FIELD OF THE INVENTION
This invention relates to a multi-layer information medium which has at least two information-storing layers, the information-storing layer being typically a recording layer.
There is a growing need for an optical disk having a higher density and a higher capacity. DVD (Digital Versatile Disk) is already commercially available, and the DVD has a storage capacity of about 4.7 GB per single side which is about seven times larger than the compact disk. Technologies enabling further increase in the amount of information recorded are under active development.
Technologies that have been used for increasing the recording capacity of an optical disk include use of a recording/reading beam having a reduced wavelength, use of an objective lens having a higher NA (numerical aperture) in the optical system irradiating the recording/reading beam, increase in the number of recording layers, and multi-value recording. Among these, three-dimensional recording by increasing the number of recording layers enables remarkable increase in the recording capacity at low cost compared to the use of shorter wavelength or use of a lens with a higher NA. The three dimensional recording medium is described, for example, in Japanese Patent Application Kokai (JP-A) 198709/1997, and JP-A 255374/1996 discloses a medium wherein a rewritable information storage layer and a read only information storage layer are laminated.
In the reading of a multi-layer recording medium including a plurality of recording layers by using an optical pickup which emits a reading beam, the optical pickup receives the beam reflected from the recording layer on which the reading beam had focused, and in addition, the beam reflected from the recording layer(s) other than the recording layer to which the reading beam had focused. This results in the signal interference between the plurality of recording layers, and cross talk is induced. As a consequence, noise is introduced in the read out signal. The influence of the beam reflected from the recording layer other than the target recording layer reduces inversely with the square of the distance between the recording layers. Therefore, increase in the distance between the adjacent recording layers is effective in reducing the noise induced. For example, when the medium is used with an optical pickup having the structure normally employed in DVD and other conventional optical disks, the recording layers are disposed at a mutual distance of at least 30 xcexcm, and preferably at least 70 xcexcm to realize the signal quality of practically acceptable level. This corresponds the Examples of the JP-A 198709/1997 wherein a transparent resin layer of 100 xcexcm thick is provided between the recording layers, and the JP-A 255374/1996 wherein two adjacent information storage layers are disposed at a distance of 30 xcexcm or more.
However, when the distance between the adjacent recording layers is increased to as large as 30 xcexcm or more, limitation in the number of recording layers in the medium will be required to avoid excessive increase in the disk thickness, and the total storage capacity of the disk will also be limited.
The transparent resin layer provided between the recording layers is also associated with a difficulty. To be more specific, formation of a transparent resin layer with a consistent thickness is difficult in spite of various attempts in forming the transparent resin layer by spin coating, resin sheet disposition and the like when the transparent resin layer formed is as thick as, for example, 30 xcexcm or more, and in particular, 70 xcexcm or more. The thick resin layer also suffers from increased internal stress and the medium is subject to warping. As a consequence, reliable provision with the optical disk of the required mechanical precision has been difficult.
In the case of the medium having a single recording layer formed on a substrate, the shape of the grooves (guide grooves) formed in the resin substrate will be transferred to the recording layer. In contrast, in the case of a medium wherein two or more recording layers are formed on the substrate with an intervening relatively thick transparent layer between the recording layers, it is quite difficult to transfer the shape of the grooves formed in the substrate to all of the recording layers since the groove depth is about 100 nm at most for optical reasons while the distance between the recording layers is at least 30 xcexcm as described above. As a consequence, formation of the grooves in the transparent resin layer by photopolymerization (2P) process will be required as described, for example, in the JP-A 198709/1997 and eminent increase in the production cost is invited.
An object of the present invention is to reduce the distance between adjacent information-storing layers in a multi-layer information medium having a plurality of data layers with no increase in the cross talk between the information-storing layers. Another object of the invention is to provide such multi-layer information medium at a low cost.
Such objects are attained by the present invention as described in (1) to (5), below.
(1) An optical information medium having at least two information-storing layers each storing recorded information and/or servo information, wherein at least one of the information-storing layers is recorded or read by the recording beam or the reading beam which has passed through other information-storing layer(s), and wherein the medium has a filter layer between the adjacent information-storing layers, wherein
in the spectral absorption characteristics in the wavelength range of 300 to 1000 nm of this filter layer, a wavelength range exhibiting an absorption of 80% or higher and a wavelength range exhibiting an absorption of 20% or lower are present.
(2) An optical information medium having at least two information-storing layers each storing recorded information and/or servo information, wherein at least one of the information-storing layers is recorded or read by the recording beam or the reading beam which has passed through other information-storing layer(s), wherein the medium has a filter layer between the adjacent information-storing layers, and wherein the medium is used in a system wherein two or more recording/reading beams each having different wavelength are used, wherein
the filter layer exhibits a relatively high absorption for the reading/recording beam used for its closest information-storing layer on the side of the light incidence and a relatively low absorption for the reading/recording beam used for its closest information-storing layer on the side of the light exit.
(3) The optical information medium according to the above (2) wherein said filters exhibits an absorption for the reading/recording beam used for its closest information-storing layer on the side of the light incidence of 80% or more and an absorption for the reading/recording beam used for its closest information-storing layer on the side of the light exit of 20% or lower.
(4) The optical information medium according to any one of the above (1) to (3) wherein at least one of said filter layers is a layer formed by UV curing a composition containing a UV-curable composition and a photopolymerization initiator.
(5) The optical information medium according to any one of the above (1) to (4) wherein at least one of said filter layers is a layer containing a dye.