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Optical characterization (Optical characterization)

Allocca, chiummo - 12:25 Friday 02 February 2018 (40509)

Arm losses measurement

Yesterday we collected some data to measure the arm losses in both FPs. This could be of particular interest after what Ettore reported when installing the NE payload.

Basically we followed the same procedure as reported in 40074, with hundreds of urad of misalignement for the input test mass not concerned with the measurement.

We started around 17.00 LT with North arm and collected some lock-unlocks both by unlocking the IMC (could be useful for finesse measurements) and with meta-unlocking the arm (see attached picture). Furthermore we collected also some shutter closings to check the residual offset at the time of the measurement.

One thing that we immediately noticed is that the DC level of B1p_DC is different when we have aligned NI or WI.

Analysis will follow.

Below some useful GPS:

N arm

misalign WI by 800urad mar ty (was -1.9 ->800)
misalign WE by 50urad mar tx (was +48.506 -> 100)

Time: Feb01,18-17:13:55 -> GPS: 1201536851
begin lock-unlock IMC (NARM locked)

Time: Feb01,18-17:21:17 -> GPS: 1201537293
begin meta-lock-unlock only NARM
Time: Feb01,18-17:26:30 -> GPS: 1201537606
end NARM

Time: Feb01,18-17:32:18 -> GPS: 1201537954
new lock NARM because the previous seemed too misaligned
Time: Feb01,18-17:36:07 -> GPS: 1201538183

W arm

misalign NI by 800urad mar ty (was 1.4 ->800)
misalign WE by 50urad mar tx (was +42.4376 -> 100)

Time: Feb01,18-17:41:43 -> GPS: 1201538519
lock WARM - unlock with IMC
Time: Feb01,18-17:51:13 -> GPS: 1201539089
lock WARM -> unlock WARM
Time: Feb01,18-18:04:10 -> GPS: 1201539866

Time: Feb01,18-18:07:03 -> GPS: 1201540039
shutter
B4
Time: Feb01,18-18:08:50 -> GPS: 1201540146
B5
does it work?!?!?!
Time: Feb01,18-18:12:35 -> GPS: 1201540371
B7
Time: Feb01,18-18:14:33 -> GPS: 1201540489
B8
Time: Feb01,18-18:15:29 -> GPS: 1201540545

Images attached to this report

Comments to this report:

capocasa - 16:13 Monday 05 February 2018 (40533)

In order to exrapolate the round trip losses, I have looked at the change in the reflected power during the "meta-unlocks", where the cavity is unlocked but still resonant. Unfortunately most of the PDs do not see a very clear signal.

In particular, as already observed sometimes in the past, B4 seems to get more light when the cavity is locked. This is an unexpected behaviour which has not been understood so far.

For what concerns the west arm, I'm not able to extract any loss number (see pic 1), while for the north arm (pic 2) the change in the reflection is recorded quite clearly by the two B5 photodiodes.

They give losses of 390 ppm (B5_PD1) and 250 ppm (B5_PD2). I remark that B5_PD1 has a large offset of about -0.445 mW while B5_PD2 has an offset of -0.0093 mW.

B1p_PD1 also sees a signal, but the reflected power during the lock keeps growing and probably it cannot be trusted. Anyway, it corresponds to a much lower loss level of about 20 ppm. The offset (measured during a IMC unlock) is about -0.2e-3 mW.

Images attached to this comment

bersanetti - 18:18 Monday 05 February 2018 (40534)

B1p_PD1 had its shutter closed, so unfortunately its information is not useful. This can be checked looking at the Vbias channel (see figure), which is around 0 instead of ~ -10 V. If I remember correctly, having the Vbias off means that the shutter is closed, but the other way around is not necessarily true, as one can rearm the shutter (i.e. Vbias on) without actually opening it. Some information can be obtained looking at previous data and checking whether the channels [Bench]_[PD]_shutter_[open/close]_count incremented or not.

Images attached to this comment

capocasa - 1:44 Tuesday 06 February 2018 (40536)

capocasa - 1:48 Tuesday 06 February 2018 (40537)

Yes, Diego is right. Actually the signal I was referring to in entry 40533 is the one from B1p_PD2.