Optical: systems and elements – Holographic system or element – Hardware for producing a hologram
Reexamination Certificate
2002-07-31
2004-04-13
Dunn, Drew (Department: 2872)
Optical: systems and elements
Holographic system or element
Hardware for producing a hologram
C359S010000, C359S024000, C359S025000, C430S001000, C365S125000
Reexamination Certificate
active
06721076
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the general field of holographic storage systems and methods. More specifically the invention relates to a system and method for reflective holographic storage with associated multiplexing techniques.
BACKGROUND
General holographic storage systems are discussed in “Holographic Memories”, by Demetri Psaltis et. al.,
Scientific American
, November 1995, which is hereby incorporated by reference. Holography is also discussed in the text Holographic Data Storage, by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Eds., copyright 2000, Springer-Verlag which is hereby incorporated by reference. The basic principles of holography involve the recording of an interference pattern formed between two beams of light, referred to as an object beam and a reference beam. The object beam is encoded with data in a two dimensional pattern. The reference beam is used to form the interference pattern with the encoded object beam and is subsequently used to reconstruct the data by illuminating the recorded pattern.
In a volume holographic storage medium, a large number of holograms can be stored in the same volume region using multiplexing techniques. There are several techniques for multiplexing holograms, including shift multiplexing, angle multiplexing, wavelength multiplexing, correlation multiplexing and phase multiplexing. Volume holography uses a thick recording medium, where the thickness dimension is associated with Bragg selectivity in the movement of the holographic storage medium in shift multiplexing or the angle change in angle multiplexing.
Shift multiplexing is a volume holography method for storing a plurality of images within a single holographic medium. Such shift multiplexing is discussed in “Shift Multiplexing with Spherical Reference Waves”, pages 2403-2417, by George Barbastathis et al,
Applied Optics
, Vol. 35, No. 14, May 10, 1996. Shift multiplexing generally involves the high density packing of successive holograms in an x-y array. Overlapping holograms produced by shifting the medium in the grating direction are differentiated by first-order Bragg selectivity.
FIG. 7
illustrates the basic setup of a typical prior art holographic system. The holographic storage system
700
includes a laser light source
710
. The coherent light from the laser light source
710
is directed to a beam splitter
715
, such as a polarizing beam splitter cube, which splits the light from laser light source
710
into a reference beam
720
and an object beam
725
. Reference beam
720
is reflected by a turning mirror
730
to a lens
735
. Object beam
725
is directed to a turning mirror
745
which directs the object beam to a Spatial Pattern Encoder
755
, which encodes the object beam with data (an image). The object beam is then directed to a holographic storage media
750
with lens
780
. Pattern encoder
755
may be a spatial light modulator (“SLM”), or any device capable of encoding the object beam, such as a fixed mask, or other page composer. The encoded object beam
725
is then directed to lens
780
that focuses the encoded object beam
725
to a particular site on the holographic storage media
750
. Successive overlapping holograms may be recorded in a shift multiplex system by translating the holographic storage media
750
in a shift multiplex direction
788
.
During readout of holograms previously stored in the holographic storage media
750
, object beam
725
is blocked from transmission and a reference beam is projected at the same angle to the same spot on the holographic storage medium on which the desired information was previously stored. Diffraction of the reference beam with the previously stored hologram generates a reconstruction beam
782
that reconstructs the previously stored hologram. The reconstructed beam is transmitted towards imaging lens
784
that directs and images the reconstruction beam onto the plane of the optical detector
786
. Optical detector
786
may be a conventional photodiode array, charge coupled device or other suitable detector array that transforms the encoded page into digitized data. In the prior art holographic storage system
700
, spatial light modulator
755
and detector
786
are on opposite sides of holographic storage media
750
. Lens
780
and lens
784
are also on opposite sides of holographic storage media
750
, and are required to image the encoded object beam
725
onto the holographic storage media
750
and image the reconstruction beam
782
onto the detector
786
, respectively. Lens
735
is required to image the reference beam
735
onto the holographic storage media
750
.
Another prior art holographic system is described in “Holographic 3-D Disk using In-line Face-to-Face Recording”, by Kimihiro Saito and Hideyoshi Horimai. The system described utilizes a photosensitive layer with a reflecting unit underneath. A reference beam passes through a first region of the media downward and a second region upwards. The direction of the information beam is opposite to that of the reference beam. Intersection between the reference beam and information beam results in a reflection type hologram. Shift multiplexing can be utilized for multiple recording.
Angle multiplexing is a volume holography method for storing a plurality of images within a single photorefractive medium. Such angle multiplexing is discussed, for example, in “Holographic Memories”, by Demetri Psaltis et. al.,
Scientific American
, November 1995, and by P. J. van Heerden in, “Theory of Optical Information Storage In Solids,”
Applied Optics
, Vol. 2, No. 4, page 393 (1963). A typical system employing angle mutiplexing described in Holographic Data Storage, by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Eds., pages 343-397, copyright 2000, Springer-Verlag. Angle multiplexing generally involves storage of multiple pages of data in the same photorecording medium by altering the angle of the reference beam entering the media during storage of each page while maintaining the position of the object beam. Each hologram is stored in the same volume and is differentiated by Bragg selectivity. Bragg selectivity during angle multiplexing is described in Holographic Data Storage, pages 30-38 by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Eds., copyright 2000, Springer-Verlag. Any of the recorded holograms can be viewed by illuminating the photorecording medium with a reference beam set at the appropriate angle.
FIG. 8
illustrates a prior art system geometry in which the encoded object beam and the recording reference beam are counterpropagating. Such a system is described in “Volume Holographic Multiplexing Methods”, by G. Barbastathis and D. Psaltis, published in Holographic Data Storage, pages 22-59, by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Eds., copyright 2000, Springer-Verlag, which is expressly incorporated herein by reference. This geometry is often preferred in wavelength multiplexed systems because it maximizes the optical wavelength Bragg selectivity. However, the prior art system requires that the object beam optics
810
and reference beam optics
815
be on different sides of the holographic storage media
820
in order for the beams to be counterpropagating. Thus, the system is not of a compact design since components are required on both sides of the holographic storage media
820
.
Although the prior art systems offer the ability to store a large number of holograms within a holographic storage media, there are disadvantages to existing systems. Although providing for storing of multiple overlapping images, shift multiplexing requires a relatively thick recording medium. However, as the thickness of the photopolymer increases, recording of holograms is made difficult both by the absorption of light by the photosensitizer, and by the low viscosity of the photopolymer before exposure. Recording thick polymer holograms is discussed in Holographic Data Storage, pages 172-208, by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Eds., copyright 2000, Springer-Verlag. In addition, as the r
Anderson Ken
Curtis Kevin
King Brian
Boutsikaris Leo
Dunn Drew
InPhase Technologies Inc.
Morrison & Foerster / LLP
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