Low-noise, single-frequency 200 W fiber amplifier

Gravitational wave detectors require single-frequency laser sources with challenging properties regarding beam quality, polarization, and noise properties. We developed a single-frequency fiber amplifier engineering prototype based on standard step-index polarization maintaining fibers and characterized the 200 W output beam with the complete set of measurements necessary to evaluate the system's performance with respect to the application requirements. The output beam has a TEM₀₀-mode content of 94:8% at 200W and a polarization extinction ratio of 19 dB. In the crucial frequency range from 1 Hz to 100 kHz the frequency noise, relative power noise, and relative pointing noise measurements demonstrated low noise properties. In addition, the pointing noise below 100 Hz is the lowest reported for single-frequency amplifiers with 200W output power. SBS-free operation was demonstrated by monitoring the relative power noise in the MHz frequency range. The system was operated above 200W for 695 h and evaluated again after 650 h of operation. No signs of photodarkening were found.