emission predominates and we have achieved Light Amplification by Stimulated Emission of Radiation (Laser). The condition that (53.31) leads to laser action is known as population inversion. Using the basic mechanisms of matter radiation interaction two classes of optical sources have emerged. Devices using spontaneous emission (Light Emitting Diodes or LEDs) and devices using light There are three ways in which absorption or emission can take amplification (lasers). The details of their operation depend on the place. Consider again the two level system, shown in Figure 53.8, material systems used and we briefly discuss this important issue. but with electrons at level Then without external light irradiating Spontaneous emission and laser action has been observed in a the material the electrons will drop spontaneously to level E, emit- large number of material systems ranging from glasses to crystals to ting photons of energy Eg- This emission is completely random and semiconductors. Which material system is used depends on a num- without any triggering mechanics being involved. ber of requirements imposed by the application and for fibre com- Consider now that the material is irradiated with light of wave- munications the indisputable choice is that of semiconductor length No close to Ac. Then the incoming photons will be absorbed materials. Optical sources made of semiconductors offers excellent by the material and electrons will now appear at level E2. The performance both optical and electronic, small size, reliability and electrons stay at E2 for a time interval A T whose duration is a perhaps what may be the most important feature for future evolution characteristic of the material. If now the external irradiance is the possibility of integrating optical sources with the electronic removed the electrons will delay to level E1 through spontaneous functions using the same material system. emission and eventually the level E2 will be empty. Since the emitted frequency is a function of the energy bandgap a The 