Patent ID: 8570849

Claim:
A method for adjusting a tilt angle of an objective lens with respect to one of a multilayer optical disc having a plurality of recording layers and an optical axis of an optical information recording/reproducing apparatus, wherein the apparatus emits a laser beam having a wavelength λ for the multilayer optical disc, the objective lens has a numerical aperture NA, and the wavelength λ and the numerical aperture NA satisfy the following conditions: 390<λ<420 and 0.75<NA<0.90, the method comprising: selecting a predetermined transparent parallel plate to satisfy a condition −0.015<t1−t2<0.005 where t1 represents a thickness of the predetermined transparent parallel flat plate and t2 represents a design protective layer thickness at which a coma caused when off-axis light is incident on the objective lens becomes smallest, t1 and t2 having units of mm determining the tilt angle of the objective lens by examining if a spot quality of the laser beam defined through the predetermined parallel plate matches a predetermined spot quality standard; and tilting the objective lens to have the determined tilt angle based on the examining of the spot quality of the laser beam.