Abstract:
A filtering technique for a free space communication that features encoding and decoding of signals employing a filtering apparatus that includes a bulk holographic transform function. Employing the encoding and decoding technique facilitates providing a great number of channels of communication in a unit volume while preventing unwanted cross-talk between the communication channels. In addition, secure communication links between transmitters and receivers may be provided.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
   The present patent application is a continuation-in-part of U.S. patent application Ser. No. 09/648,847 filed Aug. 25, 2000 entitled SHARED MULTI-CHANNEL PARALLEL OPTICAL INTERFACE and having Robert Mays, Jr. listed as inventor, which is incorporated herein by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to an wireless, or free space, communication. Particularly, the present invention concerns discriminating techniques suited for wireless free space interconnects. 
   Reliance upon wireless technology is increasing as the need to increase computational efficiency becomes salient. Specifically, improved operational characteristics of data links employ advancements in wireless communication systems to replace conventional hardwired technology. A well-known example includes the replacement of conventional hardwired telephony with wireless cellular technology. This has generated a need for improvement methodologies that move away from traditional RF wireless technology to optical technology. 
   U.S. Pat. No. 4,057,319 to Ash et al. discloses an optical interconnect system in which individual connections are made involving the passage of light between a specific device in one array of optical devices and a specific device in another array of optical devices. This is achieved via a phase hologram plate of the transmission type fixed relative to each array. 
   U.S. Pat. No. 5,140,657 to Thylen discloses a device for optically coupling an optical fiber, forming part of an optical communication system, to an optical semiconductor laser amplifier. Specifically, the semiconductor laser amplifier has an input facet and an output facet, and the optical fiber has an end surface arranged opposite to at least one of the facets. A diffraction optics element is disposed between the end surface of the fiber and the surface of the facet in order to adapt the nearfield of the fiber end to the nearfield of the facet surface while filtering the same to reduce spontaneous emission noise. The diffraction optics element is described as being a phase hologram. 
   U.S. Pat. No. 6,072,579 to Funato discloses an optical pickup apparatus that includes first and second light sources that selectively emit one of first and second light beams, respectively. The first and second light beams are different in wavelength and are suitable for accessing first and second optical disks respectively. A coupling lens converts a corresponding one of the first and second light beams into a collimated beam. An objective lens forms a light spot on a corresponding one of the first and second optical disks by focusing the collimated beam. A holographic optical element receives a reflection beam of the light spot from one of the first and second optical disks and provides holographic effects on the reflection beam so as to diffract the reflection beam in predetermined directions of diffraction depending on the wavelength of the reflection beam. A photo detector receives the reflection beam from the holographic optical element at light receiving areas and outputs signals indicative of respective intensities of the received reflection beam at the light receiving areas, so that a focusing error signal and a tracking error signal are generated based on the signals. A drawback with the aforementioned optical interconnect systems is that each coupling device requires precise alignment of the optical elements to achieve efficient coupling of optical energy while avoiding cross-talk between adjacent channels. 
   What is needed, therefore, is an improved free space interconnect technique that reduces cross-talk between adjacent channels. 
   SUMMARY OF THE INVENTION 
   Provided is a communication system that features encoding and decoding of signal employing a filtering apparatus that includes a bulk holographic transform function. Employing the encoding and decoding technique facilitates providing a great number of channels of communication in a unit volume while preventing unwanted cross-talk between the communication channels. In addition, secure communication links between transmitters and receivers may be provided. To that end, the system includes a source of energy to propagate a signal along a communication path, a detector positioned in the communication path, and a filtering system disposed in the optical path, the filtering system having a transform function associated therewith, encode the signal, defining an encoded signal, and decode the encoded signal to retrieve the signal for detection by the detector. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified plan view of a communication system in accordance with one embodiment of the present invention; 
       FIG. 2  is a simplified plan view showing an apparatus for fabricating the filter apparatus shown above in  FIG. 1 , in accordance with one embodiment of the present invention; 
       FIG. 3  is a perspective view of a subportion of a volume of the filter apparatus discussed above in  FIGS. 1 and 2  showing a holographic transform function recorded therein; 
       FIG. 4  is a graphical representation showing charge distribution changes in the volume discussed above with respect to  FIG. 3 , in relation to the optical energy impinging thereupon and the resulting strain in the material of the volume; 
       FIG. 5  is a perspective view demonstrating a first subportion of a bulk holographic transform function discussed showing a portion of the characteristics of optical energy impinging upon the volume discussed above with respect to  FIG. 3 ; 
       FIG. 6  is a perspective view demonstrating a second subportion of a bulk holographic transform function discussed showing a portion of the characteristics of optical energy impinging upon the volume discussed above with respect to  FIG. 3 ; 
       FIG. 7  is a perspective view demonstrating a third subportion of a bulk holographic transform function discussed showing a portion of the characteristics of optical energy impinging upon the volume discussed above with respect to  FIG. 3 ; 
       FIG. 8  is a perspective view demonstrating the resulting bulk holographic transform function recorded in a volume of material from the combined characteristics shown above in  FIGS. 5-7 ; 
       FIG. 9  is a simplified plan view showing encoding and decoding of optical energy in accordance with the present invention; 
       FIGS. 10A and 10B  is perspective view of the communication system shown above in  FIG. 1 , in accordance with an alternate embodiment; 
       FIG. 11  is perspective view of an array of the filters fabricated on a photo-sheet shown above in  FIG. 10A and 10B ; 
       FIG. 12  is a cross-sectional plan view of the optical communication system shown above in  FIGS. 10A and 10B , in accordance with the present invention; 
       FIG. 13  is a cross-sectional plan view of the optical communication system shown above in  FIG. 12 , in accordance with an alternate embodiment of the present invention; 
       FIG. 14  is a simplified plan view showing sequential encoding and decoding of optical energy in accordance with the present invention; 
       FIG. 15  is a perspective view of a compound holographic transform function in accordance with the present invention that may be employed as the filter discussed above with respect to  FIG. 1 ; 
       FIG. 13  is a perspective view of the wavelength properties associated with the optical energy from which the interference pattern formed by the apparatus shown in  FIG. 9 ; 
       FIG. 14  is a perspective view of a holographic transform formed from the recording of the properties shown above with respect to  FIGS. 11-13  recorded in a subportion of a photo-sensitive sheet shown in  FIG. 9 ; 
       FIG. 15  is a perspective view of a compound holographic transformed from the recording of two holographic transforms in a sub-portion of a photo-sensitive sheet shown above in  FIG. 9 ; 
       FIG. 16  is a cross-sectional view of a filter employed in the communication system shown above in  FIG. 1 , in accordance with an alternate embodiment of the present invention; 
       FIG. 17  is a cross-sectional view of the filter employed in the communication system shown above in  FIG. 1 , in accordance with a second alternate embodiment of the present invention; 
       FIG. 18  is a cross-sectional view of the filter employed in the communication system shown above in  FIG. 1 , in accordance with a third alternate embodiment of the present invention; 
       FIG. 19  is a cross-sectional view of a substrate on which the filter discussed above with respect to FIGS.  1  and  16 - 18  is fabricated; 
       FIG. 20  is a cross-sectional view of the substrate, shown above in  FIG. 19 , undergoing processing showing a photo-resist layer disposed thereon; 
       FIG. 21  is a cross-sectional view of the substrate, shown above in  FIG. 20 , undergoing processing showing a photo-resist layer being patterned; 
       FIG. 22  is a cross-sectional view of the substrate, shown above in  FIG. 21 , undergoing processing after a first etch step; and 
       FIG. 23  is a cross-sectional view of the substrate, shown above in  FIG. 22 , undergoing processing after a second etch step. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , shown is a communication system  10  including a source of optical energy  12 , an optical detector  14  in data communication with the source of optical energy  12 , with an filtering apparatus  16  disposed therebetween. The source  12  directs optical energy  18  along a path  20  in which the optical detector  14  lies. The filtering apparatus  16  is disposed between the source  12  and the optical detector  14  and filters optical energy propagating therethrough. In this manner, filtering apparatus  16  removes from unwanted characteristics from the optical energy impinging upon optical detector  14 . 
   The unwanted characteristics that may be removed from the optical energy  18  includes amplitude wavelength and/or polarization information. To that end, filtering apparatus  16  has a bulk hologram recorded therein that defines a transform function, shown graphically as periodic lines  26  for simplicity. The transform function  26  facilitates characterizing optical energy  18  to have desired characteristics that may improve detection of information contained in the optical energy  18 , by the optical detector  14 . Specifically, optical energy  18  may function as a carrier wave and be modulated with information. Filtering is achieved by the transform function selectively allowing specified characteristics of the carrier, e.g., optical energy  18 , to pass therethrough and impinge upon optical detector  14 . 
   Referring to both  FIGS. 1 and 2 , transform function  26  is recorded as a periodic arrangement of the space-charge field of the material from which filtering apparatus  16  is fabricated. The transform function  26  is recorded employing a system  30  that includes a beam source  32  that directs a beam  34   a  into wave manipulation optics  36 , such as a ¼ waveplate  38 , so that a beam  34   b  is circularly polarized. Beam  34   b  impinges upon polarizer  40  so that a beam  34   c  propagating therethrough is linearly polarized. Beam  34   c  impinges upon a Faraday rotator  42  that changes birefringence properties to selectively filter unwanted polarizations from beam  34   c . In this manner, a beam  34   d  egressing from the rotator  42  is linearly polarized. Beam  34   d  impinges upon a beam splitter  44  that directs a first subportion  34   e  of beam  34   d  onto a planar mirror  46 . A second subportion  34   f  of beam  34   d  passes through splitter  44 . The first and second subportions  34   e  and  34   f  intersect at region  50  forming an optical interference pattern that is unique in both time and space. The material from which filtering apparatus  16  is formed, photosensitive sheet  52 , is disposed in the region so as to be exposed to the optical interference pattern. The interference pattern permeates the photosensitive sheet  52  and modulates the refractive index and charge distribution throughout the volume thereof. To that end, sheet  52  may be formed from any suitable photo-responsive material, such as silver halide or other photopolymers. Other materials from which sheet  52  may be formed include LiNbO 3 , LiTaO 3 , BaTiO 3 , KnbO 3 , Bi 12 SiO 20 , Bi 12 GeO 20 , PbZrO 3 , PbTiO 3 , LaZrO 3 , or LaTiO 3 . 
   Referring to  FIGS. 2 ,  3  and  4 , the modulation that is induced throughout the volume of the photosensitive sheet  52  is in accordance with the modulation properties of the first and second subportions  34   e  and  34   f . A subportion of the aforementioned volume is shown as  60 . A cross-section of volume  60  is shown as  64 . An interference pattern, shown for simplicity as  66 , is produced by beams  34   f  and  34   e . Interference pattern  66  induces changes in refractive indices of volume  64  based on the spatial modulation of photo-currents that results from non-uniform illumination. Charges such as electrons  68 , or holes, migrate within volume  64  due to diffusion and/or drift in an electric field present therein, referred to as photo-excited charges. The generation of photocurrents at low beam intensity depends on the presence of suitable donors. The photo-excited charges, which are excited from the impurity centers by interference pattern  66 , are re-trapped at other locations within volume  64 . This produces positive and negative charges of ionized trap centers that are re-excited and re-trapped until finally drifting out of the region of volume  64  upon which the interference pattern  66  impinges. This produces a charge distribution within volume  64 , shown by curve  70 . Charge distribution  70  creates a strain through volume  64 , shown by curve  72  that produces regions of negative charge concentration  74  and regions of positive charge concentration  76 . The resulting space-charge field between the ionized donor centers and the trapped photo-excited charges modulates the refractive indices, which is shown graphically by curve  78 . 
   Referring to FIGS.  2  and  4 - 8 , shown are exemplary data associated with the interference pattern generated by the superposition of the first and second sub-portions  34   e  and  34   f . Datum  80  shows one of the dimensions recorded in sheet  52 . Specifically, datum  80  is a three-dimensional representation of the amplitude components of the interference pattern. Datum  82  is phase components associated with the interference pattern. Datum  84  is the wavelength components associated with the interference pattern. Recorded in a sub-portion of sheet  52 , data  80 ,  82  and  84  define a hologram  88  that is defined throughout the entire bulk or volumetric thickness, v δ , measured between opposing sides of volume  64 . 
   The volumetric thickness, v δ , is defined to be the thickness required to record a complete holographic transform function. It has been determined that, for a given material, the volumetric thickness, v δ , is inversely proportion to the wavelengths of first and second sub-portions  34   e  and  34   f  that create the interference pattern. A volumetric thickness, v δ , as little as several microns was found suitable for recordation of a single holographic transform in the near-infrared optical frequencies. With the appropriate volumetric thickness, v δ , all of the physical properties associated with the photonic or electromagnetic waves of the interference pattern, e.g., spatial and temporal (phase) aspects, wavelength, amplitude, polarization, etc. are stored in volume  64 . Holographic transform  88  functions as a gateway to provide real-time and near real-time optical filtering and encoding. 
   Referring to  FIG. 9 , a wavefront  18   a  is emitted by optical energy source  12  having a signal modulated thereon, shown as curves  18   b  and  18   c . After propagating through filtering apparatus  16 , transform function  26  operates on wavefront  18   a  to rearrange the electromagnetic fields associated therewith, thereby encoding the same. Encoded wavefront  16   a  includes the modulation  18   b  and  18   c . However, in order to perceive the information associated with the modulation, the encoded wavefront  16   a  should be decoded. This requires propagating encoded signal  16   a  through a transform function that is substantially identical to transform function  26 . To that end, an additional filtering apparatus  116  having the same transform function  26  recorded therein should be placed between a detector  14  and encoded wavefront  16   a . Upon propagating through filtering apparatus  116  encoded wavefront  16   a  is unencoded, thereby rendering unencoded wavefront  18   a  and all the information contained in modulation  18   b  and  18   c . Thus, it is seen that the inverse transform of the transform function  26  is the transform function  26  itself. Thus, propagating a wavefront through an even multiple of a single transform function, the original wavefront may be maintained. Conversely, propagating wavefront through an uneven multiple of a single transform function results in an encoded wavefront, which is virtually impossible to detect, much less demodulate, without unencoding the same. In this manner, superior beam-sensor discrimination may be achieved. 
   Referring to  FIGS. 10 and 10B , beam-sensor discrimination provided by the present invention is beneficial to a multi-channel optical communication system  310 . One example of optical communication system  310  includes an array  312  of optical transmitters, shown generally as  312   a - 312   p , and an array  314  of optical detectors, shown generally as  314   a - 314   p . The optical transmitters  312   a - 312   p  generate optical energy to propagate along a plurality of axes, and the optical receivers  314   a - 314   p  are positioned to sense optical energy propagating along one of the plurality of optical axes. Specifically, the array  312  is an (XxY) array of semiconductor lasers that produce a beam that may be modulated to contain information. The array  314  may comprise of virtually any optical detector known, such a charged coupled devices (CCD) or charge injection detectors (CID). In the present example, the array  314  comprises of CIDs arranged in an (MxN) array of discrete elements. The optical beam from the each of the individual emitters  312   a - 312   p  may expand to impinge upon each of the detectors  314   a - 314   p  of the array  314  if desired. Alternatively, the optical beam from each of the individual emitters  312   a   312   p  may be focused to impinge upon any subportion of the detectors  314   a - 314   p  of the array  314 , discussed more fully below. In this fashion, a beam sensed by one of the detectors  314   a - 314   p  of the array  314  may differ from the beam sensed upon the remaining detectors  314   a - 314   p  of the array  314 . To control the wavefront of the optical energy produced by the transmitters  312   a - 312   p , the filtering apparatus  16 , discussed above with respect to  FIGS. 1-8  may be employed as an array of the filtering apparatuses  416 , shown more clearly in  FIG. 11  as array  400 . 
   Specifically, referring to  FIGS. 11 and 13 , the individual filtering apparatuses  416  of the array are arranged to be at the same pitch and sizing of the array  312 . The numerical aperture of each of the filtering apparatuses  416  of the array  400  is of sufficient size to collect substantially all of the optical energy produced by the transmitters  312   a - 312   p  corresponding thereto. In one example, the array  400  is attached to the array  312  with each lens resting adjacent to one of the transmitters  312   a - 312   p . To provide the necessary functions, each of filtering apparatuses  416  of the array  400  may be fabricated to include the features mentioned above in  FIGS. 1-8 . As a result, each of the filtering apparatuses  416   b  of the array may be formed to having functional characteristics that differ from the remaining filtering apparatuses  416  of the array. In this manner, each beam produced by the array  312  may be provided with unique properties, such as wavelength, amplitude and polarization. This facilitates reducing crosstalk and improving signal-to-noise ratio in the optical communication system  310 . 
   Specifically, the filtering apparatus  316  may include an additional array  400   b  of filtering apparatuses  416   b  that match the pitch of the individual detectors  314   a - 314   p  of the array  314 , shown more clearly in FIG.  13 . The filtering apparatuses  416   b  may be fabricated to provide the same features as discussed above with respect to array  400 , shown in  FIGS. 10A and 10B . 
   Referring to  FIGS. 10A ,  10 B,  11  and  13  each of the transmitters  312   a - 312   p  of the array  312  would then be uniquely associated to communicate with only one of the detectors  314   a - 314   p  of the array  314 . In this manner, the transmitter  312   a - 312   p  of the array  312  that is in data communication with one of the one of the detectors  314   a - 314   p  of the array  314  would differ from the transmitters  312   a - 312   p  in data communication with remaining detectors  314   a - 314   p  of the array  314 , forming a transmitter/detector pair that is in optical communication. Communication between the transmitter detector pair is achieved by having the properties of the filtering apparatuses  416  in array  400  associated with the transmitter match the properties of the filtering apparatuses  416   b  in array  400   b  associated with the detector. For example were the filtering apparatuses  416  associated with transmitter  312   a  to match the properties of filtering apparatuses  216   b  associated with detector  314   c , the optical energy produced by transmitter  312   a  could be sensed by detector  314   c . Assume no other detector has filtering apparatuses  416   b  associated therewith that have properties matching the properties of the filtering apparatuses  416  associated with transmitter  312   a . Then detector  314   c  would be the only detector of array  314  capable of sensing optical energy from transmitter  312   a . This results from the inherent properties of holographic transforms, discussed more fully below. It should be seen that in addition to filtering, the holographic transform provides security against unauthorized sensing of optical energy. In this manner, information modulated on the optical energy produced by transmitter  312   a  may only be perceived by detector  314   c . This is also discussed more fully below. 
   It should be understood, however that one of the transmitters  312   a - 312   p  might be in data communication with any number of the detectors  314   a - 314   p  by multiple filtering apparatuses  416   b  matching the properties of one of the filtering apparatuses  416 . Similarly, one of the multiple transmitters  312   a - 312   p  may be in optical communication with one or more of the detectors  314   a - 314   p  by appropriately matching the filtering apparatuses  416  to the filtering apparatuses  416   b.    
   In one example, superior performance was found by having the array  314  sectioned into (mxn) bins, with each bin corresponding to a particular polarization and/or wavelength that matched a particular polarization and/or wavelength corresponding to a transmitter  312   a - 312   p . Thus, were a beam from one or more of the transmitters  312   a - 312   p  to flood the entire (MxN) array  314  or multiple (mxn) bins, only the appropriate detectors  314   a - 314   p  sense information with a very high signal-to-noise ratio and discrimination capability. 
   Additional beam-sensor discrimination may be achieved by employing transmitters  312   a - 312   p  having different wavelengths or by incorporating up-conversion processes that include optical coatings applied to the individual transmitters  312   a - 312   p  or made integral therewith. One such up-conversion process is described by F. E. Auzel in “Materials and Devices Using Double-Pumped Phosphors With Energy Transfer”, Proc. of IEEE, vol. 61. no. 6, Jun. 1973. In addition, coating one or more filtering apparatuses  416  of array  400  with a polarizing film provides further discrimination using polarizing discrimination. The combined effect of the transform function and the polarizing improves the extinction ratio of either the transform function or the polarizing film by one order of magnitude or better. For example, a typical polarizing film providing an extinction ratio of 50 to 100 may be increased to 1,000, or better, when employed in conjunction with the transform function in accordance with the present invention. Similar improvements in the extinction ratio of a transform function is realized with this combination. To that end, the polarizing orientation of the film should match the polarizing orientation provided by the transform function. 
   Referring to both  FIGS. 2 and 13 , filtering apparatuses  416  and  416   b , with differing transform functions are formed on differing photosensitive sheets  52 . Specifically, the transform function is defined by the interference pattern formed by the first and second subportions  34   e  and  34   f  intersecting at region  50 . This interference pattern is unique in both time and space. As a result, each of the filtering apparatuses formed on the sheet  52  would have substantially identical holographic transform functions. To create filter apparatuses with differing transform functions, an additional photosensitive sheet  52  would be employed. Considering that the interference pattern is unique in both time and space, a subsequent sheet  52  disposed in region  50  would have a differing transform function recorded therein thereon than the transform function recorded on a sheet  52  at an earlier time. This is due, in part, to the time-varying fluctuations in the operational characteristics of the various components of system  30 . As a result multiple sheets  52  are formed, each of which has a transform function associated therewith that differs from the transform function associated with the remaining sheets. After forming the aforementioned multiple sheets, the filtering apparatuses on each of the sheets is segmented so that the same may be arranged proximate to one or more emitters and one or more detectors, as desired. 
   Alternatively, or in addition, the Faraday rotator  42  may be rotated to provide the lenses formed on the photosensitive sheet  52  with a holographic transform function that differs from the transform function associated with the lenses formed on a previous photosensitive sheet  52 . 
   Referring to  FIG. 13 , it should be noted that the array  312  may comprise a single emitter  412  that produces sufficient beam width to impinge upon all of filtering apparatuses  416  of array  400 . In this manner, the array  400  of filtering apparatuses  416  is employed in the aggregate to increase both the numerical aperture and enhance the signal to noise ratio, as well as to provide a multi-transform operation across the cross-section of the optical energy produced by single emitter  412 . Employing the multi-transform operation over the cross-sectional area of the optical energy takes advantage of the properties of the holographic transforms recorded in each filtering apparatuses  416 . Specifically, the holographic transforms in each of the filtering apparatuses function in the aggregate to operate on the wavefront of the optical energy as an aggregate holographic transform to vary the wavefront, defining an encoded wavefront. The encoded wavefront may be returned to the un-encoded state, i.e., decoded, by having the same propagate through a matching aggregate holographic transform. To that end, the array of detectors  314  may comprise of a single detector to facilitate unencoding of the optical energy. This makes the present invention suitable for use with free space interconnects over local area networks, wide area networks and metropolitan area networks, because multiple networks may communicate through a common volume of space without corrupting the data associated with the network. 
   Referring to  FIG. 14 , another property of the transform function concerns sequential encoding and decoding. As mentioned above, the inverse transform function of a holographic transform function is the function itself. As a result, multiple encoding may be facilitated to provide increased beam-sensor discrimination. Specifically, assuming an optical encoding system  415  comprising a first filtering apparatus including  416   a  having a first transform function H 1  and a second filtering apparatus  417   a  including a second transform function H 2 . Propagating wavefront  418   a  through first filtering apparatus including  416   a  would result in encoded wavefront  419   a . Propagating encoded wavefront  419   a  through second filtering apparatus  417   a  would further encode wavefront  419   a , yielding encoded wavefront  421   a . To decode wavefront  421   a  to yield unencoded wavefront  418   a  requires first passing wavefront  421   a  through the second transform function to yield wavefront  419   a . Thereafter, wavefront  419   a  would propagate through the first transform function to yield unencoded wavefront  418   a . To that end, a decoding system  415   b  includes a third and fourth apparatus  417   b  and  416   b , respectively. Third filtering apparatus  417   b  has a transform function associated therewith that is identical to transform function H 2 . Fourth filtering apparatus  416   b  has a transform function associated therewith that is identical to transform function H 1 . To decode wavefront  421   a , third filtering apparatus  417   b  is positioned between second filtering apparatus  417   a  and fourth filtering apparatus  416   b . In this manner, wavefront  421   a  first propagates through third filtering apparatus  417   b  to be decoded by transform function H 2  forming wavefront  419   a . Wavefront  419   a  then propagates through filtering apparatus  416   b  to be decoded by transform function H 1 , thereby yielding wavefront  418   a . Wavefront  418   a  may then be sensed by a detector (not shown) to retrieve information contained therein. 
   Reversing the order of unencoding so that the first transform operated on wavefront  421   a  would yield unintelligible information, thereby preventing any information modulated on wavefront  418   a  being unencoded. 
   Referring to  FIG. 15 , a property recognized with respect to the holographic transform functions is that two holographic transform function may be recorded in the an identical volume without interfering with each other. As a result a compound holographic transform function  700  may be recorded in which two or more independent holographic transform functions are recorded across a unit volume. Compound holographic transform function  700  is shown having two holographic transform functions  700   a  and  700   b  recorded therein. It was determined, however, that the volumetric thickness, v δ , was also defined by the number of holographic transforms recorded in a unit volume formed in a volume. Specifically, it is found that were recording and retrieval of multiple and independent holographic transforms, e.g., numbering in the hundreds and thousands, desired, then several millimeters of volumetric thickness, v δ , would be required. 
   Referring to  FIGS. 2 and 16 , to relax the alignment tolerance between optical energy source  12  and detector  14 , filtering apparatus  16  may be provided with a lensing function. In this manner, filtering apparatus  16  may concurrently refract and filter optical energy  18 . In this manner, filtering apparatus  16  defines a lens  22  having a bulk holographic transform function  26  recorded in substantially the entire volume thereof, through which optical energy will propagate. In this manner, the lens  22  and the bulk holographic transform function  26  are integrally formed in a manner described more fully below. Although the surface  28  of the lens  22  disposed opposite to the spherical arcuate surface  24  is shown as being planar, the surface  28  may also be arcuate as shown in surface  128  of lens  122  in FIG.  17 . 
   The refractory function of the filtering apparatus  16  facilitates impingement of the optical energy  18  onto the optical detector  14 . In this manner, the precise alignment of the optical detector  14  with respect to the source  12  and, therefore, the path  20  may be relaxed. 
   Referring to both  FIGS. 2 and 18 , were it desired to further control the shape of optical energy propagating through lens  22 , a lens  222  may be formed with a Fresnel lens  228  disposed opposite to the spherical surface  224 . In this manner, substantially all of the optical energy propagating through lens  222  may be focused to differing points, depending upon the wavelength of optical energy propagating therethrough. To that end, the Fresnel lens  228  includes a plurality of concentric grooves, shown as recesses  228   a    228   b  and  228   c  that are radially symmetrically disposed about a common axis  230 . Thus, lens  222  may have three optical functions integrally formed in a common element, when providing the bulk holographic transform function  226  therein. 
   To provide the aforementioned lensing function, the manufacturing process of photosensitive sheet  52  may include providing a photosensitive layer  800  adhered to a sacrificial support  802 , shown in FIG.  19 . Examples of sacrificial layers include glass, plastic and the like. The photosensitive layer  800  and sacrificial support  802  form a photosensitive substrate  804 . Typically, photosensitive layer  800  is tens of microns thick. As shown in  FIG. 20 , a photo resist layer  806  is deposited onto the photosensitive layer  800  and then is patterned to leave predetermined areas exposed, shown as  808  in  FIG. 21 , defining a patterned substrate  810 . Located between exposed areas  808  are photo resist islands  812 . Patterned substrate  810  is exposed to a light source, such as ultraviolet light. This ultraviolet light darkens the volume of photo resist layer  800  that is coextensive with exposed areas  808  being darkened, i.e., become opaque to optical energy. The volume of photosensitive layer  800  that are coextensive with photo resist islands  812  are not darkened by the ultraviolet light, i.e., remaining transparent to optical energy. Thereafter, photo resist islands  812  are removed using standard etch techniques, leaving etched substrate  814 , shown in FIG.  22 . 
   Etched substrate  814  has two arcuate regions  816  that are located in areas of the photosensitive layer  800  disposed adjacent to islands  812 , shown in FIG.  23 . Arcuate regions  816  of  FIG. 22  result from the difference in exposure time to the etch process of the differing regions of photosensitive layer  800 . 
   Referring to  FIGS. 2 ,  11  and  22 , a subsequent etch process is performed to form array  400 . During this etch process the support is removed as well as nearly 50% of photosensitive layer  800  to form a very thin array. Array  400  is then placed in the apparatus  30  and the bulk holographic transform functions are recorded in the arcuate regions  816  that define the lenses, as discussed above. The Fresnel lens may also be formed on the lenses of the array  400  using conventional semiconductor techniques. Thereafter, the lenses may be segmented from the photo resistive sheet or MxN sub-arrays of lenses may be segmented therefrom. 
   Although the invention has been described in terms of specific embodiments, one skilled in the art will recognize that various changes to the invention may be performed, and are meant to be included herein. For example, in additional to the optical communication discussed above, the present invention may be employed for RF communication using wavelengths in the range of one micron to one millimeters, inclusive. 
   In addition, instead of a transmissive filtering apparatus  16 , a reflective filtering apparatus may be employed. The present invention would be suited for use on storage media such as compact diskettes that store various information, e.g., audio content, video content, audio-visual content and the like. In this manner, a signal, either optical or RF, would propagate into the filtering apparatus and be reflected back from the filtering apparatus through a common surface. 
   Further, instead of forming the arcuate regions  816  using standard etch techniques, the same may be formed by exposing substrate  810 , shown in  FIG. 21 , to thermal energy. In one example, substrate  810  is convectionally heated, and photo resist layer  806  is patterned to control the regions of photosensitive layer  800  that may expand. 
   In another example, the photosensitive layer is heated by conduction employing laser ablation/shaping. Specifically, a laser beam impinges upon areas of photosensitive layer  800  where lens are to be formed. The thermal energy from the laser beam causes the photosensitive layer  800  to bubble, forming arcuate regions  816  thereon, as shown in FIG.  22 . In addition, the holographic transform function has been found to be effective in filtering electromagnetic energy outside of the optical spectrum, e.g., in the microwave region. Therefore, the scope of the invention should not be based upon the foregoing description. Rather, the scope of the invention should be determined based upon the claims recited herein, including the full scope of equivalents thereof.