Development of dual-layer recording-reproduction optical disks is now in progress, wherein a dual-layer recording film is formed on one side of the optical disk so as to double the recording capacity per side.
As shown in a sectional view of FIG. 1, a dual-layer recording-reproduction optical disk has, on its substrate of polycarbonate or the like, a recording film L1 layer for recording data, a space layer, a recording film L0 layer for recording data, and a cover layer for protecting the recording film L0 layer and so forth formed thereunder. An optical pickup (not shown) for irradiating a laser beam to the dual-layer recording-reproduction optical disk and receiving the reflected beam therefrom is positioned above in the drawing. Hereinafter each of the recording film L0 layer and the recording film L1 layer will be referred to simply as a recording layer in case there is no necessity of distinguishing these two layers from each other.
For the purpose of recording data in the recording layer of such a dual-layer recording-reproduction optical disk, it is necessary to format the recording layer by first dividing the same into sectors each composed of 2048 (=2K) bytes as a unit for recording and reproduction, and then recording a sector address in a header of each sector.
As to techniques for recording sector addresses and data in the recording layer, there are known a method of forming pits (small holes) by embossing or the like in manufacture of a dual-layer recording-reproduction optical disk, and another method of recording marks (phase transition areas) by irradiating a laser beam to the recording layer of a completed dual-layer recording-reproduction optical disk. Hereinafter a pit formed in the process of manufacturing a dual-layer recording-reproduction optical disk will be referred to as an “embossed pit”.
When recording any mark in and/or reading the same from the recording film L1 layer of the dual-layer recording-reproduction optical disk, as shown in FIG. 1, a laser beam emitted from an unshown optical pickup is irradiated to the recording film L1 layer via the recording film L0 layer, and a reflected beam from the recording film L1 layer is received by the optical pickup via the recording film L0 layer.
With regard to the laser beam transmittance and reflectance, any portion of the recording film layer having embossed pits or marks is different from the other portion thereof having none of such pits or marks.
Consequently, in irradiating the laser beam to the recording film L1 layer via the recording film L0 layer or in receiving the reflected beam from the recording film L1 layer via the recording film L0 layer, there occurs some change or offset in the amplitude of the irradiated or reflected beam in accordance with the presence or absence of such embossed pits or marks on the recording film L0 layer, hence raising a problem that high-precision recording or reproduction of marks in or from the recording film L1 layer is difficult.