Effects induced by 4.7 eV UV laser irradiation on pure silica core multimode opticalfibers investigated by in situ optical absorption measurements

We investigated by in situ optical absorption measurements the effects induced by 4.7 eV UV laser irradiationon pure silica core optical fibers. Laser irradiation with 100 MWcm−2 laser intensity generates in the fiber E′centers which partially decay after irradiation due to their reaction with diffusing H2. An absorption bandpeaked at 5.3 eV is observed to grow in the post-irradiation stage with a kinetics anti-correlated to the decayof the 5.8 eV band of the E′ centers. The defect absorbing at 5.3 eV is proposed to be formed by trapping onpre-existing precursors of hydrogen atoms made available by breaking of H2 on E′.We also show by repeatedirradiation experiments that the 5.3 eV-absorbing center is photochemically destroyed by 4.7 eV laser light,and we estimate the cross section of this process. Possible structural models for this defect are discussed.