Multi-layer method allows easier compatibility with Blu-ray media compared with other technology like holographic. Moreover Multi-layer drive can be developed and be brought to mass-production easier, since it offers compatibility with simple optics.
For multilayer optical discs, it has been difficult to obtain clear signals from each recording layer in a stable manner due to crosstalk from adjacent layers, transmission loss and spherical aberration..
To reduce transmittance loss, Pioneer used a conventional material (like Aluminum or silver) as a reflective material, which doesn?t allow optical laser to reach to innermost layers. By using dielectric material, transmittance loss was minimized. .
Crosstalk issues were examined by using a dual layer BD disc. Pioneer tested prototypes with different spacers of different thickness and concluded that the spacer should have a thickness of 10 micrometer or more..
Stacked layers with equally spaced would also have an affect in the quality of the signal due to crosstalk. For this reason, Pioneer applied two spacers with different thickness between the layers, greatly reducing the signal interference. On the other hand, stacking layers increase the influence of spherical aberration. Pioneer's R&D experimented with the thickness of the cover layer, measuring the produced jitter values. This was done on a single-layer Blu-ray disc, since spherical aberration has not significant effect on dual layer BD discs due to their 25 micrometer cover layers. .
However, for multi-layer disc, residual spherical aberration factor gets bigger and the development of a comprehensive spherical aberration design becomes important. .
Pioneer's first trial disc included a 16-layer 25GB/layer (400GB) optical disc with a 10 & 14 micrometer dielectric. A BD drive pickup was used. In order to support multi-layer, scope of beam expander was increased. For Spherical aberration correction, beam expander was used instead of movable collimate lens..
The second trial disc had 20 layers with each layer capable to hold 25GB, resulting to a total available capacity of 500GB. Again, a 10 & 14 micrometer dielectric was used, together with the use of object lens with less influence in residual spherical aberration..
Pioneer concluded that by adopting a dielectric material as the reflection film, incident light efficiently reaches as far as innermost layer, producing a clear eye pattern (read-out signal). The interlayer crosstalk was reduced by stacking 2 different thickness of layers alternatively. In addition, by using an improved objective lens and wide range compensator effectively decreased spherical aberration..
Other promising results include the achievement of a jitter of just 10% in all the layers of a 20-layer disc sample. For reading all the layers, Pioneer slightly increased the power of the laser, but due to the absorption of laser by dielectric material, the produced extra heat did not destruct the disc. .
We should note here that the disc was read using a conventional sligthy modified servo mechanism found in current BD players. Since the optical specifications of the objective lens, such as NA (Numerical Aperture), are the same as those for the existing BD discs, it is possible to maintain compatibility between the new 16-layer optical disc and the BD discs. .
Concluding, Pioneer announced the development of a working 16-layer BD-ROM disc with a capacity of 400GB. The company established the basic technology for the development of multi-layer discs, and would like to investigate the possibilities to create a recordable multi-layer disc in the near future.
In addition, Pioneer confirmed the feasibility of a 500GB multi-layer optical ROM disk. The company believes that the Multi-layer optical ROM disc system will offer a capacity of 1TB.