Optical disk media

The key component of the optical recording system is the recording medium, which is marked in one step and read in another step with a laser beam. Many materials have been considered for the purpose, including metal-films, organic dyes, dye-loaded polymers, metal-loaded polymers, discontinuous metal films, thermal coloration systems and bilayers (1-19). Tellurium- and gold-based optical disks have been the most widely studied materials to date. 

D. Of the items listed, the following are valid backup hardware digital audio tape, digital linear tape, and optical disk. Because hard disks are more failure-prone than the other types listed, they make a poor backup medium. 

In addition, the surface should be free of contaminating particles and occlusions that would interfere with the information retrieval process. A typical ablative-mode optical disk has the structure shown in Figme 16.14. The substrate is an optically transparent material such as polycarbonate, PMMA, polyfethylene terephthalate), or polyfvinyl chloride) topped by a subbing layer to provide an optically smooth surface for the recording layer. A metal reflector (typically aluminum) is then incorporated next to a transparent dielectric medium such as poly(a-methyl styrene) and, finally, the absorbing layer where the information pits are created is added. The latter can be a metal-polymer composite (silver particles in a gel) or a dye molecule dispersed in a polymer matrix such as squaryllium dyes, which act as infrared absorbers for GaAs lasers, typically... 

Past strategies for increasing the storage capacity of optical disks were based on a reduction of A and an increase in NA, as can be seen from Table 12.1. In principle, a reduction in the spot size can be achieved with the aid of solid immersion lenses. This as yet not practically exploited technique, operating with a hemispherical or a Weierstrass superspherical lens placed near the recording medium (< 100 nm), yields a reduced spot size, S, as is evident from Eqs. (12-2) and (12-3), respectively, where n denotes the refractive index of the lens [8],... 

The final type of storage is the optical drive. Optical drives work by using a laser rather than magnetism to change the characteristics of the storage medium (typically an aluminum-coated plastic disk). Optical drives look similar to and are used for the same applications as Zip drives (e.g. archival storage and large file transport). However, optical drives can store more information and have slower access times than Zip drives. 

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