|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.