Annealing of Long Period Gratings in Standard Hydrogen-Loaded Fiber

Long period gratings (LPG) are commonly fabricated in hydrogen-loaded fibers [1]. As classical Fiber Bragg Gratings (FBGs), LPGs have to be annealed at elevated temperature to stabilize their spectral response. Since a LPG of period λ is coupling the propagating mode with cladding modes (in this paper we focuse on LP01-LP05 coupling), the grating central wavelength λC is varying with the difference of effective indices between the two coupled modes, Δneff, by λC=Δneff Λ. As a consequence, a variation of 2 10-6 on Δneff will induce a variation of about 1nm on λC whereas for FBG, this kind of index variation has negligible effect. Taking into account the specification of the coupling induced by the LPG, we studied its thermal annealing during a long period of time (more than 100 hours) to observe index modulation change in the order of 10-6. In this paper, we present investigations on thermal annealing of LPG at temperature ranging from 110°C to 250°C, and with duration greater than 200h.