We have seen from the ray model that the rays can be launched at discrete angles inside an optical fiber. For a
particular launching angle all the rays which lie on the surface of a cone are equip-probable rays. Basically the ring of rays is simultaneously launched inside the optical fiber (as shown in the figure 2). Each ray has a wave front associated with it. The wave fronts corresponding to the rays will interfere. Somewhere the interference is destructive and somewhere it is constructive. So when the wave fronts move inside the core of the fiber they exhibit field distributions which have maxima and minima (see fig.3).
(b)
When total internal reflection takes place, the field must decay away from the core cladding boundary. If the field does
not decay, then the energy is not guided along the fiber axis and the energy is lost. Here, since we are interested in the guided fields, we accept only those field distributions which decay away from the core-cladding interface.
