After the first direct detection of gravitational waves (GWs) in 2015, the feasibility of GW-astronomy as a complementary method to carry out astrophysical and cosmological observations on a regular basis has been reaffirmed. However, ultra-sensitive detectors are required to reliably develop GW-astronomy. A major upgrade proposed to increase the sensitivity of next-generation gravitational wave detectors (GWDs) relies on cryogenically cooling of the optical components to reduce the thermal noise. This upgrade requires the substrate material to be crystalline silicon and this, in turn, requires laser sources at wavelengths above 1.5 μm. In this context, research and development of single-frequency high-power laser sources with high TEM00 mode content must be carried out. This thesis is focused on the investigation of Er3+-doped and Er3+:Yb3+-codoped fiber amplifiers (EDFAs and EYDFAs) around 1.5 μm for next-generation GWDs. Er3+ is widely used as an active dopant in fiber amplifiers in the region between 1.5 μm and 1.6 μm due to its convenient emission spectrum. However, an analytical model to describe its optical transfer functions considering the involved energy levels when pumped at 976nm (i.e., 4I11/2, 4I13/2 and 4I15/2) had not been reported yet. In this thesis, an analytical model to describe the optical-to-optical transfer functions of EDFAs is developed and used to study the gain and phase dynamics of the amplifier. The results prove the influence of the non-radiative transition in the optical transfer functions and a linear relation between the output power and the effective lifetime of the meta-stable energy level 4I13/2. The predictions of the model were in good agreement with the experimental measurements. When Er3+-doped fibers are codoped with Yb3+, the pump power absorption is dramatically enhanced, enabling higher optical efficiency and gain levels. However, at high pump power levels, EYDFAs are prone to develop excess amplified spontaneous emission in the Yb3+ emission band (i.e., between 1.0 μm and 1.1 μm), which limits the output power at 1.5 μm and can turn the system unstable. In this regard, the benefits of the so-called off-peak pumping scheme (i.e., pump at a wavelength different from that of the maximum absorption) were investigated in this thesis. A double-clad EYDFA pumped at 940nm was built and characterized considering the requirements of GWDs. For the first time, an output power of 100W in the linearly-polarized TEM00 mode at 1.5 μm was achieved in single-frequency operation using an all-fiber setup. Core-pumping fiber amplifiers effectively increases the pump absorption with respect to cladding-pumped systems due to a perfect spatial overlap of the pump light with the gain medium. Nonetheless, it requires single-mode pump sources. To date, single-mode diodes at 9xxnm only provide a maximum power of around 1 W, which relegates the implementation of core-pumped fiber amplifiers to low power systems. In this thesis, a fiber laser at 1018nm has been used to core-pump a single-mode and single-frequency EYDFA at 1556nm for the first time. The method is equivalent to an off-peak pumping scheme in which the pump wavelength is longer than the maximum absorption peak wavelength rather than shorter. Additionally, a photodarkening-like phenomenon was observed for the first time in an EYDFA at 1.5 μm. The core-pump concept with high pump power levels is a promising alternative for purely single-mode amplifiers up to tens of watts.