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this afternoon, I firstly connected the HV output of the Laselock to the FP-cavity PZT : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/248
locking the FP-cavity on the local synthesizer works better with the improved HV output range.
the ~20Hz oscillation in the beating signal of the FP-cavity/synthesizer is still there.
one can observe exactly the same oscillation on the Laser PZT signal.
I think the FP-cavity is oscillating at this frequency and the laser PZT correction signal is compensating it.
this oscillation produces also an error signal with the beating of the laser harmonic and the synthesizer.
an other possibility for this low frequency oscillation could be the hexapod stability ?
then, I switched to the 500MHz Ring Synthesizer instead of the local one.
we were able to lock all the elements, FP-cavity and Laser, in a same time.
we have Vp=3.25V of signal amplitude when not locked and dVrms = 140mVrms of rms noise once locked => rms jitter = dVrms / (2pi F Vp) ~ 13.7ps rms
with F=500MHz.
reaching a state where both loops are locked is not simple as :
- the FP-cavity motors are noisy when they move, and one direction has some backlash compensation which produces a long unlock.
- the laser motors are less noisy but too noisy to be moved w/o unlock
- moving the FP-cavity motor changes the laser PZT position and can put the system out of the locking range for the laser PZT.
the correct strategy is :
1) lock the laser on the FP-cavity using the laser motors (no lock of the FP-cavity compare to the synthesizer).
2) wait to have a quite good thermal stabilization => this can take time especially if it is the first lock of the day when the cavity is cold.
3) measure the frequency beating between FP-cavity and synthesizer.
4) pre-compensate the direction of the laser PZT with the laser motor when you will play on the FP-cavity motor
ex : a step on the right on P4 motor will move the laser PZT to the top, then you need to pre-compensate this move by placing the laser PZT voltage lower with the laser motor (Smaract).
5) do the move on P4 motor
6) check that the frequency beating between FP-cavity and synthesizer is reduced
7) redo 4) 5) 6) until the frequency beating is < 5Hz and stable
8) start the lock on the FP-cavity PZT.
due to the power drifts inducing frequency drifts, it will be difficult to work continuously more than a couple of minutes.
the procedure has to be done continously from 4) to 8)
having HV output on the laser PZT should help to improve both lockings range and inscrease the locking duration => to be checked w/o fast feedback loop (FFL) or with FFL if needed.
| Ronic Chiche wrote: |
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this morning, I added a Thorlabs lowpass filter at 1kHz after the mixer and a resistor at the Laselock output to the FP-cavity PZT.
I set the PID with P=10 and I=D~0
in attachement, a plot of the 500MHz mixer output, before and during the lock.
during the lock, one gets a residual oscillation around 20Hz... where does it come from ???
could it be some mechanical resonance, as we already had before (it was ~30Hz) : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/84
when unlocked, one gets : A*sin(dPHI) with A=3V => 6Vpp signal
when locked, one gets a 400mVpp error signal => A*dPHIpp = 0.4pp => dPHIpp = 133 mrad
dPHI = 2*pi*F500M*dt => peak-peak jitter dtpp = 42 ps => rms jitter = 15ps
this value can be measured directly on the 500MHz signals coming from the laser and the reference synthesizer.
| Ronic Chiche wrote: |
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this afternoon, I tried to lock the FP-cavity on a local 500MHz synthesizer as a frequency reference
(it's faster to change the frequency from the sythesizer than from the FP cavity !!!)
I found some parameters on the PID of the Laselock to phase-lock the reference and the FP-cavity
but the locking quality is poor => we produce some oscillations which are copied by the FP-cavity/laser lock on the laser PZT.
the error signal (FP-cavity/reference) is quite noisy, then maybe one can try to do some analog filtering at a lower frequency ?
=> one can use Thorlabs LPF at 10kHz or 1KHz.
one can try also to filter the signal going to the PZT to reduce the excitation of the PZT resonance.
=> use a variable resistor in serie with the PZT capacitance to make and RC filter (Cpzt ~ 70nF)
for reminding, the previously working PID parameters, found with the 133MHz laser/533MHz reference lock, were:
P=10, I=0.01, D=0 for mid SR, and no filtering.
once we will found good PID parameters, we will be able to switch to the Ring oscillator at 500MHz as a reference,
then work with HV from the Laselock to improve the locking range.
| Ronic Chiche wrote: |
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this afternoon, I installed the frequency detection scheme in the bunker :
- I changed the large reflected photodiode DET100 by a fast small DET10.
- a 50ohms splitter is connected to the photodiode :
* one cable is going directly to the scope for monitoring
* one cable is going to the 500MHz sharp bandpass filter to select only this harmonic.
despite the small BW of the filter, one gets 3 harmonics : 500MHz + (0 +/- 33) MHz with a bit less power on sidebands.
power on 500MHz : ~ -30dBm if the laser amplifier is at 30%
- after the BPF, one goes directly to the RF amplifier which is 2x Minicircuit amp ZX60-33LN in cascade to get ~ +3dBm
- then one goes to the level 17 mixer on the RF port.
the LO port is feeded with the 500MHz coming from the Ring reference oscillator.
in attachement is a picture of the output signal : ~ 1.5Vpp
I think the width of the signal is coming from the 33MHz sidebands which are not perfectly removed by the 20MHz BW of the scope.
one finds a frequency shift of 250kHz.
as the laser frequency has been set at 500.25MHz (on the 15th harmonic), would it mean the Ring reference oscillator was at 500MHz sharp ? => to be checked with Kevin.
| Ronic Chiche wrote: |
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Today, I connected the BNC-DB9 female prolongator cable to the FP-cavity PZT cable (DB9-DB9) and to the channel B of the Laselock.
I additionnaly connected the reflected laser beam (normally connected to scope ch2) to the spectrum analyzer @ 1GHz to observe the 30th harmonic of the laser comb.
on the 2 attached pictures, the 1st one is @0V on FP-cavity PZT and the 2nd one is @10V on the FP-cavity PZT.
as the laser is locked on the FP-cavity, its frequency follows the FP-cavity length and its frequency changes.
as expected, applying 10V on the FP-cavity PZT, increases the laser harmonic frequency @ 1GHz by about 10Hz => the relative change is 10^-9 V^-1
(as the FP-cavity length is always moving due to temperature or low frequency vibrations, there are always some FP-cavity length fluctuations, and the measurement has to be quick to get a correct evaluation).
| Ronic Chiche wrote: |
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Today, I installed a DB9-DB9 female-male cable on the PZT connector of the FP-cavity.
the PZT is connected between pins 1 and 2, with a capacitance around 70nF.
I need to make a prolongator cable BNC-DB9 female to connect it to the feedback system.
with a (dLpzt / dV) of 5nm/V, one should be able to see 37mHz/V on Frep which is equivalent to ~10Hz/10V @ 1GHz (30th Frep harmonics)
I connected a photodiode on the spectrum analyzer to measure this variation => to be done tomorrow.
I don't have any information about the polarity of the PZT on the DB9 connector but I know that the PZT length should increase with positivite voltage in normal operation.
from the Yann documentation about the PZT mount (in attached file), it should mean that the cavity length should decrease when the PZT length is increasing, and then the FP-cavity FSR should increase.
=> to be tested tomorrow.
about the PZT mount :
I understand that the HR face of the P4 mirror is the face placed on the only part with a chamfer (chanfrein), on the FP-cavity side.
in the documentation, the PZT connector orientation is misleading as it is oriented to the FP-cavity side instead of to the "outside" as one can see it in the cavity picture in attached file.
| Ronic Chiche wrote: |
- régler le problème 1:
Kevin m'a apporté un déphaseur Minicircuits JSPHS-661 (400-660MHz / 180° de phase) qui permet de déphaser le 500MHz de ~ 1ns avec une tension DC 0-10V.
on peut alors changer le signe du lock pour scanner les 2ns d'une période complète de 500MHz.
- régler le problème 2:
la synchro anneau se fait sur la RF du synthé 500MHz avec une signal de trig fabriqué à partir d'un 16MHz, issu d'une division de ce 500MHz.
en cas de perte de synchro de la cavité FP, on va relocker sur le 500MHz mais avec une phase aléatoire par rapport au 16MHz.
on peut donc remplacer ce 16MHz par le signal 33MHz issu du laser de telle façon que l'injection des électrons dans l'anneau se fera toujours avec la même phase par rapport à ce signal à 33MHz.
il faudra donc envoyer ce signal issu du laser cavité FP au système de synchro anneau, de cette façon la phase d'injection des électrons dans l'anneau par rapport au laser sera toujours la même.
mais il n'y a aucune raison que les électrons tombent exactement sur le pulse laser (avec la bonne phase).
il faudra donc scanner la phase du signal de trig pour décaler l'injection machine par rapport au signal 33MHz avec des steps ~ 1ns.
pour cela, on peut utiliser les générateurs de delais Greenfield Technology GFT1020 actuellement utilisés pour la synchro (résolution 100ps).
voir schéma attaché en pdf
| Ronic Chiche wrote: |
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pour préparer le lock cavité-anneau, j'ai un setup de lock en salle optique entre le laser OneFive 133MHz et un synthé à 533MHz (133MHz x4).
ce matin, j'ai pu locker les 2 ensembles avec le laselock avec une stabilité RMS, je pense inférieure à la ps.
ma limite de mesure du jitter temporel au scope est de ~ 2.5ps.
- une fois locké avec le laselock, je peux facilement décaler légèrement en phase les 2 signaux de façon très précise (<1ps) en jouant sur l'offset de lock,
mais je ai une plage assez petite (+/- 250ps) qui correspond grosso modo aux plages linéaires du sinus (1/4 de période) soit 500ps (F ~ 533MHz => T ~ 2ns)
en changeant le signe du lock, je peux faire des sauts du lock d'une 1/2 période, soit 1ns...
mais cela ne suffit pas à couvrir l'intégralité de la période du signal de référence.
=> 1er problème : je n'ai accès qu'aux plages "linéaires" du signal de référence.
il faudrait un petit déphaseur programmable piloté en remote pour faire des steps de 100ps environ, sur une plage de 1 ou 2ns afin d'être sur de scanner tout la période du 500MHz.
- en coupant le lock, les fréquences driftent l'une par rapport à l'autre.
et en raccrochant le lock, on peut scanner toute la période entre 2 pulses d'électrons par steps de 2ns.
puis en changeant le signe du lock, par steps de 2ns mais décalé de 1ns.
on peut donc facilement scanner la période des électrons avec des steps de 1ns et une plage de 500ps autour de chacun de ces steps.
=> 2e problème : lorsque l'on perd le lock de la cavité FP/laser involontairement, on perd l'info de la longueur de la cavité FP.
et lorsqu'on retrouvera le lock, il va se raccrocher sur une autre oscillation du 500MHz.
et donc on va perdre la phase avec les électrons à 16MHz.
=> on peut éventuellement afficher ce signal à 16MHz, en même temps que le 500MHz pour rechercher l'oscillation correspondante à la bonne phase sur le 16MHz.
mais dans tous les cas, il faudra rechercher à nouveau la phase é-/laser à chaque délock.
- Autre possibilité, faire la synchro sur le 16MHz au lieu du 500MHz.
la tentative aujourd'hui n'a rien donné car le signal est 30x moins intense => beaucoup plus de bruit.
=> le lock n'arrive pas du tout à accrocher même en filtrant énormément le signal IF avec 10kHz de BW.
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