Short-wavelength, near-diffraction-limited beam, 792 nm cladding pumped efficient thulium-doped fiber laser in a power-scalable fiber design

Thulium-doped fiber lasers (TDFLs) offer a broad 3 F 4 → 3 H emission band ~1600-2200 nm. Thanks to "two-for-one" cross-relaxation process in TDFLs [1], that enables to achieve output power > 1 kW at ~2045 nm 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. 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.