Blu-ray Disc is incompatible with HD DVD and DVD media because instead of having a 1.2mm thick disc made of a 0.6 mm disc with 0.6 mm protective coating on top (as in DVD media), Blu-ray Disc is made of a 1.1 mm disc covered by a 0.1mm protective layer.
Because of the different thicknesses of the protective layers, the data layers of both formats are at different depths from the surface and thus the components required to read or write them are different...until now.
The Nikkei Net Interactive reported over the weekend that Ricoh Technologies has developed an optical device that can read and write both next-generation optical disk formats.
The device in question is a diffraction plate that is placed between the laser and the lens, which allows changing the laser beam's spot size on the fly. This will make possible to use the same components to read both Blu-ray Disc and HD DVD media. Hardware manufacturers would only need to make the diffraction plate change to the proper state depending on the media loaded in the disc drive.