| 摘要 |
PURPOSE:To reduce the adjustment cost considerably and improve the stability with time of an optical system and make an optical head small-sized by devising the shape of an element, which divides an incident beam to photodetectors, so that photodetectors can be constituted on the same plane. CONSTITUTION:A mirror body 30 of the incident beam dividing element is arranged before the condensation point of a disc-reflected luminous flux 9 whose course is changed by a beam prism, and its rear face M, front face H, and part T are a total-reflecting face, a half-transmission mirror face, and a transmission face respectively, and therefore, the reflected luminous flux 9 is divided into a reflected luminous flux 31 and a reflected luminous flux 32. A plane detector 33 is placed near the middle between condensation points P0 and P0' of luminous fluxes 31 and 32 at the time, when a spot is focused on a disc, and consists of two groups of circular two-divided photodetectors 34 and 35. These detectors are so arranged that quantities of light incident on an inside sensitive area I or I ' and an outside sensitive area II or II' are balanced when the spot on an optical disc 7 is in focus. Consequently, a subtracting circuit and an adder are connected to detectors 34 and 35 respectively to detect the out-of-focus state. |
