Temperature-Dependent Emission Cross-Section Spectra at 1.8 $\mu \mathrm{m}$ of the Thulium-Doped Fibers Cooled Down to Cryogenic Temperatures

Thulium fiber lasers are important $2-\mu \mathrm{m}$ sources of radiation [1] with a wide range of applications e.g. in industry, medicine, or defense. However, their power is an order of magnitude below their potential and the main limiting factor is associated with thermal effects in the fibers, which significantly affect both spectroscopic and laser properties [2], [3]. The emission peak around 1800 nm showed a significant change in its shape at cryogenic temperature when compared to the other measurements presented in [3]. Therefore, we present here emission spectra measurements carried out using a liquid nitrogen cryostat to cool the temperature of fiber down to 77 K continuously in order to clarify the trends of emission changes at low temperatures.