Audio-band Coating Thermal Noise Measurement for Advanced LIGO with a   Multi-mode Optical Resonator
  In modern high precision optical instruments, such as in gravitational wave detectors or frequency references, thermally induced fluctuations in the reflective coatings can be a limiting noise source. This noise, known as coating thermal noise, can be reduced by choosing materials with low mechanical loss. Examination of new materials becomes a necessity in order to further minimize the coating thermal noise and thus improve sensitivity of next generation instruments. We present a novel approach to directly measure coating thermal noise using a high finesse folded cavity in which multiple Hermite-Gaussian modes co-resonate. This method is used to probe surface fluctuations on the order 10^-17 m\rtHz in the frequency range 30-400 Hz. We applied this technique to measure thermal noise and loss angle of the coating used in Advanced LIGO. 
S. Gras, H. Yu, W. Yam, D. Martynov, and M. Evans
physics.ins-det astro-ph.IM physics.optics