Patent ID: 7821905

Claim:
An optical pickup device comprising: a light source that emits a light beam of a predetermined wavelength of about 405 nm; an objective lens being a plastic lens provided with, on at least one surface, diffraction means of a zone diffractive structure suppressing generation of aberration to be caused by a temperature change, and has a numerical aperture of 0.82 or larger for gathering the light beam emitted from the light source with respect to an optical disk; and a collimator lens disposed between the light source and the objective lens, and derives a substantially-collimated light by converting an angle of divergence of the light beam emitted from the light source, wherein the objective lens is formed to satisfy equation (1), where ΔSA3T is an amount of change of third-order spherical aberration to be generated in the objective lens in response to a temperature change ΔT (° C.) in a range from −10° C. to 75° C., and ΔSA3λ is an amount of change of third-order spherical aberration to be generated in the objective lens in response to a wavelength change Δλ (nm) within a range of ±5 nm observed in the light beam emitted from the light source, equation (3) is satisfied when a length of an optical path increased by the diffractive structure of the diffraction means is represented by an optical-path difference function φ (h) calculated by equation (2), where h (mm) is a height from an optical axis, Cn is an nth-order coefficient of an optical-path difference, and λ (nm) is a wavelength of an incoming light beam, equation (4) is satisfied, where ΔSA3 is an amount of change of third-order axial spherical aberration to be generated in the objective lens in response to the wavelength change Δλ (nm), and ΔSA5 is an amount of change of fifth-order axial spherical aberration, and with the objective lens, the generation of the spherical aberration to be caused by the temperature change can be suppressed, and when the light beam emitted from the light source is changed in wavelength, the generation of the spherical aberration can be suppressed by moving the collimator lens (Δ SA 3 T/ΔT )×(Δ SA 3λ/Δλ)<0 (1) φ ( h )=( C 2× h 2 +C 4 ×h 4 +C 6 ×h 6 +C 8 ×h 8 +C 10 ×h 10 + . . . )×λ/10 6 (mm) (2) C 2 ×C 10>0 (3) Δ SA 3×Δ SA 5>0 (4).