The attachment shows the frequency control signal (IMC-F) and the transmitted intensity (IM4 trans) before and after turning on the HPO. 2 W into the modecleaner, with the interferometer unlocked.
Although Sheila's most recent DARM spectrum shows coherence with both frequency and intensity noise on the laser, I would hazard a guess that the frequency noise is not really the issue here, since it is only a factor of (at most) a few worse than before.
Keita, Evan
Here is a plot of transmission RIN spectra for for the front-end, the HPO, the refcav, the ISS, and the IMC.
Most of the intensity noise transmitted through the IMC between 10 and 100 Hz is coherent with the HPO spectrum, not with the front-end spectrum. The front-end spectrum is also essentially unchanged from before the HPO turn-on (this is not shown in the attachment).
The HPO performance is only slightly worse than what was reported in the HPO paper (Winklemann et al. 2011, fig. 12b). However, this RIN is only reduced by a factor of 10 by the time it gets through the IMC.
The ISS RIN spectra are as expected (see, e.g., Kwee et al. 2011, fig. 7), but these signals seem to be completely unrelated to the intensity noise that is actually transmitted to the downstream parts of the PSL and ISC systems. The out-of-loop diode reports a RIN that is better than 10–6/Hz1/2, but obviously this isn't true for either the refcav or IMC singals. Note in particular the lack of coherence between the out-of-loop sensor and the IMC transmission. (These spectra use Keita's new antiwhitening filters.)
The refcav transmission spectrum is more or less identical to (and coherent with) the IMC transmission spectrum.