Rana suggested including some additional terms to the cost function to penalize high sensitivity to deviations in the layer thickness (L). So the list of terms contributing to the cost function now reads:
- Thermal noise - we use the proxy function from E0900068-v3 to do this
- Deviation from target T @1064nm, p-pol
- Deviation from target T @532nm, p and s-pol
- HR Surface field
- The ratio
 with dL/L = 1%, evaluated at 1064nm p-pol and 532nm p and s-pol (only the latter two for the AR side)
I did not include other sensitivity terms, like sensitivity to the refractive index values for the low and high index materials (which are just taken from GWINC).
There is still some arbitrariness in how I chose to weight the relative contributions to the cost function, but after some playing around, I think I have a solution that I think will work. Here are the spectral reflectivity and layer thickness plots for the HR and AR sides respectively.
HR side: for a 1% increase in the thickness of all layers, the transmission changes by 5% @ 1064nm p-pol and 0.5% @ 532nm s and p-pol
 
AR side: for a 1% change in the thickness of all layers, the transmission changes by <0.5% @ 532nm s and p-pol
  (substrate to the right of layer 38)
I've also checked that we need 19 layer pairs to meet the spec requirements, running the code with fewer layer pairs leads to (in particular) large deviations from the target value of 50ppm @ 1064nm p-pol.
Do these look reasonable?
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