Optical scanning device with coma correction for improved focus tracking signal

摘要

An optical scanning device (1) includes: a radiation source (4) for supplying a radiation beam (8); a lens system (5) for transforming the radiation beam to a scanning spot (14); and a detection system (6) that includes a quadrant detector (20), an astigmatism generating element (18) for generating a first amount of coma (W₃₁^a) so as to transform the radiation beam to a first astigmatic radiation beam (21), and a coma correcting element (19) for generating a second amount of coma (W₃₁^b) so as to compensate the first amount of coma. The 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 the correction surface along the optical axis of the lens system, “r” and “θ” designate polar coordinates, and “A” designates a first constant dependent on the second amount of coma.