we have had this new issue (watchdog alarm switching off the amplifier) for several days.

Alphanov mentionned it could come from the input oscillator stability (modelock loss for example).

Today,

- I tried to start the amplifier at 0% but it ended immediately in a "watchdog alarm" which stopped the amplifier
the input average power detected by the software was around 5.2mW, far above the 2mW needed by the amplifier.

- I checked the 33MHz oscillator at the output of the fiber connected to the Shafter-Kirchhoff fiber coupler with the Labbuddy fiber photodiode and a scope.
see the picture below : the peak are clean and at 33MHz... then, the Onefive oscillator is not faulty.

- after plugging back the fiber to the amplifier input, the input average power detected by the software increased a bit around 5.5 - 5.6mW.
I tried to start again the amplifier at 0% and now, it worked without any alarm !!!
then, the origin of the "watchdog alarm" was maybe the threshold on this input power.

Then, I launched the "Alphanov" software and changed again the "threshold level", previously set at 1V (see previous post below), to 0.96V !
with this threshold level, the Alphanov software detects at the beginning a frequency of 66MHz and then decreases slowly to 33MHz => to be understood.
it should allow also more room before triggering again the "watchdog alarm" if the input average power decreases a bit => to be confirmed by Alphanov.

With Daniele after, we took a readings of the beam with a lens of 750 mm.

From the observed data, we can state that the beam is not completely Gaussian M2 > 1, due to the variation of the ellipticity of the beam along the focused path 

readings have been saved and will be added after processing them.

Also adding to the previous comment about the error, after closing the bunker and starting the measurement we did not have issue for around 20 min then the same error appeared (alarm triggered by Watchdog) it shows when the input average power detected by the software was around 5.474 mW , I did a reset of the software then turned the amplifier on and the reading is 5.515mW it works for about 2 minutes (enough to take 2 readings) then it switches off. In addition, there was a drop in the power reading on the PD_preamp2 (which seems related to watchdog issue) we had to do this many times in order to finish taking the readings .

This morning,

- with the "Alphanov" software => Central panel => "system" button => Watchdog "Period Max" was at 100ns and has been set to 200ns.
(Alphanov told me to change the value from 50ns to 100ns but it was already at 100ns, then I set it to 200ns).
=> to be checked with Alphanov : what is the meaning of this parameter.

- it seems the watchdog alarm is related to the "PD_IN" parameter value of the "LAL" software (5.5mW is OK, but 5.2mW triggers the alarm).
my understanding is the Threshold Voltage, which detects the input signal to measure the repetition rate, could be at a too high level... even very close to the max level of the signal.
then if the input decreases a little bit, the signal goes below this Threshold Voltage => the software does not detect a signal any more => it triggers the watchdog alarm and the amplifier stops
(which stops also the preamplifier, related to the PD_CRI of the "Alphanov" software which goes to ~0W. PD_Preamp2 is a copy of PD_CRI but in the "LAL" software).

then, I changed back the Threshold Voltage from 0.96V (to detect properly 33MHz rep rate) to 0.7V which is close to the original value (0.67V) but for which the software detects ~100MHz rep. rate !
with this Threshold level, I did a test by detuning very slightly the input power by unscrewing a little bit the fiber-fiber coupler => PD_IN = 4.9mW => no alarm triggered !
then I screwed back the coupler => PD_IN = 5.6mW.

I did a power test with 3rd stage of the amplifier at 20% during 15 minutes => no alarm.

conclusion : there is an issue with the rep rate detection.
at 33MHz, one needs to put the threshold at the limit of the signal which fires an alarm if the signal decreased a little bit.
by lowering the threshold => no more alarm (I hope) but a wrong rep rate value !

This morning with Daniele, we did a second measurement of the beam profile using a different lens of focal 400 mm.

at a power amplification of 20%

the amplifier stayed on for around one hour and a half with no errors. 

tomorrow will attemt to do one at higher amplification percentage.

data will be added after processing.

On Monday 20th of February, Daniele and Viktor took some beam profile measurement data @ 50% of power amplification.

data and analysis can be found in the attached file.

This morning with Victor we took a reading of the beam profile using a 400 mm lens at 70 % and 100 % amplification.

Will process the data and add after

Note : after an hour and half of amplifier on at 70% we increased the power to 100%, and after ~ 10 minutes the amplifier turned off and showed an error on the software called MMD3

and an error of the power mentioning " Le laser a été arrêté parce que sa puissance est trop éloignée de la puissance nominale"

"The laser has been stopped because its power is too far from the nominal power"

after that, the amplifier worked in varying intervals until it switched off because of the previously mentioned error.

solution could be related to the configuration file values ??????

this morning with Manar,
we checked the Alphanov amplifier issue : "The laser has been stopped because its power is too far from the nominal power".

the photodiode voltage measuring the output power has drifted again and then, the estimated output power was wrong, triggering an alarm issue.
I changed the "calibration" file to fix the issue : see the attached file
 

This morning with Manar, in order to prepare the installation of the news FP cavity mirrors,

- we installed the Koheras below the optical table
- we put a fiber prolongator at the end of the Koheras fiber and we put a mounted collimator at the end of the fiber.
we added a mirror at 45° to inject the beam to injection axis of the table.
- as the beam was still slightly divergent, we added a 750mm lens ~20cm after the collimator to improve the beam collimation.
now the collimated beam is going approximatively through the middle all of the iris in the optical path.

we finished by ~2h of cleaning of the optical table and elements, which were very dirty.
it is still a bit dirty. see the attached file to get the numbers provided by the particle counter after the cleaning.

This morning with Ronic , in preparation for the mirror installation, we removed additional mechanics and cables from the table. 

We organized and cleaned the permanently installed cables on the table away from the area of the cavity opening. 

We opened the two top vents and left the shutters open to circulate clean air , (note the shutters surrounding the laser and amplifier are half closed for safety)

Note: before installing the mirrors, preferably the shutters should be opened a day before to have clean air in the surrounding of the cavity openings

and avoid any cleaning or air disturbance.

We took reading of the beam profile of the amplifier without a lens @ 30 cm from the 2nd wedge for amplification percentages

20-30-40-50-60-70-80-90-100 % another additional 3 readings will be taken at different locations for all amplification percentages.

All the equipments have been switched off on Friday 17th due to a power shut down on Saturday 18th :
CW and Onefive oscillators, computers, laser amplifier, cavity mirror motors, chiller, etc...

As the general local network for the FP-cavity devices is 192.168.xxx.xxx, I changed the IP adress of the smaract laser motors (Frep and CEP) controller.

For that, I used the embedded web server as described in the manual in attached file.

the new adresses are :
IP :             192.168.1.10
Gateaway : 192.168.1.1
Subnet :      255.255.255.0
Port :          5000

Today, I restarted all the equipements :

Chiller                                                 : ok, temperature converged quickly to the set temperature : 25°C
Cavity mirrors motors controller         : I had to configure the general network as 192.168.xxx.xxx in order to operate the software designed by Didier Jehanno
Computer                                           : ok, now, it IP address is 192.168.1.1
Laser amplifier controller                    : I had a LOT of issues in restarting the LAL software. I had to restart several times the amplifier controller in order to access it by software. I quickly check the power increase to 50% after it has been correctly seeded
CW oscillator                                     : ok, Koheras restarted without any issue.
OneFive oscillator                             : the power ON was ok but it did not modelock. I tried several gently knocks on the case but without success (one clearly see the effect on the scope, the oscillator "tries" to modelock).
                                                            fortunately, after about 1h, the oscillator modelocked alone (check with Buddy fiber photodiode + scope).... the power is the same as before. the Alphanov software measures 5.4mW (it was 5.5 mW the 2/1/2023)

Smaract controller for OneFive        : ok, I changed its IP address to 192.168.1.10. I tested the PC control in moving forth and back the "CEP" motor by 1mm... the OneFive was still modelocked.
 

due to several "case temperature" issues, I change the "Max case temperature" in the "general config" of the "Alphanov Software" from 50°C to 55°C.
=> validated by Alphanov.

the last tries to lock the 33MHz + amplifier to the 30k Finesse FP-cavity were unsuccessful.

during a laser Frep scan using the Laselock, one observes that the main cavity resonance is not able to stay inside the PZT scan range from one scan to another (500ms-1s period)
is it the effect of a large and slow phase noise ?

some informations:

- The 33MHz laser came back at lab from repair on March 2018.
- it has been sent to Alphanov in May 2020.
- it failled and has been sent to NKT/OneFive for repair in September 2021
- it came back to lab from repair in June 2022.
- on post #92 (Feb. 2020), it seems that we already locked the 33MHz laser + CELIA amplifier to the ThomX FP-cavity.

- The PZT sensitivity for the 33MHz laser is given to 0,3Hz/V for Frep <=> 2.6MHz/V for optical frequency.
=> 10V on PZT is equivalent to 26MHz of optical frequency shift which is less than FSR !

- by comparison, the PZT sensitivity for the 133MHz laser is given to 3.9Hz/V for Frep <=> 8.5MHz/V for optical frequency.

- by comparison, the PZT sensitivity for the NKT CW laser is given 10pm/100V for Wavelength <=> 30MHz/V for optical frequency

- by comparison, the PZT sensitivity for the ThomX FP cavity (Z20H38x40C) is 4nm/V for length expansion => 8nm/V for round-trip expansion <=> 0.03Hz/V for FSR expansion <=> 260kHz/V for optical frequency !!!
the PZT expansion estimation is in attached file.

finding the right modulation/demodulation PDH phase is very difficult on the main resonance because the we get non stationnary signals with a lot of oscillations.
changing the phase, in this condition, does not really change the error signal.
Then, we moved on the first secundary resonance with less gain and less coupling.
Thus, the error signal is more similar to the theoretical PDH signal => one can adjust the modulation/demodulation PDH phase to get the maximum error signal.

then, we locked pretty easily on this first secondary resonance, with a coupling around some % when we adjust the CEP motor.

we tried to lock on the main resonance but it is too noisy and unstable.
it seems we really need high BW feedback.

I tried to add a fast analog loop on the laser intra-cavity EOM but without a clear effect.
the problem is the gain of this loop : it is difficult to produce a "high voltage" (above 10Vpp) on this EOM.
I put "my" amplifier but the voltage output is limited... commercial amplifiers will have the same issue.
we can add HV amplifiers but it takes place and it will add some noise on the signal.

A loop with an AOM could be easier to install and manage... but at the price of a loss of power before the laser amplifier...
 

today with Daniele, we locked easily (but with a noisy lock) on the secundary resonance and we tried to lock on the main resonance (with very low coupling ~10% which mean a CEP ~Pi)
the lock was possible but was very noisy.

I installed a fast loop using my small DC amplifier based on OP37 (max gain=100) modified to be AC coupled to avoid to amplify the PDH box offset.
the output votage swing of the OP37 is only ~10V. Thus, the effect of this fast loop on the lock stability is not visible !

Thus, I added the M250 Leysop HV amplifier (see attached documentation), which is able to drive an EOM with >5MHz bandwidth and ~250V swing, after my OP37 amplifier.
with this additionnal HV amplifier, now we can clearly see the effect of the EOM loop which improves the lock stability BUT, even with a poor CEP, the lock is very unstable on the main resonance.
it seems the optical phase noise is still too large and/or its BW too high to be completely compensated.

The next step is to try to remove all the possible noise sources from the optical table:
- the laptop placed on the ionic pump
- the 2 Rigol generators on the table surface
and switch off the controller of the Smaract laser cavity motors.

If it doesn't help, we can send the error signal to a spectrum analyzer to have a better view of the different harmonics involved in the residual phase noise.
could it remain some noise above the present PDH box BW (1.9MHz LP filter) ?

lastely, we can also make an optical phase noise measurement to check if the Alphanov amplifier does not add some noise.

After removing the 2 generators from the optical table, the lock is much more stable and now, it is possible to lock on the main resonance with a poor CEP but with quite good stability.
the coupling is still very low ~ 5% for that CEP but if one improves it (CEP ->0), using the laser double-wedge motor, one clearly sees an improvement of the coupling... but at the cost of the lock stability.

the reason of the poor coupling is also because the laser amplifier is used at 0%, for which we know the part of the laser signal power, compared to the total power, is low.
(a part of the beam @1030nm is not propagating in the fiber core of the amp, and then, it cannot be coupled to the FP-cavity).

the fast lock loop on the EOM has been disabled for the moment.
it has to be installed back to improve the stability at a better CEP.

at present, the FP-cavity power is estimated at ~ 90W (~270µW in transmission of ~3ppm mirrors) for ~300mW of total power coming from the laser amp.

next steps :
- in Open Loop : check what is the best coupling we can observe for CEP=0 @ P ~ 10W (laser amp at ~ 25%)
- in Closed Loop : @ P ~ 10W => measure the best transmitted power after alignement/polarization/feedback adjust => ~ 3-10kW in the cavity ?

this morning, I tested the laser+amplifier @ 30% lock on the FP cavity with and without Smaract motors.

I recorded the PDH error signal during a lock:
- blue   : with Smaract motors controller powered ON but motors are stopped
- yellow: with Smaract motors controller powered OFF

with Smaract motors controller powered ON and motors stopped, one can see a group of resonances around 10kHz (8 - 11 - 14kHz) which disappears when the controller is powered OFF.
one can see also a group of resonances around 25-30kHz for which some peaks desappear when the controller is OFF but most of them are still there... could it come from noise on the Onefive laser PZT ?
one can see also a noise reduction at low frequency with a corner frequency around 17kHz, which could be the Unity Gain Bandwidth of the feedback loop on the laser PZT (fast feedback loop on EOM was disconnected)
=> to be confirmed

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I was able to lock with a decent noise on transmission and reflection signals @ Pin=17W (30%) of input power and with a coupling ~ 20%.
I measured 31mW in transmission => Pcav ~ 10.3kW (T ~ 3ppm)
as T1=115 ppm and F=30000, the cavity gain is T1*(F/pi)^2 = 10.5k,
so, the FP cavity power should have been 17W * 20% * 10,5k = 35.7 kW !!! (maybe the formula is wrong if the coupling loss comes from the CEP detuning effect)
=> we have to check the incoming power and the formula !

so, maximum expected power in FP-cavity could be 70W * 100% * 10,5k * (10.3/35.7) = 210 kW !!! :-(

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I was able to redo the lock easily in remote in the control room (with the Smaract motors controller OFF).