Abstract:
An apparatus and a method for reading from and/or writing to holographic storage media using a coaxial arrangement of an object beam and one or more reference beams or a coaxial arrangement of a reconstructed object beam and one or more reference beams is proposed. A focus of the one or more reference beams within a holographic storage medium is shifted along the optical axis relative to a focus of the object beam or the reconstructed object beam.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an apparatus and a method for reading from and/or writing to holographic storage media, and more specifically to an apparatus and a method using a coaxial arrangement of an object beam and one or more reference beams, which achieves an improved beam overlap within a holographic storage medium. The invention further relates to a holographic storage medium suitable for use in such an apparatus or with such a method. 
       BACKGROUND OF THE INVENTION 
       [0002]    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. 
         [0003]    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. 
         [0004]    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. 
         [0005]    EP 1 624 451 discloses a holographic storage system with a coaxial arrangement, where a plurality of reference beams are arranged around the object beam. According to this solution the object beam and the reference beams are coupled in and out at the object plane and the image plane, respectively. This arrangement is a so-called split aperture arrangement, because the aperture of the Fourier objective is split into an object part and a reference part. The arrangement has the advantage that the holographic material is near the Fourier plane, where the data density is the highest. Also, the overlap between the object beam and the reference beams is good near the Fourier plane. However, half of the aperture of the Fourier objective is used for the reference beams. This means that the capacity of a single hologram of the split aperture system is only half of the capacity of a common aperture arrangement. In addition, a total overlap of the object beam and the reference beams occurs only in the Fourier plane. The overlap is only partial within a 100-200 μm thick layer in the holographic storage medium. This value depends on the diameter of the hologram and the numerical aperture of the Fourier objective. Starting at a distance of about 200-400 μm from the Fourier plane there is no overlap at all. This limits the maximum useful thickness of the holographic material. 
         [0006]    In WO2006/003077 a 12 f reflection type coaxial holographic storage arrangement with three confocally arranged Fourier planes is shown. In this arrangement the object beam and the reference beams are coupled in and out at the first and third Fourier planes, respectively. The reference beams are small spots in these planes. More precisely, they form diffraction patterns, similar to the Airy pattern. This arrangement is a so-called common aperture arrangement, because at the object plane and the image plane the object beam and the reference beams fill out the same area of the aperture. The beams fill out the entire aperture of the objectives. The disclosed arrangement allows to apply shift multiplexing, reference scanning multiplexing, phase coded multiplexing, or a combination of these multiplexing schemes. The reference beams are a pair (or pairs of) half cone shaped beams. 
         [0007]    EP 1 912 212 discloses a holographic storage system with a coaxial arrangement of one or more reference beams and an object beam or a coaxial arrangement of one or more reference beams and a reconstructed object beam. The foci of the one or more reference beams are shifted relative to the focus of the object beam within the focal plane of the object beam or the reconstructed object beam. 
         [0008]    EP 1 837 871 discloses a holographic storage medium with a holographic layer and a mirror layer. According to one embodiment the mirror layer has one or more reflective areas for reflecting one or more reference beams, whereas the remaining area of the mirror layer is transparent or absorptive. 
         [0009]    In the known holographic storage systems the object beam and the reference beams do not fully overlap. To achieve a better overlap, the holographic material needs to be placed far from the Fourier plane. As a consequence, the single hologram data density in the holographic material is low. 
       SUMMARY OF THE INVENTION 
       [0010]    It is an object of the invention to propose an apparatus and a method for reading from and/or writing to a holographic storage medium with an improved overlap of the object beam and the one or more reference beams. 
         [0011]    It is a further object of the invention to propose a holographic storage medium, which is suitable for use in such an apparatus or with such a method. 
         [0012]    According to the invention, in an apparatus for reading from and/or writing to a holographic storage medium, with a coaxial arrangement of one or more reference beams and an object beam or a coaxial arrangement of one or more reference beams and a reconstructed object beam, a focus of the one or more reference beams within the holographic storage medium is shifted along the optical axis relative to a focus of the object beam or the reconstructed object beam. 
         [0013]    Likewise, a method for reading from and/or writing to a holographic storage medium using a coaxial arrangement of one or more reference beams and an object beam or a coaxial arrangement of one or more reference beams and a reconstructed object beam has the step of focusing the one or more reference beams to a focus within the holographic storage medium that is shifted along the optical axis relative to a focus of the object beam or the reconstructed object beam. 
         [0014]    It has been found that it is not necessary to place the foci of the one or more reference beams into the Fourier plane of the object beam. If the Fourier plane and the focal plane of the one or more reference beams are shifted along the optical axis relative to each other, the overlap of the one or more reference beams and the object beam near the Fourier plane is greatly improved. This means that a desired data capacity is achieve with a reduced number of multiplexed holograms. This in turn leads to a better single hologram diffraction efficiency, as the finite dynamic range of the holographic material is shared between a reduced number of multiplexed holograms. The one or more reference beams advantageously are one cone-shaped or two half cone shaped reference beams, which are placed on or close to the optical axis. 
         [0015]    Advantageously, the one or more reference beams are focused on a small reflective area of the holographic storage medium. 
         [0016]    For this purpose, the holographic storage medium has a holographic layer and a mirror layer, which has one or more small reflective areas for reflecting the one or more reference beams, and which is transparent or absorptive otherwise. The small reflective areas favorably have a circular, oval or rectangular shape. 
         [0017]    The small reflective areas reflect the one or more reference beams, whereas the object beam is practically not reflected by the mirror layer due to the very small size of the respective reflective area. Therefore, only a forward propagating object beam is present at the location of the holographic layer. This beam interferes with both the forward and the backward propagating reference beams, and records a hologram in the holographic material. 
         [0018]    Advantageously, the holographic storage medium further has a spacer layer for separating the holographic layer and the mirror layer. The spacer layer allows do define the distance between the foci of the one or more reference beams and the focus of the object beam, and thus to optimize the overlap between the one or more reference beams and the object beam. 
         [0019]    Preferably, for readout of a hologram the one or more reference beams are phase conjugate with regard to the one or more reference beams for recording of a hologram. 
         [0020]    By using phase conjugate reference beams for readout of a holograms, a phase conjugate object beam is reconstructed. This reconstructed object beam has the same wavefront as the original object beam, but it propagates in the opposite direction. If this reconstructed object beam propagates back through all the same or identical lenses as the original object beam, the wavefront aberrations caused by the lenses are corrected when the reconstructed object beam passes through the lenses in the opposite direction. 
         [0021]    Preferably, an objective for focusing the object beam and the one or more reference beams into the holographic storage medium is optimized for the reference beams only. As the wavefront aberrations of the object beam are automatically corrected due to the phase conjugate readout, it is not necessary to optimize the lens also for the object beam. This allows to make the objective lighter and less expensive, as the objective does not need to be diffraction limited on the whole Fourier plane, i.e. as the objective has a reduced requirement regarding wavefront distortions away from the optical axis. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    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: 
           [0023]      FIG. 1  illustrates the setup of a coaxial holographic storage system according to the invention, 
           [0024]      FIG. 2  depicts the reference beams and the object beam at the position of the holographic storage medium, 
           [0025]      FIG. 3  shows a patterned mirror layer of the holographic storage medium, 
           [0026]      FIG. 4  shows the incoupling scheme for the reference beams, 
           [0027]      FIG. 5  depicts simulation results for the holographic storage system based on a first set of system parameters, and 
           [0028]      FIG. 6  shows simulation results for the holographic storage system based on a second set of system parameters. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0029]    A coaxial holographic storage system according to the invention is schematically illustrated in  FIG. 1 . For recording a hologram an object beam  20  coming from an amplitude SLM  1  is Fourier transformed by a first long focal length Fourier objective  9 . This objective is one of four identical long focal length Fourier objectives  9 ,  10 ,  11 ,  12 . Located in the Fourier plane of the first long focal length Fourier objective  9  is an aperture  2  for low pass filtering the Fourier image in order to reduce the high spatial frequency components of the image. The plane of the aperture  2  does not coincide with the focal plane  3  of the second Fourier objective  10 , where a reference beam  21  is coupled in. More details on the beam coupling will be given below with reference to  FIG. 3 . The distance between the Fourier plane and the focus of the reference beam  21  is reproduced at the position of the holographic material. Both beams  20 ,  21  pass through the second long focal length objective  10  and a high NA objective  4  and reach a holographic storage medium  5 . 
         [0030]    At the incoupling plane  3  the reference beam  21  consists of one cone shaped light beam or two half cone shaped light beams arranged at a small distance from each other. In the following the term “reference beam” shall thus be understood as “one or more reference beams”. The second objective  10  converts the reference beam  21  to one or two half-circle shaped plane waves that are almost parallel with the optical axis. The high NA objective  4  is optimized for converting these plane waves back to half cone shaped light beams, as illustrated in  FIG. 2 . The tips of these half-cone(s) are located on a patterned mirror layer  51 , which has small reflective areas  56  at the respective focus positions, but is otherwise transparent. The diameter of the small reflective areas  56 , which is determined by the numerical aperture, the laser wavelength and the precision of a servo system of the storage system, is about 1 μm. The patterned mirror layer  51  is illustrated in  FIG. 3 . In this figure the reflective areas  56  are small circular areas. Of course, these areas can also have other shapes, e.g. a rectangular or an oval shape. In addition, the mirror layer  51  may likewise be absorptive instead of transparent. The object beam  20  has a blurred focus  54  between the high NA objective  4  and the patterned mirror layer  51 , which is similar to the Fourier plane. The high NA objective  4  is not optimized for the object beam  20 , but for the reference beam  21 . A holographic layer  53  of the holographic storage medium  5  is located around this focus. A spacer layer  52  is provided for ensuring a desired distance between the holographic material and the patterned mirror layer  51 . The distance between the focus of the reference beam  21  and the focus of the object beam  20 , as well as the thickness of the spacer layer  52 , are optimized to reach the best overlap of the reference beam  21  and the object beam  20 . The holographic layer  53 , the spacer layer  52 , and the patterned mirror layer  51  are all part of the holographic storage medium  5 . Located between the high NA objective  4  and the holographic storage medium  5  is a quarter wave plate  55 , whose function will be explained later. 
         [0031]    The one or two half cone shaped light beams of the reference beam  21  form one or two full cones together with their reflected pairs. Since the object beam  20  is practically not reflected from the patterned mirror layer  51  due to the very small respective reflective area, only a forward propagating object beam  20  is present at the location of the holographic layer  53 . This beam interferes with both the forward and the backward propagating reference beams  21 , and records a hologram in the holographic material. 
         [0032]    For reading a hologram the readout reference beam is similar to the reference beam  21  during recording, except that the half cone(s) are flipped. The forward propagating half-cone(s) of the reference beam  21  is substituted by the reflected half-cone(s) of the readout reference beam and vice versa. This means that the reference beam  21  during recording and the readout reference beam during reading are phase conjugated pairs of each other. 
         [0033]    To make the implications of the use of phase conjugate beams clear, if a volume hologram is read by a readout reference beam, which is the phase conjugate version of the reference beam that was used for recording the hologram, the reconstructed object beam is the phase conjugate of the original object beam. This reconstructed object beam has the same wavefront as the original object beam, but it propagates in the opposite direction. If this reconstructed object beam propagates back through all the same or identical lenses as the original object beam, the wavefront aberrations caused by the lenses are corrected when the reconstructed object beam passes through the lenses in the opposite direction. 
         [0034]    In the optical system illustrated in  FIG. 1 , the reconstructed object beam  15  first passes through the high NA objective  4 . Then the beam is refracted by a polarization beam splitter cube  14 . After passing through the third long focal length objective  11 , the readout reference beam is stopped by an obscuration plate  6  at its focus. The Fourier transformed object is reconstructed in a plane  7 , where it is cut by a circular aperture. Finally, the fourth long focal length objective  12  performs an inverse Fourier transformation of the reconstructed object beam  15  and images the reconstructed image on a detector  8 . 
         [0035]    The incoupling of the reference beam  21  and the separation of the optical paths of the object beam  20  and the reconstructed object beam  15  are achieved using polarization. This is illustrated in  FIG. 4 . As depicted in  FIGS. 2 and 4 , there are three quarter wave plates in the system. A first one  32  is located between the incoupling plane  3  and the polarizing beam splitter cube  13 , a second one  33  before the incoupling of the reference beam  21  in the object beam path, and a third one  55  between the objective  4  and the holographic storage medium  5 . This ensures that the object beam  20  as well as the reference beam cones are circularly polarized at the incoupling plane of the reference beam  21  and also inside the holographic storage medium  5 , while they are linearly polarized along the remaining optical path. 
         [0036]    Using the optical setup of  FIG. 1 , hologram multiplexing is performed by shifting the holographic storage medium  5  to the next reflective area of the patterned mirror layer  51 . In this position the hologram overlaps with the previous hologram, which allows to increase the data density. 
         [0037]    Favorably, a random phase mask is arranged in front of the amplitude SLM  1 , which eliminates the high intensity peak in the middle of Fourier plane. This is especially useful when the 
         [0038]    Fourier plane is inside the holographic material. The phase mask modulates the phase of each pixel by 0 or π. The number of white pixels that have 0 and π phase shift should be approximately identical. 
         [0039]    In the following simulation results obtained for the proposed holographic storage system using the beam propagation method shall be presented.  FIG. 5  shows a simulation result of the detector image based on the following first set of system parameters:
   Spacer layer thickness: 700 μm   Defocus: 700 μm   Active material thickness: 300 μm   Distance between object beam and reference cone axis: 10 μm   Reference/object energy ratio: 1000/1   SLM: normal amplitude SLM, 10 μm pixel size   Reference incoupling diameter (at focus): 6 μm   Reference outcoupling diameter: 20 μm   Wavelength: 400 nm   Material refractive index: 1.5   Objective focal distance: 7 mm   Fourier plane cut diameter: 0.6×DNyquist   
 
         [0052]    The symbol error rate for these parameters was zero, no errors occurred. 
         [0053]      FIG. 6  shows a simulation result of the detector image with the following second set of system parameters:
   Spacer layer thickness: 500 μm   Defocus: 600 μm   Active material thickness: 300 μm   Distance of object beam and reference cone axis: 0 μm (coaxial)   Reference/object energy ratio: 20/1   SLM: random phase masked, 10 μm pixel size   Reference incoupling diameter (at focus): 6 μm   Reference outcoupling diameter: 20 μm   Wavelength: 400 nm   Material refractive index: 1.5   Objective focal distance: 7 mm   Fourier plane cut diameter: 0.6×Dnyquist     
         [0066]    The symbol error rate for these parameters was SER=0.04%, two errors occurred within 4800 blocks.