| 摘要 |
An optical head eliminates an disturbance which occurs in a focus error signal in association with the decentering of an optical disk when an optical spot crosses a track on a storage film surface, and in addition the optical head cancels an off-set which occurs in a tracking error signal in association with the movement of an objective lens. In the constitution of the optical head, phases of wave surfaces of ±1st order diffracted lights are shifted by an amount of lambd/2 with a common period by using a phase-inverted diffraction grating the phase of which is inverted with a period of D lambd/(2P . NA) in a radial direction of the optical disk, thus causing, on the optical disk, sub-spots to be located on the same track as a main-spot. Taking advantage of this location, focal point shift error signals of the main-spot and the sub-spots are added to each other with an appropriate gain ratio, thereby making it possible to cancel the disturbance which occurs in the focus error signal. Meanwhile, tracking error signals of the main-spot and the sub-spots according to a push-pull method are subtracted from each other with an appropriate gain ratio, thereby making it possible to cancel the off-set.
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