JP2008096799A - Hologram device and hologram recording method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a two-dimensional transformation code with which favorable recording without degradation in image quality can be performed regardless of the characteristics of an optical filter suppressing a DC component included in signal light. <P>SOLUTION: A method for recording a hologram is provided aiming to record interference fringes of signal light and separately projected reference light in a hologram recording medium 50, the signal light modulated by a two-dimensional transformation code displayed in a spatial modulator 9. By inserting an aperture (a DC cut mask) 21 that cuts off a DC component of the signal light into the optical path of the signal light after spatial modulation and by forming the two-dimensional transformation code of the spatial modulator 9 in a pattern having a shorter period than a reference modulation pattern, favorable recording without degradation of image quality can be carried out regardless of the characteristics of an optical filter suppressing the DC component included in the signal light. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a hologram recording apparatus and method for volume recording a hologram on a hologram recording medium, and more particularly to a two-dimensional modulation code for modulating signal light during recording.

  In recent years, holographic storage systems that record and reproduce large amounts of data using hologram technology have been proposed. In this holographic storage system, for example, a signal light including recording data generated by a spatial light modulator such as a liquid crystal element and a reference light set corresponding to the signal light are generated at a predetermined angle by a hologram recording medium ( The recording system records the interference fringes generated by the signal light and the reference light on the recording material, and the hologram recording medium is irradiated with the reproduction illumination light. And a reproduction system that reproduces data by generating diffracted light (reproduced signal light) corresponding to the interference fringes, receiving it by a light receiving element such as a CCD image sensor, and analyzing it. . A hologram per spatial light modulator recorded in this way is called a page.

  In the holographic storage system, a technique called multiple recording is used to improve recording density. This is different from recording on a conventional optical disk in that a large number of independent pages are recorded at one place. Typical examples of such a multiplex recording system are angle multiplex recording, shift multiplex recording, phase code multiplex recording, and the like, and many other multiplex systems are known.

The angle multiplexing method records and reproduces a large number of independent pages in one place by changing the angle of the reference beam. Shift multiplex performs multiplex recording by shifting the recording position little by little. In phase code multiplexing, when one page is recorded, recording is performed by simultaneously applying reference light from various directions. However, at that time, a phase change is given to the reference light from each direction. By combining this phase change in various ways, a large number of independent pages are recorded and reproduced in one place. In addition to the above three types, many multiplexing schemes are considered.
(For example, non-patent literature: Holographic Data Storage; HJCoufal, D.Psaltis, GTSincerbox ED; Springer; p.185 Photopolymer System)

  In order to improve the recording density in hologram recording, improvement of multiplicity is one of the important factors. However, in order to improve the multiplicity, since the dynamic range of the recording medium is finite, it is necessary to perform hologram recording with the minimum recording power necessary for reproducing the hologram. Conventionally, block data modulation methods such as 2/4 modulation (a type of two-dimensional modulation code) have been used for recording data pages, and DC contributes little to information in the frequency space where hologram recording is performed. The problem is that the components become dominant and waste the dynamic range of the media.

  As a means for suppressing the DC component, it is known to install an aperture on the optical path of the signal light of the hologram recording optical system to suppress the DC component of the signal light (for example, K. Kawano, K. Haga , J. Minabe, S. Yasuda, M. Furuki, Y. Ogasawara, K. Hayashi, and H. Yoshizawa, “Band-Pass Filtering in Holographic Data Storage”, Lasers and Electro-Optics Society, 2005. LEOS 2005. The 18th Annual Meeting of the IEEE, 22-28 Oct. 2005, 491-492), when a general block modulation code is applied to such an optical system, the image quality of the modulation code is degraded due to the characteristics of the optical filter. It became a factor of the error rate deterioration.

  The present invention has been devised in view of the above circumstances, and an object of the present invention is to achieve good recording with no deterioration in image quality by suppressing the DC component contained in the signal light while considering the characteristics of the optical filter. An object of the present invention is to provide a hologram recording / reproducing apparatus and method capable of performing the above.

  In order to achieve the above object, the present invention provides a hologram recording method for recording interference fringes between signal light modulated by a two-dimensional modulation code of a spatial modulator and reference light separately irradiated on a hologram recording medium, A filter for cutting the DC component of the signal light is inserted in the signal light optical path after spatial modulation, and the pattern of the two-dimensional modulation code displayed on the spatial modulator is a pattern having a shorter period than the reference modulation pattern To do.

  As described above, in the present invention, the DC component is suppressed from the signal light spatially modulated by the filter (a high-pass filter or a band-pass filter is used, and may also be referred to as a DC cut mask), thereby suppressing the recording power. I can do it. In addition, the two-dimensional modulation code pattern of the spatial modulator is a pattern having a shorter cycle than the reference modulation pattern, and suppresses the DC component at the time of modulation, thereby suppressing image quality deterioration due to the DC cut and providing a good image. Recording can be performed.

  According to the present invention, by inserting a high-pass filter in the signal light optical path after spatial modulation, and the pattern of the two-dimensional modulation code of the spatial modulator is a pattern having a shorter cycle than the reference modulation pattern, The recording power can be suppressed while maintaining good image recording, and the dynamic range of the recording medium can be effectively utilized.

(Example 1)
FIG. 1 is a schematic diagram showing a configuration of a hologram recording / reproducing apparatus according to an embodiment of the present invention. The hologram recording / reproducing apparatus has a configuration in which image data is multiplexed and reproduced by an angle multiplexing method, and includes a laser light source 1, a shutter 2, a half-wave plate 3, a polarizing beam splitter (PBS) 4, a shutter 5, a mirror 6, a beam extract. Hologram recording medium having pandas 7 and 13, mirrors 8 and 14, spatial modulator (SLM) 9, lenses 10, 11, 12, 16, 17 and 19, rotating mirror 15, apertures 20 and 21, and image sensor 22. 50 records and reproduces holograms. The lenses 10 and 11, the lenses 12 and 16, and the lenses 17 and 19 each constitute a 4F system.

  Next, an outline of the operation of the hologram recording / reproducing apparatus of the present embodiment will be described. First, during recording, the shutters 2 and 5 are opened. Laser light emitted from the laser light source 1 enters the PBS 4 through the shutter 2 and the half-wave plate 3, and is branched into the signal light 100 and the reference light 200 here. The signal light 100 is spatially modulated by the data page by passing through the SLM 9 through the shutter 5, the mirror 6, the beam expander 7, and the mirror 8. The spatially modulated signal light 100 passes through the lenses 10, 11, and 12 and is collected on the hologram recording medium 50. On the other hand, the reference light 200 is applied to the hologram recording medium 50 through the beam expander 13, the mirror 14, and the rotating mirror 15. As a result, interference fringes between the signal light 100 and the reference light 200 are recorded on the hologram recording medium 50. At this time, every time the data page that modulates the signal light 100 is changed, the rotation mirror 15 is rotated to change the incident angle of the reference light 200 to the hologram recording medium 50, so that the angle is set to the same recording area of the hologram recording medium 50. Multiple recording is performed by multiplexing.

  During reproduction, the shutter 2 is opened and the shutter 5 is closed. At the time of reproduction, reference light (reproduction reference light) is incident on the hologram on the recording medium from the same angle as that used for recording, hologram reproduction is performed, and the reproduced image is captured by the image sensor 22. Therefore, only the reference light 200 is irradiated onto the hologram recording medium 50 through the rotating mirror 15, and the generated reproduction signal light 300 is incident on the image sensor 22 through the lenses 16, 17, and 19, and reproduction image data is obtained here. At this time, by rotating the rotary mirror 15 and changing the irradiation angle of the reference light 200 to the hologram recording medium 50, the multiplex recorded image data is sequentially reproduced. The hologram spot size is defined by the size of the aperture arranged on the Fourier plane in the signal light optical system.

  Here, in the hologram storage, the recording density is roughly determined by the number of bits per data page, multiplicity, spot size, spot pitch, and the like. Among these parameters, in order to improve multiplicity, which is one of the parameters that characterize hologram recording, since the dynamic range of the medium is finite, the minimum necessary level that can be detected by the image sensor 22 used for reading is used. It is necessary to perform hologram recording with the power of. Hologram recording is recording in Fourier space, and the main component of a normal block modulation code is a DC component. Therefore, suppressing this DC component leads to a reduction in recording power.

  In the present embodiment, a method for creating a modulation code in which the DC component of a signal is suppressed is proposed by taking 2/4 block modulation, which is one of typical block modulation schemes, as an example. At the same time, in the recording optical system, when the recording power of the signal light 100 is reduced by arranging the aperture 20 for cutting the DC light on the Fourier plane in the optical path as shown in FIG. The present invention provides a modulation code that is effective for minimizing the image quality degradation of modulation symbols due to.

  FIG. 2 shows a data page by 2/4 block modulation, and FIG. 3 shows a data symbol by 2/4 block modulation. As shown in FIG. 3, 2/4 block modulation expresses 2 bits of data by 2 × 2 pixels. Of the 2 × 2 pixels, only one pixel is a high level signal, and the remaining three pixels are low level signals. The patterns arranged around the data page in FIG. 2 are positioning markers used when decoding modulation codes.

FIG. 4 shows a detailed example of the signal light optical path shown in FIG. When Fourier transform is applied to a normal recording data page (modulated image), it can be seen that the power of the DC component is dominant. Hologram recording is performed in a frequency space, and in order to reduce recording power in order to improve multiplicity, it is effective to suppress the DC component. In the optical system of FIG. 4, a change as shown in FIG. 5 is made to the aperture 21 that defines the spot size of the hologram, and a DC cut mask is used in which the central portion of the aperture 21 is a light shielding portion 31. That is, in this embodiment, the aperture 21 is also used as a DC cut mask function.
Since the DC component is distributed in the vicinity of the optical axis in the frequency space, the DC component of the signal light is cut by the light shielding unit 31. In other words, this aperture (DC cut mask) 21 constitutes a bandpass filter (a combination of an HPF that cuts the DC component of signal light and an LPF that defines the Fourier plane size of the signal light). . 21 constitutes a high-pass filter (HPF). For this reason, if the data page optimized for this optical filter is not displayed on the SLM 9, an image with image quality degradation is already recorded before hologram recording.

  FIG. 6 shows the result of applying a normal modulation code to the DC cut optical system shown in FIG. 4 without considering the optical filter characteristics. The image in FIG. 9A is an original image (2/4 block modulation image), and FIG. 10B is an optical HPF in which the modulation signal has a maximum frequency of 0.5 as a cutoff frequency. It is an image. As a matter of course, the DC component is largely removed from the image after the application of the HPF, only the edge of the marker is extracted, and the modulation code itself is greatly deteriorated at the location where the low frequency component is included by the adjacent modulation symbol. For this reason, robust marker recognition is often difficult in positioning using a marker in which only edges of about several pixels are imaged. If the marker recognition fails, the worst case is that the data of the corresponding page cannot be decoded at all.

  FIG. 7 shows the result of evaluating the effect on the error rate due to image degradation after application of the optical filter. In the figure, the portion where the boundary line is black indicates an error symbol, and it can be seen that this error is concentrated on the symbol that has deteriorated the image due to the optical filter because it includes a low-frequency component. This image is a reproduced image when only one data page is hologram-recorded without performing multiple recording. In the conventional optical system in which the DC cut mask of FIG. 4 does not exist when multiple recording is not performed, error-free reproduction is performed, and thus the cause of the error is image degradation of the modulated image due to the HPF configured by the DC cut mask 21 It can be seen that

  In view of the above situation, a modulation code optimized for the characteristics of the optical filter of the optical system of FIG. 1 has been devised. The specific method is shown in FIG. In the figure, the symbols shown at the top are normal 2/4 block modulation codes, and the symbols shown at the bottom are improved symbols. To suppress the DC component, the vertical and horizontal frequencies were doubled. In order to suppress intersymbol interference, uncorrelated patterns are arranged in the vertical and horizontal directions where the intersymbol distance is short. FIG. 9A shows a modulation code to which these modulation symbols are applied (an image obtained by cutting out a part of the modulation code).

  Further, as can be seen from FIG. 9, the shape of the positioning marker (upper left corner in the image) 40 including a lot of low frequency components is also changed. In order to suppress the DC component, a dot-shaped marker was devised. In general, in a positioning algorithm using a positioning marker, marker edge information can be one of important information for positioning. Therefore, it is difficult to detect a marker edge in a dot-shaped marker. Therefore, by performing shape correction processing on the detected marker image during marker detection processing, edge evaluation equivalent to that of conventional markers can be performed, and centroid detection can be performed with necessary and sufficient accuracy. It was. The shape correction processing is realized by so-called expansion / reduction processing, and the dot interval needs to be set so that the correction can be effectively performed by this processing, and this is sufficiently possible.

  FIG. 9B shows the result of applying the modulated image optimized for the optical filter to the optical system shown in FIG. As can be seen from this image, there is no deterioration of the modulation symbol due to the optical filter as seen in FIG. Similarly to FIG. 8, the result of evaluating the error rate for the hologram image reproduced by applying a new modulation code is shown in FIG. As can be seen from the figure, by using the newly proposed modulation code, the image quality deterioration of the modulation symbol due to the optical filter is suppressed, and there is no symbol error due to the image quality deterioration of the modulation symbol seen in FIG. It became.

  According to this embodiment, even in an optical system in which an aperture (DC cut mask) 21 is installed on the optical path of signal light of the hologram recording optical system in order to suppress the DC component of the signal light, the modulation pattern as a reference is used. In addition, by using a block modulation code having a pattern with a short period, hologram recording can be performed without deteriorating the image quality of the modulation symbol. As a result, it is possible to achieve a practical error rate while improving the multiplicity, and can greatly contribute to the improvement of the recording density. In addition, the positioning marker on the data page according to the present embodiment is also optimized for the characteristics of the optical system. Therefore, the image processing algorithm corresponding to this marker does not cause image quality degradation, and the conventional angle is determined. It has become possible to ensure the same positioning accuracy as the multiple optical system.

FIG. 1 is a schematic diagram showing the configuration of a hologram recording / reproducing apparatus according to an embodiment of the present invention. It is the block diagram which showed the data page example by 2/4 block modulation. It is the figure which showed the data symbol example by 2/4 block modulation. It is the figure which showed the detailed example of the signal light optical path shown in FIG. FIG. 5 is a plan view showing a configuration example of the DC cut mask shown in FIG. It is the figure which showed the example of an image after 2/4 block modulation | alteration image and optical HPF application. It is a circuit diagram showing an example of an error map of an HPF application image. It is a figure explaining optimization of a 2/4 block modulation code. It is the figure which showed the optimized 2/4 block modulation image and the image example after optical HPF application. It is a circuit diagram showing an error map example of an HPF application image of the present embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Laser light source, 2, 5 ... Shutter, 3 ... Half wave plate, 4 ... Polarizing beam splitter (PBS), 6, 8, 14 ... Mirror, 7, 13 ... Beam expander, 9 ... ... Spatial modulator (SLM) 10, 11, 12, 16, 17, 19 ... lens, 15 ... rotating mirror, 20, 21 ... aperture, 22 ... image sensor, 50 ... hologram recording medium.

Claims (9)

A hologram recording apparatus for recording interference fringes between signal light displayed on a spatial modulator and modulated by a two-dimensional modulation code and reference light separately irradiated on a hologram recording medium,
A filter for cutting a DC component of signal light in the signal light optical path after spatial modulation;
The hologram recording apparatus, wherein the spatial modulator displays a two-dimensional modulation code having a pattern having a shorter cycle than a reference modulation pattern.   2. The hologram recording apparatus according to claim 1, wherein the filter includes a light shielding plate positioned in the vicinity of the optical axis of the signal light optical path, and an opening is provided in an outer peripheral portion of the light shielding plate.   2. The hologram recording apparatus according to claim 1, wherein the two-dimensional modulation code has an uncorrelated pattern at a position where the intersymbol distance is short.   The positioning marker for positioning a reproduced image when reproducing a hologram recorded using signal light modulated by the two-dimensional modulation code has a shape that suppresses a DC component. Hologram recording device.   5. The hologram recording apparatus according to claim 4, wherein the positioning marker is formed by dots.   5. The hologram recording apparatus according to claim 4, wherein image processing for correcting the shape of the positioning marker is performed when the positioning marker is detected.   2. The hologram recording apparatus according to claim 1, wherein the two-dimensional modulation code is a 2/4 block modulation code. A hologram recording method for recording interference fringes between signal light modulated by a two-dimensional modulation code of a spatial modulator and reference light separately irradiated on a hologram recording medium,
The DC component of the signal light after spatial modulation is cut by a filter, and the pattern of the two-dimensional modulation code displayed on the spatial modulator is a pattern having a shorter period than a reference modulation pattern Hologram recording method.   9. The hologram recording method according to claim 8, wherein the two-dimensional modulation code has an uncorrelated pattern at a position where the intersymbol distance is short.

Images