1. Field of the Invention
The present invention relates to optical disks having high recording density, and more particularly, to an optical disk such as a compact disk and a video disk that allows recording/reproduction for a long period of time.
2 .Description of the Background Art
Various optical disks such as a compact disk (CD) and a video disk are developed. There are various standards such as CD-DA, CD-ROM, and CD-I even in one type of a compact disk. According to these standards, the physical dimensions such as disk outer diameter, track pitch, and minimum size of pit (shortest pit length) are predetermined.
For example, in a CD-ROM, the track pitch is 1.6 .mu.m and the shortest pit length is 0.83-0.97 .mu.m. The shortest pit length varies according to the allowance of the linear velocity (1.2-1.4 m/sec) of the optical disk. More specifically, since the frequency of a reproduction clock is 4.3218 MHz, the physical length of 3T which is the shortest pit length is 0.83 (.apprxeq.(1.2/4.3218).times.3) .mu.m when the linear velocity is, for example, 1.2 m/second.
In accordance with the technical advance in the field of short-wavelength laser, various research is carried out as to disk fabrication outside the standard range and reproduction thereof. For example, the technique of recording information at a track pitch smaller than that of the standard to store information approximately two times the standard capacity into an optical disk of an outer diameter identical to that of the standard is known. In this case, reproduction from such an optical disk of high density recording can be effected by reducing the diameter of the beam spot employed in reproduction.
One method of increasing the recording density of an optical disk is to compress the physical amount such as the track pitch and the pit length. For example, a double recording density can be achieved by setting the track pitch and the pit length to 1/2 respectively.
However, crosstalk occurs between adjacent tracks at the time of reproduction if the track pitch is too small. Furthermore, if the pit length is reduced, the level of a reproduced signal becomes smaller as the pit has a length more approximating to the shortest pit length. Although the level of the reproduced signal can be easily compensated for, it is difficult to remove the crosstalk component from the reproduced signal.
If the recording density is improved on physical basis as described above, the standard deviation of the jitter of the reproduced signal is increased at the time of reproduction. The frequency of readout error becomes higher as this standard deviation is greater.