Patent attributes
A diffractive part for an optical scanning device is described. The device is for scanning a first information layer by means of a first radiation beam having a first wavelength and a first polarisation, a second information layer by means of a second radiation beam having a second wavelength and a second polarisation, and a third information layer by means of a third radiation beam having a third wavelength and a third polarisation. The first, second and third wavelengths differ from each other. The device includes a radiation source for supplying the first, second and third radiation beams, an objective lens system for converging the radiation beams on the respective information layer, and a diffractive part arranged in the optical path of the radiation beams. The diffractive part includes a first diffractive grating structure having a series of steps of predetermined height to introduce a phase change that is an integral multiple of 27E for both the second and third wavelengths but not the first wavelength, and a second diffractive grating structure having a series of steps of predetermined height to introduce a phase change that is an integral multiple of 27π for the first and third wavelengths but not the second wavelength.
