MIF (General)

kentaro.komori - 1:24 Tuesday 22 April 2025 (33501)

Re-tuning of INP2 pitch and PRC2 pitch

[Tanaka, Komori]

Abstract:

We successfully recovered the relatively good sensitivity recorded on April 6th and 7th by re-tweaking the INP2 pitch and PRC2 pitch.

Details:

As noted in klog:33500, the most sensitive alignment appears to be INP2 pitch, and adjusting PRC2 pitch is necessary in conjunction.
We explored various offsets for these degrees of freedom and found relatively optimal values:
INP2 pitch offset: -1.6
PRC2 pitch offset: +0.14

With these settings, we were able to recover a sensitivity level comparable to the best sensitivity previously recorded around 100 Hz on April 6th and 7th.
However, we did not observe any further improvement in the noise around 100 Hz or enhancement in overall sensitivity beyond the previous best.

We left the interferometer operating with these offsets, but the DC readout almost failed several times after a loss.
Therefore, we temporarily removed the offsets to allow for relocking, and then re-applied them.
After doing so, the lock became stable again.
One possible issue is misalignment to the OMC, so we plan to perform the OMC RF sideband alignment procedure with these pitch offsets applied.

Comments to this report:

takaaki.yokozawa - 4:54 Tuesday 22 April 2025 (33502)

I performed the OMC alignment with RF signal.
And compared the spectrum (blue) no offset in INP2 pitch and PRC2 pitch (red) suitable offset in INP2 pitch and PRC2 pitch

Images attached to this comment

33502_1745265248_Screenshot from 2025-04-22 04-54-02.png

takafumi.ushiba - 12:18 Tuesday 22 April 2025 (33505)

Summary of the proposal on the next commissioning:

1. Measure TF from CARM to DARM to confirm the alignment doesn't change CARM to DARM coupling so much.
2. Injection test (acoustic and/or shaker) with updated IFO alignment to check if there are another noise source around 100-120.

Discussion:

I compared the REFL PDA3 RF45 signals before and after tweaking INP2/PRC2 offsets (fig1).
Top left, bottom left, and top right graphs show spectrum of REFL PDA3 RF45 signals, spectrum of DARM displacement sensitivity, and coherence btween DARM and REFL PDA3 RF45 signals, respectively.
Blue and red colors represents before and after tweaking, respectively.

Followings are what I noticed:
1. REFL PDA3 RF45 signals have less reduction of the bump around 117Hz, and so on, compared with the DARM.
2. Coherence between REFL PDA3 RF45 and DARM became small though there is still bumps around 100-120 Hz.
3. Bump around 520 Hz was not reduced in both REFL PDA3 RF45 and DARM, which also results in no change of the coherence.

The fact of #1 implies that the bump in DARM was reduced not by the reduction of the noise source itself (for example amount of scattered light) but by couling reduction from noise source to DARM.
Inaddition, the fact of #3 implies that CARM to DARM coupling doesn't seem reduced because the noise around 520Hz was well matched in the CARM noise projection reported in klog33460.
So, DARM noise which has a coherence with REFL signals is unlikely to the CARM couling noise.
To confirm this, it would be nice if we could repeat the TF measurement from CARM to DARM in the same manner with klog33460.

By the way, the fact of #2 implies that there might be another noise source around 100-120 Hz, which cannot be seen in CARM sensors.
So, it would be better to have injection test again with the updated alignment of IFO.

Images attached to this comment

33505_1745291381_Screenshot from 2025-04-22 12-05-16.png

tomotada.akutsu - 12:46 Tuesday 22 April 2025 (33507)

Regarding the monitors, did you also check POP_FORWARD/BACKWARD and POS (I do not remember if we had a PD for POS or not)? Apart from these, RF-relevant POP signals might have some independent bumpy information.