From the power transmitted behind the end mirror (0.5 mw) and behind the output mirror (8.5 mW) it seems that the end mirror transmission is about 17 time smaller than the output mirror. Assuming that the output mirror transmission is 0.8%, one would deduce that the end mirror transmission is of the order of 0.05% in good agreement with the expected value. If so, the large reflection should come from an asymmetry between the input and output mirrors (which seems difficult to imagine).
On the other hand there is a strange "large" peak in the middle of the FSR. Where is that coming from? Is there a polarization problem? In the entry it is written that the polarization is OK.
Question: did somebody already check that the input and output mirrors are mounted with the HR side facing the inside of the cavity?