Some permanent calibration lines on the mirrors around 55 Hz and around 355 Hz allow to monitor the arm cavity pole in Hrec

(https://wiki.virgo-gw.eu/Commissioning/Calibration/HistoryOfCalibrationLineFrequencies). 

These lines allow to derive the ITF optical gain. In particular, the OG as extracted from the BS line is proportional to the OG as extracted from the ETM lines though the arm cavity Finesse: G_BS = G_ITF/(2F/pi)

( see https://arxiv.org/pdf/1401.6066.pdf for the details). 

Using the data from ER11b over 1 day (start GPS = 1182352218), an hystogram of the Finesse has been made for the North (figure 1) and West (figure 2) arm. The Finesse distribution as extracted from the 55 Hz frequency line is shown in the first subplot, while the distribution as extracted from the 358 Hz line is shown in the second subplot. The values as extracted from the two lines is slightly different (order of 1-2%), but the main advantage of using this method is that it allows to monitor the Finesse variation in time, more than the absolute value of the Finesse itself.

More analysis and refinements will follow.