Patent ID: 6344935
Filing Date: 2002-02-05
Classification: G11B

Abstract:
An optical system of an optical pick-up that is capable of using at least two types of optical discs having different recording density, said optical system comprising:a light source portion for selectively emitting a first laser beam having a first wavelength and a second laser beam having a second wavelength, wherein said second wavelength is longer than said first wavelength; and an objective lens for converging said laser beam from said light source portion onto an recording layer of said optical disc, said objective lens being provided with a transmittance controlling phase grating in a peripheral region that is outside of the transmitting region of the laser beam having a predetermined NA required for said optical disk having lower recording density, wherein said transmittance controlling phase grating is formed as a plurality of concentric ring-shaped grooves to have a wavelength dependence such that the transmittance for said first laser beam is higher than the transmittance for said second laser beam, and wherein each of said grooves has a rectangular cross-section in a plane containing an optical axis of said objective lens, side walls of said grooves are substantially parallel to said optical axis, recessed surfaces of said grooves and protuberant surfaces between said grooves are substantially coincident with the macroscopic shape of said lens surface, respectively.