The present invention is particularly relevant for optical data storage and optical disc apparatuses for reading and/or recording data from and/or on multi-layer optical discs.

BACKGROUND OF THE INVENTION Conventional optical scanning devices usually comprise a clamper on which a disc, such as a DVD (DVD stands for Digital Versatile Disc) is fixed before scanning. The optical scanning device comprises a spinning motor in order to rotate the clamper and the information carrier. The rotating information carrier is scanned by an optical beam, in order to read information written in spirally shaped tracks, or to write information in spirally shaped grooves.

European patent applications 03290470. 8, 03290471.6 and 03290473.2, filed on February 2003, the 27th, are dedicated to information carriers comprising a plurality of information layers which optical properties depend on a potential difference applied between two electrodes. Such information carriers can have a relatively large number of information layers. Actually, by appropriately selecting the potential differences applied to the information layers, one information layer of the information carrier can have optical properties suitable for scanning this information layer by means of an optical beam having a wavelength, whereas the other information layers can be transparent at the wavelength of the optical beam, thus not perturbing the scanning of the scanned information layer. In these patent applications, ROM, WORM and RW information carriers are described (ROM stands for Read Only Memory, WORM for Write Once Read Many and RW for ReWritable).

Hence, the expression"scanning"means either reading or writing data from or to the information carrier.

An example of such an information carrier is described in Fig. la and lb, and corresponds to an information carrier described in European patent application 03290470.8.

Such an information carrier comprises a first information layer 11, a first electrolyte layer 12, a first counter electrode 13, a spacer layer 14, a second information layer 15, a second electrolyte layer 16 and a second counter electrode 17. Such an information carrier might comprise more than two information layers. For example, such an information carrier

might comprise 10,20 or up to 100 or more information layers. For example, an information carrier comprising 6 information layers is depicted in Fig. lb. Such an information carrier might comprise information layers which optical properties cannot be changed by means of a potential difference. For example, the information carrier can comprise a ROM, a WORM or a RW information layer with non-switchable optical properties, said information layer being used as last information layer in the information carrier. This is particularly useful in an information carrier implementing the BD standard (BD stands for Blu-Ray Disc).

The information layers 11 and 15 comprise pits and lands, which are obtained by means of conventional techniques, such as embossing and printing.

This information carrier is intended to be scanned by an optical beam, which has a wavelength 1. The first and second electrolyte layers 12 and 16, the first and second counter electrodes 13 and 17 as well as the spacer layer 14, are chosen to be transparent at the wavelength 1, or at least to have a very small absorption at this wavelength, in order not to interact with the optical beam.

In the example of Fig. la and lb, the first and second information layers 11 and 15 comprise an electrochromic material. Other information carriers are described in the abovementioned patent applications, such as information carriers with information layers comprising liquid crystal materials.

An electrochromic material is a material having optical properties, which can change as a result of electron uptake or loss. Electrochromic materials are known from those skilled in the art. For example, the publication"Electrochromism: Fundamentals and Applications", written by Paul M. S. Monk et. al. and published in 1995, describes the properties of electrochromic materials. Preferably, the electrochromic materials used in an information carrier in accordance with the invention are thiophene derivatives, such as poly (3,4- ethylenedioxythiophene), also called PEDT or PEDOT and described, for example, in "Poly (3,4-ethylenedioxythiophene) and Its Derivatives: Past, Present and Future", by L. Bert Goenendaal et. al. , published in Advanced Materials 2000,12, No. 7.

In the example of Fig. la, the electrochromic material of the first and second information layers 11 and 15 is the same, and has a reduced state and an oxidized state. The electrochromic material is chosen to have a high absorption and reflection at the wavelength 1 when it is in its reduced state, and a low absorption and reflection at the wavelength 1 when it is in its oxidized state.

When the first information layer 11 is scanned for reading information from this first information layer 11, a potential difference V1 is applied between the first information layer

11 and the first counter electrode 13, the first information layer 11 being at a higher potential than the first counter electrode 13. A current flows from the first information layer 11 to the first counter electrode 13, whereas electrons are transported from the first counter electrode 13 to the first information layer 11. Electrons are absorbed by the electrochromic materials, which becomes reduced. For reasons of electrical neutrality, positive ions from the first electrolyte layer 12 are absorbed by the first information layer 11 or negative ions are expelled by the first information layer 11, and negative ions from the first electrolyte 12 are absorbed by the first counter electrode 13 or positive ions are expelled by the first counter electrode 13. Hence, the first counter electrode is an ion-accepting and donating electrode.

The potential difference V1 is chosen so that, when applied, the absorption and reflection of the first information layer 11 becomes relatively high at the wavelength 1.

Then, once the absorption and reflection of the first information layer 11 is high, information can be read from this information layer using conventional read-out techniques, such as the phase difference read-out principle used, for example, for read-out of CD-ROM, or alternatively by the reflection or absorption difference between marks and non-marks.

Once the information of the first information layer 11 has been read, the second information layer 15 is scanned. First, the first information layer 11 is made transparent by applying a potential difference-V1 between the first information layer 11 and the first counter electrode 13, which is a reverse potential difference compared to V1. As a consequence, the electrochromic material of the first information layer 11 becomes oxidized, in which state it has a low absorption and reflection at the wavelength 1. Then, the second information layer 15 is made absorbent, by applying a potential difference V2 between the second information layer 15 and the second counter electrode 17. In this example, V2 is equal to Vl, because the first and second information stacks comprise the same electrochromic material.

Once the absorption of the second information layer 15 is high, information can be read from this information layer. The first information layer 11 does not perturb read-out of information, because the first information layer 11 is made transparent. As a consequence, it is possible to address only one information layer, while the rest of the information carrier is transparent or has a low absorption and reflection. The desired layer is addressed by applying the suitable potential differences between the information layers and the counter electrodes of the different information stacks.

The information layers thus have optical properties, which depend on a potential difference applied between two electrodes. In the case of Fig. la and lb, the two electrodes

are the information layer and the counter electrode. In other cases, an information layer can be placed between two electrodes.

As a consequence, potential differences have to be applied to such an information carrier. This is not possible with a conventional optical scanning device in which the information carrier rotates during scanning.

SUMMARY OF THE INVENTION It is an object of the invention to provide an optical scanning device which is able to scan an information carrier comprising a plurality of information layers which optical properties depend on a potential difference applied between two electrodes.

To this end, the invention proposes an optical scanning device for scanning an information carrier comprising a plurality of information layers which optical properties depend on a potential difference applied between two electrodes, said optical scanning device comprising a rotating part comprising means for receiving said information carrier, said receiving means comprising a plurality of contacts for connecting said electrodes, and a plurality of receivers, each receiver corresponding to a given information layer, and a fixed part comprising an energy source adapted to transfer energy to the receiver corresponding to a selected information layer, the rotating part comprising means for applying a potential difference between the contacts connected to the electrodes corresponding to the selected information layer.

According to the invention, the information carrier is fixed to the rotating part. The rotating part comprises a plurality of contacts to which the electrodes are connected. As the rotating part and the information carrier rotate during scanning, it is not possible to apply potential differences to the information layers by means of wires connected to the fixed parts of the optical scanning device. As a consequence, the rotating part comprises the means for applying potential differences. In order to select an information layer to which a potential difference has to be applied in order to change its optical properties, a plurality of receivers are used in the rotating part, each receiver corresponding to a given information layer. When an information layer is selected, energy is transferred to the receiver corresponding to the selected information layer, by means of an energy source located in a fixed part of the optical scanning device. A circuit in the fixed part can control the energy source so that energy is transferred to the receiver corresponding to the selected information layer. As a consequence, no wire is used between the fixed part of the optical scanning device and the information carrier, which allows the information carrier to rotate freely.

Preferably, the means for applying a potential difference are adapted to apply a potential difference corresponding to said transferred energy between said two contacts. The potential difference applied to an information layer is thus provided by the energy source.

This has the advantage that no auxiliary energy source is required in the rotating part, which simplifies the manufacturing process of the rotating part.

Advantageously, the optical scanning device comprises an induction coil mounted on the rotating part and means for applying a magnetic flux through the induction coil in order to create an inductive current, the means for applying a potential difference being adapted to apply a potential difference corresponding to said inductive current between said two contacts. In this case, an induction coil mounted on the rotating part provides the electrical energy necessary to apply potential differences. In this case, the receivers are only used for selecting the information layer to which a potential difference has to be applied. The potential difference applied to an information layer is provided by the induction coil. This allows saving power, because the energy required for selecting an information layer is less that the energy required for applying a potential difference to this information layer. Alternatively, a battery is used in the rotating part, and the induction coil is used in order to recharge said battery.

In a first embodiment of the invention, the energy source comprises at least one radiation source and the receivers are photosensitive detectors. Preferably, the radiation source is a radiation source which is used for scanning the information carrier. In this case, the same radiation source is used in order to scan the information carrier and transfer energy to the receiver corresponding to the selected information layer. This simplifies the optical scanning device, as only one radiation source is required.

Advantageously, the radiation source is powered by a pulse generator with a powering period, said powering period being an integral multiple of the period of rotation of the rotating part. The radiation source can be the radiation source used for scanning the information carrier, or an auxiliary radiation source. In order to send the radiation to the same photosensitive detector, the radiation source is fired once a rotation of the rotating part, or once every two or three rotations of the rotating part. In this way, only the photosensitive detector corresponding to the selected information layer receives the radiation emitted by the radiation source.

Preferably, the photosensitive detectors are arranged in a circle and the energy source comprises a plurality of radiation sources arranged in a circle, said radiation sources being fired in such a way that the photosensitive detector corresponding to the selected information

layer is successively illuminated by all the radiation sources during one rotation of the rotating part. With this configuration, it is possible to transfer energy to a photosensitive detector continuously during the rotation of the clamper. As a consequence, it is possible to continuously apply a potential difference between the appropriate contacts, in order to change the optical properties of the selected information layer, which corresponds to the continuously illuminated photosensitive detector. Hence, the change of optical properties of the selected information layer is relatively rapid.

In a second embodiment of the invention, the receivers are conductive rings and the energy source comprises at least one conductive brush adapted to contact at least one conductive ring. According to this embodiment, an electrical contact is used in order to transfer energy between the fixed part and the rotating part. This can be realized by means of a slip contact. Preferably, the conductive ring comprises a conductive fluid. Such a conductive fluid can easily rotate with the rotating part, while being in contact with a fixed brush or electrode.

In a third embodiment of the invention, the receivers are primary conductors and the energy source comprises at least one secondary conductor, the transfer of energy between said secondary conductor and a primary conductor being realized by capacitive coupling.

In a fourth embodiment of the invention, the receivers are induction coils and the energy source comprises at least one electromagnetic part, the transfer of energy between said electromagnetic part and an induction coil being realized by inductive coupling.

These and other aspects of the invention will be apparent from and will be elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail by way of example with reference to the accompanying drawings, in which: - Fig. la and lb show an information carrier for use with an optical scanning device in accordance with the invention; - Fig. 2 shows a rotating part and an information carrier in accordance with the invention; - Fig. 3 shows an optical scanning device in accordance with the invention; - Fig. 4 shows an optical scanning device in accordance with an advantageous embodiment of the invention; - Fig. 5a shows a first optical scanning device in accordance with a first embodiment of the invention and Fig. 5b is a projection from a viewpoint P of Fig. 5a ;

- Fig. 6 shows a preferred embodiment of the optical scanning device of Fig. 5a ; - Fig. 7a shows a second optical scanning device in accordance with a first embodiment of the invention and Fig. 7b is a projection from a viewpoint P of Fig. 7a; - Fig. 8a shows a third optical scanning device in accordance with a first embodiment of the invention, Fig. 8b is a projection from a viewpoint P of Fig. 8a and Fig. 8c is a top view of the energy source of Fig. 8a ; - Fig. 9a shows a first optical scanning device in accordance with a second embodiment of the invention and Fig. 9b is a projection from a viewpoint P of Fig. 9a; -Fig. 10a shows a second optical scanning device in accordance with a second embodiment of the invention and Fig. 10b is a projection from a viewpoint P of Fig. 10a ; - Fig. 11 shows a preferred embodiment of the optical scanning device of Fig. 9a; - Fig. 12a shows an optical scanning device in accordance with a third embodiment of the invention and Fig. 12b is a projection from a viewpoint P of Fig. 12a ; - Fig. 13 shows an optical scanning device in accordance with a fourth embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION A rotating part and an information carrier in accordance with the invention are depicted in Fig. 2. A clamper 202 is adapted to fixedly receive an information carrier 201. The clamper 202 is mounted on a rotation axis 209, which is connected to a spinning motor, not shown on Fig. 2. The clamper 202 and the rotation axis 209 form the rotating part.

The information carrier 201 comprises a plurality of electrodes, such as electrodes 203 and 204. The information carrier 201 comprises a plurality of information layers, which optical properties depend on a potential difference applied between two electrodes. In the example of Fig. 2, only a part of the information carrier 201 is represented, which corresponds to an inner part of said information carrier 201. An information layer can be an electrode, as described in Fig. 1, or an information layer can be located between two electrodes.

The clamper comprises contacts, such as contacts 207 and 208, which are adapted to connect the electrodes of the information carrier 201. In this example, a first electrode 203 comprises a first connection 205, which is connected to a first contact 207, and a second electrode 204 comprises a second connection 206, which is connected to a second contact 208.

The contacts of the clamper 202 are connected to means for applying potential differences between two contacts, which applying means are comprised in the rotating part, as will be described in more details in the following Figures.

In the example of Fig. 2, the clamper 202 has a staircase shape, which allows connecting the electrodes to the contacts of the clamper. Other shapes of the clamper 202 can be provided, such as a clamper having a ring shaped surface with contacts arranged in a circle on this surface. In this case, the information carrier has connections arranged in a circle on one of its surfaces in the inner region of the information carrier, which connections are connected to the electrodes of the information carrier. In the following, the invention is described in relation with a clamper such as the clamper described in Fig. 2. Of course, the invention applies to any clamper, as soon as the clamper comprises contacts to which the electrodes of an information carrier can be connected.

An optical scanning device in accordance with the invention is depicted in Fig. 3. This optical scanning device comprises a clamper 301 and an energy source 300. The clamper 301 is mounted on a rotation axis 305, which is connected to a spinning motor. The clamper 301 and the rotation axis 305 form a rotating part. The energy source 300 is fixed in the optical scanning device. The clamper 301 comprises eight contacts 311 to 318. The rotating part comprises four receivers 321 to 324, which are mounted on the rotation axis 305 in this example. The energy source 300 comprises a generator 341 and four energy transferring means 331 to 334. The generator 341 controls switches in order to send energy to one of the transferring means 331 to 334.

Each receiver corresponds to a given information layer. In the example described hereinafter, the information carrier comprises four information layers. A first information layer is located between electrodes connected to contacts 317 and 318, a second information layer between electrodes connected to contacts 315 and 316, a third information layer between electrodes connected to contacts 313 and 314 and a fourth information layer between electrodes connected to contacts 311 and 312. The first receiver 321 corresponds to the first information layer, the second receiver 322 to the second information layer, and so on. Let it be assumed that the third information is selected, i. e. that a potential difference has to be applied between contacts 313 and 314 in order to change the optical properties of the third information layer. The generator 341 switches the switch connected to the energy transferring means 333 on, so that energy is sent to said energy transferring means 333, which energy is transferred to the third receiver 323. Hence, the energy source 300 is adapted

to transfer energy to the receiver corresponding to a selected information layer. The amount of energy to be transferred depends on the time and the potential difference needed in order to change the optical properties of an information layer, which depend on the structure of the information carrier and the materials used in said information carrier.

Then, the transferred energy can be used, as depicted in Fig. 3, in order to apply a potential difference between the contacts corresponding to the selected information layer. In this case, the means for applying a potential difference are wires connected to the receivers of the rotating part.

The rotating part might comprise additional means for applying a potential difference, such as a battery mounted in the rotating part. In this case, the transferred energy is only used in order to know which information layer is selected. The rotating part comprises a circuit, which is adapted to select the contacts between which a potential difference has to be applied, as a function of the receiver to which energy has been transferred. This circuit controls switches, for example transistors, so that the potential difference is applied between the selected contacts.

This allows reducing power consumption in the fixed part, as a small amount of energy needs to be transferred to a receiver so that the circuit in the rotating part can detect that the information layer corresponding to this receiver has been selected.

Fig. 4 shows an optical scanning device in accordance with an advantageous embodiment of the invention, with an induction coil mounted on the clamper. The clamper 301 comprises an induction coil 401 mounted on it. The optical scanning device further comprises a fixed magnet 402, which creates a magnetic field B. During scanning of the information carrier, the clamper 301 rotates. As a consequence, the magnetic flux created by the magnetic field inside the induction coil 401 varies, so that an inductive current is created in the induction coil 401. This inductive current is used by a circuit 403, which supplies said inductive current between the two contacts corresponding to the selected information layer.

In this case, no battery is needed in the rotating part. Alternatively, a battery is used in the clamper 301, and the inductive current is then used in order to recharge said battery.

A first optical scanning device in accordance with a first embodiment of the invention is depicted in Fig. 5a and 5b. This optical scanning device comprises the clamper 301 mounted on the axis of rotation 305, and a first, second, third and fourth radiation sources

505 to 508. The clamper 301 comprises the eight contacts 311 to 318, and a first, second, third and fourth photosensitive detectors 501 to 504. Fig. 5b is a projection from a viewpoint P of Fig. 5a. The radiation sources 505 to 508 are, for example, lasers or LEDs (LED stands for Light Emitting Diode). The photosensitive detectors 501 to 504 are, for example, photodiodes.

The optical scanning device further comprises the generator 341 adapted to send energy to the radiation sources 505 to 508, so that the radiation source that receives said energy emits a radiation. The generator 341 as well as the switches used to select the radiation source to which energy is sent are not represented on Fig. 5a.

The example described hereinafter is the same as the example described in Fig. 3.

When the optical properties of the first information layer have to be changed, that is to say the first information layer is selected, the first radiation source 505 is fired, i. e. energy is sent to this first radiation source 505. The first radiation source 505 emits a radiation, which is absorbed by the first photosensitive detector 501. This radiation is converted into a current, and a potential difference is thus created between the poles of the first photosensitive detector 501. As a consequence, a potential difference is applied between contacts 317 and 318, and the optical properties of the first information layer are changed. When the second information layer is selected, the second radiation source 506 is fired, and so on.

A radiation source can emit a continuous radiation. In this case, the radiation is converted into a current only when the corresponding photosensitive detector is illuminated.

In this example, a photosensitive detector is illuminated once per rotation of the clamper 301, during a time which depends on an area of the photosensitive detector. In order to save power, the radiation can be emitted only when the photosensitive detector is above the radiation source. This can be achieved by powering the radiation source by a pulse generator with a powering period, said powering period being an integral multiple of the period of rotation of the clamper 301. For example, if the powering period is equal to the period of rotation of the clamper 301, the photosensitive detector is illuminated once per single rotation of the clamper 301. If the powering period is twice the period of rotation of the clamper 301, the photosensitive detector is illuminated once every two rotations of the clamper 301.

Instead of the first, second, third and fourth radiation sources 505 to 508, a translatable radiation source might be used, which can be placed in front of either the first, the second, the third or the fourth photosensitive detectors 501 to 504. The generator 341 is adapted to control the position of said translatable radiation source. When the first information layer is selected, the generator 341 places the translatable radiation source in

front of the first photosensitive detector 501, and energy is sent to said translatable radiation source.

In this latter case, the energy source is translatable relatively to the information carrier. However, when the energy source transfers energy to the appropriate photosensitive detector, it remains in a fixed position. As a consequence, the expression"fixed part"should not be understood as a part that is completely fixed, but as a part that can be fixed while the rotating part rotates, i. e. a part that does not rotate with the rotating part.

Fig. 6 shows an optical scanning device in accordance with a first embodiment of the invention, wherein the radiation source is a radiation source which is used for scanning the information carrier. The optical scanning device comprises a laser source 601 and an objective lens 602. The laser source 601 produces an optical beam, which is then focussed on a scanned information layer, by means of the objective lens 602.

In this embodiment, the laser source 601 is also used for transferring energy to a photosensitive detector corresponding to a selected information layer. Actually, the laser source 601 is a part of an optical pick-up unit, which is translatable relatively to the information carrier. Hence, the laser source 601 can be used in order to replace the four radiation sources 505 to 508 of Fig. 4. When the first information layer is selected, the radiation source 601 is placed in front of the first photosensitive detector 501, and emits a radiation. When the second information layer is selected, the radiation source 601 is placed in front of the second photosensitive detector 502, and so on. Once a proper quantity of energy has been transferred to a photosensitive detector, the laser source 601 is translated in order to scan the information carrier. The quantity of energy that has to be transferred depends, inter alia, on the properties of the materials used in the information carrier, and of the structure of the rotating part. For example, if the rotating part further comprises a battery, as explained in Fig. 3 and 4, the quantity of energy that need to be transferred is relatively low.

The radiation emitted by the radiation source 601 might be focussed on a photosensitive detector, by means of the objective lens 602, or by means of an additional lens. It is also possible to send the radiation as such to the photosensitive detector.

Fig. 7a shows a second optical scanning device in accordance with a first embodiment of the invention and Fig. 7b is a projection from a viewpoint P of Fig. 7a. This optical scanning device comprises the clamper 301 mounted on the axis of rotation 305, and a radiation source 705. The clamper comprises eight photosensitive detectors 701a, 701b, 702a,

702b, 703a, 703b, 704a and 704b. The photosensitive detectors are arranged in a circle on the bottom surface of the clamper 301.

In this embodiment, the radiation source 705 can be, for example, a laser diode or a LED. Alternatively, the radiation source 705 can be the laser source, which is used for scanning the information carrier.

The example described hereinafter is the same as the example described in Fig. 3. The contacts 311 and 312 are connected to the poles of the photosensitive detector 704a, the contacts 313 and 314 are connected to the poles of the photosensitive detector 703a, the contacts 315 and 316 are connected to the poles of the photosensitive detector 702a and the contacts 317 and 318 are connected to the poles of the photosensitive detector 701a.

When the first information layer is selected, the photosensitive detector 701a is illuminated by the radiation source 705. The position of the photosensitive detector 701a with respect to the radiation source 705 is determined by means of a tacho comprised in the clamper 301. Most of the optical scanning devices comprise such a tacho. If the optical scanning device does not comprise any tacho, a synchronisation pattern is provided on the clamper 301, indicating a reference point of the clamper 301. The positions of the photosensitive detectors with respect to this reference point are known, as well as the angular velocity of the rotating part. The synchronisation pattern is detected by means of the optical pick-up unit. Hence, the position of the photosensitive detector 701a can easily be determined.

The radiation source 705 is powered by a pulse generator with a powering period, said powering period being an integral multiple of the period of rotation of the rotating part. As a consequence, only the photosensitive detector 701a is illuminated when the information carrier rotates. Then, the radiation is converted into a current, and a potential difference is thus created between the poles of the first photosensitive detector 701a. As a consequence, a potential difference is applied between contacts 317 and 318, and the optical properties of the first information layer are changed.

When the second information layer is selected, the photosensitive detector 702a is illuminated by the radiation source 705. This can be done by delaying the emission of the radiation during a time corresponding to the angle between the photosensitive detectors 701a and 702a. In the example of Fig. 7a and 7b, this angle is 90 degrees. Then, the radiation source is powered by a pulse generator with a powering period, said powering period being an integral multiple of the period of rotation of the rotating part.

In the example of Fig. 7a and 7b, the clamper 301 further comprises four additional photosensitive detectors 701b to 704b. The photosensitive detector 701b corresponds to the first information layer, the photosensitive detector 702b corresponds to the second information layer, the photosensitive detector 703b corresponds to the third information layer and the photosensitive detector 704b corresponds to the fourth information layer. The contacts 311 and 312 are connected to the poles of the photosensitive detector 704b, the contacts 313 and 314 are connected to the poles of the photosensitive detector 703b, the contacts 315 and 316 are connected to the poles of the photosensitive detector 702b and the contacts 317 and 318 are connected to the poles of the photosensitive detector 701b.

However, the polarity of the photosensitive detectors 701b to 704b is inverted compared to the polarity of the photosensitive detectors 701a to 704a. This means for example that the potential difference applied between the contacts 317 and 318 when the photosensitive detector 701b is illuminated is a reverse potential difference compared to the potential difference applied between the contacts 317 and 318 when the photosensitive detector 701a is illuminated. This allows applying positive and negative potential differences between the electrodes of the information carrier. Actually, this might be required because in certain information carriers with a plurality of information layers which optical properties depend on a potential difference applied between two electrodes, different potential differences have to be applied, depending on the desired change in optical properties. For example, it might be necessary to apply a positive potential difference to an information layer in order to make it absorbent and reflective, and a negative potential difference in order to make it transparent.

A third optical scanning device in accordance with a first embodiment of the invention is depicted in Fig. 8a, 8b and 8c. Fig. 8b represents a bottom view of the clamper 301 of Fig. 8a and Fig 8c represents a top view of the energy source of Fig. 8a. This optical scanning device comprises the clamper 301 mounted on the axis of rotation 305, and an energy source comprising a set of radiation sources 801 to 808 arranged in a circle. The clamper comprises four photosensitive detectors 701a, 702a, 703a and 704a. The photosensitive detectors are arranged in a circle on the bottom surface of the clamper 301.

The example described hereinafter is the same as the example described in Fig. 7. In order to select the second information layer, the radiation source located below the photosensitive detector 702a emits a radiation. If the clamper is in a position corresponding to the position depicted in Fig. 8b, the radiation source 801 emits a radiation. As a

consequence, a potential difference is applied between contacts 315 and 316, as explained hereinbefore. However, the clamper 301 rotates, so that after a certain time, which depends on the rotation frequency of the clamper 301, the photosensitive detector 702a exits the area illuminated by the radiation source 801, which is represented in dotted line in Fig. 8b. When the photosensitive detector 702a exits the area illuminated by the radiation source 801, the radiation source that is directly adjacent to the radiation source 801 in the sense of rotation of the clamper 301 is fired, whereas the radiation source 801 stops emitting a radiation. In this example, if the sense of rotation of the clamper 301 is clockwise, the radiation source 808 is fired. This is repeated throughout a whole rotation of the clamper 301, so that after a complete rotation, the radiation source 801 is again fired. This can be achieved by means of a pulse generator controlling the radiation sources 801 to 808, which fires the radiation sources with a time delay synchronous to the rotation of the clamper 301.

As a consequence, the photosensitive detector corresponding to the information layer to be selected is successively illuminated by all the radiation sources during one rotation of the clamper. Hence, current is created in said photosensitive detector during almost all the rotation of the clamper. It is even possible to obtain a continuous current in said photosensitive detector, if the areas illuminated by adjacent radiation sources slightly overlap. Consequently, as the speed of change of optical properties of the corresponding information layer depends on the current created in the photosensitive detector, the optical device of Fig. 8a to 8c allows obtaining rapid changes of optical properties.

Fig. 9a shows a first optical scanning device in accordance with a second embodiment of the invention. Such an optical scanning device comprises a first, a second, a third, a fourth and a fifth conductive rings 921 to 925, a first, a second, a third, a fourth and a fifth brushes 931 to 935 and the generator 341. Fig. 9b is a projection from a viewpoint P of Fig. 9a.

The brushes are for example carbon brushes. A conductive ring and a carbon brush, such as the first conductive ring 921 and the first brush 931, form a slip contact. Such a slip contact is described, for example, in patent US 4,398, 113, granted August 9, 1983. Instead of a brush 931, another conductive ring can be used, which is fixed relatively to the first conductive ring 921, and which is in electrical contact with said first conductive ring 921. In order to achieve this, a ball bearing can be used between the two conductive rings. The ball bearing is lubricated by a conductive oil or grease which may contain electrically conductive particles such as carbon particles or metal particles or a conductive polymer.

The first brush 931 is in electrical contact with the first conductive ring 921, the second brush 932 is in electrical contact with the second conductive ring 922, and so on. The fifth brush 935 is connected to a ground of the generator 341, and the first, third, fifth and seventh contacts 311,313, 315 and 317 are connected to the fifth conductive ring 925.

When the first information layer is selected, the generator 341 switches the switch corresponding to the first brush 931, and energy is sent to said first brush 931, which means in this case that a potential difference is applied between the first brush 931 and the ground.

As a consequence, a potential difference is applied between the contacts 318 and 317, and the optical properties of the first information layer are changed.

Instead of the first, second, third and fourth brushes 931 to 934, a translatable brush might be used, which can be in electrical contact with either the first, the second, the third or the fourth conductive ring 921 to 924. The generator 341 is adapted to control the position of said translatable brush. When the first information layer is selected, the generator 341 places the translatable brush in electrical contact with the first conductive ring 921, and energy is sent to said translatable brush.

Fig. 10a shows a second optical scanning device in accordance with a second embodiment of the invention and Fig. 10b is a projection from a viewpoint P of Fig. 10a.

This optical scanning device comprises the same elements as the optical scanning device depicted in Fig. 9a and 9b, but the conductive rings 921 to 925 are mounted on the clamper 301, instead of the rotation axis 305. The conductive rings 921 to 925 have different diameters, so that they can be mounted on a same surface of the clamper 301.

The functioning of such an optical scanning device is the same as the functioning described in Fig. 9a and 9b. Instead of the first, second, third and fourth brushes 931 to 934, a translatable brush might be used, which can be in electrical contact with either the first, the second, the third or the fourth conductive ring 921 to 924. The generator 341 is adapted to control the position of said translatable brush.

Fig. 11 shows a preferred embodiment of the optical scanning device of Fig. 9a and 9b. Only one conductive ring and one brush or electrode, corresponding to the first conductive ring 921 and the first brush 931, have been represented, for reasons of convenience. In this preferred embodiment, the other conductive rings and brushes of Fig. 9a and 9b are identical to the conductive ring and brush represented on Fig. 11.

In this preferred embodiment, the conductive ring 921 is made of a conductive fluid 1104. The conductive fluid is encapsulated in an isolated part 1101, which is fixed in the optical scanning device. Sealing rings 1105 are used in order to allow the rotation axis 305 to rotate relatively to the isolated part 1101.

An electrode 1102 is plunged into the conductive fluid 1104, and is connected to the generator 341, not represented in Fig 11, via a switch. When the first information layer is selected, this switch is switched on and a potential difference is thus applied between the electrode 1102 and the ground. As the electrode 1102 is in electrical contact with the conductive fluid 1104, which is connected to the contact 318, said potential difference is applied between the contacts 317 and 318. The conductive fluid 1104 can be a conducting fluid, or a suspension of a fluid carrier with metallic particles or carbon particles. For example, a polymeric matrix with copper particles embedded in the matrix can be used as conductive fluid 1104.

Fig. 12a shows an optical scanning device in accordance with a third embodiment of the invention. In this embodiment, the optical scanning device comprises a first, a second, a third and a fourth primary conductors 1221 to 1224 and a first, a second, a third and a fourth secondary conductors 1231 to 1234. The secondary conductors 1231 to 1234 have two poles, one of the poles being connected to a ground, not shown on Fig. 12a, the other one connected to the generator 341, via a switch. The primary conductors 1221 to 1224 are mounted on the rotation axis 305 and the secondary conductors 1231 to 1234 are fixed parts of the optical scanning device. An insulator is placed between a primary and a secondary conductor.

Hence, the optical scanning device comprises four insulators 1241 to 1244. An insulator is, for example, air or a thin film of insulating oil. A primary conductor, an insulator and a secondary conductor form a capacitive ring. Fig. 12b is a projection from a viewpoint P of Fig. 12a.

The first primary conductor 1221 has two poles, one connected to contact 318, the other one to contact 317. The second primary conductor 1222 also has two poles, one connected to contact 316, the other one to contact 315, and so on.

When the first layer is selected, the generator 341 switches the switch corresponding to the first secondary conductor 1231 on, and a potential difference is applied between the poles of the first secondary conductor 1231, which is arranged in such a way that a capacitive coupling occurs between the first secondary conductor 1231 and the first primary conductor 1221. As a consequence, energy is transmitted to the first primary conductor 1221, and thus

received by said first primary conductor 1221. A potential difference is thus applied between the poles of the first primary conductor 1221. Hence, a potential difference is applied between contacts 318 and 317, and the optical properties of the first information layer are changed.

As capacitive coupling does not require any contact between the secondary conductors 1231 to 1234 and the primary conductors 1221 to 1224, it is easily implemented in an optical scanning device in accordance with the invention, which requires an energy transfer between a fixed part and a rotating part.

Instead of the first, second, third and fourth secondary conductors 1231 to 1234, a translatable secondary conductor might be used, which can be arranged so that a capacitive coupling occurs with either the first, the second, the third or the fourth primary conductors 1221 to 1224. The generator 341 is adapted to control the position of said translatable secondary conductor. When the first information layer is selected, the generator 341 places the translatable secondary conductor around the first conductive ring 1221, and energy is sent to said translatable secondary conductor.

Fig. 13 shows an optical scanning device in accordance with a fourth embodiment of the invention. In this embodiment, the receivers are induction coils and the energy source comprises the generator 341 and electromagnetic parts, the transfer of energy between an electromagnetic part and an induction coil being realized by inductive coupling. Only one induction coil 1321 and one electromagnetic part 1331 have been represented, for reasons of convenience. The optical scanning device further comprises three electromagnetic parts and three induction coils, each corresponding to a given information layer.

The induction coil 1321 is mounted on the rotation axis. The electromagnetic part 1331 is a fixed part of the optical scanning device. The electromagnetic part 1331 is connected to the generator 341 via a switch, so that a magnetic field is created when a potential difference is applied to said electromagnetic part 1331.

When the first layer is selected, the generator 341 switches the switch corresponding to the electromagnetic part 1331 on, and an alternative potential difference is applied to said electromagnetic part 1331. The electromagnetic part 1331 converts this alternative potential difference into an alternative magnetic field. This alternative magnetic field creates a variation of the magnetic flux through the induction coil 1321, thus creating an inductive current inside said induction coil 1321. This inductive current is applied between contacts

317 and 318. As a consequence, a potential difference is applied between said contacts 317 and 318, and the optical properties of the first information layer are changed.

In this embodiment, the rotation of the induction coil 1321 does not play any role, as the inductive current is created by the variation of the magnetic flux inside the induction coil 1321, which variation is due to the alternative magnetic field. As a consequence, the transferred energy does not depend on the speed of rotation of the rotating part, which is an advantage.

Instead of the four electromagnetic parts, a translatable electromagnetic part might be used, which can be arranged so that it creates an alternative magnetic field inside the induction coil corresponding to the selected information layer. The generator 341 is adapted to control the position of said translatable electromagnetic part.

Any reference sign in the following claims should not be construed as limiting the claim. It will be obvious that the use of the verb"to comprise"and its conjugations does not exclude the presence of any other elements besides those defined in any claim. The word"a" or"an"preceding an element does not exclude the presence of a plurality of such elements.