Patent ID: 7116495

Claim:
An optical system for use in an optical pickup apparatus which comprises a light source to emit a light flux having a wavelength λ, the optical system to converge the light flux onto an optical information recording medium, a focusing device to focus the converged light flux on the optical information recording medium, and a photo-detector to detect a light flux reflected from the optical information recording medium; the optical system comprising: an objective lens; a plastic coupling lens to change a divergent angle of the light flux emitted from the light source; and an optical element, provided between the objective lens and the plastic coupling lens, to pass light flux from the plastic coupling lens and to reflect light flux from the objective lens to the photo-detector; wherein the optical system is structured such that the light flux emitted from the light source passes through the coupling lens, the optical element and the objective lens in this order when the light flux is converged onto the optical information recording medium and the reflected light flux reflected from the optical information recording medium passes through the objective lens and is reflected by the optical element to the photo-detector without passing through the coupling lens when the reflected light flux is detected by the photo-detector, wherein a ring-shaped diffractive structure is formed on at least one surface of the coupling lens, wherein the coupling lens has a positive refractive power as a whole, and wherein a change of an image forming magnification of the coupling lens as a single optical element is negligible even when temperature of the optical pickup apparatus is changed.