Optical cavity reaching 10⁻¹⁶ frequency stability with compact optical circulator based in-coupling optics

Future space missions will benefit from highly stable and compact optical frequency references. While many promising technologies are currently under investigation, optical cavities are a well-suited technique for applications in which relative references are required. To improve the frequency stability of optical cavities, a key step in combining high performance with compactness and robustness is the further development of in-coupling optics. Here, we present our work of using a fiber-coupled circulator based in-coupling for a high-finesse optical cavity. Implementing the new, to the best of our knowledge, in-coupling board to an extensively characterized crossed cavity set-up allows us to identify possible differences to the commonly used free-beam technique. With a frequency stability of 5.5 × 10⁻¹⁶ Hz⁻¹/² at 1 Hz and with only a slight degradation in frequency stability below the mHz range, no circulator-caused instabilities were observed.