Download MASTER THESIS

2.1 Dispersion Theory
An optical signal is distorted when it travels along the fibre. This is mainly a consequence of dispersion,
which is one of the most important problems in the optical fibre. The dispersion effect causes a different
delay to each of the optical signal’s components, so that, at the detector, these components are registered
with different arrival times. All this produces a distorted signal with respect to the transmitted one. The
phenomenon is illustrated in the figure 1 below, where each pulse broadens and overlaps with its
neighbours becoming indistinguishable at the receiver.
Figure 1. Dispersion effect
The phenomenon of dispersion consists of two different types:
Intermodal or Modal dispersion, appears in multi-mode fibres where the optical signal propagates in many
“modes”, each one following a different trajectory inside the fibre’s core, as following from rays theory,
see Figure 2 . In this way, all the modes, from a single pulse, experience different delays generating a pulse
spread. The strength of this effect strongly depends on the refractive index profile of the fibre in and
around the fibre core.
Figure 2. Intermodal or Modal dispersion
15