| 摘要 |
PURPOSE:To simplify the constitution, by bringing the spot center of a reflected beam with a position shifted from the boudary of two photoelectric elements, amplifying the output of the photoelectric elements with different amplification factors and obtaining a focus error signal. CONSTITUTION:An optical beam 2 from a light source 1 irradiates an information medium 8, passes through a lens 12 through polarized beam and is made incident to a photodetector 11. The photodetector 11 consists of four photodetectors 11a-11d. The center of a diameter of an irradiated spot 13 of a reflected beam 10 is shifted from the boundary of the photodetector. The amplification factor of the outputs of the photodetectors 11c and 11d with larger amount of photodetection is made smaller than that of the photodetectors 11a and 11b. When a gap between the information medium 8 and a converging lens 6 is changed, the diameter of the spot 13 is changed and a focus controlling signal is obtained from the difference of quantity of light of the photodetectors. |
