Objective optical system for optical recording media and optical pickup device using it

An objective optical system for focusing light from a light source onto optical recording media, the objective optical system comprising, in order from the light source side along an optical axis:

• an aperture control filter with a diffractive optical function;
• and
• an objective lens;
• wherein
• the aperture control filter includes, in a one piece structure, a glass substrate, an aperture control structure on the light source side of the glass substrate, and a diffractive optical structure on the recording media side of the glass substrate that provides the diffractive optical function of the aperture control filter;
• the objective optical system is configured to receive a light beam of a first wavelength λ1 from its light source side and focus diffracted light diffracted by said diffractive optical structure at a first numerical aperture NA1 onto a desired portion of a first optical recording medium having a substrate thickness T1, to receive a light beam of a second wavelength λ2 from its light source side and focus diffracted light diffracted by said diffractive optical structure at a second numerical aperture NA2 onto a desired portion of a second optical recording medium having a substrate thickness T2, and to receive a light beam of a third wavelength λ3 from its light source side and focus diffracted light diffracted by said diffractive optical structure at a third numerical aperture NA3 onto a desired portion of a third optical recording medium having a substrate thickness T3;
• and
• the following conditions are satisfied:
• λ1<λ2<λ3
• NA1≦NA2>NA3
• T1≦T2<T3.