Direct-read-after-write optical disks are of write-once and erasable types. Of these types of optical disks, a write-once optical disk may have its data by recording surface made from tellurium (Te) or bismuth (Bi) so that pits are formed thereon by fusing the surface by focusing a laser beam thereon, or made from Sb2Se3, TeOx or an organic dye-based thin film so that optical reflectivity is changed by focusing the laser beam on the surface.
A CD-R, which is a write-once optical disk, includes a number of pregrooves for guide purposes. The pregrooves radially wobble slightly at a center frequency of 22.05 kHz. Address information at a time of recording called ATIP (Absolute Time In Pregroove) is FSK-modulated with a maximum deviation of 1 kHz. to be multiplexed and recorded in the pregrooves.
The signal recording format of a CD-R disk includes a power calibration area (PCA) for recording and measuring an optimum write power, a program memory area (PMA) for temporarily recording signal recording information or skip information in a writing process, a lead-in area, a program area, and a lead-out area, which are arranged in an order described from the center of the disk.
With respect to the CD-R disk that is a write-once optical disk, an OPC (Optimum Power Control) operation is performed prior to recording so as to set a laser beam to its optimum write power. For this purpose, the power calibration area includes 100 test areas (partitions) each formed of 15 frames.
Such a setting of the optimum write power is necessary because disks differ in their recording characteristic depending on their manufactures. If the optimum write power of a disk is not obtained, a reproduced signal may have its jitter or error rate considerably worsened in some cases.
According to a conventional method, one write power is assigned to one frame, and recording is performed on the test area with powers of 15 levels from the minimum to the maximum power. Thereafter, the peak value (P) and the bottom value (B) of the envelop of each RF (high-frequency) signal reproduced from the test area are detected. Then, the write power of a level at which it is determined that a value β obtained from β=(P+B)/(P−B) exceeds a predetermined value (for instance, 0.04) is considered as an optimum write power, and thereafter, signal recording is performed.
One test area (15 frames) in the power calibration area corresponds to approximately 1.7 rotations of the disk.
However, disk cost reduction has caused the signal recording surfaces of disks to contain deflection or eccentricity, or to suffer from unevenness of the application of signal recording surface materials. Further, multi-speed or high-speed rotations of a disk drive unit bring the rotation frequency of a disk closer to the resonance frequency of the lens actuator of an optical pickup, thus causing resonance. These causes a variation in a cycle of one rotation of the disk, thus exerting the following effects on the OPC operation for setting the optimum write power.
The center of the optical axis of the laser beam is deviated from the center of an objective lens so that the amount of the laser beam emitted from the objective lens changes, thus causing a variation in efficiency for light utilization. This is particularly noticeable if a rotational system has a large eccentricity. Further, if the signal recording surface has a large deflection, the incident angle of the laser beam varies to cause a variation in the amount of the laser beam effective for recording. That is, there has been the problem that this periodic variation prevents the optimum write power from being set with good accuracy.