Optical scanning device

摘要

An optical device ( 1 ) for scanning three information layers ( 2, 2', 2 '') by means of three radiation beams ( 4, 4', 4 '') having three respective wavelengths (lambda₁, lambda₂, lambda₃) and polarizations (p₁, p₂, p₃), wherein the three wavelengths substantially differ from each other. The device comprises a radiation source ( 7 ) for emitting the three radiation beams, an objective lens system ( 8 ) for converging the three radiation beams beam on the positions of the three respective information layers, and a phase structure ( 24 ) having a non-periodic stepped profile. Furthermore, the structure includes birefringent material sensitive to the three polarizations and the stepped profile is designed for introducing three wavefront modifications (DeltaW₁, DeltaW₂, DeltaW₃) for the three wavelengths, respectively, wherein one of the wavefront modifications is of a type different from the others and one of the polarizations differs from the others.