To investigate the reliability of the photodiode shutter, I looked at the B7/8_PD1 shutters that are closed during a lock sequence. The first figure is showing the counter of the number of closure over a bit less than a month. This counter is reset when the monitoring program is restarted. We are close to one thousand cycles over one month.

The second figure is showing a typical lock sequence. The top plot shows the power seen by both photodiodes (PD2 was rescale by a factor 13 to compensate for the splitter and match PD1). When PD1 reach about 8 mW, its shutter is closed, so there is no more power on this photodiode, and then the locking sequence continue with more power on PD2. The bottom plot is the shutter count. Since this count is sample only every 5 seconds by the slow frame builder the step of the counter do not exactly time match the power drop on PD1.

The third figure is showing the power measured before a step in the shutter counter and after the step (taking few seconds of margin given the poor timing resolution of the shutter counter) for all closure events with at least 1 mW before closure. This corresponds to 954 closures. For all cases, the power after closure is very small showing a success rate of 100% for this shutter.

The last figure is the same plot for B8_PD1. We have the same number of events (the shutters are closed when both arms are locked), with again 100% of success.

This kind of analysis is more difficult to do for the B1 photodiode since after a closure, there is no more power. But the fact that B1_PD2 was not damaged given the many unlock events we had, while B1_PD1 was damaged after a few unlocks, combined with the B7/B8 results, is showing that either a shutter (and its driving electronic) is working reliably for a thousand of cycles (or more) or it will show problem very quickly.