| 摘要 |
PROBLEM TO BE SOLVED: To position-control an objective lens by an astigmatic method in the interchangeable device for high and standard density disks. SOLUTION: A disk class detecting circuit 11 makes a discrimination to a newly inserted disk to determine whether it is a high density disk or not. If it is a standard density disk, a beforehand set offset value is superimposed to the focus signal outputted from a light receiving element 8 by an offset superimposing circuit 12 and outputted. Thus, while converging the laser beams to a standard density disk using an objective lens 5, to which the optical characteristics for a high density disk are optimally designed, the central section of the laser means is in a focused condition, the peripheral section is not focused due to the adverse effect of the spherical aberration and the astigmatic method is made applicable. |
