Finesse evolution in a bichromatic cavity under blue and red resonant illumination in vacuum

Abstract

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.

This paper tests whether illumination-induced loss in one reflection band of a bichromatic optical cavity affects its other band, and whether red illumination damages blue-band performance. A vacuum Fabry–Pérot cavity with $\mathrm{SiO_2}/\mathrm{Ta_2O_5}$ coatings supports resonances at 455 and 671 nm. Finesse is tracked by cavity ring-down during prolonged resonant illumination and subsequent recovery in air. Under 455 nm illumination, the blue finesse falls from $(7.7\pm0.7)\times10^3$ to $(1.9\pm0.2)\times10^3$ after 11 hours of accumulated illumination. A second run monitoring both wavelengths shows blue finesse decreasing from $(4.82\pm0.07)\times10^3$ to $(2.4\pm0.2)\times10^3$ over 9.3 hours, while red finesse remains near $1.6\times10^4$. No measurable degradation occurs during intervals without circulating light. Continuous 671 nm illumination for 11.3 and 13.5 hours produces no measurable degradation at either wavelength. Blue illumination in air partially restores blue finesse, from approximately $2.0\times10^3$ to $2.8\times10^3$ over 20.4 hours, without a measurable change at 671 nm. An Avrami-type loss model with fixed exponent $n=3$ describes blue degradation better than the tested first-order oxygen-depletion model. This is compatible with nucleation-and-growth kinetics but does not identify the microscopic mechanism; hydrocarbon contamination is suggested rather than established. Recovery kinetics are likewise phenomenological. Stability conclusions apply to the tested coating and illumination conditions. The proposed blue operating intensity of roughly $5\,\mathrm{kW/cm^2}$ is inferred from the degraded steady state rather than established by a systematic damage-threshold measurement.