Impact of LIGO O5 Test Mass Coating and FROSTI Actuations on Interferometer Performance

The current thermal compensation system of LIGO, designed to mitigate thermal aberrations on the test masses resulting from high laser power coating absorption, requires further development to better achieve the sensitivity goals for future upgrades. Under the effects of non-uniform thermal absorption such as point defects, higher-order laser modes scattered from imperfections in the optics become near co-resonant with the fundamental Gaussian beam, particularly the 7-th order modes, exacerbating interferometer losses and degrading the sensitivity. FROSTI (Front Surface Type Irradiator) can break this modal degeneracy by actively shifting the higher-order mode resonances while having a negligible impact on the fundamental mode. We focus on quantifying the potential impact of the proposed FROSTI actuations along with the effects of new mirror coatings for LIGO A+ to help achieve its sensitivity goal.