Optical card storage is one of the most compact form of data storage, which stores information on plastic card of credit-card size. This method of optical storage has been intensively explored for commercial applications. However, the capacity of this storage is strictly limited. The capacity is given by the total size of memory area divided by the extent of laser-beam spot, and the minimum size of spot is as large as a half of light wavelength due to the light-wave diffraction. To overcome this limitation, we record pits into multiple layers in a volume medium in three-dimensions rather than only in the surface of the medium[1-2]. In this paper, we describe the multi-layered optical card system, which we developed.
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