The 5-inch (130 mm) disk has a holographic system that writes and reads data with a blue laser. The laser beam splits into two beams, a signal beam that convey data and a reference beam. Data to be stored is encoded into data pages, which are written in multiplex in the storage media.Read more at Storage Pipeline
Hitachi developed 200GB holographic disk

Hitachi said it will begin sampling a holographic disk with a 200GB capacity in Fall 2005.