Nonlinear transmission coefficient of ytterbium-holmium fiber at the wavelength 978 nm

An experimental and theoretical investigation of the nonlinear transmission coefficient of ytterbium-holmium codoped silica fiber (YHF) at continuous-wave 978-nm pumping is reported. An analysis of the fiber absorption and luminescence spectra under 978-nm pumping reveals a variety of processes participating in the mixed (Yb³⁺, Ho³⁺) system: the energy transfer Yb³⁺ → Ho³⁺, the up-conversion and excited-state absorption in Ho³⁺, and the luminescence quenching due to the presence of Yb³⁺-Yb³⁺ ion pairs. These processes are shown to notably affect the transmission coefficient of YHF, which is reflected in a pronounced modification of its dependences on pump power, fiber length, and dopant concentrations, in comparison with the case of purely ytterbium-doped fiber (YF). A modeling of the experimentally measured dependences of the YHF transmission coefficient on pump power and fiber length allows us to obtain the coefficients addressing the energy transfer process Yb³⁺ → Ho³⁺ in YHF. A comparison of YHF and YF at the same pumping conditions reveals that, in YHF, a considerable part of the pump power absorbed in the Yb³⁺ subsystem is transferred into the Ho³⁺ subsystem; this results in an effective population of its ⁵ I ₇ state and makes YHF promising for 2-μm lasing and amplifying.