Patent ID: 6873590
Filing Date: 2005-03-29
Classification: G11B

Abstract:
1. An objective lens for an optical pick-up that converges light beams of at least two different wavelengths onto recording layers of optical discs of at least two different recording densities, respectively, said objective lens comprising: a refractive lens whose at least one surface is divided into a common area through which a light beam of a low NA, which is necessary and sufficient for an optical disc having low recording density, passes and a high NA exclusive area through which a light beam of a high NA, which is necessary only for an optical disc having high recording density, passes, wherein a diffractive lens structure having a plurality of concentric ring areas with minute steps at the boundaries therebetween is formed in at least said high NA exclusive area, said diffractive lens structure formed in said high NA exclusive area is designed to maximize the diffraction efficiency of second or higher diffraction light at the wavelength corresponding to said optical disc having high recording density.