| 摘要 |
PURPOSE:To obtain a compact optical head at a low cost by using a Fresnel lens for a part of an optical member placed on the way of an optical path. CONSTITUTION:One (track shift detecting system) of reflected beams divided into two systems by a half mirror 30 is made incident on a photodetector 32 through a Fresnel lens 31 stuck to the mirror 30. This detecting signal contains an information signal. The other beam (out-of-focus detecting system) from the mirror 30 passes through a Fresnel lens 33 stuck to the mirror 30, and thereafter, it is drawn out asymmetrically with respect to the optical axis by a knife edge 34, made incident on a photodetector 35, and by its detecting signal, an objective lens 26 is maintained in a focused state. Since the Fresnel lenses 31, 33 are made of plastic, a titled optical head is made compact and low in cost. |
