JP5415540B2 - Angle control method - Google Patents

Description

  The present invention relates to an angle control method for hologram recording / reproduction.

  Optical information recording media represented by CDs (Compact Discs), DVDs (Digital Versatile Discs), BDs (Blu-ray Discs), etc. have hitherto been used mainly for shortening the wavelength of laser light and the numerical aperture (NA) of objective lenses. Increasing the recording density has responded to the increase in recording density. However, all of them are said to be approaching the limit due to technical reasons and the like, and there is a demand for an increase in recording density by other means and methods.

  Among various proposals, in recent years, volume recording type high-density optical recording (hereinafter referred to as “holographic memory”) using holography and development of a recording / reproducing apparatus for holographic memory have been developed for practical use. ing. Holographic memory recording methods generally use information light generated by spatially modulating laser light with a spatial modulator such as a liquid crystal element or digital micromirror device, and information at the same wavelength as the information light. Reference light generated from the same light source as the light is irradiated to the same location in the recording medium, and at that time, light interference fringes generated by the information light and the reference light are recorded in the recording medium.

  When reproducing the holographic memory, the information light at the time of recording is reproduced by irradiating only the reference light, and the information modulated at the time of recording can be acquired. In contrast to the so-called surface recording method, in which recording marks are recorded on the recording surface, such as a DVD, the holographic optical disk is a volume recording method capable of recording in the thickness direction of the information recording layer. Larger recording density can be obtained.

  In the case of a DVD or the like, the recording mark generally represents ON / OFF bit data. In the case of a holographic memory, information light is collectively modulated by a relatively large amount of information and recorded as interference fringes. This set of information is a modulation pattern of information light held in a recording medium, which is a minimum unit of recording / reproduction in the form of a two-dimensional barcode composed of black and white dots, and is called page data.

  One of the methods for increasing the recording density of the holographic memory is an angle multiplex recording method. This multiplex recording method is a method of recording a plurality of page data while shifting the irradiation angle of the laser beam at the same place in the holographic memory.

  In order to read page data recorded by angle multiplexing, the desired page data can be read only by reproducing the relative angle between the laser beam during recording and the holographic memory recording medium during reproduction. doing.

  However, as the storage capacity increases, the recording angle step becomes minute, and there is a problem that strict angular accuracy is required during reproduction. As is generally considered, in the control using only the position information from the encoder of the drive mechanism itself, it is very difficult to satisfy the required accuracy considering the influence of the error of the medium mounting position and the error between the recording device and the playback device. It is difficult to.

  In order to solve the above problems, various conventional techniques have been invented.

  For example, Patent Document 1 proposes a method of generating an angle error signal based on reproduction light from adjacent page data reproduced at the same time.

  For example, Non-Patent Document 1 describes that a good signal can be obtained by controlling the relative angle between the reference light and the medium so that the reproduction light intensity received by a CMOS sensor or the like is maximized. .

  However, in the method of Non-Patent Document 1, it takes time to optimize the reproduction light intensity.

JP 2008-16170 A

2006 Optical Data Storage Topical Meeting Conference Proceedings MP4 “The Angle Align method of Reference Beam for Holographic Data Storage”

  Accordingly, the present invention provides a recording / reproducing angle control method of a holographic memory that can be quickly controlled to an optimum reading angle.

The angle control method of the present invention includes a step of irradiating the optical information recording medium on which a hologram generated by interference between information light and reference light is formed, extracting the reproduction light, and the optical information recording medium surface. And rotating the angle between the optical axis of the reference light around the X axis represented by the intersection of the optical information recording medium surface and the plane formed by the information light and the reference light, and the information The first angle around the X axis and the second angle around the Y axis at which the intensity of the reproduction light is maximized are obtained by rotating around the Y axis perpendicular to the plane formed by the light and the reference light. Any one of a step, a step of setting the first angle around the X-axis to an angle offset by a predetermined offset angle, an angle at which the optical information storage medium and the reference light are incident, The optical information recording medium is shaped while rotating around the Y axis. Irradiating the hologram with the reference light to extract the reproduction light, and detecting the reproduction light with a photodetector provided with two light receiving areas equally divided symmetrically with respect to the optical axis center, A step of extracting an angle error signal by calculating a difference between signals relating to received light intensity output from the two light receiving areas detected by a photodetector; and the optical information recording medium so as to make the angle error signal zero. Alternatively, the method includes a step of controlling the angle by rotating any one of the angles of incidence of the reference light around the Y axis.

  According to the present invention, the holographic memory can be controlled to an optimum reading angle in a short time.

1 is a schematic diagram of a holographic memory according to a first embodiment of the present invention. The figure explaining the 1st Embodiment of this invention. The figure explaining the 1st Embodiment of this invention. The figure explaining the 1st Embodiment of this invention. The figure explaining the 1st Embodiment of this invention. The figure explaining the 1st Embodiment of this invention. The figure explaining the 1st Embodiment of this invention. The figure explaining the 1st Embodiment of this invention. The figure explaining the 1st Embodiment of this invention. The figure explaining the 1st Embodiment of this invention. The figure explaining the 1st Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings to be described below, the same reference numerals indicate the same parts, and duplicate descriptions are omitted.

(First embodiment)
FIG. 1 is a schematic view showing a holographic memory according to the present embodiment.

  A collimator lens for shaping the laser light is omitted.

  The holographic memory 10 according to this embodiment includes a laser light source 20, quarter-wave plates 30 and 40, polarizing beam splitters 50, 60, 70 and 75, and lenses 80, 90, 110, 120, 130 and 140. 145, condenser lens 100, mirror 150, spatial light modulator 160, shutter 170, imager 180, photodetector 185, conjugate mirror 190, holographic recording medium (optical information recording medium, hereinafter referred to as recording) 200).

  The laser light source 20 is used for recording / reproducing of the holographic memory. The laser light source 20 generally uses a combination of a green or blue-violet semiconductor laser and an external resonator (not shown) for stabilizing the wavelength. In addition, other wavelengths and laser light such as so-called DFB laser, SHG laser, solid laser, or gas laser may be used.

  The quarter-wave plates 30 and 40 convert linearly polarized light into circularly polarized light.

  The polarization beam splitters 50, 60, 70, and 75 separate the polarization components of light, partially transmit, and partially reflect. The polarization beam splitter 50 separates the light emitted from the light source 20 into the reference light R and the information light I. The reference light R is also used as reproduction light S when reproducing the holographic memory.

  The spatial light modulator 160 generates the information light I by intensity-modulating the incident light into a lattice-like binary pattern of bright and dark. As the spatial light modulator 160, a liquid crystal element, a digital micromirror device, a reflective FLCOS (Ferroelectric Liquid Crystal On Silicon), or the like can be used. The spatial light modulator 160 can switch between light transmission and light shielding according to the pixel value.

  The shutter 170 is opened during recording in the holographic memory and closed during reproduction.

  The conjugate mirror 190 inverts and reflects the phase component of the light.

  The imager 180 can be a two-dimensional image sensor such as a CCD or CMOS, or a one-dimensional linear image sensor. Moreover, it can also comprise so that an imaging tube may be used.

  The photodetector 185 can be a two-divided or four-divided photodetector. A CCD sensor or a CMOS sensor can also be used. The light incident on the photo detector 185 is adjusted so as to enter the center of the photo detector 120.

  The recording medium 200 is a transmission type recording medium, and has a structure in which a hologram recording material 210 is sandwiched between two substrates 220 and 230 as shown in FIG.

  The substrates 220 and 230 are made of a light transmissive material such as glass, polycarbonate, or acrylic resin that is transmissive to the wavelength of the laser light source 20 to be used.

  The hologram recording material 210 is a material on which a hologram is formed by causing the information light I and the reference light R to interfere with each other. The hologram recording material 210 is generally formed of a radical polymerization type material called a photopolymer, and includes a radical polymerizable compound, a photo radical polymerization initiator, a matrix material, and the like.

  A dichroic mirror (not shown) may be provided between the recording medium 200 and the image pickup device 180.

  FIG. 3 shows a block diagram when processing is performed after the reproduction light S is detected by the imaging device 180 and the photodetector 185. Information light I is omitted.

  At the time of reproduction, the light emitted from the light source 20 is extracted as the reference light R by the polarization beam splitter 50 and guided to the recording medium 200 via the mirror 150. The reproduction light S obtained from the recording medium 200 is guided to the image pickup device 180 and the photodetector 185 via the polarization beam splitter 70.

  Since the reproduction light S detected by the imager 180 reads information in the form of a two-dimensional barcode, it takes time for data processing. On the other hand, since the photodetector 185 can perform processing quickly in order to detect only the intensity of the reproduction light S, the photodetector 185 is controlled to θy that maximizes the intensity of the reproduction light S. Data processing is performed by the imager 180.

  At this time, the angle error signal generation unit 240 generates an angle error signal from the signal detected by the photodetector 185. The angle control unit 250 controls the angle to an appropriate angle using the angle error signal. The angle control unit 250 controls the angle of the recording medium 200 or the mirror 150 for changing the angle of the reference light R so as to make the angle error signal zero based on a command from the host CPU 260. The angle error signal will be described later.

  As shown in FIG. 4, the photodetector 185 is divided into two parts in the vertical direction. From the photodetector 185, a current signal Ia corresponding to the light intensity of the reproduction light S in the upper half divided into two and a current signal Ib corresponding to the light intensity of the reproduction light S in the lower half are output.

  The current signals Ia and Ib are input to the angle error signal generation unit 240, converted into voltage signals by the current amplifier, and the angle error signal Ve is generated by the difference calculation circuit.

  The X ′, Y ′, and Z ′ coordinate axes are taken with respect to the photodetector 185. The Z ′ axis is the optical axis direction of the reproduction light. When the X′Z ′ plane and the XZ plane of the recording medium 200 are in the same plane, the Y ′ axis is an axis parallel to the Y axis of the recording medium 200. The X ′ axis is an axis orthogonal to the Y ′ axis in the light receiving surface of the photodetector 185 and is parallel to the direction of rotation of the recording medium 200 around the Y axis.

  The intersection of the X ′ axis and the Y ′ axis indicates the optical center of the photodetector 185. In other words, the X ′ axis corresponds to a dividing line in which two light receiving regions of the upper half and the lower half of the photodetector 185 are provided.

  Next, the operation principle will be described.

(Recording to holographic memory)
In the present embodiment, a two-beam optical system is used, which is a system in which the information light I and the reference light R are incident on the holographic memory recording medium 200 so as to overlap the recording well medium 200 through separate lenses or the like.

  In the present embodiment, as shown in FIG. 5, the xyz coordinate axis is taken. The thickness direction of the recording medium 200 is the z-axis direction. Here, the X-axis direction and the Y-axis direction exist in the recording medium 200 plane. Note that the recording medium 200 may have a rectangular shape as shown in FIG. 5 or a disk shape. When a rectangular shape is used, a lithium niobate crystal can be used. In the case of a disk shape, a photopolymer described later can be used.

  The recording medium 200 performs recording by angle multiplexing by rotation around the Y axis (θy rotation). Further, not only the rotation of the y axis but also a rotation driving mechanism (normal disk rotation) around the z axis and a rotation driving mechanism around the x axis (θx rotation) for fine adjustment can be used. As the rotation driving mechanism of the recording medium 200, for example, a small spherical ultrasonic motor or the like can be used.

  At the time of recording, as shown in FIG. 1, the light emitted from the laser light source 20 passes through the quarter wavelength plate 30 and enters the polarization beam splitter 50.

  Part of the light incident on the polarization beam splitter 50 is transmitted and part of it is reflected. The light diameter of the reflected light is adjusted by the lenses 80 and 90, a part of the light is reflected by the polarization beam splitter 60, and enters the spatial light modulator 160. The light incident on the spatial light modulation 160 is reflected as information light I toward the polarization beam splitter 70.

  The light reflected toward the polarization beam splitter 70 passes through the polarization beam splitter 70 and is condensed on the recording medium 200 by the condenser lens 100.

  At this time, the shutter 170 is open.

  On the other hand, the light transmitted through the polarization beam splitter 50 is reflected by the mirror 150 and the diameter of the light is adjusted by the lenses 110 and 120, and the reference light R is irradiated to the position where the information light I is condensed on the recording medium 200. The

  At this time, interference fringes (holograms) are formed on the hologram recording material 210 in the recording medium 200. This is because the photopolymerization initiator in the photopolymer constituting the hologram recording material 210 is activated by absorbing photons.

  That is, the monomer is consumed by initiating / promoting the polymerization of the monomer at the interference fringe bright part. When the monomer is consumed, the monomer is moved and supplied from the dark part of the interference fringe to the bright part, resulting in a density difference between the bright part and the dark part of the interference fringe pattern. Thereby, refractive index modulation according to the intensity distribution of the interference fringe pattern is formed, and recording is performed on the recording medium 200.

(Reproduction of holographic memory)
Next, an operation for reproducing θy by angle control will be described.

  FIG. 6 shows a flowchart for performing reproduction by angle control of θy according to the present embodiment.

  In step S10, the holographic recording medium 200 or the reference light R is moved to an area where the page data is angle-multiplexed recorded (hereinafter referred to as a book). For example, for the movement at this time, a servo mark is recorded in advance on the recording medium 200, and the servo mark is detected (using a mark detector (not shown)) to move to a predetermined book. it can.

  The holographic recording medium 200 holds information light I at the time of hologram recording and interference fringes (holograms) of the reference light R as a refractive index distribution.

In step S20, initial adjustment of the angles θx and θy is performed. The angle between the reference light R and the holographic recording medium 200, θx, and θy are finely adjusted so that any one of the page data recorded by angle multiplexing can be reproduced. Here, since the purpose is to obtain the initial position, θx and θy are adjusted using the hill climbing method or the like so that the intensity of the reproduction light S is maximized. Reference angles θx and θy after adjustment are θx ₀ and θy ₀ , respectively.

In step S30, as shown in Equation 1 below, is offset [theta] x _ofs the [theta] x from the reference angle [theta] x _0. The offset indicates a difference from the reference position. At this time, the recording medium 200 may be tilted using a drive mechanism (not shown) to change θx. Further, θx may be changed by changing the incident angle of the reference light R with respect to the recording medium 200.

7 and 8 are schematic views showing the relationship between the medium and the reference light when viewed from the X-axis direction. 7 shows the positional relationship when the recording medium 200 at the time of reproduction is tilted by Δθx from the position of the recording medium 200 at the time of recording. FIG. 8 shows the position when the reference light at the time of reproduction is tilted by Δθx from the reference light at the time of recording. It is a relationship. In either case, θx is defined by the angle between the reference beam and the medium when projected onto the YZ plane.

And The optimum offset amount at this time depends on the numerical aperture NA of the lens, the thickness of the hologram recording material 210 constituting the recording medium 200 shown in FIG. The optimum offset amount is the minimum amount that the amplitude of the angle error signal obtained when the offset amount is gradually increased exceeds a certain threshold, and the bit error rate of the reproduced data is below a predetermined value. Any value may be used as long as the maximum θx offset amount is within the range. The bit error rate is a ratio of error data in reproduced data. Alternatively, the angle error signal may be determined so that the amplitude is maximized.

The angle error signal Ve detected by the photodetector 185 is expressed by the following Expression 2 using a voltage signal Va corresponding to the upper half light intensity and a voltage signal Vb corresponding to the lower half light intensity.

Further, by dividing by the sum signal of the light intensity of the reproduction light S as shown in the following expression 3, the offset caused by the change in the light amount can be canceled out.

Expressions 2 and 3 can be changed as appropriate depending on the number and arrangement of the photodetectors 185.

  FIG. 9 shows a simulation result of the luminance distribution when the light of the reproduction light S is detected by the photodetector 185. The numbers (1) to (7) correspond to the angle Δθy shown on the horizontal axis. Here, Δθy indicates a deviation from θy during recording. Therefore, an appropriate angle at the time of reproduction is θy = 0 ° and Δθy = 0 °.

  FIG. 9A shows the result when Δθy is changed from −0.03 ° to 0.03 ° in the state where θx = 0 °.

  It can be seen that the luminance distribution when Δθy is changed from 0 ° to −0.03 ° is substantially the same as the luminance distribution when Δθy is changed from 0 ° to 0.03 °.

  In this state, it is difficult to distinguish whether Δθy is negative or positive based on the luminance distribution.

  FIG. 9B shows a result when Δθy is changed from −0.03 ° to 0.03 ° in the state where θx = 0.01 °.

  It can be seen that the luminance distribution when Δθy is changed from 0 ° to −0.03 ° is brighter in the upper part and darker in the lower part.

  On the other hand, the luminance distribution when Δθy is changed from 0 ° to 0.03 ° shows that the upper part is dark and the lower part is bright.

  In this way, by changing the rotation angle around the X axis of the recording medium 200 or the incident angle of the reference light R to change θx, it is possible to easily grasp which direction Δθy is inclined to plus or minus. be able to. That is, by changing θx, the difference between the bright part and the dark part appears above and below the photodetector 185, so that the angle error signal can be easily detected.

  In step S40, it is determined whether the sum signal of the light intensity of the reproduction light S has reached a certain detection value.

  In step S50, if the detection value determined in step S40 has not been reached, θy is driven in any direction by open control to search for page data. The open control is a control that mechanically moves to θy during recording without using a servo signal. At this time, an in-plane intensity distribution is generated in the reproduction light S. The reproduction light S is received by the photodetector 185, and an angle error signal is obtained by calculating the difference.

  In step S60, if the detection value determined in step S40 has been reached, it is determined whether or not the angle error signal is zero for θy.

  FIG. 10 shows the sum signal of the angle error signal and the light intensity detected using this embodiment.

  The horizontal axis represents the angle Δθy, the left vertical axis represents the angle error signal, and the right vertical axis represents the total light intensity signal detected by the photodetector 185. The broken line shown in FIG. 10 indicates the difference (angle error signal) between the upper half current signal Ia and the lower half current signal Ib detected by the photodetector 185, and the solid line indicates the total light intensity signal detected by the photodetector 185. Indicates.

  The value (the point indicated by the arrow 270) at which the total signal of the light intensity is the maximum is the value of θy at which the image pickup device 180 can reproduce the reproduction light most clearly.

  The offset of θx was set to 0.01 °, NA was set to 0.65, and the thickness (z-axis direction) of the hologram recording material 210 was set to 1.5 mm.

  In FIG. 10, Δθy at which the angle error signal becomes zero is shifted in the positive direction. This is because the Bragg condition is changed by adding a slight offset to θx.

  Further, the angle at which the angle error signal (broken line in FIG. 10) is zero (the point indicated by the arrow 280) is the angle at which the image pickup device 180 can reproduce the reproduction light most clearly. That is, when the angle error signal becomes zero, the sum signal of the light intensity is maximum.

  Therefore, if only the angle error signal is monitored so that the angle error signal becomes zero, the optimum reproduction angle can be controlled.

  For example, if an angle error signal is detected at a point indicated by an arrow 290, the optimum reproduction angle can be controlled by setting Δθy in a negative direction. Here, setting Δθy in a negative direction indicates that the recording medium 200 is rotated in the Y axis.

  FIG. 11 shows the angle error signal and the sum signal of the light intensity detected by using the present embodiment by angle multiplex recording in which recording is performed while rotating θy. FIG. 11 shows the sum signal of the angle error signal and the light intensity of the three page data.

  The horizontal axis indicates the angle of θy. The vertical axis is the same as in FIG.

  In the case of angle multiplex recording, the angle of θy can be controlled by detecting the plus or minus of the angle error signal.

  The angle error signal is controlled to be zero at the angle where the page data exists.

  For example, when detecting the page data of the second page adjacent from the page data of the first page, the angle error signal is changed by changing θy until the intensity of the sum signal of the light intensity falls below a predetermined threshold and again exceeds the threshold. And the angle of θy is controlled.

  At this time, instead of using the sum signal of the light intensity, the signal of the region having the largest intensity change with respect to the angle among the signals of the photodetector 185 in which the region receiving the reproduction light S is divided is used as the sum signal. Even when the data are adjacent and the sum signal is not separated, a sum signal separated for each page data can be obtained.

  For example, when a result as shown in FIG. 9B is obtained, the angle of θy is controlled by detecting a reproduction signal using a photodetector divided into four parts and using the signal obtained from the lower left area as a sum signal. It can be carried out.

  Of course, without detecting the intensity of the sum signal of the light intensity, the position information by the encoder of the drive mechanism, the position information recorded on the recording medium 200, etc. are used to move to the angle of the adjacent page data, and the angle The angle θy can also be controlled by detecting an error signal.

  In step S61, if the value determined in step S60 has not been reached, the recording medium 200 is rotated on the Y axis to rotate θy. Then, Step S60 is repeated again.

  In this way, the angle of θy can be controlled by performing a closed servo loop, that is, detecting how the angle error signal or the light intensity summation signal periodically increases or decreases, Furthermore, the desired page data can be reproduced accurately and quickly.

  In step S70, the page data is imaged by the imager 180 and reproduced.

  When reproducing the recording medium 200, the shutter 170 is closed to block the information light I, and only the reference light R is incident on the recording medium 200.

  At this time, the reference light R is transmitted through the recording medium 200 and the quarter-wave plate and reflected by the conjugate mirror 190 to be incident on the recording medium 200 again.

  When the reference light R is incident on the recording medium 200, the wavefront of the information light I is reproduced, and the information light I passes through the condenser lens 100 as the reproduction light S and enters the polarization beam splitter 70.

  Part of the reproduction light S incident on the polarization beam splitter 70 is transmitted and part of it is reflected. The reproduction light S reflected by the polarization beam splitter 70 is adjusted in light diameter by the lenses 130 and 140, enters the imager 180, and can read data.

  On the other hand, the reproduction light S reflected by the polarization beam splitter 70 passes through the lens 145 and is detected by the photodetector 185.

  The page data in the book is moved to the next book when the reproduction of the page data in the book is completed by repeating the operations in steps S40 to S70 by changing θy by a small angle by open control as appropriate.

  In this way, by controlling the angle error signal, it is possible to quickly control the angle of θy, and it is possible to accurately reproduce data without unevenness in luminance.

  DESCRIPTION OF SYMBOLS 10 ... Holographic memory, 20 ... Laser light source, 30, 40 ... 1/4 wavelength plate, 50, 60, 70 ... Polarizing beam splitter, 80, 90, 110, 120, 130, 140, 145 ... Lens, 100 ... Collection Optical lens 150 ... Mirror 160 ... Spatial light modulator 170 ... Shutter 180 ... Imaging device 185 ... Photo detector 190 ... Conjugate mirror 200 ... Holographic recording medium (optical information recording medium, recording medium) 210 ... Hologram recording material, 220, 230 ... substrate, 240 ... angle error signal generation unit, 250 ... angle control unit, 260 ... host CPU, 270, 280, 290 ... arrow, Ia, Ib ... current signal, I ... information light, R ... Reference light, S ... Reproduction light

Claims (4)

Irradiating the optical information recording medium on which the hologram generated by the interference between the information light and the reference light is formed with the reference light to extract the reproduction light;
An angle between the optical information recording medium surface and the optical axis of the reference light,
Rotating around the X-axis represented by the intersection of the optical information recording medium surface and the plane formed by the information light and the reference light, and around the Y-axis orthogonal to the surface formed by the information light and the reference light To obtain a first angle around the X axis and a second angle around the Y axis, at which the intensity of the reproduction light is maximized,
Setting the first angle around the X axis to an angle offset by a predetermined offset angle;
Reproducing light by irradiating the hologram formed on the optical information recording medium while rotating one of the optical information storage medium and the incident angle of the reference light around the Y axis. A step of taking out
The reproduction light is detected by a photodetector provided with two light receiving areas equally divided symmetrically with respect to the center of the optical axis, and a difference between signals relating to the received light intensity output from the two light receiving areas detected by the photodetector. Calculating an angle error signal by calculating
To be so that the angle error signal to zero, the optical information recording medium, or, one of the incident angle of the reference beam, is rotated about the Y axis, and controlling the angle,
An angle control method comprising:   2. The angle control method according to claim 1, wherein the photodetector is a two-divided or four-divided photodetector. When forming the hologram, the reproduction light is extracted by irradiating the optical information recording medium with the reference light from a direction opposite to the back surface of the optical information recording medium surface irradiated with the reference light. The angle control method according to claim 1. 2. The angle multiplexing recording according to claim 1 , wherein either one of the optical information recording medium and the incident angle of the reference light is angle-multiplexed while the Y axis is rotated at a predetermined angular interval. Angle control method.

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