WP6522

During the maintenance period yesterday, I have measured the spectra of IMC WFS DCs with and without the IMC locked. The purpose of this measurement was to cross-check the calibration factor of 1/3 which previously estimated by Daniel (31631, Nov 2016).

A conclusion:

I agree with Daniel that the IMC WFS DC signals should be scaled by a factor of 1/3 when the IMC is locked.

[Some plots]

• The 1st attachment: raw spectra of the IMC WFS DCs
  • (orange) when IMC was unlocked with the MC2 significantly misaligned. Pin = 2 W. (It had to be low power in order not to saturate the ADCs.)
  • (red) when IMC was locked. Pin = 20 W. The main interferometer was left unlocked.
• The 2nd attachment: calibrated spectra of the IMC WFS DCs.
  • (blue) when unlocked.
  • (red) when locked.
  • Also, I have included an extra factor of sqrt( 8 / pi) to get all the spectra calibrated in the unit of fractional amplitude defined as |E_{10}/ E_{00}| or |E_{01} / E_{00}|.

A side note: depending on whether the IMC is locked or not, the spectral shapes vary. This is not so surprising because when the IMC is locked, the reflection port must be dominated by non-00 modes. So the signals should contain relatively large amount of cross terms from various HOMs. So for the reason, I don't have a good explanation why they have to be a factor of 1/3. Even so, it is interesting to see some peaks come and go.