I looked into the uncertainty in coating thickness of the QWL SiO2/Ta2O5 coating The thickness of  4.53 +/- 0.07 um (~1.5%)seems to be appropriate.

The thermal noise level is directly proportional to the coating thickness, so we want to estimate its uncertainty. The error in the thickness is from

• The uncertainties in nL and nH: since the physical thickness is lambda/(4*n), the error in n goes to the error in d.
• Manufacturing process.

The errors in nL and nH are quite small, nL ~ 1.45 +/ 0.01, nH ~ 2.06+/- 0.01. (From the literature). I also looked around the error in IBS thickness control, they are usually better than 0.1 nm, IBS, but that is the current technology. In literature around 2000s, 2% error seems to be the number estimated for the thickness control (Sullivan 2000, Badoil 2007). As a quick check, I used the same assumption for error propagation similar to that of AlGaAs coating. The result gives ~ 4.53 +/- 0.07 um for coating thickness.

Note that the error here is smaller than the difference in coating thickness for the coatings with or without half wave cap.

For 28 Layer (with cap), the coating thickness is 4.53 um,  for 28 layer QWL, the coating thickness is 4.35 um. But after digging up all the information from REO, and peter king they agree that it is 28 QWL with half wave cap.  I tried to compare the calculation and the photothermal TF measurement, but the effect is too small to be conclusive about the structure. So the biggest error might come from the fact that the coating has cap or not. The error is about 4%.