Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2000-05-31
2004-01-13
Korzuch, William (Department: 2653)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
Reexamination Certificate
active
06678220
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an information recording apparatus for recording information by a local physical change in a recording medium due to energy irradiation, and exclusively relates to an optical disk device.
2. Description of the Related Art
Currently available optical disk-medium and device combinations can be roughly classified into a combination of magneto-optical disks and a magneto-optical disk device of the type of forming a mark by a reversal magnetic domain on a recording layer by heating the recording layer, and a combination of phase-change disks and an optical disk device of the type of forming a mark by an amorphous domain on a recording layer while changing the cooling speed of the recording layer by controlling the quantity of recording energy input at heating. As an example of a method for improving recording density of information to be recorded on such recording media, there is a method in which marks holding information are reduced as a whole and, at the same time, the distance between adjacent marks is shortened. In either type, it is, however, essential that the mark shape is controlled accurately. That is, if the mark length along a scanning direction of optical spot (tracking direction) is too short, jitter of a reproduced signal increases so that reliability of reproduced information cannot be secured. If the mark length along the tracking direction is contrariwise too long, jitter of the reproduced signal also increases. If the mark width along a direction perpendicular to the scanning direction (perpendicular to tracks) of optical spot is too narrow, the amplitude of the reproduced signal decreases so that reliability of reproduced information cannot be secured. If the mark width in the perpendicular direction is contrariwise too wide, crosstalk in reproduction process with respect to adjacent tracks increases so that reliability of reproduced information cannot be secured and, accordingly, there is a risk of cross-erase that adjacent recorded marks may be destroyed when things come to the worst. As described above, there is an important theme in terms of improvement of recording density of such recording media that a mark holding is formed into a desired shape (length and width) which is neither too large nor too small.
It is, however, very difficult to form stably and accurately a small mark having a size not larger than about a half of the optical spot size when the mark is to be reduced in size for the sake of high-density recording. This is because such a small mark cannot but be formed stably in a spatially gentle temperature gradient portion which is near a peak temperature portion among temperature rise portions of the recording layer by the optical spot. Hence, the peak temperature in a recording mode varies for every mark because of the change of the temperature of the recording layer before recording energy irradiation and the change of the intensity of recording energy. That is, the effective recording sensitivity of the recording layer varies. As a result, the mark shape varies greatly. In the case of an optical disk of the type of controlling the mark shape by recording energy intensity waveform, the peak temperature of the recording layer is apt to vary particularly due to variation of thickness of the recording layer of the recording medium and in accordance with the recording pattern. Reduction of the diameter of the optical spot by use of a short wavelength laser light source is direct to solve the aforementioned problem. Reduction of the wavelength of a semiconductor laser diode which is a typical laser light source is, however, attended with technical difficulty. Hence, there is the existing state of affairs that no technique has satisfied the demand for increase of recording density.
To solve these problems, in JP-A-5-298737 (hereinafter referred to as a first related art), a test-write zone is provided on a recording medium so that a signal reproduced from marks recorded in the test-write zone is evaluated for the purpose of forming small marks by the same optical spot stably and accurately. That is, when recording is performed while recording power is changed, leading and trailing edges of a mark shift in reverse directions along a tracking direction. The first related art asserts that the recording condition (especially, recording power) can be optimized when the length of the mark in the tracking direction is detected by use of the aforementioned technique.
In JP-A-1-292603 (hereinafter referred to as a second related art), for the same purpose as that of the first related art, a magneto-optical recording medium is heated periodically and intermittently by light pulses at regular intervals and, at the same time, a magnetic field applied to the heated region is inverted at a high speed. That is, the direction of magnetization of the approximately circular heated region can be determined in accordance with heat by every light pulse. Hence, when this operation is repeated while the heated position is shifted along the tracking direction, the direction of magnetization of a crescent-shaped domain can be substantially determined in accordance with heat by every light pulse. The second related art asserts that the mark length in the tracking direction and the mark width in a direction perpendicular to the tracking direction can be controlled independently so that a very small mark compared with the diameter of the optical spot can be formed stably regardless of the mark width.
The first related art, however, utilizes the characteristic of so-called “light intensity modulation method” in which the mark shape is determined only by a heating procedure by means of an optical spot. That is, the phenomenon that leading and trailing edges of each mark shift in reverse directions when recording is performed while recording power is changed is peculiar to the light intensity modulation method. The first related art cannot be applied to a “magnetic field modulation method” or a “laser-strobe magnetic field modulation method” in which leading and trailing edges of each mark shift by the same distance in the same direction along the tracking direction even in the case where recording is performed while recording power is changed. Moreover, substantially in the first related art, the shifting of the leading and trailing edges of the mark caused by the change of recording power is merely detected but the mark width is not detected directly. Accordingly, it was difficult to control the mark width accurately, that is, to determine recording power accurately. Hence, it was impossible to perform recording/reproduction using small marks at narrow track intervals, so that the first related art was disadvantageous in terms of improvement of recording surface density.
In the second related art, a very small mark compared with the diameter of the optical spot can be formed and, at the same time, the mark width and the shortest mark length can be changed individually. This effect is however effective only on magneto-optical recording media. Moreover, the conventional test-write method as disclosed in the first related art cannot be used in the second related art because leading and trailing edges of each mark shift by the same distance in the same direction along the tracking direction even in the case where recording is performed while recording power is changed, as described above. Hence, it was impossible to control the mark width sufficiently, so that it was difficult to reduce the track pitch.
For the aforementioned reason, in the respective related art, it was therefore impossible to form small marks with sufficient accuracy, so that it was impossible to improve recording surface density.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an information recording apparatus capable of forming marks carrying information with a shape desired to enhance recording areal density.
According to an aspect of the present invention, there is provided an information recordi
Kenyon & Kenyon
Korzuch William
Le Kimlien T.
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