Enhancement of cladding modes couplings in tilted Bragg gratings owing to cladding etching

We analyze the evolution of the cladding modes couplings in tilted fiber Bragg gratings when the fiber diameter and the refractive index of the surrounding medium are modified. To decrease the fiber diameter, we make use of hydrofluoric acid and to modify the external refractive index, we use a set of index oils. We also investigate a new way to correlate the spectral evolution of tilted Bragg gratings with respect to a change of the external refractive index.