Abstract:
This invention relates to an improved design for the transmit and receive optical elements of a waveguide-based optical touch screen sensor, where at least one converging lens is incorporated within the body of each transmit and receive element. The optical elements of the improved design are more mechanically robust, easier to incorporate into the touch screen assembly, and are less susceptible to stray light and the ingress of foreign matter. In one embodiment the converging lens collimates the light into a plane wave. In another embodiment the converging lens focuses the light to an external point. In yet another embodiment, each transmit and receive element also includes at least one diverging lens. The transmit and receive elements and associated waveguides preferably comprise photo-patternable polymers.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to improved designs for transmit and receive optical elements of a waveguide-based optical touch screen sensor. 
       BACKGROUND TO THE INVENTION 
       [0002]    Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field. 
         [0003]      FIG. 1  illustrates the operation of an optical touch screen sensor  10  described in U.S. Pat. Nos. 5,914,709, 6,181,842 and 6,351,260, and U.S. patent application No. 2002/0088930 A1 and 2004/0201579 A1 (the contents of which are incorporated into this specification by way of cross-reference). In this optical touch screen sensor design, integrated optical waveguides  11 ,  12  are used to launch an array of light beams  13  across a screen, then collect them at the other side of the screen and conduct them to a position-sensitive detector A touch event  14  (eg by a finger or stylus) is detected as a shadow  15 , with position determined from the particular beam(s) blocked by the touching object. The touch screen sensors are usually two dimensional and rectangular, with two arrays (X, Y) of transmit waveguides along adjacent sides of the screen, and two corresponding arrays of receive waveguides along the other two sides of the screen. As part of the transmit side, in one embodiment a single optical source (such as a light emitting diode (LED) or a vertical cavity surface emitting laser (VCSEL)) launches light into a plurality of waveguides that form both the X and Y transmit arrays. In another embodiment, a separate optical source is used for each of the X and Y transmit arrays. In an existing design for the transmit side, the waveguide arrays guide light from the optical source to tows offenses  16  that expand the guided light beams in the horizontal (i.e. X, Y) plane, then collimate them in the horizontal plane as they are launched across the screen face. Collimation in the vertical plane may be achieved with an external vertical collimating lens (VCL), for example a cylindrical lens, not shown in  FIG. 1 . The receive side is essentially identical, and on each side the arrays of waveguides and lenses are positioned within the bezel of the screen. To minimise the width of the bezel, it is desirable for the transmit and receive elements to be as short as possible. For reasons of cost and ease of fabrication, it is highly preferred to form the waveguides and lenses out of a photo-patternable polymer material. Optical touch screens typically operate with infrared light to avoid interfering with the display, however visible light may be used if required. 
         [0004]    The transmit and receive elements of the existing design as shown in U.S. Pat. No. 5,914,709, U.S. Pat. No. 6,181,842 and U.S. 2004/0201579 A1 encounter difficulties with collimation in the vertical plane, where for ease of assembly it is convenient to use a single VCL for all transmit or receive elements in each array along the sides of the optical touch screen. The placement of a VCL and a conventional transmit element  20  on a common base  28  is shown in  FIG. 2   a  (plan view) and  2   b  (side view), with the end of substrate  26  butted against the back of VCL  23 . Common base  28  could alternatively be placed on the top surface of transmit element  20 . Transmit element  20  would normally be sandwiched between lower and upper cladding layers (as shown in  FIG. 4   b ), but these have been omitted for simplicity. It can be seen that it is difficult for the entire curved end face  21  of transmit element  20  to be positioned at the focal plane  22  of VCL  23 . Therefore while emerging rays  24  can be perfectly collimated in the vertical direction, this is not the case for rays  25 . On the transmit side, the unavoidable spread of the beam in the vertical direction from incomplete collimation is simply a source of stray light. On the receive side however, the problem is potentially more serious because of the possibility of out-of-plane stray light entering the receive elements (this effect can be seen by reversing the direction of light rays  25  in  FIG. 2   b ) 
         [0005]    It can also be seen that optimal placement of focal plane  22  with curved end face  21  depends critically on gap  27  between the apex of curved end face  21  and the end of substrate  26 . A simple approach in achieving the placement is to butt substrate  26  against VCL  23 , which can be achieved in several ways well known in the art (for example a pick-and-place machine with a vision system). Nevertheless, distance  27  is governed by the amount that the substrate  26  protrudes past the apex of curved end face  21 , and its accuracy depends on the tool used to cut the substrate. By means of alignment marks, a dicing saw typically can cut silicon wafers with an accuracy of approximately 10 μm, which may be sufficient for the present application. However for reasons of cost, it may be preferable to use plastic substrates, and unlike silicon where the only dimensional variable is thermal expansion (which is relatively easy to control), the dimensions of plastic substrates are also known to depend on humidity and thermal and mechanical history, which are far more difficult to control. For these reasons, accurate inter-layer registration is a known problem in the fabrication of multilayer plastic devices such as flexible displays. 
         [0006]    Yet another problem with the transmit and receive elements of the existing design is that curved end face  21 , being a reflective surface, must be an interface with a large refractive index difference, such as an air/polymer interface. Therefore when an upper cladding (highly desirable for optical isolation and mechanical protection of the waveguides) is being deposited, it has to be patterned so that it does not cover the curved end face, as discussed in U.S. patent application No. 2005/0089298 A1 (incorporated herein by reference in its entirety). However there is then a risk that the exposed curved end face could be damaged, for example during assembly of the optical touch screen sensor. The fact that curved end face  21  is an optical surface also means that gap  27  between transmit element  20  and VCL  23  cannot be filled with a transparent adhesive, which would aid in connecting the two components and prevent foreign matter from entering gap  27  and blocking the light. 
         [0007]    It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. 
       SUMMARY OF THE INVENTION 
       [0008]    To this end, a first aspect of the invention provides an input device for an electronic device comprising: 
         [0009]    at least one light source; 
         [0010]    at least one light detector to detect light intensity at a plurality of light detecting elements; 
         [0011]    an input area defining a plane; 
         [0012]    and a waveguide structure including a plurality of waveguides with associated slab regions composed of a first material with first refractive index, wherein: 
         [0013]    each of said associated slab regions has a substantially straight end face and contains at least one converging lens; 
         [0014]    said light source couples light into a first set of waveguides with associated first set of slab regions of said waveguide structure; 
         [0015]    said first set of waveguides directs the light into said associated first set of slab regions; 
         [0016]    said converging lenses focus the light in the plane of the input area to produce a first grid of light beams; 
         [0017]    and said first grid of light beams traverses the input area in a first direction and is directed to the light detecting elements of said light detector by a second set of waveguides with associated second set of slab regions of said waveguide structure. 
         [0018]    Preferably, the first set of waveguides with associated first set of slab legions sends beams of light across the input area towards corresponding waveguides of the second set of waveguides with associated second set of slab regions. 
         [0019]    Preferably, the converging lens is composed of a second material with refractive index different to that of said first material, and is selected from a Luneburg lens, a Fresnel lens, a chirped grating or an in-plane lens. 
         [0020]    More preferably, the converging lens is an in-plane lens. 
         [0021]    In one embodiment, the second material has refractive index less than the refractive index of said first material, and the converging lens is bi-concave, plano-concave or meniscus concave in shape. Preferably, the second material is air. In an alternative embodiment, the second refractive index is greater than said first refractive index, and the converging lens is bi-convex, plano-convex or meniscus convex in shape. 
         [0022]    Preferably, each light beam is collimated into a plane wave 
         [0023]    Alternatively, each light beam is focused to a point located within said input area 
         [0024]    Preferably, each associated slab region additionally contains at least one diverging lens. More preferably, each associated slab region contains one diverging lens and one converging lens in a beam expander configuration. 
         [0025]    In a further preferred embodiment, said light source couples light into a third set of waveguides with associated third set of slab regions of said waveguide structure; 
         [0026]    said third set of waveguides directs the light into said associated third set of slab regions; 
         [0027]    said converging lenses focus the light in the plane of the input area to produce a second grid of light beams; 
         [0028]    and said second grid of light beams traverses the input area in a second direction, different to the first direction, and is directed to the light detecting elements of said light detector by a fourth set of waveguides with associated fourth set of slab regions of said waveguide structure. 
         [0029]    Preferably, the third set of waveguides with associated third set of slab regions sends beams of light across the input area towards corresponding waveguides of the fourth set of waveguides with associated fourth set of slab regions. 
         [0030]    Preferably, said input area is quadrilateral, said first and third sets of waveguides with associated first and third sets of slab regions are arranged along adjacent first and third edges of the input area, and said second and fourth sets of waveguides with associated second and fourth sets of slab regions are arranged along adjacent second and fourth edges of the input area. 
         [0031]    More preferably, said input area is rectangular, and the second direction is substantially perpendicular to the first direction. More preferably, the end faces of the first, second, third and fourth sets of slab regions associated with the first, second, third and fourth sets of waveguides terminate are substantially parallel to the corresponding edges of the input area. More preferably the first direction is substantially perpendicular to the first and second edges of the input area, and the second direction is substantially perpendicular to the third and fourth edges of the input area. 
         [0032]    Preferably, a user provides input to the electronic device by interacting with the input area. More preferably, the user interacts with the input area with a finger or stylus. 
         [0033]    Preferably, said waveguide structure is a photolithographically defined structure. 
         [0034]    Preferably, said first material is a dielectric material. More preferably, the dielectric material is a polymer. 
         [0035]    In one embodiment the input device additionally comprises first and second external lenses proximate to the ends of the first and second sets of slab regions associated with the first and second sets of waveguides, to collimate the first grid of light beams in the direction perpendicular to the input area plane. 
         [0036]    In a further embodiment, the input device additionally comprises third and fourth external lenses proximate to the ends of the third and fourth sets of slab legions associated with the third and fourth sets of waveguides, to collimate the second grid of light beams in the direction perpendicular to the input area plane. 
         [0037]    In a further embodiment the input device additionally comprises: 
         [0038]    first and second external lenses proximate to the ends of the first and second sets of slab regions associated with the first and second sets of waveguides, to collimate the first grid of light beams in the direction perpendicular to the input area plane; and 
         [0039]    third and fourth external lenses proximate to the ends of the third and fourth sets of slab regions associated with the third and fourth sets of waveguides, to collimate the second grid of light beams in the direction perpendicular to the input area plane, wherein the end faces of the first, second, third and fourth sets of slab regions associated with the first, second, third and fourth sets of waveguides are located in the focal planes of the first, second, third and fourth external lenses. 
         [0040]    Preferably, the input device additionally comprises a transparent material between the ends of the first, second, third and fourth sets of slab regions and the first second, third and fourth external lenses. More preferably, the transparent material has refractive index substantially equal to the refractive index of said first material. 
         [0041]    Preferably, said transparent material is an adhesive, to attach each external lens to its respective set of waveguides with associated slab regions. More preferably, said transparent material has refractive index substantially equal to the refractive index of said first material. 
         [0042]    Advantageously, optical elements according to at least a preferred embodiment of the present invention are more mechanically robust, easier to incorporate into the touch screen assembly and are less susceptible to stray light and the ingress of foreign matter. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0043]    The invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
           [0044]      FIG. 1  illustrates the operation of a waveguide-based optical touch screen sensor incorporating lenses to provide in-plane focusing of the light beams; 
           [0045]      FIGS. 2   a  (plan view) and  2   b  (side view) show the positioning of a vertical collimating lens with respect to the end face of a conventional transmit element; 
           [0046]      FIG. 3  shows a transmit element containing a converging lens according to a first embodiment of the present invention; 
           [0047]      FIGS. 4   a  (plan view) and  4   b  (side view) illustrate the fabrication of a transmit element incorporating a converging lens, using photo-patternable polymers; 
           [0048]      FIG. 5  shows a transmit element according to the first embodiment of the invention, containing a piano-concave lens composed of air; 
           [0049]      FIG. 6  illustrates the emission of a sheet of light from an array of adjacent transmit elements; 
           [0050]      FIG. 7  shows another transmit element according to the first embodiment of the invention, containing a bi-concave lens composed of air; 
           [0051]      FIG. 8  shows a transmit element containing a converging lens according to a second embodiment of the invention; 
           [0052]      FIG. 9  shows a pair of transmit and receive elements, each containing a converging lens according to the second embodiment of the invention; 
           [0053]      FIG. 10  shows a transmit element according to the second embodiment of the invention, containing a biconcave lens composed of air; 
           [0054]      FIG. 11  shows a transmit element according to a third embodiment of the invention, containing a converging lens and a diverging lens; and 
           [0055]      FIGS. 12   a  (plan view) and  12   b  (side view) show the positioning of a vertical collimating lens with respect to the end face of a transmit element according to a first embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0056]    The waveguide-based optical touch screen sensor technology disclosed in U.S. Pat. Nos. 5,914,709, 6,181,842 and 6,351,260, and U.S. patent application Nos. 2002/0088930 A1 and 2004/0201579 A1, has potential application to a variety of consumer electronics devices, including mobile phones, computers, games, and personal digital assistants (PDAs). To be acceptable for such devices, it is essential that the various components be fabricated and assembled at an acceptable cost. Compared to an approach with paired arrays of optical sources and detectors, as disclosed for example in U.S. Pat. Nos. 3,764,813 and 4,301,447, this waveguide-based technology requires only one optical source and one detector, providing a significant cost advantage. With the waveguides and associated collimating optics being the enabling components of this touch screen sensor technology, it is essential to be able to mass produce them in a low cost manner, a requirement that can only be satisfied with polymer materials. Photo-patternable polymers that can be processed using a photolithography/wet development method are particularly preferred because of the ease and mild conditions (eg UV exposure followed by solvent development) by which they can be patterned, and the relatively low cost of the processing equipment. 
         [0057]    Examples of photo-patternable polymers include acrylates and siloxanes. One particularly suitable class of materials is UV curable siloxane polymers, synthesised for example by a condensation reaction as disclosed in U.S. Pat. Nos. 6,800,724 and 6,818,721 (each of which is incorporated herein by reference in its entirety). Siloxane polymers have excellent adhesion to a variety of substrate materials, including silicon, glass and plastics. A photoinitiator or thermal initiator may be added to increase the rate of curing. Examples of commercially available photoinitiators include 1-hydroxy-cyclohexyl-phenyl-ketone (Irgacure 184), 2-methyl-1[4-methylthio)phenyl]-2-morpholinopropan-1-one (Irgacure 907), 2,2-dimethoxy-1,2-diphenylethan-1-one (Irgacure 651), 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1 (Irgacure 369), 4-(dimethylamino)benzophenone, 2-hydroxy-2-methyl-1-phenyl-propan-1-one (Darocur 1173), benzophenone (Darocur BP), 1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one (Irgacure 2959), 4,4′-bis(diethylamino) benzophenone (DEAB), 2-chlorothioxanthone, 2-methylthioxanthone, 2-isopropylthioxanthone, benzoin and 4,4′-dimethoxybenzoin. For curing with visible light, the initiator may for example be camphorquinone. A mixture of two or more photoinitiators may also be used. For example, Irgacure 1000 is a mixture of 80% Darocur 1173 and 20% Irgacure 184. For thermal curing, organic peroxides in the form of peroxides (eg dibenzoyl peroxide), peroxydicarbonates, peresters (t-butyl perbenzoate), perketals, hydroperoxides, as well as AIBN (azobisisobutyronitrile), may be used as initiators. 
         [0058]    Other additives, such as stabilisers, plasticisers, contrast enhancers, dyes or fillers may be added to enhance the properties of the polymer as required. 
         [0059]    Thin films of polymer material suitable for waveguide fabrication by photo-patterning can be deposited on a substrate by a variety of methods, including spin coating, dip coating, meniscus coating, extrusion coating and slot coating. These thin films can then be photo-patterned with light either through a mask, for example in a mask aligner or stepper, or by a laser direct writing procedure. Exposure through a mask is generally preferred for high fabrication throughput. 
         [0060]    As explained previously, there are several problems with the existing designs of transmit and receive elements of a waveguide-based optical touch screen sensor. These problems include: incomplete collimation in the vertical plane that may allow stray light to enter the receive optics; difficulty in cutting the substrate with sufficient accuracy for simple butt-placement of a vertical collimating lens; the risk of mechanical damage to the exposed curved end faces; and the fact that an adhesive cannot be placed between the transmit or receive elements and the vertical collimating lenses. 
         [0061]    The following section will concentrate on modified designs for the transmit elements, however it will be appreciated that the receive elements are in general mirror images of the transmit elements. Unless stated otherwise, all design modifications for the transmit elements apply equally well to the receive elements. 
         [0062]    With reference to  FIGS. 2   a  and  2   b , all of the abovementioned problems are caused by the fact that in the existing design, collimation in the horizontal plane occurs at curved end face  21 , which is an optical surface. In the present invention, if the horizontal collimation were to be performed by one or more converging lenses incorporated within the body of the transmit element, the end face could be made straight and butted against the vertical collimating lens. Many classes of converging lenses suitable for planar waveguides are known in the art, including: 
         [0000]    geodesic lenses comprising a spheroidal indentation in the waveguide surface (“Geodesic optical waveguide lens analysis”, W. H. Southwell,  J. Opt. Soc. Am  67, 1293-1299 (1977);
 
Luneburg lenses comprising a shaped overlay of high index material (U.S. Pat. No. 4,856,861; U.S. Pat. No. 4,979,788);
 
chirped gratings (U.S. Pat. No. 4,262,996; U.S. Pat. No. 4,440,468); and
 
Fresnel lenses of various types (U.S. Pat. No. 4,367,916; U.S. Pat. No. 4,445,759; “Ion-milled waveguide lenses and lens arrays in GaAs”, T. Q. Vu &amp; C. S. Isai,  J. Lightwave Technol.  7, 1559-1566 (1989)).
 
         [0063]    In terms of fabrication however, the most convenient converging lenses are simple in-plane convex or concave lenses composed of a transparent material with refractive index different to that of the waveguide material. In-plane lenses that converge or diverge light in planar waveguides are well known in the art, where the lens material has either higher or lower refractive index than the surrounding waveguide (“A new guided-wave lens structure”, M. M. Minot and C. C. Lee,  J. Lightwave Technol.  8, 1856-1865 (1990); “Design of low-loss tapered waveguides using the telescope structure compensation”, C. W. Chang, M. L. Wu and W. F. Hsieh,  IEEE Photon. Technol Lett  15, 1378-1380 (2003); JP 56078814A; U.S. Pat. No. 4,755,014; U.S. Pat. No. 5,253,319; U.S. Pat. No. 6,980,718; U.S. Pat. No. 6,935,764) In the present application, where the transmit elements are composed of polymer, the material used for the lenses must be compatible with the polymer, particularly in terms of processing conditions (eg deposition temperature) Generally, it is preferable for the refractive index contrast to be as large as possible, to minimise the length of the transmit element. It is particularly preferred that the in-plane lens(es) be composed of air, because the refractive index contrast is relatively large (polymer index n polymer ˜1.513, air index n air ˜1), and the processing is simple (no additional processing steps are required). Since the refractive index of air is less than that of polymer, an in-plane converging lens composed of air will be concave (eg bi-concave, piano-concave or meniscus concave) in shape. Alternatively, the in-plane converging lens may be composed of a material of higher refractive index, in which case it will be convex (eg bi-convex, plano-convex or meniscus convex) in shape. 
         [0064]    The present invention and U.S. patent application No. 2006/0088244A1 (incorporated by reference in its entirety) both describe in-plane converging lenses included within a slab region, but they differ in that the included lenses of the present invention perform the required collimation of signal light by themselves (so that the slab region has a straight end face), while in U.S. 2006/0088244 A1 the slab region has a curved end face that contributes to the collimation process. It should be understood that the transmit and receive elements of the present invention may also include one or more diverging lenses, as can the transmit and receive elements of U.S. 2006/0088244 A1. 
         [0065]    A transmit element  30  incorporating a converging lens according to a first embodiment of the present invention is shown in  FIG. 3 . Transmit waveguide  31  guides light  32  into polymer slab waveguide region  33  at point  34 , whereupon it spreads with divergence angle 2φ and encounters converging lens  35  that collimates the light into rays  36 , parallel to optical axis  37 , that exit end face  38  to form transmitted plane wave  39 . Crucially, end face  38  is straight, and can be readily cut with a dicing saw, laser cutter or the like. End face  38  is also perpendicular to optical axis  37 , so that rays  36  encounter it at normal incidence and are not refracted. Preferably, transmit waveguide  31  is symmetrically located with respect to converging lens  35 . More preferably, transmit waveguide  31  and converging lens  35  are symmetrically located with respect to polymer slab waveguide region  33 . Converging lens  35  can have a variety of shapes, so long as it has sufficient width to capture all light rays within divergence angle 2φ and the correct curvature to collimate the rays along optical axis  37 . 
         [0066]    A method for fabricating a transmit element  30  incorporating a converging lens is described in the following non-limiting example, with reference to  FIGS. 4   a  and  4   b    
       EXAMPLE 1 
       [0067]    Following the procedure disclosed in U.S. Pat. No. 6,818,721, a lower refractive index polymer A was prepared with a viscosity of 2500 cP (at 20° C.) and a refractive index (measured at 20° C. on an Abbé refractometer with room light) of 1.483. A higher refractive index polymer B was prepared with a viscosity of 2200 cP (at 20° C.) and a refractive index of 1.509 (at 20° C.). A suitable photoinitiator was added to both polymer A and polymer B 
         [0068]    Polymer A was spin coated onto silicon wafer  40  and cured with UV light from a mercury lamp, to form lower cladding layer  41  with thickness 20 μm and refractive index 1.485 (at 20° C. and 850 nm). Polymer B was spin coated onto lower cladding layer  41  to form core layer  42 , and patterned with UV light through a mask. The unexposed polymer B material was then dissolved in isopropanol to form input waveguide  31  and transmit element  30  incorporating converging lens  35  composed of air. Exposed core layer  42  had thickness of 11 μm and a refractive index of 1.513 (at 20° C. and 850 nm). Finally, a protective upper cladding layer  43  was deposited by spin coating and UV curing a second layer of polymer A. Note that it is necessary to pattern upper cladding layer  43  in the same manner as for the core layer (as disclosed in U.S. patent application No. 2005/0089298 A1), to avoid in-filling converging lens  35  with cured polymer A. Although converging lens  35  is filled temporarily with uncured polymer A, this material is removed in the subsequent development step. The exact positioning of opening  44  in patterned upper cladding layer  43  is not particularly important, so long as converging lens  35  remains uncovered. If additional mechanical protection is required, a cover plate  45  may be fixed above converging lens  35 , however this may not be necessary since in the assembled touch screen sensor; transmit element  30  will generally be located within the bezel of the screen. 
         [0069]    It will be appreciated that if converging lens  35  is composed of air, no additional process steps are required to incorporate it within transmit element  30 . It requires nothing more than a modification of the core layer and upper cladding layer mask designs, and is therefore preferred for ease of fabrication. It would be possible however, at the expense of additional process steps, to fill converging lens  35  with some other curable polymer C, with refractive index significantly different from polymer B, either before or after upper cladding layer  43  is deposited and patterned. It will be further appreciated that, with appropriate mask design, a lens  35  of virtually any shape can be incorporated within transmit element  30 . 
         [0070]    All subsequent examples describe exemplary transmit elements fabricated by the process described in Example 1. Unless stated otherwise, input waveguide  31  has a width of 8 μm and a height of 11 μm. With these dimensions, and with a relatively large core/cladding refractive index difference (ie difference between refractive indices of cured polymers A and B) of 0.028, it will be appreciated by those skilled in the art that input waveguide  31  will be multi-moded (ie it will support several optical modes) at a typical operating wavelength of 850 nm. These parameters also fix the divergence angle 2φ, which is measured experimentally to be approximately 16° 
         [0071]    The following two non-limiting examples describe transmit elements  30  according to a first embodiment of the invention, where a single included converging lens  35  composed of air is used to collimate the transmit light into a plane wave (as shown in  FIG. 3 ). The elements are designed for a wavelength of 850 nm, and for simplicity a geometrical ray optics approach is used in these and all subsequent examples, ie point  34  (where light  32  from input waveguide  31  enters slab waveguide region  33 ) is assumed to be a point source, and the diverging light rays are assumed to form a spherical wavefront within slab region  33 . In practice, the finite width and (generally) multimode nature of input waveguide  31  results in a complex wavefront, however this does not affect the essence of the invention. 
       EXAMPLE 2 
       [0072]      FIG. 5  shows a transmit element  30  according to the first embodiment of the invention, containing a plano-concave lens  50  comprising a concave front surface  51  and a planar back surface  52 . In this example, plano-concave lens  50  is composed of air and extends across the full width of polymer slab waveguide region  33 , thereby splitting it into first slab waveguide region  53  and second slab waveguide region  54 . Such a design may be advantageous for a wet development fabrication process, to aid the passage of solvent and the removal of unexposed material. It will be appreciated that second slab waveguide region  54  allows end face  38  to be straight but has no focusing function, and can therefore be of any length. For example it may be made arbitrarily short to minimise the overall length of transmit element  30 . In practice, second slab waveguide region  54  should have a length  55  of at least 30 μm, to allow sufficient margin for the dicing process that forms end face  38 . To minimise the overall length, gap  59  between first slab waveguide region  53  and second slab waveguide region  54  should be made as small as possible within the limits of the fabrication process. For example, with photo-patternable polymers processed using a photolithography/wet development method, gap  59  should be at least 5 to 10 μm. Note that with other materials and/or other photolithography tools, gap  59  could possibly be made smaller. First slab waveguide region  53  has length  56  of 2670 μm and width  57  of 750 μm, chosen such that light diverging in first waveguide slab region  53  within angle 2φ=16° will fill concave front surface  51 . It is a well known result of geometrical optics (“Optics”, E. Hecht, 2 nd  ed, Addison-Wesley (1987), p. 129-132) that if light rays emanating from a point source in a medium of higher refractive index n 2  pass into a medium of lower refractive index n 1  through an ellipsoidal interface, they will emerge perfectly collimated provided the interface is a portion of an ellipse chosen such that its eccentricity is equal to n 1 /n 2  and the point source is located at its farther focus. In the present case therefore, concave front surface  51  needs to be a portion of an ellipse with eccentricity=n air /n core ˜1/1.513 and with farther focus at point  34 . 
         [0073]    With this particular design of first slab waveguide region  53  and plano-convex lens  50 , collimated light rays  58  will be emitted from the entire width of second slab waveguide region  54 , so that when several transmit elements are placed adjacent to each other in an array, as shown in  FIG. 6 , they will emit an essentially uninterrupted sheet of light U.S. patent application No. 200410201579 A1 teaches that this type of output is preferable for an optical touch screen sensor since, compared to an alternative configuration with discrete beams separated by considerable dark regions, a sheet of light minimises the required dynamic range of the photodetectors associated with the X, Y receive arrays, enhances the grey scale interpolation of the position sensing algorithms, and minimises the chance that a thin touching object could be missed by the beams. A configuration with discrete beams also complicates the manufacturing process, because the receive side waveguides need to be critically aligned (in the horizontal plane) with the transmit side waveguides, whereas with a sheet of light, the horizontal positioning of the receive side waveguides is non-critical. 
         [0074]    Nevertheless, other considerations may indicate that a sheet of light is not the optimal configuration. For example, as pointed out in U.S. patent application No. 2006/0188196 A1 (incorporated herein by reference in its entirety), polymer materials typically have large thermo-optic coefficients (ie their refractive index varies significantly with ambient temperature) and so the refraction at lens surface  51  (governed by Snell&#39;s law) will be temperature dependent. Therefore while an elliptical concave front surface  51  of plano-concave lens  50  will collimate the light perfectly at one particular temperature, it will not do so at any other temperature, representing a source of optical power loss and possible cross-talk into adjacent receive elements. To allow for this temperature effect, it may be preferable to have a smaller “fill factor” for each transmit element, so that only some fraction of piano-concave lens  50  is illuminated by light diverging within first slab waveguide region  53 . 
       EXAMPLE 3 
       [0075]      FIG. 7  shows another transmit element  30  according to a first embodiment of the invention, containing bi-concave lens  70  composed of air and comprising concave front surface  71  and concave back surface  72 . Bi-concave lens  70  extends across the full width of polymer slab waveguide region  33 , splitting it into first slab waveguide region  73  and second slab waveguide region  74 . Concave front surface  71  is designed to be an are of a circle centred on point  34 , so that light rays emanating from this point encounter front surface  71  at normal incidence and pass through without being refracted. The light rays then encounter concave back surface  72 , which is designed to collimate them into output rays  75 . In another well known result of geometrical optics (“Optics”, E. Hecht, 2 nd  ed, Addison-Wesley (1987), p. 129-132), if light rays emanating from a point source in a medium of lower refractive index n 1  pass into a medium of higher refractive index n 2  through a hyperboloidal interface, they will emerge perfectly collimated provided the interface is a portion of a hyperbola chosen such that its eccentricity is equal to n 2 /n 1  and the point source is located at its farther focus. In the present case therefore, concave back surface  72  needs to be a portion of a hyperbola with eccentricity=n eff /n air  and farther focus at point  34 , where n eff  is the effective refractive index of the polymer/air path between point  34  and surface  72 . 
         [0076]    A transmit element  80  incorporating a converging lens according to a second embodiment of the present invention is shown in  FIG. 8 . Transmit waveguide  81  guides light  82  into polymer slab waveguide region  83  at point  84 , whereupon it spreads with divergence angle 2φ and encounters converging lens  85  that focuses the light into rays  86  that exit end face  87  and converge to an external point  88 . Preferably, as shown in  FIG. 9 , external point  88  is located on optical axis  89 , midway across screen area  90 . This symmetrical arrangement retains several preferred aspects of the first embodiment, where receive element  91  is the mirror image of transmit element  80  and each receive element is paired with a transmit element. Since the width of a touch screen is typically of order 100 mm whereas the length of transmit element  80  is of order 1 mm, the distance from point  84  to converging lens  85  is approximately two orders of magnitude smaller than the distance from converging lens  85  to external point  88 . Compared with the first embodiment, this second embodiment may be advantageous because it may be more resilient to temperature-induced variations in the polymer refractive index, as discussed above in Example 2 and in U.S. patent application No. 2006/0188196 A1. Specifically, temperature changes will move the focal point (ie the image) to and fro slightly across the screen, but will not cause a large variation in the amount of light captured at the receive elements. 
       EXAMPLE 4 
       [0077]      FIG. 10  shows transmit element  80  according to the second embodiment of the present invention, containing a bi-concave lens  100  composed of air and comprising a concave front surface  101  and a concave back surface  102 . Bi-concave lens  100  extends across the full width of polymer slab waveguide region  83 , splitting it into first slab waveguide region  103  and second slab waveguide region  104 . Concave front surface  101  is ellipsoidal in shape, comprising a portion of an ellipse with eccentricity=n air /n core ˜1/1.513 and with farther focus at point  84 . Concave back surface  102  is also ellipsoidal in shape, comprising a portion of an ellipse with eccentricity=n air /n core ˜1/1.513 but with farther focus at external point  88 . It will be appreciated that the local curvature of concave back surface  102  is much smaller than the local curvature of concave front surface  101 , since its farther focus is much more distant. 
         [0078]    It should be noted that light rays  105  are no longer perpendicular to end face  87 , and will therefore be refracted there. However since the angle of incidence will be extremely close to zero, this additional refraction will be small and can be compensated for (if required) with a minor adjustment to the shape of bi-concave lens  100 . 
         [0079]    A transmit element  110  according to a third embodiment of the present invention is shown in  FIG. 11 . In this case, in addition to the converging lens  111 , transmit element  110  also contains a diverging lens  112 . Those skilled in the art will recognise that diverging lens  112  and converging lens  111  are positioned in a beam expander configuration, described for example by Chang et al (“Design of low-loss tapered waveguides using the telescope structure compensation”,  IEEE Photon Technol. Lett.  15, 1378-1380 (2003)) As disclosed in U.S. patent application No. 2006/0088244 A1, diverging lens  112  serves to increase the divergence angle of light within transmit element  110 , thereby reducing its overall length. As illustrated in  FIG. 11 , converging lens  111  is designed to collimate diverging rays  113  into plane wave  114  propagating parallel to optical axis  115  in the same manner as in the first embodiment of the present invention. Alternatively, converging lens  111  could be designed to focus diverging rays  113  to an external point on optical axis  115  in the same manner as in the second embodiment of the present invention. 
         [0080]    The advantages of the transmit and receive elements of the present invention over those of the prior art will now be explained, with reference firstly to  FIGS. 12   a  and  12   b  which show an assembly of a transmit element  30  according to the first embodiment and a vertical collimating lens (VCL)  23 , and secondly to  FIGS. 2   a  and  2   b  which show an assembly of a conventional transmit element  20  and a VCL  23 . For clarity, the upper cladding layer has been omitted from  FIGS. 12   a  and  12   b . A first advantage is that end face  38  is butted directly against VCL  23 , eliminating any gap  27  between them VCL  23  and transmit element  30  will both be bonded to common base  28 , and preferably also bonded to each other at the interface between end face  38  and VCL face  120 . Preferably, the applied transparent adhesive has refractive index substantially similar to that of core layer  42 , to minimise reflection losses. More preferably, the refractive indices of core layer  42 , VCL  23  and the transparent adhesive are all substantially similar. Alternatively, an index matching fluid can be placed at the interface between end face  38  and VCL face  120 . 
         [0081]    A second advantage is that the whole of end face  38  can be placed at the focal plane  22  of VCL  23 , so that emitted light rays  121  are perfectly collimated in the vertical plane, preventing out-of-plane stray light from being emitted from transmit element  30  or from entering the corresponding receive element. Those skilled in the art will understand that when light  122  propagating through cove layer  42  traverses converging (air) lens  35 , the lack of vertical confinement will result in some out-of-plane divergence, represented by ray  123 . However such rays will be blocked by screen bezel  124  or by lower cladding layer  41  and substrate  40 , and therefore will not be a source of stray light to the receive optics. 
         [0082]    A third advantage is that in the assembly process, end face  38  of transmit element  30  is automatically located at the focal plane  22  of VCL  23 . This is in contrast to the situation shown in  FIG. 2   a , where gap  27  is a critical dimension that depends on the accuracy of the dicing process. 
         [0083]    It will be appreciated that all of these advantages stem from the feature that end face  38  of transmit element  30  is no longer a refractive surface, and can instead be made straight and perpendicular to optical axis  37 . This feature is shared by transmit elements  80  and  110  of the second and third embodiments and by the receive elements that are generally mirror images of the transmit elements. 
         [0084]    Those skilled in the art will realise that if the in-plane lenses preferred in the present invention are composed of a lower refractive index material, and in particular air, some degree of out-of-plane optical loss will occur due to reduced vertical confinement as light traverses the lenses. By way of illustration, in the piano-concave air lens structure of Example 2 shown in  FIG. 5 , the spacing between first slab waveguide region  53  and second slab waveguide region  54  will be approximately 85 μm at the extremities, assuming a gap  59  of 10 μm. The optical loss can be estimated by assuming a single Gaussian mode diverging out of a slab waveguide (first slab waveguide region  53 ) into free space, and calculating its overlap with a second slab waveguide (second slab waveguide region  54 ) after traversing a gap of a certain length. For the waveguide parameters used in this specification (wavelength 850 nm and slab height of 11 μm), the loss is estimated to be 0.016 dB after a 10 μm gap and 1.1 dB after an 85 μm gap. 
         [0085]    While a 1.1 dB (ie ˜23%) out-of-plane loss is not ideal, it is believed to be acceptable considering the simplicity and cost-effectiveness of fabricating basic in-plane concave air lenses. There are of course many means for avoiding the out-of-plane loss, using other types of converging lenses that are all within the scope of this invention. One approach would be to construct the in-plane lenses from a material with higher refractive index than the slab. Another approach would be to fabricate an air lens with a more complicated shape, such as a Fresnel lens or a chirped grating, where the air gap is shorter. Yet another approach would be to use a geodesic lens or an overlay-type lens such as a Luneburg lens or an overlay-type Fresnel lens. Compared to a simple in-plane air lens, however, all of these alternatives require further fabrication process steps, and many of them are complicated structures that require an extremely high degree of fabrication precision. 
         [0086]    Although the invention has been described with reference to specific examples it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.