Many technology roadmaps are possible for phase change optical recording (See for example Fig. 6.1). However, many companies are emphasizing the necessity for strong compatibility among at least three generations (under 3.5 GB, about 4.7 GB and about 15 GB). Compatibility would make it difficult to change the thickness of the substrate, to change the penetration depth to the recording layer or to use very high NA lenses (>0.65). Since all the roadmaps use a blue laser for the 15 GB generation, compatibility also requires that the disks designed for red lasers must be readable with blue. Toshiba presented results demonstrating that this is possible.
There is close to a consensus that a red laser will be used for the 4.7 GB generation, but the margins are very thin. Similarly, the expectation is that a single recording layer with a blue laser and 0.65 NA will be sufficient for achieving about 15 GB, but again the margins are thin. Alternate plans would include using 2 layers for the 15 GB generation and 4 layers for 30 GB. Matsushita has a two-layer, erasable disk working in its lab. However, some companies, for example, Sony, have completely incompatible proposals. Sony presented an open eye pattern for data recorded at a capacity of 12 GB using a 515 nm laser, NA = 0.85, and a thin substrate with 0.1 µm depth to the recording layer. The panel also saw proposals using partial response maximum likelihood (PRML), optical super resolution, a thermal management buffer layer to improve the writing, and cross track cancellation to allow for higher track density.
Fig. 6.1. Roadmap for phase change disks.