In the last weeks the IMC cavity losses had been evaluated as a function of the position of the beam on the terminal mirror, see logbook entry 31641. The measurement had been performed by injecting a white power noise at the pumping diodes level ,thus on the input beam power, and evaluating the frequency of the IMC pole.

From this measurement is then possible to evaluate the finesse of the cavity and the losses.

We decided then to find a way to constantly measure online the frequency of the cavity pole in order to better study the influence of various parameters on the cavity pole.

To do that two excitation lines has then been added at  f1 = 127 and f2 = 2727 Hz allowing to compute the frequency of the pole as:

where TF|f_i is the Transfer Function between the IMC input power and the IMC transmitted power at the f_i frequency.

The signal computation has been implemented in the BC DSP and it runs online with the name Bs_IMCpole.

The reliability of the signal has been evaluated by reconstructing the pole offline (making TFs with matlab for time intervals of 100 sec) and comparing with it the online data.

In Figure 1 the online pole signal and the reconstruction has been compared for 1 night data in which the beam on the IMC terminal mirror has been scanned.

The frequency of the IMC pole and then the losses have been evaluated by scanning the mirror for an entire weekend, figure 2 and 3, where it is visible that the frequency of the pole and the losses increase approaching the outer sides of the mirror. Unfortunately for that weekend the computation has been done offline since a problem in the computation of the online pole frequency signal was present, figure 4. The correct computation of the pole has been implemented on the 7th of October at 16:16 UTC.

Moreover the behavior in time of IMC Tra can be reconstructed starting from the PSL TRA DC and the frequency of the IMC pole, to compute the cavity Finesse. The comparison between the IMC TRA measured and reconstructed is visible in figure 5.

Furthermore the accuracy in the evaluation of the cavity pole is improved using the online data with respect to the offline reconstruction, see figure 6 and 7 where the blue histograms are the online data and the red one are the reconstructed ones.

Concluding this logbook entry would like to highlight, more than the measurement results, the development of an online diagnostic tool to evaluate the frequency of the pole, then the losses of the cavity, during the IMC commissioning and beyond.