A magnetooptical disk has two axially space-apart translucent recording layers. Each translucent recording layer is axially closer to an outer surface of the disk than to the other recording layer. This geometry enables closely axially disposing a magnetic field biasing means to each of the recording...http://www.google.de/patents/US5420834?utm_source=gb-gplus-sharePatent US5420834 - Optical disk recorders using disks coatings close to outer surfaces with optical path extending through a thick transparent substrate
