third possibility is the most important. Consider again the two number of semiconductor material systems have evolved to satisfy level system but this time there are electrons at level E2 and the the transmission requirements. The first system to be developed was material is irradiated. Then the electrons at level E, interact with the the ternary Ga Al As/GaAs 1). The subscript X is radiation, drop to level E, and photons are released in phase with referred to as the mole fraction and by varying it one varies the those of the input light. This phenomena is called stimulated wavelength at which the device operates. The extremes of the emission and under certain conditions can lead to light amplifica- operation are obtained with x = 0, 1 = 0.9um and with 0.4, a = Absorption Light Emission Light Figure 53.8 The intreaction of radiation and matter: (a) random emission; (b) absorption and random emission; (c) stimulated emission and amplification 53/10 Fibre optic communications 0.65mm. The GaAIAs system therefore covers the 0.8um fibre window, the first optical window to be used. A study of the fibre loss Contact verses wavelengths will highlight the simple fact that operation of 0.8um is not the optimum. The silica fibre has the zero dispersion of around 1.3um and the global loss minimum at 1.55pm. To operate at these two windows a quarternary compound semiconduc- tor has been developed, In X Ga 1-x As y P Pi-y Now there are two mole Contact parameters and the bandgap energy corresponds to wavelength ranging from 0.92um to 1.65pm. This material therefore covers both the 1.3um and 1.55pm windows. Other materials have been investigated but they have never reached the maturity of the two outlined above. 53.2.2 Light emitting diodes and lasers The operation of LED and lasers is based on the same principle, that of population inversion (negative temperature), but the structures used are radically different. Al y Ga 1-yAl Al X Ga 1-xAs Population inversion is obtained in a semiconductor p-n junction Active layer by heavil