JP5394275B2 - Wavefront controller operation amount control method and hologram reproducing apparatus - Google Patents

Description

  The present invention relates to a method for controlling the amount of operation of a wavefront controller arranged to optimize the wavefront of reference light during reproduction in a hologram recording / reproducing system that records and reproduces image information using a hologram. The present invention relates to an operation amount control method for a wavefront controller suitable for compensating for interference fringe distortion caused by shrinkage of a hologram recording medium, and a hologram reproducing apparatus using the same.

  In recent years, as a next-generation optical information recording / reproducing method aiming at high-speed and large-capacity recording / reproducing, research and development of a hologram recording / reproducing method has been actively conducted.

  A conventional hologram recording / reproducing apparatus is configured as shown in FIG. 11, for example, and the outline thereof will be described here. The coherent laser beam emitted from the laser light source 201 is expanded in beam diameter by a beam expander composed of a diverging lens 202 and a collimating lens 203, passes through a half-wave plate 204, is deflected by a mirror 205, and then a polarized beam. The light is branched into two light beams by the splitter 206.

  The light beam traveling downward in the figure from the polarization beam splitter 206 passes through the shutter 207 and the polarization beam splitter 208 in the open state and is irradiated onto the spatial light modulation element 209, and is spatially modulated by the spatial light modulation element 209, Signal light carrying digital image information, which is called page data, in which white and black binary pixels are two-dimensionally arranged. The polarization direction of the signal light emitted from the spatial light modulator 209 is changed from the incident state. The signal light is reflected to the left in the drawing by the polarization beam splitter 208 and optically Fourier transformed by the lens 210 to be holograms. Irradiated to a recording medium (hereinafter simply referred to as “recording medium”) 211.

  On the other hand, the light beam traveling leftward in the drawing from the polarization beam splitter 206 is used as reference light (recording reference light), passes through the mirror 222 and the open shutter 224, and the signal light in the recording medium 211 passes. The place is irradiated at a different angle from the signal light.

  When the signal light and the reference light are simultaneously irradiated, interference fringes are generated in the volume inside the recording medium 211, and hologram recording is performed by transferring the fringe distribution to the recording area of the recording medium 211 in the form of a refractive index distribution or the like. In the case of the angle multiplex recording method, different page data are displayed on the spatial light modulation element 209, and different from each other by changing the angle (rotation angle) of the recording medium 211 with respect to the signal light and the reference light little by little. It becomes possible to multiplex-record page data in the same area in the recording medium 211, and high-density information storage is possible.

  When reproducing the page data recorded on the hologram recording medium 211, the shutter 207 is closed and the recording medium 211 is irradiated with only the reference light (reference light at the time of reproduction) at the same angle as that during recording. Even if a plurality of page data are recorded in the recording area, only desired page data can be selectively taken out and imaged by a CCD (imaging device) 213 via the lens 212.

  In the case of a photosensitive resin material (photopolymer) that is often used as a hologram recording medium, a hologram (interference fringes) is recorded by generating a refractive index difference inside the recording medium. At this time, medium shrinkage occurs due to photopolymerization. In addition, when a temperature difference occurs between signal recording and signal reproduction, the recording medium may contract or expand due to the influence of the temperature difference. When the recording medium contracts or expands, so-called interference fringe distortion occurs, in which the interference fringes recorded in the medium are distorted. Even if reference light similar to that at the time of recording is incident on the distorted interference fringes, it becomes difficult to reproduce the recorded bit data without error, so that the SNR (Signal to Noise Ratio) of the reproduced data is lowered.

  Recently, a method for optically compensating for interference fringe distortion has been studied in order to improve reproduction data deterioration (SNR reduction) caused by interference fringe distortion caused by such shrinkage (including expansion) of the recording medium. Yes. In the compensation method using adaptive optics, which is one of the optical compensation methods, a wavefront controller is provided in the optical path of the reference light so as to create an optimal reference light wavefront for optically compensating for interference fringe distortion. (See Patent Document 1 and Non-Patent Document 1 below). As a result, interference fringe distortion can be compensated, and deterioration of reproduced data can be improved.

  As the above-described wavefront controller, a deformable mirror having a reflecting mirror surface formed of a metal thin film or the like has been proposed (see Patent Document 1 and Non-Patent Document 1 below). The reflecting mirror surface is supported from the back side by a plurality of pin members that are configured so that the amount of protrusion from the substrate is variable, and by adjusting the amount of protrusion for each pin member, the mirror surface supported by each pin member Each region is operated so that the overall shape changes.

  The amount of movement of the reflecting mirror surface (the amount of protrusion of each pin member) is obtained by imaging page data reproduced by the wavefront formed by the reflecting mirror surface, evaluating the image based on luminance, SNR, etc. Accordingly, the procedure is changed at high speed so as to obtain an optimum wavefront while repeating the procedure of changing the operation amount so as to obtain a more appropriate wavefront.

  As a control method at that time, in the techniques described in Patent Document 1 and Non-Patent Document 1 described below, a genetic algorithm is considered in consideration that the output response of reproduction page data to the input of reproduction reference light becomes nonlinear. A control method using (Generic Algorithm) is adopted.

Japanese Patent Application No. 2009-093989

T. Muroi, et. Al., “Optical Compensation of Distorted Data Image by Interference Fringe Distortion in Holographic Data Storage”, Appl. Opt., Vol.48, Iss. 19, pp. 3681-3690, 2009

  For practical application of the hologram recording / reproducing system, reduction of the time required for optimizing the reference light wavefront has been an issue. In order to solve such a problem, the inventor of the present application paid attention to the operating range (operable range) of the reflecting mirror surface in the wavefront controller.

  In other words, if the operating range is set wider than necessary, the calculation and search of the wavefront to obtain the optimal reference light wavefront will result in the calculation and search of unnecessary areas. There is a possibility that the time required until the calculation becomes longer, or if the calculation and search of the wavefront are forcibly terminated in a certain time, the accuracy of the calculated wavefront is lowered.

  On the other hand, if the operating range is too narrow, the calculation and search of the wavefront to obtain the optimal reference light wavefront cannot be performed in the indispensable region, and the optimal solution of the reference light wavefront is obtained. There is a risk that the wavefront accuracy may be greatly reduced even if it is not possible.

  The present invention has been made in view of such circumstances, appropriately set the operating range of the reflecting mirror surface in the wavefront controller for compensating for interference fringe distortion caused by contraction (including expansion) of the hologram recording medium, An object of the present invention is to provide a method for controlling the amount of operation of a wavefront controller and a hologram reproducing apparatus capable of reducing the time required for optimizing the wavefront of reference light during reproduction and improving the wavefront accuracy.

In order to achieve the above object, an operation amount control method for a wavefront controller according to the present invention includes a hologram recording medium on which interference fringes are recorded by irradiating a focused signal light together with a reference light during recording, and the hologram recording In a hologram reproducing apparatus comprising a wavefront controller disposed in an optical path of reproduction reference light to compensate for interference fringe distortion caused by contraction of a medium, when optimizing the wavefront of the reproduction reference light An operation amount control method for controlling an operation amount of a reflecting mirror surface of the wavefront controller,
Based on the relative positional relationship between the signal beam, the recording reference beam and the hologram recording medium, the Bragg condition based on the relative positional relationship, and the shrinkage rate of the hologram recording medium, the wavefront of the reproducing reference beam is optimized. Predicting the maximum value of the amount of movement of the reflecting mirror surface necessary for the conversion to the maximum, and setting the maximum value to an operation limit value for limiting the operating range of the reflecting mirror surface in the process of optimizing the wavefront It is characterized by.

The operation amount control how the wavefront control device according to the present invention, and based on the relative positional relationship, the magnitude of the angle between the axis of the recording time of the reference light of the light beam constituting the signal light is minimal becomes light and the angle theta _K1 of the interference fringes K1 formed by the said recording time of the reference light, beam the magnitude of the angle between the axis of the recording time of the reference light of the light rays which constitute the signal light is maximized And a recording interference fringe angle calculating step for obtaining an angle θ _K2 of the interference fringe K2 formed by the recording reference light,
When distortion occurs due to shrinkage of the hologram recording medium based on the Bragg condition, the shrinkage rate of the hologram recording medium, and the interference fringe angles θ _K1 and θ _K2 obtained in the recording interference fringe angle calculation step Reproducing reference light angle calculation step for obtaining optimum reproduction reference light angles φ _R1 and φ _R2 for reproducing the interference fringes K1 and K2, respectively,
Respective angular differences Δφ ₁ (= φ _R1 −φ _R ) and Δφ ₂ (= φ _R2 −φ _R ) between the obtained angles φ _R1 and φ _R2 and an intermediate angle φ _R obtained by arithmetically averaging them are obtained. An angle difference calculating step;
The amount of operation D _{DM1 of the} reflecting mirror surface required to compensate for the obtained angle difference Δφ ₁ and the amount of operation D _{DM2 of the} reflecting mirror surface required to compensate for the angle difference Δφ ₂ are calculated. and, including the operation limit value setting step to the operation restriction value towards a large formed by comparing them, as steps.

Further, in the case where the hologram recording medium is a recording of a plurality of interference fringes by an angle multiplex recording method,
When the interference fringes recorded at the time when the rotation angle of the hologram recording medium becomes maximum are reproduced, the maximum value D _{DMM1 of the} amount of movement of the reflecting mirror surface and the interference fringes recorded at the time when the rotation angle becomes minimum The maximum value D _DMM2 of the operation amount of the reflecting mirror surface in the case of reproducing is calculated by the above-described operation control method of the wavefront controller, and the larger one is compared as the operation limit value. it can.

  The hologram reproducing apparatus according to the present invention includes a wavefront controller in which the operation limit value is set by the operation amount control method of the wavefront controller according to the present invention.

  “Reproduction” in the hologram reproducing apparatus of the present invention indicates that it has a hologram reproducing function, and does not mean that it does not have a hologram recording function. That is, an apparatus having both a hologram recording function and a reproducing function is also included in the concept of the hologram reproducing apparatus of the present invention.

  According to the method for controlling the operation amount of the wavefront controller and the hologram reproducing apparatus according to the present invention, the relative positional relationship between the signal light, the recording reference light, and the hologram recording medium, the Bragg condition based on the relative positional relationship, and the hologram recording medium Based on the shrinkage rate of the mirror, the maximum value of the amount of movement of the reflecting mirror surface necessary for optimizing the wavefront of the reference light during reproduction is predicted, and the operation of the reflecting mirror surface in the process of optimizing the wavefront is estimated based on the maximum value By setting the operation limit value to limit the range, it is possible to appropriately set the operation range without making the operation range of the reflector surface unnecessarily wide or too narrow. Wavefront accuracy can be improved while shortening the time (compensation time) required to optimize the wavefront of light.

  In addition, by reducing the compensation time (improving compensation speed) and improving the wavefront accuracy, it is possible to improve the SNR of the reproduced data, thereby enabling good reproduction of the data multiplexed and recorded on the hologram recording medium at high density. Therefore, recording / reproduction with a larger capacity than the conventional one is possible.

It is the schematic which shows the structure of the hologram reproduction apparatus which concerns on one Embodiment. It is sectional drawing which shows schematic structure of a wavefront controller. It is a conceptual diagram which shows the relative positional relationship of signal light, recording reference light, and a hologram recording medium, and the angle of the interference fringe recorded (when signal light is parallel light). It is a conceptual diagram which shows the relationship between the shrinkage rate of a hologram recording medium, and interference fringe distortion. It is a conceptual diagram which shows the irradiation angle of the reference light at the time of reproduction | regeneration at the time of medium shrinkage | contraction (when signal light is parallel light). It is a conceptual diagram which shows the optical path in the medium which satisfy | fills the Bragg condition at the time of medium contraction. It is a conceptual diagram which shows the relative positional relationship of signal light, the recording reference light and the hologram recording medium, and the angle of the interference fringe to be recorded (when the signal light is focused light). It is a conceptual diagram which shows the irradiation angle of the reference light at the time of reproduction | regeneration at the time of medium shrinkage | contraction (when signal light is focused light). It is a figure which shows the relationship between the incident angle of the signal beam | light to a hologram recording medium, and the maximum operation amount of a thin film mirror. The figure which shows the page data image (a) reproduced | regenerated using the reference light of the same wave front as the time of recording, and the page data image (b) reproduced | regenerated using the reference light of the wave front compensated by applying this invention It is. It is the schematic which shows the structure of the conventional hologram reproduction apparatus.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of a wavefront controller operation amount control method and a hologram reproducing apparatus according to the present invention will be described below in detail with reference to the drawings.

  First, a hologram reproducing apparatus according to an embodiment of the present invention will be described with reference to FIG. This hologram reproducing apparatus is configured as a recording / reproducing apparatus having a hologram recording function and a reproducing function, and a hologram recording medium (hereinafter sometimes simply referred to as “recording medium”) 11 is photosensitive. It is composed of a resin material (photopolymer), and an angle multiplex recording method is adopted. The mechanism (flow of recording / reproducing operation) will be described below.

  The coherent laser beam emitted from the laser light source 1 is expanded in beam diameter by a beam expander composed of a diverging lens 2 and a collimating lens 3, passes through a half-wave plate 4, is deflected by a mirror 5, and then is a polarized beam. The light is branched into two light beams by the splitter 6.

  The light beam traveling downward in the figure from the polarization beam splitter 6 passes through the shutter 7 and the polarization beam splitter 8 in the open state, and is irradiated onto the spatial light modulation element 9, and is spatially modulated by the spatial light modulation element 9, The signal light carries page data composed of a digital image in which white and black binary pixels are two-dimensionally arranged. The polarization direction of the signal light emitted from the spatial light modulator 9 is changed from that of the incident light, and is reflected leftward in the drawing by the polarization beam splitter 8 and optically Fourier transformed by the lens 10 to be holograms. The recording medium 11 is irradiated.

  On the other hand, the light beam traveling from the polarizing beam splitter 6 toward the left in the figure is used as reference light (recording reference light), passes through the wavefront controller 21 and the open shutter 24, and the signal light in the recording medium 11 is transmitted. The passing place is irradiated at an angle different from that of the signal light, whereby interference fringes are recorded in the recording medium 11.

  The recording medium 11 is mounted on a rotary stage (not shown) and can rotate by a predetermined angle with respect to the reference light and the signal light. By changing the angle little by little, angle multiplex recording for recording a plurality of different page data in the same area of the recording medium 11 is possible.

  When the page data recorded on the hologram recording medium 11 is reproduced, the shutter 7 is closed and the recording medium 11 is irradiated with only the reference light (reference light at the time of reproduction), thereby performing multiple recording in the same recording area. Of the plurality of page data, only desired page data is selectively reproduced and imaged by a CCD (imaging device) 13 via the lens 12.

The wavefront controller 21 is arranged to compensate for interference fringe distortion caused by the shrinkage (including expansion) of the hologram recording medium. As shown in FIG. A deformable mirror that is supported from the back side by a plurality of pin members (only _two pin members P ₁ and P ₂ farthest from the central axis C ₁ are shown in FIG. 2) that can be moved back and forth in the figure. Yes, by adjusting the amount of protrusion from the substrate 52 for each pin member, each region of the thin film mirror 51 supported by each pin member is moved in the vertical direction, and the shape of the entire thin film mirror 51 changes. Is configured to do.

  The movement amount of the thin film mirror 51 (the protruding amount of each pin member) is controlled by the control method using a genetic algorithm in the measurement control device 40 shown in FIG. That is, the measurement control device 40 evaluates the reproduction data imaged by the CCD 13, outputs a control signal corresponding to the evaluation result to the wavefront controller 21, and changes the operation amount of the thin film mirror 51. It is performed repeatedly using a genetic algorithm to optimize the wavefront of the reference light during playback.

  In the genetic algorithm, a group (individual group) consisting of a plurality of individuals is assumed, a gene as an element that defines the trait of each individual is defined, and gene duplication, crossover, mutation, etc. are repeatedly executed. . Based on the fitness, individuals that are most suitable for the environment will form the next generation. Specifically, the state of the thin film mirror 51 is an individual, and the protrusion amount of each pin member (protrusion position with respect to the substrate 52) is a gene. As the fitness, the brightness of reproduction data, SNR, and the like are used.

  In addition, in the control of the operation amount of the thin film mirror 51, the operation amount control method of the wavefront controller according to the embodiment of the present invention is applied, and the operation range of the thin film mirror 51 is set to an appropriate one. .

  Hereinafter, the operation amount control method of the wavefront controller according to the embodiment of the present invention will be described. Here, for easy understanding, the case where the signal light is parallel light will be described first, and the case of the embodiment in which the signal light is focused light will be described based on this description. 3 to 8 illustrate the principle of the method of the present invention. The relative positional relationship between the illustrated signal light, recording reference light, recording medium, reproduction reference light, and interference fringes is shown in FIG. Does not necessarily match.

As shown in FIG. 3, let d be the thickness of the recording medium during recording (before shrinkage). Further, both the signal light and the reference light are parallel lights, and each angle of the signal light, the recording reference light, the interference fringes recorded in the medium, and the recording medium with respect to the axis A ₁ (can be arbitrarily set) in the figure. Are θ _S , θ _R , θ _K, and θ _M , respectively.

  From the angle of the signal light and the reference light, the interference fringe angle is calculated by the following equation (1).

As shown in FIG. 4, when the recording medium contracts (the contraction rate is α), the recording medium thickness d contracts to (1−α) d, thereby changing the angle of the interference fringes. When the angle of this time is phi _K, it satisfied the relationship of the formula (2), from which the angle phi _K, can be calculated by the following equation (3). In addition, when the manufacturer of the recording medium publishes the measured value, the published value can be used as the shrinkage rate of the recording medium. In addition, a plurality of (about 30 pages) page data is recorded by the angle multiplex recording method using the signal light and the reference light, both of which are parallel light, and the contraction rate is obtained from the state of each data when they are reproduced. Is also possible. Since the shrinkage rate depends on the material of the recording medium, once measured, the shrinkage rate of other recording media made of the same material can be used as the same.

As shown in FIG. 5, when reproducing the distorted interference fringes, it is necessary to irradiate the reference beam to match the angle phi _K distorted interference fringes at an angle phi _R. The angle phi _R can be obtained by the signal light, the Bragg condition based on the relative positional relationship between the recording time of the reference light and the recording medium.

  That is, as shown in FIG. 6, in order to satisfy the Bragg condition, the sum of the optical path lengths of the signal light in the recording medium and the recording reference light needs to be an integral multiple of the wavelength. Therefore, the relationship of the following equation (4) needs to be established before the medium contraction, and the relationship of the following equation (5) needs to be established after the medium contraction.

  From the above equations (4) and (5), the relationship of the following equation (6) is derived.

And rearranging by substituting the above equation (3) in the above equation (6), the reference light angle phi _R of the interference fringes for satisfying the Bragg condition distorted, can be obtained by the following equation (7).

  Therefore, the compensation angle of the reference light during reproduction (difference between the angle of the reference light during recording and the angle of the reference light during reproduction) Δθ required to compensate for interference fringe distortion caused by the shrinkage of the recording medium is expressed by the following equation: It can be obtained by (8).

  Next, an operation amount control method (hereinafter referred to as “method of the present embodiment”) of the wavefront controller when the signal light is focused light and corresponds to the embodiment shown in FIG. 1 will be described.

  The method of the present embodiment optimizes the wavefront of the reference light during reproduction based on the relative positional relationship between the signal light, the reference light during recording and the recording medium, the Bragg condition based on the relative positional relationship, and the shrinkage rate of the recording medium. The maximum value of the amount of operation of the reflecting mirror surface (thin film mirror 51) necessary to perform the prediction is predicted, and the maximum value is set as an operation limit value for limiting the operating range of the reflecting mirror surface in the process of optimizing the reference light wavefront. Specifically, it is performed according to the following procedure.

<1> As shown in FIG. 7, when the relative positional relationship between the signal light, the recording reference light, and the recording medium is set, based on this relative positional relationship, reference is made to the recording light among the light beams constituting the signal light. The angle θ _K1 of the interference fringe K1 formed by the light beam L _S1 having the smallest angle with the optical axis and the recording reference light, and the recording reference light among the light beams constituting the signal light. a ray L _S2 of the magnitude of the angle between the axis a _R is maximum, the angle in the angle theta _K2 (FIG. 7 during recording reference beam and interference fringes K2 formed by the axis a of the signal light _S is the starting line, and the clockwise angle in the figure is displayed as positive and the counterclockwise angle is displayed as negative) (interference fringe angle calculation step during recording).

When the signal light is focused as shown in FIG. 7, the incident angle of the signal light varies depending on the position. Angle theta _S1 signal described above with respect to the axis _{A S} of the light beam _{L S1} and light _{L S2,} and theta _S2, (the angle between theta _R relative to the axis _{A S)} These rays _{L S1} and light _{L S2} and a reference beam The angles θ _K1 and θ _K2 of the interference fringes K1 and K2 formed by the above equation (1) can be obtained from the above equations (1) by the following equations (9) and (10).

<2> Based on the Bragg condition based on the relative positional relationship, the shrinkage rate of the recording medium, and the interference fringe angles θ _K1 and θ _K2 obtained in the recording interference fringe angle calculation step, The optimum reproduction reference light angles φ _R1 and φ _R2 for reproducing the interference fringes K1 and K2 when distorted are obtained (reproduction reference light angle calculation step).

As shown in FIG. 8, when the signal light is a focused light, the incident angle of the signal light varies depending on the position, so the distortion pattern of the interference fringes varies depending on the location. Therefore, reproduction for compensating the distorted interference fringes is performed. The angle of the hour reference light also varies depending on the position. The angles φ _R1 and φ _R2 of the reproduction reference light optimal for reproducing the interference fringes K1 and K2 when distorted due to the shrinkage of the recording medium are obtained from the above equation (7) and the following equations (11), ( 12).

<3> Respective angle differences Δφ ₁ (= φ _R1 −φ _R ) and Δφ between the angles φ _R1 and φ _R2 obtained in the reference light angle calculation step during reproduction and the intermediate angle φ _R obtained by arithmetic averaging of these angles ₂ (= φ _R2 −φ _R ) is obtained (angle difference calculating step).

The intermediate angle phi _R is determined by the following equation (13), the angle difference [Delta] [phi ₁ and [Delta] [phi ₂ are determined respectively by the following formulas (14) and (15).

<4> Reflector mirror operation amount D _DM1 required to compensate for the angle difference Δφ ₁ obtained in the angle difference calculation step, and the reflector mirror surface required to compensate for the angle difference Δφ ₂ . The operation amount D _DM2 is calculated, and these are compared, and the larger one is set as the operation limit value for limiting the operation range of the reflecting mirror surface (operation limit value setting step). The significance of this is as follows.

That is, when the way the distortion plays a hologram from different interference fringes on the location, as shown in FIG. 8, as the intermediate angle φ axis A of the _R to the reproduction time of the reference beam _{R 'is} positioned at the beginning, the reflecting surface mirror And the relative angle of the recording medium. However, only by adjusting this relative angle, the reproduction-time reference light reflected from the reflecting mirror becomes parallel light traveling along the axis A _R ′. angle of reproduction reference beam incident on the position of the K1 and K2 be the same as the intermediate angle phi _R. And the intermediate angle phi _R, between the angle phi _R1 and phi _R2 interference fringes K1 and K2 of the optimal reproduction reference beam to reproduce each case distorted, as described above angular difference [Delta] [phi ₁ and [Delta] [phi ₂ has occurred. Therefore, in order to satisfactorily reproduce the interference fringes K1 and K2 when distorted, it is necessary to compensate for the angle differences Δφ ₁ and Δφ ₂ .

These angle differences Δφ ₁ and Δφ ₂ are finally compensated by operating the reflecting mirror surface (thin film mirror 51) by control using a genetic algorithm, but the operating range of the reflecting mirror surface is more than necessary. If it is set to a large value, the reflecting mirror surface will be operated in excess of the amount of motion that is finally required in the control process, and this may cause a long time for compensation.

Therefore, the mirror mirror movement amounts D _DM1 and D _DM2 necessary for compensating the angle differences Δφ ₁ and Δφ ₂ described above are calculated in advance, and the larger one is compared as the operation limit value. is there. In the method of the present embodiment, as shown in FIG. 9, it is assumed that the pin members P ₁ and P ₂ of the wavefront controller 21 are pin members that operate to compensate for the angle differences Δφ ₁ and Δφ ₂ , respectively. The operation amounts D _DM1 and D _DM2 are calculated. Central axis distance from _{C 1} to pin member _P 1, _{P 2} and (mutually equal) and _{P DM,} the pin member _P 1, the pin member _P 1 by the operation amount _{D DM1, D DM2} of _{P 2} of the thin-film mirror 51, Assuming that the angle of the thin film mirror 51 at each position of P ₂ (the angle with respect to the substrate 52 perpendicular to the central axis C ₁ ) is equal to the angle difference Δφ ₁ , Δφ ₂ , D _DM1 and D _DM2 are expressed by the following equation (16): And (17), respectively.

The calculated sizes of D _DM1 and D _DM2 are compared. When D _DM1 > D _DM2 , D _DM1 is set as the limit value of the operating range of the thin film mirror 51, and when D _DM1 <D _DM2 , D _DM1 is set. _DM2 is the limit value of the operating range of the thin film mirror 51. That is, the operation range _{D D} of thin-film mirror _51, in the case of D _{DM1> D DM2} is represented by the following formula _(18), in the case of D DM1 _{<D DM2} is expressed by the following formula (19) It will be.

When the angle multiplex recording method is employed, the angle θ _M of the recording medium in the above-described procedure changes for each of a plurality of page data to be multiplex recorded. Therefore, when the page data recorded at the time when the rotation angle of the recording medium becomes maximum is reproduced, the maximum value D _DMM1 of the operation amount of the thin film mirror 51 and the page data recorded at the time when the rotation angle becomes minimum are obtained. The maximum value D _DMM2 of the operation amount of the thin film mirror 51 in the case of reproduction is calculated according to the above-described procedure, and the larger one is compared to be the operation limit value of the thin film mirror 51. That is, the operation range _{D D} of thin-film mirror _51, in the case of _{D DMM1>} D _DMM2 is represented by the following formula _(20), in the case of _D DMM1 _{<D DMM2} is expressed by the following formula (21) It will be.

  By limiting the operating range of the thin film mirror 51 in this way, unnecessary calculations and searches can be reduced, so that the compensation time can be shortened and the compensation accuracy can be improved.

  Hereinafter, the result of verifying the effect when the present invention is applied will be described. The verification of the effect was performed for the case where the hologram reproducing apparatus having the mode shown in FIG.

The angle of the recording medium 11 when recording / reproducing (the angle θ _M ) was set from 90 ° to 100 °. The focal length of the lens 10 was 50 mm, and the image size during recording was 7.7 mm. At this time, the angles of the both ends of the signal light (the angles θ _S1 and θ _S2 ) were 8.75 ° and −8.75 °. In addition, the angle of the reference light during recording (the above angle θ _R ) was 40 °.

  FIG. 9 shows the incident angle of the signal light and the maximum operation amount of the reflecting mirror surface 51 required to compensate for interference fringe distortion when the angle of the recording medium 11 is 90 °, 95 °, and 100 °. Shows the relationship. When the angle of the recording medium is 90 °, the operation amount of the thin film mirror 51 is maximized, and the operation amount at this time is determined to be 2 μm.

  As a result, the operating range of the thin film mirror 51 is set to 0 to 2 μm, and in this operating range, the wavefront of the reference light during reproduction is optimized using the genetic algorithm disclosed in the above-mentioned Patent Document 1 and Non-Patent Document 1. went.

  As shown in FIG. 10, the page data image (a) reproduced using the reference light having the same wavefront as that at the time of recording and the operating range of the thin film mirror 51 are limited to 0 to 2 μm and compensated using a genetic algorithm. Comparing with the page data image (b) reproduced using the reference light having the wave front, the reproduced data is obtained in (b) even from the area where the reproduced image is dark and no reproduced data is obtained in (a). It was. From this, it was confirmed that even when the operating range of the wavefront controller is limited by applying the present invention, the interference fringe distortion can be optically compensated and the deterioration of the reproduction data can be improved.

  In addition, when the wavefront of the reference light during reproduction is optimized using the same genetic algorithm without limiting the operating range of the thin film mirror 51, the control is performed when the present invention is applied. While the number of steps was 64, when the operation limit value was not set, the number of control steps reached 192. Therefore, when the present invention is applied to limit the operating range of the wavefront controller, the number of control steps can be greatly reduced, and the time required to optically compensate for interference fringe distortion. It was confirmed that can be shortened.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to the above-mentioned embodiment.

  For example, wavefront controllers are not limited to deformable mirrors, but include liquid crystal panels, liquid crystal lenses that correct specific Zernike coefficients, or optical systems that combine multiple elements such as acousto-optic modulators and electro-optic modulators. But it can also be configured.

  Further, the control method of the wavefront controller is not limited to that using a genetic algorithm, and other methods such as feedback control and feedforward control can also be used.

1,201 Laser light source 2,202 Diverging lens 3,203 Collimating lens 4,204 Half-wave plate 5,205,222 Mirror 6,8,206,208 Polarizing beam splitter 7,24,207,224 Shutter 9,209 Spatial light Modulation element 10, 12, 210, 212 Lens 11, 211 Hologram recording medium 13, 213 CCD
21 Wavefront controller 40 Measurement control device 51 Thin film mirror 52 Substrate P ₁ , P ₂ pin member K ₁ , K 2 Interference fringe

Claims (3)

A hologram recording medium in which interference fringes are recorded by irradiating the focused signal light together with the reference light during recording, and in the optical path of the reference light during reproduction in order to compensate for interference fringe distortion caused by contraction of the hologram recording medium In a hologram reproducing apparatus comprising a wavefront controller disposed, an operation amount control method for controlling an operation amount of a reflecting mirror surface of the wavefront controller when optimizing a wavefront of the reference light during reproduction,
Based on the relative positional relationship between the signal beam, the recording reference beam and the hologram recording medium, the Bragg condition based on the relative positional relationship, and the shrinkage rate of the hologram recording medium, the wavefront of the reproducing reference beam is optimized. Predicting the maximum value of the amount of motion of the reflecting mirror surface necessary to achieve, and setting the maximum value to an operation limit value for limiting the operating range of the reflecting mirror surface in the process of optimizing the wavefront ,
Based on the relative positional relationship, of the interference fringes K1 formed by the light beam constituting the signal light and having the smallest angle with the axis of the recording reference light beam and the recording reference light beam. The angle θ of the interference fringe K2 formed by the light beam having the maximum angle between the angle θ _K1 and the axis of the recording reference light beam among the light beams constituting the signal light and the recording reference light beam A recording interference fringe angle calculating step for _{obtaining K2} ;
When distortion occurs due to shrinkage of the hologram recording medium based on the Bragg condition, the shrinkage rate of the hologram recording medium, and the interference fringe angles θ _K1 and θ _K2 obtained in the recording interference fringe angle calculation step Reproducing reference light angle calculation step for obtaining optimum reproduction reference light angles φ _R1 and φ _R2 for reproducing the interference fringes K1 and K2, respectively ,
Respective angular differences Δφ ₁ (= φ _R1 −φ _R ) and Δφ ₂ (= φ _R2 −φ _R ) between the obtained angles φ _R1 and φ _R2 and an intermediate angle φ _R obtained by arithmetically averaging them are obtained. An angle difference calculating step;
The amount of operation D _{DM1 of the} reflecting mirror surface required to compensate for the obtained angle difference Δφ ₁ and the amount of operation D _{DM2 of the} reflecting mirror surface required to compensate for the angle difference Δφ ₂ are calculated. An operation amount control method for a wavefront controller, comprising: an operation limit value setting step in which the larger of these values is set as the operation limit value as a procedure . In the case where the hologram recording medium has a plurality of interference fringes recorded by an angle multiplex recording method,
When the interference fringes recorded at the time when the rotation angle of the hologram recording medium becomes maximum are reproduced, the maximum value D _{DMM1 of the} amount of movement of the reflecting mirror surface and the interference fringes recorded at the time when the rotation angle becomes minimum the maximum value D _DMM2 operation amount of the reflecting mirror surface in the case of reproducing, according to claim 1 Symbol respectively calculated by the operation control method of the wavefront controller of mounting, the operation limit value towards the larger formed by comparing these An operation amount control method for a wavefront controller, characterized in that: Hologram reproducing apparatus characterized in that it comprises a wavefront controller the operation limit value is set by the operation amount control method of wavefront controller of claim 1 or 2.