Objective lens and optical head device provided with the same

An objective lens for an optical head device which condenses two or more laser beams having wavelengths corresponding to two or more optical recording mediums having different thicknesses of transparent substrates on recording surfaces of the optical recording mediums via one optical condensing system to record information on the recording surfaces and/or reproduce the information on the recording surfaces, the objective lens comprising:

• a refractive surface including:
• a middle region which is formed in a middle portion of the refractive surface centering on an optical axis of the objective lens and which is used with respect to all of the optical recording mediums;
• one or more peripheral regions which are concentrically formed outside the middle region;
• and an aperture limiting ring band which is concentrically formed between the middle region and the peripheral region or between the peripheral regions;
• wherein the middle region includes:
• a plurality of middle ring-band refractive surfaces having aspherical shapes which are mutually different in refractive power;
• and stepped portions formed toward an optical axis direction in boundaries of the plurality of middle ring-band refractive surfaces;
• radial direction positions of the stepped portions are used as positions to correct spherical aberrations with respect to all of the optical recording mediums, and heights of the stepped portions and the aspherical shapes of the middle ring-band refractive surfaces excluding the middle ring-band refractive surfaces disposed adjacent to the aperture limiting ring band are formed in such a manner as to correct the spherical aberrations with respect to all of the optical recording mediums;
• the aperture limiting ring band is formed in the vicinity of a numerical aperture required in recording/reproducing the information with respect to one of the optical recording mediums in order to prevent the laser beam corresponding to the optical recording medium from being condensed on the recording surface of the optical recording medium;
• the peripheral region comprises one or more peripheral ring-band refractive surfaces;
• and
• the middle ring-band refractive surface and the peripheral ring-band refractive surface disposed adjacent to the aperture limiting ring band, or the peripheral ring-band refractive surfaces disposed adjacent to an inner peripheral side and an outer peripheral side of the aperture limiting ring band are formed into aspherical shapes optimized with respect to the specific optical recording medium.