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 [9,11]) 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. © 2019 SPIE.