Abstract:
The invention provides a method and apparatus for providing a high information capacity, high data rate and short access time simultaneously. The method and apparatus include a multilayer waveguide holographic carrier, a multilayer waveguide holographic data storage system, a multilayer waveguide hologram reading method with random data access, and a process and apparatus for recording matrix waveguide hologram layers and assembling a multilayer carrier.

Description:
FIELD OF THE INVENTION 
   The present invention relates to volume holographic data storage and more particularly, to waveguide multilayer holographic data storage systems for providing a high throughput of data storage. 
   BACKGROUND 
   The logic of evolution of modern information technologies dictates a necessity to create data storage systems with a high information capacity, a high data rate and small access time, i.e. a high throughput system. Many researchers use the CRP (capacity-rate product) factor for the throughput estimation where CRP=Capacity[GB]×Data Rate[Mbps] (High Throughput Optical Data Storage Systems  An OIDA Preliminary Workshop Report  April 1999. Prepared for Optoelectronic Industry Development Association by Tom D. Milster). 
   A more objective factor, being proposed for use in this invention, is CARP (capacity-access-rate product), which is the capacity in GB, divided by access time in ms and multiplied by the data rate in Mbps. We have CARP={C[GB]/A[ms]}×Data Rate[Mbps]. A comparison of CARP factors gives the possibility to estimate objectively the advantages of any data storage system in terms of throughput. 
   It is clear that a need exists for systems in future applications where CRP&gt;10 5  and CARP&gt;10 6 . That is, for example, a memory system with &gt;1 GB information capacity, &gt;100 Mbps data rate and &lt;1 ms access time. At the same time, it is clear that it is necessary to ensure a minimum quality of recorded and readout signals, that is to provide a desired value of the signal/noise ratio and thereby to maintain a desired value of the error probability. 
   Holographic methods are considered the most prospective for high throughput data storage. More specifically, the data page oriented random access holographic memory is in the first place as a high throughput system. However, there have been difficulties and problems in the development of the high throughput system up to the present day. The high data rate for optical data storage systems depends on the light source power, sensitivity of photodetector, the number of information parallel input-output channels, and also on the conveying speed of the carrier or optical reading head, when using a design with moving mechanical parts. 
   For holographic storage a large number of parallel data channels is provided due to data presentation as two-dimensional pages of digital binary or amplitude data. Moreover, the highest data rate is provided when there are no moving mechanical parts, such as a rotating disk carrier. 
   Short random access time of a memory system is a result of applying a high-speed addressing system such as electro- or acousto-optical deflectors and using a recording-reading schema, which provides for transferring read images from different microholograms to a photodetector without any mechanical movement. 
   Use of a volume information carrier in optical (including holographic) data storage for providing a high information capacity and high information density is well known, as in U.S. Pat. No. 6,181,665 issued Jan. 30, 2001 to Roh. But existing methods of optical (holographic) data storage based on a volume carrier do not obtain high capacity and short random access time simultaneously in accordance with the circumstances indicated below. 
   There are several methods of volumetric holographic carrier applications. The first is using angle multiplexed volume holograms, which provide for the superimposing of data pages of Fourier or Fresnel holograms in the volume photorecording medium. Each of the holograms is recorded with a separate angle of the reference beam. The same angle of the readout beam is required for data page reading. Examples include Roh, U.S. Pat. No. 6,072,608 issued Jun. 6, 2000 to Psaltis et al., U.S. Pat. No. 5,896,359 issued Apr. 20, 1999 to Stoll, and U.S. Pat. No. 5,696,613 issued Dec. 9, 1997 to Redfield et al. 
   A second method is using encrypted holograms for holographic data storage as in U.S. Pat. No. 5,940,514 issued Aug. 17, 1999 to Heanue et al. In the Heanue system orthogonal phase-code multiplexing is used in the volume medium and the data is encrypted by modulating the reference beam. 
   This method has a number of limitations. The main problem is a deficiency of the volumetric medium in meeting the necessary requirements. For example, ferroelectric crystals do not exhibit sufficiently great stability, and photopolymers have too large a shrinkage factor. 
   A third method is using holograms recorded in a multilayer medium as described by “Holographic multiplexing in a multilayer recording medium”, Arkady S. Bablumian, Thomas F. Krile, David J. Mehrl, and John F. Walkup,  Proc. SPIE , Vol. 3468, pp. 215-224 (1998) and by Milster. One or more holograms (a hologram matrix) are recorded in each layer of the volume carrier. A readout of each hologram is made by a separate reading beam. A limitation of this method is a low layer count, the number of layers being limited by the noise from neighboring holograms located on other layers. 
   The last method is using waveguide multilayer holograms. See “Medium, method, and device for hologram recording, and hologram recording and reproducing device”, Mizuno Shinichi (Sony Corp.) JP09101735A2, Publication date: Apr. 15, 1997. Waveguide holograms are recorded in thin films of a multilayer carrier. Known methods of multilayered waveguide hologram recording and reading do not provide a high data density and small access time simultaneously. 
   International Publication No. WO 01/57602 discloses the recording of holograms in a wave guide layer formed in a structure containing multiple wave guide layers. An optical system allows the writing of holograms in the wave guide layer and subsequent reading of the written holograms. However, the memory system does not provide a combination of very low access time and high data density simultaneously because the data carrier tape or data storage card moves during readout. Any mechanical movement in a data storage system results in a relatively long data access time. 
   The analysis of known methods and apparatus in the field of holographic data storage permit to draw a conclusion: at the present time there is no high throughput holographic data storage system approach providing a high value of the CARP factor. 
   It is an objective of this invention to provide a holographic storage system with a high CARP factor. 
   SUMMARY 
   The present method offers an integrated approach to solving a problem of providing a high information capacity, high data rate and short access time simultaneously. The required characteristics of a system are provided by a tightly bounded information carrier construction technique and new methods of data accessing, reading and recording. 
   The present invention includes a multilayer waveguide holographic carrier, a multilayer waveguide holographic data storage system, a multilayer waveguide hologram reading method with random data access, and a process and apparatus for recording matrix waveguide hologram layers and assembling a multilayer carrier. The multilayer wave guide hologram reading method incorporates an electronic moving window provided by a spatial light modulator (SLM) or charge coupled device (CCD) on the surface of the multilayer wave guide. The hologram pitch is related to the SLM or CCD element size. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention itself both as to organization and method of operation, as well as objects and advantages thereof, will become readily apparent from the following detailed description when read in connection with the accompanying drawings: 
       FIG. 1   a  shows a multilayer waveguide holographic carrier with end surface couplers for a reference beam; 
       FIG. 1   b  shows a multilayer waveguide holographic carrier with diffraction grating couplers for a reference beam; 
       FIG. 2   a  illustrates a method of putting a reference beam into a waveguide layer of a data storage carrier through an end surface coupler and radiation from reconstructed holograms; 
       FIG. 2   b  illustrates a method of putting a reference beam into a waveguide layer of data storage carrier through a diffraction grating coupler and radiation from reconstructed holograms; 
       FIG. 3  shows a data page image pattern to be stored holographically in a focusing plane; 
       FIG. 4  shows a hologram layer with a superimposed hologram; 
       FIG. 5  illustrates a system with random data access for retrieving holographically stored data from a multilayer waveguide carrier; 
       FIG. 6  illustrates a geometrical relationship between waveguide holograms in a hologram layer and a photodetector array; 
       FIG. 7  illustrates a system for retrieving holographically stored data from a multilayer waveguide carrier utilizing a phase conjugate reference beam; 
       FIG. 8  illustrates a system for superimposed waveguide hologram reading; 
       FIG. 9  illustrates a system for encrypted waveguide hologram reading; 
       FIG. 10  illustrates a system for waveguide hologram reading by a laser matrix; 
       FIG. 11  represents a schematic view of a process and apparatus for recording a matrix of waveguide Fourier (quasi Fourier) holograms in a photorecording layer by using a diffraction grating coupler; 
       FIG. 12  represents a schematic view of a process and apparatus for recording a matrix of waveguide Fourier (quasi Fourier) holograms in a photorecording layer by using SLM disposed in a convergent beam; 
       FIG. 13  represents a schematic view of a process and apparatus for recording a matrix of waveguide Fourier (quasi Fourier) holograms in a photorecording layer by using a random phase mask; 
       FIG. 14  represents a schematic view of a process and apparatus for recording a matrix of waveguide Fourier (quasi Fourier) holograms in a layer by using a small angle input of a reference beam; 
       FIG. 15  represents a schematic view of the single layer matrix waveguide Fresnel hologram recording process and apparatus; and 
       FIG. 16  illustrates a system for multiplexed waveguide hologram recording. 
   

   DETAILED DESCRIPTION 
   Multilayer Holographic Data Storage Carrier 
     FIGS. 1   a  and  1   b  show a multilayer holographic waveguide data storage carrier  10 . It comprises layer groups each containing a hologram layer  11   i  where i is the current layer index and cladding layer  12   i . Holograms  14   i,k  are located along row axis  01   ij  where j is the current row index and k is the current hologram index. Holograms are non-overlapping in each of the rows. 
   In the first variant shown in  FIG. 1   a , hologram layer  11   i  in each group is at the same time a waveguide layer having end surface coupler  15   i . In the second variant shown in  FIG. 1   a , the hologram layer  11   i  and waveguide layer  13   i  with a diffraction grating coupler  16   i  (seen in  FIG. 1   b ) in each of the groups are made separately and attached to each other with an optical contact therebetween to provide transmission of the guided wave into the hologram layer. In both variants there is a cladding layer on the outer surface of the waveguide layer, with a similar function to prior art cladding layers. 
   In  FIGS. 1   a  and  1   b  h =  is the size of a hologram in the row direction and d =  is the pitch of a hologram in the row direction. h ⊥  and d ⊥  are the size and pitch of the holograms respectively in the transverse direction. h is the thickness of a hologram layer and d is the pitch of the layers. 
   As shown in  FIGS. 2   a  and  2   b , a readout beam  20  penetrates into a waveguide layer through coupler  15   i  (or  16   i ). Then, the readout beam propagates along respective row ij as a guided wave  21   ij  and reconstructs radiation beams  22   ijk  from all its holograms simultaneously. Reconstructed radiation from each hologram propagates towards an output surface  02  and is restricted in its spatial angle γ. 
   When holograms have a specified spatial angle γ of radiation, the hologram pitch p =  between adjacent holograms is established so as to provide an intersection of said radiation at plane  03  and in the area above this plane. All reconstructed radiation beams form focused data page images at parallel plane  04 . 
     FIG. 3  shows a data page image pattern  51  in the focusing plane  04 . Data pixels  17   mn  have sizes s = , s ⊥  and pitches t = , t ⊥  and are disposed as a 2-D matrix. m and n are current pixel indices along rows and columns respectively. All data page images have the same orientation. M and N are quantities of data pixels in the respective direction. 
     FIG. 4  shows a hologram layer with superimposed holograms. The angle between non-parallel row axes  01   ij  and  01 ′ij is α. Some holograms relating to different non-parallel intersecting rows are recorded so to be at least partially superimposed. The angle between any of two nearest non-parallel hologram rows is established to be not less than the angle selectivity of said superimposed holograms. 
   Readout Method and System 
     FIG. 5  illustrates a system for retrieving holographically stored data from the multilayer waveguide carrier. The system includes a multilayer holographic waveguide data storage carrier  10  and a layer and row access unit  30 . The layer and row access unit  30  is made up of a laser  31  for generating a beam of coherent radiation and a beam former  32  for forming a beam  24 , which is deflected by angular deflector  33  and becomes beam  25  passing through an optical element (lens)  34  to a selected layer  11   i  and, through the respective coupler  15   i  (or  16   i ), into the selected layer along the required hologram row. 
   A hologram access unit  40  made in the form of a “moving window” is arranged in the region between planes  02  and  03  (see  FIG. 2   a ) and intended for separating radiation  22   ijk  from any hologram  14   ijk  to gain access thereto and block radiation from other reconstructed holograms. 
   A multielement photodetector  50  faces towards the output surface  02  of the carrier, intended for receiving reconstructed radiation  22   ijk  from said hologram, disposed at plane  04  of focus of this radiation and optically coupled with a pixel pattern  51  (see  FIG. 3 ) of data stored by the hologram. 
   Lastly, a computer  60  is connected through respective interface units to control inputs of the layer and row access unit  61 , hologram access unit  62  and the photodetector  63  to control their coordinated operation. 
     FIG. 6  illustrates a geometrical relationship between waveguide hologram  14   ijk  in a hologram layer and photodetector array  50 . 
   The photodetector array pixel quantity Q =  in one direction, which is parallel to the hologram rows and data rows, must be Q = =P = /p = ≧(q = −1)h = /p = +M=[h = (q = −1)+Mp = ]/p =  where: 
   P =  is the linear size of detector array along rows, P = =(q = −1)h = +Mp = ; 
   h =  is the hologram pitch along a row; 
   q =  is the number of holograms in the row; 
   p =  is the pitch of detector pixels along a row; and 
   M is the number of pixels of readout data in a data page row. 
   Respectively, the photodetector array pixel quantity in other direction, which is perpendicular to hologram and data page rows, must be Q ⊥ =Q ⊥ /p ⊥ ≧h ⊥ (q ⊥ −1)/p ⊥ +N, where:
         Q ⊥  is the linear size of detector array along columns;   h ⊥  is the hologram pitch along a column;   q ⊥  is the number of holograms in the column;   p ⊥  is the pitch of detector pixels along the column; and   N is the number of pixels of readout data in a data page column.       

   L = =(q = −1)h = +d =  is the linear size of the hologram row in the selected direction. The pitch of data page image pixels is equal to or larger than the detector pixel pitch in which case it is a whole number multiple of it. 
     FIG. 7  illustrates a system for retrieving holographically stored data from a multilayer waveguide carrier utilizing a phase conjugate reference beam  20 *. In comparison with  FIG. 5 , a conjugate coupler  15 *i is used and the photodetector is disposed at conjugate plane  04 *. 
     FIG. 8  illustrates a system for superimposed waveguide hologram reading. Holograms from non-parallel rows are read by readout beams  20  and  20 ′ having an angle • between them. An additional deflector is used in the layer and row access unit to provide the required additional angular deviation of reading beam  20  in a plane which is parallel to layer  11   i . For example, it is possible to use a rotated optical plate  35  in addition to deflector  33  (made as a rotated mirror provided with a rotary actuator controlled by computer through the respective interface). 
     FIG. 9  illustrates a system for encrypted waveguide hologram reading. A multichannel phase spatial light modulator  41  and cylindrical lens  36  are used respectively for readout beam encoding (encryption) and directing the encoded beam  27   ij  into waveguide layer  11   i.    
     FIG. 10  illustrates a system for waveguide hologram reading by a laser matrix. Laser matrix  37  and optical fibers  38   ij  are used for forming a separate readout beam for each hologram row. The computer controls each laser of matrix  37  through an interface  65 . 
   Waveguide Hologram Recording Process and Apparatus 
   Holograms can be recorded as Fourier (or quasi Fourier) or Fresnel holograms of a two dimensional matrix of digital (binary or multilevel) or analog signals. Hologram matrices are recorded on separate layers. Then the hologram layers (and waveguide layers when used separately) and cladding layers are sandwiched together forming an optical contact between them, thus producing the multilayer waveguide holographic data storage carrier. 
   Fourier (or Quasi Fourier) Hologram Recording 
     FIG. 11  represents a schematic view of a process and apparatus for recording a matrix of waveguide Fourier (or quasi Fourier) holograms in a photorecording layer by using a diffraction grating coupler. A monochromatic light source, such as a laser, generates a beam of coherent radiation that is split into a first (signal) beam  70  and a second beam which is used to form a reference beam  28  by optical means  32 , as shown in  FIG. 11 . A signal collimated beam  71  expanded by standard optical means  80 , such as lenses, passes through (or reflects from) a spatial light modulator (SLM)  42 . The data page is displayed by SLM  42 . Computer  60  forms control signals which arrive at SLM  42  through interface  66 . Beam  72 , modulated in amplitude (or phase, or polarization) according to the control signals, is focused at the plane  06  near the photorecording medium  17  by an optical element (lens)  81  following which it illuminates a local area of the photorecording medium  17 . Thus, this local area is illuminated by an image of the Fourier (or quasi Fourier) transformation function of the data page. The layer of photorecording medium  17  is laminated on an optically transparent hard substrate  18  (for example, glass). 
   Simultaneously, reference beam  28  is transformed by diffraction grating reference beam coupler  73  into guided reference wave  29 . Wave  29  then illuminates the same local area. 
   A diaphragm  83  may be located close to the photorecording medium surface for preventing parasitic illumination of the photorecording medium. 
   The optical system for forming the transformed data page image to be recorded in the medium  17  may be realized by different methods, which depend upon the character of the readout beam as described below: 
   1) Readout Beam is the Analog of a Reference Beam. 
   In this case, the distance between plane  07  (where the optical element  81  is located) and plane  08  (where the SLM  42  is located) is such that the reconstructed data page image will be located at the same distance from the photorecording medium as the distance from the hologram to the detector plane of the readout device. At the same time, the pitch of data page pixel images must be equal to, or a whole number multiple of the pitch of photodetector pixels. This means, for example, that if the pitch of readout data pixel images at the plane  04  of photodetector  50  ( FIG. 6 ) is equal to the pitch of pixels displayed by the SLM, then a distance V between plane  08  and plane  07  is equal to the double focus length ( 2 F) of lens  81 . F is the distance between planes  06  and plane  07 . 
   Different layers  11   i  ( FIG. 5 ) of multilayer holographic carrier  10  are located at different distances Gi ( FIG. 6 ) from the photodetector plane  04  ( FIG. 5 ). Therefore, it is necessary to provide a condition: •Fi+Gi=constant. In this case, reconstructed data images from all layers of the carrier will have an identical scale. 
   Parallel plate  82  ( FIG. 11 ) of optically transparent material (or a special phase compensator) is used to compensate for any difference in the optical distance from different layers to the detector plane. The thickness and refractive index of this plate must be such as to provide an optical analog of carrier layers located between given layer  11   i , ( FIG. 6 ) and photodetector plane  04  ( FIG. 6 ). 
   2) Readout Beam (such as 20*,  FIG. 7 ) is Phase Conjugate to the Reference Beam. 
   In this case, as shown in  FIG. 12 , SLM  42  is in the convergent beam from lens  81  in the immediate proximity of plane  07 . 
   Note: the readout of these type of holograms does not provide for using any image forming optics between hologram plane  01   i  ( FIG. 6 ) and photodetector plane  04  ( FIG. 6 ). 
     FIG. 13  represents a schematic view, which is the same as in  FIG. 11 , except for the use of a random phase mask  43  to provide a more uniform Fourier image distribution in hologram recording plane  05   i . It is possible to use a phase spatial light modulator as a phase mask  43 . 
   Hologram Recording Procedure 
   As shown in  FIG. 11 , guided reference wave  29  propagates in photorecording film layer  17  as in a waveguide. Simultaneously, the modulated signal beam (Fourier or quasi Fourier image) is directed along the line normal to the photorecording film layer. Holograms are recorded by sequentially shifting the photorecording layer after each recording along a distance in the specified direction which is equal to the pitch size h =  of the holograms to be recorded. Two-coordinate positioner  90  is used to make the shifting and is controlled by computer  60  through interface  67 . The pitch (h ⊂  and h ⊥ ,  FIG. 1   a,b ) of holograms must be divisible by a whole number of photodetector pixels p =  and p ⊥  ( FIG. 6 ). Recorded holograms are arranged in hologram rows forming a matrix in the photorecording layer. 
     FIG. 13  illustrates variants of the recording procedure using a carrier, which contains two different layers: a photorecording (photosensitive) layer  17  and a waveguide layer  19 . In particular, the reference beam is directed into waveguide layer  19  by a prism coupler  86 . 
   As shown in  FIG. 12  and  FIG. 14 , the reference beam  28  is directed at a small angle β to the photorecording layer  17 . If the photorecording layer does not have a hard substrate, it is possible to place this layer between optical plates  84  and  85  by using immersion layers  87  and  88  having a refractive index close to that of the photorecording layer. 
   Fresnel Holograms Recording 
   In this case, the readout is to be made by the conjugate reference beam. The recording procedure is the same as described above, but, as shown in  FIG. 15 , optical elements, such as focusing lens  81  and collimating lens  89 , form a Fresnel image of SLM data page  42  in the hologram recording plane  05   i.    
   Formation of a diffraction grating to couple the reference beam to the waveguide layer. 
   Grating coupler  16   i  ( FIG. 1   b ) is recorded by a holographic method on the periphery of the photorecording layer  11   i  ( FIGS. 1   a ,  1   b ), which is also a waveguide layer, or it is formed on the periphery of separate waveguide layer  13   i  ( FIGS. 1   a ,  1   b ) by stamping, etching or other known methods. 
   Superimposed Hologram Recording 
   The recording procedure is the same as described above, but as shown in  FIG. 16 , at least two superimposed hologram  91  and  91 ′ are recorded sequentially in the overlapping area with different propagation directions  29  and  29 ′ of the reference beam in the hologram recording plane  05   i . A minimum angle • between reference beam directions is necessary to provide the independent readout of holograms by the appropriate readout beam. 
   Encrypted Hologram Recording 
   The recording procedure is the same as described above, but the reference beam is formed by the same method as that used for forming a readout encoded beam  27   ij  ( FIG. 9 ). 
   Accordingly, while this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the scope of the invention.