60/167,894,60/167,895,60/167,896, and 60/167,900, all filed November 30,1999, whose disclosures are hereby incorporated by reference in their entireties into the present disclosure.

FIELD OF THE INVENTION The present invention is directed to media, systems and methods for fluorescent multilayer data storage and more particularly to such media, systems and methods which are designed to increase capacity, data rate, or both.

BACKGROUND OF THE INVENTION The increasing demand for optical data storage capacity and data rate is far beyond the performance of contemporary optical recording devices. To meet this demand, volumetric methods like holographic, two-photon recording and fluorescent multilayer technology are suggested. A high data rate in these systems is supported by parallel reading by using a CCD photo-matrix. The recent tremendous progress in the speed of CCD devices opens a possibility to realize devices with data rates up to 10 Gb/s. One serious problem limiting this data rate is the speed of mechanical scanning of a carrier or optical readout head to support such a high readout data rate. It would be highly desirable to overcome the aforesaid mechanical problem.

Another problem, which limits the information capacity, is depth of focus. In order to obtain Terabyte capacity for a standard size of device, one must realize an

optimal design of optics and carrier geometry supporting high focusing depth. Such an optimal design has so far been absent from the art.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical information medium having high capacity.

Another object of the present invention is to provide an apparatus capable of reading information at a high data rate.

Another object of the present invention is to provide principles of optimal design of optics and media geometry in order to obtain a high capacity for a standard size of recording medium.

The first object of the present invention can be attained by a fluorescent multilayer optical medium, which is realized in the form of an optical card or disk having dozens of information layers. In the case of an optical card, the information field in each layer has a plurality of individual pages or tracks which include information pits. The size of each individual information pit is a tradeoff between the 2-D data density and the number of layers. This optimum is closely related to the optimal numerical aperture of the objective, providing maximal information capacity.

The second object of the present invention can be attained by a playing apparatus using the above-described optical medium. In the case of a multilayer card, the apparatus includes two (or more) optical readout heads with a high frame rate (up to several kHz) CCD matrix as a photosensitive element. The Gb/s data rate is provided by scanning into depth by one (or several) heads and laterally moving another head (or several heads) to the next column of pages. In the case of an optical disk, the readout heads have high-speed CCD arrays, and reading is provided by

scanning some of the optical heads along the radial directions of a continuously rotating disk and focusing into depth of other heads. To combine the high data rate with high sensitivity, the detector photosensitive matrix is organized by the so-called TDI (time delay and integration) method. The commercially available TDI sensor is a linear array of 512,1024 or 2048 elements containing 32,48,72,96,144 adjacent columns that are internally coupled together and sequenced by external timing logic.

The array captures a line of image data over a period of time and then transmits the line to a data-capture host. Each element across the array is exposed simultaneously.

Each column is exposed sequentially over a brief time interval. As a carrier is continuously moved past the TDI camera sensor array and lens, the charge developed in each element of the TDI column is transferred on to the next column in synchronization with the carrier motion. When compared to a typical line scan sensor, TDI operation increases the total array exposure time by a factor 32,64,72,96, or 144 (depending on the version).

To provide a long exposure time, the present invention further provides a new method of following the moving carrier. In this method, an actuator with an objective lens and a mirror follows a certain part of the information field (page) on a moving carrier and after certain exposure time returns back and starts to read a next page.

The third object of the present invention is attained by numerical modeling of the focusing depth limited by aberrations for different value of numerical aperture.

This procedure provides the optimal numerical aperture supporting maximal information capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 Principle of reading information from a multilayer optical card by using several optical heads.

Fig. 2 Readout head.

Fig. 3 Principle of reading information from a rotating multilayer optical disk by using a line scan matrix.

Fig. 4 Reading information by scanning in depth and with simultaneous lateral shift.

Fig. 5 Parallel reading from several layers.

Fig. 6 Reading information from a continuously moving optical card by following the carrier.

Fig. 7 Dependence of total capacity (number of layers) on numerical aperture of objective.

Fig. 8 Principle of reading information from a card by the use of two scanning mirrors.

Table 1 Dependence of capacity on wavelength, numerical aperture and encoding standard (CD or DVD) DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Various preferred embodiments of the present invention are described hereinafter with reference to the accompanying drawings.

Fig. 1 demonstrates the principle of reading information stored in the multilayer optical card 101 by several readout heads 103 under the control of step- movers 105, each of which can move along the X and Y axes. In order to provide a high data rate, one of the readout heads 103 reads page by page from a certain column. The time of moving from layer to layer is much shorter than the time of

lateral moving from column to column. During the reading of the certain column, other heads 103 are moving laterally to the next columns. Let us consider a numerical example. The card has 50 layers with size 16x16 cm2. The distance between layers is 20 u. The bit size is 0.4x0.4 12. Hence the data density is 80 MB/cm2. The page size is 400x400 t2. The page has 1000x1000 bits. There are two optical heads 103; one is active, and one is positioning. The time to move from layer to layer is 0.6 ms. The time to move from column to column is 50 ms. The average access time (time of positioning) is 50 ms. The light wavelength is 0.5 p. The laser power is 10 mW. The numerical aperture of the imaging objective is NA = 0.5. The photosensitive CCD matrix has 2000x2000 pixels, and the frame rate is 1000 frames/s. The number of photoelectrons per pixel is 10,000. The oversampling is 4 pixels per pit. The light integration time is 0.4 ms. The matching of the image is obtained by software. It is easy to calculate that this device has an information capacity of 1 TB and a readout data rate of 1 Gb/s. Indeed, the reading of one page takes 0.4 ms. During the next 0.6 ms, a head 103 moves to the next layer. Hence, to read one page with an information capacity of 1 Mb takes 1 ms, so that the readout rate is 1 Gb/s. After reading of the 50'1'page, the head 103 moves to the next column; meanwhile, the other head 103 starts to read a different column.

Fig. 2 is a readout head according to embodiment of a second aspect of the present invention. The optical head has a laser or LED source 201, collimator 202, dichroic mirror 203, objective 204, corrector 205 (optional) compensating aberrations, fluorescent multilayer card 206, connected with mechanical mover, filter 207 for filtering of exciting light, second collimator 208, forming a parallel beam, sphero- cylindric lens 209 forming rectangular shape of light spot, and CCD matrix 210. The

auto focusing is realized by wobbling the micro-objective 204 in the vertical direction.

The oscillation produces a signal indicating a focusing error. The scheme of synchrony detection forms the servo-signal, which controls the actuator of the micro- objective. In another variant of the autofocusing system, special marks on the optical card are imaged onto a special group of pixels of the CCD matrix. The readout rate of this group of pixels can be significantly higher than the frame rate of the information part of CCD matrix, and hence the autofocusing become faster.

Fig. 3 demonstrates the principle of reading from a continuously rotating multilayer disk 301. The information is stored in the form of circular or spiral tracks 303 like in a CD or DVD. The parallel reading is realized by using several readout heads 305. In each head 305, the exciting light of a laser or LED is formed in a light strip 307. A CCD array or TDI matrix 309 is used as the photosensor. The objective 311 images information stored in several tracks onto the CCD or TDI array. The focusing is based on a conventional astigmatic auto-focusing system. Each head 305 is movable in a radial direction R by a step-mover 313. Alternately, the head could extend in such a length that it does not have to be moved in the radial direction at all.

Let us consider a numerical example. The 20-layer disk with a diameter of 300 mm has an information capacity of about 1 TB. The four-output TDI sensor with 4096 elements per array and 96 columns (stages) has a 200 MHz data rate. Each track takes two pixels and simultaneously reads 2048 tracks, so that a total data rate is 100Mb/s. That corresponds to a linear speed of about 25 cm/s for the DVD format.

To obtain a 1 Gb/s data rate, one uses ten optical heads 305, each having a TDI sensor.

In another embodiment, the carrier is an optical card which moves continuously along the X coordinate until the boundary of the information field, after which the optical head is shifted along the Y coordinate to another strip of the information field, and a stage with the optical card moves back with constant speed.

Using the same TDI sensor as in the previous case, the average data rate is about 100 Mb/s.

Another embodiment of reading from a continuously rotating multilayer disk uses a high-speed CCD matrix. As shown in Fig. 4, in the disk 401 having multiple layers 403, the information is stored in the form of a plurality of pages 405 written along spiral tracks. The pages 405 on neighboring layers 403 are shifted relative to one another. The reading is realized by scanning in the depth. The exciting sources produce pulses synchronized with the sequence of frames in the CCD matrix. After emitting an exciting pulse for reading of the next page, the readout head moves relative the disk simultaneously in vertical and horizontal directions, and the next exciting pulse is emitted when the readout head is positioned above the pages of the neighboring layer. After reading of the last layer, the integral lateral shift due to rotating of the disk is equal to the distance between neighboring columns of pages.

The process repeats with the reading of the next column.

Let us consider a numerical example. The CCD matrix has 2000x2000 pixels, and the frame rate is 2 kHz. The oversampling is 4 pixels per pit. Refocusing from layer to layer takes 0.5 ms. The disk has 300 mm diameter and 30 layers. The size of a page is 400 . Reading of 30 layers takes 30 ms. During this time, the lateral shift due to rotation is 400 t, so that the linear rotating speed is 1.3 cm/s. Thus, the average date rate is 1 Gb/s.

Another embodiment of reading from a continuously rotating multilayer disk uses a CCD or CMOS array that is oriented along the track. The tracks are written in radial directions on the disk; such tracks are shown in Fig. 3 as 303a. Let us consider a numerical example. The typical scanning rate is of 80 kHz. During the scanning time the disk moves for a distance much less than the width of a pit; thus, its motion is negligibly slow. For the pit's width of 0.5 micron, the linear velocity must be less than 0.5 cm/s. The imaging length of 400 micron provides the reading of 600 bits per scanning period. The readout bit rate is 48 Mb/s.

Another embodiment for a reproducing apparatus for a multilayer fluorescent disk 501, shown in Fig. 5, has a plurality of readout heads 503, of which each reads its own layer 505. The individual readout head 503 has the same design as a readout head in a apparatus for reproducing information from fluorescent multilayer disk patented in our United States Patent No. 6,009,065, issued December 28,1999. In the particular case of a 300 mm disk having 20 layers with encoding in DVD format and a data rate of 10 Mb/s, the total data rate is 20Mb/s. Thus, an uncompressible movie can be stored by de-multiplexing and writing on different layers and can be reproduced by subsequent multiplexing.

Usually, the intensity of the fluorescent signal is several orders of magnitude less than the intensity of the exciting light. Reading a weak signal requires a highly sensitive photomatrix or a long exposure time. The straightforward solution to this problem is step-like motion of the carrier or the readout head. However, the step-like motion is undesirable for highly precise mechanics. The TDI technology is capable of following the uniformly moving carrier and increasing the exposure time. For a given linear velocity, the exposure time is limited by the number of columns (stages).

Increasing the stage number is a serious challenge for technology because of the increase in noise and deterioration of spatial resolution due to the blooming effect.

To increase exposure time, an embodiment of the present invention provides a novel principle of mechanical scanning, which will be explained with reference to Fig.

6. One element of this embodiment is that the drive 600, in which the carrier (disk or card) 602 is read by the optical head 604, implements a combination of uniform continuous motion of the carrier (disk, or card) 602 and a periodical motion of the actuator 606 with the objective 608. For example, let us consider the uniform motion of an optical card 602. The information is stored in the form of a plurality of pages having information fluorescent pits. The optical head 604 has an actuator 606 with an objective 608, a nonmovable CCD matrix 610 and a second imaging lens 612. The actuator 606 can move in all three directions. Thus, in the XY plane it could perform an oscillation motion around a non-shifted position with an amplitude of about 1 mm with a period range from 1 msec up to 100 msec. At a certain moment, a certain information page is imaged on the CCD matrix 610. The actuator 606 moves with the same velocity as the optical card 602. The optical card 602 moves continuously along the X coordinate. The actuator 606 with a mirror 614 and objective 608 moves synchronously with the optical card 602 along the X coordinate and additionally can move along the Z coordinate providing focusing. The platform with the optical head 604 including the actuator 606 with objective lens 608 and mirror 614, imaging lens 612, beamsplitter 616, laser 618, dichroic or other filter 620 and CCD camera 610 realizes a step-like motion along the Y coordinate, providing a shift from one line to another line of pages. In another embodiment, the optical card is fixed, and the

platform with the optical head can move along the X direction continuously and makes steps in the Y direction.

The autofocusing is provided by wobbling of the objective around the position of optimal focusing, summation of the signal over the entire matrix or a special part of the matrix, and feedback of the differential signal to a servo system. For effective focusing, the frequency of the sequence of servo pulses must be higher than the period of lateral oscillation of the actuator. That is possible in two cases. In the first case, the frame rate is higher than the frequency of periodical motion of the actuator. In the second case, the focusing signal is integrated from a special part of the CCD matrix, and the scan rate of this part is higher than the frame rate of the essential part of the matrix.

Autotracking can be realized by reading special tracking marks written along the upper and lower sides of page rows. These marks are imaged to special rows of pixels on the CCD matrix. The signal from these rows of pixels is fed back to a tracking servo-system. For effective tracking, the readout rate of these rows must be higher than the frequency of periodical motion of actuator.

In another embodiment, the optical head is fixed, and the optical card moves continuously in the X direction and makes steps in the Y direction. The distance between individual pages is determined by the accuracy of positioning of the actuator and could be comparably small.

In another embodiment, during the period of following a certain column of pages, the information is read in depth by focusing from one layer to another.

The technique based on following the optical card actuator can be applied to a rotating disk as well. In this case the information pages are written along spiral tracks,

and the optical head is involved in radial motion following the spiral tracks. The actuator with the mirror and the objective lens follows the moving information page.

To obtain the maximal information capacity, new principles of optimal design must be applied. To date, the progress in capacity has been based on increasing the data density in each information layer. This tendency leads to a demand for a small size of the light spot, which is determined by ratio R/NA, where X is the wavelength of exciting light and NA is the numerical aperture. Using a shorter wavelength and a higher numerical aperture has traditionally supported the progress in capacity.

However, a high numerical aperture is not compatible with the demands of volumetric optical storage because of low tolerance to thickness of the information media. The aberrations affect the intensity distribution at the focal plane, and the depth of spatial resolution deteriorates very rapidly with increasing numerical aperture NA. Thus, the effective number of information layers varies strongly with NA. For example, if NA=0.65, and the distance between layers is 40 n, as it is in the DVD standard, only two layers are possible. The results of simulations shown in the table and in Fig. 7 demonstrate the existence of a maximum in the total capacity (or number of layers) as a function of numerical aperture. For example, in the case of the CD encoding format, the optimal numerical aperture is close to 0.4, and for a distance between layers of 20 , the number of layers is 50 with total capacity of 50 GB. Correction of aberration eliminates the maximum. The application of moderate numerical apertures leads to softening of demand for optics and accuracy of multilayer technology. In particular, it makes it possible to realize a large size (300 mm and more) multilayer disk, because of high tolerance to angle deviations, variations of thickness, and other parameters.

Yet another preferred embodiment is shown in Fig. 8. A laser or other light source 801 emits a beam 802, which passes through a collimating lens 803 and is reflected by a first scanning (turning, movable) mirror 804. From the first scanning mirror 804, the beam passes through a cylindrical lens 805 and is directed by a dichroic mirror 806 onto a second scanning mirror 807 and thence is focused by a microobjective 808 onto one of the information layers of a fluorescent optical card 809. The fluorescence excited by the beam 802 in the card 809 is collected by the same microobjective 808 and is directed by the second scanning mirror 807 through the dichroic mirror 806 onto a filter 810, which filters out parasitic reflections at the wavelength of the laser beam 802. Downstream from the filter 810, the filtered fluorescent light 811 is focused by a lens 812 and is directed by a third scanning mirror 813 onto a CCD matrix 814.

Data are read through step-wise moving of the card 809 in the X direction and by movement of a movable part 815 of the optical head (including the above- described elements 801 and 803-808) in the Y direction. The focusing of the laser beam 802 into a specific layer of the card 809 is accomplished by movement of the microobjective 808 in the Z direction. Data are read page by page.

The system of Fig. 8 can be varied in any of the following ways. The two scanning mirrors 804 and 807 can be replaced by a single scanning mirror which undergoes angular oscillation about two perpendicular axes. Optical elements can be moved or replaced with other optical elements; some, such as the mirror 813, can be eliminated. The moving part 815 of the optical head can include the mirror 807 and the microobjective 808 only. The filter 810 can be eliminated if the refractive indices

of the layers of the card 809 are matched perfectly, so as to eliminate parasitic reflection.

While various preferred embodiments have been set forth in detail above, those skilled in the art who have reviewed the disclosure will readily appreciate the other embodiments can be realized within the scope of the present invention. For example, numerical examples are illustrative rather than limiting. Also, the principles described above can be justified for non-fluorescent optical storage as well and could use reflection, absorption, polarization, etc. Also, embodiments disclosed separately can be combined. Therefore, the present invention should be construed as limited only by the appended claims.