Jennie W, Sheila

While Camilla changed the SRCL tuning (nominal for SRCL1_OFFSET is -191 counts) for squeezer tests today Sheila and I wanted to check if DHARD sensors are still sensitive to DARM. As there is coupling from DARM to DHARD and then DHARD back to DARM, this changes the DARM OLG and thus the sensing function.

I tried to check on whether the sensing function was affected by DHARD currently by looking at the coherence between low frequency PCAL lines below 20 Hz in DARM. The only one we still use is 17.1Hz, there where others used to monitor the low frequency sensing function and the SRC detuning, but these are no longer in use. The other calibration lines still in use below 20Hz are injections on the ETMX suspension.

Francisco looked at this in 2024 in alog #80267, for different offset values of the AS A WFS cwhich are used as a sensor in DHARD. For a setting of 0, which matches the current setting of this offset, the coherence (bottom graph) between DARM and DHARD looks like it is bellow 1 at 17.1 Hz, the SRCL1_OFFSET at the time was -290 counts.

I compared short spectra of DARM with coherences from CAL-DELTAL_EXTERNAL_DQ to ASC-DHARD_P_SM_DQ and ASC-DHARD_Y_SM_DQ. See this image.

For the detunings used, the 17.1Hz line only seems to show up strongly in the -90 count offset, see image of the darm spectra in the top left graph, DARM coherence with DHARD_P in the middle row left graph, DARM coherence with DHARD_Y in the bottom left graph, and the DHARD_P spectra in the top right graph. From the spectra it looks like the 17.1 Hz line is almost buried in the noise for most of the detunings.

Since all these measurements only used ~ 3 mins of data I decided to compare the hour long period from this entry where Camilla had the SRCL1 offset at -290 counts, to a period after commissioning was over when it was back at the nominal of -191 counts.

See this image where I have also added a graph of DHARD_Y spectrum in the middle right of the image.

Purple is the -290 count SRCL offset and Yellow is the -191 count offset. The coupling of the PCAL line to DHARD P and Y does not really change between the two different detunings, but since there is some coupling to DHARD we might want to revisit the AS A WFS offset. Also the coherence from DARM to DHARD_Y (bottom left graph in my first image below) for the same SRCL1 OFFSET (-290 counts, see this image) as Francisco's measurement  was 0.6, compared to the 0.1 coherence he found so that might be a reason to keep looking at this issue.

Finally I include an ndscope of the optical spring frequency (f_S) and the coupled cavity pole frequency (f_C) changing with the detuning. NB: For future me, that optical spring frequency channel defaults to zero when the calculated value is very small I think.