Abstract:
The present invention relates to an apparatus for reading from and/or writing to holographic storage media, and more specifically to an apparatus for reading from and/or writing to holographic storage media having an improved overlap between one or more reference beams and an object beam or a reconstructed object beam. According to the invention, the apparatus has a common aperture arrangement of a reference beam and an object beam or a reconstructed object beam, wherein the focus of the reference beam is shifted relative to the focus of the object beam or the reconstructed object beam within a focal plane of the object beam or the reconstructed object beam inside or close to the holographic storage medium.

Description:
This application claims the benefit, under 35 U.S.C. §119, of European Patent Application No. 06122284.0 on 13 Oct. 2006. 
     FIELD OF THE INVENTION 
     The present invention relates to an apparatus for reading from and/or writing to holographic storage media, and more specifically to an apparatus for reading from and/or writing to holographic storage media having an improved overlap between one or more reference beams and an object beam or a reconstructed object beam. 
     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 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 (SLM) and detected with a detector array. 
     Currently mainly two solutions for holographic storage systems are discussed. In the collinear system, as disclosed for example in EP 1 624 451, separate parts of the objective lens aperture are used for the object beam and the reference beam, respectively. This system uses a kind of shift multiplexing as a multiplexing method. In the off-axis recording system, as disclosed for example in U.S. Pat. No. 6,721,076, the object beam and the reference beam do not share the same optical path. In this system angle and polytopic multiplexing are used. 
     For both solutions the overlapping of the reference beam and the object beam in the holographic storage medium is not optimal. This has the consequence that a significantly lower data density is obtained. 
     In holographic data storage the achievable capacity is strongly related to the so called M-number (M#) of the storage material. This number can be calculated as M#=M·√{square root over (η)}. Here M denotes the number of holograms that can be multiplexed with a given diffraction efficiency □. This equation can only be applied when the overlap of the object beam and the reference beam is optimal. In practice, the overlap is always only partial. Thus a factor L ol  has to be introduced, which describes the material consumption of the non-overlapping beams, so that M#=M√{square root over (η)}·L ol . The factor L ol  is about 2 for collinear and off-axis recording. For further details see Curtis et al., “M/# requirements for Holographic Data Storage”, Proceedings of the ODS conference 2006, pp. 9-11. In conclusion, a non-optimum overlap of the object beam and the reference beam in the holographic storage medium decreases the achievable capacity if a specific material and optical setup is given. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to propose a solution for improving the overlap of the object beam and the reference beam in an apparatus for writing to holographic storage media. 
     According to the invention, this object is achieved by an apparatus for reading from and/or writing to a holographic storage medium, with a coaxial arrangement of a reference beam and an object beam or a reconstructed object beam, wherein the focus of the reference beam is shifted relative to the focus of the object beam or the reconstructed object beam within a focal plane of the object beam or the reconstructed object beam inside or close to the holographic storage medium. The reference beam fills approximately the same aperture as the object beam, but is focused to a different point or region to allow a separation of the reconstructed object beam and the reference beam. The apparatus according to the invention in this way has an improved overlap of the reference beam and the object beam/reconstructed object beam within the holographic storage medium. This improved overlap allows to increase the data density. 
     Preferably, the apparatus has two or more reference beams or a ring shaped reference beam. Using two or more reference beams or a ring shaped reference beam has the advantage that a further improved overlap is achieved, especially if the reference beams are arranged symmetrically around the object beam/reconstructed object beam. The two or more reference beams or the ring shaped reference beam are advantageously generated by a diffractive element or a refractive element. Of course, also other types of elements may be used, e.g. reflective elements. Examples of a diffractive element are a linear or rotationally symmetric sinusoidal or binary grating, or a suitable holographic element. Diffractive elements have the advantage that in addition to generating multiple reference beams they also allow to influence the shape of the reference beams. Examples of a refractive element are a biprism or an inverted biprism, or a cone or an inverted cone. Of course, the cone or the inverted cone can also be approximated by a multi facet prism, i.e. the curved surface may be approximated by plane polygons. In this case the ring shaped reference beam is approximated by a plurality of reference beams, whose foci are located on a ring. Refractive elements have the advantage that they can be manufactured at low cost. 
     Advantageously, an aperture is provided for blocking the one or more reference beams. Such an aperture only pass the reconstructed object beam, whereas the one or more reference beams are blocked. Blocking of the reference beams ensures that they do not have any negative effect during data detection by the detector array. At the same time the aperture can be used for filtering the reconstructed object beam if a filtering is needed. 
     The apparatus is either adapted to read from and/or write to a transmission type holographic storage medium, or to a reflection type holographic storage medium. In case of a transmission type holographic storage medium the improved overlap of the object beam/reconstructed object beam and the one or more reference beams is easier to achieve than for a reflection type holographic storage medium. The reflection type holographic storage medium in turn has the advantage that a plurality of optical components are used for reading as well as for writing, i.e. the number of components is reduced. In this case, however, the diameters of the one or more reference beams and their tilting angles relative to the object beam or the reconstructed object beam are chosen such that the incoming one or more reference beams do not overlap with reflected reference beams within the holographic storage medium. This is achieved, for example, by using half cone shaped reference beams. Otherwise conjugate holograms could be produced in the holographic storage medium. 
    
    
     
       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  illustrates an optimum overlap of an object beam and a reference beam inside a holographic storage medium, 
         FIG. 2  depicts a nearly optimum overlap of an object beam and partial reference beams achieved by the solution according to the invention, 
         FIG. 3  illustrates an aperture for blocking the partial reference beams, 
         FIG. 4  shows a transmission type apparatus for reading from and writing to holographic storage media during the writing operation, 
         FIG. 5  shows the apparatus of  FIG. 4  during the reading operation, 
         FIG. 6  shows a perspective view and a side view of two types of bi-prisms used as partial beam generating element, 
         FIG. 7  illustrates a diffractive element used as partial beam generating element, 
         FIG. 8  depicts the distribution of the object beam and the partial reference beams in the focal plane of the reference beams, 
         FIG. 9  illustrates the overlap of the object beam and the partial reference beams in a plane perpendicular to the plane defined by the partial reference beams, 
         FIG. 10  depicts the distribution of the object beam and a ring type reference beam in the focal plane of the reference beam, 
         FIG. 11  illustrates two types of refractive elements used as partial beam generating element to obtain the ring type reference beam, 
         FIG. 12  illustrates a diffractive element used as partial beam generating element to obtain the ring type reference beam, 
         FIG. 13  shows a reflection type apparatus for reading from and writing to holographic storage media during the writing operation, 
         FIG. 14  shows the apparatus of  FIG. 13  during the reading operation, 
         FIG. 15  illustrates the overlap of an object beam and an incoming and reflected reference beam in a reflection type medium, and 
         FIG. 16  compares the situation of  FIG. 15  with the situation in a transmission type medium. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As illustrated in  FIG. 1 , apparently an optimum overlap of an object beam  5  and a reference beam  6  is achieved if the reference beam  6  fills at least the same aperture as the object beam  5  inside the transmission type holographic storage medium  15 , i.e. if it fills at least the same volume. This solution, however, has the disadvantage that during read-out it is not possible to separate a reconstructed object beam  19  and the reference beam  6 . The object beam  5  is typically much weaker in intensity than the reference beam  6  in order to allow a high number of multiplexed holograms. This makes the read-out of the reconstructed object beam  19  much more difficult. The signal generated by the reference beam  6  is superimposed on the much weaker signal of the reconstructed object beam  19 . In off-axis and collinear recording the reconstructed object beam  19  is automatically separated from the reference beam  6 , as both beams  6 ,  19  propagate with different angles at the position of the image plane. 
       FIG. 2  schematically depicts how a nearly optimum overlap of an object beam  6  and partial reference beams  6  is achieved, according to the present invention, by shifting the focus of each reference beam  6  or the focus of each of a plurality of partial reference beams  6   a ,  6   b  relative to the focus of the object beam  5  within the focal plane  25  of the object beam  5 . The beams  5 ,  6   a ,  6   b  are separated in the focal plane  25 , while they still overlap very well in a region a little bit away from the focal plane  25 . This region is sometimes called the “Fresnel region”. The recordable volume of the holographic storage medium  15  preferably lies inside this region. The object beam  5  and the reference beams  6   a ,  6   b  can be separated during read-out by using an appropriate aperture or spatial filter at or near the focal plane of the object beam  5  and the reference beams  6   a ,  6   b , which blocks the reference beams  6   a ,  6   b . Contrary to the foci of the reference beams  6   a ,  6   b  the focus of the object beam  5  does not form a small spot, but an area. This is due to the fact that in a the object beam  5  coming from an SLM is not a plane wave, but a superposition of multiple plane waves having different propagation directions. Each of these plane waves has its focus at a different location of the focal plane  25 . 
       FIG. 3  shows an example of how the reference beams  6   a ,  6   b  are blocked during read-out by using an aperture  22  behind the holographic storage medium  15 . According to the invention the recording is realized only in the Fresnel region, i.e. not in the direct vicinity of the focal region  25 , there is a nearly full overlap of the object beam  5  and the reference beams  6   a ,  6   b . Still, a reconstructed object beam  19  is reliably separated from the reference beams  6   a ,  6   b.    
     An exemplary setup of an apparatus for reading from and writing to holographic storage media is shown in  FIG. 4 . In contrast to  FIGS. 1 to 3 , for simplification only the principal rays of the light beams are illustrated. A source of coherent light, e.g. a laser diode  1 , emits a light beam  2 , which is collimated, expanded and filtered by a beam expander and filter arrangement  3 . The light beam  2  is then divided into two separate light beams  5 ,  6  by a beam splitter  4 . The first light beam  5 , the so called “object beam”, passes a beam shutter  7  and is directed by two mirrors  8 ,  9  towards a spatial light modulator (SLM)  10 . The SLM  10  modulates the light beam  5  to imprint a 2-dimensional data pattern. The object beam  5  is filtered by a pair of Fourier lenses  11 ,  13  and a spatial filter  12 , which filters out the high frequency components of the object beam  5 . The object beam  5  is then focused into a holographic storage medium  15 , e.g. a holographic disk or card, by an objective lens  14 . The second light beam  6 , the reference beam, also passes a beam shutter  16  before it impinges on a partial beam generating element  17 , e.g. a bi-prism or a diffractive element. The partial beam generating element  17  generates two or more partial reference beams  6   a ,  6   b  from the reference beam  6 . The partial beam generating element  17  is designed in such a way that the foci of the two partial reference beams  6   a ,  6   b  lie besides the focal area of the object beam  5 . The partial reference beams  6   a ,  6   b  are coupled into the optical path of the object beam  5  by a beam coupling element  18 , e.g. a beam splitter, and focused into the holographic storage medium  15  by the objective lens  14 . At the intersection of the object beam  5  and the partial reference beams  6   a ,  6   b  an interference pattern appears, which is recorded in a photo-sensitive layer of the holographic storage medium  15 . 
     As shown in  FIG. 5 . the stored data are retrieved from the holographic storage medium  15  by illuminating a recorded hologram with the partial reference beams  6   a ,  6   b  only. For this purpose the object beam  5  is blocked by the beam shutter  7 . The partial reference beams  6   a ,  6   b  are diffracted by the hologram structure and produce a copy of the original object beam  5 , the reconstructed object beam  19 . This reconstructed object beam  19  is collimated by an objective lens  20  and directed onto a 2-dimensional array detector  24 , e.g. a CCD-array. A further pair of Fourier lenses  21 ,  23  and a further spatial filter  22  block the partial reference beams  6   a ,  6   b . The spatial filter  22  is advantageously also used for filtering out the high frequency components of the reconstructed object beam  19 . The array detector  24  allows to reconstruct the recorded data. 
     Of course, in an apparatus for writing only the elements of the reading path can be omitted. Similarly, in an apparatus for reading only the elements of the path of the object beam  5  can be omitted. 
     The geometry of a bi-prism  17 ″ as the partial beam generating element  17  is shown in  FIG. 6 .  FIG. 6   a ) shows a 3-dimensional view of two different types of bi-prisms  17 ″, whereas in  FIG. 6   b ) cuts through the bi-prisms  17 ″ are illustrated. As can be seen, the bi-prisms  17 ″ divide the incoming reference beam  6  into two reference beams  6   a ,  6   b  having different propagation directions. 
     An exemplary form of a diffractive element  17 ′ as the partial beam generating element  17  is shown in  FIG. 7 . In the bottom of the figure a perspective view of the element  17 ′ is shown. Above this perspective view a magnified top view and a cut through the surface along a line  AA . In this example the diffractive element  17 ′ is a transmissive diffraction grating. The beam entrance surface  30  of the element is plane, whereas the beam exit surface  31  of the element has a sinusoidal or a binary (two step) surface structure. If the depth of the grating is chosen to be approximately half of the wavelength of the laser light, then most of the incoming light  6  is diffracted into rays  6   a ,  6   b  that propagate at a certain angle relative to the optical axis. The angle and thus the position of the spot in the focal plane  25  can be controlled by choosing the appropriate grating period of the grating. 
       FIG. 8  shows the distribution of the object beam  5  and the partial reference beams  6   a ,  6   b  as a top view onto the focal plane  25 . The spots of the two reference beams lie beside the object beam  5 . 
     In the above described solution the improved overlap is realized in the plane spanned by the optical axes of the two partial reference beams  6   a ,  6   b . The overlap in a plane perpendicular to this plane is illustrated in  FIG. 9 . 
     As a further improvement the bi-prism or the sinusoidal or binary grating is preferably substituted by an optical element which generates a focus ring  6   c , instead of two spots in the focal plane  25 . This is illustrated in  FIG. 10 . In this case the overlap is improved in the whole volume, because the overlap depicted in the plane shown in  FIG. 2  is achieved for the whole hologram volume. 
     A 3-dimensional view of two different types of refractive optical element  17 ″ as the partial beam generating element  17  is diagrammatically shown in  FIG. 11 . The refractive optical element  17 ″ has the shape of a cone ( FIG. 11   a )) or an inverse cone ( FIG. 11   b )) with one plane side and one cone-shaped side, or an approximated shape of a cone or an inverse cone using a polygon approximation. In the latter case instead of a focal ring in the focal plane  25  multiple foci arranged on a ring are generated in the focal plane  25 . 
     A diffractive optical element  17 ′, which consists of a sinusoidal or binary surface grating  17 ′ with a rotational symmetry, is illustrated in  FIG. 12 . The function principle is the same as for the linear sinusoidal or binary grating structure of  FIG. 7 . The only difference is that, due to the rotational symmetry of the element, the beams are diffracted in such a way that the propagation angle relative to the optical axis is the same for all partial beams. 
     Though in the above description a transmission type holographic storage system has been explained, the idea is also applicable to reflection type holographic storage systems. Reflection type means that the data is recorded and read from only one side of the holographic storage medium  15 . As a result some parts of the optical setup, for example the objective lens  14 , can be used for both recording and reading of the data. This simplifies the setup and makes it more compact. An exemplary setup of a reflection type holographic storage system is shown in  FIGS. 13 and 14  during recording and reading, respectively. In the figures the same reference signs as in  FIGS. 4 and 5  are used to designate the same elements. The holographic storage medium  15  has a reflective layer  27  behind the storage layer such that during reading a reconstructed object beam  19  is reflected back to the objective lens  14 . The main difference compared to the transmission type system is that only half or less of the aperture of the objective lens  14  is illuminated by the incoming reference beams  6   a ,  6   b  to avoid the recording of conjugated holograms. This is preferably controlled by an additional aperture  30  positioned in front of the partial beam generating element  17  which generates the reference beams  6   a ,  6   b . The illuminated aperture of the incoming reference beams is chosen such that there is no overlap of the incoming and the reflected reference beams. This principle is illustrated in  FIG. 15 . For simplification only one reference beam  6   a  is shown. Favorably, more than one reference beam is used to achieve a good overlap of the object beam  5  and the reference beams  6   a ,  6   b . As can be seen, the reflective layer  27  is located in the focal plane below the hologram storage layer  28 . In the figure the hologram storage layer  28  is separated from the reflective layer  27  by a spacer layer  29 . For comparison  FIG. 16  shows the same situation for the transmission type holographic storage medium  15 . It can be seen that for a reflection type holographic storage medium  15  the incoming reference beam  6   a  and the reflected reference beam  6   d  have to fill the same aperture as the incoming reference beam  6   a  for the transmission type medium  15 . This is preferably realized by a higher tilt of the incoming reference beam  6   a  towards the optical axis. The diameter of the reference beams  6   a ,  6   b  and their tilting angles are chosen such that the incoming reference beams  6   a ,  6   b  and the reflected reference beams  6   d  do not overlap, e.g. by using half cone reference beams  6   a ,  6   b.