Abnormal Shift of Center Wavelength in Annealing Long-Period Gratings

During annealing process of long-period gratings, the center wavelengths of some modes shift to red wavelength while those of others shift to blue compared with their original written ones, and the magnitudes of the shifts are related to the decay of the grating strength. The red shifts of center wavelength are more evident in lower-order modes of the gratings. We suggest that part of the photodissociated-H2 combines continuously with Ge-defect centers within 2 h after UV-exposure, and with which the observed abnormal shifts can be explained successfully.