Comparison of Frequency Stability Measurements of a Quad-bore Ultra-low Ex-pansion Cavity*

Precision spectroscopy benefits from identifying the dominant sources of noise affecting frequency-locked lasers. An ultra-stable Fabry-Perot with four bores coated for a wide range of wavelengths can be used to lock multiple lasers with correlated noise and frequency drift. However, lasers locked to the quad-bore cavity are still susceptible to frequency instabilities between cavity modes of separate bores. We compare a collection of methods that attempt to characterize the differential noise between lasers locked to the fundamental modes of separate cavity bores using the Pound-Drever-Hall method. We use an Allan variance analysis of this data and a comparison to an external frequency reference to distinguish and characterize the frequency drift of the cavity as well as the differential noise between the various cavity modes used for laser locking.