Patent Application
20080273444
The present invention relates to a holographic record carrier such as an optical disk or card or the like with which information is optically recorded or reproduced, and more particularly, to a holographic record carrier which has a recording layer irradiated with an optical beam for recording information thereon or reproducing information therefrom.
A hologram has drawn attention because of its ability to record two-dimensional data at a high density, for use in high density information recording. The hologram is characterized by volumetrically recording a wave front of light, which carries out recording of information on a recording medium made of a photosensitive material such as a photo-refractive material as changes in refractive index as a refraction grating. Multiplex recording on the holographic record carrier can dramatically increase the recording capacity. There are included angle multiplexing, phase coding multiplexing and the like in the multiplex recording in which information can be recorded multiple times by changing the incident angle or phase of interfering light waves even in a multiplexed hologram region. For example, a recording and reproducing system which utilizes the holographic record carrier as a disk has been developed (see Laid-open Japanese Patent Application Kokai No. 11-311937).
In the developed holographic recording system, reference light is converged on the reflective film through the recording layer as a spot, and the reference light reflected by the reflective film diverges to pass through the recording layer again, and simultaneously, information light, which carries information to be recorded, is passed through the recording layer at the same area. In this way, in the recording layer, the reflected reference light interferes with the information light to form an interference pattern to volumetrically record hologram as a refraction grating within the recording layer. The holograms of the interference pattern are recorded in the recording layer adjacent to each other, overlapping in sequence. Then, the reference light is irradiated to detect and demodulate reproduced light restored from each hologram to reproduce recorded information.
In the case that the reference light and information light coaxially impinge from the same side of the recording layer, it is difficult to separate the reference light reflected on the reflective film from the reproduced light from the holograms during reproduction of information. This causes the performance of reading a reproduced signal to be degraded.
To solve these problems, the holographic recording system shown in Laid-open Japanese Patent Application No. 11-311937 is provide with an objective lens immediately preceded by a bisect azimuth rotator which is a rotator having a pupil divided into two areas, which have respective optical rotating directions different by 90° from each other to prevent the reference light from impinging on a photodetector.
However, the conventional method involves a problem that the bisect azimuth rotator and the objective lens must be integrally driven. The conventional method also has a problem of a deteriorated recording characteristic from reproduced light corresponding to the vicinity of the division boundary of the bisect azimuth rotator.
In the case that a hologram is recorded in such a holographic record carrier of reflective type, four kinds of hologram are recorded by interference due to four light beams of entering reference light and signal light and reflected reference light and signal light, thereby wastefully using holographic recording layer performance.
Further, when reproducing information, it is difficult to separate diffracted light caused by a reproduced hologram from reflected reference light, because the reference light is reflected from the reflective film of the holographic record carrier. Accordingly, readout performance of the reproduced signal is deteriorated. Further, since a reflected hologram image is recorded, the reproduced signal is deteriorated.
It is therefore an exemplary object of the present invention to provide a holographic record carrier, a recording/reproducing and hologram device capable of providing recording and reproduction stability.
It is therefore an exemplary object of the present invention to provide a holographic record carrier, a recording/reproducing method therefor, and a hologram apparatus which are capable of stably recording or reproducing information.
A holographic record carrier according to the present invention, which is irradiated with light for recording information thereon and reproducing information therefrom, comprises:
a holographic recording layer for storing optical interference patterns as diffraction gratings therein produced by coherent components of reference light and signal light;
a reflective layer disposed on one side of said holographic recording layer opposite to a side which is irradiated with light; and
a plurality of non-reflective regions arranged on said reflective layer at the same intervals as record-intervals of said diffraction gratings.
In the following, embodiments of the present invention will be described with reference to the drawings.
In a Holographic Recording/Reproducing Apparatus, holographic recording is performed by using a first light beam causing a reference light and a signal light interfering with each other, and at the same time using a servo beam of laser light with a different wavelength from the first light beam to carry out a servo control (focusing and tracking) on relative positioning of a holographic record carrier and a pickup device particularly an object lens thereof. The following description is an example of such device.
The holographic record carrier 2 comprises a substrate 3 with transferred tracks, a reflective layer 5, a separation layer 6, a holographic recording layer 7, and a protective layer 8 which are laminated on the substrate 3 from an opposite side to a side from which reference light impinges. As such, the reflective layer 5 is arranged over or on the opposite side of the substrate 3 to the optical irradiation side which is directed to the holographic record carrier. The reflective layer 5 has marks M, each serving as a non-reflective region, formed in the same interval as a multiple interval Px of the hologram. The holographic recording is performed while matching laser light (servo beam SB) used of the servo control with each mark M so that both optical axes of the servo beam SB the first light beam FB are nearly coaxial with each other. Each of the marks may be a pinhole PH (
The holographic recording layer 7 stores an optical interference pattern as a refraction grating (hologram) produced by components of the coherent reference light and signal light included in the first light beam FB. The first light beam FB including the components of the reference light and signal light to record the hologram is used when information recording. On the other hand during information reproduction the first light beam FB consisting of the reference light component alone is used. Further, in the case of phase encoding multiple reproducing, the first light beam FB includes a phase modulation pattern and a reference light component, although it does not include the signal light component. The holographic recording layer 7 for preserving an optical interference pattern is made of a photo-sensitive material such as a photo-refractive material, a hole burning material, a photo-chromic material or the like.
The reflective layer 5 is made, for example, of a metal film, a phase-change film, a dye film or the like or a combination thereof, which is set to reflect the first light beam FB for the holographic recording. A position decision servo control (focusing servo and x- and y-directions servo controls) is conducted on the basis of detections of irradiation and reflection of the servo beam SB of holographic recording.
The substrate 3 is made may be, for example, glass, polycarbonate, amorphous polyolefin, polyimide, plastics such as PET, PEN, PES, ultraviolet curing acrylic resin, and the like. The substrate 3 has a plurality of tracks T formed on the main surface in the form of grooves that extend spaced away from each other without intersection. The reflective layer 5 functions as a guiding layer. The separation layer 6 and protective layer 8 are made of an optically transparent material, and function to planarize the laminate and protect the holographic recording layer and the like.
The servo beam SB is focused on the pinhole PH in order to read the servo track or pit formed on the substrate 3. The pinhole PH may be filled with a material having a property to transmit the reference light component (Zero-order beam) of the first light beam FB.
As shown in
The servo control is conducted by driving an objective lens by an actuator in accordance with a detected signal, using a pickup which includes a light source for emitting a light beam, an optical system including an objective lens for converging the light beam on the reflective layer 5 as a light spot and leading its reflected light to a photodetector, and the like. The diameter of the light spot is set to be narrowed down to a value determined by the wavelength of the light beam and the numerical aperture (NA) of the objective lens (a so-called diffraction limit which is, for example, 0.82λ/NA (λ=wavelength), but is determined only by the wavelength of light and the numerical aperture when aberration is sufficiently small as compared with the wavelength). In other words, the light beam-radiated from the objective lens is used such that it is focused when the reflective layer lies at the position of its beam waist. The width of the grooves is determined as appropriate in accordance with an output of the photodetector which receives the reflected light from the light spot such as a push-pull signal.
The foregoing embodiment has shown a holographic record carrier, the structure of which has the reflective layer 5 and the holographic recording layer 7 laminated with intervention of a separation layer. In addition to such holographic record carrier, in another embodiment, such a separation layer may be omitted. Moreover, a still another embodiment as an exemplary modification includes a holographic record carrier which has a separation layer of substrate 3 interposed between the reflective layer 5 and holographic recording layer 7.
The holographic recording/reproducing apparatus of
The holographic recording/reproducing apparatus comprises a controller circuit 37 which is connected to first laser source driving circuit 25a, second laser source driving circuit 25b, spatial light modulator driving circuit 26, reproduced signal processing circuit 27, servo signal processing circuit 28, focusing servo circuit 29, x-direction movement servo circuit 30x, y-direction movement servo circuit 30y, pickup position detecting circuit 31, slider servo circuit 32, rotation encoder 33, a rotation detector 34, and spindle servo circuit 35. The controller circuit 37 conducts a focusing servo control, an x- and y-direction movement servo control, a reproduced position (position in the x- and y-direction) control, and the like related to the pickup through the foregoing circuits connected thereto based on signals from these circuits. The controller circuit 37, which is based on a microcomputer that is equipped with a variety of memories for controlling the overall apparatus, generates a variety of control signals in accordance with manipulation inputs from the user from an operation unit (not shown) and a current operating condition of the apparatus, and is connected to a display unit (not shown) for displaying an operating situation and the like for the user. The controller circuit 37 is also responsible for processing such as encoding of data to be recorded, input from the outside, and the like, and supplies a predetermined signal to the spatial light modulator driving circuit 26 for controlling the recording sequence. Furthermore, the controller circuit 37 performs demodulation and error correction processing based on signals from the reproduced signal processing circuit 27 to restore data recorded on the holographic record carrier. In addition, the controller circuit 37 decodes restored data to reproduce information data which is output as reproduced information data.
The recording/reproducing optical system comprises a first laser source LD1 for recording and reproducing holograms, a first collimator lens CL1, a first half mirror prism HP1, a second half mirror prism HP2, a polarizing spatial light modulator SLM, a reproduced signal detector including an image sensor IS comprised of an array such as a CCD, a complimentary metal oxide semiconductor device, or the like, a third half mirror prism HP3, and a fourth half mirror prism HP4.
The servo system comprises an objective lens actuator 36 for servo-controlling (movements in the x-, y-, z-directions) of the position of a light beam with respect to the holographic record carrier 2, a second laser source LD2, a second collimator lens CL2, a diffraction optical element GR such as a grating or the like for generating a multi-beam for a servo light beam, a polarization beam splitter PBS, a quarter wavelength plate ¼λ, a coupling lens AS, and a servo signal detector including a photodetector PD.
The common system comprises a dichroic prism DP and the objective lens OB.
As shown in
The first laser source LD1 is connected to the first laser source driving circuit 25a, and has its output adjusted by the first laser source driving circuit 25a such that the intensity of an emitted light beam is increased for recording and decreased for reproduction. The second laser source LD2 is connected to the second laser source driving circuit 25b.
The polarizing spatial light modulator SLM of reflection type has a function of electrically transmitting or blocking a part or all of incident light with a liquid crystal panel or the like having a plurality of pixel electrodes that are divided in a matrix shape or the like. The polarizing spatial light modulator SLM, which is connected to the first laser source driving circuit 25a, modulates and reflects an light beam so as to have a polarization component distribution based on page data to be recorded (two-dimensional data of information pattern such as bright and dark dot pattern or the like on a plane) from the spatial light modulator driving circuit 26 to generate signal light. Further, instead of the polarizing spatial light modulator SLM, in case that a transparent liquid crystal panel having a plurality of pixel electrodes divided into a matrix is used as the spatial light modulator, the modulator is arranged between the first and second half mirror prisms HP1 and HP2.
The reproduced signal detector including the image sensor IS is connected to the reproduced signal processing circuit 27.
Further, the pickup device 23 is provided with the objective lens actuator 36 for moving the objective lens OB in the optical axis (z direction) parallel direction, and in a track (y direction) parallel direction, and in a radial (x direction) direction perpendicular to the track.
The photodetector PD is connected to the servo signal processing circuit 28, and has the shape of light receiving element divided for focusing servo and x and y direction movement servo generally used for optical disks. The servo scheme is not limited to an astigmatism method, but can employ a push-pull method. The output signal of the photodetector PD, such as a focus error signal and a tracking error signal etc. is supplied to the servo signal processing circuit 28.
In the servo signal processing circuit 28, a focusing driving signal is generated from the focus error signal, and is supplied to the focusing servo circuit 29 through the controller circuit 37. The focusing servo circuit 29 drives the focusing section of the objective lens actuator 36 mounted in the pickup device 23, so that the focusing section operates to adjust the focus position of an optical spot irradiated to the holographic record carrier.
Further, in the servo signal processing circuit 28, x and y direction movement driving signals are generated from x and y direction movement error signals, and supplied to the x-direction movement servo circuit 30x and y-direction movement servo circuit 30y, respectively. Thus the x-direction movement servo circuit 20x and the y-direction movement servo circuit 30y drive the objective lens actuator 36 mounted on the pickup 23 according to the x- and y-direction movement driving signals. Therefore, the objective lens is driven by the amount of driving current according to the driving signal along the x, y and z axes, and then the position of the focal point incident on the holographic record carrier is displaced. Accordingly, it is possible to fix a relative position of the focal point with respect to a moving holographic record carrier and then to guarantee time to form the hologram when recording data.
The controller circuit 37 generates a slider driving signal based on a position signal from the operation panel or the pickup position detecting circuit 31 and the x direction movement (tracking) error signal from the servo signal processing circuit 28, and supplies the slider driving signal to the slider servo circuit 32. The slider servo circuit 32 moves the pickup device 23 in the radial direction of the disk in response to a driving current carried with the slider driving signal by the pickup actuator 24.
The rotation encoder 33 detects a frequency signal indicative of a current rotating frequency of the spindle motor 22 for rotating the holographic record carrier 2 through the turn table, generates a rotational speed signal indicative of the spindle rotational signal corresponding thereto, and supplies the rotational speed signal to the rotation detector 34. The rotation detector 34 generates a rotational speed position signal which is supplied to the controller circuit 37. The controller circuit 37 generates a spindle driving signal which is supplied to the spindle servo circuit 35 to control the spindle motor 22 for driving the holographic record carrier 2 to rotate.
The objective lens actuator 36 comprises an actuator base 42 which can swing in the y-direction by a piezo element 39 which is coupled to a support 38 secured to a pickup body (not shown). Within the pickup body, there are the aforementioned optical parts required for making up the pickup such as the prism 45 for reflecting a light beam from the laser at right angles for leading the light beam to the objective lens OB, and the like. The light beam passes through an opening 42c and the objective lens OB, and is converged to spot light which is irradiated to an information recording surface of the medium on the turn table.
As shown in
The lens holder 48 is spaced apart from and sandwiched between a pair of magnetic circuits. Each magnetic circuit comprises a magnet 55 facing the lens holder 48, and a metal plate 56 for supporting the magnet 55, and is secured on the actuator base 42. The lens holder 48 is formed with a pair of throughholes which are positioned to sandwich the objective lens OB in parallel with the optical axis of the objective lens OB and the central axis of the coil inside the focusing coil 50 of the lens holder 48 in a direction in which the longitudinal supporting members 53 extend. A yoke 57, which extends from the metal plate 56 of the magnetic circuit, is inserted into each through hole without a contact therebetween. The focusing coil 50 and tracking coil 51 are positioned within a magnetic gap of the magnetic circuit which is made up of the magnet 55 and yoke 57.
The focusing coil 50, tracking coil 51, and piezo element 39 are controlled by the focusing servo circuit 29, x-direction movement servo circuit 30x, and y-direction movement servo circuit 30y, respectively. Since parallel magnetic flux crossing perpendicularly to the respective coils can be generated in the magnetic gap, driving forces in the x- and z-directions can be generated by supplying predetermined currents to the respective coils to drive the aforementioned movable optical system in the respective directions.
In this way, voice coil motors are used to drive the objective lens OB in the x- and y-directions, and the objective lens OB is driven for the y-direction together with the actuator base using a piezo element or the like. Other than the foregoing structure, the actuator may use voice coil motors for all the axes.
Description will be made on a recording and reproducing method for recording or reproducing information by irradiating a holographic record carrier with an light beam using the holographic recording and reproducing apparatus described above.
During recording, as shown in
The signal beam transmits the second half mirror prism HP2, and impinges on the polarizing spatial light modulator SLM along the normal of the reflective surface. The signal light modulated in a predetermined manner by and reflected from the polarizing spatial light modulator SLM again impinges on the second half mirror prism HP2 and directs to the fourth half mirror prism HP4.
The reference beam is reflected by the third half mirror prism HP3, and directs to the fourth half mirror prism HP4.
The reference light and the signal light are combined so as to be substantially coaxial by using the fourth half mirror prism HP4. The two combined light beams pass through the dichroic prism DP, and are converged on the holographic record carrier 2 by the objective lens OB for recording a hologram.
During information reproduction, on the other hand, light is separated into a reference beam and a signal beam by the first half mirror HP1, in a manner similar to the recording, as shown in
Since reproduced light (two-dot chain line) generated from the holographic record carrier 2 transmits the objective lens OB, dichroic prism DP, fourth half mirror prism HP4, and third half mirror prism HP3, and impinges on the image sensor IS. The image sensor IS delivers an output corresponding to an image formed by the reproduced light to the reproduced signal processing circuit 27 which generates a reproduced signal that is supplied to the controller circuit 50 for reproducing recorded page data. In addition, an image forming lens may be provided between the third half mirror prism HP3 and the image sensor IS.
Here, a position decision servo control is performed with respect to the holographic record carrier or hologram disk 2 in both recording and reproduction of the hologram. According to the position decision servo control, three axes actuator (objective lens actuator 36) is capable of driving the objective lens along the x, y, and z-directions, by an error signal operated and obtained based the output of the photodetector PD.
During both recording and reproduction, the second laser source LD2 for servo control emits coherent light at a different wavelength from the first laser source LD1, as shown in
Further, the z-direction servo (focusing servo) control along the z-direction may be performed by the astigmatic method, three-beam method, spot size method and push/pull method that are used in a conventional light pickup or a combination thereof may be used.
With the astigmatism method, for example, a central portion of the photodetector PD comprises light receiving elements 1a-1d having a light receiving surface equally divided into four for receiving a beam, for example, as shown in
In accordance with output signals of the respective light receiving elements 1a-1d of the photodetector PD, the servo signal processing circuit 28 generates an RF signal Rf and a focus error signal. When the signals of the light receiving elements 1a-1d are labeled Aa−Ad, respectively, in this order, the focus error signal FE is calculated by EF=(Aa+Ac)−(Ab+Ad), and the tracking error signal TE is calculated by TE=(Aa+Ad)−(Ab+Ac). These error signals are supplied to the controller circuit 37.
In the present embodiment, a position decision servo control with the holographic record carrier 2 is always performed by the servo beam SB. Simultaneously, the reproduction of the hologram is performed using the first light beam FB (reference light) and recording of the same is performed using the first light beam FB (reference light and signal light).
As shown in
The recording of the hologram is performed by interfering components of the reference light and signal light of the first light beam FB in the holographic recording layer 7. Since the modulation signal (signal light component) modulated in the polarizing spatial light modulator SLM is a diffraction light component more than 1st order, it has a considerable area in the vicinity of a condensing spot (Fourier surface). Accordingly, a beam is substantially reflected from the reflective layer 5. Meanwhile, since the reference light (or Zero-order component) is unmodulated DC light, it has a spot size that is determined by the number of the openings and wavelength of the objective lens OB. Further, when a pinhole PH is somewhat larger than the spot size, the reference light passes through the pinhole PH.
As shown in
As shown in
The holographic device shown in
In the reproduction operation, a first light beam consisting of non-modulated laser light, that is, Zero-order light (reference light component) in the transparent polarizing spatial light modulator SLM is condensed on the holographic record carrier 2 through the objective lens OB, the reproduced wave is reconstructed and returned to a pickup through the objective lens OB. The component reflected from the fourth half mirror prism HP4 is incident on the image detection sensor IS. The image detection sensor IS transfers an output corresponding to an image generated using the reproduced light to the reproduction signal detection processing circuit 27, provides the control circuit 50 with the reproduction signal generated there and reproduces page data that has been recorded. The configuration of the servo beam SB (servo control) is identical to the configuration shown in
As shown in
A position determination servo control with the holographic record carrier 2 is always performed by using the servo beam SB and at the same time, recording of the hologram is performed using a first light beam FB. The servo control may be performed by irradiating a plurality of servo beams on a vicinity pinhole PH.
As shown in
As shown in
As to the servo beam SB, a time axis servo control is simultaneously performed where the objective lens OB is also followed along the y-direction using a mark Y along the y-direction by adding the servo beam SB to the same tracking servo control as is in Example 1. Since the servo control by the servo beam SB is the same with the Example 1, a detailed description thereof will be omitted.
Although the mark Y having the same holographic recording interval in the extending direction in the track T of the Example 3 shown in
As shown in
As shown in
As described above, according to the present embodiment of the present invention, since the reference light is always prevented from being returned to the non-reflective region such as the pinhole PH on the reflective layer, the diffraction light from a reproduced hologram can be divided. When recording the hologram, since only the reference light effectively becomes non-reflective, a surplus hologram such as a reflected image is not recorded. As a result, the holographic recording layer is not deteriorated unnecessarily. Further, since the reference light is not returned to the detector when reproducing a hologram, it is possible to receive the diffraction light only from the hologram needed to reproduce the signal. As a result, a SN ratio (signal to noise ratio) of data reproduction is improved so that it is possible to perform stable reproduction.
Besides, though the foregoing embodiment includes the holographic record carrier 2 as shown in
Furthermore, in the embodiment described above, a case where holographic recording, mark record and servo control are performed is explained using the first light beam FB and the servo beam SB (second light beam) from the first and second laser light source LD1 and LD2, the first and second beams having different wavelengths with each other. In addition to such embodiment, it is possible to use first and second light sources LD1 and LD2 can project laser light having the same wavelength. In this case, for example, while performing the servo control by controlling the light intensity of the servo beam SB below the level so as not to reach holographic recording, the first light beam FB is irradiated only when the holographic recording is needed.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-098241 | Mar 2004 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2005/005051 | 3/15/2005 | WO | 00 | 1/30/2007 |
