Optical: systems and elements – Holographic system or element – Fourier transform holography
Reexamination Certificate
2002-09-27
2004-12-14
Boutsikaris, Leo (Department: 2872)
Optical: systems and elements
Holographic system or element
Fourier transform holography
C359S031000, C359S032000, C359S035000, C365S216000
Reexamination Certificate
active
06831762
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 holographic storage with optical folding.
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.
There are several typical holographic storage geometries and imaging systems. A Fourier-plane, or 4-F, geometry is one such imaging system, where the spatial light modulator is Fourier-transformed-onto the holographic material and the reconstruction in turn is Fourier-transformed onto the detector array. In this architecture, a first fourier transform lens of focal length f
1
is inserted between the spatial light modulator and the holographic storage media, and a second similar fourier transform lens of focal length f
2
is inserted between the holographic storage media and the detector array. The spatial light modulator and the first principal plane of the first lens are separated by a distance f
1
, and the second principal plane of the first lens and holographic storage media are separated by a distance f
1
. On the detector side of the storage media, the holographic storage media and the second principal plane of the second lens are separated by a distance f
2
, and the first principal plane of the second lens and the detector array are separated by a distance f
2
. Thus, the principal planes of the two lenses are separated by the sum of their focal lengths, with a 2-D input and output plane located one focal length in front of and behind the lens pair. The magnification of the 4-F system is given by f
2
/f
1
. 4-F systems are described in Holographic Data Storage, by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Eds., pages 28-30 and 429-431, copyright 2000, Springer-Verlag. The distance between the spatial light modulator and the first fourier transform lens, and the distance between the second fourier transform lens and the detector array is commonly referred to as the back focal length (or BFL). In certain designs, high performance fourier transform lens are utilized which operate with a high modulation transfer function. Such lenses provide a high signal-to-noise ratio at the storage medium and the detector array, resulting in a lower bit error rate at readout. However, the back focal length of such lenses is large, requiring a larger physical space in prior art 4-F systems.
FIG. 10
illustrates the basic setup of a typical prior art 4-F holographic system. The holographic storage system
700
includes a laser light source
710
. The coherent light from the laser light source
710
is split into a reference beam and an object beam. The reference beam and object beam are directed to a holographic storage medium to record and recall holographic information. Light generated by laser light source
710
is directed to a beam splitter
715
, such as a 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 pattern encoder
755
, which encodes the object beam with data. The object beam is then directed to a holographic storage media
750
with a lens
780
of focal length f
1
. 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 pattern encoder
755
receives digitized data and imposes that pattern onto the object beam
725
, such that the object beam
725
comprises an array of dark and light spots. 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
. Pattern encoder
755
is located a distance BFL
1
, (back focal length) from lens
780
, and holographic storage media
750
is located a distance FFL
1
, (front focal length) from lens
780
.
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
directs and images the reconstruction beam onto the plane of the optical detector
786
. Imaging lens
784
has a focal length f
2
. Imaging lens
784
is located a distance FFL
2
from holographic storage media
750
, and optical detector
786
is located a distance BFL
2
from imaging lens
784
. 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.
Although the prior art 4-F holographic systems offer the ability to accurately store holograms within a holographic storage media, there are disadvantages to existing systems. Existing systems require that the pattern encoder, first fourier transform lens, holographic storage media, second fourier transform lens, and optical detector be separated by a distance approximately f, requiring a certain physical space to house the optical components. Such systems often do not fit in standard drive envelopes.
Thus, there has been a need for improvements in the design of holographic storage systems. More specifically, there has been a need for more compact holographic storage systems.
SUMMARY OF THE INVENTION
The present invention provides a solution to the needs described above through a system and method/for holographic storage with optical folding.
In a first embodiment of the invention, the invention presents a system for recording and reading out holograms in a storage medium. The system comprises a pattern encoder, a first fourier transform lens with a focal length f
1
, a second fourier transform lens with a focal length f
2
, a detector array, a first prism located between the pattern encoder and the first fourier transform lens, and a second prism located between the second fourier transform lens and the detector array. The optical length between the pattern encoder and the first fourier transform lens through the first prism is equal to the back focal length BFL
1
, and the optical path length between the second fourier transform lens and the detector array through the second prism is equal to the back focal length BFL
2
.
A further embodiment of the invention presents a method for directing an object beam in a holographic system to a storage medium. The method comprises encoding an object beam with data utilizing a pattern encoder and directing the encoded object beam to a fourier transform lens with focal length f
1
from the pattern encoder through a prism, where the optical path length between the spatial light modulator and the fourier transform lens through the prism is equal to back focal length BFL
1
. The encoded object beam is then fourier transformed to a holographic storage medium located a distance equal to front focal length FFL
1
from the fourier transform lens.
An embodiment of the inv
Schuck Miller
Shuman Curt
Tackitt Michael
Wegner Aaron
Boutsikaris Leo
InPhase Technologies Inc.
Morrison & Foerster / LLP
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