Excitation dependent radiation-induced losses in step-index fibers with undoped core

The radiation-induced loss of multimode step-index fibers with undoped synthetic silica core has been reduced significantly over the years. Most of the efforts focussed on material modifications and improvements of the undoped core material having either high or low OH-content. With increasing radiation resistance due to core material improvements, influences of the core-cladding interface and of the cladding material became more important. These effects can be analyzed with an improved measurement system using different excitation conditions at the frontface of the step-index fibers: low order meridional rays have high intensity in the center of the core material and low intensity in the core-cladding transition region, in contrast to high order skew or helix rays having opposite intensity profiles. The measurement system including the specific excitation conditions is described. Applying this setup, radiation-induced losses during and after contiuous gamma irradiation were investigat ed with regard to differences of the specific excitation conditions. The measured values at 10 krad total dose differ by more than a factor of two.