I used the XARM as a reference to measure the frequency noise after the MC. It's huge around 4kHz--hundreds of times larger than the frequency noise the MC servo is actually squashing. This presents a real problem for our noise performance.
An elog search reveals that this noise has been present (although not calibrated till now) for years. We're not sure what's causing it, but suspicion falls on the piezojena input PZTs.
I didn't bother too much about it before because we previously had enough common mode servo oomph to squash it below other DARM noises, and I didn't worry too much about stuff at 4kHz.. Now that we have a weaker FSS and thus much weaker CM servo, we can't squash it, and the most interesting feature of our IFO is at 4kHz.
I'll measure the actual voltage noise going to the PZTs. I remember doing this before and concluding it was ok, but can't find an elog entry. So this time maybe I'll do it right.
This level of frequency noise has not changed, but we now have increased common mode servo gain and so it's not as huge of a deal, although we should still probably do something about it.
Attached is a plot of the piezojena noise measurement, estimated into Hz, along with another measurement of frequency noise as described above.
To get the piezojena voltage noise into Hz, I estimated the PZTs within have a flat 2 micron/V response (based on a rough knowledge of their geometry and assuming a 10 milliradian / 150V steering range). This is the voltage noise with the PZTs operating in closed loop mode, which is how we normally run them. This plot also ignores the transfer function of the Pomona box, as we are mainly looking at noise in the kHz band. I think this plot shows that these PZTs are a good candidate for creating this frequency noise, especially near their mechanical resonances (the manual says the unloaded resonances are in the 3-4kHz range).
I've been operating one DOF of the piezojenas in open loop mode for a couple of weeks now, and the feared drift has not been a problem at all. If we plan to keep using these after the upgrade, we should definitely put some big resistors in series at the outputs and operate them in open loop mode.
Also attached is a plot of RF DARM noise, with a frequency noise spectrum. That spectrum is a REFL 2I spectrum put into DARM units using a measured TF (driving MC_L and measuring REFL 2I and DARM_ERR), and then put into meters using the same DARM calibration as used for the DARM curve.