ted as a bundle of rays, called modes, which is a concept of geometric optics. The concept of normalised frequency belongs to the formal analysis of the light propagation in fibre and is defined as in Equation 53.4. (53.4) Now the combination of the concepts of mode and normalised frequency enable us to derive a fundamental characteristic of light propagation in fibre. It was stated earlier that the fibre will accept and propagate all the rays entering the acceptance cone. The ques- tion now arises as to how many modes of propagation can be P1> > 0 cr supported by the fibre. Assuming that the refractive index of the core is uniform then the number of modes is given by Equation 53.5. Figure 53.1 Principles of light guiding: (a) light refraction; (53.5) (b) the critical angle Qcr for the limiting case of zero refraction; (c) total internal reflection for >Ocr 53/4 Fibre optic communications Cladding Refractive index Figure 53.2 The geometry of a fibre waveguide: (a) physical shape; (b) the fibre internal geometry along the axis AA1 For example with A = 0.1, n. = 1.5 and a = 25um the number of Usually the wavelength of operation is decided from other con- modes at 0.85pm is 311 and 936 at 1.55um. This regime of propa- siderations and the single mode propagation conditions are satisfied gation is known as multimode propagation and implies that a num- by changing either An or a or both. Since very small An leads to high ber of rays propagate simultaneously. The alternative to multimode bend losses both parameters are used in designing for single mode is single mode or monomode propagation, that is M = 1. The propagation. condition for single mode propagation is given by Equation 53.6. In order to preserve the mechanical strength of the fibre, the core diameter is reduced to around 8-10um but the cladding is main- tained at 60-125um. Since the standard multimode fibre has a core 2 - ) 1/2 (53.6) diameter of 50um but a cladding diameter of 125um it is not possible to identify the propagation conditions from a visual ins