30 W ultra-stable laser light at 2128 nm for future gravitational-wave observatories

Thermal noise of the dielectric mirror coatings can limit laser-optical high-precision measurements. Coatings made of amorphous silicon and silicon nitride could provide a remedy for both gravitational-wave detectors and optical clocks. However, the absorption spectra of these materials require laser wavelengths around 2 $\mu$m. For GW detectors, ultra-stable laser light of tens or hundreds of watts is needed. Here, we report the production of nearly 30 W of ultra-stable laser light at 2128 nm by frequency conversion of 1064 nm light from a master oscillator power amplifier system. We achieve an external conversion efficiency of (67.5 $\pm$ 0.5) % via optical parametric oscillation and a relative power noise in the range of $10^{-6}$/$\sqrt{\text{Hz}}$ at 100 Hz, which is almost as low as that of the input light and underlines the potential of our approach.