| 摘要 |
PURPOSE:To obtain a stable focus error signal against the deflection of a spot in its dividing direction by specifying the width of a photodetecting area in the dividing direction of a photodetecting surface, the maximum deflecting range of the spot and the relation between the spot radiuses. CONSTITUTION:A photodetecting surface of a photodetector 6 is divided into surfaces A, B and C. Then 2r+dc>a+b+c is satisfied for the photodetecting area widths (a), (b) and (c) set in the dividing directions of faces A-C. In this case, the maximum spot deflecting range where the value of a focus error signal does not exceed the maximum allowance value of the spot deflection is defined as (dc) when the center of a light beam spot on the photodetecting surface is deflected toward the dividing direction from the center of the photodetecting surface in a focused state and the spot radius is referred to as (r). In such a way, a stable focus error signal is obtained even in case the center of the light beam spot moves toward the dividing direction of the photodetecting surfaces from the center of the photodetecting surface in a focused state with no focus error signal produced in response to the distance between both centers. |
