Patent ID: 7839751

Claim:
An optical system of an optical pick-up that utilizes a first laser beam and a second laser beam different in wavelength, respectively for at least two types of optical discs according to different standards, so as to record information to and/or reproduce information from the respective optical discs, comprising: an objective lens; and an optical element including at least a phase shift surface among the surfaces thereof, and located closer to a light source than the objective lens; wherein: t1≦t2 is satisfied, where t 1 represents a thickness of a cover layer of a first optical disc for which the first laser beam is employed for recording or reproducing the information, and t 2 a thickness of a cover layer of a second optical disc for which the second laser beam, having a longer wavelength than the first laser beam, is employed for recording or reproducing the information; NA1≧NA2 is satisfied, where NA 1 represents a numerical aperture of the objective lens necessary for recording or reproducing the information on the first optical disc, and NA 2 the numerical aperture necessary for recording or reproducing the information on the second optical disc; the phase shift surface includes a first region that converges the first laser beam on a recording surface of the first optical disc, and the second laser beam on a recording surface of the second optical disc, after the laser beams are transmitted through the optical element and the objective lens; the first region includes at least an annular zone structure having an annular zone group including steps each of which gives an additional optical path length to the first laser beam generally by two wavelengths, with respect to an inner refracting surface at a boundary between adjacent refracting surfaces, and a return step portion located at an outer position of the annular zone group; and the return step includes a step that gives an additional optical path length to the first laser beam by three wavelengths with respect to an inner refracting surface located inside of the return step, in an opposite direction to the step of the annular zone group.