Abstract:
An EET (“eight-to-ten”) method has been proposed for two-dimensional spatial encoding of information stored in two- or three-dimensional, in particular fluorescent optical carriers. The method specifically ensures the same writing density as DVD carriers with EFM (“eight-fourteen modulation”) modulation code but for 0.8×0.4μ information pit (fluorescent mark), i.e. as in CD data carriers. The larger—as compared to the DVD format—pit size enables a simpler technology for manufacturing fluorescent multilayer carriers, for instance of ROM type, and a stronger fluorescent signal in reading. The high writing density is ensured through virtually 100% filling of the information layer area with fluorescent marks in a gap-free manner. In addition, this allows application of the parallel data reading procedure and a ten-fold higher reading speed than in DVD systems. Increasing the size of the channel bit to 0.4 μm—which is 1.5 and 3 times higher than for CD and DVD formats, respectively—allows a significant reduction in the frequency band and hence in photoreceiver noises. For equal values of the reading radiation wavelength and numerical aperture of the objective lens used, the proposed ETT (“eight-to-ten”) method of two-dimensional encoding in fluorescent carriers enables a significant lower magnitude of reading error probability in contrast to existing optical information carriers of DVD-type. It is also applicable to other write-once and rewritable optical data carriers based on various physical and chemical principles of forming information pits.

Description:
This application claims the benefit of Provisional application Ser. No. 60/254,541, filed Dec. 12, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention. 
     The invention relates to optical memory systems and more particularly to methods of 2D spatial encoding of information for high-density writing in one- or multilayer optical, especially fluorescent data storage systems. 
     2. Description of the Prior Art. 
     The available optical memory systems utilize 2D carriers, generally with one or two information layers. Most of the prior art in the field of optical information recording are based on generating changes in the intensity of reflected laser radiation in local microregions (pits) of the information layer. These changes contain stored information and can result from interference effects on a microrelief surface of optical discs of a CD- Read-Only-Memory (ROM)-type, burning of holes in the metal film, dye bleaching, local melting of dye-containing polymers in widely used CD-Write-Once-Read-Many (WORM) systems, change in reflection coefficient in phase-change CD-Rewritable (RW) systems, etc. 
     FIG. 1 schematically presents geometry of two-dimensional spatial distribution of information pits on the surface of CD and DVD optical information carriers. Their spatial distribution in CD- and DVD-ROM can be characterized by such parameters as typical pit sizes (shortest pit length l, width w, depth d, and track pitch p) and channel bit length. 
     Numerical values of these and other CD- and DVD-ROM parameters are given in Table 1. [“Information Storage Materials ”, pp. 36, 42]. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 From CD to DVD 
               
             
          
           
               
                   
                 Parameter 
                 CD 
                 DVD 
               
               
                   
                   
               
             
          
           
               
                   
                 Wavelength λ, nm 
                 780 
                 650 
               
               
                   
                 Numerical aperture NA 
                 0.45 
                 0.60 
               
               
                   
                 Shortest pit length, nm 
                 831 
                 399 
               
               
                   
                 Depth, μm 
                 0.13-0.15 
                 0.11-0.12 
               
               
                   
                 Track pitch, μm 
                 1.6 
                 0.74 
               
               
                   
                 Channel bit length, nm 
                 277 
                 133 
               
               
                   
                 Modulation code* 
                 EFM 
                 EFM** 
               
               
                   
                 Physical bit density, Mbit/cm 2   
                 106 
                 508 
               
               
                   
                 Reference velocity CLV, m/s 
                 1.2 
                 4.0 
               
               
                   
                 Spot size λ/2NA, μm 
                 0.9 
                 0.55 
               
               
                   
                 Capacity, GB 
                 0.65 
                 4.7 
               
               
                   
                   
               
               
                   
                 *For EFM (“eight-fourteen modulation”) one has 17 channel bits (14 modulation and 3 merging bits) for 8 data bits. Each channel bit corresponds to 1/3 of the minimum mark length. The physical bit density equals to 1/(track pitch × channel bit length × 17/8). For EFM** the factor 17/8 is replaced by 16/8.  
               
             
          
         
       
     
     As can be seen from Table 1, transfer to the DVD-format considerably increases the density and consequently the volume of stored information. However, as can be seen from FIG.  1  and Table 1, information pits occupy yet only part of the information layer area which significantly reduces the values of density and volume of stored information as compared to maximum permissible magnitudes. 
     To increase the writing density such methods are used as transfer to shorter-wavelength radiation sources in combination with high-NA objective lens (see Table 1 and as an example [I. Ichimura et al, SPIE, 3864, 228]), a reduced track pitch and increasing the groove depth of the land groove recording type optical disk [S. Morita et al, SPIE, 3109, 167]. For high-density data storage, new media and techniques for data reading therefrom [T. Vo-Diny et al, SPIE, 3401, 284], pit-depth modulation [S. Spielman et al, SPIE, 3109, 98], and optical discs having square information pits arranged in symmetrical patterns [Satoh et al, U.S. Pat. No 5,572,508]. 
     Data writing density as high as more than several terabits per cubic centimeter can be ensured by three-dimensional (monolithic) photosensitive media exhibiting various photophysical or photochemical non-linear effects at two-photon absorption. The best reading/writing mode for such 3D WORM or WER information carriers is cooperative two-photon absorption by photosensitive components and photoreaction products themselves via an intermediate virtual level as in the case of photochromic [D. Parthenopoulos et al, Science, 245, (1989), 843] or photobleacing [P. Cheng et al, Scanning, 18, (1996), 129] materials or registration of a change in refractive index as in the case of photorefractive crystals [Y. Kawata et al, Opt. Lett., 23, (1998), 756] or polymers [D. Day et al, SPIE, 3864, 103] and photopolymers [R. Borisov et al, Appl. Phys., B 67, (1998), 1]. 
     Generally, such reading/writing mode allows local recording of data as marks (pits) analogous to information pits in conventional CD- or DVD-ROM, with varied optical properties in the volume of information medium. 
     Practical realization of this principle however is impeded by the large overall size needed for such writing of femtosecond laser radiation sources and the extremely low photosensitivity of the media themselves. The latter is dictated predominantly by the low magnitudes of cross sections of two-photon absorption currently known. 
     That same reason rules out the application of 1-10-mW small-size semiconductor lasers for two-photon data writing. Besides, the design of the 3D system based on this principle is rather complicated. 
     To increase the volume of information stored the application of multilayer bilateral information carriers would be technically more justifiable. However, they impose certain restrictions on the design and properties of the recording medium, data reading and writing procedures, especially in the depth, thereby creating additional difficulties. 
     In the reflection mode operation, each information layer of the multilayer optical data carrier must have a partially reflecting coat. This attenuates the intensity of both reading and reflecting, information-carrying, beams as a result of direct and reverse motion through the carrier up to a specified information carrying layer and back to the photoreceiver. In addition, as both beams are coherent, they are prone to difficult-to-read diffraction and interference distortions on pits and grooves of the information carrying layers occurring on their way. 
     In this case, preference should be given to multilayer optical information carriers with fluorescent reading that need no partially reflecting coats. Said information carriers ensure considerably reduced diffraction and interference distortions due to the noncoherence of fluorescent radiation, longer wavelength thereof in contrast to reading laser radiation as well as transparence and homogeneity (identity of refraction indices for some layers) of the optical medium with respect to the incident laser and fluorescent radiations. Consequently, multilayer fluorescent optical information carriers have advantages over reflecting ones. In addition, fluorescent reading enables a higher signal-to-noise ratio as compared to the absorption method. 
     Currently the general demand to all types, in particular fluorescent, of multilayer information carriers as optical discs and cards, tapes or cylinders is that they must ensure maximum possible volume and density of recorded information and maximum possible data reading rate. These requirements are met by minimizing the size of information pits and increasing the recording density thereof in each separate information layer while increasing the total number of layers, as well as by switching over to shorter-wavelength optical radiators as the information density storable in N-dimensional memory systems (N=1, 2, 3) is inversely proportional to the wavelength to the power N. However reducing the size of information pits and accordingly increasing the writing density thereof may lead to a lower intensity of the reading information signal and higher crosstalks due to the “excitation” of the adjacent information layers the reading radiation is passing through. As a result, the reading signal-to-noise ratio goes down. 
     The purpose of the present invention is to eliminate the above drawbacks through application of a new method of spatial encoding in information layers and parallel data reading therefrom. 
     SUMMARY OF THE INVENTION 
     The subject of the present invention is a new ETT (“eight-to-ten”) method of two-dimensional spatial encoding of information stored in two- or three-dimensional, in particular fluorescent optical carriers. The method specifically ensures the same writing density as DVD carriers with EFM (“eight-fourteen modulation”) modulation code (see Table 1) but for a 0.8×0.4 μm information pit (fluorescent mark), i.e. as in CD data carriers. The larger—as compared to the DVD format—pit size enables a simpler technology for manufacturing fluorescent multilayer carriers, for instance of ROM type, and a stronger fluorescent signal in reading. The high writing density is ensured through virtually 100% filling of the information layer area with fluorescent marks in a gap-free manner. The proposed method allows application of the parallel data reading procedure and a ten-fold higher reading speed than in DVD systems. Increasing the size of the channel bit to 0.4 μm—which is 1.5 and 3 times higher than for CD and DVD formats, respectively—allows a significant reduction in the frequency band and hence in photoreceiver noises. 
     For equal values of the reading radiation wavelength and numerical aperture of the objective lens used, the proposed ETT method of two-dimensional encoding in fluorescent carriers enables a significantly lower magnitude of reading error probability in contrast to existing optical information carriers of DVD-type. 
     The proposed method is also applicable to other one- and multilayer optical data carriers based on various physical and chemical principles of forming information pits, such as photorefractive crystals and polymers, photopolymers, magnetooptical, phase-change, persistent spectral-hole-burning recording systems, as well as to other one- and multilayer optical data carriers of ROM-, WORM- and WER- type. 
     Said method is applicable to various forms of optical memory, for example, as an optical disc, optical memory card, optical memory tape or drum (cylinder), etc. 
     The subject of the invention is writing of one information byte in a field (microregion) consisting of ten (2×5) square elements (“(2×5)-field”) of specific size in which each said square element contains or does not contain any changes (different from those layer regions that do not carry information) in optical properties (absorption and reflection factors, refraction index, birefringence factor, etc.) testifying either presence or absence of an information pit therein. One information byte has an area of 10S, where S=a×a is the area of one square element and a is the square side. Adjacent bytes are positioned on the plane close to each other without gaps. 
     Another subject of the invention is a representation of all 256 combinations composing an information byte on the information layer plane by “(2×5)-fields” of two types wherein the first 222 combinations are represented by fields in which each information-carrying square element (pit) has inside a “(2×5)-field” at least one identical adjacent element positioned transversely or lengthwise while each square element comprising no information pit has inside the field identical adjacent element (parity condition). 
     With appropriate modification, the method can be also utilized for three-dimensional volumetric data encoding as volumetric bytes recorded within a specified microvolume using the two-photon procedure, each said byte consisting of N number of cubic elements of certain size. Data can be read by either one- or two-photon procedure. 
     A further subject of the invention is the use for writing C&amp;D information of the remaining 52 combinations each represented by one of two complementary “(2×5)-fields”, wherein the parity condition can be violated only for the top left or bottom left square element of the field consisting of (2×5) square elements. 
     The subject of this invention is the selection while writing from the pair of “(2×5)-fields’ of such a “(2×5)-field” that when joined to the left field thereof enables meeting the parity condition inside each lengthwise strip consisting of joined to one another fields (bytes). In this case, the minimal regions of the information layer containing any changes (distinct from the layer regions containing no information) in optical (for example, fluorescent) properties, are composed of two adjacent square elements with varied optical (for example, fluorescent) properties (i.e. information pits (or fluorescent marks)) and consequently measure a×2a. The layer&#39;s minimal regions free of information pits have the same size. 
     One more subject of the present invention is a possibility of gap-free filling of virtually entire area of the data-carrying layer with the proposed ETT-coded information pits. 
     Further, the invention concerns parallel data reading from an optical carrier with the ETT code by means of one- or two-dimensional photodetector array, for instance CCD cameras, enabling mutual longitudinal (and transverse, if necessary) motion relative to each other at a rate timed with both the size of the channel bit and the frame operating rate of the photodetector (CCD camera) array. In so doing, the adjacent element pairs in each transverse column belonging to different “(2×5)-fields” are identified simultaneously. Both elements are considered information-carrying pits when the signals arrived from individual photodetectors “covering” respective square elements of the information layer exceed some level L 1  and are not considered such when both signals do not exceed some level L 2 &lt;L 1 . When the above-mentioned terms are not met, the square element with a higher signal level is identified as an information pit, while the other element, accordingly, is not considered an information pit. The L 1  and L 2  magnitudes are preset by the technical parameters of both the information-carrying medium and the optical reading device. 
     Another subject of the invention is an optical memory system of ROM type based on a multilayer fluorescent optical card and parallel data reader switching on reading radiation with a wavelength such that it excites fluorescence of the information pits, a dichroic mirror transmitting reading and reflecting fluorescent (information-carrying) radiation, an optical system shaping requisite spatial configuration of the reading beam in the location of a given information layer and fluorescent image thereof in the plane of the linear photodetector array (the linear array of 10 CCD cameras). 
     The optical card has 10 information layers of size 10 cm×10 cm, each of approximately 6 GB in capacity (or 60 GB in a card). Each layer consists of 250 information strips 400 μm wide and is provided with C&amp;D (&lt;&lt;control and display&gt;&gt;) information tracks allowing timing and autotracking of the linear CCD array as it moves across and along the optical card. Each of the CCD cameras has 1000×1000 pixels of size 8 μm and is capable of comparing a signal from each pixel with two levels, and signals from adjacent pixels between themselves. When the linear CCD array operates in the data pit-to-CCD pixel mode (at 20-fold magnification of the camera) at the rate of 30 frames per second, the whole linear array can read information at 3·10 8  pit/s, which in compliance with the ETT-code makes 30·10 6  B/s or approximately 220 Mb/s. In this case the rate of the linear CCD array is about 12 mm/s, which is almost 100 times slower than one-time data reading from a CD ROM optical disc. 
     The attached figures and examples illustrating the proposed invention will make its specific features and advantages more demonstrative. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically illustrates the geometry of two-dimensional spatial distribution of information pits on the surface of a CD or DVD optical information carrier. 
     FIG. 2 schematically illustrates the geometric configuration of four adjacent information bytes recorded by means of a fluorescent substance using the ETT code. 
     FIG. 3 is a binary-ETT code conversion table. 
     FIG. 4 is a schematic presentation of an embodiment of the optical pickup for parallel reading of information generated by the ETT method of two-dimensional spatial encoding in a fluorescent multilayer optical card. 
     FIG. 5 is a schematic presentation of optical communication among elements of the information layers of the optical card and the CCD-camera array. 
     FIG. 6 schematically illustrates the movements of the CCD-camera array and the fluorescent optical card relative to each other. 
     FIG. 7 is the original computer image of an ETT-coded fluorescent optical card fragment for λ=0.65 μm and NA=0.65 that can be used as a photomask for fabrication of one of the information layers in a CD ROM multilayer fluorescent optical disc. 
     FIG. 8 is a computer image of that same fragment of the fluorescent card generated by the reading optical system in the plane of the CCD-camera array. 
     FIG. 9 is a picture of that same fragment of the fluorescent card that has been read by a CCD camera. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 is a schematic illustration of the geometry of two-dimensional spatial distribution of information pits as four adjacent bytes  20 , which are ETT-coded on the surface of, for example, fluorescent data carrier. Here, in compliance with the proposed invention, one data byte is written in field (microregion)  21  consisting of ten (2×5) square elements (hereinafter conventionally called as a “(2×5)-field”) having a certain size, wherein each of said square elements contains or does not contain any changes (different from the layer regions carrying no information) in optical properties (factors of absorption, reflection, refraction, birefringence, etc.) testifying the absence or presence of the information pit therein. 
     Without loosing generality of the proposed invention, hereinafter the multilayer fluorescent data carrier is exemplified by a ROM optical card wherein the fact of the presence or absence of fluorescence in a given square element is used as said optical changes. 
     For example, depending on the information written, each of said square elements could be either filled, as element  22 , or not filled, as element  23 , with a substance fluorescing at absorption of reading radiation. Thus, an information byte occupies an area of 10 S, where S=a×a is the area of one square element and a is the side of the square. Adjacent bytes are located on the plane close to each other, with no gaps, in the manner as shown in FIG.  2 . 
     All 256 combinations composing an information byte are represented on the information layer plane by fields of (2×5) square elements of two types. The first 222 combinations are represented by fields characterized by such property that each square element  22  filled with a fluorescent substance (information pit (fluorescent mark)) has inside its “(2×5)-field” at least one identical adjacent element positioned transversely or lengthwise while each square element  23  free of fluorescent substance has inside the field identical adjacent element. Two top and left bottom bytes in FIG. 2 satisfy this condition hereinafter referred to as parity condition. 
     The remaining combinations are represented each by one of two complementary fields wherein the parity condition can be violated only with regard to either left top or left bottom square element of the “(2×5)-field” (right bottom byte in FIG.  2 ). Totally, there are 52 such pairs of fields reserved for using as C&amp;D combinations in excess of 256 ones needed to compose an information byte. At data recording, from the pair of fields one is chosen such that when joined to the left field therefrom it ensures satisfaction of the parity condition inside each longitudinal strip consisting of fields (bytes) joined one to another. Consequently, a minimal region filled with a fluorescent substance (or any other material having changes in optical properties other than ones inherent in regions carrying no information) consists of two adjacent fluorescent elements (information pits (fluorescent marks)) and thus has size a×2a. The layer&#39;s minimal region free of information pits has the same size. FIG. 3 presents a conversion table for binary and ETT codes. 
     Using the proposed ETT method of two-dimensional data encoding, information fills in a gap-free manner virtually the entire area of information layers with fluorescent marks (information pits) thus providing an opportunity to use parallel reading techniques by means of one- or two-dimensional photodetector array, for instance a Charge-Coupled Device (CCD)-camera array. 
     In this case, identification of fluorescent and non-fluorescent square elements in each information layer of the multilayer fluorescent carrier, for instance fluorescent card, takes place layer by layer as it moves under the Linear CCD array (or vice versa, as the Linear CCD array moves along the card) lengthwise (and transversely, if necessary) at a rate timed with both the magnitude of the channel bit and the operating frame rate of the photodetector array. In so doing, pairs of adjacent elements of each transverse column (the bottom element of the top strip and the top element of the bottom strip in FIG. 2) are identified simultaneously. 
     Both elements are considered information-carrying pits when the signals arrived from individual pixels of the CCD camera covering respective square elements of the fluorescent information layer exceed some level L 1 . When both signals do not exceed some level L 2 &lt;L 1 , then both elements are not pits. When the above-mentioned terms are not met, the square element with a higher signal level is identified as an information pit, while the element with a lower signal level is not an information pit. The L 1  and L 2  magnitudes are preset a priori. They depend on the channel bit length, correlation between the information pit (fluorescent mark) size and the CCD camera&#39;s elementary pixel size, reader wavelength, numerical aperture of the lens and magnification factor thereof. For a given reading device, they can be assumed as known. 
     Assume that I p  and I n  are fluorescent signals in locations where an information pit is present or absent, respectively. It has been proved that identification accuracy C=(I p −L 1 )=(L 2 −I n ) for the ETT method of two-dimensional data encoding exceeds the respective value for DVD systems in a broad range of changes in reader parameters and consequently the reading error probability for ETT-coded information read by the CCD camera is lower than that for information read from a DVD disc. 
     FIG. 4 schematically presents one of the embodiments of proposed optical pickup for reading data generated using the ETT method of two-dimensional encoding in the fluorescent carrier made as a multilayer optical card. 
     Optical pickup  40  is designated for reading data stored in one of information layers  41  of fluorescent carrier  42 . Said optical pickup comprises source of reading radiation  43  with such a wavelength that it excites information pit fluorescence, dichroic mirror  44 , transmitting reading radiation  55  and reflecting fluorescent (information-carrying) radiation  53 , optical system  45  shaping requisite spatial configuration  56  of reading beam  55  in the location of a given information layer and fluorescent image thereof in the plane of location of one- or two-dimensional photodetector array  46 . Constructionally, reading involves movement of the reading head comprising items  43 ,  44 ,  45  and  46 , and fluorescent optical card  42  relative to each other. On the layout in FIG. 4 the reading head is immobile while the optical card is by means of device  47  set to linear motion relative to said reading head lengthwise and transversely (directions “A”) and rotationally (direction “B”) for their angular positioning relative to each other. 
     As a source of reading radiation  43 , there can be used either one CW diode laser or a matrix of light-emitting diodes (LED) (organic or solid-state) or a matrix of vertical cavity surface emitting lasers (VCSEL) integrated with microelectronic circuitry controlled by computer as well as controllable transparency, etc. 
     As photodetector matrix  46 , there can be used a multitude of photodiodes, phototransistors and other photosensitive elements. The most preferable are Charge-Coupled Devices (CCD cameras)  51 . 
     Optical system  45  incorporates at least linear array  48  of microlenses  49  and servo mechanism  50  enabling focusing error check and autotracking as well as microlens linear array movement along axis Z within 1-3 mm for reading data from given fluorescent layer  41  of multilayer carrier  42 . In addition, said optical system enables operation of the reading head in the controllable magnification mode. The number of microlenses  49  can be equal to the number of CCD cameras  51  in photoreceiver linear array  46 . 
     As can be seen from FIGS. 4 and 5, on reading linear array  48  of microlenses  49  irradiates only certain parts  52  of information layer  41 . Images of information pits in these parts are transferred to certain scale onto the plane of linear array  46  of CCD cameras  51  by means of fluorescent radiation  53  excited by reading radiation source  43  using microlenses  49 . To read the remaining within the linear array part  54  of information pits, the carrier or the reading head move transversely relative to each other, as shown by an arrow in FIG.  5  and more clearly in FIG. 6 (view from top). 
     Card  60  can have width H equal to the length of linear array  61  of CCD cameras  62 . The length of card  60  may be arbitrary. Each information layer of the card consists of N information strips of width h. Each strip  63  is equipped with special C&amp;D (“control and display”) track  64  with periodically written thereon information pits  65  enabling timing and autotracking of linear array  61  of CCD cameras  62  as it moves relative to card  60 . A diagram of continuous joint movement of the first two CCD cameras of the linear array along and across optical card  60  is shown by dotted line in FIG.  6 . The synchronous movement of all other CCD cameras of linear array  61  takes place in a similar way. 
     The device schematically presented in FIG. 4 operates as follows. Upon transmission through dichroic mirror  44 , reading radiation beam  55  is focused by means of optical system  45  in the plane of information-carrying layer  41  subjected to reading as intensity-uniform spatial configuration  56  matching the configuration of photodetector linear array  46  constructed to certain scale. Fluorescent radiation  53  induced by reading radiation  55  from source  43  is collected by that same optical system  45  and upon reflection from dichroic mirror  44  is presented to certain scale in the plane of photodetector linear array  46 . Electric signals  57  arriving from the photodetector linear array, following their identification as described above, are converted to data and sent to a customer while control and display information signals are utilized to ensure normal operation of servo mechanism  50 , actuator  47  and power control unit for reading radiation source  43 . 
     The advantages of the proposed encoding method can be exemplified as follows. 
     When using the proposed in the present invention ETT method of two-dimensional data encoding for fabrication of a multilayer fluorescent optical pickup wherein&#39;the recording density in each information layer is equal to the recording density in DVD RON systems, the fluorescent information carrier should have the following parameters: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 Size of square element filled with fluorescent 
                 S = 0.4 μm × 0.4 μm 
               
               
                 substance (or not filled) 
               
               
                 Size of minimal region filled with fluorescent 
                 0.4 μm × 0.8 μm 
               
               
                 substance (or not filled) 
               
               
                 Size of information byte 
                 1.6 μm 2   
               
               
                 Size of channel bit 
                 0.4 μm 
               
               
                   
               
             
          
         
       
     
     This means that for above parameters the proposed ETT method of two-dimensional data encoding on fluorescent carriers enables writing density 60 MB/cm 2 , which is characteristic of DVD systems. 
     Let us assume that one information layer of the optical card has width 10 cm equal to the length of a linear array of 10 CCD cameras. If the layer length is 10 cm, said layer contains approximately 6 GB of information (or approximately 60 GB of information in 10 layers). Each layer consists of 250 information strips 400 μm wide and 10 cm long. Each CCD camera has 1000×1000 pixels and is capable of comparing the signal from each pixel with two levels and the signals from adjacent pixels between each other. If a CCD-camera pixel is equal to 8 μm, the approximate size of each camera is 1 cm×1 cm and that of the whole linear array 10 cm×1 cm. With a 20-fold magnification, the camera pixel size coincides with the magnified pit size. In this case, if the camera generates 30 frames per second, the whole linear array can read at the rate of 3·10 8  pit/s which in terms of the ETT code is 30·10 6  B/s, or approximately 220 Mb/s. 
     The rate of the linear CCD array movement above the optical card (or vice versa) is found from the assumption that the way of 400 μm (the field accepted by one camera as data pit-to-CCD pixel) covered within the time interval between two consecutive frames is {fraction (1/30)}s. Thus, the movement rate is approximately 12 mm/s, which is about 10 times less than the rate of one-time data reading from a CD optical disc. 
     A possible embodiment is when a CCD camera pixel is 4 μm, i.e. one information pit is read in 4 pixels of the CCD camera (to be more exact, ¼ of pit is read in one pixel). In this case, the reading rate slows down 4 times making approximately 55 Mb/s but the reading accuracy significantly improves resulting in a considerably reduced reading error probability. 
     FIGS. 7-9 show the original computer image of an ETT-coded fragment of the fluorescent optical card at λ=0.65 μm and NA=0.65 (FIG.  7 ), computer image of that same fragment generated by the optical system of the reading device in the plane of the linear CCD array (FIG. 8) and the real image of the same fragment read by the CCD camera. Subsequent processing of the latter image allows reproduction of the original image of said fragment with probability of 1. 
     Consequently, the proposed ETT-method of two-dimensional encoding of data on fluorescent carriers enables the same recording density as the EFM-encoding techniques used in DVD memory systems (see Table 1). In addition, the ETT-encoding method ensures provides fluorescent memory systems with other advantages: 
     1. The size of the minimal region filled with a fluorescent substance comparable with the size of the CD-format pit (and twice as big as the size of the minimal information pit of the DVD-format) facilitates the process for manufacturing a multilayer fluorescent information carrier and enables a higher fluorescent signal on reading thus facilitating operation of the reader; 
     2. The size of the ETT channel bit more than the CD one by a factor of 1.5 (and three-fold larger than that of the DVD format) allows a significant reduction in the frequency band and accordingly the reader photoreceiver noise; 
     3. Virtually 100% filling of the information layer area with fluorescent marks (information pits) permits application of parallel reading methods and enables an increase in the reading rate by tens of times in comparison with the DVD systems; 
     4. For equal wavelengths of the reading radiation and the objective lens used, a considerably lower reading error probability is enabled in contrast to the CD and DVD optical data carriers. 
     The proposed ETT (“eight-to-ten”) method of two-dimensional data encoding can be used not only in one- or multilayer fluorescent information carriers but also in carriers based on other physical and chemical principles, such as photorefractive crystals, photopolymers, magnetooptical, phase-change and persistent spectral-hole-burning recording systems, as well as in a number of other two- and three-dimensional carriers of ROM-, WORM- and WER- type. 
     It is to be noted that in the proposed invention the fluorescent information carrier has been discussed as an optical card allowing a simplest possible realization of the design of an optical pickup with parallel data reading. However, with some technological changes in the reading device, the proposed encoding method is applicable to other forms of optical memory, for example, such as an optical disc, optical memory plate, optical memory tape, or optical memory drum (cylinder), etc. 
     With appropriate modification, said method can be also used for three-dimensional volumetric data encoding as volumetric bytes recorded within a specified microvolume using the two-photon procedure, each said byte consisting of N number of cubic elements of certain size. Data can be read by means of either one- or two-photon procedure.