WO2005008637A2

WO2005008637A2 - Multi-stack information carrier with photochromic materials

Info

Publication number: WO2005008637A2
Application number: PCT/IB2004/002289
Authority: WO
Inventors: Martinus Van Der Mark; Rifat Hikmet; Robert Hendriks
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-07-18
Filing date: 2004-07-08
Publication date: 2005-01-27
Also published as: TW200509096A; WO2005008637A3

Abstract

The invention relates to an information carrier for scanning information by means of an optical beam (OB) having a wavelength, said information carrier comprising at least two information stacks. Each stack is a waveguide comprising two cladding layers (11, 14) and a waveguide core (13) between said two cladding layers. The waveguide comprises an information layer (12) comprising a photochromic material which optical properties at the wavelength of the optical beam can be switched by means of a light beam (LB) propagated into said waveguide.

Description

Multi-stack information carrier with photochromic materials

FIELD OF THE INVENTION The present invention relates to a multi-stack optical information carrier. The present invention also relates to a scanning device for scanning a multi-stack optical information carrier. The present invention also relates to a method of reading from and a method of recording on a multi-stack optical information carrier. 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-stack optical discs.

BACKGROUND OF THE INVENTION In the field of optical recording, increasing the capacity of the information carrier is the trend. An already investigated way for increasing the data capacity consists in using a plurality of information layers in the information carrier. For example, a DVD (Digital Video Disc) can comprise two information layers. Information is recorded on or read from an information layer by means of an optical beam, using local refractive index variations or the presence of surface relief structures. However, the number of information layers in such an information carrier is limited. First, because the light intensity of the optical beam decreases with each additional addressed layer. Actually, when the optical beam has to pass many layers for addressing a layer, interaction takes place in the non-addressed layers, reducing the intensity of the optical beam. Additionally, the local refractive index variations of the written information patterns in the non-addressed layers cause refraction and scattering of the traversing light-beam, leading to deteriorated writing and reading. Hence, conventional optical data storage techniques are not suitable for multi-layer information carriers, in particular for information carriers comprising more than three layers.

SUMMARY OF THE INVENTION It is an object of the invention to provide an information carrier, which can comprise an increased number of layers. To this end, the invention proposes an information carrier for scanning information by means of an optical beam having a wavelength, said information carrier comprising at least two information stacks, wherein each stack is a waveguide comprising two cladding layers and a waveguide core between said two cladding layers, said waveguide comprising an information layer comprising a photochromic material which optical properties at the wavelength of the optical beam can be switched by means of a light beam propagated into said waveguide. According to the invention, the information layers comprise a photochromic material, which optical properties can be switched by propagating a light beam into a waveguide. The waveguide comprises two cladding layers and a waveguide core, which are arranged in such a way that the light beam can be confined by total internal reflection into said waveguide. Hence, the optical properties of the photochromic material of a selected information stack can be switched by propagating a light beam into said information stack. As a consequence, by appropriately switching the optical properties of the information layers, it is possible to scan one information layer having optical properties suitable for interacting with the optical beam, whereas the optical properties of the other information layers are chosen so that the interactions between these non-addressed layers and the optical beam are reduced. As a consequence, the number of layers might be increased. In an advantageous embodiment of the invention, the light beam has a wavelength that differs from the wavelength of the optical beam. In order to propagate the light beam into a waveguide, the cladding layers and the waveguide core have different refractive indices at the wavelength of the light beam. Actually, in order to achieve total internal reflection of the light beam in the waveguide, the waveguide core needs to have a refractive index that is higher than the refractive index of the cladding layers. However, it is preferable that the waveguide cores and the cladding layers have substantially equal refractive indices at the wavelength of the optical beam, in order to avoid parasitic reflections at the interfaces between the cladding layers and the waveguide cores, during scanning of an information layer. According to this advantageous embodiment, it is possible to design the information carrier so that the cladding layers and the waveguide cores have substantially equal refractive indices at the wavelength of the optical beam, whereas they have substantially different refractive indices at the wavelength of the light beam. In a preferred embodiment of the invention, a cladding layer of an information stack serves as cladding layer of another information stack. This reduces the number of layers of the stacks. Hence, the information carrier is less bulky, and the manufacturing process of the information carriers is simplified. In another advantageous embodiment of the invention, the photochromic material can be locally degraded by means of the optical beam in order to write information in the information layer. According to this embodiment, information might be written by a user on the information carrier. In another preferred embodiment of the invention, the information carrier comprises a mirror for directing the light beam into the information stacks. This allows in particular using the laser source used for scanning the information layers as source for generating the light beam used for switching the optical properties of the information layers. In another advantageous embodiment of the invention, an information stack comprises at least a first and a second section arranged so that the light beam propagated in the first section is not propagated in the second section. According to this embodiment, it is possible to switch the optical properties of the second section by propagating the light beam into said second section, while the first section is being scanned by the optical beam. This limits the time required for switching the optical properties of an information layer before it can be scanned. The invention also relates to an optical scanning device for scanning an information carrier by means of an optical beam having a wavelength, said information carrier comprising at least two information stacks, wherein each stack is a waveguide comprising two cladding layers and a waveguide core between said two cladding layers, said waveguide comprising an information layer comprising a photochromic material which optical properties at the wavelength of the optical beam can be switched by means of a light beam propagated into said waveguide, said optical scanning device comprising means for generating the optical beam, means for focussing said optical beam on an information layer, means for generating the light beam and means for directing said light beam into the waveguide corresponding to a selected information layer. Advantageously, the optical device comprises a clamper for receiving the information carrier, and the means for directing said light beam comprise a mirror mounted on said clamper. Preferably, the optical scanning device further comprises a bleaching light source for producing a bleaching light having a wavelength that differs from the wavelength of the light beam, said bleaching light source being adapted to illuminate the whole information carrier. The bleaching light is used for switching the optical properties of the information layers which optical properties had been switched by propagation of the light beam in the corresponding waveguides. The invention also relates to a method of reading information from an information carrier by means of an optical beam having a wavelength, said information carrier comprising at least two information stacks, wherein each stack is a waveguide comprising two cladding layers and a waveguide core between said two cladding layers, said waveguide comprising an information layer comprising a photochromic material which optical properties at the wavelength of the optical beam can be switched by means of a light beam propagated into said waveguide, said method comprising the steps of propagating a light beam into the information stack from which information shall be read and focussing the optical beam on the information layer of said stack. The invention further relates to a method of recording information on an information carrier by means of an optical beam having a wavelength, said information carrier comprising at least two information stacks, wherein each stack is a waveguide comprising two cladding layers and a waveguide core between said two cladding layers, said waveguide comprising an information layer comprising a photochromic material which optical properties at the wavelength of the optical beam can be switched by means of a light beam propagated into said waveguide, said method comprising the step of focussing the optical beam on the information layer of the information stack on which information shall be recorded in order to locally degrade the photochromic material. 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, lb and lc show a first ROM information carrier in accordance with the invention;

- Fig. 2a and 2b show a second and a third ROM information carrier in accordance with the invention;

- Fig. 3 shows a WORM information carrier in accordance with the invention;

- Fig. 4 shows an information carrier in accordance with an advantageous embodiment of the invention;

- Fig. 5 shows a first optical scanning device in accordance with the invention; - Fig. 6 shows a second optical scanning device in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION A first ROM information carrier in accordance with the invention is depicted in Fig. la. Such an information carrier comprises a first cladding layer 11, a first information layer 12, a first waveguide core 13, a second cladding layer 14, a second information layer 15, a second waveguide core 16 and a third cladding layer 17. In this example, the first information layer 12 is between the first cladding layer 11 and the first waveguide core 13, the second information layer 15 is between the second cladding layer 14 and the second waveguide core 16. The first cladding layer 11, the first waveguide core 13 and the second cladding layer 14 form a first waveguide. This first waveguide comprises the first information layer 12. The second cladding layer 14, the second waveguide core 16 and the third cladding layer 17 form a second waveguide. This second waveguide comprises the second information layer 15. An information carrier in accordance with the invention might comprise more than two information stacks. For example, an information carrier in accordance with the invention might comprise 10, 20 or up to 100 or more information stacks. For example, an information carrier in accordance with the invention, which comprises 6 information stacks, is depicted in Fig. lb. An information layer comprises pits and lands that comprise a photochromic material.

This information carrier is a ROM (Read Only Memory) information carrier, which means that a user cannot record information on this carrier. The information is recorded during a manufacturing process and cannot be erased. Such an information carrier is manufactured by means of conventional techniques, such as described in patent application WO 98/50914. For example, a stamper comprising a plurality of convexities is applied to the second waveguide core 16. This results in providing a pattern on the surface of said second waveguide core 16, said pattern being similar to the convexities of the stamper. Then, a layer comprising the photochromic material is deposited onto the surface of the patterned layer. This layer comprising the photochromic material is chosen to have good adhesion properties to the patterned layer. A part of this layer penetrates into the pits of the patterned layer and another part remains on the surface of the lands of the patterned layer. This other part is eventually eliminated by means of a suitable solvent. Hence, the second information layer 15 is obtained. Then, the second information layer 15 is coated with the second cladding layer 14, which is then coated with the first waveguide core 13. A stamper comprising a plurality of convexities is then applied to this first waveguide core 13, and the operations described above are repeated in order to obtain the first information layer 12. These operations can then be repeated in order to obtain an information carrier comprising a plurality of information stacks. Such an information carrier might also be manufactured by means of an injection molding technique, as described in WO 98/50914. This information carrier is intended to be scanned by an optical beam OB, which has a wavelength 11. The first, second and third cladding layers 11, 14 and 17, as well as the first and second waveguide cores 13 and 16, are chosen to be transparent at the wavelength 11, or at least to have a very small absorption and reflection at this wavelength, in order not to interact with the optical beam. The first and second information layers 12 and 15 comprise a photochromic material. A photochromic material is a material having optical properties, which can change as a result of absorption of light having a suitable wavelength. Photochromic materials are known from those skilled in the art. For example, the publication "Photochromism: Memories and Switches", published in May 2000 in Chemical Reviews 100, describes the properties of photochromic materials. A photochromic material has at least two different molecular forms having different absorption spectra. A first form of the photochromic material can be changed to a second form by absorption of light having a suitable wavelength. This change leads to a change in refractive index and absorption of the photochromic material. The change of the form of the photochromic material is reversible. This means that the second form of the photochromic material can be turned to the first form by absorption of light having a suitable wavelength that differs from the wavelength used for changing the first form to the second form. For certain photochromic materials however, the change from the second form to the first form can be induced by a thermal mechanism at ambient temperature. Preferably, the photochromic materials used in an information carrier in accordance with the invention are fulgides and diarylethenes. For example, a diarylethene comprises an open form, which does not absorb in the visible part of the spectrum. By irradiation with Ultra- Violet (UN) light, a ring closure occurs, leading to a closed form that absorbs in the visible part of the spectrum. The closed form is thermally stable, and can be transformed back to the open form, by irradiating the closed form with a light having a wavelength in the visible part. The wavelengths that are necessary to induce these transformations, as well as the wavelengths at which the open and the closed form absorb, depend on the nature of the photochromic material, and can be tuned chemically. These transformations can be induced a large number of times, for example ten thousand times. An example of diarylethene which can be used in an information carrier in accordance with the invention is l,2-bis(5-phenyl-2- methylthien-3-yl)-cyclopentene (B-DTCP). In the example of Fig. la, the photochromic material of the first and second information layers 12 and 15 is the same, and is chosen to have a low absorption and reflection at the wavelength 11 when it is in its first form, and a high absorption and reflection at the wavelength 11 when it is in its second form. When the first information layer 12 is selected for reading information from this first information layer 12, a light beam LB, having a wavelength 12, is propagated into the first waveguide. The wavelength 12 corresponds to the wavelength at which the first form of the photochromic material is changed to the second form. Fig. lc shows the propagation of the light beam into an information stack. Fig. lc shows a part of the information carrier, located at the outer side of said information carrier. The light beam is directed in the information stack with a certain angle, so that total internal reflection occurs inside the waveguide. This angle should lie within the numerical aperture of the waveguide. The numerical aperture depends on the refractive indices of the cladding layers and the waveguide core, which depend on the wavelength 12. In order to assure that total internal reflection can occur inside the waveguide, the refractive index of the waveguide core should be higher than the refractive index of the cladding layers, at the wavelength 12. The wavelengths 11 and 12 can be equal. In particular, the optical beam that is used for scanning the information layers can also be used for switching the optical properties of the information layers, as will be explained in Fig. 6. However, it is preferable that the wavelengths 11 and 12 are different. Actually, if the wavelengths 11 and 12 are different, it is possible to use waveguide cores and cladding layers having substantially equal refractive indices at the wavelength 11, in order to avoid parasitic reflections at the interfaces between the cladding layers and the waveguide cores, during scanning of an information layer. This is difficult if the wavelengths 11 and 12 are equal, because the waveguide cores and the cladding layers need to have different refractive indices at the wavelength 12, as explained hereinbefore. It is however possible to avoid parasitic reflections by suitably choosing the thickness and refractive index of each layer, even if the difference in refractive index at the wavelength 11 between layers is substantial. When the light beam with wavelength 12 is propagated into the first information stack, the first form of the photochromic material absorbs said light beam, and is thus transformed into its second form. The absorption and reflection of the first information layer 12 thus become relatively high at the wavelength 11. Then, once the absorption and reflection of the first information layer 12 are high, the light beam does not need to be further propagated into the first waveguide. This is true if the second form of the photochromic material is thermally stable. If the second form of the photochromic material is not thermally stable, it might be necessary to propagate the light beam inside the first waveguide as long as the first information layer 12 is selected. This consumes a lot of power. As a consequence, it is preferable to use a photochromic material having a thermally stable second form, such as B-DTCP. As the absorption and reflection of the first information layer 12 are 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 pits and lands. The second information layer 15 does not perturb read-out of information, because the second information layer 15 is made transparent to the wavelength 11. 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. Once the information of the first information layer 12 has been read, the second information layer 15 is selected. First, the first information layer 12 can be made transparent. If the second form of the photochromic material is not thermally stable, this is achieved by stopping propagating the light beam into the first waveguide. If the second form of the photochromic material is thermally stable, this is achieved by propagating a light beam inside the waveguide, said light beam having a wavelength 13 adapted to induce the transformation from the second form to the first form of the photochromic material. Instead of propagating a light beam with wavelength 13 inside the waveguide, it is possible to illuminate the whole information carrier with a bleaching light source having a wavelength 13, as will be explained in more details in Fig. 6. Such a bleaching light source has the effect of transforming the second form of the photochromic materials of the information carrier into the first form. In other words, the bleaching lamp has the effect of making all the information stacks transparent to the wavelength 11. Then, the second information layer 15 is made absorbent and reflective, by propagating a light beam with wavelength 14 into the second waveguide. In this example, 14 is equal to 12, because the first and second information stacks comprise the same photochromic material. If different photochromic materials are used in the first and second information layers 12 and 15, 14 might differ from 12. Once the absorption and reflection of the second information layer 15 are high, information can be read from this second information layer 15. In the example of Fig. la, the second cladding layer 14 serves as cladding layer for the first information stack, as well as for the second information stack. This has the advantage that it limits the number of layers in the information carrier, thus making said information carrier less bulky and the manufacturing process simpler. Alternatively, the cladding layers are each used in only one information stack. In this case, the infomiation stacks are separated by spacers. Figs. 2a shows a second ROM information carrier in accordance with the invention.

Such an information carrier comprises a first cladding layer 21, a first waveguide core 22, a first information layer 23, a first spacer layer 24, a third cladding layer 25, a second waveguide core 26, a second infomiation layer 27 and a second spacer layer 28. The first information layer 23 and the first spacer layer 24 form a second cladding layer, the second information layer 27 and the second spacer layer 28 form a second cladding layer. Hence, in this second information carrier, the first and second spacer layers 24 and 28 are used as parts of cladding layers. The first and second spacer layers 24 and 28 are chosen to be transparent at the wavelength 11, or at least to have a very small absorption and reflection at this wavelength, in order not to interact with the optical beam. The manufacturing process of such an information carrier is similar to the manufacturing process described in Fig. 1. The stampers are applied to the first and second spacer layers 24 and 28. In this second ROM information carrier, the light beam is propagated in the waveguide cores. For example, if the first information layer 23 is selected, the light beam is propagated in the first waveguide core 22. However, in a waveguide comprising a waveguide layer between two cladding layers, a small part of the light propagated in the waveguide core penetrates into the adjacent cladding layers. This means that the first information layer 23 absorbs a part of the light propagated in the first information stack, thus allowing changing the optical properties of the first information layer 23. Hence, in the example of Fig. 2a, a weak coupling between the light beam and the photochromic material takes place, whereas a strong coupling takes place in the information carrier of Fig. la. It is advantageous to have a weaker coupling. Actually, when the coupling between the light beam and the photochromic materials is strong, a large amount of damping occurs due to light absorption in the selected information layer. As a consequence, switching the optical properties of such an information layer is more time and power consuming. However, the first and second cladding layers 21 and 25 are chosen to be absorbing at the wavelength 12, or at least to have a small transmission at this wavelength. Actually, when the light beam at wavelength 12 is propagated into a waveguide, scattering occurs by the information layer of this waveguide. For example, scattering of light by the first information layer 23 occurs when the light beam at wavelength 12 is propagated into the first waveguide. The scattered light could reach the second information layer 27, and an unwanted switching of the optical properties of the second information layer 27 could occur. This is why the second cladding layer 25 is chosen to be absorbent at the wavelength 12, in order to block the scattered light that arises mainly as a result of scattering by the first information layer 23. This is the same for the first cladding layer 21. However, as a consequence of the first cladding layer 21 being absorbent at wavelength 12, a part of the light beam at wavelength 12 propagated into the first waveguide is absorbed in the first cladding layer 21. Hence, switching of the optical properties of the first information layer 23 requires a relatively long time and is relatively highly power consuming. The third ROM information carrier of Fig. 2b solves these problems.

Figs. 2b shows a third ROM information carrier in accordance with the invention. Such an information carrier comprises a first confining layer 201, a first cladding layer 202, a first waveguide core 203, a first coupling layer 204, a first information layer 205, a first spacer layer 206, a second confining layer 207, a third cladding layer 208, a second waveguide core 209, a second coupling layer 210, a second information layer 211 and a second spacer layer 212. The first coupling layer 204, the first information layer 205 and the first spacer layer 206 forai a second cladding layer. The first cladding layer 202, the first waveguide core 203 and the second cladding layer form a first waveguide. The second coupling layer 210, the second information layer 211 and the second spacer layer 212 form a fourth cladding layer. The third cladding layer 208, the second waveguide core 209 and the fourth cladding layer form a second waveguide. The first and second spacer layers 206 and 212 are chosen to be transparent at the wavelength 11, or at least to have a very small absorption and reflection at this wavelength, in order not to interact with the optical beam. The first and second confining layers 201 and 207 have the function of blocking the stray light which arises mainly as a result of scattering by the information layer 205. The confining layers are chosen to be transparent at the wavelength 11, or at least to have a very small absorption and reflection at this wavelength, in order not to interact with the optical beam. Further, the confining layers are chosen to be absorbing at the wavelength 12, or at least to have a small transmission at this wavelength. As explained before, this is necessary to prevent undesired switching of more than one information layer at a time. Hence, in the information carrier of Fig. 2b, the first cladding layer 202 does not need to be absorbent at the wavelength 12, as is the case for the first cladding layer 21 of Fig. 2a. As a consequence, the first cladding layer 202 is chosen transparent at the wavelength 12. This reduces the absorption by the first cladding layer 202 of the light beam propagated in the first waveguide. As a consequence, switching the optical properties of the information layers of the information carrier of Fig. 2b requires less time and is less power consuming than for the information carrier of Fig. 2a. The manufacturing process of such an information carrier is similar to the manufacturing process described in Fig. 1. The stampers are applied to the first and second spacer layers 206 and 212. In this second ROM information carrier, the light beam is propagated in the waveguide cores. For example, if the first information layer 205 is selected, the light beam is propagated in the first waveguide core 203. However, in a waveguide comprising a waveguide core between two cladding layers, a small part of the light propagated in the waveguide core penetrates into the adjacent cladding layers. This means that the first information layer 205 absorbs a small part of the light propagated in the first information stack, thus allowing changing the optical properties of the first information layer 205. Coupling of light between the waveguide layer 203 and the information layer 205 can only take place if the thickness of the first coupling layer 204 is small compared to the penetration depth of light in the second cladding layer. Hence, in the example of Fig. 2b, a weaker coupling between the light beam and the photochromic material takes place as compared to the example of Fig. 2a. As explained before, it is advantageous to have a weaker coupling, and in the example of Fig. 2b, the coupling can be adjusted according to the thickness of the first coupling layer 204. The coupling will decrease with increasing thickness. Also, the coupling can be adjusted by changing the refractive index difference between the first waveguide core 203 and the first coupling layer 204. A further advantage of the information carrier of Fig. 2b is that the refractive index of the information layers is allowed to be larger than the refractive index of the waveguide cores and thus larger than the refractive index of the cladding layers. This is beneficial, because all the light that penetrates into a cladding layer also penetrates into the adjacent information layer, thus avoiding loss of light in the cladding layers.

Fig. 3 shows a WORM (Write Once Read Many) information carrier in accordance with the invention. This information carrier comprises a first cladding layer 31, a first information layer 32, a first waveguide core 33, a second cladding layer 34 a second information layer 35, a second waveguide core 36 and a third cladding layer 37. The first and second information layers 32 and 35 comprise a photochromic material which can be locally degraded by means of the optical beam OB at the wavelength 11 in order to write information in the information layers 32 and 35. In order to locally degrade the photochromic material, a relatively high power of the optical beam is required. The high power is absorbed by the photochromic material and changes its material properties, for example by melting, annealing, photochemical reactions, thermal damaging or deterioration. This relatively high power is used during writing of information on the information carrier, whereas a smaller power is used during reading, the latter being not able to degrade the photochromic material. In order to write information on the first information layer 32, the optical beam having the relatively high power is focussed on the first information layer 32, in order to locally degrade the photochromic material, for writing marks. In Fig. 3, the marks where the photochromic material is degraded are represented by dotted lines. The depth of the marks in the information layers can be chosen by varying the power of the optical beam, or by varying the time during which the optical beam is focussed on a mark. Having different depth of marks allows multilevel recording. In single-level recording, typically two reflection states or levels are used, whereas in case of multi-level recording, more reflection levels are defined to represent data. In order to write information on the second information layer 35, the optical beam having the relatively high power is focussed on the second information layer 35, in order to locally degrade the photochromic material, for writing marks. In order to read information from the second information layer 35, this second information layer 35 is made absorbent and reflective at the wavelength 11, by propagating the light beam at wavelength 12 inside the second information stack. The second information layer 35 becomes absorbent and reflective, except where marks have been written, because the photochromic material in these marks is degraded so that the photochromic process cannot occur anymore in these marks. Hence, the difference in absorption and reflection between the marks and the non-marked areas in the second information layer 35 is used for reading information from the second information layer 35. In order to read information from the first information layer 32, the second information layer 35 is made transparent at the wavelength 11, for example by propagating the light beam at wavelength 13 inside the second information stack. Hence, the whole second information layer 35, including the marks, becomes transparent. Hence, the second information layer 35 does not perturb the scanning of the first information layer 32. Then, the first information layer 32 is made absorbent at the wavelength 11, by propagating the light beam at wavelength 12 inside the first information stack. The first information layer 32 becomes absorbent and reflective, except where marks have been written. Information can then be read from the first information layer 32. It should be noticed that such degradable photochromic materials can be used in information carriers having the layers described in Fig. 2a and Fig. 2b.

Fig. 4 shows an information carrier in accordance with an advantageous embodiment of the invention. Such an information earner comprises an information stack comprising a first cladding layer 41, an information layer 42, a waveguide core 43 and a second cladding layer 44. This information carrier comprises other information stacks, which are not represented in Fig. 4. The information stack is divided into a first and a second section 47 and 48, by means of a separator 45. The information carrier further comprises a central hole 46. The separator 45 is arranged so that a light beam propagated in the first section 47 is not propagated in the second section 48, and vice versa. When the information layer 42 is selected for reading information from this information layer, the optical properties of the part of the information layer 42 located in the first section 47 are switched, by propagating a first light beam LB1 in the first section 47. Fig. 6 describes in more detail how a first light beam LB1 is propagated in the first section 47. The time required for switching the optical properties of an information layer depends on the quantity of photochromic material in this information layer. Now, the quantity of photochromic material in the first section 47 is less than in an information layer comprising only one section, such as the first information layer 23 of Fig. 2a. As a consequence, the switching of the optical properties of the part of the information layer 42 located in the first section 47 requires less time. Once the optical properties of the part of the information layer 42 located in the first section 47 have been switched, this part can be scanned by the optical beam. During scanning of this part, the optical properties of the part of the information layer 42 located in the second section 48 are switched, by propagating a second light beam LB2 in the second section 48. As the scanning of the part of the information layer 42 located in the first section 47 typically requires a few minutes, the time available for switching the optical properties of the part of the information layer 42 located in the second section 48 is relatively high. As a consequence, the second light beam LB2 can have a relatively low power. Hence, dividing the information stacks into two sections is particularly advantageous, because it allows reducing the time required for switching the optical properties of the information layers before scanning, because switching is done in parallel to scanning, as well as the power required for switching said optical properties.

Fig. 5 shows a first optical scanning device in accordance with the invention. Such an optical device comprises a radiation source 501 for producing an optical beam 502, a collimator lens 503, a beam splitter 504, a servo controlled objective lens 505, a detector lens

506, detecting means 507, measuring means 508 and a controller 509. This optical device is intended for scanning an information carrier 510. The information carrier 510 comprises two infomiation stacks 511 and 512, each comprising an information layer. During a scanning operation, which can be a writing operation or a reading operation, the information carrier 510 is scanned by the optical beam 502 produced by the radiation source 501. The collimator lens 503 and the objective lens 505 focus the optical beam 502 on an information layer of the information carrier 510. The collimator lens 503 and the objective lens 505 are focussing means. During a scanning operation, a focus error signal is detected, the magnitude of which corresponds to an error of positioning of the optical beam 502 on the information layer. This focus error signal might be used in order to correct the axial position of the objective lens 505, in order to compensate for a focus error of the optical beam 502. A signal is sent to the controller 509, which drives an actuator in order to move the objective lens 505 axially. The focus error signal and the data written on the information layer are detected by the detecting means 507. The optical beam 502, reflected by the information carrier 510, is transformed to a parallel beam by the objective lens 505, and then reaches the detector lens 506 by means of the beam splitter 504. This reflected beam then reaches the detecting means

507. The optical device further comprises a clamper 520 for receiving the information carrier 510. The optical scanning device further comprises a light beam source 531, a light beam collimator 532 and a light beam lens 533, for generating and directing a light beam 534. The light beam 534 is directed into the information stack comprising a selected information layer, in order to switch the optical properties of this information layer. The light beam source 531, the light beam collimator 532 and the light beam lens 533 can be translated in order to propagate the light beam 534 in the information stack comprising the selected information layer. A servo loop similar to that of components 501 to 509 can be added in order to ensure proper positioning of the light beam 534 onto the selected information layer. In the example of Fig. 5, the first information layer in the first information stack is selected. The light beam 534 is thus propagated into the first information stack 511. Once the optical properties of the corresponding information layer are switched, this first information layer is scanned. It is important to notice that the optical properties of the second information layer in the second information stack 512 can be switched during scanning of the first information layer. Actually, the scanning of the first information layer is not perturbed by the second information layer, whatever the optical properties of the second information layer, as long as the distance between the first and second information layer is sufficiently high, typically more than 5 micrometers, in order to avoid crosstalk. This allows scanning the second information layer immediately after the first information layer has been scanned, without waiting for the switching of the optical properties of the second information layer. Moreover, as the time available for switching the optical properties of the second information layer is relatively high, the power of the light beam source 531 can be reduced during propagation of the light beam 534 into the second information stack 512. It should be noticed that in another embodiment, the signal corresponding to information written in the information carrier 510 can be detected in transmission by a second objective lens, a second detector lens and second detecting means, which are placed opposed to the objective lens 505, compared to the information carrier 510. It should also be noticed that in another embodiment, the information carrier 510 can have a mirror at the back of the whole carrier, that reflects the beam transmitted through all information stacks, including the addressed one. In this case, the optical scanning device as shown in Fig 5 can be used to read the data.

Fig. 6 shows a second optical scanning device in accordance with the invention. Such an optical scanning device comprises a clamper 620 comprising a mirror 621, a first light beam source 631, a first light beam collimator 632, a first light beam lens 633, a second light beam source 634, a second light beam collimator 635 and a second light beam lens 636. The first light beam source 631 generates a first light beam 602, which is directed into a first or a second information stack 611 or 612 of an information carrier 610, by means of the first light beam collimator 632, the first light beam lens 633 and the mirror 621. The second light beam source 634 generates a second light beam 603, which is directed into the first or the second information stack 611 or 612 by means of the second light beam collimator 635 and the second light beam lens 636. This information carrier is intended to be scanned by an optical beam 601. The radiation source for generating said optical beam 601, as well as other elements represented in Fig. 5, are not represented in Fig. 6. The optical scanning device further comprises a bleaching light source 604. In this example, the information carrier comprises two sections, as depicted in Fig. 4. Of course, this second optical scanning device in accordance with the invention can be used for scanning information carriers comprising only one section. When the first information layer in the first information stack 611 is selected, the optical properties of the part of the first information layer located in the first section are switched, by propagating the first light beam 602 into the first information stack 611. This is achieved by means of the mirror 621, which is arranged so that the first light beam 602 enters the first information stack 611 with an angle that allows for total internal reflection inside the first information stack 611. As depicted in Fig. lc, the angle should lie within the numerical aperture of the waveguide. The numerical aperture depends on the refractive indices of the cladding layers and the waveguide core of the first information stack 611, which depend on the wavelength of the first light beam 602. If the first information stack 611 comprises only one section, the first light beam 602 can also be used for switching the optical properties of the first information layer. It should be noticed that the mirror 621 can be a part of the information carrier 610, instead of a part of the clamper 620. The mirror 621 is preferably at an angle of 45 degrees so that the light beam is deflected by 90 degrees. In particular, the mirror may be the result of total internal reflection at the interface of the information carrier with the surrounding air. During scanning of the part of the first information layer located in the first section, the optical properties of the part of the first information layer located in the second section are switched, by propagating the second light beam 603 inside the first information stack 611. Instead of the first light beam 602, the optical beam 601 can be used for switching the optical properties of the first information layer. Actually, the optical beam 601 can be translated relatively to the information carrier 610. This is possible if the optical beam has the same wavelength as the first light beam, i.e. if the wavelength 11 is equal to the wavelength 12. However, it is preferable, as explained in Fig. la, that the wavelength 11 differs from the wavelength 12. In this case however, it is also possible to use the optical beam 601 for switching the optical properties of the first information layer, if said optical beam 601 has two different wavelengths. This is in particular the case in many optical scanning devices, for assuring compatibility between different formats of information carriers. For example, in an optical scanning device intended to scan a DVD and a BD (BD stands for Blu-Ray Disc), the radiation source that generates the optical beam has two different wavelengths, one at 405 nanometres, the other one at 650 nanometres. In this case, the wavelength at 405 nanometres can be used for switching the optical properties of the first information layer and the wavelength at 650 nanometres can be used for scanning the first information layer. Once the first information layer has been scanned, the bleaching light source 604 generates a light at wavelength 13 suitable for making the first information layer transparent to the wavelength of the optical beam 601. As a consequence, the scanning of the second information layer in the second information stack 612 is not perturbed by the first information layer. It should be noticed that the bleaching light source can be arcanged for illuminating only a part of the information earner 610, such as the first section of said information carrier. 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.

Claims

1 An information carrier for scanning information by means of an optical beam (OB) having a wavelength, said information carrier comprising at least two information stacks, wherein each stack is a waveguide comprising two cladding layers (11, 14) and a waveguide core (13) between said two cladding layers, said waveguide comprising an information layer (12) comprising a photochromic material which optical properties at the wavelength of the optical beam can be switched by means of a light beam (LB) propagated into said waveguide.

2 An information carrier as claimed in claim 1, wherein said light beam has a wavelength that differs from the wavelength of the optical beam.

3 An information carrier as claimed in claim 1, wherein a cladding layer of an information stack serves as cladding layer of another information stack.

4 An information carrier as claimed in claim 1, wherein the photochromic material can be locally degraded by means of the optical beam in order to write information in the information layer.

5 An information carrier as claimed in claim 1, said information carrier comprising a mirror (621) for directing the light beam into the information stacks.

6 An information carrier as claimed in claim 1, wherein an information stack comprises a first and a second section (47, 48) arranged so that the light beam propagated in the first section is not propagated in the second section.

7 An optical scanning device for scanning an information carrier (510) by means of an optical beam (502) having a wavelength, said information carrier comprising at least two information stacks (511, 512), wherein each stack is a waveguide comprising two cladding layers and a waveguide core between said two cladding layers, said waveguide comprising an information layer comprising a photochromic material which optical properties at the wavelength of the optical beam can be switched by means of a light beam (534) propagated into said waveguide, said optical scanning device comprising means (501) for generating the optical beam, means (503, 505) for focussing said optical beam on an information layer, means (531) for generating the light beam and means (532, 533) for directing said light beam into the waveguide corresponding to a selected information layer.

8 An optical scanning device as claimed in Claim 7, said optical device comprising a clamper (620) for receiving the information carrier, wherein the means for directing said light beam comprise a mirror (621) mounted on said clamper. 9 An optical scanning device as claimed in Claim 7, said optical scanning device further comprising a bleaching light source (604) for producing a bleaching light having a wavelength that differs from the wavelength of the light beam, said bleaching light source being adapted to illuminate the whole information carrier. 10 A method of reading information from an information carrier by means of an optical beam having a wavelength, said information carrier comprising at least two information stacks, wherein each stack is a waveguide comprising two cladding layers and a waveguide core between said two cladding layers, said waveguide comprising an information layer comprising a photochromic material which optical properties at the wavelength of the optical beam can be switched by means of a light propagated into said waveguide, said method comprising the steps of propagating a light beam into the information stack from which information shall be read and focussing the optical beam on the information layer of said stack. 11 A method of recording information on an information carrier by means of an optical beam having a wavelength, said information earner comprising at least two information stacks, wherein each stack is a waveguide comprising two cladding layers and a waveguide core between said two cladding layers, said waveguide comprising an information layer comprising a photochromic material which optical properties at the wavelength of the optical beam can be switched by means of a light propagated into said waveguide, said method comprising the step of focussing the optical beam on the information layer of the information stack on which information shall be recorded in order to locally degrade the photochromic material.