Method for reproducing hologram

A method for reproducing a hologram includes: irradiating a recording disc with a first reference beam and a second reference beam, both having a parallel light flux, in different directions at a same incident angle to form a hologram having an unslanted grating pattern in which a grating vector is parallel to a light incident surface of the recording disc; irradiating the hologram with the first reference beam or the second reference beam to extract reproduced light; and detecting a position where an intensity of the reproduced light is maximum.

CROSS-REFERENCE TO THE RELATED APPLICATION(S)

The present disclosure relates to the subject matters contained in Japanese Patent Application No. 2008-318877 filed on Dec. 15, 2009, which are incorporated herein by reference in its entirety.

FIELD

The present invention relates to a method for reproducing a hologram.

BACKGROUND

Since a CD (Compact Disc) is developed, capacity of an optical disc has been expanded while setting a shorter wavelength of a laser and a higher numerical aperture of an objective lens as main development targets. As a result of the emergence of a BD (Blu-ray Disc) using a blue-violet laser having a wavelength of 405 nm band and an objective lens having a numerical aperture of 0.85, the capacity of an optical disc is approaching near the limit. The reasons of the above are that, when the wavelength is 400 nm or shorter, substrate absorption becomes prominent, and that the numerical aperture of the objective lens is near 1 which is the physical limit.

In order to further increasing the capacity, as a successor of the above-described optical storage, a hologram recording/reproducing apparatus has been proposed.

A hologram recording/reproducing apparatus performs recording on the principle that signal beam and reference beam emitted from a light source are caused to interfere with each other in a recording medium, to record information three-dimensionally in the form of minute interference fringes (hologram). In the apparatus, plural sets of information can be multiply recorded in the same place of a recording medium. Therefore, the capacity can be significantly increased as compared with the two-dimensional recording of a current optical disc in which information is recorded in the form of pits or marks in a plane.

In accordance with the remarkable progress of the technical level of key components which are necessary for constructing a system of a hologram recording/reproducing apparatus, such as a spatial light modulator and an image pickup device, commercialization of a hologram recording/reproducing apparatus and widespread use subsequent thereto are becoming a real possibility.

In practical application of a hologram recording/reproducing apparatus, however, there is a difficulty of degradation of a reproduced image due to temperature difference. The difficulty is caused by a phenomenon that anisotropic thermal expansion of a hologram, and a change of the refractive index of a recording material occur with a change of the temperature, and the reference beam in reproduction does not satisfy the Bragg condition.

As a method which may solve the difficulty, there has been proposed a configuration in which the shift quantity of the reproduction wavelength is determined based on the temperature difference between recording and reproduction, and the oscillation wavelength of a variable wavelength laser is shifted. An example of such configuration is disclosed in JP-A-2006-267554 (corresponding U.S. publication is: US 2006/0232841 A1).

In the configuration disclosed in JP-A-2006-267554, although a certain improvement can be expected, however, it seems that a recording/reproducing method which is more accurate is necessary in view of configuring a stable system. Prior to compensation of a reproduced image in reproduction involving a temperature difference, therefore, a position servo control must be first accurately performed on a recorded portion. In this case, a method in which the position servo control is performed by using an external sensor may be employed. In the method, however, the stability is low in view of a temporal change of a recorded hologram, apparatus compatibility, etc. Therefore, it is preferred that a servo control is performed while a servo signal is produced by a recorded hologram itself. In the servo signal, with respect to uncertain variations such as disturbances against the temperature change and design errors, naturally, the system characteristic is requested to maintain the present status (hereinafter, this is referred to as robust). However, a simple method which can be used in a practical level has not been proposed.

SUMMARY

According to a first aspect of the invention, there is provided a method for reproducing a hologram, the method including: irradiating a recording disc with a first reference beam and a second reference beam, both having a parallel light flux, in different directions at a same incident angle to form a hologram having an unslanted grating pattern in which a grating vector is parallel to a light incident surface of the recording disc; irradiating the hologram with the first reference beam or the second reference beam to extract reproduced light; and detecting a position where an intensity of the reproduced light is maximum.

According to a second aspect of the invention, there is provided a method for reproducing a hologram, the method including: irradiating a recording disc with a first reference beam and a second reference beam, both having a parallel light flux, in different directions at a same incident angle to form a first hologram having an unslanted grating pattern in which a grating vector is parallel to a light incident surface of the recording disc; irradiating a position of the recording disc where the first hologram is formed with a first signal beam or a second signal beam which is produced as a binarized pattern by a spatial light modulator to cause interference with the first reference beam or the second reference beam to thereby to form a second hologram; irradiating the first hologram formed in the recording disc with the first reference beam or the second reference beam to extract reproduced light; detecting a position where an intensity of the reproduced light is maximum while rotating the recording disc about an axis that is perpendicular to the grating vector of the unslanted grating pattern, the axis being on the light incident surface of the recording disc; detecting a position where an intensity of the reproduced light is maximum while rotating the recording disc in a direction perpendicular to the grating vector; and irradiating the detected position with the first reference beam or the second reference beam to reproduce the second hologram formed at the position where the first hologram is formed.

According to a third aspect of the invention, there is provided a method for reproducing a hologram, the method including: irradiating a recording disc with a first reference beam and a second reference beam, both having a parallel light flux, in different directions at a same incident angle to form a first hologram having an unslanted grating pattern in which a grating vector is parallel to a light incident surface of the recording disc; irradiating a position of the recording disc where the first hologram is formed with a first signal beam or a second signal beam which is produced as a binarized pattern by a spatial light modulator to cause interference with the first reference beam or the second reference beam to thereby to form a second hologram; irradiating the first hologram formed in the recording disc with the first reference beam or the second reference beam to extract reproduced light; detecting a position where an intensity of the reproduced light is maximum while rotating the recording disc in a direction perpendicular to the grating vector of the unslanted grating pattern; detecting a position where an intensity of the reproduced light is maximum while rotating the recording disc about an axis that is perpendicular to the grating vector, the axis being on the light incident surface of the recording disc; and irradiating the detected position with the first reference beam or the second reference beam to reproduce the second hologram formed at the position where the first hologram is formed.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments of the invention will be described with reference to the drawings. In the following description, the same or similar components will be denoted by the same reference numerals, and the duplicate description thereof will be omitted.

First Embodiment

FIG. 1is a diagram of a hologram reproducing apparatus according to a first embodiment of the present invention, andFIG. 2is a diagram of a recording medium and an optical system.

As shown inFIG. 1, the hologram reproducing apparatus according to the first embodiment includes a recording disc101, a first reference beam104and a second reference beam106which are emitted from a light source (not shown), a first signal beam105, a second signal beam107, spatial light modulators108,109, image pickup devices111,112, and photodetectors113,114. In the light source which emits the first reference beam104and the second reference beam106, a plurality of light sources may be used, or a light source which is split by abeam splitter may be used.

A hologram is recorded into the recording disc101by means of interference between reference beam and signal beam. In the embodiment, two sets of the first reference beam104and the first signal beam105, and the second reference beam106and the second signal beam107are used. An example of a hologram reproducing apparatus which uses two sets of reference beam and signal beam is disclosed in JP-A-2004-354565.

The spatial light modulators108,109modulate intensity of light from the light source to a binary pattern consisting of bright and dark points, to produce the first signal beam105and the second signal beam107.

As the spatial light modulators108,109, preferably, a DMD (digital micro-mirror device), ferroelectric liquid crystal, or the like may be used.

The image pickup devices111,112are used for reproducing an information hologram (this term may be used in the description for a hologram for main data to be distinguished from a servo hologram which will be described later), and detect reproduced light which is generated by irradiating the recording disc101with the first reference beam104or the second reference beam106, and causing diffraction in the recording disc101. As the image pickup devices111,112, preferably, a CCD or a CMOS may be used. The photodetectors113,114detect the signal light intensity of the reproduced light from the recording disc101. As the photodetectors113,114a photodiode may be used.

As shown inFIG. 2, the recording disc101has a configuration in which a recording medium102is sandwiched by substrates103. As the recording medium102, a photopolymer is mainly used. A photopolymer is a photosensitive material in which photopolymerization of a polymerizable compound (monomer) may be used, and is a gelatinous material which contains as primary components a monomer, a photopolymerization initiator, and a matrix that has a porous structure, and that plays a role of holding the volume before and after recording. A photopolymer is being developed with the objectives of high sensitivity and improvement of multiple performance.

As the substrates103, typically, polycarbonate, amorphous polyolefin, glass, or the like may be used. The primary objectives of the substrates103are to hold the shape of the photopolymer which is a gelatinous material, and to protect the photopolymer of the recording medium102from damage and dust.

In order to simplify the drawings, an objective lens is omitted inFIG. 1, and only the central ray of the first signal beam105is shown. As shown inFIG. 2, actually, a configuration in which light is converged on the recording disc101by using objective lenses110a,110bmay be used. As the objective lenses110a,110b, a three-group three-element configuration is shown. The two objective lenses110a,110bare opposed to each other across the recording disc101in a so-called tandem arrangement. In the case where the objective lenses110a,110bhaving a high numerical aperture are used in the tandem arrangement, the performance as a multi-group lens is ensured with the objectives of reduction of the curvature of field, assuring the operating distance, and the like. While the objective lenses having the three-group three-element configuration are shown in the embodiment, it is a matter of course that the lenses are not restricted to this. With respect to the second signal beam107, the identical configuration is employed, and hence its illustration is omitted inFIG. 2.

Next, an operation in the case where recording is performed on the recording disc101will be described.

The recording in the hologram reproducing apparatus to which the embodiment is applied is divided into two steps of “recording of servo hologram” and “recording of information hologram” which is normally performed. Hereinafter, description will be made in the sequence of “recording of servo hologram”, “recording of information hologram”, and “reproducing method”.

Recording of Servo Hologram

The recording of a servo hologram is performed for the following reason. Because anisotropic thermal expansion of a hologram, and a change of the refractive index of the recording material occur with a change of the temperature, the reference beam in reproduction does not satisfy the Bragg condition. First, the reason will be described with reference toFIGS. 3 to 5.

As shown inFIG. 3, the recording disc101has the configuration in which the recording medium102is sandwiched by the two substrates103as described with reference toFIG. 2. The recording disc101is irradiated with reference beam201and signal beam203. The reference beam201is a parallel light flux. By contrast, a configuration where the signal beam203is converged on the recording disc101by an objective lens205is usually employed. When paying attention to the signal beam203from one pixel of a spatial light modulator202, in a target optical arrangement, the light can be deemed a spherical wave from a point light source inFIG. 3, and signal beam204after passing through the objective lens205can be deemed a parallel light flux as illustrated. When a complex hologram which is recorded as result of interference between the reference beam201that is a parallel light flux, and the signal beam204that is a converging light flux is element reduced, the hologram can be considered as a superposition of holograms of parallel light fluxes.

With reference toFIG. 4, a hologram which is recorded as result of interference between the reference beam201and signal beam204both of which are parallel light fluxes will be discussed. InFIG. 4, the reference symbol K indicates a grating vector206in recording. Usually, a hologram is a refractive index modulation grating. In this case, the grating vector206in recording is a vector perpendicular to a constant refractive index plane.

In the case where the recording disc101is heated, thermal expansion of the recording medium102is anisotropic because the recording medium102is sandwiched by the substrates103, and the boundary conditions are different among the upper, lower, right, and left sides. The phenomenon that the thermal expansion is anisotropic is described in a related-art document 1 listed below.

As shown inFIG. 5, thermal expansion (the arrow207shown inFIG. 5) in the in-plane direction of the recording disc101, and that (the arrow208shown inFIG. 5) in the thickness direction of the recording medium are different from each other. This is described in a document listed below. According to the physical model described in the related-art document 2, the coefficient of linear expansion in the in-plane direction is given by the coefficient of linear expansion of the substrates, and the coefficient of linear expansion in the thickness direction of the recording medium is given by the coefficient of linear expansion of the recording medium. Here, it is assumed that the recording medium102is a photopolymer, and the substrates103are glass substrates.

According to, for example, a related-art document 3 listed below, the coefficient of linear expansion of a photopolymer is 5×10−4[/K], and that of glass is 7×10−6[/K]. Since the coefficient of linear expansion of the recording medium102is larger by one digit or more than that of the substrates103, the expansion in the thickness direction is much larger than that in the in-plane direction of the disc.

As shown inFIG. 5, due to the anisotropic thermal expansion, the grating vector K (206) in recording is changed to a grating vector K′ (209) after the temperature change. Moreover, also the refractive index change of the photopolymer due to the temperature change is added. Therefore, even when the recording disc is irradiated with the reference beam201which is identical with that in recording, the Bragg condition is not satisfied, with the result that the signal of the reproduced light210is degraded.

In addition, in order to increase the capacity of a hologram recording/reproducing apparatus of the two-flux type, it is target to reduce the angle step of the angle multiplexing to increase the multiplicity. Therefore, it is necessary to increase the thickness of the medium to reduce the angular selectivity. As the medium is made thicker, however, degradation of the reproduced signal in reproduction involving a temperature difference is more conspicuous.

With reference toFIGS. 6,8, and9, next, a method of recording the servo hologram, and that of recording the is information hologram in the embodiment of the invention will be described.FIGS. 6,8, and9show a manner of recording a hologram301in the recording disc101, and also an unslanted grating pattern302of the hologram301.

For the sake of convenience in description, an xyz orthogonal coordinate system is set as illustrated. Namely, the disc face of the recording disc101is set as the xy plane, and the thickness direction (in the figures, the downward direction) of the recording disc101is set as the z axis.

Before a target information hologram is recorded, the recording disc101is irradiated with the first reference beam104and the second reference beam106, and the hologram301is recorded into the recording medium102as shown inFIG. 6. Since the hologram301is recorded by using a parallel light flux, the hologram301in which the noise level is low, or which is excellent is obtained.

At this time, it is necessary to form an unslanted grating pattern302in the recording medium102.

Reproduced light from the unslanted grating pattern302is robust to a temperature change. This is because, as described with reference toFIGS. 3 to 5, thermal expansion due to a temperature change in the thickness direction of the recording medium is large, but, in the case of the unslanted grating pattern302, the direction change of the grating vector K is small.

In order to form the unslanted grating pattern302, the illumination of the first reference beam104and the second reference beam106must be performed in a mirror image relationship with respect to the xz plane.

When the incident angle of the first reference beam104to the recording disc is indicated by θ1and the angle of orientation from the x axis is indicated by φ1, the ray vector R1of the first reference beam is given by the following Equation (1).
R1=(sin θ1cos φ1, sin θ1sin φ1, cos θ1)   (1)

On the other hand, when the incident angle of the second reference beam106to the recording disc is indicated by θ2and the angle of orientation from the x axis is indicated by φ2, the ray vector R2of the second reference beam is given by the following Equation (2)

Namely, θ2=θ1and φ2=−φ1are attained, and the first reference beam104and the second reference beam106are in a mirror image relationship with respect to the xz plane. When this condition may be used, a grating vector in which only the y component is non-zero can be formed. Namely, the grating vector K is given by the following Equation (3) and formed as a grating vector in which only the y component is non-zero.
K=(0, Ky, 0)   (3)

In the recording disc101, the recording medium102is sandwiched by the both substrates103, and the boundary conditions are different among the upper, lower, right, and left sides. During recording, therefore, the recording disc contracts by about 0.1%. However, also the contraction direction is anisotropic, and the, thickness direction of the substrates103is dominant. Therefore, the unslanted grating pattern302is robust also to the contraction.

In the recording system of the hologram reproducing apparatus in the embodiment, the angle multiplexing (hereinafter, referred to as θyangle multiplexing) system in which the recording disc101is rotated about the y axis, and the shift multiplication system in which multiplication is performed while the beam spot position on the recording disc101is shifted by the target rotation of the recording disc101are used in combination.

The hologram reproducing apparatus in the embodiment is assumed to have: a driving mechanism for rotation about the y axis; a driving mechanism for rotation about the z axis (the target disc rotation); and a driving mechanism for rotation about the x axis for fine adjustment.FIG. 7shows a configuration example of the disc rotating mechanism. In the example, a small spherical ultrasonic motor303is shown. In order to hold a recording disc304, a semispherical rotor305may be used. A mechanism in which the three axes are friction driven by three stators306is employed. When rotations about the three axes are combined with one another, it is possible to realize an arbitrary rotation.

Recording of Information Hologram

Next, a method of recording information into the recording medium102will be described.FIG. 8is a diagram showing the case where recording is performed on the recording disc101with simultaneously irradiating the recording disc with the first signal beam105and the first reference beam104. The first signal beam105is encoded with a binary pattern consisting of bright and dark points, by the spatial light modulator108. The information holograms are recorded by the θyangle multiplexing system using the first reference beam104.

As shown inFIG. 9, similarly with the above-described method of recording the information hologram, next, recording is performed on the recording disc101with simultaneously irradiating the recording disc with the second signal beam107and the second reference beam106. The second signal beam107is encoded with a binary pattern consisting of bright and dark points, by the spatial light modulator109. The information holograms are recorded by the θyangle multiplexing system using the second reference beam106.

Although, in the above, the description has been made in the sequence of “recording of servo hologram” and “recording of information hologram”, the recording sequence is not restricted to this. Alternatively, “recording of information hologram” and “recording of servo hologram” may be performed in this sequence, or “recording of information hologram” may be interrupted and “recording of servo hologram” may be performed.

Reproducing Method

Referring toFIGS. 10 and 11, next, a method for reproducing the servo hologram and information hologram which are recorded in the recording disc101in the embodiment will be described.

For the sake of convenience in description, an xyz orthogonal coordinate system is set as illustrated. Namely, the disc face of the recording disc101is set as the xy plane, the thickness direction (in the figures, the downward direction) of the recording disc101is set as the z axis.

FIG. 10is a diagram showing the method for reproducing the servo hologram and information hologram which are recorded is the recording disc101. The figure shows reproduced light401from the information hologram, and reproduced light402from the servo hologram. The reproduced light401and the reproduced light402are generated by irradiating the hologram301formed in the recording disc101, with the first reference beam104, to cause diffraction therefrom.

As shown inFIG. 10, the recording disc101is irradiated only with the first reference beam104, and the image pickup device111receives the reproduced light401diffracted from the information hologram301recorded in the recording disc101, while θy-rotating the recording disc101. Thereafter, the received reproduced image is decoded to obtain information.

At this time, in accordance with the irradiation of the first reference beam104, also the reproduced light402from the servo hologram301is generated simultaneously with the reproduced light401. The reproduced light402is received by the photodetector114. The information received by the photodetector114may be used as position information for the servo control.

As shown inFIG. 11, similarly withFIG. 10, next, the recording disc101is irradiated only with the second reference beam106, and the image pickup device112receives the reproduced light403diffracted from the information hologram301recorded in the recording disc101, while θq-rotating the recording disc101. The reproduced light404from the servo hologram301is received by the photodetector113.

According to the embodiment, it is possible to provide a hologram reproducing apparatus in which positioning for reproduction that is robust to a temperature change is enabled. Therefore, the spot position which is θy-angle multiplexing recorded can be highly accurately detected, and, particularly, positional deviation in the disc tangential direction can be detected with a high degree of accuracy. Optical devices which are to be newly disposed are only the photodetectors and circuit components associated therewith. Therefore, the embodiment can be easily embodied. The reproduced light from the information hologram, and that from the servo hologram advance through the different optical paths, and hence a crosstalk-free system is realized. Furthermore, it is requested only to write at least one servo hologram on an angle multiplexing recorded spot of about several hundred multiplications. Therefore, a waste of M/# (which is an index indicating the multiple performance of a recording medium) and reduction of the recording capacity due to recording of the servo hologram are in a negligible range.

Second Embodiment

FIG. 12is a diagram of a hologram reproducing apparatus which uses phase conjugate reproduction according to a second embodiment of the invention.

In the second embodiment, a total of three optical paths for the first signal beam105which is caused to irradiate the recording disc101by an objective lens (not shown), and the first reference beam104and second reference beam106which are parallel light fluxes are configured. In order to avoid complication of the figure, the objective lens is not shown, and only the central ray of the first signal beam105is shown. In order to attain a phase conjugate reproduction system, λ/4 plates502,505and mirror503,506are placed on the side which is opposite across the recording disc101. Furthermore, the apparatus includes shutters501,504. In order to separate phase conjugate reproduced light, the apparatus further includes polarizing beam splitters507,508,509on the light incident side. Examples of a phase conjugate reproduction system are disclosed in the following publications:

JP-A-2006-317886; and

In the embodiment, multiplex recording is performed while rotating the recording disc101, or recording is performed by using “θyangle multiplexing”, and “shift multiplication” which is performed while the beam spot position on the recording disc is shifted by the target rotation of the disc. Namely, recording is performed in the same manner as the first embodiment. Hereinafter, recording of a servo hologram, recording of an information hologram, and a method for reproducing them will be sequentially described.

FIGS. 13 to 15are diagrams showing a method of recording the servo hologram, and that of recording the information hologram in the second embodiment.

Recording of Servo Hologram

Before a target information hologram is recorded, as shown inFIG. 13, the recording disc101is simultaneously irradiated with the first reference beam104which is a parallel light flux, and the second reference beam106which is a parallel light flux, and the servo hologram301is recorded into the recording medium102. In a similar manner as the first embodiment, the two kinds of reference beam are symmetrically placed so as to form in a mirror image relationship with respect to the xz plane, thereby recording an unslanted grating pattern302in the recording medium.

Since the unslanted grating pattern302is recorded by using two parallel light fluxes, the unslanted grating pattern is a hologram in which the noise level is low, or which is excellent.

Recording of Information Hologram

As shown inFIG. 14, next, the recording disc101is irradiated with the first signal beam105through the spatial light modulator108and the beam splitter509. Namely, the light modulator108causes information which is encoded with a binary pattern consisting of bright and dark points to be carried on light, and then the light is converged on the recording disc101. At the same time, the recording disc101is irradiated with the first reference beam104, and recorded into the disc by the θyangle multiplexing. As shown inFIG. 15, then, the recording disc101is irradiated with the first signal beam105through the spatial light modulator108and the beam splitter509. Thereafter, the reference beam is switched to the second reference beam106, the same place of the recording disc101is irradiated with the reference beam, and recording is performed by the θyangle multiplexing.

In a similar manner as the description in the first embodiment, the description has been made in the sequence of “recording of servo hologram” and “recording of information hologram”, but the recording sequence is not restricted to this. Alternatively, “recording of information hologram” and “recording of servo hologram” may be performed in this sequence, or “recording of information hologram” may be interrupted and “recording of servo hologram” may be performed.

Reproducing Method

Next, referring toFIGS. 16 and 17, a method for reproducing the servo hologram and information hologram which are recorded in the recording disc101as described above will be described.

FIG. 16is a diagram showing the method for reproducing the servo hologram and information hologram which are recorded in the recording disc101.

In reproduction, the recording disc101is irradiated only with the first reference beam104. It is assumed that the first reference beam104is P-polarized.

As shown inFIG. 16, only the first reference beam104impinges on the recording disc101through the beam splitter507. Then, the recording disc101is irradiated with reflected light701which becomes S-polarized after passing through the recording disc101and reciprocating through the shutter501, the λ/4 plate502, and the mirror503. Here, the shutter501is set to the open state, and the laser of the first reference beam104is allowed to pass through in a state where it reciprocates through the incident path. By the reflected light701which impinges from the side of the λ/4 plate502and the mirror503, diffracted light which is phase conjugate is generated from the hologram301recorded in the recording disc101. The diffracted light is formed as reproduced light702from the information hologram, and as reproduced light703from the servo hologram, respectively. The reproduced light703from the servo hologram is diffracted light from the servo hologram which is recorded by simultaneous irradiation with two sets of the first reference beam104and the second reference beam106.

The reproduced light702from the information hologram is reflected by the polarizing beam splitter509, and received by the image pickup device111. Thereafter, a reproduced image received by the image pickup device111is decoded to obtain information.

On the other hand, the reproduced light703from the servo hologram is reflected by the polarizing beam splitter508, and received by the photodetector114. There is a possibility that reproduced light704produced by the first reference beam104during incidence becomes noises. Therefore, the shutter504may be set to the close state to cut off the light. When the noises are at a non-problematic level, the shutter504may be set to the open state.

The other second reference beam106operates in a similar manner as the above-described operation. As shown inFIG. 17, only the second reference beam106impinges on the recording disc101through the polarizing beam splitter508. The recording disc101is irradiated with reflected light705which becomes S-polarized after passing through the recording disc101and reciprocating through the shutter504, the λ/4 plate505, and the mirror506.

By the reflected light705which impinges from the side of the λ/4 plate505and the mirror506, diffracted light which is phase conjugate is generated from the hologram301recorded in the recording disc101. The diffracted light is formed as reproduced light706from the information hologram, and as reproduced light707from the servo hologram, respectively.

The reproduced light707from the servo hologram is diffracted light from the hologram which is recorded by simultaneous irradiation with two sets of the first reference beam104and the second reference beam106.

The reproduced light706from the information hologram is reflected by the polarizing beam splitter509, and received by the image pickup device111. Thereafter, the received reproduced image is decoded to obtain information.

On the other hand, the reproduced light707from the servo hologram is reflected by the polarizing beam splitter507, and received by the photodetector113. There is a possibility that reproduced light708produced by the first reference beam104during incidence becomes noises. Therefore, the shutter501may be set to the close state to cut off the light. When the noises are at a non-problematic level, the shutter501may be set to the open state.

According to the embodiment, it is possible to provide a hologram reproducing method in which positioning for reproduction that is robust to a temperature change is enabled. Therefore, the spot position which is θy-angle multiplexing recorded can be highly accurately detected, and, particularly, positional deviation in the disc tangential direction can be detected with a high degree of accuracy. The embodiment is characterized also in that the embodiment may be applied also to phase conjugate reproduction which is advantageous to reduction in size of an apparatus.

The present invention is not limited to the first and second embodiments which are described above, and various changes or combinations in design may be made without departing the spirit and the scope of the present invention. The reproducing apparatuses of the embodiments may be provided with mechanism and components for recording disc101.

Hereinafter, examples of the present invention will be described.

A servo hologram and an information hologram are recorded in the method which has been described in the first embodiment, and the recording disc101is irradiated with the first reference beam104as shown inFIG. 10. For the reproduced signal received by the second photodetector114, the characteristics of the reproduced signal from the servo hologram301are checked. The characteristics of the reproduced light which is detected by the first photodetector113when the recording disc101is irradiated with the second reference beam106are identical with the above characteristics, and the description of the characteristics is omitted.

The intensity of the reproduced signal from the servo hologram301is subjected to the electromagnetic field analysis by using the RCWA (Rigorous Coupled-Wave Analysis) method. In an analysis in the case where two fluxes are in the same plane, Kogelnik's Coupled-Wave Theory, which is an approximate theory, may be applied. However, a precision analysis in the case where the optical path of the first reference beam104is not in the same plane as in the invention is beyond the range of Coupled-Wave Theory. Therefore, numerical calculation is performed by using the RCWA method that is a rigorous analysis method from which approximation is eliminated from Coupled-Wave Theory. Details of the Kogelnik's Coupled-Wave Theory are described in the following document.

The analysis conditions are set to appropriate specification values, or the laser wavelength of 405 nm and the recording medium thickness of 1,000 μm. Both the incident angles of the first reference beam104and the second reference beam106are set to 50 deg., and the angles of orientation are set to φ1=45 degrees and φ2=−45 degrees, respectively. Therefore, an unslanted grating pattern, i.e., a hologram in which the grating vector is in the recording medium plane can be recorded. The analysis technique and conditions are identical with those in (Example 2), (Example 3), and (Example 4) below.

FIG. 18shows an optical arrangement for measuring the characteristics of the reproduced signal from the servo hologram301with respect to disc rotation801, andFIG. 19shows results of the checks on the characteristics of the reproduced signal from the servo hologram301with respect to the disc rotation801. The horizontal axis of the graph indicates the deviation angle of the disc rotation801from the position where the servo hologram is recorded, and the vertical axis indicates the light receiving amount in the second photodetector114. As shown inFIG. 19, the reproduced signal approximately has a waveform of sinc2(x), and, when a deviation of the disc rotation801from the recorded position is ±0.05 deg., the intensity of the reproduced signal is lowered to the vicinity of zero. It is seen that, when the signal may be used, the position where the servo hologram is recorded can be highly accurately detected. With using the characteristics, therefore, the position where the signal intensity is maximum may be searched, and the rotation may be performed under servo control.

As shown inFIG. 20, a servo hologram601is recorded in each spot in the method which has been described in the first embodiment.FIG. 21shows results of the checks on the characteristics of the reproduced signal from the servo hologram601with respect to the disc rotation801. In the case where the recording disc is irradiated with the second reference beam106, the signal characteristics of the reproduced light received by the first photodetector113are identical with the characteristics, and hence description thereof is omitted.

The horizontal axis indicates the angle of the disc rotation801from the position where the servo hologram601is recorded, and the vertical axis indicates the light receiving amount in the second photodetector114. It is seen that, when the signal may be used, the recorded position can be highly accurately detected.

In a hologram recording/reproducing system, in order to realize high-density recording, it is necessary to increase the numerical aperture of an objective lens and the thickness of a recording medium. In accordance with this, the tolerance for the disc rotation801(deviation in the disc tangential direction) is extremely reduced. From the above, it is seen that the reproduction positioning method in the embodiment is very useful.

A servo hologram and an information hologram are recorded in the method which has been described in the first embodiment. The characteristics of the reproduced signal from the servo hologram301with respect to x-axis rotation1001in the recording disc101are checked for the reproduced signal which is received by the second photodetector114when the recording disc101is irradiated with the first reference beam104as shown inFIG. 22. Here, the rotation axis about which the e angle multiplexing is performed is the y axis, and the x axis is perpendicular to the axis. In the case where the recording disc101is irradiated with the second reference beam106, the signal characteristics of the reproduced signal received by the first photodetector113are identical with the characteristics, and hence description thereof is omitted.

FIG. 23shows results of the checks on the characteristics of the reproduced signal from the servo hologram301with respect to the x-axis rotation1001. The horizontal axis indicates the deviation angle of the x-axis rotation1001from the recorded position, and the vertical axis indicates the light receiving amount in the second photodetector114. As shown inFIG. 23, the reproduced signal approximately has a waveform of sinc2(x), and, when a deviation of the x-axis rotation1001from the recorded position is ±0.04 degrees, the signal intensity is lowered to the vicinity of zero. It is seen that, when the signal may be used, the recorded rotation position can be highly accurately detected.

A servo hologram and an information hologram are recorded in the method which has been described in the first embodiment. The characteristics of the reproduced signal from the servo hologram301with respect to y-axis rotation1101in the recording disc101are checked for the reproduced signal which is received by the second photodetector114when the recording disc101is irradiated with the first reference beam104as shown inFIG. 24. Here, the rotation axis about which the θyangle multiplexing is performed is the y axis, and the x axis is perpendicular to the axis. In the case where the recording disc101is irradiated with the second reference beam106, the signal characteristics of the reproduced signal received by the first photodetector113are identical with the characteristics, and hence description thereof is omitted.

FIG. 25shows results of the checks on the characteristics of the reproduced signal from the servo hologram301with respect to the y-axis rotation1101. The horizontal axis indicates the angle of the y-axis rotation1101, and the vertical axis indicates the light receiving amount in the second photodetector114. In the embodiment, in the hologram reproducing apparatus, the θyangle multiplexing is performed while conducting the y-axis rotation1101. As shown inFIG. 25, the intensity of reproduced signal is not substantially changed with the change ±10 degrees of the y-axis rotation1101. This seems to be a natural result of the spatial arrangement of the holograms recorded by two sets of reference beam, and the spatial arrangement of the reference beam. When the characteristics are used, it is possible to always monitor the servo signal in the case where information recorded by the θyangle multiplexing is reproduced while conducting the y-axis rotation1101.

It is to be understood that the invention is not limited to the specific embodiments described above and that the invention can be embodied with the components modified without departing from the spirit and scope of the invention. The invention can be embodied in various forms according to appropriate combinations of the components disclosed in the embodiments described above. For example, some components may be deleted from the configurations described as the embodiments. Further, the components described in different embodiments may be used appropriately in combination.