  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.
