we report on a 126w fiber laser operating at 1018 nm with an optical efficiency of 75%. the optimal length for such a fiber laser is theoretically analyzed using steady-state rate equations including amplified spontaneous emission. excellent agreement on the maximum output power is achieved between the numerical result and the experimental counterpart. furthermore, a monolithic tandem pumped fiber amplifier is established by using conventional 30/250 mu m double clad ytterbium-doped fiber, and 185w output power with 85% optical efficiency is realized.