Patent ID: 7023788

Claim:
An objective lens for converging a beam emitted by a light source on a recording surface of an optical disc, having compatibility with at least two types of optical discs of different data densities by employing diffracting structure having annular zones formed on at least one surface of the objective lens, the surface on which the diffracting structure is formed including: an inner area which focuses the beam on the recording surface of each optical disc substantially with no aberration both when a first beam of a first wavelength for information recording/readout of a first optical disc is incident thereon and when a second beam of a second wavelength for information recording/readout of a second optical disc having data density higher than that of the first optical disc is incident thereon; and an outer area which focuses the second beam on the recording surface of the second optical disc substantially with no aberration while forming a wavefront that is substantially continuous with a wavefront of part of the second beam that passed through the inner area, wherein the outer area includes at least one special annular zone which is formed so that part of the first beam that passed through the special annular zone will be substantially in phase with part of the first beam that passed through the inner area, and wherein a convergence angle θ [deg] of part of the first beam incident on the outermost part of the inner area measured after emerging from the objective lens and a design numerical aperture NA ref as an NA (Numerical Aperture) necessary for the information recording/readout of the first optical disc satisfy: 0.9<sin θ/ NA ref <1.0, and wherein an effective NA of the objective lens for the first beam is set substantially equal to the numerical aperture NA ref .