Abstract:
A method for forming a structure comprising multiple parallel surfaces having a precise separation is disclosed. Precise separation and parallelism of the surfaces is achieved through the use of an adhesive mixture that comprises a plurality of spacers having a dimension substantially equal to the desired separation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The underlying concepts, but not necessarily the language, of the following cases are incorporated by reference:
       (1) U.S. patent application Ser. No. 11/366,730, filed Mar. 2, 2006;   (2) U.S. patent application Attorney Docket Number 123-060US, filed Oct. 29, 2007; and   (3) U.S. Provisional Patent Application Attorney Docket Number 123-071US, filed on even date herewith.
 
If there are any contradictions or inconsistencies in language between this application and one or more of the cases that have been incorporated by reference that might affect the interpretation of the claims in this case, the claims in this case should be interpreted to be consistent with the language in this case.
       
 
     
    
     FIELD OF THE INVENTION 
       [0005]    The present invention relates to displacement sensors in general, and, more particularly, to optical displacement sensors. 
       BACKGROUND OF THE INVENTION 
       [0006]    Many structures require two or more plates that are held parallel to one another with a precise separation between them. Examples include liquid crystal displays, plasma displays, and optically resonant cavities, such as a Fabry-Perot interferometer or Fabry-Perot etalon. An optically resonant cavity is a well-known structure that is defined by two partially reflective parallel surfaces that are separated by a precise distance. This distance is referred to as the cavity length. For light of a particular wavelength, the reflectivity and transmissivity of the optically resonant cavity are functions of the cavity length. A Fabry-Perot etalon has a fixed cavity length, while a Fabry-Perot interferometer has a cavity length that can vary. 
         [0007]    A Fabry-Perot interferometer is used as the basis of many optical displacement sensors, wherein its cavity length varies in response to an environmental stimulus, such as acceleration, vibration, pressure, temperature, sound, etc. In some of these sensors, one of the surfaces of the optically resonant cavity is a surface of a membrane that moves in response to the environmental stimulus. When the movable membrane moves in response to the environmental stimulus, the reflectivity and transmissivity of the Fabry-Perot interferometer changes. Photodetectors detect the light reflected and/or transmitted by the Fabry-Perot interferometer and generate electrical signal(s) based on the intensity of the detected light. An electrical signal based on the environmental stimulus is thereby generated. 
         [0008]    The optical performance of a Fabry-Perot interferometer-based sensor can be highly dependent upon the initial separation (i.e., initial cavity length) and parallelism of the two surfaces that define the optically resonant cavity. The alignment of these surfaces during fabrication can represent one of the dominant factors in the cost of producing such a device. 
         [0009]    One conventional fabrication method relies on the use of support structure that has multi-axis alignment capability. This support structure aligns and holds the multiple surfaces while adhesives are applied and cured to permanently fix them in their relative positions. Unfortunately, active alignment of the surfaces can be a time-consuming process. In addition, trapped air bubbles and internal stresses in the adhesives can lead to movement of the surfaces during and/or after the adhesives are cured. 
         [0010]    Another conventional fabrication method relies on the monolithic integration of the surfaces. This typically entails the use of integrated circuit processing equipment in a semiconductor fabrication facility. Although such structures can exhibit exceptional alignment and parallelism, the costs associated with such equipment and facilities can be prohibitive. 
         [0011]    Another conventional approach relies on forming alignment features, such as Vee grooves and trapezoidal holes, in each of the surfaces to be aligned. These alignment features are used to trap precision spacers, such as glass spheres or optical fibers, which determine the separation of the surfaces. Unfortunately, this approach has several drawbacks. First, the spacers can be very difficult to handle and insert into the alignment features. Second, the spacers must typically be fixed in the alignment features prior to assembling the multiple surfaces. As a result, minute volumes of an adhesive must be dispensed at each spacer location. Once the spacers are in place with the adhesive, a partial cure of the adhesive is performed to keep the spacers in place during the rest of the assembly process. Since the spacers are usually quite light, the adhesive tends to displace the spacers, at least slightly, from their respective alignment features. This results positional error. In addition, the need to form alignment features as well as the need to add an additional adhesive step increases the overall cost of this fabrication method. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention enables parallel plate structures having precise separation without some of the costs and disadvantages for doing so in the prior art. Parallel plate structures in accordance with the present invention include optically resonant cavities such as etalons and interferometers. For example, embodiments of the present invention are particularly well-suited for use in optical displacement sensors such as microphones, high-sensitivity pressure sensors, vibration sensors, and accelerometers. 
         [0013]    Embodiments of the present invention, like the prior art, use spacers having a precise dimension to determine the spacing between surfaces of two layers. Unlike the prior art, however, some embodiments of the present invention embed such spacers within an uncured adhesive to form a mixture that can be easily and controllably applied. This mixture is disposed between the two layers and thinned so that the two layers are separated by a single layer of the spacers. As a result, the separation between the two layers is made substantially equal to the precise dimension of the spacers. 
         [0014]    Some embodiments of the present invention comprise an optically resonant cavity. In some embodiments, one of the surfaces that define the optically resonant cavity is a surface of a membrane that is physically adapted to move in response to an environmental stimulus. 
         [0015]    Some embodiments of the present invention comprise a pair of optically resonant cavities that are mechanically and optically coupled. One surface of each of the optically resonant cavities is a surface of a membrane that is physically adapted to move in response to an environmental stimulus. The membrane interposes the other surface of each of the optically resonant cavities so that the cavity lengths of the two cavities are mechanically coupled. 
         [0016]    Some embodiments of the present invention comprise an array of optically resonant cavities. In some embodiments, each cavity in the array is physically adapted to respond to the same environmental stimulus. In some embodiments, at least two cavities in the array are physically adapted to respond to different environmental stimuli. In some embodiments, each cavity in the array is optically resonant for the same wavelength of light. In some embodiments, cavities in the array are optically resonant for different wavelengths of light. 
         [0017]    An embodiment of the present invention comprises a method comprising: providing a first layer comprising a first surface; providing an adhesive mixture, wherein the adhesive mixture comprises an adhesive in an uncured state and a plurality of spacers, and wherein each of the plurality of spacers is characterized by a dimension that is substantially equal to a first thickness; applying the adhesive mixture to the first layer; and curing the adhesive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  depicts a method for forming a parallel-layer structure in accordance with an illustrative embodiment of the present invention. 
           [0019]      FIG. 2A  depicts a cross-sectional diagram of details of a parallel-layer structure, prior to a thinning operation, in accordance with the illustrative embodiment of the present invention. 
           [0020]      FIG. 2B  depicts a cross-sectional diagram of details of a parallel-layer structure, after a thinning operation, in accordance with the illustrative embodiment of the present invention. 
           [0021]      FIG. 3A  depicts a method for forming an optical displacement sensor in accordance with the illustrative embodiment of the present invention. 
           [0022]      FIG. 3B  depicts sub-operations suitable for forming a beam splitter as part of optical displacement sensor  400  in accordance with the illustrative embodiment of the present invention. 
           [0023]      FIG. 4  depicts details of an optical displacement sensor in accordance with the illustrative embodiment of the present invention. 
           [0024]      FIG. 5  depicts a cross-sectional diagram of details of a beam splitter as part of optical displacement sensor  400  in accordance with the illustrative embodiment of the present invention. 
           [0025]      FIG. 6  depicts a cross-sectional diagram of details of a beam splitter in accordance with an alternative embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]      FIG. 1  depicts a method for forming a parallel-layer structure in accordance with an illustrative embodiment of the present invention. Method  100  is described herein with additional reference to  FIGS. 2A and 2B . 
         [0027]      FIGS. 2A and 2B  depict cross-sectional diagrams of details of a parallel-layer structure, prior to and after a thinning operation, respectively, in accordance with the illustrative embodiment of the present invention. Structure  200  comprises first layer  202 , adhesive layer  204 , and second layer  206 . 
         [0028]    Referring now to  FIGS. 1 and 2A , method  100  begins with operation  101 , wherein first layer  202  is provided. First layer  202  is a layer of polymer material having a thickness within the range of approximately 15 microns to approximately 5000 microns. In some embodiments, layer first layer  202  has a thickness that is substantially equal to 100 microns. Materials suitable for use in first layer  202  include, without limitation, Mylar, polyesters, Kapton, polyimides, nylons, Rayon, and polyethylenes, acrylics, polycarbonates, polypropylenes, vellums, or other cellulose based membranes. In some embodiments, first layer  202  comprises a material other than a polymer. Non-polymeric materials suitable for use in first layer  202  include metals, ceramics, semiconductors, glasses, dielectrics, and the like. In some embodiments, first layer  202  is a layer of material that is disposed on a substrate. 
         [0029]    At operation  102 , adhesive layer  204  is applied to the first layer  202 . Adhesive layer  204  is a mixture of adhesive  208 , in an uncured state, and spacers  210 . Adhesive layer  204  is applied only to the outer portions of first layer  202  so that a cavity is formed after subsequent operation  103 . 
         [0030]    Adhesive  208  is a liquid epoxy that cures at room temperature. In some embodiments, adhesive  208  comprises an adhesive other than a liquid epoxy that is curable at room temperature. Adhesives suitable for use as adhesive  208  include, without limitation, UV-curable epoxies, thermo-set epoxies, and the like. The viscosity of adhesive  208  is within a range that enables it to mix with spacers  210 . In some embodiments, adhesive  208  is a gel in its uncured state. 
         [0031]    Spacers  210  are spheres of a substantially incompressible material that have a diameter within the range of approximately 5 microns to approximately 5000 microns. In some embodiments, spacers  210  have a diameter substantially equal to 150 microns. Materials suitable for use in spacers  210  include, without limitation, glasses, ceramics, metals, plastics, and the like. It will be apparent to one of ordinary skill in the art that, in practice, some slight variation in the diameter of spacer  210  is expected. Given sufficient bonding area and a sufficient number of spacers, however, deleterious effects on the parallelism of first layer  202  and second layer  206  can be made tolerable. In some embodiments, spacers  210  are spheres of a substantially resilient material. Such resilient spacers can be compressed, if desired, to afford the ability to tune the size of gap g 0 . Resilient materials suitable for use in spacers  210  include, without limitation, rubber, and plastics, such as styrene, butadiene, divinylbenzene, vinyl, Teflon, and the like. 
         [0032]    In some embodiments, spacers  210  are cylinders that have a diameter within the range of approximately 5 microns to approximately 5000 microns. In some embodiments, spacers  210  are a mixture of spacers of suitable shapes that include spacers having a dimension that is substantially equal to a desired gap g 0 . 
         [0033]    At operation  103 , second layer  206  is disposed on adhesive layer  204 . Second layer  206  is analogous to first layer  202  in its thickness and suitable materials. After operation  103 , surfaces  214  and  216  are parallel only to the extent that adhesive  208  is applied uniformly around the perimeter of first layer  202 . 
         [0034]    Referring now to  FIGS. 1 and 2B , at operation  104 , pressure is applied to first layer  202  and second layer  206  to compress adhesive layer  204  to a thickness that is substantially equal to the diameter of spacers  210 . The thickness of adhesive layer  204  after operation  104  is substantially equal to the diameter of spacers  210 . The diameter of spacers  210 , therefore, determines the size of gap g 0  of cavity  212 . It should be noted that the parallelism of surfaces  214  and  216  is dependent upon the uniformity of the diameters of spacers  210  and the extent to which adhesive layer  204  is thinned to the diameter of the spacers. 
         [0035]    In some embodiments, adhesive layer  204  is applied to first layer  202  as a substantially solid preform of adhesive that contains spacers  210 . After operation  103 , energy, such as heat, is applied to the epoxy to enable it to soften (or melt) and adhere to first layer  202  and second layer  206 . Operation  104 , therefore, would occur while adhesive layer  204  was in its softened (or melted) state. In such embodiments, the preform may be formed by the application of a mixture of an adhesive and spacers in a mold that provides the preform with its desired shape. It will be clear to those of ordinary skill in the art, after reading this specification, how to make and use adhesive preforms that comprise spacers  210 . 
         [0036]    At operation  105 , adhesive layer  204  is cured to harden adhesive  208  and thereby physically constrain spacers  210  and fix gap g 0 . 
         [0037]      FIG. 3A  depicts a method for forming an optical displacement sensor in accordance with the illustrative embodiment of the present invention. Method  300  is described herein with additional reference to  FIG. 4 . 
         [0038]      FIG. 4  depicts details of an optical displacement sensor in accordance with the illustrative embodiment of the present invention. 
         [0039]    Method  300  begins at operation  301 , wherein beam splitter  408  is provided. The sub-operations suitable for the formation of beam splitter  408  are described below and with respect to  FIGS. 3B and 5 . Beam splitter  408  is an optical element that receives input light  404  and distributes it into reflected signal  410  and transmitted signal  414  based on an environmental stimulus—specifically input sound. The operation of beam splitter  408  is described in detail in U.S. patent application Attorney Docket 123-060US, filed Oct. 29, 2007, and U.S. patent application Ser. No. 11/366,730, filed Mar. 2, 2006, both of which are incorporated herein by reference. Although the illustrative embodiment comprises a beam splitter that is responsive to acoustic energy, it will be clear to one of ordinary skill in the art, after reading this specification, how to make and use alternative embodiments of the present invention wherein a beam splitter is responsive to a different environmental stimulus, such as mechanical energy (e.g., acceleration, vibration, etc.), pressure, thermal energy, nuclear energy, the presence of a chemical, and the like. 
         [0040]    Input light  404  is provided by light source  402 , which is a laser diode capable of emitting monochromatic light at 850 nanometers (nm) with a spectral-width of less than ten (10) nanometers, and preferably less than three (3) nanometers. 
         [0041]    Lens  406  is a plano-convex lens that is suitable for collimating light emitted by a source  402  in well-known fashion. Lens  406  includes access hole  424 , which facilitates the propagation of acoustic energy toward beam splitter  408 . In some embodiments, lens  406  does not include access hole  424 . Lens  406  is aligned to source  402  such that the output of source  402  is received off the central axis of lens  406 . Lens  406  collimates the output of source  402  into input light  404  and directs input light  404  toward the focal point of lens  406 . Lens  406  also receives reflected signal  410  from beam splitter  408  and focuses the optical energy of reflected signal  410  toward detector  412 . The configuration of lens  406 , with respect to source  402 , beam splitter  408 , and detector  412 , is often referred to as a “pupil-division” configuration. 
         [0042]    In some embodiments of the present invention, lens  406  is not present. In these embodiments, source  402  comprises a collimating lens and a non-orthogonal angle is formed by the direction of propagation of the output of source  402  and beam splitter  408 . 
         [0043]    Detectors  412  and  416  are photodetectors sensitive to the wavelength of the output light from source  402 . Each of detectors  412  and  416  measure the intensity of the light that is incident on it and transmits an electrical signal indicative of that intensity to processor  422 . It will be clear to those skilled in the art, after reading this specification, how to make and use detectors  412  and  416 . Detector  412  receives reflected signal  410  and detector  416  receives transmitted signal  414 . 
         [0044]    Processor  422  is a general-purpose processor that is capable of reading data and instructions from a memory, of executing instructions, of writing data to a memory, of receiving data from detectors  412  and  416 , and of providing electrical signal  426 , which is based on electrical signals  418  and  420 . Processor  422  receives electrical signals  418  and  420  and performs signal processing based on those signals. It will be clear to those skilled in the art, after reading this specification, how to make and use processor  422 . 
         [0045]      FIG. 3B  depicts sub-operations suitable for forming a beam splitter as part of optical displacement sensor  400  in accordance with the illustrative embodiment of the present invention. 
         [0046]      FIG. 5  depicts a cross-sectional diagram of details of a beam splitter as part of optical displacement sensor  400  in accordance with the illustrative embodiment of the present invention. Beam splitter  408  comprises first optically resonant cavity  510  and second optically resonant cavity  522 . Surface  506  of first layer  502  and surface  508  of second layer  504  collectively define first optically resonant cavity  510 . In similar fashion, surface  520  of third layer  516  and surface  518  of second layer  504  collectively define second optically resonant cavity  522 . Optically resonant cavities  510  and  522  function in cooperative fashion to collectively distribute input light  404  into reflected signal  410  and transmitted signal  414 . 
         [0047]    Operation  310  begins with sub-operation  306 , wherein first layer  502  is provided. First layer  502  is a layer of material that is translucent for a wavelength of light contained in input light  404 . First layer  502  is approximately n*λ/4-thick, where λ is the wavelength (within layer  502 ) of light provided by source  402  and n is an odd-integer. In some embodiments of the present invention, layer  502  comprises layer of silicon-rich silicon nitride (SiRN) having a thickness substantially equal to 100 nanometers (nm). The thickness of layer  502  is determined so as to provide suitable performance of beam splitter  408  for input light  404 . It will be appreciated by those skilled in the art that first layer  502  may have other thicknesses and comprise other materials. 
         [0048]    First substrate  512 - 1  is a 500 micron-thick silicon wafer. First substrate  512 - 1  provides a mechanical platform for layer  502 . First substrate  512 - 1  is substantially transparent for a wavelength of light contained in input light  404 . In some embodiments of the present invention, first substrate  512 - 1  is a material other than silicon. Suitable materials for first substrate  512 - 1  include, without limitation, glass, III-V compound semiconductors, II-VI compound semiconductors, ceramics, and germanium. 
         [0049]    Anti-reflection coating  514  is disposed on the surface of first substrate  512 - 1  that is distal to second layer  504 . It will be clear to those skilled in the art how to make and use anti-reflection coating  514 . 
         [0050]    At operation  307 , first adhesive layer  204 - 1  is applied to the perimeter of first layer  502 . First adhesive layer  204 - 1  comprises equal volumes of a UV-curable epoxy and spherical spacers having a diameter of approximately 150 microns. The viscosity of adhesive layer  204 - 1  is selected as gel-like to mitigate problems associated with its flow along first layer  502  during fabrication. Adhesive layer  204 - 1  is analogous to adhesive layer  204  described above and with respect to  FIGS. 1 and 2 . 
         [0051]    At operation  308 , second layer  506  is disposed on first adhesive layer  504 . Second layer  506  is a layer of silicon nitride having a thickness substantially equal to 100 nm. Second layer  506  is analogous to first layer  502  and, therefore, can comprise layers having the same thicknesses and the same materials as first layer  502 . 
         [0052]    Second layer  506  is formed on second substrate  512 - 2 , which has been etched to form a membrane region in the areas of optically resonant cavities  510  and  522 . Second substrate  512 - 2  is a silicon substrate having a thickness substantially equal to 500 microns. Second substrate  512 - 2  is analogous to first substrate  512 - 1 . 
         [0053]    In some embodiments, beam splitter  408  comprises a single optically resonant cavity  510 . In these embodiments, operation  301  would not comprise sub-operations  309  and  310 , but would instead move directly to sub-operation  311  after sub-operation  308 . 
         [0054]    At operation  309 , second adhesive layer  204 - 2  is applied to the perimeter of second substrate  512 - 2 . Second adhesive layer  204 - 2  is analogous to adhesive layer  204  and comprises equal volumes of UV-curable epoxy and spherical spacers that have a diameter substantially equal to 150 microns. Together, substrate  512 - 2  and adhesive layer  204 - 2  define gap g 2 . By virtue of second substrate  512 - 2  and second adhesive layer  204 - 2 , gaps g 2  and g 1  are not equal. It will be clear to those skilled in the art, however, after reading this specification, how to make and use embodiments of the present invention wherein gaps g 1  and g 2  are substantially equal. 
         [0055]    In some embodiments, the diameter of the spacers contained in second adhesive layer  204 - 2  is a different diameter than the diameter of the spacers contained in first adhesive layer  204 - 1 . In some embodiments, the shape of the spacers contained in second adhesive layer  204 - 2  is different than the shape of the spacers contained in first adhesive layer  204 - 1 . 
         [0056]    At operation  310 , third layer  516  is disposed on second adhesive layer  204 - 2 . Third layer  516  is a layer of silicon nitride having a thickness substantially equal to 100 nm. Third layer  516  is analogous to first layer  502  and, therefore, can comprise layers within the same thicknesses range and the same materials as first layer  502 . 
         [0057]    Third layer  516  is formed on third substrate  512 - 3 , which is a silicon substrate having a thickness substantially equal to 500 microns. Third substrate  512 - 3  is analogous to first substrate  512 - 1 . 
         [0058]    Anti-reflection coating  524  is disposed on the surface of third substrate  512 - 3  that is distal to second layer  504 . It will be clear to those skilled in the art how to make and use anti-reflection coating  524 . 
         [0059]    By virtue of etched substrate  512 - 2  and a lack of adhesives  204 - 1  and  204 - 2  in the regions of optically resonant cavities  510  and  522 , second layer  504  comprises a membrane region that is capable of motion in response to an environmental stimulus—specifically acoustic energy contained the input sound. As a result, cavity length g 1  of optically resonant cavity  510  and cavity length g 2  of optically resonant cavity  522  are a function of the input sound. Typically, second layer  504  comprises through holes to facilitate its motion although, for clarity, these are not shown in  FIG. 5 . 
         [0060]    At operation  311 , pressure is applied to the stack of layers to thin first adhesive layer  204 - 1  and second adhesive layer  204 - 2  to a single layer of spacers each. As a result, the initial cavity length (i.e., the cavity length in the absence of an environmental stimulus) of optically resonant cavities  510  and  522  are made equal to gap g 1  and gap g 2 , respectively. 
         [0061]    At operation  312 , first adhesive layer  204 - 1  and second adhesive layer  204 - 2  are cured to harden their respective adhesives and constrain their respective spacers. 
         [0062]      FIG. 6  depicts a cross-sectional diagram of details of a beam splitter in accordance with an alternative embodiment of the present invention. Beam splitter  600  comprises first optically resonant cavity  608  and second optically resonant cavity  616 . Each of first optically resonant cavity  608  and second optically resonant cavity  616  are resonant at a different wavelength. In addition, first optically resonant cavity  608  and second optically resonant cavity  616  are sensitive to different environmental stimuli. In some embodiments, first optically resonant cavity  608  and second optically resonant cavity  616  are resonant at the same wavelength. In some embodiments, first optically resonant cavity  608  and second optically resonant cavity  616  respond to the same environmental stimulus. 
         [0063]    Optically resonant cavity  608  is defined by first surface  604  and second surface  606 , which are separated by gap g 3 . The initial cavity length of optically resonant cavity  608 , therefore, is equal to gap g 3 . The instantaneous cavity length of optically resonant cavity  608  is based on environmental stimulus stimulus 1 . 
         [0064]    First surface  604  is a first region of a surface of first layer  502 . 
         [0065]    Second surface  606  is a surface of second layer  602 . Second layer  602  is disposed on first adhesive layer  204 - 3 , which is analogous to adhesive layer  204  described above and with respect to  FIGS. 1 and 2 . Second layer  602  is analogous to second layer  504  of beam splitter  408 , as described above and with respect to  FIG. 5 . The spacers contained in adhesive layer  204 - 3  are glass spheres that have a diameter substantially equal to 150 microns. As a result, the initial cavity length of optically resonant cavity  608  is equal to gap g 3 , which is substantially equal to 150 microns. 
         [0066]    Optically resonant cavity  616  is defined by third surface  612  and fourth surface  614 , which are separated by gap g 4 . The initial cavity length of optically resonant cavity  616 , therefore, is equal to gap g 4 . The instantaneous cavity length of optically resonant cavity  616  is based on environmental stimulus stimulus 2 . 
         [0067]    Third surface  612  is a second region of the surface of first layer  502 . 
         [0068]    Fourth surface  614  is a surface of third layer  610 . Third layer  610  is disposed on second adhesive layer  204 - 4 , which is analogous to adhesive layer  204  described above and with respect to  FIGS. 1 and 2 . Third layer  610  is analogous to second layer  504  of beam splitter  408 , as described above and with respect to  FIG. 5 . The spacers contained in adhesive layer  204 - 3  are glass spheres that have a diameter substantially equal to 100 microns. As a result, the initial cavity length of optically resonant cavity  608  is equal to gap g 4 , which is substantially equal to 100 microns. 
         [0069]    Input light  618  is a collimated beam of light that contains light characterized by multiple wavelengths, including those for which optically resonant cavities  608  and  616  are optically resonant. The diameter of the beam of input light  618  is sufficient to flood illuminate both optically resonant cavities. 
         [0070]    In operation, first optically resonant cavity  608  distributes light characterized by a first wavelength into reflected signal  620  and transmitted signal  626 . In similar fashion, second optically resonant cavity  616  distributes light characterized by a second wavelength into reflected signal  632  and transmitted signal  638 . 
         [0071]    Photodetectors  622 ,  628 ,  634 , and  640  provide electrical signals  624 ,  630 ,  636 , and  642 , respectively, to processor  422 . Electrical signals  624 ,  630 ,  636 , and  642  are based on the optical energy in reflected signal  620 , transmitted signal  626 , reflected signal  632 , and transmitted signal  638 , respectively. 
         [0072]    Processor  422  provides an output signal based on some or all of electrical signals  624 ,  630 ,  636 , and  642 . 
         [0073]    It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.