Abstract

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 λ_C is varying with the difference of effective indices between the two coupled modes, Δneff, by λ_C=Δneff Λ. As a consequence, a variation of 2 10^-6 on Δneff will induce a variation of about 1nm on λ_C 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.

© 1997 Optical Society of America