CD and digital versatile disk DVD

Since its release in 1982, the compact disk has taken the world by storm, and billions of CDs have been manufactured [5, 6]. Most of them are of the readonly memory (ROM) type, made from transparent polycarbonate (see Chart 12.1) and providing almost perfect resolution. 

In the cases of both CD-ROM and DVD, the information is binary coded bitwise in the form of pits and lands (see Fig. 12.1). Commencing at the inside, spirally arranged tracks of pits and lands are engraved into the disk. 

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], 

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Optical memory devices such as compact disks (CDs) and digital versatile disks (DVDs) are becoming essential items of audio and visual media as well as of external computer memory media. In these devices, a laser beamis used to record and read information. Because the laser spot can be focused to within a 1 fxm scale, optical memory can access higher density and capacity than conventional magnetic memories can. 

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