1. Field of the Invention
The present invention relates to an optical disk apparatus, particularly to the optical disk apparatus which is suitably used to reproduce information from a so-called Low-to-High type optical disk in which a reflectance of a recorded portion is higher than that of an unrecorded portion.
2. Description of the Related Art
Currently, HD DVD-R (High Definition Digital Versatile Disc-Recordable) is being prepared for the market. In HD DVD-R, there is proposed a so-called Low-to-High type optical disk in which the reflectance of the recorded portion is higher than the reflectance of the unrecorded portion.
In such optical disks, because a recording layer is made of an organic coloring material, the Low-to-High type optical disk is easily affected by light from the outside compared with a High-to-Low type optical disk in which the recording layer is made of an inorganic material. On the other hand, at least 1000000-time reproductions are demanded in the HD DVD standard. Even in the Low-to-High type optical disk, it is necessary to ensure high reliability for recording information. Therefore, in the optical disk drive apparatus, it is necessary that a laser power be properly set during recording such that degradation of a recording film property and cross erase are not generated due to excessive light irradiation.
FIGS. 8A and 8B schematically show operations when the information is recorded in or reproduced from the Low-to-High type HD DVD-R. As shown in FIGS. 8A and 8b, as with the High-to-Low type HD DVD-R, the recording is performed only to grooves in the Low-to-High type optical disk. Because the optical disk is not rewritable, the recording is sequentially performed to the track one by one from an inner circumference to an outer circumference of the optical disk.
As shown in FIG. 8A, when the recording is finished to a groove (G1), the recording is performed to a groove (G2) adjacent to the groove (G1). In this case, the recorded groove (G1) is partially irradiated with a periphery of a light spot when the recording is performed to the groove (G2). Laser beam intensity in the light spot concentrates on a central portion of the light spot. Therefore, even if the recorded groove (G1) is partially irradiated with the periphery of the light spot, a recording mark is not formed on the groove (G1).
However, at this point, the laser beam leaks slightly into the groove (G1). The leakage of the laser beam affects the recording layer of the groove (G1) to generate the degradation of the recording layer in terms of light stability. Additionally, when the groove is irradiated with the high-power laser beam in the recording operation, the degradation is generated in the recording film property, which possibly results in a problem of reproduction durability.
In the reproduction operation, the optical disk is irradiated with the laser beam having a certain level of laser power lower than that in the recording. This causes the recording layer in the groove to be exposed to deteriorate the recording layer in light resistance. Particularly, an irradiation frequency with the reproduction laser beam is increased in an area, such as a data lead-in area and a file information storage area, which is read in each reproduction and frequently accessed by an optical pickup. Therefore, in the area, the recording layer deterioration is easily generated compared with other areas. However, when the area cannot be reproduced, information necessary to reproduce main information cannot be obtained to disable the information reproduction of the whole optical disk.