Finesse evolution in a bichromatic cavity under blue and red resonant illumination in vacuum
The long-term performance of high-finesse optical cavities depends on the stability of their mirror coatings under vacuum and under continuous exposure to high circulating power. In cavities supporting multiple wavelengths, a central question is whether degradation at one wavelength compromises performance at another. Understanding this interplay is essential for those cavity quantum electrodynamics experiments that need diverse optical fields for cavity stabilization, trapping, and light--matter coupling. Here, we investigate wavelength-dependent degradation and recovery of a bichromatic cavity with high-reflectivity coating at 455 nm and 671 nm in vacuum. Cavity ring-down measurements reveal a decrease in the 455 nm finesse from 7.7*10^3 to 1.9*10^3 after 11 hours of continuous exposure to high circulating power. Simultaneous measurements show no measurable degradation at 671 nm. Continuous operation at 671 nm, instead, produces no measurable degradation at either wavelength under the tested conditions. Comparison of oxygen depletion and Avrami-type loss models indicates kinetics compatible with nucleation and growth, without uniquely identifying the microscopic mechanism. We demonstrate partial recovery of the finesse with illumination at 455 nm in air. Our results indicate a wavelength dependence of laser-induced optical damage in bichromatic cavities and provide an estimate of the blue light intensity that can be safely used in such cavities.