| 摘要 |
PROBLEM TO BE SOLVED: To provide an optical head having a signal recording and reproducing performance practically usable without means of limiting opening a at low cost, in an optical head to record/reproduce optical information recording media having light sources of two different sorts of wavelengths and base materials of different thickness. SOLUTION: On the surface of a lens or an optical part in an optical passage, means of changing optical phase distribution 14a-14c in a region a little near to the outer peripheral of an entrance pupil of an objective lens 14 are provided. The boundary of the region is 75% of an effective diameter of the objective lens in the case of finite optical system and likewise 60% of the same in the case of infinite optical system. This region is constituted by dividing into plural ringlike sections and alternatively arranging a section giving phase difference in level and a section giving no such difference. The amount of phase difference in level then is set so as to be an integral multiple of the wavelength for a light of shorter wavelength and to be around 180 deg. for a light of long wavelength. |
