Patent ID: 7158468

Claim:
An optical system for an optical head of an optical disc drive, the optical disc drive being capable of using at least a first disc and a second disc, a protective layer of the second disc being thicker than that of the first disc, a data density of the second disc being lower than that of the first disc, said optical system comprising: a light source that selectively emits a first laser beam having a first wavelength and a second laser beam having a second wavelength, the second wavelength being longer than the first wavelength; and an objective lens that converges the first beam and the second beam on the first disc and the second disc, respectively, the second beam being incident on said objective lens as a diverging light beam, the first beam being incident on said objective lens as a beam of which a degree of divergence is less than that of the second beam, wherein one surface of said objective lens is divided into a common area at a central portion and a high NA exclusive area at a peripheral portion surrounding said common area, a diffraction lens structure including a plurality of concentric annular zones having minute steps being formed in said high NA exclusive area, said diffraction lens structure varies a spherical aberration depending on the wavelength of light passed therethrough, said diffraction lens structure compensating for aberrations of a portion of the first beam passed therethrough, said diffraction lens structure causes aberrations to a portion of the second laser beam passed therethrough, the first laser beam passed through said common area and said high NA exclusive area being converged on the first disc to form a beam spot, a portion of the second laser beam passed through said common area being converged on the second disc to form a beam spot, the portion of the second laser beam passed through said high NA exclusive area being diffused on the second disc.