Fujifilm Corp developed a new recording method for optical discs by using two-photon absorption to generate heat.
Fujifilm confirmed that the method realizes a recording density of 25 Gbytes per layer, which is equivalent to the recording density of Blu-ray Disc (BD), and 20 layers per side of a disc. 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 the disc by 2015.
Moreover, the new recording method has a potential to realize a 15-Tbyte disc in the future and a cost as low as a magnetic tape.
As for the capacity, the new method uses a two-photon absorption phenomenon, which is suited for multilayer discs. Because the reaction caused by the two-photon absorption can be limited to the small area of the focal point of a laser light, it is possible to increase the number of recording layers.
There are other researches on the use of a two-photon absorption phenomenon. But, unlike the past researches, Fujifilm combined a two-photon absorption phenomenon with the "Heat-mode Recording" method. The Heat-mode Recording is a technology that exploits a phenomenon in which an irreversible change is caused by applying a laser light with a high energy density to instantaneously increase the temperature of a minute area of a recording material.
A recorded mark having a convex shape is formed on the part where the laser light is applied. This time, Fujifilm used a Ti/S laser (405nm, 76MHz, 2 picoseconds). The numerical aperture (NA) of the optical system is 0.85.
The recording element made with the new method consists of (1) a recording layer on which the convex-shaped mark is formed by applying the laser light and (2) ultraviolet curable resin and adhesive material layers that sandwich the recording layer. 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.
In the recorded area, surface scattering and diffraction occur due to the convex-shaped mark. Therefore, "High-to-Low Polarity" (two values), which lowers reflectance ratio more than in the area where no data is recorded, is used for modulation. The recording layer and the ultraviolet curable resin layer have almost the same refractive index so that reflection does not occur on the interface between the two layers.
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