Patent ID: 7813252

Claim:
An optical pick-up used for recording information to and/or reproducing information from at least three types of optical discs, by selectively using one of three types of light beams including a first light beam having a shortest wavelength of λ 1 , a second light beam having a wavelength of λ 2 longer than the wavelength of the first light beam and a third light beam having a longest wavelength of λ 3 , the at least three types of optical discs including a first optical disc having a highest recording density, a second optical disc having a recording density lower than that of the first optical disc, and a third optical disc having a lowest recording density, when protective layer thicknesses of the first, second and third optical discs are represented by t 1 , t 2 and t 3 , respectively, the protective layer thicknesses satisfying a condition of t 1 ≦t 2 <t 3 , the optical pick-up comprising: a first light source that emits the first light beam when the first optical disc is used; a second light source that emits the second light beam when the second optical disc is used; a third light source that emits the third light beam when the third optical disc is used; a first coupling lens; and an objective lens formed to converge each of the first, second and third light beams onto the first, second and third optical discs, respectively, wherein the objective lens has a step structure including a plurality of concentrically formed refractive surface zones divided by steps, the step structure having a function of giving, at each step, an optical path length difference of approximately 2λ 1 to the first light beam; wherein the first coupling lens causes the first light beam to be incident on the objective lens as a converging beam, wherein if f 1 and M 1 respectively represent a focal length and magnification of the objective lens defined when the first optical disc is used, and f 2 and M 2 respectively represent a focal length and magnification of the objective lens defined when the second optical disc is used, the objective lens is positioned to satisfy a condition (1): −0.35 <f 2 ×M 2 −f 1 ×M 1<−0.07 (1).