Temperature-Dependent Emission Spectra Around 800 nm and 1800 nm of the Thulium-Doped Fibers Pumped at 1565 nm

Two-micron sources of radiation are in high demand for their widespread utilization in areas such as defence, industrial, or biomedical applications. Thulium-doped fiber lasers offer access to a wide emission spectral range in this region and moreover enable the so-called two-for-one cross-relaxation process leading up to the theoretical doubling of the overall quantum efficiency up to 2 [1]. However, their maximum reachable power is an order of magnitude below their potential. One of the main limiting factors is associated with thermal effects in the fiber since the temperature can exceed 500 K during the laser operation [2], which significantly affects both spectroscopic and laser properties [3], [4].