this afternoon, we put back the 1/4 and 1/2 waveplates and after a long alignment/angle tuning/CEP optimization process, we hardly got 80-81kW in the cavity for 33% amplifier ratio. the transmission photodiode is then ~ 7.5 divisions (500mV/division) on the scope.
BUT during the 1/2 and 1/4 waveplate angle tuning process, we clearly saw:
- a reflection signal level which is very sensitive to the input polarization
- a coupling which can be almost zero with ~70kW inside the cavity
- the more important: ~9.5 divisions (500mV/division) on the scope with only 70-75kW in the powermeter
=> which device should we believe ?
the problem is all these photodiodes (reflection, error signal, transmission) and powermeter are aligned with mirrors which seem to be sensitive to the polarization state of the beam... and for the "output" beam side of the cavity, we cannot remove the mirrors because of the mechanics of the cavity which prevent a direct view of the ports. for the "input" beam side of the cavity, the output port is used for the cavity beam size measurement with a Basler camera.
we have to think to how to solve this issue !
| Ronic Chiche wrote: |
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I put back and aligned the 1/2 waveplate only and I get 71kW max after its optimization and optimizing the CEP and alignment (without walking procedure).
this waveplate change a lot the reflected power seen on the CH2 of the scope.
to be continued this afternoon...
| Ronic Chiche wrote: |
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we did a long term run (25 mn) with the powermeter located at the FPC position at 33% amplifier ratio.
the initial power was 18W .
then, it goes to 18.5W in 10 minutes, then 5 minutes later, it goes to 18.3W and stay stable until the end the run.
| Ronic Chiche wrote: |
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this morning, with Daniele, we checked the amplifier power, right after the CVBG (the power meter has to be placed on a metal plate above the large table hole), and just at the input of the FP cavity, after the 1/4 and 1/2 waveplates (the power meter has to be placed on flat beam dump+ Thorlabs beam dump + V metallic mount to be at the right height).
we compared with the power measured the 9th of september : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/356
the 3 collumns are the measurement close to the CVBG on 9th of september / same position today / just befor the FPC today
amplifier ratio (%) power after compressor (W) (new) power @ CVBG (new) power @ FPC ratio FPC/CVBG power (%)
10 0.91 0.93 0.83 89.2
20 8.6 8.6 8.0 93.0
30 16.7 16.9 15.8 93.5
40 25.5 25.6 24.0 93.7
50 34.5 34.6 32.5 93.9
60 42.5 42.5 39.5 92.9
70 50.0 50.0 45.0 90.0
we observed with the viewer the beam on the powermeter.
at 70% of amplifier ratio, the beam size is as big as the powermeter detector.
then, the 93% to 90% transport efficiency drop could come from this "too small" powermeter detector.
we observed also at this power ratio (70%) that the 1/4 and 1/2 waveplates were not perfectly centered and we burn a part of the platic mount at this power.
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