Patent ID: 7184385

Claim:
An optical scanning device for scanning a first information layer by means of a first radiation beam in a first operating mode and a second information layer by means of a second radiation beam in a second operating mode, the device comprising: a radiation source for emitting said first and second radiation beams consecutively or simultaneously, an objective lens system for converging said first and second radiation beams on the positions of said first and second information layers, respectively, and a phase structure arranged in the optical path of said first and second radiation beams, the structure having an optical axis and including a central part (P 1 ) and at least one marginal part (P 2 ) for forming a non-periodic stepped profile in the radial direction, characterized in that one (P 2 ) of said central and marginal parts (P 1 , P 2 ) is divided into at least a first angular segment (AS 2,1 ) and a second angular segment (AS 2,2 ) having a first step height (h 2,1 ) and a second, different step height (h 2,2 ), respectively, for introducing in said first operating mode a first wavefront modification ΔW 2,1,1 and a second wavefront modification ΔW 2,2,1 , respectively, and in said second operating mode a third wavefront modification ΔW 2,1,2 and a fourth wavefront modification ΔW 2,2,2 , respectively, where the difference (ΔW 2,1,2 +ΔW 2,2,2 )−(ΔW 2,1,1 +ΔW 2,2,1 ) is asymmetric in respect of said optical axis ( 19 ).