This morning with Daniele, we operated the FP-cavity to be sure there is no issue due to the Linac section changing operation :
- opening/closing the valves due to air pressure break down
- opening/closing the bunker roof
- large temperature change
- etc...

the temperature in the bunker dropped to 18°C instead of 21.5°C, so we had to do some FP-cavity alignment.
but after obtaining resonances and changing the CEP, we got more than 50kW for 33% of amplifier ratio => OK !

This morning, I locked again the FP-cavity at more than 50kW with 33% of laser amplifier ratio.

the temperature is measured today at 20.5°C despite the fact the air conditionning is still OFF.
the temperature, rising from 18°C, started monday at 11AM roughly.
maybe, the difference comes from the external doors of the Igloo which have been closed recently ? to be confirmed.
as the temperature is measured inside the airflow housing, all the powered equipements are disspating some heat, rising the inside temperature compared to the bunker temperature.
EDIT : Sophie Chance told me the water temperature cooling of ThomX was defective... the water temperature went to 29°C ! it is being fixed by Dalkia.
careful, we don't have a temperature measurement of the water going to the chiller...

to find resonances, I moved the OC/OP/OCH.01-MOT06 motor from position -770 000 steps to position -788 600 steps which corresponds to ~100µm !
it can be explained by the temperature change and also because during the lock last week, I let the MOT06 motor in position after the cavity was at 50kW.
I had also to change the CEP motor position from -170µm to -420µm and to do some alignment (with walking procedure).

accessing the motor OC/OP/OCH.01-MOT03 from the ATK panel, produces an error : "connection to device failed"... to be solved by Kevin ?

from now on, the transmission factor for calculating the FP intra cavity power is 1/1.75ppm ~ 570 000 (changed in the Powermeter software)

this morning the temperature of the water cooling circuit is still very high said Sophie, and the laser amplifier temperature "Temp Amp1" is at 29°C instead of ~23-25°C
then, when I start the laser amplifier, it stops rapidly with a warning error.
I will check the chiller status as the Dalkia should fix the water temperature problem at some time (Sophie + Alice working on this point)

the air temperature in the bunker is back at 19.5°C from last Friday afternoon and is stable at less than 0.5°C.

after restarting the chiller, the "Temp Amp1" is now at 25°C before starting the power in the amplifier, it rises at 28.5°C after some minutes
I found the resonances with the motor OC/OP/OCH.01-MOT06 at position -783 460 steps.
and I had to tune the CEP to get the maximum resonances at -198µm.

with the new transmission factor, I got almost 90kW for 33% laser amplifier ratio after a simple alignment procedure.

the motor OC/OP/OCH.01-MOT03 is sill in error.

this morning, I had to restart the chiller. Temp Amp 1 is at 27.5°C after having started the amplifier at 33% ratio.

I removed the 100kohms resistor on the Transmission channel, on the scope (now, it is 1Mohms), to get more sensitivity.

the air temperature is around 21°C and is stable during the last days.

I don't see any power coming from the amplifier despite it is at 33%... I think the MPS prevent us to work and the amplifier shutter is blocking the beam.
I will send an email to Sophie to check that point.

EDIT : Kevin showed me how to acknowledge the error on the MPS
=> go to "Detail" and then acknowledge the "vacuum error".

I saw the FP-cavity resonances but I didn't have time to optimize them as the water temperature is still too high and the chiller is not working good enough.
=> the amplifier did a safety stop.
one needs to wait this issue is fixed.

OC/OP/OCH.01-MOT06 at position -790 000 steps.
CEP motor = -300µm but far from the optimum CEP position.

this morning, the computer was restarted and I had to launch all the applications.
no problem except the powermeter for which I had to do a power switch OFF and ON remotely.

the chiller was in error and I had to restart it also.

the air temperature seems to be not properly controlled (see the picture)
one can see a constant temperature drop from Tuesday 9 April 10:30am.

it seems the water cooling is also not working properly as I saw the laser amplifier temperature (Temp Amp 1) increased constantly during the run from 24°C to 30°C (in 30 mintues).
=> it's not normal, it should be regulated at 25°C !

I got 89kW in the FP-cavity with 33% of laser amplifier ratio after tuning the CEP and the alignment.

OC/OP/OCH.01-MOT06 roughly at position -782 000 steps.
CEP motor = -425µm

the OC/OP/OCH.01-MOT03 is still faulty => to be fixed by Kevin.

this morning, the chiller was in error and I had to restart it.

the air temperature is around 21°C.

the water cooling has been repaired but the initial Temp Amp 1 is at 28°C.

I got 86kW in the FP-cavity with 33% of laser amplifier ratio after tuning the CEP and the alignment.
but the amplifier stopped after an error after 30 minutes => the Temp Amp1 reached 30°C => it's not normal.
the chiller was again in error. I tried to restart it but I got an ERROR14 : Thermostat error, the flow rate is zero !!!
the tubes for the inner circuit going to the amplifier are a little hot by touching them with my hand.
the tubes for the outer circuit going to the ThomX water circuit is hot for the "blue" tube et cold for the "yellow" tube.
it means there is no flow in the outer circuit.
Jean-Noel just told me it's normal because he stopped the ThomX water pump because of a too high temperature => Dalkia should fix the problem today.

OC/OP/OCH.01-MOT06 roughly at position -792 600 steps.
CEP motor = -211µm

the OC/OP/OCH.01-MOT03 is still faulty => to be fixed by Kevin.

Yesterday, the ThomX water circuit should have been repared,
and yesterday aftenoon, Daniele restarted the chiller and I checked it this morning without any error.
BUT the pump of the ThomX water circuit is again in default...
Dalkia has been called to fix the problem.

despite this point, I restarted the laser amplifier this morning.
everything seems as usual and I can see some small resonance at the output of the Fabry-Perot cavity
BUT the reflection photodiode level doen't change when the amplifier is ON.
I need to check if the photodiode switch is set ON or if it is misaligned or dead.... to be checked.
in between, I prefer to stop the laser amplifier.

edit : the photodiode problem was coming from the PhD power supply which was OFF.
I just turned it ON.

the air temperature is between 21°C and 22°C.

I tried to lock the cavity but it was very instable with only short time locks.
I had to increase the parameters of the PID:
P = 0.07
I = 0.0007
D = 0.706
after that, the lock was stable but we can see something in transmission (dark blue curve) which seems to be a degeneracy.

maybe because of the water cooling temperature, after stopping the laser amplifier, I was not able to restart it.
I always got an error (an alarm was triggered by bad sequence) when I want to switch ON the preamplifier at 0%.
the Temp Amp 1 is about 25-26°C... it is stange.
to be tested after the water cooling is repaired.

today, I tried to restart the amplifier at 0% (without 3rd stage) but it didn't start... still the same error message.

but the PD_IN power and the PD_PULSE frequency seems correct.... I will try to power the amplifier OFF and ON later to see if one can reset this error.

edit : finally, a the end of the day, I switched OFF and ON several times the amplifier but I always get the same error message "bad sequence error".
I tried to use the Alphanov software to see if we can get more information about the error :
the watchdog LED is RED => I have to check if it is normal or not before starting the amplifier
and the PD_CRI LED is RED => normal because the preamp stage is not started (and it does not want to start...)

PD_IN is at 3.1mV but a previous post says that the amplifier worked with 2.7mV.
we can try to increase to 3.2-3.3 mW and see if it works....

edit : it seems we already had this kind of error before.
Guillaume suggested to check the average power of the seeder at the output of the fiber, the repetition rate and the stability of the signal.
previous post (https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/179) have shown an input power (PD_IN) around 5.7mW (see the attached image) !
this power was maybe obtained without any EOM which divide the power by ~2.
=> we can try to remove it temporarilly to check if the amplifier is able to restart in this condition... to be done

today with Daniele, we disconnected the EOM to increase the injected power of the amplifier, we were at 4.8mW but we still have the "bad sequence" error => email to Guillaume
we tried also several time again, to stop and restart the amplifier, but it didn't help.
I connected back the EOM and the power dropped to 2.7mW instead of 3.1mW => fiber injection alignement to be done

last week, we measured the transmission of a plan mirror from the same batch than the ThomX mirrors and we got ~ 1.75ppm instead of 3ppm which was the calculated value by the LMA.

previously, we put 1/3ppm = 333 333 as multiplication factor.
from today, we will put 1/1.75ppm ~ 570 000 as multiplication factor.

then, previously we got 50kW, now we should get 85.5kW instead in the same conditions.

Today, we restarted the FP-cavity locking, after 3 weeks without using it.
those 3 weeks have been used to set the machine ring at the new (better) frequency : 500.067MHz (corresponding to a change of -183kHz)
or 33.3378MHz in fondamental frequency (corresponding to a change of -12.2kHz)

the power supply of FP-cavity photodiodes was OFF maybe due to some electrons losses.
we had to switch it OFF and ON to restart it.

the computer was restarted and we had to restart the applications and set their parameters.
in attachement, the parameters of the Kangoo software.
we had several "warnings" from the laser amplifier software => the amplifier stopped
but maybe it's because of the electrons not sent in the dump which pertubate the monitoring signal levels of the amplifier => to be confirmed

we had to do some alignment.
with 33% of  amplifier ratio, we had 49kW inside the FP-cavity.
we tried to move a bit the L-shape mirror but without any significant effect to improve the intra-cavity power.

we didn't start the laser and FP cavities length increase procedure.

surprinsingly, the beating frequency between laser cavity and 33.3378MHz is only 140Hz and not 12kHz as expected.... to be investigated
 

this morning I solved the problem of the 33MHz frequency beating which should be ~12kHz between the generator (at the new frequency 33.3378MHz) and the laser cavity frequency (still at the old frequency which is 33.35MHz)...
the generator providing the 33MHz frequency has 2 channels, and only one was set at the proper frequency !

so, I set both channels at  33.3378MHz and now I measure properly ~12kHz of frequency beating.

now, we can move the laser cavity and the FP cavity in confidence.

this morning, I finished the FP-cavity alignment with the new ring RF frequency.
I get back 50kW @ 33% amplifier ratio.

I'm also able to lock very smoothly on the 500MHz ring RF frequency.
so, we are ready to produce X-ray.

warning : apparently, when one changes the 33MHz generator frequency with the "Nicolas" script, it does not change both channels of the generator, which is mandatory for us.
=> to be fixed : freq(CH2) = freq(CH1)

new optimized lock parameters in attached file (Kangoo parameters)

no more "Alphanov amplifier warnings" when one works without the ring machine => it is important/mandatory to send the electrons in the dump.

 

at the beginning of the procedure, the frequency gap between the new Ring 33MHz frequency (33.3378MHz) and the laser/FP cavities frequency was 12.33kHz
=> the Smaract motor position was at +100µm
=> the FP cavity motor Mot.03 position was at -358 720 steps
the PDin photodiode was at 3.151mW @ 33% amplifier ratio
the PDpulse photodiode was at 33.384MHz

after several moves (each time, one corrects the CEP / alignment to keep ~ 47kW inside the FP-cavity)
we can move the laser cavity at 300nm/s without any laser modelock loss
we move the FP cavity at the same speed (300nm/s = 50 steps/s with 1step  = 6nm)

now, we did roughly half of the travel : dF @ 33MHz = 5.3kHz
=> the Smaract motor position was at +1075µm
=> the FP cavity motor Mot.03 position was at -200 000 steps
the PDin photodiode was at 3.178mW @ 33% amplifier ratio
the PDpulse photodiode was at 33.377MHz

This afternoon, I did the 2nd half of the travel: dF @ 33MHz = 0Hz
=> the Smaract motor position is now at +1750µm
=> the FP cavity motor Mot.03 position stayed at -200 000 steps
=> the FP cavity motor Mot.06 position is now at -790 000 steps
the PDin photodiode was at 3.191mW @ 33% amplifier ratio
the PDpulse photodiode was at 33.372 / 33.371MHz

the FP-cavity power is ~47kW @ 33% amplifier ratio => to be improved

there is no signal beating at 500MHz, only at 33MHz => to be investiguated => fixed

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.

This morning with Daniele, we changed the laser/RF locking scheme :

1) we use the laser 33MHz internal photodiode to do some beating with the 33MHz RF generator (A Rigol generator locked on the 500MHz Ring oscillator).
this beating is only for monitoring purpose, in order to manually adjust the laser frequency at the beginning of a run.

2) the previous scheme was using a fast photodiode in reflection of the FP cavity to do some beating with the 500MHz Ring RF generator,
but the beating signal was becoming very noisy when the FP-cavity was locked.

so, we moved this fast photodiode in transmission of the FP-cavity, in addition to the slow one (already connected to the scope to monitor the transmitted power).
this fast photodiode is used to extract the 500MHz harmonics to make the beating with the 500MHz Ring RF generator.

now, this beating signal is very clean when the FP-cavity is locked (no signal when the FP-cavity is not locked, obviously).

=> the acquisition of the lock of the 2 feedback loops seems much more easy and stable.
BUT we did a jitter measurement and it is still around 10-15ps rms and seems to be dominated by the 20Hz noise oscillation !!!

some previous measurements showed that this oscillation should come from the laser or from the Alphanov amplifier (not from the FP-cavity).
we have to redo a beating test with the Alphanov amplifier @ 500MHz and with the laser @ 33MHz...

laser cavity :

when one decreases the laser motor position, the laser repetition rate increases (laser cavity length decreases).
=> +/- laser motor step  =>  +/- laser cavity roundtrip length => -/+ laser repetition rate => -/+ laser harmonic @500MHz 
=> +/- 100nm => +/- 200nm => -/+ 0.7Hz @33.33MHz => -/+ 10Hz @500MHz

here is the natural variation of the laser cavity frequency beating with RF @500MHz over 1h (~1.6s / iteration)
one can see some oscillations equivalent to ~1µm of roundtrip length with ~10 minutes period and maybe a slower drift or oscillation with ~2µm of roundtrip range over the hour.
I mention that I moved the laser "PZT" motor before taking the data : could it be the reason of the 10-20min oscillations ?

during the same time, here is a probe temperature curve (the probe in stuck on the end flange, close to X-hutch, of the FP-cavity, inside the housing... not close to the laser position).
the temperature variation range is ~2.5/100 °C which induces on inox (relative length thermal effect : 17e-6 /K) a length variation of 4µm of roundtrip (10m) which could be compatible to the laser cavity length variation measured.

Now, I placed a temperature probe stuck on the laser housing itself.

fig. 1

one can compare the temperature measured at the surface of the laser housing and the beating frequency with the 500MHz reference oscillator
one sees a possible very long term correlation but there is no correlation at the minute level when we see the frequency oscillation after t=2000s.

the laser housing temperature seems not to induce directly a frequency variation.

fig 2 / 3

we applied 15W on the heating wire rolled around the FP-cavity flange.
in red, we see the temperature increasing on the probe rolled around the wire, reaching almost 30°C.
we heat the inside of the housing (airflow stopped) during more than 30 minutes
in green, we don't see any variation (even if one makes a zoom) of the temperature of the probe stuck on the laser housing.

in same time, on fig 3, one can see the frequency drift.
there is no correlation between the oscillations and the temperature.

CONCLUSION :
the laser frequency fluctuations does not seem to come from the outside temperature.
 

the last thing we did with Daniele, is to start/stop the airflow on top of the housing (closed) to see a possible pressure effect on the laser frequency drift.

fig 1 : in green, the temperature measured with the probe stuck on the laser housing.
one can clearly see the 2 "start - wait ~10mins - stop" we did at 16h50 then at 17h30.
the air temperature blowed by the airflow is cooler than the housing temperature and we see the effect on the probe.

fig 2 : this is the laser frequency drift during the 1st airflow start/stop
the airflow has been turned on at 100 iterations and stopped at 500 iterations (~5 mins)
we don't see any correlation

fig 3 : this is again the laser frequency drift during the 2nd airflow start/stop
the airflow has been turned on at 1850 iterations and stopped at 2550 iterations (~10 mins)
we don't see any correlation

CONCLUSION : neither external temperature change or pressure variations can explain the 10-20min period oscillations observed on the laser frequency variations.
it can be either the laser temperature regulation or the RF reference oscillator temperature regulation (due to the oven of the quartz)

today, I swapped the 500MHz RF reference oscillator for a Siglent 500MHz DDS oscillator.

see the attached plot : the beating frequency with the 500MHz laser harmonics produces the same behavior as before.
so, the oscillations should come from the laser temperature regulation.

long-term correlation, over 5-6 days, between the temperature measured in the bunker, outside of the housing (blue curve) and the temperature measured with a probe stuck on the laser housing, inside of the FP-cavity housing (green).

it's a perfect correlation with almost the same temperature scale : 1°C outisde the housing => 1°C of laser housing

thus, a stabilization of the temperature, inside of the housing, could help to reduce the frequency drifts of the laser.

this morning, I did a 500MHz beating frequency (Laser vs RF) drift test with the Smaract motors configured with "Sensor Mode" OFF over 50 minutes (~ 1500 acquisitions)

compared with previous measurements, one observes a much more smooth "exponential-like" drift compared to "Sensor Mode" ON (see previous posts).

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...

this morning, with Vincent Chaumat, we made a beating (phase detection) between the laser 33MHz and the 33MHz coming from the Rigol generator locked by the 10MHz link onto the 500MHz ring generator.

once the laser and the FP-cavity are locked on the RF reference, the 33MHz phase detection is stabilized and confirm the laser/FP-cavity lock on the RF reference.

we plan to make a drift measurement and a jitter measurement to estimate this lock quality.
the 33MHz frequency being quite small, the quality of this estimation will be poor but it can give a "worse case" estimation.

 today, I locked the laser on the FP-cavity and the FP-cavity on the RF reference with 47kW inside the FP-cavity.

equivalent jitter :

when lock is OFF : sine signal V=V0*sin(phi(t)) with V0=300mV => Vrms = 300/sqrt(2)) = 210mV rms

when lock is ON : noise 10mV < dV < 20mV 
for low phase values : dV=V0*dphi

when the RF locking is done, this voltage is about dV = 20mV rms => dphi = dV/V0 = 67 mrad
dphi = 2*pi*f0*dt => jitter dt = dphi/(2*pi*f0) = dV * T0/(2*pi*V0)

T0/(2*pi*V0) ~ 1ps/mV => jitter ~ 10-20 ps rms

previous measurements were exhibiting a jitter lower than 5ps rms : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/299

previously, we have connected motor M4 with an IcePap controller.
see this post : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/278

due to the backlash behavior of the FP-cavity motors, it is impossible to work in both directions on one motor when the FP-cavity is locked.
so, we decided to use also the motor M1 to work in the other direction.
then, normally M1 will work in one direction and M4 in the other direction.

motor M1 (or P1Z) which was on the axis 9 on the ISP controller has been assigned to OC/OP/OCH.01-MOT.06

motor M4 (or P4Z) which was on the axis 12 on the ISP controller has been assigned to OC/OP/OCH.01-MOT.03

this double motors scheme works good.

if motor P4 is used by increasing value, the FP-cavityPZT signal (pink signal on scope) goes HIGHER
if motor P1 is used by increasing value, the FP-cavityPZT signal (pink signal on scope) goes LOWER

!!! CAREFUL !!!
after each session, it is mandatory to put the motors back to their initial position to avoid FP-cavity misalignment.
on P1 : - 900 000 steps
on P4 : - 360 000 steps

at the begining of each session, it will be mandatory to avoid any backlash for the motors :

- for the P4 motor : one can go 2k steps DOWN, then 2k steps UP to go back to -360 000 and use the steps UP without any backlash
- for the P1 motor : one can go 2k steps UP, then 2k steps DOWN to go back to -900 000 and use the steps DOWN without any backlash

today, I used the P1 motor to do some steps DOWN to find back the main resonance at the begining of the session.

this morning, we locked the laser on the FP-cavity with about 48-49kW stable in the FP-cavity (for 32% of amplifier ratio).
and then we locked the FP-cavity on the RF reference (500MHz beating lock + 33MHz beating for starting lock search).

the starting lock search voltage has been chosen between -20mV and +20mV.

then we lock with an uncertainty of 180°... but we can check the lock phase with the "Synchro oscilloscope" (192.168.229.21).
if necessary, we can stop the lock and let the phase drift and then relock with the right phase.

Kevin did a buckets scan (30ns by 2ns steps) to find the right bucket and also a fine phase tuning scan (2ns scan).
unfortunately, the fine phase was close to 0, so the Laselock did the fine phase adjustement by changing the SetPoint of the RF loop to : -720mV

at this moment, we got relatively stable X-rays at an approximated flux of 1e10 photons/s

we still need to optimize phase, table position and improve the jitters...
 

this morning, I added a DET36 photodiode in reflection (I used a thin beam splitter) to better measure the coupling over the full beam size.
this DET36 photodiode is sent to the "laser locking" oscilloscope with the other signals :
CH1 :transmission
CH2 : reflection
CH3 : error signal  or  FP-cavity PZT (depending on the Laselock Monitor 1 signal configuration)
CH4 : laser PZT (Laselock Monitor 2 signal)

I kept the DET10 photodiode in reflection, but slightly misaligned to get the 500MHz laser harmonic without too much amplitude variation when locked (the lock  behavior changes almost the center of the beam).
the DET10 photodiode is sent to the "RF locking" oscilloscope on CH1

this morning, I plugged back the 4 power cables which supply all the equipments.

all the equipments have restarted without problem, but the shutter of the laser amplifier needs a ThomX safety system grant to let the beam propagate to the cavity...
and then continue the commissioning of the restart after Christmas shutdown.

the laser coupled into the amplifier is still measured at ~3mW at the input, on the Alphanov software, once the Amp stage are started at 0% of power ratio.

this afternoon, Kevin restarted the 3rd safety system of ThomX to grant the opening of the beam shutter => now the shutter can be opened.

after adjusting the phase for the PDH system, I was able to find back the laser/CFP lock.

after some minimal alignement, one gets back 50kW for 30.5% of laser amplifier ratio, as before the Christmas shutdown.

I didn't try to lock the FP cavity on the 500MHz RF reference clock as I'm not sure if it has been set to 500.25MHz (as before shutdown) or at 500.1MHz (the future RF frequency) which is very far and will require to move the FP cavity on a long rage.
=> I will ask to Vincent Chaumat to confirm the present RF frequency.

this morning, I set the Ring frequency at 500.25MHz and after switching ON the power supply of the amplifiers for the RF signal coming from the laser,
I saw the beating frequency  (between ring RF and laser harmonic frerquencies at 500.25MHz) that I canceled, playing with the Smaract motor.

Now, I need to adjust the FP-cavity length to follow the laser cavity length but the IcePap DS (controlling the FP-cavity motor) freezes too often and it is almost impossible to make the adjustment.
Kevin sent a GLPI ticket to try to solve this problem.... waiting for IT answer...

on last friday (12/01/2024), the IcePap DS issue was solved and I was able to control the FP-cavity motors properly.
I was able to quickly lock either the laser on the FP-cavity and the FP-cavity on the RF reference as before.

then, we can close these posts on restarting the equipments after the Christmas shutdown

we plan to use Electrically Tunable Lenses EL-10-30-C NIR to build the ThomX telescope but they need to be compatible with the powerful laser beam.

for CW regime, on the datasheet (see attached file), the optical damage threshold is 10 kW/cm².
with 70W laser power and 1mm² beam surface (which is much smaller than the real beam size), we are at 7kW/cm².

for pulsed regime, with Daniele, we can imagine that the inner material is water (n ~ 1.3 in the datasheet).
One found a paper about laser induced electric breakdown in water (see attached paper) which give a breakdown field of about 1e8 V/m (see Fig. 1 of the paper) for 7ns pulse width.
with 70W average power, 33MHz repetition rate, 10ps pulse width 1mm² beam surface, one has an intensity of ~ 2e11 W/m² (or 2.7 GW/cm²)
I = E² / 377 => E = 8.7e6 V/m which is also much smaller than the breakdown value.

from the fig. 4 of the paper, it seems the breakdown threshold increases a lot for shorter pulses from 30 GW/cm² (~10ns pulses) to ~150 GW/cm² (~10ps pulses)
so, one can expect a good behavior in pulsed regime too.

can anyone check the validity of these rough calculations ???

this morning, I shut off all the equipements (especially the computer) except the ones powered by the UPS : laser controller, laser peltier controller, laser motors controller.
and I disconnected the cables from the power plugs of the wall (to prevent any possible overvoltage).

Jean-Noël Cayla will need to shut off the electrical shelf (the one powering the UPS) during the afternoon, I will check at this moment if the laser controller is still properly powered thanks to the UPS.

this afternoon, I checked visually what happens when the AC power of the UPS is broken down => the laser controller has still its "emission" button ON... no AC interruption.
so, I guess the UPS switched properly the output AC line from the input AC line to the internal batteries.

as the CFP motors displacements induce a cavity unlock, we try to change the cavity length by changing the temperature of elment of the cavity.

yesterday, we tried to put a "heating cable" borrowed to the vacuum group to change the temperature of one bellows between the FP-cavity and the electron ring.
we chose a below because it is flexible and should not apply a too strong force on the cavity vessels.
we heated the cable at ~30°C but we didn't see a clear effect on the FP-cavity frequency measurement.

then, we put a heating cable around the X-ray output flange of the FP-cavity vessel and we saw a clear effect : a relatively fast (at a "second" level) frequency change.
the problem is the heating system is not remotely driven.

the cable is R=55ohms impedance and can reach 450°C for 1kW dissipated.
so today, we will try to use 2 remotely controlled Siglent SPD3303X power supplies.
they can reach V=120V DC => P=V²/R=260W => we could reach more than 100°C

several days ago, we installed a heating cable around the output flange (close to the X-hutch) of the FP-cavity vessel.
we are able to heat this cable by applying a DC voltage coming from 2 DC voltage supplies in serie (2 devices with 2x 0-30V / 3A => 0-120V / 3A).
currently, we apply at maximum 7V DC on each channel => 28V DC on the cable. its temperature reach ~ 30°C after 1/2h or more.

this temperature increase on one vessel of the FP-cavity, changes very slowly the length of the FP-cavity.
see this post (red curve on fig. 2) for a typical temperature curve measured (exponential-like curve) on the heating cable itself : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/290

the length change rate of the FP-cavity, due to this heating, is difficult to estimate because it depends on the part of this exponential curve you make the measurement.
but globally the drift is about 50nm every 5 minutes, which is roughly equivalent to 10 steps of the FP-cavity motor every 5 minutes.

today, I removed one of the DC voltage supply (as with 30V, we are already able to 30°C on the heating cable).

the remaining DC voltage supply is configured in series (2x30V = 60V max) and is at the IP address : 192.168.1.101

this afternoon, I started the heating cable after the warm-up of the FP-cavity :

14h20 : start to fill 50kW in the cavity
15h : start of the heating cable @10V (2x 5V) (iteration 2300)
15h16 : start of the heating cable @60V (2x 30V) (iteration 3400)
15h20 : start of the heating cable @20V (2x 10V) (iteration 3600)

the offset frequency, for 20V of voltage on the DC voltage supply, is 175Hz @500MHz (equivalent to 3.5µm)
 

last friday, I locked the cavity and then, switched ON the heating cable after waiting the cavity reaches a steady state in frequency drift : see the figure.
blue : temperature of the probe on the heating cable
red : freqeuncy drift between FP-cavity (laser locked on it) and RF reference oscillator.

@ 10min : filling the FP-cavity with 50kW
@ 50min : "long" lock loss of about 1min => jumb in frequency => it is strange to see the cavity does not reach the same frequency steady state after the lock loss than before the lock loss.
(just before the small frequency jump, the frequency is increasing a bit because of the lock has been lost and the laser is drifting by itself in open loop. the jump, after that, corresponds to the moment where the laser is locked back to the FP-cavity)
​@ 55min : switch ON the heating cable with 20V on the voltage supply => the probe stuck on the cable shows the temperature increases => and a bit later, one can see the frequency decreasing.

surprisingly, we don't reach a steady state ...
I suspect that filling the FP-cavity with power (without switching ON the heating cable) could first inscrease the frequency and later could decrease it if several parts of the mechanics are involved and if the thermal conduction is slow because of some isolation embbedded in the mechanics (for example ceramics balls).

it is possible to make the frequency drifting sensitivity test of the heating cable without locking the cavity at 50kW.
one can work in open loop, without power in the FP-cavity, and follow the frequency drift by keeping the main resonance at the same relative position to the laser PZT.
 

on last tuesday, I did a long-term measurement of the frequency drift when the heating cable is powered ON at 20V (2x 10V) @ t=0mn.

the freqeuncy drift (between the laser, locked in open loop on the FP-cavity, and the RF reference oscillator) acquisition was made by the National Instrument software.
the sampling rate of this acquisition is not set by the user but depends on several parameters of the acquisition... and then can be subject to change.

on can see on the data, in blue, a possible sampling rate change @ t~30mn.
@ t~100mn : the frequency sign changes but the measurement gives always the absolute value.

in orange, the fit is not very good... possibly due to the sampling rate change.

the conclusion we can have is only this frequency drift action has a very long time constant ~ 2h
and could be used to compensate, as soon as the power is in the FPcavity, the frequency drift coming from this power.
 

today with Fatematuj, we did a long-term frequency drift measurement between the FP-cavity loaded with 50kW and the RF reference frequency... and without any other thermal source (no heating cable !)
we had a small lock loss at t ~ 120mn.

we see at the begining an exponential behavior of the FP-cavity reaching a thermal steady state until t ~ 40mn, and then we see the frequency slowly drifting in the reverted direction...
which is the evidence of the thermal source reaching a different mechanical part of the FP-cavity for which an increase of temperature induces a reverted change in frequency.

the very long delay between these 2 opposite behaviors could come from the ceramic balls on which the big mechanical mounts of the mirrors are placed.

we have to redo this measurement with a longer period to see if and when we are reaching a steady state !

Today, after laser was modelocked, we checked the power inside the fiber coming from the Alphanov Strecher box.
it was almost zero, at the nW level.

after doing some very rough alignment, we clearly saw some power (with the powermeter) correlated with the position of mirrors, when turning the alignment knobs.
this is a first step but the output power in the fiber is still very low, about 20nW !

an additionnal 2mm L-shaped hex key is needed to do some walking alignment on the mounts.

This morning with Viktor, we started the alignment of the CVBG and fiber.

we did a better alignment of the 2-pass CVBG.
we are able to the see a spot after PBS (after the 2 CVBG pass) which means the alignment is OK even if it can be improved.

then, we started the fiber injection alignment with the 2 last mirrors (7 & 8 on Alphanov documentation).
we saw that if we unswcrew the fiber to play on the focal position, we are able to improve a lot the power in the fiber.
2-3µW with the fiber screwed => 500µW with the fiber unscrewed.
it means the beam is not enough focused.
I will ask Guillaume Machinet his advice when injecting the 33MHz... do we need to replace the long focal lens just after the Isolator ?
and with which value approximatively ?

we played also with the Schafter+Kirchhoff mount of the fiber colimator (see attached documentation).
we loosen the 2 screws around the eccentric key which adjust the focal position + play on this eccentic key + tighten the 2 screws again.
now, we reached 330µW with the fiber properly screwed.

we have to check the available power before the fiber injection but we have very few place to place the powermeter.
maybe with a small mirror ?

In preparation for tomorrow morning, I did a test with a 45° mount to have a power pickup inside the injection box.

I used the 133MHz laser.
output power measured with the powermeter : 45mW
output power measured with the powermeter + OD2 : 2mW
output power measured with the 45° mount with BB1-E03 mirror with the powermeter + OD2 : 2mW

as expected the BB1-E03 mirrors have a very good reflectance for AOI=45°
the specs give >99% @ 1030nm for both S and P polarizations.

This morning with Manar, we measured the power at different points:
- direct measurement at the output of the 33MHz laser : 35 mW
inside the injection box :
- after the Isolator + focalization lens : 35 mW
- just at the input of the strecher CVBG : 35 mW
- atfer optimizing the alignment of the double path CVBG, in between the 2 fiber injection coupling mirrors : 5,7 mW
(the power is measured after going through the quarter-waveplate and PBS)

I found an old post from Loic claiming that with 27mW input power, we got 9.6 mW after the PBS instead of 5,7 mW !
https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/70
we have to check if the laser wavelength shifted, if the alignment could be improved, if the quarter-waveplate has the right angle,....

then, we tried to couple this 5,7mW inside the fiber using the schafter-kirchoff mount (SKM) but it is a nightmare.
changing the focus and the internal fiber angle is very sensitive, not always predictable and rarely reproductible...
I have to ask Guillaume how he used this mount...
the best power we saw in the fiber is 1mW but after screwing the fixing screws of the SKM, we lose almost all the alignment and we have something around 100-300µW...

I used the 33MHz spectrum measurement : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/153
fitted by P=P0*sech²((f-f0)/df)
f0 = c/1030m
df = (c/1030nm^2) * 2.5nm

and the strecher CVBG measured data (in attached file with reference D24-02)
to estimate the expected power to be coupled into the fiber.

the corresponding plot shows 3 curves:
- black : the 33MHz laser spectrum "manually fittted" with the sech² function (mentionned at the begining of this post) to match the measured spectrum from the elog.
- blue : the strecher CVBG reflectivity curve from the Excel measured datasheet in attachement.
- red : the corresponding output power after a double path into a CVBG (the reflectivity is applied twice).

with this simulation, one can estimate the power after CVBG to be 12.5 mW for 35 mW of input power.
or 9.6 mW after CVBG for 27 mW of input power which is exactly what has been measured in a previous post by Loic : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/70

Manar + Victor, today in the morning we aligned the CVBG of the stretcher.

The double path using the mirror mounts, reached up to 5.4 mW just before the injection into the fibers

then we rotated the quarter wave plate and the power increased by a factor of ~ 2 to 10.3 mW

we started with the injection into the fiber by alignment of the 2 mirror mounts.

1) fiber not connected and have everything pass through the mount

2) connect fiber loosely and increase the power in fiber bit by bit until fiber fixed fully

reached power up to 438 uW

alignment not finished using the mounts, there is still also schafter-kirchoff mount.

This morning with Manar, we continued the strecher CVBG alignment and fiber injection procedure :

- we checked the power at the output of the laser and confirmed the measurement after the Isolator : ~ 36 mW
- we tried to improve the power after the strecher CVBG by rotating the quarter-wave plate but it seems we were already at the maximum : ~ 10.5 mW
- we aligned the 2 fiber injection mirrors + schafter-kirchoff mount z-axis : we saw 5 mW at the output of the fiber but it is very difficult to keep the power after swcrewing the 2 z-axis fixing screws of the schafter-kirchoff mount.

we decided to replace the 750mm focusing lens installed by a 1000mm lens.

- we aligned again the 2 fiber injection mirrors + schafter-kirchoff mount z-axis : now, we have ~ 6.2 mW stable at the output of the fiber after screwing the 2 z-axis fixing screws of the schafter-kirchoff mount.

the procedure for aligning the schafter-kirchoff mount is :
- unscrew the z-axis and tilt fixing screws (5 screws in total).
- improve the injection with the z-axis knob and the tilt screws
- tighten very softly all the fixing srews once the optimization is finished => you will lose a part of the alignment but not completely (~ 1 mW level)
- redo a part of the alignment with the 2 aligning mirrors => you should find back the values after optimization.

we let the power-meter at the output of the fiber to check in the next days if the injected power in the fiber changes or not....

it is important to not "kick" the fiber injection box or put anything on it ! as the schafter-kirchoff mount adjustment is soooo touchy...

this morning : still 6.2 mW at the output of the fiber.

the powermeter has been removed and the fiber connected to amplifier input fiber (no EOM connected in between).

Monday 7/11 morning, the power in the fiber was still 6.2 mW

Today, the power measured at the input of the amplifier (PD_IN on the LAL amplifier software) is 2.5mW instead of >3mW generally measured

The power coming from the NKT/Onefive Origami oscillator is still >37mW (measured directly with the powermeter at the laser output without OD2).

thus, the problem should come from:

- the strecher/fiber alignment.

- or maybe from a wavelength shift of the oscillator

today, the Alphanov amplifier stopped because of the too low injected power : ~2.5mW (measured by the Alphanov software)
it was still working at ~2.7mW.
normally, the injected power is ~3.1mW (equivalent to 6.2mW measured directly at the fiber output).
I checked the power coming from the OneFive oscillator : it is still 36mW, identical to the power measured the first day we installed it.

by moving the "focus" knob of the mount, we saw this 3.1mW back but it was impossible to get it in a stable way.
so we think the "strecher box alignment" is still good and we decided to realign the Schafter-Kirchhoff fiber mount.

up to now, we lost all the coupling in the mount, then we improved the power at the fiber output to 4.2mW, then we lost everything again.
we will continue this afternoon.

this afternoon, I was able to go back to 5.4mW in the fiber output (3.2-3.3mW in the Alphanov software).
IN CASE OF SMALL LOSS OF POWER IN THE FIBER : DON'T TOUCH THE SCHAFTER-KIRCHHOFF MOUNT
it is too sensitive... open the Strecher box and do the alignment improvement with the 2 final (alignment) mirrors in the box.
even if it is mandatory to adjust the Schafter-Kirchhoff mount, you will have to finish the alignment with the 2 final (alignment) mirrors of the Strecher box.

Now, all the Schafter-Kirchhoff mount screws are well tighten !
So next time, use the Strecher box final mirrors to improve the injection into the fiber.

after the new fiber alignment, I was able to lock "as usual" the cavity to 47kW @30% of laser amplifier ratio.

This morning with Ronic, using a pickup mirror, we measured the power in different locations along the path of the stretcher box

Output power from oscillator : ~35mW

After the isolator : ~ 35 mW

just before the CVBG : ~ 35 mW

just before the fiber injection : 3 mW

After improving on the mirrors directly correlated to the first line in CVBG and the second reflection line (mirror 4 and 6 on the Alphanov documentation)

we managed to increase the power output just before the fiber to 5.7 mW

To confirm it is indeed the second reflection we see, when cutting the second line and the power drops to almost sero (~70 uW)

We will check the CVBG documentation if it is the maximum power we can obtain after the it due to it's bandwidth !!!!

for the fiber injection :

First without connecting the fiber we checked power after Schafter+Kirchhoff mount we measure 5.5 mW (all power pass through)

when connecting the fiber, we get 300 uW after a quick alignment on the power meter + OD2.

The issue of being too much sensitive arises from the Schafter+Kirchhoff mount, after adjusting the tilt screws and Black knob we got 1.05mW in the fiber for a quick moment.

we had at one point a stable 811 uW into the fiber, but when fixing the mount screws the power drops significantly to 150 uW and it is very difficult to reproduce.

The Schafter+Kirchhoff mount is very difficult to align, waiting on advice from Guillaume Machinet.

the 2 files describe the specfications for the16  mirrors ordered (4 for ThomX + spare, 4 for SBOX + spare) and the measurements made by the LMA.

this morning I monitored the frequency shift between the FP-cavity frequency (the laser is locked on it) and the RF reference frequency @ 500MHz,
during the warm-up time, for 50kW stored in the cavity (the recording started ~ 9h20).

one observes on fig.1 a simple exponential behavior with 500Hz frequency shift @500MHz (equivalent to ~ 10µm) for 50kW stored. 
the time constant T of the exponential curve is 12.5 minutes and the stable region starts at 5T ~ 1h.

the small drop at the end of the curve ( ~ @10h30) could come from the external temperature which started to drift before (see fig.2)

today for the first time, we were able to make a long term lock of all the elements (Laser on FP cavity and FP-cavity on RF reference) during 20 minutes @50kW without any single loss of lock.

the success of this long term lock was coming from the possibility to drive the Smaract motor in piezo-scan mode and to apply a heating on one vessel of the FP-cavity to let its frequency slowly drift always in the same direction.
then, the FP-cavity motor can follow this drift without any loss of the lock.

even when we lost the lock, it was quite easy to get it back because of the frequency drift, always in the same direction due to the FP-cavity heating with the heating cable.

a good strategy to make this double lock :
1) adjust the laser frequency to the RF reference frequency in using the Smaract motor in "closed loop" mode until reaching a beating < 10Hz.
2) search for FP-cavity resonance in using the ISP motor of the FP-cavity
3) once the laser is locked on the FP-cavity, the FP-cavity length changes => work on the FP-cavity motor to follow the drift until one reachs a thermal equilibrium.
4) once the thermal equilibrium is reached, one can switch on the heating of the heating cable to continue the slow "thermal" drift in the same direction 5V of total voltage should be enough for 1h of work.
5) in the same time, one need to cancel the frequency offset between the RF reference and the FP-cavity using the FP-cavity motor and one need to follow the laser cavity fluctuations using the Smaract motor in "closed loop" mode.
6) once the FP-cavity is at the same length than the RF-reference, one can close the second loop (when both PZT are in the middle of their range).
7) at that point, one need to swap the Smaract motor control to "piezo scan" mode => one follows only the laser fluctuations ~ 1µm peak-peak
8) one always follows the FP-cavity drift using the IcePap controller on the FP-cavity motor, 10 steps by 10 steps => it should work during 1h.
9) when one reachs the thermal equilibrium again, one can add 5V to the heating cable voltage.
 

today, I did a 20 minutes "double lock" run w/o any single lock loss.

the attached file shows the recorded RMS beating voltage between 500MHz signals (laser harmonic and RF reference).
the end of the plot after iteration ~1.8k shows the lock loss to the RF reference.

when the lock is not working, one gets a sine signal V=V0*sin(phi(t)) with V0=288mV => Vrms = 288/sqrt(2)) = 204mV rms
(this signal is not on the figure)

for low phase values : V=V0*dphi
when the locking is good, this voltage is about V = 3.5mV rms => dphi = 12mrad rms
a full beating signal period (500MHz => 2ns) corresponds to 2pi, so dphi = 12mrad rms => jitter dt = 4ps rms

this afternoon, we did a new long-term "double lock" run w/o any single lock loss during 1/2h.

we did 2 acquisitions :

fig 1 : phase measurement between the 33MHz signal coming from the laser and the 500MHz RF.
this plot doesn't last 1/2h.
the measured jitter is ~45ps, at the limit of the scope resolution.

fig 2 : same phase measurement between the two 500MHz signals (laser and RF)
the lock is lost at the end of the plot.
the measured voltage noise is Vrms ~ 2.5mV rms => jitter ~  2.8ps rms

the conversion factor between jitter and voltage is 1,1 ps / mV

!!! CAREFUL !!! heating the FP-cavity with the heating cable works in the opposite direction of the heating due to power in the cavity !

current Smaract motors parameters : 
- Closed loop Max frequency : 5000
- Signal Amplitude threshold : 2047
- High voltage threshold : 511

when one drives the Smaract motors in "closed loop" mode, one can get a displacement as small as 50nm... but at the price of a delock of the laser/FP-cavity.

when one drives the motor in "open loop" mode, 1 step is equivalent to 4µm !!! it is much larger than the laser PZT range.

when one drives the motor in "Piezo Scan" mode with a speed of 1V/s, one can move the motor without losing the laser/Fp-cavity lock.
the PZT voltage range of 100V (max value) is roughly equivalent to 2-3µm of round trip length, which is enough to manage several "fast" (10-20 minutes) oscillations of the laser frequency :
see these posts to get some info on the laser frequency oscillations : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/289