Objective lens for optical pick-up

An optical pick-up comprising:

• a first laser diode which emits a shorter wavelength laser beam;
• a second laser diode which emits a longer wavelength laser beam, whose light emitting point is placed in the vicinity of that of said first laser diode;
• and
• an objective lens which converges the shorter wavelength laser beam emitted by said first laser diode on a first optical disc having a thinner cover layer and high data density and converges the longer wavelength laser beam emitted by said second laser diode on a second optical disc having a thicker cover layer and low data density,
• wherein said objective lens is configured to provide axial astigmatism corresponding to wavefront aberration of 0.01λ [rms] or more when the shorter wavelength laser beam is converged on the first optical disc, and to provide abaxial third order coma so that a sign thereof when the shorter wavelength laser beam is converged on the first optical disc will be opposite to the sign thereof when the longer wavelength laser beam is converged on the second optical disc, in a virtual state in which the optical axis of said objective lens coincides with the normal of the optical disc,
• wherein said objective lens being an anamorphic lens which forms two focal lines when either of the first and second laser beams is incident on said objective lens,
• wherein light emitting points of said first and second laser diodes are arranged so that light converging points of the laser beams emitted by said first and second laser diodes are aligned in a direction parallel to one of the two focal lines formed nearer to said objective lens when parallel light is incident on said objective lens in the direction of the optical axis of the optical system of said optical pick-up, and
• wherein the optical axis of said objective lens is tilted relative to the normal to the optical disc in a plane including said focal line nearer to said objective lens.

An optical disc device, comprising:

• a spindle motor which spins an optical disc;
• an optical pick-up;
• and
• a driving mechanism which slides at least part of said optical pick-up in the radial direction of the optical disc,
• said optical pick-up including:
• a first laser diode which emits a shorter wavelength laser beam;
• a second laser diode which emits a longer wavelength laser beam, whose light emitting point is placed in the vicinity of that of said first laser diode;
• and
• an objective lens which converges the shorter wavelength laser beam emitted by said first laser diode on a first optical disc having a thin cover layer and high data density and converges the longer wavelength laser beam emitted by said second laser diode on a second optical disc having a thick cover layer and low data density,
• wherein said objective lens is formed to provide axial astigmatism corresponding to wavefront aberration of 0.01λ [rms] or more when the shorter wavelength laser beam is converged on the first optical disc, and to provide abaxial third order 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 the longer wavelength laser beam is converged on the second optical disc in a virtual state in which the optical axis of said objective lens coincides with the normal of the optical disc,
• wherein said objective lens being an anamorphic lens forming two focal lines;
• wherein light emitting points of said first and second laser diodes are arranged so that focal points formed by the laser beams emitted by said first and second laser diodes will be aligned in a direction parallel to one of the two focal lines formed nearer to said objective lens when parallel light is incident on said objective lens in the direction of the optical axis of the optical system of said optical pick-up,
• wherein the optical axis of said objective lens is tilted relative to the normal of the optical disc in a plane including said focal line nearer to said objective lens, and
• wherein the position of said optical pick-up relative to the optical disc is set so that said focal line nearer to said objective lens will be parallel to a tangential line to the optical disc when seen in the direction of the optical axis of said objective lens.