| 摘要 |
PURPOSE:To execute a stable focusing control by providing a collimator lens on a position where the symmetry property to an axis orthogonal to a guide track is not influenced by a lens aberration, of a circuit pattern in a beam spot projected to a photodetector. CONSTITUTION:In a beam spot 52 projected to a 4-split photodetector 27, a part 53 which becomes a shadow is generated due to a diffraction by a guide track 23c. Even if this diffracted pattern 53 is in a focused state, a position relation to an information track 23b of an optical disk 23 is shifted. However, when the position is set so that an aberration of a collimator lens 22a and an objective lens 24a is negated, by turning a collimator lens unit 22, the pattern 53 becomes symmetrical to a boundary line Y-Y, namely, a state that no crosstalk is generated, even if the position relation of a beam spot 51 and the information track 23 is in any state. That is to say, by specifying the position relation of the guide track 23c, the collimator lens 22a and the objective lens 24a, a focusing control can be executed stably. |
