Design of long‐period gratings: Necessity of a three‐layer fiber geometry for cladding mode characteristics

In this paper, we show that a complete three‐layer analysis is necessary to characterise the cladding modes in order to obtain grating period, resonances, and coupling length in the design of long‐period gratings (LPGs), and that the simplified two‐layer fiber geometry used by many authors leads to incorrect designs. This is illustrated by design calculations corresponding to actual applications of long‐period gratings as sensors and gain equalisation filters for EDFA.