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Entry  Mon Aug 25 03:56:17 2014, tara, HowTo, NoiseBudget, optimization for ETM with a-Si/SiO2 coatings  opt2_aSi.pngopt2_RT.png
    Reply  Wed Aug 27 03:21:53 2014, rana, Summary, optic, optimization for ETM with a-Si/SiO2 coatings  aSi_120_Layers_60000.pdfaSi_R_60000.pdfaSi_120_TOnoise_60000.pdf
Message ID: 1486     Entry time: Wed Aug 27 03:21:53 2014     In reply to: 1484
Author: rana 
Type: Summary 
Category: optic 
Subject: optimization for ETM with a-Si/SiO2 coatings  

 I filled in more values for a-Si at 120 K into the wiki that Matt Abernathy set up. Then I ran the optimization code for Brownian noise only:

 aSi_120_Layers_60000.pdf

The above plot shows the comparison between the optimized aLIGO coating (silica:tantala at 300K) v. the a-Si coating at 120 K.

 aSi_R_60000.pdf

Then, finally, I compared the TO and Brownian noise of the two designs using the plotTO120.m script:

 aSi_120_TOnoise_60000.pdf

The dashed curves are silica:tantala and the solid lines are a-Si:silica. The Brownian noise improvement is a factor of ~6. A factor of ~1.6 comes from the temperature and the remaining factor of ~3.9 comes from the low loss and the lower number of layers.

I think this is not yet the global optimum, but just what I got with a couple hours of fmincon. On the next iteration, we should make sure that we minimize the sensitvity to coating thickness variations. As it turns out, there was no need to do the thermo optic cancellation since the thermo-elastic is so low and the thermo-refractive is below the Brownian almost at all frequencies.

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