ted as follows: Transverse Electrical (TE) modes, having the electrical field vector "transverse," thus orthogonal to the fiber section radius Transverse magnetic (TM) modes, having the magnetic field vector "transverse," thus orthogonal to the fiber section radius HE modes, where no field is "transverse" The propagation description through the modes represented in Equation 4.1 is accurate, but difficult to manage for our purpose. In order to work with a simpler mode set, the solution of Maxwell equations in an orthogonal coordinate system can be attempted [3]. By exploiting again the weakly guiding property of the fiber, two sets of modes can be derived that due to the adopted coordinate system are linearly polarized (LP). These modes (called LP) can be divided, looking at the electrical field, into x-polarized and y-polarized modes, where x and y are the orthogonal coordinates in the plane of the fiber section and due to the fiber cylindrical symmetry, every x-polarized mode is degener- ate with a y-polarized mode. Naturally, as far as the adopted approximations are valid, LP modes can be represented as a sum of TE, TM, and HE modes. An LP mode, having, for example, linear polarization along the x axis, can be repre- sented with the expression (4.2) where x is the axis unitary vector 1,m are the mode identification numbers LP modes are much easier to use when the fiber propagation has to be considered for telecommunication purposes. The description of fiber propagation via LP modes is less accurate than via TE, TM, and HE modes with the effect that LP modes with the same mode index are not exactly orthogonal in real fibers, but a coupling exists. However, in studying fiber propagation, many other approximations are done (e.g., lin- ear fiber axis, perfect circular section, abrupt index change between the core, and the clad- ding) that creates much more modes coupling both among LP and among TE, HE, and TM modes, SO that the LP approximation can be adopted without any problem. Telecommunication Networks Differen