Single-frequency Yb3+ and Er3+:Yb3+ fiber amplifiers (YDFA/EYDFA) in MOPA configuration operating at 1064nm and around 1550nm are promising candidates to fulfill the challenging requirements on laser sources for the next generation of interferometric gravitational wave detectors (GWDs). They offer high beam quality, long-term stability and allow for excellent thermal management. We developed an engineering fiber amplifier prototype at 1064nm emitting around 200W of linearly-polarized light in the TEM00 mode. The system consists of three modules: the seed source, the pre-amplifier and the main amplifier. The modular design ensures reliable long-term operation, decreases system complexity and simplifies maintenance procedures and repair. In addition, commercial available fibers increase the flexibility of the entire system. We also developed and characterized a fiber amplifier prototype at 1556nm that emits 100W of linearly-polarized light in the TEM00 mode. The EYDFA is pumped of-resonantly at 940nm to enhance the Yb3+-to-Er3+ energy transfer efficiency and enable a higher amplified spontaneous emission (ASE) threshold. In addition to that, we performed measurements to study phase to intensity noise coupling via the Kramers-Kronig relation above the stimulated Brillouin scattering (SBS) threshold, as it was proposed based on numerical simulations. This effect is based on an asymmetric gain spectrum, which we measured experimentally and used for the reconstruction of the broadband excess intensity noise.