The Influence of Adjacent Preformat Pits on Magneto-Optical Signals

Fluctuations in magneto-optical (MO) signals have been observed in 3.5-in. 640 MB MO disks which have the zone constant angular velocity (ZCAV) format. They were found to occur at the border of each band (zone) when there were preformat pits in the adjacent band. The fluctuations were observed in those MO disks whose substrates were injection molded polycarbonate (PC). The degree of these MO signal fluctuations depended on the injection molding conditions of the substrates and the phase shift of the optical head. It is thought that such phenomena can be attributed to tilted ellipsoids of substrate birefringence that occur around preformat pits, and which are caused by stresses induced around the preformat pits during the injection molding process.