  Cavity ring-down is an essential test to measure ultra-high quality factor
(UHQ) optical cavities, which is, however, frequently misinterpreted due to
lacking of a specified analysis guideline. Here we clarify the basic property
of cavity ring down and present a step-by-step method that enables extraction
of the overall quality factor, as well as the intrinsic loss and coupling state
of UHQ cavities with better fidelity and simplicity than prior schemes. Our
work can facilitate acurrate design and characterization of UHQ cavities for
ultra-low noise lasers, high finesse reference cavities, and ultra-narrow
optical filters.
