Two types of resonances in long-period gratings induced by arc discharges in boron/ germanium co-doped fibers.

We demonstrate that under certain conditions it is possible to fabricate in a B/Ge co-doped fiber an arc-induced long-period grating whose spectrum contains a dual set of resonances. These two sets of resonances are formed by distinct mechanisms and are caused by coupling to cladding modes of different symmetries. They behave differently at high temperatures: the set produced by symmetric perturbation disappears during annealing at a temperature of 800 degrees C, while the other set produced by an antisymmetric mechanism can withstand temperatures above 1000 degrees C.