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20Hz oscillation in the locking between Laser and FP-cavity, posted by Ronic Chiche at ThomX igloo about mechanics | lasers and optics | detectors and electronics

20Hz oscillation in the locking between Laser and FP-cavity, posted by Ronic Chiche at ThomX igloo about mechanics | lasers and optics | detectors and electronics

20Hz oscillation in the locking between Laser and FP-cavity, posted by Ronic Chiche at ThomX igloo about mechanics | lasers and optics | detectors and electronics

Message ID: 259 Entry time: Fri Sep 15 18:11:07 2023 In reply to: 258 Reply to this: 324

Author:	Ronic Chiche
Status:	Under Process
Type:	issue
Category:	mechanics \| lasers and optics \| detectors and electronics
Location:	ThomX igloo
Title:	20Hz oscillation in the locking between Laser and FP-cavity

this afternoon, we did 2 tests to better understand this 20Hz oscillation:

- we locked the amplified laser directly to the 500MHz ring reference oscillator, without any intermediate locking to the FP-cavity => no change
the 20Hz oscillation is still present in the correction signal of the laser PZT.

- we switched OFF the controller of the hexapod => no change.

conclusion:
the 20Hz oscillation is coming from the laser cavity
or is coming from "outside" and could be measured, maybe at a higher level, with an external "noises & vibrations measurement system".

Ronic Chiche wrote:

this morning, we tried to find the origin of the 20Hz oscillation.

- we switched OFF the laser Smaract motors controller => no change

- then, we addionally disconnected the FP-cavity PZT cable from the Laselock (we put a charge of 1kohm) => no change

- then, we switched ON the laser Smaract motors controller and switched OFF the FP-cavity motors controllers => no change

in conclusion, we don't really know where this instabillity comes from.
the amplitude is roughly 1Vpp (when the oscillation is at its maximum) on the laser PZT <=> length oscillation of ~20nm pp

could it come :
- from the air cooling regulation with pressure variation ?
- from vibrations of the hexapod below the table ?
or is it from inside of the laser or FP cavities ?

see these posts for the first measurements on this issue: https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/257

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