The master laser shows increase in the power high frequency noise these are similar to the increases in the EOM correction noise. To investigate this we have monitored the output power of the laser used in the clean room at LAPP. That laser is similar to the Virgo master laser (maybe even indentical). It is a Mephisto free space laser at 1064nm.
Figure 1 shows the spectrogram over 1 night of the LAPP laser output power normalized by the median spectrum. Between minute 50 and 100 there is a small increase in broadband noise above a few kHz. At a few hundred Hz there many peaks which are moving up and down in height, these are expected to be mechanical resonances of various mounts on the optical bench.
Figure 2 shows the spectrogram over a weekend, with the noise eater switched off. There are many more times when the noise at high frequency increases, and there is also a series of harmonics of ~230Hz moving up and down at the same time.
Figure 3 shows for spectra of:
- Yellow, noise eater enabled, noise low
- Purple, noise eater enabled, noise high
- Blue, noise eater disable, noise low
- Red, noise eater disable, noise high
In all cases there is no external control loops applied to the laser (but there DAC cables attached to the PZT, temperature and current inputs of the laser with active signals sent during various measurements on working days).
Clearly the noise eater is reducing the amplitude noise by a large amount, especially the ~230Hz harmonics, and the broad bump at a few kHz. A plausible explanation is that the master laser has its amplitude noise level fluctuating over time scale of hours, the noise eater has sufficient gain to mostly remove this noise level fluctuation, but not completely. As the noise eater acts on the pumping current, there might be some cross talk to the master laser frequency which is causing the increase EOM corrections to compensate.