Patent ID: 8045426

Claim:
An optical head device comprising: a first light source for emitting a first light beam; a second light source for emitting a second light beam having a wavelength longer than a wavelength of the first light beam; a third light source for emitting a third light beam having a wavelength longer than the wavelength of the second light beam; a light flux separating section for selectively reflecting or transmitting the first light beam, the second light beam, and the third light beam to be incident in an identical direction; a first objective lens for collecting the first light beam transmitted through the light flux separating section on a first optical information recording medium, and having a first numerical aperture; a second objective lens for collecting the second light beam reflected on the light flux separating section on a second optical information recording medium, for collecting the third light beam reflected on the light flux separating section on a third optical information recording medium, and having a second numerical aperture different from the first numerical aperture; and an aberration corrector disposed between the first light source and the light flux separating section, and for correcting a third-order spherical aberration which is generated depending on a thickness of a light transmissive layer from a surface of the first optical information recording medium, corresponding to an optimal substrate thickness of the first objective lens, to an intended information recording surface, assuming that the light transmissive layer thickness of the first optical information recording medium which minimizes a residual third-order spherical aberration at the time of incidence of parallel light into the first objective lens is defined as the optimal substrate thickness of the first objective lens, wherein the first objective lens and the second objective lens are disposed in the order of the second objective lens and the first objective lens from an incident side of the first light beam, the second light beam, and the third light beam, along a direction tangential to the first optical information recording medium, the second optical information recording medium, and the third optical information recording medium, and the optimal substrate thickness of the first objective lens is defined in such a manner that a variation of a third-order astigmatism which is generated on the information recording surface of the first optical information recording medium farthest from the first objective lens at the time of correcting the third-order spherical aberration, and a variation of a third-order astigmatism which is generated on the information recording surface of the first optical information recording medium closest to the first objective lens at the time of correcting the third-order spherical aberration are set equal to each other.