WO2001041131A2

WO2001041131A2 - Fluorescent multilayer data storage system

Info

Publication number: WO2001041131A2
Application number: PCT/US2000/042407
Authority: WO
Inventors: Sergei Magnitskii; Eugene Levich; Andrey Tarasishin; Boris Chernobrod; Alexej Lezhnev; Zeev Orbakh; A. Dovgan
Original assignee: Trid Store Ip, Llc
Priority date: 1999-11-30
Filing date: 2000-11-30
Publication date: 2001-06-07
Also published as: WO2001041131A3; AU3970901A; WO2001041131A9

Abstract

In an optical medium such as a fluorescent multilayer disk (206), both reading speed and storage capacity are increased in various ways. Multiple heads (103) can be provided, in which case one head reads information while another moves from one reading position to the next. An optical head can read from several tracks on a disk simultaneously by focusing the exiting light into a light strip and using a photo-sensor array (210). The information pages in each layer can be staggered from those in adjacent layers. The number of layers in a medium can be selected by maximizing capacity with respect to numerical aperture. To increase exposure time, a reading head can follow the medium for one reading operation and then return to its initial position for the next reading operation.

Description

FLUORESCENT MULTILAYER DATA STORAGE WITH HIGH CAPACITY

A.ND HIGH DATA RATE

REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Nos.

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 refocusing 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 wich size 16x16 cm². The distance between layers is

20 μ. The bit size is 0.4x0.4 μ². Hence the data density is 80 MB/cm². The page size is

400x400 μ². 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 μ. 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 1Mb takes 1 ms, so that the readout rate is 1 Gb/s. After reading of the

50^th 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 sp ot, and CCD matrix 210. The autofocusing 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

31 1 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

lOOMb/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 μ. Reading of 30 layers takes 30 ms. During this time, the lateral shift

due to rotation is 400 μ, 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 refocusing 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 λ/NA, where λ 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 μ, 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

μ, 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 81 1 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 a "e 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.

Claims

What is claimed is:

1. An apparatus for reproducing optical information stored in a multilayer medium

as fluorescent pits organized in a plurality of information pages arranged in columns,

said apparatus comprising:

a plurality of optical heads, each of said plurality of optical heads comprising a

light source which excites a certain one of the plurality of information pages, a photo¬

sensor matrix, an objective for imaging information stored in said certain one of the

plurality of pages onto the photo-sensor matrix, and a step-mover for moving the

optical head in a plurality of spatial coordinates; and

positioning means for positioning the plurality of optical heads relative to the

medium such that some of the plurality of optical heads function as positioning heads

and others of the plurality of optical heads function as active heads to read the

information from the plurality of pages by refocusing from page to page in each

column in a vertical direction, wherein the positioning means controls the heads such

that when an active optical head finishes reading from a last one of a plurality of

layers in the medium, the active optical head becomes a positioning head and starts to

move laterally towards a next one of the columns.

2. The apparatus of claim 1 , wherein the light source comprises a laser.

3. The apparatus of claim 1 , wherein the light source comprises an LED.

4. The apparatus of claim 1 , wherein the light source comprises a matrix of

LED's.

5. The apparatus of claim 1, wherein the photo-sensor matrix comprises a CCD

matrix.

6. The apparatus of claim 1 , wherein the photo-sensor matrix comprises a CID

matrix.

7. An apparatus for reproducing information from a continuously rotating

fluorescent multilayer optical disk in which the information is stored in tracks,

the apparatus comprising:

a plurality of optical readout heads, each of the plurality of optical heads

comprising a photo-sensor array, a light source for emitting exciting light, a

cylindrical lens for forming the exciting light from the light source into a light

strip for illuminating a plurality of the tracks simultaneously, and a lens array for

imaging the information from the plurality of tracks onto the photo-sensor array;

and

means for spinning the disk at a speed consistent with a readout rate of the

plurality of optical heads.

8. The apparatus of claim 7, wherein the photo-sensor array comprises a CCD

array.

9. The apparatus of claim 7, wherein the light source comprises a laser.

10. The apparatus of claim 7, wherein the light source comprises an LED.

1 1. The apparatus of claim 7, wherein the photo-sensor array operates on a TDI

technique with a plurality of stages.

12. The apparatus of claim 7, wherein different ones of the optical heads read the

information from different layers of the disk simultaneously.

13. The apparatus of claim 7, wherein different ones of the optical heads read the

information from a single layer of the disk simultaneously.

14. An optical medium for storing information in a plurality of layers, each of the

plurality of layers being organized into a plurality of pages having a page size,

the pages in each of the layers being laterally offset from the pages on adjacent

ones of the layers by a lateral shift which is less than the page size.

15. The optical medium of claim 14, wherein the optical medium is an optical disk

16. An apparatus for reading information stored in multilayer continuously

rotating disk, the disk storing information in a plurality of layers, each of the

ones of the layers by a lateral shift which is less than the page size, the

apparatus comprising:

at least one optical readout head having a photo-sensor matrix and a plurality of

excitation sources for emitting a sequence of excitation pulses which are synchronized

with a sequence of frames in the photo-sensor matrix, the head scanning the medium

in depth from layer to layer, and

rotating means for rotating the disk at a speed such that when the head finishes

reading of a page from a last layer, the head is positioned over a page on a first layer.

17. The apparatus of claim 16, wherein the photo-sensor matrix comprises a CCD

matrix.

18. The apparatus of claim 16, wherein the excitation sources comprise lasers.

19. The apparatus of claim 16, wherein the excitation sources comprise LED's.

20. An optical disk for storing information in a plurality of tracks, each of the

plurality of tracks extending in a radial direction on the disk.

21. An apparatus for reading information from an optical disk in which

information is stored in a plurality of tracks, each of the plurality of tracks

extending in a radial direction along the disk, the apparatus comprising:

at least one optical head for reading information from the optical disk, the head

comprising a photo-sensor array which is oriented along the radial direction so as

to be oriented along one of the tracks; and

means for rotating the disk at a rotating velocity which is slow enough that a

motion of an image of the track on the photo-sensor array is negligible during a

scanning period.

22. The apparatus of claim 21, wherein the photo-sensor array comprises a

CCD array.

23. A method of making an optical storage medium to maximize a storage

capacity of the medium, the storage capacity being a function of a parameter,

the method comprising:

(a) selecting a value of the parameter at which the function has a maximum; and

(b) constructing the optical storage medium in accordance with the value of the

parameter selected in step (a).

24. The method of claim 23, wherein the parameter comprises a numerical

aperture of an apparatus for reading the medium.

25. The method of claim 24, wherein step (b) comprises constructing the medium

to have an optimal number of layers for a fixed distance between the layers.

26. The method of claim 23, wherein at least one of a pit size, a distance between pits

and a distance between layers varies from layer to layer.

27. A method of reading information from a continuously moving carrier, the method

comprising:

(a) moving a reading head from an initial position to follow the moving carrier

during a reading operation; and

(b) then returning the reading head back to the initial position to begin a next

reading operation.

28. The method of claim 27, wherein:

the carrier comprises a plurality of layers; and

during the reading operation of step (a), the reading head reads the information

from more than one of the layers.

29. An apparatus for reading information from an optical medium, the apparatus

comprising:

a light source for emitting reading light;

scanning optics for scanning the reading light on the medium; and

reading optics for receiving information-bearing light from the medium and for

reading the information from the information-bearing light.

30. The apparatus of claim 29, wherein the scanning optics comprise a scanning

mirror.

31. The apparatus of claim 30, wherein the scanning optics comprise two scanning

mirrors.

32. The apparatus of claim 30, wherein the reading optics comprise a scanning mirror.