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Entry  Mon Nov 12 22:56:10 2012, tara, Notes, NoiseBudget, Thermoelastic noise in spacer and substrate cavity_TE_v2.png
    Reply  Tue Nov 13 01:56:24 2012, tara, Notes, NoiseBudget, Thermoelastic noise in spacer and substrate 
       Reply  Wed Nov 14 00:25:48 2012, tara, Notes, NoiseBudget, ฺBrownian noise in spacer and substrate IMG_1975.jpg
       Reply  Wed Nov 14 19:46:47 2012, tara, Notes, NoiseBudget, Thermoelastic noise in spacer and substrate substrateTE_compare.pngTE_cavity_v2.mph
          Reply  Tue Nov 20 11:50:29 2012, rana, Notes, NoiseBudget, Thermoelastic noise in spacer and substrate 
             Reply  Wed Nov 21 20:16:55 2012, rana, Notes, NoiseBudget, Thermoelastic noise in spacer and substrate 
                Reply  Mon Nov 26 15:36:46 2012, rana, Notes, NoiseBudget, Thermoelastic noise in spacer and substrate 
                   Reply  Tue Nov 27 05:30:03 2012, rana, Notes, NoiseBudget, Thermoelastic noise in spacer and substrate 
          Reply  Tue Nov 27 05:38:01 2012, tara, Notes, NoiseBudget, Thermoelastic noise in spacer and substrate TE_Brownian_compare2.pngTE_Brownian_compare2.fig
Message ID: 1076     Entry time: Wed Nov 14 19:46:47 2012     In reply to: 1074     Reply to this: 1079   1083
Author: tara 
Type: Notes 
Category: NoiseBudget 
Subject: Thermoelastic noise in spacer and substrate 

Found the problem. My noise budget code was wrong. So after I fixed it, the TE noise in substrate result from COMSOL agrees pretty well with the analytical result (within 20%).

substrateTE_compare.png

The result from COMSOL is plotted in dashed-black line. The result from Cerdonio is plotted in dashed pink.  Since my simulation uses the adiabatic assumption (used in BGV and Liu&Thorne paper), the results agree at high frequency. So I think the calculation is correct. I'll check some options (changing spot size, changing material) to see if TE noise can be made lower for AlAs/GaAs samples.

I attached my COMSOL file below. This is done in 3D model. It could have been done in 2-D axis symmetric setting, but I used 3-D for spacer sagging before, so I just used the same geometry I had.

 Quote:

I realized that the mesh size was too large, even with the finest mesh for default setting. So I reduced the mesh size around the beam area and the results got closer to the analytical prediction. It is still a factor of 2 below the prediction. I'll see if I can hunt down this problem . I think it will be a good idea to verify my model by using my model to calculate Brownian noise and comparing with the result reported by Braunschwig group. 

When I defined mesh size in COMSOL, I used the predefined value provided by COMSOL. The finest mesh has maximum element size ~500 um, and minimum ~5um. Since the beam size is ~ 180 um, the maximum element size should be ~10 um. So I changed the values around, defined new area for smaller mesh until the results did not change much. I ran the simulation a few time to make sure that the solution converges. Right now my substrate has 3 regions

  1. a cylinder at the center where the beam hits the mirror, Radius = 2x180um, depth=2x180um ,   Min/Max mesh size = 12.6/27 um
  2. an outer region, radius = 5x180um, depth = 5x180 um, min/max mesh size = 10/50 um
  3. The rest of the substrate, fine mesh

I tried to change the mesh size/boundary size a bit to get the result accurate enough without taking too much time. The TE estimation still a factor of 2 below the analytical estimate.

 

 

Attachment 2: TE_cavity_v2.mph  7.844 MB  Uploaded Wed Nov 14 21:01:26 2012
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