Sheila, Georgia
Today while LLO was commissioning we took some time to look into output jitter. The end goal is to have a projection of OMC ASC to DARM for the noise budget, following similar methods to Koji or Carl
Today we only mostly ran excitations to the POS_Y loop, with various offsets and excitation levels. We also did a couple of excitations to the POS_X loop.
- There is a strange offset in the POS_Y loop, causing extra coupling which Craig and I saw back in March
- We can manually add offsets to the loop to minimise the coupling of a fixed amplitude excitation to DARM, see first attachment
- However: the offset for minimal DARM coupling is different to the offset for highest DARM optical gain, see second attachment
- The "coupling co-efficient" in the top plot of the second attachment is just sqrt(DARM_inj^2 - DARM_quiet^2) / QPD_inj , i.e. the ratio of the excitation measured in DARM to the excitation measured in the QPD
- We also made the measurements for a "Koji-type-2" upper limit projection of the ASC to DARM, third attachment, using an offset to enhance the coupling of an excitation.
However our excitation is too small to make a reasonable darm projection (using this data we can say that the output jitter is not above DARM, which is not very useful)Edit: see comment for updated upper limit projection - We were worried about large amplitude excitations dominating the RMS motion of the OMC and changing the coupling co-efficient, but further analysis is required.
Other things to note:
- During this test the Sensemon BNS range jumped to 120 Mpc, this wasn't a real range increase but actually a miscalibration while we were playing with the optical gain
I had a look at DARM with and without the large offset in POS_Y, using the online time-dependent-correction-factor corrected channel (in the above alog the 3rd attachment is instead just using CAL-DELTAL_EXTERNAL).
The top plot of the attachment shows DARM with and without this offset, and with and without an excitation. The optical gain reduction with the alignment offset is obvious above 60 Hz, however I do not see any extra jitter coupling. I also plotted the coherence between one of the QPD channels and DARM (all the QPD channels looked very similar), and don't see anything there.
If I understand correctly: Adding the alignment offset increased our excitation coupling by a factor of 2, so given that we don't see extra jitter coupling with the offset our ambient noise is at least a factor of 2 below DARM.
Note: we were limited to offsets between -1 and 6 in the POS_Y degree of freedom: going outside these bounds saturated the OMC suspension.
