JP2009098360A - Hologram recording device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hologram recording device which can record a hologram while compensating specified spatial frequency components. <P>SOLUTION: Coherent light from a light source 10 is modulated in accordance with digital data by a spacial optical modulator 16, to be signal light S. A part of spatial frequency components included in the signal light S is passed through by a filter part 22, and is Fourier-transformed by a Fourier transformation lens 24, and a light recording medium 26 is irradiated with the Fourier-transformed signal light. In the filter part 22, the main opening passing through Nyquist frequency among the spatial frequency components included in the signal light S and an auxiliary opening passing through harmonics with a frequency of integral multiple of a specified spacial frequency whose regeneration rate is made low on regeneration are formed, and, by the harmonics passing through the auxiliary opening, the spatial frequency components whose regeneration rate is low can be compensated. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a hologram recording apparatus.

  The hologram recording / reproducing apparatus records and reproduces digital data by recording / reproducing signal light, which is a digital image of binary digital data “0, 1”, etc., for example, “bright, dark” by a spatial light modulator, as a hologram. Reproduction is performed. In the case of a Fourier transform hologram, the signal light is Fourier-transformed by a lens, irradiated with the reference light that interferes with the signal light and generates interference fringes, and the generated interference fringes are recorded as a hologram on the optical recording medium. . Further, when data is reproduced, reference light is irradiated onto the optical recording medium, diffracted light is received by a photodetector, a digital image is reproduced, and digital data is acquired.

  In such a hologram recording / reproducing apparatus, in order to improve the recording density, it is desirable to reduce the irradiation area to the recording medium. For this reason, it is desired to reduce the spatial frequency band of the signal light applied to the optical recording medium. Theoretically, the original data can be completely restored if a band equal to or larger than the Nyquist size among the spatial frequency components included in the original data page can be detected (see Non-Patent Document 1 below). Therefore, in order to realize a high recording density, it is desirable to limit the band of the signal light with a Nyquist size (remove high frequency components).

  However, in an actual hologram recording / reproducing apparatus, the quality of the hologram reproduction image is affected by the light transmission characteristics and the recording characteristics of the optical recording medium. That is, due to the light transmission characteristics of the hologram recording / reproducing apparatus, a specific spatial frequency component contained in the outgoing light of the spatial light modulator is subjected to a recording / reproducing process and a photodetector (CMOS) compared to other spatial frequency components. The amplitude is greatly attenuated before reaching the camera or the like, and the S / N ratio of the reproduced image is deteriorated. Further, due to the recording characteristics of the optical recording medium, it is difficult for a specific spatial frequency component of the spatial frequency component of the signal light to be recorded as a hologram, and a necessary spatial frequency component is less likely to appear in a reproduced image. As a result, the spectral distribution of the reproduced image deviates from the spectral distribution of the original data, and the S / N ratio of the reproduced image deteriorates.

From the above, it is possible to improve the S / N ratio of a reproduced image by compensating for the spatial frequency component having a low reproduction rate during hologram reproduction.
IBM J. RES. DEVELOP. VOL. 44NO. 3 MAY 2000 J. Ashley et al.

  An object of the present invention is to provide a hologram recording apparatus capable of recording a hologram while supplementing a specific spatial frequency component.

  In order to achieve the above object, a hologram recording apparatus according to claim 1 includes a signal light generating means for generating signal light modulated in accordance with digital data, and a pixel pitch of the signal light generating means. A filter means having a main opening having a size, a distance corresponding to a specific spatial frequency around the main opening, and an auxiliary opening arranged away from the main opening, and a Fourier transform of the signal light that has passed through the filter means And an irradiating optical system for irradiating the optical recording medium together with the reference light.

  The invention according to claim 2 is characterized in that, in the invention according to claim 1, the auxiliary aperture allows a harmonic having a frequency that is an integral multiple of a spatial frequency component having a low reproduction rate during reproduction to pass through.

  According to a third aspect of the present invention, in the first or second aspect of the present invention, the main opening and the auxiliary opening are configured such that light transmittance increases from the peripheral portion toward the central portion. Features.

  According to a fourth aspect of the present invention, in the invention according to any one of the first to third aspects, the filter means is formed with four auxiliary openings at the vertical and horizontal positions of the main opening which is rectangular. It is characterized by.

  According to a fifth aspect of the present invention, in the invention according to any one of the first to third aspects, the filter means has eight auxiliary openings on the upper and lower sides, the right and left sides of the main opening, and the extension of the diagonal line. A hologram recording apparatus characterized by being formed in pieces.

  According to the first aspect of the present invention, it is possible to record a hologram while maintaining a small recording area while compensating for a specific spatial frequency component as compared with the case where the present configuration is not provided.

  According to the invention of claim 2, the S / N ratio of the reproduced image can be improved as compared with the case where the present configuration is not provided.

  According to the third aspect of the present invention, the side lobe caused by the diffracted light generated at the peripheral edge portions of the main opening and the auxiliary opening can be reduced as compared with the case where this configuration is not provided.

  According to the fourth and fifth aspects of the invention, the S / N ratio of the reproduced image can be improved as compared with the case where the present configuration is not provided.

  Hereinafter, the best mode for carrying out the present invention (hereinafter referred to as an embodiment) will be described with reference to the drawings.

  FIG. 1 shows a configuration example of an embodiment of a hologram recording / reproducing apparatus according to the present invention. In FIG. 1, when recording signal light as a hologram, coherent light from the light source 10 is converted into parallel light having a wide aperture by the lenses 12 and 14 and is incident on the spatial light modulator 16.

  The spatial light modulator 16 is constituted by, for example, a liquid crystal panel, and displays a digital image (binary image) in which binary digital data “0, 1” is “bright, dark”, for example, by a computer (not shown). As a result, the light that has passed through the spatial light modulator 16 is intensity-modulated according to the value of each pixel of the binary image to become the signal light S. The signal light S passes through the relay lenses 18 and 20 and the filter unit 22 provided therebetween, is Fourier-transformed by the Fourier transform lens 24, and is applied to the optical recording medium 26. The filter unit 22 functions as a filter that passes a part of the spatial frequency component included in the signal light S. Details thereof will be described later.

  Further, the reference light R has the same optical axis as the signal light S, and is irradiated onto the optical recording medium 24 from the outside thereof. This reference light R is also converted into parallel light by the lenses 12 and 14 from the coherent light from the light source 10 and is incident on the outer peripheral region of the spatial light modulator 16. Similar to the signal light S, the reference light R that has passed through the outer peripheral region of the spatial light modulator 16 passes through the relay lenses 18 and 20 and the filter unit 22, and is Fourier-transformed by the Fourier transform lens 24 to irradiate the optical recording medium 26. Is done.

  Through the above steps, the signal light S after Fourier transform and the reference light R interfere with each other in the optical recording medium 26, and the signal light S is recorded on the optical recording medium 26 as a hologram.

  FIG. 2 shows a display pattern of the spatial light modulator 16. In FIG. 2, the binary image illustrated in FIG. 3 is displayed in the center area A, and the reference light R is transmitted through the outer peripheral area B. Here, FIG. 3 is an example of a part of the binary image displayed on the spatial light modulator 16, but as described above, the binary image has binary digital data “0, 1” “bright”. , Dark ”. In FIG. 3, a white and black rectangular pattern is a binary image displayed as “bright, dark”, and each white and black rectangular area is composed of, for example, d × d = 2 × 2 pixels. Yes. In the present embodiment, this rectangular area is referred to as a pixel unit, and the length d of one side thereof is described as “pixel pitch”.

  The example of FIG. 1 is a so-called collinear method in which the reference light R and the signal light S are irradiated to the optical recording medium 26 by a coaxial optical system. Alternatively, a two-wave system may be used in which the optical recording medium 26 is irradiated as reference light R ′ that passes through a different optical path from the signal light S by a simple reflecting mirror.

  Next, in FIG. 1, when information is reproduced from the diffracted light of the hologram, the coherent light from the light source 10 is converted to only the reference light R by the spatial light modulator 16, and Fourier-transformed by the Fourier transform lens 24 to perform optical recording. The medium 26 is irradiated. In this case, the spatial light modulator 16 shown in FIG. 2 is controlled such that the signal light S that passes through the central area A of the display pattern is blocked and only the reference light R passes through the outer peripheral area B.

  The diffracted light from the hologram generated by the irradiation of the reference light R is converted into parallel light by the inverse Fourier transform lens 28 and received by the photodetector 30 constituted by a CMOS camera or the like. The output signal of the photodetector 30 that has received the diffracted light is input to an information acquisition unit 32 realized by a computer or the like, and information contained in the hologram is acquired.

  FIG. 4 shows a configuration example of the filter unit 22 described above. In FIG. 4, the filter portion 22 has a main opening 22a and an auxiliary opening 22b.

The main opening 22a is a rectangular opening having a size that allows a Nyquist-sized spatial frequency band to pass through among the spatial frequency components included in the signal light S. When the Nyquist size is N, the main opening 22a is a square opening with N sides. here,
N = λf / d
It is. Here, λ is the wavelength of the signal light S, f is the focal length of the relay lens 18, and d is the pixel pitch shown in FIG.

  In general, when Fourier transforming light with a pixel pitch d, the Fourier spectrum of the light spreads in the vertical and horizontal directions around the focal point of the lens. At this time, the spatial frequency component of the signal light S collected on a rectangular region having one side of λf / d (Nyquist size N) is the Nyquist frequency of the signal light S. Therefore, when the main opening 22a is the Nyquist size, the Nyquist frequency of the signal light S Fourier-transformed by the relay lens 18 is passed.

  On the other hand, on the high frequency side of the Nyquist frequency, there are those having low transfer characteristics in the hologram recording / reproducing apparatus shown in FIG. These spatial frequency components have a low reproduction rate during reproduction, and the S / N ratio of the reproduced image is deteriorated. Therefore, in order to compensate for these spatial frequency components, an auxiliary opening 22b through which harmonics that are integral multiples of the spatial frequency components pass is provided in the filter unit 22. Thereby, the harmonic of the specific spatial frequency component is irradiated onto the optical recording medium 26, and the amplitude of the attenuated specific spatial frequency component can be recovered.

In FIG. 4, four auxiliary openings 22 b are formed at positions above, below, left, and right of the rectangular (square) main opening 22 a. Assuming that the spatial frequency components having a low reproduction rate during reproduction are the spatial frequency μ in the X-axis direction and the spatial frequency ν in the Y-axis direction, these spatial frequency components are X ₀ = μλf, Y ₀ = in the Fourier plane. A bright spot is created at a position of νλf. Further, a spatial frequency component having a frequency m times (m is an integer) of these spatial frequencies creates a bright spot at a position of X ₀ = mμλf, Y ₀ = mνλf on the Fourier transform plane. Therefore, the filter unit 22 disposed on the Fourier transform plane of the relay lens 18 is provided with an auxiliary opening 22b at a position of X ₀ = mμλf, Y ₀ = mνλf in order to compensate for the spatial frequency components of the frequencies μ and ν. It is preferable to pass harmonics. In the example shown in FIG. 4, four auxiliary openings 22b are provided at the position of m = 2 (± 2X ₀ , ± 2Y ₀ ). As described above, the auxiliary opening 22b is arranged away from the main opening 22a by a distance corresponding to the spatial frequency of the signal light S.

  Further, the size of the auxiliary opening 22b needs to be equal to or greater than the beam waist (position where the harmonic beam becomes the narrowest). Preferably, it is 1 to 1.2 times the beam waist, and more preferably 1.2 to 1.5 times. When the harmonic frequency has a band from (μ−Δμ / 2, ν−Δν / 2) to (μ + Δμ / 2, ν + Δν / 2), the size of the auxiliary opening 22b is set to ΔX = Δμλf, Let ΔY = Δνλf.

In the example shown in FIG. 4, the number of auxiliary openings 22b is four, but the present invention is not limited to this. For example, m may be an integer of 2 or more, and the auxiliary opening 22b may be provided at a position of (± mX ₀ , ± mY ₀ ).

  Further, FIG. 5 shows another configuration example of the filter unit 22, and the auxiliary opening 22 b is provided on the extension of the diagonal line in addition to the vertical and horizontal positions of the rectangular main opening 22 a. . In this case, the auxiliary openings 22b provided on the extension of the diagonal line are arranged so that the positions on the X axis and the Y axis are the same as the auxiliary openings 22b provided on the upper, lower, left and right sides of the main opening 22a. . As described above, in the example of FIG. 5, eight auxiliary openings 22b are provided.

  6A and 6B show modified examples of the main opening 22a and the auxiliary opening 22b. In FIG. 6A, for convenience of explanation, the main opening 22a and the auxiliary opening 22b are represented by one figure, but it means that the main opening 22a and the auxiliary opening 22b have the same size. is not. In FIG. 6A, the interval between the lines described inside the main opening 22a and the auxiliary opening 22b indicates the light transmittance. The wider the interval between the lines, the higher the transmittance. FIG. 6 (b) shows the transmittance in the direction of arrow A in FIG. 6 (a). In this embodiment, as shown in FIGS. 6A and 6B, the transmittance in the vicinity of the peripheral portions of the main opening 22a and the auxiliary opening 22b increases from the peripheral portion toward the central portion. ing. As a result, when the change in transmittance at the peripheral portions of the main opening 22a and the auxiliary opening 22b is steep, side lobes due to diffracted light generated at the peripheral portion can be reduced, and the S / N ratio of the reproduced image is improved.

It is a figure which shows the structural example of one Embodiment of the hologram recording / reproducing apparatus concerning this invention. It is a figure which shows the display pattern of a spatial light modulator. It is a figure which shows the example of a part of binary image displayed on a spatial light modulator. It is a figure which shows the structural example of a filter part. It is a figure which shows the other structural example of a filter part. It is a figure which shows the modification of a main opening and an auxiliary opening.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 Light source, 12, 14 lens, 16 Spatial light modulator, 18, 20 Relay lens, 22 Filter part, 22a Main aperture, 22b Auxiliary aperture, 24 Fourier transform lens, 26 Optical recording medium, 28 Inverse Fourier transform lens, 30 light Detector, 32 information acquisition part.

Claims (5)

Signal light generating means for generating signal light modulated according to digital data;
A filter means having a main opening having a size corresponding to a pixel pitch of the signal light generating means, and an auxiliary opening disposed around the main opening at a distance corresponding to a specific spatial frequency and separated from the main opening;
Irradiation optical system for Fourier transforming the signal light that has passed through the filter means and irradiating the optical recording medium together with the reference light;
A hologram recording apparatus comprising:   2. The hologram recording apparatus according to claim 1, wherein the auxiliary aperture passes harmonics having a frequency that is an integral multiple of a spatial frequency component having a low reproduction rate during reproduction.   3. The hologram recording apparatus according to claim 1, wherein the main opening and the auxiliary opening are configured such that light transmittance increases from a peripheral part toward a central part. .   4. The hologram recording apparatus according to claim 1, wherein four auxiliary openings are formed in the filter means at positions above, below, left, and right of the main opening that is rectangular. Hologram recording device.   The hologram recording apparatus according to any one of claims 1 to 3, wherein the filter means has eight auxiliary openings formed vertically, horizontally, and diagonally on the main opening. A hologram recording apparatus characterized by the above.

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