The measurement of the reflectivities of the arms on and off resonance give a direct way to estimate the round trip losses experienced by the circulating field, and yesterday night we performed it as it has been done many times (see for instance 41555).

In the attached histograms, a distribution of reflectivity values are reported as measured by using B1p_PD1 photodiode, with different conditions (arm on resonance, off resonance, and only ITM correctly aligned). For the West arm (see firs plot), the outcomes are pretty consistent:

when only the WI is aligned we have a nice estimate of the mirror reflectivity that compares well with nominal value (and, by the way, gives a value of Finesse Warm = 454), the reflectivity off resonance is centered around 1 (as expected) and the reflectivity on resonance is comparable with Warm RTL = 60ppm, as reported in many other measurements.

Unfortunately, the same was not true for the North arm measurement. In the second picture it seems that, apart from a wider spread of the values, there is some kind of overall shift towards higher reflectivities (in particular, larger than 1 in the off resonance case that is unfortunately non-physical). We are trying to understand if the problem is in the processing or in the measurement itself.