As a part of the innovative design of the 140 GHz gyrotron amplifier loaded with confocal waveguide, the design process of the quasi-optical input launcher is comprehensively demonstrated. Owing to the relative insensitivity of the Guassian optics to the frequency fluctuation, the bandwidth of this type of power coupler could be drastically larger than the typical inserted waveguide coupler. The input launcher comprised of the antenna coupler, the mirror transformer and corrugated waveguide guarantees 6.8 GHz bandwidth with respect to >50% power injection efficiency, solving the problem of parasitic resonances and unexpected efficiency shrink simultaneously.