In Sony's system (used on their proprietary Optical Disc Archive system which is predicated on Archival Disc, Free slots itself primarily based on Blu-ray) the drive has four optical pickups, two on each facet of the disc, with each pickup having two lenses for a total of 8 lenses and laser beams. The ExpressCard standard specifies two kind components, ExpressCard/34 (34 millimetres (1.3 in) large) and ExpressCard/fifty four (fifty four mm (2.1 in) huge, in an L-shape)-the connector is similar on both (34 mm large).
Both layers are accessible from the identical aspect, but require the optics to vary the laser's focus. Because the depth of the pits is approximately one-quarter to one-sixth of the laser's wavelength, the mirrored beam's section is shifted in relation to the incoming beam, inflicting mutual destructive interference and reducing the mirrored beam's intensity. In Zen's system (developed in conjunction with Sanyo and licensed by Kenwood), a diffraction grating is used to split a laser beam into 7 beams, which are then centered into the disc