Highly efficient generation of coherent light at 2128 nm via degenerate optical-parametric oscillation.

Cryogenic operation, in conjunction with new test-mass materials, promises to reduce the sensitivity limitations from thermal noise in gravitational-wave detectors. Currently, the most advanced materials under discussion are crystalline silicon as a substrate with amorphous silicon-based coatings. However, they require operational wavelengths around 2 µm to avoid laser absorption. Here we present a light source at 2128 nm based on a degenerate optical parametric oscillator to convert light from a 1064 nm nonplanar ring-oscillator. We achieve an external conversion efficiency of (87.1±0.4)% at a pump power of 52 mW in periodically poled potassium titanyl phosphate (internal efficiency was 93%). With our approach, light from the established and existing laser sources can be efficiently converted to the 2 µm regime while retaining the excellent stability properties.