Optical information recording/reproducing apparatus and objective lens for the same

An optical information recording/reproducing apparatus for recording information to and/or reproducing information from two types of optical discs including first and second optical discs differing in recording density by selectively using two types of light beams including first and second light beams respectively having first and second wavelengths, the first light beam being a substantially collimated light beam and the second light beam being one of a substantially collimated beam and a diverging light beam,

• when λ1 (unit:
• nm) represents the first wavelength and λ2 (unit:
• nm) represents the second wavelength, λ1 and λ2 satisfying conditions:
• 390<λ1<420;
• and
• 640<λ2<700
• when t1 (unit:
• mm) represents a protective layer thickness of the first optical disc for which information recording or information reproducing is performed by using the first light beam, and t2 (unit:
• mm) represents a protective layer thickness of the second optical disc for which information recording or information reproducing is performed by using the second light beam, t1 and t2 satisfying conditions:
• 0.05<t1<0.15;
• and
• 0.50<t2<0.70,
• when NA1 represents a numerical aperture required for the information recording or information reproducing for the first optical disc, and NA2 represents a numerical aperture required for the information recording or information reproducing for the second optical disc, NA1 and NA2 satisfying a condition:
• NA1>NA2,
• the optical information recording/reproducing apparatus comprising an objective lens,
• wherein:
• at least one of optical surfaces of the objective lens comprises a diffraction surface having a diffraction structure defined by an optical path difference function:
• φ(h)=(P2×h2+P4×h4+P6×h6+P8×h8+P10×h10+P12×h12)mλ
• where P2, P4, P6 . . . represent coefficients of the 2nd order, 4th order, 6th order, h represents a height from an optical axis, m represents a diffraction order at which a diffraction efficiency of an incident light beam is maximized, and λ represents a design wavelength of the incident light beam;
• the diffraction surface has a first region contributing to converging the first light beam onto a recording surface of the first optical disc and converging the second light beam onto a recording surface of the second optical disc;
• the first region has a diffraction structure configured such that each of a diffraction order at which a diffraction efficiency is maximized for the first light beam and a diffraction order at which a diffraction efficiency is maximized for the second light beam is 1st order;
• the diffraction surface has a second region located outside the first region;
• the second region is configured to contribute to converging the first light beam onto the recording surface of the first optical disc and not to contribute to convergence of the second light beam;
• when P2 represents a 2nd order coefficient of the optical path difference function defining the diffraction structure in the first region, and f2 (unit:
• mm) represents a focal length of the objective lens with respect to a diffracted light beam of the second light beam having a diffraction order at which the diffraction efficiency is maximized for the second light beam passing through the first region, the objective lens satisfies a condition:
• 35<P2×f2<200 (1).

An objective lens for an optical information recording/reproducing apparatus for recording information to and/or reproducing information from two types of optical discs including first and second optical discs differing in recording density by selectively using two types of light beams including first and second light beams respectively having first and second wavelengths, the first light beam being a substantially collimated light beam and the second light beam being one of a substantially collimated beam and a diverging light beam,

• when λ1 (unit:
• nm) represents the first wavelength and λ2 (unit:
• nm) represents the second wavelength, λ1 and λ2 satisfying conditions:
• 390<λ1<420;
• and
• 640<λ2<700
• when t1 (unit:
• mm) represents a protective layer thickness of the first optical disc for which information recording or information reproducing is performed by using the first light beam, and t2 (unit:
• mm) represents a protective layer thickness of the second optical disc for which information recording or information reproducing is performed by using the second light beam, t1 and t2 satisfying conditions:
• 0.05<t1<0.15;
• and
• 0.50<t2<0.70,
• when NA1 represents a numerical aperture required for the information recording or information reproducing for the first optical disc, and NA2 represents a numerical aperture required for the information recording or information reproducing for the second optical disc, NA1 and NA2 satisfying a condition:
• NA1>NA2,
• wherein:
• at least one of optical surfaces of the objective lens comprises a diffraction surface having a diffraction structure defined by an optical path difference function:
• φ(h)=(P2×h2+P4×h4+P6×h6+P8×h8+P10×h10+P12×h12)mλ
• where P2, P4, P6 . . . represent coefficients of the 2nd order, 4th order, 6th order, h represents a height from an optical axis, m represents a diffraction order at which a diffraction efficiency of an incident light beam, and λ represents a design wavelength of the incident light beam;
• the diffraction surface has a first region contributing to converging the first light beam onto a recording surface of the first optical disc and converging the second light beam onto a recording surface of the second optical disc;
• the first region has a diffraction structure configured such that each of a diffraction order which a diffraction efficiency is maximized for the first light beam and a diffraction order at which a diffraction efficiency is maximized for the second light beam is 1st order;
• the diffraction surface has a second region located outside the first region;
• the second region is configured to contribute to converging the first light beam onto the recording surface of the first optical disc and not to contribute to convergence of the second light beam;
• when P2 represents a 2nd order coefficient of the optical path difference function defining the diffraction structure in the first region, and f2 (unit:
• mm) represents a focal length of the objective lens with respect to a diffracted light beam of the second light beam having a diffraction order at which the diffraction efficiency is maximized for the second light beam passing through the first region, and λB1 (unit:
• nm) represents a blazed wavelength in the first region, the objective lens satisfies a condition:
• −0.010<(λB1−λ2)/(P2×f2×λ2)<−0.001 (3).

An optical information recording/reproducing apparatus for recording information to and/or reproducing information from two types of optical discs including first and second optical discs differing in recording density by selectively using two types of light beams including first and second light beams respectively having first and second wavelengths, the first light beam being a substantially collimated light beam and the second light beam being one of a substantially collimated beam and a diverging light beam,

• when λ1 (unit:
• nm) represents the first wavelength and λ2 (unit:
• nm) represents the second wavelength, λ1 and λ2 satisfying conditions:
• 390<λ1<420;
• and
• 640<λ2<700
• when t1 (unit:
• mm) represents a protective layer thickness of the first optical disc for which information recording or information reproducing is performed by using the first light beam, and t2 (unit:
• mm) represents a protective layer thickness of the second optical disc for which information recording or information reproducing is performed by using the second light beam, t1 and t2 satisfying conditions:
• 0.05<t1<0.15;
• and
• 0.50<t2<0.70,
• when NA1 represents a numerical aperture required for the information recording or information reproducing for the first optical disc, and NA2 represents a numerical aperture required for the information recording or information reproducing for the second optical disc, NA1 and NA2 satisfying a condition:
• NA1>NA2,
• the optical information recording/reproducing apparatus comprising an objective lens,
• wherein:
• at least one of optical surfaces of the objective lens comprises a diffraction surface having a diffraction structure defined by an optical path difference function:
• φ(h)=(P2×h2+P4×h4+P6×h6+P8×h8+P10×h10+P12×h12)mλ
• where P2, P4, P6 . . . represent coefficients of the 2nd order, 4th order, 6th order, h represents a height from an optical axis, m represents a diffraction order at which a diffraction efficiency of an incident light beam is maximized, and λ represents a design wavelength of the incident light beam;
• the diffraction surface has a first region contributing to converging the first light beam onto a recording surface of the first optical disc and converging the second light beam onto a recording surface of the second optical disc;
• the first surface has a diffraction structure configured such that each of a diffraction order which the diffraction efficiency is maximized for the first light beam and a diffraction order at which the diffraction efficiency is maximized for the second light beam is 1st order;
• the diffraction surface has a second region located outside the first region;
• the second region is configured to contribute to converging the first light beam onto the recording surface of the first optical disc and not to contribute to convergence of the second light beam;
• when P2 represents a 2nd order coefficient of the optical path difference function defining the diffraction structure in the first region, and f2 (unit:
• mm) represents a focal length of the objective lens with respect to a diffracted light beam of the second light beam having a diffraction order at which the diffraction efficiency is maximized for the second light beam passing through the first region, and λB1 (unit:
• nm) represents a blazed wavelength in the first region, the objective lens satisfies a condition:
• −0.010<(λB1−λ2)/(P2×f2×λ2)<−0.001 (3).