Conversion of 30 W laser light at 1064 nm to 20 W at 2128 nm and comparison of relative power noise

All current gravitational wave (GW) observatories operate with Nd:YAG lasers with a wavelength of 1064 nm. The sensitivity of future GW observatories could benefit significantly from changing the laser wavelength to approximately 2 µm combined with exchanging the current room temperature test mass mirrors with cryogenically cooled crystalline silicon test masses with mirror coatings from amorphous silicon and amorphous silicon nitride layers. Laser light of the order of ten watts with a low relative power noise (RPN) would be required. Here we use a laboratory-built degenerate optical parametric oscillator to convert the light from a high-power Nd:YAG laser to 2128 nm. With an input power of 30 W, we achieve an output power of 20 W, which corresponds to an external conversion efficiency of approximately 67%. We find that the RPN spectrum marginally increases during the wavelength conversion process. Our result is an important step in the development of low-noise light around 2 µm based on existing low-noise Nd:YAG lasers.