Objective lens for optical discs

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 suitable for information recording/readout of a first optical disc is incident thereon and when a second beam of a second wavelength suitable for information recording/readout of a second optical disc having data density relatively 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 objective lens is placed so that the first beam will be incident thereon as a parallel beam,
• wherein the outer area includes at least one first annular zone which is formed so that part of the first beam that passed through the first annular zone will be substantially in antiphase with part of the first beam that passed through the inner area,
• wherein focal length fl of the objective lens for the first wavelength, height H of the outermost part of the inner area measured from the optical axis of the objective lens, and a design numerical aperture NAref as an NA (Numerical Aperture) necessary for the information recording/readout of the first optical disc satisfy:
• 1.0<H/(fl·NAref)<1.1, and
• wherein an effective NA of the objective lens for the first beam is substantially equal to the design numerical aperture NAref.