[Fujimoto, Tanaka, Komori]

Abstract:

We measured the transfer functions and dark noises of the IMC common-mode servo (CMS).
The input-referred noise of the IMC CMS seems reasonable, and we found several issues to be resolved in the measurement.

Details:

As a first step toward performing the full CARM noise budget, we measured the transfer functions and dark noises of the IMC CMS using settings as close as possible to those during observation mode.
The power spectra were taken with Mokulab in two frequency ranges: 10 Hz–1 kHz and 1 kHz–100 kHz.
We first measured the dark noise of Mokulab itself (Fig. 1) and compared it with the dark noise of the IMC CMS.

For the measurement of the dark noise of the IMC CMS slow path in the 1 kHz–100 kHz range, we inserted an SR560 because the noise was otherwise hidden beneath the Mokulab noise floor due to the generic filter with a DC gain of 0.1.
The SR560 was configured with a first-order high-pass filter at 300 Hz and a gain of 100 above that frequency.
The input-referred noise of the SR560 is negligible and is also plotted in Fig. 1.
The measured dark noises of the CMS channels are compared as well.

Ideally, the dark noise should be measured with the three boost filters in the common path enabled to fully reproduce the real operating condition.
However, enabling the boost filters caused circuit saturation, so we instead measured the noise without the boost filters and will compensate for their effect later.

The measured transfer functions of the slow and fast paths are shown in Fig. 2.
The corresponding input equivalent noise spectra for both paths are shown in Fig. 3.

Our next steps will be:

• Resolving the mismatch between the spectra measured in the two different frequency bands.

• Resolving the discrepancy between the measured and modeled transfer functions.

• Performing similar measurements for the CARM CMS.

• Measuring the power dependence of the RFPD noise floor to experimentally determine the shot noise equivalent power.