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Message ID: 2094     Entry time: Thu Oct 15 01:21:31 2009
Author: rana 
Type: Summary 
Category: COC 
Subject: Thermal Lensing in the ITM 

Thermal lensing formula:


from (T090018 by A. Abramovici (which references another doc).

In the above equation:

w        1/e^2 beam radius

k        thermal conductivity (not the wave vector) = 1.3 W / m/ K

alpha    absorption coefficient (~10 ppm/cm for our glass)

NP       power in the glass (alpha*NP = absorbed power)

dn/dT    index of refraction change per deg  (12 ppm/K)

d        mirror thickness (25 mm for all of our SOS)

I'm attaching a plot showing the focal length as a function of recycling cavity power for both our current MOS and future SOS designs.

I've assumed a 10 ppm/cm absorption here. It may actually be less for our current ITMs which are made of Heraeus low absorption glass - our new ITMs are Corning 7980-A (measured to have an absorption of 13 ppm/cm ala the iLIGO COC FDD). I expect that our thermal lens focal length will always be longer than 1 km and so I guess this isn't an issue.

Attachment 2: aa.png  32 kB  Uploaded Thu Oct 15 02:53:38 2009  | Hide | Hide all
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