## 2 **Introduction**  
Thulium-doped systems are drawing a great deal of interest for numerous laser applications in the near and mid-infrared regions of the spectrum, since they provide broad emission bands covering the empty region (1400-2700 nm) between neodymium and erbium systems. In this respect, transitions originating from the \({}^{3}\)F\({}_{4}\) (\({}^{3}\)F\({}_{4}\)\(\rightarrow\)\({}^{3}\)H\({}_{4}\) and \({}^{3}\)F\({}_{4}\)\(\rightarrow\)\({}^{3}\)H\({}_{5}\) transitions corresponding to 1.5-\(\upmu\)m and 2.5-\(\upmu\)m emissions) and \({}^{3}\)H\({}_{4}\) (\({}^{3}\)H\({}_{4}\)\(\rightarrow\)\({}^{3}\)H\({}_{6}\) transition corresponding to 1.9-\(\upmu\)m emissions) energy states are worth studying. For this purpose, we choose to employ a 800-nm excitation scheme, which leads two broad emission bands centered near 1.9 \(\upmu\)m and 1.5 \(\upmu\)m. Employing this pumping scheme is further beneficial because there are many commercial pump sources available at these wavelengths and this pumping scheme provides an additional advantage, namely a \"two-for-one\" process which favors 1.9-\(\upmu\)m emission at higher active ion concentrations. The broad emission band around 1.9 \(\upmu\)m can be utilized to build new mid-infrared laser sources suitable for spectroscopic, chemical, and atmospheric sensing applications [1]. In addition, strong water absorption occurs at these wavelengths, making thulium-doped gain media very attractive in the development of lasers for biomedical applications such as tissue welding and ablation [2; 3]. Therefore, investigation of novel laser materials, especially thulium-doped hosts which fluoresce around 2 \(\upmu\)m, deserves a great deal of attention, considering the ever-increasing demand and need for new laser sources operating in the near- and mid-IR regions of the spectrum. So far, detailed studies have been conducted on germinate, silicate, and fluoride glass hosts together with various crystal gain media doped with rare earths for 2-\(\upmu\)m laser applications [4; 5; 6]. Among other laser gain media, tellurite glasses (with network former TeO\({}_{2}\)) offer several benefits, such as high chemical stability, easy and low-cost production due to their amorphous nature, and a wide transparency range from 0.355 to 5 \(\upmu\)m [3; 7; 8]. Tellurite glasses are known to be moisture-resistant, thermally and mechanically stable materials, making them attractive for fiber laser applications as well [9; 10]. Furthermore, tellurite glass hosts have relatively low phonon energies (700-750 cm\({}^{-1}\)). Hence, non-radiative decay is relatively low which leads to relatively high luminescence quantum efficiencies. To date, most of the effort has been spent on Nd\({}^{3+}\)- and Er\({}^{3+}\)-doped tellurite glass systems [11; 12; 13; 14; 15; 16] and there are relatively fewer studies conducted on the Tm\({}^{3+}\)-doped tellurite glasses [17; 18; 19; 20; 21; 22; 23; 24; 25]. Recently, lasing and mode-locked operation from bulk thulium-doped tellurite glasses were reported [20; 26; 27; 28]. Since Tm\({}^{3+}\)-doped tellurite glasses offer a great potential for the development of 2-\(\upmu\)m laser sources in bulk as well as fiber configurations, there is interest in further exploring the spectroscopic properties of the tellurite glass hosts doped with thulium. In this study, we performed a comparative spectroscopic analysis of three different tellurite glasses with various Tm\({}^{3+}\) concentrations in order to investigate the dependence of the emission cross section and quantum efficiency, which are among the most critical parameters for laser applications. In particular, we studied tellurite glasses with the compositions Tm\({}_{2}\)O\({}_{3}\):TeO\({}_{2}\)-ZnO, Tm\({}_{2}\)O\({}_{3}\):TeO\({}_{2}\)-Nb\({}_{2}\)O\({}_{5}\), and Tm\({}^{3+}\):TeO\({}_{2}\)-K\({}_{2}\)O-Nb\({}_{2}\)O\({}_{5}\). We investigated the influence of doping concentration as well as the host composition, both of which affect the cross relaxation rate and the luminescence quantum efficiency.