| 摘要 |
In a device for detecting a focus error in controlling a position in an optical axis, a laser beam is split into first and second beam components, and one of the first and second beam components is diverged or converged. Then, the first and second beam components are superposed on each other, providing a single laser beam. The single laser beam is applied to an optical data-recording medium through an objective lens. The first laser beam component is focused on a first focusing point that is set at one side of the pinhole. The second beam component is focused on a second focusing point that is set at the other side of the pinhole.
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