| 摘要 |
PURPOSE:To prevent a focusing position from being varied due to the drive of an objective lens by restricting a reflected light beam to a proper diameter and a proper shape by a diaphragm on the way of leading the reflected light beam to a photodetector. CONSTITUTION:A light beam radiated from a semiconductor laser light source 1 and passed through a beam splitter 2 is restricted at its effective diameter by a circular aperture diaphragm 11 and then made incident upon an objective lens 2. When the objective lens 3 is driven so as to form a minimum beam spot on an information carrying face 10 in accordance with the movement of an information carrying face 10, reflected light from the carrying face 9 is reversely advanced in an optical path as it is and made incident upon the beam splitter 2 as a light beam having the same diameter as that of the aperture 11. The light beam reflected by the beam splitter 2 is made incident upon a sensor lens 5 and then made incident upon a sensor 6 and the diaphragm 11 is arranged on a fixed position from the light source 1. Even when the information carrying body 10 is moved to a position 10' and the objective lens 3 is moved to a position 3', the diameter of the light beam 71 made incident upon the sensor 6 is not changed when the sensor 6 is positioned on a focusing position.
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