Patent ID: 6785203
Filing Date: 2004-08-31
Classification: 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, a data recording density of said first optical disc being greater than a 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, a first region being defined on said objective lens, said first region providing a numerical aperture appropriate for converging said second laser beam on said second optical disc, coma of said objective lens in said first region being compensated better when said second laser beam is converged on said second optical disc than when said first laser beam is converged on said first optical discs, the objective lens satisfying the following condition: 02/SC1|<0.2, wherein, SC1 represents an offense, SC against sine condition at the peripheral portion of said first region, taking the protective layer of said first optical disc into account, when the said first laser beam is converged on said first optical disc, wherein SC2 represents an offense, SC against sine condition at the peripheral portion of said first region, taking the protective layer of said second optical disc into account, when said second laser beam is converged on the second optical disc, and wherein the offense SC against the sine condition is defined by the following formula: SC=nH1/(nâ€² sin Uâ€²)âˆ’f(1âˆ’m) wherein,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.