Ultrahigh-finesse optical cavities

As the finesse of a Fabry-Perot optical cavity increases to about 20,000, the fringe width (approximately 10 kHz for a 50-cm cavity) is sufficiently narrow for transverse-mode splittings to be resolved by using a highly stabilized ring dye laser. A perturbative theory interprets this effect as a slight deviation of the cavity from cylindrical symmetry, the magnitude of the asymmetry being at the level of a few tenths of a nanometer. The understanding of these splittings will permit accurate optical frequency measurements by the recently proposed optical-radio-frequency divider.