| 摘要 |
PURPOSE:To obtain a focus and a tracking error signal by excluding a half- mirror while employing a Fresnel prism system and a heterodyne system, and using only one sensor. CONSTITUTION:A reflected beam from a disk D is made incident to a Fresnel prism 8 as it is without being split into two by a half-mirror to illuminate only one sensor 11. The sensor 11 is split into S1-S8. The focus error signal SF is obtained from SF=(S1+S4+S5+S8)-(S2+S3+S6+S7). When there is no tracking shift, the shape of the beam on the sensor 11 is in a focusing-point state as shown in a figure (a-1)(SF=0), and a figure (a-2)(SF>0) or figure (a-3)(SF<0) is obtained according to whether the disk is farther than focal length or not, obtaining the signal SF. The tracking error signal ST, on the other hand, is obtained from ST=(S1+S2+S7+S8)-(S3+S4+S5+S6). An illuminance distribution is as shown in figures (b-1, b-2, and b-3). Slanting line parts are low illuminance parts. |
