| 摘要 |
An optical head device is provided with an objective lens (5) for collecting light emitted from a light source on an optical recording medium (6); a diffraction optical element (8a) for diffracting the reflection light of the emitted light reflected by the recording medium (6) and enters through the objective lens (5); and a photodetector (10a) for receiving a light flux of the reflection light diffracted by the diffraction optical element (8a). The light flux has a first light flux group passed through an area in the diffraction optical element (8a), and a second light flux group passed through a part of the area. The photodetector (10a) detects the first light flux group and the second light flux group by separate photodetector sections (14a to 14h). Based on the detection results from the separate photodetector sections (14a to 14h), a focus error signal to be used for focus servo and a protection layer thickness shift signal indicating a protection layer thickness shift of the optical recording medium (6) are detected.
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