Core-diameter adjustment,in analogy to doping management,is proposed in this letter for balancing thermal load and nonlinear efects.In this scheme,the core-to-cladding ratio increases with increasing core diameter along the direction of signal and pump propagation.An all-fber-integrated Yb-doped master oscillator power amplifcation(MOPA) is successfully demonstrated.Two segments of fber with diferent core diameters but same inner-cladding diameters and doping levels are spliced together and used as gain fbers in the last stage.A maximum average output power of 200 W at an overall slope efciency of 71% is achieved from the MOPA with a pulse energy of 2 mJ and peak power as high as 80 kW. Core-diameter adjustment, in analogy to doping management, is proposed in this letter for balancing thermal load and nonlinear efects. In this scheme, the core-to-cladding ratio increases with increasing core diameter along the direction of signal and pump propagation. An all-fber-integrated Yb-doped master oscillator power amplifation (MOPA) is successfully demonstrated. Two segments of fber with diferent core diameters but same inner-cladding diameters and doping levels are spliced ​​together and used as gain fbers in the last stage. A maximum average output power of 200 W at an overall slope efciency of 71% is achieved from the MOPA with a pulse energy of 2 mJ and peak power as high as 80 kW.