this morning, with Vincent :

- we added the 10Hz trigger to the CH2 on the remote scope (192.168.229.21) to check the synchronization at the right timing.
CH1 : 33MHz RING
CH2 : LINAC/RING synchro signal
CH3 : 500MHz RING
CH4 : 33MHz FPC

we successfully lock both laser on FPC (87kW for 33% amp ratio) and FPC on 500MHz RF.
we saw the result on the scope.
suprisingly, the FPC/RF lock seems much robust than before.

the only problem is the MOT.06 which make the lock being lost at almost every move.
one reason could be the rust on the mechanics... we can try to change the position of both plan mirror motors to work in a proper region.
before, it was working well at -900 000 steps on MOT.06, now it is -780 00.
it's a quite long travel...

end of this posts thread

after trying to increase the power and after moving S3 to try to change the size of the beam on the mirrors, the FPC power suddenly droped.

so, we need to remove M1 to check if it has been damaged.

we placed an iris in reflection of the FPC to keep the alignment for the new mirror M1.
the alignment has been done with an iris and the amplifier at 10% (to get a quite round beam) and the beam profiler.
once the iris is almost closed, the alignment is good, using the diffractions rings.

this morning, with Daniele, we took some images of the M1 mirror HR surface :

1- large image span of the mirror surface (~center) before being cleaned :
one can see a lot of spots

2- zoom in on some spots in the middle of the mirror.
Daniele saw 2 suspicious spots among many.
difficult to be sure if it's only some dust or if it's some damage

3- large image span of the mirror surface after being cleaned with the spin coater machine
only the 2 suspicious spots remain, all others have been washed.

4- zoom in on the 2 suspicious spots
if one compares with the image before cleaning, one can see the spots are much larger

5&6- zoom in on the 2 suspicious spots separately after a 2nd cleaning process with the spin coater,
it seems the spots are even a bit larger than before

conclusion : it seems that these spots are presently showing some damage on the mirror surface.
it is strange that there are 2 in the same time.
one possible explanation is that, as we had to move the z-axis motor before, and we know that the mechanics are rusty, it's possible that it released some metallic particles on the mirror surface.
and 2 were in the beam area.... 

This afternoon, we installed a new M1 mirror, the only one for which the box was not already opened.

as the mirror should be "brand new", as the LMA has packaged it, we decided not to clean it with ultra-pure water and to install it out of the box.

we realigned it with the iris previously installed for that purpose.

primary and turbo pumping are in process. one has to wait until the vacuum level is low enough to start the ionic pumping and to reopen the isolating valve.

let's cross our fingers...

today, with Daniele, we did a simple polarization measurement in transmission of the FP cavity beam.

1) we locked the FP cavity and measured 82.5kw (for 33% amplifier ratio) with the power-meter moved a little bit further (this is maybe the reason why the power is a little bit lower than usually measured).

2) then, we installed a high power PBS CCM1-PBS25-1064-HP(/M) with a transmission of 89% of the P-polarization @ 1030nm
We measured 66.5kW in transmission of the cube.

3) then we installed a half waveplate to find the optimum angle => 77kW measured at an angle of 342°
(almost no power has been observed in the vertical polarization state of the PBS).

Then we rotated the waveplate to find the previous measurement => 66.5kW at an angle of 348°
(we checked that adding the waveplate in the path almost does not change the measured power)

Conclusions :
- the FP cavity polarization is almost horizontal (P-polarization) and linear (77kW ~ 89%*82.5kW)
- the angle of the polarization is roughly 2*(348-342)° = 12°

Today we did several long runs (~1h each) at ~23kW, 46kW, 66kW with different PID parameters which seems better.

P = 0.1
I  = 0.0015
D = 1.5

which implies a different fast loop gain.
For these new PID parameters, it was impossible to use 33% amplifier ratio => to much power on the PID at the diffuser limit (axis 18).
so, we added a NE02 optical attenuator on the mobile diffuser => we can't use the old recipies anymore.

This morning with Viktor, we opened the 2 vessels and removed the 4 FP-cavity mirrors.

all the mirrors have been put in plastic boxes with labels to indicate which mirror it is,

and with all the HR coatings face down.

the mirrors are in the PLIC room in the "THOMX MIRRORS" box.

Today with Daniele, we did 6 long runs at different power (23kW, 46kW, 66kW, 73kW, 92kW, 92kW)

All the lock loss in between these several runs are due to FPC locking parameters change.
most of the few lock losses during the stable power duration, are due to 20Hz oscillation noise or because we forgot to center properly the PZT in its range (operator faults).

the 4 first runs (23kW, 46kW, 66kW, 73kW) are using the PID : (P=0.1 / I=0.0015 / D=1.5) without the additionnal optical attenuator placed after the diffuser.

the 5th run (92kW) is using the PID : (P=0.1 / I=0.0015 / D=1.5) with the additionnal optical attenuator placed after the diffuser.

the 6th run (92kW) is using the PID : (P=0.05 / I=0.0005 / D=0.6) and obviously a different diffuser position) with the additionnal optical attenuator placed after the diffuser.

surprisingly, we never saw any high frequency noise during the day !

to be noticed : the electron machine was OFF / the day was sunny without wind / almost nobody was working in the bunker.

today, long run directly at full power (33% amplifier ratio) => 98kW in the FPC at the very begining of the run.
(P=0.05 / I=0.0005 / D=0.6)

10-40 min : almost misalignment effect (CEP did not change so much)
                   I increased 3 times the amplifier ratio to 34%, 35%, 36% to compensate the misalignment and keep the power almost constant.

then I did a full tuning (CEP + alignment)

45-110 min : misalignment effect after ~30min of warming up of the FPC => much more stable.
                    I corrected twice (@ 85min) the alignment to compensate a bit the power loss

globally, the FPC seems stable => all the lock losses come from the 20Hz noise and are recovered very quicly by the locking.
I never saw the high frequency noise which can produce long lock losses.

maybe it's time to add the gain x3 on the laser PZT channel to get some room on the 20Hz noise compensation.

during the last days we changed the FP-cavity - RF reference locking system to force the right bucket:
in addition to the 500MHz beating which is the signal input to the PID, we added a 33MHz beating signal which is the trigger signal of the PID.
then, the lock is triggered only when this trigger signal is close to 0.

as the peak input signal is Ap = 0.5V, the beating signal is Ap*sin(phi)
one needs a phi determination below 2pi/15 which correspond to a particular 500MHz bucket (500MHz/33MHz = 15).

so, Ap*sin(+/- 2pi/30) = +/- 0.2*Ap = +/- 0.1V
we should be able to put a trigger intervalle value between -0.1V and +0.1V to always fall into the same bucket.
we did a test with -0.05V et +0.05V and we always fall into the same bucket => we will have to test the intervalle -0.1V / +0.1V to check if it is ok or not.

as the crossing is around 0V, there is 2 possible locking trigger phase position with Pi intervalle.
for the moment, there is no way to get rid of this problem. one can only let the phase drift until it reaches the right phase to start the lock.

if one tries to detect the maximum or minimum value to fall into the same bucket, the voltage level is very sensitive :
Ap*sin(pi/2 +/- pi/30) =  0.995 * Ap => it has to be sensitive at 0.5% !

we changed also some locking condition on the 2 Laselock PID regulators : see the attached picture.
search and relock : reset mode with 0V for both => when the 2 PID are losing the lock, they search a new lock starting with the same 0V value on the PZT (it can prevent they are out of range in some cases)
with these condition, it seems the lock is quicker to come back.

we can check the phase bucket with the remote oscilloscope at the IP address : 192.168.229.21 (see attached picture)
blue : trigger signal @ 10Hz (will be 50Hz in next ThomX update)
red : reference 33MHz signal, phase linked with the 500MHz of the ring
yellow : laser 33MHz signal (need to have always the same phase relation with the reference 33MHz when locked to RF reference)
 

the rms jitter with the RF reference is still ~15ps => to be improved...

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.

• 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

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.

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).

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.

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.

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.

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.

this afternoon, we were able to lock the laser on the FP-cavity and the FP-cavity on the Ring reference oscillator in a same time from the control room.

the procedure when laser, cavity and reference oscillators are far in frequency, is :
1) start the amplifier to get the laser signal in the reference photodiode used to measure the 33MHz.
2) look at the beating frequency with the reference oscillator and quickly cancel it using the Smaract motors.
one need to remember the direction and distance corrected by the Smaract motors.
3) compensate the motion in the FP-cavity using these rules :
         - 600nm with the Smaract motors <=> 100 steps on the FP-cavity motors
         - decreasing the value on the Smaract motors <=> increasing the value on the FP-cavity motors
4) start the lock between the laser and FP-cavity
5) adjust both cavity length step by step until reaching < 5Hz of beating
6) start the lock between FP-cavity and reference oscillator

we still see the 20Hz noise both in the PDH error signal and in the laser PZT feedback signal.
it could mean :
    either the FP cavity is stable but the Laser cavity is oscillating (noise source) and badly compensated by its own PZT
    or the FP cavity is oscillating (noise source) and the Laser cavity is oscillating too (due to feedback) but badly compensated with the laser PZT
the bad compensation in both case could come from a mechanical resonance which produces a phase jump that the PID is not able to properly compensate, even with a high gain (integration).
=> to be checked with a behavorial simulation
the noise source in the laser cavity could be the Smaract motors.
=> we need to switch them off to compare the noise with and without

after a sudden FP-cavity Finesse increase, we had to install the Fast Feedback Loop in order to continue to lock the cavity.

this afternoon, I was able to lock both the FP-cavity and the Laser on the 500.25MHz oscillator reference.

I had to change the CEP to reduce the effective Finesse and help to get a better stabililty => P ~ 25kW inside the FP-cavity for 30% of laser amp. power ratio.

the 20Hz oscillation is still there on the PZT signal and can be seen also on the reflected signal when the amplitude of the PZT signal is large or when the lock is lost or almost lost.
this 20Hz signal is modulated by a higher frequency of the reflected signal => to be investigated...

the posts, specific to the 20Hz oscillation noise, are here : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/258

this post closes this thread.