JP2006209081A - Hologram recording/reproducing device and optical unit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hologram recording/reproducing device whose optical system can be simplified even when a phase modulating element is used for both signal light and reference light and which can suppress occurrence of noise and a decrease in quantity of light. <P>SOLUTION: A spatial light modulator generates the signal light and reference light traveling in a common light path and, for example, the phase modulating element is used for both those signal light and reference light to constitute the hologram recording/reproducing device with the one phase modulating element. Consequently, even when the phase modulating element is used for both the signal light and reference light, the optical system can be simplified. A first relay lens system including a first shielding plate having a pinhole of Nyquist size at a focal position thereof is used and the phase modulating element is disposed behind the 1st relay lens system and at a conjugate plane of the signal light and reference light, even when the phase modulating element is used for both the signal light and reference light, the occurrence of noise and the decrease in quantity of light can be suppressed. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a hologram recording / reproducing apparatus and an optical unit.

  Development of hologram recording devices that record data using holography is in progress.

  In the hologram recording apparatus, two modulated signal lights (data superimposed) and unmodulated reference light are generated from laser light, and these are irradiated to the same place on the hologram recording medium. As a result, the signal light and the reference light interfere on the hologram recording medium, a diffraction grating (hologram) is formed at the irradiation point, and data is recorded on the hologram recording medium.

  By irradiating the recorded hologram recording medium with reference light, diffracted light (reproduced light) is generated from the diffraction grating formed during recording. Since the reproduction light includes data superimposed on the signal light at the time of recording, the recorded signal can be reproduced by receiving this with a light receiving element.

  In order to record a large amount of information on the hologram recording medium, a large number of holograms may be formed on the hologram recording medium. In this case, the hologram is not necessarily formed at different locations on the hologram recording medium, and so-called multiplex recording in which a plurality of holograms are formed at the same location (or an overlapping region) of the hologram recording medium is possible.

Here, development of a holographic recording apparatus that increases the storage capacity by using phase correlation multiplexing, which is a type of multiplex recording, is underway (see, for example, Patent Document 1).
Japanese Patent Laid-Open No. 11-242424

  In Patent Document 1, phase correlation multiplexing is realized by using a phase mask with respect to the reference light. However, suppression of the DC component of the hologram signal recorded using the phase mask also for signal light is realized. Can be considered.

  However, when a phase mask is used for both the signal light and the reference light, both a phase mask for the signal light and a phase mask for the reference light are required, and parts for these are also required, resulting in a complicated optical system. There is a problem of becoming.

  In addition, when these phase masks are used, noise is likely to occur. On the other hand, if a filter for removing such noise is used, the amount of light irradiated on the hologram recording medium is reduced. There is.

  In order to solve such a problem, the present invention can simplify the optical system even when a phase modulation element is used for both the signal light and the reference light, and can generate noise and reduce the amount of light. An object of the present invention is to provide a hologram recording / reproducing apparatus and an optical unit capable of suppressing the above-mentioned problem.

  In order to solve such a problem, a hologram recording / reproducing apparatus and an optical unit according to the present invention include a laser light source that emits laser light, and signal light and reference light that share an optical path from the laser light emitted from the laser light source. A spatial light modulator to be generated and a first shielding plate provided with a pinhole having a size of 1 to 2 times the Nyquist size at the focal position, and the signal light generated by the spatial light modulator and the reference A first relay lens system through which light passes; a phase modulation element disposed on a conjugate plane of the signal light and reference light that has passed through the first relay lens system; and the signal light and reference that have passed through the phase modulation element An objective lens for condensing light onto a hologram recording medium, and a light receiving system for receiving return light from the hologram recording medium, which is disposed between the phase modulation element and the objective lens It is intended to include a.

  In the present invention, a signal light and a reference light having a common optical path are generated by a spatial light modulator, and the phase modulation element is used for both the signal light and the reference light, for example. Can be configured as one. As a result, even when the phase modulation element is used for both the signal light and the reference light, the optical system can be simplified.

In addition, a first relay lens system having a first shielding plate provided with a pinhole having a size of 1 to 2 times the Nyquist size at the focal position is used, and a phase modulation element is provided at the subsequent stage of the first relay lens system. In addition, since the signal light and the reference light are arranged on the conjugate plane, even when the phase modulation element is used for both the signal light and the reference light, it is possible to suppress the generation of noise and a decrease in the light amount. If the pinhole is smaller than 1 times the Nyquist size, the amount of light decreases, and if it exceeds 2 times, the noise increases.
(1) A second shielding plate having a pinhole of 1 to 2 times the Nyquist size is provided at the focal position, and a second light through which the signal light and the reference light that have passed through the phase modulation element pass. A relay lens system may be further provided.

Thereby, noise generated in the phase modulation element can be removed. Since the pinhole is 1 to 2 times larger than the Nyquist size, it is possible to suppress a decrease in the light amount.
(2) The phase modulation element may be configured such that a phase pattern for the signal light and a phase pattern for the reference light are different.

Thereby, the mutual interference between holograms can be controlled independently by the change of the phase pattern with respect to the reference light, and the multiplicity can be further increased.
(3) The light receiving system includes a polarizing beam splitter disposed on an optical path between the phase modulation element and the objective lens, and a light receiving element that receives the laser light reflected by the polarizing beam splitter. It may be.
By using a reflective optical system, the optical unit can be downsized compared to the transmissive type.

(4) A means for always controlling the distance between the objective lens and the lens closer to the objective lens of the second relay lens system to be constant may be further provided.

  As described above, according to the present invention, it is possible to simplify the optical system even when the phase modulation element is used for both the signal light and the reference light by configuring the single phase modulation element. become. In addition, a first relay lens system having a first shielding plate provided with a pinhole having a size of 1 to 2 times the Nyquist size at the focal position is used, and a phase modulation element is provided at the subsequent stage of the first relay lens system. In addition, since the signal light and the reference light are arranged on the conjugate plane, even when the phase modulation element is used for both the signal light and the reference light, it is possible to suppress the generation of noise and a decrease in the light amount.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(Configuration of hologram recording / reproducing apparatus)

  FIG. 1 is a schematic diagram of a hologram recording / reproducing apparatus centering on an optical unit according to an embodiment of the present invention. When the term recording / reproducing apparatus is used in the following description, unless otherwise specified, a recording apparatus (an apparatus for recording on a hologram recording medium) or a reproducing apparatus (an apparatus for reproducing a hologram recording medium), and It includes all recording devices and playback devices (devices that perform recording and playback).

  As shown in FIG. 1, the hologram recording / reproducing apparatus records and reproduces information on a hologram recording medium 101, and includes an optical unit 100 and a control unit 150.

  The optical unit 100 includes a recording / reproducing light source 111, a beam expander 112, a mirror 113, a half-wave plate 114, a polarization beam splitter 115, a spatial light modulator 116, a first relay lens system 117, a phase modulation element 118, a polarization beam. Splitter 119, second relay lens system 120, quarter-wave plate 121, mirror 122, objective lens 123, image sensor 124, dichroic mirror 125, servo light source 126, collimator lens 127, grating 128, beam splitter 129, mirror 130 A condensing lens 131, a cylindrical lens 132, and a light receiving element 133.

  The hologram recording medium 101 has a disk shape (disk shape) when viewed from above, and includes a protective layer 102, a recording layer 103, a groove 104, and a reflective layer 105, and records interference fringes due to signal light and reference light. It is.

  The protective layer 102 is a layer for protecting the recording layer 103 from the outside world.

  The recording layer 103 records the interference fringes as a change in refractive index (or transmittance), and any material that changes the refractive index (or transmittance) according to the intensity of light can be used. It can be used regardless of whether it is an organic material or an inorganic material.

  As the inorganic material, for example, a photorefractive material whose refractive index changes according to the exposure amount by an electro-optic effect such as lithium niobate (LiNbO3) can be used.

  As the organic material, for example, a photopolymerization type photopolymer can be used. In the photopolymerization type photopolymer, in the initial state, monomers are uniformly dispersed in the matrix polymer. When this is irradiated with light, the monomer is polymerized at the exposed portion. As the polymer is formed, the monomer moves from the surroundings, and the concentration of the monomer changes depending on the location.

  As described above, when the refractive index (or transmittance) of the recording layer 103 changes in accordance with the exposure amount, interference fringes caused by interference between the reference light and the signal light are regarded as changes in the refractive index (or transmittance). Recording on the hologram recording medium 101 is possible.

  The hologram recording medium 101 is rotated by driving means such as a spindle motor 500, and can record the image of the spatial light modulator 116 as a large number of holograms. The optical unit 100 is moved in the radial direction of the hologram recording medium 101 by driving means (not shown).

  Since the hologram recording medium 101 moves, recording / reproduction on the hologram recording medium 101 is performed along a track formed in the moving direction.

  The groove 104 is provided for performing servo control such as tracking and focusing on the hologram recording medium 101. That is, the groove 104 is formed along the track of the hologram recording medium 101, and the tracking servo and the focus servo are controlled by controlling the condensing position and the condensing depth of the servo light so as to correspond to the groove 104. Done. Further, when the groove 104 in which the address signal is modulated is provided, the address signal is read.

  The recording / reproducing light source 111 is a laser light source and includes, for example, a laser diode (LD) having a wavelength of 405 [nm], and converts the laser light into parallel light by, for example, a collimating lens.

  The beam expander 112 is an optical element that expands the beam diameter of the laser light emitted from the recording / reproducing light source 111 to a predetermined size.

  The mirror 113 is an optical element for reflecting the laser light that has passed through the beam expander 112 toward the polarization beam splitter 115.

  The polarization beam splitter 115 reflects the laser light incident from the mirror 113 via the half-wave plate 114 toward the spatial light modulator 116 and transmits the light reflected by the spatial light modulator 116.

  The spatial light modulator 116 is an optical element that superimposes data by spatially (in this case, two-dimensionally) modulating signal light. It is possible to use a reflection type element such as a DMD (Digital Micro Mirror), a reflection type liquid crystal, or a GLV (Grating Light Value) element for the spatial light modulator. Note that a structure using a transmissive liquid crystal element which is a transmissive element can also be used. In the present embodiment, the reference light is formed by the spatial light modulator 116 so as to surround the outer periphery of, for example, a circular signal light. That is, on the spatial light modulator 116, a reflection area (area for reference light) that simply reflects a signal is provided so as to surround a circular modulation area (area for signal light). Thereby, the signal light and the reference light having a common optical path can be generated from the laser light emitted from the recording / reproducing light source 111.

  FIG. 2 shows an example pattern of signal light and reference light generated by the spatial light modulator 116 and passed through the polarization beam splitter 115.

  The first relay lens system 117 includes a pair of lenses 141 and 142 and a shielding plate 143.

  The lens 141 and the shielding plate 143 are spatial filters that normalize a laser beam carrying a signal to a Gaussian distribution.

  The lens 141 is an optical element that condenses the laser light that has passed through the polarization beam splitter 115 on the shielding plate 143. By this lens 141, the laser light that has passed through the polarization beam splitter 115 is converted into a Fraunhofer diffraction image (Fourier transform image) at the focal position of the lens 141 and imaged.

  The shielding plate 143 is disposed at the focal position of the lens 141, is provided with a pinhole 144 having a size that is 1 to 2 times the Nyquist size, and 0th-order light and ± 1st-order light pass through the pinhole 144.

  The lens 142 converts light that has passed through the pinhole 144 of the shielding plate 143 into parallel light.

  The phase modulation element 118 is an optical element for giving the reference light a random phase or a certain phase pattern, and may be called a phase mask. The phase modulation element 118 may be ground glass, a diffuser, or a spatial phase modulator. It is also possible to use a hologram element in which a phase pattern is recorded. Light having a phase pattern is generated by reproduction from the hologram element.

  3A to 3D are plan views showing examples of patterns of the phase modulation element 118. FIG.

  FIG. 3A shows a phase modulation element 118 having a circular phase modulation region 301 for signal light and a ring-shaped phase modulation region 302 for reference light provided so as to surround the phase modulation region 301.

  FIG. 3B shows a phase modulation element 118 having only a circular phase modulation region 303 for signal light.

  FIG. 3C shows a phase modulation element 118 having only a ring-shaped phase modulation region 304 for reference light.

  FIG. 3D shows a phase modulation element 118 having a circular phase modulation region 305 shared by signal light and reference light.

  The polarization beam splitter 119 is an optical element that reflects parallel light incident from the phase modulation element 118 and reflects reflected light from the hologram recording medium 101 that has passed through the second relay lens system 120 toward the image sensor 124.

  The second relay lens system 120 includes a shielding plate 147 provided with a pair of lenses 145 and 146 and a pinhole 148. These configurations are the same as those of the first relay lens system 117. The second relay lens system 120 can cut noise generated in the phase modulation element 118. However, the shielding plate 147 may not be provided.

  The objective lens 123 condenses both the signal light and the reference light, and also the laser light from the servo light source 126 on the hologram recording medium 101 via the quarter wavelength plate 121 and the mirror 122, and the hologram recording medium 101. It is an optical element for making the reflected light from the light into parallel light.

  The image sensor 124 is an element for inputting an image of reproduction light, and is configured by, for example, a two-dimensional CCD.

The dichroic mirror 125 is an optical element for setting light used for recording / reproduction (laser light from the recording / reproducing light source 111) and light used for servo (laser light from the servo light source 126) to the same optical path. The dichroic mirror 125 transmits the recording / reproducing light from the recording / reproducing light source 111 and responds to the servo from the servo light source 126 in response to the difference in laser light wavelength between the recording / reproducing light source 111 and the servo light source 126. Reflects light. The surface of the dichroic mirror 125 is subjected to a thin film treatment such that the recording / reproducing light is totally transmitted and the light used for servo is totally reflected.
The servo light source 126 is a light source for servo control such as tracking servo and focus servo and reading of an address signal, and emits laser light having a wavelength different from that of the recording / reproducing light source 111. The servo light source 126 is, for example, a laser diode, and uses, for example, 650 nm having low sensitivity with respect to the hologram recording medium 101 as an oscillation wavelength.

  The collimating lens 127 is an optical element that converts the laser light emitted from the servo light source 126 into parallel light.

  The grating 128 is an optical element for dividing the laser light emitted from the collimating lens 127 into three beams, and is configured by two elements. The laser beam is divided for servo control and address signal reading.

  The beam splitter 129 is an optical element that transmits the laser light emitted from the grating 128 and reflects the return light that is reflected from the hologram recording medium 101 and returned.

  The mirror 130 is an optical element that reflects the return light from the beam splitter 129 toward the light receiving element 133.

  Here, the beam splitter 129 and the mirror 130 are integrated by a prism, but may be separate optical elements.

  The condensing lens 131 is an optical element for condensing the return light from the mirror 130 onto the light receiving element 133.

  The cylindrical lens 132 is an optical element for converting the beam shape of the laser light collected from the condensing lens 131 from a circle to an ellipse.

The light receiving element 133 is an element, such as a photodiode, for receiving return light and outputting a tracking error signal for tracking servo control, a focus error signal for focus servo control, and an address signal.
(Details of actuator part)

  FIG. 4 is a schematic sectional view showing an example of an actuator unit 400 for driving the mirror 122 and the objective lens 123 to execute tracking servo and focus servo.

  As shown in FIG. 4, the actuator unit 400 is attached to the actuator unit main body 401 and the actuator unit main body 401, and laser light (signal light, reference light) incident on the hologram recording medium 101 in a parallel or nearly parallel state. , The laser light emitted from the servo light source 126) changes the optical path in a direction perpendicular to or nearly perpendicular to the hologram recording medium 101, and the return light from the hologram recording medium 101 is parallel to the hologram recording medium 101. Alternatively, the mirror 122 that changes to a direction close to parallel, the objective lens 123 held by the actuator unit body 401, and the objective lens 123 perpendicular to the surface of the hologram recording medium 101 in order to perform focus servo of the objective lens 123. Or a focus actuator that drives in a direction close to vertical And over motor 402, in order to perform the tracking servo, and a tracking actuator 403 for driving the actuator body 401 with respect to the radial direction of the hologram recording medium 101.

  The focus actuator 402 and the tracking actuator 403 execute focus servo and tracking servo based on the control from the control unit 150 that receives the tracking error signal and the focus error signal from the light receiving element 133. That is, each of the focus actuator 402 and the tracking actuator 403 (see FIGS. 8 and 10) is displaced by the focus command voltage 803 and the tracking command voltage 804 (see FIG. 1), thereby condensing the light spot at a desired position. And control its position.

  In such a configuration of the actuator unit 400, the tracking actuator 403 drives the actuator unit body 401 with respect to the radial direction of the hologram recording medium 101 during the tracking operation, so that the light beam can be positioned at a predetermined position in the tracking direction. By adopting such a tracking method, the laser beam can always be guided to the center of the objective lens when positioning the light beam, so that aberration can be suppressed. In the case of performing hologram recording / reproduction, since it is highly possible that a writing or reading error occurs due to the occurrence of such aberration, it is significant to suppress the occurrence of aberration. Further, the focus actuator 402 drives the objective lens 123 in a direction perpendicular or nearly perpendicular to the surface of the hologram recording medium 101 to condense a light beam at a desired position in the hologram recording medium. Details of the operation of the servo system that performs focus servo and tracking servo will be described later.

  (Operation of hologram recording / reproducing device)

The outline of the operation of the hologram recording / reproducing apparatus will be described below.
A. When recording

  An outline of the operation of the hologram recording / reproducing apparatus during recording will be described.

  The laser beam emitted from the recording / reproducing light source 111 is made to have a desired beam diameter by the beam expander 112. The laser light passes through the mirror 113 and the half-wave plate 114, is reflected by the polarization beam splitter 115, and enters the spatial light modulator 116.

  The signal light and the reference light generated by the spatial light modulator 116 are the polarization beam splitter 115, the first relay lens system 117, the phase modulation element 118, the polarization beam splitter 119, the second relay lens system 120, the dichroic mirrors 125, 1 The light passes through the / 4 wavelength plate 121, is reflected by the mirror 122, passes through the objective lens 123, and is condensed at substantially the same location on the hologram recording medium 101.

Thereby, interference fringes are formed on the hologram recording medium 101. As a result, the information spatially modulated by the spatial light modulator 116 is recorded on the hologram recording medium 101 as a hologram.
B. During playback

  An outline of the operation of the hologram recording / reproducing apparatus during reproduction will be described.

  Only the reference light is incident on the hologram recording medium 101 during reproduction.

  The laser beam emitted from the recording / reproducing light source 111 is made to have a desired beam diameter by the beam expander 112. The laser light passes through the mirror 113 and the half-wave plate 114, is reflected by the polarization beam splitter 115, and enters the spatial light modulator 116.

  By making all signal light regions in the spatial light modulator 116 “dark”, only the reference light is polarized beam splitter 115, first relay lens system 117, phase modulation element 118, polarization beam splitter 119, and second relay lens. The light passes through the system 120, the dichroic mirror 125, and the quarter-wave plate 121, is reflected by the mirror 122, passes through the objective lens 123, and enters the hologram recording medium 101.

  When the reference light enters the hologram recording medium 101, diffracted light (reproduced light) is generated from the hologram recorded on the hologram recording medium 101.

The generated reproduction light follows an optical path opposite to that of the incident light, passes through the objective lens 123, the mirror 122, the quarter wavelength plate 121, the dichroic mirror 125, and the second relay lens system 120, and is reflected by the polarization beam splitter 119 to capture an image. The light enters the element 124 and is converted into an electrical signal corresponding to the spatial two-dimensional data in the spatial light modulator 116 by the imaging element 124. The output from the image sensor 124 is binarized by a signal processing unit (not shown) and converted into time-series binarized data.
(Confirmation of effect of hologram recording / reproducing device)

  Next, the result of an experiment conducted to confirm the effect of the hologram recording / reproducing apparatus according to this embodiment will be shown.

  FIG. 5A is a diagram schematically showing the relationship among the spatial light modulator 116, the phase modulation element 118, and the imaging element 124 in the optical unit shown in FIG. However, the shielding plate 147 in the second relay lens system 120 is omitted here.

  FIGS. 5B and 5C are examples in which the phase modulation element 118 is disposed in front of the spatial light modulator 116. In FIG. 5B, the first and second relay lens systems are provided with shielding plates, whereas in FIG. 5C, the shielding lens system is shielded by the relay lens system between the phase modulation element 118 and the spatial light modulator 116. The difference is that no plate is provided.

  FIG. 6 is a graph showing the results of measuring SNR (signal-noise ratio) for these optical systems.

  In FIG. 6, Case 1 indicates the measurement result for the optical system in FIG. 5A, Case 2 indicates the measurement result for the optical system in FIG. 5B, and Case 3 indicates the measurement result for the optical system in FIG. 5C.

These results revealed the following.
(1) The SNR is improved when the phase modulation element 118 is provided after the spatial light modulator 116 than when the phase modulation element 118 is provided before the spatial light modulator 116. (Case 1 VSCase 2, Case 3)
(2) The SNR is improved when the shield plate is provided in the relay lens system subsequent to the phase modulation element 118 as compared with the case where the shield plate is not provided. (Case2VSCase3)
(Other embodiments)

  In this embodiment, the distance between the objective lens 123 and the lens 146 of the second relay lens system 120 near the objective lens 123 is controlled to be constant.

  As shown in FIG. 4, in the actuator unit 400, focus servo and tracking servo are applied to the rotating hologram recording medium 101, so that the position of the objective lens 123 inevitably moves. At this time, wavefront aberration accompanying the movement of the objective lens 123 occurs in the hologram to be recorded and reproduced. This embodiment prevents the occurrence of such wavefront aberration.

  FIG. 7 shows an example of the wavefront aberration of the image on the image sensor 124 with respect to the position of the objective lens 123.

  As shown in FIG. 7, most of the wavefront aberration is caused by defocus (deviation from a predetermined focal point). Assuming that 0.07λ is the standard of the wavefront aberration allowed for recording and reproduction, it can be used only for defocus in the range of ± 60 μm at most. It can also be seen that the wavefront aberration increases sensitively with respect to the position of the objective lens 123 even within that range. Actually, a tolerance of ± 100 μm or more is necessary corresponding to the amount of movement of the objective lens 123 in the focus direction according to the warp of the hologram recording medium 101.

  Therefore, the distance Lol between the objective lens 123 and the lens 146 of the second relay lens system 120 in the vicinity of the objective lens 123 is controlled to be constant, and the deterioration of the wavefront aberration is suppressed even if the objective lens 123 moves. To be able to.

  FIG. 8 is a schematic cross-sectional view for realizing this.

  In FIG. 8, the quarter wave plate 121 and the dichroic mirror 125 existing in FIG. 1 are not shown for convenience. Even if optical components such as the quarter-wave plate 121 and the dichroic mirror 125 are present in the middle or not, the effect according to the present embodiment is similarly obtained.

  Moreover, the space | interval Lol here points out the physical length along an optical axis.

  As shown in FIG. 8, in this embodiment, the lens 146 of the second relay lens system 120 is mounted on the distance control actuator 801, and the distance Lol from the objective lens 123 is always controlled to be constant.

  FIG. 9 shows the wavefront aberration with respect to the movement of the objective lens 123 when the distance Lol is controlled to be constant. From the figure, it can be seen that the wavefront aberration hardly changes with the movement of the objective lens 123 of ± 300 μm. At this time, since only one (one side) of the relay lens moves, there is a concern about a change in image magnification. Therefore, FIG. 9 also shows the change in image magnification. The change in image magnification is as small as 0.025% at ± 300 μm, and hardly affects the recording / reproduction of the hologram.

  FIG. 10 shows a configuration example of a servo for controlling the distance between the objective lens 123 and the lens 146 of the second relay lens system 120 near the objective lens 123 to be constant.

  The microcomputer 802 takes in the focus command voltage 803 and the tracking command voltage 804 for the focus actuator 402 and the tracking actuator 403 from the control unit 150, and calculates the movement amount of the objective lens 123 in the microcomputer 802.

  Then, a command voltage for moving the lens 146 of the second relay lens system 120 by the same amount as the movement amount is obtained and supplied to the interval control actuator 801. As a result, the distance Lol between the lens 146 of the second relay lens system 120 and the objective lens 123 is controlled to be always constant in synchronization with the movement of the objective lens 123. Details of the control system for controlling the interval Lol will be described later.

  As the distance control actuator 801, an electromagnetic coil method, a piezo method, or the like can be used.

  As described above, stable hologram recording and reproduction can be performed while performing focus and tracking servo.

(About control system)
In the hologram recording / reproducing apparatus of the present embodiment, a control system is used for each part, and a description will be given here. 1 and 8, the operations of the focus servo system, the tracking servo system, and the spindle servo system as the servo system (a control system that constitutes a feedback control system and the variable is a position) will be described. A control system in which the distance Lol between the lens 146 of the relay lens system 120 and the objective lens 123 is always constant will be described in detail with reference to FIGS. 10, 11, and 12.

  As described above, the recording / reproducing operation is performed by specifying the recording / reproducing area on the hologram recording medium 101. The recording / reproducing area is specified by the action of the focus servo system, the tracking servo system, and the spindle servo system. Therefore, this point will be described in order.

  The focus servo system, tracking servo system, and spindle servo system are composed of a servo optical part for detecting a servo error signal of each optical unit 100, a control part 150, an actuator part 400 of the optical unit 100, and a spindle motor 500. Yes. The servo optical part uses a servo light source 126, a collimating lens 127, a grating 128, a beam splitter 129, a mirror 130, a condensing lens 131, a cylindrical lens 132, and a light receiving element 133 as servo-dedicated optical parts. A dichroic mirror 125, a mirror 122, and an objective lens 123 are used as a common optical unit for recording and reproduction.

  First, a light beam emitted from the servo light source 126 (hereinafter abbreviated as a servo beam) passes through the collimator lens 127, the grating 128, the beam splitter 129, and the dichroic mirror 125, and passes through the mirror 122 and the objective lens 123. Then, the light is reflected by the reflection layer 105 on which the groove 104 of the hologram recording medium 101 is formed, and again passes through the objective lens 123, the mirror 122, the dichroic mirror 125, and the beam splitter 129, and then the mirror 130, the condensing lens 131, and the cylindrical lens. 132 to the light receiving element 133.

  The light receiving element 133 is an optical detector composed of a plurality of divided optical detectors, and generates an electrical signal corresponding to the amount of light received by each of the optical detectors. The control unit 150 has a calculation unit composed of a CPU (Central Processing Unit) or a DSP (Digital Signal Processor) as a main unit, and receives this electric signal and calculates it, for example, by the stigma method. A focus error signal for focus servo control is generated, and a tracking error signal for tracking servo control is generated by a push-pull method, for example. Further, for example, the control unit 150 detects information on the position recorded in the groove 104 based on a signal from the light receiving element 133 or based on a signal from the imaging element 124 to generate a spindle error signal.

  Then, the control unit 150 performs an operation such as phase compensation, and supplies a focus command voltage 803 based on the focus error signal to the focus actuator 402 to focus the servo light beam as a light spot on the reflection layer 105. Further, the control unit 150 performs an operation such as phase compensation, supplies a tracking command voltage 804 based on the tracking error signal to the tracking actuator 403, and arranges this light spot at a predetermined position in a direction orthogonal to the groove 104. Thus, positioning in the radial direction of the hologram recording medium 101 is performed. Further, the control unit 150 performs operations such as phase compensation, supplies a spindle command voltage 805 based on the spindle error signal to the spindle motor 500, and places the light spot at a predetermined position along the groove 104. Positioning in the direction perpendicular to the radial direction.

  By determining the relative position between the optical unit 100 and the hologram recording medium 101 using the servo light beam as described above, the light beam from the recording / reproducing light source 111 is also a desired one of the hologram recording medium 101 as described above. Will be placed at the position. The reason is that the optical part for servo and the optical part for recording / reproducing are integrally formed as the optical unit 100, so that the optical path of the beam from the servo light source 126 and the optical path of the light beam from the recording / reproducing light source 111 are as follows. This is because the mutual relationship with is precisely defined. As a result, desired recording data can be written as a hologram in a predetermined area of the hologram recording medium 101, and a reproduction signal can be obtained from the hologram in the predetermined area of the hologram recording medium 101.

  Next, a control system in which the distance Lol between the lens 146 of the relay lens system 120 and the objective lens 123 is always constant will be described. As described above, the microcomputer 802 fetches the focus command voltage 803 and the tracking command voltage 804 from the control unit 150, and the microcomputer 802 calculates the movement amount of the objective lens 123, and the second relay lens system by the same amount as the movement amount. A command voltage for moving the lens 146 of 120 is obtained and supplied to the distance control actuator 801, and the distance Lol between the lens 146 and the objective lens 123 of the second relay lens system 120 is controlled to be always constant. However, specific examples thereof will be described in detail with reference to FIGS. 10, 11, and 12.

  The entire optical unit 100 shown in FIG. 1 moves in the inner and outer peripheral directions in accordance with the change in the recording / reproducing radial region in the hologram recording medium 101. However, since the optical unit 100 includes many optical components as described above, its weight is heavy, and it is difficult to increase the positional accuracy and response speed of positioning in the tracking direction. Therefore, as an example of the present embodiment, the actuator unit 400 including the lightweight mirror 122 is used, and in the entire optical unit 100 that moves in a large range in the tracking direction, a smaller range in the tracking direction independently. The position accuracy in the tracking direction and the response speed are increased. That is, the tracking servo system is a so-called two-stage servo system that is divided into bands.

  Here, when the actuator unit 400 including the mirror 122 moves, for example, in the left-hand direction when viewed from the front of FIG. 10 by the driving force of the tracking actuator 403, the size of the interval Lol is set to a predetermined value. It will be larger than the distance. Conversely, when moving in the right hand direction when viewed from the front of FIG. 10, the size of the interval Lol is smaller than a predetermined distance. As a result, in any case, the wavefront aberration is deteriorated.

  The actuator unit 400 including the mirror 122 is fixedly incorporated as a part of the entire optical unit 100 without moving separately in the optical unit 100, and the entire optical unit 100 is recorded as a hologram. When moving in the inner and outer circumferential directions according to the change in the recording / reproducing radial area on the medium 101, the interval Lol is always kept constant in the tracking direction. The wavefront aberration does not deteriorate.

  Further, the influence on the interval Lol caused by the action of the focus servo will be described. When the distance between the hologram recording medium 101 and the mirror 122 changes, the focus servo acts to keep the distance between the objective lens 123 and the hologram recording medium 101 constant. As a result, the distance between the mirror 122 and the objective lens 123 changes, and the distance Lol also changes. For example, in FIG. 10, when the hologram recording medium 101 moves relative to the optical unit 100 as viewed from the front, the objective lens 123 is also moved in accordance with the action of the focus servo. Moving downward, the size of the interval Lol becomes larger than a predetermined distance. Further, when the hologram recording medium 101 moves upward as viewed from the front of the paper, the objective lens 123 is also moved upward according to the action of the focus servo, and the size of the interval Lol is determined in advance. It becomes smaller than the predetermined distance. As a result, in any case, the wavefront aberration is deteriorated.

  FIG. 11 shows processing in the control unit 150 and the microcomputer 802 in the form of a block diagram. Here, symbol Vf is a voltage value of the focus command voltage 803, symbol Vt is a voltage value of the tracking command voltage 804, and symbol Vr is a voltage value of the interval control command voltage supplied to the interval control actuator 801. Gf is a gain that is multiplied by the voltage Vf of the focus command voltage 803, and a symbol Gt is a gain that is multiplied by the voltage Vt of the tracking command voltage 804. The focus error signal generation is, for example, calculation processing based on the stigma method, and the tracking error signal generation is calculation processing based on the push-pull method. The focus servo processing and tracking servo processing are, for example, phase compensation and gain adjustment processing.

  The sensitivity of the focus actuator 402 can be expressed as Sf (μm / V), the sensitivity of the tracking actuator 403 can be expressed as St (μm / V), and the sensitivity of the interval control actuator 801 can be expressed as Sr (μm / V). (Expression 1) is for maintaining the size of the interval Lol at a predetermined size when the voltage Vf is input to the focus actuator 402 having the sensitivity Sf and the voltage Vt is input to the tracking actuator 403 having the sensitivity St. It is a formula for obtaining the voltage Vr.

(Formula 1)
Vr = (Vf × Sf / Sr) + (Vt × St / Sr)

  That is, the control system in which the interval Lol is always constant is configured as a feedforward control system, and the first item of (Equation 1) cancels the change from the predetermined size of the interval Lol when the focus actuator 402 moves. The second item represents a voltage for canceling a change from a predetermined size of the interval Lol when the tracking actuator 403 moves.

  In (Expression 1), it is assumed that there is no so-called offset, and when the voltage Vf and the voltage Vt are zero, the interval Lol is a predetermined separation distance. If there is an offset, the offset voltage is subtracted from the voltage Vf and the voltage Vt, and (Equation 1) may be applied. Similarly, when there is an offset with respect to the voltage Vr, a voltage obtained by subtracting the offset from the calculation result of (Equation 1) is applied to the distance control actuator 801 to keep the distance Lol at a predetermined distance. And can.

  In the present embodiment, the microcomputer 802 is used. However, the contents of processing in the microcomputer 802 may be taken in as processing by the control unit 150 and the microcomputer 802 may be omitted. Further, when any one or all of the focus actuator 402, the tracking actuator 403, and the interval control actuator 801 have a non-linear characteristic with respect to the voltage applied to the actuator, a ROM (Read Only Memory) is provided for each input voltage. By recording a displacement correction value and referring to the value, it is possible to correct the interval Lol accurately.

  By adopting the control system as described above, the size of the interval Lol can be set to a predetermined value even when either or both of the focus actuator and the tracking actuator move, so that the recording does not deteriorate the wavefront aberration. A playback device can be provided.

  That is, an example of a control system provided in the recording / reproducing apparatus has the following configuration. A distance control actuator 801 is provided to move the position of the lens 146 closer to the objective lens 123 of the second relay lens system 120 in the optical axis direction, and the objective lens 123 is moved substantially perpendicularly to the surface of the hologram recording medium 101. Based on a focus command voltage 803 applied to the focus actuator 402, the distance control actuator 801 is controlled so as to keep the distance between the objective lens 123 and the lens 146 constant. In this case, only the first item is adopted as the voltage Vr shown in (Equation 1).

  Another example of the control system provided in the recording / reproducing apparatus has the following configuration. A distance control actuator 801 is provided to move the position of the lens 146 closer to the objective lens 123 of the second relay lens system 120 in the optical axis direction, and the objective lens 123 is moved substantially perpendicularly to the surface of the hologram recording medium 101. Based on a focus command voltage 803 applied to the focus actuator 402 for tracking and a tracking command voltage 804 applied to the tracking actuator 403 for moving the objective lens 123 substantially horizontally with respect to the surface of the hologram recording medium 101, The distance control actuator 801 is controlled so that the distance between the objective lens 123 and the lens 146 is constant. In this case, the first item and the second item are adopted for the voltage Vr shown in (Expression 1). Further, when the tracking servo system is not a two-stage servo system, only the actuator unit 400 in the optical unit 100 changes the positional relationship with the spindle motor 500. However, the voltage Vr shown in (Equation 1) adopts the first item and the second item.

  FIG. 12 shows the results of an experiment performed based on (Expression 1). The horizontal axis represents the amount of change in the optical path length (interval Lol), and the change in the optical path length includes both movement of the mirror 122 and the objective lens 123. The vertical axis represents the S / N (signal-to-noise ratio) of the reproduction signal, and the curve indicated by the control ON indicates the characteristics when the above-described control for keeping the interval Lol constant is employed. The curve indicated by the control OFF indicates the characteristics when the above-described control for keeping the interval Lol constant is not employed, and the influence of the wavefront aberration is reflected in the S / N of the reproduction signal. Yes. That is, when the control is turned on, the S / N of the reproduction signal is improved.

(Other processing performed by the control unit)
Although it has been described above that the control unit 150 has a function as a part of the control system, the control unit 150 also has a function of controlling other parts of the recording / reproducing apparatus. The control unit 150 controls the optical unit 100 during both recording and reproduction. That is, at the time of writing, a two-dimensional pattern based on the information (recording data) is displayed on the spatial light modulator 116 by a signal from the control unit 150 according to the information (recording data). To display. On the other hand, at the time of reproduction, a two-dimensional pattern is written in the reference light display area of the spatial light modulator 116, and the output from the array-type photodetector 124 is processed by the control unit 150. Further, at the time of recording, information to be recorded is taken from the external device, and at the time of reproduction, reproduction information processed by the control unit is output to the external device.

  In addition, this invention is not limited to said embodiment, It can change and implement variously within the range of the technical idea of the invention. For example, as an example of the hologram recording medium, the disk type recording medium has been described. However, the shape of the recording medium is not limited to the disk type, and may be, for example, a card type.

1 is a schematic diagram of a hologram recording / reproducing apparatus according to an embodiment. It is a figure which shows the example of a pattern of the signal beam | light and reference light which were produced | generated by the spatial light modulator and passed through the polarization beam splitter. It is the top view which showed the example of the pattern of a phase modulation element. It is a typical sectional view showing an example of an actuator part. It is the schematic which shows the various optical systems of the experiment conducted in order to confirm the effect of a hologram recording / reproducing apparatus. It is a graph which shows the experimental result performed in order to confirm the effect of a hologram recording / reproducing apparatus. It is a figure which shows the example of the wave aberration of the image on an image pick-up element with respect to the position of an objective lens. It is a typical sectional view of the composition concerning other embodiments. It is a graph which shows the wavefront aberration with respect to the movement of an objective lens when the space | interval of an objective lens and the lens of the 2nd relay lens system near this objective lens is controlled uniformly. It is a figure which shows the structure of the control system which concerns on other embodiment. It is a figure explaining the process of the control system which concerns on other embodiment with a block diagram. It is an experimental result explaining the effect of the control system concerning other embodiments.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Optical unit, 101 Hologram recording medium, 111 Recording / reproducing light source, 116 Spatial light modulator, 117 1st relay lens system, 118 Phase modulation element, 119 Polarization beam splitter, 120 2nd relay lens system, 123 Objective lens, 124 Image sensor, 141 lens, 142 lens, 143 shielding plate, 144 pinhole, 145 lens, 146 lens, 147 shielding plate, 148 pinhole, 801 interval control actuator, 803 focus command voltage, 804 tracking command voltage

Claims (8)

A laser light source for emitting laser light;
A spatial light modulator that generates signal light and reference light having a common optical path from laser light emitted from the laser light source;
A first relay lens having a first shielding plate provided with a pinhole of 1 to 2 times the Nyquist size at a focal position, and through which the signal light and the reference light generated by the spatial light modulator pass The system,
A phase modulation element disposed on a conjugate surface of the signal light and the reference light that has passed through the first relay lens system;
An objective lens that focuses the signal light and the reference light that have passed through the phase modulation element on a hologram recording medium;
A hologram recording / reproducing apparatus comprising: a light receiving system that is disposed between the phase modulation element and the objective lens and receives return light from the hologram recording medium. The hologram recording / reproducing apparatus according to claim 1,
A second relay lens system having a second shielding plate provided with a pinhole having a size of 1 to 2 times the Nyquist size at a focal position, through which the signal light and the reference light that have passed through the phase modulation element pass; A hologram recording / reproducing apparatus further comprising: The hologram recording / reproducing apparatus according to claim 1,
The hologram recording / reproducing apparatus, wherein the phase modulation element has a phase pattern for the signal light and a phase pattern for the reference light different from each other. The hologram recording / reproducing apparatus according to claim 1,
The light receiving system is
A polarizing beam splitter disposed on an optical path between the phase modulation element and the objective lens;
A hologram recording / reproducing apparatus comprising: a light receiving element that receives the laser light reflected from the polarizing beam splitter. The hologram recording / reproducing apparatus according to claim 2,
A hologram recording / reproducing apparatus, further comprising means for constantly controlling a distance between the objective lens and a lens closer to the objective lens of the second relay lens system. The hologram recording / reproducing apparatus according to claim 2,
An interval control actuator for moving the position of the lens closer to the objective lens of the second relay lens system in the optical axis direction;
Based on a focus command voltage applied to a focus actuator for moving the objective lens substantially perpendicularly to the surface of the hologram recording medium, the one closer to the objective lens and the objective lens of the second relay lens system A hologram recording / reproducing apparatus, wherein the distance control actuator is controlled so that the distance from the lens is constant. The hologram recording / reproducing apparatus according to claim 2,
An interval control actuator for moving the position of the lens closer to the objective lens of the second relay lens system in the optical axis direction;
A focus command voltage applied to a focus actuator for moving the objective lens substantially perpendicularly to the surface of the hologram recording medium and for moving the objective lens substantially horizontally relative to the surface of the hologram recording medium The distance control actuator is controlled based on a tracking command voltage applied to the tracking actuator so that a distance between the objective lens and a lens closer to the objective lens of the second relay lens system is constant. Hologram recording / reproducing apparatus. A laser light source for emitting laser light;
A spatial light modulator that generates signal light and reference light having a common optical path from laser light emitted from the laser light source;
A first relay lens having a first shielding plate provided with a pinhole of 1 to 2 times the Nyquist size at a focal position, and through which the signal light and the reference light generated by the spatial light modulator pass The system,
A phase modulation element disposed on a conjugate surface of the signal light and the reference light that has passed through the first relay lens system;
An objective lens that focuses the signal light and the reference light that have passed through the phase modulation element on a hologram recording medium;
An optical unit comprising: a light receiving system that is disposed between the phase modulation element and the objective lens and receives return light from the hologram recording medium.

Images