[Stefan, Chris, Kiwamu]
We spent some time today at ISCT1 to locate the forest-type noise at around 1 kHz which we saw in the ALS CARM signal (see alog 6900) and also did some hardware works.
We did find a couple of suspicious optics on the table although we are still studying how they pollute the ALS CARM signal. We have stick foam in the mount of every reflective optic on the table to help their mechanical damping ability. The work continues.
Noise hunting at ISCT1
To do noise hunting on ISCT1 we did a speaker technique --- we hooked up a small speaker at the output of the common mode servo while keeping the green beatnote within the PLL's readout range by ezcaservo acting on the ALS_COMM VCO. The ALS CARM noise sounded mostly high frequency-ish although we didn't confirm how exactly high the frequency was. Then we bought the speaker close to every optic to see if they resonate due to the acoustic (positive) feedback at exactly the same frequency as that of the ALS CARM noise. We found a couple of optics which do resonate with the CARM sound. They are :
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ALS-M9
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ALS-M10
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ALS-BS5
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ALS-BS3
We are suspicious about these optics. They may be the ones introducing the forest noise at around 1 kHz. We have stick foam in the mount of every reflective optic to damp unwanted mechanical vibration. We will take another noise spectrum tomorrow to see how this improves it.
SHG mystery
There has been a mystery that somehow the SHG efficiency dropped by a factor of 4 or so at some point (approximately from 0.5 mW to 0.1 mW level) and it didn't come back to the high value. As a part of the ISCT1 works we inspected the doubling oven to see if there is an obvious clipping or something stupid. However we didn't find a thing. We did the following things:
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A knife edge measurement for measuring the beam sptial jitter of the frequency-doubled PSL light
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We placed a knife-edge type object to partially block the beam and placed a PD in the downstream. We didn't see forest type structure around 1 kHz. Though it is possible that the electronics noise was so high that it covered possible noises around this region.
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Alignment of the SHG green power monitor PD
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Placement of a razer blade to dump a stray light
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Re-alignment of the SHG oven to maximize the power
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This didn't help increasing the SHG power.
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Re-adjustment of the oven temperature set point.
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We ended up a usual set point of 35 deg. No improvements in the SHG power.
After all these works we measured the SHG power again and it was measured to be 0.140 uW which is still low compared with the past. It is still unclear what happened and we are not quite sure how this can be related to the ALS CARM noise.
REFL alignment
To evaluate the residual noise in the PSL frequency we needed a length sensor of the Y arm, namely the interferometer reflection (REFL) RFPD. We started rearranging the optics to guide the REFL beam to the RFPD. The work is ongoing. Currently the REFL beam is dumped. To prevent a damage on the RFPD due to an accidental power increment we need the trigger function of the mechanical shutter working.
