Abstract:
The invention relates to a method for recording data in a holographic recording medium ( 14 ) in a holographic data storage system. This method comprises the steps of: —recording a first hologram (H 1 , H 1 ′) through a first side ( 141 ) of said holographic recording medium, —rotating said holographic recording medium ( 14 ), —recording a second hologram (H 2 , H 2 ′) through a second side ( 142 ) of same holographic recording medium.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for recording data in a holographic recording medium in a holographic data storage system. 
         [0002]    The invention is particularly relevant to increase data storage capacity in holographic data storage systems, namely reflective holographic data storage systems using a reflective spatial light modulator. 
       BACKGROUND OF THE INVENTION 
       [0003]    The holographic data storage technology is known to have the potential to obtain large storage densities, fast parallel access, rapid searches in large data bases and data encryption. Furthermore, this technology is suitable for compact and portable data storage. 
         [0004]    However, it is a constant concern to increase the data capacity of the holographic storage medium used in the holographic data storage systems. 
         [0005]    For that purpose, one solution consists in carrying out multiplexing in order to get as many holographic patterns as multiplexing channels. 
         [0006]    Multiplexing can be achieved by using various methods. 
         [0007]    One method is called “angle multiplexing”. According to this method, the angle of the reference beam with respect to the recording medium is varied so that a plurality of holograms is recorded at a same location of the recording medium, each corresponding to a given angle of the reference beam. 
         [0008]    In “wavelength multiplexing”, the wavelength of the radiation beam is tuned in order to record different holograms at the same location of the holographic medium. 
         [0009]    “Shift multiplexing” consists in recording a set of holograms by shifting the recording medium with respect to the optical unit. Once a hologram has been recorded at a given location of the recording medium, the recording medium is shifted over a distance that is less that the width of the hologram. Shift multiplexing is only possible when spherical waves interfere. 
         [0010]    Naturally, combinations of different multiplexing methods may be possible. 
         [0011]    However, although multiplexing leads to good results, it is still desirable to obtain an even higher storage capacity of the holographic recording medium. 
       SUMMARY OF THE INVENTION 
       [0012]    It is an object of the invention to provide a method for recording data in a holographic recording medium in a holographic data storage system which would contribute to a further increase of the data storage capacity of said holographic recording medium while being compatible with certain multiplexing methods. 
         [0013]    To this end, the invention proposes a method for recording data in a holographic recording medium in a holographic data storage system, comprising the steps of:
       recording a first hologram through a first side of said holographic recording medium,   rotating said holographic recording medium,   recording a second hologram through a second side of same holographic recording medium.       
 
         [0017]    It can be readily understood that the method according to the invention leads to an increase by a factor of two of the storage capacity of the recording medium. 
         [0018]    In a particular embodiment, said holograms are wavelength multiplexed holograms. 
         [0019]    This is allowed in case the maximum number of wavelength multiplexed holograms is limited by the wavelength range of the tunable laser systems and not by material properties, e.g. the dynamic range of the material. 
         [0020]    In a preferred embodiment, said first wavelength multiplexed holograms are recorded at wavelengths positioned in between the recording wavelengths of said second wavelength multiplexed holograms. 
         [0021]    In an alternative embodiment, said holograms are shift multiplexed holograms. However, in that case spherical waves are required, as above mentioned. 
         [0022]    These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The invention will now be described in more detail by way of example with reference to the accompanying drawings, in which: 
           [0024]      FIG. 1   a  shows a holographic data storage system after recording data through one side of the holographic recording medium; 
           [0025]      FIG. 1   b  shows the holographic recording medium after rotation; 
           [0026]      FIG. 1   c  shows the holographic data storage system after recording data trough the two sides of the holographic recording medium; 
           [0027]      FIG. 2  shows the embodiment of the holographic data storage system during reading-out; 
           [0028]      FIG. 3  shows a holographic data storage system designed for shift multiplexing. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    The optical device depicted in  FIG. 1   a  is a holographic data storage system intended to record data in and read data from a holographic recording medium  14 . 
         [0030]    This optical device further comprises a radiation source  10 , a collimator  11 , a polarizing beam splitter  12 , a quarter wave plate  13 , a reflective spatial light modulator  15  and detecting means  16 . The optical device also comprises means for receiving the recording medium, which are not shown in  FIGS. 1   a  and  1   c . These receiving means are, for example, a table such as those conventionally used in CD or DVD players. 
         [0031]    During recording, which is represented in  FIG. 1   a , the radiation source  10  generates a radiation beam, which is transformed into a parallel beam by means of the collimator  11 . This parallel beam is then directed towards the recording medium  14  by means of the polarizing beam splitter  12 . After the parallel beam has passed through the polarizing beam splitter  12 , it has a linear polarization. This linearly polarized beam then passes through the quarter wave plate  13 , which creates a circularly polarized beam. This latter beam passes through the recording medium  14  and reaches the reflective spatial light modulator  15 . The signal thus reflected is circularly polarized and carries the information sent to the reflective spatial light modulator  15 . This reflected signal then reaches the recording medium  14  through a first side  141  and an interference takes place with the circularly polarized beam that has just passed through the quarter wave plate  13 . This interference creates an information pattern in the recording medium  14 , and a first hologram H 1  is thus recorded. Interference can take place between the beam coming from the reflective spatial light modulator  15  and the beam that has just passed through the quarter wave plate  13 , because these two beams have the same polarization. The beam that has just passed through the quarter wave plate  13  plays the role of a reference beam, whereas the beam coming from the reflective spatial light modulator  15  plays the role of a signal beam. 
         [0032]    The reflective spatial light modulator  15  may be, for example, a reflective ferroelectric Liquid Crystal on Silicon (FLCOS) spatial light modulator. Such a spatial light modulator is commercialized, inter alia, by the companies “Boulder Nonlinear Systems” and “Displaytech”. The reflective spatial light modulator  15  may also be a reflective Digital Micromirror Device (DMD) spatial light modulator. Such a spatial light modulator is commercialized, inter alia, by the company “Productivity Systems”. The reflective spatial light modulator  15  may also be a combination of a transmissive spatial light modulator and a mirror, although this solution is less preferred, because the efficiency of a transmissive spatial light modulator is lower than the efficiency of a reflective spatial light modulator. 
         [0033]    In order to increase the storage capacity of the recording medium  14 , wavelength multiplexing can be carried out consisting in recording at least another hologram at the same location by modifying the wavelength of the radiation beam. The optical device in accordance with  FIGS. 1   a  and  1   c  is particularly suitable for wavelength multiplexing. Actually, in order to record a relatively high number of holograms at a same location of the recording medium  14 , the wavelength selectivity should be as low as possible. The wavelength selectivity represents the gap between two successive wavelengths that can be used for recording two holograms with acceptable crosstalk. Details about wavelength multiplexing can be found in H. J. Coufal, D. Psaltis, G. T. Sincerbox (Eds.), ‘Holographic data storage’, Springer series in optical sciences, (2000). 
         [0034]    Once a first hologram H 1  has been recorded in the recording medium  14 , another first hologram H 1 ′ may be recorded by moving the recording medium  14  with respect to the optical pick-up unit comprising the detecting means  16 , the polarizing beam splitter  12 , the quarter wave plate  13  and the reflective spatial light modulator  15 . Alternatively, the optical pick-up unit is moved with respect to the recording medium  14 , in a direction parallel to the recording medium  14 . 
         [0035]    After recording of the first holograms H 1  and H 1 ′, the holographic recording medium  14  is rotated as shown on  FIG. 1   b  and placed again in the optical device for recording two second holograms H 2  and H 2 ′ represented on  FIG. 1   c . The way the second holograms are recorded in the recording medium  14  is the same as that used for recording the first holograms H 1  and H 1 ′, except that in  FIG. 1   c  recording is effected through a second side  142  of the recording medium  14 . 
         [0036]    During read-out, which is represented in  FIG. 2 , the radiation source  10  generates a radiation beam having a given wavelength, which is transformed into a parallel beam by means of the collimator  11 . This parallel beam is then directed towards the recording medium  14  by means of the polarizing beam splitter  12 . After the parallel beam has passed through the polarizing beam splitter  12 , it has a linear polarization. This linearly polarized beam then passes through the quarter wave plate  13 , which creates a circularly polarized beam. This latter beam reaches the recording medium  14 , and is reflected by the information patterns recorded in said recording medium  14  as first hologram H 1  and second hologram H 2 ′. A first reconstructed signal beam is thus created which carries the information corresponding to the first hologram H 1  recorded with the same wavelength. This reconstructed signal beam passes through the quarter wave plate  13 , which creates a beam with a linear polarization perpendicular to the polarization of the beam that has just been deflected by the polarizing beam splitter  12 . As a consequence, this linearly polarized reconstructed signal beam passes through the polarizing beam splitter  12  and thus forms a real image on the detecting means  16 . The recorded first hologram H 1  is thus read-out. 
         [0037]    It should be noted that the second hologram H 2 ′ also gives rise to an image which is a virtual image formed together with the real image obtained from the first hologram H 1 . Whereas in the real holographic image every pixel of the spatial light modulator  15  is imaged on a pixel of the detecting means  16 , every pixel of the virtual image is smeared out over a large number of pixels on said detecting means  16 , resulting in a DC offset. Thus, the capacity increase of a factor  2  is obtained at the expense of the signal-to-noise ratio. 
         [0038]    This cross-talk between the real and the virtual images can be decreased by recording the holograms through one side at wavelengths which are positioned in between the wavelengths of the holograms recorded through the other side of the recording medium  14 , e.g. at . . . 400 nm, 402 nm, 404 nm, . . . through one side and at . . . , 401 nm, 403 nm, 405 nm, . . . through the other side. 
         [0039]    Naturally, for reading-out said second holograms H 2 , H 2 ′ the recording medium  14  has to be rotated so as to convert the corresponding virtual images into real images formed on the detecting means  16 . 
         [0040]    The storage capacity of the recording medium  14  can also be increased by using shift multiplexing.  FIG. 3  shows an optical device for that purpose. 
         [0041]    This optical device comprises, in addition to elements already depicted with reference to  FIGS. 1   a  and  1   c , a first lens  21  and a second lens  22 . The first lens  21  is arranged between the polarizing beam splitter  12  and the recording medium  14 , and the second lens  22  is arranged between the recording medium  14  and the reflective spatial light modulator  15 . 
         [0042]    During recording, the radiation beam that has passed through the quarter wave plate  13  is focused in the recording medium  14  by means of the first lens  21 . A spherical wave beam is thus focused in the recording medium  14 . This spherical wave beam is then made parallel by means of the second lens  22 , and then reaches the reflective spatial light modulator  15  where a signal beam is created. On the way back from the spatial light modulator  15 , the signal beam is focused on the recording medium  14  by means of the second lens  22 . As a consequence, a spherical wave signal beam interferes with a spherical wave reference beam inside the recording medium  14 , and an information pattern is created, which corresponds to a first hologram to be recorded. 
         [0043]    The fact that spherical wave beams interfere inside the recording medium  14  allows for shift multiplexing. Shift multiplexing consists in recording a second set of holograms by shifting the recording medium with respect to the optical pick-up unit. Once a first hologram has been recorded at a given location of the recording medium, the recording medium is moved over a distance that is less that the width of a hologram, and then another first hologram may be recorded. Shift multiplexing is only possible when spherical waves interfere. 
         [0044]    After the holographic recording medium is rotated, a set of second holograms can be recorded the same way as said first holograms. 
         [0045]    Advantageously, a combination of shift multiplexing and wavelength multiplexing may be used for recording data in the recording medium  14 . 
         [0046]    During read-out, a spherical wave beam is sent towards the recording medium  14  and is reflected by the information pattern. As explained in  FIG. 1   c , a reconstructed signal beam is created, which then reaches the detecting means  16 . Read-out of a hologram that has been recorded with a given wavelength and a given position of the recording medium  14  with respect to the optical pick-up unit is performed by placing the recording medium  14  at the same position and generating a radiation beam having the same wavelength. It should be noted that the medium  14  needs not to be rotated after recording before it can be read out. A real image is obtained without rotating the medium  14  because the virtual image is converted into a real image by the first lens  21 . 
         [0047]    Any reference sign in the following claims should not be construed as limiting the claim. It will be obvious that the use of the verb “to comprise” and its conjugations does not exclude the presence of any other elements besides those defined in any claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements.