JP4975007B2 - Hologram reproduction wavefront control method and apparatus - Google Patents

Description

  The present invention relates to a holographic memory system, and more particularly to a hologram reproduction wavefront control method and apparatus for controlling the wavefront shape of reference light for reproduction based on information obtained from a reproduced image.

  In a digital hologram, generally, image data is driven by a spatial light modulation element such as a liquid crystal display element to generate modulation data, and laser light is modulated by the modulation data to generate object light. The object light is optically Fourier transformed and interfered with the recording reference light to generate interference light, and interference fringes carrying image data are recorded on the hologram recording medium. During reproduction, reproduction reference light is irradiated onto the hologram recording medium on which the interference fringes are recorded, and diffracted light from the interference fringes is converted back to parallel light by a lens and received by a two-dimensional light receiving element (CCD or CMOS sensor). By doing so, the reproduced image data is read.

  By the way, in a holographic memory system taking into consideration a medium removable type (removable), there is a discrepancy in the size of the reproduced image, rotation of the image, etc. due to uncertainty in the positional accuracy of the hologram recording medium. There is.

  Conventionally, as a countermeasure against such a phenomenon, a predetermined sync pattern (synchronization signal) serving as an index (marker) such as an image position or size is superimposed on image data during recording, and this sync pattern is reproduced during reproduction. Is known, and the reproduction image is rotated and enlarged / reduced (see Patent Documents 1 and 2 and Non-Patent Document 1 below).

  In addition, in a hologram recording medium made of a photopolymer (photosensitive resin) material, an interference fringe carrying image data due to a refractive index difference generated inside the recording medium due to a photopolymerization reaction when irradiated with object light and reference light. Is recorded, but volume shrinkage may occur at that time. When this volume shrinkage occurs, a Bragg mismatch (a state that does not satisfy the Bragg diffraction condition) occurs during reproduction, so that the SNR (Signal to Noise Ratio) of the reproduced image is lowered and a part of the image becomes dark. If it is remarkable, the reproduction of the image itself becomes impossible.

  As a technique corresponding to such a decrease in the SNR of a reproduced image caused by volume shrinkage of the hologram recording medium, for example, those described in Patent Documents 3 and 4 below are known. The applicant of the present application has also devised the technique described in Patent Document 5 below and disclosed it to the Patent Office.

Japanese Patent No. 3737292 JP 2008-139125 A JP 2006-343702 A JP 2008-96783 A Japanese Patent Application No. 2007-255322 Michiichi Hashimoto et al. “High-precision detection method of marker position in hologram memory”, Science Technique MR2004-70, CPM2004-183 (2005-03)

  The techniques described in Patent Documents 3 to 5 maximize the SNR by controlling the irradiation angle and wavefront shape of the reference light for reproduction based on the image evaluation while evaluating the state of the reproduced image. For example, the SNR of the reproduced image itself is used as a specific index when performing image evaluation.

  However, since the SNR of the reproduced image by the hologram is calculated by statistical processing (processing using standard deviation or average value) with the brightness of each pixel of the reproduced image as a sample value, the number of samples (number of pixels to be sampled) ) Will not be able to calculate an accurate value, and therefore has the following problems.

  That is, if an accurate SNR is calculated by increasing the number of samples, it is possible to perform correct image evaluation. However, as the number of samples increases, more time is required to calculate the SNR. There is a problem that it is difficult to quickly maximize the SNR of the reproduced image by speeding up the wavefront control of the reference light.

  In addition, there is a desire to perform image evaluation on a partial area of a reproduced image. However, in the conventional method that requires the statistical processing described above that requires a large number of samples, an image of a partial area in the reproduced image is used. There is also the problem that evaluation is difficult.

  The present invention has been made in view of such circumstances, and can perform image evaluation on an entire area or a partial area of a reproduction image in a short time, and based on the image evaluation, the wavefront of reproduction reference light It is an object of the present invention to provide a hologram reproduction wavefront control method and apparatus capable of quickly maximizing the SNR of a reproduction image by controlling.

  In order to solve the above-described problems, the hologram reproduction wavefront control method and apparatus according to the present invention provide a pattern signal superimposed on image data to have a function as a conventional sync pattern (matching function of rotation and enlargement / reduction). In addition, a function as an index for image evaluation is further provided, and the pattern signal is used as an index for performing wavefront control of the reproduction reference light.

  That is, in the first hologram reproduction wavefront control method according to the present invention, a reproduction image reproduced by irradiating a reproduction reference beam onto a hologram recording medium on which image data on which a pattern signal having a predetermined spatial frequency is superimposed is recorded. Frequency analysis of a predetermined region, image evaluation is performed based on the amplitude value of the second harmonic component of the pattern signal obtained by the frequency analysis, and the wavefront of the reproduction reference light is minimized so that the amplitude value is minimized It is characterized by controlling.

In addition, the first hologram reproduction wavefront control device according to the present invention provides a reproduction image reproduced by irradiating a reproduction reference light onto a hologram recording medium on which image data on which a pattern signal having a predetermined spatial frequency is superimposed is recorded. A frequency analysis means for frequency analysis of a predetermined region;
Wavefront control means for performing image evaluation based on an amplitude value of a second harmonic component of the pattern signal obtained by the frequency analysis, and controlling a wavefront of the reproduction reference light so that the amplitude value is minimized; It is characterized by comprising.

  In addition, the second hologram reproduction wavefront control method according to the present invention provides a reproduction image reproduced by irradiating a reproduction reference light onto a hologram recording medium on which image data on which a pattern signal having a predetermined spatial frequency is superimposed is recorded. Analyzing the predetermined region by frequency, and performing image evaluation based on the ratio of the amplitude value of the fundamental wave component of the pattern signal to the amplitude value of the second harmonic component of the pattern signal obtained by the frequency analysis, The wavefront of the reference light for reproduction is controlled so that the value of the ratio becomes maximum.

In addition, the second hologram reproduction wavefront control device according to the present invention provides a reproduction image reproduced by irradiating a reproduction reference light onto a hologram recording medium on which image data on which a pattern signal having a predetermined spatial frequency is superimposed is recorded. A frequency analysis means for frequency analysis of a predetermined region;
Image evaluation is performed based on the ratio of the amplitude value of the fundamental wave component of the pattern signal to the amplitude value of the second harmonic component of the pattern signal obtained by the frequency analysis, and the value of the ratio is maximized And a wavefront control means for controlling the wavefront of the reference light for reproduction as described above.

  The first and second hologram reproduction wavefront control devices described above preferably include recording means for recording the pattern signal on the hologram recording medium by superimposing the pattern signal on the image data.

  The “predetermined area” includes a case where the entire area of the reproduction image is indicated in addition to an arbitrary partial area in the reproduction image.

  As the “frequency analysis”, various analysis methods such as Fourier transform and wavelet transform can be used, but it is preferable to use Fourier transform from the viewpoint of speeding up.

  According to the hologram reproduction wavefront control method and apparatus according to the present invention, the amplitude value of the second harmonic component of the pattern signal obtained by frequency analysis, or the fundamental wave component of the pattern signal with respect to the amplitude value of the second harmonic component. By using the value of the ratio of the amplitude values as an index for image evaluation, it is possible to perform image evaluation on the entire region or a partial region of the reproduced image in a short time.

  In addition, the SNR of the reproduced image is quickly maximized by controlling the wavefront of the reproduction reference light so that the amplitude value of the second harmonic component is minimized or the ratio value is maximized. This makes it possible to record and reproduce images through the hologram recording medium with high accuracy.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram showing an arrangement example of pattern signals used in the present invention, and FIG. 2 is a conceptual diagram showing an outline of a hologram reproduction wavefront control method according to an embodiment of the present invention.

  As shown in FIG. 2, in the method of the present embodiment, first, the pattern signals 32 </ b> A to 32 </ b> E shown in FIG. 1 are superimposed on the image data 31 recorded on the hologram recording medium 43. The image data 31 and the pattern signals 32 </ b> A to 32 </ b> E are formed by the parallel light applied to the spatial light modulator (SLM) 41 being modulated by the spatial light modulator 41.

  As shown in FIG. 1, the pattern signal 32A superimposed on the center of the image data 31 is a pixel block of a predetermined unit repeated at a predetermined pitch and arranged in a vertical and horizontal cross shape. It is comprised so that it may have. On the other hand, the pattern signals 32B to 32E respectively superimposed on the four corners of the image data 31 are obtained by repeating the same pixel blocks as the pattern signal 32A at the same pitch as the pattern signal 32A and arranging them in a vertical and horizontal pattern. It is comprised so that it may have the same single spatial frequency.

  It is also possible to use other pattern signals 32F and 32G as shown in FIGS. 3A and 3B in place of the pattern signals 32A to 32E. In addition, a single spatial frequency can be used. It is possible to use various pattern signals configured to have Further, the setting of the pitch of the pixel block in the pattern signals 32A to 32G and the pattern signal 32H described later is appropriately determined according to the expected image quality of the reproduced image. If the reproduced image quality is expected to be relatively good, a fine pitch (for example, every several pixels) can be used, but if the reproduced image quality is expected to be poor, a coarser pitch ( For example, the pitch is set at intervals of several tens of pixels.

  Next, object light carrying image data (hereinafter referred to as “patterned image data”) on which pattern signals 32A to 32E are superimposed is optically Fourier transformed by a Fourier transform lens (FTL) 42. The hologram recording medium 43 is irradiated to a predetermined position, and the same position on the hologram recording medium 43 is irradiated with the recording reference light from another direction. Due to the optical interference between the object beam and the recording reference beam, an interference pattern carrying information of a Fourier transform image (Fraunhofer diffraction image) of the image data with pattern is formed, and the interference pattern is recorded on the hologram recording medium 43. The

  At the time of reproduction, the object light is shielded by the shutter 47, and only the reproduction reference light is irradiated to the recording medium. The irradiated reference light for reproduction is diffracted by the interference fringes recorded in the hologram recording medium 43, and this diffracted light is optically inverse Fourier transformed by the Fourier transform lens 44, so that the CCD, CMOS sensor, etc. A reproduced image is formed on the two-dimensional image sensor 45. The reproduced image data can be reproduced by capturing the reproduced image with a two-dimensional image sensor and restoring the bit data.

  By the way, in the general conventional method, the reproduction reference light having the same wavefront shape as the recording reference light is used. However, when a photosensitive resin such as a photopolymer is used as the hologram recording medium 43, The following problems arise.

  That is, the hologram recording medium 43 may shrink in volume upon curing due to a photopolymerization reaction during recording. When this volume shrinkage is isotropic, the reproduction position changes as a whole to several degrees to several degrees, but the image data itself can be reproduced. However, this volume shrinkage is usually not isotropic, in which case the SNR of the reproduced image data is significantly reduced, and in the extreme case, part or all of the recorded image data cannot be reproduced. Sometimes.

  Therefore, in the method of the present embodiment, the image data of the reproduced image is subjected to frequency analysis and image evaluation is performed, and the wavefront (specifically, the shape and traveling direction of the wavefront) of the reproduction reference light is controlled based on the image evaluation. In the following, two methods effective in image evaluation and wavefront control will be described.

<First method>
In this first method, first, frequency analysis is performed on image data corresponding to a predetermined region of a reproduced image. The predetermined area may be the entire area of the image, but a part of the area where the pattern signals 32A to 32E are superimposed, for example, an area at the center of the image including the pattern signal 32A, or a pattern signal 32B. It is also possible to select an area in the upper left corner of the image including the image as a predetermined area. In addition, it is preferable to use a fast Fourier transform (FFT) for frequency analysis.

Next, the amplitude value A (f ₂ ) of the second harmonic component of the pattern signal (a part or all of the pattern signals 32A to 32E, the same applies hereinafter) obtained by this frequency analysis (see FIG. 6 described later). The image evaluation of the predetermined area is performed based on the above, and the wavefront of the reproduction reference light is changed so that the amplitude value A (f ₂ ) is minimized (the reciprocal 1 / A (f ₂ ) is maximized). Feedback control. Note that the wavefront control of the reproduction reference light is performed using the wavefront controller 46.

<Second method>
This second method is the same as the first method in terms of frequency analysis. The difference is that the amplitude value A (f ₁ ) of the fundamental wave component of the pattern signal with respect to the amplitude value A (f ₂ ) of the second harmonic component of the pattern signal obtained by frequency analysis (see FIG. 6 described later). The image evaluation of the predetermined area is performed based on the ratio value A (f ₁ ) / A (f ₂ ), and the wavefront of the reproduction reference light is feedback-controlled so that the value of this ratio is minimized.

  FIG. 4 shows an example of a reproduced image 51 (FIG. 4A) before applying the method of the present embodiment and a reproduced image 52 (FIG. 4B) after application. The reproduced image 51 is dark as a whole due to a decrease in SNR due to the volume shrinkage of the hologram recording medium, whereas the reproduced image 52 is improved because the SNR is improved and the entire image becomes bright and the bits are easily discriminated. ing. In this case, when applied to the calculation formula for SNR calculation, there was an SNR improvement effect of 1.2 dB.

  The image evaluation was performed based on the image data in the evaluation area 53 having a long and narrow rectangular shape shown on the reproduced image 51. In this evaluation area 53, as shown in FIG. 5, a pattern signal 32H in which pixel blocks of a predetermined unit are repeated at a predetermined pitch and arranged in a line in the horizontal direction is superimposed.

FIG. 6 shows an example of a frequency analysis result obtained by performing fast Fourier transform on the image data in the evaluation region 53. As shown in FIG. 6, among the repetitive frequency components, the fundamental wave component of the pattern signal 32H becomes the maximum, and the second harmonic component appears at a frequency twice as high. The signal amplitude value of the fundamental wave component at this time is A (f ₁ ), and the signal amplitude value of the second harmonic component is A (f ₂ ).

FIG. 7 shows the relationship between the image evaluation values (the above-mentioned 1 / A (f ₁ ) and A (f ₁ ) / A (f ₂ )) changed by the feedback control of the method of this embodiment and the SNR of the reproduced image. Correlation is shown (normalized so that each maximum value is approximately 1). As shown in FIG. 7, 1 / A (f ₁ ) and A (f ₁ ) / A (f ₂ ), which are indices for image evaluation in the method of the present embodiment, are correlated with the SNR of the reproduced image, and the SNR It can be seen that the wavefront control of the reference light for reproduction can be effectively performed by using these indexes for image evaluation instead of.

  Next, an embodiment of a hologram reproduction wavefront control apparatus according to the present invention will be described. FIG. 8 is a schematic configuration diagram of a hologram reproduction wavefront control apparatus according to an embodiment of the present invention.

  The hologram reproduction wavefront control device shown in FIG. 8 is configured as a hologram recording / reproduction device capable of recording and reproducing image data to / from the hologram recording medium 11. Hereinafter, the device will be described as an information recording function and an information reproduction function. Separately described.

<Information recording function>
As shown in FIG. 8, the coherent laser beam emitted from the laser light source 1 is expanded in beam diameter by a beam expander composed of a diverging lens 2 and a collimating lens 3, is transmitted through a half-wave plate 4, and is reflected by a reflecting mirror 5. After being reflected, the light beam is split into two light beams by the polarization beam splitter 6 and functions as signal light (object light: actually signal light by the spatial light modulator 8) and reference light, respectively.

  The reference light is reflected by the wavefront controller 26 and applied to a predetermined area on the hologram recording medium 11 through a shutter 23 for appropriately passing / blocking the light flux. On the other hand, the signal light is applied to the spatial light modulator 8 via the polarization beam splitter 7.

  As the spatial light modulation element 8, a light valve such as a liquid crystal display panel or DMD (Digital Micromirror Device) is used. Each pixel on the spatial light modulation element 8 passes light (reflects in the example of FIG. 8) / blocks light according to binary digital information on image data (page data), thereby spatially modulating light. . Signal light carrying page data information is generated by this modulation processing.

  Further, the apparatus according to the present embodiment is configured to superimpose a pattern signal having a predetermined spatial frequency (see the above-described pattern signals 32A to 32H) on a predetermined area of page data when generating signal light.

  The signal light emitted from the spatial light modulator 8 has a polarization direction that is different from the incident state, is reflected by the polarization beam splitter 7, passes through the shutter 9, and is optically Fourier transformed by the Fourier transform lens 10. Then, the hologram recording medium 11 is irradiated. At this time, since the reference light is simultaneously irradiated from another angle to the region irradiated with the signal light in the hologram recording medium 11, interference fringes are generated in the volume inside the recording medium 11, and this fringe distribution is represented by a refractive index. Hologram recording is performed by transferring to a recording area of the recording medium 11 in the form of distribution or the like. Lenses 22 and 24 are disposed before and after the shutter 23 in the optical path of the reference light.

  Also, different page data is displayed on the spatial light modulator 8 and the incident angle of the reference light on the hologram recording medium 11 is changed little by little, so that different page data are multiplexed and recorded in the same area in the recording medium 11. It becomes possible to store information with higher density.

<Information playback function>
Next, at the time of information reproduction, the reference light that has passed through the shutter 23 is irradiated to a predetermined region of the hologram recording medium 11 at the same incident angle and position as at the time of information recording. The shutter 9 is in a closed state.

  That is, when reproducing the desired page data, first, the reference light incidence conditions are set so as to be the same as those at the time of recording the information on which the desired page data is recorded, and the reference light is incident on the hologram recording medium 11. As a result, reproduction light (diffracted light) carrying desired page data information is output from the hologram recording medium 11, and this reproduction light enters the CCD 13 via the Fourier transform lens 12 and is imaged.

  The reproduced image data imaged by the CCD 13 is input to the measurement control device 25 (which constitutes wavefront control means together with the wavefront controller 26 in the present embodiment device), and the measurement control device 25 is based on the frequency analysis of the reproduced image data. Image evaluation is performed, and the wavefront controller 26 is driven based on the image evaluation to control the wavefront shape and irradiation direction of the reproduction reference light. As the image evaluation and wavefront control methods, the first method or the second method described above is applied.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to the above-mentioned embodiment, It is possible to employ | adopt a various aspect.

  For example, the above-described embodiment has a function of recording image data on a hologram recording medium and a function of superimposing a pattern signal of a predetermined spatial frequency on the image data, but does not have such a recording function. It is also possible to adopt an aspect.

  Further, the arrangement of members constituting the optical system of the apparatus can be changed as appropriate.

The figure which shows the example of arrangement of a pattern signal Conceptual diagram of hologram reproduction wavefront control method according to one embodiment The figure which shows other pattern signals (A) and (B) Diagram showing other pattern signals The figure which shows the reproduction | regeneration image (A) before application of this invention, and the reproduction | regeneration image (B) after application Graph showing frequency analysis result by fast Fourier transform A graph showing a correlation between an image evaluation value (1 / A (f ₁ ) and A (f ₁ ) / A (f ₂ )) by feedback control and an SNR of a reproduced image 1 is a schematic configuration diagram of a hologram reproduction wavefront control device according to an embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser light source 2 Divergent lens 3 Collimating lens 4 Half wave plate 5 Mirror 6,7 Polarizing beam splitter 8,41 Spatial light modulator 9,23,47 Shutter 10,12,22,24,42,44 Lens 11,43 Hologram Recording medium 13 CCD
25 Measurement control device 26, 46 Wavefront controller 31 Image data 32A to 32H Pattern signal 45 Two-dimensional imaging device 51, 52 Reproduced image 53 Evaluation region

Claims (5)

  The pattern obtained by frequency-analyzing a predetermined region of a reproduced image reproduced by irradiating a reproduction reference light onto a hologram recording medium on which image data on which a pattern signal of a predetermined spatial frequency is superimposed is recorded, and obtained by the frequency analysis A hologram reproduction wavefront control method characterized by performing image evaluation based on an amplitude value of a second harmonic component of a signal and controlling a wavefront of the reproduction reference light so that the amplitude value is minimized. Frequency analysis means for performing frequency analysis on a predetermined region of a reproduced image reproduced by irradiating a reproduction reference light onto a hologram recording medium on which image data on which a pattern signal of a predetermined spatial frequency is superimposed is recorded;
Wavefront control means for performing image evaluation based on an amplitude value of a second harmonic component of the pattern signal obtained by the frequency analysis, and controlling a wavefront of the reproduction reference light so that the amplitude value is minimized; A hologram reproduction wavefront control device comprising:   The pattern obtained by frequency-analyzing a predetermined region of a reproduced image reproduced by irradiating a reproduction reference light onto a hologram recording medium on which image data on which a pattern signal of a predetermined spatial frequency is superimposed is recorded, and obtained by the frequency analysis An image is evaluated based on the ratio of the amplitude value of the fundamental wave component of the pattern signal to the amplitude value of the second harmonic component of the signal, and the reference light for reproduction is adjusted so that the value of the ratio becomes maximum. A hologram reproduction wavefront control method comprising controlling a wavefront. Frequency analysis means for performing frequency analysis on a predetermined region of a reproduced image reproduced by irradiating a reproduction reference light onto a hologram recording medium on which image data on which a pattern signal of a predetermined spatial frequency is superimposed is recorded;
Image evaluation is performed based on the ratio of the amplitude value of the fundamental wave component of the pattern signal to the amplitude value of the second harmonic component of the pattern signal obtained by the frequency analysis, and the value of the ratio is maximized And a wavefront control means for controlling the wavefront of the reference light for reproduction as described above. 5. The hologram reproduction wavefront control apparatus according to claim 2, further comprising recording means for superimposing the pattern signal on the image data and recording it on the hologram recording medium.