at of a single in practice through a number of mechanisms. The first and obvious mode with 2.0dB/km to 2.5 dB/km at 850nm and 0.55dB/km to 0.6 one is that high order modes, that is those with the maximum delay, dB/km at 1300nm. The difference of course lies in bandwidth. The Optical sources 53/9 bandwidth of graded index 50/125um multimode fibre is around tion. This is a fundamental concept and its understanding is invalu- 600MHz.km and 1500MHz.km at 850nm and 1300nm respectively. able in understanding the operation of optical sources. The equivalent r.m.s. dispersion is 333.3ps/km and 125ps/km re- Consider again the two level system with a total number of atoms spectively and it is easy to see the advantage of single mode N of which n, are at E1 and at E2. Under thermal equilibrium the transmission. number of atoms at level E. is given by Equation 53.32, where k is Boltzmann's constant (1.38 X and T the absolute tem- perature in Kelvin. 53.2 Optical sources (53.32) 53.2.1 The basics of light generation Matter absorbs or emits radiation by electron transitions between Since T > 0 then If now external light irradiates the energy levels which characterise the material. According to quan- material the probabilities of absorption (E1-E2)or emission (E2 tum mechanics this interaction takes place only in discrete quan- E1) are proportional to the intensity of light and the number of tities of energy. That is assuming that two energy levels E1 and E2 atoms at levels E, and E2, that is, the probability of absorption is exist with E2 > E1 then the minimum energy which can be absorbed approximately equal to In, and the probability of emission is ap- or emitted is Eg (where Eg = E2 - E1) which corresponds to a proximately equal to In2 characteristic wavelength 2c given by Equation 53.31, where h is Since the material absorbs the radiation. Now consider that Planck's constant (6.6261 c is the speed of light (2.999 somehow but since the probability of absorption is as before X 10 8/m/s) and a the wavelength of the radiation. 