Abstract:
The present invention relates to a phase mask for holographic data storage, and to a method and an apparatus for reading from and/or writing to holographic storage media using such a phase mask. 
     According to the invention, the phase mask has a plurality of phase cells, whose size is equal to an integer multiple of the size of the pixels of a spatial light modulator of the apparatus. The phase cells have a phase variation on sub-cell scale, which is inverse for essentially half the number of phase cells.

Description:
This application claims the benefit, under 35 U.S.C. §119, of European Patent Application No. 07114691.4 filed Aug. 21, 2007. 
     FIELD OF THE INVENTION 
     The present invention relates to a phase mask for holographic data storage, and to a method and an apparatus for reading from and/or writing to holographic storage media using such a phase mask. 
     BACKGROUND OF THE INVENTION 
     In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam, the so-called ‘object beam’, is modulated by a spatial light modulator (SLM) and carries the information to be recorded. The second beam serves as a reference beam. The interference pattern leads to modifications of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. This results in the reconstruction of the recorded object beam. 
     One advantage of holographic data storage is an increased data capacity. Contrary to conventional optical storage media, the volume of the holographic storage medium is used for storing information, not just a few layers. One further advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift multiplexing, etc. Furthermore, instead of storing single bits, data are stored as data pages. Typically a data page consists of a matrix of light-dark-patterns, i.e. a two dimensional binary array or an array of grey values, which code multiple bits. This allows to achieve increased data rates in addition to the increased storage density. The data page is imprinted onto the object beam by the spatial light modulator and detected with an array detector. 
     As described above, in page-oriented holographic data storage a pixelated spatial light modulator is used for modulating the object beam intensity with information. This intensity distribution is usually Fourier transformed by an objective lens. The Fourier transform, i.e. the spectrum of a pixelated data pattern has a high central intensity peak, hereafter referred to as DC-peak. The actual information is distributed around this peak on a much lower level, typically −60 dB. The DC-peak of the object beam can cause an undesired saturation of the photosensitive medium. The envelope of the surrounding intensity distribution can be described by a 2-dimensional sinc-function (sin (x)/x), which results from the usual square-like shape of the pixels. The full information about the SLM pixel pattern is located below the so-called Nyquist limit which lies at half the distance to the first zero of the sinc-function. 
     In order to suppress the DC-peak it has been proposed to apply a phase modulation in addition to the intensity modulation. For example, in M. J. O&#39;Callaghan: “Sorting through the lore of phase mask options—performance measures and practical commercial designs”, Proc. SPIE Vol. 5362 (2004), pp. 150-159, different types of pixelated and non-pixelated phase masks are discussed. Typically, however, a binary phase mask is used for this purpose, which introduces a phase shift of 0 or π with respect to the laser wavelength. The phase cells, i.e. the areas with constant phase, have a size of one or more pixels of the SLM. The spatial distribution of 0 and π cells is random or pseudo random, the total number of 0 and π cells is essentially the same. 
     A phase mask with a cell size of one SLM pixel suppresses the DC-peak quite well. However, a large fraction of intensity is still located above the Nyquist limit. This fraction is redundant and does not contain necessary data information. Furthermore, because of the sinc-like envelope, the intensity distribution is not flat within the central region of the Fourier plane or in the holographic medium, respectively. 
     This drawback is overcome by a phase mask having a phase modulation with a lower spatial resolution than the SLM. Such a phase mask lead to a narrower intensity distribution in the Fourier plane. Therefore, it reduces the unnecessary intensity above the Nyquist limit, whereas the important intensity below the Nyquist limit is increased. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to propose a phase mask for holographic data storage, which further improves the intensity distribution of a light beam in a Fourier plane of a holographic storage system. 
     According to the invention, this object is achieved by a phase mask for holographic data storage having a plurality of phase cells, wherein the phase cells have a phase variation on sub-cell scale. 
     The invention proposes to apply a special phase distribution within each phase cell. If the cell size is the same as the SLM pixel size, this means a phase variation on a sub-pixel scale. The special phase distribution on sub-cell scale affects the intensity distribution in the Fourier plane in such way that its envelope becomes flatter. As a consequence the illumination of the holographic storage medium is more balanced and a possible saturation is avoided. This allows to achieve stronger holograms with a better signal-to-noise ratio (SNR) or a larger number of multiplexed holograms, i.e. an increased capacity. 
     Preferably, the size of the phase cells is equal to an integer multiple of the size of the pixels of the SLM. As stated above, when the phase cell size is equal to the size of the SLM pixels, the envelope of the intensity distribution in the Fourier plane become flatter. If the size of the phase cells is increased to an integer multiple of the SLM pixel size, for example if one phase cell is associated to four SLM pixels, the intensity above the Nyquist limit is reduced, whereas the intensity below the Nyquist limit is increased. This further improves the illumination of the holographic storage medium. The size of the phase cells is preferably identical in perpendicular directions within the plane of the phase mask, e.g. each phase cell is associated to 2×2, 3×3, etc. SLM pixels. Of course, it is likewise possible to use different sizes in both directions, e.g. 1×2, 2×3, etc. SLM pixels. 
     Advantageously, a special pattern of the phase variation is at least one of a circular, rectangular, ring-like, polygon-like, or pixelated pattern, or a combination of these patterns. Such patterns offer the advantage that they can be relatively easy manufactured. Of course, different phase cells can have different phase variations, i.e. different patterns. 
     Preferably, the phase variation is inverse for half the number of phase cells. This solution automatically ensures a balanced occurrence of π and 0 phase shifts. It is likewise feasible that the inverse pattern differs from the regular pattern. 
     Advantageously, the phase mask introduces a binary or a multi level phase shift. Alternatively, is introduces random distributed phase shifts, e.g. Gaussian distributed. The spatial distribution of 0 and π cells is random or pseudo random. Likewise, also the spatial distribution of phase cells with different patterns is random or pseudo random. 
     Preferentially, the phase mask is a transparent optical element with a varying surface height or a varying refractive index. This allows to generate the necessary phase shifts very easily. Alternatively, the phase mask is a reflective optical element with a varying surface height. This is especially useful when a reflective element is needed in the optical path anyway. 
     According to a further refinement of the invention, the phase mask has a switchable phase distribution or switchable phase shifts. In this way the phase mask can be adapted to different types of holographic storage media, or to different operating conditions. 
     Advantageously, an apparatus for reading from and/or writing to holographic storage media includes a phase mask according to the invention in an optical path of an object beam. Such an apparatus modifies the phase distribution of the object beam upon writing to the holographic storage medium. This leads to an improved, more homogeneous illumination of the holographic storage medium. This allows to achieve stronger holograms with a better signal-to-noise ratio (SNR) or a larger number of multiplexed holograms, i.e. an increased capacity. 
     Preferably, a further phase mask is included in an optical path of a reference beam. This has the advantage that a good overlap of the object beam and the reference beam inside the holographic storage medium is ensured. The further phase mask may have the same phase variation as or a different phase variation than the phase mask included in the optical path of the object beam. 
     Advantageously, the phase mask is an integral part of the SLM. In this way the SLM generates both the data page and the modified phase distribution, so that no additional component is necessary. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding the invention shall now be explained in more detail in the following description with reference to the figures. It is understood that the invention is not limited to this exemplary embodiment and that specified features can also expediently be combined and/or modified without departing from the scope of the present invention. In the figures: 
         FIG. 1  shows an SLM with a data pattern, 
         FIG. 2  shows the resulting intensity distribution in the Fourier plane, 
         FIG. 3  illustrates a phase mask having a cell size equal to the SLM pixel size, which introduces a phase shift of 0 or π for each pixel, 
         FIG. 4  illustrates the resulting intensity distribution in the Fourier plane, 
         FIG. 5  illustrates a phase mask having a cell size two times larger than the SLM pixel size, which introduces a phase shift of 0 or π for each pixel, 
         FIG. 6  shows the resulting intensity distribution in the Fourier plane, 
         FIG. 7  illustrates a first embodiment of a binary phase mask according to the invention, 
         FIG. 8  shows the resulting intensity distribution in the Fourier plane, 
         FIG. 9  illustrates a second embodiment of a binary phase mask according to the invention, 
         FIG. 10  shows the corresponding intensity distribution in the Fourier plane, 
         FIG. 11  illustrates a third embodiment of a binary phase mask according to the invention, 
         FIG. 12  shows the corresponding intensity distribution in the Fourier plane, 
         FIG. 13  shows the intensity distribution in the Fourier plane without the sub-cell phase variation for comparison, 
         FIG. 14  illustrates a fourth embodiment of a binary phase mask according to the invention, 
         FIG. 15  shows the corresponding intensity distribution in the Fourier plane, and 
         FIG. 16  depicts an apparatus for reading from and/or writing to a holographic storage medium using a phase mask according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows an SLM  1  with a data pattern consisting of a plurality of pixels  2 . In this figure, “off-pixels” are symbolized by black pixels, whereas “on-pixels” are symbolized by white pixels. For page oriented data storage usually a 2-dimensional modulation scheme is applied. A common approach is to divide each data page  1  into a set of sub pages or blocks consisting, for example, of 4×4 or 5×5 pixels  2 . During modulation the user data is transformed into a set of blocks. A known modulation uses three on-pixels in each 4×4 SLM block (4×4-3 modulation). The number of combinations of three on-pixels in a 4×4 SLM pixel block is equal to the selection of 3 out of 16, i.e. the number of combinations equals 560. This corresponds to a capacity of ˜9.1 bits of user data per pixel block. 
       FIG. 2  shows the intensity distribution in the Fourier plane resulting from the distribution of pixels  2  of the data page  1  of  FIG. 1 . Illustrated is a cut through the 2-dimensional distribution in logarithmic scale. As can be seen, the Fourier transform has a high central intensity peak. This DC-peak of the object beam is likely to cause an undesired saturation of the photosensitive medium. 
     Illustrated in  FIG. 3  is a phase mask  3  having a plurality of phase cells  4 , which introduce a phase shift of 0 or π for each pixel  2 . The spatial distribution of the phase shift of 0 or π is random. The size of the phase cells  4  of the phase mask  3  is the same as the size of the pixels  2  of the SLM  1 . Of course, if the size of the pixels  2  at the position of the phase mask  3  is different from the size of the pixels  2  at the position of the SLM  1 , e.g. due to imaging, the size of the phase cells  4  is adapted accordingly. In other words, a single phase cell  4  is assigned to each pixel  2 . 
       FIG. 4  illustrates the intensity distribution in the Fourier plane resulting from the distribution of pixels  2  of the data page  1  of  FIG. 1  modulated with the phase mask  3  of  FIG. 3 . Shown is a cut through the 2-dimensional distribution in linear scale. The phase mask with a cell size of one SLM pixel suppresses the DC-peak quite well. However, a large fraction of intensity is still located above the Nyquist limit. In addition, the intensity distribution within the central region of the Fourier plane is not flat. 
       FIG. 5  shows a phase mask  3  having a cell size two times larger than the SLM pixel size. This means that a single phase cell  4  is assigned to four pixels  2  of the SLM  1 . Again, the phase cells  4  introduce a phase shift of 0 or π for each pixel. The spatial distribution of the phase shift of 0 or π is random. 
       FIG. 6  depicts the intensity distribution in the Fourier plane resulting from the distribution of pixels  2  of the data page  1  of  FIG. 1  modulated with the phase mask  3  of  FIG. 5 . Shown is a cut through the 2-dimensional distribution in linear scale. The phase mask  3  reduces the intensity above the Nyquist limit, i.e. outside the interval [−1, 1], whereas the intensity below the Nyquist limit is increased. 
     A first embodiment of a phase mask  3  according to the invention is illustrated in  FIG. 7 . Each phase cell causes a phase shift of 0 or π, i.e. the phase mask  3  is binary. However, the phase shift varies within each phase cell  4 . Inside a circular area of each phase cell  4  the phase shift is different from the phase shift in the surrounding area. If the phase shift within the circular area is π, the phase shift in the surrounding area is 0, and vice versa. The phase mask  3  is still binary. Of course, the invention can likewise be realized with a multi-level phase mask. 
       FIG. 8  depicts the intensity distribution in the Fourier plane resulting from the distribution of pixels  2  of the data page  1  of  FIG. 1  modulated with the phase mask  3  of  FIG. 7 . Shown is a cut through the 2-dimensional distribution in linear scale. As can be seen, the resulting intensity distribution has a flatter envelope. 
     A second embodiment of a phase mask  3  according to the invention is illustrated in  FIG. 9 . Here a square-shaped pattern is used instead of the circular pattern of  FIG. 7 . 
     The intensity distribution in the Fourier plane resulting from the distribution of pixels  2  of the data page  1  of  FIG. 1  modulated with the phase mask  3  of  FIG. 9  is depicted in  FIG. 10 . Shown is a cut through the 2-dimensional distribution in linear scale. The resulting intensity distribution has an even flatter envelope. 
     A third embodiment of a phase mask  3  according to the invention is illustrated in  FIG. 11 . Here the sub-cell phase variation of  FIG. 7  using a circular-shaped pattern is combined with the low-resolution phase mask  3  depicted in  FIG. 5 . 
     The intensity distribution in the Fourier plane resulting from the distribution of pixels  2  of the data page  1  of  FIG. 1  modulated with the phase mask  3  of  FIG. 11  is depicted in  FIG. 12 . Shown is a cut through the 2-dimensional distribution in linear scale. The intensity distribution is concentrated below the Nyquist limit and at the same time has a flat envelope. 
     For comparison  FIG. 13  depicts the corresponding intensity distribution in the Fourier plane without the sub-cell phase variation. The intensity distribution is still concentrated below the Nyquist limit, but the envelope is no longer flat. 
       FIG. 14  shows a fourth embodiment of a phase mask  3  according to the invention. In this embodiment the sub-cell phase variation of  FIG. 9  using a square-shaped pattern is combined with the low-resolution phase mask  3  depicted in  FIG. 5 . 
     The intensity distribution in the Fourier plane resulting from the distribution of pixels  2  of the data page  1  of  FIG. 1  modulated with the phase mask  3  of  FIG. 14  is depicted in  FIG. 15 . Shown is a cut through the 2-dimensional distribution in linear scale. Again the intensity distribution is concentrated below the Nyquist limit. At the same time the envelope is even flatter. 
     In  FIG. 16  an apparatus  7  for reading from and/or writing to a holographic storage medium  16  is shown schematically. A source of coherent light, e.g. a laser diode  8 , emits a light beam  9 , which is collimated by a collimating lens  10 . The light beam  9  is then divided into two separate light beams  13 ,  14 , i.e. the object beam  13  and the reference beam  14 . In the example the division of the light beam  9  is achieved using a first beam splitter  11 . However, it is likewise possible to use other optical components for this purpose. A spatial light modulator (SLM)  1  modulates the object beam  13  to imprint a 2-dimensional data pattern. Located behind the SLM  1  is a phase mask  3 , which adds a sub-cell scale phase variation to the data pixels of the data pattern. A further, identical or different phase mask (not shown) can likewise be included in the reference beam path. Both the object beam  13  and the reference beam  14  are focused into a holographic storage medium  16 , e.g. a holographic disk or card, by an objective lens  15 . At the intersection of the object beam  13  and the reference beam  14  an interference pattern appears, which is recorded in a photo-sensitive layer of the holographic storage medium  16 . 
     The stored data are retrieved from the holographic storage medium  16  by illuminating a recorded hologram with the reference beam  14  only. The reference beam  14  is diffracted by the hologram structure and produces a copy of the original object beam  13 , the reconstructed object beam  17 . This reconstructed object beam  17  is collimated by the objective lens  9  and directed onto a 2-dimensional array detector  19 , e.g. a CCD-array, by a second beam splitter  18 . The array detector  19  allows to reconstruct the recorded data.