This morning the shift was dedicated to the Fmoderr investigation using the digital demodulation :

The first step was to add a big offset (+0.2V ) on IMC perturb, which is equivalent to shift the IMC PDH locking point.

Then we scan the IMC length from about -1mm to +1mm more or less centered on the usual Fmoderr tuning point,

Which is also equivalent to make Fmoderr in DC and not at 1111Hz.

Thanks to the digital demodulation, we were able to see the 3 modulations together (see attached plot 1).

The glitches are due to IMC unlock during the IMC end mirror scan.

The channel IMC_TRA_6MHz_50Hz_TIME is the analogically demodulated one and amplified (at 8MHz and not 6 as named) and used until now (always in DC not with 1111Hz mod/demod).

The non expected result is that the minimum of the magnitude is not the same for each modulation frrequency.

This can be explained (or not) by additional residual amplitude modulation (RAM) coming from phase modulation process.

In any case this implies unbalanced transmitted sidebands.

Once done, we tried the opposite thing : try to tune the IMC PDH locking point using these signals :

We intentionally mistuned the IMC FSR (IMC length out of about 500um from fmoderr) and we scanned the IMC_perturb with Pstab open (see attached plot 2).

Again, the minimum of the transmitted modulated power for each frequency is not achieved for the same locking point...

We try to choose a locking point for the IMC for which the 56MHz is minimized (-0.6V on perturb) and try to relock,

but the induced effect on the IMC reflection shape perturbed the automatic alignment avoiding any ITF dependency observation of this parameter.

The conclusion is that  the current  Fmoderr point is maybe not the "good one" so we implemented an offset on ACL (now =0) in order to explore it.

It's also the case for the IMC PDH locking point (need more time to fix AA dependencies).

We also want to check if these signals are stable when modulation depth is changing (RAM strongly dependent on EOM crystal temperature).

No configuration changed with respect to the last ITF lock...