_Applied_  
_sciences_  
**Comparative Spectroscopic Investigation of Tm\({}^{3+}\):Tellurite Glasses for 2-\(\upmu\)m Lasing Applications**  
**[PERSON] \({}^{1,2,3,*}\), [PERSON] \({}^{1}\), [PERSON] \({}^{4}\), [PERSON] \({}^{5}\), [PERSON] \({}^{5}\) and [PERSON] \({}^{1,6,*}\)**  
\({}^{1}\) Laser Research Laboratory, Departments of Physics and Electrical-Electronics Engineering,  
Koc University Rumelifeneri, Sariyer, Istanbul 34450, Turkey; [EMAIL_ADDRESS]  
\({}^{2}\) Center for Free-Electron Laser Science, Deutsches Elektronen Synchrotron (DESY),  
Notkestrasse 85, 22607 Hamburg, Germany  
\({}^{3}\) Physics Department, University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany  
\({}^{4}\) Teknofil. Inc., Zekeriyakoy, Istanbul 34450, Turkey; [EMAIL_ADDRESS]  
\({}^{5}\) Dipartimento di Biotecnologie, University of Verona and INSTM UdR Verona, CaVignal,  
Strada Le Grazie 15, 37134 Verona, Italy; [EMAIL_ADDRESS] (A.S.); [EMAIL_ADDRESS] (M.B.)  
\({}^{6}\) Koc University Surface Science and Technology Center (KUYTAM), Koc University Rumelifeneri,  
Sariyer, Istanbul 34450, Turkey; [EMAIL_ADDRESS] (A.S.);  
\({}^{*}\) Correspondence: [EMAIL_ADDRESS] (H.C.); [EMAIL_ADDRESS] (A.S.);  
Tel.: +49-40-8998-6358 (H.C.); +90-212-338-1429 (A.S.)  
Received: 10 January 2018; Accepted: 21 February 2018; Published: 27 February 2018  
**Abstract:** We performed a comparative spectroscopic analysis on three novel Tm\({}^{3+}\):tellurite-based glasses with the following compositions Tm\({}_{2}\)O\({}_{3}\):TeO\({}_{2}\)-ZnO (TeZnTm), Tm\({}_{2}\)O\({}_{3}\):TeO\({}_{2}\)-Nb\({}_{2}\)O\({}_{5}\) (TeNbTm), and Tm\({}^{3+}\):TeO\({}_{2}\)-K\({}_{2}\)O-Nb\({}_{2}\)O\({}_{5}\) (TeNbRTm), primarily for 2-\(\upmu\)m laser applications. Tellurite glasses were prepared at different doping concentrations in order to investigate the effect of Tm\({}^{3+}\) ion concentration as well as host composition on the stimulated emission cross sections and the luminescence quantum efficiencies. By performing Judd-Ofelt analysis, we determined the average radiative lifetimes of the \({}^{3}\)H\({}_{4}\) level to be 2.55 \(\pm\) 0.07 ms, 2.76 \(\pm\) 0.03 ms and 2.57 \(\pm\) 0.20 ms for the TeZnTm, TeNbTm and TeNbKTm samples, respectively. We clearly observed the effect of the cross-relaxation, which becomes significant at higher Tm\({}_{2}\)O\({}_{3}\) concentrations, leading to the quenching of 1460-nm emission and enhancement of 1860-nm emission. Furthermore, with increasing Tm\({}_{2}\)O\({}_{3}\) concentrations, we observed a decrease in the fluorescence lifetimes as a result of the onset of non-radiative decay. For the \({}^{3}\)H\({}_{4}\) level, the highest obtained quantum efficiency was 32% for the samples with the lowest Tm\({}_{2}\)O\({}_{3}\) ion concentration. For the 1860-nm emission band, the average emission cross section was determined to measure around 6.33 \(\pm\) 0.34 \(\times\) 10\({}^{-21}\) cm\({}^{2}\), revealing the potential of thulium-doped tellurite gain media for 2-\(\upmu\)m laser applications in bulk and fiber configurations.  
**Keywords:** glass lasers; tellurite glass; thulium; thulium-doped laser glasses; solid-state spectroscopy; lanthanide ion-doped glasses; 2-micron lasers