craig.cahillane@LIGO.ORG - posted 19:43, Wednesday 18 July 2018 - last comment - 14:14, Saturday 21 July 2018(42964)
ALS DIFF Locked
Sheila, Craig, Hang After Georgia and Sheila locked ALS COMM this morning, we proceeded to lock ALS DIFF. Our first attempts to lock failed as the ETMY ESD linearization was off. When we switch it on ALS DIFF locking worked. We were also able to get the ISC_LOCK guardian to reach LOCKING_ALS. The plot shows the spectrum of H1:ALS-C_DIFF_PLL_CTRL_OUT_DQ, which is already calibrated into micrometers just like ALS COMM. ALS DIFF RMS = 6.8 Hz = 48 pm
Images attached to this report
Non-image files attached to this report
Comments related to this report
There is a more than 3 orders of magnitude difference below 0.1Hz. How?
Blue is an old reference with the ALS DIFF not locked, shouldn't have included it. A better comparison is in Alexa's thesis, figure 6.8, where she cites 40 pm RMS in loop noise, or 5.6 Hz RMS. Her plot is at the error point of the PLL, while ours is at the control point, so this isn't a direct comparison. I would make a direct comparison if I knew the TF from DIFF PLL ERR to CTRL, but the medm screen is broken and only the gods of interferometry know where the foton files live.
Congrats, team!
Lisa Barsotti showed me an old lock of ALS_DIFF made by Kiwamu and Sheila in the pre-O1 days. Their ALS_DIFF frequency noise was 60 pm RMS, or 8.5 Hz RMS. Our number is not much better, even after our optics replacement significantly increased the finesse of the arms. Looking at Kiwamu's first attachment in the linked alog compared with ours, most of the RMS came from high frequency. Our high frequency RMS seems to be around two times lower, so the new arms are doing something. Low frequency noise hits us harder, yielding a similar RMS.