Patent ID: 7843792

Claim:
An objective lens used for an optical information recording/reproducing device for recording information to and/or reproducing information from three types of optical discs including a first optical disc having a highest recording density, a second optical disc having a second highest recording density and a third optical disc having a lowest recording density, by selectively using one of three types of light beams including first, second and third light beams, when wavelengths of the first to third light beams are respectively represented by λ 1 (nm), λ 2 (nm) and λ 3 (nm), λ 1 <λ 2 <λ 3 being satisfied, when a thickness of a protective layer of the first optical disc requiring use of the first light beam is represented by t1 (mm), a thickness of a protective layer of the second optical disc requiring use of the second light beam is represented by t2 (mm), and a thickness of a protective layer of the third optical disc requiring use of the third light beam is represented by t3 (mm), t1≦t2<t3 being satisfied, when a numerical aperture necessary for recording information to or reproducing information from the first optical discs is represented by NA 1 , a numerical aperture necessary for recording information to or reproducing information from the second optical discs is represented by NA 2 , and a numerical aperture necessary for recording information to or reproducing information from the third optical discs is represented by NA 3 , a relationship NA 1 >NA 3 and NA 2 >NA 3 being satisfied, wherein: the objective lens comprises a first optical member and a second optical member of materials different from each other and cemented together at a cementing surface; the cementing surface comprises a first phase shift structure having a plurality of refractive surface zones concentrically formed about an optical axis of the objective lens; the first phase shift structure satisfies conditions (1) and (2): 0.85<Φ2/Φ1<1.15 (1) 0.10<|(Φ3−Φ2)/Φ1|<0.50 (2) where Φ1 =m(λ 1)×(λ 1 /( n 2(λ 1 )− n 1(λ 1 ))), Φ2 =m(λ 2 )×(λ 2 /( n 2(λ 2 )− n 1(λ 2 ))), Φ3 =m(λ 3 )×(λ 3 /( n 2(λ 3 )− n 1(λ 3 ))), m(λ 1 ), m(λ 2 ), and m(λ 3 ) are diffraction orders at which diffraction efficiencies of the first through third light beams are maximized, respectively, n1 (λ 1 ), n1 (λ 2 ), and n1 (λ 3 ) are refractive indexes of the first optical member, respectively, and n2 (λ 1 ), n2 (λ 2 ), and n2 (λ 3 ) are refractive indexes of the second optical member, respectively.