Over the weekend I ran a transfer function on the BS with optical lever damping filters off, and FM2 of both L2P and L2Y filters on. These two filters were installed last week (alog 14046 and alog 14022). This transfer function was taken to determine the residual motion of the BS in yaw and pit given a drive in length on the M2 stage. We can compare this to the previous rediula motion with FM1 filters on for both L2Y and L2P as measured by Stefan alog 9394. The attach screen shot shows a side by side comparion. The red traces are all YAW/L and the blue traces are all PIT/L. And the left most windows, or the traces with square symbols are the TF with FM2 (current version), while the right most windows, or the traces with the circle symbols are the TF with FM1 on (old version). The L1Y coupling improved by about 16dB at low frequency, and 24dB at high frequency. Meanwhile, in L2P the high frequency magnitude is about the same (it's actually slightly worse by about 2dB), but at low frequency we have imporoved by a factor of about 19dB. The new filter for L2P was intended to mostly correct the low frequency motion, so this result is about as expected. Also, in the process of creating the new filters, we did not concern ourselves with any of the resonances, which is why you don't see much of a change (and it may be a bit worse) compared to the older values. Jeff and I also checked that the BS coil balancing did not change between Stefan's measurement and mine, indicating that the improvement clearly came from the coupling filter change. I was unsure why the phase between the new and old version are no longer the same, given the that old and new filters have the same phase. Kiwamu explained to me that this came from whether we were pushing too much or too little to compensate for the L2Y/P coupling. The phase can change by 180 when we adjust for this.
Next, I plan on inspecting the power spectrum of the BS oplev with the PRMI locked on the sideband. One question we have though, is what constitutes good enough residual motion?
To confirm our results I also repeated the measurements taken by Rana in alog 14028. The first screenshot shows the BS oplev pit and yaw power spectrum under a few different configurations. The dashed green and brown traces are spectra taken by Rana with PRMI locked and the older L2Y/P coupling filters. Meanwhile, the pink and light blue traces are the spectra with PRMI locked and the new coupling filters. We see an imporovement in the power spectrum with the new filters at about 0.1-0.3Hz in Yaw, and and 0.001-0.8Hz in Pit. The red and blue traces are the spectra with PRMI unlocked and the new filters. The second screenshot shows the coherence of the oplevs with the MICH loop. The trace color representation is as before. One can see a reduction in the coherenece for both Pit and Yaw with the new filters in place.
Note: everything can be found under /ligo/home/alexan.staley/Public/BS_Coupling/
