Optical: systems and elements – Holographic system or element – Fourier transform holography
Reexamination Certificate
1998-03-30
2001-08-28
Schuberg, Darren (Department: 2872)
Optical: systems and elements
Holographic system or element
Fourier transform holography
C359S020000, C359S011000, C369S103000
Reexamination Certificate
active
06281993
ABSTRACT:
BACKGROUND OF THE INVENTION
Optical information processor and storage of pixelated data pattern in a Fourier plane of a lens is achieved by a plurality of optical elements operating with a spatial light modulator. A laser light source illuminates a data pattern with a wavefront that is created by a phase shifting element to effect a phase shift. The optical information can be effectively processed or stored, for instance, in a holographic data storage system.
1. Field of the Invention
This invention is concerned with optical information processing or storage of a pixelated data pattern. In particular, the invention is directed to phase shifting this data in a Fourier plane of an optical system, preferably a lens. This invention is directed to the illumination of the data pattern with a wavefront that is created by a particular phase shifting element to effect optical data processing and optical data storage, particularly holographic data storage.
2. Description of the Related Art
Digital holographic optical data storage devices for high density and high data rate storage devices and video server is highly desirable. It is also desirable to process optical data in a manner that is highly efficient and effective even when there is no storage involved. Such systems are optical correlators, and it is desirable for these to permit for efficient high density correlation.
A known storage device involves storing the data as a series of Fourier transform holograms in a photorefractive or photochromic crystal or photopolymer. Each data page is constructed using a spatial light modulator (SLM) which is a liquid crystal display or a flexible silicon mirror array. The digital 1's and 0's that are to be stored are represented by individual pixels of the SLM respectively. Due to the regular spacing of pixels on the SLM and to the fact that coherent light is used in recording the holograms, large intensity spikes appear in the Fourier transform plane where the holograms are recorded. These spikes are extremely detrimental to the hologram recording process and significantly reduce the dynamic range of the material available for recording holograms and, therefore, substantially reduce the amount of data that can be stored per unit volume.
The desired distributions of intensity in the Fourier transform plane for the most likely devices namely having Gaussian distributions or uniform distributions. These distributions may be achieved under certain critical limiting circumstances with a phase shifting element consisting of randomly distributed phase pixels having either 0 or a fraction of &pgr; phase shifts. However, the phase pixels are desirably of the same size and periodicity as the SLM pixels and must accurately register with the SLM pixels. They must also be very accurate multiples of &pgr; phase shifts for the best performance.
This type of discrete phase step mask should either be in very close proximity or be imaged to be in very close proximity with the SLM to avoid diffraction effects which can deteriorate the reconstructed image of the data. It is difficult and expensive to make and align such a system. Several variations of this general type of phase shifting element have been proposed but all have similar difficulties.
The invention provides a set of phase shifting element designs which avoid most of the difficulties of manufacturing, positioning and alignment of random phase shifting elements but which allow the light intensity in the Fourier plane to be distributed in a more uniform manner. These phase shifting elements should be inexpensive, easily aligned and should be capable of being incorporated directly on the SLM or can be designed to work with a reflective SLM.
During the 1970s and early 1980s, considerable work had been done on random phase masks for Fourier transform holographic storage. A number of researchers investigated the use of discrete random phase steps. C. B. Burckhardt, “Use of a Random Phase Mask for the Recording of Fourier Transform Holograms of Data Marks,” Appl. Optics, 1970, 9, 695-700; B. Hill, “Some Aspects of a Large Capacity Holographic Memory,” Appl Optics, 1972, 11, 182-191; W. C. Stewart, “Random Phase Data Mask: Fabrication Tolerances and Advantages of Four Phase Level Marks,” Appl. Optics, 1972, 11, 604-608; W. J. Dallas, “Deterministic Diffusers for Holography,” Appl. Optics, 1973, 12, 1179-1187; and Y. Nakayama, “Linear Recording of Fourier Transform Holograms Using a Pseudorandom Diffuser,” Appl. Optics, 1982, 21, 1410-1418. The contents of all the references herein are incorporated by reference herein.
Two level masks consisting of randomly distributed phase mask with 0 and &pgr; phase shifts for each data pixel were studied. In addition, four, six and higher level masks were studied and compared with the two level masks. These studies showed that the random phase mask could smooth the Fourier transform of an image and retrieve the data with good image fidelity if the phases were placed at the data mask location and if the phase pixels are exactly aligned and registered with the data mask. However, it is extremely difficult to place convectional phase masks exactly at the SLM and alignment and spacing is an issue that causes the image to be degraded by diffraction effects.
Continuous random phase masks were studied by D. G. Esaev, “Continuous Random Phase mask,” Sov. Phys. Tech. Phys., 1977, 9, 1150-1152, which is incorporated by reference herein. Esaev showed that continuous random phase pixels would accomplish the same smoothing. A. Iwamoto, “Artificial Diffuser for Fourier Transform Hologram Recording,” Appl. Optics, 1980, 19, 215-220, which is incorporated by reference herein, proposed a phase mask of random pixels but where the phase variation is of a higher spatial frequency than the data pixels and where the phase variation is continuous and smooth rather than having discrete steps. However, the phase pixels in both these masks have the same alignment, registration and interference difficulties as the two and multiple level masks.
This field of phase masks research seems to have been abandoned during the 1980s and has not been of interest until the 1990s when the enabling technologies required for holographic storage have finally become available.
The invention seeks to use phase shifting elements in holographic storage so as to provide an enhanced storage system, product and procedure.
SUMMARY OF THE INVENTION
To overcome the problems discussed above, and to overcome other problems in the art, the present invention discloses a phase shifting device which is provided for use in Fourier transform holographic data storage. This phase shifting element has linear variations and phase may be placed at different positions in the optical train so that it does relate to the SLM effectively. The phase shifting element of the invention is directed to creating a substantially linear phase shift and revolution symmetry around the optical axis of illumination of a data mask. This phase shifting element includes an axicon. An axicon is a device which relates to a cone configuration.
Other phase shifting elements in the category of the invention include a multi-fold phase shifting element. This can be a mask which is substantially linear in its characteristics. Thus, a linear phase shifting element would act to cause a linear phase shift, namely, a shift of the peak at the Fourier plane. The invention is concerned with a mask which effects a linear phase shift.
Ideally, such a phase shifting device creates a substantial revolution symmetry. The phases could be 2, 3, 4, 5, 6, 7, 8 fold, etc., namely, an integer number. The device for effecting this infinite fold increase is termed the axicon.
The axicon, in a side view cross-section, is formed by infinite straight edges which extend from the upstream side of the phase shifting element towards the pointed side, namely, the downstream side. This configuration can be seen in the evolvement of the phase shifting element from the two-fold version to the multi-fold phase shifting element format.
Bernal Maria-Pilar
Coufal Hans Jürgen
Grygier Robert Keith
Jefferson Carl Michael
Oesterschoze Egbert
Assaf Fayez
Berthold Thomas R.
International Business Machines - Corporation
Martin Robert B.
Schuberg Darren
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