I saved the traces of old and new noise budgets as txt files and then plotted them together to understand the differences between the codes. One can refer to CTN:2250 for a summary of calculations of old noise budget.

Coating Brownian Noise:

• Old noise budget uses a simple formula from Harry et al. (2001) Eq 21.
• New noise budget uses analysis of Hong et al . PRD 87, 082001 (2013) which takes into account different bulk and shear loss angles for each layer.
• In this run, I used the same values of bulk and shear loss angles of both AlGaAs and GaAs, equal to 2.41e-5 +/- 0.482e-5 rad. This is the same value used in old calculations as well.
• It is interesting to see that there is almost no difference (<5% mostly) in the prediction of two methods even though they were done with vastly different methods.
• In the future, however, I would try to use different values. But there is no proper source for them. I can try using the coating loss angle values mentioned in Penn et al JOSA B Vol 36, Issue-4 (2019).

Coating Thermo-optic Noise:

• Both new and old methods use the same formula from  Evans et al. (2008), Eq 4.
• But to calculate the effective coefficient of thermo-refractive effect, the old method used a complex matrix calculation which I'm unable to follow. The value was 8.61e-5 1/K.
• The new noise budget follows Evans et al. (2008) Appendix B to the best of my knowledge and generates a slightly different value of  8.2e-5 1/K.
• However this changes the coating thermo-optic contribution by almost 50%!
• IF ANYONE IS WILLING TO LOOK INTO THIS DISCREPANCY FOR A SANITY CHECK, I'LL BE REALLY HAPPY. CONTACT ME FOR MORE SPECIFICS.

Photothermal Noise:

There is no distinction in the calculation of this contribution after the changes made in CTN:2388, but there is still a difference (which reaches to around 20% above 1 kHz). This is maybe because of interpolation errors when taking the differences. But I'm not sure about this. This can take a sanity check from others as well.

No differences in:

The following calculations had no differences between the two noise budgets and the contribution differences are also none or minimal. 

• Substrate Thermo-elastic Noise
• Substrate Brownian Noise
• Seismic Noise
• PDH Shot Noise
• PLL Oscillation Noise
• PLL Readout Noise
• Residual NPRO Noise