Patent attributes
An optical system has an optical functional surface including a common region used for conducting information recording and/or reproducing for both of a first optical information recording medium and a second optical information recording medium. The common region comprises a refractive surface of an imaginary basic aspherical surface and a optical path difference providing structure in which plural ring-shapes zones are separated around the center of an optical axis and neighboring ring-shaped zones are displaced to each other in a direction of an optical axis so as to cause an optical path difference obtained by multiplying a predetermined wavelength λs (λ1<λs<λ2) with almost an integer. The refractive surface of the imaginary basic aspherical surface is structured such that a spherical aberration becomes under on the first information recording medium and a spherical aberration becomes over on the second information recording medium.
