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Describes the systems and materials for three-dimensional optical data storage, which could exceed the classical limit in density of conventional optical data storage, gaining capacity/ density thirty times higher than that of a current compact disk. Experimental results demonstrate that with the proposed method at least thirty layers are successfully recorded and read in 300 /spl mu/m-thick photopolymer film. An illustration is shown of three dimensional optical data storage system, with which bit data are recorded inside of a thick volumetric medium, but not on the surface. The memory material must be photosensitive for writing, but transparent and refractile for reading. We used a photopolymerizable monomer-mixture as a read-only memory; thirty layers with bit data were written in a 300 /spl mu/m thick film with the separation of 10 /spl mu/m between layers. As an erasable three-dimensional memory, we used photorefractive crystals. We succeeded in recording ten layers in a lithium niobate crystal. Photochromic organic materials have also been intensively investigated for three-dimensional memory. A problem with photochromic memory is that the recorded data are gradually erased in reading because reading is also made by photon absorption. We use a near-infrared semiconductor laser for phase-data reading because near-infrared photons are not absorbed by photochromic dyes but phase-delayed due to the difference in refractive index between two isomers of the photochromic dye.