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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: 1484     Entry time: Mon Aug 25 03:56:17 2014     Reply to this: 1486
Author: tara 
Type: HowTo 
Category: NoiseBudget 
Subject: optimization for ETM with a-Si/SiO2 coatings  

 I used optimization codes for ETM. The optimization reduce the PSD of Brownian noise by ~ 3/4 (in units of [m^2/Hz]) from QWL structure.

 Since we have not had all the material parameters for aSi:H at 120K with 1550nm, the optimization here is for room temperature with 1550 nm (for Brownian noise only). 

 opt2_aSi.png

opt2_RT.png

fig1: optical thickness for ETM with minimized BR noise. The transmission is 5.4 ppm and the reflected phase is ~ 179 degree.

Parameters/configuration used in the optimization:

  • T = 300 K   (room temp)
  • wavelength = 1550 nm;
  • Si substrate, n = 3.5;
  • Low index material : fused silica, loss = 0.4e-4, n = 1.444;
  • High index material: aSi:H, loss = 1e-6, n = 3.48; 
  • The coating has SiO2 cap (air-coating surface) for protection
  • Spot radius = 6 cm.
  •  This optimization is only for Brownian noise, we can do another optimization once the thermo-optical properties are known (thermal expansion, dn/dT)

It is remarkable that 5ppm transmission can be achieved with just 17 layers of coatings due to the largely different values between nL and nH. This makes the total thickness down to ~ 3 um.

BR noise from the optimized coating is  3.3x 10^-42 [m^2/Hz] at 100 Hz. This is converted to the strain of ~ 5x10^-25 [1/sqrt Hz] for 4 km interferometer. 

Note: for QWL structure, with 14 layers + half wave cap of SiO2 (total of 15 layers), the transmission is ~5.2 ppm and the coating Brownian noise is 4.2x10^-42 [m^2 /Hz]. So the optimization reduced the PSD of BR noise by ~ 25%. 

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