Estimation of energy transfer parameters in thulium- and ytterbium-doped silica fibers

Silica-based thulium-doped fibers sensitized by ytterbium are being developed for applications in fiber amplifiers and lasers at various wavelengths (around 800 nm, 1470 nm and 2 mu m). Several studies have been performed to design and optimize thulium- and ytterbium-doped fiber (TYDF) amplifiers and lasers at the above mentioned wavelengths. Although some papers dealing with modeling of such a system exist, the parameters used in the simulations, like energy transfer coefficients, have not been experimentally determined to date. In this paper we present an estimation of the energy transfer coefficients by comparison of the measured emission of three TYDF samples with numerical simulations of the respective emission using a spectrally and spatially resolved model of TYDF. We found that the energy transfer coefficients are higher than those reported in Tm/Yb-doped fluoride based crystals. This fact together with the possibility of increasing the energy transfer efficiency, by improvement of excited level lifetime of thulium by high alumina codoping, makes thulium/ytterbium co-doped silica fibers promising for applications in fiber lasers and amplifiers.