Hitachi has developed a new technology that is capable of storing up to 500GB of data onto discs that have a super-resolution film on their substrate.
The company said that the new technology can be applied to the next-generation optical discs. Hitachi's achievement takes advantage of a new signal processing technology that is capable of increasing the S/N (signal-to-noise ratio) ratio of the playback signal of a multi-layer "super-resolution" optical disc. This paves the way to the development of optical systems capable of storing up to 500GB of data on 12cm discs.
A Super resolution optical disc uses a nonlinear optical thin film formed on a substrate. The nonlinear optical thin film comprises an amorphous alloy or a mixture of the amorphous alloy and an oxide glass component. The film has a refractive index which changes in response to a change of intensity of a laser beam incident thereto.
This absorption-saturation phenomenon allows transmission of light having intensity above its absorption saturation limit of the film, but absorbs light having intensity below this level. Spatial intensity of a laser beam used for writing and reading information has a Gaussian distribution. Therefore, when the laser beam transmits the super-resolution film, light is absorbed at the skirts having low intensity but transmits the super-resolution film at the center where intensity is high. In consequence, the beam diameter after transmission can be reduced. By reducing the diameter of the laser beam (narrowing), the recording density is increased.
In addition, Hitachi developed a new signal processing circuit that was essential in order to filter the low-frequency noise that was introduced in the reading signal. The noise was related to the low-temperature areas of the super-resolution substrate.
Hitachi said that the new method is able to increase the recording density by four times, compared to the density achieved in current optical media. The technology can be applied to multi-layer media also, according to the company.