Tonight we power up to 20 watts and were locked for 5 hours. I measured the CARM OLG (UGF ~ 15kHz) and error spectrum from 0.5 Hz to 5 MHz in response to Stefan and Hang looking for CARM saturations. (PDFs 1 and 2) They may have been correct to worry: the noise is worst at 100 kHz, a full factor of 10 above where it is at 8 kHz, with peaks a factor of 10 above that. I'll run some RMS calculations later, but high frequency definitely dominates this spectrum. We'll need to think about the saturation limits of the common mode board, and the kind of signals we are sending through it to control the laser. I also ran some bandlimited frequency noise injections with different DARM offsets. I found that as I reduced the DARM offset, the frequency noise coupling to DARM got worse. (PNG 1) This result was counterintuitive to me. It makes sense for the high frequency noise around 4.1 kHz which masks the OMC dither to cause the low frequency DARM noise to increase with reduced DARM offset, since there is less light in the OMC. But I would have expected the linear Freq to DARM coupling to decrease with reduced DARM offset. This prompted me to run a suite of injections which will be analyzed at a later date.
I am particularly puzzled by the scaling. Frequency noise as calculated by T1500461 contains terms that scale linearly with the DARM offset and terms that scale like the third power. The later seems to dominate below a few Hertz. The term that scales linearly would show up as a constant in the sensitivity plot. On the other hand, terms due to higher order modes in the PRC should look like a constant sensing noise, and therefore scale like the inverse of the DARM offset in the sensitivity plot. This scaling at lower frequencies seems roughly inverse if one compares the 4pm and 11.5pm offsets. However, it completely fails to explain the rather significant improvement from 11.5pm to 16pm.
At high frequencies the scaling looks more constant, but this coupling cannot be explained with what's in T1500461 and is thought to be due to higher modes.