Sony managed to read and write on seven layers on holographic medium using the Micro-reflector method, further improving the company's previous achievements
that stored data in 4 layers of a volumetric optical disc.
In the Micro-Reflector recording technology, a laser light emitted from a blue violet semiconductor laser diode is split into two so that one irradiates the front side of a volumetric optical disc medium as a reference light, while the other is emitted to the back side as a recording light. By precisely aligning the focal points of the two laser beams using servo technology, the two counter-propagating light beams focus on the same point on the disc's holographic recording material. Their interference creates a diffraction-limited size fringe corresponding to a 1 bit of information. Changing the focal points results in recording in more layers. During reproduction, light is emitted on the front side of the medium.
However, Micro-Reflector multi-layer recording does not avoid the issues met in multi-layer recording systems in general. The reproduction signal gets weaker as the system tried to read the deeper layers. In addition, a slow data rate is also listed as a problem.
Sony made progress in increasing the data transmission speed, memory density per layer and the number of recording layers. Tech-On
publication reports that Sony managed to record data on a volumetric disc spinning at 1050rpm, 15 times faster than the company's previous demonstration, offering an equivalent data transmission of 3Mbps in case of 1-7PP modulated data (already implemented by the Blu-ray Disc).
Storage density was also increased to Gbytes per layer, for a 12cm disc.
Sony said that it calculated the error rates of reproduced signals on a 7-layer medium, with the maximum reported error rate to be 4.1 ? 10-4.
The company made the announcement at ISOM' 07, an international conference on optical memory, which took place Oct 21-25, 2007, in Singapore.
Sony also announced the results of recording and reading data on a 10-layer disc, suggesting promising eye-pattern signals.
Sony aims at the production of a a 500 Gbytes (25 Gbytes x 20 layers) 12cm disc by 2010.