Novel growth phenomena in fibre Bragg gratings under low irradiation power

Some new phenomena in the writing process of Bragg gratings in B/Ge co-doped non-hydrogenated optical fibre are observed at relatively low irradiation power. The grating reflectivity continues to grow even after the irradiation is turned off. Meanwhile the Bragg wavelength continues to shift to the blue. Several hours later, there are no noticeable changes in the grating characteristics.