Abstract:
Depth sensing imaging pixels include pairs of left and right pixels forming an asymmetrical angular response to incident light. A single microlens is positioned above each pair of left and right pixels. Each microlens spans across each of the pairs of pixels in a horizontal direction. Each microlens has a length that is substantially twice the length of either the left or right pixel in the horizontal direction; and each microlens has a width that is substantially the same as a width of either the left or right pixel in a vertical direction. The horizontal and vertical directions are horizontal and vertical directions of a planar image array. A light pipe in each pixel is used to improve light concentration and reduce cross talk.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of U.S. Provisional Patent Application Ser. No. 61/522,876, filed Aug. 12, 2011, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates, in general, to imaging systems. More specifically, the present invention relates to imaging systems with depth sensing capabilities and stereo perception, although using only a single sensor with a single lens. 
       BACKGROUND OF THE INVENTION 
       [0003]    Modern electronic devices such as cellular telephones, cameras, and computers often use digital image sensors. Imagers (i.e., image sensors) may be formed from a two-dimensional array of image sensing pixels. Each pixel receives incident photons (light) and converts the photons into electrical signals. 
         [0004]    Some applications, such as three-dimensional (3D) imaging may require electronic devices to have depth sensing capabilities. For example, to properly generate a 3D image for a given scene, an electronic device may need to identify the distances between the electronic device and objects in the scene. To identify distances, conventional electronic devices use complex arrangements. Some arrangements require the use of multiple cameras with multiple image sensors and lenses that capture images from various viewpoints. These increase cost and complexity in obtaining good stereo imaging performance. Other arrangements require the addition of lenticular arrays that focus incident light on sub-regions of a two-dimensional pixel array. Due to the addition of components, such as complex lens arrays, these arrangements lead to reduced spatial resolution, increased cost and complexity. 
         [0005]    The present invention, as will be explained, addresses an improved imager that obtains stereo performance using a single sensor with a single lens. Such imager reduces complexity and cost, and improves the stereo imaging performance. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0006]    The invention may be best understood from the following detailed description when read in connection with the accompanying figures: 
           [0007]      FIG. 1  is a schematic diagram of an electronic device with a camera sensor that may include depth sensing pixels, in accordance with an embodiment of the present invention. 
           [0008]      FIG. 2A  is a cross-sectional view of a pair of depth sensing pixels covered by one microlens that has an asymmetric angular response, in accordance with an embodiment of the present invention. 
           [0009]      FIGS. 2B and 2C  are cross-sectional views of a depth sensing pixel that may be asymmetrically sensitive to incident light at negative and positive angles of incidence, in accordance with an embodiment of the present invention. 
           [0010]      FIG. 2D  shows a cross-sectional view and a top view of a pair of depth sensing pixels covered by one microlens, in accordance with an embodiment of the present invention. 
           [0011]      FIG. 3  is a diagram of illustrative output signals of a depth sensing pixel for incident light striking the depth sensing pixel at varying angles of incidence, in accordance with an embodiment of the present invention. 
           [0012]      FIG. 4  is a diagram of illustrative output signals of depth sensing pixels in a depth sensing pixel pair for incident light striking the depth sensing pixel pair at varying angles of incidence, in accordance with an embodiment of the present invention. 
           [0013]      FIG. 5A  is a diagram of a depth sensing imager having a lens and an object located at a focal distance away from the lens, showing how the lens focuses light from the object onto the depth sensing imager, in accordance with an embodiment of the present invention. 
           [0014]      FIG. 5B  is a diagram of a depth sensing imager having a lens and an object located at more than a focal distance away from the lens, showing how the lens focuses light from the object onto the depth sensing imager, in accordance with an embodiment of the present invention. 
           [0015]      FIG. 5C  is a diagram of a depth sensing imager having a lens and an object located less than a focal distance away from the imaging lens, showing how the lens focuses light from the object onto the depth sensing imager, in accordance with an embodiment of the present invention. 
           [0016]      FIG. 6  is a diagram of illustrative depth output signals of a depth sensing pixel pair for an object at varying distances from the depth sensing pixel, in accordance with an embodiment of the present invention. 
           [0017]      FIG. 7  is a perspective view of one microlens covering two depth sensing pixels, in accordance with an embodiment of the present invention. 
           [0018]      FIG. 8  is a diagram showing a top view of two sets of two depth sensing pixels of  FIG. 7  arranged in a Bayer pattern, in accordance with an embodiment of the present invention. 
           [0019]      FIG. 9  is diagram of a cross-sectional view of two sets of two depth sensing pixels, showing light entering one light pipe (LP) in each set, in accordance with an embodiment of the present invention. 
           [0020]      FIG. 10  is diagram of a side view of the two sets of two depth sensing pixels shown in  FIG. 9 . 
           [0021]      FIG. 11  is plot of the relative signal response versus the incident angle of light entering left and right pixels in each set of pixels shown in  FIG. 9 , in accordance with an embodiment of the present invention. 
           [0022]      FIG. 12  is a top view of sets of left and right pixels arranged in a Bayer pattern, in accordance with an embodiment of the present invention. 
           [0023]      FIGS. 13A and 13B  are top views of sets of left and right pixels arranged differently so that each forms a Bayer pattern, in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    An electronic device with a digital camera module is shown in  FIG. 1 . Electronic device  10  may be a digital camera, a computer, a cellular telephone, a medical device, or other electronic device. Camera module  12  may include image sensor  14  and one or more lenses. During operation, the lenses focus light onto image sensor  14 . Image sensor  14  includes photosensitive elements (i.e., pixels) that convert the light into digital data. Image sensors may have any number of pixels (e.g., hundreds, thousands, millions, or more). As examples, image sensor  14  may include bias circuitry (e.g., source follower load circuits), sample and hold circuitry, correlated double sampling (CDS) circuitry, amplifier circuitry, analog-to-digital converter (ADC) circuitry, data output circuitry, memory (e.g., buffer circuitry), address circuitry, etc. 
         [0025]    Still and video image data from camera sensor  14  may be provided to image processing and data formatting circuitry  16  via path  26 . Image processing and data formatting circuitry  16  may be used to perform image processing functions such as data formatting, adjusting white balance and exposure, implementing video image stabilization, face detection, etc. Image processing and data formatting circuitry  16  may also be used to compress raw camera image files, if desired (e.g., to Joint Photographic Experts Group, or JPEG format). In a typical arrangement, which is sometimes referred to as a system-on-chip, or SOC arrangement, camera sensor  14  and image processing and data formatting circuitry  16  are implemented on a common integrated circuit. The use of a single integrated circuit to implement camera sensor  14  and image processing and data formatting circuitry  16  may help to minimize costs. 
         [0026]    Camera module  12  (e.g., image processing and data formatting circuitry  16 ) conveys acquired image data to host subsystem  20  over path  18 . Electronic device  10  typically provides a user with numerous high-level functions. In a computer or advanced cellular telephone, for example, a user may be provided with the ability to run user applications. To implement these functions, host subsystem  20  of electronic device  10  may have input-output devices  22 , such as keypads, input-output ports, joysticks, displays, and storage and processing circuitry  24 . Storage and processing circuitry  24  may include volatile and nonvolatile memory (e.g., random-access memory, flash memory, hard drives, solid state drives, etc.). Storage and processing circuitry  24  may also include microprocessors, microcontrollers, digital signal processors, application specific integrated circuits, etc. 
         [0027]    It may be desirable to form image sensors with depth sensing capabilities (e.g., for use in 3D imaging applications, such as machine vision applications and other three dimensional imaging applications). To provide depth sensing capabilities, camera sensor  14  may include pixels such as pixels  100 A, and  100 B, shown in  FIG. 2A . 
         [0028]      FIG. 2A  shows an illustrative cross-section of pixels  100 A and  100 B. Pixels  100 A and  100 B may contain microlens  102 , color filter  104 , a stack of dielectric layers  106 , substrate layer  108 , a photosensitive area, such as photosensitive area  110 A and  110 B formed in substrate layer  108 , and pixel separating areas  112  formed in substrate layer  108 . 
         [0029]    Microlens  102  may direct incident light towards a substrate area between pixel separators  112 . Color filter  104  may filter the incident light by only allowing predetermined wavelengths to pass through color filter  104  (e.g., color filter  104  may only be transparent to wavelengths corresponding to a green color). Photo-sensitive areas  110 A and  110 B may serve to absorb incident light focused by microlens  102  and produce image signals that correspond to the amount of incident light absorbed. 
         [0030]    A pair of pixels  100 A and  100 B may be covered by one microlens  102 . Thus, the pair of pixels may be provided with an asymmetric angular response (e.g., pixels  100 A and  100 B may produce different image signals based on the angle at which incident light reaches pixels  100 A and  100 B). The angle at which incident light reaches pixels  100 A and  100 B may be referred to herein as an incident angle, or angle of incidence. 
         [0031]    In the example of  FIG. 2B , incident light  113  may originate from the left of a normal axis  116  and may reach a pair of pixels  100 A and  100 B with an angle  114  relative to normal axis  116 . Angle  114  may be a negative angle of incident light. Incident light  113  that reaches microlens  102  at a negative angle, such as angle  114 , may be focused towards photosensitive area  110 A, and pixel  100 A may produce relatively high image signals. 
         [0032]    In the example of  FIG. 2C , incident light  113  may originate from the right of normal axis  116  and reach the pair of pixels  100 A and  100 B with an angle  118  relative to normal axis  116 . Angle  118  may be a positive angle of incident light. 
         [0033]    Incident light that reaches microlens  102  at a positive angle, such as angle  118 , may be focused towards photosensitive area  110 B. In this case, pixel  100 B may produce an image signal output that is relatively high. 
         [0034]    Due to the special formation of the microlens, pixels  100 A and  100 B may have an asymmetric angular response (e.g., pixel  100 A and  100 B may produce different signal outputs for incident light with a given intensity, based on an angle of incidence). In the diagram of  FIG. 3 , an example of the image output signals of pixel  100 A in response to varying angles of incident light are shown. As shown, pixel  100 A may produce larger image signals for negative angles of incident light and smaller image signals for positive angles of incident light. In other words, pixel  100 A produces larger image signals as the incident angle becomes more negative. 
         [0035]      FIG. 2D  illustrates an adjacent pair of pixels ( 100 A and  100 B) with the same microlens, in which pixel  100 A is formed on the right side of the pair, and pixel  100 B is formed on the left side of the pair. An adjacent pair of pixels, such as pixels  100 A and  100 B, may be referred to herein as pixel pair  200 . The pixel pair  200  may also be referred to herein as pixel type 1 and pixel type 2. 
         [0036]    Incident light  113  that reaches pair of pixels  100 A and  100 B may have an angle of incidence that is approximately equal for both pixels. In the arrangement of  FIG. 2D , incident light  113  may be focused by microlens  102 A onto photosensitive area  110 B in pixel  100 A and photosensitive area  110 B in pixel  100 B. In response to receiving incident light  113 , pixel  100 A may produce an output image signal that is high and pixel  100 B may produce an output image signal that is high by the microlens design. 
         [0037]    The respective output image signals for pixel pair  200  (e.g., pixels  100 A and  100 B) are shown in  FIG. 4 . As shown, line  160  may reflect the output image signal for pixel  100 A and line  162  may reflect the output image signal for pixel  100 B. For negative angles of incidence, the output image signal for pixel  100 A may increase (because incident light is focused onto photosensitive area  110 A of pixel  100 A) and the output image signal for pixel  100 B may decrease (because incident light is focused away from photosensitive area  110 B of pixel  100 B). For positive angles of incidence, the output image signal for pixel  100 A may be relatively small and the output image signal for pixel  100 B may be relatively large (e.g., the output signal from pixel  100 A may decrease and the output signal from pixel  100 B may increase). 
         [0038]    Line  164  of  FIG. 4  may reflect the sum of the output signals for pixel pair  200 . As shown, line  164  may remain relatively constant regardless of the angle of incidence (e.g., for any given angle of incidence, the total amount of light that is absorbed by the combination of pixels  100 A and  100 B may be constant). 
         [0039]    Pixel pairs  200  may be used to form imagers with depth sensing capabilities.  FIGS. 5A ,  5 B and  5 C show illustrative image sensors  14  with depth sensing capabilities. As shown, image sensor  14  may contain an array of pixels  201  formed from pixel pairs  200  (e.g., pixel pairs  200 A,  200 B,  200 C, etc.). Image sensor  14  may have an associated camera lens  202  that focuses light originating from a scene of interest (e.g., a scene that includes an object  204 ) onto the array of pixels. Camera lens  202  may be located at a distance D F  from image sensor  14 . Distance D F  may correspond to the focal length of camera lens  202 . 
         [0040]    In the arrangement of  FIG. 5A , object  204  may be located at distance D 0  from camera lens  202 . Distance D 0  may correspond to a focused object plane of camera lens  202  (e.g., a plane located at a distance D o  from camera lens  202 ). The focused object plane and a plane corresponding to image sensor  14  may sometimes be referred to as conjugate planes. In this case, light from object  204  may be focused onto pixel pair  200 A, at an angle θ 0  and an angle −θ 0 . The image output signals of pixels  100 A and  100 B of pixel pair  200  may be equal (e.g., most of the light is absorbed by pixel  100 A for the positive angle and most of the light is absorbed by pixel  100 B for the negative angle). 
         [0041]    In the arrangement of  FIG. 5B , object  204  may be located at a distance D 1  from camera lens  202 . Distance D 1  may be larger than the distance of the focused object plane (e.g., the focused object plane corresponding to distance D 0 ) of camera lens  202 . In this case, some of the light from object  204  may be focused onto pixel pair  200 B at a negative angle −θ 1 (e.g., the light focused by the bottom half pupil of camera lens  202 ) and some of the light from object  204  may be focused onto pixel pair  200 C at a positive angle θ 1  (e.g., the light focused by the top half pupil of camera lens  202 ). 
         [0042]    In the arrangement of  FIG. 5C , object  204  may be located at a distance D 2  from camera lens  202 . Distance D 2  may be smaller than the distance of the focused object plane (e.g., the focused object plane corresponding to distance D 0 ) of camera lens  202 . In this case, some of the light from object  204  may be focused by the top half pupil of camera lens  202  onto pixel pair  200 B at a positive angle θ 2  and some of the light from object  204  may be focused by the bottom half pupil of camera lens  202  onto pixel pair  200 C at a negative angle −θ 2 . 
         [0043]    The arrangements of  FIGS. 5A ,  5 B and  5 C may effectively partition the light focused by camera lens  202  into two halves split by a center plane at a midpoint between the top of the lens pupil and the bottom of the lens pupil (e.g., split into a top half and a bottom half). Each pixel in the paired pixel array  201  may receive different amounts of light from top or bottom half of the lens pupil, respectively. For example, for an object at distance D 1 , pixel  100 A of  200 B may receive more light than pixel  100 B of  200 B. For an object at distance D 2 , pixel  100 A of  200 B may receive less light than  100 B of  200 B. The partitioning of the light focused by camera lens  202  may be referred to herein as lens partitioning, or lens pupil division. 
         [0044]    The output image signals of each pixel pair  200  of image sensor  14  may depend on the distance from camera lens  202  to object  204 . The angle at which incident light reaches pixel pairs  200  of image sensor  14  depends on the distance between lens  202  and objects in a given scene (e.g., the distance between objects such as object  204  and device  10 ). 
         [0045]    An image depth signal may be calculated from the difference between the two output image signals of each pixel pair  200 . The diagram of  FIG. 6  shows an image depth signal that may be calculated for pixel pair  200 B by subtracting the image signal output of pixel  100 B from the image signal output of pixel  100 A (e.g., by subtracting line  162  from line  160  of  FIG. 4 ). As shown in  FIG. 6 , for an object at a distance that is less than distance D 0 , the image depth signal may be negative. For an object at a distance that is greater than the focused object distance D 0 , the image depth signal may be positive. 
         [0046]    For distances greater than D 4  and less than D 3 , the image depth signal may remain constant. Pixels  100 A and  100 B may be unable to resolve incident angles with magnitudes larger than the magnitudes of angles provided by objects at distances greater than D 4 , or at distances less than D 3 . In other words, a depth sensing imager may be unable to accurately measure depth information for objects at distances greater than D 4 , or at distances less than D 3 . The depth sensing imager may be unable to distinguish whether an object is at a distance D 4  or a distance D 5  (as an example). If desired, the depth sensing imager may assume that all objects that result in an image depth signal equivalent to distance D 2  or D 4  are at a distance of D 2  or D 4 , respectively. 
         [0047]    To provide an imager  14  with depth sensing capabilities, two dimensional pixel arrays  201  may be formed from various combinations of depth sensing pixel pairs  200  and regular pixels (e.g., pixels without asymmetric angular responses). For a more comprehensive description of two dimensional pixel arrays  201 , with depth sensing capabilities and with regular pixels (e.g., pixels without asymmetric angular responses), reference is made to Application Ser. No. 13/188,389, filed on Jul. 21, 2011, titled Imagers with Depth Sensing Capabilities, having common inventors. That application is incorporated herein by reference in its entirety. 
         [0048]    It should be understood that the depth sensing pixels may be formed with any desirable types of color filters. Depth sensing pixels may be formed with red color filters, blue color filters, green color filters, or color filters that pass other desirable wavelengths of light, such as infrared and ultraviolet light wavelengths. If desired, depth sensing pixels may be formed with color filters that pass multiple wavelengths of light. For example, to increase the amount of light absorbed by a depth sensing pixel, the depth sensing pixel may be formed with a color filter that passes many wavelengths of light. As another example, the depth sensing pixel may be formed without a color filter (sometimes referred to as a clear pixel). 
         [0049]    Referring now to  FIG. 7 , there is shown a perspective view of an embodiment of the present invention. The pixel pair  302  is similar to the pixel pair  200  shown in  FIG. 2D . The pixel pair includes left and right pixels, or as sometimes referred to as pixel type-one and pixel type-two. As shown in  FIG. 7 , a single microlens  300  (same as  102  in  FIG. 2D ) is positioned above the left and right pixels so that the single microlens spans across both pixels in the horizontal direction. 
         [0050]    Several pixel pairs  302  are shown in  FIG. 8 . Each pixel pair includes a single color filter of a CFA (color filter array) that forms a Bayer pattern. Pixel pair  302 A forms two color filters for green. Pixel pair  302 B forms two color filters for blue. Pixel pair  302 C forms two green filters. Similarly, pixel pairs  302 D,  302 E,  302 F,  302 G and  302 H form pairs of color filters producing a Bayer pattern. 
         [0051]    Referring now to  FIG. 9 , there is shown an asymmetric pixel configuration that includes microlens  300  and pixel pair  302 , similar to the pixel configuration of  FIG. 7 . It will be appreciated that  FIG. 9  shows four pixels, namely, pixels  316 A and  316 B forming one pair of pixels on the left side of the figure and pixels  316 A and  316 B forming another pair of pixels on the right side of the figure. As shown, each microlens  300  covers two pixels in the horizontal direction. A planarization layer  310  is disposed under each microlens  300 . Below planarization layer  310 , there is shown a color filter which spans across two pixels  316 A and  316 B. Thus, color filter  312  is similar in length to the length of microlens  300  and covers a pixel pair (or a set of pixels). 
         [0052]    Disposed between each color filter  312  and each pixel pair  316 A and  316 B are two light pipes (LPs). Each LP improves the light concentration that impinges upon each respective pixel. The LP improves, not only the light concentration, but also reduces cross-talk and insures good three dimensional performance, even with very small pixel pitches, such as 1.4 microns or less. 
         [0053]    As shown on the left side of  FIG. 9 , light enters pixel photosensitive area  316 B by way of LP  314 B. Similarly, on the right side of  FIG. 9 , light enters LP  314 A and pixel photosensitive area  316 A. It will be appreciated that LP  314 B, on the left side of the figure, includes most of the light, because the light passing through microlens  300  is angled at a negative angle with respect to a vertical line through microlens  300 . In a similar way, the light on the right side of the figure, enters LP  314 A, because the light passing through microlens  300  is angled at a positive angle with respect to a vertical line through microlens  300 . 
         [0054]      FIG. 10  shows the same pixels as in  FIG. 9 , except that a side-view is shown of the pixel pair. As shown, microlens  300  only spans one pixel in the vertical direction, or the column direction of a pixel array. Accordingly, microlens  300  is effective in reducing cross-talk in the vertical direction of the pixel array. Also shown in the figure is a side-view of LP  314  and pixel photosensitive area  316 . In addition, light is shown concentrated in LP  314  and passing into pixel photosensitive area  316 . 
         [0055]      FIG. 11  shows the relative signal response versus the incident angle of light entering a pixel pair. As shown, the right pixel (or pixel  314 B on the left side of  FIG. 9 ) responds strongly, when the light enters at a negative angle with respect to a vertical line passing through microlens  300 . On the other hand, when the left pixel (or pixel  314 A on the right side of  FIG. 9 ) receives light at a positive angle with respect to a normal passing through microlens  300 , the pixel also responds strongly. At normal incidence, however, the responses of the left and right pixels are relatively low. It will be appreciated that if the two pixels forming each pixel pair is summed in the horizontal direction, a normal image may be formed. On the other hand, since the left and right pixels form asymmetric pixel angular responses, the present invention obtains depth sensing capabilities. 
         [0056]    It will now be understood that an asymmetric angular response stereo sensor is provided by the present invention. By having a 2×1 CFA pattern, as shown in  FIG. 8 , the present invention may process the color normally for two separate images and obtain two separate Bayer patterns, as shown in  FIG. 12 . Accordingly, the two pixel pairs shown on the left side of  FIG. 12  may be separated into two images (the left image has two pixels and the right image has two pixels). 
         [0057]    For example, the first pixel pair provides a green color; when the pair is separated into left and right images, the present invention provides a single green pixel for the left image and a single green pixel for the right image. Similarly, when the two right pixels providing red colors are separated into left and right images, the present invention forms a left image with a red color and a right image with a red color. Thus, a 2×1 CFA pattern enables the present invention to form a normal Bayer color process for two separate images (left and right Bayer images), as shown in  FIG. 12 . 
         [0058]    Referring next to  FIGS. 13A and 13B , there are shown two different CFA/microlens arrangements, namely arrangement 1 in  FIG. 13A  and arrangement 2 in  FIG. 13B . It will be appreciated that each arrangement includes microlenses that cover 2×1 pixels, as shown in  FIG. 7 . The microlenses, however, are shown zigzag-shifted relative to each other by one pixel in neighboring rows. These arrangement result in no resolution loss in the horizontal direction and would be valuable for HD video format. 
         [0059]    In arrangement 1 shown in  FIG. 13A , the first and second rows&#39; CFA pattern is GRGRGR . . . , and the third and fourth rows&#39; CFA patterns is BGBGBG . . . . The 2×1 microlens for the first and third rows start from the first column, whereas the microlens for the second and fourth rows start one column earlier, or later. Therefore, the left image pixel array is formed by pixels L1, L2, L3, L4, L5, L6, L7 and L8. Similarly, the right image pixel array is formed by pixels R1, R2, R3, R4, R5, R6, R7 and R8. The first Bayer pattern for the left image is formed by Gr=L1 in the first row, R=L2 in the second row, B=L1 in the third row, and Gb=L2 in the fourth row. The first Bayer pattern for the right image is formed by Gr=R1 in the second row, R=R2 in the first row, B=R1 in the fourth row, and Gb=R2 in the third row. 
         [0060]    In arrangement 2, shown in  FIG. 13B , the first and third rows are an all green CFA, the second row is an all red CFA, and the fourth row is an all blue CFA. The 2×1 microlens for the first and third rows start from the first column, whereas the microlens for second and fourth rows start one column earlier, or later. Therefore, the left image pixel array is formed by pixels L1, L2, L3, L4, L5, L6, L7 and L8. Similarly, the right image pixel array is formed by pixels R1, R2, R3, R4, R5, R6, R7 and R8. The first Bayer pattern for the left image is formed by Gr=L1 in the first row, R=L2 in the second row, Gb=L1 in the third row, and B=L2 in the fourth row. The first Bayer pattern for the right image is formed by Gr=R1 in the first row, R=R2 in the second row, Gb=R1 in the third row and B=R2 in the fourth row. 
         [0061]    Referring again to  FIGS. 9  and  FIG. 10 , it will be understood that each microlens covers two pixels in the horizontal direction, but only covers one pixel in the vertical direction. Furthermore, the radius of curvature of each microlens in both directions are different due to processing limitations. The microlens material includes an optical index (n) that varies in range between 1.5 and 1.6. Furthermore, the LP may be filled by material having a higher optical index (n greater than 1.6) than its surrounding oxide material, in which the latter may have an optical index of 1.4 or 1.5. In this manner, the light is maintained within the LP. 
         [0062]    Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.