Optical memory devices compact discs

25 The information on a compact disc is laid on a helix which starts close to the centre and moves outwards the laser beam is focused to a spot of diameter 1 jum, smaller than the pitch of the helix (after Bayer AG). 

26 Steps in the mastering and replication of a coiTipact disc (after Bayer AG). 

These requirements called for a glas polymer with a reasonably high T. Polycarbonate was found to be the ideal tem. Development work Bayer AG showed that standard grades were not satisfactory. A specific resin was developed with a low RMM (to decrease viscosity and to reduce moulded-in stress). The RMM was not reduced enough to lower the toughness to an inappropriate level. 

The standard audio GD is of diameter 120 nun and has a storage capacity of 600 megabytes it could store the entire text of the Encyclopaedia Britannka, equivalent to 1200 floppy disks. The development of polycarbonate as the material for larger optical memory discs (133 mm, 2(X) mm, and 300 nun) is in progress. 

In the CD, digital information is stored on a transparent polycarbonate disc in the form of microscopic indentations in the surface. The pitted surface is metallized for laser beam reflection and then coated with a protective lacquer. The laser beam enters from below (see Fig. 8.24). When the beam hits a pit there is no reflection when it hits a flat element between the pits it is reflected. The no-reflection/reflection sequence delivers a series of zero/one signals to a photodetector which converts the digital information to analogue music signals. 

The pits lie along a helical track (see Fig. 8.25). The pits are of depth 0.1 i m the track pitch is 1.6 m and the pit length is 1 to 3.3 pm. The laser beam reaches a sharp focus (a spot of diameter 1 pm) on the metallized track. When 

The drive sprocket is injection moulded in rubber-toughened PA6.6, which is resistant to repeated impact over a wide range of temperatures, and which shows excellent abrasion resistance. The plastic sprocket absorbs the shock of gear changes better than metal, causes less wear on the chain, and provides a quieter ride. The polymer is resistant to lubricants and other liquids. Reduced friction improves power transmission and,coupled with a lower weight, results in better performance. 

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Dithiolene complexes, and more specifically the nickel derivatives, are involved in materials used for optical data storage, such as compact disc or laser disc read-only memory (CD- or LD-ROM), and also in copiers or photography related devices. In the latter case, it is the IR-absorbing property that is exploited. Some of these compounds can be found in reviews by Mueller-Westerhoff et al. (353, 354). Recent patents are given as examples in references (458—481). In the field of optical storage, short reviews were published in 1988 (482) and 1990 (362, 483). Here, the dithiolene complexes act as inhibitors of the laser-induced fading of the colored thin layers of the optical discs. They also act as an antioxidant and increase the photostability of the cyanine dyes that constitute the recording layer. Contrary to what was observed for the two... 

CD-ROM Acronym for compact disc read-only memory. A high-capacity, optical storage device that uses compact disc technology to store large amounts of information, up to 650MB (the equivalent of approximately 300,000 pages of text), on a single 4.72" disk. 

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