Abstract:
A light guide array for outputting light with improved uniformity and collimation includes a supporting material and a plurality of light guides formed in the supporting material. Each of the light guides has an entrance aperture for receiving light and an exit aperture for outputting light. The light guides can be solid pipes or hollow tunnels passing through the supporting material. The supporting material can be a metal, such as Al, Au, Ni, a semiconductor material, such as silicon, poly-silicon, SiC, GaAs, or an optically transparent material. Semiconductor fabrication techniques can be used to build the array. The array can be incorporated into an optical projection system to improve performance.

Description:
TECHNICAL FIELD  
         [0001]    The invention relates generally to optical systems that convert non-homogeneous light from light sources into substantially homogeneous and collimated illumination. More particularly, the invention relates to an optical system, including a compact light guide array for delivering substantially uniform and collimated light to an area such as that of a light valve.  
         BACKGROUND  
         [0002]    Light valves based on liquid crystal display technology, as well as MEMS (Micro-electro-mechanical systems) technology, have been used in various systems and applications, which include but are not limited to projectors, projection TVs, camcorders, digital still cameras, internet appliances, cell phones and headsets. In most of light valve applications, low cost, compactness and lightweight of the illumination system is desired. In addition, a uniform, bright and stable image is an important requirement in such applications.  
           [0003]    [0003]FIGS. 1A and 1B show examples of prior art straight  10  and tapered  20  light guide integrators, respectively, that increase light uniformity in various known illumination systems.  
           [0004]    The straight light guide  10  can be a solid glass rod with polished surfaces, or alternatively, a hollow tunnel with reflective surfaces. The light enters the entrance aperture  1  and emerges from the exit aperture  2  more uniformly after experiencing multiple reflections, in case of hollow light tunnels, or multiple total internal reflections, in case of solid light rods. The light uniformity at the exit aperture  2  increases with the increase in the length L of the light guides  10 ,  20 .  
           [0005]    As shown in FIG. 1A, the entrance  1  and exit  2  apertures of the straight light guide  10  have equal W 1 ×W 2  cross-sectional areas.  
           [0006]    The tapered light guide  20  of FIG. 1B delivers more uniform and collimated light in comparison with straight light guides of FIG. 1A. A tapered light guide  20  usually has unequal cross-sectional areas A 1 , A 2  at its entrance and exit apertures  3 , 4 . Entrance  3  and exit  4  apertures can have unequal sizes and similar aperture shapes such as square, rectangular or circular, as well as unequal sizes and different aperture shapes. Such light guides have been described in U.S. Pat. No. 6,332,688, to Magarill, U.S. Pat. No. 5,829,858 to Levis and U.S. Pat. No. 4,765,718, to Henkes.  
           [0007]    Known light integrating technologies suffer from inefficiency in light coupling and lack of compactness. Therefore, there is a need for a compact, lightweight, efficient and cost-effective illumination system to provide uniform and collimated light over a predefined area, such as the active area of a light valve.  
         SUMMARY OF THE INVENTION  
         [0008]    It is an advantage of the present invention to provide a relatively compact, light weight, efficient and cost-effective illumination system capable of producing a light beam, of selected cross-section and uniform intensity, which emits into a selected numerical aperture of emission. Furthermore, the illumination system can efficiently couple light from sources having a wide variety of sizes and shapes into light valves of various shapes and sizes.  
           [0009]    A feature of the invention is the focusing of the light beam into an array of micro-guides capable of setting the numerical aperture as well as providing the desired uniformity. Micro-tunnels (hollow micro light guides) as well as micro-pipes (solid micro light guides) of selected shapes and sizes can be used to form arrays of selected shapes and sizes.  
           [0010]    One embodiment of the invention uses straight and/or tapered micro-tunnels to form an array of micro light guides that provides uniform light distribution over a certain area with a desired cone angle (numerical aperture) population.  
           [0011]    Another embodiment uses arrays with straight and/or tapered micro-pipes instead of micro-tunnels in order to provide the same function.  
           [0012]    A further embodiment of the invention provides a projection system utilizing micro-guides such as those disclosed and described herein.  
           [0013]    Additional embodiments of the invention provide fabrication methods of such micro-guide arrays.  
           [0014]    Other embodiments, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional features, embodiments and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1A shows a perspective view of a prior art straight light guide.  
         [0016]    [0016]FIG. 1B shows a perspective view of a prior art tapered light guide.  
         [0017]    [0017]FIG. 2A shows a top down view of a two-dimensional rectangular micro-tunnel array in accordance with an embodiment of the present invention.  
         [0018]    [0018]FIG. 2B shows a cross sectional view of a straight micro-tunnel array along line A of FIG. 2A.  
         [0019]    [0019]FIG. 2C shows a perspective view of a straight micro-tunnel that can be included in the array of FIG. 2B.  
         [0020]    [0020]FIG. 2D shows a cross sectional view of a straight micro-tunnel array along line A of FIG. 2A, where the micro-tunnel length l is smaller than the frame thickness t.  
         [0021]    [0021]FIG. 2E shows a cross sectional view of a tapered micro-tunnel array along line A of FIG. 2A, where the entrance aperture is smaller than the exit aperture and the micro-tunnel length l is smaller than the frame thickness t.  
         [0022]    [0022]FIG. 2F shows a perspective view of a tapered micro-tunnel of FIG. 2E, where the dimensions of entrance and exit apertures are a 1  and a 2 , respectively.  
         [0023]    [0023]FIG. 2G shows a cross sectional view of a tapered micro-tunnel array along line A of FIG. 2A, where the entrance aperture is larger than the exit aperture and the micro-tunnel length l is smaller than the frame thickness t.  
         [0024]    [0024]FIG. 2H shows a perspective view of a tapered micro-tunnel of FIG. 2G, where the dimensions of entrance and exit apertures are b 1  and b 2 , respectively.  
         [0025]    [0025]FIG. 21 shows a top view of a one-dimensional rectangular micro-tunnel array.  
         [0026]    [0026]FIG. 2J shows a top view of an array with rings of micro-tunnels.  
         [0027]    [0027]FIG. 2K shows a top view of a two-dimensional rectangular micro-pipe array in accordance with another embodiment of the invention.  
         [0028]    [0028]FIG. 2L shows a cross sectional view of a straight micro-pipe array along line A of FIG. 2K.  
         [0029]    [0029]FIG. 2M shows a cross sectional view of a tapered micro-pipe array along line A of FIG. 2K, where the entrance aperture is smaller than the exit aperture and the micro-pipe length l is smaller than the substrate thickness t.  
         [0030]    [0030]FIG. 2N shows a cross sectional view of a tapered micro-pipe array along line A of FIG. 2K, where the entrance aperture is larger than the exit aperture and the micro-pipe length l is smaller than the substrate thickness t.  
         [0031]    [0031]FIG. 3A shows a cross sectional view of a first illumination system that uniformizes and collimates a light beam in accordance with an embodiment of the invention.  
         [0032]    [0032]FIG. 3B shows a cross sectional view of a second illumination system that uniformizes and reduces collimation of a light beam in accordance with an embodiment of the invention.  
         [0033]    [0033]FIG. 3C shows a cross sectional view of a projector system in accordance with an embodiment of the invention.  
         [0034]    [0034]FIG. 4A shows a cross sectional view of a straight micro-tunnel array of FIG. 2B and illustrates its function of splitting a light beam into sub-beams. The incident light beam has a uniform angle of incidence.  
         [0035]    [0035]FIG. 4B shows a cross sectional view of a straight micro-tunnel array of FIG. 2B and illustrates its function of splitting a light beam into sub-beams. The first half of micro-tunnel array receives one part of light beam with angle of incidence +θ/2 and the second half receives second part of light beam with angle of incidence −θ/2.  
         [0036]    [0036]FIGS. 5A-5H show fabrication steps of micro-tunnel arrays using SOI wafers in accordance with an embodiment of the invention.  
         [0037]    [0037]FIGS. 6A-6F show alternative fabrication steps of micro-tunnel arrays using various types of starting materials in accordance with an embodiment of the invention.  
         [0038]    [0038]FIGS. 7A-7D show fabrication steps of micro-pipe arrays in accordance with an embodiment of the invention.  
         [0039]    It is to be understood that the drawings are solely for purposes of illustration and not as a definition of the limits of the invention. Furthermore, it is to be understood that the drawings are not necessarily drawn to scale and that, unless otherwise stated, they are merely intended to conceptually illustrate the structures and methods described herein. 
     
    
     DETAILED DESCRIPTION  
       [0040]    Described herein are: micro-tunnel and micro-pipe arrays, their fabrication methods, and optical systems employing the arrays.  
         [0041]    [0041]FIGS. 2A-2J show straight and tapered micro-tunnel arrays in accordance with various embodiments of the invention. FIG. 2A shows a plan view of a two-dimensional micro-tunnel array system  30 , which consists of micro-tunnels  32  arranged in two dimensions (x and y) and outputs a relatively uniform distribution of light over an area. Although the invention is not so limited, a micro-tunnel array  30  can have micro-tunnels  32  numbering from a few to millions, with each micro-tunnel being distinct in terms of size and shape of its cross section. For simplicity of illustration, the array  30  is shown to have twelve micro-tunnels  32  arranged in three rows and four columns. Each micro-tunnel  32  has a w 1 ×w 2  cross-section area, and the array  30  has a cross section area of W 1 ×W 2 .  
         [0042]    The invention is not limited to any particular values for w 1 , w 2 , W 1  and W 2 . However, w 1  and w 2  can range from few microns to few millimeters, and W 1  and W 2  can range from few millimeters to few centimeters. These dimensions are usually limited by available fabrication techniques and can be selected based on the particular application of the array.  
         [0043]    [0043]FIG. 2B shows a cross sectional view of FIG. 2A taken along line A. The micro-tunnels  32  of FIG. 2B are of the straight type (as shown in the example given in FIG. 2C) but can be also of the tapered type (examples shown in FIGS. 2F and 2H). As shown in FIG. 2B, the micro-tunnels  32  length and the frame  35  thickness t are equal. The supporting frame  35  material can be, but is not limited to glass, Al, Au, Ni, silicon, poly-silicon, SiC and/or GaAs. The frame  35  is preferably a supporting material thick enough to support the whole array  30 . The light incident  33  upon the entrance aperture of the array  30  is partly transmitted  34  through the micro-tunnels  32 . The rest of the light is reflected and/or absorbed by the frame surface  31   a . The light that enters the micro-tunnels  32  may experience some or no reflections at all depending on the length of micro-tunnel  1  and angle of incidence of the light beam  33  upon the entrance aperture of each micro-tunnel  32 .  
         [0044]    [0044]FIG. 2C shows a perspective view  36  of an example of a straight micro-tunnel  32  with a w 1 ×w 2  cross sectional area and a length 1, which can be included in the array  30  of FIG. 2A. The internal surface of each micro-tunnel  32  is either reflective or coated with a reflective material such as aluminum, silver and/or dielectric mirror (i.e., alternating layers of low-index and high index dielectric materials such as SiO 2  and TiO 2 ). This internal reflective coating can be a cold mirror coating, thus leading to the removal of a portion or all of infrared (IR) heat from the incident light beam  33  without the use of transmissive heat filters, which can reduce the visible light in the beam. Deposition techniques, such as sputtering, thermal or e-beam evaporation, plating and/or electroplating can be used to coat the internal micro-tunnel  32  surface and/or the outer frame surfaces  31   a  and  31   b  with a reflective layer(s).  
         [0045]    The cross-section (i.e., entrance aperture) of each micro-tunnel  32  in an array  30  can have a size and shape independent of other micro-tunnels  32  within the array  30 . In general, there is no constraint on the shape of entrance aperture of a micro-tunnel. Shapes such as square, triangular, rectangular, circular, oval, and combinations of various shapes within an array  30  are contemplated by the invention. In addition, the cross-section (i.e., entrance aperture) of the entire array  30  can have various sizes and shapes independent of the individual sizes and shapes of micro-tunnel entrance apertures.  
         [0046]    A micro-tunnel array  80  can be one-dimensional, as shown in FIG. 21. The micro-tunnels  82  are arranged in one dimension along the y-axis. This array  80  delivers more uniform light distribution along the y-axis in this case without impacting light uniformity along the x-axis.  
         [0047]    [0047]FIG. 2J shows an array  90  with rings of micro-tunnels  92  arranged along the radius r, which is capable of delivering uniform light along x- and y-axes.  
         [0048]    The ratio of the total cross-section area of the micro-tunnels to the total area of the array defines the micro-tunnel density of the array. For example, the micro-tunnel density of array  30  of FIG. 2A is (12×w 1 ×w 2 )/(W 1 ×W 2 ). This ratio can be increased by reducing k 1  and/or k 2 , i.e., the frame  31  area surrounding the micro-tunnels  32 .  
         [0049]    The ratio of transmitted light  34  to incident light  33  defines the transmission ratio of the micro-tunnel array  30 . The transmission ratio increases as the micro-tunnel density increases and both can be equal when the received light  33  and/or the micro-tunnels  32  are uniformly distributed over the array area W 1 ×W 2 .  
         [0050]    The length l (preferably several microns to several hundreds of microns) of the straight micro-tunnels  32  can be smaller than the frame  35  thickness t (preferably several hundreds of microns to several millimeters), as shown in FIG. 2D. In this case, portions of the frame  35  can be made thick enough to provide extra support to the whole micro-tunnel array  30 . A perspective view of this straight micro-tunnel is similar to that shown in FIG. 2C, except with a micro-tunnel length l less than the frame  35  thickness t.  
         [0051]    Micro-tunnels  32  of array  30  can be tapered rather than straight as shown in FIGS. 2E and 2G. The entrance aperture a 1  of the tapered micro-tunnel  32  of FIG. 2F is smaller than the exit aperture a 2 . In FIG. 2E, the micro-tunnel length l is shown smaller than t but can be extended to the full thickness t of the frame  35 .  
         [0052]    The tapered micro-tunnel  32  can be reversed as shown in FIGS. 2G and 2H such that the entrance aperture b 1  of the tapered micro-tunnel  32  is larger than the exit aperture b 2  and the length of the micro-tunnel is l&lt;t. This type of micro-tunnel array delivers light which is less uniform than the received light.  
         [0053]    In alternative embodiments of the invention shown in FIGS. 2K-2N, the homogenization and collimation of light is performed using a micro-pipe array  130  rather than a micro-tunnel array. The array  130  consists of solid micro-pipes  132 , which are made of an optically transmissive material with optically smooth back and front surfaces. In order to have total internal reflection (i.e., the reflection of the total amount of incident light at the boundary between two medium) within a micro-pipe  132 , the index of refraction of the micro-pipe  132  itself has to be larger than that of the material  135  surrounding the micro-pipe  132  and angle of incidence of light needs to be greater than the critical angle. Both the micro-pipes  132  and surrounding materials  135  can be optically transparent, thus allowing received light  33  to be fully transmitted  34  through the array  130 , except for Fresnel reflections, i.e., the reflections of a portion of incident light at a discrete interface between two medium having different refractive indices.  
         [0054]    Light incident  33  upon the entrance aperture of the micro-pipe  132  is guided within the micro-pipe  132  through total internal reflection. Light incident outside micro-pipe  132  entrance apertures gets transmitted through without experiencing total internal reflection.  
         [0055]    Micro-pipes  132  of array  130  can be straight (FIG. 2L) and/or tapered (FIGS. 2M-2N) with their length being l&lt;t. In principal, there is no constraint on shape and size of the overall micro-pipe array  130  or the shapes and sizes of micro-pipes  132  within the array  130 . The arrays of FIGS. 2A-2J can also be implemented using micro-tunnel arrays combined with micro-pipe arrays.  
         [0056]    In accordance with a further embodiment of the invention, illumination systems  100  and  200  receive light beams  103  and  203  and use micro-tunnel arrays of FIGS. 2E and 2G, respectively, as shown in FIGS. 3A-3B. Alternatively, micro-pipe arrays of FIGS. 2M and 2N can be used instead of micro-tunnel arrays of FIGS. 2E and 2G, respectively, to implement systems  100 ,  200  of this embodiment.  
         [0057]    [0057]FIG. 3A shows an illumination system  100  that produces uniform and more collimated light beam when compared to illumination system  200  of FIG. 3B. In FIG. 3A, an exemplary light beam  103  with a cone angle θ is received by micro-tunnel array  105  from a light source  102  housed in an elliptical mirror  101 . Micro-tunnel array  105  homogenizes and collimates received light beam  103 , delivering uniform light beam  104  with a cone angle α&lt;θ.  
         [0058]    Illumination system  200  of FIG. 3B receives a light beam  203  with a cone angle θ and produces a uniform and less collimated light beam  204  with a cone angle α&gt;θ.  
         [0059]    [0059]FIG. 3C shows projection system  125  that uses a transmissive LCD (liquid crystal display) panel  131 . Other panel types such as reflective LCOS (liquid crystal on silicon) and DMD (digital micro mirror) can be used. The projection systems  125  include a light source  102  housed in an elliptical mirror  101  and a micro-tunnel array  105  that homogenizes and collimates received light beam  103 .  
         [0060]    As shown in FIG. 3C, the light exits the micro-tunnel array  105  and is transmitted through relay optics, such as a condenser lens  127 . The condenser lens  127  illuminates and focuses, in a superimposing manner, the light output from the light guide array onto the image gate (entry face) of a light valve (LCD panel)  131 . The light beam passed through the light valve  131  is focused by a field lens  128  into the aperture of a projection lens  132 . The image displayed on the light valve  131  is projected onto a screen  133 .  
         [0061]    The homogenization of a light beam by a straight micro-tunnel array  30  is illustrated in FIGS. 4A-4B, which show cross sectional views of FIG. 2A along line A with exemplary received  33  and delivered  34  light beams. As shown in FIGS. 4A-4B, a straight micro-tunnel array  30  splits the light beam  33  received from a non-homogeneous light source into a large number of sub-beams A-C equal to the number of the micro-tunnels  32  in the array  30 . For illustration simplicity, the light beam  33  is illustrated as having ten rays  1 - 10 , each of which have a uniform angle of incidence  0 . In addition, the exemplary array  30  has only three rows of micro-tunnels  32 . An array  30  can have any suitable number of pipes or tunnels, up to and including hundreds of thousands or even millions of micro-tunnels  32  or pipes.  
         [0062]    The transmitted light  34  includes three sub-beams A, B and C emerging from the corresponding micro-tunnels  32 , as shown in FIGS. 4A-4B. Sub-beam A consists of rays  2  and  3 , sub-beam B consists of rays  5  and  6  and sub-beam C consists of rays  8  and  9 . Rays  1 ,  4 ,  7  and  10  are reflected and/or absorbed by the frame surface  31   a . Each sub-beam experiences a number of reflections within the corresponding micro-tunnel depending on the micro-tunnel length  32  and angle of incidence θ/2 of light rays. The sub-beams A, B and C exit micro-tunnels  32  with a cone angle α and spread over the light valve area (not shown) leading to a highly uniform distribution of light. In this case, the cone angle α of light beam  34  is equal to angle θ of received beam  33 . It is possible to decrease the cone angle α (i.e., resulting in more collimated light) by using tapered micro-tunnels of FIGS. 2E-2F, rather than the straight micro-tunnels of FIGS. 2C-2D.  
         [0063]    In case of micro-pipe arrays  130 , the above discussion applies considering that light rays experience total internal reflection within the micro-pipes rather than reflections.  
         [0064]    The micro-tunnel  30  and micro-pipe  130  arrays disclosed herein (FIGS. 2A-2N) have at least four advantages over known light guides  10  and  20 . First, the micro-tunnel  30  and micro-pipe  130  arrays provide a superior level of compactness and light-weight. The length (l&lt;t) of the these arrays  30  and  130  is lower than the length of known light guides  10  and  20  by one to three orders of magnitude resulting in very compact light-weight systems. Second, the micro-tunnel  30  and micro-pipe  130  arrays can provide higher level of light uniformity due to the large number of virtual sources formed by these arrays  30  and  130 . Images of these virtual sources are superimposed on top of each other forming an extremely uniform distribution of light over a certain area, such as the light valve area. Third, higher coupling efficiency between the light source and the light valve can be provided by the micro-tunnel  30  and micro-pipe  130  arrays resulting in more efficient use of light by the light valve. In this case, the micro-tunnel  30  and micro-pipe  130  arrays lower the cone angle population of the light beam delivered to the light valve thus enhancing coupling efficiency. Fourth, the micro-tunnel  30  and micro-pipe  130  arrays can be mass produced in an integrated circuit (IC) like process, thus providing a cost effective solution.  
         [0065]    Micro-tunnel and micro-pipe arrays disclosed herein can be made using various processes including, but not limited to, standard photolithography, silicon surface micromachining, silicon bulk micromachining, LIGA, HEXSIL, electroforming of high aspect ratio structures, nano-technology techniques and combinations of two or more of these methods.  
         [0066]    An exemplary method of fabricating micro-tunnel arrays  30  is shown in FIGS. 5A-5E. A cross sectional view of a structure for forming the array illustrated along line B of FIG. 2A is shown in FIGS. 5A-5E.  
         [0067]    [0067]FIG. 5A shows a first layer  51 , an insulating layer  52  and an optional substrate  55 . Layer  51  can be made of metals such as Al, Au and Ni, but can also be made of single-crystalline, poly-crystalline or amorphous semiconducting and/or insulating materials such as Si, SiC, Ge, GaAs, InP, diamond, and combinations thereof. Layer  52  is preferably thermally grown silicon oxide but can be made of other insulating materials (e.g. silicon nitride) that can act as an etch stop during the etching of substrate  55 . Substrate layer  55  can be silicon and can be, but is not limited to, a combination of one or more of the following materials: single-crystalline silicon, poly-crystalline silicon, amorphous silicon, single-crystalline silicon carbide, poly-crystalline silicon carbide, single-crystalline silicon germanium, poly-crystalline silicon germanium, single-crystalline diamond, poly-crystalline diamond, glass, ceramic, metal (e.g., Au, Al, and Ni), silicon nitride, and silicon oxide.  
         [0068]    The structure shown in FIG. 5A is preferably a silicon-on-insulator (SOI) structure. For such SOI structures, standard integrated circuits (IC) fabrication tools and procedures, such as photolithography, Reactive Ion Etching (RIE), deep RIE, doping, diffusion, annealing, ion implantation, metal deposition, and growth and deposition of silicon oxide, silicon nitride, and the like can be used in the fabrication process.  
         [0069]    [0069]FIG. 5B shows a cross sectional view of a patterned layer  53 , which defines a micro-tunnel array. The layer  53  is deposited on top of layer  51  and patterned as shown in FIG. 5B. This patterned layer  53  acts as a masking layer during the process of etching layer  51 . This masking layer  53  can be silicon oxide, silicon nitride, photoresist or another suitable material.  
         [0070]    [0070]FIG. 5C shows a cross sectional view of micro-tunnel array  57  after etching layer  51  in the areas that have no masking protection using, for example, Reactive Ion Etching (RIE) or deep Reactive Ion Etching (DRIE). A portion of the optional substrate layer  55  and the masking layer  53  are removed as shown in FIG. 5D using wet etching techniques such as KOH or dry etching techniques such as DRIE. The insulating layer  52  is subsequently removed as shown in FIG. 5D using appropriate etch techniques such as Reactive Ion Etching (RIE) or a timed wet etching to clear the micro-tunnels exit apertures  58 . At this point, a reflective layer  54  is deposited on the micro-tunnels internal side-walls and front side of the array. In addition, the backside of the array may be coated with this reflective layer as shown in FIG. 5E. The reflective layer  54  can be Al, Au, silver, dielectric mirror or a combination of a metal layer and dielectric layers. Deposition techniques include, but are not limited to evaporation, sputtering, plating and chemical vapor deposition.  
         [0071]    The order of the fabrication steps described above can be changed and the process can still obtain the desired structure at the end of the fabrication process. For example, one can start the fabrication process by removing a portion of the optional substrate layer  55  and subsequently removing the insulating layer  52  from the etched areas of the back side using wet etch or dry etch techniques. At this stage, a cross sectional view of the structure is shown in FIG. 5F. A masking layer  53  is then deposited and patterned as shown in FIG. 5G. The layer  51  is then etched, the masking layer  53  is removed, and reflective layer  54  is subsequently deposited as shown in FIG. 5H.  
         [0072]    Micro-tunnel arrays can be fabricated using other starting materials, as shown in FIGS. 6A and 6B. FIG. 6A shows an n-type layer  61  grown on top of a p-type substrate  65 . FIG. 6B shows n-type layer  71 , highly-doped p-type silicon layer  72  and n-type or p-type substrate  75 . Layer  72  serves as an etch stop during the backside etch of the substrate  75 .  
         [0073]    For substrates of FIG. 6A, the fabrication process preferably starts with etching a portion of the optional substrate layer  65  using wet etch methods such as electrochemical KOH. In an electrochemical KOH etch process, one layer  61  is biased in order to protect it during the etch process of a second layer  65 . In this case, the etch process removes a portion of layer  65  and stops at the n-type silicon layer  61  as shown in FIG. 6C. If a highly doped layer p-type layer  72  (FIG. 6B) is present, electrochemical KOH etching will not be necessary and regular KOH etch will stop at layer  72  as shown in FIG. 6D.  
         [0074]    In addition or alternatively, a starting material can consist of a single layer  81  such as a p-type or n-type silicon as shown in FIG. 6E. In this case, the length of the micro-tunnels will be equal to the substrate thickness and there is no need to remove material from the backside of the substrate.  
         [0075]    Alternatively, as shown in FIG. 6F, part of the substrate can be removed using timed wet or dry etch techniques to allow making micro-tunnels with length l less than the substrate thickness t. A timed etch eliminates the need for an etch stop layer. At this stage, the fabrication steps described in FIGS. 5G-5H can be used to complete the fabrication process of structures shown in FIGS. 6C-6F.  
         [0076]    Fabrication steps of micro-pipe arrays  130  are shown in FIGS. 7A-7D. FIG. 7A shows a cross sectional view of an optically transparent substrate  150  taken along line B of FIG. 2K. The top and bottom sides of the substrate can be coated with an anti-reflective layer to reduce Fresnel reflections and enhance transmission efficiency of light.  
         [0077]    A layer  151  such as silicon oxide, silicon nitride or photoresist is deposited on top of substrate  150  and patterned as shown in FIG. 7B. This patterned layer  151  defines the micro-pipe array  130 . An ion implantation and/or diffusion steps are subsequently performed leading to ions penetrating the substrate  150  surface in areas not covered by layer  151  to a depth l smaller than the substrate  150  thickness t. The layer  151  acts as an implantation or diffusion stop for preventing ions from penetrating areas of the substrate  150  located below layer  151  during the implantation or diffusion process. The layer  151  is then removed, resulting in a micro-pipe array  130  as shown in FIG. 7D. The index of refraction of these implanted regions  155  is decreased (or increased) in comparison to the non-implanted regions of the substrate  150 .  
         [0078]    A wide range of elements such as phosphorus, boron, or nitrogen can be implanted or diffused into the non-masked regions of the substrate  150 . For example, it is possible to initially use ion implantation at room temperature to implant the substrate with the desired material at a certain depth below the substrate surface and then heat the substrate to a high temperature (e.g. 700-1200° C.) to diffuse the implanted material into a higher depth. Alternatively, the substrate can be coated and patterned with a certain material such as Al and Ni and then heated to a high temperature to diffuse the coated/patterned material into the substrate eliminating the need for a masking/stop layer.  
         [0079]    The micro-tunnel and micro-pipe arrays disclosed herein have broad applications, including, but not limited to, projection displays such as projection TV, digital TV, home theater and monitors; direct-view displays and micro-displays used in gaming consoles, camcorders, cameras, cell phones, internet appliances, and headsets; lithography and photomask generation equipment; laser thermal processing; microscopy; fiber optic illumination; medical instrumentation and portable patient monitoring; GPS/navigation units; indicators on a car&#39;s dashboard; barcode scanners and test-and-measurement equipment.  
         [0080]    While specific embodiments of the present invention have been shown and described, it will be apparent to those skilled in the art that the disclosed invention may be modified in numerous ways and may assume many embodiments other than those specifically set out and described above. Accordingly, the scope of the invention is indicated in the appended claims, and all changes that come within the meaning and range of equivalents are intended to be embraced therein.