Nowadays, the main limiting factor of power scaling of high-power thulium fiber lasers is high heat load causing thermal mode instability (TMI) and other thermally induced difficulties. Increased temperature of the fiber core also leads to large changes of its spectroscopic parameters. In this work, we present the experimental measurements of the temperature dependence of the fluorescence lifetime of the thulium-doped fibers. The results of the temperature-dependent fluorescence lifetime, absorption and emission cross-section spectra, and energy transfer coefficient k3011, which characterizes so-called “two-for-one” cross-relaxation process 3H4, 3H6 → 3F4, 3F4, were employed to develop a numerical model for simulation and optimization of high-power thulium-doped fiber lasers and amplifiers with temperature-dependent fiber characteristics.
We report on the first measurement of the temperature dependent silica thulium-doped fiber absorption and emission cross-sections in the spectral range 700–2200 nm for a temperature range −196–300 °C. Using this data, we simulated the effect of temperature-induced shift of cross-sections on high power thulium-doped fiber laser (TDFL) performance when clad-pumped at 790 nm. The simulations show considerable impact on TDFL threshold and efficiency for shorter fiber length. The effect is most apparent for amplifier setup, especially with strong amplified spontaneous emission.
Thulium-doped fiber lasers have been extensively investigated as the most promising source of efficient laser emission at wavelengths around 2 μm, i. e., in the eye-safer spectral region and in the atmospheric window as well. It allows for wide range of applications including medicine, defense, distance measurement or materials processing. To enhance pump absorption efficiency along the active double-clad fiber, good overlap of the pump light and doped fiber core should be achieved along the fiber length. The overlap can be increased by breaking the circular symmetry of the inner cladding by shaping its cross-section. Further mode-mixing and better pump absorption can be achieved by coiling and twisting of double-clad fibers. In this work we present experimental measurement of 792 nm pump cladding absorption of a series of double-clad thuliumdoped fibers with respect to their bend radius, the inner cladding cross-sectional shape and twist rate. With these fibers, we assembled a set of fiber lasers with different resonator setups and tested their performance. Twisting was introduced to fiber during drawing from an octagonal, CO2 laser-shaped or mechanically grinded preform so that the twist remained frozen in the drawn fiber. We have shown that the fiber twist significantly improves the pump absorption even in the case of straight or coiled fibers with large coil radii. We provide a preliminary comparison of two fiber laser resonators.
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