The output power of fiber-based single-frequency amplifiers, e.g. for gravitational wave detectors, is typically limited by nonlinear effects (e.g. stimulated Brillouin scattering). In general, to reduce the impact of nonlinearities, the mode area of the fiber core is enlarged. Chirally-coupled-core (3C®) fibers have been specifically designed to enable single-mode operation with a large mode area core. 3C®-fibers consist of a step-index fiber structure, whose signal core is additionally chirally surrounded by one or more satellite cores. Because of the phase matching and the helical geometry, the higher order modes are pulled out of the signal core, which enables a high-purity modal content in the core. We present a robust and monolithic fiber amplifier based on an ytterbium-doped 3C®-fiber in combination with commercially available standard fibers. For the realization of such a monolithic system, a mode field adapter (MFA) was designed and installed between a standard polarization-maintaining fiber and an active 3C®-fiber for the first time. The MFA was tested regarding the guiding modal content by means of a S2-system. Overall, the fiber amplifier achieves a polarization extinction ratio of 17.6 dB and an optical output power of 100.1W in a linearly polarized TEM00-mode. To our knowledge, the fundamental mode content of 98.9\% is the highest TEM00-mode content of fiber amplifiers reported at this power level. This work emphasizes the high potential of fiber amplifiers based on 3C®-fibers as laser sources for the next generation of GWDs and demonstrates that compact and robust amplifiers can be realized using 3C®-fibers.