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From the background information supplied by the inventors, news correspondents obtained the following quote: "The present invention relates to an optical pickup device and an optical disc apparatus.
"JP-A-2006-344344, page 26, FIGS. 3 and 5, has disclosed this technical field as background art. This document discloses as an objective that 'a desirable signal is obtained with high accuracy from an optical disc having a plurality of recording layers.' As solving means to achieve the above-mentioned objective, the document discloses that 'the optical beam of a P-polarized light emitted from an optical source unit (51) is reflected by an optical disc (15) to be turned into an S-polarized light and incident on a lens (61). At 1/4 wavelength plates (62, 63), an optical phase difference of +1/4 wavelength is added to the optical beam incident on +X side of an optical axis, and the optical phase difference of -1/4 wavelength is given to the optical beam incident
Supplementing the background information on this patent, VerticalNews reporters also obtained the inventor's summary information for this patent: "An optical pickup device displaces an objective lens to adjust it in a focusing direction by detecting a focusing error signal, and also detects a tracking error signal to displace the objective lens in a radius direction of an optical disc and perform a tracking adjustment, for a purpose of correctly irradiating a spot on a predetermined recording track in the optical disc. A positional control for the objective lens is therefore performed by the above-mentioned signals. Among the above-mentioned servo signals, the tracking error signal has a serious problem because the dual layer disc has two recording layers. In the case of dual layer disc, stray light reflected from an untargeted recording layer is incident on a light receiving part on which the signal light reflected from the targeted recording layer is also incident. When both the signal light and stray light are incident on the light receiving parts, the two lights are interfered with each other and a fluctuation component caused by the interference is detected as a tracking error signal.
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