Monolithic fiber amplifiers for high power single-frequency and single-mode laser beam generation

The next generation of ground-based gravitational wave detectors requires several hundred watts of optical power to increase the detector sensitivity. In addition, high demands are placed on the beam quality and noise properties of the laser beam. Optical amplifiers based on ytterbium-doped optical fibers are being researched as a promising technology for power scaling. However, existing laser systems cannot meet the requirements on the beam properties, the long-term stability and the output power. In this thesis, a monolithic fiber amplifier concept has been developed for use in gravitational wave detectors. These amplifiers rely on fiber components for fiber-based pump and signal coupling to the gain fiber, which enables alignment-free and low-maintenance operation. Excellent beam characteristics at the 200 W optical power level are demonstrated and conclusively prove for the first time that fiber amplifiers can produce a high power beam that is in accordance with the requirements for the free-running laser system for gravitational wave detectors. The generated 200 W laser beam shows a very high TEM${}_{00}$-mode content of up to 96 %, a high linear polarization extinction ratio of 19 dB, and low fluctuations of power, beam pointing, and frequency in the frequency range of 1 Hz – 100 kHz. Several amplifier systems are fabricated within defined tolerances and reproducible optical properties of the output beam are demonstrated. Constant laser beam properties are confirmed by long-term operation (approximately 700 h and 4150 h) at 200 W and by subsequent characterization. Using two of these fiber amplifiers, an optical output power of 400 W is generated via coherent beam combination with optical properties comparable to the individual 200 W beams. The achieved output power represents the current power record with noise properties being in line with gravitational wave detector requirements. The central components of these monolithic fiber amplifiers are fiber optic components, which enable fiber-based coupling of optical signals into or out of the fiber. Driven by the fiber amplifier development, a novel process is developed that enables fiber optic component manufacturing using CO${}_2$-laser radiation. The laser beam is used for the micromachining of optical fibers to obtain lateral optical access to the concealed waveguiding structures of the fiber. Low-loss access to the fiber's pump cladding (<0.05 dB), access to the fiber core (<0.27 dB), and selective ablation for surface enhancement is demonstrated. The laser machining produced optical surfaces with roughness values of Ra<15 nm. Based on this process development, the research and fabrication of novel and low-loss (<0.13 dB) evanescent field-based fiber couplers for beam monitoring is shown.