We reconstructed the history of the cavity transmission. 

At the beginning of June we locked again the FC, after more than 6 months (entry #1385), at that time the injected power was 34 mW and the PD in transmission read a bit more than 4 V (which corresponds to ~ 7000 counts). (entry #1404)

After then we decided to reduce the injected power to 12 mW and the transmitted power was reduced accordingly ( ~2300 counts). (Not reported on logbook!)

Recently Yuhang found that by reducing the gain of the locking servo the transmission could reach 5000 counts. (entry #1644) We didn't remeasure the power at that time. 

Few days ago, after some realignment work on the bench (entry #1647), we found out the transmission was back to 2200 counts. (entry #1655). In that occasion (even if it is not reported on the logbook)  the AOM was also realigned and this brought to a large increasing of the power injected into FC (30 mW). The power was reduced by tweaking the half waveplate after the main laser.

Last Thursday (26/09) we spent half a day to carefully check and asses the alignment level (which is reported in entry #1674). The conclusion of that work is that the cavity seems well aligned. 

It seems to me that this mysterious change in the cavity transmission is connected to the change in the injected power, which is largely affected by AOM alignment condition. Also, the only straightforward reason why a reduction in the locking servo gain should bring an increasing of the transmitted power is that the loop was oscillating, and this is likely to be due to an increased power.

Anyway, I think that from now on we should better monitor (and report on the logbook) the level of power injected into FC and the cavity transmission.