| 摘要 |
<p>The appts. comprises a lens (7) having lens areas forming four quadrants (I to IV) sectioned by two planes, each including the optical axis (Z) and being perpendicular to each other. The focal lengths of the lens areas of the first and third quadrants (I,III) are equal, as are the focal lengths of the lens areas of the second and fourth quadrants (II,IV). The focal length of the lens areas in the first and third quadrants is different from that of the lens areas in the second and fourth quadrants. The respective lens areas have symmetry about the optical axis Z. The lens is located within the optical path of the beam reflected from an optical disc and a photodetector having four divided detecting portions in correspondence with the respective lens areas for detecting the reflected beam passed through the lens. By measuring the difference in output between first and third and second and fourth quadrants the converging state of the lower beam on the record medium surface is detected. This enables focus control.</p> |
