After tweaking the alignment into the pre-modecleaner, I measured the power noise spectra for both photodiodes PDA and PDB when either was used as the in-the-loop sensor. The plot PDA.png is for when PDA was used as the in-the-loop sensor. PDB.png is for when PDB is used as the in-the-loop sensor. When the ISS is off, in both cases the free-running power noise measurements agree quite well. However the in-the-loop and out-of-the-loop spectra disagree quite badly when PDB is used as the in-the-loop sensor. The in-the-loop noise suggests there's enough gain in the loop in this case with the level being ~1.0E-8. The out-of-the-loop spectrum is more than a factor of 10 higher, which suggests there's a difference in the two optical paths (most likely beam pointing). Not sure why at this stage. I would probably use PDA as the photodiode to be used as the loop sensor (for now).