Numerical Investigation of Emission Properties and Pump Noise Transfer Functions of an Yb3+:Er3+:Tm3+:Ho3+ Co-Doped Glass

Rare-earth doped glasses exhibit optical properties that depend on the specific rare-earth used, and therefore, rare-earth doped active devices are suitable for many different applications. In this article, for the first time to our knowledge, we investigate the emission, the gain spectrum and the properties of a 980-nm-pumped Yb3+:Er3+:Tm3+:Ho3+ co-doped germanate glass. In such a complex quadruply-doped active glass, the pumping process operates via both direct pumping (to ytterbium and erbium) and energy transfer processes (ytterbium to the other three rare-earths, erbium to thulium and holmium and thulium to holmium), which also act as a de-excitation processes. The interplay of all processes relies on the relative rare-earth concentrations and the relative weights. The optimization study has been performed by means of a numerical model based on rate equation system, that takes into account eleven energy levels and relevant energy transfer phenomena between the different ions. The emission and gain properties have been studied at different rare-earth concentrations, in order to optimize the emission and gain spectra. With an optimal set of concentrations, a broadband emission spectrum with a proper power spectral density and a bandwidth at -10dB of 660nm has been achieved by joining the emission bandwidth of erbium at 1550nm, thulium at 1800nm and holmium at 2050nm. Furthermore, the pump noise effect on the output of the system has been investigated by numerically calculating the pump noise transfer function. We show that thulium and holmium emission level populations are quite insensitive to pump intensity noise due to filtering by energy transfer processes.