Investigation of photodarkening in tandem-pumped Yb-doped fibers.

Investigation of photodarkening (PD) in Yb-doped fibers tandem-pumped at 1018 nm is reported. For a homemade Yb-doped aluminosilicate double-clad fiber (YADF), the transmitted power of a 633 nm probe beam is reduced by 2.4% over 2 hours for the tandem pumping configuration at 1018 nm, which is significantly smaller than 33.3% for a laser diode (LD) pumping at 976 nm. A tandem-pumped Yb fiber amplifier also shows a much smaller decrease in the amplified output power over time than a LD-pumped Yb fiber amplifier. Based on fluorescence spectra of the YADF, we can not only associate PD of the YADF to intrinsic oxygen deficiency centers or Tm impurities but also confirm the impact of the excited Yb ion density on PD. The benefits of the tandem pumping in a high-power Yb fiber laser system will be discussed.