Modal degeneracy in thermally loaded optical resonators

We characterize thermally-induced effects in an optical resonator illuminated with high-power laser radiation. From thermoelastic deformation of a mirror’s surface, we observe degeneracy of higher-order spatial modes with the fundamental mode that leads to power transmission limits and thermally-induced power fluctuations using a ring resonator with calibrated absorption loss. A model used to predict the thermal distortion shows reasonable agreement with experiments. Predictions for two future upgrades to the Laser Interferometer Gravitational-wave Observatory (LIGO) show that coating absorption should be less than 3.8 ppm and 0.44 ppm for Enhanced and Advanced LIGO Fabry-Perot cavities, respectively, to avoid the first higher-order modal degeneracy. c © 2007 Optical Society of America

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