Energy transfer coefficients in thulium-doped silica fibers

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Thulium-doped fiber lasers (TDFL) are currently in focus of intense research worldwide with a great application potential in a spectral region around 2 μm. Their broad utilization includes among others medicine, defense or material processing. TDFL are in foreground of the interest especially thanks to a thulium energy level structure which enables a so-called two-for-one cross-relaxation (CR) process. This CR process presents a way to generate two photons at 2 μm from one pump photon at around 790 nm, and thus it allows to efficiently generate emission at 2 μm from available high brightness laser diodes emitting around 790 nm. Although the CR process is very promising and high-power fiber lasers based on it have already been presented, there are still reserves in its practical exploitation. In order to push the practical limits, reliable theoretical models are necessary. Among all parameters needed for the modelling, those describing energy transfers (ET) between thulium levels pose the main uncertainty. In this contribution, we present a method of energy transfer coefficients evaluation using rate equation modelling. This approach was based on a set of rate equations relating populations of energy levels with spectroscopic data. The coefficients were derived from fluorescence measurements by fitting fluorescence decay curves with theoretical equations. Studied fibers were pumped at two wavelengths – 793 nm and 1620 nm. Fluorescence curves were collected at 800 nm and 2 μm. All combinations of pumping and fluorescence measurements were examined for various pump power in a range up to 70 mW. Calculated energy transfer coefficients will be used in theoretical investigations and optimization of thulium-doped silica-based fiber lasers.