Based on an external cavity using Littrow configuration, a high-power, narrow-linewidth and broadband-tunable Yb<sup>3+</sup>-doped double-clad fiber (YDCF) laser was demonstrated. Numerical simulations were performed to optimize the design
of the laser. A wide wavelength tuning range from 1046 nm to 1121 nm was achieved. The 3-dB linewidth of the laser
was up to 0.5 nm. An output power of over 20W at 1089 nm with a slope efficiency of 60% was achieved.