TDK Corp developed a write-once optical disc with a capacity of 320 Gbytes. It has ten layers, and each layer has a storage capacity of 32 Gbytes. The new optical disc has a larger capacity than the Blu-ray disc, which has a capacity of 25 Gbytes per layer. And it is possible to write and read data on and from the disc by using a blue-violet semiconductor laser with an oscillation wavelength of 405nm and an objective lens with a numerical aperture (NA) of 0.85, which are currently used for the Blu-ray disc.