Abstract:
A high rate optical correlator is implemented on a substrate in which all of the optical devices are referenced to the flat surface of the substrate for optical alignment purposes by mounting the devices thereon. With the substrate surface as a reference point, alignment of the optical pieces is achieved to within a wavelength to eliminate the possibility of a “no correlation” result due to optical misalignment of the optical pieces. Additionally for the active elements, namely the laser, detector and spatial light modulators, interconnection of these devices and to drive sources is accomplished via direct coupling through the substrate so that the devices can communicate with each other through the silicon, thus to eliminate wire bonding and reduce pin count for the approximate 100,000 optical interconnects for a 256/256 array. Moreover, an epoxy frame which is milled at its top surface is used to mount an optical element over an active element for the alignment thereof.

Description:
RELATED APPLICATIONS 
     The present application claims priority from U.S. Provisional Patent Application No. 60/168,488 filed Dec. 2, 1999. 
    
    
     FIELD OF INVENTION 
     This invention relates to optical correlators and more particularly to a method and apparatus for solving alignment and interconnect problems. 
     BACKGROUND OF THE INVENTION 
     Optical correlators have existed in the past to provide an indication of correlation between a sample image and a reference image to provide information as to the correspondence between the sample image and the reference image. 
     One type of optical correlator is a van der Lugt image correlator which involves the utilization of a laser source, a pair of spatial light modulators, a detector and a number of optical elements for redirecting light from the laser and to provide for a Fourier transform and an inverse Fourier transform so that an optical correlation can be made. 
     One of the most serious problems with the implementation of a van der Lugt image correlator is the alignment of the optical pieces. It has been found that a misalignment of even a few wavelengths can cause a discrepancy in the correlation result. So highly accurate is the image correlation that a misalignment can cause one portion of the sample image to be shifted only minutely with respect to a corresponding location on the reference image. The result of a misalignment of even a small amount degrades the correlation obtained between the reference image and the sample image. 
     If the reference image is not aligned with the sample image then for any given area there may be no correlation, when there would be a positive correlation if the alignment were perfect. If one does not obtain a correlation where it is supposed to be, then applications such as the inspection of a semiconductor devices, analysis of mammography images and pap smears, signal identification and other applications of ontical correlation will suffer. 
     Moreover, if the alignment is not perfect, there may be false correlations across the extent of the sample image, yielding false results overall. 
     In one application in order to inspect a significant area, the correlator may analyze as many as 256/256 pixels. With correlation being determined on a pixel by pixel basis, the amount of pin outs required to interconnect all the active devices can exceed 100,000. Not only is this physically difficult with external wiring, the reliability of such a device is in question. 
     Both optical correlation systems and their components are well known as can be seen by U.S. Pat. No. 5,920,430 for Lens List Joint Transform Optical Correlator for Precision Industrial Positioning Systems; U.S. Pat. No. 5,619,496 for Method and Apparatus for Optical Pattern Recognition; U.S. Pat. No. 5,488,504 for Hybridized Asymmetric Fabry-Perot Quantum Well Light Modulator; and U.S. Pat. No. 5,951,627 for “Photonic FFT Processor”. 
     However, none of the aforementioned patents address the problems of alignment and intraconnection for optical correlators. 
     SUMMARY OF THE INVENTION 
     In order to obtain near perfect alignment and to provide a simplified system for interconnecting the active devices of an optical correlator, in the subject invention all of the optical pieces and the active devices are mounted on or in a semiconductor substrate, with the optical alignment being referenced to the flat surface of the substrate. In one embodiment, the active devices are either embedded in the semiconductor substrate or mounted on top of it, with the surface of the substrate providing a datum plane from which alignment is established. Thus, for instance, prisms, polarizing beamsplitters, spatial light modulators and detector arrays are all referenced to the datum plane established by the surface of the semiconductor substrate. 
     Moreover all optical elements such as traditional lenses, Fourier transform lenses, or other optical elements are mounted directly to the surface of the semiconductor substrate which serves as a reference or datum plane, thus providing the alignment required. 
     Mounting the optical pieces on the semiconductor substrate means for instance that the output of a laser when redirected via a prism, through a beamsplitting device and imaged onto another prism from whence it is redirected to the surface of a spatial light modulator provides an accurately controllable alignment axis for the beam. Because of the alignment provided by the surface of the substrate the beam reflected by the spatial light modulator is directed back along this accurately determined optical axis where it is redirected by a reflective beamsplitter along a further accurately controlled axis where it impinges upon a second prism, there to be redirected onto the surface of a second spatial light modulator. 
     The accuracy with which light from the first spatial light modulator is directed onto the second spatial light modulator is indeed critical because while the first spatial light modulator carries the sample image, the second spatial light modulator carries the reference to which the sample image is to be compared. 
     Any misalignment between the optical axis on which the light travels from the first spatial light modulator to the second spatial light modulator severely impacts the accuracy of the correlation. This is because locations on the sample will not correspond to the corresponding locations on the reference. 
     Having established a mechanism by which an alignment can be preserved so that on a pixel by pixel basis the images can be compared, there is nonetheless the necessity of interconnecting the spatial light modulators to drive sources which are offchip. There is also the necessity for connecting to the detector array so that some offchip device can measure the degree of correlation. Alternatively, a correlation engine may be embedded into the substrate to which the detector must be connected. 
     In a further aspect of the subject invention, a mounting technique utilizes an epoxy frame, the top surfaces of which are polished flat to provide a plane parallel to the datum plane established by the surface of the substrate. This frame is used to mount optical elements above an active device and still provide accurate alignment. 
     In the subject invention, interconnection to the arrays of pixels which exist on the spatial light modulators and indeed to the CCD detector elements are carried through embedded electrical circuits within the substrate. This eliminates the large number of connections which would be necessary and, for a 256/256 array would eliminate external connections which could number as many as 100,000. 
     Not only is the internal interconnection of the active devices of the correlator simplified through the utilization of the embedded circuits within the semiconductor substrate, pathlinks can be reduced significantly. 
     In one embodiment, in the subject invention a so-called smart CMOS platform is provided to solve the connection problem mentioned above. 
     Thus in one embodiment the subject image correlator includes a silicon substrate with the following elements mounted to the surface of the substrate or embedded in it: a laser diode, a first prism, a first beamsplitter, a second beamsplitter, an input spatial light modulator, a first detector array, an inverse Fourier transform lens, second beamsplitter, and a filter spatial light modulator. In addition a Fourier transform lens is positioned between the two beamsplitter, with all the devices being integrated directly onto a silicon chip. 
     In one embodiment the detector array is preferably a pixilated detector array using MED pixels, where MED stands for modulator/emitter/detector. Alternatively other technologies such as silicon photodiode or CCD array technology are within the scope of the subject invention. Passive components, namely the prisms, beamsplitters and lenses, can be integrated directly into subsystems, also referenced to the surface of the substrate for convenient alignment and assembly. Alternatively, the Fourier transform lens may be replaced with a holographic lens. 
     Note, if the two dimensional detector arrays are replaced with linear arrays, then the correlator can be used for spectral analysis applications including voice recognition. 
     With the ability to provide appropriate alignment utilizing the van der Lugt correlator architecture with multiple quantum well spatial light modulators, the large increase in correlation rate between an image candidate and a reference is preserved due to the small size of the correlator and the enormous processing speeds which are achievable due to the small size. The subject system enables real time correlation of single reference images and near real time correlation with multiple reference images utilizing data delivery by the CMOS circuitry which is embedded in the substrate as well as the utilization of multiple quantum well spatial light modulators. 
     In summary, a high rate optical correlator is implemented on a substrate in which all of the optical devices are referenced to the flat surface of the substrate for optical alignment purposes by mounting the devices thereon. With the substrate surface as a reference point, alignment of the optical pieces is achieved to within a wavelength to eliminate the possibility of a “no correlation” result due to optical misalignment of the optical pieces. Additionally for the active elements, namely the laser, detector and spatial light modulators, interconnection of these devices and to drive sources is accomplished via direct coupling through the substrate so that the devices can communicate with each other through the silicon, thus to eliminate wire bonding and reduce pin count for the approximate 100,000 optical interconnects for a 256/256 array. Moreover, an epoxy frame which is milled at its top surface is used to mount an optical element over an active element for the alignment thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features of the subject invention will be better understood in connection with the Detailed Description in conjunction with the Drawings of which: 
     FIG. 1 is block diagram of an optical comparator to be implemented on a substrate in accordance with the subject invention; 
     FIG. 2 is a diagrammatic illustration of the mounting of optical pieces on a substrate both from a top view and in an isometric view; 
     FIG. 3 is a diagrammatic representation of the physical mounting of a optical elements on the surface of a silicon wafer to implement the optical correlator of FIG. 1; and, 
     FIGS. 4A-4D indicate method steps for mounting active devices on the substrate of FIG. 3, indicating the mounting of an optical element above an active device and the alignment of the optical axis thereof through the utilization of an epoxy frame which has been mechanically milled such that the top surfaces of the frame are parallel to the datum plane associated with the top surface of the substrate. 
    
    
     DETAILED DESCRIPTION 
     In the subject invention, in one embodiment, a system is described that uses flip-chip mounted. GaAs based spatial light modulators to enable the implementation of a high-speed correlator on a chip. The illustrated embodiment exploits optoelectronic flip-chip techniques to provide high-speed spatial light modulation with a significant increase in frame rate over that currently available. 
     Referring now to FIG. 1, a block diagram of a van der Lugt optical comparator is shown, which is one type of comparator which may be implemented by the subject method of mounting optical elements on the surface of a silicon substrate. 
     In FIG. 1, the subject correlator  10  includes a laser diode  12 , an objective lens  14 , a pinhole  16 , a collimating lens  18 , a diaphragm  20 , a first beamsplitter  22 , a rectangular slit  24 , and an input multiple quantum well based spatial light modulator  26 . Spatial light modulator  26  is provided with a sample image  28 . 
     The system further includes a first CCD camera  30 , an inverse Fourier transform lens  32 , a second beamsplitter  34 , and a filter multiple quantum well based spatial light modulator  36 . Reference images are Fourier transformed and provided as illustrated at  37  to modulator  36 . In addition, the system includes a second CCD camera  38 , an imaging lens  40 , and a mirror  42 . Finally, the system includes a Fourier transform lens  44 , a lens  46 , an optical fiber  48 , and a spectrometer  50 . 
     In one embodiment, the laser diode operates at 860 nm, but the subject invention would work equally well if it operated in the range of 400 nm to 1600 nm. The pinhole is 25 um in diameter. The collimating lens has a focal length of 300 mm. The combination of the objective lens  14 , the pinhole  16 , and the collimating lens  18  form a beam expander with a spatial filter. Preferably, the Fourier transform lens  44  has a focal length of 231 mm, and that of the inverse Fourier transform lens  32  is 250 mm. The imaging lens  40  has a focal length of 225 mm. Both beamsplitters  22  and  34  are 50:50 beamsplitters. 
     In the present invention, the spatial light modulators are formed of arrays of multiple quantum well (MQW) GaAs based devices. The multiple quantum well spatial light modulator has a flip chip design in which a CMOS substitute has a ball grid array of solid balls. On top of this substrate is an array of multiple quantum well devices. A quartz cover is provided on epoxy standoffs in one embodiment. Alternatively the cover could be made to touch the top of the pixels and so would not be resting on epoxy standoffs. 
     Note that the multiple quantum level devices can switch as quickly as an electrical signal to them can be changed. The bandwidth is approximately 100 GHz. This means that the maximum frame rate is 100 billion frames/second instead of 10,000 frames/sec that is the best case with liquid crystal based spatial light modulators. The current state of the art with GaAs based devices is 300,000 frames/sec, where the lateral data rate into the devices from the CMOS circuitry is the limiting factor. 
     Since in the existing art the limiting factor is how the spatial light modulator is constituted, the use of a multiple quantum well device clearly means that there is an increase in the number of frames per second that can be processed. In fact, there is an increase in several orders of magnitude in the speed of the correlator in the present invention over the prior art. Thus, the subject invention solves the need to implement an optical image correlator that is significantly faster than are correlators made with liquid crystals. 
     The system depicted in FIG. 1 is a van der Lugt image correlator and it is based on Fourier transform techniques that compare converted input images with reference images provided by the filters. Filters are created by Fourier transforming reference images, and converting them to binary amplitude data. 
     System operation begins with the image  28  to be identified being coupled to input modulator  26 . Image  28  is first illuminated by a collimated laser beam from laser  12 . The modulated image is reflected onto Fourier transform lens  44  where it is converted to a Fourier transformed image. The transformed image is then directed to modulator  36  which contains a Fourier transformed rendition of the image to be recognized. The identification process involves multiplying the Fourier transform of the input image with the Fourier transformed reference image. The output then passes through inverse Fourier transform lens  32  and is displayed on CCD camera  30 . A positive correlation appears as a bright spot, or a correlation peak. The second CCD camera, camera  38 , allows the operator to see the input image. 
     As mentioned above, optical correlation is performed using reference filters. The Fourier transform filter is designed using amplitude encoded binary phase only principals (BPOF) with the BPOF filters used because of their high discrimination capability. 
     Thus, the present method for operating a high-speed optical correlator consists of displaying the image to be identified on the input spatial light modulator; illuminating the image with a collimated laser beam; passing the modulated image through a Fourier transform lens; projecting the transformed image onto the modulator which contains a reference filter of the image to be recognized, thus multiplying the Fourier transformed input image with the reference filter; passing the output through an inverse Fourier transform lens; and displaying that image on a CCD camera. Rapid presentation of reference images for correlation is provided by repeating the above steps with different reference images until a positive correlation is found. 
     It will be appreciated that optical image correlation is based on a two dimensional projection of a three dimensional object. It depends strongly on the filter image being quite close in orientation to the orientation of the image being identified. With the use of multiple quantum well devices, the extraordinarily high frame rate allows virtually every conceivable orientation of candidate images to be correlated with an image, and for that comparison to be done within seconds, i.e., in real time. 
     As a result a high-speed optical correlator is provided that can perform correlations at orders of magnitude higher speed than previous systems. 
     Another novel aspect is an optical image correlator with the functional capability of 300,000 frames/sec and expandability to billions of frames per second. 
     However implementation of such a correlator depends on critical alignment of the optical elements. How this is accomplished is now explained: 
     Referring now to FIG. 2, the semiconductor substrate onto which the optical elements are to be either embedded or mounted on is illustrated by reference character  50 , and in one embodiment is only one inch by one inch in dimension. 
     As can be seen from the top view, a laser  52  is utilized to illuminate spatial light modulator  54  through a polarizing beamsplitter  56 . The output of the spatial light modulator  54  is redirected by beamsplitter  56  through a Fourier transform lens  58  and is redirected by a polarizing beamsplitter  60  to a second spatial light modulator  62 . The output of spatial light modulator  62  is transmitted through an inverse Fourier transform lens  64  to a detector  66 . 
     As can be seen from the isometric view, the laser and detector may be embedded in the silicon chip, as can be the spatial light modulators. Alternatively the spatial light modulators may be built up above and on top the silicon chip, with prisms  70 ,  72 ,  74  and  76  mounted on top of these active devices to redirect the light traveling horizontally to a vertical direction and vice versa. 
     It will be appreciated that the horizontal optical light paths, here shown at  80  and  82 , are critical in the alignment of images from spatial light modulator  54  to spatial light modulator  62 . These prisms and in fact the mounting and orientation of the beamsplitters are critical to determining the light path direction. The light path direction is critical not only along horizontal paths  80  and  82 , but also along horizontal path  84 . 
     As mentioned hereinbefore, with misalignment there can be an offsetting of the image from spatial light modulator  54  onto spatial light modulator  62  which materially affects the correlation obtainable. 
     As can be more accurately seen in FIG. 3, the criticality of the light paths are key to the accuracy of the correlation or in fact whether there will be a correlation. Here it can be seen that datum plane  90  is established by the polished surface of silicon wafer  50 , which in a preferred embodiment is optically flat. 
     This datum plane establishes the location of prism  70  above laser  52  due to the fact that the bottom edge  92  of the prism fits directly on the top surface  94  of silicon wafer  50 . This insures that the light from laser  52  is directed exactly along light path  80  through, in one embodiment, an optics module  96 . In one embodiment, optics module  96  includes objective lens  14 , pinhole  16 , collimating lens  18  and diaphragm  20  of FIG.  1 . 
     The light beam exits the optics module  96  and impinges upon polarizing beamsplitter  56  which again has a bottom edge  98  which determines the orientation of the polarizing beamsplitter relative to datum plane  90 . 
     Thereafter the optical beam exits the polarizing beamsplitter and impinges upon prism  72 , whereupon it is redirected onto spatial light modulator  54 . 
     Spatial light modulator  54  is positioned on the datum plane via its lower edge  100 , as will be described in connection with FIGS. 4A-4C. 
     Moreover the alignment of prism  72  with respect to datum plane  90  is established through the utilization of a frame to be described in connection with FIGS. 4A-4D so that the orientation of the prism is in fact determined through the utilization of datum plane  90 . 
     Likewise polarizing beamsplitter  82  is located on surface  94  with a lower edge  102  providing for the alignment orientation of this optical element. 
     In the same way as spatial light modulator  54 , spatial light modulator  62  is mounted on surface  94  with its lower edge  104  referenced to surface  94 . Likewise prism  74  is referenced to the datum plane through the techniques described in FIGS. 4A-4D. 
     It will also be noted that prism  76  has a lower edge  106  which rests on the surface of the silicon chip, namely surface  94 , with detector  66  embedded therebeneath. 
     It will appreciated that both the Fourier transform lens and the inverse Fourier transform lens can be mounted in housings to provide for accurate alignment of their optical axes along paths  84  and  82  respectively. 
     Referring now to FIG. 4A in order to accurately align an optical element above an active device, an active device  110  is provided with a ball grid array  112  of exceedingly accurately sized balls. The ball grid array serves to connect the active device to the surface  114  of a substrate  116  in which are embedded active elements, one of which is illustrated by embedded drive  120 . 
     The ball grid array serves to connect an active device on the surface of the substrate to either embedded devices within the substrate or interconnection circuits. 
     Here it will be seen that the bottom surface  122  of active device  110  is parallel to datum plane  122  provided by the polished surface of substrate  116 . 
     Referring now to FIG. 4B an epoxy frame  130  is deposited around active device  110  with the tops of the frame  132  extending above a plane  134  which is parallel to plane  122 . 
     As illustrated in FIG. 4C, the top surfaces of frame  130  are milled down to plane  134 , with the plane of the top surface of the frame being parallel to datum plane  122 . This provides an extremely accurate surface onto which any optical elements above the active device may be mounted. 
     As shown in FIG. 4D, a prism  140  is mounted to top surface  132  of frame  130 , thus establishing an optical path  142  which is parallel not only to plane  134  but also to datum plane  122 . 
     What can be seen is that the mechanical milling of the frame top surface establishes a reference plane for the alignment of the optical element on top of it by virtue of the placement of prism  140  on top of this highly accurate surface. 
     Having now described a few embodiments of the invention, and some modifications and variations thereto, it should be apparent to those skilled in the art that the foregoing is merely illustrative and not limiting, having been presented by the way of example only. Numerous modifications and other embodiments are within the scope of one of ordinary skill in the art and are contemplated as falling within the scope of the invention as limited only by the appended claims and equivalents thereto.