Abstract:
Disclosed are a holographic memory medium for use in a holographic memory system, which can prevent generation of stray light of a gating beam at the time of recording information, and a recording apparatus which uses this memory medium. This memory medium records an information signal carried by a signal beam as a coherent reference beam and the signal beam enter the memory medium with a gating beam being present. The memory medium has at least a first surface and a second surface parallel to each other and comprises an antireflection coat, formed on the first surface, for preventing reflection of the gating beam. The recording apparatus records an information signal using a holographic memory medium and comprises a light absorbing member of absorbing the gating beam coming through the memory medium.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a holographic memory medium and a recording apparatus using this medium. 
     2. Description of the Related Art 
     A volume holographic memory system is one of digital information recording systems which use the principle of holography. This volume holographic memory system converts digital data or an information signal to a two-dimensional dot beam pattern with a contrast, adds a reference beam to this pattern, yielding an optical interference pattern, and records this optical interference pattern as two-dimensional data on a recording medium. For the recording medium is used a photorefractive crystal such as LN which has a cylindrical shape, a rectangular parallelepiped or the like. 
     As reproducing an information signal, the reference beam alone is irradiated on the recording medium to thereby reproduce an optical interference pattern and performs inverse transform on the optical interference pattern, yielding a dot beam pattern. The original data can be acquired by reconverting this signal to digital data by using an electronic circuit. 
     Because the same reference beam as used in the recording operation is irradiated on a recording medium in a reproducing operation, recorded information may be erased at the same time as information is reproduced. As one solution to such a problem, the beam power of the reference beam to be used in a reproducing operation is set lower than that needed in a recording operation. However, repeating the reproducing operation erases recorded information slightly though it occurs. In this respect, there has been a demand for a system which has less reproduction deterioration. 
     A two-color holographic memory system performs recording and reproducing operations using a combination of lights of different wavelengths by utilizing the intermediate level of optical excitation in a recording medium which is comprised of a photorefractive crystal. Specifically, a gating beam whose wavelength differs from that of the reference beam is used in such a way that information is recordable only when this gating beam and the reference beam are both irradiated on the recording medium. A reproducing operation is carried out by irradiating the reference beam alone on the recording medium while cutting off the gating beam. This prevents reproduction deterioration. 
     In the above two-color holographic memory system, if the gating beam is irradiated on the recording medium at a position different from a predetermined target position where information is to be recorded, the information signal recorded at the irradiated portion may be erased. 
     For instance, as the gating beam irradiated on the recording medium passes through the recording medium and emerges from the opposite side, part of the gating beam may be reflected at the emerging surface of the recording medium and may return inside the recording medium. At the surface of the recording medium where the gating beam has been irradiated, part of the gating beam may be reflected there and further reflected by the components of the recording apparatus to reach the recording medium again. Those stray lights may undesirably erase non-target recorded signals. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a holographic memory medium which can prevent generation of stray light of a gating beam at the time of recording information, and a recording apparatus which uses this memory medium. 
     To achieve the above object, according to one aspect of this invention, there is provided a holographic memory medium for recording an information signal carried by a signal beam as a coherent reference beam and the signal beam enter the memory medium with a gating beam being present, which memory medium has at least a first surface and a second surface parallel to each other and comprises an antireflection coat, formed on the first surface, for preventing reflection of the gating beam. 
     According to another aspect of this invention, there is provided a recording apparatus for recording an information signal using a holographic memory medium, which comprises light absorbing member of absorbing the gating beam coming through the memory medium. 
    
    
     BRIEF EXPLANATION OF THE DRAWINGS 
     FIG. 1 is a structural diagram illustrating a recording apparatus embodying this invention; 
     FIG. 2 is a perspective view showing a memory medium according to this invention; 
     FIG. 3 is a perspective view showing another memory medium according to this invention; 
     FIG. 4 is a structural diagram illustrating another recording apparatus embodying this invention; 
     FIG. 5 is a perspective view showing a different memory medium according to this invention; 
     FIG. 6 is a perspective view showing a further memory medium according to this invention; and 
     FIG. 7 is a structural diagram illustrating a further recording apparatus embodying this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. 
     According to a holographic memory device embodying this invention as illustrated in FIG. 1, which can prevent generation of stray light of a gating beam, a recording medium  10 , comprised of a photorefractive crystal such as LN, is cylindrical and has its crystal axis  10 a arranged in parallel to the direction of an arrow A. That is, the recording medium  10  has parallel top and bottom surfaces laid perpendicular to the direction of the arrow A. Located above the recording medium  10  is a gating-beam source  11  which emits a gating beam  11   a  downward in parallel to the center axis  10 a of the recording medium  10 . This gating-beam source  11  can irradiate a beam spot at any desired position in the recording medium  10  and has power high enough to cause optical excitation in the recording medium  10 . For example, a super luminescent diode or the like is preferable as the beam source. 
     The recording medium  10  is supported at near the lower end portion from the sides. The recording medium  10  is securely fitted in a through hole formed in a ring-shaped gear  21  at the center. The peripheral edge of the ring-shaped gear  21  is held vertically by a holding block  22  via bearing means (not shown) in such a way that the ring-shaped gear  21  is slidable. Accordingly, the recording medium  10  is rotatable together with the ring-shaped gear  21  about the center axis  10   a  of the recording medium  10  in the direction of an arrow B. Unillustrated rotation means is engaged with the ring-shaped gear  21  whose rotation is controlled by a controller  41 . 
     The holding block  22  can also be moved by unillustrated parallel moving means in the vertical direction indicated by the arrow A. The controller  41  also controls the positioning of the holding block  22 . 
     A laser beam emitted from another beam source or a laser-beam source  23  is split into a signal beam  23   a  and a reference beam  23   b  by a beam splitter  24 . After the beam size is expanded by a beam expander  25 , the signal beam  23   a  enters a spatial light modulator (hereinafter called “SLM”)  26 . 
     Record data is sent to the SLM  26  after it is converted by an encoder  42  to an information signal of a sequence of data of a unit page corresponding to a two-dimensional page. The SLM  26  forms a dot matrix upon reception of this data. As the signal beam  23   a  passes the SLM  26 , it is optically modulated in the state where it includes the information signal. Further, the signal beam  23   a  passes through a Fourier transform lens  27  so that the information signal undergoes Fourier transform, and so converges in the recording medium  10  as to form crossover light. 
     The reference beam  23   b,  split by the beam splitter  24 , is sequentially reflected and guided toward the recording medium  10  by reflecting mirrors  28   a  and  28   b.  The reflecting mirror  28   b  is so adjusted that the reference beam  23   b  crosses the signal beam  23   a  in front of or at the back of the position of the crossover light formed by the signal beam  23   a.  The position of the gating-beam source  11  is so adjusted that the gating beam  11   a  is irradiated in the vicinity of this crossing position. 
     The signal beam,  23   a  and the reference beam  23   b  interfere with each other at the crossing position, thus forming an interference pattern. This interference pattern is recorded on the recording medium  10 . 
     Recording information signals using spatial multiplexing and angular multiplexing can increase the recording density of the recording medium  10 . When the recording medium  10  is moved horizontally in the direction of the arrow A by the controller  41 , the position of the interference pattern formed by the reference beam and the signal beam with respect to the recording medium  10  changes to thereby ensure spatial-multiplexing based recording. As the recording medium  10  is rotated in the direction of the arrow B by the controller  41 , the recording plane of the interference pattern is rotated, thereby ensuring angular-multiplexing based recording. 
     In a mode of reproducing recorded information, the gating-beam source  11  is turned off to stop irradiating the gating beam toward the recording medium  10 . Further, the signal beam  23   a  is shielded by the SLM  26  so that it does not reach the recording medium  10 . As only the reference beam  23   b  is irradiated on the recording medium  10 , the interference pattern recorded in the recording medium  10  is reproduced as its diffracted light. This diffracted light is led through an inverse Fourier lens  30  for inverse Fourier transform, the diffracted light is converted to a pattern of light intensities. The light intensity pattern is supplied to a CCD (Charge Coupled Device)  31  where it is converted to an analog electric signal. This analog electric signal is then sent to a decoder  43 . The decoder  43  converts the analog electric signal to a digital signal which is reproduced data. 
     As shown in FIG. 3, the recording medium  10  does not have to have a cylindrical shape. The recording medium  10  has only to be columnar with parallel top and bottom surfaces. For instance, the recording medium  10  may be a rectangular parallelepiped. In this case, the multiplexing recording is accomplished by making only the parallel movement of the recording medium  10  in the direction of the arrow B, not the rotational movement in the direction of the arrow A, under the control of the controller  41 . 
     As shown in FIGS. 2 and 3, an antireflection (AR) coat  12  which suppresses reflection of the gating beam  11   a  is formed on the surface of the recording medium  10  where the gating beam  11   a  enters and the top surface of the recording medium  10 . A light-absorptive coat  13  which is capable of absorbing the gating beam  11   a  is formed on the surface of the recording medium  10  from which the gating beam  11   a  emerges or the bottom surface of the recording medium  10 . 
     This structure can prevent such a phenomenon that part of the gating beam  11   a  irradiated on the recording medium  10  is reflected at the top of the recording medium  10  and is further reflected irregularly by the components of the recording apparatus to reach the recording medium  10  again. This structure can also suppress such a phenomenon that when the gating beam  11   a  once entered in the recording medium  10  passes through the recording medium  10  and emerges from the bottom thereof, part of the gating beam  11   a  is reflected there and is confined inside the recording medium  10 . 
     In a holographic memory device according to another embodiment of this invention shown in FIG. 4, the antireflection coat  12  is formed on both of the top surface of the recording medium  10  where the gating beam  11   a  enters and the bottom surface of the recording medium  10  from which the gating beam  11   a  emerges, as shown in FIG. 5 or FIG.  6 . The shape of the recording medium  10  is not limited to a cylindrical shape or a rectangular parallelepiped shape, but can take any columnar shape with parallel top and bottom surfaces. A reflecting mirror  32  is disposed on the path on which the gating beam  11   a  emerges from the recording medium  10 . This reflecting mirror  32  reflects and guides the gating beam  11   a  in a direction different from the direction toward the recording medium  10 . A light-absorbing plate  33 , which has the light-absorptive coat  13  formed on its surface, is disposed perpendicular to the light reflected by the reflecting mirror  32  and absorbs the gating beam  11   a  coming from the reflecting mirror  32 . 
     This structure can prevent such a phenomenon that part of the gating beam  11   a  irradiated on the recording medium  10  is reflected at the top of the recording medium  10  and is further reflected irregularly by the components of the recording apparatus to reach the recording medium  10  again. This structure can also suppress such a phenomenon that when the gating beam  11   a  once entered in the recording medium  10  passes through the recording medium  10  and emerges from the bottom thereof, part of the gating beam  11   a  is reflected there and is confined inside the recording medium  10 . Further, as the gating beam  11   a  is absorbed by the light-absorbing plate  33  located remote from the recording medium  10 , the recording medium  10  is not affected by the heat that is generated by the light-absorptive coat  13  absorbing the gating beam  11   a.  This embodiment is preferable because of its ability to prevent heat-based deterioration of the recording medium  10 . 
     It is to be noted that the light-absorbing member of the recording apparatus in FIG. 4 can suppress the generation of stray light whichever one of the recording media shown in FIGS. 2,  3 ,  5  and  6  is used. 
     In a holographic memory device according to a further embodiment of this invention shown in FIG. 7, the recording medium  10  is cylindrical as shown in FIG.  5 . The gating-beam source  11  is located on one side of the recording medium  10 . The gating beam  11   a  enters the recording medium  10  from one side at a given angle to the center axis  10   a  of the recording medium  10 , crosses the center axis  10   a  and comes out of the recording medium  10  from the opposite side to the incident side. 
     As shown in FIG. 5, the antireflection coat  12  is formed on the top and bottom surface of the recording medium  10  excluding the side surface. The reflecting mirror  32  is disposed on the path on which the gating beam  11   a  passing through the recording medium  10  emerges therefrom. This reflecting mirror  32  reflects the gating beam  11   a  in a direction different from the direction toward the recording medium  10 . The light-absorbing plate  33  is disposed approximately perpendicular to the path of the reflected gating beam  11   a.  Formed on the light-absorbing plate  33  is the light-absorptive coat  13  which absorbs the gating beam  11   a.    
     This structure can suppress such a phenomenon that when the gating beam  11   a  once entered in the recording medium  10  from one side portion passes through the recording medium  10  and emerges from the opposite side portion, part of the gating beam  11   a  is reflected there and is confined inside the recording medium  10 . Further, as the gating beam  11   a  is absorbed by the light-absorbing plate  33  located remote from the recording medium  10 , the recording medium  10  is not affected by the heat that is generated by the light-absorptive coat  13  absorbing the gating beam  11   a.  This embodiment is preferable because of its ability to prevent heat-based deterioration of the recording medium  10 . 
     As apparent from the above, the holographic memory medium according to this invention and a recording apparatus using this medium can suppress stray light that is produced as the gating beam is scattered at the surface of the recording medium, and can therefore advantageously prevent information signals recorded on the recording medium from being unintentionally erased by such stray light.