Fujifilm developed a new recording method for optical discs, which takes advantage of the two-photon absorption phenomenon to apply heat-mode recording on multiple recording layers of an optical disc.
Generally, two-photon absorption is the simultaneous absorption of two photons of identical or different frequencies in order to excite a molecule from one state to a higher energy electronic state. The energy difference between the involved lower and upper states of the molecule is equal to the sum of the energies of the two photons.
The ability of two-photon excitation to address molecules deep within an optical medium without affecting other areas makes it possible to store and retrieve information in the volume of a substance rather than only on a surface as is done on the DVD or BD. Therefore, 3D optical data storage has the possibility to provide media that contain terabyte-level data capacities on a single disc.
Fujifilm made a step further and combined the two-photon absorption phenomenon with the "Heat-mode Recording" method. Recording marks are formed on the redording material due to highly-concetrated heat coming from a laser light with a high energy density.
As the Tech-On website reports, Fujitsu used a Ti/S laser (405nm, 76MHz, 2 psec pulse) and an optical system with a numerical aperture of 0.85.
The optical disc consisted of a recording layer (2-photon absorption and polymer materials,) on which convex-shaped marks were formed by applying the laser light. The recording layer was sandwiched by a ultraviolet curable resin layer and an adhesive material layer.
Obviously, the recorded data is read out by detecting the change of the amount of light reflected on the interface between the recording layer and the adhesive material layer.
Fujitsu recorded 25GB data on a single layer disc and confirmed the read-out of a 17PP-random encoded signal (1T=75nm, 2T~8T marks, Tp=320nm) - the same applied to currently available Blu-ray discs. However, a higher frequency laser and a more sensitive detector will be required in order to improve the reported 22% jitter of the eye pattern read-out signal.
By changing the irradiation time of a laser light, the height of the convex-shaped marks on the recording layer can be controlled, meaning that more than two state values can be represented by each mark, pushing further the information stored on the disc by 2 or 3 times.
Therefore, with the method, it is possible to make a double-sided optical disc whose storage capacity is 1 Tbyte. The company aims to commercialize these disc by 2015.
Moreover, the new recording method has a potential to realize a 15-Tbyte multi-layered disc in the future.
Fujifilm added that manufacturing the new disc is cheaper and faster than the currently available BD discs.
The two-photon absorption disc has a cost as low as that of a magnetic tape and an access speed of about several ten to hundred milliseconds, Fujifilm said.
However, the playback signals have a reflectance ratio of just 0.5% - the 1/40 that of BD.
Fujifilm will showcase the technology results at Inter BEE 2012, which will take place from Nov 14 to 16, 2012, Chiba Prefecture, Japan.
