In order to figure out what is going on with the IMC cavity pole, I made a comparison between a recent and past IMC open-loop transfer functions.

If the cavity pole really dropped to 7.7 kHz from 8.8 kHz, this would cause a measurable change in the open-loop transfer function. According to the comparison, I did not see a large change. This result supports the hypothesis that the IMC cavity pole stayed (almost) the same.

The plot below shows the IMC open-loop transfer functions: one is taken yesterday shown in in blue, and the other was measured on February 7th with the same servo board configuration but with a higher laser power.

As shown in the plot, they look similar to each other except for the gains. The difference in the gains must be due a combination of the different laser power and non-carefully adjusted gain on the IMC servo board. In addition, there is a large discrepancy at high frequencies above 80 kHz, but this is pretty much out of our interested frequency band. So we don't care about it.

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If the cavity pole really dropped from 8.8 to 7.7 kHz, this would decrease the response above 7.7 kHz by about 12 % in amplitude and also give an extra retardation in the phase by 3-4 degrees at around 8 kHz. In order to check if there is such change between the two transfer functions, I took a ratio of the two TFs -- the plot below shows a transfer function of (recent TF) / (past TF).

Even though the data does not look super smooth, we can already see that there is no 12% drop above 8 kHz in the amplitude or no 3-4 degrees retardation in its phase at around 8 kHz. It rather looks as if the IMC stayed almost the same since February. Also I am attaching a matalb fig file of the same plot in case somebody wants to play with it.