2. Bragg gratings in guided optics

2.1 Notion of mode coupling

When we consider an ideal optical fiber, light propagates in different orthogonal modes, and there can be no direct coupling of energy between them: these are the guide's "proper" modes. On the other hand, in a real fiber, intermodal coupling can occur because no fiber, due to technological manufacturing constraints, has a perfectly cylindrical geometry, nor a refractive index that is completely constant longitudinally. In this case, the coupling is "assisted" by disturbances in the guide: fluctuations in index, diameter, etc. However, as we shall see, it is precisely a longitudinal variation in refractive index that is used in a controlled and periodic way to obtain a Bragg grating.

In any unimodal fiber, the coupling of part of...