| 摘要 |
PROBLEM TO BE SOLVED: To correct a color aberration generated due to the difference of a wavelength and/or a spherical aberration generated due to the difference in the thickness of a transparent substrate, by forming a diffraction ring belt on at least one optical face which constitutes a condensing optical system. SOLUTION: A diffraction optical system is a single lens both faces of which are aspherical. A diffraction face is formed on at least one face. By using the same action as the correction of a spherical aberration, a diffraction ring belt is formed on an optical face. A color aberration on an axis is corrected with reference to the difference of a wavelength. A light source 111 on one side radiates a laser beam at a wavelength of 400 nm. A circularly polarized parallel luminous flux is narrowed down by a diaphragm 17, and it is condensed on an information recording face 22 via the transparent substrate 21 of a first optical disk 20 by using an objective lens 16. The luminous flux which is modulated by an information bit so as to be reflected is transmitted again through the objective lens 16, the diaphragm 17, a quarter-wave plate 14 and a collimator 13, it is incident on a polarization beam splitter, it is reflected, it is given an astigmatism by a cylindrical lens 18, and it is incident on a photodetector 30. |
