| 摘要 |
An optical scanning device ( 1 ) for scanning three information layer ( 2, 2', 2'' ) with three respective radiation beams ( 4, 4', 4'' ) having three respective wavelengths (lambda<SUB>1</SUB>, lambda<SUB>2</SUB>, lambda<SUB>3</SUB>) and polarizations (p 1 , p 2 , p 3 ). The three wavelengths differ from each other. At least one of the three polarizations differs from the others. The device comprises a diffractive part ( 24 ) including a pattern of pattern elements which have one stepped profile for forming three diffracted beams ( 15, 15', 15'' ) from the three radiation beams, the part comprising birefringent material, sensitive to the three polarizations. The stepped profile is designed such that the heights (h<SUB>j</SUB>) of the steps of a pattern element introduce phase changes that equal at least two different multiples of 2pi for one (lambda<SUB>1</SUB>) of the three wavelengths and equal at least two different phase changes modulo 2pi for one (lambda<SUB>2</SUB>) of the two other wavelengths. |
