Radiation-induced optical fibre loss in the far IR

It was observed that the loss of a Ge-doped graded index (GI) fibre during gamma irradiation grows stronger at a longer wavelength (about 1300 nm) than at a shorter one (830 nm). The same is known from Ge-doped and undoped single mode (SM) fibres at about 1550 and 1300 nm. During longer irradiations (<or=2 d) it was found that the loss of the GI fibre at 830 nm was reduced by photobleaching, leading to approximately the same loss at 830 and 1300 nm during irradiation with higher light powers (>or=10 muW). The stronger loss increase at 1550 nm of a Ge-doped SM fibre continued for more than 70 days, leading to an about 1.5 times higher loss at 1550 nm than at 1300 nm. The reason seems to be an long-lived absorption peak in the far IR.