WO2006043223A1

WO2006043223A1 - An optical information carrier

Info

Publication number: WO2006043223A1
Application number: PCT/IB2005/053391
Authority: WO
Inventors: Herman Bulle; Andrei Mijiritskii
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-10-20
Filing date: 2005-10-17
Publication date: 2006-04-27
Also published as: TW200627444A

Abstract

The present invention relates to an optical information carrier with a substrate and information layers having optically readable effects which effects represent information readable by a light beam (10) entering the carrier from an entrance side. The information layers comprises a first (52) and a second layer (54) of information with a first and a second area density of readable effects, respectively, that are readable by the same optical method. The first area density of readable effects is larger than the second area density of readable effects, and the second layer of information is located closer to the entrance side than the first layer of information. Thereby, a precise and reliable multi-layered optical information carrier is obtained.

Description

An optical information carrier

FIELD OF THE INVENTION

The present invention relates to an optical information carrier, comprising a substrate and information layers having optically readable effects which effects represent information readable by a light beam entering the carrier from an entrance side, the information layers comprising a first and a second layer of information.

BACKGROUND OF THE INVENTION

In order to meet the demand of increasing information storage capacity the available optical media, presently e.g. digital versatile disc (DVD) and the Blu-ray Disc (BD), show a constant improvement in storage capacity. Dual-layer optical media or even multi-layer optical media have been introduced as a result. In addition, hybrid media such as SA-CD and BD-DVD have been disclosed.

A common problem for a multi- layer optical media is to limit the influence or disturbance of the light being transmitted through a layer of information to and from another layer of information resided further down into the multi- layer optical media.

Some solutions have used different reproducing and/or recording methods for the different layers, such as different wavelength for the various information layers and magneto-optical storage for some layers and conventional optical storage for other layers. These solutions however complicate the design of the optical drives for reproducing and/or recording the information, and possibly the manufacturing of the optical media itself.

Alternatively, a single reproducing and/or recording method for the different layers may be applied. Some of these solutions apply a larger storage density, i.e. small pits or marks, of an upper layer relative to deeper layers to increase the disc capacity. However, the smaller pits or marks require an enhanced precision during reading and/or recording. In particular, for the rewritable and recordable optical media this is complicated due to the needed thermal balancing of the writing process. Although not encountered for the current optical information systems yet, it is foreseen that further capacity increase by reducing the wavelength of the reproducing/recording light beam and, therefore, by reducing the pit/mark size may lead to a problem with read-only media as well. The course for the problem can be the depth and shape of the pits with respect to the required mirror/reflector thickness.

Hence, an improved optical storage medium would be advantageous, and in particular a more efficient and/or reliable optical storage medium would be advantageous.

SUMMARY OF THE INVENTION

Accordingly, the invention preferably seeks to mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination.

This object and several other objects are obtained in a first aspect of the invention by providing an optical information carrier comprising a substrate and information layers having optically readable effects and/or being adapted for recording optically readable effects, said effects represent information readable by a light beam entering the carrier from an entrance side, the information layers comprising a first layer of information, said first layer having a first area density of readable effects, and

- a semitransparent second layer of information, said second layer having a second area density of readable effects, wherein said first and second layers of information comprise optically readable effects readable by the same optical method, and wherein the first area density of readable effects is larger than the second area density of readable effects, and the second layer of information is located closer to the entrance side than the first layer of information.

The invention is particularly, but not exclusively, advantageous for obtaining a simplified optical information carrier for multi-layer information reproduction and/or recording. In particular, the carrier according to the present invention provides an efficient way of enhancing the data capacity of such a carrier with a low error rate due to e.g. inter- symbol interference and radial cross-talk in the reading and/or writing of readable effects. It should be emphasized that the optical information carrier according to the present invention is not limited to dual layer carriers. Rather, the invention may also be applied to multilayered carrier, i.e. having 3, 4, 5, 6, 7, 8, 9, 10 etc. layers of information. For such embodiments, layers of information located in between the entrance side and a layer of information to be read or written should of course also be semi-transparent. Thus, the said first layer of information according to the first aspect of the invention may also be semi-transparent for such embodiments. The first and the second layers of information may be readable by the same associated optical system having one wavelength of the light beam and one numerical aperture (NA) in order to facilitate a simple way of reading the information. This is a particular beneficial as it provides a simple optical method for retrieving the information. Alternatively, the invention may an apply magneto-optically method for reading and writing information from and to the first and the second information layers.

The optical readable effects may be protrusions in a reflective layer in the plane of optical information carrier as with presently known standards for read-only media of various formats. Alternatively, the optical readable effects may be areas of alternating reflection i.e. pit or marks as with presently known standards for rewritable or writable media of various formats. Thus, the present invention is easily integrated with standard formats. The first and the second information layers may comprise a reflective layer where at least the reflective layer of the second information layer may be semitransparent. As mentioned above for the first layer of information may also comprise a semi-transparent reflective layer if necessary.

The readable effects may have a length substantially equal even to an integer times a reference length, the reference length for the second layer of infoπnation being larger than the reference length of the first layer for information in order to obtain a higher area density of readable effect for the second layer of information. The reference length is also known as the channel bit length and the downscaling of the reference length provides an easy way of increasing the area density of readable effects. Alternatively or additionally, the readable effects may have an average width, the average width of the readable effects in the second layer of information being larger than the average width of the readable effects in the first layer of information. Similarly, the downscaling of the average width provides an relatively simple way of increasing the area density of readable effects.

The first and the second layer of information may be arranged according to a standard format chosen from the group of the following standard formats: compact disc (CD), digital versatile disc (DVD), and Blu-ray disc (BD), so that the present invention is readily implemented with hitherto known technology. The first and the second layer of information may comprises a guiding track intended to guide the light beam in recording of optically readable effect, i.e. by indicated the ideal position of the recorded marks. According to the invention, the area density of the guiding track of the first layer of information may be larger than the area density of the guiding track of the second layer of information. In particular, the guiding track - also called the pre-groove - is typically an essentially spiral-shaped track when viewed from above the carrier. As a measure for the area density of such an spiral guiding track, the distance between subsequent turns of the track may be applied. This distance is also known as the track pitch distance. Thus, it should be noted that even though no readable effects are recorded, e.g. a unrecorded information carrier, the relative arrangement of the pre-grooves of the information layers may be adapted for utilizing the present invention.

In a second aspect, the present invention relates to an optical apparatus adapted for reproducing and/or recording information from/to an optical information carrier according to the first aspect, the optical apparatus comprising: - a holding means to fixate and rotate the information carrier, a light source capable of emitting a light beam for reading information as readable effects and/or recording information as readable effects, and photodetection means capable of detecting reflected light from the optical information carrier and transform it into electrical signals. The first and second aspects of the present invention may each be combined with any of the other aspects. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE FIGURES The present invention will now be explained with reference to the accompanying Figures, where

Figure 1 shows a cross-sectional view of an embodiment of a multi-layered optical information carrier according to the invention,

Figure 2 shows a cross-sectional view of a recording mark, and Figure 3 shows results obtained from a first and a second information layer according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows a cross-sectional view of an embodiment of a rewritable multi- layered optical information carrier according to the invention. The carrier comprises a substrate 51 for supporting the carrier. In the embodiment shown in Figure 1, the carrier comprises a first layer of information 52 and a second layer of information 54. The first and the second layer of information 52 and 54 are separated by a space layer 53 of an appropriate polymer, e.g. polycarbonate (PC) or similar. Above the second layer of information 54 a cover layer 55 is found. The cover layer 55 serves the dual purpose of mechanically protecting the information layers 52 and 54 and allowing sufficient transmission of the relevant wavelengths for recording and reproducing of information to and from the carrier.

The upper part of the cover layer 55 is also the entrance side 56 for a light beam 10, e.g. a laser beam, that is focussed by a lens 15 on an information layer 52 or 54. In Figure 1, the light beam 10 is focussed onto the first layer of information 52.

The first and the second layers of information 52 and 54 have a stacking structure with recording layer 523 and 544 being embedded from above and below by dielectric layers, 522 and 524, and 543 and 545, respectively, as shown in Figure 1. Additionally, the first and the second layers of information 52 and 54 comprise a reflective layer 521 and 542 for reflecting the light beam 10 back to an optical recording or reproducing device (not shown) via the lens 15. For this embodiment, the reflective layer 542 of the second information layer 54 is of a thickness so as to allow transmittance of the light beam 10 to the first layer of information 52 as shown in Figure 1. Trie reflective layers are preferably made of metals, such as Al, Au, Ag, Cu or an alloy thereof. For Blu-ray media the reflective layers 521 and 542 have a thickness in the interval from 20-300 and 2-30 nm, respectively.

Recording on a rewritable carrier is realized by creating amorphous marks 100 (see Figure 2) in the crystalline matrix of the recording layer 523 and 544. This is achieved by melt-quenching the recording material i.e. heating it up above its melting temperature and subsequently cooling down very quickly below its crystallization temperature. During cooling down some recrystallization of the molten material usually takes place. The recording process is done with a sequence of high-power laser pulses that are used for melting, separated by low-power gaps that are used for quenching. In Figure 2 a cross-sectional view of such a recording mark 100 is shown. The outer line 103 denoted the area that is initially molten upon writing. The area 101 shows the final mark after rapid cooling i.e. quenching. The area 102 denotes the amount of the mark 100 that is recrystallized during the cooling down. As the area 102 should be minimized in order to provide a stable and precisely defined recording mark 100, it is important that a sufficiently high cooling rate is obtained. During the recording process, the reflective layers 521 and 542 serves an additional purpose as heat sinks for the recording layers 523 and 544, respectively. Thus, for high cooling rates, the reflective layers 521 and 542 should have a relative large thickness. However, this should be balanced by the fact that at least the upper reflective layer 542 should have a sufficiently small thickness to be semi-transparent for the light beam 10.

In Figure 3, experimental results are shown from the first and the second information layers 52 and 54 with different thickness of the reflective layer 521 and 542. The thickness of the reflective layer 521 and 542 for the lower and upper panel are 120 ran and 10 nm, respectively.

The upper panel represents the data recorded and read-out from a semi- transparent rewritable second information layer 54. This data layer comprises a relatively thin heat-sink or reflective layer 542, and therefore possesses a low cooling rate. The lower panel corresponds to data recorded and read-out from a rewritable information layer 52 comprising a relatively thick heat-sink or reflective layer 521, thus, it possesses a high cooling rate. In the experiment discussed, both information layers 52 and 54 comprise the same recording material.

The vertical and horizontal axes in the plots of Fig. 3 indicate the value of the actual size of the marks 100 and spaces separating the marks 100, respectively. The values are given in the T-units, where T is a channel-bit length of the system. In the ideal situation, all bit lengths should be exact multiples of T. In practice, a distribution of lengths occurs. The clouds of data points in Figure 3 represent a measured 2r..8!Tset of bit lengths in accordance with the Blu-ray Disc format. The area of the clouds represents the spread in the actual mark 100 and space sizes. Spread in recording mark 100 size leads to errors in detection of the information bits and, therefore, to a data retrieval failure. For a given data density, if a mark 100 is shorter (longer) than expected than the succeeding (or preceding) space will be longer (shorter), and vice versa. If a nT mark or space, where n is an integer, is rather a (niV^Tmark or space — inter-symbol interference (ISI) takes place, which manifests itself as bit misdetection.

As can be seen from Fig. 3, in the case of the first information layer 52 in the lower panel, a much smaller spread in recorded mark 100 size (and spaces between them) can be achieved. Since absolute magnitude of mark 100 (space) variation is determined by the media morphology and its thermal performance, the spread in the actual bit lengths does not scale down with reduction of the channel-bit length. In the plots of Fig. 3, this effect will be apparent as a reduction in the distance between the bit length distribution clouds while the clouds areas will remain the same. This inevitably leads to an increase in jitter and inter- symbol interference (ISI). Thus, reduction in channel-bit length will be accompanied with an increase in bit error rate. As it will be appreciated, the present invention facilitate further downscaling of the size of the marks 100 in a simple and efficient manner by having the second layer of information 54 with the lowest area density of readable effects closer to the entrance side 56 relative to the first layer of information 52.

Although the present invention has been described in connection with the specified embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. In the claims, the term comprising does not exclude the presence of other elements or steps. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. In addition, singular references do not exclude a plurality. Thus, references to "a", "an", "first", "second" etc. do not preclude a plurality. Furthermore, reference signs in the claims shall not be construed as limiting the scope.

Claims

CLAIMS:

1. An optical information carrier comprising a substrate and information layers having optically readable effects and/or being adapted for recording optically readable effects, said effects represent information readable by a light beam (10) entering the carrier from an entrance side (56), the information layers comprising - a first layer of information (52), said first layer having a first area density of readable effects, and

- a semitransparent second layer of information (54), said second layer having a second area density of readable effects, wherein said first and second layers of information (52,54) comprise optically readable effects readable by the same optical method, and wherein the first area density of readable effects is larger than the second area density of readable effects, and the second layer of information (54) is located closer to the entrance side (56) than the first layer of information (52).

2. An optical information carrier according to claim 1, wherein said first and the second layers of information (52,54) are readable by the same associated optical system having one wavelength of the light beam (10) and one numerical aperture (NA).

3. An optical information carrier according to claim 1, wherein the optical readable effects are protrusions in a reflective layer in the plane of optical information carrier.

4. An optical information carrier according to claim 1, wherein the optical readable effects are areas of alternating reflection.

5. An optical information carrier according to claim 4, wherein the first and the second information layers (52,54) comprise a reflective layer (521,542), wherein at least the reflective layer (542) of the second information layer (54) is semitransparent.

6. An optical information carrier according to claim 1 , wherein the readable effects have a length substantially equal even to an integer times a reference length, the reference length for the second layer of information (54) being larger than the reference length of the first layer for information (52).

7. An optical information carrier according to claim 1, wherein the readable effects have an average width, the average width of the readable effects in the second layer of information (54) being larger than the average width of the readable effects in the first layer of information (52).

8. An optical information carrier according to claim 1, wherein the first and the second layer of information (52,54) are arranged according a standard format chosen from the group of the following standard formats: compact disc (CD), digital versatile disc (DVD), and Blu-ray disc (BD).

9. An optical information carrier according to claim 1, wherein the first and the second layer of information (52,54) comprises a guiding track intended to guide the light beam (10) in recording of optically readable effect, wherein the area density of the guiding track of the first layer of information (52) is larger than the area density of the guiding track of the second layer of information (54).

10. An optical apparatus adapted for reproducing and/or recording information from/to an optical information carrier according to claim 1, the optical apparatus comprising:

- a holding means to fixate and rotate the information carrier, - a light source capable of emitting a light beam (10) for reading information as readable effects and/or recording information as readable effects, and

- photodetection means capable of detecting reflected light from the optical information carrier and transform it into electrical signals.