| 摘要 |
PURPOSE:To attain sure focusing even for an optical disk with a small difference in the reflectance between a base and a recording film by radiating a laser them through the base of the optical disk so as to detect the increase in the laser output due to the rapid increase in the reflected luminous quantity. CONSTITUTION:A base 12 of a laser 1 is reflected in a base splitter 2 and rediated onto a recording face 6 through a lambda/4 plate 3, a lens 4 and a base 5. Its reflected light is collected onto a detector 10 through a convex lens 7,the beam splitter 2 and a half mirror 8. The correction of spherical abberation due to the difference in the refraction between air and the base is applied to the lens 4 to form a minimum spot with non-abberation onto the recording face only. Part of the reflected light focused onto the recording face is returned to the laser 1 to feedback coupling to the laser 1 and the laser power is increased momentarily. Thus, a low frequency component 19 in the signal of a detector 11 is eliminated and the result is binarized at a position of L3 level by a comparator circuit 18, then the moment when the laser is focused on the recorded face is discriminated easily and stable focus locking is attained even with a disk having a small difference in the reflectance between the base and the recording film.
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