JP2014086102A - Random phase mask for fourier transform hologram - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a simpler and more efficient method for making a random phase mask by solving problems in a method utilizing a random phase mask to give a random phase modulation to a signal light on a Fourier transform type hologram in which a spatial distribution effect for a light intensity distribution of the random phase mask is too strong and the light intensity distribution at a focal position is unnecessarily-widely spread so that a recording spot on a recording medium expands to pose an obstacle for high recording density; and a random phase mask having an appropriate distribution effect requires enormous calculation processing for its designing.SOLUTION: In the present invention, an effective area of a phase modulation element is divided into minute areas and shapes and sizes of phase modulation areas are spatially randomly distributed in a randomly decided minute area. Random spatial frequency components are generated in a surface of the element and a smooth and contracted light intensity distribution at a focal point of a lens is obtained. Calculation for preparation is simple and routinized and recalculation is not required unless a spatial resolution on a page data to be recorded changes.

Description

  The present invention relates to a random phase modulation pattern applied to page data recording by a Fourier transform hologram and a method for producing the random phase modulation mask.

With the development of an advanced information society, a recording system that records and reproduces a large amount of information in a short time is required.
A recording method using an optical disk such as a DVD has dramatically increased the recording capacity due to the multi-layered structure, but is reaching a limit point in terms of recording speed and reproduction speed.
This is based on the principle of sequentially recording / reproducing data strings developed in time series by one laser head as the disk rotates.

On the other hand, Fourier transform holograms that can collectively record data arrays called page data are known (Non-Patent Documents 1 and 2).
This is a technique in which signal light formed by page data is collected by a lens and is collectively recorded in a recording medium as a Fourier transform image, and can also be collectively reproduced as page data during reproduction.
That is, since it can be recorded / reproduced in parallel as page data, it is considered as a promising candidate for the next generation recording medium.
However, there is a problem in this Fourier transform hologram recording method. Specifically, since the signal light having recorded information is condensed by the lens, a very steep light intensity distribution is generated at the focal position, deviating from the dynamic range in the photosensitive of the recording medium, and the recorded information is deteriorated. is there.

As a technique for solving this problem, a technique has been developed in which a random phase is given to the signal light together with the recording information, and the light intensity distribution by the light collection at the focal position is made smooth (Non-Patent Document 3).
It is widely recognized that this improves the hologram recording characteristics, and is known as an essential element technology in the hologram recording system (Patent Documents 1 and 2, and Non-Patent Document 1).

  However, the smooth light intensity distribution at the condensing position obtained by the random phase increases the distribution of the signal light on the recording medium, that is, the recording spot, and the number of recording spots on the recording medium having a finite area. There is a problem of decreasing (Non-Patent Document 4).

  As a method for solving this problem, a preprocessing calculation process such as an iterative Fourier transform method has been developed, and it has become possible to adjust the size of the recording spot (Non-Patent Document 5).

  However, with regard to the random phase, it is not suitable for high-speed parallel recording with original page data to perform the pre-processing calculation process for each recording spot on the hologram recording system. There is a need for a random phase distribution generation technique that does not require (Patent Document 3).

Patent Document 3 provides a random phase mask pattern that does not require a large amount of calculation, restricts the light intensity distribution in condensing signal light, and narrows the recording spot.
However, it is a specification that requires high-definition manufacturing technology on the sub-wavelength scale based on the principle, and simple manufacturing is difficult.

In Patent Document 4, in a collinear method that is a special form of a Fourier transform hologram, a smooth intensity distribution can be obtained by giving a random phase to both signal light and reference light, and in particular, a pseudo-random configuration is adopted. In some cases, an effect of narrowing the light intensity distribution at the time of condensing is obtained as compared with the case of a completely random phase.
However, the design using the multi-level phase level makes the production process multistage and is not suitable for simple production.

  In the Fourier transform hologram that realizes high-speed parallel recording of page data in this way, a random phase that smooths the focused signal light is necessary, but the recording spot spreads, so this is narrowed. However, there has been a problem that a large amount of calculation processing, a multi-stage manufacturing process, and the like are required.

Japanese Patent Application No. 2006-71382 Japanese Patent Application No. 2007-212577 Japanese Patent Application No. 2006-167648 Japanese Patent Application No. 2004-295169

H.J.Coufal, 2 others edited, "Holographic Data Storage", Springer, 2000, 10 May 1996 / vol.35, No.14 / APPLIED OPTICS, pp2389-2398 “High-density recording in photopolymer-based holographic three-dimensional disks” March 1970 / vol.9, No.3 / APPLIED OPTICS, pp695-700 “Use of a Random Phase Mask for the Recording of Fourier Transform Holograms of Data Masks” January 1972 / vol.11, No.1 / APPLIED OPTICS, pp182-191 “Some Aspects of a Large Capacity Holographic memory” July 1988 / vol.5, No.7 / Journal of Optical Society of America A, pp.1058-1065 “Iterative Fourier-transform algorithm applied to computer holography”

  Therefore, there is currently a demand for a technique for narrowing the light intensity distribution and balancing the quality of the hologram and the high recording density while having a smooth light intensity distribution due to the effect of the random phase.

In the present invention, first, the following random phase modulation pattern can be provided.
(1) A binary random phase modulation pattern used for a random phase mask for optical phase modulation used for Fourier transform type holographic optical information recording / reproduction, wherein the random phase modulation pattern is adjacent to the same pixel The square area is divided into square micro-areas, and the micro-area is an invalid area that is not phase-modulated, or a square phase-modulation area that is smaller than the micro-area and has the same pixel size is defined on each side of the square. It is determined at random whether to arrange in parallel with each side of the micro area, and the phase modulation area determined at random with respect to the micro area is included in the micro area so that all of the phase modulation area is included in the micro area. A random phase modulation pattern characterized by being randomly arranged with respect to a region.

Next, in the present invention,
(2) A binary random phase modulation pattern used for a random phase mask for optical phase modulation used for Fourier transform type holographic optical information recording / reproduction, wherein the random phase modulation pattern includes the same pixel Divided into square micro-regions of the size, and with respect to the micro-regions, the phase-modulable regions of any size and the same pixel size are set as invalid regions that are not phase-modulated. It is determined at random whether to arrange in parallel with each side of the minute region, and the phase-modulable region determined at random with respect to the minute region is included in the minute region in its entirety or a part thereof Are arranged randomly with respect to the minute region so that they overlap each other (except when the whole region of the minute region overlaps). In the random phase modulation pattern, a random phase modulation pattern characterized in that a part of the phase-modulable region that overlaps all of the included regions is a phase modulation region,
Can provide.

Next, the present invention
(3) A random phase mask characterized by being optically phase-modulated with the random phase modulation pattern described in (1) or (2),
Can provide.
further,
(4) Fourier transform hologram type optical information recording / reproducing apparatus, characterized in that the random phase mask described in (3) is used,
Can provide.

Finally, the present invention
(5) A method for producing a binary random phase modulation pattern for use in a random phase mask for optical phase modulation used for Fourier transform type hologram optical information recording / reproduction, wherein the random phase modulation patterns are adjacent to each other. Each of the squares is divided into square micro-regions having the same pixel size, and the micro-regions are invalid regions that are not phase-modulated, or square phase-modulation regions that are smaller than the square micro-regions and have the same pixel size. Whether the side is arranged in parallel with each side of the minute region is randomly determined, and the phase modulation region randomly determined for the minute region is included in the minute region so that the entire region is included in the minute region. A method for producing a random phase modulation pattern, characterized by being randomly arranged with respect to a minute region;
When,
(6) A method for producing a binary random phase modulation pattern used for a random phase mask for optical phase modulation used for Fourier transform hologram optical information recording / reproduction, wherein the random phase modulation patterns are adjacent to each other. It is divided into square micro-regions of the same pixel size, and regarding the micro-region, an invalid region that is not phase-modulated or a square phase-modulable region of the same pixel size with an arbitrary size is assigned to each side of the square. It is randomly determined whether to arrange in parallel with each side of the minute region, and the phase-modulable region randomly determined with respect to the minute region is entirely included in the minute region or a part thereof. Labels that are randomly arranged with respect to the minute region so as to overlap (except when overlapping in the entire region of the minute region). In a method for producing a random phase modulation pattern, a method for producing a random phase modulation pattern, wherein a part of the phase-modulable region that overlaps all of the included regions is defined as a phase modulation region,
Can provide.

  In the random phase mask pattern of the present invention, when the effective area of the random phase mask is divided into minute regions, the phase modulation region is included in the minute region determined at random, and the entire region is included in the minute region. Therefore, a random spatial frequency component is generated in the element surface, and a smooth and narrow light intensity distribution can be obtained at the condensing position of the lens.

  In addition, when a part of the region overlaps the minute region, the shape and size of the phase modulation region in the minute region can be modulated more randomly, and a smoother distribution can be obtained.

  Phase modulation is binary modulation with binary values of 0 and Δφ, and the manufacture of multi-level phase modulation elements requires extremely complicated manufacturing processes and processing accuracy, while in the actual manufacturing process of the present invention, It has become possible to produce in a simple process.

  In addition, the randomization calculation for preparation may be simple by a routine that does not include repetition by condition determination, and recalculation is not necessary unless the spatial resolution in the recorded page data is changed.

FIG. 1 is an overall view showing random phase modulation using a spatial light modulator. FIG. 2 is a diagram showing an outline of a random phase mask pattern produced using a spatial light modulator. FIG. 3 is a schematic diagram of a small region of a random phase mask pattern. FIG. 4 is a comparison diagram of Fourier transform images of random phase distribution. FIG. 5 is a comparison diagram of intensity distributions of Fourier transform images of the conventional example and the random phase distribution of the present application. FIG. 6 is a diagram showing the measurement of the light intensity distribution (Fourier transform image) at the condensing position by the lens. FIG. 7 is a comparison diagram of measurement results of the light intensity distribution at the lens condensing position (origin position). FIG. 8a is a comparative view of the light intensity distribution at the condensing position by the lens. FIG. 8b is a diagram for explaining the recording density of data.

Broadly classifying means for giving spatially random phase distribution to light, there are two types: a method using a spatial light modulator and a method using a photomask.
The former has the feature that the spatial distribution of the phase can be changed at any time by computer control (FIG. 1).
The latter uses a photomask prepared in advance so as to give a specific random phase distribution, and is suitable when it is not necessary to change the random phase distribution at any time, resulting in a great cost advantage.
However, although both can be used properly depending on whether the distribution needs to be changed over time or not, there is no difference between them when it comes to the function of giving a random phase to light.

Therefore, in the present invention, a method for producing a random phase modulation pattern (random phase mask pattern) using a spatial phase modulator will be described below.
The design of the random phase distribution is performed in accordance with the requirements for use in hologram recording.
As the biggest restriction, it is preferable that the spatial resolution of the random phase modulation is equal to or higher than the spatial resolution of the page data that is information recorded by the hologram.

A specific design procedure will be described with reference to FIG.
(Manufacturing Method A) (A-1) First, the element of the spatial light modulator is divided into w × w pixel regions (hereinafter referred to as micro regions). In FIG. 2, w = 200 for illustration.
The effective number of pixels of the spatial light modulator used in this study is about 800 in the horizontal direction (x-axis direction) and 600 in the vertical direction (y-axis direction), so it was divided into 4 × 3 = 12 areas. become.

(A-2) Next, it is randomly determined whether to apply Δφ phase modulation to each minute region or to make it zero.
The minute region to which the phase change amount 0 is given becomes a blank on the pattern.
Randomization may be performed by appropriately setting the probability that the above-described 12 minute regions become blank regions, such as 50%.

  (A-3) A v × v pixel region (hereinafter referred to as a phase modulation region: v <w) is arranged at the center of the minute region for each minute region to which phase modulation is to be applied. As described above, the size of w and v is preferably higher than the resolution of page data at the time of hologram recording.

(A-4) Next, movement amounts δx and δy in the x-axis and y-axis directions of each phase modulation region are determined at random.
Randomization may be set as appropriate in addition to setting all possible combinations of possible movement amounts δx and δy in the x-axis and y-axis directions to equal probabilities.

(A-5) In order to move from the center position, positive and negative movement amounts are respectively defined. The maximum absolute value δmax of δx and δy is assumed to be δmax = (w−v) / 2 on the premise that even a part of the phase modulation region does not protrude from the minute region. Therefore, for each phase modulation region, δx and δy are randomly determined in the ranges of −δmax ≦ δx ≦ + δmax and −δmax ≦ δy ≦ + δmax, respectively, and move in each minute region.
A phase change amount Δφ to be given to each phase modulation region moved at random is determined.

  (A-6) Data of the random phase modulation pattern determined as described above is generated by a calculation program in the computer, transmitted to the spatial light modulation, and a phase change is given to the incident light.

(Production Method B) Another production method has been found by expanding the production method A of the random phase modulation pattern described above.
The manufacturing method is almost the same, but the steps (A-3), (A-4), and (A-5) are different.
This will be specifically described with reference to FIG.

In the manufacturing method A, a constraint condition of v <w and a maximum value δmax of absolute values of δx and δy are determined for a v × v pixel region so that each phase modulation region does not protrude even a minute region. However, it is assumed here that this restriction is removed, and the phase modulation region (phase modulation possible region) may be smaller or larger than the minute region, and a part thereof may protrude.
However, the phase modulation of the region (hatched portion on the right in FIG. 3) that protrudes from the minute region to which each phase modulation region (phase modulation possible region) belongs is invalid (0).

Therefore, the phase modulation regions in the adjacent minute regions do not affect each other, and the shape and the size change in the belonging minute region by protruding.
The determination of the movement amount (A-4) and (A-5) changes as follows.

(B-4) Next, movement amounts δx and δy in the x-axis and y-axis directions of each phase modulation region (phase modulation possible region) are randomly determined.
In order to move from the center position, positive and negative movement amounts are respectively defined.
Since the phase modulation possible region may protrude from the minute region, the maximum value δmax of the absolute value of δx and δy is arbitrarily set in principle, but the minute region is completely protruded and the minute region is blank. Consider the region so that there is no bias in randomness.
For each phase-modulable region, δx and δy are randomly determined in the ranges of −δmax ≦ δx ≦ + δmax and −δmax ≦ δy ≦ + δmax, respectively, and move in each minute region.

(B-5) With respect to the maximum movement distance δmax, if δmax is set to be larger than the condition that δmax> (w−v) / 2 + v, ie, (w−v) / 2 + v, where the phase-modulable region completely protrudes from the minute region This increases the probability that a blank will be generated in a minute region, and causes a bias in randomness.
Further, under the condition that the minute region is completely included in the phase modulation possible region, that is, when v ≧ w, for example, in the case of horizontal movement, δx in the range of 0 ≦ δx, δy ≦ (v−w) / 2, When δy is set, the probability that a state in which phase modulation is performed over the entire small region is increased, and this causes a bias in the randomness of the phase modulation region over the entire w × w region.
Therefore, it is only necessary to move in a random manner so that the combinations of the movement amounts δx and δy all have equal probabilities within the range considering the above-described case.
A phase change amount Δφ to be given to a region (phase modulation region) included in a minute region among the phase modulation possible regions that are randomly moved is determined.
Random phase modulation pattern manufacturing method B is used to eliminate the regularity that the size of the phase modulation region is constant in all the minute regions in random phase modulation pattern manufacturing method A and to further increase randomness. It is an added method.

In this example, verification was performed with a random phase modulation pattern having a size of 600 × 600 pixels.
FIG. 5 shows an extracted pixel size of 40 × 40.
A random phase modulation pattern in which the micro area is w = 6 and the phase modulation area is v = 4 was produced.
In the randomization of the blank area, the probability of blanking in the entire random phase pattern was set to 50%.
In addition, with respect to the random arrangement of the phase modulation (possible) regions in each micro area of the manufacturing method A and the manufacturing method B, each arrangement has a substantially equal probability. Randomization was done with care not to be.

In recording a Fourier transform hologram, when signal light is collected by a lens, it is desired to smooth the light intensity distribution at the focal position and at the same time narrow the spatial distribution appropriately.
The fact that the random phase modulation pattern produced by the above procedure solves the above problem is verified by examining the light intensity distribution at the focal point when the light modulated by the random phase distribution is collected by the lens. can do.

It is known that the calculation of the light intensity distribution at the lens focal position may be performed by spatially Fourier transforming the phase distribution of incident light, that is, the random phase modulation distribution here.
In order to verify the effect of the present invention, a result of calculating a Fourier transform image of a general random phase distribution and the random phase distribution of the present invention is compared.

As shown in FIG. 4, when a Fourier transform image is calculated for a general random phase distribution, the light intensity distribution is distributed over a wide range.
On the other hand, when the production method A of the present invention is used, the overall lightness is smooth and the band is narrowed in the center, and each phase modulation region is spatially randomly distributed, so that an ideal light intensity distribution is obtained. Something close to is obtained.

However, there is a partial strength.
When the production method B of the present invention is used, a smoother distribution is obtained, and the effect of randomly changing the shape and size of the phase modulation distribution is obtained.

In FIG. 5, in order to clarify these effects, I (x) = ∫I (x, y) dy is calculated and plotted with the light intensity distribution of the Fourier transform image of FIG. 4 as I (x, y). The results of comparison are shown.
It can be confirmed that a smooth Fourier change image narrowed by the production method A of the present invention is obtained, and that a smoother distribution is obtained by the production method B.

For the above pattern, the random phase distribution shown in FIG. 4 is transmitted from the computer to the spatial light modulator (Hamamatsu Photonics, LCOS-SLM) as shown in FIG. 6, and the actual laser light (Toptica Photonics, 405 nm) is transmitted. Phase modulation was applied, light was collected by a lens (focal length 300 mm), and the light intensity distribution at the light collection position was measured.
This light intensity distribution corresponds to the Fourier transform image shown as the calculation result in FIG.
FIG. 7 shows the light intensity distribution actually measured by the photodetector.

When a general phase distribution is used, light is distributed over a wide range.
When the phase distribution according to the manufacturing method A of the present invention is used, although the condensing point is partially seen, the intensity distribution is smooth and narrow as a whole.

  Furthermore, when the production method B of the present invention is used, a smoother intensity distribution is obtained. These results are completely consistent with the calculation results shown in FIG.

To further clarify these results in FIG. 8a, I (x) = ∫I (x, y) dy is calculated and plotted with the light intensity distribution of FIG. 7 as I (x, y). The comparison result is shown.
In FIG. 8a, similarly to the result shown in the calculation result of FIG. 5, a narrowed light intensity distribution is generated by the manufacturing method A of the present invention, and a smoother light intensity distribution is obtained by the manufacturing method B. Is shown.

With reference to FIG. 8b, a rough comparison of recording densities is shown next.
In consideration of the non-linear photosensitive characteristics of the medium, there is a region where the light intensity at both ends (about 2500 pixels in diameter) of a general random phase distribution spread and the same light intensity of the manufacturing method B of the present invention are exposed. Assuming that it contributes effectively to the generation of a reconstructed image, the production method B of the present invention records data in an area having a diameter of about 1000 pixels (about 750 to about 1750 on the x axis in FIG. 8b) from FIG. 8b. Can be estimated.

Since the number of bits of page data recorded per focused spot does not change, the recording density is increased according to the spot area ratio, (2500/1000) ² = 6.25, that is, at least about 6 times higher than the conventional example. It can be seen that the recording density is achieved by the method according to the present invention.

At present, regarding a random phase modulation pattern applied to page data recording by a Fourier transform hologram and a method for manufacturing the random phase modulation mask, an optical element for imparting a spatially promising random phase change has multiple values (multiple values). Many of them have been proposed.
However, the random phase mask of the present invention expresses a desired effect in binary (binary), can greatly reduce the difficulty of manufacturing, and can be expected to have superiority in industrial applications. .
When the present invention is used, the calculation of randomization for preparation is simple by a routine that does not include repetition by condition determination, and recalculation is not necessary unless the spatial resolution in the page data to be recorded is changed. The manufacturing process became simpler.

DESCRIPTION OF SYMBOLS 1 Incident light 2 Output light 3 Spatial light modulator 4 Beam splitter 5 Computer 6 Photo detector 7 Phase distribution data 8 Transmission 9 Lens

Claims (6)

A random phase modulation pattern consisting of two values used for a random phase mask for optical phase modulation used for Fourier transform type hologram optical information recording / reproduction,
The random phase modulation pattern is divided into adjacent square microregions of the same pixel size,
Whether the micro area is an invalid area that is not phase-modulated, or a square phase modulation area that is smaller than the micro area and has the same pixel size is arranged with each side of the square parallel to each side of the micro area Is determined at random,
The random phase modulation pattern characterized in that the phase modulation areas determined randomly with respect to the micro area are randomly arranged with respect to the micro area so that all of the phase modulation areas are included in the micro area. A random phase modulation pattern consisting of two values used for a random phase mask for optical phase modulation used for Fourier transform type hologram optical information recording / reproduction,
The random phase modulation pattern is divided into adjacent square microregions of the same pixel size,
Whether the micro area is an invalid area that is not phase-modulated or a square phase-modulable area having an arbitrary size and the same pixel size is arranged with each side of the square parallel to each side of the micro area Is determined at random,
The phase-modulable region determined at random with respect to the minute region is entirely included in the minute region or partially overlaps (except when overlapping in the entire region of the minute region). In the random phase modulation pattern randomly arranged with respect to the minute region,
A random phase modulation pattern characterized in that a part of the phase-modulable region that overlaps all of the included regions is a phase modulation region.   A random phase mask that is optically phase-modulated with the random phase modulation pattern according to claim 1.   4. A Fourier transform hologram optical information recording / reproducing apparatus using the random phase mask according to claim 3. A method for producing a random phase modulation pattern consisting of two values used for a random phase mask for optical phase modulation used for Fourier transform type hologram optical information recording / reproduction,
The random phase modulation pattern is divided into adjacent square microregions of the same pixel size;
With respect to the minute area, it is set as an invalid area that is not phase-modulated, or a square phase-modulated area that is smaller than the square minute area and has the same pixel size is arranged with each side of the square parallel to each side of the minute area. Is decided at random,
A method for producing a random phase modulation pattern, wherein the phase modulation regions determined randomly with respect to the minute region are randomly arranged with respect to the minute region so that the entire region is included in the minute region. A method for producing a random phase modulation pattern consisting of two values used for a random phase mask for optical phase modulation used for Fourier transform type hologram optical information recording / reproduction,
The random phase modulation pattern is divided into adjacent square microregions of the same pixel size;
Whether the micro area is an invalid area that is not phase-modulated or a square phase-modulable area having an arbitrary size and the same pixel size is arranged with each side of the square parallel to each side of the micro area Is randomly determined,
The phase-modulable region determined at random with respect to the minute region is entirely included in the minute region or partially overlaps (except when overlapping in the entire region of the minute region). In addition, in a method for producing a random phase modulation pattern that is randomly arranged with respect to the minute region,
A method for producing a random phase modulation pattern, characterized in that a part of the phase-modulable region that overlaps all of the included regions is defined as a phase modulation region.

Images