Operational optimization of a fully-crystalline optical cavity

Optical cavities with crystalline optical coatings offer reduced thermal noise and lower drift rates, but must be operated at low intracavity power due to power-dependent coating noise.[1] We use a modified PDH scheme where an AOM generates an FM-triplet.[2] This allows cavity stabilization to a sub-ppm fraction of the cavity linewidth, while coupling <100nW of light into the cavity. Precision cryogenic thermometry stabilises the cavity at a zero-crossing of the thermal-expansion coefficient of silicon near 16K, suppressing temperature driven fluctuations. Comparison with a Sr clock [3] allows characterization of the long-term stability of the cavity, and has previously shown drift at the level of a few Hz per day. Improvements in cavity performance suggest that optical local oscillators could replace traditional hydrogen masers where long-term stability is required, as in timescale applications.[4]



[1] Kedar et al., Optica 10, 464-470 (2023); Yu et al., Phys. Rev. X 13, 041002 (2023)

[2] Kedar et al., Optica 11, 58-63 (2024)

[3] Bothwell et al., Metrologia 56, 065004 (2019);

[4] Milner et al., Phys. Rev. Lett. 123, 173201 (2019).