Question

Discus modal propagation in Fibres

Answer 1

1. In an optical fiber, we see that the signal is traveled from source to destination by total internal reflection inside a core. The ray model does not predict whether, after total internal reflection there will be some field in the cladding. Also, it does not predict that rays can be launched at only discreet angles in an optical fiber. So there are certain limitations to ray model.

2. For a complete description of light propagation inside an optical fiber, we have to go in for a more rigorous model, called the WAVE MODEL. Here we shall discuss the propagation of light, treating light as an electromagnetic wave.

3. Energy gets guided inside an optical fiber i.e. the energy is confined inside the core and the field exponentially dies down in the cladding from the core-cladding interface.

4. In a practical fiber the cladding surrounded by a protecting layer. Generally, by the time the field reaches that layer, the field dies down significantly so that the protecting layer does not affect the propagation of the wave significantly.

      In other words, we can take the cladding to be of an infinite size in our analysis. So whole propagation of light is governed by the core-cladding interface and the interface between the cladding and other protecting layers does not affect the propagation.