Objective lens, optical system and optical pickup apparatus

An optical pickup apparatus, comprising:

• a light source;
• and
• an optical system to converge a light flux from the light source onto an information recording plane of the optical information recording medium so as to conduct recording and/or reproducing information;
• the optical system comprising:
• a chromatic aberration correcting element having a ring-shaped zonal structure including plural ring-shaped zones on at least one optical surface thereof in which neighboring ring-shaped zones are divided with a stepped section in such a way that one of the neighboring ring-shaped zones located apart from the optical axis has a longer optical path than the other one located closer to the optical axis, and
• an objective lens to converge a light flux from the chromatic aberration correcting element onto an information recording plane of an optical disk and having a ring-shaped zonal structure including plural ring-shaped zones on at least one optical surface thereof in which neighboring ring shaped zones are divided with a stepped section shaped in the optical axis direction in such a way that the stepped section causes a optical path difference between light fluxes having passed through the neighboring ring-shaped zones;
• wherein the ring-shaped zonal structure of the chromatic aberration correcting element corrects a deviation of a focal point caused by the objective lens due to a wavelength fluctuation of an incident light flux coming into the optical system;
• and
• the ring-shaped zonal structure of the objective lens corrects a spherical aberration caused by the objective lens due to a wavelength fluctuation of an incident light flux coming into the optical system.

An objective lens for use in an objective lens, comprising:

• a ring-shaped zonal structure including plural ring-shaped zones on at least one optical surface thereof in which neighboring ring-shaped zones are divided with a stepped section shaped in the optical axis direction in such a way that the stepped section causes a optical path difference between light fluxes having passed through the neighboring ring-shaped zones;
• wherein the following formula is satisfied:
• SA1>SA2
• where, when it is defined that a refractive lens has not the ring-shaped zonal structure and has the same design wavelength, the same material, the same focal length, the same image-side numerical aperture, the same magnification, the same lens thickness and the same back focus as those of the objective lens, SA1 is a wavefront aberration (λrms) when a light flux having a wavelength longer by 10 nm than the design wavelength comes into the refractive lens with a magnification larger by a predetermined value than the above magnification, and SA2 is a wavefront aberration (λrms) when a light flux having a wavelength longer by 10 nm than the design wavelength comes into the objective lens with the magnification larger by the predetermined value than the above magnification.