Sony Corp. presented its achievements on data reading/writing on a medium equivalent to four-layer disc with the use of Micro-Reflector recording, a type of holographic recording. Thus far, the company has unveiled the results only on a single-layer medium.
In the Micro-Reflector recording, light beams are irradiated on both sides of the recording medium. Two light beams are then interfered on the recording layer by aligning their focal points, thereby recording an interference fringe which corresponds to 1-bit information capacity. The information is reproduced by emitting light on the front side of the medium. Interference fringes with different depths can be recorded by changing the depth of the focal points as in the case of recording on a multilayered medium.
In addition to multilayer recording, the company points out advantages in that the method is less likely to be influenced by the expansion/contraction of photopolymer due to heat, thus preventing signal reading errors. This is because the interference fringes are smaller than those used in other holographic recording methods where recording is performed on a page basis. The company expects that the Micro-Reflector system will easily facilitate reductions in equipment cost because it can utilize existing blue-violet semiconductor laser diodes and eliminate the need of a spatial phase modulator, CMOS sensor or other parts.
Four layers totaling 5 GB
The recording density per one layer is 1.25 GB in terms of a 12 cm-disk capacity, totaling 5 GB for four layers. The distance between adjacent layers is 50 μm. When writing on a four-layer medium, the recording and reference lights both had a power of 4.5 mW. In the previous experiment using a single-layer medium, both lights exhibited a power of 3.5 mW. The power of the reference light during data reading operation was reduced from 0.67 mW on a single-layer medium to 0.5 mW.
The experimental system used in the latest report is identical to that used in the previous experiment. A light emitted from a blue-violet semiconductor laser diode with a wavelength of 405 nm is split into two, so that a recording medium is irradiated on both sides with the resultant lights. In order to increase the number of recording layers, Sony said it simply added optical parts to change the focal point depth.
Other conditions remained the same as with the experiment using a single-layer medium. For example, a numerical aperture (NA) of the objective lens used for both recording and reference lights is 0.5. The system involves 17 PP modulation employed for the Blu-ray Disc and other media.
The recording medium is composed of a 250 μm-thick photopolymer recording layer sandwiched between two 600 μm-thick cover layers. The photopolymer is manufactured by Nippon Paint Co., Ltd.
One of the problems of Micro-Reflector multilayer recording is that the reproduction signal gets weaker in the deeper layer. Although the company did not measure the specific value, Sony said that judging from the eye pattern, "the amplitude of the reproduction signal in the fourth layer is about half that in the first layer." Slow data transfer rate was also listed as a problem.
The company intends to solve these problems in the future to achieve 500 GB recording density per a capacity of one optical disc by recording on a medium with twenty layers each having 25 GB density.