| 摘要 |
PROBLEM TO BE SOLVED: To surely execute recording and reproducing of information on recording media varying in substrate thicknesses and to reduce the size over the entire part by using an objective lens varying in the focal lengths between a central part and a peripheral part so that light beams may be made incident effectively on the respective central and peripheral parts. SOLUTION: The objective lens 4 has the focal lengths varying in the central part 4a and the peripheral part 4b and condenses the light beams from the semiconductor laser 1 to the recording media 5, 6 of a large substrate thickness and a small substrate thickness. At this time, all of the laser beams are made incident only on the central part 4a of the lens without the beam conversion by a beam shape converting means 3. The light beams are made incident on the recording medium 6 by converting the beams to the beams of a ring band shape in section by the means 3 and making all of these beams only on the peripheral part 4b of the lens 4. The light beams are effectively utilized for the recording media 5, 6. As a result, the exit power of the semiconductor laser 1 is controlled, by which the information is surely recorded to and reproduced from the recording media 5, 6 varying in the substrate thickness and the size over the entire part is reduced. |
