some additional information:
the beat noise was measured as the feedback signal to the VCO of the PLL, so the calibration factor does not change with changing optical power, alignment, mixers etc.
It's a convenient way to change individual things in the setup and be able do directly compare the measurements without lots of calibration.
We checked the following things:
The feedback signal is only valid until about 10k. Tara will measure the UGF again on Monday but the signal above 10k exactly scales with power on PD or gain settings while below it stays constant which is exactly what we expect when having enough loop gain in the PLL loop.
Looking more into detail in the spectrum we looked from some tens of Hz to 10k and then tried to excite the spectrum. We could clearly identify the individual resonance peaks from e.g. the beam splitter mount of the beat setup. We know that the way it is set up is very bad but nevertheless we expect mainly lots of resonance peaks but not this hump shaped spectrum.
Now the interesting part is that if we excite the surface of the optical table by touching it softly with e.g. a balldriver we can excite those resonances. What i found very interesting is if you excite the bottom of the table we excite the hump very broadband. You only have to touch it barely like tipping with your fingertips and the whole hump increases. So my guess is that we have a scatter source somewhere which would also explain the shape (at least from my experience).
So the plan for Monday is to have a closer look on that, then checking all the wholes in the foam insulation and probably make them little bigger (right now they are 1/2 inch). Other things are reducing the power from the laser (something we wanna do anyways in the long term), replacing the PMC which scatters a lot of light (you actually don't need an IR viewer to see that, detector card is enough) by a good one and getting a symmetric layout with two periscopes (simple ones like we have now on the other side of the cavity, we casn replace them later by real good ones) for the beat at a lower beam height to reduce all resonances to better see where/what the underlying noise floor looks like.
The current PD for the 160 MHz beat signal is 120MHz. We use a 2 GHz PD to compare the results between two PDS,
and there is no difference. 120 MHz PD seems to be working fine for us. However, the beat signals at freq above ~5 kHz we have seen so far are not real signal from cavities' noise.
We have checked several parts on the PSL setup to search for excess noise, we have not checked the PD for beat signal, so
we try this measurement.
We use a 2 GHz PD to see the beat signal from another port of the BS. The attached figure has
two traces of the beat signal, the one on top from 120MHz PD and the one on the background from 2GHz PD .
The results are similar up to ~10 kHz.
The difference at high f comes from different bandwidth and gain setup for PLL loop, because
it changes with gain setup on SR560. So, the beat noise results shown so far are valid only up to 10kHz.
At higher f, it just the PLL loop.