Various optical discs, formed in a disc shape and onto which optical recording and/or reproduction are performed, are currently in use as optical recording/reproduction media. Such optical discs include read-only optical discs, in which embossed pits corresponding to data are formed in advance in the disc substrate; magneto-optical discs, which utilize a magneto-optical effect to perform data recording; and phase-change optical discs, which use phase changes in the recording film to perform data recording.
Of these discs, in optical discs enabling writing, such as magneto-optical discs and phase-change optical discs, grooves are conventionally formed in the disc substrate along recording tracks. Here a “groove” is a so-called guide groove formed along the recording track mainly in order to enable execution of tracking servo; the intervals between the ends of the groove apertures are called lands.
In the case of an optical disc in which grooves are formed, conventionally a tracking servo is configured using a tracking error signal based on the push-pull signal obtained from light reflected and diffracted by the grooves. Here a push-pull signal is obtained by detecting light reflected and diffracted by a groove using two photodetectors positioned symmetrically with respect to the groove center, and taking the difference in outputs of the two photodetectors.
Conventionally in the case of such optical discs, by improving the reproduction resolution of the optical pickup mounted in the reproducing device, high recording densities have been achieved. Improvement of the reproduction resolution of the optical pickup has been optically realized mainly by shortening the wavelength λ of the laser light used for data reproduction, and increasing the numerical aperture NA of the objective lens which condenses the laser light on the optical disc.
Conventionally, in each of the formats of so-called CD-R or write-once compact discs (CDs) and rewritable MDs (Mini-Discs) which are magneto-optical (MO) discs, write-once DVD-R type DVDs (Digital Versatile Discs), and so-called DVD+RW or DVD-RW (both registered trademarks for optical discs) which are rewritable DVDs, a groove recording format in which grooves are recorded has been proposed. In each of the formats of ISO system magneto-optical discs, land recording formats in which recording on lands is performed have been proposed.
On the other hand, in DVD-RAM (Random Access Memory) and similar formats, as a method of realizing optical discs with high densities a so-called land/groove recording method has been proposed in which on both grooves and lands data are recorded to double the track density compared with the conventional one.
Land/groove recording methods are also being studied for use in high-density optical discs being developed in recent years as next-generation optical discs, such as DVR (Digital Video Recordable) or Blu-ray Discs, and μ-Discs or the like which are small-sized MDs, and using the methods efforts are being made to raise recording densities.
However, when performing land/groove recording on DVD-RAM or other media, if the focus point is not separately adjusted during recording and reproduction for recording on lands and recording on grooves, optimal recording/reproduction characteristics are not obtained, and so there is the disadvantage of a more complex optical system.
Further, in “ISOM 2000 Simulation of Heat Generation and Conduction on Land/Groove Disc”, it is reported that recording beam shapes are different in recording on lands and in recording on grooves, and clearly it is difficult to obtain uniform land recording/reproduction characteristics and groove recording/reproduction characteristics, so that there is the problem of regions with different recording/reproduction characteristics existing in the same optical recording/reproduction medium.
Further, in Blu-ray Discs and other high-density optical discs, the recording/reproduction characteristics are satisfactory near the reading surface, which in the case of Blu-ray Discs is on the lands nearer the light irradiation side, though the recording/reproduction characteristics are poor away from the reading surface, which in the case of Blu-ray Discs is the grooves farther from the light irradiation side.
In DVD-ROM (Read-Only Memory) discs and similar, at present it is possible to directly record signals in such a land/groove recording format; however, satisfactory and uniform recording/reproduction characteristics in recording on lands and in recording on grooves are desired.
Further, as described above, in Blu-ray Discs and other high-density optical discs the groove portions are away from the reading surface, and it is difficult to obtain satisfactory recording/reproduction characteristics in these groove portions.
On the other hand, in processes to manufacture an optical recording/reproduction medium, a method is conceivable in which the depression/protrusion pattern to be formed in the substrate is inverted for manufacturing. That is, in conventional processes to manufacture an optical recording/reproduction medium, photolithography or other means are used to form a minute depression/protrusion pattern in the photosensing layer of the glass master, and then plating or other means are employed to form a master stamper of, for example, nickel.
Then, either by an injection molding method in which this master stamper is positioned in a die or other equipment and a resin is injected, or by a so-called 2P (photo-polymerization) method in which an ultraviolet-curing resin, for example, is applied to the substrate and this stamper is pressed into the resin layer to form the desired depression/protrusion pattern, the substrate of the optical recording/reproduction medium in which the predetermined minute depression/protrusion pattern is formed in the surface can be formed.
Thus as described above, when groove portions are provided on the side farther from the reading light and satisfactory recording/reproduction characteristics cannot be maintained, a replica of the above-described master stamper, that is, a so-called mother stamper may be formed by transcription using electroplating or other methods to invert the depression/protrusion pattern and to obtain a configuration of the groove pattern on the substrate provided on the side closer to the reading light, thereby improving the recording/reproduction characteristics.
However, when a groove recording or land recording format is adopted, if an attempt is made to achieve high recording densities similar to the case of a land/groove recording format, then the track density must be set to double that of the case of a land/groove recording format, that is, the track pitch must be reduced to half; consequently the amplitude of the push-pull signal or other tracking servo signal is reduced, so that stable tracking and the reproduction of wobble signals become difficult.
For example, a land/groove recording format may have a track pitch of 0.60 μm, that is, a land width of 0.30 μm and a groove width of 0.30 μm, and the push-pull signal amplitude is approximately 90%.
However, when attempting to achieve a similar recording density using a groove recording format, if the track pitch is set to 0.32 μm, the push-pull signal amplitude is approximately 18%.
In conventional optical discs, the track pitch is set to the track pitch corresponding to a cutoff frequency of an optical pickup of a reproducing device, that is, approximately 3/2 to twice a spatial frequency. The cutoff frequency is the frequency at which the reproduction signal amplitude is substantially zero; if the wavelength of the laser light used for data reproduction is λ, and the numerical aperture of the objective lens focusing the laser light on the optical disc is NA, then the cutoff frequency is represented by 2NA/λ.
In the case of DVRs and the above-described Blu-ray Discs, the numerical aperture NA is 0.85±0.05, and the wavelength λ of reproduction light is 405±10 nm. If the numerical aperture NA is for example 0.85 and the reproduction light wavelength λ is for example 406 nm, then the cutoff frequency (2NA/λ) is 4187 lines/mm, and the track pitch corresponding to this is 0.239 μm.
If the track pitch of DVR and Blu-ray Discs is 0.32 μm, then this is approximately 4/3 (0.32/0.239=1.339) of the track pitch corresponding to the cutoff frequency 0.239 μm.
The reason for conventionally setting the track pitch to approximately 3/2 to twice the track pitch corresponding to the cutoff frequency is that, in order to obtain stabilized tracking servo operation and to reproduce a stable wobble signal, it is required to obtain a tracking servo signal amplitude at a sufficient level.
The high-density optical discs of recent years use a push-pull signal as the tracking error signal; however, in order to perform stable tracking servo operation, the push-pull signal amplitude ratio must be approximately 0.135 or more. Further, it is desirable that measures be taken to enable stable reproduction of the wobble signal.
On the other hand, apart from recording on grooves, a format is proposed in which TOC (Table of Contents) and other information is recorded as pits. However, because the density of pits in the circumferential direction is approximately half of that of grooves, there is the disadvantage that the tracking servo signal amplitude (push-pull signal amplitude) is reduced to half.
That is, if an attempt is made to increase the density with the above-described DVR and Blu-ray Disc track pitch being set to 0.32 μm, the push-pull signal amplitude for pits falls to approximately 9% (18%/2), so that tracking servo operation becomes difficult. Also, the pit depth at which the push-pull signal amplitude is maximum is approximately ⅛ of the wavelength λ of the optical pickup, and pit widths are approximately half the track pitch, so that the pit push-pull signal amplitude is improved to 12%, but tracking servo operation becomes unstable.