The wavelength dependent measurement of the relaxation oscillation frequency in a laser oscillator allows the study of the three- and four-level nature of the active material. Accordingly we present a tunable thulium doped fiber laser that covers the transition from three- to four-level operation at 2025 nm, characterized by the vanishing occupation of the terminal laser level. The laser is tunable from 1864nm to 2075nm with a linewidth below 0.3 nm. A maximum output power of 3.8W at 1930nm was achieved with a slope efficiency of 33.6% and an absorbed pump power of 17.5W. Absorption and emission cross sections were derived over the tuning range by an intracavity measurement method.
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