Abstract
The high-power operation of fiber lasers was enabled mainly by the invention of cladding pumping within a double-clad fiber structure. Various cross-sectional shapes of double-clad fibers as well as unconventional coiling methods have been investigated both experimentally and theoretically in order to enhance the absorption of the multimode-pump. With enhanced pump absorption efficiency, the double-clad fiber of shorter length can be used in the fiber devices and in such a way the unwanted effects of background losses and nonlinear effects can be mitigated. In this paper we report on numerical modelling of optical pump absorption in double-clad octagonal active fiber of different fiber geometry and layouts. Namely we investigate the effect of the bending radii, twist rate of the fiber, doped core area (holmium is considered in this as a doping ion) and pump beam shape. The numerical model is based on FEM-BPM method. The optimized geometries and layouts shall finally result in a highly efficient laser of small footprint without the need of water cooling with great potential for application with low power consumption, tightly limited space and weight requirements. Optimized design will also minimize risk of damage of the fiber during operation of the fiber laser.
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