Objective lens, optical head, optical disk device, and information processing device

An objective lens, comprising:

• a first area substantially concentrically formed on a laser light source side surface of the objective lens with respect to an optical axis;
• a second area formed on an outside of the first area;
• and
• a third area formed on an outside of the second area,
• a diffraction structure is formed on the first area, the second area, and the third area,
• the first area, the second area, and the third area being configured to converge, on an information recording surface of a first information recording medium having a light transmissive layer of a first thickness t1 [mm], laser light of a diffraction order having a largest diffraction efficiency, out of laser light of a first wavelength λ1 [nm] to be diffracted by the diffraction structure formed on the first area, the second area, and the third area,
• the first area and the second area being configured to converge, on an information recording surface of a second information recording medium having a light transmissive layer of a second thickness t2 (t2>t1) [mm], laser light of a diffraction order having a largest diffraction efficiency, out of laser light of a second wavelength λ2 (λ2>λ1) [nm] to be diffracted by the diffraction structure formed on the first area and the second area,
• the first area being configured to converge, on an information recording surface of a third information recording medium having a light transmissive layer of a third thickness t3 (t3>t2) [mm], laser light of a diffraction order having a largest diffraction efficiency, out of laser light of a third wavelength λ3 (λ3>λ2) [nm] to be diffracted by the diffraction structure formed on the first area,
• assuming that third-order coma aberration [mλ/deg] per unit angle generated by tilt of the first information recording medium in recording information on the first information recording medium or reproducing information from the first information recording medium is CMD1, third-order coma aberration [mλ/deg] per unit angle generated by tilt of the third information recording medium in recording information on the third information recording medium or reproducing information from the third information recording medium is CMD3, third-order coma aberration [mλ/deg] per unit angle generated by tilt of the objective lens in recording information on the first information recording medium or reproducing information from the first information recording medium is CML1, and third-order coma aberration [mλ/deg] per unit angle generated by tilt of the objective lens in recording information on the third information recording medium or reproducing information from the third information recording medium is CML3,
• a sine condition deviation amount SC1 in recording information on the first information recording medium or reproducing information from the first information recording medium is expressed by CMD1+CML1,
• a sine condition deviation amount SC3 in recording information on the third information recording medium or reproducing information from the third information recording medium is expressed by CMD3+CML3, and
• the sine condition deviation amount SC1 and the sine condition deviation amount SC3 satisfy:
• SC1>0 and
• SC3<0, and
• assuming that a focal length [mm] in recording information on the first information recording medium or reproducing information from the first information recording medium is f1, a focal length [mm] in recording information on the third information recording medium or reproducing information from the third information recording medium is f3, a numerical aperture in recording information on the first information recording medium or reproducing information from the first information recording medium is NA1 (where 0.8≧NA1≧0.9), and a numerical aperture in recording information on the third information recording medium or reproducing information from the third information recording medium is NA3 (where 0.45≧NA3≧0.52, and NA3/NA1<2/3),
• the focal length f1 and the focal length f3 satisfy:
• f3/f1≧(2*NA1)/(3*NA3).