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VIS (SR2)
terrence.tsang - 14:42 Thursday 16 May 2019 (8885) Print this report
SR2 optical lever noise hunt

With Mark,

A measurement of the residual motion SR2 optic shows that it has ~1 µm or urad of longitudinal/tilt rms displacement and ~10 µm/s rms velocity in longitudinal, ~10 µrad/s rms angular velocity in pitch and ~1 µrad/s rms angular velocity in yaw. The measurement was across 0 - 15Hz and was done with all local controls on. Since the oplev is known to be noisy, I turned off the TM damping control and did the same measurement again and it didn't seem to work and in fact there is no change in the result at all. I came to conclusion that the optical lever signals are just too noisy because the measured rms velocity drastically reduced to ~1 µm/s if the measurement was only across 0 - 1 Hz (the pendulum mode is at <1 Hz so I expect any motion in L should happen at below 1 Hz). Same measurement was done for the BS and the result is about the same.

To rule out electronic noise we measure the QPD vertical and horizontal signal before the division by the QPD sum. In figure 1, PIT_IN1 is the vertical channel, YAW_IN1 is the horizontal channel and the SUM_IN1 is the sum. If the noise is electronic noise, we should expect the noise level in both horizontal channel and vertical channel to be at around the same level. But, as shown in figure 1, the horizontal and vertical noise level is clearly separated from the sum as if there the beamspot has real horizontal and vertical displacements. We tweaked the gain of the whittening filter and we also tried enabling the built-in gain of 5 of the QPD circuit board and no obvious improvements were observed, as in, there is still a huge gap between the horizontal/vertical noise level and the sum noise level. Then, we blocked the beam from entering the lower viewport and did the same measurement again. This time the horizontal and vertical noise level became similar to that of the sum noise level (see figure 2). 

Today, the air conditioners and HEPA fans were turned off for a short window for a separate measurement so we used this opportunity to do the same measurement again. Figure 3 shows the same measurement, with dotted lines as old measurements. As can be seen from the figure, the noise levels have obvious improvements at ~>1 Hz and it actually reveals some mechanical resonances at >10 Hz. So, I decided to measure the residual motion under this circumstance. Figure 4 and figure 5 show the residual rms displacement and rms velocity up to 5 Hz. dotted lines are the measurements with fans off while solid lines are the measurements with fans back on. As shown in the figure, turning off the fans drastically improve the longitudinal and pitch noise levels (But mysteriously worsen yaw).

However, the noise level is still too high for us to measure the real residual motion of the optic. We can only make a pessimistic conclusion about the residual motion. In terms of displacement, we can say that the residual displacement/angular displacements are < 1 µm or µrad. But, we can't say for sure that the suspension meets the type B residual velocity requirement. The best result we get is ~3 µm/s and I am quite sure that a huge portion of this is noise.

In the long term, we need to suppress the air flow around the optical levels so to reduce the displacement noise induced by the fans. There is also a possibility that the noise is caused by beam jitters. But, we still need further investigations to confirm.

As far as I know, the optical lever signal is also highly susceptible to any force that is acted on the suspension's white outer frame where the optical lever setup is physically based on. A push on the frame is sufficient to cause the beam to move out of QPD linear range. I discovered this half a year ago but I am not sure if this is relevant.

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