From a better analysis of the photodiodes signals, the 155kHz line seem to be visible on B7 PD1 (figure 1), while no signal is visible on PD2. The same happens also for B8 PD1. This is quite surprising because the shutter in front of PD1 should be closed in dark fringe. Further investigation are needed to better clarify why the signal is not visible on B7_PD2.