| 摘要 |
PURPOSE:To miniaturize and lighten a device and to facilitate the adjustment of an optical axis by constituting two flenel zone plates corresponding to a position being in line symmetry to a dividing line, providing bisected photodetectors in two focal positions respectively and arranging them so that an optical axis is on the prolongation of the dividing line. CONSTITUTION:The bisected Fresnel zone plate 20 is arranged so that whose dividing line 20L is intersected with the optical axis, and the bisected photodetectors 21a, 21b are arranged on the focal positions of respective 1st- order diffraction beams so that the optical axes of reflected beams are on the prolongation of respective dividing lines. The detective signals H, I, J, K of the light receiving parts 21h, 21i, 21j, 21k of the bisected photodetectors 21a, 21b are processed by adders 10a, 10b and a subtracter 11 and a focus error signal F is outputted. |
