TCS power meters replaced witn photodiodes

In the both bench we (Maurizio and I) have installed the Photodiodes to the replacement of Power Meters.
The Photodiode 13300 has been mounted in the NI bench and the Photodiode 13249 has been mounted in the NI bench.
Both PDs are equipped with low noise pre-amplifiers with a gain of 100 from DC to 30kHz (assembled in the E-lab of TOV).

We report the calibration for both Photodiodes:
13300: PNI(W) = 5.086 * V(V) + 16.32
13249: PWI(W) = 4.387 * V(V) + 5.916

This makes it possible to have the noise spectra of the CO₂ lasers and to make coherence with the dark fringe.
We had a small problem with the WI installation: the noise spectrum looked terrible with a huge bump at about 1.5kHz. We realized that each preamplifier wants his own power supply...

Later the IFO was locked with low TCS power and we started investigating the coupling of the CO₂ laser noise into the dark fringe.

We report two noise spectra and coherence plots.

Figure #1: NI power 1.0W, WI power 0.3W. No coherence with the dark fringe.



Figure #2: NI power 1.5W, WI power 0.3W. Some coherence with the dark fringe around 20Hz with the NI laser.

The pre-amplifier connected to the NI photodiode was not working properly, giving a laser noise 10 times higher than the WI even if the power on the photodiodes is almost the same. So it has been temporarily replaced with a standard VIGO preamplifier. The main difference is that the gain of the VIGO pre is ten times higher (to be taken into account when measuring noise spectra) and has no DC offset.