The influence of the cross-sectional shape of the inner cladding on the absorption characteristics of double-clad fiber lasers has been investigated by wave coupling theory. Wave equations are solved taking into account mode coupling effects between eigenmodes. The coupling between eigenmodes arises from the nonuniform absorption in the doped core region and leads to nonexponential absorption of the pump power. A characteristic parameter is proposed to compare the absorption characteristics among various cross-sectional shapes of double-clad fibers. It is found that a triangular cross section has the best absorption characteristics among the cross-sectional shapes examined.
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