OPTICAL SCANNING DEVICE

摘要

An optical scanning device for scanning a first optical record carrier, a second, different, optical record carrier and a third, different, optical record carrier, each record carrier having an information layer. The scanning device includes a radiation source system arranged to produce first, second and third radiation beams for scanning the first, second and third record carriers with first, second and third wavelengths, respectively; and a diffraction structure (34) arranged to introduce a first, second and third wavefront modification into the first, second and third radiation beams, respectively. The diffraction structure includes a surface having a plurality of steps (38), each step having a step height (h_j) which controls a form of the first, second and third wavefront modifications, characterized in that the diffraction structure is formed from a material having an Abbe number of less than 50, whereby dispersion is provided between the first, second and third wavelengths in order to further control the form of the first, second and third wavefront modifications.