Optical disks commonly known as CDs, DVDs and the like have become widespread and are popular with consumers as high-capacity, low-price information recording media capable of random access. Recently, accompanying the increasing sophistication of personal computers and audio-visual devices, the amount of information handled by users has increased dramatically, and optical disks, as high-capacity information recording media, need to increase their capacity even more in response.
Recording onto an optical disk is performed by focusing laser beam with an objective lens to form optical spots on the recording medium, and forming recording marks on the information recording surface according to the optical spots. Further, information is reproduced by detecting, with optical spots, the difference in reflectance between the recording marks and other parts.
In this way, since information is recorded and reproduced on an optical disk using optical spots, the recording density of information is restricted by the size of the optical spots. Consequently, by reducing the size of optical spots, the information recording density of optical disks can be improved, and the recording capacity per disk can be increased. The diameter of an optical spot formed on the information recording surface is known to be proportionate to λ/NA, where λ is the laser beam wavelength and NA is the objective lens numerical aperture. Therefore, effective ways to improve the information recording density of an optical disk are to shorten the laser beam wavelength and increase the numerical aperture of the objective lens.
Recently a high density optical disk having a recording capacity of approximately 25 GB per information recording surface has been developed by applying technology for reducing optical spots, using a blue laser diode of wavelength λ=405 nm and an objective lens having numerical aperture NA=0.85, instead of the red laser diode of λ=650 nm and objective lens with NA=0.6 that had been used in DVDs (recording capacity=4.7 GB per information recording surface).
Adjustment of the distance between the information recording surface and the focal point on the optical disk is performed by altering the target position of the focus servo. By controlling the focal point position so that the focus error signal becomes zero, the focus servo constantly makes the focal point follow the recording surface. In order to adjust the distance between the recording surface and the focal point, an offset (focus offset) is added to the focus error signal. Accordingly, by altering the target position of the focus servo, the average distance between the recording surface and the focal point can be adjusted.
However, increasing the NA of the objective lens in order to increase recording density gives rise to the problem of optical spot distortion due to spherical aberration. Since the spherical aberration is approximately proportional to NA4×d, where d is the disk cover layer thickness error, in an optical disk system using a high NA objective lens, a large spherical aberration occurs by even a slight cover layer thickness error, causing readout signal quality to deteriorate markedly. Therefore, in the disk manufacturing process it was necessary to control the cover layer thickness with higher precision than in the case of conventional DVDs and the like, and this gave rise to issues of increasing manufacturing costs.
For the above-mentioned reasons, in the case of an optical disk system using a high NA objective lens, it is not sufficient simply to adjust the focus offset as in conventional DVDs, and a means to correct the spherical aberration in order to deal with cover layer thickness error for each disk is also essential.
Technology for correcting spherical aberration is used whereby in the optical system of an optical pickup, a spherical aberration compensation means is provided in the laser beam path, and by adding a prescribed amount of spherical aberration to the light beams passing through, the spherical aberration generated by the disk cover layer thickness error when light is focused onto the recording surface by an objective lens, and the spherical aberration added in advance by the spherical aberration compensation means cancel each other out. As a spherical aberration compensation means used in this way,    (1) a spherical aberration compensation means that comprises a 2-group objective lens, which alters the distance between each objective lens using an actuator, and    (2) a spherical aberration compensation means that comprises liquid crystal elements split into a plurality of regions, which controls the refractive index of each region by applying a voltage, and other such means are known, and are described in Patent document 1 and 2 for example.
There are various commonly known examples of indices for determining the spherical aberration to be applied in advance by the spherical aberration compensation means (hereinafter simply referred to as “spherical aberration”). For example, using the technology described in Patent document 1, adjustment of the focus offset and spherical aberration is performed according to the variation of the readout signal (RF signal) amplitude obtained when reproducing data recorded on the disk. However, with respect to a disk that does not have any data recorded (no recording marks), in other words a blank disk, even if one tried to optimize the focus offset and spherical aberration, since a readout signal cannot be obtained, this could not be performed.
Further, since the technology described in Patent document 2 also performs adjustment of the focus offset and spherical aberration using the data readout signal amplitude or both the data readout signal amplitude and the tracking error signal amplitude, there must be some data recorded on the information recording surface of the optical disk. In other words, this was still problematic in that if the optical disk did not have any data recorded on it, the necessary readout signal could not be obtained, and adjustment of the spherical aberration could not be achieved.
As technology to resolve this problem, technology for performing adjustments of the focus offset and spherical aberration based on the tracking error signal or wobble signal, which can be obtained even from a blank disk, is known, and described in the following patent documents for example.    [Patent document 1] JP-A No.11388/2000    [Patent document 2] JP-A No.222838/2001    [Patent document 3] JP-A No.233917/2003    [Patent document 4] JP-A No.168225/2003    [Patent document 5] JP-A No.171630/2004    [Patent document 6] JP-A No.241081/2004    [Patent document 7] JP-A No.241102/2004
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