Virgo Logbook

This morning, Richard changed the temperature of the NI TCS laser by 1 degree. This had a considerable effect on the etalon of the NI, as shown by B7_DC in fig 1. Also according to Michele's mirror temperature (fig 2), the NI got about 150 mK colder (extrapolating a bit). This should correspond to a loss of 100 mW of TCS power according to Gabriele's calibration of 1.3 K/W. We checked the power sent to the mirror with the big power-meter, and indeed we measured 800 instead of 900 mW.

Since the DC power of the TCS is stabilized by the rotator, this means that the NI mirror and the power-meter do not see the same relative change in power. In theory, this effect cannot be caused by a change of the polarization state of the laser, since both the power-meter and the NI mirror are behind the same polarizer. (Also note that the out-of-loop diode sees an increase of powerat the same moment, but this can be explained by a change in polarization since it takes a part of the beam with a different polarization.)

The only explanation we can think of is that the pointing of the beam has changed. It could be that the sensitivity of power-meter depends on the position of the beam. Another explanation might be that there is more power in 'central spot', which is blocked by the mask. Fig 3 shows the IR image when the laser is stabilized at 21 degrees and fig 4 at 22 degrees. The images would be compatible with both explanations. In the end, we put back the old temperature.