Fiber design for high-power low-cost Yb:Al-doped fiber laser operating at 980 nm

We investigated an Yb:Al-doped depressed-clad hollow optical fiber (DCHOF) for cladding-pumped 980-nm laser operation. With a careful design, the nonzero fundamental-mode cutoff characteristics of a DCHOF allows the competing 1030-1060 nm emission to be filtered out, despite being quite close to 980 nm. The laser yielded over 3 W of output power in a diffraction limited beam (M ² ∼ 1.09) from a DCHOF with an innercladding diameter of 120 μm. This is large enough for pumping with standard fiber-coupled multimode diode sources. By reducing the inner-cladding size to 90 μm, and hence, lowering the 980-nm threshold, the output power was scaled up to 7.5 W. However, we believe that the M² -parameter degrades to 2.7, as a result of increased cladding-mode lasing, as the cladding thickness is reduced.