Patent ID: 7142489

Claim:
An optical disk device comprising: a light source; an objective lens for focusing light emitted from the light source on an optical disk; an optical splitter for diffracting the light reflected by the optical disk; and a photodetector on which the light diffracted by the optical splitter is focused, wherein the optical splitter is divided into n(n≧2) regions AK(k=1, 2, . . . , n) by a straight line that intersects with an optical axis, the photodetector is divided into at least two detection regions A and A′, the light emitted from the light source is focused on any one of a plurality of signal planes of the optical disk by the objective lens, light reflected from the signal plane on which the light emitted from the light source is focused (the focused plane) and light reflected from a signal plane located in proximity to the focused plane (a proximity plane) pass through the objective lens to turn into light beams a and a′, respectively, that enter the optical splitter, 1st-order diffracted light beams ak and ak′ having a common diffraction optical axis are derived from the light beams a and a′ that have entered the regions Ak of the optical splitter, respectively, and are projected on the photodetector, distributions of the 1st-order diffracted light beams ak and ak′ on the photodetector are approximately inverted with respect to an intersection point of the diffraction optical axis and a detection plane, and the 1st-order diffracted light beam ak is mostly within the detection region A and the 1st-order diffracted light beam ak′ is mostly within the detection region A′.