Design of thulium-doped fiber with microstructured core and air cladding

Performance of high power fiber lasers is known to be affected by adverse effects related to material nonlinearity and thermally-induced degradation. The general strategy to mitigate these effects is to enlarge the cross section of the active fiber, i.e. to increase core area (mode field diameter) as well as cladding area. Since this could be difficult to achieve with a standard fiber design, e.g. due to technological limitations on achievable dopant concentration, specialty optical fibers like ones with microstrucured core¹ were proposed as an attractive alternative. We report here on the new design of thulium-doped fiber with segmented core formed by 19 sub-cores and with air-cladding. The proposed fiber is compatible with the step index fiber with core/cladding diameter 25/250μm. The mode field parameters are determined numerically by finite element modal analysis. Potential losses due to fiber bending are estimated using refractive index transformation.