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Thermally induced transverse mode instability (TMI) has been recognized as one of the major limits to average power scaling of single-mode fiber laser. Mitigating the thermal load in single-mode high-power fiber lasers by operating lasing closer to the pump wavelength is one of the effort directions. Here, we demonstrate 220w single–mode output power at 1018nm from an ytterbium-doped all-solid photonic bandgap fiber (ASPBF) pumped at 976nm. The quantum defect is only 4.1%, helping to mitigate the thermal load. The ASPBF fiber has the multiple-cladding-resonant design, leading to better higher-order modes (HOM) suppression in its ~50µm core. The large core/cladding ratio also benefits the 1018nm lasing, providing the higher cladding pump absorption so shorter fiber length is needed with better ASE suppression at longer wavelength. In addition, the use of a phosphosilicate host in this fiber also enhances ytterbium gain at 1018nm, leading to a reduction in the required inversion, further increasing efficiency. In the laser test, one end of fiber is spliced to a high-reflective fiber-Bragg-grating at 1018nm and the other end is right-angle cleaved. ~62% and ~77% lasing efficiency has been achieved around maximum power with respective to the launched and absorbed pump power. The M2 was measured at 130W as 1.06 and 1.17 with respective to the x and y axis.
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