We have used the same technique than the one presented in VIR-0232A-10 and in the previous logentry (31558).

First, we have injected a white noise at the level of the slave laser pumping diodes power supplies (0-10kHz).

We also measured the cavity pole injecting 2 lines (at 127 and 2727 Hz) at the level of the slave laser power supplies (see entry 31665).

The TF is computed between IMC_TRA_DC and PMC_TRA_DC.

In the following table, we have summarized the results obtained with the 2011 dihedron (installed from May to November) and AdV dihedron #1 installed in November (see logentry 31696).

The difference between the two FSR probably comes from the difference of angle between the two dihedron mirrors (31664).

It represents a difference half-length of the IMC about 1.7mm.

Conclusion:

The round trip losses in the cavity look to be of the order of 520ppm with the new dihedron. To be compared with 625ppm of the old dihedron.

The IMC throughput loss has been decreased by 8% installing the new dihedron (mainly related to the Finesse reduction).

Moreover, some doubts subsist on the old dihedron mirrors transmittance and was resent to the LMA to confirm results obtained with the old dihedron.

Unfortunately, the total losses budget can only be measured when the beam exiting the IMC cavity and travelling through all the SIB1 optical components will be detectable (between SIB1 and PR).