All-Fiber Power-Scaling of a Small-core Thulium-doped Fiber Laser at 1931 nm

Lasers based on thulium (Tm$^{3+}$)-doped fibers allow for laser emission from 1900 nm to 2100 nm, making them suitable for applications such as free-space telecommunications, remote sensing, nonlinear frequency conversion, defence, medical surgery, and as a pump source for holmium (Ho$^{3+}$)-doped fiber or crystal lasers. Typically, Ho 3+: yag crystal lasers are pumped around 1908 nm because the strongest absorption occurs at this wavelength. Another absorption peak with about half the absorption strength exists around 1931 nm, making this wavelength also interesting [1], especially since Tm$^{3+}$-doped fiber lasers can be realized more efficiently at this wavelength compared to 1908 nm. However, due to strong water absorption lines around 1931 nm only a narrow window of about 0.5 nm has to be targeted.