Patent ID: 7613098

Claim:
An objective optical system for focusing light from a light source onto at least two different types of optical recording media having different substrate thicknesses in order to record information on or reproduce information from the at least two different types of optical recording media, wherein the objective optical system comprises: a variable refractive power element with refractive power that may be varied so that the objective optical system can focus light beams having the same or very nearly the same wavelength onto said at least two different types of optical recording media having different substrate thicknesses in order to record information on or reproduce information from the at least two different types of optical recording media; and an objective lens; wherein the refractive power of said variable refractive power element may be varied so as to compensate for differences in spherical aberration of the objective optical system for foci of the light focused on said at least two different types of optical recording media due to their different substrate thicknesses; said variable refractive power element comprises a two-liquid lens element that includes a conductive aqueous liquid and a non-conductive oily liquid that have refractive indices different from each other and are sealed in a cylindrical container having transparent axial end faces; and the refractive power of said variable refractive power element is changeable by changing a voltage applied to said container that changes the profile of the interface between said aqueous liquid and said oily liquid according to the light beam being focused.