Patent ID: 6785215
Filing Date: 2004-08-31
Classification: G11B

Abstract:
An objective lens for an optical head, said objective lens converging first and second laser beams on first and second optical discs, respectively, the first laser beam having a shorter wavelength than the second laser beam, a protective layer of the first disc being thinner than a protective layer of the second optical disc,wherein said objective lens is compensated for astigmatism with respect to the first beam, which is incident on said objective lens as an off-axis beam at a predetermined incident angle, by providing on-axis astigmatism corresponding to wave front aberration greater than or equal to 0.01&lgr; (rms), and wherein said objective lens is configured such that coma is compensated better in a case where the first laser beam is converged on the first optical disc than in a case where the second laser beam is converged on the second optical disc.