Solution:

Mode coupling is a concept for describing and calculating light propagation in certain situations, Gyolving nonlinear interactions.

Individual modes do not normally propagate throughout the length of the fiber. This result in a mode conversion which is known as mode coupling.

Coupled mode equations obtained from Maxwell's equations can be used for the analysis of mode coupling. Mode coupling affects the transmission properties of fiber which is a serious cause for concern when used for long-distance communication.

These will have the effect of coupling energy traveling in one mode to another depending on the specific perturbation.

Ray theory aids the understanding of this phenomenon, as shown in Figure 1.13, which illustrates two types of perturbation. It may be observed that in both cases the ray no longer maintains the same angle with the axis.

Thus individual modes do not normally propagate throughout the length of the fiber without large energy transfers to adjacent modes, even when the fiber is exceptionally good quality and is not strained or bent by its surroundings.

This mode conversion is known as mode coupling or mixing. It is usually analyzed using coupled mode equations which can be obtained directly from Maxwell’s equations.