Investigation of substrate birefringence effects on optical-disk performance.

Substrates for magneto-optical disks are produced economically and efficiently by injection molding of polycarbonate plastics. Unfortunately, most plastics are birefringent, with different refractive indices in the plane of the disk (lateral birefringence) and perpendicular to the plane of the disk (vertical birefringence). One manifestation of media birefringence is the existence of a focus offset between the two distinct best-focus positions for data detection and tracking. This focus offset degrades overall system performance as the compromise focus position between the two best-focus points reduces the operating margin of the individual data and tracking channels. We present detailed numerical modeling results on the role of substrate birefringence in causing this focus offset.