Objective lens and optical information recording/reproducing device having the same

An optical information recording/reproducing device for recording information to and/or reproducing information from three types of optical discs, by selectively using one of three types of light beams including a first light beam having a first wavelength λ1 (nm), a second light beam having a second wavelength λ2 (nm) and a third light beam having a third wavelength λ3 (nm),

• the at least three types of optical discs including a first optical disc for which information recording or information reproducing is executed by using the first light beam, a second optical disc for which information recording or information reproducing is executed by using the second light beam, and a third optical disc for which information recording or information reproducing is executed by using the third light beam,
• the first, second and third wavelengths λ1, λ2 and λ3 satisfying a condition:
• λ1<λ2<λ3,
• when numerical apertures required for information reproducing or information recording on the first, second and third optical discs are defined as NA1, NA2 and NA3, respectively, the numerical apertures satisfying following relationships:
• (NA1>NA3);
• and
• (NA2>NA3),
• when protective layer thicknesses of the first, second and third optical discs are represented by t1 (mm), t2 (mm) and t3 (mm), respectively, the protective layer thicknesses are t1≈0.6 mm, t2≈0.6 mm and t3≈1.2 mm,
• the optical information recording/reproducing device comprising:
• light sources respectively emitting the first, second and third light beams;
• and
• an objective lens,
• the objective lens comprising a first area contributing to converging the third light beam onto a record surface of the third optical disc,
• the first area including a phase shift structure having a plurality of refractive surface zones concentrically formed about a predetermined axis,
• the phase shift structure including a first step group and a second step group,
• wherein:
• an optical path length difference given by the first step group to an incident light beam between adjacent refractive surface zones in the first step group is different from an optical path length difference given by the second step group to the incident light beam between adjacent refractive surface zones in the second step group;
• positions of steps formed in the first and second step groups are defined by first and second optical path difference functions, respectively;
• an i-th optical path difference function φi(h) is expressed by:
• φi(h)=(Pi2h2+Pi4h4+Pi6h6+Pi8h8+Pi10h10+Pi12h12)miλ
• where Pi2, Pi4, Pi6 . . . (i:
• natural number) represents coefficients of the 2nd order, 4th order, 6th order, . . . of the i-th optical path difference function, h represents a height from an optical axis, mi represents a diffraction order at which diffraction efficiency is maximized, and λ represents a design wavelength of the incident light beam;
• the 2nd order coefficient P12 for a first optical path difference function φ1(h) is a positive value;
• the 2nd order coefficient P22 of a second optical path difference function φ2(h) is a negative value;
• the first step group is configured such that an optical path length difference ΔOPD1 (nm) given by each step of the first step group to the first light beam satisfies a condition:
• 2N1+1.10<|ΔOPD1/λ1|<2N1+1.40 (1)
• where N1 is a natural number or zero;
• the second step group is configured such that an optical path length difference ΔOPD2 (nm) given by each step of the second step group to the first light beam satisfies a condition:
• 2N2−0.10<|ΔOPD2/λ1|<2N2+0.10 (2)
• where N2 is a natural number;
• and
• each of the first, second and third light beams is incident on the objective lens as a collimated beam.