At the beginning of 2016 the first direct detection of a gravitational wave by a laser-based Michelson interferometer has been reported. So far, the number of confirmed detections has raised to three and will certainly increase further. Thereby, mankind has an amazing new tool to observe and study the universe, namely by gravitational wave astronomy. However, in order to develop their full potential a more sensitive generation of earth-bound gravitational wave detectors will be required in the future. Current detectors are operating with solid-state laser systems at 1064nm but higher output power levels and longer wavelengths will be required by next-generation detectors. The requirements with respect to the laser sources, e.g. on the beam quality and the linewidth, are very challenging and unique. In recent years, it has been demonstrated that single-frequency fiber amplifiers are a very attractive concept as next-generation gravitational wave detectors laser sources. This paper reviews, highlights, and summarizes the corresponding research results, in particular regarding power scaling, noise properties, and coherent beam combination.

We report on the design and fabrication of an Er³⁺:Yb³⁺ triple clad fiber and on the power scaling of a single frequency fiber amplifier at 1.5 μm based on that fiber. In addition, we report on mode content measurements in order to reveal the overlap of the amplifier output with the TEM₀₀ mode. The triple clad design was used to enable high output power levels, a good slope efficiency and an excellent beam quality. A maximum single frequency output power of 61 W at 1.5 μm could be achieved with the aid of the co-seeding method, which was used to suppress parasitic processes at 1.0 μm. With a scanning ring cavity the mode content of the amplifier output was analyzed with respect to the TEM modes. For all output power levels the TEM₀₀ content was above 90 %.

Understanding the gain dynamics of fiber amplifiers is essential for the implementation and active stabilization of low noise amplifiers or for coherent beam combining schemes. The gain dynamics of purely Er³⁺ or Yb³⁺ doped fiber amplifiers are well studied, whereas no analysis for co-doped systems, especially for Er³⁺:Yb³⁺ co-doped fiber amplifiers has been performed, so far. Here, we analyze for the first time the gain dynamics of Er³⁺:Yb³⁺ co-doped fiber amplifiers theoretically and experimentally. It is shown that due to the energy transfer between the Yb³⁺ and Er³⁺ ions a full analytical solution is not possible. Thus, we used numerical simulations to gain further insights. Comparison of experimental and numerical results shows good qualitative agreement. In addition, we were able to determine the Yb³⁺-Er³⁺ transfer function of the energy transfer experimentally.

Integrated fiber components enable compact and robust laser systems but are usually not available with specialty fibers. However, specialty fibers allow power scaling of fiber lasers and amplifiers with all advantages of these fiber designs. In this context, chirally-coupled-core fibers show promising properties for optical fiber components. In this paper, we present the development of a highly-integrated signal and pump combiner in chirally-core-fibers using a side-pumping technology. Combining up to four fiber-coupled pump diodes, a pump-light limited power handling of 600 W can be achieved with an efficiency of 78%. The combiner was tested in a side-pumped single-frequency all-fiber amplifier but can also be implemented in almost any fiber laser or amplifier.

Long-period fiber gratings (LPGs) are well known for their sensitivity to external influences, which make them interesting for a large number of sensing applications. For these applications, fibers with a high numerical aperture (i.e., fibers with highly germanium (Ge)-doped fused silica fiber cores) are more attractive since they are intrinsically photosensitive, as well as less sensitive to bend- and microbend-induced light attenuations. In this work, we introduce a novel method to inscribe LPGs into highly Ge-doped, single-mode fibers. By tapering the optical fiber, and thus, tailoring the effective indices of the core and cladding modes, for the first time, an LPG was inscribed into such fibers using the amplitude mask technique and a KrF excimer laser. Based on this novel method, sensitive LPG-based fiber optic sensors only a few millimeters in length can be incorporated in bend-insensitive fibers for use in various monitoring applications. Moreover, by applying the described inscription method, the LPG spectrum can be influenced and tailored according to the specific demands of a particular application.

SiO₂ fibers doped with rare-earth-doped nanocrystals are promising to overcome the strong SiO₂ multi-phonon quenching and could yield novel laser gain materials. So far, no attention has been paid to the question how well the nanocrystals can suppress the quenching depending on the properties of the SiO₂ host glass. Here, a novel analytical model was applied to study the impact of the glass purity and composition on the quenching efficiency. Only a few experimentally accessible material and design parameters are required to compute the average quenching rate inside the nanocrystals. It is demonstrated that sufficiently low levels of quenching can only be expected for SiO₂ free of impurities or dopants that increase the multi-phonon absorption. This indicates that high-purity aluminosilicate glasses, in contrast to phosphosilicate and borosilicate glasses, are ideal hosts.