Patent ID: 7830772

Claim:
An objective optical system used for an optical information recording/reproducing device for recording information to and/or reproducing information from at least two types of optical discs, by selectively using one of two types of light beams including a first light beam having a first wavelength λa (nm) and a second light beam having a second wavelength λb (nm), the at least two types of optical discs including an optical disc Da for which information recording or information reproducing is executed by using the first light beam and an optical disc Db for which information recording or information reproducing is executed by using the second light beam, the first and second wavelengths λa and λb satisfying a condition: 1.8 <λb/λa< 2.0, when protective layer thicknesses of the optical discs Da and Db are represented by ta (mm) and tb (mm), respectively, the protective layer thicknesses satisfying a condition of ta<tb, when numerical apertures required for information reproducing or information recording on the optical discs Da and Db are defined as NAa and NAb, respectively, the numerical apertures satisfying a relationship NAa>NAb, the objective optical system comprising: an optical element configured to have an annular zone structure on at least one surface of the optical element, the annular zone structure including a plurality of annular zones configured to have at least one step formed, at a boundary between adjacent ones of the plurality of annular zones, to extend in a direction of an optical axis of the objective optical system, the at least one step being provided to cause a predetermined optical path length difference between a light beam passing through an outside of the boundary and a light beam passing through an inside of the boundary, the predetermined optical path length difference given to the first light beam by the at least one step being approximately equal to an odd multiple of the first wavelength λa, Abbe number of material of the optical element satisfying a condition of 15<νd<35.