| 摘要 |
PURPOSE:To obtain a sufficient focus servo gain even in the vicinity of focusing, by selecting and detecting only a variation of the outside peripheral area of a luminous flux which follows up a focal shift of the surface to be irradiated of the luminous flux which has passed through the surface to be irradiated. CONSTITUTION:A reflected luminous flux Tb is set so that 10%-30% of the whole reflected luminous flux is made incident on a focal shift detecting area 7a, when a luminous flux from an objective lens 4 has formed the smallest light spot on an information recording medium 5 (namely, a focused state). In this state, by excluding the center part of the luminous flux Tb which is not related directly to a minute focal shift, a focus error signals is obtained from only a variation portion of the outside peripheral portion of the luminous flux Tb. In such a way, without adding an optical member particularly, the sensitivity of a focus control signal can be improved. |
