| 摘要 |
PURPOSE:To prevent the deterioration in quality of a focusing error caused by the aberration of an optical system by interposing a turnable parallel plate tilted from an optical axis to a divergent or converging part of light in an optical path. CONSTITUTION:The astigmatism in a direction in parallel or not orthogonal to an information track remaining in the optical system is removed. That is, the direction of the astigmatism is controlled by turning a parallel plate 12 around the optical axis 13 and the quantity of astigmatism is controlled by varying a tilt angle theta of the parallel plate or using the parallel plate different in plate thickness (t) and refractive index (n). Thus, it is prevented that the aberration of the optical system influences the diffraction pattern by a guide track projected onto a photodetector, resulting in deteriorating the quality of a focusing error signal. |
