Sony Corp wrote and read data on a medium equivalent to a seven-layer disc by using "Micro-Reflector method," a multilayer recording technology using holographic recording.

The company calculated the error rates of reproduced signals after signal conditioning such as equalizing. Until now, the company wrote and read on discs of up to four layers and announced only the eye patterns of reproduced signals.

This time, the achievement was announced at ISOM' 07, an international conference on optical memory, which took place Oct 21-25, 2007, in Singapore. At the conference, Sony made speeches about speeding up data transmission speed, improving memory density per layer and increasing the number of recording layers even more.

In the Micro-Reflector method, lights are shed on both sides of a holographic medium. The focus points of the lights meet and interact with each other in a recording layer to form interference fringes. An interference fringe has an information amount of one bit. To read the data, a light is shed on one side of the medium.

Even though interference fringes are recorded on holographic media, the company defines the technology as a multilayer recording technology, not as a holographic recording technology. It is because interference fringes can be recorded in layers of various depths as if they are recorded on multilayer media if the depth of focal point is changed.

The new recording technology uses only one layer. Therefore, it can simplify the structure of record media more than common multilayer recording technologies that have multiple recording layers.

The error rates ares of practical use

The announced error rate is different in each layer, and the maximum error rate is 4.1 × 10-4.

"If error corrections are applied, the signals can be reproduced without any problem," Sony said.

The method of signal conditioning is "very similar to that for DVDs," the company said. "1-7PP" is used to encode data.

The memory density per layer is 1.2 Gbytes calculated in terms of a 12cm disc. In the experiment, a small disc of about 8cm in diameter was used, and interference fringes were recorded on a concentric area of about 200μm in the diametrical parallel direction.

Improvements in data transmission speed, memory density and the number of recording layers

Sony is making efforts to speed up data transmission speed, improve memory density per layer and increase the number of recording layers. The company reported the results of the experiment where data were recorded at a rotational speed of 1050rpm, 15 times faster than before to improve the data recording speed.

The data were recorded at 1.388MHz, 15 times as high frequency as before. And a reconstructed waveform was a substantially sinusoidal waveform, Sony said. The disc rotation speed is 70rpm when data are being read. The data were not encoded this time.

The data transmission speed is equivalent to 3Mbps if reproduced signals are considered as the reproduced signals of the shortest recording mark encoded by the 1-7PP, the company said.

As for the improvement of memory density, Sony achieved the memory density of 3 Gbytes per layer calculated in terms of a 12cm disc, 2.5 times as large as before. When reproduced signals were observed after signal conditioning, favorable eye patterns were found, the company said. The error rate was not calculated.

To improve the memory density, the channel clock frequency of recording data in the 1-7PP was enhanced to 920kHz, 2.5 times higher than before.

Sony also announced the experimental results of recording and reading data on a 10-layer disc. The 1-7PP was used to encode data. The error rate was not calculated, but the eye patterns of reproduced signals were observed. In each layer, favorable eye patterns were observed, the company said. The memory density per layer is equivalent to 1.2 Gbytes calculated in terms of a 12cm disc.

Sony aims to realize a memory capacity of 500 Gbytes (25 Gbytes x 20 layers) in a 12cm disc by about 2010.