JP4930980B2 - Optical information recording method and recording medium - Google Patents

Description

  The present invention relates to an optical information recording method for recording interference fringes due to interference between information light and reference light as a hologram on a recording medium, and a recording medium on which interference fringes are recorded.

  Holographic recording in which information is recorded on a recording medium using holography is generally performed by superimposing information light carrying image information constituting recording light and recording reference light inside the recording medium. This is done by writing an interference pattern that can be generated on a recording medium. At the time of reproducing the recorded information, the image information is reproduced by diffraction by the interference pattern by irradiating the recording medium with the reference light for reproduction (see Patent Document 1).

  In recent years, volume holography, particularly digital volume holography, has been developed and attracted attention for practical use for ultra-high density optical recording. Volume holography is a method of writing interference patterns in three dimensions by actively utilizing the thickness direction of the recording medium. Increasing the thickness increases the diffraction efficiency and increases the recording capacity using multiple recording. There is a feature that can be planned. Digital volume holography is a computer-oriented holographic recording method that uses a recording medium and a recording method similar to those of volume holography, but restricts image information to be recorded to a binarized digital pattern. In this digital volume holography, for example, image information such as an analog picture is once digitized, developed into two-dimensional digital pattern information, and recorded as image information. At the time of reproduction, the two-dimensional digital pattern information is read and decoded, so that the original image information is restored and displayed. As a result, even if the S / N ratio (signal to noise ratio) is somewhat poor at the time of reproduction, the original information is reproduced very faithfully by performing differential detection or encoding binary data and performing error correction. It becomes possible.

  As an optical recording / reproducing apparatus for performing such holographic recording, the information light and the reference light are applied to the recording medium from the same surface side of the recording medium by the objective lens so that the optical axis of the information light and the optical axis of the reference light are coaxial. An apparatus for irradiation has been proposed (Patent Document 1).

  In this optical recording / reproducing apparatus, a spatial light modulator for information light and a spatial modulation pattern for reference light are displayed on a spatial light modulator (sometimes abbreviated as SLM) having a large number of pixels arranged in a lattice pattern. Information light and reference light carrying two-dimensional digital pattern information are generated by changing the state of light phase, intensity, wavelength and the like for each pixel by an optical modulator. A liquid crystal display device may be used as the spatial light modulator, but a DMD (digital micromirror device) may also be used. The DMD has a plurality of reflective pixels, and can reflect light by changing the reflection direction for each pixel.

  Since the spatially modulated information light and reference light are irradiated by the objective lens so as to converge on the recording medium, the Fourier-transformed frequencies of the information light spatial modulation pattern and the reference light spatial modulation pattern The components form interference fringes.

  FIG. 19 shows the configuration of the pickup of the optical information recording / reproducing apparatus using the DMD. The pickup 101 irradiates the optical information recording medium 151 with reference light and information light, and receives reproduction light from the optical information recording medium 151. The pickup 101 includes a laser light source 103, a collimator lens 105, a mirror 107, a DMD 109, a polarization beam splitter 111, relay lenses 113 and 115, an aperture 127, a mirror 117, a quarter wavelength plate 119, an objective lens 121, a ring mask 123, A photodetector 125 is provided.

  When information is recorded by the optical information recording / reproducing apparatus of FIG. 19, laser light is emitted from the laser light source 103, converted into parallel rays by the collimator lens 105, and reflected by the mirror 107 toward the DMD 109. The DMD 109 has a plurality of minute mirrors as pixels, and the tilt angle of the mirror can be changed for each pixel. If the information light spatial modulation pattern and the reference light spatial modulation pattern are displayed on a plurality of pixels of the DMD 109, the reflected light 133 is spatially modulated by the information light spatial modulation pattern and the reference light spatial modulation pattern. It becomes information light and reference light.

  The information light and the reference light pass through the polarization beam splitter 111 and propagate so as to form an image on the entrance pupil plane of the objective lens 121 by the pair of relay lenses 113 and 115. On the way, a part of the diffracted light is removed by the opening 127 arranged in the focal plane between the pair of relay lenses 113 and 115, reflected by the mirror 117 toward the objective lens 121, and the quarter-wave plate 119. Pass through. The objective lens 121 causes information light and recording reference light to interfere with each other in the hologram recording layer 153 of the optical information recording medium 151 to form holography.

  Further, when information is reproduced by the optical information recording / reproducing apparatus of FIG. 19, only the reference modulation spatial modulation pattern is displayed on the DMD 109 to generate reproduction reference light, and the reproduction reference light is used as the objective lens. 121 irradiates the interference fringes formed on the hologram recording layer 153 of the optical information recording medium 151. The reproduction light and reproduction reference light reproduced from the interference fringes are reflected by the reflection layer 155 of the optical information recording medium 151, and in the reverse direction, the objective lens 121, the quarter-wave plate 119, the mirror 117, and a pair of relay lenses. 113 and 115 are propagated. Here, since the polarization of the reproduction light and the reproduction reference light is changed by 90 ° by passing through the quarter-wave plate 119 twice, it is reflected toward the photodetector 125 by the polarization beam splitter 111. Then, the reference light for reproduction is removed by the ring mask 123, and the reproduced light is incident on the photodetector 125.

  The optical information recording medium 151 for recording interference fringes is provided with a hologram recording layer 153 and a reflection layer 155 between two substrates 151a and 151b. The hologram recording layer 153 is mixed with a monomer having photosensitivity to the information light and the reference light, and a part of the monomer reacts with the polymer by interference fringes formed by the interference of the information light and the reference light. Thus, optical characteristics such as refractive index are changed, and interference fringes (holograms) are recorded. That is, a part of the monomer in the hologram recording layer is consumed by recording the interference fringes. Further, when the monomer concentration in the recorded portion decreases, the monomer diffuses and moves from the surroundings.

JP 2005-32307 A

  FIG. 20 shows the configuration of a conventional spatial modulation pattern 141 for information light. The information light spatial modulation pattern 141 is formed by arranging a plurality of block units 143, and each block unit 143 includes a reference mark 147 and a symbol unit 145 of a certain amount of two-dimensional pattern information.

  The symbol unit 145 of the two-dimensional pattern information is generated by encoding information to be recorded for each unit of a certain amount of information. In FIG. 20, symbol unit 145 includes information to be recorded every 8 bits, in which 3 pixels out of 4 × 4 pixels are turned on pixels 145 a (shown by shading in FIG. 20). It is encoded by arrangement.

  The reference mark 147 is a two-dimensional pattern having a predetermined shape, and the position of the entire spatial modulation pattern, the direction of the entire spatial modulation pattern, the position of each symbol unit 145 of the two-dimensional pattern information in the spatial modulation pattern, or each symbol unit. It is a standard for the orientation of the. Conventionally, the reference mark 147 is represented by turning on all of the 4 × 4 square pixels. The symbol unit 145 in FIG. 20 is turned on only in 3 × 4 × 4 pixels. Therefore, if the 4 × 4 pixel is turned on, the symbol unit 145 can be recognized separately from the symbol unit 145. It can be a standard.

  In FIG. 20, a block unit 143 has a 4 × 4 on-pixel 145a at the center as a reference mark 147, and is surrounded by two pixels, thereby a region corresponding to 6 × 6 symbol units (24 × 24 pixels). Thirty-two symbol units 145 are arranged so as to be filled. In FIG. 20, in the spatial modulation pattern 141 for information light, there are four block units 143 at the top, six at the second, eight at the third to sixth, six at the seventh, six at the seventh, Four are arranged on the stage. Only the leftmost block unit 143a (indicated by a dotted line) in the uppermost stage has a configuration in which only the reference mark 147 is arranged at the center, and no symbol unit of two-dimensional pattern information is arranged around the reference mark 147. For this reason, the leftmost block unit 143a in the uppermost stage can be distinguished from other block units, and can be used as a reference for the position and orientation of the spatial modulation pattern for information light 141 itself. For example, when the information light spatial modulation pattern 141 is detected during reproduction, alignment is performed so that the block unit 143a is located at the upper left, and then the block unit 143a is decoded and reproduced in order from the block unit immediately to the right. Can do.

  Furthermore, since the symbol units are arranged so that each block unit 143 has a square shape centered on the reference mark 147, each symbol unit 145 can be identified by specifying the relative position to the reference mark 147. Can be specified.

Since the spatial light modulator is an aggregate of minute pixels having a periodic structure, it forms a kind of diffraction grating. A diffraction grating has a diffraction grating period of a, a diffraction light angle of θ _d , an incident light angle of θ ₀ , a diffraction order of m (m = 0, ± 1, ± 2), and a light wavelength of λ. ,
Expression 1: a (sin θ _d −sin θ ₀ ) = mλ
Satisfy the relationship.

In the optical information recording / reproducing apparatus in FIG. 19, the light 131 from the mirror 107 is incident on the DMD 109 with an incident angle θ ₀ and is emitted vertically from the DMD 109 (θ ₁ = 0 °). Has been. This relationship is shown in FIG. 21. The interval between the pixels 135 of the DMD 109 becomes the period a of the diffraction grating, and the angle θ _d of the diffracted light 132 is determined by Equation 1 from the interval a of the pixels 135, the wavelength λ of the incident light 131, and the incident angle θ _0. It does not depend on the inclination angle Ψ of. On the other hand, the outgoing angle θ ₁ of the reflected light 133 (overlapped with the third-order diffracted light in FIG. 21) is calculated from the incident angle θ ₀ of the incident light 131 and the inclination angle ψ of the pixel 135 of the DMD 109: ₁ = θ ₀ -2Ψ
It depends on. In FIG. 21, since the outgoing angle θ ₁ of the reflected light 133 is 0 °, the outgoing angle θ ₁ is not shown.

  FIG. 21 shows a case where all the pixels 135 of the spatial light modulator 109 are turned on, and the interval “a” of the pixels 135 is the period of the diffraction grating. However, in the case of a spatial light modulator that displays a spatial modulation pattern for information light or a spatial modulation pattern for reference light, the spatial modulation pattern for information light or the spatial modulation pattern for reference light is a combination of on pixels and off pixels. Since it is formed, it has various periodic components, and the period of the diffraction grating is not limited to the interval a between adjacent pixels. However, since the pattern displayed in the spatial light modulator 109 is configured with the pixel 135 as a reference unit element, the interval a between the pixels is minimized as the period of the diffraction grating. Therefore, the spatial frequency in a state where all the pixels 135 are turned on is referred to as a basic spatial frequency in the spatial light modulator.

Conventionally, the optical axis (θ ₁ ) of the information light, the recording reference light, and the reproduction reference light reflected and generated by the DMD 109 coincides with a part of the traveling direction (θ _d ) of the diffracted light 132. (Paragraph 0029 of Patent Document 1). In other words, the configuration is such that a (sin θ ₁ −sin θ ₀ ) = mλ is satisfied and the incident angle θ ₀ is −a · sin θ ₀ = mλ.

When a transmissive liquid crystal display device or the like is used as the SLM, light is incident on the SLM perpendicularly (θ ₀ = 0 °) and light emitted perpendicularly (θ ₁ = 0 °) is used. Therefore, the incident angle = the outgoing angle, and the 0th-order diffracted light and the optical axis coincided.

  In the Japanese Patent Application No. 2005-312513, the present inventor compared the conventional optical information recording / reproducing apparatus with an optical axis of information light directed from the spatial light modulator toward the objective lens being diffracted by the spatial light modulator. An optical information recording apparatus configured so as not to coincide with the next (m = 0, ± 1, ± 2,...) Diffracted light has been proposed. When the optical axis of the information light is configured not to match the diffracted light, the bright spot generated by the fundamental spatial frequency of the spatial light modulator Fourier-transformed by the objective lens can be arranged in the region with the strongest light intensity. Thus, the intensity of the interference fringes can be increased, and the hologram size can be reduced.

  The present invention provides an optical information recording method and recording medium capable of further reducing the hologram size and increasing the recording density in the case where the optical axis of the information light does not coincide with the diffracted light. The issue is to provide. Another object of the present invention is to provide an optical information recording method and a recording medium that can improve the reliability of recording and reproduction as compared with the prior art.

  In order to solve the above problems, the optical information recording method of the present invention is spatially modulated by the recording reference light and the spatial modulation pattern for information light displayed on the spatial light modulator having a plurality of pixels. In the optical information recording method of recording interference fringes between the recording reference light and the information light in the hologram recording layer of the recording medium by irradiating the recorded information light with the objective lens so as to converge on the recording medium, The optical axis of the information light from the spatial light modulator toward the objective lens does not coincide with the mth-order (m = 0, ± 1, ± 2...) Diffracted light diffracted by the spatial light modulator. The information light spatial modulation pattern includes a plurality of reference marks, and a spatial frequency in a region of the reference marks includes a spatial frequency smaller than a basic spatial frequency of the spatial light modulator.

  Further, in the optical information recording method, the direction of the optical axis of the information light is preferably in the range of the traveling direction of the (m + 0.2) th to (m + 0.8) th diffracted light. In the spatial light modulator, the plurality of pixels are arranged in a grid, and the direction of the optical axis of the information light is about (m + 0.5) with respect to the diagonal direction of the grid formed by the plurality of pixels. It is preferable to coincide with the traveling direction of the next diffracted light.

  Furthermore, in the optical information recording method, it is preferable that the spatial frequency in the region of the reference mark includes a spatial frequency that is half the basic spatial frequency of the spatial light modulator. The reference mark preferably includes a pattern in which pixels having different attributes are alternately arranged.

  Furthermore, in the optical information recording method, the direction of the optical axis of the information light is preferably perpendicular to the spatial light modulator.

  The recording medium of the present invention is characterized in that interference fringes are recorded on the hologram recording layer by the optical information recording method.

In the optical information recording method of the present invention, by shifting the optical axis of the outgoing light toward the objective lens from the m-th order diffracted light, bright spots with a plurality of fundamental spatial frequencies can be arranged in the strongest region, The intensity of interference fringes can be increased. In addition, even if the output angle θ ₁ from the spatial light modulator varies due to an error in the tilt angle of the reflection surface of the pixel and the position of the shape component pattern in the Fourier plane is shifted, it depends on a plurality of basic spatial frequencies. Since there are bright spots, strong intensity can be maintained in the hologram recording area, and the reliability is improved.

  Furthermore, in the past, a region of very strong light intensity is localized at the center, so that the monomer concentration in the hologram recording layer of the optical information recording medium is partially depleted, which may hinder multiple recording. However, in the optical information recording method of the present invention, since a plurality of bright spots are arranged in a region having a high light intensity, the light intensity distribution in the hologram recording region can be averaged as compared with the conventional case. . As a result, the monomer concentration also decreases on average, and the multiple recording characteristics can be improved, so that the recording capacity can be increased.

  Further, in the optical information recording method of the present invention, all the spatial frequency characteristics are shown by the four bright spots located in the vicinity of the optical axis, so the hologram recording area is reduced to a size including these four bright spots. be able to. Therefore, since the size of each hologram can be reduced, more holograms can be recorded accordingly, and the recording capacity can be increased.

  In addition, in the optical information recording method of the present invention, the spatial modulation pattern for information light includes a plurality of reference marks, and the spatial frequency in the reference mark region is smaller than the basic spatial frequency of the spatial light modulator. As a result, the bright spot generated by Fourier transform of the reference mark exists inside the bright spot with the four basic spatial frequencies, and the reference mark does not disappear even if the hologram size is reduced, improving the reliability. be able to. Further, the size of the hologram can be further reduced, and the recording capacity can be improved.

  First, the relationship between the incident light 21, the emitted light 22, and the diffracted light 23 in the spatial light modulator 11 of the present invention will be described with reference to FIG. The optical information recording / reproducing apparatus of the present invention displays the information light spatial modulation pattern and the recording reference light spatial modulation pattern on the spatial light modulator 11 during recording, spatially modulates the information light, A recording reference beam is generated. Note that the recording reference light does not have to be spatially modulated. In this case, only the information light spatial modulation pattern may be displayed on the spatial light modulator 11. The information light and the recording reference light are irradiated so as to converge on the recording medium (not shown) by the objective lens 13 and interfere with each other in the hologram recording layer of the recording medium, so that the information light and the recording reference light are recorded. Light interference fringes are recorded on the hologram recording layer. Next, when reproducing the interference fringes recorded using the spatially-modulated recording reference light, the spatial light modulator 11 generates the recording reference light spatial modulation pattern and generates the recording reference light. The same reproduction reference light spatial modulation pattern is displayed, and the light is spatially modulated to generate reproduction reference light. The reproduction reference light is irradiated by the objective lens 13 so as to converge on the recording medium, and interferes with interference fringes recorded on the hologram recording layer of the recording medium to generate reproduction light. The generated reproduction light is propagated by the optical system including the objective lens 13, and the spatial modulation pattern is detected by the photodetector.

In the present invention, the optical axis of the outgoing light 22 (information light, recording reference light or reproduction reference light) directed from the spatial light modulator 11 toward the objective lens 13 is diffracted by the spatial light modulator 11 (mth order). m = 0, ± 1, ± 2...) is not matched with the diffracted light 23. This means that the emitted light 22 is located between the m-th order and the (m + 1) -th order diffracted light 23. In FIG. 1, the outgoing light 22 is located between the −1st order diffracted light and the 0th order diffracted light. Further, the incident angle of the light 21 incident on the spatial light modulator 11 is θ ₀ , the emission angle of the light 22 emitted from the spatial light modulator 11 is θ ₁ , the wavelength of the light is λ, and a plurality of light beams in the spatial light modulator 11 Where the period of the pixel 12 is a and the diffraction order of the diffracted light 23 is m (m = 0, ± 1, ± 2...)
Formula 3: mλ ≠ a (sin θ ₁ −sin θ ₀ )
The apparatus is configured to satisfy the following relational expression. In FIG. 1, the reflection type spatial light modulator is exemplified, but the same applies to a transmission type spatial light modulator.

Here, the direction of the optical axis of the emitted light 22 is preferably within the range of the traveling direction of the (m + 0.2) th to (m + 0.8) th diffracted light 23, and particularly about (m + 0.5) th. The traveling direction of the diffracted light 23 is preferably. That is, it is preferable to satisfy the relational expression of (m + 0.2) λ ≦ a (sin θ ₁ −sin θ ₀ ) ≦ (m + 0.8) λ , and particularly about (m + 0.5) λ = a (sin θ ₁ −sin θ ₀ ). It is more preferable that the relational expression is satisfied.

In FIG. 1, the angle θ ₁ of the emitted light 22 is emitted obliquely for easy understanding, but the two-dimensional pattern information is displayed when the objective lens 13 performs Fourier transform on the two-dimensional pattern information. Since the display surface of the spatial light modulator 11 is preferably parallel to the entrance pupil plane (perpendicular to the optical axis) of the objective lens 13, the exit angle θ ₁ is 0 °, that is, the light of the exit light 22. It is preferable that the axis is perpendicular to the spatial light modulator 11. In this case, since sin θ ₁ = 0, Equation 3 becomes mλ ≠ −a · sin θ ₀ , and the incident angle θ ₀ is (m + 0.2) λ ≦ −a · sin θ ₀ ≦ (m + 0.8) λ It is preferable to satisfy the following relational expression, and it is more preferable that the relational expression of about (m + 0.5) λ = −a · sin θ ₀ is satisfied.

Furthermore, in the case of a DMD having a reflection type pixel, if the angle of the reflection surface of the pixel is Ψ, the emission angle θ ₁ = θ ₀ -2Ψ, so the period a and the inclination angle of the pixel 12 of the spatial light modulator 11 When ψ satisfies the relational expression mλ ≠ −a · sin 2ψ, the optical axis of the emitted light 22 is perpendicular to the spatial light modulator 11 and does not coincide with the diffracted light. The period a and the tilt angle Ψ of the pixel of the spatial light modulator 11 preferably satisfy the relational expression of (m + 0.2) λ ≦ −a · sin2Ψ ≦ (m + 0.8) λ , particularly about (m + 0.5). It is more preferable to satisfy the relational expression of λ = −a · sin2Ψ.

Figure 2 shows a Fourier transform image of the conventional optical information recording and reproducing apparatus angle theta ₁ of the emitted light matches the m-th order diffracted light, Figure 3, the angle theta ₁ of the emitted light is m order diffracted light The Fourier-transform image in the optical information recording / reproducing apparatus of this invention which does not correspond is shown.

  As shown in the upper left of FIG. 2, when all the pixels 135 of the spatial light modulator 109 having a plurality of pixels 135 arranged in a grid are turned on, when Fourier transform is performed by the objective lens, It is converted into a pattern 137 as shown. In FIG. 2, since all the pixels 135 of the spatial light modulator 109 are in the ON state, the pattern displayed on the spatial light modulator 109 is a pattern composed of the fundamental spatial frequency, and the Fourier-transformed pattern 137 is This indicates the frequency component of the fundamental spatial frequency. The bright spot 138 in the pattern 137 is referred to as a “bright spot by the basic spatial frequency”.

The bright spot 138 by each fundamental spatial frequency is m-th order diffracted light in each direction, and the position of each bright spot 138 is the diffraction angle θ _d in the above-described formula 1: a (sin θ _d −sin θ ₀ ) = mλ. Sought by. In FIG. 2, since the optical axis of the outgoing light toward the objective lens coincides with the mth-order diffracted light, a bright spot 138 based on the fundamental spatial frequency is located on the optical axis (center).

  In analyzing the Fourier transform of the light spatially modulated by the spatial light modulator 109, the components of the spatial light modulator 109 are an arrangement component 135 a caused by the arrangement of the pixel 135 and a shape component caused by the shape of the pixel 135. 135b. In FIG. 2 and FIG. 3, “*” between the arrangement component and the shape component is a convolution calculation symbol.

The arrangement component 135a is subjected to Fourier transform to obtain an arrangement component pattern 137a having bright spots 138a arranged in a similar shape to the arrangement component 135a, and the shape component 135b is subjected to Fourier transformation to obtain an intensity distribution due to the shape of the pixel. The shape component pattern 137b has. In FIG. 2, a shape component pattern 137b for a square pixel 135 is shown. In the intensity distribution of the shape component pattern 137b, an area having the strongest intensity is located in the central portion, and an area periodically arranged in a direction facing each side of the square of the pixel 135 (up, down, left and right in FIG. 2). The strength gradually decreases as the distance from the center portion increases. Further, in the diagonal direction of pixel 135 (oblique direction in FIG. 2), there is a region only in the vicinity of the central portion, and the light intensity in the region gradually decreases as the distance from the central portion increases. The position of the shape component pattern 137b is obtained by the emission angle θ ₁ in the above-described formula 2: θ ₁ = θ ₀ −2Ψ.

  The pattern 137 is a pattern in which the arrangement component pattern 137a and the shape component pattern 137b are overlapped and multiplied by the intensity. This point will be described with a one-dimensional Fourier transform waveform. As shown in FIG. 4, the shape component pattern 41 obtained by Fourier transforming a rectangular wave corresponding to the shape component has a periodic peak corresponding to the arrangement component. When the arrangement component patterns 42 are overlapped and multiplied by the intensity, a composite pattern 44 is obtained. That is, the intensity of the shape component pattern 41 at the peak position of the arrangement component pattern 42 becomes the composite pattern 44. In the composite pattern 44, the shape component pattern 41 is indicated by a dotted line. The synthesized pattern 44 corresponds to the Fourier transform pattern 137 in the conventional optical information recording / reproducing apparatus in that the center of the shape component pattern 41 and the center of the arrangement component pattern 42 coincide.

In FIG. 2, since the angle θ _{1 of the} emitted light matches the angle θ _d of the mth-order diffracted light, the center of the arrangement component pattern 137a matches the center of the shape component pattern 137b. The Fourier transform pattern 137 has the strongest light intensity in the m-th order diffracted light positioned at the center, and the (m ± 1, 2,...) Next bright spots 138 away from the center (m. As the difference increases, the strength decreases. Further, in the diagonal direction (diagonal direction in FIG. 2), since the shape component pattern 137b exists only in the vicinity of the center portion, each (m ± 1, 2,...) Next bright spot of the Fourier transform pattern 137 is also the center portion. The intensity decreases as the distance from the center increases (the difference from m increases).

  Next, as shown in the upper left of FIG. 3, in the recording / reproducing method of the present invention, when all the pixels 12 of the spatial light modulator 11 having a plurality of pixels 12 arranged in a grid are turned on, the objective lens Is transformed into a pattern 31 as shown in the upper right of FIG. In FIG. 3, the spatial light modulator 11 is rotated by 45 ° about the optical axis. This is because each pixel 12 of the DMD rotates about the vertical diagonal line in FIG. This is because the rotating shaft is arranged vertically in the apparatus. When the spatial light modulator 11 is rotated, the Fourier-transformed pattern 31 is also rotated, but the shape of the pattern 31 itself is not changed, and the pattern 31 in FIG. 3 also shows a frequency component of the basic spatial frequency. The bright spots 33 in the pattern 31 are also bright spots based on the fundamental spatial frequency.

  The spatial light modulator 11 in FIG. 3 can also be decomposed into the arrangement component 12 a resulting from the arrangement of the pixels 12 and the shape component 12 b resulting from the shape of the pixels 12, as in FIG. 2. When the arrangement component 12a is Fourier transformed, an arrangement component pattern 31a having bright points 33a arranged in a similar shape to the arrangement component 12a is obtained, and when the shape component 12b is Fourier transformed, the intensity distribution resulting from the shape of the pixel is obtained. The shape component pattern 31b has. The arrangement component pattern 31a and the shape component pattern 31b in FIG. 3 are the same as the arrangement component pattern 137a and the shape component pattern 137b in FIG. 2 rotated by 45 °.

  As shown in FIG. 3, when the optical axis of the outgoing light toward the objective lens is configured not to match the m-th order diffracted light, the bright spot 33a of the arrangement component pattern 31a is shifted from the optical axis. This corresponds to the relationship of the composite pattern 45 when the arrangement component pattern 43 having no peak at the center is overlapped with the shape component pattern 41 of FIG. In the synthesized pattern 45, the shape component pattern 41 is indicated by a dotted line, and it can be seen that two peaks near the center both show strong intensity.

In FIG. 3, the position of the m + 0.5th order diffracted light in the diagonal direction of pixel 12 (lateral direction 15 in FIG. 3 and oblique 45 ° in FIG. 2) coincides with the optical axis. The optical axis is the center of the four bright spots 33a. Here, the period a in Expression 3 for obtaining the diffraction order in the diagonal direction: mλ ≠ a (sin θ ₁ −sin θ ₀ ) is the interval between the rotation axes 14 (half the diagonal line of the pixel) as shown in FIG. It becomes. That is, if the length of one side of the pixel is L, it is 2 ^1/2 · L / 2.

On the other hand, since the position of the shape component pattern 31b is obtained by the emission angle θ ₁ in the above-described equation 2: θ ₁ = θ ₀ -2Ψ, the center of the shape component pattern 31b is the optical axis of the emitted light at the emission angle θ _1. Matches. For this reason, the pattern 31 obtained by superimposing the arrangement component pattern 31a and the shape component pattern 31b is strong at the bright spots 33 by the four basic spatial frequencies in the vicinity of the optical axis, and each basic space as the distance from them increases. The intensity of the bright spot 33 due to the frequency becomes weak.

  In this way, when the optical axis of the outgoing light toward the objective lens is configured not to match the m-th order diffracted light, a plurality of bright spots 33 based on the basic spatial frequency of the spatial light modulator are arranged in the strongest region. And the intensity of the interference fringes can be increased.

Further, even if the emission angle θ ₁ varies due to an error in the tilt angle of the reflection surface of the pixel and the position of the shape component pattern 31b on the Fourier plane is shifted, there are bright spots 33 with a plurality of basic spatial frequencies. Therefore, a strong intensity can be maintained in the hologram recording area, and the reliability is improved.

  Furthermore, in the past, a region of very strong light intensity is localized at the center, so that the monomer concentration in the hologram recording layer of the optical information recording medium is partially depleted, which may hinder multiple recording. However, in the present invention, since the bright spots 33 having a plurality of fundamental spatial frequencies are arranged in the region where the light intensity is strong, the light intensity distribution in the hologram recording region can be averaged as compared with the conventional case. . As a result, the monomer concentration also decreases on average, and the multiple recording characteristics can be improved, so that the recording capacity can be increased.

  As another advantage, if the optical axis of the outgoing light toward the objective lens is configured not to match the m-th order diffracted light, the hologram size can be reduced. In the pattern 137 of FIG. 2, the hologram recording area 139 is indicated by a white circle. In FIG. 2, in order to obtain all the spatial frequency characteristics, it is necessary to include at least the central bright spot 138 and the bright spots 138 located in the four directions in the hologram recording area 139. On the other hand, as shown in the pattern 31 of FIG. 3, in the present invention, since all the spatial frequency characteristics are shown by the four bright spots 33 located in the vicinity of the optical axis, the four bright spots 33 are included. The hologram recording area 34 (indicated by white circles) can be reduced to the size. Therefore, since the size of each hologram can be reduced, more holograms can be recorded accordingly, and the recording capacity can be increased.

Specifically, in the DMD, when the DMD emits perpendicularly to the objective lens (θ ₁ = 0) and the optical axis of the emitted light is (m + 0.5) as the traveling direction of the next diffracted light, (m + 0. 5) The relational expression of λ = −a · sin2Ψ is satisfied. For example, in the case of a 0.7 inch XGA (1024 × 768 pixel) type DMD manufactured by Texas Instruments, the pitch of square pixels is 13.68 μm, and therefore (m + 0.5) -order diffracted light in the diagonal direction. Is selected, the pixel period a is 2 ^1/2 · 13.68 μm / 2 = 9.65 μm. FIG. 5 shows the tilt angle and the incident angle of the pixel that is the traveling direction of the (m + 0.5) -th diffracted light in the diagonal direction when light having a wavelength of 532 nm and light having a wavelength of 410 nm are used in this DMD. Since the DMD has an inclination angle of 11 to 13 °, when using light having a wavelength of 532 nm, the inclination of the pixel is set so as to be in the traveling direction of the −7.5th-order light. When the angle is 12.18 °, the incident angle to the DMD is 24.36 °, and light having a wavelength of 410 nm is used, the inclination angle of the pixel is set to 11.87 so that the traveling direction of the −9.5th order light is obtained. The incident angle to DMD is 23.74 °.

  FIGS. 6 to 9 use light having a wavelength of 532 nm, and the incident angle is 24.4 ° with respect to a DMD having a tilt angle of 11.4 °, 12.0 °, 12.2 °, and 12.4 °. A Fourier transform image when light is incident at an angle is shown. In addition, the upper stage is a case where all the pixels are turned on, and the lower stage is a case where pixels in an on state and an off state (shaded hatching) are randomly arranged.

  In FIG. 6, since the tilt angle of the pixel 62 of the DMD 61 is 11.4 °, the emission angle is 24.4 ° −2 × 11.4 ° = 1.6 °. The diffraction order at an output angle of 1.6 ° is 9.65 μm × (sin (1.6 °) −sin (24.4 °)) / 532 nm = −7.00 order light. Therefore, in FIG. 6, the optical axis of the emitted light coincides with the diffracted light, as in the conventional optical information recording / reproducing apparatus. In FIG. 6, in the upper Fourier transform pattern 63, the optical axis is located at the bright spot with the fundamental spatial frequency on the left side from the center, and the light intensity is strong at that position. However, in the essential hologram recording area (white circle) 64, the light intensity is weak and the interference fringes are weak. Further, in the lower Fourier transform pattern 65 in which the ON pixels 62a and the OFF pixels 62b are randomly arranged, each bright spot is blurred and the light intensity distribution is widened. However, the light intensity is still weak in a part of the hologram recording area (white circle) 64, and it is difficult to record and reproduce reliable information.

  In FIG. 7, since the inclination angle of the pixel 72 of the DMD 71 is 12.0 °, the emission angle is 24.4 ° −2 × 12.0 ° = 0.4 °. The diffraction order at an output angle of 0.4 ° is 9.65 μm × (sin (0.4 °) −sin (24.4 °)) / 532 nm = −7.37 order light. Therefore, in FIG. 7, the optical axis of the emitted light does not coincide with the diffracted light. In FIG. 7, the Fourier transform pattern 73 in the upper stage has a slightly strong light intensity at the left bright spot among the bright spots with four basic spatial frequencies near the center, but the other three bright spots have sufficiently high light intensity. Interference fringes can be recorded in the entire hologram recording area (white circle) 74. The lower Fourier transform pattern 75 in which the on-pixels 72a and the off-pixels 72b are randomly arranged is also slightly leftward from the center, but the light intensity distribution spreads over the entire hologram recording area (white circle) 74. Therefore, interference fringes can be recorded, and reliable information recording / reproduction can be performed.

  In FIG. 8, since the inclination angle of the pixel 82 of the DMD 81 is 12.2 °, the emission angle is 24.4 ° −2 × 12.2 ° = 0 °. The diffraction order at an exit angle of 0 ° is 9.65 μm × (sin (0 °) −sin (24.4 °)) / 532 nm = −7.49th order light. Therefore, in FIG. 8, the optical axis of the emitted light does not coincide with the diffracted light. In FIG. 8, the upper Fourier transform pattern 83 has bright spots that are equally strong at the four basic spatial frequencies in the vicinity of the center, and records uniform interference fringes over the entire hologram recording area (white circle) 84. It is possible. Even in the lower Fourier transform pattern 85 in which the ON pixels 82a and the OFF pixels 82b are randomly arranged, the light intensity distribution spreads over the entire hologram recording region (white circle) 84, so that interference fringes are recorded evenly. It is possible to record and reproduce information with reliability.

  In FIG. 9, since the tilt angle of the pixel 92 of the DMD 91 is 12.4 °, the emission angle is 24.4 ° −2 × 12.4 ° = −0.4 °. The diffraction order at the exit angle of −0.4 ° is 9.65 μm × (sin (−0.4 °) −sin (24.4 °)) / 532 nm = −7.62 order light. Therefore, in FIG. 9, the optical axis of the emitted light does not coincide with the diffracted light. In FIG. 9, the Fourier transform pattern 93 in the upper stage has a slightly strong light intensity at the right bright spot among the bright spots by the four basic spatial frequencies near the center, but the other three bright spots have sufficiently high light intensity. The interference fringes can be recorded in the entire hologram recording area (white circle) 94. The lower Fourier transform pattern 95 in which the ON pixels 92 a and the OFF pixels 92 b are randomly arranged is also slightly deviated to the left of the center, but the light intensity distribution spreads over the entire hologram recording region (white circle) 94. Therefore, interference fringes can be recorded, and reliable information recording / reproduction can be performed.

  In addition, the range of the said inclination angle is a design value in one product, and is not limited to the said numerical value. For example, if the tilt angle is 10.5 °, the direction of travel of the 6.5th order light can be used when using light with a wavelength of 532 nm, and 8.5 when using light with a wavelength of 410 nm. Since it can be set as the traveling direction of the next light, a highly versatile DMD can be manufactured. Also, if the pixel spacing is changed, the angle must be redesigned.

  In the above description, a spatial light modulator having reflective pixels such as a DMD has been mainly described. However, the present invention can also be applied to a transmissive spatial light modulator. FIG. 10 shows the relationship between the incident light 21, the outgoing light 22, and the diffracted light 23 in the transmissive spatial light modulator 11.

First, also in the transmissive spatial light modulator 11, the incident angle of the incident light 21 is θ ₀ , the emission angle of the light 22 emitted from the spatial light modulator 11 is θ ₁ , the wavelength of the light is λ, and the spatial light When the period of the plurality of pixels 12 in the modulator 11 is a and the diffraction order of the diffracted light 23 is m (m = 0, ± 1, ± 2...), Equation 3: mλ ≠ a (sin θ ₁ −sin θ ₀ ) The apparatus is configured to satisfy the relational expression. However, in FIG. 10, the exit angle θ ₁ is not shown because it is 0 °. Therefore, the transmissive spatial light modulator 11 has a refracting means 16. In FIG. 10, incident light 21 enters the spatial light modulator 11 at an incident angle θ ₀ . The outgoing light 22 modulated by the plurality of pixels 12 of the spatial light modulator 11 passes through the refracting means 16 and is emitted at the outgoing angle θ ₁ . In the transmissive spatial light modulator 11, since the incident light 21 is refracted by the refracting means 16, θ ₁ = θ ₀ −θ _r when the angle of the emitted light 22 with respect to the incident light 21 is θ _r . On the other hand, since the plurality of pixels 12 of the spatial light modulator 11 function as a diffraction grating, the diffraction angle θ according to Equation 1: a (sin θ _d −sin θ ₀ ) = mλ (m = 0, ± 1, ± 2...) _d diffracted light 23 is generated.

As shown in FIG. 10, the angle θ ₁ of the outgoing light 22 is preferably 0 °, that is, the optical axis of the outgoing light 22 is preferably perpendicular to the spatial light modulator 11. In this case, since sin θ ₁ = 0, Equation 3 becomes mλ ≠ −a · sin θ ₀ , and the incident angle θ ₀ is (m + 0.2) λ ≦ −a · sin θ ₀ ≦ (m + 0.8) λ It is preferable to satisfy the following relational expression, and it is more preferable that the relational expression of about (m + 0.5) λ = −a · sin θ ₀ is satisfied. Here, since θ ₁ = θ ₀ −θ _r , when θ ₁ = 0, θ ₀ = θ _r , and the above equation 3 becomes mλ ≠ −a · sin θ _r , and the incident light of the emitted light 22 The angle θ _{r with} respect to 21 preferably satisfies the relational expression of (m + 0.2) λ ≦ −a · sin θ ₀ ≦ (m + 0.8) λ . From another viewpoint, if the angle θ _{r of the} emitted light 22 with respect to the incident light 21 satisfies the relational expression mλ ≠ −a · sin θ _r , the optical axis of the emitted light 22 is relative to the spatial light modulator 11. The configuration is vertical and does not coincide with the diffracted light.

  As the refracting means 16, an angled edge or a phase plate can be used. If an angle is formed on the edge of a transparent member having a refractive index different from that of the surrounding material, a difference in thickness is caused because the thickness is partially different. Further, the phase plate is a transparent member in which a portion having a different refractive index is provided, and a phase difference is caused by the thickness of the portion having a different refractive index. In FIG. 10, the refracting means 16 is disposed on the exit surface side of the spatial light modulator 11, but the refracting means 16 may be disposed on the incident surface side, or as a function inside the spatial light modulator 11. The refracting means 16 may be realized.

  Furthermore, in the optical information recording method of the present invention, the spatial modulation pattern for information light includes a plurality of reference marks, and the spatial frequency in the region of the reference mark includes a spatial frequency smaller than the basic spatial frequency of the spatial light modulator. Like that. From another viewpoint, it is sufficient that the periodicity of the pixels in the reference mark region is larger than the periodicity of the pixels of the spatial light modulator. For example, the spatial frequency of a pattern in which on pixels and off pixels are alternately arranged is half the basic spatial frequency.

  FIGS. 11A to 11C each show a spatial modulation pattern of 8 pixels arranged linearly in the upper stage, and show the light intensity and the periodicity t in the lower stage. The spatial modulation pattern and the periodicity t It is a figure for demonstrating the relationship of these. In FIGS. 11A to 11C, the spatial modulation pattern is composed of binary values of an on state and an off state.

FIG. 11A shows a spatial modulation pattern 1101 in which all eight pixels are in an ON state, and has a period t ₁ with an interval “a” between adjacent pixels, so that the spatial frequency is a fundamental spatial frequency. In FIG. 11A, the light intensity between pixels is 0 because of the gap between the pixels of the spatial light modulator. FIG. 11B shows a spatial modulation pattern 1102 in which on-pixels and off-pixels are alternately arranged, and has a period t ₂ of an interval of 2 pixels (2 × a). Since the period t ₂ of the spatial modulation pattern 1102 in FIG. 11B is twice the period t ₁ of the spatial modulation pattern 1101 in FIG. 11A, the spatial frequency of the spatial modulation pattern 1102 is half the basic spatial frequency. Become. FIG. 11C shows a spatial modulation pattern 1103 in which two ON pixels and two OFF pixels are alternately arranged. A period t _{3 of} an interval of 4 pixels (4 × a) and an interval a of 1 pixel. And a period t ₁ . Of these, since the period t ₃ is four times the period t ₁ , the spatial modulation pattern 1103 includes a spatial frequency that is a quarter of the fundamental spatial frequency.

  When the period of the spatial modulation pattern increases, the spatial frequency decreases, and when a spatial modulation pattern having a large period is Fourier transformed, a bright spot is generated between the bright spot and the bright spot by the basic spatial frequency. In other words, in the present invention, a bright spot generated by Fourier transform of the reference mark (hereinafter referred to as “bright spot by the reference mark”) is generated between the bright spot by the basic spatial frequency.

  12A and 12B, as in FIG. 3, the position of the m + 0.5th order diffracted light in the diagonal direction of the pixel coincides with the optical axis, that is, the optical axis shines with four fundamental spatial frequencies. A spatially transformed pattern 1201 in which all the pixels are in an on state and a spatial modulation pattern 1202 in which the pixels are in an on state in a lattice shape in the state of being at the center of a point 33 is shown.

  In the case of the spatial modulation pattern 1201 in which all the pixels in FIG. 12A are turned on, the spatial modulation pattern 1201 is composed of the fundamental spatial frequency. It becomes. As shown in FIG. 20, in the conventional spatial light modulation pattern for information light, the reference mark 147 is a pattern in which all the 4 × 4 square pixels are turned on (indicated by shading in FIG. 20). . For this reason, the spatial frequency in the region of the reference mark 147 becomes the basic spatial frequency, and the bright spot by the reference mark obtained by Fourier transform becomes the pattern 1203 in FIG. For this reason, when the size of the hologram 1207 is reduced in the pattern 1203 of FIG. 12A, first, the bright spot by the reference mark is out of the range of the hologram 1207, so the SN ratio of the reference mark first deteriorates. . If the size of the hologram 1207 is further reduced, the reference mark disappears from the reproduced spatial modulation pattern and information cannot be reproduced.

  On the other hand, as shown in FIG. 12B, in the case of the spatial modulation pattern 1202 in which the pixels are turned on in a lattice shape, the spatial frequency is half of the basic spatial frequency. A bright spot is also generated at the center position of a square (see the dotted line 1206 in FIGS. 12A and 12B) formed by bright spots 1205 having four basic spatial frequencies. For this reason, when a lattice-like spatial modulation pattern as shown in FIG. 12B is used as a reference mark, the spatial frequency in that region is half the basic spatial frequency, and the bright spot by the reference mark is shown in FIG. It becomes like the pattern 1204 of B). As a result, the bright spot by the reference mark exists inside the bright spot by the four basic spatial frequencies, and even if the size of the hologram 1207 is reduced, the reference mark does not disappear and the reliability can be improved. Further, the size of the hologram can be further reduced, and the recording capacity can be improved.

  In particular, it is preferable that the bright spot by the reference mark is located on the optical axis because the reference mark can be reproduced more reliably. For example, as shown in FIG. 12B, the position of the m + 0.5th order diffracted light in the diagonal direction of the pixel coincides with the optical axis, that is, the optical axis becomes the center of the bright spot 33 with four fundamental spatial frequencies. In the state, the reference mark may include a spatial frequency that is half the basic spatial frequency.

  FIG. 13A shows a conventional spatial light modulation pattern for information light 1301 and a spatial light modulation pattern for reference light 1302, and FIG. 13B shows a spatial light modulation pattern for information light according to an embodiment of the present invention. 1303 and a spatial light modulation pattern 1302 for reference light are shown. 13A and 13B, information light spatial light modulation patterns 1301 and 1303 are arranged at the center, and an annular reference light spatial light modulation pattern 1302 is arranged around the information light spatial light modulation patterns 1301 and 1303. In the center, the overall configuration of the spatial light modulation patterns 1301 and 1303 for information light is such that the information to be recorded is encoded for each unit of a certain amount of information to generate a symbol unit of two-dimensional pattern information, and a certain amount of symbol unit And the reference mark are formed as one block unit, and a plurality of block units are arranged.

  Then, in the conventional spatial light modulation pattern for information light 1301 in FIG. 13A, as is clear from the partially enlarged view shown on the left side of FIG. In the spatial light modulation pattern for information light 1303 in FIG. 13B, the reference mark 1306 turns on the pixels in a grid pattern as is clear from the partially enlarged view shown in the left of FIG. It is a pattern that is in a state. 13 (A) and 13 (B) is a block unit portion in which only the reference marks 1305 and 1306 are arranged and the symbol unit is not arranged, and the spatial light modulation pattern 1301 for information light, This is a reference point for the position and orientation of 1303 itself.

  14 and 15 show spatial light modulation patterns when the recorded interference fringes are reproduced by actually changing the size (also referred to as aperture size) of the aperture (indicated by reference numeral 227 in FIG. 18). The size of the aperture affects the size of the interference fringes to be recorded (= hologram size). When the aperture is large, the interference fringes recorded are large, and when the aperture is small, the interference fringes recorded are small. 14 uses the information light spatial modulation pattern 1301 and the reference light spatial modulation pattern 1302 of FIG. 13A, and FIG. 15 uses the information light spatial modulation pattern 1303 and the reference light spatial modulation of FIG. 13B. A hologram was recorded using pattern 1302. 14 and 15, the five reproduced images in the upper stage are recorded and reproduced in order from the left under the conditions where the aperture size is 8.4 mm, 8.0 mm, 7.6 mm, 7.2 mm, and 6.8 mm. The four reproduced images in the lower stage are recorded and reproduced in order from the left under the conditions where the aperture size is 6.4 mm, 6.0 mm, 5.6 mm, and 5.2 mm.

  When the conventional optical information recording method of FIG. 14 is used, the reference mark can be confirmed in the upper stage, but the reference mark disappears in the lower stage where the aperture size is 6.4 mm or less, and the reference mark is reproduced. Not be able to. When the optical information recording method of the present invention shown in FIG. 15 was used, the reference mark could be confirmed in both the upper and lower stages, and the reference mark could be reproduced even if the aperture was made small.

  FIG. 16 is a diagram showing the relationship between the size of the opening (aperture size) and the bit error rate of the reproduced reference mark. If the aperture is reduced, the size of the recorded hologram is also reduced. In FIG. 16, in the case of the conventional reference mark in which all the 4 × 4 square pixels are turned on (circled line), the bit error rate rapidly increases when the size of the opening is 6.4 mm or less. , You can not play at all. On the other hand, in the case of the reference mark in which the pixels are turned on in a lattice shape (■ broken line), the bit error rate does not increase so much even when the size of the opening is 6.4 mm or less, and becomes 4 mm. However, the bit error rate could be suppressed to about 0.2.

  FIGS. 17A to 17H show another embodiment of the reference mark of the present invention. In FIGS. 17A to 17H, white pixels are in an on state and shaded pixels are in an off state. In FIGS. 17A to 17D, 4 × 4 pixels are used as reference mark regions, but the present invention is not limited to 4 × 4 pixels. In FIG. 17F, the reference mark area is 3 × 4 pixels, and in FIGS. 17G and 17H, the reference mark area is 5 × 5 pixels.

  FIG. 18 shows a configuration of the pickup 201 of the optical information recording / reproducing apparatus of the present invention using a reflective spatial light modulator. The pickup 201 of the present invention is the same as the pickup 201 of the optical information recording / reproducing apparatus of the present invention. The light source 203 for recording / reproduction, the collimator lens 205, the mirror 207, the spatial light modulator 209, the polarization beam splitter 211, the relay lenses 213, 215, An aperture 227, a mirror 217, a quarter-wave plate 219, an objective lens 221, a ring mask 223, and a light detection means 225 are provided.

  When information is recorded or reproduced in the pickup 201 shown in FIG. 18, the light emitted from the recording / reproducing light source 203 is converted into parallel rays by the collimator lens 205 and reflected by the mirror 207 toward the spatial light modulator 209. . In the present invention, the wavelength of the light emitted from the recording / reproducing light source 203 and the mirror 207 so that the outgoing light 232 of the spatial light modulator 209 does not coincide with the diffracted light 233 (shown by a dotted line) by the spatial light modulator 209. The incident angle of the incident light 231 with respect to the spatial light modulator and the period of the pixels of the spatial light modulator are configured. When a spatial light modulator having a reflective pixel as shown in FIG. 18 is used, the inclination angle of the reflective surface of the pixel is also set so that the emitted light 232 of the spatial light modulator does not coincide with the diffracted light 233. Composed. Hereinafter, each component of the optical information recording / reproducing apparatus will be described in order.

  The recording / reproducing light source 203 emits light for forming information light for recording information and reference light for recording and light for forming reference light for reproduction for reproducing information. As the light source 203, for example, a semiconductor laser that generates a coherent linearly polarized light beam can be used. As the recording / reproducing light source 203, a shorter wavelength is advantageous in order to perform high-density recording, and it is preferable to employ a blue laser (for example, a wavelength of 532 nm) or a green laser (for example, a wavelength of 410 nm). A solid-state laser can also be used as the light source 203.

  The collimator lens 205 makes the divergent beam from the recording / reproducing light source 61 almost parallel.

  The mirror 207 reflects the light from the recording / reproducing light source 203 and directs it to the spatial light modulator 209. With the mirror 207, the incident angle of the light 231 incident on the spatial light modulator 209 can be adjusted. For example, the position or angle of the mirror may be changed with respect to light having different wavelengths so that the emitted light 232 of the spatial light modulator can be adjusted so as not to coincide with the diffracted light 233.

  The spatial light modulator 209 can be a transmissive or reflective spatial light modulator that has a plurality of pixels and can modulate the phase or / and intensity of the emitted light for each pixel. As the spatial light modulator 209, a DMD (digital micromirror device) or a matrix type liquid crystal element can be used. The DMD can spatially modulate the intensity by changing the reflection direction of the incident light for each pixel, or spatially modulate the phase by changing the reflection position of the incident light for each pixel. The liquid crystal element can spatially modulate the intensity and phase of incident light by controlling the alignment state of the liquid crystal for each pixel. For example, the phase of the light can be spatially modulated by setting the phase of the emitted light for each pixel to one of two values that differ from each other by π radians. In FIG. 18, the spatial light modulator 209 is a reflection type, and is disposed so as to reflect incident light and emit it perpendicularly to the spatial light modulator.

  Information light can be generated by displaying the spatial modulation pattern for information light on the display surface of the spatial light modulator 209 and spatially modulating the light with the displayed spatial modulation pattern for information light. The spatial modulation pattern for information light includes two-dimensional pattern information obtained by encoding information to be recorded, and is expressed by two-dimensionally arranging pixels whose attributes are changed in multiple stages. For each pixel of the spatial modulation pattern for information light, for example, the state of light such as phase and intensity is changed in multiple stages. In the following description, a method of changing the light intensity for each pixel in two steps of on (white pixel) and off (black or shaded pixel) will be described. However, the method is not limited to this method. Absent. For example, the phase of the light may be changed, or may be changed to three or more stages instead of two stages.

  The recording reference light may be spatially modulated or may not be spatially modulated. When the recording reference light is spatially modulated, multiple recording can be performed by changing the spatial modulation pattern of the recording reference light, and information illegality can be obtained by using the spatial modulation pattern of the recording reference light as a key. Access security can be improved. When spatially modulating the reference light, for example, irradiate the spatial light modulator displaying the reference light spatial modulation pattern to generate the reference light spatially modulated by the reference light spatial modulation pattern. it can. When information light and recording reference light are generated by a single spatial light modulator 209, an information light region and a reference light region are provided on the display surface of the spatial light modulator 209, respectively, and spatial modulation for information light is performed. The pattern and the spatial modulation pattern for recording reference light may be displayed. The information light region is preferably disposed near the center of the spatial light modulator 209, and the reference light region is preferably disposed in an annular shape so as to surround the information light region. A spatial light modulator that generates information light and recording reference light may be provided separately. For example, the light from the light source 203 is divided by a beam splitter or the like, one light is spatially modulated by a first spatial light modulator to generate information light, and the other light is modulated by a second spatial light modulation. The reference light may be generated by spatial modulation by a device. In this case, since the information light spatial modulation pattern and the reference light spatial modulation pattern need to be imaged on the entrance pupil plane of the objective lens 221, the spatial light modulator that generates the information light and the space that generates the reference light. The light modulator has a conjugate relationship, and is propagated to the entrance pupil plane of the objective lens 221 by a pair of relay lenses 213 and 215.

  When a plurality of pixels of the spatial light modulator 209 are arranged in a lattice pattern, the optical axis of the outgoing light 23 emitted from the spatial light modulator 209 shifts the diffraction order in the diagonal direction of the grating by the plurality of pixels. As shown in the arrangement component pattern 31a of FIG. 3, the optical axis moves in the diagonal direction of the bright spot on the Fourier plane, which is preferable because the distance from the optical axis to each adjacent bright spot can be easily equalized. In particular, when the exit angle of the optical axis of the emitted light 23 is equal to the (m ± 0.5) order diffraction angle in the diagonal direction of the grating by the pixel, the distances to the four adjacent bright spots are equal, The intensity of the Fourier transform image can be flattened. Even when the plurality of pixels of the spatial light modulator 209 are arranged in a non-lattice arrangement such as a honeycomb, the optical axis does not coincide with the mth-order diffracted light. By doing so, a plurality of bright spots can be arranged in a strong region in the Fourier transform image. If the arrangement is changed, the arrangement component pattern 31a in FIG. 3 is changed.

  The plurality of pixels of the spatial light modulator 209 may have a shape other than a square shape. For example, it may be a rectangle, rhombus, hexagon, triangle, or circle. In this case, the shape component pattern 31b of FIG. 3 changes according to the shape.

  The polarization beam splitter 211 has a semi-reflective surface that reflects or transmits linearly polarized light (for example, P-polarized light) and transmits or reflects linearly polarized light (for example, S-polarized light) perpendicular to the polarized light. In FIG. 18, a polarization beam splitter 211 transmits information light, recording reference light, or reproduction reference light emitted from the spatial light modulator 209, and uses the reproduction light and the recording medium generated from the hologram recording layer of the recording medium. The reflected reference light for reproduction is reflected toward the light detection means 225.

  The pair of relay lenses 213 and 215 are arranged between the spatial light modulator 209 and the objective lens 221 so as to form an image displayed on the spatial light modulator 209 on the entrance pupil plane of the objective lens 221. Is arranged. That is, the distance from the spatial light modulator 209 to the first relay lens 213 is the focal length f1 of the first relay lens 213, and the distance from the second relay lens 215 to the entrance pupil plane of the objective lens 221 is the second. The distance between the first and second relay lenses 213 and 215 is the sum of the focal distance f1 of the first relay lens 213 and the focal distance f2 of the second relay lens 215. Are arranged as follows.

  In FIG. 18, a pair of relay lenses 213 and 215 are arranged between the objective lens 221 and the light detection means 225, and the reproduction generated from the hologram recording layer 253 of the recording medium 251 by the reproduction reference light. The light detection lens 225 is again arranged to form an image on the exit pupil plane of the light objective lens 221. That is, the distance from the exit pupil plane of the objective lens 221 to the second relay lens 215 becomes the focal length f2, and the distance from the first relay lens 213 to the light detection means 225 becomes the focal length f1, and the first and second The relay lenses 213 and 215 are arranged such that the distance between them is the sum of the focal length f1 and the focal length f2.

  The arrangement of the pair of relay lenses 213 and 215 is changed by appropriately arranging other optical elements. For example, if a magnifying lens is arranged between the first relay lens 213 and the light detection means 225, the distance from the first relay lens 213 to the entrance pupil plane of the magnifying lens is arranged to be the focal length f1. The

  The opening 227 is disposed at a focal position between the first and second relay lenses 213 and 215, and removes higher-order diffracted light. The size of the hologram recording area can be adjusted by the size of the opening 227. In the present invention, since the optical axis of the outgoing light 232 from the spatial light modulator 209 is configured not to coincide with the diffracted light 233 of the mth order (m = 0, ± 1, ± 2...), The hologram Recording area can be reduced. Furthermore, since the spatial frequency in the area of the reference mark includes a spatial frequency smaller than the basic spatial frequency of the spatial light modulator, it can be reproduced even if the hologram recording area is reduced. Therefore, the opening 227 may be reduced to reduce the hologram recording area. In FIG. 18, when the optical axis of the outgoing light 232 coincides with the traveling direction of the m + 0.5th order diffracted light, the mth order and m + 1st order diffracted light 233 in the vicinity thereof are allowed to pass, and the diffraction is separated by a higher order The light may be blocked. The means for adjusting the recording area of the hologram is not limited to the opening 227.

  The mirror 217 reflects the traveling direction of light toward the objective lens 221 and is not necessary depending on the configuration of the optical system.

  The quarter-wave plate 219 is a phase plate that changes the optical path difference of polarized light that vibrates in directions perpendicular to each other by a quarter wavelength. The P-polarized light is changed to circularly polarized light by the quarter-wave plate 219, and further, when this circularly polarized light passes through the quarter-wave plate 219, it is changed to S-polarized light. With this quarter-wave plate 219, the reproduction reference light and the reproduction light at the time of reproduction can be separated by the polarization beam splitter 211.

  The objective lens 221 irradiates the recording medium 251 with information light and recording reference light imaged on the entrance pupil plane at the time of recording, and causes the hologram recording layer 253 to interfere and record, and at the time of reproduction. Is to irradiate the recording medium 251 with the reproduction reference light imaged on the entrance pupil plane, and to input the reproduction light generated from the hologram recording layer 253 of the recording medium 251 to form an image on the exit pupil plane. In FIG. 18, the objective lens 221 is shown as a single lens, but a compound lens may be used.

  The ring mask 223 is for removing reproduction reference light reflected by the reflective layer 255 of the recording medium 251 together with reproduction light during reproduction.

  The light detection means 225 has a plurality of light receiving pixels, and can detect the intensity of light received for each light receiving pixel. As the light detection means 225, a CCD solid-state image sensor or a MOS solid-state image sensor can be used. Further, as the light detection means 225, a smart optical sensor in which a MOS type solid-state imaging device and a signal processing circuit are integrated on one chip (for example, a document “O plus E, September 1996, No. 202, Nos. 93 to 93”). (See page 99). Since this smart optical sensor has a high transfer rate and a high-speed calculation function, it is possible to perform high-speed reproduction by using this smart optical sensor, for example, reproduction at a transfer rate on the order of G (giga) bits / second. Can be performed.

  The recording medium 251 has a hologram recording layer 253 on which interference fringes are recorded. In FIG. 18, the recording medium 251 further includes a first substrate 251a, a reflective layer 255, and a second transparent substrate 251b. As the recording medium 251, a disk-shaped or card-shaped one can be used, and recording and reproduction may be performed while the recording medium 251 is rotated, or may be fixed at the time of recording and reproduction. When the recording medium 251 uses a disk-shaped recording medium and performs recording and reproduction while rotating, a disk drive mechanism used in a CD drive or a DVD drive can be used. It is preferable because compatibility with a drive or a DVD drive can be easily provided.

  In addition, it is preferable that information for positioning is recorded in advance on the recording medium 251 and a feedback mechanism is used for positioning the irradiation position because more accurate positioning can be performed. For example, pits may be formed as positioning information on the surface of the reflective layer 255 of the recording medium 251, and the positioning information may be recorded in advance. In addition, when using light of a wavelength different from that for recording or reproduction as the light for reading the positioning information, the light for recording or reproduction is used separately from the reflection layer for the light for reading the positioning information for forming the pits. A wavelength selective reflection layer for reflection may be provided. For example, if a wavelength selective reflection layer that reflects light for recording or reproduction and transmits light for reading positioning information is formed between the reflective layer and the hologram recording layer, the positioning information is superimposed on the recording / reproducing area. Recording can be performed, and the pickup device can be arranged on the same side of the recording medium, so that the recording / reproducing apparatus can be downsized.

  The operation as the optical information recording apparatus will be described. Light emitted from the light source 203 is converted into parallel light by the collimator lens 205 and reflected by the mirror 207 toward the spatial light modulator 209. Information light and reference light for recording are generated by the spatial modulation pattern for information light and the spatial modulation pattern for reference light displayed on the spatial light modulator 209. In the optical information recording method of the present invention, the spatial modulation pattern for information light has a plurality of reference marks, and the spatial frequency in the area of the reference mark is smaller than the basic spatial frequency of the spatial light modulator 209. To include. Furthermore, the optical axis (light beam in the center in the figure) of the light traveling from the spatial light modulator 209 to the objective lens is arranged so as not to coincide with the traveling direction of the mth-order diffracted light 233. In FIG. 18, since the information light is arranged at the center and the annular recording reference light is arranged around it, the optical axis of the information light and the optical axis of the recording reference light are located on the same line. And does not coincide with the traveling direction of the m-th order diffracted light 233.

  The information light and the recording reference light pass through the polarization beam splitter 211, and a pair of relay lenses 213 and 215 forms an image of the spatial modulation pattern displayed on the spatial light modulator 209 on the entrance pupil plane of the objective lens 221. Propagated to be. On the way, high-order diffracted light is removed by the opening 227, reflected by the mirror 217 toward the objective lens 221, and passes through the quarter-wave plate 219. Interference fringes of information light and recording reference light are recorded on the hologram recording layer 253 of the recording medium 251.

  Thus, the recording medium on which interference fringes are recorded by the recording method of the present invention can reproduce information by reproducing the reference mark even when the hologram size is small. Reliability is improved and recording capacity is also improved.

  Further, the operation as the optical information reproducing apparatus will be described. Light emitted from the light source 203 is converted into parallel light by the collimator lens 205 and reflected by the mirror 207 toward the spatial light modulator 209. Then, reproduction reference light is generated by the reference light spatial modulation pattern displayed on the spatial light modulator 209. Note that the reference light spatial modulation pattern of the reproduction reference light at the time of reproduction is a reference light spatial modulation pattern of the recording reference light irradiated when information recorded on the recording medium is recorded. Here, the optical axis (light beam in the center in the figure) of the light traveling from the spatial light modulator 209 toward the objective lens does not coincide with the traveling direction of the mth-order diffracted light 233. In FIG. 18, an annular reproduction reference beam is arranged, and its optical axis does not coincide with the traveling direction of the m-th order diffracted beam 233.

  The reproduction reference light emitted from the spatial light modulator 209 passes through the polarization beam splitter 211 and is displayed on the spatial light modulator 209 on the entrance pupil plane of the objective lens 221 by the pair of relay lenses 213 and 215. The spatial modulation pattern for light is propagated so as to form an image. On the way, high-order diffracted light is removed by the opening 227, reflected by the mirror 217 toward the objective lens 221, and passes through the quarter-wave plate 219. Then, the recording medium 251 is irradiated by the objective lens 221 and is diffracted by the interference fringes recorded on the hologram recording layer 253 of the recording medium 251 to generate reproduction light having the same information as the information light at the time of recording.

  The reproduction light is reflected by the reflection layer 255 of the recording medium 251 and exits from the recording medium 251 toward the objective lens 221, and the objective lens 221 forms an information light spatial modulation pattern on the exit pupil plane. Is propagated by the pair of relay lenses 215 and 213 so as to form an image again on the light detection means 255. In the meantime, the reproduction light passes through the quarter-wave plate 219 and is reflected toward the polarization beam splitter 211 by the mirror 217. Since the reproduction light passes through the quarter-wave plate 219 twice compared to the reproduction reference light at the time of irradiation, the polarization direction is shifted by 90 °. Reflected towards. Then, the reproduction reference light is removed by the ring mask 223, and the spatial modulation pattern of the reproduction light is detected by the light detection means 225. The detected information is sent to a control means (not shown), and the control means decodes and reproduces the information.

  In addition, this invention is not limited to embodiment mentioned above, A various change is possible as needed. For example, in FIG. 18, the hologram is formed by irradiating the recording medium with the information light and the reference light from the same surface side of the recording medium so that the optical axis of the information light and the optical axis of the reference light are coaxial. However, a two-beam interference type hologram may be formed by irradiating the recording medium with the information light and the reference light so that the optical axis of the information light and the optical axis of the reference light intersect at a certain angle. In this case, it is preferable that at least one of the light spatially modulated by the spatial light modulator is configured such that the light emitted from the spatial light modulator does not coincide with the mth-order diffracted light as described above.

The figure explaining the relationship of the light in the spatial light modulator of this invention Fourier transform image in a conventional optical information recording / reproducing apparatus Fourier transform image in optical information recording / reproducing apparatus of the present invention The figure explaining the composition of Fourier transform by one-dimensional Fourier transform waveform Table showing one condition of DMD that can implement the present invention Fourier transform image in comparative example Fourier transform image in the present invention Fourier transform image in the present invention Fourier transform image in the present invention The figure explaining the relationship of the light in the transmissive | pervious spatial light modulator of this invention (A) thru | or (C) is a figure for demonstrating the relationship between a spatial modulation pattern and the periodicity t. (A) And (B) is a figure which shows the Fourier-transform image in each spatial modulation pattern. (A) is a conventional (B) is a diagram showing a spatial light modulation pattern of an embodiment of the present invention. Reproduced image of hologram recorded by conventional method An image obtained by reproducing a hologram recorded by the optical information recording method of the present invention Diagram showing the relationship between the size of the opening and the bit error rate of the reproduced reference mark (A) thru | or (H) is a figure which shows other embodiment of the reference mark of this invention. Schematic configuration diagram showing the configuration of the pickup of the optical information recording / reproducing apparatus of the present invention Schematic configuration diagram showing the configuration of a pickup of a conventional optical information recording / reproducing apparatus The figure explaining the relationship of the light in the conventional spatial light modulator The figure explaining the relationship of the light in the conventional spatial light modulator

Explanation of symbols

11 spatial light modulator 12 pixel 13 objective lens 21 incident light 22 outgoing light 23 diffracted light 1302 spatial modulation pattern for reference light 1303 spatial modulation pattern for information light 1306 fiducial mark

Claims (7)

Irradiate the reference light for recording and the information light spatially modulated by the spatial modulation pattern for information light displayed on the spatial light modulator having a plurality of pixels so as to converge on the recording medium by the objective lens. In the optical information recording method for recording interference fringes between the recording reference light and the information light in the hologram recording layer of the recording medium,
The optical axis of the information light from the spatial light modulator toward the objective lens does not coincide with the mth-order (m = 0, ± 1, ± 2...) Diffracted light diffracted by the spatial light modulator.
The spatial modulation pattern for information light includes a plurality of reference marks, and a spatial frequency in a region of the reference mark includes a spatial frequency smaller than a basic spatial frequency of the spatial light modulator. Method. The wavelength of the light incident on the spatial light modulator is λ, the incident angle of the light incident on the spatial light modulator is θ ₀ , the emission angle of the information light emitted from the spatial light modulator is θ ₁ , When the period of the plurality of pixels in the spatial light modulator is a,
( M + 0.2) λ ≦ a (sin θ ₁ −sin θ ₀ ) ≦ (m + 0.8) λ
The optical information recording method according to claim 1, wherein the following relational expression is satisfied . The wavelength of the light incident on the spatial light modulator is λ, the incident angle of the light incident on the spatial light modulator is θ ₀ , the emission angle of the information light emitted from the spatial light modulator is θ ₁ , When the period of the plurality of pixels in the spatial light modulator is a,
( M + 0.5) λ = a (sin θ ₁ −sin θ ₀ )
The optical information recording method according to claim 1, wherein the following relational expression is satisfied .   4. The optical information recording method according to claim 1, wherein a spatial frequency in a region of the reference mark includes a spatial frequency that is half of a basic spatial frequency of the spatial light modulator. 5.   The optical information recording method according to any one of claims 1 to 3, wherein the reference mark includes a pattern in which pixels having different attributes are alternately arranged.   6. The optical information recording method according to claim 1, wherein a direction of an optical axis of the information light is perpendicular to the spatial light modulator.   7. A recording medium, wherein interference fringes are recorded on the hologram recording layer by the optical information recording method according to claim 1.