  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.
