Optical data storage by selective localized alteration of a format hologram

Digital data bits are stored at storage locations at plural depths within a holographic storage medium as selective, localized alterations in a format hologram. Micro-localized regions of a reflection format hologram extending throughout the medium are deleted by focusing a high-power laser beam at desired storage locations. The deletion regions have a lower reflectivity than the surrounding parts of the format hologram. Tunable-focus storage and retrieval heads, as well as dynamic aberration compensators, are used for multi-depth access. Storage and retrieval may each be achieved with a single head.

FIELD OF THE INVENTION
 The present invention relates to the field of holographic data storage, and
 in particular to a system and method for storing data as micro-localized
 alterations in a format hologram.
 BACKGROUND OF THE INVENTION
 In conventional holographic storage, data is stored as holograms resulting
 from the interference of a signal and reference beam. During storage, both
 the reference and signal beams are incident on the storage medium. During
 retrieval, only the reference beam is incident on the medium. The
 reference beam interacts with a stored hologram, generating a
 reconstructed signal beam proportional to the original signal beam used to
 store the hologram.
 For information on conventional volume holographic storage see for example
 U.S. Pat. Nos. 4,920,220, 5,450,218, and 5,440,669. In conventional volume
 holographic storage, each bit is stored as a hologram extending over the
 entire volume of the storage medium. Multiple bits are encoded and decoded
 together in pages, or two-dimensional arrays of bits. Multiple pages can
 be stored within the volume by angular, wavelength, phase-code, or related
 multiplexing techniques. Each page can be independently retrieved using
 its corresponding reference beam. The parallel nature of the storage
 approach allows high transfer rates and short access times, since as many
 as `10.sup.6 bits within one page can be stored and retrieved
 simultaneously.
 Conventional page-based volume holographic storage generally requires
 complex, specialized components such as amplitude and/or phase spatial
 light modulators. Moreover, ensuring that the reference and signal beams
 are mutually coherent over the entire volume of the storage medium
 generally requires a light source with a relatively high coherence length,
 as well as a relatively stable mechanical system. Mechanical stability and
 coherence-length requirements have hindered the development of
 inexpensive, stable, and rugged holographic storage devices capable of
 convenient operation in a typical user environment.
 In U.S. Pat. No. 4,458,345, Bjorklund et al. describe a bit-wise volume
 holographic storage method using signal and reference beams incident on a
 rotating disk in a transmission geometry. The signal and reference beams
 are incident from the same side of the disk. The angle between the
 reference and signal beams can be altered to store holograms at various
 depths within the medium. A separate photodetector is used to retrieve
 data stored at each depth. The interaction of light with the medium is
 localized through two-photon recording.
 In U.S. Pat. No. 5,659,536, Maillot et al. describe a system in which
 multiple holograms are stored at each location in a disk through
 wavelength multiplexing. Each hologram spans the depth of the medium. In
 U.S. Pat. No. 5,289,407, Strickler et al. describe a multi-layered,
 non-holographic, index-perturbation optical storage system. Bits are
 stored as localized perturbations in the index of refraction of a
 photopolymer, caused by the high intensity at the focus of a single laser
 beam.
 SUMMARY OF THE INVENTION
 Briefly, and in general terms, the present invention provides a
 multi-depth, bit-wise optical data storage and/or retrieval system and
 method having improved storage density, and in which the optical
 components used for storage and retrieval can be relatively simple,
 inexpensive, and robust.
 With the present invention, a format hologram is first stored in a
 holographic storage medium, and data are then stored as selective,
 microlocalized alterations of the format hologram. The alterations are
 stored at a plurality of depths within the medium, thereby allowing the
 storage of multiple data layers.
 Storing the format hologram, which requires maintaining mutual coherence
 between two light beams, can be performed in controlled conditions in a
 factory environment. Data storage and retrieval can then be performed in a
 user environment using a relatively simple and robust device. Storage or
 retrieval can be performed using a single light beam incident on the
 medium, and using a single optical head.
 In a presently preferred embodiment, the format hologram is a reflection
 hologram stored throughout the volume of the medium, and having
 substantially planar fringes perpendicular to the depth of the medium. The
 reflection hologram is capable of reflecting light traveling along the
 depth of the medium. Partial or complete deletion of the reflection
 hologram generates storage locations of lower reflectivity than the
 surrounding intact regions of the hologram.
 Other features and advantages of the invention will become apparent from
 the following detailed description, taken in conjunction with the
 accompanying drawings, which illustrates by way of example the invention.

DETAILED DESCRIPTION
 FIG. 1 shows a schematic side view of a holographic storage medium 22
 containing a substantially planar format hologram (holographic grating) 27
 stored within medium 22, according to a preferred embodiment of the
 present invention. The fringes of hologram 27 are marked 28. For clarity,
 the spacing between fringes 28 is exaggerated in FIG. 1 relative to the
 thickness of medium 22; medium 22 preferably comprises a larger number of
 fringes than shown. Medium 22 is formed of a structurally homogeneous
 planar layer of a photopolymer having a thickness preferably on the order
 of hundreds of .mu.m, for example about 100-200 .mu.m or less. For
 information on photopolymers see for example Lessard and Manivannan (ed.),
 Selected Papers on Photopolymers, SPIE Milestone Series, v. MS-114, SPIE
 Optical Engineering Press, Bellingham, Wash., 1995. Formatting optics (not
 shown) in optical communication with medium 22 generate two plane-wave
 light beams 31a-b incident on opposite (top and bottom) planar input
 surfaces 50a-b of medium 22, respectively. Surfaces 50a-b are transverse
 to the depth 25 of medium 22. Beams 31a-b have identical wavelengths and
 are mutually coherent. Beams 31a-b each contain single plane-wave
 components. The interference of beams 31a-b within medium 22 generates
 hologram 27. Hologram 27 is preferably a phase hologram, characterized by
 a periodic variation in the real component of the index of refraction.
 Hologram 27 is preferably an elementary hologram, i.e. a hologram written
 by two plane-wave beams. The variation of hologram 27 along depth 25 can
 be characterized by a single spatial frequency. The fringes 28 of hologram
 27 are mutually parallel, and are regularly spaced apart along the depth
 25 of medium 22. Hologram 27 is preferably substantially uniform across
 medium 22 in the plane orthogonal to depth 25, and is recorded in one step
 for the entire volume of medium 22.
 FIG. 2-A shows a perspective schematic view of a presently preferred
 optical data storage and/or retrieval system 20 of the present invention.
 A disk-shaped storage device 21 comprises medium 22 as well as packaging
 elements for mechanically protecting medium 22 and for mounting device 21.
 Hologram 27 is stored within medium 22, and is insensitive to ambient heat
 and light levels within medium 22. Device 21 is detachably mounted on a
 rotary holder 24. Holder 24 continuously rotates medium 22 at high
 velocity about an axis of rotation coinciding with depth 25. Multiple
 storage subvolumes of medium 22 are stacked along depth 25. Each subvolume
 contains plural concentric data tracks 23. Adjacent data tracks at one
 depth are separated along a generally radial direction 15, while storage
 locations along a data track are separated along a circumferential
 direction 17.
 A head/arm assembly 10 is used to access desired storage locations within
 medium 22. Head/arm assembly 10 and holder 24 are connected to a fixed
 housing (not shown). Head/arm assembly 10 comprises a movable carriage
 assembly 11 and fixed, generally radial, mutually parallel rails 12.
 Carriage assembly 11 is movably mounted on rails 12. Carriage assembly 11
 is capable of linear motion along rails 12 along radial direction 15,
 relative to medium 22. Carriage assembly 11 comprises a voice coil
 actuator for controlling its coarse tracking positioning along rails 12,
 with respect to medium 22. Carriage assembly 11 faces top input surface
 50a of medium 22.
 FIG. 2-B shows a schematic view of the optics 26 of a preferred storage
 system of the present invention. Optics 26 are used to generate a storage
 light beam 30a, and to direct storage beam 30a onto desired storage
 locations within medium 22. Optics 26 are mechanically coupled to holder
 24 such that storage beam 30a is incident on medium 22 through input
 surface 50a when device 21 is mounted on holder 24. Optics 26 comprise a
 light source 34 for generating beam 30a, and a tunable-focus storage head
 46 in optical communication with light source 34. Storage head 46 directs
 and focuses beam 30a onto desired storage locations 52 within medium 22.
 Light source 34 is preferably a laser with a high enough output power to
 allow altering format hologram 27 in a detectable and localized manner.
 Storage head 46 comprises a high numerical aperture (N.A.) objective lens
 48 facing medium 22, and a dynamic aberration compensator 39 in the light
 path between light source 34 and objective lens 48. Objective lens 48
 generally has a N.A. higher than 0.25, in particular higher than about
 0.4, preferably about 0.5. High numerical apertures are desirable since
 they allow relatively short depths of field, and consequently relatively
 close spacings between adjacent storage locations 52 along the depth of
 the medium. High numerical apertures also allow relatively small spot
 sizes at the focus of beam 30a, and consequently small spacings along a
 track 23 and between tracks 23. Increasing numerical apertures above about
 0.5 or 0.6 may lead to substantially increased complexity in the optics
 required for storage and retrieval, and to relatively stringent tolerances
 on mechanical components.
 Lens 48 is mounted on a dual-axis actuator 47, which controls the focusing
 and fine-tracking position of lens 48 relative to medium 22. The focusing
 actuator controls the vertical (in-depth) motion of lens 48 relative to
 medium 22, both coarsely for accessing different depth layers and finely
 for maintaining lens 48 focused on a desired depth layer. Fine-tracking
 positioning is performed along the radial direction of medium 22, i.e.
 across tracks 23.
 Dynamic aberration compensator 39 dynamically compensates for the variable
 spherical aberration introduced in beams 30a-b by medium 22. The spherical
 aberration in each beam depends on the depth accessed by the beam.
 Aberration compensators are known in conventional optical recording.
 Various dynamic aberration compensators have been described for
 conventional pit-based storage, for example in U.S. Pat. No. 5,202,875
 (Rosen et al). While aberration compensator 39 is shown for clarity as
 separate from objective lens 48 and actuator 47, aberration compensator 39
 may be integrated with lens 48.
 During storage, lens 48 focuses beam 30a at storage locations 52 at desired
 depths within medium 22. Beam 30a causes selective micro-localized
 alterations in hologram 27 at the chosen storage locations 52. A
 description of preferred characteristics of such alterations can be found
 below with reference to FIGS. 3-A and 3-B.
 FIG. 2-C schematically illustrates the optics 126 of a preferred retrieval
 system of the present invention. During retrieval, optics 126 are used to
 generate an input light beam 130a, to direct input beam 130a onto desired
 storage locations within medium 22, and to direct an output beam 130b
 reflected by medium 22 towards a detector 58. The intensity of input beam
 130a is low enough so that beam 130a does not cause substantial deletion
 of format hologram 27.
 Optics 126 are mechanically coupled to holder 24 such that input beam 130a
 is incident on medium 22 through input surface 50a when device 21 is
 mounted on holder 24. Optics 126 comprise a light source 134 for
 generating beam 130a, and a tunable-focus retrieval head 146 in optical
 communication with light source 134. Retrieval head 146 directs and
 focuses beam 130a onto desired storage locations 52 within medium 22, and
 captures and directs output beam 130b to detector 58.
 Light source 134 comprises a laser. Retrieval head 146 comprises a
 high-N.A. objective lens 48 facing medium 22, a dynamic aberration
 compensator 39 in the light path between light source 34 and objective
 lens 48, an optical detector 58 in optical communication with medium 22,
 and beam separation components 38 for directing beam 130a toward medium 22
 while directing output beam 130b toward detector 58. Beam separation
 components 38 separate beams 130a and 130b. Beam separation components 38
 are conventional. Beam separation components 38 comprise a polarizing beam
 splitter (PBS) 54 and a quarter-wave plate 56 situated in the optical path
 of beams 130a-b, between light source 134 and medium 22. Polarizing beam
 splitters and quarter wave plates are used instead of simple
 beam-splitters for reducing losses at the separation elements.
 Detector 58 is a confocal, depth-selective detector comprising spatial
 filtering optics for allowing detector 58 to selectively access only
 storage locations at desired depths within medium 22. Spatial filtering
 optics are well known. The spatial filtering optics preferably include an
 appropriately placed pinhole for selectively allowing only rays reflected
 from an accessed storage location to be directed to detector 58. The
 pinhole blocks stray light from non-accessed regions of medium 22, which
 would otherwise be incident on detector 58.
 During retrieval, input beam 130a is reflected by medium 22 to generate
 output beam 130b. Beams 130a-b are substantially counterpropagating, and
 both pass through surface 50a. Output beam 130b is captured by lens 48 and
 directed by optics 126 to detector 58. The intensity of output beam 130b
 is indicative of the interaction between input beam 130a and medium 22 at
 the accessed storage location 52.
 FIG. 3-A illustrates a localized alteration 62 in hologram 27, stored at
 the focus of storage beam 30a at storage location 52. Hologram 27 is
 magnified for clarity of presentation; in the preferred embodiment,
 alteration 62 extends over tens of fringes 28. Fringes 28 are
 substantially planar, and locally define the direction of back-reflection
 of format hologram 27 at each location 52. The direction of reflection is
 preferably the same throughout medium 22.
 Alteration 62 may be represented as a micro-localized variation in the
 amplitude and/or phase of hologram 27. Alteration 62 is preferably a
 deletion in hologram 27, such that the reflectivity of medium 22 at
 storage location 52 is less than the reflectivity of the surrounding
 intact parts of hologram 27. The depth of alteration 62 can be defined as
 the depth over which its associated index variation is within a given
 factor (e.g. a factor of 2) of the variation at the focus of storage beam
 30a.
 Alteration 62 is preferably stored at the diffraction limit of high-N.A.
 optics. Alteration 62 preferably extends over a depth of less than a few
 tens of microns (e.g. &lt;50 .mu.m), in particular about 20 .mu.m or less.
 The depth of alteration 62 is preferably comparable to the Rayleigh range
 of storage beam 30a. Alteration 62 preferably has a spot (in plane) size
 of less than a few microns (e.g. &lt;5 .mu.m), in particular approximately
 one to two .mu.m. An alteration length of 1 .mu.m corresponds to a readout
 time of tens of ns for a medium speed of tens of m/s. The spot size
 characterizes the width of a track. The spot size may limit the minimal
 intertrack spacing, as well as the data density along a track. Adjacent
 tracks are preferably spaced by a distance at least on the order of the
 alteration spot size. Adjacent alterations along a track are also
 separated by a distance at least on the order of the alteration spot size.
 During storage, beam 30a is focused at high intensity at storage location
 52. The localized high intensity causes the localized deletion of hologram
 27 at storage location 52. During retrieval, beam 130a is focused within
 medium 22 at the depth of location 52. If beam 130a is focused on an
 intact region of hologram 27, the resulting intensity of reflected output
 beam 130b is relatively high. If beam 130a is focused on a deleted part of
 hologram 27, the intensity of the resulting reflected output beam 130b is
 relatively low.
 During readout, input beam 130a is Bragg-matched at its focus to hologram
 27. That is, the wavelength of input beam 130a at its focus is equal to
 the wavelength of hologram 27. As is apparent to the skilled artisan, any
 substantial Guoy shift within beam 130a, shrinkage within medium 22, or
 background index changes during storage within medium 22 are taken into
 account for Bragg-matching beam 130a to hologram 27. For information on
 the Guoy shift see for example Siegman, Lasers, University Science Books,
 Mill Valley, Calif., 1986, p. 682-685. Shrinkage and background index
 changes in photopolymers are well characterized in the art.
 FIG. 3-B shows a side sectional view through medium 22, illustrating a
 preferred relative arrangement of alterations 62 in depth. Multiple
 plate-shaped subvolumes 66 of alterations 62 are stacked along the depth
 of medium 22. Adjacent subvolumes are separated by a center-to-center
 distance on the order of the alteration depth or depth of focus of input
 beam 30a. During storage of alterations 62, storage beam 30a is maintained
 focused within a subvolume at a constant depth, as medium 22 is moved
 relative to beam 30a along a track 23 within the subvolume. During
 retrieval, medium 22 is moved at constant velocity with respect to input
 beam 130a.
 Consider two alterations 62', 62" situated in different (e.g. adjacent)
 subvolumes, at different depths within medium 22. When storage beam 30a is
 focused at the location of alteration 62', the out-of-focus parts of
 storage beam 30a also illuminate and may partially delete the region of
 hologram 27 surrounding the storage location of alteration 62". The
 out-of-focus light used for accessing the location of alteration 62' can
 reduce the contrast achievable for reading out of alteration 62". The
 degradation of optical properties at one depth due to data storage at
 other depths within medium 22 can be characterized by the "scheduling
 loss" of the system. Scheduling losses can limit the number of data layers
 that may be stacked along the depth of medium 22. An optically non-linear
 storage material can be used to reduce scheduling losses. Scheduling
 losses can also be reduced by offsetting vertically-adjacent tracks or
 alterations in the radial direction, such that alterations in adjacent
 depth layers are not vertically aligned.
 FIG. 4 illustrates a side sectional view of medium 22 in an alternative
 embodiment of the present invention. A format hologram 27' stored within
 medium 22 comprises a, plurality of distinct storage subvolumes 80 stacked
 along depth 25. Each storage subvolume 80 is characterized by the presence
 of reflective fringes of format hologram 27, while the space between
 adjacent subvolumes is characterized by a relative lack of variation in
 the index of refraction within medium 22. The variation of hologram 27'
 along depth 25 is characterized by two closely-spaced spatial frequencies.
 The difference between the two frequencies defines the spacing between
 adjacent storage subvolumes 80. Format hologram 27' can be stored through
 the interference of two pairs of beams incident on medium 22, each pair at
 a distinct frequency. Each pair is similar to the beam pair described with
 reference to FIG. 1.
 In one embodiment, the wavelength of the input beam used during retrieval
 is different from that of the storage beam, and chosen to minimize the
 effect of the input beam on the format hologram. The two wavelengths are
 chosen such that the storage medium is more photosensitive at the storage
 wavelength than at the retrieval wavelength. Using wavelengths
 corresponding to different medium photosensitivities can facilitate
 altering the format hologram at the storage wavelength while ensuring that
 the retrieval light does not substantially alter (e.g. delete) the format
 hologram.
 Parallel readout can be accomplished by using a light source comprising a
 plurality of mutually incoherent lasers aligned in close proximity. The
 lasers generate spatially separated, mutually incoherent input beams. The
 input beams are imaged onto a radial line such that each input beam is
 focused on one of a number of adjacent tracks within the medium. A
 detector comprising multiple independent aligned detecting elements is
 then used for data retrieval. Each of the reconstructed output beams is
 incident on one of the detecting elements. Since the input beams are
 mutually incoherent, they do not interfere even if their corresponding
 tracks are closely spaced.
 It will be clear to one skilled in the art that the above embodiments may
 be altered in many ways without departing from the scope and spirit of the
 invention. The storage medium material need not be a photopolymer. For
 example, various storage materials known in the art can be suitable for
 the present invention, including photopolymers, photosensitive glasses,
 and photorefractive materials. Various mechanisms for the interaction
 between the storage light beam and the medium are suitable for altering
 the format hologram. For example, the storage light beam may selectively
 alter the format hologram by physically altering the medium structure. The
 alteration may directly depend on the maximal light intensity (power/area)
 within the medium, or on the fluence (energy/area) of the light beam. The
 material may be sensitized by illumination at one wavelength for
 selectively altering the format hologram using localized light of another
 wavelength.
 The format hologram need not be completely uniform in plane, nor completely
 regular in its depth variation. The format hologram may comprise distinct,
 independently-recordable and addressable sectors. The format hologram need
 not be a phase hologram, and may be an absorption hologram. The
 alterations in the format hologram need not be uniform round spots. As the
 disk continuously rotates, a continuous alteration can be written, and the
 intensity of the writing beam can be varied in time to store information
 as micro-localized variations in the format hologram according to a
 suitable modulation code.
 Multiple discrete amplitude/phase levels for the alterations can be used
 for digital gray scale storage. Continuous levels can be used for analog
 storage. Deletions of the format hologram can be complete or partial
 deletions. Various track arrangements, both in plane and in depth, can be
 used; such an arrangement includes a 3-D Cartesian array. The storage
 medium need not be disk-shaped; data may be stored in a Cartesian
 geometry, with the heads controlled by x-y stages. Various mounting and
 actuating (e.g. rotary/linear, horizontal and vertical) arrangements for
 the heads may be suitable. The storage medium may be moved relative to a
 vertically-fixed head to bring different depths in focus. The storage
 medium need not be packaged in a disk-like storage device; various other
 storage devices (e.g. cartridges or cards) may be suitable. Various types
 of lasers can be used as light sources, including diode, solid state, and
 other types of lasers. The light source may include a non-linear
 frequency-converter in addition to a laser.
 It will be apparent from the foregoing that, while particular forms of the
 invention have been illustrated and described, various modifications can
 be made without departing from the spirit and scope of the invention.
 Accordingly, it is not intended that the invention be limited, except as
 determined by the following claims and their legal equivalents.