NAOJ GW Elog Logbook 3.2
Since we have new IRMC servo working. I measured again the noise spectrum of homodyne when there is only LO incidence. I did the measurement after common mode noise rejection. The result is shown in the attached figure. There is three main difference.
1. New servo has more flat low-frequency noise.
2. New servo shows much more peaks in the low-frequency region. It looks like electronic noise. As pointed by Matteo, this is because of the ground loop.
3. The peaks at the high frequency of these two situations are different.
After connecting the power of homodyne and spectrum analyzer(this can remove the ground loop noise), I measured noise spectrum of homodyne again. During this measurement, there is only an infrared beam incident and has a power value of 1.3mW.
The result is shown in the attached figure.