Theoretical study of pumping absorption in a co-linear side-pumping coupler

The coupling and absorption properties of a co-linear side-pumping scheme have been calculated numerically by the Beam Propagation Method. In the co-linear side-pumping scheme which is composed of a multi-mode pump fiber and a core-doped double-cladding fiber, the coupling and absorption properties of each incident mode are different, and the absorption efficiency of the pump light depends on power distributions of the launched modes. We got higher absorption efficiency when high-order mode has bigger power percentage in total input light. Compared with the end-pumping scheme, co-linear side-pumping needs bigger absorption coefficient to reach high pump light absorption efficiency.