Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2000-04-15
2003-06-03
Tran, Thang V. (Department: 2653)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044370, C369S094000
Reexamination Certificate
active
06574174
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains generally to systems and methods for optical data storage. More specifically, the present invention relates to an optical data storage system utilizing multi-layered optical storage media comprising a single dedicated servo layer and a plurality of data layers, with each data layer providing an areal density comparable to that of conventional single data layer media. Separate servo and read-write laser beams operate at dual foci within the media, and separate dedicated and embedded servo systems, associated with the servo and read-write beams, provide focus and tracking error correction.
2. The Background Art
Optical information storage technologies have provided increasing storage densities over the years. The demand for greater optical storage densities has been persistent, and various approaches to increased optical storage densities have been considered. Conventional far-field techniques for reading and writing optical media utilize a laser beam focused onto the data plane of an optical medium by an objective lens. For a laser beam of wavelength &lgr; and an objective lens with a numerical aperture NA, a read/write spot size of approximately &lgr;/2NA is obtained. Conventional techniques currently allow single data layer optical media having storage capacities of between about 2.6 GB and about 4.77 GB in currently used 120 mm DVD optical disks.
Diffraction limitations imposed on the read/write spot size by the light wavelength and numerical aperture (NA) of the focusing optics provide limitations on optical media storage capacity. Increasing the NA of the focusing objective lens to greater than approximately 0.6 results in rapid increases sensitivity to tolerances and results in beam aberrations. Use of shorter wavelength semiconductor lasers will allow increased storage densities in the future, but shorter wavelength laser devices have so far tended to have limited output powers, limited operational temperature ranges, and are subject to materials limitations which have so far resulted in poor reliability and relatively rapid deterioration. The shorter wavelength lasers also reduce wavelength tolerance.
One approach to increased optical storage densities has been through development of near-field optical data storage techniques, which require the use of radiation source apertures and distances on the order of generally less than the wavelength &lgr; of the radiation source to allow high storage densities. One near-field technique involves use of a solid immersion lens (SIL) positioned between the objective and the optical medium to provide an increase in NA which is proportional to the refractive index of the SIL material. The use of a SIL, however, is subject to the refractive index limitation of SIL materials. Still another near-field method utilizes tapered optical fibers with metallized sides. While tapered fibers have provided small spot sizes, they are severely limited in output power, and are subject to catastrophic breakdown at the emission aperture. Perhaps the most important drawback to near-field technologies, however, is imposed by the necessary close spacing of the optical medium and light aperture, which requires the use of a flying head. The flying optical head, using a SIL or tapered fiber, adds cost and complexity to storage systems, and the flying height of the head can result in head/disk contact and poor reliability. These problems do not occur with far-field systems.
Another approach to increased optical data storage density has been through use of multiple data layers on a single substrate. This is most easily achieved by placing a single data storage layer on each side of a substrate to provide a dual sided optical medium having effectively twice the storage density of a single-sided optical medium. Dual sided media, however, inconveniently require that the optical disk be “flipped” in order to read the opposite side. Dual optical heads can be used with the media to avoid flipping the medium, but result in substantially higher drive costs.
A more attractive multi-layer optical medium would utilize multiple data layers which are addressable from a single side of the optical medium. However, the reading and writing of an underlying data layer through an overlying outer data layer or layers on a single sided medium introduces numerous complexities. Reduced optical transmission to an underlying data layer through overlying layers, potential cross-talk between adjacent data layers, low signal-to-noise rations, and spherical aberration introduced by the thickness of multiple layers, have presented serious limitations to multi-layered optical media. Heretofore, the only commercially useful single side, multi-layer optical medium has involved dual stamped substrates which are sandwiched together with a spacing of about 60 microns, with substantial de-rating (by a factor of two or more) of the inner and/or outer substrate being required to avoid spherical aberration. The de-rating of the inner data layer results in only a limited increase in areal storage density compared to single side, single layer media. Further, the optical transmission and spherical aberration considerations noted above have limited such media to only two data layers.
There is accordingly a need for an optical data storage system and method that utilizes multiple data layers on a single substrate which allows the same storage capacity on each data layer as is available in single data layer optical media, which provides more than two data layers addressable from a single side of the medium, which provides good optical transmission to underlying data layers through outerlying data layers, which avoids cross-talk between adjacent data layers, and which does not require spherical aberration correction. The present invention satisfies these needs, as well as others, and generally overcomes deficiencies found in currently available optical data storage systems.
SUMMARY OF THE INVENTION
The present invention is an optical information storage system using optical storage media including multiple data layers or stacks wherein each of the multiple data stacks has a storage density comparable to a conventional single layer optical disk. The optical media of the invention thus provide a high areal storage density.
In general terms, the invention comprises an optical medium having a single dedicated servo reference layer and multiple data stacks which each contain an embedded servo format, a servo laser beam positioned to maintain a first focus point on the dedicated servo reference layer, a read-write laser beam positioned to maintain a second focus point on one of the data stacks, a first, dedicated servo system which provides focus and tracking error correction according to error signals generated from the dedicated servo layer, and a second, embedded servo system which provides focus and tracking error correction according to error signals generated from the data stacks. The dedicated servo layer, in different embodiments of the invention, may be positioned either below or above the data stacks in the optical medium, or interposed between data stacks. The data stacks may comprise discrete physical data layers or “virtual” data layers defined by a format hologram. The servo and read-write lasers may differ in wavelength and/or polarization.
By way of example, and not of limitation, in one presently preferred embodiment the optical medium comprises a dedicated servo layer together with a lower or innermost data stack proximate to the servo layer, and at least one overlying or outer data stack positioned above or outside the innermost data stack. More preferably the medium comprises first, second, third and fourth data stacks positioned above the dedicated servo layer, with the first data stack being outermost, and the fourth data stack being innermost and located adjacent the dedicated servo layer. Each data stack comprises a layer of read-write material surrounded by or positioned between at least two dielectric layers
Amble James R.
Daiber Andrew J.
Ferrier Herman A.
Ghose Sanjoy
Hesselink Lambrerrtus
Sierra Patent Group Ltd.
Siros Technologies, Inc.
Tran Thang V.
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