On Friday morning we worked on the tuning of the OMC mode matching. During the first part of the shift, the ITF was locked in Low noise 3, and we tried different longitudinal positions of the SR lens, monitoring at the same time the north arm gain given by the calibration line. Then the ITF was locked in low noise 1 and we performed a few OMC scans in order to evaluate the mode matching for two different positions of SR.
From 07:41 to 07:43 utc, we increased the amplitude of the NE calibration line at 62.5 Hz, up to 2e-06 (which is more than a factor 10 larger than the usual amplitude of this line). This was needed in order to be able to monitor the north arm optical gain in a relatively fast way.
Initially the position of SR (which had been set during the previous shift of Nov 21) was X = -200 um. Then we started moving the SR longitudinal position. Below is the list of steps performed:
- 1228205534 07 Dec 2018 08:11:56 UTC Moved SR to X=+100 um (ramp time = 200s).
-
ITF unlocked at 08:14:22 UTC
-
ITF relocked at 08h43 UTC
-
1228207906 07 Dec 2018 08:51:28 UTC SR moved to X = +400
-
1228208554 07 Dec 2018 09:02:16 UTC SR moved to X = +700
-
1228209343 07 Dec 2018 09:15:25 UTC going to X = +1300
-
1228210034 07 Dec 2018 09:26:56 UTC going to X = +100 (ramp time = 600s)
-
1228211247 07 Dec 2018 09:47:09 UTC going to X = -200 (ramp time = 200s)
-
1228211898 07 Dec 2018 09:58:00 UTC going to X = -500
-
1228212434 07 Dec 2018 10:06:56 UTC going to X = -800
-
1228213011 07 Dec 2018 10:16:33 UTC going to X = -1400 (ramp = 300s)
-
1228213793 07 Dec 2018 10:29:35 UTC going to X = 400 (ramp = 600s) (Warning : glitch a few s before 10h44 utc)
-
1228215303 07 Dec 2018 10:54:45 UTC going to X = 300 (ramp = 60s)
-
1228215767 07 Dec 2018 11:02:29 UTC going to X = 200
-
1228216208 07 Dec 2018 11:09:50 UTC going to X = 100
-
1228216624 07 Dec 2018 11:16:46 UTC going to X = 0
-
1228217094 07 Dec 2018 11:24:36 UTC going to X = -100
-
1228217508 07 Dec 2018 11:31:30 UTC going to X = -200
-
1228217938 07 Dec 2018 11:38:40 UTC going to X = 500
-
1228218990 07 Dec 2018 11:56:12 UTC going to X = 400 (ramp = 400) – interrupted
- 1228219057 07 Dec 2018 11:57:19 UTC going to X = 400 (ramp = 60)
The scan of SR longitudinal position is shown in Fig.1 The best optical gain seems to have been reached for SR F0 near +400 um, but further analysis is required.
The ITF was then locked in low noise 1. The node ITF_LOCK was set to pause, and we opened the OMC shutter using the DET_MAIN node. This allowed us to perform a few scans of the OMC:
- Scan of a whole FSR with SR F0 X = 400 um, started à 13h51m30 utc: see Fig.2
- Slower scan with SR X = 400 um, at 14h01m30 utc : see Fig.3. Looking at the ratio of the first order mode (Fig.4) over the carrier TEM00 in the B1s2_DC signal, one finds 45%, but the amplitude of the first order mode seems to vary very much in time (see for example differences over the whole FSR in Fig.2). The ratio between the second order mode (Fig.5) over the carrier TEM00 gives ~5%.
- Slow scan with SR X = 100 um, at 14h20 utc: see Fig.6. The ratio between the first order mode (Fig.7) over the carrier TEM00 in the B1s2_DC signal gives 11%. The ratio between the second order mode (Fig.8) over the carrier TEM00 gives ~8%.
At first sight, it seems that the mode matching is better tuned for SR F0 X = 400 um (with respect to SR F0 X = 100 um), however the alignment seems to be worsed. In any case a more detailed analysis will be performed using also the signal demodulated at the DARM line frequency which has the advantage of not being spoiled by the side bands.
At the end of the shift we left SR F0 X at +100 um.
