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AdV-SGD (FDS commissioning)
ciani, conti, d'angelo - 17:55 Tuesday 23 November 2021 (53956) Print this report
Investigation on lines around 40 Hz in HD diff audio (stray light) (spoiler: the culprit is LO_M2 mirror!)

We set out to investigate the origin of the line(s) around 40 Hz that shows up in the homodyne detector differential channel. After a number of tests, we concluded that the element vibrating at that frequency is the LO_M2 actuated mirror (on the LO line just before entering the HD). The two motors sticking out of the back are probably responsible for lowering the resonant frequency, which appears to be instead in the range of 100-200 Hz for the other mounts. We have not yet investigated the complete coupling mechanism (i.e. where the stray light originates and recouples).

In the following, we report the full list of tests (to be more thouroughly analized later).

We started by closing the shutters of Green, SC, and BAB (only LO to the Homodyne), plus we put the Delay Line on in order to have the LO light circulating only on EQB1 bench.

We first looked at the spectra of EQB1_HD_DIFF_AUDIO with the bench in different environmental conditions (box, HVAC, lights)
One minute stretches of data (first number is GPS start time, data good for at least 60 s):

1321697970 air off, EQB1 box closed, light on
1321698139 air on, EQB1 box closed, light on
1321699508 air off, open (section 18 - above galvo), light on (above sec 17)
1321699625 air off, open (section 18 - above galvo, light off
1321700020 air off, open sec 17->19, lights on (only above sec 17)

We note that the bump below 10 Hz disappears when HVAC is off and the box closed, while it partially survives even with HVAC off if the box is open (plots will follow).

We then investigated the origin of the approx. 43 Hz line in the EQB1_HD_DIFF_AUDIO channel. All GPS start times are followed by  minute of test data.

1. GPS 1321698379: Tapping horizontally on table side facing SQB1, in a point close to the green rotator (and air off, EQB1 box closed, light on): 43 Hz gets excited.

For the subsequent tests, we kept air off, open sec 17->19, lights.

2. Tests with a light dumper:
1321700314 put an rectangular glass IR dumper on HD box out port towards cameras
1321700431 removed the dumper, as a double check
1321700568 put IR dumper on HD box SQZ input port
1321704720 IR dump on HD SQZ input, inside the HD box before the BS (by hand, also touching box and table)

We didn't see any significant effect on the 40 Hz noise.

3. " mechanical dumping" tests (press a finger on top of the optics mount and seeing if the 40 Hz noise is dumped):
1321701487 finger pressing on the waveplate mount at HD box SQZ input 
1321701856 HD lid removed
1321701981 hands on HD box walls (wall at SQZ input and its opposite wall)
1321702097 hands on HD box walls (wall at LO input and its opposite wall)
1321702306 finger pressing on HD BS mount (top)
1321702500 finger light touching on HD BS mount (top)
1321703199 finger pressing on mirror mount on translation stage inside HD box
1321703381 finger light tapping on mirror mount on translation stage inside HD box

Again, we see no clear effect on the 40 Hz noise.

5. Tapping (by using Allen key gently touching the optic mount) tests:
1321703544 on HD BS
1321703656 on HD SQZ input Lambda mount
1321704349 on HD lens in reflection from LO on the BS
1321704463 on HD lens in transmission from LO on the BS
1321705568 tool tapping on actuated mirror LO_M2 before HD

These tests showed that the 40 Hz noise is clearly excited when tapping on LO_M2, while tapping on other optics mount excites structures above 100 Hz.

Comments to this report:
bonavena, ciani, conti, d'angelo - 18:08 Wednesday 24 November 2021 (53971) Print this report

We post the plots relating to the different tests.

In the figure 1 we can see how the air affect the system.
The purple line represent the system in the following configuration: air off, EQB1 box closed, light on.
In the blue line the air was switched on, EQB1 box closed, light on.
We note how the air excites different structures above the 100Hz and how the low frequecies noise (<10Hz) disappeares when the air is switched off.

In the figure 2 the purple line is: air off, EQB1 box closed, light on.
In the blue curve the air was off, the box was opened in the sec 17, 18 and 19. The lights were on only above the sec 17.
Even with the air off, opening some sections, the noise at low frequecies reappears.

1. The figure 3 confirms that tapping horizontally on table side facing SQB1 the peak at 43 Hz gets excited (the purple reference curve is the same of the previous plots: air off, EQB1 box closed, light on).

For the following configurations the reference purple curves represent the system with: air off, open sec 17->19, lights on (only above sec 17)

2. The analysis on the tests with a light dumper confirm that we don't see effect on the 43Hz (no figures are attached) 

3. The analysis of the "mechanical dumping" tests confirm the two structures around 110 and 190 Hz:

  • in figure 4 the blue curve shows how removing the HD lid a peak at 110 Hz is excited.
  • in figure 5 pressing the HD BS mount there is a peak around 190 Hz.

5. Figure 6 confirms how the 43 Hz noise is clearly excited when tapping on LO_M2. 

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