. Again, full details of the photolithography processing have not been provided here, but those are understood in the art.

Figure 14 shows a flowchart 1400 illustrating the formation of a capping layer in an example embodiment, using in-situ partial UV exposure of an additional layer. Here, the flowchart starts at step 1402 with provision of a component generated from the formation of the inclined guiding channel layer (see Figure 10). At step 1404, optional cleaning and conditioning are allowed for, as well as in step 1406 allowing for deposition of an optional adhesion promoting layer. At step 1408, the material(s) for formation of the capping layer are coated onto the inclined guiding channel layer or optionally on the deposited adhesion promoting layer. At step 1410, baking is performed, followed at step 1411 by UV exposure with reduced energy dosage by, for example, partial exposure or using a grey mask. After an optional post exposure bake at step 1412, developing is performed at step 1414, followed by an optional baking at step 1416. Figure 15a) shows a flowchart 1500 illustrating the formation of a capping layer in other example embodiments, more particular formation of an external capping layer. Here, the flowchart starts at step 1502 with the provision of a transparent/support substrate or a transparent substrate with an organic light transmissive device at a backside thereof. At step 1504, optional cleaning and conditioning are allowed for, as well as in step 1506 allowing for deposition of an optional adhesion promoting layer. At step 1508, the material(s) for formation of the capping layer are coated onto the transparent/support substrate or the transparent substrate with an organic light transmissive device at a backside thereof, or optionally on the deposited adhesion promoting layer. The formation, dimensioning, and shaping of the capping layer can then be performed for example directly as part of the coating of the material(s) for the capping layer (unexposed layer, step 1510), or by a photolithography processing, step 1512, or imprinting processing, step 1514, or hot embossing/thermal compression processing, step 1516. The processes 1510, 1512, 1514, and 1516 are each understood in the art, and will not be described in detail herein.

In relation to the above flowcharts in Figures 9 to 15, it is again noted that the multilayer structure in example embodiments can have one or more transparent substrates between any one of the layers of the multilayer structures.

Where a supporting substrate was utilized in the formation of the multilayer structure, detachment of the multilayer structure from the supporting substrate may be performed for example, but not limited to, using an energy assisted process in which the supporting substrate is removed under provision of photons and/or heat, or using a mechanical process such as peel off and/or tear of processes.

Figure 15b) shows a flow-chart 1550 illustrating a method of forming either a waveguide, or waveguide with inclined light coupling channel or capping layer, the method involving molding according to an example embodiment. The method of forming either a waveguide, or waveguide with inclined light coupling channel or capping layer, comprises at step 1552 installing and closing a mold in a mold cavity of an injection machine. The method may include at step 1554 feeding a chosen raw material for the waveguide, or waveguide with inclined light coupling channel or capping layer to a feeder of the injection machine. The method may include at step 1556 heating up to melt the raw material. Further, the method may include at step 1558 injecting the molten raw material to the mold under pressure. This may be followed at step 1560 by cooling down of the molten raw material and mold. Then finally, at step 1562, the mold may be opened and the waveguide, or waveguide with inclined light coupling channel or capping layer may be ejected.

Process Integration

It will be appreciated by a person skilled in art that the multilayer structures in example embodiments can be fabricated using either a layer-by-layer (monolithic) process integration, or by bonding individually formed components of the multilayer structure, or by a combination of layer-by-layer and bonding. As such, in the flowcharts in Figures 9, 10, and 14, the "substrate" can be either the previous layer formed as part of a layer-by-layer (monolithic) process, or a separate substrate for formation of the (individual) component/layer. If the individual component/layer is detached from the substrate before bonding (or after bonding if the substrate is not at the intermediate layer between two bonded components), the substrate can be opaque or transparent. On the other hand, if the substrate is to be incorporated into the multilayer structure to be formed, a transparent substrate is used (compare for example Figure 8e), where the multilayer structure 840 can be fabricated for example by a combination of layer-by-layer (monolithic) formation of the waveguide layer 802 and the light guiding channel layer 804, followed by bonding to the capping layer 806 formed, separately, on the transparent substrate 842 having the organic light transmissive device layer 808 formed on a backside thereof. The bonding methods can be, for example, but not limited to, pick-n-place, compression, flat plate lamination, roller lamination etc. The bonding can be achieved, for example, but not limited to, by UV illumination and / or heat up, through pressure- sensitive adhesive molecule layer, pressure sensitive adhesive tape, etc.

The described embodiments can provide a solution for light coupling into and out of a planar waveguide with "large coupling area" and offer solutions for a large thickness range.

The thickness of the waveguide in example embodiments ranges between about 1μΐη - ΙΟΟΌμιη, preferably between about 5μπ - 250μΐη. The vertical distance between the light source and/or photo detector and the waveguide can be as wide as required in example embodiments and monolithic integration is possible but not a necessity.

In the following, enhancer structures, in example embodiments formed as part of the capping layer (compare e.g. Figures 8a), c), e) and f)) will be described. Figure 16 shows a schematic diagram illustrating coupling of light from a light source 1602 into a thin-film planar waveguide 1604 via the inclined coupling channels e.g. 1606. For illustrative purposes, in Figure 16 light coupling is illustrated only in areas where the light is incident in the regions of the top surfaces e.g. 1608 of each channel e.g. 1606, but it will be appreciated that light incident in the areas of the "gaps" between the inclined channels can be propagated/directed by way of the capping layer of 1610 into adjacent inclined channels, as described above with reference to Figures 6a) to 6c).

For an inclination angle Θ, of 45° in an example embodiment, Figure 17 shows simulation results of the coupling efficiency (CE) as a function of the incident light angle, 0 inci_dent- As can be seen in Figure 17, there exists a maximum at about 78.8° in the coupling efficiency, noting that, in addition to the dependency on the angle <9_1st, the results will depend on the reflective indices of the material(s) for the inclined channels e.g. 1606 and the waveguide 1604. It is noted, however, that the curve 1700 is not dependent upon the material of the capping layer 1610. The curve 1700 shown in Figure 5 can be expressed as:

where Θ _1st = 45°, as mentioned above, and for the incident light angle in degrees. Returning to Figure 16, in the example embodiment, the light source 1602 is a non-directional light source, having for example an intensity profile 1800 as shown in Figure 18. The light source in an example embodiment may be in the form of an organic light emitting device (OLED). Formation of an OLED is understood in the art, and will not be described here in any detail. Also shown in Figure 18 are different example light vectors 1802 to 1804, for different 0 incident values.

Figure 19 shows a coupling efficiency profile 1900 corresponding to profile 1700 in Figure 17 using the same coordinate system compared to the light source intensity profile of Figure 18, and centred around 0 incident ⁼ 78.8°.

In example embodiments of the present invention, it has been recognised that the coupling efficiency of the multi-layer structures incorporating the inclined coupling channels can be improved by using enhancer structures, formed for example in the capping layer. More particular, in preferred example embodiments, the enhancer structures are designed such that, where a non-directional light source is used for coupling, but the coupling structure has a certain window (or direction) of highest coupling efficiency, refraction and reflection effects are minimised for that window (or direction).

As schematically indicated in Figure 20a), to minimise the reflection (or structural) effect against the preferred window (or direction), in an example embodiment against Θ _inCident = 78.8°, parallel enhancer structure elements e.g. 2002 are preferred, with regard to the window (or direction) of maximum coupling efficiency, indicated in Figure 20a) by arrows e.g. 2004.

On the other hand, to minimise the refractive effect against the window (or direction) of maximum coupling efficiency, here 0 incident = 78.8°, enhancer structure elements e.g. 2006 perpendicular to the window (or direction), indicated again as arrows e.g. 2004, are preferred, as shown in Figure 20b).

Figure 21 shows a schematic drawing illustrating a multi-layer structure 2100 comprising an enhancer structure 2102 formed in a capping layer 2104. For ease of illustration, only one inclined channel 2106 is shown in Figure 21 , but it will be appreciated that the multi-layer structure 2100 comprises a plurality of inclined channels in a preferred embodiment. The enhancer structure 2102 includes enhancer structure elements e.g. 2108 parallel to the preferred window (or direction), here 0_inCident = 78.8°. Furthermore, the enhancer structure 2102 comprises enhancer structure elements e.g. 2110 perpendicular to the window (or direction) of highest coupling efficiency, here # incident = 78.8°. In Figure 21 , the relevant pitch is the width of the enhancer elements e.g. 2110 which are substantially perpendicular to the incident direction, and is about 5 pm in this example embodiment.

It was found from simulation results that the multi-layer structure 2100 advantageously provides an increase in overall coupling efficiency as compared to a similar structure without the enhancer structure 2102, for the same non-directional light source. Flat resting areas 2112a, b are provided on either side of the enhancer structure 2102 in this example embodiment, for facilitating mounting of the light source 2114 (or other elements such as a photodetector).

In Figure 22, in another embodiment, each inclined light coupling channel e.g. 2206 is subjected to only one enhancer structure element 2210 perpendicular to the window (all direction) of maximum coupling efficiency, here 0 incident = 78.8°. It was found from simulation results, that the multi-layer structure 2200 can similarly improve the overall coupling efficiency compared to a similar multi-layer structure without the enhancer structure 2202. Flat resting areas 2212a, b are provided on either side of the enhancer structure 2202 in this example embodiment, for facilitating mounting of the light source 2214 (or other elements such as a photodetector).

Figure 23 shows a flowchart 2300 illustrating a method of coupling light from a light source into a planar waveguide according to an example embodiment. At step 2302, the light source is provided above the planar waveguide. At step 2304, one or more inclined light guiding channels are provided in a thin-film layer between the planar waveguide and the light source. At step 2306, light from the light source is coupled into the guiding channels such that the light coupled into each guiding channel undergoes at least one internal reflection. At step 2308, the light exiting each guiding channel is coupled into the planar waveguide for propagation in the waveguide. At step 2310, a capping layer is provided between the light source and the light guiding channels in the thin-film layer, wherein the light source is non- directional, and the capping layer is configured for reducing refraction and reflection of incident light within a window or direction of maximum coupling efficiency of the coupling from the guiding channels into the waveguide.

It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

Claims

1. A method of coupling light from a light source into a planar waveguide, the method comprising the steps of:

providing the light source above the planar waveguide;

providing one or more inclined light guiding channels in a thin-film layer between the planar waveguide and the light source;

coupling light from the light source into the guiding channels such that the light coupled into each guiding channel undergoes at least one internal reflection; coupling the light exiting each guiding channel into the planar waveguide for propagation in the waveguide; and

providing a capping layer between the light source and the light guiding channels in the thin-film layer;

wherein the light source is non-directional, and the capping layer is configured for reducing refraction and reflection of incident light within a window or direction of maximum coupling efficiency of the coupling from the guiding channels into the waveguide.

2. The method as claimed in claim 1 , wherein each guiding channel comprises a first end surface facing the light source and substantially parallel to the planar waveguide, and a second end surface facing, and substantially parallel to, the planar waveguide.

3. The method as claimed in claims 1 or 2, wherein the capping layer is configured for directing light incident in an area between adjacent first surfaces of the guiding channels towards one of the adjacent first surfaces.

4. The method as claimed in claim 3, wherein the capping layer is configured such that light incident in the area between the adjacent first surfaces of the guiding channels is directed towards the one of the adjacent first surfaces via at least one total internal reflection at a top surface of the capping layer.

5. The method as claimed in claim 3, wherein the capping layer is configured for manipulating an angle of incidence of the light incident in the area between adjacent first surfaces of the guiding channels into the one of the adjacent first surfaces

6. The method as claimed in any one of the preceding claims, wherein the capping layer comprises an enhancer structure comprising first enhancer elements substantially perpendicular to the window or direction, and second enhancer elements substantially parallel to the window or direction.

7. The method as claimed in any one of the preceding claims, further comprising:

providing a second set of one or more inclined light guiding channels in a thin-film layer on the planar waveguide;

coupling light from the waveguide into the guiding channels such that the light coupled into each guiding channel undergoes at least one total internal reflection; and

coupling the light out of each guiding channel.

8. The method as claimed in claim 7, further comprising providing a detector on the second set of guiding channels.

9. The method as claimed in claim 8, wherein each guiding channel of the second set comprises a first end surface facing the detector and substantially parallel to the planar waveguide, and a second end surface facing, and substantially parallel to, the planar waveguide.

10. A multi-layer structure for coupling light from a light source into a planar waveguide, the structure comprising:

the planar waveguide;

one or more inclined light guiding channels in a thin-film layer on the planar waveguide disposed such that the light coupled into each guiding channel undergoes at least one total internal reflection and such that the light exiting each guiding channel is coupled into the planar waveguide for propagation in the waveguide; and

a capping layer on the light guiding channels in the thin-film layer;

1 . The structure as claimed in claim 10, wherein each guiding channel comprises a first end surface facing the light source and substantially parallel to the planar waveguide, and a second end surface facing, and substantially parallel to, the planar waveguide.

12. The structure as claimed in claims 10 or 1 1 , wherein the capping layer is configured for directing light incident in an area between adjacent first surfaces of the guiding channels towards one of the adjacent first surfaces.

13. The structure as claimed in claim 12, wherein the capping layer is configured such that light incident in the area between the adjacent first surfaces of the guiding channels is directed towards the one of the adjacent first surfaces via at least one total internal reflection at a top surface of the capping layer.

14. The structure as claimed in claim 12, wherein the capping layer is configured for manipulating an angle of incidence of the light incident in the area between adjacent first surfaces of the guiding channels into the one of the adjacent first surfaces

15. The structure as claimed in any one of claims 10 to 14, wherein the capping layer comprises an enhancer structure comprising first enhancer elements substantially perpendicular to the window or direction, and second enhancer elements substantially parallel to the window or direction.

16. The structure as claimed in any one of claims 10 to 15, further comprising:

a second set of one or more inclined light guiding channels in a thin-film layer on the planar waveguide, disposed for coupling light from the waveguide into the guiding channels such that the light coupled into each guiding channel undergoes at least one total internal reflection, and for coupling the light out of each guiding channel.

17. The structure as claimed in claim 16, further comprising a detector on the second set of guiding channels.

18. The structure as claimed in claim 17, wherein each guiding channel of the second set comprises a first end surface facing the detector and substantially parallel to the planar waveguide, and a second end surface facing, and substantially parallel to, the planar waveguide.

19. The structure as claimed in any one of claims 10 to 18, further comprising the light source.

20. The structure as claimed in any one of claims 10 to 19, further comprising one or more transparent layers between any one of the layers of the structure.

21. The structure as claimed in any one of claims 10 to 20, further comprising one or more adhesion promoting layers between any one of the layers of the structure.

22. A method for fabricating a multi-layer structure as claimed in any one of claims 10 to 21.

23. The method as claimed in claim 22, comprising one or more of a group consisting of photolithography processing, UV exposure processing, laser excitation processing, unexposed layer processing, imprinting processing, hot embossing/thermal compression processing, and molding processing.

24. The method as claimed in claims 22 or 23, comprising layer-by-layer processing, bonding processing, or a combination of both.