2ND-ORDER DIFFRACTION EFFICIENCY OF BRAGG GRATINGS WRITTEN WITHIN GERMANOSILICATE FIBERS

Second order diffraction has been observed from Bragg gratings written within germanosilicate glass fibres. Experimental results are reporte d on the evolution of the first and the second order Bragg grating ref lectivities as function of the uv laser shot number used for the inscr iption. Second order diffraction can be detected once the first order grating reflectivity has begun to saturate. Possible explanations of t he second order grating formation are discussed. Our experimental resu lts are consistent with a saturation in the photo-induced change in re fractive index.