| 摘要 |
PROBLEM TO BE SOLVED: To miniaturize the device by embodying the effective numerical apertures NA of plural objective lenses with a single kind of opening member. SOLUTION: The opening member 6 is so arranged that the exit light from the objective lens 4 transmits through the aperture 6a of the opening member 6. The device is provided with an opening member driving device 7 for moving the opening member 6 is parallel with the optical axis direction of the luminous flux. At the time of detecting the information on an information recording medium 5a having a high recording density, the opening member driving device 7 moves the opening member 6 to a position where the luminous flux diameter of the exit light is not limited. On the other hand, at the time of detecting the information of the information recording medium 5b having low recording density, the opening member driving device 7 moves the opening member 6 to a position where the luminous flux diameter of the exit light is limited. In this way, the luminous flux diameter is limited according to the information recording medium to be used, by which the effective numerical aperture NA of the objective lens 4 is changed, and a light spot of adequate size is obtd. |
