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Thu May 8 02:06:13 2008
Thermal Lensing in the ITMs and BS may be a problem
The iLIGO interferometers start to see thermal lensing effects with ~2W into the MC, a recycling
gain of ~50, and a beam waist on the ITMs of ~3.5 cm.
At the 40m, the laser power into the MC is 1/2 as much, the recycling gain is 4-5x less, but the
beam on the ITM has a 3 mm waist. So the power in the ITM bulk is 10x less but
the power density
is 100x more
. Seems like the induced lens in the ITM bulk might be larger and that if there's
significant absorption on the ITM face (remember our Finesse is 4-5x higher) the beam size in the
arm cavity may also change enough to measure.
Someone (like Andrey) should calculate how much the beam sizes change with absorbed power.