I compared our beat measurement with results from Numata2003 and TNI. They agree well. I'm quite certain that we reach Brownian thermal noise from coatings.

 To make sure that what we measure is real Coating Brownian noise (It could be something else, i.e thermal noise in the support, spacer , or optical bond), we should compare our result to previous measurements to make sure that the numbers agree.

 Numata etal and TNI reported coating thermal noise measurement from suspended cavities (no spacer). They adjusted loss in the coatings to fit the measurement.  Phi coatings as reported in Numata is 4e-4 while TNI gives phi perp = phi_para = 2.7e-4.  Both agree with our result, see the plot below.  This means that our result is comparable with what they measured. It should be an evidence to support that we see real coating thermal noise, not contribution from something else (spacer, optical bond between the mirrors and the spacer).

Another evidence is from our previous measurement from 8" cavity.

• The measurement also agrees with Numata's 2003 result, with phi coatings = 4e-4, see PSL:1018.
• And the signal scales correctly with a factor of ~ 9 (from shorter cavity, and from smaller spotsize^2), seeT1200057. Had it been noise from optical bonding/ spacer (independent from spotsize), the scale factor would have been 8/1.45 ~ 5.5. The scale from substrate Brownian will also be different because of 1/w_spot dependent. Thermoelastic/ thermoopitc will have different slope.

So It is clear that our beat measurements from both 8" and 1.45" cavities are coating Brownian noise limited (around 50Hz-1kHz).