JP2005352097A - Hologram recording method, hologram reproducing method, hologram recording apparatus, hologram reproducing apparatus and hologram recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hologram recording method and an apparatus for recording and reproducing a hologram while suppressing decrease in the SN ratio of a necessary frequency component having lower intensity than that of a frequency component having the maximum intensity in Fourier transformed signal light, without saturating the frequency component having the maximum intensity in the Fourier transformed signal light within the dynamic range of an optical recording material. <P>SOLUTION: The hologram recording method is carried out by controlling reference light to have the intensity distribution which reduces the intensity of a frequency component having the maximum intensity in the Fourier transformed signal light, and allowing the reference light to interfere with Fourier transformed signal light so as to decrease the intensity of the frequency component having the maximum intensity in the Fourier transformed signal light. The hologram recording apparatus is equipped with a light intensity converting means which converts the intensity distribution of the reference light into intensity distribution which decreases the intensity of the frequency component having the maximum intensity in the Fourier transformed signal light. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a hologram recording method, a hologram reproducing method, a hologram recording apparatus, a hologram reproducing apparatus, and a hologram recording medium.

  2. Description of the Related Art In recent years, holographic optical memory has attracted attention as a means for recording and reproducing various kinds of information such as images and sounds at high speed in computers and the like. The hologram optical memory has a hologram recording medium formed of an optical recording material for recording and reproducing information, and can record and reproduce information three-dimensionally on the hologram recording medium.

  That is, a recording medium such as an optical disk sequentially records information only on a specific surface in the recording medium, and sequentially reproduces an array of two-dimensional data recorded on the specific surface. In, by irradiating the hologram recording medium with light, a multiple data structure in which a plurality of two-dimensional data arrays are stacked in the hologram recording medium can be formed. As a result, a large amount of information can be recorded in the three-dimensional direction of the hologram recording medium, and a large-capacity hologram optical memory can be realized. For example, since information can be recorded not only on a specific surface in the hologram recording medium in the hologram optical memory but also in the thickness direction of the hologram recording medium, the thickness of the hologram recording medium can be increased by increasing the thickness of the hologram optical recording medium. The capacity can be increased.

  In such a hologram optical memory, information recorded at a three-dimensional position of the hologram recording medium can be accessed by irradiating light to the three-dimensional position in the hologram recording medium. For this reason, light can be irradiated to a specific surface in the hologram recording medium, and information can be recorded on the specific surface at a time as a two-dimensional data array. On the other hand, by irradiating light to each two-dimensional data array recorded three-dimensionally in a hologram recording medium, information consisting of each two-dimensional data array is read and reproduced at a time. be able to. As a result, if a hologram optical memory is used, information can be recorded / reproduced on / from the hologram recording medium at high speed.

  Thus, according to the hologram optical memory, a large amount of information can be recorded and reproduced at high speed.

  Information recording and reproduction with respect to the hologram recording medium can be performed using a well-known hologram recording and reproduction method. That is, splitting light generated from a light source such as a laser, irradiating a hologram recording medium made of an optical recording material with one light beam as reference light, and irradiating an object such as a spatial light modulator with the other light beam To obtain signal light having information and irradiate the hologram recording medium with the signal light. Then, the reference light and the signal light are caused to interfere with each other in the hologram recording medium, and the interference fringes of the reference light and the signal light are recorded on the hologram recording medium. In this way, the information (intensity and phase) of the signal light is recorded on the hologram recording medium, and the information on the signal light is reproduced by irradiating the hologram recording medium on which the information of the signal light is recorded with the reference light. Can do.

  For example, digital hologram recording will be specifically described as an example of a hologram recording method. FIG. 1 is a diagram for explaining a conventional digital hologram recording apparatus and recording method. As shown in FIG. 1, the conventional digital hologram recording apparatus includes a laser light source 101, a half mirror 102, mirrors 103 and 104, a beam expander 105, a spatial light modulator 106, and a lens system 107. When recording information on the hologram recording medium 110 using a conventional digital hologram recording apparatus, first, the hologram recording medium 110 on which no hologram is recorded is installed in the digital hologram recording apparatus. Next, one light beam is generated from the laser light source 101, and this light beam is divided into two light beams by the half mirror 102. One of the two light beams is reflected by the half mirror 102 toward the mirror 103 in a direction perpendicular to the light beam incident from the laser light source. The light beam reflected by the mirror 103 is further reflected by the mirror 104 and applied to the hologram recording medium 110 as reference light. The other light beam split by the half mirror 102 passes through the half mirror 102 and enters the beam expander 105. The diameter of the light beam incident on the beam expander 105 is expanded by the beam expander 105, and the light beam having the expanded beam diameter is applied to the spatial light modulator 106. The light beam applied to the spatial light modulator 106 is modulated by the spatial light modulator 106 and enters the lens system 107 in parallel as signal light. In the digital hologram recording apparatus, the spatial light modulator 106 modulates the light beam to form signal light corresponding to a two-dimensional data array in which “1” and “0” are two-dimensionally arranged. Therefore, a light beam having information of a two-dimensional data array enters the lens system 107 as parallel light. The lens system 107 is an optical system in which a light beam having information of a two-dimensional data array is diffracted by Franhofer diffraction. When the signal light having the information of the two-dimensional data array modulated by the spatial light modulation element 106 passes through the lens system 107, the signal light is Fourier transformed, and the hologram recording medium 110 is irradiated with the Fourier transformed signal light. Is done. Then, the reference light applied to the hologram recording medium 110 and the Fourier-transformed signal light interfere with each other, and interference fringes between the reference light and the Fourier-transformed signal light are recorded on the hologram recording medium 110.

  FIG. 2 is a diagram showing an intensity distribution of reference light irradiated on a hologram recording medium in a conventional digital hologram recording apparatus. In FIG. 2, the horizontal axis indicates the spatial position expressed in arbitrary units, and the vertical axis indicates the intensity of the light beam expressed in arbitrary units. In general, the reference light has the highest intensity at the center of the beam with respect to a spatial position, and has an intensity distribution that gradually decreases from the center of the beam toward the periphery of the beam.

  FIG. 3 is a diagram illustrating an example of a two-dimensional data array formed by the spatial light modulation elements. As shown in FIG. 3, the spatial light modulator 106 forms various two-dimensional data arrays composed of “1” data represented by white squares and “0” data represented by black squares. The light incident on the spatial light modulator 106 is modulated in accordance with such a two-dimensional data array formed by the spatial light modulator 106. Then, light corresponding to the two-dimensional data array formed by the spatial light modulator 106 enters the lens system 107 as signal light, and the two-dimensional data array formed by the spatial light modulator 106 is converted into the lens system. A Fourier transform is performed by 107.

  FIG. 4 is a diagram showing the intensity distribution of the signal light Fourier-transformed by the lens system at the recording position of the hologram recording medium. In FIG. 4, the horizontal axis indicates a spatial position expressed in arbitrary units, and the vertical axis indicates the intensity of the light beam expressed in arbitrary units. In general, the Fourier-transformed signal light has a peak having the highest intensity at the center of the beam with respect to a spatial position, and has a plurality of peaks symmetrically around the peak having the maximum intensity. . As the peaks of the Fourier transformed signal light move away from the center of the beam, the intensity of the peaks decreases. Here, the peak having the highest intensity at the center of the beam is the highest at the center of the beam due to a low-frequency component (called a DC component) in the pattern of the two-dimensional data array formed by the spatial light modulator. The plurality of peaks appearing around the peak having intensity are caused by high frequency components in the pattern of the two-dimensional data array. The peak farther from the center of the beam corresponds to the higher frequency component of the two-dimensional data array. That is, in the signal light subjected to Fourier transform, generally, the peak intensity of the DC component corresponding to the low frequency component in the two-dimensional data array pattern is higher than the peak intensity of the high frequency component in the two-dimensional data array pattern. large. Next, the Fourier-transformed signal light having an intensity distribution composed of a plurality of peaks as shown in FIG. 4 includes the reference light and hologram in FIG. 2 having an intensity distribution that gradually decreases from the center of the beam toward the periphery. Interference occurs at the recording position of the recording medium 110.

  FIG. 5 is a diagram showing an intensity distribution of interference fringes formed by interference of reference light and signal light at a recording position of a hologram recording medium in a conventional digital hologram recording apparatus. In FIG. 5, the horizontal axis indicates the spatial position expressed in arbitrary units, and the vertical axis indicates the intensity of the light beam expressed in arbitrary units. The interference fringe intensity distribution formed by the interference of the reference light and the signal light at the recording position of the hologram recording medium has an intensity distribution in which the Fourier transformed signal light has a plurality of peaks, and the reference light is Since it has an intensity distribution that gradually decreases from the center of the beam toward the periphery, it has a very high intensity distribution near the center of the beam, and a relative to the periphery of the peak that has a very high intensity distribution near the center of the beam. It has a plurality of peaks with a very low intensity distribution. Here, in the intensity distribution of interference fringes, the difference between the peak intensity near the center of the beam and the peak intensity near the periphery of the beam is the peak intensity near the center of the beam in the Fourier transformed signal light and the periphery of the beam. It becomes larger than the difference with the intensity of the peak in the vicinity.

  As described above, when interference fringes between a Fourier-transformed signal light having an intensity distribution composed of a plurality of peaks and a reference light having an intensity distribution gradually decreasing from the center of the beam toward the periphery are recorded, a two-dimensional The light intensity corresponding to the low frequency component in the data array pattern is very large, and the light intensity corresponding to the high frequency component in the two-dimensional data array pattern is relatively very small. Here, when the interference fringes between the reference light and the signal light are recorded on the hologram recording medium, it is sufficient that all the low frequency components and necessary high frequency components in the two-dimensional data array pattern can be recorded. The optical recording material forming the hologram recording medium has a limited dynamic range with respect to the intensity of light to be recorded. As a result, at present, it is impossible to record all of the low frequency components and necessary high frequency components in the two-dimensional data array pattern on the hologram recording medium. That is, if an attempt is made to record a high frequency component in a two-dimensional data array pattern on a hologram recording medium with a sufficient SN ratio, the intensity of light corresponding to the low frequency component in the two-dimensional data array pattern is excessively exposed. As a result, it becomes saturated and information of light corresponding to the low frequency component cannot be recorded. Conversely, when recording on a hologram recording medium without saturating light corresponding to the low frequency component in the two-dimensional data array pattern, the dynamic range of the optical recording material of the hologram recording medium is almost low. Will be surrounded by light equivalent to. As a result, the light intensity corresponding to the high-frequency component in the two-dimensional data array pattern is weak, and the light intensity distribution corresponding to the high-frequency component is not sufficiently recorded, and the SN related to the light intensity corresponding to the high-frequency component is not recorded. The ratio will drop. Therefore, when the hologram recording medium is irradiated with the reference light and the information of the signal light is reproduced, the SN ratio of the high-frequency component in the two-dimensional data array pattern also decreases, and the high-frequency component in the two-dimensional data array pattern The reproducibility of information will be reduced.

  Therefore, in the conventional digital hologram recording apparatus as shown in FIG. 1, the distance between the lens system 107 and the hologram recording medium 110 is slightly increased or decreased to focus the Fourier-transformed signal light on the hologram recording medium. There is a method of slightly defocusing from the recording position at 110. When the focal point of the Fourier-transformed signal light is defocused from the recording position on the hologram recording medium 110, the intensity of the signal light at the recording position on the hologram recording medium 110 can be reduced, and the pattern of the two-dimensional data array By reducing the light intensity corresponding to the low frequency component, it is possible to prevent the light intensity corresponding to the low frequency component from being saturated within the dynamic range of the hologram recording medium 110. However, since the focus of the Fourier-transformed signal light is defocused from the recording position on the hologram recording medium 110, the intensity distribution of the Fourier-transformed signal light is widened, and the information on the signal light on the hologram recording medium 110 is spread. The recording density is lowered. Further, in the intensity distribution of the signal light subjected to Fourier transform, the S / N ratio relating to both the light intensity corresponding to the low frequency component and the light intensity corresponding to the high frequency component in the two-dimensional data array pattern also decreases.

  Further, in Patent Document 1, the light after the Fourier transform of the signal light of the binary two-dimensional digital data image by the lens and the reference light are formed on a part of the light shielding body arranged in front of the optical recording medium. There is provided an optical recording method of recording a predetermined shape and hologram in an optical recording medium by transmitting the light transmitting portion having a predetermined shape and size corresponding to the Fourier transform image of the signal light and irradiating the optical recording medium. It is disclosed. According to this method, since the Fourier-transformed signal light is passed through the light-shielding body having the light transmitting portion, the intensity of the low-frequency component in the Fourier-transformed signal light can be reduced, and the low-frequency in the Fourier-transformed signal light can be reduced. Both the frequency component and the intensity of the high frequency component can be included in the dynamic range of the optical recording material. However, since not only the intensity of the low frequency component in the Fourier transformed signal light but also the intensity of the high frequency component is reduced equally, the ratio between the intensity of the low frequency component and the intensity of the high frequency component in the Fourier transformed signal light is: The signal-to-noise ratio is constant regardless of the presence or absence of the light-shielding body, and the SN ratio related to the intensity of the high-frequency component remains insufficient. Therefore, even if the optical recording method disclosed in Patent Document 1 is used, it is difficult to improve the reproducibility of high-frequency component information in a two-dimensional data array pattern.

  In particular, the lens system 107 that transmits signal light and the hologram recording medium 110 that records signal light have a frequency response characteristic to the frequency in the two-dimensional data array pattern, and the two-dimensional data array pattern. Since the frequency at is not infinite, the intensity distribution of the light recorded on the hologram recording medium is affected by the frequency component in the pattern of the two-dimensional data array. For this reason, it is possible to record the intensity distribution of the signal light subjected to Fourier transform over as many frequency components as possible from low frequency components to high frequency components in the two-dimensional data array pattern. Is desirable for accurately recording and reproducing. However, as in the method disclosed in Patent Document 1, when the difference between the intensity of the low frequency component and the high frequency component in the Fourier transformed signal light is large, the low frequency component in the Fourier transformed signal light is It is difficult to increase the S / N ratio related to the high-frequency component in the signal light that is included in the dynamic range of the optical recording material and Fourier-transformed, and it may be difficult to accurately record and reproduce the two-dimensional data array pattern.

Therefore, the hologram is recorded by suppressing the decrease in the SN ratio of the necessary high frequency component in the Fourier transformed signal light without saturating the low frequency component in the Fourier transformed signal light within the dynamic range of the optical recording material. Hologram recording method and hologram recording apparatus capable of performing the above processing without saturating the low-frequency component in the Fourier-transformed signal light within the dynamic range of the optical recording material, and the necessary high-frequency component in the Fourier-transformed signal light. A hologram reproducing method and hologram reproducing apparatus capable of reproducing a hologram while suppressing a decrease in the S / N ratio, and a low-frequency component in Fourier-transformed signal light without being saturated within the dynamic range of the optical recording material, SN of necessary high frequency component in signal light subjected to Fourier transform Hologram recording medium on which a hologram has been recorded by suppressing a decrease in is required.
JP 2000-66565 A

  The present invention requires that the maximum intensity frequency component in the Fourier-transformed signal light is less than the maximum intensity frequency component in the Fourier-transformed signal light without saturating the frequency component in the dynamic range of the optical recording material. An object of the present invention is to provide a hologram recording method and a hologram recording apparatus capable of recording a hologram while suppressing a decrease in the SN ratio of a frequency component.

  Further, the present invention provides an intensity smaller than the maximum intensity frequency component in the Fourier transformed signal light without saturating the maximum intensity frequency component in the Fourier transformed signal light within the dynamic range of the optical recording material. An object of the present invention is to provide a hologram reproducing method and a hologram reproducing apparatus capable of reproducing a hologram while suppressing a decrease in the SN ratio of necessary frequency components.

  Furthermore, the present invention provides an intensity smaller than the maximum intensity frequency component in the Fourier transformed signal light without saturating the maximum intensity frequency component in the Fourier transformed signal light within the dynamic range of the optical recording material. An object of the present invention is to provide a hologram recording medium on which a hologram is recorded while suppressing a decrease in the SN ratio of necessary frequency components.

  The invention according to claim 1 is a hologram recording method for recording a hologram on the hologram recording medium by irradiating the hologram recording medium with reference light and signal light Fourier-transformed through a Fourier transform optical system, wherein the reference light includes: It has an intensity distribution that reduces the intensity of the frequency component having the maximum intensity in the Fourier-transformed signal light, and interferes with the reference light and the Fourier-transformed signal light to maximize the maximum intensity in the Fourier-transformed signal light. The intensity of the frequency component is reduced.

  According to a second aspect of the present invention, in the hologram recording method according to the first aspect, the frequency component having the maximum intensity in the Fourier-transformed signal light is a frequency component having the lowest frequency in the Fourier-transformed signal light. It is characterized by that.

  According to a third aspect of the present invention, in the hologram reproducing method, the reference light having an intensity distribution for reducing the intensity of the frequency component having the maximum intensity in the signal light Fourier-transformed through the Fourier transform optical system and the Fourier-transformed signal light. The hologram is reproduced by irradiating the hologram recording medium on which the hologram is recorded by irradiating with the reference light.

  According to a fourth aspect of the present invention, in the hologram recording apparatus for recording a hologram on the hologram recording medium by irradiating the hologram recording medium with the reference light and signal light Fourier-transformed through a Fourier transform optical system, the intensity of the reference light It has a light intensity conversion means for converting the distribution into an intensity distribution that reduces the intensity of the frequency component having the maximum intensity in the Fourier-transformed signal light.

  A fifth aspect of the present invention is the hologram recording apparatus according to the fourth aspect, wherein the reference light is a light beam, and the diameter of the light beam incident on the light intensity converting means is enlarged. It further has a beam expander.

  A sixth aspect of the present invention is the hologram recording apparatus according to the fourth or fifth aspect, wherein the light intensity converting means includes a transparent substrate and a metal film provided on the transparent substrate. To do.

  A seventh aspect of the present invention is the hologram recording apparatus according to the fourth or fifth aspect, wherein the light intensity conversion means includes a transparent substrate and a dielectric multilayer film provided on the transparent substrate. And

  The invention according to claim 8 is the hologram recording apparatus according to any one of claims 4 to 7, further comprising a laser light source provided with an external resonator.

  According to a ninth aspect of the present invention, in the hologram recording apparatus according to any one of the fourth to seventh aspects, a semiconductor laser light source is provided.

  The invention according to claim 10 irradiates the signal light Fourier-transformed through the Fourier transform optical system and the reference light having an intensity distribution for reducing the intensity of the maximum intensity frequency component in the Fourier-transformed signal light. In the hologram reproducing apparatus for reproducing the hologram from the hologram recording medium on which the hologram is recorded, reference light irradiating means for irradiating the hologram recording medium with the reference light is provided.

  According to an eleventh aspect of the present invention, in the hologram recording medium, the reference light having an intensity distribution for reducing the intensity of the frequency component having the maximum intensity in the signal light Fourier-transformed through the Fourier transform optical system and the Fourier-transformed signal light. The hologram is recorded by irradiating.

  According to the present invention, an intensity smaller than the maximum intensity frequency component in the Fourier transformed signal light is obtained without saturating the maximum intensity frequency component in the Fourier transformed signal light within the dynamic range of the optical recording material. It is possible to provide a hologram recording method and a hologram recording apparatus capable of recording a hologram while suppressing a decrease in the SN ratio of necessary frequency components.

  In addition, according to the present invention, the maximum intensity frequency component in the Fourier transformed signal light is smaller than the maximum intensity frequency component in the Fourier transformed signal light without being saturated in the dynamic range of the optical recording material. It is possible to provide a hologram reproducing method and a hologram reproducing apparatus capable of reproducing a hologram while suppressing a decrease in the SN ratio of a necessary frequency component having intensity.

  Further, according to the present invention, the maximum intensity frequency component in the Fourier-transformed signal light is smaller than the maximum intensity frequency component in the Fourier-transformed signal light without saturating in the dynamic range of the optical recording material. It is possible to provide a hologram recording medium on which a hologram is recorded while suppressing a decrease in the SN ratio of a necessary frequency component having strength.

  Next, embodiments of the present invention will be described with reference to the drawings.

  First, the hologram recording method according to the present invention will be described.

  A hologram recording method according to the present invention is a hologram recording method for recording a hologram on a hologram recording medium by irradiating the hologram recording medium with reference light and signal light Fourier-transformed through a Fourier transform optical system. The intensity of the frequency component of the maximum intensity in the converted signal light has an intensity distribution that reduces the intensity of the frequency component of the maximum intensity, and the intensity of the frequency component of the maximum intensity in the signal light subjected to Fourier transform by interfering with the reference light and the signal light subjected to Fourier transform Reduce.

  In the hologram recording method according to the present invention, the light beam generated from the light source is divided into two light beams, and one of the two light beams is used as the reference light. The other of the two light beams irradiates an object such as a spatial light modulator and a liquid crystal device, and the light reflected from or transmitted through the object is parallel to the Fourier transform optical system as signal light having information on the object. Incident as light. When the hologram is a digital hologram, a spatial light modulation element such as a DMD (digital micromirror device) or a liquid crystal device is used as the object, and the spatial light modulation element is driven, A two-dimensional data array corresponding to the binary values “1” and “0” is formed in the spatial light modulator. Then, the light passing through the spatial light modulation element is modulated by the spatial light modulation element to form signal light having binary two-dimensional data. When a spatial light modulation element is used as an object, the diameter of the light beam incident on the spatial light modulation element is smaller than the size of the light modulation part of the spatial light modulation element (mirror surface of DMD or liquid crystal screen of liquid crystal device). In this case, the diameter of the light beam incident on the spatial light modulation element may be enlarged by a beam expander or the like.

  Next, the signal light incident parallel to the Fourier transform optical system is Fourier transformed through the Fourier transform optical system. The Fourier transform optical system is a lens optical system in which incident signal light is diffracted by Franhofer diffraction. In the digital hologram, the two-dimensional data array corresponding to the binary values “1” and “0” is Fourier-transformed, and the two-dimensional data array corresponding to the binary values “1” and “0” is converted. It is converted into signal light having an intensity distribution corresponding to the frequency component included in the pattern.

  Both the reference light and the Fourier-transformed signal light are applied to the hologram recording medium including the optical recording material. Note that the light beam generated from the light source may be used directly as the reference light, or the light beam generated from the light source may be collected by an appropriate lens system and irradiated onto the hologram recording medium. In addition, since the Fourier transform optical system is a lens system that collects parallel light, the signal light that has undergone Fourier transform is also collected at the focal point of the Fourier transform optical system. The recording position of the hologram recording medium is arranged at the focal position of the Fourier transform optical system. In particular, when a spatial light modulation element is used as an object, the range of the two-dimensional data array formed on the spatial light modulation element is larger than the optical recording range of the hologram recording medium. It is necessary to reduce the image of the two-dimensional data array formed on the spatial light modulator. That is, in order to improve the recording density of information in the hologram recording medium, the signal light is condensed at the recording position of the hologram recording medium by the Fourier transform optical system. Then, the reference light and the Fourier-transformed signal light interfere with each other in the hologram recording medium, and interference fringes of the reference light and the Fourier-transformed signal light are recorded on the hologram recording medium.

  Here, the intensity distribution of the signal light subjected to the Fourier transform with respect to the spatial position varies depending on the information of the signal light coming from the object, but usually has a maximum intensity frequency component and a plurality of lower intensity frequency components.

  In the hologram recording method according to the present invention, the reference light has an intensity distribution that reduces the intensity of the frequency component having the maximum intensity in the Fourier-transformed signal light. Thereby, the Fourier-transformed signal light having the maximum intensity of the frequency component and the reference light having an intensity distribution that reduces the intensity of the maximum-intensity frequency component in the Fourier-transformed signal light are caused to interfere with each other. Thus, the intensity of the maximum intensity frequency component in the Fourier-transformed signal light is reduced by the signal light. That is, the optical recording material in which the intensity of the frequency component with the maximum intensity in the Fourier-transformed signal light and the intensity of the frequency component with the maximum intensity in the interference fringe between the reference light and the Fourier-transformed signal light are included in the hologram recording medium It can be reduced so as not to saturate within the dynamic range.

  In addition, since the reference light does not have an intensity distribution that reduces the intensity of a plurality of frequency components whose intensity is lower than the maximum intensity frequency component in the Fourier-transformed signal light, the maximum intensity in the Fourier-transformed signal light The intensity of the frequency component having an intensity smaller than that of the frequency component is not reduced even if the Fourier-transformed signal light and the reference light interfere with each other. Thereby, it is possible to adjust the ratio between the intensity of the frequency component having the maximum intensity and the intensity of the frequency component having a smaller intensity than the frequency component having the maximum intensity in the interference fringes between the reference light and the signal light subjected to Fourier transform. Therefore, the S / N ratio of the frequency component whose intensity is lower than the maximum intensity frequency component in the interference fringe between the reference light and the Fourier-transformed signal light is not reduced. Thus, with respect to hologram recording, the dynamic range of the optical recording material of the hologram recording medium can be used effectively.

  In order to generate reference light having an intensity distribution that reduces the intensity of the frequency component having the maximum intensity in the Fourier-transformed signal light from the light beam generated from the light source, a part of the light beam generated from the light source is shielded. A mask or the like can be used. More specifically, the mask for shielding a part of the light beam generated from the light source is Fourier-transformed in the light beam generated from the light source so as to reduce the intensity of the maximum intensity frequency component in the Fourier-transformed signal light. It has a function of reducing the intensity of the portion corresponding to the maximum intensity frequency component in the signal light. Such a mask may be inserted into the optical path of the reference light.

  According to the hologram recording method of the present invention, the reference light has an intensity distribution that reduces the intensity of the frequency component having the maximum intensity in the Fourier-transformed signal light, and causes the reference light and the Fourier-transformed signal light to interfere with each other. By reducing the intensity of the maximum intensity frequency component in the Fourier transformed signal light, the Fourier transform is achieved without saturating the maximum intensity frequency component in the Fourier transformed signal light within the dynamic range of the optical recording material. It is possible to provide a hologram recording method capable of recording a hologram while suppressing a decrease in the SN ratio of a necessary frequency component having an intensity smaller than the maximum intensity frequency component in the signal light.

  In addition, according to the hologram recording method of the present invention, since the image point of the Fourier-transformed signal light does not need to be defocused from the recording position of the hologram recording medium, the interference fringes of the reference light and the Fourier-transformed signal light However, the recording density of the interference fringes of the reference light and the Fourier transformed signal light is not reduced.

  Furthermore, in the hologram recording method according to the present invention, a part of the Fourier-transformed signal light is shielded to reduce the intensity of the maximum intensity frequency component in the Fourier-transformed signal light. When the converted signal light interferes, a part of the light beam generated from the light source is shielded so that the reference light has an intensity distribution that reduces the intensity of the maximum intensity frequency component in the Fourier-transformed signal light. doing. For this reason, disturbance due to light shielding does not occur in the Fourier-transformed signal light. On the other hand, the reference light may be disturbed by light shielding (for example, diffraction of light by a mask), but the same reference light is used when recording and reproducing interference fringes between the reference light and the Fourier-transformed signal light. (If diffraction is generated by the same mask), the Fourier-transformed signal light is recorded on the hologram recording medium even if the intensity distribution of the reference light recorded on the hologram recording medium is disturbed by light shielding. Since the contribution of the reference light in the interference fringes is canceled by using the same reference light, it is reproduced accurately.

  In the hologram recording method according to the present invention, in order to reduce the intensity of the frequency component having the maximum intensity in the Fourier transformed signal light, the Fourier transformed signal light recorded on the hologram recording medium for a predetermined time is used. The intensity decreases. Therefore, when the intensity of the Fourier-transformed signal light recorded on the hologram recording medium for a predetermined time is insufficient, the intensity of the light beam of the light source is increased, or the reference light and the Fourier-transformed signal What is necessary is just to extend the time for recording (exposing) the light interference fringes on the hologram recording medium.

  In the hologram recording method according to the present invention, reducing the intensity of the maximum intensity frequency component in the Fourier-transformed signal light can be achieved by reducing the intensity of the maximum intensity frequency component in the Fourier-transformed signal light to zero. Good. In the claims and specification of the present application, when the intensity of the maximum intensity frequency component in the Fourier transformed signal light is set to 0, the intensity of the maximum intensity frequency component in the Fourier transformed signal light is expressed as Fourier For the standard of zero intensity in the converted signal light, both the case where it is completely zero and the case where it can be regarded as substantially zero are included. Of course, in the hologram recording method according to the present invention, the intensity of the maximum intensity frequency component in the Fourier transformed signal light is not reduced to 0, but the intensity of the maximum intensity frequency component in the Fourier transformed signal light is appropriately set. You may irradiate the hologram recording medium with the reference light which decreases.

  In the hologram recording method according to the present invention, the intensity of the frequency component of the maximum intensity in the Fourier-transformed signal light is set to 0, so that the Fourier-transformed signal light in the interference fringe between the reference light and the Fourier-transformed signal light Intensity distributions of frequency components derived from frequency components different from the maximum intensity frequency component in can be recorded at the recording position of the hologram recording medium with a high S / N ratio within the dynamic range of the optical recording material in the hologram recording medium. it can. That is, in order to record the intensity distribution of the frequency component derived from the frequency component different from the maximum intensity frequency component in the Fourier-transformed signal light in the interference fringe between the reference light and the Fourier-transformed signal light, the hologram recording medium The dynamic range of the optical recording material can be effectively used.

  In the hologram recording method according to the present invention, the maximum intensity frequency component in the Fourier-transformed signal light is preferably the lowest frequency component in the Fourier-transformed signal light.

  In the hologram recording method according to the present invention, when the frequency component of the maximum intensity in the Fourier transformed signal light is the lowest frequency component in the Fourier transformed signal light, the lowest frequency component in the Fourier transformed signal light. Therefore, the intensity of the frequency component derived from the frequency component of the lowest frequency in the Fourier-transformed signal light in the interference fringe between the reference light and the Fourier-transformed signal light may be reduced. As a result, in the interference fringes between the reference light and the Fourier-transformed signal light, the intensity distribution of the frequency component derived from a frequency component different from the frequency component of the lowest frequency in the Fourier-transformed signal light can be recorded on the hologram recording medium. Recording can be performed at a recording position of the hologram recording medium with a high S / N ratio within the dynamic range of the material. That is, in order to record the intensity distribution of the frequency component derived from the frequency component different from the lowest frequency component in the Fourier transformed signal light in the interference fringe between the reference light and the Fourier transformed signal light, the hologram recording medium The dynamic range of the optical recording material can be effectively used.

  For example, in a digital hologram, regarding the two-dimensional data array of “1” and “0”, the low frequency component in the pattern of the two-dimensional data array of “1” and “0” is the information of the two-dimensional data array. In order to record the interference fringes having on the hologram recording medium, the importance is relatively low. For this reason, the intensity of the low-frequency component of the Fourier-transformed signal light having the information of the two-dimensional data array is reduced by the reference light having an intensity distribution that reduces the intensity of the low-frequency component of the Fourier-transformed signal light. May be. Thereby, the intensity | strength of the frequency component originating in the low frequency component of the signal light by which the Fourier transform was carried out in the interference fringe of the reference light and the signal light by which the Fourier transform was carried out can be reduced. Accordingly, in the interference fringes between the reference light and the Fourier-transformed signal light, the intensity distribution of the frequency component derived from the frequency component different from the low-frequency component of the Fourier-transformed signal light is expressed as the dynamic range of the optical recording material in the hologram recording medium. Can be recorded within the range.

  In addition, when the frequency component of the lowest frequency in the signal light subjected to Fourier transform is unnecessary, the intensity of the frequency component of the lowest frequency in the signal light subjected to Fourier transform is determined as the frequency component of the lowest frequency in the signal light subjected to Fourier transform. The reference light having an intensity distribution that makes the intensity of the light intensity zero may be set to zero. Here, setting the intensity of the frequency component of the lowest frequency in the signal light subjected to Fourier transform to 0 is substantially 0 even when it is completely 0 with respect to the criterion of 0 intensity in the signal light subjected to Fourier transform. This includes both cases where it can be considered to exist.

  For example, in a digital hologram, for a two-dimensional data array of “1” and “0”, the data signal inversion in the data array is set to “1”, and the data signal is not inverted between the data boundaries in the data array. When a two-dimensional data array modulation method (NRZI method: non-return-to-zero inverted) that assigns “0” to “0” is employed, the non-inversion of the data signal between the data boundaries in the data array The frequency component of the lowest frequency in the Fourier-transformed signal light corresponding to is unnecessary. When modulating a two-dimensional data array using such a modulation method, a reference light that reduces the intensity of the frequency component of the maximum intensity and the minimum frequency in the Fourier-transformed signal light is used as the Fourier-transformed signal light. In the interference fringes between the reference light and the Fourier-transformed signal light, the maximum intensity of the Fourier-transformed signal light and the intensity of the frequency component of the lowest frequency corresponding to the frequency component of the lowest frequency Can be reduced. Therefore, it is possible to record the intensity distribution of the required higher frequency components in the interference fringes between the reference light and the Fourier-transformed signal light within the dynamic range of the optical recording material in the hologram recording medium.

  In addition, when the Fourier transform optical system is a lens system having a circular aperture, the signal light is diffracted by Franhofer diffraction and Fourier transformed through the circular aperture. The Fourier-transformed signal light has an intensity distribution corresponding to the frequency component of the signal light at the recording position of the hologram recording medium placed at the focal position of the Fourier transform optical system. Here, the frequency component of the lowest frequency included in the signal light has the same intensity distribution as the diffraction image by the circular aperture, and is distributed in a so-called Airy disk range. Therefore, by using the reference light in which the intensity of the portion corresponding to the diameter of the Airy disk of the frequency component of the lowest frequency included in the signal light is reduced, the intensity of the frequency component of the lowest frequency included in the signal light is changed to the reference light. Can be effectively reduced. The diameter of the Airy disk having the lowest frequency component contained in the signal light varies depending on the wavelength of the signal light and the size of the aperture of the Fourier transform optical system.

  Next, the hologram recording apparatus according to the present invention will be described.

  A hologram recording apparatus according to the present invention is a hologram recording apparatus for recording a hologram on a hologram recording medium by irradiating the hologram recording medium with reference light and signal light Fourier-transformed through a Fourier transform optical system, and the intensity distribution of the reference light Is converted into an intensity distribution that reduces the intensity of the frequency component having the maximum intensity in the Fourier-transformed signal light.

  According to the hologram recording apparatus of the present invention, since the light intensity conversion means for converting the intensity distribution of the reference light into an intensity distribution that reduces the intensity of the frequency component of the maximum intensity in the Fourier-transformed signal light, Without saturating the frequency component of the maximum intensity in the signal light subjected to Fourier transform within the range of the dynamic range of the optical recording material, the necessary frequency component having an intensity smaller than the frequency component of the maximum intensity in the signal light subjected to Fourier transform. It is possible to provide a hologram recording apparatus capable of recording a hologram while suppressing a decrease in the SN ratio.

  More specifically, the hologram recording apparatus according to the present invention includes a light source that generates a light beam, a splitting optical system that divides the light beam emitted from the light source into two light beams, and hologram recording using one of the two light beams as reference light. Optical system for irradiating the medium, optical system for illuminating the object with the other of the two light beams to obtain parallel signal light, Fourier transform optical system for Fourier transforming the signal light from the object, and recording position on the hologram recording medium Is set to the focal position of the Fourier transform optical system.

  The light source that generates the light beam is preferably a laser light source because the light beam is coherent light.

  In particular, the hologram recording apparatus according to the present invention preferably has a laser light source provided with an external resonator. For a laser light source including an external resonator, light is resonated in the external resonator to emit laser light, and the coherent length (interference distance) of the laser light depends on the length of the external resonator. Therefore, when using a long external resonator such as a known solid-state laser, it is possible to obtain a laser beam having a large coherent length and high coherence, and the optical path difference between the reference light and the signal light cannot be accurately adjusted. Even so, interference fringes between the reference light and the signal light can be obtained. For this reason, it becomes easy to constitute a hologram recording apparatus.

  On the other hand, the hologram recording apparatus according to the present invention preferably has a semiconductor laser light source. When a semiconductor laser is used as a light source for generating a light beam, the hologram recording apparatus can be miniaturized because the semiconductor laser is a small laser light source.

  A half mirror can be used as a splitting optical system that splits a light beam emitted from a light source into two light beams. As an optical system for irradiating the hologram recording medium with one of the two light beams as reference light, a single mirror or a plurality of mirrors can be used as appropriate. As an optical system that obtains parallel signal light by irradiating an object with the other of the two light beams, a collimator lens or an afocal optical system that converts light coming from the object into parallel light can be used. Here, when the size of the portion of the object that irradiates light is larger than the diameter of the light beam that irradiates the object, the diameter of the light beam is set to be equal to or larger than the size of the portion of the object that irradiates light. It is necessary, and preferably the same size as the size of the portion of the object that irradiates light. In order to widen the diameter of the light beam, a beam expander composed of an afocal optical system can be used.

  Here, the object is not particularly limited, but in the digital hologram, a spatial light modulation element such as a DMD (digital micromirror device) or a liquid crystal device is used. In other words, white and black two-dimensional patterns corresponding to “1” and “0” are formed on the image display surface of the spatial light modulator. The white and black two-dimensional patterns form a two-dimensional data array composed of “1” and “0”. The light transmitted through the object including the spatial light modulation element or the light reflected from the object enters the Fourier transform optical system as signal light that is parallel light parallel to the optical axis. That is, signal light having information of a two-dimensional data array composed of “1” and “0” is Fourier transformed by the Fourier transform optical system. In addition, when using a spatial light modulation element as an object, a control mechanism for controlling the spatial light modulation element is necessary.

  A Fourier transform optical system that Fourier-transforms signal light from an object is not particularly limited as long as it is an optical system that focuses parallel light, but is usually a focusing optical system including one or more lenses. In the Fourier transform optical system, since the lens usually has a circular aperture, the signal light of parallel light is diffracted by Franhofer diffraction to Fourier transform the signal light. Further, the Fourier transform optical system may have an opening for removing an unnecessary portion of the signal light.

  With the setting mechanism of the hologram recording apparatus according to the present invention, the recording position on the hologram recording medium is set to the focal position of the Fourier transform optical system.

  The hologram recording apparatus according to the present invention has light intensity conversion means for converting the intensity distribution of the reference light into an intensity distribution that reduces the intensity of the frequency component of the maximum intensity in the Fourier-transformed signal light. The light intensity conversion means is inserted into an optical path through which the reference light passes. The light intensity converting means has a portion that transmits a predetermined portion of the light beam generated from the light source and reflects or partially transmits (reduces) another portion of the light beam generated from the light source. For example, the light intensity conversion means includes a transparent substrate and a predetermined portion that reflects the reference light provided on the transparent substrate or having a lower transmittance of the reference light than the transparent substrate provided on the transparent substrate. This predetermined portion varies depending on the wavelength of the light generated from the light source and the shape and size of the aperture of the Fourier transform optical system that Fourier transforms the signal light.

  In the hologram recording apparatus according to the present invention, the light intensity conversion means preferably includes a transparent substrate and a metal film provided on the transparent substrate. The light intensity conversion means reflects a part of the light beam generated from the light source by the metal film provided on the transparent substrate or transmits the light beam through the metal film provided on the transparent substrate. That is, when the thickness of the metal film provided on the transparent substrate is thick, a part of the light beam generated from the light source can be reflected by the metal film and provided on the transparent substrate. When the metal film is thin, a part of the light beam generated from the light source can be transmitted through the metal film. The intensity of the reference light transmitted through the metal film is reduced by the metal film. In addition, in order to make the intensity distribution of the reference light an intensity distribution that reduces the intensity of the frequency component of the maximum intensity in the Fourier-transformed signal light, the range of the portion that reduces the intensity in the light beam generated from the light source is It is determined by the size of the metal film provided on the transparent substrate. The range of the portion where the intensity of the light beam generated from the light source is reduced varies depending on the wavelength of the light generated from the light source and the aperture of the Fourier transform optical system that Fourier transforms the signal light. For example, the range of the portion that reduces the intensity of the light beam generated from the light source has the diameter of the Airy disk of the Fourier-transformed signal light if the aperture of the Fourier transform optical system that Fourier transforms the signal light is a circular aperture It is good also as a disk type. Here, as a material of the metal film, it is preferable to use a metal material such as aluminum and silver having high reflectivity. By using such a metal material having a high reflectance, the intensity of a part of the light beam generated from the light source can be effectively reduced. Further, the remaining portion of the light beam generated from the light source is passed through a transparent substrate. The intensity of the remaining portion of the light beam generated from the light source is hardly reduced. Examples of the transparent substrate include a glass substrate and a plastic substrate that transmit a light beam generated from a light source. Therefore, the difference between the intensity of the portion incident on the metal film and the intensity of the portion incident on the transparent substrate in the light beam generated from the light source can be increased. In this way, the light intensity conversion means has the transparent substrate and the metal film provided on the transparent substrate, so that the intensity distribution of the reference light can be set by setting the size and thickness of the metal film. The intensity distribution of the frequency component having the maximum intensity in the Fourier-transformed signal light can be easily converted to an intensity distribution. Note that the metal film can be manufactured by sputtering a metal on a transparent substrate.

  In the hologram recording apparatus according to the present invention, the light intensity conversion means preferably includes a transparent substrate and a dielectric multilayer film provided on the transparent substrate. The light intensity converting means transmits a part of the light beam generated from the light source through the dielectric multilayer film provided on the transparent substrate. By designing the material (refractive index or dielectric constant) and thickness of each film constituting the dielectric multilayer film, the transmittance of the reference light transmitted through the dielectric multilayer film can be appropriately set. As a result, a part of the light beam generated from the light source is reduced through the dielectric multilayer film so that the reference light has an intensity distribution that reduces the intensity of the frequency component having the maximum intensity in the Fourier-transformed signal light. Can be made. In addition, in order to make the intensity distribution of the reference light an intensity distribution that reduces the intensity of the frequency component of the maximum intensity in the Fourier-transformed signal light, the range of the portion that reduces the intensity in the light beam generated from the light source is It is determined by the size of the dielectric multilayer film provided on the transparent substrate. The range of the portion where the intensity of the light beam generated from the light source is reduced varies depending on the wavelength of the light generated from the light source and the aperture of the Fourier transform optical system that Fourier transforms the signal light. For example, the range of the portion that reduces the intensity of the light beam generated from the light source has the diameter of the Airy disk of the Fourier-transformed signal light if the aperture of the Fourier transform optical system that Fourier transforms the signal light is a circular aperture It is good also as a disk type. Further, the remaining part of the reference light is passed through a transparent substrate. The transparent substrate hardly reduces the intensity of the remaining portion of the reference light. Examples of the transparent substrate include a glass substrate and a plastic substrate that transmit a light beam generated from a light source. Therefore, the difference between the intensity of the portion incident on the dielectric multilayer film and the intensity of the portion incident on the transparent substrate in the light beam generated from the light source can be increased. Thus, since the light intensity conversion means has a transparent substrate and a dielectric multilayer film provided on the transparent substrate, the intensity distribution of the reference light can be set by setting the shape and thickness of the dielectric multilayer film. The intensity distribution of the frequency component having the maximum intensity in the Fourier-transformed signal light can be easily converted to an intensity distribution. The dielectric multilayer film can be produced by sputtering a dielectric that forms the dielectric multilayer film on a transparent substrate.

  The hologram recording apparatus according to the present invention may further include a beam expander that expands the diameter of the light beam incident on the light intensity conversion means when the reference light is a light beam. As a beam expander that expands the diameter of the light beam incident on the intensity conversion means, an afocal optical system that converts the light beam incident on the intensity conversion means into a parallel light beam having a larger diameter can be used. . By having a beam expander that expands the diameter of the light beam incident on the light intensity conversion means, the size of the light intensity conversion means can be increased, and the light intensity conversion means can be easily manufactured. That is, the size of a part of the reference light such as a metal film and a dielectric multilayer film provided on a transparent substrate that reflects or partially transmits (decreases) the reference light beam diameter. The light intensity conversion means can be easily manufactured. The reference light whose beam diameter has been enlarged by the beam expander is passed through the light intensity conversion means, and the reference light that has passed through the light intensity conversion means has the same diameter as the light beam generated from the light source by the lens system that reduces the beam diameter. Is converted again into reference light having a beam diameter of 2 mm and irradiated onto the hologram recording medium.

  Next, the hologram recording medium according to the present invention will be described.

  The hologram recording medium according to the present invention irradiates a hologram by irradiating signal light that has undergone Fourier transform through a Fourier transform optical system and reference light having an intensity distribution that reduces the intensity of the maximum intensity frequency component in the signal light that has undergone Fourier transform. Is a hologram recording medium on which is recorded.

  The hologram recording medium according to the present invention irradiates a hologram by irradiating signal light that has undergone Fourier transform through a Fourier transform optical system and reference light having an intensity distribution that reduces the intensity of the maximum intensity frequency component in the signal light that has undergone Fourier transform. Is the hologram recording medium on which is recorded, so that the maximum intensity frequency component in the Fourier-transformed signal light is not saturated in the dynamic range of the optical recording material, and the maximum intensity frequency component in the Fourier-transformed signal light It is possible to provide a hologram recording medium on which a hologram is recorded while suppressing a decrease in the SN ratio of a necessary frequency component having a smaller intensity.

  Note that the hologram recording medium is made of an optical recording material such as bismuth silicate and lithium niobate that has a large change in dielectric constant upon exposure, and a photopolymer that produces a refractive index difference upon exposure.

  Next, a hologram reproducing apparatus according to the present invention will be described.

  The hologram reproducing apparatus according to the present invention irradiates a hologram by irradiating a signal light that has undergone Fourier transform through a Fourier transform optical system and a reference light having an intensity distribution that reduces the intensity of the maximum intensity frequency component in the signal light that has undergone Fourier transform. Is a hologram reproducing apparatus that reproduces a hologram from a hologram recording medium on which is recorded, and includes reference light irradiation means for irradiating the hologram recording medium with the reference light. The reference light irradiating means includes light intensity converting means included in the hologram recording apparatus according to the present invention, and the reference light having the intensity distribution for reducing the intensity of the frequency component of the maximum intensity in the Fourier-transformed signal light. Irradiate the hologram recording medium. Thereby, the hologram recorded on the hologram recording medium according to the present invention can be reproduced. In the hologram reproducing device, for example, a detecting unit such as a CCD for detecting the hologram reproduced by the reference light may be provided.

  According to the hologram reproducing apparatus of the present invention, since it has the reference light irradiating means for irradiating the hologram recording medium with the above-mentioned reference light, the frequency component of the maximum intensity in the Fourier-transformed signal light is in the range of the dynamic range of the optical recording material. A hologram reproducing apparatus capable of reproducing a hologram while suppressing a decrease in the SN ratio of a necessary frequency component having an intensity smaller than the maximum intensity frequency component in the Fourier-transformed signal light without being saturated with can do.

  Next, the hologram reproducing method according to the present invention will be described.

  According to the hologram reproducing method of the present invention, a hologram is obtained by irradiating a signal beam Fourier-transformed through a Fourier transform optical system and a reference beam having an intensity distribution that reduces the intensity of a frequency component having the maximum intensity in the Fourier-transformed signal beam. Is a hologram reproducing method for reproducing a hologram by irradiating the hologram recording medium on which is recorded with the reference light described above.

  According to the hologram reproducing method of the present invention, the signal light Fourier-transformed through the Fourier transform optical system and the reference light having an intensity distribution that reduces the intensity of the frequency component having the maximum intensity in the Fourier-transformed signal light are irradiated. The hologram recording medium on which the hologram is recorded is irradiated with the above reference light to reproduce the hologram, so that the frequency component of the maximum intensity in the Fourier transformed signal light is saturated within the dynamic range of the optical recording material. There is provided a hologram reproduction method capable of reproducing a hologram while suppressing a decrease in the SN ratio of a necessary frequency component having an intensity smaller than the maximum intensity frequency component in the Fourier-transformed signal light. it can.

  The hologram recording apparatus and the hologram reproducing apparatus according to the present invention may be combined, that is, the hologram recording apparatus may be used as a hologram reproducing apparatus to form a hologram recording / reproducing apparatus. In such a hologram recording / reproducing apparatus, the hologram recording method and the hologram reproducing method according to the present invention can be implemented. Specifically, in the hologram recording apparatus according to the present invention, when reproducing a hologram recorded on the hologram recording medium according to the present invention, an object that blocks light incident on the object is used, or in the optical path of the signal light What is necessary is just to insert the shielding body which interrupts | blocks light into. In this way, only the reference light is applied to the hologram recording medium according to the present invention, and the hologram recorded on the hologram recording medium is reproduced. The hologram recording apparatus according to the present invention may be provided with detection means for detecting the reproduced hologram.

  The hologram recording method, hologram recording apparatus, hologram reproducing method, hologram reproducing apparatus, and hologram recording medium according to the present invention can be used, for example, in a hologram optical memory capable of recording and reproducing a large amount of information at high speed.

  FIG. 6 is a diagram for explaining a specific example of the hologram recording apparatus and the recording method according to the present invention. The hologram recording apparatus according to the present invention shown in FIG. 6 is an example of a digital hologram recording apparatus, and is represented by a solid line in FIG. 6 provided on a laser light source 1, a half mirror 2, mirrors 3 and 4, and an optical path of signal light. The beam expander 5, the spatial light modulator 6, the lens system 7 represented by the solid line in FIG. 6 provided on the optical path of the signal light, and the mask made of the glass substrate 8 and the aluminum film 9. The lens system 7 may include a converging lens, a pinhole, and a collimator lens in order to remove unnecessary portions of signal light incident from the spatial light modulator. That is, the focal point of the converging lens and the collimator lens is set at the pinhole position, the converging lens converges the parallel signal light into the pinhole as the converging light, and the collimating lens emits the diverging light passing through the pinhole As parallel light, the parallel light is incident on a lens (system) that focuses the hologram recording medium.

  Further, a beam expander 5 represented by a dotted line in FIG. 6 provided on the optical path of the reference light and a lens system for reducing the beam diameter represented by the dotted line in FIG. 6 provided on the optical path of the reference light as necessary. You may have. When the hologram recording apparatus according to the present invention includes the beam expander 5 and the lens system 7 represented by dotted lines in FIG. 6 provided on the optical path of the reference light, the beam diameter of the reference light can be enlarged. The size of the aluminum film 9 can be increased, and the mask can be easily manufactured. The light beam expanded by the beam expander 5 passes through the mask, and the diameter of the light beam is returned to the original by the lens system 7.

  When recording information on the hologram recording medium 10 using the hologram recording apparatus according to the present invention, first, the hologram recording medium 10 on which no hologram is recorded is installed in the hologram recording apparatus. Next, one light beam is generated from the laser light source 1, and this light beam is divided into two light beams by the half mirror 2. One of the two light beams is reflected by the half mirror 2 toward the mirror 3 in a direction perpendicular to the light beam incident from the laser light source. The light beam reflected by the mirror 3 passes through the mask made of the glass substrate 8 and the aluminum film 9, is further reflected by the mirror 4, and is applied to the hologram recording medium 10 as reference light. Here, the light beam reflected by the mirror 3 is modulated when passing through the mask made of the glass substrate 8 and the aluminum film 9.

  The other light beam divided by the half mirror 2 passes through the half mirror 2 and enters the beam expander 5. The diameter of the light beam incident on the beam expander 5 is expanded by the beam expander 5, and the light beam having the expanded beam diameter is irradiated onto the spatial light modulator 6 as parallel light. The light beam applied to the spatial light modulator 6 is modulated by the spatial light modulator 6 and enters the lens system 7 in parallel as signal light. In the digital hologram recording apparatus, the spatial light modulator 6 modulates the light beam to form signal light corresponding to a two-dimensional data array in which “1” and “0” are two-dimensionally arranged. FIG. 3 is a diagram illustrating an example of a two-dimensional data array formed by the spatial light modulation elements. As shown in FIG. 3, the spatial light modulator 6 forms various two-dimensional data arrays composed of “1” data represented by white squares and “0” data represented by black squares. The light incident on the spatial light modulation element 6 is modulated in correspondence with such a two-dimensional data array formed by the spatial light modulation element 6. In this way, the light beam having the information of the two-dimensional data array enters the lens system 7 as parallel light.

  Next, light corresponding to the two-dimensional data array formed by the spatial light modulator 6 enters the lens system 7 as signal light, and the two-dimensional data array formed by the spatial light modulator 6 Fourier transformed by the system 7. The lens system 7 is an optical system in which a light beam having information of a two-dimensional data array is diffracted by Franhofer diffraction. FIG. 4 is a diagram showing the intensity distribution of the signal light Fourier-transformed by the lens system at the recording position of the hologram recording medium. In FIG. 4, the horizontal axis indicates a spatial position expressed in arbitrary units, and the vertical axis indicates the intensity of the light beam expressed in arbitrary units. In general, the Fourier-transformed signal light has a peak having the highest intensity at the center of the beam with respect to a spatial position, and has a plurality of peaks symmetrically around the peak having the maximum intensity. . As the peaks of the Fourier transformed signal light move away from the center of the beam, the intensity of the peaks decreases. Here, the peak having the highest intensity at the center of the beam is caused by a low-frequency component (referred to as a DC component) in the pattern of the two-dimensional data array formed by the spatial light modulator 6, and is the highest at the center of the beam. A plurality of peaks appearing around a peak having a high intensity is caused by a high-frequency component in a two-dimensional data array pattern. The peak farther from the center of the beam corresponds to the higher frequency component of the two-dimensional data array. That is, in the signal light subjected to Fourier transform, generally, the peak intensity of the DC component corresponding to the low frequency component in the two-dimensional data array pattern is higher than the peak intensity of the high frequency component in the two-dimensional data array pattern. large. The hologram recording medium 10 is irradiated with the Fourier-transformed signal light.

  In the hologram recording apparatus according to the present invention as shown in FIG. 6, the light beam reflected by the mirror 3 is modulated when passing through the mask made of the glass substrate 8 and the aluminum film 9. FIG. 7 is a plan view of a mask made of a glass substrate and an aluminum film in the hologram recording apparatus according to the present invention. The mask has a glass substrate 8 having a square shape and a disk-shaped aluminum film 9 provided on the glass substrate 8. In FIG. 7, the diameter of the light beam incident on the mask is represented by a dotted line. Since a predetermined portion of the light beam that passes through the mask passes through the glass substrate 8 having a square shape, the intensity of the light beam that passes through the glass substrate 8 hardly changes. On the other hand, another predetermined portion of the light beam passing through the mask is reflected by the disc-shaped aluminum film 9 or passes through the aluminum film 9. When a predetermined portion of the light beam passing through the mask passes through the aluminum film 9, the intensity of the predetermined portion of the light beam is significantly reduced. The thickness and diameter of the disk-shaped aluminum film 9 are such that only the intensity of the frequency component of the maximum intensity in the Fourier-transformed signal light is reduced. When the aperture of the Fourier transform optical system for Fourier transforming the signal light is circular, the diameter of the disk-shaped aluminum film 9 may be the diameter of the Airy disk in the signal light subjected to Fourier transform.

  The light beam modulated by the mask made of the glass substrate 8 and the aluminum film 9 is applied to the hologram recording medium 10 as reference light. FIG. 8 is a diagram showing the intensity distribution of the reference light applied to the hologram recording medium in the hologram recording apparatus according to the present invention. In FIG. 8, the horizontal axis indicates the spatial position expressed in arbitrary units, and the vertical axis indicates the intensity of the light beam expressed in arbitrary units. As shown in FIG. 8, the reference light applied to the hologram recording medium 10 has an intensity distribution that reduces the intensity of the frequency component having the maximum intensity in the Fourier-transformed signal light. That is, the reference light has a relatively reduced intensity so as to reduce the intensity of the highest intensity frequency component in the Fourier transformed signal light at the center of the beam with respect to the spatial position, In the vicinity of the center of the beam having a reduced intensity, there is an intensity distribution that gradually decreases from the vicinity of the center of the beam toward the periphery of the beam.

  Next, a Fourier-transformed signal light having an intensity distribution composed of a plurality of peaks as shown in FIG. 4, and a reference light having an intensity distribution for reducing the intensity of the maximum intensity frequency component in the Fourier-transformed signal light, Interferes at the recording position of the hologram recording medium 10. Then, interference fringes between the reference light irradiated on the hologram recording medium 10 and the Fourier-transformed signal light are recorded on the hologram recording medium 10. FIG. 9 is a diagram showing the intensity distribution of interference fringes obtained by the interference of the reference light and the signal light at the recording position of the hologram recording medium in the hologram recording apparatus according to the present invention. In FIG. 8, the horizontal axis indicates the spatial position expressed in arbitrary units, and the vertical axis indicates the intensity of the light beam expressed in arbitrary units. In the hologram recording apparatus according to the present invention, the Fourier-transformed signal light is Fourier-transformed by interfering with the reference light having an intensity distribution that reduces the intensity of the maximum intensity frequency component in the Fourier-transformed signal light. The maximum intensity frequency component in the signal light can be reduced. Therefore, in the intensity distribution of the interference fringes formed by the interference of the reference light and the signal light at the recording position of the hologram recording medium, the intensity near the center related to the low frequency component in the Fourier transformed signal light is significantly increased. It is possible to suppress the interference fringe intensity saturation derived from the intensity of the low frequency component in the signal light Fourier-transformed within the dynamic range of the hologram recording medium. Further, since the intensity near the periphery of the high-frequency component in the signal light subjected to Fourier transform is not reduced by the reference light, the intensity of the interference fringes derived from the intensity of the high-frequency component in the signal light subjected to Fourier transform is related to the presence or absence of the mask. The SN ratio of the interference fringe intensity derived from the intensity of the high frequency component in the Fourier-transformed signal light hardly changes. Therefore, the intensity distribution of the reference light using the mask is an intensity distribution that reduces the frequency component of the maximum intensity in the Fourier-transformed signal light, so that in the intensity distribution of the interference fringes between the reference light and the signal light, the hologram Interference fringes derived from the intensity of high frequency components in the signal light subjected to Fourier transform while avoiding saturation of the intensity of the low frequency components in the signal light subjected to Fourier transform within the dynamic range of the recording medium It is possible to suppress a decrease in the SN ratio of the intensity.

  The hologram recording medium in which the interference fringes between the reference light and the signal light are recorded as a hologram in this way is placed at a predetermined position of the hologram recording apparatus shown in FIG. 6, and the spatial light modulation element 6 is controlled to control the lens system. By blocking the signal light entering 7 or inserting a shielding plate in the optical path of the signal light, the hologram recording medium can be irradiated with the reference light that has passed through the mask. Thereby, the hologram recorded on the hologram recording medium can be reproduced.

  In the hologram recording apparatus according to the present invention shown by the solid line in FIG. 6, hologram recording was performed according to the hologram recording method according to the present invention. As the laser light source, a second harmonic (SHG) (wavelength 532 nm) of a YAG laser was used. Also, using a DMD as a spatial light modulator, create a two-dimensional random pattern of “1” (reflecting surface shown in white) and “0” (non-reflecting surface shown in black) as shown in FIG. did. As the hologram recording medium, a hologram recording medium formed of a photopolymer was used. Further, as a mask, a mask in which a disk-shaped aluminum film was formed by sputtering links in the center of a slide glass substrate as shown in FIG. 7 was used. As described above, when a hologram recorded on the hologram recording medium is reproduced, it is confirmed that the low frequency component in the interference fringe between the reference light and the signal light is not saturated and the SN ratio of the high frequency component does not decrease. I was able to.

  Although the embodiments and examples of the present invention have been specifically described above, the present invention is not limited to these embodiments and examples, and the embodiments and examples of the present invention are not limited thereto. Can be changed or modified without departing from the spirit and scope of the present invention.

  The present invention requires that the maximum intensity frequency component in the Fourier-transformed signal light is less than the maximum intensity frequency component in the Fourier-transformed signal light without saturating the frequency component in the dynamic range of the optical recording material. The present invention can be applied to a hologram recording / reproducing method and a hologram recording / reproducing apparatus capable of recording / reproducing a hologram while suppressing a decrease in the SN ratio of frequency components.

  Further, the present invention provides an intensity smaller than the maximum intensity frequency component in the Fourier transformed signal light without saturating the maximum intensity frequency component in the Fourier transformed signal light within the dynamic range of the optical recording material. The present invention can be applied to a hologram recording medium on which holograms are recorded while suppressing a decrease in the SN ratio of necessary frequency components.

It is a figure explaining the conventional digital hologram recording device and recording method. It is a figure which shows intensity distribution of the reference light irradiated to the hologram recording medium in the conventional digital hologram recording device. It is a figure which shows the example of the two-dimensional data arrangement | sequence formed with a spatial light modulation element. It is a figure which shows intensity distribution of the signal light Fourier-transformed by the lens system in the recording position of a hologram recording medium. It is a figure which shows intensity distribution of the interference fringe formed by a reference light and signal light interfering in the recording position of the hologram recording medium in the conventional digital hologram recording device. It is a figure explaining the specific example of the hologram recording apparatus and recording method by this invention. It is a top view of the mask which consists of a glass substrate and aluminum film used for the hologram recording apparatus by this invention. It is a figure which shows intensity distribution of the reference light irradiated to the hologram recording medium in the hologram recording apparatus by this invention. It is a figure which shows intensity distribution of the interference fringe obtained by a reference light and signal light interfering in the recording position of the hologram recording medium in the hologram recording apparatus by this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1,101 Laser light source 2,102 Half mirror 3, 4, 103, 104 Mirror 5, 105 Beam expander 6, 106 Spatial light modulation element 7, 107 Lens system 8 Glass substrate 9 Aluminum film 10, 110 Hologram recording medium

Claims (11)

In the hologram recording method of recording a hologram on the hologram recording medium by irradiating the hologram recording medium with a reference light and signal light Fourier-transformed through a Fourier transform optical system,
The reference light has an intensity distribution that reduces the intensity of the maximum intensity frequency component in the Fourier-transformed signal light,
A hologram recording method, comprising: interfering the reference light and the Fourier-transformed signal light to reduce the intensity of the maximum intensity frequency component in the Fourier-transformed signal light.   2. The hologram recording method according to claim 1, wherein the frequency component having the maximum intensity in the Fourier-transformed signal light is a frequency component having the lowest frequency in the Fourier-transformed signal light.   Hologram recording medium on which holograms are recorded by irradiating signal light that has undergone Fourier transform through a Fourier transform optical system and reference light having an intensity distribution that reduces the intensity of the maximum intensity frequency component in the signal light that has undergone Fourier transform And irradiating the reference light to reproduce the hologram. In a hologram recording apparatus for recording a hologram on the hologram recording medium by irradiating the hologram recording medium with reference light and signal light Fourier-transformed through a Fourier transform optical system,
A hologram recording apparatus comprising: a light intensity converting unit that converts the intensity distribution of the reference light into an intensity distribution that reduces the intensity of the frequency component having the maximum intensity in the Fourier-transformed signal light. The reference light is a light beam,
The hologram recording apparatus according to claim 4, further comprising a beam expander that expands a diameter of the light beam incident on the light intensity conversion unit.   6. The hologram recording apparatus according to claim 4, wherein the light intensity conversion means includes a transparent substrate and a metal film provided on the transparent substrate.   6. The hologram recording apparatus according to claim 4, wherein the light intensity conversion means includes a transparent substrate and a dielectric multilayer film provided on the transparent substrate.   The hologram recording apparatus according to claim 4, further comprising a laser light source including an external resonator.   The hologram recording apparatus according to claim 4, further comprising a semiconductor laser light source. Hologram recording medium on which holograms are recorded by irradiating signal light that has undergone Fourier transform through a Fourier transform optical system and reference light having an intensity distribution that reduces the intensity of the maximum intensity frequency component in the signal light that has undergone Fourier transform In the hologram reproducing apparatus for reproducing the hologram from
A hologram reproducing apparatus comprising reference light irradiation means for irradiating the hologram recording medium with the reference light.   A hologram is recorded by irradiating signal light that has undergone Fourier transform through a Fourier transform optical system and reference light having an intensity distribution that reduces the intensity of the maximum intensity frequency component in the Fourier transformed signal light. Hologram recording medium.

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