| Quote from #5894 |
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(Things to be done)
[DONE] 1.1 Measurement of the arm fluctuation => to allow re-designing the servo shape
[DONE] 1.2 temporary SR560 servo
[ONGOING]1.3 Sanity checks on the modulation depth, reflectivity, PD dark noise and etc.,
[DONE] 1.4 Make the servo more robust
[DONE] 1.5 Some modifications on the medm screens
[NOTYET] 1.6 Activation of the temperature feedback through the realtime digital control
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Some updates on the Y end green PDH lock
(Measurement of the Y arm fluctuation)
In order to design the PDH box's servo shape we wanted to measure the Y arm fluctuation.
Here is the spectrum taken by looking at the control signal before the laser PZT.
The scale of the Y axis is calibrated by using the PZT response of 5 MHz/V.
Above 10 Hz the spectrum shows 1/f noise which I believe the laser frequency noise.
(Temporary servo setup)
We have found that the servo shape was not enough ( #5890) to well-suppress the fluctuation shown above. Since the Newfocus fast servo box only makes 1/f shape, the error signal wasn't suppressed within the linear range.
So I have added an SR560 in the other input of the Newfocus servo box to make the filter shape 1/f^2.
Then the lock became more solid and the reflected DC light in time series is now much flat if the alignments are good.
I will post the servo shape and diagram later.
(Sanity checks)
I looked at the reflected DC light when the laser was kept locked.
The reflectivity of the Y arm cavity went down to about 30% and this is good because it is supposed to be 27.5% when it is locked according the spec.
This means the mode-matching is not so bad. |