Interaction of modes in a fiber structure based on a section of double-cladding fiber

We study interaction of modes in a fiber-optical structure based on a double-cladding fiber section with small core (SM630). Mode analys is used to explain the mechanism of formation of the dips that are observed in the transmission spectra of the structure. It is shown that the dips are formed at wavelengths where the propagation constants of two modes are very close to each other.