(E). Audio , Video & Compact Disc :

The entertainment industry, too uses lasers.(A certain sales representative was heard reporting that his Company's sales of HeNe lasers to disco clubs exceeded those to professional users). Compact disc audio , video & data storage :(Fig.21)

It uses a GaAlAs laser for writing very high density digital data on a thin metallic layer. For reading, the same laser is used, in combination with a PIN photo diode. To achieve a capacity of 1 Gbyte for 12-inch disc (equal to that of some 10-100 magnetic discs of similar size) and a user error rate of around 10-12, exacting engineering problems had to be solved, embracing not only optical and electronic but also mechanical, metallurgical, logistic and control engineering disciplines. The development took several years. The result is impressive.

Recording Principle: All information, whether pictorial, verbal, alphabetical or numerical, is reduced to strings of binary zeros and ones (Fig.22).

Writing it onto the recording medium a fine layer of a bismuth compound - is by means of ablation : a tiny circular matt area obtained by burning out a hole in the otherwise high-shine bismuth layer represents a one, while the unburnt and therefore highly reflective digital location will denote a zero.

The data are recorded on a continuous, tightly wound spiral, pre grooved in the polymer backing prior to the deposition of the bismuth layer , thus providing an optical 'handrail' to the write/read head (Fig.23). Dimension: The data holes have a diameter of 0.6 mm and the between tracks distance is 1.6 mm. This corresponds to a bit density of 3x1010 bits/mm2 (over half a million bits per pinhead area). On the top of both surfaces glass layer of 1.1mm are placed for strengthening / protective purpose.

Figure 23.: The diagram depicts the basis of readout from an optical disk. The laser beam is focused on the surface of CD containing information in forms of pits. The reflected laser beam is focused on the photodiode which extract the information to electrical digital pulses. Even dust particles on the protective layer do not affect the readout .