All-fiber cascaded combiners for high-power adjustable-ring mode laser beam with a flattop central beam

•We first presented all-fiber cascaded combiners for ARM beam laser with a flattop central beam. The cascaded combiners are consist of a 3×1 combiner and a (6+1)×1 combiner in series.•By selecting appropriate structural parameters of the cascaded combiners, the first 3 × 1 combiner can form a multi-mode laser beam, which is input to the center port of the second (6 + 1) × 1 combiner to obtain an ARM beam with a uniformly distributed central beam.•The cascaded combiners yield an 8.8 kW central output laser with a transmission efficiency of 96.2 % and M2 factor of 5.24, as well as a ring output laser with an average efficiency higher than 98 %.