EP2074616A2 - Setupand methods for storing data in and reading out data from a holographic storage arrangement. - Google Patents

Setupand methods for storing data in and reading out data from a holographic storage arrangement.

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Publication number
EP2074616A2
EP2074616A2 EP07826722A EP07826722A EP2074616A2 EP 2074616 A2 EP2074616 A2 EP 2074616A2 EP 07826722 A EP07826722 A EP 07826722A EP 07826722 A EP07826722 A EP 07826722A EP 2074616 A2 EP2074616 A2 EP 2074616A2
Authority
EP
European Patent Office
Prior art keywords
holographic storage
reference beam
grating
storage medium
storage arrangement
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP07826722A
Other languages
German (de)
French (fr)
Inventor
Frank Schuurmans
Jan Frederik Suijver
Martinus Van Der Mark
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Koninklijke Philips NV
Original Assignee
Koninklijke Philips Electronics NV
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Koninklijke Philips Electronics NV filed Critical Koninklijke Philips Electronics NV
Priority to EP07826722A priority Critical patent/EP2074616A2/en
Publication of EP2074616A2 publication Critical patent/EP2074616A2/en
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
    • G11B7/004—Recording, reproducing or erasing methods; Read, write or erase circuits therefor
    • G11B7/0065—Recording, reproducing or erasing by using optical interference patterns, e.g. holograms
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/02—Details of features involved during the holographic process; Replication of holograms without interference recording
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/02—Details of features involved during the holographic process; Replication of holograms without interference recording
    • G03H1/0252—Laminate comprising a hologram layer
    • G03H1/0256—Laminate comprising a hologram layer having specific functional layer
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
    • G11B7/12—Heads, e.g. forming of the optical beam spot or modulation of the optical beam
    • G11B7/135—Means for guiding the beam from the source to the record carrier or from the record carrier to the detector
    • G11B7/1353—Diffractive elements, e.g. holograms or gratings
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
    • G11B7/12—Heads, e.g. forming of the optical beam spot or modulation of the optical beam
    • G11B7/135—Means for guiding the beam from the source to the record carrier or from the record carrier to the detector
    • G11B7/1365—Separate or integrated refractive elements, e.g. wave plates
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
    • G11B7/24—Record carriers characterised by shape, structure or physical properties, or by the selection of the material
    • G11B7/2403—Layers; Shape, structure or physical properties thereof
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/26—Processes or apparatus specially adapted to produce multiple sub- holograms or to obtain images from them, e.g. multicolour technique
    • G03H2001/2625—Nature of the sub-holograms
    • G03H2001/264—One hologram being a HOE
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H2223/00—Optical components
    • G03H2223/20—Birefringent optical element, e.g. wave plate

Definitions

  • the present invention relates to a setup and to methods for storing data in and reading out data from a holographic storage medium.
  • a two-dimensional spatial light modulator (SLM) pattern containing digital information ('O's and 'Ts) is projected onto a holographic storage medium.
  • the most common configuration is the so called 4f Fourier configuration, in which the distance between the SLM and a first lens is one focal distance fi of this lens, the distance from this lens to the medium is fi, the distance from the medium to a second lens is one focal distance f 2 of this second lens, and finally the distance from this second lens to a detector array is again f 2 .
  • fi f 2 .
  • An illustration of such a setup is given in Figure 4.
  • the light emitted by a laser source (not shown, indicated by reference numeral 112) is split into two beams.
  • a first beam 114 is directed towards a reflective spatial light modulator 116, e.g. LCoS device, by means of a polarizing beam splitter 136.
  • a second beam is used as a reference beam 122.
  • the two-dimensional data page generated by the reflective spatial light modulator 116 (R- SLM) is reflected back towards an imaging lens 118 which focuses the light into a holographic storage medium 126.
  • the light focused from the lens 118 into the holographic storage medium 126 is known as the signal beam 120.
  • the holographic storage medium 126 is part of a holographic storage arrangement 110 which, in the simple case as illustrated, comprises two protective layers 138, 140 between which the holographic storage medium 126 is sandwiched.
  • the signal beam 120 interferes with the reference beam 122 which results in the modulation of the refractive index in the holographic storage medium 126. This modulation represents the stored data.
  • the medium is illuminated with only the reference beam 122 which results, by means of scattering in the holographic storage medium 126, in the reconstruction of the data page wave front originally carried by the signal beam 120.
  • the scattered light beam 142 is imaged with a lens 144 onto a detector array 124, e.g. CMOS or CCD array.
  • the distances from the R-SLM 116 to the first lens 118 corresponds to the focal distance of this lens 118 and is equal to the distance from the lens 118 to the holographic storage medium 126.
  • the distance from the holographic storage medium 126 to the second lens 144 and the distance from the second lens 114 to the detector array 124 are identical to the first mentioned distances; hence the name 4f configuration.
  • a well-known issue with the configuration shown in Figure 4 is the lack of compactness, as there is bulky and large optics on both sides of the holographic storage medium 126.
  • a known improvement in prior art is to perform phase conjugate read out (see for example: Ken Anderson et al., High speed holographic data storage at 100 Gbit/in 2 , Tech. Digest ISOM/ODS 2005). Using phase conjugation, the read out is done with the phase conjugate of the reference beam as used during recording, i.e. the reference beam during read out is counter-propagating the reference beam during data storage.
  • FIG. 5 An illustration of such a setup is given in Figure 5. While the recording phase is similar as described with reference to Figure 4, using the reference beam 122, the read out phase is different.
  • the holographic medium is illuminated with the counter-propagating reference beam 132.
  • the resulting scattered beam travels to and through the lens 118.
  • the polarizing beam splitter 136 projects the scattered beam to the detector array 124 which is, in contrast to the set up of Figure 4, positioned on the same side of the holographic storage arrangement 110 as the spatial light modulator 116, the polarizing beam splitter 136, the focusing lens 118 and the light source 112.
  • a setup for storing data in and reading out data from a holographic storage medium comprising a light source for emitting light having a wavelength ⁇ , a modulator for modulating the light emitted by the light source so as to form a signal beam, projection means for focusing the signal beam into the holographic storage medium, a light source for emitting a reference beam into the holographic storage medium, the reference beam having a wavelength ⁇ , wherein the setup comprises a grating and a ⁇ /4-layer arranged such that the incident reference beam is diffracted by the grating, upon which the diffracted beam again travels through the ⁇ /4-layer.
  • the complete optics is positioned on the same side of the holographic storage medium which is especially useful as regards the compactness of the setup.
  • the grating constant is ⁇ /2sin ⁇ , ⁇ being the nominal incidence angle of the incident reference beam.
  • ⁇ being the nominal incidence angle of the incident reference beam.
  • the incidence angle ⁇ ' of the reference beam is variable with respect to the nominal incidence angle ⁇ of the reference beam. On this basis, an angle multiplexing is made possible, which permits to store more information into the same volume of the holographic storage medium.
  • the grating constant is ⁇ /2sin ⁇ for different incidence angles ⁇ ' of the reference beam during storing, and the incident reference beam during read out is aligned with the diffracted reference beam during recording.
  • the grating constant is ⁇ /2sin ⁇ for different in- cidence angles ⁇ ' of the reference beam during storing, and the diffracted reference beam during read out is aligned with the incident reference beam during recording.
  • the grating is adapted to be diffractive during read out and essentially transparent during recording. No diffracted reference beam is generated during storing due to the absence of the grating, while during read out the diffracted reference beam can be used for reading the data from the holographic medium storage. As explained in the detailed description, this permits to increase the data density in the holographic medium.
  • the grating may have photochromic properties.
  • a selectively transparent element is arranged between the projection means and the grating, the setup comprising means to render the selectively transparent element essentially transparent during read out and essentially non-transparent during recording.
  • An aspect of the present invention further relates to a holographic storage arrange- ment for storing data on the basis of light having a wavelength ⁇ , the holographic storage arrangement comprising a holographic storage medium, a grating, and a ⁇ /4-layer between the holographic storage medium and the grating.
  • the holographic storage arrangement may form an integral arrangement comprising the holographic storage medium, the grating, and the ⁇ /4-layer.
  • the holographic storage arrangement is capable of being used with a reference beam generated by a setup for storing data in and reading out data from a holographic storage arrangement, the reference beam having a nominal incidence angle ⁇ , and the grating having a grating constant of ⁇ /2sin ⁇ .
  • An aspect of the present invention further relates to a method of storing data in a holographic storage arrangement, said method comprising the steps of: emitting light having a wavelength ⁇ , modulating the light so as to form a signal beam, providing a holographic storage arrangement comprising a holographic storage medium, a grating, and a ⁇ /4-layer between the holographic storage medium and the grating, focusing the signal beam into the holographic storage arrangement, emitting a reference beam into the holographic storage arrangement, the reference beam having a wavelength ⁇ , wherein the incident reference beam, after traveling through the holographic storage medium and through the ⁇ /4-layer, is diffracted by the grating, upon which the diffracted beam again travels through the ⁇ /4-layer and through the holographic storage medium, and wherein data are stored in the holographic storage medium by interference of the diffracted beam with the signal beam.
  • a method of reading out data from a holographic storage arrangement comprises the steps of: emitting a reference beam into the holographic storage arrangement, the reference beam having a wavelength ⁇ , scattering the reference light beam off the holographic storage medium, and detecting the scattered reference beam, wherein the reference beam used during read out is aligned with the diffracted reference beam used during storing. Consequently, the counter-propagating nature of the diffracted beam is used during the recording phase while during the read out phase only a conventional read out is performed on the basis of the incident reference beam.
  • the present invention relates to a method of storing data in a holographic storage arrangement, said method comprising the steps of: emitting light having a wavelength ⁇ , modulating the light so as to form a signal beam, providing a holographic storage arrangement comprising a holographic storage medium, a grating, and a ⁇ /4-layer between the holographic storage medium and the grating, focusing the signal beam into the holographic storage arrangement, emitting a reference beam into the holographic storage arrangement, the reference beam having a wavelength ⁇ , wherein data are stored in the holographic storage medium by interference of the incident reference beam with the signal beam.
  • a method of reading out data from a holographic storage arrangement comprises the steps of: emitting a reference beam into the holographic storage arrangement, the reference beam having a wavelength ⁇ , wherein the incident reference beam, after traveling through the holographic storage medium and through the ⁇ /4-layer, is diffracted by the grating, upon which the diffracted beam again travels through the ⁇ /4-layer and through the holographic storage medium, scattering the diffracted reference beam off the holographic storage medium, and detecting the scattered reference beam, wherein the diffracted reference beam used during read out is aligned with the incident reference beam used during recording.
  • the read out is based on the diffracted reference beam being aligned with the incident reference beam used during recording.
  • the grating may be diffractive during read out and essentially transparent during recording.
  • a selectively transparent element is arranged between the holographic recording medium and the grating, the selectively transparent element being essentially transparent during read out and essentially non-transparent during recording.
  • Figure 1 shows a setup for storing data in and reading out data from a holographic storage medium according to an embodiment of the present invention.
  • Figure 2 shows a holographic storage arrangement according to an embodiment of the present invention.
  • Figure 3 shows holographic storage arrangements for illustrating in-plain angle multiplexing.
  • Figure 4 shows a setup for storing data in and reading out data from a holographic storage arrangement according to prior art.
  • Figure 5 shows a further setup for storing data in and reading out data from a holographic storage arrangement according to prior art.
  • Figure 1 shows a setup for storing data in and reading out data from a holographic storage medium according to the present invention.
  • the recording situation is illustrated in part (a) of Figure 1.
  • a light source not shown but indicated by reference numeral 12
  • polarized light 14 is projected into a polarizing beam splitter 36.
  • the polarization directions of the light involved are shown as small arrows or circles fixed at the origin of larger arrows indicating the direction of the light.
  • the light is reflected to the spatial light modulator 16 by the polarizing beam splitter 36.
  • the spatial light modulator 16 reflects the light into the opposite direction. This light may contain two different polarizations which represent the "0"s and the "l”s, i.e. the data to be transferred to a holographic storage medium 26.
  • the light propagates through a ⁇ /2-layer 48 and a lens 18 to be rotated in polarization and to be projected in the form of a signal beam 20 into the holographic storage arrangement 10.
  • a detector array 24 is arranged perpendicular to the spatial light modulator 16, which will be used during read out.
  • the holographic storage arrangement 10 comprises the holographic storage medium 26, a grating 28, and a ⁇ /4-layer 30 sandwiched between the holographic storage medium 26 and the grating 28.
  • the holographic storage medium 26 is covered by a protective layer 50.
  • the holographic storage medium 26 may be separate from the grating 28, and the ⁇ /4-layer 30.
  • the holographic medium 26 may be inserted by a user into a setup comprising the grating 28, and the ⁇ /4- layer 30.
  • the setup will further comprise means for receiving said holographic medium 26.
  • This receiving means may have any form suitable for arranging the holographic medium between the lens 18 and the ⁇ /4-layer 30 such as a tray used in conventional optical storage such as in a CD player.
  • a reference beam 22 is projected into the holographic storage arrangement 10.
  • This reference beam 22 has the wavelength ⁇ and may be generated by the same light source 12 as the signal beam 20, or by a different light source. Therefore the light source for generating the signal beam 20 and the light source for generating the reference beam 22 may be one and the same light source. Due to an appropriate choice of the grating constant, the reference beam 22 is diffracted into an angle such that the diffracted reference beam 34 counter-propagates the incident reference beam 22 in the em- bodiment shown in Fig. 1. By interference of the incident signal beam 20 and the diffracted reference beam 34, a holographic pattern, e.g. a spatial refractive index modulation, is formed in the holographic storage medium 26.
  • a holographic pattern e.g. a spatial refractive index modulation
  • the diffracted reference beam 34 has the same polarization as the signal beam 20, thanks to the ⁇ /4-layer 30, and may thus interfere with the signal beam 20.
  • the read out situation is illustrated in part (b) of Figure 1.
  • a reference beam 32 is projected into the holographic storage arrangement 10.
  • the reference beam 32 is in this embodiment the phase conjugate of the diffracted reference beam 34 used during recording.
  • the reference beam 32 is therefore scattered off the holograms through the lens 18, ⁇ /2-layer 48, beam splitter 36 and to the detector array 24.
  • Figure 2 shows a holographic storage arrangement according to an embodiment of the present invention.
  • the counter propagation of the incident reference beam 22 and the diffracted reference beam 34 is detailed.
  • This counter-propagating situation can be achieved by selecting the grating constant to be ⁇ /2sin ⁇ , when ⁇ is the nominal incident angle of the reference beam 22 inside the holographic medium. Then, the first order of the diffracted light beam 34 counter-propagates the incident reference beam 22.
  • Figure 3 shows holographic storage arrangements for illustrating in-plain angle multiplexing.
  • multiplexing is performed to store more information into the same volume. This is most commonly done by sequentially writing multiple pages by varying the incident angle of the reference beam.
  • the incident angle is incremented by steps of, for instance, 0.1 degrees so as to record different data pages.
  • the incident angle may be varied around a nominal incident angle being the mean value of the possible incident angles. As an example, the incident angle may be varied between 40 and 60 degrees, the nominal incident angle being in that case 50 degrees.
  • the grating will diffract light exactly in the counter-propagating direction if the grating has been designed with a grating constant of ⁇ /2sin ⁇ and ⁇ is the nominal incident angle.
  • the diffracted light will come out at a different angle.
  • Part (a) of Figure 3 shows the situation where the incident angle is smaller than the nominal incident angle ⁇
  • part (b) of Figure 3 shows an incident angle that is larger than the nominal incident angle ⁇ .
  • still correct recording and read out is possible. This can be achieved by recording and reading out at different angles of the reference beam.
  • the incident direction of the reference beam in the read out phase is matched to the direction of the diffracted light in the recording phase for a specific page.
  • the incident direction of the reference beam in the read out phase differs from the direction of the incident reference beam in the recording phase. In this way, all pages can be read out without loss of fidelity.
  • the grating will be designed with a pitch that is matched to the nominal angle ⁇ , i.e. the angle in the middle of the range of angles accessed by the reference beam.
  • the grating is present both during recording and read out. This may have the disadvantage that during recording both the incident and the diffracted reference beams are propagating through the photo-sensitive holographic medium, whereas only one of the two beams is used for storing the data and subsequent reading the data. In the case illustrated in Figure 1, only the information written by the diffracted incident beam 34 is read by the incident reference beam 32 during read out. Conse- quently, the dynamic range of photo-polymeric holographic material may be consumed by the presence of two beams, resulting in lower storage capacity. In order to solve this issue, a further embodiment is considered.
  • the diffracted reference beam is used for data recording, but the incident reference beam interferes with the signal beam in order to write the data into the holographic storage medium. Reading out the data is then achieved by a diffracted reference beam counter-propagating the incident reference beam used for recording.
  • the lowering of the storage capacity due to the presence of two reference beams in the holographic medium 26 during recording can be avoided. It is possible to make the grating switchable, i.e. only present in the read out phase and not in the recording phase. Only upon read out the grating is switched on, and the incident reference beam is first diffracted off the grating to subsequently scatter off the volume hologram to construct the signal on the detector.
  • the optical reflection and absorption of a photochromic layer can be controlled by the amount of light impinging on it at another wavelength.
  • a grating with electrochromic properties may also be used.
  • the grating in the "off-state, is essentially transparent and no diffraction takes place.
  • the grating is activated ("on"-state) during the read-out phase by coupling a light source to it of a different wavelength than the wavelength used for reading and writing data on the hologram. This is a non-contact method for switching the hologram between a "write” and "read” state.
  • a non-switchable grating is hidden behind a switchable mirror or absorber.
  • This may be a photochromic or electrochromic mirror for instance.
  • the efficiency of the grating can be optimized independently of the switching feature, allowing a larger optical contrast between the 'on' and the 'off state.

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  • Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • General Physics & Mathematics (AREA)
  • Holo Graphy (AREA)
  • Optical Recording Or Reproduction (AREA)
  • Optical Head (AREA)

Abstract

A setup for storing data in and reading out data from a holographic storage medium comprises a light source (12) for emitting light (14) having a wavelength λ, a modulator (16) for modulating the light emitted by the light source, projection means (18) for forming a signal beam (20) from the modulated light and for focusing the signal beam into the holographic storage medium (26) and a light source (12) for emitting a reference beam (22, 32) into the holographic storage medium, the reference beam having a wavelength λ. The setup comprises a grating (28) and a λ/4-layer (30) arranged such that the incident reference beam (22, 32) is diffracted by the grating, upon which the diffracted beam (34) again travels through the λ/4-layer.

Description

SETUP AND METHODS FOR STORING DATA IN AND READING OUT DATA FROM A HOLOGRAPHIC STORAGE ARRANGEMENT
Field of the invention
The present invention relates to a setup and to methods for storing data in and reading out data from a holographic storage medium.
Background of the invention
In holographic data storage a two-dimensional spatial light modulator (SLM) pattern containing digital information ('O's and 'Ts) is projected onto a holographic storage medium. The most common configuration is the so called 4f Fourier configuration, in which the distance between the SLM and a first lens is one focal distance fi of this lens, the distance from this lens to the medium is fi, the distance from the medium to a second lens is one focal distance f2 of this second lens, and finally the distance from this second lens to a detector array is again f2. Typically fi = f2. An illustration of such a setup is given in Figure 4. The light emitted by a laser source (not shown, indicated by reference numeral 112) is split into two beams. A first beam 114 is directed towards a reflective spatial light modulator 116, e.g. LCoS device, by means of a polarizing beam splitter 136. A second beam is used as a reference beam 122. The two-dimensional data page generated by the reflective spatial light modulator 116 (R- SLM) is reflected back towards an imaging lens 118 which focuses the light into a holographic storage medium 126. The light focused from the lens 118 into the holographic storage medium 126 is known as the signal beam 120. The holographic storage medium 126 is part of a holographic storage arrangement 110 which, in the simple case as illustrated, comprises two protective layers 138, 140 between which the holographic storage medium 126 is sandwiched. For recording, the signal beam 120 interferes with the reference beam 122 which results in the modulation of the refractive index in the holographic storage medium 126. This modulation represents the stored data. During read out, the medium is illuminated with only the reference beam 122 which results, by means of scattering in the holographic storage medium 126, in the reconstruction of the data page wave front originally carried by the signal beam 120. The scattered light beam 142 is imaged with a lens 144 onto a detector array 124, e.g. CMOS or CCD array. The distances from the R-SLM 116 to the first lens 118 corresponds to the focal distance of this lens 118 and is equal to the distance from the lens 118 to the holographic storage medium 126. Further, the distance from the holographic storage medium 126 to the second lens 144 and the distance from the second lens 114 to the detector array 124 are identical to the first mentioned distances; hence the name 4f configuration. A well-known issue with the configuration shown in Figure 4 is the lack of compactness, as there is bulky and large optics on both sides of the holographic storage medium 126. A known improvement in prior art is to perform phase conjugate read out (see for example: Ken Anderson et al., High speed holographic data storage at 100 Gbit/in2, Tech. Digest ISOM/ODS 2005). Using phase conjugation, the read out is done with the phase conjugate of the reference beam as used during recording, i.e. the reference beam during read out is counter-propagating the reference beam during data storage.
An illustration of such a setup is given in Figure 5. While the recording phase is similar as described with reference to Figure 4, using the reference beam 122, the read out phase is different. Upon read out, the holographic medium is illuminated with the counter-propagating reference beam 132. The resulting scattered beam travels to and through the lens 118. The polarizing beam splitter 136 projects the scattered beam to the detector array 124 which is, in contrast to the set up of Figure 4, positioned on the same side of the holographic storage arrangement 110 as the spatial light modulator 116, the polarizing beam splitter 136, the focusing lens 118 and the light source 112. However, although indeed the overall system according to Figure 5 is more compact than the system according to Figure 4, still some optics needs to be on the "other side" of the holographic storage arrangement, as the reference beam used upon read out needs to be projected from the other side of the holographic storage arrangement and, e.g. for in-plain angle multiplexing, steered. It is therefore desired to provide a system which is more compact. Summary of the invention
In accordance with an aspect of the invention, there is provided a setup for storing data in and reading out data from a holographic storage medium, said setup comprising a light source for emitting light having a wavelength λ, a modulator for modulating the light emitted by the light source so as to form a signal beam, projection means for focusing the signal beam into the holographic storage medium, a light source for emitting a reference beam into the holographic storage medium, the reference beam having a wavelength λ, wherein the setup comprises a grating and a λ/4-layer arranged such that the incident reference beam is diffracted by the grating, upon which the diffracted beam again travels through the λ/4-layer.
By employment of an appropriate grating it is possible to use the reference beam diffracted by the grating for either interfering with the signal beam and producing the refractive index modulation within the holographic storage medium, or for reading out data previously stored due to the interference of the incident reference beam and the signal beam. Thanks to the grating, the complete optics is positioned on the same side of the holographic storage medium which is especially useful as regards the compactness of the setup.
According to an embodiment, the grating constant is λ/2sinθ, θ being the nominal incidence angle of the incident reference beam. With such a grating the first order diffracted light is counter-propagating the reference beam in case this beam is incident under an angle θ. According to another embodiment, the incidence angle θ' of the reference beam is variable with respect to the nominal incidence angle θ of the reference beam. On this basis, an angle multiplexing is made possible, which permits to store more information into the same volume of the holographic storage medium.
A possibility for implementing multiplexing is that the grating constant is λ/2sinθ for different incidence angles θ' of the reference beam during storing, and the incident reference beam during read out is aligned with the diffracted reference beam during recording. Thus, even with a grating that is designed to generate the phase conjugate of a given refer- ence beam, but not of all reference beams used during storing, the present invention can be realized by aligning the beams that need to counter propagate in the sense of the present invention.
According to a further embodiment, the grating constant is λ/2sinθ for different in- cidence angles θ' of the reference beam during storing, and the diffracted reference beam during read out is aligned with the incident reference beam during recording.
According to a further embodiment, the grating is adapted to be diffractive during read out and essentially transparent during recording. No diffracted reference beam is generated during storing due to the absence of the grating, while during read out the diffracted reference beam can be used for reading the data from the holographic medium storage. As explained in the detailed description, this permits to increase the data density in the holographic medium.
In this context the grating may have photochromic properties.
Further, it is possible that a selectively transparent element is arranged between the projection means and the grating, the setup comprising means to render the selectively transparent element essentially transparent during read out and essentially non-transparent during recording.
An aspect of the present invention further relates to a holographic storage arrange- ment for storing data on the basis of light having a wavelength λ, the holographic storage arrangement comprising a holographic storage medium, a grating, and a λ/4-layer between the holographic storage medium and the grating.
Particularly, the holographic storage arrangement may form an integral arrangement comprising the holographic storage medium, the grating, and the λ/4-layer. According to an embodiment, the holographic storage arrangement is capable of being used with a reference beam generated by a setup for storing data in and reading out data from a holographic storage arrangement, the reference beam having a nominal incidence angle θ, and the grating having a grating constant of λ/2sinθ.
An aspect of the present invention further relates to a method of storing data in a holographic storage arrangement, said method comprising the steps of: emitting light having a wavelength λ, modulating the light so as to form a signal beam, providing a holographic storage arrangement comprising a holographic storage medium, a grating, and a λ/4-layer between the holographic storage medium and the grating, focusing the signal beam into the holographic storage arrangement, emitting a reference beam into the holographic storage arrangement, the reference beam having a wavelength λ, wherein the incident reference beam, after traveling through the holographic storage medium and through the λ/4-layer, is diffracted by the grating, upon which the diffracted beam again travels through the λ/4-layer and through the holographic storage medium, and wherein data are stored in the holographic storage medium by interference of the diffracted beam with the signal beam.
On this basis a method of reading out data from a holographic storage arrangement can be provided, wherein the method comprises the steps of: emitting a reference beam into the holographic storage arrangement, the reference beam having a wavelength λ, scattering the reference light beam off the holographic storage medium, and detecting the scattered reference beam, wherein the reference beam used during read out is aligned with the diffracted reference beam used during storing. Consequently, the counter-propagating nature of the diffracted beam is used during the recording phase while during the read out phase only a conventional read out is performed on the basis of the incident reference beam.
According to a further embodiment, the present invention relates to a method of storing data in a holographic storage arrangement, said method comprising the steps of: emitting light having a wavelength λ, modulating the light so as to form a signal beam, providing a holographic storage arrangement comprising a holographic storage medium, a grating, and a λ/4-layer between the holographic storage medium and the grating, focusing the signal beam into the holographic storage arrangement, emitting a reference beam into the holographic storage arrangement, the reference beam having a wavelength λ, wherein data are stored in the holographic storage medium by interference of the incident reference beam with the signal beam.
In this context a method of reading out data from a holographic storage arrangement is proposed, wherein the method comprises the steps of: emitting a reference beam into the holographic storage arrangement, the reference beam having a wavelength λ, wherein the incident reference beam, after traveling through the holographic storage medium and through the λ/4-layer, is diffracted by the grating, upon which the diffracted beam again travels through the λ/4-layer and through the holographic storage medium, scattering the diffracted reference beam off the holographic storage medium, and detecting the scattered reference beam, wherein the diffracted reference beam used during read out is aligned with the incident reference beam used during recording.
Hence, while the recording is performed on a conventional basis by interference of the incident reference beam with the signal beam, the read out is based on the diffracted reference beam being aligned with the incident reference beam used during recording.
In connection with the last mentioned embodiment the grating may be diffractive during read out and essentially transparent during recording.
Further, it is possible that a selectively transparent element is arranged between the holographic recording medium and the grating, the selectively transparent element being essentially transparent during read out and essentially non-transparent during recording.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
Brief description of the drawings
Figure 1 shows a setup for storing data in and reading out data from a holographic storage medium according to an embodiment of the present invention. Figure 2 shows a holographic storage arrangement according to an embodiment of the present invention. Figure 3 shows holographic storage arrangements for illustrating in-plain angle multiplexing.
Figure 4 shows a setup for storing data in and reading out data from a holographic storage arrangement according to prior art. Figure 5 shows a further setup for storing data in and reading out data from a holographic storage arrangement according to prior art.
Description of embodiments of the invention
Figure 1 shows a setup for storing data in and reading out data from a holographic storage medium according to the present invention. The recording situation is illustrated in part (a) of Figure 1. From a light source (not shown but indicated by reference numeral 12) polarized light 14 is projected into a polarizing beam splitter 36. The polarization directions of the light involved are shown as small arrows or circles fixed at the origin of larger arrows indicating the direction of the light. The light is reflected to the spatial light modulator 16 by the polarizing beam splitter 36. The spatial light modulator 16 reflects the light into the opposite direction. This light may contain two different polarizations which represent the "0"s and the "l"s, i.e. the data to be transferred to a holographic storage medium 26. The light propagates through a λ/2-layer 48 and a lens 18 to be rotated in polarization and to be projected in the form of a signal beam 20 into the holographic storage arrangement 10. In addition to the spatial light modulator 16, a detector array 24 is arranged perpendicular to the spatial light modulator 16, which will be used during read out.
In the embodiment of Fig. 1, the holographic storage arrangement 10 comprises the holographic storage medium 26, a grating 28, and a λ/4-layer 30 sandwiched between the holographic storage medium 26 and the grating 28. The holographic storage medium 26 is covered by a protective layer 50. It should be noted that the holographic storage medium 26 may be separate from the grating 28, and the λ/4-layer 30. In other words, the holographic medium 26 may be inserted by a user into a setup comprising the grating 28, and the λ/4- layer 30. To this end, the setup will further comprise means for receiving said holographic medium 26. This receiving means may have any form suitable for arranging the holographic medium between the lens 18 and the λ/4-layer 30 such as a tray used in conventional optical storage such as in a CD player.
In addition to the signal beam 20, a reference beam 22 is projected into the holographic storage arrangement 10. This reference beam 22 has the wavelength λ and may be generated by the same light source 12 as the signal beam 20, or by a different light source. Therefore the light source for generating the signal beam 20 and the light source for generating the reference beam 22 may be one and the same light source. Due to an appropriate choice of the grating constant, the reference beam 22 is diffracted into an angle such that the diffracted reference beam 34 counter-propagates the incident reference beam 22 in the em- bodiment shown in Fig. 1. By interference of the incident signal beam 20 and the diffracted reference beam 34, a holographic pattern, e.g. a spatial refractive index modulation, is formed in the holographic storage medium 26. Actually, the diffracted reference beam 34 has the same polarization as the signal beam 20, thanks to the λ/4-layer 30, and may thus interfere with the signal beam 20. The read out situation is illustrated in part (b) of Figure 1. During read out, only a reference beam 32 is projected into the holographic storage arrangement 10. The reference beam 32 is in this embodiment the phase conjugate of the diffracted reference beam 34 used during recording. The reference beam 32 is therefore scattered off the holograms through the lens 18, λ/2-layer 48, beam splitter 36 and to the detector array 24. Figure 2 shows a holographic storage arrangement according to an embodiment of the present invention. Here, the counter propagation of the incident reference beam 22 and the diffracted reference beam 34 is detailed. This counter-propagating situation can be achieved by selecting the grating constant to be λ/2sinθ, when θ is the nominal incident angle of the reference beam 22 inside the holographic medium. Then, the first order of the diffracted light beam 34 counter-propagates the incident reference beam 22.
Figure 3 shows holographic storage arrangements for illustrating in-plain angle multiplexing. In holographic data storage, multiplexing is performed to store more information into the same volume. This is most commonly done by sequentially writing multiple pages by varying the incident angle of the reference beam. The incident angle is incremented by steps of, for instance, 0.1 degrees so as to record different data pages. For instance, the incident angle may be varied around a nominal incident angle being the mean value of the possible incident angles. As an example, the incident angle may be varied between 40 and 60 degrees, the nominal incident angle being in that case 50 degrees. However, only for the nominal incident angle, the grating will diffract light exactly in the counter-propagating direction if the grating has been designed with a grating constant of λ/2sinθ and θ is the nominal incident angle. For other angles, as illustrated in Figure 3, the diffracted light will come out at a different angle. Part (a) of Figure 3 shows the situation where the incident angle is smaller than the nominal incident angle θ, while part (b) of Figure 3 shows an incident angle that is larger than the nominal incident angle θ. However, according to embodiments of the invention, still correct recording and read out is possible. This can be achieved by recording and reading out at different angles of the reference beam. In the recording and reading situation as illustrated in Figure 1, the incident direction of the reference beam in the read out phase is matched to the direction of the diffracted light in the recording phase for a specific page. For an incident angle of the reference beam used during recording that differs from the angle for which the grating has been designed to obtain exact counter- propagation of the diffracted beam, the incident direction of the reference beam in the read out phase differs from the direction of the incident reference beam in the recording phase. In this way, all pages can be read out without loss of fidelity. Typically, the grating will be designed with a pitch that is matched to the nominal angle θ, i.e. the angle in the middle of the range of angles accessed by the reference beam.
In the situations described above, the grating is present both during recording and read out. This may have the disadvantage that during recording both the incident and the diffracted reference beams are propagating through the photo-sensitive holographic medium, whereas only one of the two beams is used for storing the data and subsequent reading the data. In the case illustrated in Figure 1, only the information written by the diffracted incident beam 34 is read by the incident reference beam 32 during read out. Conse- quently, the dynamic range of photo-polymeric holographic material may be consumed by the presence of two beams, resulting in lower storage capacity. In order to solve this issue, a further embodiment is considered. According to this further embodiment not the diffracted reference beam is used for data recording, but the incident reference beam interferes with the signal beam in order to write the data into the holographic storage medium. Reading out the data is then achieved by a diffracted reference beam counter-propagating the incident reference beam used for recording. On basis of this further embodiment, the lowering of the storage capacity due to the presence of two reference beams in the holographic medium 26 during recording can be avoided. It is possible to make the grating switchable, i.e. only present in the read out phase and not in the recording phase. Only upon read out the grating is switched on, and the incident reference beam is first diffracted off the grating to subsequently scatter off the volume hologram to construct the signal on the detector. This can, for example, be achieved by a grating with photochromic properties. At one wavelength, the optical reflection and absorption of a photochromic layer can be controlled by the amount of light impinging on it at another wavelength. A grating with electrochromic properties may also be used.
In a first embodiment, in the "off-state, the grating is essentially transparent and no diffraction takes place. The grating is activated ("on"-state) during the read-out phase by coupling a light source to it of a different wavelength than the wavelength used for reading and writing data on the hologram. This is a non-contact method for switching the hologram between a "write" and "read" state.
Alternatively, in a second embodiment, a non-switchable grating is hidden behind a switchable mirror or absorber. This may be a photochromic or electrochromic mirror for instance. In such an embodiment the efficiency of the grating can be optimized independently of the switching feature, allowing a larger optical contrast between the 'on' and the 'off state.
Equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.
Any reference sign in the following claims should not be construed as limiting the claim. It will be obvious that the use of the verb "to comprise" and its conjugations does not exclude the presence of any other elements besides those defined in any claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

Claims

Claims
1. A setup for storing data in and reading out data from a holographic storage medium (26), said setup comprising a light source (12) for emitting light (14) having a wavelength λ, a modulator (16) for modulating the light emitted by the light source so as to form a signal beam, projection means (18) for focusing the signal beam into the holographic storage me- dium (26), a light source (12) for emitting a reference beam (22, 32) into the holographic storage medium, the reference beam having a wavelength λ, wherein the setup comprises a grating (28) and a λ/4-layer (30) arranged such that the incident reference beam (22, 32) is diffracted by the grating, upon which the diffracted beam (34) again travels through the λ/4-layer.
2. The setup according to claim 1, wherein the grating has a grating constant of λ/2sinθ, θ being a nominal incidence angle of the incident reference beam.
3. The setup according to claim 2, wherein the incidence angle θ' of the reference beam is variable with respect to the nominal incidence angle θ of the reference beam.
4. The setup according to claim 3, wherein the grating constant is λ/2sinθ for different incidence angles θ' of the reference beam (22) during storing, and the setup is ar- ranged such that the incident reference beam during read out is aligned with the diffracted reference beam during recording.
5. The setup according to claim 3, wherein the grating constant is λ/2sinθ for different incidence angles θ' of the reference beam (22) during storing, and the setup is arranged such that the diffracted reference beam during read out is aligned with the incident reference beam during recording.
6. The setup according to claim 1, wherein the grating (28) is adapted to be dif- fractive during read out and essentially transparent during recording.
7. The setup according to claim 6, wherein the grating (28) has photochromic properties.
8. The setup according to claim 1, wherein a selectively transparent element is further arranged between the projection means and the grating, the setup comprising means to render the selectively transparent element essentially transparent during read out and essentially non-transparent during recording.
9. A holographic storage arrangement (10) for storing data on the basis of light having a wavelength λ, the holographic storage arrangement comprising a holographic storage medium (26), a grating (28), and a λ/4-layer (30) between the holographic storage me- dium and the grating.
10. The holographic storage arrangement (10) according to claim 9, wherein the holographic storage arrangement forms an integral arrangement comprising the holographic storage medium (26), the grating (28), and the λ/4-layer (30).
11. The holographic storage arrangement (10) according to claim 9, wherein the holographic storage arrangement (10) is capable of being used with a reference beam generated by a setup for storing data in and reading out data from a holographic storage arrangement, the reference beam having a nominal incidence angle θ, the grating having a grating constant of λ/2sinθ.
12. A method of storing data in a holographic storage arrangement, said method comprising the steps of: emitting light (14) having a wavelength λ, modulating the light so as to form a signal beam, providing a holographic storage arrangement (10) comprising a holographic storage medium (26), a grating (28), and a λ/4-layer (30) between the holographic storage medium and the grating, focusing the signal beam into the holographic storage arrangement, emitting a reference beam (22) into the holographic storage arrangement, the refer- ence beam having a wavelength λ, wherein the incident reference beam, after traveling through the holographic storage medium and through the λ/4-layer, is diffracted by the grating, upon which the diffracted reference beam (34) again travels through the λ/4-layer and through the holographic storage medium, and wherein data are stored in the holographic storage medium by interference of the diffracted reference beam (34) with the signal beam (20).
13. A method of reading out data from a holographic storage arrangement, the data having been recorded by the method according to claim 12, said method comprising the steps of: emitting a reference beam into the holographic storage arrangement, the reference beam having a wavelength λ and being aligned with the diffracted reference beam (34) used during storing, scattering the reference beam (32) off the holographic storage medium, and detecting the scattered reference beam.
14. A method of storing data in a holographic storage arrangement, said method comprising the steps of: emitting light having a wavelength λ, modulating the light so as to form a signal beam, providing a holographic storage arrangement comprising a holographic storage medium, a grating, and a λ/4-layer between the holographic storage medium and the grating, focusing the signal beam into the holographic storage arrangement, emitting a reference beam into the holographic storage arrangement, the reference beam having a wavelength λ, wherein data are stored in the holographic storage medium by interference of the in- cident reference beam with the signal beam.
15. A method of reading out data from a holographic storage arrangement, the data having been recorded by the method according to claim 14, said method comprising the steps of: emitting a reference beam into the holographic storage arrangement, the reference beam having a wavelength λ, wherein the incident reference beam, after traveling through the holographic storage medium and through the λ/4-layer, is diffracted by the grating, upon which the diffracted beam again travels through the λ/4-layer and through the holographic storage medium, the diffracted reference beam being aligned with the incident reference beam used during recording. scattering the diffracted reference beam off the holographic storage medium, and detecting the scattered reference beam.
16. The method according to claims 14 and 15, wherein the grating is diffractive during read out and essentially transparent during recording.
17. The method according to claims 14 and 15, wherein a selectively transparent element is arranged between the holographic recording medium and the grating, the selec- tively transparent element being essentially transparent during read out and essentially non- transparent during recording.
EP07826722A 2006-10-16 2007-10-12 Setupand methods for storing data in and reading out data from a holographic storage arrangement. Withdrawn EP2074616A2 (en)

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EP07826722A EP2074616A2 (en) 2006-10-16 2007-10-12 Setupand methods for storing data in and reading out data from a holographic storage arrangement.
PCT/IB2007/054157 WO2008047282A2 (en) 2006-10-16 2007-10-12 Setupand methods for storing data in and reading out data from a holographic storage arrangement.

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CN102346424A (en) * 2010-07-30 2012-02-08 张树森 Dynamic reproduction method for grating encrypted message
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