Abstract:
The present invention relates to a compact apparatus for reading from and/or writing to holographic storage media, and to a filter for use in such an apparatus. 
     According to the invention, a polarizing filter for a light beam has:
       a substrate with an outer zone and an inner zone, the outer zone and the inner zone having different optical properties for a first direction of polarization and a second direction of polarization, and   a wave plate attached to the substrate for influencing the polarization of the light beam, wherein a part of the wave plate corresponding to either the outer zone or the inner zone is switchable.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a compact apparatus for reading from and/or writing to holographic storage media, and to a filter for use in such an apparatus. 
       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 ‘signal 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 signal 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 single or few 2-dimensional 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 signal beam by the spatial light modulator (SLM) and detected with a detector array. 
       SUMMARY OF THE INVENTION 
       [0004]    It is an object of the invention to propose an optical element that allows to realize a compact apparatus for reading from and/or writing to a holographic storage medium, and an apparatus for reading from and/or writing to a holographic storage medium using such optical element. 
         [0005]    According to the invention, this object is achieved by a polarizing filter for a light beam, having:
       a substrate with an outer zone and an inner zone, the outer zone and the inner zone having different optical properties for a first direction of polarization and a second direction of polarization, and   a wave plate attached to the substrate for influencing the polarization of the light beam, wherein a part of the wave plate corresponding to either the outer zone or the inner zone is switchable.       
 
         [0008]    The wave plate is switchable between a state in which it influences the polarization and a state in which is does not influence the polarization. Alternatively, it is switchable between two or more states in which it influences the polarization in different ways. The polarizing filter can be regarded as a polarization sensitive pinhole and functions simultaneously as a Fourier filter and a beam-combiner. The filter is advantageously used in an apparatus for reading from and/or writing to holographic storage media. The filter is arranged in a Fourier plane of the apparatus and allows to efficiently combine signal and reference beam during recording, and to separate the reference beam and the reconstructed signal beam during readout. 
         [0009]    The polarizing filter either is a transmission type filter or a reflection type filter. In the case of a transmission type filter the outer zone has a low transmissivity for the first direction of polarization and a high transmissivity for the second direction of polarization, whereas the inner zone has a high transmissivity for the first direction of polarization and a low transmissivity for the second direction of polarization. The switchable wave plate in this case is a half wave plate. The transmission type filter has the advantage that it can be easily integrated in the optical path of an apparatus for reading from and/or writing to holographic storage media. When the filter is a reflection type filter, the outer zone has a low reflectivity for the first direction of polarization and a high reflectivity for the second direction of polarization, and the inner zone has a high reflectivity for the first direction of polarization and a low reflectivity for the second direction of polarization. In this case the switchable wave plate is a quarter wave plate. The reflection type filter has the advantage that the polarization-dependent reflectivity can easily be realized with coatings. 
         [0010]    Preferably, a part of the wave plate corresponding to the inner zone is either a hole or is not switchable. Alternatively, only the part of the wave plate corresponding to the inner zone is switchable, whereas a part corresponding to the outer zone is not switchable or omitted. Advantageously, the wave plate is realized as a liquid crystal element. In this way the wave plate is inexpensive and easy to manufacture. 
         [0011]    The outer zone and the inner zone preferably have sub-wavelength gratings. Such gratings exhibit a very strong polarization dependence. They hence allow to realize the polarization dependent properties in an efficient way. 
         [0012]    Advantageously, the sub-wavelength gratings are designed such that the non-transmitted or non-reflected light is refracted at an angle sufficiently large to avoid stray light in the optical system, e.g. an angle of 10° or more. This means that the non-transmitted or non-reflected light is refracted into other diffraction orders than the zeroth order. The avoidance of stray light improves the contrast of the recorded holograms and the contrast of the reconstructed object beams. Of course, the chosen angle depends on the optical setup used in an apparatus for reading from and/or writing to a holographic storage medium. 
         [0013]    According to a further aspect of the invention, an apparatus for reading from and/or writing to a holographic storage medium has
       a 4f-system for imaging a signal beam and/or a reference beam into the holographic storage medium, and   a polarizing Fourier filter according to the invention for separating a reconstructed signal beam and the reference beam. The apparatus requires only a single 4f-imaging system and can hence be realized in a very compact way. The polarizing filter, which is located in the Fourier plane of the 4f-system, still allows a reliable separation of the reference beam and a reconstructed signal beam.       
 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    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: 
           [0017]      FIG. 1  shows a transmission type polarizing pinhole Fourier filter according to the invention, 
           [0018]      FIG. 2  schematically depicts an holographic storage apparatus during writing, 
           [0019]      FIG. 3  depicts the same apparatus during reading, 
           [0020]      FIG. 4  depicts a modified optical setup, in which transmission type SLMs are used, and 
           [0021]      FIG. 5  shows a reflection type polarizing pinhole Fourier filter according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0022]    In  FIG. 1  a polarizing pinhole Fourier filter according to the invention is shown. It is one of the principal components of the proposed system. The polarizing pinhole Fourier filter  17  consists of a transparent substrate with two different zones  17   a ,  17   b . In the outer zone  17   a  the transmission is 0 for a polarization in a first direction, e.g. the x-direction, and 1 for a polarization in a second direction, e.g. the y-direction. In the circular central zone (aperture)  17   b  the transmission is 1 for a polarization in the first direction and 0 for a polarization in the second direction. Of course, the dependence of the transmission on the direction of polarization of the outer zone  17   a  and the inner zone  17   b  can likewise be reversed. The zones  17   a ,  17   b  can for example be manufactured with the help of sub-wavelength gratings, which show a very strong polarization dependence. Preferably, the sub-wavelength gratings are designed in such a way that the non-transmitted radiation is deflected by a sufficiently large angle, such that the deflected light does not pass through the aperture of any of the following components of the optical path. In the above example the first direction of polarization is perpendicular to the second direction of polarization. In an alternative embodiment the two directions of polarization are right and left circular polarization. 
         [0023]    The polarizing pinhole Fourier filter  17  further has an electrically controlled half wave plate (λ/2 plate)  17   c , which is attached to the substrate and covered by a cover layer  17   d . The electrically controlled half wave plate  17   c  is preferably formed by a liquid crystal element. The half wave plate  17   c  in  FIG. 1  is designed in such a way that the central part, which corresponds to the inner zone  17   b , is a hole. According to an alternative solution the central part of the half wave plate  17   c  is not influenced by the applied electric field, i.e. it is not switchable. In both cases the polarization state of light passing through the inner zone  17   b  is not influenced by the electrically controlled half wave plate  17   c . As a further alternative it is likewise possible to place a switchable zone in the inner zone  17   b , whereas the outer part of the half wave plate  17   c  is not switchable. 
         [0024]    An apparatus for reading from and/or writing to a holographic storage medium is schematically depicted in  FIG. 2 . A light beam  28  emitted by a laser  1  is shaped by a beam-shaper  2 , if necessary, and collimated by a collimator lens  3 . A first polarizing beam-splitter (PBS)  5  splits an incoming light  28  beam into a reference beam  25  and a signal beam  24 . A rotatable half wave plate  4  located before the first polarizing beam-splitter (PBS)  5  is used to set the intensity ratio of the reference beam  25  and the signal beam  24 . The direction of polarization of the reference beam  25  is rotated by a second half wave plate  10 . The reference beam  25  passes a second polarizing beam-splitter  11  before being sent onto a spatial light modulator  13 . The spatial light modulator  13  allows to realize different multiplexing techniques and preferably consists of at least a phase modulator, which has a plurality of pixels, e.g. 256×256 or more pixels, and introduces a phase shift of π/2 into the reflected beam. A first quarter wave plate (λ/4 plate)  12  is provided between the second polarizing beam-splitter  11  and the spatial light modulator  13 . After having passed the first quarter wave plate  12  twice, the reference beam  25  is reflected by the second polarizing beam-splitter  11  towards a third half wave plate  14  and a third polarizing beam-splitter  15 . A lens pair  16 ,  18  forms a so-called 4-f imaging system. The polarizing Fourier filter  17  of  FIG. 1 , is situated in the Fourier plane between the two lenses  16  and  18 . The spatial light modulator  13  modifies the reference beam  25  in such way that separate foci are generated at the position of the polarizing Fourier filter  17 , e.g. by generating multiple partial reference beams. In the simplest case the spatial light modulator  13  is a diffractive element. In this case only shift-multiplexing is employed. The filter  17  blocks the 0-order components of the reference beam  25  and transmits only the higher order components. The polarization direction of the transmitted components can be rotated by 90° with the integrated electrically controlled half wave plate  17   c , depending on whether a hologram is being recorded or read. Finally, the reference beam  25  is sent though a second quarter wave plate  19  and focused with an objective lens  20  into a holographic storage medium  22 . 
         [0025]    The light transmitted through the first polarizing beam-splitter  5  represents the signal beam  24 . The signal beam  24  continues through a fourth polarizing beam-splitter  7  and impinges on a reflective amplitude modulator (or spatial light modulator, SLM)  9 . Again, the optical setup can easily be modified in such way that a transmission type SLM  9  may be used. A third quarter wave plate  8  ensures that the reflected signal beam is deviated by the fourth polarizing beam-splitter  7  towards a switchable fourth half wave plate  23 . The signal beam  24  is then deviated by the third polarizing beam-splitter  15  and imaged by the lens pair  16 ,  18  into the pupil of the objective lens  20 . The signal beam  24  is then focused into the holographic storage medium  22  after passing the second quarter wave plate  19 . The signal beam is low-pass Fourier filtered by the polarizing Fourier filter  17 . 
         [0026]    In the following the recording process shall be explained. For recording a hologram the SLM  9  transfers the data to be recorded onto the signal beam  24 . The signal beam  24  is low-pass filtered by the polarizing Fourier filter  17 , passed through the optional second quarter wave plate  19  and focused into the holographic medium  22 . In the medium the signal beam  24  has a circular polarization. The wave front of the reference beam  25  is set by the phase and amplitude SLM  13 . Then the reference beam  25 , whose direction of polarization is orthogonal to the one of the signal beam  24 , is high-pass filtered by the polarizing Fourier filter  17 . The electrically controlled half wave plate  17   c  of the filter  17  is switched off, i.e. it does not modify the direction of polarization of the signal beam  24 . The optional second quarter wave plate  19  converts the polarization of the reference beam  25  into circular polarization. The polarization direction is opposite to the one of the signal beam  24 . Therefore, the only interfering rays responsible for the generation of the hologram are the combinations of the reflected signal beam  24  and the incoming signal beam  24  and the incoming signal beam  24  and the reflected reference beam  27 . The reflection takes place at a reflective layer  21  of the holographic storage medium  22 . 
         [0027]    Advantageously, an amplitude modulator is also integrated into the spatial light modulator  13 . As any wave front is uniquely determined by its spatial amplitude and phase distribution in a single plane, a combination of a phase and an amplitude modulator permits a maximum number of multiplexing states. Furthermore, with the amplitude modulator it is possible to generate halve cone reference beams by switching off the light in halve of the aperture of the objective lens  20 . This is possible, as the modulator  13  and the pupil plane of the objective lens  20  lie in conjugate planes that are formed by the lenses  16  and  18 . 
         [0028]    The readout process is shown in  FIG. 3 . The identical reference beam  25  as the one used during the recording process is generated by the SLM  13 . Again, the reference beam  25  is high-pass filtered by the polarizing Fourier filter  17 . However, this time the electrically controlled half wave plate  17   c  is switched on, so that the polarization is rotated by 90°. The beam is sent through the second quarter wave plate  19  into the holographic storage medium  22 . This means that the circular polarization direction is opposite to the circular polarization direction used during the recording process. Consequently, the circular polarization direction of the reproduced signal beam  26  is also inversed. After passing through the second quarter wave plate  19 , the polarization of the reproduced signal beam  26  is linear in a direction that permits the passage through the polarizing Fourier filter  17 . The beam  26  is deviated by the third polarizing beam-splitter  15  and passed through the switchable fourth half wave plate  23 , which is set in a state that permits the reproduced signal beam  26  to pass trough the fourth polarizing beam-splitter  7 . Finally, the reproduced signal beam  26  is imaged onto an array detector  6 . The reflected reference beam  27  is blocked by the polarizing Fourier filter  17 . It is likewise possible to write to the holographic storage medium  22  with the electrically controlled half wave plate  17   c  being switched on. In this case it has to be switched off during reading. 
         [0029]    In this exemplary setup the distance from the array detector  6 , the SLM  13 , and the SLM  9  to the lens  16  corresponds to the focal length of the lens  16 . Otherwise, the polarizing Fourier filter  17  does not work correctly. 
         [0030]    It is also possible to operate the system without the second quarter wave plate  19 . In this case the electrically controlled half wave plate  17   c  of the polarizing Fourier filter  17  is switched on during recording and during read-out, so that the directions of polarization of the reference beam  25  and the signal beam  24  are parallel. This means that in such an alternative embodiment the electrically controlled half wave plate  17   c  is replaced by a non-switchable half wave plate. 
         [0031]      FIG. 4  depicts a modified optical setup, in which transmission type SLMs  9 ,  13  are used instead of reflective SLMs. Though reflective SLMs allow a fast switching, this modified optical setup has the advantage that the first quarter wave plate  12  and the third quarter wave plate  8  are no longer needed. The fourth PBS  11  is replaced by a mirror. 
         [0032]    As an alternative to the transmission type polarizing Fourier filter  17 , also a reflection type polarizing Fourier filter can be realized. This is depicted in  FIG. 5 . In this case the polarizing Fourier filter  17  is slightly inclined with respect to the optical axis, in order to enable a separation of the reflected light beams from the incoming light beams  25 ,  26 . As a consequence, the inner zone  17   b  has an elliptical shape. In addition, the electrically controlled half wave plate  17   c  is replaced by an electrically controlled quarter wave plate  17   e . For reflecting the light beams  25 ,  26  the polarizing Fourier filter  17  is provided with a mirror layer  17   f . In  FIG. 5 , the mirror layer  17   f  replaces the cover layer  17   d . It may of course also be arranged on the cover layer  17   d.    
         [0033]    Of course, some of the optical components used in the above described embodiments may be replaced by other components with corresponding functions, or may even be omitted in some configurations.