Bragg fibres with an intermediate layer

This paper examines the effect of an intermediate layer between the core and periodic cladding of a Bragg fibre on its optical properties. It is shown that any TE_0k mode of a standard Bragg fibre can be transformed into the fundamental mode of a Bragg fibre with an intermediate layer by adjusting the thickness of the first cladding layer. Varying the thickness of the intermediate layer, one can tune the mode composition of the transmitted light in a periodic manner. The periodic variation of the optical loss with the thickness of the intermediate layer is due to resonances and antiresonances in this layer. The resonances correspond to twomode light propagation through the fibre, whereas the antiresonances, to single-mode light propagation.