Bragg gratings in guided optics
Fiber Bragg Grating sensors - Photo-writing processes, and specifications

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...