PARTICIPANTS: Yuhang, Yuefan, Eleonora

In the past days we have monitored the cavity round trip losses. We computed them from the cavity reflectivity with the tecniques described here.

In the actual setup the losses are measured using the IR reflected beam, sensend by a TAMA fotodiode. The reflected beam is filtered whith a bandpass filter in order to get rid of the residual green and it is focused on the photodiode using a 2 inch lens with f = 30 mm. (See first attachment for the setup scheme) 

With this setup we have found that the reflectivity (ratio between reflected power in lock and out of lock) changes from day to day and takes values between 0.88% and 0.82%. It corresponds to a variation in the RTL between 40 ppm and 75 ppm.

The change can be due to the different alignment condition (the beams impinges on different points of the mirror which scatter differently) and/or to some other factor affecting the measurement and not yet understood. 

In the attached plotes there are some measurments from the last days. Unfortunately not all the measurements from which we deduce the RTL variation reported before have been recorded.