Fluorescent optical memory

Details Fluorescent optical memory Fluorescent optical memory

2000-03-21

2001-09-18

McPherson, John A. (Department: 1756)

Radiation imagery chemistry: process, composition, or product th

Imaging affecting physical property of radiation sensitive...

Radiation sensitive composition or product or process of making

C430S270150, C430S139000, C430S321000, C430S945000, C369S275300, C369S284000, C369S288000, C428S064400

Reexamination Certificate

active

06291132

ABSTRACT:

FIELD OF INVENTION
The present invention relates to optical memory storage of digital information at high volume density of data. In particular the present invention refers to an optical memory device which employs a fluorescent medium, to a method of manufacturing such a memory device and particular construction thereof implementing said fluorescent medium.
BACKGROUND OF THE INVENTION
There is a growing demand for a cheap and reliable memory device for the storage of digital information for computers, video systems, multimedia etc. This device should have a data storage capacity in excess of 10
11
bytes, fast access time, high transfer rate and long term stability. Today the capacity of available digital information storage means based on optical and magnetic methods is limited to about 5·10
8
bytes per square inch. Two-dimensional (2-D) memory devices such as optical and magneto-optical discs, magnetic discs and magnetic tapes are well known and represent most popular memory carriers for the storage of digital information. In optical 2-D memory devices the information is usually written as local variations of thickness, reflectivity, refractive index, or absorption coefficient of the medium. Storage devices, based on optical methods have advantages over magnetic ones because of less strict requirements of the components and environment. The possibility for parallel writing of information, i.e., simultaneous recording of information over the medium's surface, is another advantage of optical memory carriers, which is especially important for mass production. These carriers are usually formed as optical discs suitable for reading (CD-ROM) or write-once-read-many (CD-WORM) modes of operation. Their description can be found, for example, in
The Compact Disc Handbook
by Ken C. Pohlmann.
Unfortunately, the known-in-the-art two-dimensional optical memory devices have an important intrinsic disadvantage associated with the fact that their ultimate pixel capacity is diffraction limited by a factor of 1/&lgr;
2
where &lgr; is the wavelength of light employed in modern lasers. A certain increase in capacity can be achieved by special measures such as a “super resolution” at a fraction of a wavelength. However, implementation of this measure is associated with the necessity for very precise and sophisticated optical, mechanical and electronic equipment as well as in a high quality medium, which obviously makes this approach expensive and less feasible.
The main efforts to create a 3-D optical memory disc have been directed at developing of CD/CD-ROM-like optical devices, where reading is based on the modulation of a reflected beam. The modulation is the result of interference on the recorded pattern (pits) for CD/CD-ROM or variation of reflectivity for CD-R devices. The great advantages of this concept and method which has been developed since the 1970s by the electronic and computer community result in simple, reliable technology for the mass production of cheap optical carriers as well as pick-ups to play back the stored audio, video, and data information. A known method of improving the capacity of 2-D optical memory carriers is the stacking of two or more discs. The commercially available carriers implementing this approach are known as DVD's and are described, for example, in
Scientific American
July 1996. The disadvantage of this approach is associated with multiple reflections which occured between reflective surfaces, and led to power losses during the propagation of reading and reflected beams through the layers. The interference of light beams reflected from different layers results in beam distortions due to optical aberrations. The aberrations appear when the optical path within the storage media is changed to read different planes of stored information. High quality optical adhesives are required to assemble a stack of discs so as to reduce the influence of aberrations, bubbles, separations and inclusions and to ensure that there will not be any mechanical, thermal and chemical impact on the surface of the stacked discs. Due to the above mentioned requirements the information storage capacity of commercially available multistacked discs is limited in practice to 10
10
bytes. These carriers are composed of 2 discs with 2 information layers in each. The DVD optical discs are attached together at their back sides, and it is possible to achieve a maximum total storage capacity of around 2·10
10
bytes.
An alternative method of optical data storage is based on three-dimensional (3-D) recording. It is obvious that 3-D recording can dramatically increase the storage capacity of the device. There are known-in-the-art 3-D recording methods, based, for example, on 3-D volume storage by virtue of local changes of the refractive index of optical media. The 3-D writing to and reading from the bulk media has been widely reported (J. H. Stricler, W. W. Webb, Optics Lett., 16, 1970, 1991; H. Ueki, Y. Kawata, S. Kawata, Applied Optics, 35, 2457, 1996; Y. Kawata, R. Yuskaitis, T. Tanaka, T. Wilson, S. Kawata, Applied Optics, 35, 2466, 1996). The operation of such device is based on using local changes of the refractive index of the optical media. These local variations of refractive index result in the birefiingence and variations of polarization of the reading beam transmitted through the media. The variations are detectable and can be interpreted as a binary code. Among the drawbacks of this approach is the very weak signal value requiring a high power laser and highly sensitive detectors. The 3-D regular structure of the information carrier acts as a birefringent material at a macro scale introducing the non informative depolarization and defocusing of the transmitted beam. Variations of the refractive index introduce the phase modulation located in the adjacent layers, diffraction and power losses. The measurement of the transmitted beam requires two optical heads (transmitting and receiving) from both sides of the caer. This solution is very complicated and expensive since it requires simultaneous alignment of the heads to a diffraction limited spot, especially while taking into account the variation of the required optical path, medium inhomogeneity, and carrier/heads movement perturbations. The data recording is possible only in a sequence fashion, bit by bit using a laser—thus, it does not allow the implementation of cheap replication methods, such as mask lithography. The solidification/polymerization process results in uncontrollable material deformation during the recording procedure. The associated stresses may cause information distortion. Thus, all the above-mentioned drawbacks will put obstacles in the way of converting this approach to be realized into the practical 3D-memory device.
There are different works and patents related to this field, e.g., J. Russell, U.S. Pat. Nos. 4,163,600; 4,219,704; 5,278,816; and M. Best et al, U.S. Pat. Nos. 5,586,107; 5,255,262;. In order to provide a stable, reliable reading from different stacked discs, layers or surfaces, several methods have been invented, including layers with different reflective spectra, read by different wavelength lasers (Frisiem A. et al, U.S. Pat. No. 5,526,338), objective lens aligrnent from one layer to another (H. Rosen et al, U.S. Pat. No. 5,202,875), spherical aberration correction achieved simultaneously for different layer depth (W. Imaino, U.S. Pat. No. 5,373,499), changeable layer's transmission, reflectivity and/or polarization (J. Russell; U.S. Pat. No. 5,465,238), guide beam and scanned beam optical head configuration together with special guide plane utilization (A. Holtslag, U.S. Pat. No. 5,408,453). In Ota's patent U.S. Pat. No. 5,559,784 several methods of non fluorescent multilayer disc manufacturing are described, including mechanical, spin coating and photolithography techniques. A structure of multilayer optical discs is suggested, and a method of data recording in the form of refractive pits within a completely transparent material is described. This approach seems

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