Simulations of Pump Absorption in Tandem-Pumped Octagon Double-Clad Fibers

The cladding pumping within a double-clad fiber structure is an effective technique to convert high-power multimode beam into high-power, almost diffraction limited beam. Since the pump efficiency is limited by the presence of higher skew-ray modes, geometrical perturbations are used to scramble the modes and to achieve a higher overlap of the electromagnetic field with the doped core. In this paper, the combined effect of fiber coiling and twisting is investigated in the double-clad fiber structure with the octagonal shape of an inner cladding. Electromagnetic field propagation through fibers with different cross-section areas, bending radii and twisting rates is numerically simulated using finite element beam propagation method. Holmium-doped double-clad fiber pumped in tandem configuration at wavelength 1950 nm is selected as an example. Nontrivial dependence of pump absorption on bending radius with the presence of local maxima is revealed. The observed impact of fiber coiling and twisting increases with the size of the cross-section area of the inner cladding.