Patent ID: 6487161
Filing Date: 2002-11-26
Classification: G02B,G11B

Abstract:
An objective lens that converges a first laser beam having a first wavelength on a first optical disc having a first protective layer, and a second laser beam having a second wavelength on a second optical disc having a second protective layer, said first wavelength being shorter than said second wavelength, data recording density of said first optical disc being greater than data recording density of said second optical disc, a thickness of said first protective layer being smaller than a thickness of said second protective layer, said objective lens consisting of a single lens element, said objective lens including:a first surface located on a laser emitting side, said first surface being rotationally symmetrical about a first rotational axis; and a second surface located on an optical disc side, said second surface being rotationally symmetrical about a second rotational axis, said first rotational axis and said second rotational axis being deviated relative to each other, wherein said objective lens satisfies the following condition under a hypothetical condition where said first rotational axis coincides with said second rotational axis: â€ƒ4.01/SC2â€ƒwherein, SC1 represents an offence SC against sine condition at the peripheral portion of said first region when the said first laser beam is converged on said first disc, wherein SC2 represents an offence SC against sine condition at the peripheral portion of said first region when said second laser beam is converged on the second disc, the offence SC against sine condition being defined by the formula below: SC=nH1/(nâ€² sin Uâ€²)âˆ’f(1âˆ’m) â€ƒwherein, n represents a refractive index on the beam incident side medium, nâ€² represents a refractive index on the beam emerging side medium, Uâ€² represents an angle of the emerging beam with respect to the optical axis, m represents a paraxial magnification, H1 represents a ray height on a principal plane, and f represents a focal length.