Dynamics of Coupling Peaks by H2 Diffusion in Long-Period Grating Filters

It is now well known that germanium-doped silica fibers excellent photosensitivity. A UV source changes the refractive index of the core that contains germanium. A common type of defect formed in germanium-doped silica is GeO or oxygen deficient germania related defect [1], which is bonded to three oxygen atoms, and one bond is made to a silicon/germanium atom. This has been identified as giving rise to absorption centered around 240 nm. But, typically UV induced index changes have been limited to 3×10−5 for standard single mode fibers doped with 3% germania [2]. Increasing GeO2 doping concentration and reducing the amount of oxygen used in the fabrication of the preform also enhance the fiber photosensitivity [2]. In this case, the peak index changes are usually about 5×10−4 or less. More recently, two techniques have been proposed to enhance the photosensitivity of germanium-doped silica fibers : exposure of the material to the flame of an oxygen-hydrogen burner(flame brushing method) [3], and hydrogen loading at low temperature and high pressures [4]. Lemaire [4] showed 5.9×10−3 peak-to-peak index change in the case of 3% GeO2 loaded with 3.3% H2. In long-period grating filter fabrication, hydrogen treatment is needed to get high Δn for coupling a fundamental guided mode to the cladding leaky modes efficiently. Hydrogen treated germano-silicate fiber is unstable because of H2 diffusion process. We present the growth and decay behavior of the coupling peaks induced by H2 diffusion and annealing process for stabilization.