Virgo Logbook

AdV-DET (Commissioning)

gouaty - 23:31 Tuesday 01 October 2019 (47061)

Measurement of scattered light coupling factors on the suspended benches

This afternoon, we injected a low frequency line (0.1Hz) on the suspended benches (SDB1, SDB2, SNEB, SWEB, SPRB) in order to have a new measurement of the scattered light coupling factors. Each time the line was injected through the top stage of the suspension, and along the longitudinal degree of freedom (Z coordinate of the bench).

Below is the list of collected data:

From 15h00 to 15h17 utc, nothing was done on the interferometer. These data can be used as reference (the B5 PD1 photodiode had its shutter closed during this period).
SDB1 injections (the B5 PD1 photodiode had its shutter closed during these injections):
- From 15h18m00 to 15h23m00 utc, line amplitude corresponds to a motion of +/-17 um for the bench. The DARM spectrum during the injection is compared to the reference sensitivity in Fig.1
- At 15h23m37 utc we opened the tracking control of SDB2, while keeping the same level of injection on SDB1 for 3 min (+/-17 um).
- From 15h27m00 to 15h30m20 utc, line amplitude corresponds to a motion of +/-20 um for the bench (SDB2 tracking still open). See Fig.2
- From 15h30m40 to 15h34m40 utc, line amplitude corresponds to a motion of +/-14 um for the bench (SDB2 tracking still open). See Fig.3
- At 15h34m50 utc we closed the tracking control of SDB2, while keeping the same level of injection on SDB1 until 15h38m32 utc.
Taking another reference time without injections: 15h39m00 – 15h42m00 utc
At 15h42m14 utc the shutter of the B5 PD1 photodiode was opened. We then took another 3 min of reference data without injections (15h42m20-15h45m40 utc).
At 15h45m43 utc, the SDB2 tracking loop was open again. Taking another period of reference data (15h45m50 - 15h51m10 utc).
SDB2 injections:
- From 15h54m10 to 15h57m00 utc, line amplitude corresponds to a motion of +/-6 um for the bench.
- From 15h57m05 to 16h01m00 utc, line amplitude corresponds to a motion of +/-12 um for the bench. See Fig.4
- From 16h01m10 to 16h05m40 utc, line amplitude corresponds to a motion of +/-18 um for the bench. See Fig.5
- Taking 2 min of data without injections (16h06m00 - 16h08m20 utc) and SDB2 tracking loop still open.
At 16h08m21 utc, the SDB2 tracking loop was closed.
Opening SNEB SBE tracking loop at 16h14m01 utc.
At 16h22m30 utc : we started to inject the line on SNEB, but the interferometer unlocked (probably because an unwanted broad band noise was incidentally injected at the same time). The SNEB SBE tracking loop was then closed back.
ITF back in Low noise 3 SQZ at 16h51 utc. Taking 8 min of quiet reference data.
SNEB injections:
- Injecting line at 17h09m30 utc with amp = 30, SNEB moving by +/-9 um, see Fig.6
- Injecting line at 17h12m17 utc (4min) with amp = 60, SNEB moving by +/-18 um, see Fig.7
- Injecting line at 17h16m38 utc (3min30) with amp = 80, SNEB moving by +/-24 um, see Fig.8
Reference quiet time: 17h21m00 - 17h25m00 utc
SWEB injections:
- Injecting line at 17h30m30 utc with amp = 100, SWEB moving by +/-13 um
- Injecting line at 17h35m30 utc with amp = 150, SWEB moving by +/-20 um, see Fig.9
- Injecting line at 17h39m10 utc with amp = 190, SWEB moving by +/-27 um, see Fig.10
- Injection stopped at 17h43m50 utc
Reference quiet time: 17h44m00 - 17h49m30 utc
SPRB injections:
- Injecting line at 17h56m25 utc with amp = 200, SPRB moving by +/-100 um, see Fig.11
- Injecting line at 18h00m55 utc with amp = 300, SPRB moving by +/-150 um, see Fig.12
- Injection stopped at 18h04m40 utc

The SPRB injection did not have any visible impact in DARM, and only a tiny effect on B4_DC.

The interferometer unlocked at 18h09m38 utc due to the timing issue, and it was not possible to relock until the end of the shift.

Data will be analyzed later.

Images attached to this report

Comments to this report:

mwas - 17:00 Wednesday 02 October 2019 (47072)

Using the modeled scattered light response transfer function the measurements can be checked against the model, and the backscattered light measured by scalling the model to match the measurement.

For SDB2: Figure 1 and 2 shows that for the two injections the same back scatter coefficient of 0.0004 PPM fits well the data. Given the expected filtering of 37dB by the Faraday isolator on SDB1 it corresponds to actually 2PPM of perfectly backscattered light.

For SNEB: Figure 3 and 4 show that 0.03PPM of perfectly back scattered light fits well the measurements.

For SWEB: Figure 5 and 6, show that 0.005PPM of perfectly back scattered light fits well the measurements. This means that SWEB back scatters 6 times less light than SNEB. It would be good to check if there is an alignment issue on SNEB which could explain the extra back scattering, or if there are any other reasons why SNEB could be back scattering more light.

For SDB1: Hrec_hoft_20000Hz is misleading, as the scattered light arches must also be visible in other degrees of freedom (MICH?), and h(t) tries to subtract it, which doesn't work well as the injection is changing with time. The reliable comparison is with hrec_hoft_raw_20000Hz (as is already done for SNEB and SWEB as Hrec_hoft_20000Hz already tries to subtract scattered light by using the B7_DC and B8_DC signals). Figure 7, 8 and 9 show 3 different amplitude of injection, all are fitted with 0.002PPM of back scattered light. However there is a peak at the end of the scattered light shelf in the measured sensitivity that is not reproduced by the projection. This is probably due to either problems in the SDB1 local controls or in the filtering of them to do the projection. Figure 10 shows that by replacing the measured SDB1 motion with a perfect sine wave at 0.1Hz, a nice match can be obtained of that feature.

The code for these projections is located in /users/mwas/SBE/scatterProj_20191001.

Images attached to this comment

47072_1570026866_Screenshot from 2019-10-02 13-28-57.png

47072_1570026873_Screenshot from 2019-10-02 13-29-30.png

47072_1570026986_Screenshot from 2019-10-02 13-37-47.png

47072_1570026992_Screenshot from 2019-10-02 13-38-15.png

47072_1570026996_Screenshot from 2019-10-02 13-38-49.png

47072_1570027003_Screenshot from 2019-10-02 13-39-25.png

47072_1570027858_Screenshot from 2019-10-02 16-47-21.png

47072_1570027863_Screenshot from 2019-10-02 16-49-45.png

47072_1570027870_Screenshot from 2019-10-02 16-50-28.png

47072_1570028238_Screenshot from 2019-10-02 16-56-54.png

mwas - 13:21 Thursday 12 December 2019 (47924)

Sheon looked at these results as part of the DET scattered light technical noise review. His keen eyes noticed that also the second order scattering shelf (correponding to a beam that bounce twice from the moving bench), is also visible.

This can be included in the model, and adds another free parameters, the fraction of scattered light that see the given bench twice. The result are as follow:

SDB1 0.002ppm (first order) and 2e-7ppm (second order)
SDB2 0.0004ppm and 3e-8ppm (it is not clear if it is a measurement or an upper limit)
SNEB 0.03ppm and 1e-6ppm
SWEB 0.005ppm and 2e-6ppm

Note that for SWEB the second order scattering is 2 times larger than for SNEB, if though the first order scattering is 6 times smaller than for SNEB.

Images attached to this comment