Virgo Logbook

Since the installation of the new monolithic mirrors, I tried various times to have a look at the new etalon effect, but the result was never really clear (CHRoCC??). With the long, stable locks of the last few weeks, this is now finally possible. Figures 1 and 2 show the usual etalon plots for the NI and WI. On the X-axis is no longer the temperature according to Michele's monitor, but the raw frequency of the butterfly-modes, which is calculated with a dedicated server since last year. The Y-axis shows the normalized arm-transmissions (B7/B5 and B8/B5).

The curves show a jump after the installation of the new master laser. Similar jumps have been seen in the past when the laser locks to a different resonance of the reference cavity, which is not unexpected for a totally new laser.

To make the plots, I used about 1 month of data. Since the plots are still a bit noisy (alignment might not have been very reproducible some weeks ago), I fitted the curves by hand. Accurate numbers can hopefully be obtained after the science run. Some preliminary numbers:

North Input:
FSR = 0.183 Hz of the butterfly mode
Variation in transmission: 5.4% peak-peak
Jump after ML substitution: 0.18 FSR

West Input:
FSR: 0.185 Hz of the butterfly mode
Variation in transmission: 7.5% peak-peak
Jump after ML substitution: 0.13 FSR

If I remember correctly, the expected variation in transmission, based on the nominal reflectivity of the AR-coating should be around 7%.

If the old calculations are correct, a jump of 1 FSR of the RFC should correspond to 0.18 degrees for the OMC and 0.46 FSR for the etalon. Figure 3 shows that the temperature of the OMC hardly changed. The easiest solution to these constraints is that the laser jumped by 9 FSR of the RFC or 9 * 470e6 = 4.2GHz, which yields 9 * 0.18 - 1.6 = 0.02 degrees for the OMC and 9 * 0.46 - 4 = 0.14 FSR for the etalon.