Abstract:
Holographic data is stored in a cylindrical crystal by directing a signal beam with data encoded therewith axially through an end face of the crystal, which signal beam interferes with a reference beam directed radially through the cylindrical side surface of the crystal. By rotating the crystal about its axis, numerous holograms are recorded therein an annular layer and by indexing the crystal axially the annular layers are stacked to further increase the storage capacity of the crystal. The holograms are read from the crystal by focusing a reference beam therethrough in a radial director for diffraction with the stored holograms to produce a defracted reference beam which emerges axially from the crystal. The diffracted reference beam is then read with a detector in the form of a CCD camera.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a cylindrical medium for storing holographic data and to methods of and apparatus for manipulating data using the medium. More particularly, the present invention is directed to a cylindrical storage medium in which rotation of the medium and apparatus, one with respect to the other, provides a method of manipulating holographic data.  
         BACKGROUND OF THE INVENTION  
         [0002]    There is a constant demand to increase the storage capacity of data systems. As has been set forth in U.S. Pat. No. 6,101,161 issued Aug. 8, 2001, this is especially the case for storing large amounts of data such as, for example, the data required for motion picture images. There are of course a meriad of other needs for data storage, such as storage for library text and other massive amounts of information, which information can for example relate to anything from scientific data to financial data. In the &#39;161 patent, a cylindrical storage medium is indexed rotationally to store holographic image data in the form of pixel arrays generated from a spacial light modulator system (SLM). Multiple holograms are stored in the cylindrical crystal using angular multiplexing by rotating the crystal about its z-axis. Attempts to successfully store and retrieve data from systems such as that of the &#39;161 patent have proved illusive. A primary reason for this is the complicated cylindrical optics for imagery into and out of the cylinder. Another reason for this difficulty is that extraordinarily polarized laser beams appear to be necessary to store holographic gratings in the x,y, plane of cylindrical crystals  
           [0003]    In addition, prior art approaches do not identify a range of angles for angularly positioning each hologram in a cylindrical crystal. Moreover, the prior art does not teach or suggest that a cylindrical crystal may be moved axially in the direction of its c-axis to record holograms at different axial locations in the cylindrical crystal so as to vastly increase the storage capacity of a single cylindrical crystal.  
         SUMMARY OF THE INVENTION  
         [0004]    In view of the aforementioned considerations, the present invention is directed to a cylindrical crystal formed around an axis wherein the cylindrical crystal has holograms stored in annular arrays therein about the axis thereof, with the annular arrays being stacked in an axial direction.  
           [0005]    The present invention is also directed to a method of storing holographic data in a cylindrical crystal wherein the cylindrical crystal is formed about an axis and has an axially facing surface and a cylindrical peripherial surface. In accordance with the method, a reference beam is focused through one of the surfaces and a signal beam containing the data to be stored is focused through the other of the surfaces. The signal and reference beams interfere within the crystal to form a hologram therein containing the data. The method further includes rotating the crystal about the axis to form additional angularly spaced holograms.  
           [0006]    In addition, the method includes translating the cylindrical crystal axially to store additional layers of angularly spaced holograms within the crystal.  
           [0007]    In accordance with an apparatus for writing holograms into the cylindrical crystal, the apparatus includes a source of laser light focused through a polarizing device and a beam splitter for dividing the light into a signal beam and a reference beam. A first optical path is provided for directing the signal beam through an SLM and into the crystal in a first direction with respect to the axis to the crystal and a second optical path is provided for directing the reference beam into the crystal in direction transverse to the reference signal for interference with the signal beam to form and store the holograms within the crystal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is an enlarged perspective view of a crystal configured in accordance with the principals of the present invention;  
         [0009]    [0009]FIG. 2 is an enlarged graphical illustration showing a grating vector in the crystal of FIG. 1;  
         [0010]    [0010]FIG. 3 is a diagramatical illustration of an apparatus for writing a hologram into the crystal of FIG. 1 to thus practice a method of the present invention;  
         [0011]    [0011]FIG. 4 is a graph plotting holographic efficiency as a function of angle so as to determine angular spacing of holograms written within the crystal of FIG. 1;  
         [0012]    [0012]FIG. 5 is a diagramatical view of apparatus for reading holograms out of the crystal of FIG. 1 using a reference beam;  
         [0013]    [0013]FIG. 6 is a diagramatical view showing an apparatus for reading holograms out of the crystal of FIG. 1 by using a phase conjudate beam developed from the reference beam;  
         [0014]    [0014]FIG. 7 is an enlarged perspective view of a cylindrical crystal configured as a disk, and  
         [0015]    [0015]FIG. 8 is an enlarged perspective view of a cylindrical crystal configured as a rod.  
     
    
     DETAILED DESCRIPTION  
       [0016]    Referring now to FIG. 1, there is shown a cylindrical crystal  10  configured in accordance with the principals of the present invention. The cylindrical crystal  10  is preferably made of photorefractive crystal material such as lithium niobate (LiNbO 3 ) or lithium borate (Li 2 B 2 O 4 ). While these materials are preferred and available, other photorefractive materials maybe used, such as other glasses, for example SiO 2 : Ge, or organic polymer materials and crystals. It is a practice to dope photorefractive crystals in order to enhance their holographic storage characteristics and the crystal  10  may utilize such enhancements.  
         [0017]    The crystal  10  is formed about a z-axis  12  and has a cylindrical peripheral surface  14  which extends in the direction of the z-axis and an axially facing, polished x-y surfaces  15  and  16  which extends transverse, or more particularly, perpendicular to the z-axis. As is seen in FIG. 2, when a hologram is created within the cylindrical crystal  10 , a grating vector K is disposed obliquely with respect to the x,y plane of the crystal, the grating vector K being comprised of a radial vector Kr and an axial vector Kz.  
         [0018]    Referring now to FIG. 3 there is shown an apparatus  20  configured in accordance with the principals of the present invention for writing holograms into the crystal  10  in accordance with the methods of the present invention. The apparatus  20  comprises a source  22  for a laser beam  24 , which laser beam is passed through a pair of colluminating lenses  26  and  28  and then through a polarizer  30 . The colluminized and polarized laser beam  24  is then split by a beam splitter  32  into a signal beam  34  and a reference beam  36 . The signal beam  34  passes through an open shutter  38  and is reflected by a reflector  40  into a spacial light modulator (SLM) which encodes data or information onto the signal beam  34  preferably in the form of light and dark pixels or pixel arrays. A spherical lens  44  aligned with the c-axis of the crystal  10  (the c-axis being perpendicular to the axially facing surfaces  15  and  16  and aligned with the z-axis of the crystal) focuses the signal beam into the polished, axially facing flat surface  15  of the cylindrical crystal  10 . By sending the signal beam  34  through the axially facing flat surfaces  15  and  16 , there is no requirement for special cylindrical lenses to prepare an ordinary polarized signal beam.  
         [0019]    The reference beam  36  is directed by an optical path, comprising a reflector  50  and a cylindrical lense  52 , radially through the cylindrical peripheral surface  14  of the crystal  10  in the direction of arrow  53 . The distance between the cylindrical lens and the crystal  10  is chosen so that the reference beam is collimated in the crystal. Thus the signal beam  34  and reference beam  36  are transverse with respect to one another as they pass into the cylindrical crystal  10 . Once the beams  34  and  36  intersect within the cylindrical crystal  10 , they interfere to form a hologram  56  inside of the crystal containing the information provided by the SLM i.e. information encoded in light and dark pixel arrays. The grating vector of the hologram  56  lies in a plane parallel to the z-axis  12  shown in FIGS. 1 and 2.  
         [0020]    In noncentrosymmetric crystals, such as for example 4 mm symmetry crystals, the electro-optic tensor has circular symmetry about the z-axis  12  of the crystals. Therefore, all of the holograms  56  that can be multiplexed by rotating the cylindrical crystal  10  about the z-axis  12  have the same electro-optic coupling given by the following equation:  
         Δ                 n     =       -     1     2        2                n   o   3          r   13          E   sc                             
 
         [0021]    where R 13  is the electro-optic tensor element n 0  is the ordinary refractive index and E sc  is the light induced, space charge field parallel to the grating vector K (see FIG. 2).  
         [0022]    In 3 m symmetry crystals the electro-optic tensor is not circularly symmetric about the z-axis for holograms written as illustrated in FIG. 2. In this case, the electro-optic coupling is given by the following equation:  
         Δ                 n     =       -       r   22       2        2                n   o   3        sin                   ϕ        [       4        cos   2        ϕ     -   1     ]            E   sc                             
 
         [0023]    where R 22  is the electro-optic tensor element and φ is the angle between the x-axis and the plane of the grating.  
         [0024]    Given this theoretical background, it is seen that a plurality of holograms  56  can be stored at different angular spacings in the cylindrical crystal  10  by rotating the cylindrical crystal  10  about its axis  12 . In the apparatus of FIG. 3, this is accomplished by utilizing a precision rotator  60  driven by a stepper motor  61  which indexes the cylindrical crystal  10  through relatively small angles θ.  
         [0025]    As is seen in FIG. 4 where efficiency η is plotted as a function of Δθ, it is clear that a hologram is possible wherever the curve  64  peaks. These peaks occur at a very small angle which in the illustrated embodiment 12.2 −4  rad. This results in a storage density for a plurality of holograms  56  which is extremely high. It is not necessary to store holograms at these close angular spacings if the precision rotator stepper motor  60  cannot index in such small steps, or if it is inconvenient or otherwise difficult to position holograms this close together. The angular indexing steps θ can be substantially larger and still produce an enormous number of holograms in a 360° rotation of the cylindrical crystal  10 .  
         [0026]    The curve  64  of FIG. 4 which plots holographic density as a function of Δθ is derived from the following equation (3) for determining the efficiency η each hologram:  
       η   =       (     1     1   +     (       K                 Δθ       2      κ       )         )          {           sin   2     (     {     κ                   L        [     1   +     (       K                 Δθ       2      κ       )       ]         )     }                   and        
          κ   2       =     (       2      πΔ                 n     λ     )                                 
 
         [0027]    where Δθ is the angle from the Bragg angle, K is the grating wave vector, L is the interaction length of the signal and reference beams, λ is the laser wavelength and Δn is the change in the refractive index given by Eq. (1) for 4 mm symmetry crystals and Eq. (2) for 3 m symmetry crystals. For efficiencies of η=2.5×10 −3  the second null occurs 80−6×10 −4  rad away from perfect Bragg matching as shown in FIG. 4.  
         [0028]    If the adjacent holograms are spaced every second null, the storage density for one layer is  
       N   =         2      π     δθ     =     10   ,   470                             
 
         [0029]    where N is the number of holograms.  
         [0030]    Clearly, the number of holograms N is substantial when one considers that each hologram  56  can be a page storing 1 Mb per page as determined by light and dark pixels of pixel arrays provided by the SLM. The theoretical limit at one axial location within the cylindrical crystal  10  is 10,470×1 Mb which equals 10,470 Mbs or 10,470 Gb per 360° rotation of the cylindrical crystal, provided that the precision rotor  60  can accurately achieve a null every 12×10 −4  rad away from perfect Bragg matching of FIG. 4.  
         [0031]    The capacity of the cylindrical crystal  10  can then be multiplied by indexing the cylindrical crystal  10  in the axial direction with a linear stepping actuator  65  on which the precision rotor  60  is mounted. For example, if the cylindrical crystal  10  has sufficient thickness or axially length to be indexed axially a distance of 2 mm and it is then rotated 360° while being written into, an additional 10.470 Gb of data can be stored. Since this can be done five times with a cylindrical crystal  10  having an effective axial length of 1 cm i.e. 10 mm, then the cylindrical crystal can store 52.350 Gb of data. Such a cylindrical crystal  10  is shown in the enlarged view of FIG. 7 wherein the cylindrical crystal is shown as a disk  66  having a thickness “L” in the direction of axis  12 . The disk  66  has stacked layers  67  of individual holograms  56  recorded in annular arrays  68  therein.  
         [0032]    If the cylindrical crystal  10  is configured as a rod  70  as shown in FIG. 8 with an axial length “L” of 10 cm then the capacity is 523.50 Gb which is a substantial amount of data. This storage capacity does not necessarily all have to be used to have enormously useful storage capacity for the cylindrical crystal  10 . As with the disk  66  of FIG. 6, the rod  70  of FIG. 8 stores stacked layers  67  of individual holograms  56  recorded in annular arrays  68  therein.  
         [0033]    After data has been stored in the cylindrical crystal  10 , it is of course desirable to read the data out from the crystal in order to utilize the data. This is accomplished by the readout apparatus of FIGS. 5 and 6. Referring first to FIG. 5, there is shown a readout apparatus  75  which is substantially identical to the writing apparatus  20  of FIG. 3, but includes a detector  76 , in the form of a charge coupled device (CCD) such as a CCD camera. When the readout apparatus  75  of FIG. 4, the shutter  38  is closed so that only a reference beam  36  is generated. Reference beam  36  is again focused through the cylindrical lens  52  into the cylindrical crystal  10  and is diffracted by the holograms  56  within the cylindrical crystal to produce diffracted beam  36 ′. The diffracted reference beam  36 ′ emerges axially from the cylindrical crystal  10  and is reflected by a reflector  78  in a direction transverse, i.e. normal to the axis  12  of the cylindrical crystal  10 . The diffracted reference beam  36 ′, reflected by the reflector  78  is then focused through a spherical lens  80  into the detector  76 , which is preferably a CCD camera. In the readout apparatus  75  of FIG. 4, it is necessary to rotationally index the cylindrical crystal  10  about the axis  12  during readout and this is again accomplished by the precision rotator  60 . It is also necessary to index the cylindrical crystal  10  axially in the direction of its z-axis  12  in order to read holograms of different layers  66 . This is accomplished by the linear stepping actuator  65  which shifts the crystal  10 , and perhaps the stepping motor  61  in an assembly therewith, in axial increments to read stacked layers  66  axially distributed in the cylindrical crystal.  
         [0034]    Referring now to FIG. 6, where a readout apparatus  90  is shown, modification to the apparatus as shown in FIG. 3 is minimal because the precision rotator  60  which provides rotational stepping and the linear stepping actuator  65  need not be moved. In the readout apparatus  90 , a phase conjugate mirror  91  is positioned radially with respect to the axis  12  of the cylindrical crystal  10  so that the reflected reference beam  36 ′ that passes through the cylindrical crystal can be focused by a lens  92  onto the phase conjugate mirror to create a phase conjugate diffracted beam  36 ″. The phase conjugate beam  36 ″ diffracts from the hologram and the diffracted beam emerges axially from the cylindrical crystal  10  and is reflected by a beam splitter  94  through a cylindrical lens  95  which focuses the diffracted beam  36 ″ into the detector  76 , which is again preferably in the form of a CCD camera.  
         [0035]    The readout apparatuses  75  and  90  represent one of what might be numerous readouts at various locations, such as for example customer&#39;s facilities. Since readouts do not need a reference beam  34 , beam splitter  32 , shutter  38 , mirror  40 , SLM  42  or spherical lens  44 , these devices may be deleted for readouts.  
         [0036]    In the illustrated embodiment of the invention, the cylindrical crystal  10  is angularly and axially indexed because this is a preferable approach to practicing the method and apparatus of the invention. However, it is for the purposes of this invention only necessary that the signal in reference beams and the cylindrical crystal  10  have relative motion. Accordingly, the cylindrical crystal  10  may be held stationary while the apparatus is moved relative thereto. This may be accomplished by mechanically moving the apparatus around and along the cylindrical crystal  10  or by optically steering the signal and reference beams  34  and  36  with respect to the cylindrical crystal  10 . In order to write into the cylindrical crystal  10 , a combination of relative motions is also possible within the scope of this invention, wherein the apparatus is mechanically moved, the signal and reference beams are optically steered and the cylindrical crystal is moved, either rotationally or axially or both rotationally and axially.  
         [0037]    While in the preferred and illustrated embodiment, signal beam  34  is introduced axially into the cylindrical crystal  10  and the reference beam  36  is introduced radially into the cylindrical crystal, in another embodiment, the reference beam  36  might be introduced axially and signal beam  34  introduced radially to write holograms into the cylindrical crystal.  
         [0038]    In the illustrated and preferred embodiment, crystal  10  is a cylindrical crystal, however, since a cylinder is a polygon with an infinite number of sides, it is within the scope of this invention to have a crystal  10  which is a polygon that rotates about the axis  12  or any other shape that rotates about an axis. For example, crystal  10  could be of an oval configuration.  
         [0039]    From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing form the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.