Abstract

Thulium-doped fiber lasers (TDFLs) offer a broad ³F₄ ➔³H₆ emission band ~1600-2200 nm [1,2]. Thanks to “two-for-one” cross-relaxation process in TDFLs [1], that enables to achieve output power > 1 kW at ~2045 nm [1–3] with 0.79-μm cladding-pumping. Efficient cross-relaxation requires high Tm-concentration ( > 3wt%) [1, 2, references therein]. This results in a high core refractive index (thus higher V-number) that degrades the beam quality. It also increases the thermal load per unit length, although this can be reduced by a large cladding-to-core area ratio that increases the absorption length of the fiber. A long fiber also shifts the emission to long wavelengths with more signal re-absorption. For short-wavelength emission the re-absorption must be reduced in some other way, which also ensures adequate total pump absorption [4-6]. Simplest technique is to reduce the area ratio, but then excessive thermal load or inefficient cross-relaxation (if the Tm-concentration is low to reduce the thermal load per unit length) hampers power-scaling [1,6].

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