Optical: systems and elements – Holographic system or element – Using modulated or plural reference beams
Reexamination Certificate
1999-07-29
2001-02-20
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Holographic system or element
Using modulated or plural reference beams
C359S010000, C359S024000, C359S001000
Reexamination Certificate
active
06191875
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to holographic storage, in particular holographic storage processes such as phase correlation multiplexing.
2. Discussion of the Related Art
Holographic memory systems involve the three-dimensional storage of holographic representations (i.e., holograms) of data elements as a pattern of varying refractive index and/or absorption imprinted into the volume of a storage medium. Holographic memory systems are characterized by their high density storage potential and the potential speed with which the stored data is randomly accessed and transferred.
In general, holographic storage memory systems operate by combining a data-encoded object beam with a reference beam to create an interference pattern in a photosensitive storage medium. (See, e.g., D. Psaltis et al., “Holographic Memories,”
Scientific American,
November 1995, the disclosure of which is hereby incorporated by reference.) The interference pattern induces material alterations in the medium that create a hologram. The formation of the hologram in the storage medium is a function of the relative amplitudes and polarization states of, and phase differences between, the object beam and the reference beam. It is also highly dependent on the incident beams' wavelengths and the angles at which the object beam and the reference beam are projected into the storage medium.
Holographically stored data is reconstructed by projecting into the medium a reference beam at the same angle, wavelength, phase, and position as the reference beam used to produce the hologram. The hologram and the reference beam interact to reconstruct the stored object beam. The reconstructed object beam then is detected, e.g., using a photodetector array, and the recovered data is post-processed for delivery to output devices.
Typically, the dynamic range of the holographic storage medium is larger than what is needed to store a single hologram with an acceptable signal-to-noise ratio. Therefore, it is often desirable to multiplex a number of holograms at one location to attain greater storage density. One technique for multiplexing is phase correlation multiplexing, as discussed in co-assigned U.S. Pat. No. 5,719,691, the disclosure of which is hereby incorporated by reference. In phase correlation multiplexing, correlation sensitivity and Bragg selectivity are used to differentiate overlapping holograms within a medium. Correlation selectivity relies on the differences in amplitude, phase, and angle content of the reference beam generated by the relative shift of the storage medium with respect to the reference beam. See also co-assigned U.S. Pat. No. 5,703,705, relating to tilt multiplex holography.
While holographic processes such as phase correlation multiplexing provides useful results, variations and improvements in such processes are actively being sought to provide increased storage density and more accurate readout of stored holograms.
SUMMARY OF THE INVENTION
The invention provides an improved process for holography, in which the reference beam used for storage and readout has a correlated phase content, as specifically defined below. In particular, it was found that reference beams having random phase content limited the attainable storage density and the accuracy of readout of stored holograms due to uncontrolled fluctuations in the correlation selectivity. By contrast, a reference beam having correlated phase content provides accurate shift selectivity, the ability to tailor the reference beam spectrum to minimize crosstalk between neighboring holograms, and mitigation of limitations inherent in the holographic system or holographic medium. Improved storage density and readout accuracy are thereby attained.
In one embodiment, the reference beam having correlated phase content is provided by directing a plane wave, coherent beam of light through particular optics, e.g., a phase mask having an linearly-varying distribution of pixels (referred to as a chirped mask). Such masks provide correlation by introducing complex phase content onto the wavefront of an optical beam by generating spatially dependent optical delay and amplitude modification. It is also possible to provide a reference beam having correlated phase content by other techniques, e.g., using an array of point sources such as a laser array, optionally coupled with an optical device such as a lenslet array.
REFERENCES:
patent: 5703705 (1997-12-01), Curtis
patent: 5719691 (1998-02-01), Curtis
patent: 5892601 (1999-04-01), Curtis et al.
patent: 5940514 (1999-08-01), Heanue
Curtis Kevin Richard
Littlewood Peter B.
Mitra Partha Pratim
Wilson William Larry
Boutsikaris Leo
Lucent Technologies - Inc.
Rittman Scott J.
Spyrou Cassandra
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