Sensitive diffraction-loss measurements of transverse modes of optical cavities by the decay-time method

A simple method is proposed to measure the diffraction losses for the first few modes of stable low-loss optical resonators now available with the state-of-the-art coatings. The optical decay time of each cavity mode is isolated, permitting the determination of the cavity’s diffraction losses. The sensitivity of the method, which might reach the parts-in-106 range, emphasizes the need for more detailed theoretical calculations for high-quality laser cavities, in which low diffraction losses can limit the performance.