Effects of Ge Co-Doping on P-Related Radiation-Induced Absorption in Er/Yb-Doped Optical Fibers for Space Applications

This study demonstrates that phosphorus-related radiation-induced absorption (RIA) bands such as P-OHC and P1 in erbium/ytterbium-doped optical fibers (EYDFs) are effectively decreased by germanium (Ge) co-doping. The suppression of the generation of both P-OHC during X-ray irradiation of glass preforms and P1 during gamma-ray irradiation of optical fibers was measured for Ge concentrations between 0 wt% and 6.7 wt%. For the 10 krad X-ray irradiation of glass preforms, it was found that the RIA decreased from 5.6 dB/cm to 1.3 dB/cm when the Ge concentration was increased from 0.79 wt% to more than 4.4 wt%. Furthermore, in multimode optical fibers (MMFs) exposed to 100 krad of gamma-ray irradiation, the RIA decreased from 3.7 dB/m to 0.90 dB/m when the Ge concentration was increased from 0.0 wt% to 4.0 wt%, whereas it was 0.35 dB/m for a single-mode optical fiber (SMF) with a Ge concentration of 6.7 wt%. These results demonstrate that Ge concentrations up to 4.4 wt% are strongly correlated with improved radiation tolerance for both glass preforms and optical fibers. This effect was more prominent for SMFs than MMFs and became saturated with excess co-doping in both cases. Therefore, Ge co-doping may represent a good solution for reducing P-related RIA in EYDFs for space applications.