This is a technology being developed by Constellation 3D that is projected to replace DVDs and CD-ROMs in the near future.
These discs use many layers of flourescent material that can be focused upon to read or write data. The laser lights up the flourescent bits on the focused layer, which will glow at a slightly lower frequency than the laser light and are picked up by an optical sensor.
Current techniques would allow the same data density as a DVD which is about 4.7GB per layer, but FMDs are capable of many more layers than DVDs (which only have 2 layers per side). FMD media containing up to a hundred layers are currently feasible, increasing the potential capacity of a single disc to hundreds of gigabytes, and can be pushed even further up to a terabyte when blue diode lasers become available.
| __ | A
.-' `-. | -- focusing lens
`-.__.-' | selects one layer
\ / V
. .. . .. .. . . . ... .\/ . ... .
. . . .... . . . ..... . . .. . .. .
. .. . ... .... .... .. . ... .. . multiple layers
...dozens of more layers...
. . . .. . . .. . .. . . ... .. .
. .. . ... . . ... .. . . .. ... .