The present invention relates to a hologram reproducing device and a hologram reproducing method for reproducing an image based on a hologram recorded on a hologram recording medium.
One hologram recording method is a so-called angular multiplexing recording method in which the angle of incidence of a reference light beam with respect to a hologram recording medium is varied and a hologram is recorded each time the angle of incidence becomes a prescribed recording angle. Patent Document 1 discloses a conventional hologram reproducing method for reproducing an image based on a hologram from a hologram recording medium on which holograms have been recorded in multiplexed fashion by a recording medium of this kind. In the method disclosed in Patent Document 1, an image based on a hologram is reproduced in the following manner.
For example, as illustrated in
Examples of methods for aligning the angle of incidence of the reference beam with the recording angles θ1, θ2 and θ3 in this way are a peak hold determination method or a differential determination method. In a peak hold determination method, the received light quantity of a reproduction beam is measured while varying the angle of incidence of the reference beam at a uniform angular sweep rate by means of an incidence angle variable irradiator, and the angle of incidence of the reference beam at which an optimal peak state is obtained is confirmed when this received light quantity changes from increase to decrease. A differential determination method measures the received light quantity of a reproduction beam while variably controlling the angular sweep rate at which the angle of incidence of the reference beam is altered, and confirms the angle of incidence of the reference beam at which the optimal peak state is obtained, through change prediction, by determining the rate of change of the received light quantity with a differential circuit.
Patent Document 1: Japanese Laid-open Patent Publication No. 2006-171589
However, in a conventional hologram reproducing method, there has been margin for improving the peak hold method or differential determination method. In other words, in a peak hold determination method, even if the received light quantity has exceeded a peak state, the angle of incidence of the reference beam must be changed for some time, and therefore the incidence angle variable irradiator must perform a retrogressive operation. This places a large load on the incidence angle variable irradiator, and also causes a problem in that the operation of aligning the angle of incidence with the recording angle cannot be carried out at high speed. On the other hand, in a differential determination method, it is not possible significantly to speed up the operation of aligning the angle of incidence with a recording angle, since the angular sweep rate is controlled variably, and there is also a problem in that a calculation circuit, such as a differential circuit, or the like, is required, and therefore the circuit composition becomes complicated.
The present invention has been proposed in view of the circumstances described above. It is an object of the present invention to provide a hologram reproducing device and a hologram reproducing method whereby a reproducing operation can be carried out at high speed, by means of a simple structure.
In order to achieve the object stated above, embodiments of the present invention incorporate the following technical means.
The hologram reproducing device provided by a first aspect of the present invention is a hologram reproducing device for reproducing an image based on a hologram by irradiating a reference beam onto a hologram recording medium on which holograms have been recorded in multiplexed fashion by changing the incidence direction of the reference beam during recording, the incidence direction of the reference beam being changed during reproduction in a similar fashion to during recording, and by receiving a reproduction beam from the hologram recording medium, the hologram reproducing device comprising: a reproduction reference beam splitter for splitting the reference beam, during reproduction, into a main beam forming a main incidence direction with respect to the hologram recording medium, and sub-beams forming incidence directions spatially shifted from the main beam; an incidence direction variable irradiator for irradiating the main beam and the sub-beams onto the hologram recording medium while varying the incidence directions of the main beam and the sub-beams; a servo beam detector for detecting as servo beams a portion of the reproduction beam which is generated corresponding to the sub-beams, while the angle of incidence of the main beam and the sub-beams changes; a beam receiving unit for receiving the main portion of the reproduction beam generated corresponding to the main beam; and a reproducer for identifying the incidence direction of the main beam at which the received light quantity of the beam receiving unit becomes equal to or greater than a prescribed level, when the intensity of the reference beam is kept uniform, on the basis of the servo beam detection results, and reproducing an image obtained by the beam receiving unit when the main beam coincides with the identified incidence direction.
Preferably, the incidence direction variable irradiator is configured such that the incidence direction of the main beam is varied within a prescribed reference plane which intersects with the hologram recording medium, and the reproduction reference beam splitter is configured so as to generate, as the sub-beams, a first sub-beam and a second sub-beam which are shifted with respect to each other when projected to the reference plane.
Preferably, the first sub-beam and the second sub-beam are generated on both sides of the main beam, the main beam being central between same, when projected onto the reference plane.
Preferably, the first sub-beam and the second sub-beam are also generated on both sides of the main beam, the main beam being central between same, when projected onto a plane which is perpendicular to the reference plane.
Preferably, the servo beam detector comprises a mechanism for carrying out calculation processing using a first photosensor and a second photosensor which detect at the least the intensity of the servo beams corresponding respectively to the first sub-beam and the second sub-beam, and performing feedback control so as to achieve a prescribed target value, as a result of the calculation processing.
Preferably, the medium used as the hologram recording medium is a medium on which a hologram has been recorded each time the angle of incidence of the reference beam becomes a prescribed recording angle, the recording angles being situated at uniform angular intervals apart, and the angles formed respectively between the main beam and the first sub-beam and the second sub-beam, when the first and second sub-beams are projected to the reference plane, are set to be a different value from the uniform angular interval.
The hologram reproducing method provided by a second aspect of the present invention is a hologram reproducing method for reproducing an image based on a hologram by irradiating a reference beam onto a hologram recording medium on which holograms have been recorded in multiplexed fashion by changing the incidence direction of the reference beam during recording, the incidence direction of the reference beam being changed during reproduction in a similar fashion to during recording, and by receiving a reproduction beam from the hologram recording medium, the method comprising: a reproduction reference beam splitting step of splitting the reference beam, during reproduction, into a main beam forming a main incidence direction with respect to the hologram recording medium, and sub-beams forming incidence directions spatially shifted from the main beam within a prescribed reference plane which intersects perpendicularly with the hologram recording medium; an incidence direction variable irradiation step of irradiating the main beam and the sub-beams onto the hologram recording medium while varying the incidence directions of the main beam and the sub-beams; a servo beam detection step of detecting a portion of the reproduction beam which is generated corresponding to the sub-beams, while the angle of incidence of the main beam and the sub-beams changes; a beam receiving step of receiving the main portion of the reproduction beam generated corresponding to the main beam; and a reproduction step of identifying the incidence direction of the main beam at which the received light quantity in the beam receiving step becomes equal to or greater than a prescribed level, when the intensity of the reference beam is kept uniform, on the basis of the servo beam detection results, and reproducing an image obtained by the beam receiving unit when the main beam coincides with the identified incidence direction.
Preferred embodiments of the present invention will be described below with reference to the drawings.
The hologram reproducing device A includes a recording light optics system for irradiating a recording beam S, a reference light optics system for irradiating a recording reference beam R or recording reference beam R′ while variably controlling the incidence direction with respect to a hologram recording medium H, and a reproduction light optics system for reproducing an image based on a hologram from a reproduction beam P. Outside the drawings, there are provided: a light source that emits laser light, a collimating lens which converts laser light into parallel light, a beam splitter for splitting the laser light into a reproduction beam S and reference beams R and R′, and a light quantity ratio adjusting device.
The recording light optics system is constituted by a spatial light modulator 1, a first half mirror 2, a relay lens 3 and an object lens 4. The reference light optics system is constituted by a recording mirror 10, a reproduction reference beam splitter 11, a reproduction mirror 12, and an incidence direction variable irradiator 20. The incidence direction variable irradiator 20 varies the incidence direction of the recording reference beam R and the reproduction reference beam R′, and is constituted by a U-shaped arm member 21 and a drive motor 22. The reproduction optics system is constituted by a second half mirror 30, a reproduction condensing lens 31, a beam receiving unit 32, a servo condensing lens 40, a servo beam detector 50 and a reproducer 60. As illustrated in
As illustrated in
For example, during recording, a laser beam emitted from a light source (not illustrated) is converted into parallel light by a collimating lens (not illustrated) and is then split into a recording beam S and a recording reference beam R by a beam splitter. The recording beam S is directed to the spatial light modulator 1, while the recording reference beam R is directed to the recording mirror 10. In a similar fashion to this, during reproduction, a laser beam from a light source is passed through a collimating lens and split by a beam splitter into a recording beam S and a reproduction reference beam R′, and the reproduction reference beam R′ is directed via the reproduction reference beam splitter 11 to the reproduction mirror 12. During reproduction, by the action of a light quantity ratio adjuster, almost all of the light emitted from the light source is formed into the reproduction reference beam R′, and all of the pixels of the spatial light modulator 1 are set to a shielded state in such a manner that not even the very faintest recording beam S is incident on the hologram recording medium H.
The spatial light modulator 1 consists, for example, of a transmissive liquid crystal display having a plurality of pixels, and modulates laser light input thereto into a recording beam S having a two-dimensional pixel pattern corresponding to the information to be recorded. The recording beam S emitted from the spatial light modulator 1 is directed to the relay lens 3 by passing through the first half mirror 2, and further passes through the object lens 4 to be irradiated onto a prescribed position on the hologram recording medium H. The object lens 4 is disposed in such a manner that the optical axis thereof has a prescribed inclination with respect to the hologram recording medium H.
The recording mirror 10 is fixed to one end of the arm member 21 which is disposed on the upper surface side of the hologram recording medium H, and reflects the recording reference beam R which has traveled on a light path substantially perpendicular to the hologram recording medium H, in an oblique downward direction toward a prescribed position. The reproduction mirror 12 is fixed to the other end of the arm member 21 which is disposed on the lower surface side of the hologram recording medium H, and reflects the recording reference beam R′ which has traveled on a light path substantially parallel to the hologram recording medium H, in an oblique upward direction toward a prescribed position. The arm member 21 is rotated through a prescribed range by means of a drive motor 22 which has an axis of rotation disposed in the radial direction of the hologram recording medium H. By this means, the recording mirror 10 and the reproduction mirror 12 are caused to swing in unison in a prescribed reference plane which is perpendicular to the hologram recording medium H. This prescribed reference plane is the plane indicated by the thin double-dotted lines in
During recording, the recording mirror 10 is made to swing, and in accordance with this, a recording beam S is irradiated each time the angle of incidence of the recording reference beam R, for example, becomes a recording angle θ1, θ2 or θ3, these recording angles being separated by a uniform angular interval Δθ as illustrated in
The reproduction reference beam splitter 11 consists of a diffraction grating or prism, for example. As illustrated in
As illustrated in
During reproduction, the reproduction mirror 12 is caused to swing and accordingly, the angle of incidence of the main beam R0 and the projected angle of incidence of the first sub-beam R1 and the second sub-beam R2 are changed. Nearly all of the reproduction reference beam R is occupied by the main beam R0, and when the angle of incidence of this main beam R0 coincides with a recording angle θ1, θ2 or θ3, a reproduction beam P of sufficient light quantity is produced. This reproduction beam P includes the main portion P0 which is generated corresponding to the main beam R0, and also includes the first servo beam P1 and the second servo beam P2 which are generated corresponding to the first sub beam R1 and the second sub-beam R2.
Here, as illustrated in
As illustrated in
As illustrated in
The beam receiving unit 32 is constituted by a CCD, for example; the light reception timing is controlled by the reproducer 60, and the beam receiving unit 32 outputs a hologram image obtained by focusing the reproduction beam, to the reproducer 60. The first photosensor 51 and the second photosensor 52 detect the intensity of the first servo beam P1 and the second servo beam P2, and output signals corresponding to the respective beam intensities (beam intensity signals) to the differential amplifier 54. The differential amplifier 54 finds the difference between the two beam intensity signals and outputs a signal corresponding to this difference (a differential signal), to the reproducer 60. The detection signal from the main beam photosensor 53 is input to the reproducer 60. The reproducer 60 causes the beam receiving unit 32 to output a hologram image each time the differential signal reaches a zero level, and reads out the originally recorded information by reproducing this hologram image. Furthermore, the reproducer 60 carries out prescribed processing on the basis of the detection signal from the main beam photosensor 53.
Next, the reproduction operation of the hologram reproducing device A will be described.
As illustrated in
If the reproduction reference beam R′ is irradiated in this way, then reproduction beam P is generated in accordance with this. The first and second servo beams P1 and P2 corresponding to the first and the second sub-beams R1 and R2 are included in this reproduction beam P. Furthermore, the light forming the main portion P0 corresponding to the main beam R0 is also included in the reproduction beam P. The first servo beam P1 is detected by the first photosensor 51 and the second servo beam P2 is detected by the second photosensor 52. Beam intensity signals corresponding to the first and second servo beams P1 and P2 are output from the first and second photosensors 51 and 52, and the differential amplifier 54 outputs a differential signal corresponding to the differential between these beam intensity signals (S2).
In this case, if the beam intensity detected by the first photosensor 51 coincides with the beam intensity detected by the second photosensor 52, then a differential signal of zero level is output from the differential amplifier 54 (S3: YES). This is a state where, as illustrated by the graph and the example indicated by (B) in
If the angle of incidence of the main beam R0 coincides with the recording angle θ2 in this way (S3: YES), then the incidence direction variable irradiator 20 temporarily halts the operation of varying the incidence direction of the reproduction reference beam R′. By this means, the reproduction reference beam R′ is irradiated in a state where the angle of incidence thereof matches the recording angle θ2 (S4).
In this case, in the beam receiving unit 32, a main portion P0 of the reproduction beam having sufficient light quantity is obtained, and after confirming that the output of the first photosensor 51, the second photosensor 52 and the main beam photosensor 53 have reached a prescribed level, it is considered that the beam has been received successfully. For example, if the beam intensity of the reproduction reference beam R′ is uniform during irradiation, then it is considered that the reproduction beam has been received successfully, provided that the detection level of the main beam photosensor 53 obtained when the incidence direction variable irradiator 20 has been moved is equal to or greater than a prescribed value (Vpr), which is higher than the average value (S5: YES). In this, a preferable mode is one in which the received light quantity is a maximum (peak value) as indicated by the central state in
At S3, if the light intensity detected by the first photosensor 51 is greater or lower than the light intensity detected by the second photosensor 52, then a differential signal having a negative level or a positive level is output from the differential amplifier 54 (S3: NO).
In this case, if the differential signal has a negative level, then as indicated by the graph and the example illustrated in (A) in
On the other hand, if the differential signal has a positive level, then as indicated by the graph and the example illustrated in (C) in
Consequently, according to the hologram reproducing device A of the present embodiment, it is possible to obtain images based on holograms corresponding to the recording angles θ1, θ2 and θ3, just by providing a simple mechanism for generating first and second sub-beams R1 and R2 and detecting the first and second servo beams P1 and P2, and therefore it is possible to carry out a reproduction operation more quickly than in the prior art.
In the hologram reproducing device illustrated in
The recording prism 10′ and the reproduction prism 12′ are caused to swing in unison in a Bragg plane (not illustrated). The main axis of the recording reference beam R which is incident on the hologram recording medium H from the recording prism 10′, and the optical axis of the object lens 4, are contained in this Bragg plane Bs. During recording, the recording prism 10′ is made to swing, and in accordance with this, a recording beam S is irradiated each time the angle of incidence of the recording reference beam R becomes a recording angle θ1, θ2, θ3, for example.
A reflective diffraction grating, for example, is formed on a reflective surface 12′A of the reproduction prism 12′, and a reproduction reference beam splitter is realized by this reflective surface 12′A. More specifically, the reproduction reference beam R′ incident on the reproduction prism 12′ is split into a main beam R0 which forms a main incidence direction with respect to the hologram recording medium H and first and second sub-beams R1 and R2 forming incident directions which are spatially shifted from this main beam R0. The main axis of the main beam R0 is contained in the Bragg plane, whereas the main axes of the first and second sub-beams R1 and R2 are not parallel with any of a plane parallel to the hologram recording medium H, or the Bragg plane, or the fractal plane. The reproduction prism 12′ of this kind is caused to swing during reproduction and accordingly, the angle of incidence of the main beam R0 and the projected angles of incidence of the first and second sub-beams R1 and R2 are changed. Similarly to the embodiment described above, the main beam R0 having an incidence direction as illustrated in
By forming a diffraction grating on the incident surface and emission surface of the reproduction prism 12′, or by providing a separate prism on the incident surface and the emission surface, it is possible to provide a reproduction reference beam splitter for splitting the light into the main beam R0 and the first and second sub-beams R1 and R2.
It is also possible to achieve a simple structure for generating first and second sub-beams R1 and R2 by means of a hologram reproducing device which includes a reproduction prism 12′ of this kind, and it is possible to accelerate the reproduction operation by detecting the first and second servo beams P1 and P2.
The present invention is not limited to the respective embodiments described above.
For example, the main axes of the sub-beams are contained in the Bragg plane and may be one sub-beam having a prescribed angle of advance with respect to the main beam. In this case, the intensity of the servo beam generated corresponding to the sub-beam is detected, and the angle of incidence of the reproduction reference beam is changed from the time that the intensity of the servo beam becomes a maximum until reaching an angle corresponding to the angle of advance described above. In so doing, the angle of incidence of the main beam reaches a state which precisely matches the recording angle, and a reproduction beam of a sufficient received light level can be obtained.
Furthermore, the light quantity of the sub-beams was taken to be 10% as a proportion of the light quantity of the main beam, but is of course not limited to this. For example, since it is possible to adjust the light quantity of the servo beam corresponding to the sub-beam by shifting the sub-beam in the fractal direction, and furthermore, since the distance between the respective photosensors and the incidence position of the main beam can also be changed by this means, then it is possible to select more optimal design conditions.
Number | Date | Country | |
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Parent | PCT/JP2007/069441 | Oct 2007 | US |
Child | 12724905 | US |