Patent ID: 7170681

Claim:
An optical scanning device for scanning a first information layer by means of a first radiation beam having a first wavelength and a first polarization, a second information layer by means of a second radiation beam having a second wavelength and a second polarization, and a third information layer by means of a third radiation beam having a third wavelength and a third polarization, wherein said first, second and third wavelengths substantially differ from each other and at least one of said first, second and third polarizations differs from the others, the device comprising: a radiation source for supplying said first, second and third radiation beams consecutively or simultaneously, an objective lens system for converging said first, second and third radiation beams on the positions of said first, second and third information layers, respectively, and a diffractive part arranged in the optical path of said first, second and third radiation beams, the part including a pattern of pattern elements which have substantially one stepped profile for forming a first diffracted radiation beam, a second diffracted radiation beam and a third diffracted radiation beam from said first, second and third radiation beams, respectively, the part comprising birefringent material sensitive to said first, second and third polarizations, characterized in that said stepped profile is designed such that the heights of the steps of a pattern element introduce phase changes that substantially equal at least two different multiples of 2π for said first wavelength and at least two substantially different phase changes modulo 2π for said second wavelength.