Patent ID: 7602691
Filing Date: 2009-10-13
Classification: G11B

Abstract:
1. An optical scanning device ( 1 ) for scanning a first information layer ( 2 ) by means of a first radiation beam ( 4 ) having a first wavelength (λ 1 ) and a first polarisation (p 1 ), a second information layer ( 2 ′) by means of a second radiation beam having a second wavelength (λ 2 ) and a second polarisation (p 2 ), and a third information ( 2 ″) layer by means of a third radiation beam ( 4 ″) having a third wavelength (λ 3 ) and a third polarisation (p 3 ), wherein said first, second and third wavelengths (λ 1 , λ 2 , λ 3 ) differ from each other, the device comprising: a radiation source ( an objective lens system ( a diffractive part ( a first diffractive grating structure comprising a series of steps of predetermined height to introduce a phase change that is an integral multiple of 2π for both said second and third wavelengths (λ a second diffractive grating structure comprising a series of steps of predetermined height to introduce a phase change that is an integral multiple of 2π for said first and third wavelengths (λ