Patent ID: 7164624

Claim:
An optical scanning device ( 1 ) for scanning an information layer ( 2 ), the device including a radiation source ( 6 ) for supplying a first radiation beam ( 14 ), a lens system ( 7 ) for transforming said first radiation beam to a scanning spot ( 18 ) in the position of said information layer, the lens system having an optical axis (OO′), and a detection system ( 8 ) including: an astigmatism generating element ( 9 ) for generating a first amount of astigmatism (W 1 ) so as to transform said first radiation beam to a first astigmatic radiation beam ( 29 ) having a first focal line (F 1 ) and a second focal line (F 2 ) which is further from said astigmatism generating element than said first focal line, said first amount of astigmatism (W 1 ) being represented by a vector (W o,1 , θ 1 ) in a reference plane (XY) perpendicular to said optical axis, where W o,i represents the magnitude of W i and θ i represents the angle between said first focal line and a reference axis (X) which is perpendicular to said optical axis; an astigmatism correcting element ( 27 ) for generating a second amount of astigmatism (W 2 ) so as to transform said first astigmatic radiation beam to a second astigmatic radiation beam ( 30 ) having a third amount of astigmatism (W 3 ), said second amount of astigmatism (W 2 ) being represented by a vector (W o,2 , θ 2 ) in said reference plane, and said third amount of astigmatism (W 3 ) being represented by a vector (W o,3 , θ 3 ) in said reference plane, and a detector ( 28 ) for transforming said third radiation beam to an electrical signal, characterized in that W 3 is adapted to said detector and that W o,2 and θ 2 comply substantially with the following equation: ( W o,1 , 2θ 1 )+( W o,2 , 2θ 2 )=( W o,3 , 2θ 3 ).