Optical scanning device

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 (W31a) so that the first astigmatic radiation beam includes coma aberration,
• a coma correcting element (19) for generating a second amount of coma (W31b) 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 (19A) having a shape defined by a function H(r,θ) that includes the term A.r3.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.