Patent ID: 7215605

Claim:
An optical scanning device ( 1 ) for scanning an information layer ( 2 ) of an optical record carrier ( 3 ), the device including: a radiation source ( 4 ) for supplying a radiation beam ( 8 ), a lens system ( 5 ) for transforming said radiation beam to a scanning spot ( 14 ) in the position of said information layer, this system having an optical axis (OO′), and a detection system ( 6 ) including: an astigmatism generating element ( 18 ) for transforming said radiation beam to a first astigmatic radiation beam ( 21 ), this element further generating a first amount of coma (W 31 a ) so that the first astigmatic radiation beam includes coma aberration, a coma correcting element ( 19 ) for generating a second amount of coma (W 31 b ) so as to compensate said first amount of coma, thereby transforming said first astigmatic radiation beam to a second astigmatic radiation beam ( 22 ) that is substantially free from coma aberration, and a quadrant detector ( 20 ) for transforming said second astigmatic radiation beam to an electrical signal, characterized in that said coma correcting element includes a correction surface ( 19 A) having a shape defined by a function H(r,θ) that includes the term A.r 3 .cos(θ) wherein: “H” is the position of said correction surface along the optical axis of said lens system, “r” and “θ” designate polar coordinates in a cross-section of said first astigmatic radiation beam, and “A” designates a first constant dependent on said second amount of coma.