| 摘要 |
PURPOSE:To accurately focus the light on each of plural recording layers of a layered structure by providing a second laser light source for information recording/reproducing and erasing independently of the servo mechanism of a disk substrate and changing the distance between this laser light source and a fixed lens provided in front of this light source. CONSTITUTION:A second laser light source (the second light source) 13 is provided on an electrostrictive element 11, and the light of the light source 13 is converted to parallel rays of light by a fixed lens 9, and an objective lens 10 is driven by the servo mechanism and is focused on a second recording layer 4. An electric field is applied to the electrostrictive element 11 to expand this element in the thickness direction, and the distance between the second light source 13 and the fixed lens 9 is shortened to set the light in the out-of- focus state. Consequently, the focus position is shifted forward because the position of the objective lens 10 is not changed, and the light is focused on a first recording layer 2. The focus position is controlled by the voltage applied to an electrode 12 of the electrostrictive element 11. Thus, the position of the objective lens 10 is not changed but the position of the second light source is changed to focus the light.
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