Objective lens, light converging optical system, optical pickup apparatus, and recording/reproducing apparatus

An objective lens for recording information on and/or reproducing information from an optical information recording medium, comprising:

• a diffractive structure including ring-shaped diffractive zones on at least one surface thereof,
• wherein the objective lens is a single lens made of plastic material, at least one surface thereof is an aspheric surface, and the following formula is satisfied;
• λ≦500 nm
• where λ represents a wavelength used for recording on and/or reproducing from the optical information recording medium, and further the following conditional formula is satisfied;
• 2.0≦fD/f≦30.0
• where fD is a focal length (mm) of only a diffractive structure defined by fD=1/Σ(−2·ni·b2i) when an optical path difference added to a transmitted wavefront by a ring-shaped diffractive zone formed on an ith surface is expressed by an optical path difference function defined by Φbl=ni−(b2l·hi2·b4l−hi4+b6l−hi6+ . . . ) (here, ni represents the diffraction order of a diffracted ray having the maximum light amount among diffracted rays generated at the ring-shaped diffractive zone formed on the ith surface, hi represents a height (mm) from an optical axis, and b2l, b4l, b6l . . . represent respectively 2nd order, 4th order, 6th order, . . . optical path difference function coefficients (called also diffractive surface coefficient)), and
• f represents a focal length (mm) of the total objective lens system wherein refractive power and diffractive power by the diffractive structure are combined.