PHYSICAL MECHNISM OF OPTICAL STORAGE DEVICES
The possibility of optical dynamic random access memory (ORAM) is mentioned in relation to memory hierarchy. To overcome the density growth limit many kinds of attempts have been made including introduction of super resolution, flying head recording, near field recording and dynamic spatial phase control recording. In the future, optical storage with a high recording density of 100 Gbit per square-inch and new amplification or enhancing mechanisms for writing and reading of tiny marks will be important.
Phase-change materials are some of the most promising materials for data-storage applications. They are already used in rewriteable optical data storage and offer great potential as an emerging non-volatile electronic memory. The crystalline state often shows an octahedral-like atomic arrangement, frequently accompanied by pronounced lattice distortions and huge vacancy concentrations. This can be attributed to the chemical bonding in phase-change alloys, which is promoted by p-orbitals. From this insight, phase-change alloys with desired properties can be designed. This is demonstrated for the optical properties of phase-change alloys, in particular the contrast between the amorphous and crystalline states. In dual-layer optical disks, light reflected from an adjacent layer overlaps and interferes with light reflected from the target layer on the photodetector. We confirmed a jitter of approximately 10% for both media at a recording density corresponding to 8.5 GB on a 120 mm disk. The laser power required to record on the first medium was 12 mW, and the second medium, 13 mW. It was also confirmed that the new rewritable dual-layer phase-change optical disk possesses the potential for operation with a blue laser.
As a result, the signal obtained from outputs of the photodetector fluctuates. The patterned phase plate weakens the intensity of the main beam from the adjacent layer which enters the sub-beam detecting sections of the photodetector and restricts the interference between sub-beams from the target layer and the main beam from the adjacent layer. Beam intensity distributions on the photodetector and push–pull signals have been investigated through numerical simulations and experiments. The results demonstrate that interlayer crosstalk can be sufficiently reduced using the patterned phase plate.
Here propose the multilayer recording of shift-multiplexed holograms with a coaxial transmission-type configuration, which can effectively utilize the recording medium in the thickness direction. In this method, two shift-multiplexing layers are formed in a single recording layer, in which the focal point of the Fourier transform lens lies in two interfaces between the recording material and the substrates. We confirmed experimentally that this dual-focusing method improved the error rate of the data by one order of magnitude.
The multimode optical waveguides in the glass layers are made by ion exchange from salt melt. The glass layers can be laminated in between standard PCB base materials. The coupling elements are made of thin glass too. An optical pickup head for providing multi-dimensional multi-level (MDML) optical recording is proposed and designed according to the MDML optical recording concept which uses both intensity and polarization of light. Through dynamic testing separately with a commercial digital versatile disc rewritable (DVD-RW) disc and another disc with 0.6 mm planar substrate sputtered with magneto-optical film, the MDML optical pickup head has been proven to provide multi-level modulations of at least two physical properties of light to achieve MDML recording, e.g., multi-level reflective recording with two-level polarization recording. On the basis of the MDML optical pickup head, research on the MDML optical disc can be carried out. In addition to the MDML recording function, the MDML optical pickup head is compatible with DVD formats.