Thermal loading and modal frequency degeneracy in optical resonators for the Laser Interferometer Gravitational-wave Observatory (LIGO)

High-power lasers increase gravitational-wave detection sensitivity in the shot-noise-limited range of interferometric systems such as LIGO, the Laser Interferometer Gravitational-wave Observatory. However, thermally induced distortion of mirrors from high circulating power degrades the performance of optical resonators. The maximum power resonating in cavities for the future upgrade to LIGO (Advanced LIGO) will reach 800 kW. At these power levels, low-loss optical coatings and substrates become susceptible to thermal distortions capable of degrading LIGO's performance. The ability to predict when thermal distortion will significantly impact a resonator's beam quality is critical for future generations of LIGO. This work presents experimental results of thermal loading in an optical resonator as well as simulations for predicting thermal performance of any interferometer configuration.;A Fabry-Perot ring cavity known as a modecleaner uses calibrated absorption loss to measure the effects of high circulating power similar to what will be observed in resonant systems for Advanced LIGO. An additional modecleaner with low absorption loss is also tested. Results show that power coupling from the fundamental mode to frequency-degenerate higher-order modes significantly degrades the resonant fundamental mode necessary for gravitational-wave detection. Models of thermal distortion including thermal lensing and thermoelastic surface deformation are used to simulate the behavior of various resonant interferometer configurations. Comparisons between modal frequency degeneracy data and simulations are found to be in reasonable agreement, allowing degeneracy predictions for LIGO resonators most susceptible to thermal loading. Finally, mitigation of thermal effects is discussed, as well as solutions for designing interferometers utilizing power levels greater than those of Advanced LIGO.