Patent ID: 7602691

Claim:
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 ( 7 a, 7 b ) for supplying said first, second and third radiation beams ( 4 , 4 ′, 4 ″); an objective lens system ( 8 ) for converging said radiation beams on the respective information layer; and a diffractive part ( 24 ) arranged in the optical path of said radiation beams ( 4 , 4 ′, 4 ″), the diffractive part comprising: 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 2 , λ 3 ,) but not for said first wavelength (λ 1 ); and 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 1 , λ 3 ) but not for said second wavelength (λ 2 ).