WO1999009551A1

WO1999009551A1 - Reading/recording method and apparatus for three-dimensional information carrier

Info

Publication number: WO1999009551A1
Application number: PCT/IL1998/000375
Authority: WO
Inventors: Boris Glushko; Vladimir Krupkin
Original assignee: Omd Devices, L.L.C.
Priority date: 1997-08-13
Filing date: 1998-08-12
Publication date: 1999-02-25
Also published as: IL121537A0; AU8644498A

Abstract

A method and an apparatus for reading information in a three-dimensional information carrier are presented. The information carrier is formed with a plurality of spaced-apart data regions, each surrounded by surrounding regions, wherein the data regions are made of a substantially fluorescent material, and the surrounding regions are made of a substantially optically transparent material with respect to a predetermined incident radiation. At least one pair of spatially separated, diffraction limited scanning beams of the incident radiation is produced. The at least one pair of beams is directed onto the information carrier and focused onto a focal plane of a light directing optics in a manner to provide their spatial separation in the focal plane and their overlap at any other location inside the carrier. The light directing optics provides a predetermined propagation of the at least one pair of beams within the carrier defined by spatial distribution of an integral intensity of the beams, so as to produce an output fluorescent radiation generated substantially by the fluorescent regions located in said focal plane.

Description

Reading/Recording Method and Apparatus for Three-Dimensional Information Carrier

FIELD OF THE INVENTION

This invention is in the field of scanning techniques and relates to a method and an apparatus for reading/recording in a three-dimensional information carrier. The invention is particularly useful for reading/recording in optical memory devices, non-destructive remote control and testing, diagnostics, etc.

BACKGROUND OF THE INVENTION

Three-dimensional information carriers, such as optical disks or optical cards, have been developed to significantly increase the amount of stored information, as compared to conventional two-dimensional optical memory devices. A three-dimensional optical memory device is typically formed of information layers, each having a plurality of spaced-apart data regions. The capacity of this device is proportional to the third order of a reading radiation wavelength. For example, the total thickness of a three-dimensional optical device can be about 1 mm and can consist of information layers having thickness of 0.01 mm. Thus, the storage capacity of this device is 100 times greater than the capacity of a single layer. A three-dimensional optical memory device is disclosed for example in U.S. Patent No. 4,090,031. The device comprises a substrate and a plurality of data layers provided on one side of the substrate. Each of the layers comprises data tracks formed of lines of data spots. The data spots are formed of either binary coded digital information, or frequency or pulse length modulated analog information, which is photographically recorded. According to one approach proposed in this patent, the data spots are substantially radiation-reflective. Unfortunately, the conventional reading techniques when applied to such a ''reflective" information carrier fail to provide successful read-out of the stored information. Indeed, when accessing the addressed layer by incident radiation, unavoidable multiple reflection and diffraction from upper and lower information layers occur. This results in undesirable crosstalk between the data regions of different layers, affecting the signal-to-noise ratio. According to an alternative approach disclosed in the above patent, the data regions are made of different photoluminescent materials having different optical properties. To read out the stored information, the device is scanned by "white" light of many frequencies so as to illuminate different layers by reading beams of different wavelengths. Accordingly, different colored filters are accommodated in front of numerous detectors, each associated with a corresponding one of the information layers. It is evident that this approach significantly complicates the manufacture of both the information carrier and reading device used therewith.

U.S. Patent No. 5,559,784 discloses a reading apparatus for reading out information stored in a multilayer recording medium. Here, multiple information layers are located on an optically transparent substrate, two adjacent information layers being spaced by a transparent intermediate layer. Each information layer comprises recording regions having a first refractive index spaced by regions having a second refractive index substantially different from the first one. Two alternating techniques are proposed to read out the information stored in such medium. According to one technique, illumination and detection units are accommodated at opposite sides of the medium. Scanning radiation impinges onto the medium from above, and refracted radiation is collected from below. It is understood that practically speaking, a reading device to implement this technique would be inconvenient in operation. By the other technique, radiation reflected from data regions, rather than that refracted, is collected.

Both of the above techniques apply two scanning beams for illumination of the medium, wherein the beams have the same frequencies and phase difference of 180°. The beams overlap (i.e. their optical paths coincide) everywhere except for the addressed in-focus layer. Thus, owing to the fact that the overlapping beams pass the same recording regions located in out-of-focus layers, these regions identically affect the beam's phases. As a result, the contribution of these regions to the beam's phase would be compensated, when reaching a common sensing means. As to the recording regions located in the in-focus layer, they also affect the beam's phase. But, due to the beams spatial separation in the in-focus layer, they cause a certain phase difference of each of the beams, which can be detected. The above technique requires the interference of two reflected beams to be effected on the common sensing surface, which is difficult to be carried out in practice. Additionally, each of the incident beams, as well as each of the reflected beams, interacts with data regions located in the optical path of the beams propagating towards the addressed layer and the sensor, respectively. These data regions affect the beams phase, resulting in that the detected signal, on the one hand, is too weak, and, on the other hand, contains too much "noise" to be successfully filtered out. SUMMARY OF THE INVENTION

There is accordingly a need in the art to overcome the disadvantages of the conventional reading/recording techniques by a novel scanning method and apparatus for recording and reading in a three-dimensional information carrier.

It is a major object of the present invention to provide such an apparatus that enables to eliminate or at least substantially reduce an undesirable crosstalk between data regions located in different layers of the information carrier. It is a further object of the present invention to provide such an apparatus that enables to significantly reduce absorption of reading radiation and significantly increase read out signals.

There is thus provided according to one aspect of the present invention, an apparatus for reading information in a three-dimensional information carrier formed with a plurality of spaced-apart data regions, each surrounded by surrounding regions, wherein the data regions are made of a substantially fluorescent material and the surrounding regions are made of a substantially optically transparent material with respect to a predetermined incident radiation, the apparatus comprising: (a) an illumination unit producing at least one pair of spatially separated diffraction limited coherent scanning beams of said incident radiation; (b) a light directing optics that defines a focal plane inside the information carrier and provides a spatial separation of the beams in said focal plane, the light directing optics being capable of providing a predetermined propagation of said at least one pair of beams within the carrier defined by spatial distribution of an integral intensity of said at least one pair of beams, so as to produce an output fluorescent radiation generated substantially by the fluorescent regions located in said focal plane; (c) a detector unit capable of receiving the output fluorescent radiation and generating data representative thereof.

5 The term "integral intensity " signifies total intensity resulting from an interference of the at least one pair of beams. The main idea of the present invention is based on the following. At least one pair of spatially separated diffraction limited coherent scanning beams of the certain incident radiation is provided. The beams are directed through a common optics onto the focal ι o plane in a manner to be spatially separated in the focal plane and to overlap at any location out of the focal plane. In other words, an interference of the beams occurs everywhere along their optical path, but except for the closest vicinity of the focal plane. As known, the propagation of each of the beams causes certain phase variations. The beams propagate along the same optical

15 path and their phase difference at each location depends on an initial phase difference, if any, and the optical path passed. The interference of the coherent beams of the same frequency, due to the phase variations, causes a total (integral) intensity to vary between maximum and minimum values. Thus, the interference of the beams provides a certain distribution of the

20 integral intensity. The beams' propagation towards the carrier represents a certain pattern formed by minimum and maximum values of the integral intensity. The incident radiation of the maximum and minimum integral intensities, while impinging on the fluorescent regions, would produce and would not produce, respectively, the fluorescent radiation. Hence, by

25 preventing the incident radiation of maximum integral intensity from reaching the inside of the carrier, or by providing the minimum integral intensity within a substantially wide area, the generation of the fluorescent radiation by the fluorescent regions located in the out-of- focus planes can be avoided. In other words, the present invention provides the effective interaction (i.e. the interaction producing the fluorescent radiation) of the scanning beams with the recording fluorescent regions located in the in-focus plane only.

The illumination unit comprises a light emitting means for generating at least one pair of incident beams having the same wavelength and intensity. This may be implemented by placing a pair of spaced-apart, diffraction limited apertures in the proximity of a coherent light source. The focusing optics images the apertures onto the focal plane.

By one embodiment of the invention, the illumination unit comprises a phase plate accommodated in the proximity of one of the apertures of said at least one pair of apertures. The phase plate shifts the phase of the corresponding beam by 180°. Accordingly, the interference of the beams, which occurs in all out-of-focus location inside the carrier, compensates the beams' amplitudes, i.e. provides the minimum, zero-intensity. When such "zero-intensity" radiation impinges onto the fluorescent region, it does not excite any fluorescent radiation in this region. On the contrary, in the in-focus layer, where the scanning beams are spatially separated, each of them produces the output fluorescent radiation, when interacting with the recording region. By providing an appropriate numerical aperture of the reading apparatus, the zero integral intensity of the incident radiation can be provided within a substantially wide region. Thus, the predetermined propagation of the beams inside the carrier is such that the minimum integral intensity is provided within a substantially wide area.

By another embodiment of the invention, the beams initially have the same phase. A mask, having a pattern formed by transmitting and blocking regions with respect to the incident radiation, is located in the optical path of the beams. These regions are aligned similar to the spatial distribution of the integral intensity of the incident radiation. The mask prevents the maximum intensity radiation from reaching the inside of the carrier, and allows the minimum intensity radiation to pass therethrough. Thus, the predetermined propagation of the beams inside the carrier is that within the areas of minimum integral intensity.

The light directing optics comprises a beam splitter for separating the output fluorescent radiation and directing it towards the sensing means. The detector unit comprises a sensing means and a filtering means.

The filtering means includes a spectral filter of a known kind for allowing the passage of the output fluorescent radiation and preventing any other radiation spectrum from reaching the sensing means. The sensing means comprises at least two sensing elements spaced from each distance substantially equal to the distance between two adjacent fluorescent regions. The detector unit may also comprise a confocal aperture accommodated in the proximity of the sensing means.

The illumination unit may comprise an array of apertures, in which case, the sensing means comprises an array of sensing elements. This enables a plurality of recording regions located in the focal plane to be read out simultaneously.

The information carrier may be an optical memory device, such as disk, card, etc., and may be manufactured by any suitable technique, for example such as disclosed in co-pending U.S. Patent Application Serial No. 08/956,052 assigned to the assignee of the present application. Alternatively, the information carrier may be a biological tissue carrying body, wherein the above apparatus enables to locate this tissue within a diagnostic region inside the body.

According to another aspect of the present invention, there is provided a method for reading information in a three-dimensional information carrier formed with a plurality of spaced-apart data regions, each surrounded by surrounding regions, wherein the data regions are made of a substantially fluorescent material, and the surrounding regions are made of a substantially optically transparent material with respect to a predetermined incident radiation, the method comprising:

(i) providing at least one pair of spatially separated diffraction limited coherent scanning beams of said incident radiation; (ii) directing said at least one pair of beams onto a focal plane of a light directing optics located inside the carrier, so as to provide a spatial separation of the beams in said focal plane, wherein said directing provides a predetermined propagation of said at least one pair of beams within the carrier defined by spatial distribution of an integral intensity of said at least one pair of beams, producing thereby an output fluorescent radiation generated substantially by the fluorescent regions located in said focal plane; and (iii) detecting the output fluorescent radiation and generating data representative thereof.

More specifically, the present invention is used for reading information in a three-dimensional optical memory disk and is therefore described below with respect to this application.

BRIEF DESCRIPTION OF THE DRAWINGS: In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic block diagram illustrating the main components and operational principles of a reading apparatus according to one embodiment of the invention; Figs. 2a to 2d are four experimental graphs, respectively, showing the main operational principles of the present invention, as compared to the conventional techniques;

Fig. 3 is a schematic block diagram illustrating the main components and operational principles of a reading apparatus according to another embodiment of the invention;

Figs. 4a to 4d illustrate four different arrangements of apertures, respectively, useful in the apparatus of Fig. 1; and

Fig. 5 is a schematic block diagram illustrating the main components and operational principles of a reading apparatus according to yet another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is first being made to Fig. 1 illustrating, by way of a block diagram, an apparatus, generally designated 1, for reading information stored in a disk 2 where information regions are distributed in a three-dimensional fashion. The disk 2 comprises several information layers, for example, three layers L_1? L₂ and L₃. The adjacent information layers are spaced by intermediate layers L⁽¹⁾ and L⁽²⁾, respectively, made of substantially optically transparent material. Recorded information stored in the data layers is in the form of a pattern having a plurality of spaced-apart data regions, generally at

Rf, containing a fluorescent material, which are spaced by substantially optically transparent regions R_t. It should be noted, although not specifically shown, that a suitable drive means is typically provided for driving the rotation of the disk 2 about its axis, and for driving a reciprocating movement of the disk with respect to the apparatus 1.

The reading apparatus 1 comprises an illumination unit 4, a light directing optics 6 and a detector unit 8. The illumination unit 6 includes a light source 10 that generates reading radiation B_r of the certain wavelength λ and intensity , and a pair of diffraction limited spaced-apart apertures 12a and 12b accommodated in the proximity of the light source 10. The apertures 12a and 12b are aligned along a line normal to the optical axis OA of the light directing optics 6. The arrangement of the apertures 12a and 12b is more specifically illustrated in Fig. 4a. As shown, the apertures 12a and 12b have identical shape (circular) and dimensions and are spaced from each other a certain distance d equal to the wavelength of the incident radiation, that is d= . The passage of the emitted radiation B_r through the apertures 12a and

12b results in a pair of diffraction limited coherent beams 14a and 14b of the same wavelength λ and intensity I. It should be noted, although not specifically shown, that the apertures can be replaced by a holographic element (e.g. grating), fiber optics or the like splitting means. A phase-shifting plate 16 is accommodated proximate to one of the apertures, for example the aperture 12b, in the optical path of the beam 14b and shifts the beam's phase by 180°. Alternatively, although not specifically shown, the aperture 12b may itself represent the phase-shifting plate 16.

The light directing optics 6 comprises a beam splitter 18, which is transparent with respect to the incident radiation B_r, thereby transmitting it towards the disk 2 and reflecting fluorescent radiation B_f returned from the disk 2. Converging lenses 20 and 22 are accommodated at opposite sides of the beam splitter 18. The light source and the addressed layer, for example the layer L₂, are positioned in the focal planes of the lenses 20 and 22, respectively. Thus, the lens 20 directs the reading radiation B_r in the form of two substantially overlapping parallel beams 14a and 14b onto the beam splitter 18, while the lens 22 focuses the beams onto a focal plane 24 coinciding with the addressed layer L₂. The converging lenses 20 and 22 define the focal plane 24 inside the disk 2, and are designed and positioned in a manner to image the apertures 12a and 12b onto the focal plane 24, providing two spaced-apart images 26a and 26b.

The interaction between the reading radiation B_r and fluorescent data regions Ri and R2_f ⁽²⁾ located in the focal plane 24 produces output fluorescent radiation B_f having certain spectral characteristic different from that of the reading radiation. The fluorescent radiation B_f is collected by the lens 22, reflected by the beam splitter 18 and propagates towards the detector unit 8. A similar lens 28 is accommodated in the optical path of the fluorescent radiation Bf reflected from the beam splitter 18, so as to focus the fluorescent radiation onto the sensing surface, generally at S, of the detector unit 8.

The detector unit 10 comprises a sensing means in the form of two sensing elements 30a and 30b defining together the sensing surface S, and a spectral filter 32 located in front of the sensing elements 30a and 30b. The filter 32 may be of any known kind executing direct detection, namely operating so as to allow the propagation therethrough of the known spectrum of the fluorescent radiation, and to prevent any other radiation (reading) from reaching the sensing means. Each of the sensor elements 30a and 30b operates in a conventional manner for receiving light signals and providing electrical output representative thereof. The sensing elements 30a and 30b are spaced from each other similar to the adjacent fluorescent regions Rif and R2_f within the information layer. A confocal aperture 34 is optionally provided, being accommodated in the closest proximity of the sensing surface S, and serving for spatial rejection of unwanted radiation coming from non-addressed layer, including both the residual reading and fluorescent output. In the present example of Fig. 1, the aperture 34 has relatively large dimensions and transmits total fluorescence coming from the disk 2 onto the sensing surface S. The reading apparatus 1 operates in the following manner. The incident beams 14a and 14b scan the disk 2. Since the small apertures 12a and 12b are spatially separated by an appropriately small distance, and since the beams 14a and 14b pass through the common focusing optics (i.e. lenses 20 and 22) appropriately designed, the beams 14a and 14b overlap substantially everywhere, except for the focal plane 20, where they are spatially separated being aligned similar to the apertures 12a and 12b. Since the coherent beams 14a and 14b have the same wavelength λ and intensity / and has the phase difference of 180°, the interference of the beams yields the compensation of their intensities. Hence, the integral radiation produced by the overlapped beams 14a and 14b has zero-intensity. The interaction between such zero-intensity radiation with any fluorescent region does not affect this region. As shown in Fig. 1, the overlapped incident beams 14a and 14b pass the fluorescent regions located in the out-of-focus layers Li and L producing no fluorescence in these locations. The interaction of each of the incident beams separately with any fluorescent region does affect this region, producing the output fluorescent radiation component Rf. The incident beams 14a and 14b are spatially separated only in the focal plane 20, i.e. in the addressed layer L . Therefore, when the rotation of the disk 2 causes the interactions between the in-focus fluorescent regions Rif (2) and R2 ²⁾ with the beams 14a and 14b, respectively, two fluorescent radiation components Bι ²⁾ and B2_f ⁽²⁾ are produced. These fluorescent radiation components are collected by the lens 18 and reflected by the beam splitter 18 to be focused onto the sensing surface S by the imaging lens 28. The spectral filter 32 allows the passage of the fluorescent radiation therethrough and filters out any other radiation spectrum. The confocal

(2) aperture 34 combines both fluorescent radiation components Ri_f and R2 to be detected. This approach sacrifices the resolution for simplicity and for strengthening the received fluorescent signal. It should be noted, although not specifically shown, that the spectral filter 32 may be integral with the sensing elements 30a and 30b, rather than being a stand-alone unit as in the present example.

Turning now to Figs. 2a to 2d, there are illustrated four graphs 36a, 36b, 36c and 36d which represent experimental results obtained for the intensity distribution of the incident radiation while propagating towards the addressed layer L₂ located in the focal plane 24 of the focusing lenses 20 and 22. The X- and Y- axes are directed along the focal plane and optical axis OA of the light propagation, respectively, intersecting at the fluorescent region. The graphs 36a and 36b illustrate the propagation of two beams of incident radiation having identical wavelength (λ=0.65μm), intensity and phase, while the graphs 36c and 36d illustrate the propagation of two beams of identical wavelength (λ=0.65μm) and intensity, but having 180° phase difference. In the examples of Figs. 2a and 2c, the apertures 12a and 12b, and consequently the images 26a and 26b, are spaced from each other by the distance dj equal to the wavelength of the incident radiation, i.e. df=Λ. In the examples of Figs. 2b and 2d, the space < _> between the apertures 12a and 12b is as follows: d₂=2λ- "White" segments S_w show high intensity of the total radiation propagating towards the layer L₂, while "black" segments Sb show low intensity radiation.

As clearly seen in Figs. 2a and 2b, illustrating the case of two beams having identical wavelength, intensity and phase, the total intensity of the incident radiation is maximal at paraxial area of the optical axis OA and reduces at increased angles relative to the optical axis. On the contrary, when using the pair of beams having different phases (Figs. 2c and 2d), the intensity of the incident radiation is minimal at the paraxial area of the optical axis OA.

The absorbed energy and, consequently, the fluorescence is high when the interaction between the incident radiation and fluorescent regions occur at locations within the "white" segments, whilst being low at locations within the "black" segments. It is now clear that it is preferable to provide the propagation of the incident radiation through non-addressed (out-of-focus) layers substantially in the "black" areas. As to the numerical aperture characteristics of the reading apparatus, it is typically about 0.5 for high capacity memory devices. In this respect, the following important information is demonstrated in the graphs 36b and 36d, representing the cases of larger space (2λ) between the apertures 12a and 12b, as compared to that represented by the graphs 36a and 36d. Graph 36b shows a relatively small "white" area of high intensity reading radiation in the closest vicinity of the axis OA, and a relatively large low intensity ("black") area at the periphery region where most of the reading radiation propagates. Thus, in some cases, only this periphery region should be used for the propagation of the incident reading radiation, while blocking the propagation of the incident radiation in the paraxial area of the optical axis OA. Graph 36c shows relatively low reading radiation intensity ("black" segment) within nearly entire propagation area. This "black" area is proximate to the optical axis OA. The reading radiation propagating in this area does not affect the fluorescent regions, i.e. provides no effective interaction therewith.

In each of the above examples the beams are substantially spatially separated only in the focal plane 24, and therefore excite significant fluorescence at the focal plane 24 only. As to the out-of-focus planes, the above graphs teach that by minimizing the total (integral) intensity value of the incident radiation inside the carrier, excitation of any fluorescence in the out-of-focus layers would be avoided.

In the example of Fig. 1, the apertures 12a and 12b are aligned as shown in Fig. 4a. Fig. 4b illustrates another arrangement, generally at 40, of a pair of non-overlapping apertures 42a and 42b. This arrangement is also suitable for use in the apparatus 1, when it is desirable to sense all the fluorescence coming from the disk 2. Here, the annular aperture 42a surrounds the circular aperture 42b. Similarly, the dimensions of the apertures are close to diffraction limited. One of the apertures may represent the phase shifting plate.

Referring now to Fig. 3, there is illustrated an alternative example of a reading apparatus, generally designated 100, for reading information in the optical disk 2. The apparatus 100 is constructed and operated in accordance with the teachings described above with reference to Figs. 2a to 2d. Same reference numbers are used for identifying those components, which are identical in the examples of Figs. 1 and 3, to facilitate understanding. The apparatus 100 comprises an illumination unit 104, a light directing optics 106 and a detector unit 8. Here, the illumination unit comprises a pair of diffraction limited apertures 112a and 112b aligned similar to the apertures 12a and 12b in the apparatus 1, but being spaced from each other the distance _/_?, i.e. < _=2^. The beams 14a and 14b have the same wavelength λ, intensity / and, in comparison to those of the apparatus 1, have the same phase. Consequently, no phase-shifting plate is required. The light directing optics 106 comprises a mask 44 accommodated proximate to the lens 20 downstream thereof. The mask 44 may be positioned at any other location of the beams' overlap. The mask 44 has a pattern formed by regions 44a and 44b having transparent and opaque properties, respectively, with respect to the incident radiation and transparent for the fluorescent radiation. Alternatively, the regions 44a may be constituted by holes made in the absorbing plate. The mask is designed in a mam er to provide the transmission (regions 44a) and blocking (opaque regions 44b) of the incident radiation at those locations where minimum and maximum total intensity of the incident radiation (integral, resulting from the overlapping of the beams 14a and 14b) are expected, respectively. The distribution of the minimum and maximum intensity values can be easily taken into account, when designing a suitable arrangement of the apertures 112a and 112b and of the light directing optics 106, as described above with references to Figs. 2a-2d. The mask 44 (its regions 44b) blocks the reading radiation of maximum integral intensity, preventing it from reaching the inside of the disk.

In the above examples, the light source and the apertures constitute together a pair of point-like spaced-apart light sources producing a pair of diffraction limited coherent light beams. The sensor is equipped with a single aperture, which detects all the fluorescence coming from the disk.

Turning now to Figs. 4c and 4d, two more examples of the arrangements of apertures, designated respectively 46 and 48 are illustrated. The arrangements 46 and 48 are also suitable for use in the reading apparatus for reading out information stored in the disk 1. Each of the arrangements 46 and 48 comprises two pairs Ai and A₂ of non-overlapping apertures. In Fig. 4c, apertures 46a-46b and 46c-46d are shown, where one aperture in each pair (for example apertures 46b and 46c) may itself represent a phase-shifting plate or, alternatively, be followed, when mounted in the reading apparatus 1, by such a plate, shown at 42 in Fig. 1. In Fig. 4d, apertures 48a-48b form a first pair Ai and apertures 48c-48d form a second pair Ai. The apertures 48a-48d are aligned in a manner that each two adjacent apertures have an overlapping region, generally at 50, while the pair At is spatially separated from the pair A₂.

It should be noted that, when the use of a plurality of apertures is desired, the following two approaches are suitable. By one approach, each pair of beams (produced by the pair of apertures) could be considered separately, so as to prevent the interaction between each pair of beams and the carrier anywhere, except for the focal plane. This would result in the absence of absorption and fluorescence at non-addressed layers. To this end, each pair of beams should be independent from others, which could be achieved by making them of different wavelengths and/or polarizations. Alternatively, all the apertures are coherent, and all beams have certain constant phase relations. In this case, it is not necessary to have intensity compensating pairs of beams, but it is sufficient to have "zero" integral intensity (i.e. the field from all beams) at non-addressed layers.

In view of the above, the distribution of the incident (reading) radiation in the addressed layer is defined by composite shape of the entire apertures' arrangement. The arrangement shown in Figs. 4a, 4b and 4c above provide separate interaction of light beams passing through the apertures with the fluorescent regions. Each such region of interaction produces a fluorescent radiation component. By detecting the total fluorescence (i.e. by using a relatively large aperture in the detector unit) successful reading, with relatively low resolution and rate, is provided, while the detection of fluorescent radiation components coming from separated regions allows for parallel reading of different data regions with enhanced resolution and rate. The arrangement shown in Fig. 4d provides the interaction between the incident beams produced by each of the overlapping regions 50 with a corresponding fluorescent region located in the addressed layer. This forms a relatively smooth distribution of the incident radiation over the addressed layer.

Referring to Fig. 5, there is illustrated a reading apparatus, generally designated 200, constructed generally similar to that described above with reference to Fig. 1, but differing in the provision of an array of the pairs of apertures. Consequently, an illumination unit 204 comprises a plurality of n light emitters, generally at 210, emitting a plurality of beams of incident radiation B_r suitable to generate fluorescence when interacting with the fluorescent regions (not shown) inside the disk 1. An array of pairs of apertures 212 (for example 1000 apertures) is provided and accommodated in the closest proximity of the light emitters 210. One aperture in each pair represents a phase-shifting plate for shifting the phase of a corresponding beam at 180°. A detector unit 210 comprises a sensor 230 having an array of sensing elements, such as a charge coupled device (CCD) camera or TV camera. This construction of the reading apparatus 200 enables the entire information stored in the addressed information layer to be read out simultaneously.

It is important to note, although not specifically shown, that in either of the above described examples the disk 2 may be replaced by any other three-dimensional information carrier characterized by the following main feature:

- spaced-apart regions having fluorescent properties with respect to a certain incident radiation, each of the fluorescent regions being surrounded by optically transparent regions, with respect to this incident radiation.

For example, the information carrier may be the patient's body, and the reading apparatus may be used for detecting and locating an abnormal tissue in the body. To this end, a photosensitizer is collected in the abnormal tissue of the patient's body. The photosensitizer, when interacting with the appropriate incident radiation, produces fluorescence. The incident light (i.e. its frequency) should satisfy the following conditions:

- produce fluorescence of the photosensitizer;

- have low absorption in the tissue material, to allow for efficient focusing of the incident light components. Additionally, the above-described reading/recording technique could be used as an auto focusing technique. Indeed, the fluorescence is excited in the focal plane only. The fluorescence decreases when the information layer moves out of focus and this effect could be measured and used for auto focusing corrections. In view of the above, the advantages of the present invention are self-evident. Indeed, by providing one or more pairs of incident beams and effecting their spatial separation in the focal plane, whilst preventing their effective (fluorescence producing) interaction with the fluorescent regions

5 located in any other out-of-focus plane, the information could be successfully read out. The provision of an information carrier having a three-dimensional distribution of the fluorescent regions, on the one hand, significantly simplifies the construction of a suitable reading apparatus, and, on the other hand, enables signal-to-noise ratio in the detected radiation to be significantly _l o increased.

Those skilled in the art will readily appreciate that many modifications and changes may be applied to the invention as hereinbefore exemplified without departing from its scope defined in and by the appended claims.

Claims

CLAIMS:

1. An apparatus for reading information in a three-dimensional information carrier formed with a plurality of spaced-apart data regions, each surrounded by surrounding regions, wherein the data regions are made of a

5 substantially fluorescent material, and the surrounding regions are made of a substantially optically transparent material with respect to a predetermined incident radiation, the apparatus comprising:

(a) an illumination unit producing at least one pair of spatially separated diffraction limited coherent scanning beams of said l o incident radiation;

(b) a light directing optics that defines a focal plane inside the information carrier and provides a spatial separation of the beams in said focal plane, the light directing optics being capable of providing a predetermined propagation of said at least one pair of

15 beams within the carrier defined by spatial distribution of an integral intensity of said at least one pair of beams, so as to produce an output fluorescent radiation generated substantially by the fluorescent regions located in said focal plane;

(c) a detector unit capable of receiving the output fluorescent 20 radiation and generating data representative thereof.

2. The apparatus according to Claim 1, wherein said illumination unit comprises a light emitting device generating said incident radiation, and at least one pair of diffraction limited apertures splitting the incident radiation into said first and second scanning beams.

25 3. The apparatus according to Claim 2, wherein said illumination unit comprises a phase shifting means mounted in the optical path of one beam of said at least one pair of beams.

4. The apparatus according to Claim 1, wherein said light directing optics comprises a common focusing optics for focusing the first and second beams onto first and second spaced-apart locations in the focal plane, the beams overlapping at any other location inside the carrier.

5. The apparatus according to Claim 1, wherein the light directing optics comprises a mask having a certain pattern of transmitting and blocking channels with respect to the incident radiation, the pattern corresponding to the spatial distribution of the integral intensity of said at least one pair of beams in a plane defined by said mask.

6. The apparatus according to Claim 5, wherein said transmitting and blocking channels are aligned so as to transmit minimum and block maximum of the integral intensity radiation, respectively.

7. The apparatus according to Claim 1, wherein said light directing optics comprises a beam splitter accommodated in the optical paths of the incident and output radiation, and is capable of transmitting one of them and reflecting the other.

8. The apparatus according to Claim 1, wherein said detector unit comprises a sensor means for receiving light signals and generating data representative thereof, and a filtering means allowing the passage of said output fluorescent radiation onto the sensor means.

9. The apparatus according to Claim 1, wherein said detector unit comprises at least one confocal aperture.

10. The apparatus according to Claim 1, wherein

- said illumination unit produces an array of pairs of said scanning beams, the pairs differing from each other in at least one beams' parameter;

- said detector unit comprises an array of sensing elements.

11. The apparatus according to Claim 10, wherein said at least one parameter is wavelength.

12. The apparatus according to Claim 1, wherein

- said illumination unit produces an array of scanning beams, having a certain phase relation; - said detector unit comprises an array of sensing elements.

13. The apparatus according to Claim 1, wherein said information carrier is an optical memory device.

14. A method for reading information in a three-dimensional information carrier formed with a plurality of spaced-apart data regions, each surrounded by surrounding regions, wherein the data regions are made of a substantially fluorescent material, and the surrounding regions are made of a substantially optically transparent material with respect to a predetermined incident radiation, the method comprising: (i) providing at least one pair of spatially separated diffraction limited coherent scanning beams of said incident radiation;; (ii) directing said at least one pair of beams onto a focal plane of a light directing optics located inside the carrier, so as to provide a spatial separation of the beams in said focal plane, wherein said directing provides a predetermined propagation of said at least one pair of beams within the carrier defined by spatial distribution of an integral intensity of said at least one pair of beams, producing thereby an output fluorescent radiation generated substantially by the fluorescent regions located in said focal plane; and

(iii) detecting the output fluorescent radiation and generating data representative thereof.

15. The method according to Claim 14, wherein said at least one pair of scanning beams is produced by generating a parent beam of said incident radiation and passing the parent beam through a pair of spaced-apart parallel diffraction limited apertures.

16. The method according to Claim 14, wherein step (i) comprises shifting a phase of one of said at least one pair of beams.

17. The method according to Claim 14, wherein said directing comprising passing said beams through a mask having a certain pattern of transmitting and blocking channels with respect to the incident radiation corresponding to said spatial distribution of the integral intensity of said at least one pair of beams.

18. The method according to Claim 14, wherein said detecting comprises separating between the incident and output radiation.