Single-frequency Yb3+ fiber amplifiers operating at 1064~nm are promising candidates to fulfill the challenging requirements for laser sources of the next generation of interferometric gravitational wave detectors. We present the current development progress of a fiber amplifier engineering prototype and compare the optical and thermal performance to the solid-state-laser source of advanced LIGO. The fiber amplifier system consists of two monolithic fiber amplifier stages which currently deliver more than 110~W (functional prototype demonstrated 215~W) of output power. The fiber amplifier output beam has one to two orders of magnitude lower relative beam pointing and relative power noise in the lower frequency range of 1~Hz to 100~Hz compared to the solid-state-laser system. It also has a polarization extinction ratio above 21~dB and a TEM00-mode content of more than 97.8~\%~±0.6~\% at 110~W output power. Besides the optical properties, repair and maintenance procedures are improved by a modular design of the system. Each of the modules can separately be maintained and repaired or easily be replaced by a preassembled module; it therefore minimizes laser downtimes. Another advantage is the lower heat load of approximately 500~W compared to the SSL, which produces more than 4500~W of heat, both at an optical output power of 200~W. The lower heat load simplifies cooling and reduces the complexity of the modules.