Sony's researchers had previously achieved successful recording only with four-layer and five-layer discs.
Increasing the storage capacity became increasingly difficult because the recording laser could not penetrate the upper layers of data with sufficient power to change the state of lower layers. The layers are approximately 200 micrometres thick.
Sony scientists said at the International Symposium on Optical Memory in Singapore that they have overcome this problem by further development of a system with two read heads, one on each side of the disc.
Instead of directly altering the recording layers, the twin layers generate interference patterns which are used to write the data.
Sony researchers believe that refinements to this technique will enable them to move beyond seven-layer discs to densities of up to 20 layers.
The prototype discs use standard blue-violet lasers which are focused at different depths by lenses. The system uses a separate red laser to track the recording and reading position.
The tests used an 8cm disc rotating at 1,050rpm, but Sony believes that the technology can be scaled up to work with standard 12cm discs and faster rotational speeds.
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