| 摘要 |
PURPOSE:To obtain a small-sized optical pickup having a small number of components by using a plastic diffraction type optical element having a diffraction grating and specific values of saturated water absorptivity and double refractive index. CONSTITUTION:A luminous flux from a laser light source 8 is reflected on a surface of an optical disk 7, diffracted by a grating 10a for generating a focus error signal of a diffraction type optical element 10 through an objective lens 9, and made incident on a photoreceiver 11. Thus, since this optical pickup has only one optical component, the pickup is reduced in size. The element 10 is formed in a central circular part 10b of a flat plate with a linear diffraction grating 10c, and pitch of which is formed continuously variably, and has a function for imparting an astigmatism for the focus error signal to a laser beam. Its double refractive index is 18nm/mm at 0.5% or less of saturated water absorptivity. In this case, the photoreceiver 11 is divided into four PDs. Since a condensed optical spot 12 becomes elliptical, a difference of two signals obtained by adding outputs of the PD of a diagonal direction is taken, and the focus error signal can be obtained. |
