| 摘要 |
PURPOSE:To simplify a focus control mechanism by using a plate type half mirror arranged so as to reflect a luminous flux from a laser light source on its surface, and passing through the reflected light from an optical disc to generate an astigmatism. CONSTITUTION:A translucent film is formed on one face of a base being a parallel flat plate in the half mirror 20a and about 50% of a divergent luminous flux from a semiconductor laser 2 is reflected on a mirror face 20a. The reflected luminous flux is made incident to a focus lens 8 via a collimate lens 4 and the focus of the lens 8 is controlled by a focus servo mechanism 12. The luminous flux transmitting through the lens 8 is collected on a pit face of an optical disc 10 so as to be a spot, and the luminous flux reflected on the pit face is made incident again to the mirror 20a via the lenses 8 and 4. About 50% of the reflected luminous flux, i.e., about 25% of the luminous flux transmitted from the laser 2 transmits through the mirror 20a and is received by a photodetector 14. The astigmatism occurs to the luminous flux incident to the detector 14 and the focus control mechanism is simplified. |
