Abstract: An objective lens of an optical pick-up is formed to provide axial astigmatism corresponding to wavefront aberration of 0.01&lgr; [rms] or more when a shorter wavelength laser beam emitted by a first laser diode is converged on a first optical disc, and to provide coma so that its sign when the shorter wavelength laser beam is converged on the first optical disc will be opposite to its sign when a longer wavelength laser beam emitted by a second laser diode is converged on a second optical disc.

Abstract: An objective lens for an optical head converges a first laser beam having a relatively short wavelength and a second laser beam having a relatively long wavelength on first optical disc (DVD) and a second optical disc (CD), respectively. A protective layer of the first disc is thinner than a protective layer of the second optical disc. The objective lens is compensated for astigmatism with respect to the first beam, which is incident on the 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 the objective lens is configured such that the 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.

Abstract: An objective lens of an optical pick-up is formed to provide axial astigmatism corresponding to wavefront aberration of 0.01&lgr; [rms] or more when a shorter wavelength laser beam emitted by a first laser diode is converged on a first optical disc, and to provide coma so that its sign when the shorter wavelength laser beam is converged on the first optical disc will be opposite to its sign when a longer wavelength laser beam emitted by a second laser diode is converged on a second optical disc.

Abstract: Disclosed is an optical disc apparatus that is adapted for DVD, CD and CD-R. The optical system includes a laser module and an objective lens that converges a divergent laser beam emitted from the laser module onto information layers of optical discs. The laser module is provided with a pair of emission points that are in close proximity to each other, and a pair of light receiving elements. A focusing mechanism moves the objective lens so that the beam spot follows an information layer of the optical disc. The objective lens is provided with the diffractive lens structure to have a wavelength dependence such that the spherical aberration varies in response to the change of the wavelength. Thereby, the variation of the spherical aberration due to the change of the thickness of the cover layer can be canceled by changing the wavelength.

Abstract: An objective lens for an optical head converges a first laser beam having a relatively short wavelength and a second laser beam having a relatively long wavelength on first optical disc (DVD) and a second optical disc (CD), respectively. A protective layer of the first disc is thinner than a protective layer of the second optical disc. The objective lens is compensated for astigmatism with respect to the first beam, which is incident on the 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 the objective lens is configured such that the 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.