Apparatus for detecting a focusing state of an optical system

Abstract

Abstract In an apparatus for detecting a focusing state of an optical system, a laser beam generated from a laser unit (8) is transmitted through a beam splitter (10) to a convergent lens (14). The laser beam is converged by the convergent lens (14) and projected toward the surface of an optical disk (16). The laser beam reflected from the disk (16) surface is reflected by the beam splitter (10) and projected through the aperture of a light shielding plate (18) to a photodetector (2). The photodetector has first and second photosensitive regions (2-1,2-2) arranged concentrically, and when the convergent lens is in a focusing state, the levels of the first and second photo signals from the first and second photosensitive regions (2-1, 2-2) are equal to one another, and the output signal from the differential amplifier (4) is maintained at zero level. When the convergent lens (14) is approached from the position sufficiently separated from the optical disk (16) to the position of the focusing state, a positive output signal is generated from the differential amplifier (4). When the convergent lens (14) gradually approaches from the position of the focusing state to the optical disk (16), the beam spot gradually becomes small on the photosensitive regions and again starts expanding, but the laser beam does not expand at a predetermined beam diameter at the position provided with the light shileding plate (18). Therefore, the beam is retained at a predetermined value. Consequently, the output signal from the differential amplifier (4) is not varied from the negative level to the positive level.

Inventors

• 👤 Hisashi C/O Patent Division Yamada
• 👤 Seiichi C/O Patent Division Ohgoshi

Bibliographic Details