Because phase of MICH to DARM with engaging FF in klog#31704 is ~120deg. different from one in previous measurement (klog#31266), I tried to sort out of the situations to understand this difference.

Figure 1 shows the coupling from MICH to DARM. Though original transfer functions are from count at MICH feedback to DARM displacement, I converted them to from displacement equivalent value of MICH feedback to DARM displacement by using BS actuator response in klog#29974 for the easy understanding. Blue and green points represent TFs without and with FF measured in klog#31266, respectively. Blue curve seems to be well matched with theoretical coupling of differential phase at AS port coming from Michelson motion (I assumed arm Finesse is ~1400).

Red curve represents TF with FF measured today by Yokozawa-san. Green and red curves have ~120deg mismatch in phase around 50-100Hz. According to the FF gain, all MICHFF1 (MICH to DARM), MICHFF2 (MICH to PRCL), and PRCLFF1 (PRCL to DARM) were engaged in today's measurement (Fig.2). On the other hand, measurement in klog#31266 were done with engaging only MICHFF1 (Fig.3). So I thought that MICH to PRCL to DARM might be dominant in the past measurement. And then, I estimated TF of MICH to PRCL to DARM as a product of TF from MICH to PRCL and TF from PRCL to DARM in the case of the absence of MICHFF2 and PRCLFF1 (see also brown curve in Fig.1). But it couldn't explain the difference between the red and green curves. The performance of FF is enough for now. So it's not urgent issue. On the other hand, understanding one-by-one may help us to do the future activities such as optimization of FF, offline subtraction of residual of online FF, etc.

Transfer functions of MICH to PRCL which was used for estimating TF of MICH to PRCL to DARM above are shown in Fig.4. They were not measured today and only TFs measured in klog#31266 are shown in this plot. I also converted TF of count at MICH feedback to count at PRCL error point to one of displacement equivalent value of MICH feedback to displacement equivalent value of PRCL error signal by using the BS actuator response and PRCL optical gain, respectively. Strictly speaking, we should use K1:CAL-CS_PROC_PRCL_DISPLACEMENT instead of PRCL error signal for this purpose. But it should be enough because all measured data points with large coherence are larger enough than PRCL UGF. Blue and green points represent TFs without and with FF, respectively. When we actuate MICH length by BS, change in MICH length is coupled to PRCL lenght with a factor of 0.5. So the fact that gain of blue points (without MICH to PRCL FF) shows -6dB is reasonable.

TFs of PRCL to DARM are shown in Fig.5. Red curve is today's measurement with all 3 FFs by Yokozawa-san. Because the past measurement with a same configuration as green points are poor coherence, it's difficult to say there is significant difference between today's and past resutls. Blue points which was used to estimate TF of MICH to PRCL to DARM above represents the TF only with MICH to DARM FF. I have now no idea about what is the dominant pass of PRCL to DARM coupling. So I cannot show the any theoretical lines on this plot.